Pre-Heat Treatment of Gas Atomized Al 2024 Powder

and its Effects on the Properties of Cold Spray Coatings

A Thesis Presented

By

Lauren Elizabeth Randaccio

to

The Department of Mechanical & Industrial Engineering

in partial fulfillment of the requirements

for the degree of

Master of Science

in the field of

Mechanical Engineering

Northeastern University

Boston, Massachusetts

December 2019

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ABSTRACT

Cold spray (CS) is a solid-state powder consolidation process used to produce

coatings of varying thicknesses and to build free standing parts. Micron-sized particles

are accelerated at supersonic speeds toward a substrate and experience mechanical and

metallurgical bonding upon impact. CS has a wide range of applications, both reparative

and structural. Commercially available CS feedstock includes gas-atomized powders

which possess rapidly solidified dendritic microstructures. Al 2024, Al 6061, and Al 7075

are three such alloys, widely accepted for fabrication and repair of aeronautical

components for their desirable strength-to-weight ratios. However, CS deposits often

lack in ductility and fracture toughness which is unfavorable for load bearing components.

At present, there is a need to develop new procedures and standards for high-

temperature powder processing, which will increase the reproducibility of CS coatings

and allow for manipulation of properties in the as-deposited state. Existing literature on

pre-heat-treating Al 2024 powders to improve ductility of as-sprayed parts is limited.

In this work, a study was conducted to investigate the effects of pre-heat-treating

Al 2024 powder on the properties of cold spray deposits. Three unique powder samples

were studied: (i) as-received, no heat treatment, (ii) solid solution heat-treated (495 °C, 1

hr), and (iii) annealed (415 °C, 2.5 hrs). Particles were analyzed for changes in size,

microstructure, and micro-hardness. Each powder sample was then cold sprayed on to

an Al 2024-T351 substrate using Helium as the processing gas. The deposits were tested

to determine micro-hardness and tensile properties in the as-sprayed condition.

Scanning electron microscopy revealed a dendritic/cellular microstructure within the as-

received powder, which was eliminated with both solution treatment and annealing.

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Particle hardness decreased by 33.4 % after annealing, but showed no significant change

after solution treatment, implying that full solution may not have been achieved.

Particles agglomerated during both high temperature heat-treatments due to sintering

effects, resulting in a measurable increase in size distributions. Solution heat-treated

powder produced a decrease in as-sprayed ductility and no significant change in ultimate

tensile strength (UTS), while annealed powder produced a decrease in as-spayed UTS and

no significant change in ductility. Elastic modulus remained constant across all CS

deposits.

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ACKNOWLEDGMENTS

I would first like to thank my advisor, Sinan Müftü, the Principle Investigator for

this work, for presenting me with the opportunity to join Northeastern’s Cold Spray Team

so early on in my time as a Masters student. Your support and trust in me over the past

two years allowed me the freedom to complete a thesis as well as a degree program that

was dignifying to me, and for that I am forever grateful.

I am equally grateful to Ozan Ozdemir, my colleague, mentor, friend, and interim

advisor whenever Professor Müftü was unavailable. Thank you for teaching me the ins

and outs of all things cold spray. Your patience, guidance and advice throughout this

process has been, and always will be, very much appreciated.

I must also acknowledge Northeastern’s Advanced Materials Processing

Laboratory (AMPL) and Professor Teiichi Ando. Thank you for treating me as if I was one

of your students, for providing your metallurgical expertise, and for allowing me to use

various equipment in your lab. And thank you Xingdong Dan, my peer and fellow Masters-

thesis student, with whom I collaborated in many ways. I am so appreciative of the many

micro-hardness measurements you performed for me in the AMPL.

Thank you Tricia Schwartz, research engineer at our Cold Spray Lab, for assisting

me with any and all things at KRI. Traveling to Burlington, MA sometimes four days a

week to perform experiments was challenging at times, but with your expertise and

generosity of time, I was able to accomplish so much more than I could have on my own.

Your overall support and kind nature made my experience in the lab much more

enjoyable, and it was so nice to have another woman around.

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The technical expertise of Bill Fowle and Wentao Liang from Northeastern’s

Electron Microscopy Core Facilities is also very much appreciated. Thank you Wentao

especially, for spending many hours with me doing SEM at the Kostas Research Institute

(KRI).

Thank you Professor Andrew Gouldstone for your materials science expertise and

overall support of this work in its early stages. Your genuine interest in my present and

future endeavors was always very encouraging.

Thank you Joe Conahan for assisting me with all of the tensile tests performed for

this work.

And lastly, I must acknowledge my colleagues Qiyong, Enqiang, Runyang, Scott,

and Salih, who make up the rest of Northeastern’s Cold Spray Team, as well as our office-

mate Soroush, whom I had a desk next to for the entirety of my time in this group. Your

kindness and willingness to collaborate made my time in this group much more pleasant

from day to day.

This work was financially supported by the United States Army Research

Laboratory through grant number W911NF15-2-0026. Any opinions in this thesis are

those of the author and do not necessarily reflect the viewpoints of the funding agency.

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DEDICATION

This thesis is dedicated to the young women of STEM.

You can do it.

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TABLE OF CONTENTS

ABSTRACT ............................................................................................................................ iii

ACKNOWLEDGMENTS .......................................................................................................... v

DEDICATION ....................................................................................................................... vii

TABLE OF CONTENTS......................................................................................................... viii

LIST OF TABLES ..................................................................................................................... x

LIST OF FIGURES .................................................................................................................. xi

1 INTRODUCTION ........................................................................................................... 1

1.1 Overview .............................................................................................................. 1

1.2 Research motivation ............................................................................................ 2

2 LITERATURE REVIEW .................................................................................................... 4

2.1 Cold spray ............................................................................................................. 4

2.2 Room for improvement ........................................................................................ 5

2.3 Pre-processing CS powders .................................................................................. 7

2.3.1 Solid solution heat treatment ..................................................................... 11

2.3.2 Annealing .................................................................................................... 15

2.4 Summary ............................................................................................................ 17

3 MATERIALS & METHODS ........................................................................................... 18

3.1 Material .............................................................................................................. 18

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3.2 Powder heat-treatments .................................................................................... 18

3.3 Microstructure characterization ........................................................................ 21

3.3.1 Metallographic Preparation & Etching ....................................................... 21

3.3.2 Microscopy .................................................................................................. 22

3.4 Particle size measurement ................................................................................. 22

3.5 Particle velocity measurement .......................................................................... 23

3.6 Micro-hardness testing ...................................................................................... 23

3.7 CS deposition process ........................................................................................ 24

3.8 Tensile samples .................................................................................................. 25

4 RESULTS & DISCUSSION ............................................................................................. 27

4.1 Microstructure characterization ........................................................................ 27

4.1.1 Powder ........................................................................................................ 27

4.1.2 CS deposits .................................................................................................. 29

4.2 Particle size distributions ................................................................................... 31

4.3 Particle velocities ............................................................................................... 34

4.4 Micro-hardness .................................................................................................. 35

4.5 Tensile Testing .................................................................................................... 37

5 CONCLUSIONS............................................................................................................ 41

6 FUTURE WORK ........................................................................................................... 43

APPENDIX .......................................................................................................................... 45

REFERENCES ...................................................................................................................... 49

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LIST OF TABLES

Table 1. Chemical composition of Al 2024 alloy provided by powder manufacturer (Valimet Inc., Stockton, CA). The main alloying element is Copper, at 4.16 wt-%. ................................................................................................................ 12

Table 2. Sample identification codes for Al 2024 powder in three heat-treated conditions. “-CS” is added to indicate a cold spray deposit produced from that particular powder. .................................................................................... 19

Table 3. Spray parameters held constant throughout the study. ................................... 24

Table 4. Statistical analysis of variance (ANOVA) of UTS and elongation-% data. If the P-value is less than 0.05, there is a statistical confidence of 95% that one of the data sets produced a mean that is significantly different from the other. ................................................................................................................ 40

Table 5. Complete list of tensile samples and their corresponding cross-section geometries. ...................................................................................................... 48

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LIST OF FIGURES

Figure 1. Relative temperatures for solid solution and precipitation heat-treatments [1, 2]. ................................................................................................................ 11

Figure 2. Aluminum-rich end of the aluminum-copper equilibrium diagram. Line AB represents the increase in solubility of Cu in solid aluminum as temperature increases [2-4]. Alloys containing more than 5.65 wt-% Cu (line CD) are incapable of achieving complete solid solution. ......................... 12

Figure 3. Relative temperatures for various annealing treatments. Full annealing was selected for this study. ..................................................................................... 16

Figure 4. Solid solution heat-treatment vessel made from steel hydraulic tubing. Ends of the tube are flanged to mate with compression fittings and create an air-tight mechanical seal when tightened. ................................................. 20

Figure 5. Flow chart of powder life-cycle during the study. ............................................ 20

Figure 6. Standard ASTM E8/E8M-16a tensile specimen geometry [2][4] . All dimensions are in inches. ................................................................................. 25

Figure 7. Modified ASTM E8/E8M-16a tensile specimen geometry. All dimensions are in inches. .......................................................................................................... 25

Figure 8. Cold sprayed tensile specimens prior to EDM removal from substrate. Tensile bars are made up of only cold sprayed material. ................................ 26

Figure 9. SEM images of Al 2024 particles from three heat-treatment conditions: (a, b) as-received, (c, d) solution heat-treated, (e, f) annealed. Cross-sections (left) were polished and etched to reveal microstructure. Images of whole particles (right) are only representative of their cross-sectioned counterparts and are not necessarily the same particle. ................................ 27

Figure 10. Optical micrographs of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed.

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Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. Black spots in images (b) and (c) are most-likely a result of the polish-etch procedure. .................................................... 29

Figure 11. SEM images of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. ................................................................................................ 30

Figure 12. Volume based cumulative size distribution of powder particles from three heat-treatment conditions. .............................................................................. 31

Figure 13. SEM images of whole powder particles from three heat-treatment conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. ....... 32

Figure 14. ZEISS optical images of whole powder particles from three heat-treatment conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. Particles were placed on a glass slide with a light source positioned below, so particles appear as black blobs. ................................................................... 32

Figure 15. Al 2024 powder from three heat-treatment conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. Images were taken immediately following heat-treatment, prior to any sieving or breaking up of agglomerated particles. ................................................................................... 32

Figure 16. Overall frequency based (left) and volume based (right) particle size distributions of Al 2024 as-received powder. .................................................. 33

Figure 17. Overall frequency based (left) and volume based (right) particle size distributions of Al 2024 solution heat-treated powder. .................................. 33

Figure 18. Overall frequency based (left) and volume based (right) particle size distributions of Al 2024 annealed powder. ..................................................... 33

Figure 19. Particle velocities of Al 2024 as-received powder captured by HiWatch HR1 System. The average particle speed was measured to be 1027.61 m/s with a standard deviation of 237.03 m/s. ........................................................ 34

Figure 20. Micro-hardness of Al 2024 powders from three different heat-treatment conditions and their subsequent CS deposits: (left) as-received, (middle) solid solution heat-treated, and (right) annealed. ........................................... 35

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Figure 21. Representative stress-strain curves from uniaxial tensile testing. Samples were produced from four different materials: wrought Al 2024-T351 substrate (substrate), cold sprayed Al 2024 as-received powder (AR-CS), cold sprayed Al 2024 solution heat-treated powder (SSHT-CS), and cold sprayed Al 2024 annealed powder (AHT-CS). .................................................. 37

Figure 22. Average (a) ultimate tensile strength (UTS), (b) elongation-% to fracture, and (c) elastic modulus from four sets of tensile samples. ............................. 38

Figure 23. Stress-strain curves from uniaxial tensile testing. Samples were produced from four different materials: (a) wrought Al 2024-T351 substrate, (b) cold sprayed Al 2024 as-received powder, (c) cold sprayed Al 2024 solution heat-treated powder, and (d) cold sprayed Al 2024 annealed powder. ......... 39

Figure 24. High magnification SEM images of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. ............................................... 46

Figure 25. High magnification optical micrographs of CS deposits made from three different Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the spray direction, then polished and etched to reveal microstructure. Black spots in images (b) and (c) are most-likely a result of the polish-etch procedure. ............................... 47

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1 INTRODUCTION

1.1 Overview

Cold spray (CS) is an additive manufacturing (AM) technology used to produce

coatings of varying thicknesses as well as to build free standing parts. Research on cold

spray has gained a lot of traction in the past 20 years, as the process has a wide range of

applications, both reparative and structural, and it is currently being implemented in both

industry and in defense-related projects [6, 7]. Cold spray is a solid-state process that

involves high strain rate deformation of particles flowing out of a DeLaval nozzle directed

at a substrate [6]. Particles flow out of the nozzle at super-sonic speeds via an inert gas

stream, remaining well below their melting temperatures the entire time. When critical

velocities are achieved, the particles bond upon impact with the substrate surface and

then with previously deposited particles to form a solid coating or block of material [8].

Helium is often used as the processing gas to achieve high pressures and temperatures,

and therefore higher particle speeds.

Commercially available, CS feedstock materials are metal powders consisting of

fine-sized particles (5-100 µm) that have rapidly cooled and solidified via gas-atomization.

These spherical powders possess flow properties that promote high impact velocities and

result in good mechanical and metallurgical bonding, but the rapid solidification they

experience during manufacture can also produce heterogeneous and metastable

microstructures. This inconsistency among particles can have deleterious effects on the

properties of a CS deposit as well as limit the reproducibility of a coating. Materials used

as CS powders include, but are not limited to, the following metal alloy systems: copper,

aluminum, zinc, nickel, titanium, iron-based alloys, nickel-based superalloys, etc. [9].

Aluminum aerospace alloys of 2000, 6000, 7000 series possess desirable strength-to-

weight ratios and are widely used in CS repair applications for parts subject to severe

conditions and corrosion [8, 10, 11]. The alloy chosen for this study is Al 2024, an age-

hardenable alloy used in aeronautical components exposed to severe conditions, such as

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gearboxes and fuselage parts [11]. Repairing components with cold spray coatings can

extend the life of a part at a much lower cost than traditional methods, and it is often one

of the only options when dealing with thermally sensitive substrate material [8]. Another

advantage of using cold spray as a repair solution is that it has desirable properties in its

as-deposited condition, meaning that no post-processing is necessary. In fact, some

components repaired by cold spray are simply too large to be post-processed.

Cold spray is also often compared to thermal spray and other powder

consolidation methods, over which it has many advantages. Unlike in thermal spray, CS

particles are exposed only to inert gases, instead of combustion gases, preventing certain

phenomena such as powder melting, grain growth, phase changes, and excessive

substrate heating [6, 7]. CS is also a low-temperature deposition process, so it can be

used on a much wider variety of materials than thermal spray [7]. Regardless of the

thermal history of feedstock powder, CS can produce coatings with substantial levels of

adhesion and mechanical strength [12] while retaining the chemical composition of the

feedstock material [6]. Ultimate tensile strength (UTS) and elastic modulus of cold

sprayed aluminum alloys is comparable to that of wrought material, though ductility of

the in the as-deposited condition is often sacrificed [6]. This is most-likely due to the

microstructural evolution of the powder particles that occurs during high strain rate

plastic deformation upon impact [9, 12]. It just so happens that the resulting

microstructure of the CS deposit depends heavily on the initial microstructure of the

powder; consequently, many are turning research efforts to the development of pre-

processing or powder-processing methods. This work investigates pre-heat-treatment of

gas-atomized powders as a pre-processing method for cold spray.

1.2 Research motivation

Metal alloy powders used for cold spray (CS) differ greatly from their wrought

counterparts in both thermal history and microstructure. The properties that feedstock

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particles possess often correlate with the properties of the subsequent cold sprayed

deposit, thus pre-processing powders will allow for manipulation of microstructure and

mechanical properties in a CS deposit. At present, there is a need to develop new

facilities, procedures, and standards for the pre-heat-treatment of CS feedstock powders.

Aluminum alloys fall under the umbrella of age-hardenable, heat-treatable alloys, and

vast amounts of research have gone into developing standard treatments and test

methods for wrought material. Additionally, thanks to efforts by the Army Research Lab

(ARL), aluminum alloys (i.e. 6061, 7075, and 2024) have also been heavily studied and

commercialized as powders for CS, making these materials a good starting point for

powder heat-treatment research [2]. Adapting standard heat-treatment methods to gas-

atomized powders, and developing a simple, repeatable, and cost-effective system for

pre-processing CS feedstock could revolutionize the entire additive manufacturing

process. Since a smaller body of literature exists for pre-processing Al 2024 powders than

it does for Al 6061 and Al 7075, this was the alloy selected for the study.

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2 LITERATURE REVIEW

2.1 Cold spray

Cold spray (CS) is a solid-state consolidation process where powder particles are

accelerated at super-sonic speeds toward a substrate. Raw feedstock materials include

ferrous and non-ferrous alloys and are selected based on specific application

requirements. The spherical particles, ranging 5-100 µm in diameter, are injected into a

high-velocity jet stream generated by the expansion of a pressurized, pre-heated gas

through a converging-diverging, DeLaval nozzle [6, 7]. The particles are initially carried by

a separate, cold gas stream, which meets the hot gas stream at an application site just

before the nozzle throat. Thus, the particle temperatures remain well below their melting

temperature throughout the entire process. Finally, upon exiting the nozzle, the solid

particles impact the substrate and plastically deform, creating a combination of a

metallurgical and a mechanical bond with the surrounding material [7]. However,

successful deposition and bonding is only achievable if the particles reach a critical impact

velocity, which is entirely dependent on feedstock material and gas temperatures. The

resulting deposit takes the form of a solid coating or a freestanding shape [7]. Cold spray

has recently gained a lot of traction in the world of metal additive manufacturing (AM)

technologies and is already being implemented as a repair method in military and

commercial sectors. The technology also has many advantages over thermal spray,

making it very popular in the realm of powder consolidation and other forming techniques

[6, 7].

The advantages of cold spray can be grouped into two categories: (a) as-deposited

material properties and (b) manufacturing capabilities. In its as-deposited condition, a

cold sprayed material possesses desirable properties for a variety of applications. For

example, CS has the ability to produce high-density, low-porosity coatings which is

important for components that require corrosion- and/or wear-resistance in severe

conditions [6, 11, 13-16]. Additionally, CS deposits made from aluminum alloy feedstock

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show high ultimate tensile strength (UTS) values, comparable to their wrought

counterparts, making them useful for AM and structural repair of aeronautical

components [2].

In terms of manufacturing capabilities, perhaps the greatest advantage of cold

spray is its solid-state nature. The low-temperature process prevents changes in

chemistry, phase composition, and oxidation in the as-deposited material, allowing for

deposition of heat-sensitive materials like copper, titanium, and aluminum alloys [6, 7,

11, 14, 16, 17]. Avoiding deleterious high-temperature reactions and oxidation of

substrate and/or feedstock powder makes cold spray a very attractive method for wear

restoration and protective coating applications [2, 14, 18-20]. Even the aerospace

industry, with its strict criteria for material selection and processing technologies for

flight-safety critical components, has adopted cold spray. Research efforts of the U.S.

Army Research Laboratory (ARL), in collaboration with original equipment manufacturers

(OEMs), have resulted in the qualification of cold spray aluminum alloys for use in specific

applications for the repair and dimensional restoration of magnesium and aluminum

aerospace components for Army, Navy, and Air Force aircrafts [2, 18, 19]. Aluminum

2000, 6000, and 7000 series alloys are common aluminum aerospace alloys for their

generally light weight and high strength [2]. Of these, Aluminum 2024, 6061, and 7075

are the most common, and are therefore studied at length as feedstock powders for cold

spray. Aluminum-copper alloys specifically (i.e. Al 2024) are often used for components

that experience extreme conditions (i.e. gear boxes and fuselage parts). They are

reported to have high strength, low density, and somewhat high temperature stability,

but poor corrosion performance [2, 11].

2.2 Room for improvement

Tireless research is being conducted in academic and military labs to reduce the

cost of processing, increase deposition efficiency, expand the number of CS materials

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(powder and substrate), enhance the quality of particle-substrate bonding, enhance

feedstock material microstructures, and improve mechanical properties of CS coatings.

Each of these potential advances brings cold spray one step closer to full

commercialization. Both experimental and model-based research has been published on

the optimization of cold spray processing parameters such as nozzle material and

geometries [21], gas pressure and temperature, gas type, standoff distance, and raster

speed [2]. Since process parameters are what govern particle speed and impact velocity,

specific parameters must be determined for each powder-substrate combination to

ensure good deposition efficiency and optimal mechanical properties of the as-deposited

material [18]. Gas selection and usage is another extremely important parameter and is

one major area for improvement in the cold spray process. Helium is often chosen over

Nitrogen gas, as it has favorable thermal properties allowing for faster particle velocities

and critical impact velocities. That being said, Helium is a finite resource that is very

costly, so significant attention has been shifted to the research and development of

Helium recovery systems [22-24]. Recycling helium during sprays will drive economic

costs down and increase potential for higher volume depositions [24].

Attention has also been focused on developing methods for improving, or

manipulating, mechanical properties of CS coatings in their as-deposited state. Even

when cold spray deposition produces high strength material, the same material often

suffers in ductility and fracture toughness [25]. Many researchers have addressed this

challenge by adding various post-spray heat-treatments (i.e. solution heat-treatments

and annealing treatments) to their process [25-30], but as of late, researchers are also

exploring the possibilities of pre-processing methods. Additionally, a number of

experimental studies have confirmed that the microstructure and grain size of the

feedstock powder, which is largely a result of the gas-atomization process, affects the

properties of the as-deposited CS coating [9, 10, 12, 31]. During gas-atomization, molten

metal is poured through a small opening to meet one or more gas streams [32]. The liquid

metal is rapidly dispersed by the impinging jet(s) and subdivided into fine droplets that

quickly cool and solidify into highly spherical particles [33]. The flow properties that these

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powders possess are advantageous for cold spray because they allow for high impact

velocities, which result in good bonding; however, gas-atomization is extremely chaotic,

and the rapid solidification that occurs can cause irregular grain growth, localized

segregation of alloying elements, and/or formation of metastable phases within

individual powder particles [12, 33].

These non-equilibrium/metastable microstructures, combined with high strain rate

deformation, can cause a CS deposited material to experience any of the following

harmful phenomena: [9]. The non-uniformity of particle microstructures can hinder the

reproducibility of cold spray coatings and inhibit one’s ability to predict or manipulate

properties in the as-deposited condition. Additionally, altering feedstock material

properties prior to CS deposition to manipulate properties in the as-sprayed state has the

potential to eliminate the need for post-processing, again pushing CS toward

commercialization. The Worcester Polytechnic Institute (Worcester, MA) has done a

considerable amount of work on the thermomechanical modeling side of pre-processing

Al 2024 and 6061 feedstock powders [34-38]. Their studies have furthered the

understanding of secondary phase evolution and stability during thermal treatment of CS

feedstock. They have shown that solution heat-treating aluminum powders effective in

dissolving secondary phases and has a significant effect on altering the granular structures

within particles.

2.3 Pre-processing CS powders

Because cold spray is a solid-state process, and the energy transfer from particle

to substrate is largely kinetic [6], the as-deposited microstructure, along with the

mechanical and bonding properties, largely depend on the nature of feedstock powder.

As discussed in the previous sections, achieving desired properties in the as-deposited

condition by means of pre-processing is valuable for reducing production costs and lead-

time. It is also useful for repair applications that: (a) involve components that are too

8

large to be post-processed; or, (b) are time-sensitive and need to be returned to service

as soon as possible [39]. Pre-processing methods to ensure these outcomes might include

mixing, degassing, coating, or heat-treating raw feedstock powder.

Recall that CS powders must be somewhat ductile at high rates of strain to be able

to deform plastically on impact and produce dense coatings with high bond strength [6,

9], but by mixing powders, researchers have been able to successfully spray brittle

materials as well. In addition to the metal alloy systems listed in Section 1.1, both pure

metals and metal matrix composites (MMCs) have been cold sprayed successfully.

Spraying MMCs was introduced to facilitate CS deposition of brittle materials, because

hard particles lack in ductility and are difficult to deposit directly [40]. It was determined

that adding hard particles to a deformable metallic matrix could allow them to be cold

sprayed [40, 41]. This pre-processing method, referred to as powder mixing, involves

blending two or more powders that differ in some physical property. Mixing powders of

varying size-distributions has also been done. Spencer et al. [42] attempted to optimize

stainless steel cold spray coatings using this method. Powder mixing is a widely accepted

powder-treatment strategy incorporated prior to cold spray deposition [40].

Other powder-treatment methods, such as particle coating and degassing, have

been developed to combat moisture, oxidation, and residual gases found on/in gas-

atomized aluminum particles, which pose major problems for powder consolidation [43].

Typically, aluminum powders are surrounded by a thin oxide layer on their outer surface.

Thus, their critical velocities tend to be higher (600-800 m/s), to ensure the oxide layer

ruptures on impact [44, 45]. Unfortunately, oxidation is unavoidable given the conditions

of gas-atomization; it takes place during in-flight rapid solidification and every other

instance particles are exposed to the air prior to cold spraying (i.e. collection and handling,

transportation, and storage) [46]. This thin, amorphous oxide film (2-10 nm thick) can

actually impede particle bonding during cold spray and lead to delamination and/or poor

ductility of a coating [46-48], therefore significant effort has gone in to understanding

how this oxide layer develops and how it, along with other particle impurities, might be

removed or altered before cold spraying [46-49]. Both high-temperature (300-500 °C)

9

and vacuum degassing have proven to be effective surface contaminant removal and gas

desorption procedures for aluminum alloy powders [47, 48]. U.S. Pat. No. 7,141,207

proposes using a fluidized bed to apply a copper coating to aluminum particles, providing

another method for preventing surface oxide contamination during additive

manufacturing [50].

Researchers have also investigated how degassing methods might affect the oxide

layer and/or removal of other contaminants. Degassing is a type of thermal treatment

commonly used as a pre-processing method for aluminum alloy powders to remove any

harmful moisture or gases trapped inside particles during gas atomization [49].

Degassing, similar to other standard heat-treatments, involves heating the powder to a

specified temperature and holding it there for a period of time. Higher temperature

degassing of aluminum powders (350–450°C) has the potential to lower the hardness of

powders, to prevent blistering, and to improve mechanical properties in the as-deposited

condition [47, 48, 51]. Rokni et al. [51] studied the effects of degassing Al 5056 powder

at 490°C for 6 hrs, reporting that pre-processing softened the particles and eliminated

internal porosity, resulting in more uniform deformation and improved micro-tensile

properties (UTS and elongation %).

Degassing is closely related to another powder-processing method: heat-treating.

Extensive research has been done to develop heat-treatments (HTs) that favorably alter

the microstructure and mechanical properties of wrought materials [52]. The

standardized HTs for aluminum alloys, often referred to as basic temper designations (i.e.

T4, T6) exist in traditional manufacturing mainly as a final fabrication step to relieve

residual stresses within the material and to achieve higher or maximum strength [3, 5,

52]. Only in the last five years, have research groups (to be mentioned later in this review)

begun to apply these standard treatments to heat-treatable CS powders. Extensive

research has been done on the post-heat-treatment of CS deposits and how it can alter

or improve material properties, but fewer have taken the powder heat-treatment

approach – attempting to enhance material properties by altering feedstock properties

prior to spray [34]. A pre-heat-treatment approach is more challenging to implement

10

than a post-treatment. There are risks of particles sintering together and agglomerating

during heating, which would alter their flow capabilities and slow down impact velocities,

and there are certain safety limitations involved with heating combustible metal powders

that complicate experimental design. U.S. Pat. No. 9,555,474 proposes a promising

furnace-fluidized-bed assembly for high temperature powder treatment [53], but further

investigation is needed in regard to HT procedure, scalability, cost-reduction, and quality-

control.

To determine the proper heat-treatment for a material, it is important to

understand its thermal and mechanical history. For example, conventionally cast

aluminum alloys have often experienced some strain-hardening due to previous forming,

or they have already been heat-treated to a designated temper to improve strength [52].

In the case of CS powders, thermomechanical history depends on the gas-atomization

conditions (detailed in Section 1.1). Certain HTs for wrought aluminum alloys which

involve a precipitation treatment (i.e. natural aging), such as the T3 and T4 tempers, are

used to maximize strength, fracture toughness, and resistance to fatigue. This review

however, will focus on solid solution heat-treating (SSHT) and annealing, and the crucial

aspects of both: temperature, hold time, and cooling rate. These two treatments were

selected for their ability to soften a material, therefore increasing its ductility, and for

their ability to homogenize the microstructure of a material. Applying these HTs to CS

powders would enable particles to deform more readily on impact, potentially increasing

bond strength as well as ductility in the as-deposited material. Some work has been done

on solution heat-treating small batches of Al 6061, 7075, and 2024 powders to investigate

whether or not the effects prove to be advantageous for cold spray deposition [10, 12,

34, 36, 54].

11

2.3.1 Solid solution heat treatment

During solid solution heat-treatment (SSHT), an alloy is heated to a temperature

slightly below its solidus line and held there long enough that its constituents dissolve

into a solid solution. The material is then rapidly cooled, or quenched, to lock those

constituents in their solutionized state [52]. For wrought aluminum alloys, the purpose

of the HT is to put the maximum quantity of hardening solutes (i.e. copper, magnesium,

silicon, or zinc) into solid solution in the aluminum matrix, and then induce a slow

controlled precipitation so the material reaches its maximum hardness and strength [52,

54]. This final step of the SSHT is referred to as a low temperature age or precipitation

HT (Figure 1). Since the aim of solution heat-treating CS powders is to decrease particle

hardness and homogenize microstructures, the precipitation step was removed for this

study.

There are three key components in designing a solution heat-treatment: solution

temperature, hold time, and quench rate. All of these are material dependent as well as

part dependent, meaning that the size and shape of the part must also be factored in.

Solution temperature is determined by the melting temperature of a specific alloy;

therefore, it is imperative to know its chemical compositional and thermomechanical

Figure 1. Relative temperatures for solid solution and precipitation heat-treatments [1, 2].

12

behavior. Some Al alloys are more dilute in terms of maximum solubility [52] and can

tolerate higher solution temperatures, but the temperature alone does not guarantee full

solution. Al 2024 for example, the selected feedstock alloy for this study, has a solution

temperature of 493 °C and contains 4.16% Copper (Table 1). Al 2219 on the other hand,

has an even higher solution temperature of 535 °C, but it contains more than 5.65%

Copper, making complete solution impossible (Figure 2) [3, 5].

Figure 2. Aluminum-rich end of the aluminum-copper equilibrium diagram. Line AB

represents the increase in solubility of Cu in solid aluminum as temperature increases [2-4].

Alloys containing more than 5.65 wt-% Cu (line CD) are incapable of achieving complete solid

solution.

Table 1. Chemical composition of Al 2024 alloy provided by powder manufacturer (Valimet

Inc., Stockton, CA). The main alloying element is Copper, at 4.16 wt-%.

% Al:

Bal.

%C:

--

%Ca:

--

%Cr:

< 0.01

%Cu:

4.16

%Fe:

0.06

%Mg:

1.33

%Mn:

0.69

%N:

< 0.001

%Ni:

--

%O:

0.08

%Pb:

--

%S:

--

%Si:

0.05

%Sn:

--

%Ti:

0.01

%V:

--

%Zn:

< 0.01

Others,

Each:

< 0.05

Others,

Total:

< 0.15

Chemical Analysis: Al 2024 gas-atomized powder

13

As a material is heated or cooled, a series of exo- and endo-thermic reactions (also

referred to as solution and precipitation reactions) occur as elements either dissolve or

precipitate. Temperatures and rates at which these reactions occur depend on the

diffusion rates of different solutes – i.e. copper, magnesium, silicon, and zinc have

relatively high rates of diffusion in aluminum – and they can be measured directly by

Differential Thermal Analysis (DTA) [52]. Walde et al. [34] studied the microstructural

evolution of Al 2024 CS powder during thermal processing using Differential Scanning

Calorimetry (DSC) and material modeling. Due to unique powder precipitation kinetics,

they were not able to achieve successful dissolving of secondary phases via high-

temperature solution heat-treatment, but they were able to transform some phases [34].

They were heat-treating very small batches of powder, as DSC sample pans run on the

order of ~40 µL in volume, and this powder was not cold sprayed after heat-treatment.

Evans et al. [54] investigated at the effects of pre-heat-treating Al 6061 powder, but they

were mainly focused on small batch treatments and single-particle impacts and did not

perform HTs in-house.

Hold time often depends on the size of the part and its pre-existing microstructure [3,

52]. In the case of traditional casting and fabrication processes, grain size often coincides

with part or material thickness. Since CS powders possess fine-grained microstructures,

hold time can be reduced. In their study of Al 2024 powder, Walde et al. stated that due

to the higher percentage of grain boundary area, much shorter solutionization (hold)

times will be needed for powders compared to their wrought counterparts [34].

Quench rate is the dominant parameter in SSHT. Quenching is meant to preserve the solid

solution formed at the solution temperature [52]. For this to happen, the material must

be cooled fast enough so that the homogenous microstructure achieved during

solutionization is retained. A successful quench will keep solute atoms in solution as well

as retain a certain number of vacancies within the crystal lattice structure [52]. The key

is to quench fast enough so that the elements dissolved into their liquid phase during

solution treatment do not re-precipitate [55]. There are numerous strategies and media

used to quench parts and materials, but this review will focus solely on water quenching.

14

A typical water quench can be as simple as transferring a part directly from a furnace into

a container of cool, or room temperature, water, but it is crucial that the lag time between

furnace and water is minimized.

Sabard et al. [12, 31] measured the success of solution heat-treated Al 7075 and

Al 6061 powders by tracking particle hardness and particle grain patterns, by measuring

thickness of CS coatings made with as-received and SSHT powders, and by comparing the

morphology of particle deformation in those CS deposits. They report a significant

decrease in micro-hardness of both powders after solution heat-treatment as well as a

higher degree of particle deformation and fewer voids in the as-sprayed material. Story

et al. [10] solution heat-treated Al 2024, 6061, and 7075 powders based on guidelines

from the ASM Handbook Vol. 4; for reference, Al 2024 powder was heated for 75 min at

498 °C ± 6 °C and stored at -20 °C to prevent natural aging. Data from un-treated powder

was compared to data from solutionized powder, and HT success was measured in terms

of deposition efficiency, microstructural analysis of particles and coatings, and

mechanical properties of CS deposits. The solution HT was reported to decrease particle

hardness, improve deposition efficiency, and homogenize microstructures, but it did not

improve strength or ductility in comparison to as-received powder [10].

It must be noted here that fine-size aluminum powders are combustible when in

the presence of an ignition source and oxygen, so one must remove oxygen from the

equation by heating in an inert environment or vacuum [12, 43]. Aluminum powders can

also react with water to produce flammable hydrogen gas, so one must exercise caution

when performing a water quench. Sabard et al. [12] mitigated these concerns by vacuum

sealing powders inside quartz tubes. The tubes were heat-sealed so that the powder was

never exposed to air or water. SSHTs were conducted in 140 g powder batches, allowing

for easy handling and transfer from furnace to water quench where the quartz tube was

submerged for 2-5 minutes until it reached room temperature. Given the sealed nature

of the tube, it was impossible to record the exact temperature of the powder inside;

therefore, assumptions were made regarding the temperature of the particles

themselves [12, 31].

15

Story et al. [10] took a different approach to powder containment and quenching.

With an aim of increasing formability of the particles, they designed a custom tube

furnace system for SSHT. An air-tight tube was constructed from mild steel to house the

aluminum powders during treatment. To mitigate risk of particle oxidation, the tube was

flooded with helium and then sealed. An array of thermocouples was used to record the

temperature inside the tube during heat-treatments of empty tubes, but since the use of

thermocouples was not feasible during SSHT, some assumptions were made regarding

the temperature of the powders [10]. Their custom design allowed for easy and

immediate transfer of the tube into a brine-water bath, where an average powder cooling

rate of 25 °C/s was reported based on the thermal conductivity of mild steel [10]. As is

evident from these two powder quenching techniques, when the material is not directly

exposed to the water, it is difficult to control the quench rate. Particles are being cooled

by conductive heat transfer from tube material to particle, and quench rate may suffer.

2.3.2 Annealing

The second heat-treatment to address is annealing, or annealing heat-treatment (AHT).

Annealing temperatures tend to be lower than SSHT temperatures, but again, the

temperature and duration of the treatment remains largely dependent on the alloy and

the initial structure and temper of the part [3]. Even for a specific alloy there are several

types of annealing (Figure 3), but this review will focus solely on full annealing. One can

assume from here on that the terms annealing and AHT are always referring to full

annealing, which is a typical HT for wrought aluminum alloys used to render the material

to the “O” temper, producing the softest, most ductile, and most workable condition of

the material [3]. Annealing is an advantageous choice for conventionally-made alloys, as

the reduction or elimination of the effects of cold-working is accomplished by heat-

treating at 260-440°C [3].

16

Typical annealing conditions for wrought Al 2024 are 415°C for 2-3 hrs. Annealing

can be performed in open-air, since only temperatures greater than 415°C would result

in oxidation and unwanted grain growth [3]. Additionally, annealing does not involve a

quench. Both of these facts suggest annealing to be a much more time- and cost-effective

HT for CS powder than SSHT. Larger batches could be treated without increasing safety

risks. A crucial part of annealing, like most HTs, is ensuring that the entire part reaches

the annealing temperature. For wrought parts, this is easily achieved by using a hold time

of at least 1 hr [3], but more care has to be taken when adapting this treatment to gas-

atomized powders. Some sort of in-situ powder agitation is necessary to achieve

homogenous heating and to prevent agglomeration of the powder particles. Again, this

is why US Pat. No. 9,555,474 discloses using a fluidized bed for high temperature powder

treatment [53]. Ning et al. [56] studied the effects of vacuum annealing Copper feedstock

powders at both 390°C and 500°C. They reported an increase in CS deposition efficiency,

a decrease in micro-hardness of powder particles, and a decrease in critical velocity.

Figure 3. Relative temperatures for various annealing treatments. Full annealing was selected

for this study.

17

2.4 Summary

As is evident from the literature, research has only just begun to explore how pre-

processing by means of pre-heat-treating powders can alter and manipulate the

microstructure of a CS deposition. In the last 12 or so years, few have performed

annealing treatments on gas-atomized powders for cold spray, and only in the last 3 years,

performed solid solution heat-treatments on CS powders, so it is clear that this powder

heat-treatment is far from being commercialized. Readily controlling the microstructures

in aluminum alloy feedstock particles by means of heat-treatment could revolutionize the

CS manufacturing process entirely. In the future, powder manufacturers and consumers

will work together to transform gas-atomized powders into novel CS feedstock material

with predetermined, homogeneous microstructures allowing for cold sprayed coatings

with more predictable mechanical properties more reliable reproducibility. That being

said, there is need to develop a simpler, more cost-effective version of this particular pre-

processing method. Due to procedural limitations, researchers have been unable to

produce large volume sprays with heat-treated powders and unable to report a significant

improvement in mechanical properties by means of powder heat-treatment. The

following work will address both of these shortcomings. There is some literature on the

pre-heat-treatment of Al 7075, Al 6061, and Al 2024 powders, but further investigation is

needed on the correlation between powder microstructure and the mechanical

properties of as-sprayed material. Furthermore, if the ultimate goal is to commercialize

these processes, steps need to be taken to scale-up powder-treatment systems and to

incorporate quality control.

18

3 MATERIALS & METHODS

The objective of this study is to (a) investigate if the properties of CS powders, both

microstructural and mechanical, correlate with the properties of their subsequent CS

depositions, and then (b) determine if the properties of a CS deposit can be manipulated

by altering the properties of the feedstock powder prior to spraying. The following

characterization and test procedures make up a broad, yet systematic, investigation using

powder heat-treatment as the main pre-processing method and Al 2024 as the reference

material.

3.1 Material

The feedstock material used in this study was gas-atomized Al 2024 powder

(Table 1): batch no. 49-1/18-9004S (Valimet, Stockton, CA, USA). According to the

manufacturer, no heat-treatments were performed on the powder prior to shipment, and

the powder from this batch is a “modified Mil-spec” with D10: 20.15 µm, D50: 35.30 µm,

D90: 53.85 µm sizing. Throughout the study, the powder was stored and handled inside

a glovebox chamber purged with pure N2 gas (LC Technology Solutions Inc., Salisbury, MA,

USA), which maintains an inert, moisture-free, and oxygen-free environment to prevent

excess oxidation of particle surfaces as well as any other powder contamination.

3.2 Powder heat-treatments

Feedstock powder was heat-treated to three unique conditions prior to cold

spraying: as-received (AR), solid solution heat-treated (SSHT), and annealed (AHT). All

heat-treatments (HTs) were selected based on standard treatments for wrought Al 2024

found in ASTM B918/B918M-17a and ASM Metals Handbook Vol 4: Heat Treating [3, 5]

and were performed in an 1100 °C open-air box furnace (Thermo Fisher Scientific,

19

Waltham, MA, USA). Each powder sample was given a letter code (Table 2) and stored in

the glovebox between HT and cold spray. A “-CS” is added to the letter code to denote

the cold spray deposit produced from that particular powder (i.e. AR-CS is a cold spray

deposit made from as-received powder). This notation will appear throughout this

document.

A custom in-house procedure for heating and quenching combustible aluminum

powders was designed for the solid solution heat-treatment (Figure 5). While inside the

inert glovebox, 20-25 g batches of powder were mechanically sealed inside a vessel

consisting of a hydraulic steel tube (K&M Hose Services, Woburn, MA) with two flanged

ends mated with threaded compression fittings (Figure 4). The tube was then removed

from the glovebox and tightened to 88-115 Nm (65-85 ft-lb) with a torque wrench to

ensure the air-tight seal would hold during heating and quenching. A series of torque

tests were conducted on empty tubes, prior to HT, to determine the optimal torque range

necessary to maintain a seal during water quenching. After 1 hr of heating, the tubes

were removed from the furnace and immediately submerged in a room temperature

water bath. The SSHT was deemed successful if the powder came out of the tube dry

after quenching, implying that the compression fittings held their seal for the duration of

Table 2. Sample identification codes for Al 2024 powder in three heat-treated conditions. “-CS”

is added to indicate a cold spray deposit produced from that particular powder.

Temperature Time

ARRaw powder that remains “as-

received” from manufacturerRoom temp. 5-10 months

SSHTRaw powder that has been solid

solution heat-treated 495 °C 1 h

AHT Raw powder that has been annealed 415 °C 2.5 h

Sample

CodeDescription

Heat Treatment

20

the HT. Due to the constraints of the tube dimensions, 10 discrete SSHTs were performed

to produce approximately 200 g of solution heat-treated powder. All storage and

handling of powder not sealed in a steel tube was done inside the glovebox, so powder

was never exposed to oxygen or moisture prior to cold spraying. This treatment was

created with the aim of homogenizing the microstructure of the powder and decreasing

the hardness of the particles. Such an outcome would aid in particle deformation during

cold spray and potentially improve elongation properties in the subsequent CS deposit.

Figure 4. Solid solution heat-treatment vessel made from steel hydraulic tubing. Ends of the

tube are flanged to mate with compression fittings and create an air-tight mechanical seal when

tightened.

Figure 5. Flow chart of powder life-cycle during the study.

21

Since annealing is a more commonly used heat-treatment for aluminum powders,

a simpler, more conventional approach was used for the AHT. Two batches of Al 2024

powder were placed inside aluminum pans and wrapped in aluminum foil to prevent

excess oxidation. Both pans were then placed in the same open-air furnace mentioned

above. After 2.5 hours of heating, the furnace was shut off and powder was left to air

cool, yielding approximately 400 g of AHT powder. Significant agglomeration occurred

during both high-temperature heat-treatments, so large chunks of powder had to be

eliminated with gentle powder mixing and crushing, as well as sieving, prior to cold spray.

3.3 Microstructure characterization

3.3.1 Metallographic Preparation & Etching

Powder samples were taken from each heat-treatment condition (AR, SSHT, and

AHT) (Table 2). Standard metallographic preparation techniques were used to create

mirror-finish, scratch-free cross-sections for microscopic imaging and micro-hardness

testing [57]. Samples were first mounted in high edge-retention epoxy resin (Pace

Technologies, Tuscan, AZ, USA). Once the epoxy set, a 4-5 step grinding procedure was

carried out with SiC paper increasing in fineness from 320-grit to 1200-grit (Pace

Technologies). The final wet polish was conducted using 0.05 µm alumina suspension

(Pace Technologies). Both grinding and polishing were done by hand on an EXTEC® Labpol

Duo 8 Twin Grinding/Polishing Machine (Extec Corporation, Enfield, CT) at speeds ranging

from 500-700 rpm. Additionally, half of each sample was etched using Keller’s Reagent.

Swabbing the surfaces 5-6 times was sufficient to reveal the grain structure of the

powders [58].

The same grind-polish-and-etch procedure was used to produce cross-section

samples of each cold spray deposit (AR-CS, SSHT-CS, and AHT-CS). These samples were

subject to microscopic imaging, micro-hardness testing, and porosity measurements.

22

3.3.2 Microscopy

SEM analysis was conducted, with the assistance of Wentao Liang, at Northeastern

University’s Kostas Research Institute (KRI) using a Thermo Scientific™ Scios™ DualBeam™

ultra-high-resolution system operated at 5.0 kV. Since powder samples were mounted in

a non-conductive epoxy resin, the surfaces were coated with a 3-5 nm conductive

platinum layer using a Cressington 208HR High Resolution Sputter Coater prior to SEM to

prevent surface charging defects while imaging. SEM images of the powders were used

to compare grain structure of individual particles from each HT condition (Table 2).

Optical microscopy (OM) was conducted both at KRI, using a ZEISS Axioscope 7 microscope

(Carl Zeiss AG, Oberkochen, Germany), and in Northeastern’s Advanced Materials

Processing Laboratory using an Olympus-AH-2. OM and SEM images of the CS deposits

were used to compare morphology of deformed particles, area percentage porosity, and

general microstructure resulting from the as-received, solid solution heat-treated, and

annealed powder conditions.

3.4 Particle size measurement

Particle size distributions of each powder (AR, SSHT, and AHT) were measured using

digital image processing. Twelve micrographs were taken from each powder sample using

a ZEISS Axioscope 7 microscope by spreading particles onto a glass slide with a light source

positioned below. A MATLAB program was written to threshold and contrast the images

so that individual particles appeared as blobs of dark pixels on a white background. Blob

sizes were then measured and grouped into 10 µm size ranges, so that frequency-based

and volume-based size distributions could be determined. The method was developed

based on techniques found in ASTM E2109-01 standard for determining area percentage

porosity in thermal spray coatings [59].

23

3.5 Particle velocity measurement

Particle velocity during spray operation was measured using a HiWatch HR1 System

(Oseir Oy, Tampere, Finland) specifically designed for research and development of cold

spray processes. The system was mounted inside the cold spray booth so that the nozzle

could be situated within the measuring area. This is not an in-situ measurement system,

so processing conditions (Table 3) were replicated to determine the average particle

impact velocity during CS deposition. Due to limited powder supply, measurements were

taken only from the as-received Al 2024 powder sample.

3.6 Micro-hardness testing

Vickers micro-hardness testing was performed on the mirror-polished cross-

sections described in Section 3.3.1. All measurements were conducted, with the

assistance of Mr. Xingdong Dan, on a Shimadzu HMV Micro Vickers Hardness Tester in

Northeastern’s Advanced Materials Processing Laboratory. Indentations were made on

the powder samples using the minimum loading available: 98.07 mN. This loading

condition resulted in indentations that were observed repeatedly to be approximately

1/4 the diameter of a particle cross-section, implying that the results may have a larger

error than is reported here. Indentations were made on the as-deposited CS samples with

a load of 245.2 mN.

24

3.7 CS deposition process

All cold spray deposits were produced with a VRC Gen III high-pressure cold spray

system (VRC Metal Systems, Rapid City, SD, USA) in Northeastern’s Cold Spray Laboratory

at KRI. Helium was used as the carrier gas to achieve high impact velocities. Wrought Al

2024-T351 was used as the substrate material. Powders from each HT condition

(Table 2) were sieved through a -270 mesh and then deposited onto a 0.375 inch thick

substrate that was grit blasted prior to spray to enhance particle-substrate bonding. A

simple rectangular raster pattern of the robot arm was used to produce three separate

CS deposits ranging 0.11 – 0.25 inches in thickness. Each CS block was then machined in

its as-sprayed state to produce tensile bars and microstructure samples. Table 3 details

the specific spray parameters that were held constant for each of the three CS deposits.

Table 3. Spray parameters held constant throughout the study.

CS Parameter

Carrier gas Helium

Gas pressure 510 psi

Gas temperature 415 °C

Nozzle material PBI

Nozzle type VRC #71

Deposition angle 90 °

Standoff distance 25 mm

Nozzle speed 254 mm/s

Powder feed rate 0.247 kg/hr

Layer Height 0.1 mm

25

3.8 Tensile samples

Tensile samples were machined from the as-deposited cold spray material with

standard ASTM E8/E8M-16 dimensions shown in Figure 6 and Figure 7 [4]. The

geometries were machined first in Northeastern’s machine shop with the help of Mr. Ben

Macalister. The resulting dogbone samples (Figure 8) were then sent out to be cut from

the substrate via wire EDM (United Tool & Machine, Wilmington, MA). Specimens were

machined such that the tensile loading axis was oriented perpendicular to the spray

direction. A total of 19 samples were tested including 4-5 each from the three CS deposits

(AR-CS, SSHT-CS, and AHT-CS), as well as 5 samples machined from the wrought Al 2024-

T351 substrate material. A complete list of tensile samples and their cross-sectional areas

is given in Table 6.

Figure 6. Standard ASTM E8/E8M-16a tensile specimen geometry [2, 4] . All dimensions are

in inches.

Figure 7. Modified ASTM E8/E8M-16a tensile specimen geometry. All dimensions are

in inches.

26

Uniaxial tensile testing was performed at room temperature on an Instron Model

5582 static 100 kN load frame (Instron, Norwood, MA, USA). Specimens were pulled in

the direction perpendicular to the direction of spray at an extension rate of 0.03 in/min,

and strain was measured using a 1 inch gauge length extensometer (Instron). It is

important to note that some of the cold sprayed samples fractured outside the range of

the extensometer clips, but because these classify as a brittle fractures, the elongation

data was still included in results of this study (Section 4.5).

Figure 8. Cold sprayed tensile specimens prior to EDM removal from substrate. Tensile bars

are made up of only cold sprayed material.

27

4 RESULTS & DISCUSSION

4.1 Microstructure characterization

4.1.1 Powder

Figure 9. SEM images of Al 2024 particles from three heat-treatment conditions: (a, b) as-

received, (c, d) solution heat-treated, (e, f) annealed. Cross-sections (left) were polished and

etched to reveal microstructure. Images of whole particles (right) are only representative of

their cross-sectioned counterparts and are not necessarily the same particle.

(a)

(c)

(e)

(b)

(d)

(f)

28

Figure 9 a and b reveal a typical microstructure and surface structure of gas-

atomized aluminum alloy powder. The dendritic cell structure exhibited in the cross-

sectioned particle is a direct result of the rapid solidification that occurs during gas-

atomization. These cellular boundaries are also visible in the image of the whole particle,

which appears to have bumps and grooves instead of a flat surface. This analysis of the

as-received Al 2024 powder was used as the baseline for comparison to particles from the

other two HT conditions (solutionized and annealed).

Figure 9 c and d reveal the microstructure produced by means of a solid-solution

heat-treatment (490 °C for 1 hr). The intercellular grain structure is no longer observable;

thus, it was concluded that the SSHT homogenized the microstructure. Figure 9 e and f

reveal the microstructure produced by means of a full annealing treatment (415 °C for 2.5

hr). Like the SSHT, the annealing treatment has eliminated the cellular grain structure

and homogenized the particles.

29

4.1.2 CS deposits

Figure 10 and Figure 11 display the microstructure of cold spray deposits

produced from the three different Al 2024 powders. It is evident from both SEM and

optical images of the as-received (AR) powder, that the as-sprayed material possesses the

same intercellular structure that the individual particles possessed prior to CS deposition

(Figure 9a). The intercellular boundaries remain clearly visible even in their deformed

state, making it easy to distinguish deposition layers and individual particles (Figure 10a).

This is not the case, however, for the deposits made from solutionized and annealed

powders (Figure 10 b, c). Since the intercellular precipitates in the particles have been

Figure 10. Optical micrographs of CS deposits made from three different Al 2024 powders:

(a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to

the spray direction, then polished and etched to reveal microstructure. Black spots in images (b)

and (c) are most-likely a result of the polish-etch procedure.

(a) (b)

(c)

100 µm 100 µm

100 µm

30

dissolved during high temperature heat-treatments, individual particles and layers are

much less discernible. The black spots in the optical images of as-deposited solutionized

powder and as-deposited annealed powder (Figure 10 b, c) may be a result of softened

phases being ripped out during a polish and etch procedure that is perhaps too aggressive

for the material. We believe that these black spots do not necessarily correspond to

porosity within the CS deposit and that further investigation is necessary.

Figure 11. SEM images of CS deposits made from three different Al 2024 powders: (a) as-

received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut parallel to the

spray direction, then polished and etched to reveal microstructure.

(a) (b)

(c)

31

4.2 Particle size distributions

It appears from Figure 12 that powder size distribution increased significantly with

high-temperature heat-treatment, but further consideration suggests that this may not a

true representation of individual particles increasing in size. SEM images, shown in

Figure 13, show little no observable effect on overall particle size or shape. However, it

does look as though the annealed and solution treated powders have larger numbers of

agglomerated particles and satellite particulate than the as-received powder (Figure 15).

This is most-likely a result of sintering occurring during heat-treatment and is probably

the reason for the increase in measured size distribution. This effect was addressed post-

heat-treatment with gentle powder mixing/crushing and sieving, but it was impossible to

separate all of the fine-sized “moon” particulate that had adhered to the surfaces of larger

particles. Agglomerated particles are indistinguishable to the image processing MATLAB

program and therefore register as larger particles (Figure 14). This is ultimately why there

appears to be a higher percentage of large particles in the solution heat-treated and

annealed powders in Figure 12. However, regardless of whether or not these size ranges

Figure 12. Volume based cumulative size distribution of powder particles from three heat-

treatment conditions.

32

represent those of individual spherical particles, the material traveling through the cold

spray system has clearly increased in size with heat-treatment, which significantly alters

flow properties and reduces particle velocities during deposition. Figures 16-18 further

supplement the curves plotted in Figure 12 providing bar charts of both frequency based

and volume based size distributions.

Figure 13. SEM images of whole powder particles from three heat-treatment conditions: (a)

as-received, (b) solution heat-treated, and (c) annealed.

(c)(b)(a)

Figure 14. ZEISS optical images of whole powder particles from three heat-treatment

conditions: (a) as-received, (b) solution heat-treated, and (c) annealed. Particles were placed on

a glass slide with a light source positioned below, so particles appear as black blobs.

(c)(a) (b)

Figure 15. Al 2024 powder from three heat-treatment conditions: (a) as-received, (b) solution

heat-treated, and (c) annealed. Images were taken immediately following heat-treatment, prior

to any sieving or breaking up of agglomerated particles.

(c)(a) (b)

33

Figure 16. Overall frequency based (left) and volume based (right) particle size distributions

of Al 2024 as-received powder.

Figure 17. Overall frequency based (left) and volume based (right) particle size distributions

of Al 2024 solution heat-treated powder.

Figure 18. Overall frequency based (left) and volume based (right) particle size distributions

of Al 2024 annealed powder.

34

4.3 Particle velocities

A mean particle velocity of 1027.15 m/s was recorded for the Al 2024 as received

powder. Processing conditions were mimicked during this test, so we assume that during

deposition, particles impacted the substrate and surrounding material at a mean rate of

1027.61 m/s (Figure 19). Such high velocities were achieved by using Helium as the

processing gas. Due to powder supply limitations, particle velocities were not recorded

for the solution heat-treated powder or the annealed powder. However, due to particle

agglomeration and increase in particle size distribution discussed in Section 4.2, we have

assumed that the solution-treated and the annealed particle velocities were lower than

that of the as-received powder during deposition. Further HiWatch testing is necessary

to validate this assumption, but in the future, any discrepancies should be accounted for

by adjusting spray parameters (Table 3) for the heat-treated powders so that impact

velocity remains constant across all three sprays.

Figure 19. Particle velocities of Al 2024 as-received powder captured by HiWatch HR1

System. The average particle speed was measured to be 1027.61 m/s with a standard deviation

of 237.03 m/s.

35

4.4 Micro-hardness

Figure 20 illustrates how two high temperature heat-treatments affected the

micro-hardness of Al 2024 powder and corresponding cold sprayed deposits. One

observation is the correlation between hardness of powder particles and hardness of

subsequently sprayed material. If the powder increased in hardness after heat-treatment

(i.e. solution HT), the CS deposit made with that powder also showed an increase in

hardness. Similarly, if the powder decreased in hardness after heat-treatment (i.e.

annealing), the CS deposit made with that powder also showed a decrease in hardness.

Additionally, the increase in hardness from powder particles to as-sprayed material

indicates the strain hardening occurring during CS deposition.

Figure 20. Micro-hardness of Al 2024 powders from three different heat-treatment conditions

and their subsequent CS deposits: (left) as-received, (middle) solid solution heat-treated, and

(right) annealed.

36

After solid solution heat-treatment (SSHT) the average hardness of the powder

increased by 9.3 % compared to the as-received powder. Although the AR powder column

and the SSHT column (Figure 20) appear to fall with standard deviations of each other, a

statistical analysis of variance produced a p-value of 7.8E-06, confirming that the two data

sets are in fact significantly different from one another. That being said, the solution heat-

treatment was intended to decrease the hardness of the powder; therefore, this result

was not expected and eludes to a few possibilities:

(i) The particles were not quenched at a high enough rate to retain the solid

solution formed during HT, implying that elements began to reprecipitate

immediately. This age-hardening effect would explain the significant

increase in micro-hardness compared to the as-received powder.

(ii) The particles experienced some amount of solid solution strengthening

during HT, resulting in an increase in micro-hardness.

(iii) The particles were not fully solutionized during HT, implying that the

intercellular precipitates did not fully dissolve into the aluminum matrix.

Further elemental analysis is necessary.

After the annealing treatment (AHT) the average hardness of the powder decreased

by 33.4 % compared to the as-received powder, and the microhardness of as-sprayed

material decreased by 8.0 %. This result was anticipated, as full annealing of aluminum

alloys is intended to render the softest, most ductile, and most workable condition of the

material [3].

37

4.5 Tensile Testing

Five tensile samples were tested from substrate, SSHT-CS, and AHT-CS material, and

four samples were tested from the AR-CS material. Figure 21 presents one stress-strain

curve from each of the four materials, that is only representative of its larger data set.

The four curves are presented on the same axes for visual comparison.

Figure 22 displays the average values for UTS, elongation-% to fracture, and elastic

modulus. CS deposits made from the three heat-treated powders were not able to

achieve tensile strength or ductility greater than that of the wrought Al 2024-T351

material. Comparing mechanical properties of the three CS deposits, solution heat-

treating the powder produced a minor decrease in as-sprayed elongation-% but little to

Figure 21. Representative stress-strain curves from uniaxial tensile testing. Samples were

produced from four different materials: wrought Al 2024-T351 substrate (substrate), cold

sprayed Al 2024 as-received powder (AR-CS), cold sprayed Al 2024 solution heat-treated

powder (SSHT-CS), and cold sprayed Al 2024 annealed powder (AHT-CS).

38

no change in as-sprayed UTS (confirmed with ANOVA in Table 4). This was not surprising,

as the solution treatment was unsuccessful in softening powder particles.

The annealed powder, on the other hand, produced a decrease in as-sprayed UTS

but little to no change in as-sprayed elongation-% when compared to samples made from

as-received powder (confirmed with ANOVA in Table 4). More tensile samples must be

tested to clear up the discrepancies apparent from statistical analysis of variance.

Figure 22. Average (a) ultimate tensile strength (UTS), (b) elongation-% to fracture, and (c)

elastic modulus from four sets of tensile samples.

39

Additionally, Figure 22c shows that the elastic modulus remained constant across

all four Al 2024 materials. Average values ranged from 6.70E04 to 6.86E04 MPa, but all

fall within standard deviations of one another. This result was anticipated as behavior in

the elastic region is dominated by the bulk material’s response to tensile stress, which is

not being altered in this study. Aluminum is the bulk material in each of the four tested

samples. The stress-strain curves in Figure 21 reveal the brittle nature of each of the

three fractured cold sprayed materials in comparison to the ductile fracture of wrought

substrate material.

Figure 23. Stress-strain curves from uniaxial tensile testing. Samples were produced from four

different materials: (a) wrought Al 2024-T351 substrate, (b) cold sprayed Al 2024 as-received

powder, (c) cold sprayed Al 2024 solution heat-treated powder, and (d) cold sprayed Al 2024

annealed powder.

40

Table 4. Statistical analysis of variance (ANOVA) of UTS and elongation-% data. If the P-

value is less than 0.05, there is a statistical confidence of 95% that one of the data sets produced

a mean that is significantly different from the other.

41

5 CONCLUSIONS

A number of conclusions were drawn from the findings of this study regarding the

effects of pre-heat-treating Al 2024 powder on particle characteristics and on mechanical

properties of cold sprayed material. First of all, it was determined that annealing is a

much simpler pre-heat-treatment for combustible aluminum powders than solid solution

heat-treating. There is no quench step involved, allowing for larger volumes of powder

to be treated at once. It was also determined that heating Al 2024 powders to

temperatures of 415 °C and higher results in significant particle sintering and

agglomeration which should be mitigated in future works. Additionally, both annealing

and solution heat-treatments homogenized particle microstructure by eliminating the

dendritic intercellular boundaries found in as-received powder. Though the three powder

samples polished and etched quite differently, this affect was revealed with scanning

electron microscopy (SEM).

Solution heat-treating raw Al 2024 powder resulted in a 9.3 % increase in Vickers

hardness. This unanticipated result indicates that some hardening occurred within the

material, age-hardening or solution hardening, or that the quench was unsuccessful in

retaining the solid solution formed during heating. The increase in particle hardness

resulted in a minor decrease in as-sprayed elongation-%, but it did not yield any

measurable change in as-sprayed ultimate tensile strength (UTS).

Conversely, annealing raw Al 2024 powder produced a 33.4 % decrease in Vickers

hardness. This significantly softened powder yielded a decrease in as-sprayed UTS, but it

produced no significant change in elongation-%. It was concluded that annealing still has

the potential to improve material ductility in the as-deposited state, but more data is

necessary to confirm with statistical confidence.

As anticipated from micro-hardness results, the CS deposits made with solution heat-

treated powder also showed an increase in hardness, and those made with annealed

powder also showed a decrease in hardness. The increase in hardness from powder to

42

subsequent cold sprayed deposit does not correlate with powder heat-treatment but

indicates the strain hardening that occurs during CS deposition. The three CS deposits

also polished and etched very differently; therefore, the optical and SEM images of CS

cross-sections were inconclusive.

Lastly, by comparing properties of substrate material (Al 2024-T351) to the as-

received powder deposit (AR-CS), it was confirmed that cold sprayed Al 2024 produces

UTS comparable to wrought but suffers significantly in ductility. However, during this

study, neither solution heat-treating nor annealing powder was able to significantly

improve ductility in the as-deposited state.

43

6 FUTURE WORK

This study presents a correlation between the microstructure of feedstock powders

and cold sprayed deposits. Further, it presents that the microstructures of gas-atomized

aluminum alloy powder can be significantly altered via high temperature heat-treatment.

The work is limited predominantly by the size of the data set that was collected; thus, it

can be supplemented and expanded as follows:

1. To achieve full commercialization, the powder heat-treatment process needs further

refinement to increase efficiency and implement quality control. This may be

achieved by:

a. Performing the same study using an alternative HT system custom designed by

Northeastern capstone students. A prototype is currently located in

Northeastern’s Cold Spray Lab and consists of a rotating tube furnace with a built

in N2 gas quench. A rotating furnace would provide gentle agitation of the

powders, promoting uniform heating and preventing agglomeration and/or

sintering of particles. Quenching with inert gas instead of water would also

eliminate safety concerns, cutting down production time and allowing for higher

volume HTs.

b. Developing a technique to make in-situ temperature measurements of the

powders during heating and quenching to ensure particles reach desired solution

or annealing temperature.

c. Continuing to refine the design of powder heat-treatment tubes or vessels by

experimenting with various materials, heat-treating larger batches of powder, or

designing a system to HT multiple tubes at one time.

2. There is a need for investigating the effect of pre-processing additional heat-treatable

alloys (i.e. steels, tantalum, and nonferrous alloys: titanium, copper, lead, magnesium,

nickel) used as cold spray feedstock powders.

44

3. Particle deformation behavior, parametrized by the flattening parameter, can provide

valuable information regarding the bonding process in a CS deposit. An alternative

polish/etch procedure should be investigated to try and reveal individual particle

boundaries in micrographs of CS cross-sections. If these boundaries were apparent,

flattening ratios could be measured and compared across heat-treatment conditions.

4. Particle-particle and particle-substrate bonding and/or adhesion is another

measurable property that will affect mechanical properties of a CS deposit. It would

be beneficial to deposit powders onto substrate material that differs from the

feedstock material (i.e. spray Al 2024 onto Al 6061) so that the interface has greater

contrast in the micrographs. Particle-particle bonding can also be investigated by

creating FIB samples at specific locations within the deposit.

45

APPENDIX

Table 5. Standard heat-treatments for wrought alloys used to determine temperature and

hold/soak time for solid solution heat-treatment of Al 2024 powder [3, 5].

Material Temp TemperMaterial

Thickness [in]Min Max

Max quench

delay

ASTM standard 493°C (488-499°C) 0.25 - 0.50 55-65 65-75 15s

ASM Metals

Handbook495°C

each additional

0.50+30 +30 15s

ASTM standard 466°C (460-499°C) 0.25 - 0.50 60 70

ASM Metals

Handbook465, 480, 490°C 0.50 - 1.0 90 100

1.0 - 1.5 120 130

ASTM standard 529°C (516-579°C) 1.5 - 2.0 150 160

ASM Metals

Handbook530°C 2.0 - 2.5 180 190

Solid Solution Heat Treatment Soak Time [min]

Al 2024 T4

Al 7075 W, W51

Al 6061 T4

Metals

Handbook

Vol. 4

15sASTM

standard

Material Temp TemperMaterial

Thickness [in]Min Max

Max quench

delay

ASTM standard 493°C (488-499°C) 0.25 - 0.50 55-65 65-75 15s

ASM Metals

Handbook495°C

each additional

0.50+30 +30 15s

ASTM standard 466°C (460-499°C) 0.25 - 0.50 60 70

ASM Metals

Handbook465, 480, 490°C 0.50 - 1.0 90 100

1.0 - 1.5 120 130

ASTM standard 529°C (516-579°C) 1.5 - 2.0 150 160

ASM Metals

Handbook530°C 2.0 - 2.5 180 190

Solid Solution Heat Treatment Soak Time [min]

Al 2024 T4

Al 7075 W, W51

Al 6061 T4

Metals

Handbook

Vol. 4

15sASTM

standard

46

Figure 24. High magnification SEM images of CS deposits made from three different Al 2024

powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections were cut

parallel to the spray direction, then polished and etched to reveal microstructure.

(a) (b)

(c)

47

Figure 25. High magnification optical micrographs of CS deposits made from three different

Al 2024 powders: (a) as-received, (b) solution heat-treated, and (c) annealed. Cross-sections

were cut parallel to the spray direction, then polished and etched to reveal microstructure. Black

spots in images (b) and (c) are most-likely a result of the polish-etch procedure.

(a) (b)

(c)

48

Table 6. Complete list of tensile samples and their corresponding cross-section geometries.

49

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