Re-crystallization in metals by high-power ultrasonic waves

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IME 874 Graduate Seminar

Review of

Re crystallization in metals by Inducing High power Ultrasonic Waves

Reviewed by

Ajit Kumar Kameshwararao-K733Y672 May 1, 2015

Prof: Dr. Wilfredo Moscoso-Kingsley

Re-crystallization in metals by high-power ultrasonic waves

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Re crystallization in metals by Inducing High power Ultrasonic Waves

I. INTRODUCTION –ULTRASONIC’S

Ultrasound is acoustic (sound) energy in the form of waves having a frequency above the human hearing range. The highest frequency that the human ear can detect is approximately 20 thousand cycles per second (20,000 Hz). This is where the sonic range ends, and where the ultrasonic range begins. Ultrasound is used in electronic, navigational, industrial, and security applications. It is also used in medicine to view internal organs of the body Ultrasound can be used to locate objects by means similar to the principle by which radar works. High-frequency acoustic waves reflect from objects, even comparatively small ones, because of the short wavelength.

The applications of ultrasonic waves are generally divided into two groups: low and high intensity. Low-intensity applications are those wherein the objective is to obtain information about the propagation medium without producing any modification in its state. On the contrary, high-intensity applications are those wherein the ultrasonic energy is used to produce permanent changes in the treated medium. High-power ultrasound is the part of ultrasound devoted to high-intensity applications. The limit between low and high intensity is very difficult to fix, but it can be approximately established for intensity values that, depending on the medium, vary between 0.1 W=cm2 and 1 W=cm2 . The use of high-intensity ultrasonic waves in industrial processing is generally based on the adequate exploitation of a series of mechanisms activated by the ultrasonic energy such as heat, agitation, diffusion, interface instabilities, friction, mechanical rupture, chemical effects, etc. These mechanisms can be employed to produce or to enhance a wide range of processes such as plastic and metal welding, machining, metal forming, etc., in solids or cleaning, atomization, emulsification and dispersion, degassing, extraction, defoaming, particle agglomeration, drying and dewatering, sonochemical reactions, etc., in fluids

The power ultrasonic processes are very much dependent on the irradiated medium. In fact, a typical characteristic of high-intensity ultrasonic waves is their ability to produce different phenomena in different media in such a way that these phenomena seem to be opposite at times

II. PRODUCTION OF ULTRASONICS

There are different methods for the ultrasonics most commonly used methods are

1. Mechanical Method

2. Piezoelectric generator

3. Magnetostriction generator

Mechanical method is one of the oldest methods for producing ultrasonic wave’s up to 100kHz frequencies

using Galton’s whistle. It was limited due to its limited frequency range.

Piezoelectric Generator

This method is based on the piezoelectric effect and was developed by langevin in 1917. The piezo electric effect is used to provide e.m.f and the tuning is achieved by a variable condenser as shown below

In the above fig Q is the crystal placed in between the metals A and B this combination forms parallel condenser as dielectric. The metal plates are connected to the primary circuit of the transformer which is coupled to the oscillatory circuit of the triode valve. If the natural frequency of the triode valve coincides with the crystal frequency resonance will occur and the crystal is set to mechanical vibrations due to piezoelectric effect. Using a quartz crystal Ultrasonic frequencies of 540kHz can be produced. Tourmaline crystal may be used to generate frequencies of 1.5x10^8 Hz

Magnetostriction Generator

The principle of Magnetostriction effect is utilized in this for production of ultrasonics, according to this effect a bar of ferromagnetic material like nickel or iron changes its length when it is placed in the strong magnetic field applied to its length. A nickel rod placed in a rapidly varying magnetic field alternately expands and contracts with twice the frequency of the applied magnetic field. The expansion and contraction in the rod produces ultrasonic sound waves in the medium surrounding the nickel rod. The frequency of the ultrasonics produced ranges from 8kHz to 20Khz and however the range of frequencies depend on the mode of vibration of the ferromagnetic material and may vary from few hundred to 300k Hz .

To generate the ultrasonics, the following circuit devised by G.W. Pierce is used. The specimen rod AB normally invar is placed inside a solenoid parallel to its axis. A high frequency current is passed through the solenoid and consequently the rod is magnetized ad demagnetized with varying current thus producing ultrasonics.

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If the length of the rod is such that he frequency of its vibration is equal to the frequency of the applied current resonance and thereby the amplitude of the vibration is increased. As shown in the figure above 2 co axial coils A1 and A2 are wound on the same rod. The coil A1 is connected to the grid and the coil A2 is connected to the plate of the triode valve. The variable condenser is adjusted that it should produce a periodically carrying magnetic field in it. This change in the length of the rod is connected to the grid. As a result of change in the magnetic flux, an induced e.m.f is produced in the coil A1 fed to the grid. The change in the e.m.f in the grid circuit produced large oscillatory current in plate circuit and hence it increases the Magnetostriction effect in the specimen rod so ultrasonics is produced

III- Effect of ultrasonics in melts

Nucleation of the primary phase is the first step in the

transformation of molten alloys into the solid state. Generally,

any factor which increases the number of nucleation sites or

reduces growth rate, yields fine grains in the as-cast aluminum

alloys. Based on this, many techniques of grain refinement are

available in casting practices, such as rapid solidification

deliberate addition of inoculants and forced action upon melt

which includes mechanical or magneto-hydrodynamic stirring

and ultrasonic vibration. The main mechanism of these

techniques is increasing the number of nuclei by

heterogeneous nucleation during solidification. Ultrasonic

melt treatment (UST) is known to induce grain refining in

aluminum alloys. The basic principle is introduction of

acoustic waves with a frequency higher than 17 kHz into

liquid metal. High frequency and high amplitude oscillations

result in cavitation of the melt and also promote intense

mixing through agitation. The ultrasonic generator converts

the power supply (100-250 Volts, 50-60 Hz) into a 20 to 30

kHz, 800-1000 Volts electrical signal. This signal is applied to

piezo-electrical ceramics (included in the converter) that will

convert this signal into mechanical oscillations. These

oscillations will be amplified by the booster and converter,

thus creating a hammer. The converter converts electricity into

high frequency mechanical vibration. The active elements are

usually piezoelectric ceramics. The booster (optional) serves

as an amplitude transformer. Amplitude magnification or

reduction is achieved by certain design features or the

geometrical shape of the booster.

An ultrasonic generator converts ac voltage of 50 Hz

to high frequency voltage which is transmitted to the

transducer, where it transforms into mechanical vibrations.

When the transducer is submerged in liquid the vibrations

cause pressure to build up in the liquid, e.g. acoustic waves.

The generated acoustic waves at some critical intensity

generate acoustic streaming which then forms microscopic

bubbles. The bubbles expand during negative excursion and

collapse violently during positive excursion which is referred

to as Cavitation. Cavitation is formation of a gas bubble in

liquid during the rarefaction cycle. During the compression cycle

the gas bubble collapses. During the collapse, tremendous

pressure is built up. The pressure may be of the order of several

thousand atmospheres. Cavitation produces most of the

mechanical and chemical effects in the high intensity sound

application to various mediums.

Grain Refinement through Ultrasonics Grain size has a measurable effect on most mechanical

properties. For example, at room temperature, hardness, yield

strength, tensile strength, fatigue strength and impact strength

all increase with decreasing grain size. Machinability is also

affected; rough machining favors coarse grain size while

finish machining favors fine grain size. The effect of grain

size is greatest on properties that are related to the early stages

of deformation. Thus, for example, yield stress is more

dependent on grain size than tensile strength. Fine-grain steels

do not harden quite as deeply and have fewer tendencies to

crack than coarse-grain steels of similar analysis. Also, fine-

grain steels have greater fatigue resistance, and a fine grain

size promotes a somewhat greater toughness and shock

resistance. Steels made fine grained by addition of aluminum

have machinability inferior to those made without aluminum.

Also, cold working frequently alters grain size by promoting

more rapid coarsening of the grains in critically stressed areas.

The original grain size characteristics, however, can usually be

Re-crystallization in metals by high-power ultrasonic waves

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restored by stress relieving. Coarse-grain steels have better

creep and stress rupture properties because diffusion at high

temperatures is impeded by sub grain low-angle boundaries

present in coarse-grain steels

Numerous researchers have proposed that vibrating a

liquid metal at ultrasonic frequencies (typically ~20 kHz)

causes cavitation, which can reduce the grain size by (a)

inducing local changes in the melting temperature due to the

collapse of bubbles (Clapeyron equation) (b) improving

wetting of insoluble nucleant particles; and (c) breakage of

crystals as they form caused by flow from acoustic streaming

or vibration of the horn. It has been demonstrated that the

application of ultrasonic vibrations to the solidifying alloys

prevents columnar grain formation, refines the equiaxed grains

and in some circumstances leads to the formation of non-

dendritic, globular grains. Alloys having the mentioned

microstructure are reported to possess both increased ductility

and increased strength. UST has been previously used in many

metals like aluminum alloys, copper, magnesium etc.

Experimental results show that the ultrasonic grain refining

effect is not only related to the size of particles which are

refined and/or dispersed by UST, but also related to an under

cooling available for activation of these particles in the

solidification process. Athermal heterogeneous nucleation

theory is considered to explain the effect of size and

distribution of substrate particles on the grain structure with

different undercoolings. The distribution of primary particle

sizes results in the distribution of required undercoolings.

Grain refining occurs when the undercooling is large enough

to activate the refined primary intermetallics or dispersed

inoculants. Effect of UST on grain structure of Aluminum

before and after application. Fig (a) Shows the grain structure

of a normal aluminum before UST application, in Fig(b) it is

clearly evident that the grains are more refined and the grain

size reduced from 90um to 65um.

Figure (a) represents the molecular structure of an aluminum

alloy prior applying the ultrasonic waves , fig (b) shows after

application of Ultrasonic waves. It is obvious that the grain

structure has a huge impact on ultrasonics.

The next figure shows the microstructure of

Aluminum alloy A356 after application of UST vibration

ranging from 15seconds to 180seconds. The longer the

application of UST the coarser the grain size gets

Additive Manufacturing

Additive Manufacturing refers to a process by which digital

3D design data is used to build up a component in layers by

depositing material. The term "3D printing" is increasingly

used as a synonym for Additive Manufacturing. However, the

latter is more accurate in that it describes a professional

production technique which is clearly distinguished from

conventional methods of material removal. Instead of milling

a work piece from solid block, for example, Additive

Manufacturing builds up components layer by layer using

materials which are available in fine powder form. A range of

different metals, plastics and composite materials may be

used.

The technology has especially been applied in conjunction

with Rapid Prototyping - the construction of illustrative and

functional prototypes. Additive Manufacturing is now being

used increasingly in Series Production. It gives Original

Equipment Manufacturers (OEMs) in the most varied sectors

of industry the opportunity to create a distinctive profile for

themselves based on new customer benefits, cost-saving

potential and the ability to meet sustainability goals.

There are many ways product can be 3D printed, few of them

are

1) SLA:Very high end technology utilizing laser

technology to cure layer-upon-layer of photopolymer resin

(polymer that changes properties when exposed to light)

Re-crystallization in metals by high-power ultrasonic waves

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2) FDM:Process oriented involving use of thermoplastic

(polymer that changes to a liquid upon the application of heat

and solidifies to a solid when cooled) materials injected

through indexing nozzles onto a platform. The nozzles trace

the cross-section pattern for each particular layer with the

thermoplastic material hardening prior to the application of the

next layer

3) MJM:Multi-Jet Modeling is similar to an inkjet

printer in that a head, capable of shuttling back and forth (3

dimensions-x, y, z)) incorporates hundreds of small jets to

apply a layer of thermo polymer material, layer-by-layer

4) 3DP: This involves building a model in a container

filled with powder of either starch or plaster based material.

An inkjet printer head shuttles applies a small amount of

binder to form a layer. Upon application of the binder, a new

layer of powder is sweeped over the prior layer with the

application of more binder. 5) SLS: Somewhat like SLA technology Selective Laser

Sintering (SLS) utilizes a high powered laser to fuse small

particles of plastic, metal, ceramic or glass. During the build

cycle, the platform on which the build is repositioned,

lowering by a single layer thickness. The process repeats until

the build or model is completed. Unlike SLA technology,

support material is not needed as the build is supported by

unsintered material.

Selective Laser Sintering A variety of solid freeform fabrication (SFF) techniques have

been developed to produce prototype parts directly from a

computer-aided drawing (CAD) without any hard tooling, dies

or molds. An object is created by sequentially fusing thin

layers of a powder with a scanning laser beam. Each scanned

layer represents a cross section of the object's mathematically

sliced CAD model. This was developed like other

technologies to decrease cost and time. It produces a 3D part

layer by layer selectively sintering or partially melting a

powder bed by laser radiation. In direct SLS, a high-energy

laser beam directly fuses a metal or cermets powder to high

density (>90%), preferably with minimal or no post-

processing requirements. There are two kinds of selective

laser sintering process, Direct and Indirect.

In Indirect SLS a polymer coating is used for binding

metal powders and ceramics. The metal powers are coated

with polymer and when the laser melts the polymer thus

bonding the metal powder together ad forming a green part. A

post treatment is required in an oven at very high temperature.

Direct SLS processes use metal powders directly without any

binder. Direct SLS eliminates the need for post-processing.

Ongoing material and process developments have increased

the productivity of the systems and the quality of the resulting

parts. SLS can be used as a stand-alone production technique,

or combined with conventional too making processes. Direct

SLS of composite metal powder blends can be used to produce

metal parts. In such systems, a low melting point component is

melted and employed as a matrix in which the higher melting

point components sit. Direct SLS of metals is a promising

rapid prototyping technique that allows manufacturers to pro-

SLS on a DTM Sinter station 2000. The fragile green part

produced consists of steel powder held in a polymer matrix.

The second step in the process was a sintering furnace cycle.

In this step the green parts were placed into a furnace and

heated to 1120◦C with a heating rate of 2◦C/min. During the

heating phase the polymer was burned away leaving the steel

skeleton, which was then sintered traditionally to a porous

steel structure at 1120◦C for 3 h. The sintered parts were then

cooled to room temperature at a cooling rate of 3◦C/min. The

atmosphere in the furnace was inert gas with a mix of 30%

hydrogen and 70% nitrogen recommended. Finally this

structure was infiltrated with molten bronze, which was

absorbed from the base of the parts by a wicking action, which

drew the bronze through the porous matrix resulting in a near

fully dense composite. The furnace was held at 1050◦C for 3

h. The heating rate, cooling rate, and the atmosphere inside the

furnace were the same as for the sintering cycle.

Application of UST during DMLT

Application of Ultrasonic waves during Direct metal laser sintering process: As shown in the below fig

the microstructure of “bronze, nickel and copper “obtained by DMLS process.UST has a potential application in refining the grain structure and if UST can be applied during the DMLS process the grain size can be further refined, as the temperature reaches near melting point when the laser beam contacts the metal powder and an instant fusion reaction takes place thus binding the metal powders together. UST has a huge impact on grain size especially during the phase transformation, application of UST at the correct time and for right duration can change the microstructure of the metal resulting in desired properties such as increased hardness, yield strength, malleability etc.

Conclusions and Future Work

Ultrasonic treatment of metals is very promising area of

research over the last decade. Engineering applications of

ultrasonic waves is growing day by day, a recent and very

interesting application of sound waves was used as a fire

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retardant by two students as a part of their project. In this

paper the potential use of ultrasonics was discussed and it is

evident that UST technique is used in metal processing for

grain refinement. The results showed that the ultrasonic

treatment is effective in controlling the morphology and size of

grains when applied between near melting point temperatures. A proposal of using ultrasonic treatment during direct metals

laser sintering was made. Future work should include the use

of UST during the direct metal laser sintering process of 3D

printed part.

REFERENCES

[1] Eskin, G.I., “Principles of Ultrasonic Treatment:Application of Light

Alloy Melts,”Adv. Perform.Mater., vol. 4, issue 2, pp. 223-232 (1997)

[2] G. I. Eskin: ‘Ultrasonic Treatment of Light Alloy Melts’, 1–64,135–240; 1998,

Amsterdam, Gordon & Breach [3] Jian X, Xu C, Meek T T, Han Q. Effect of ultrasonic vibration

on the solidification structure of A356 alloy. AFS Trans., 2005,

113: 131-137

[4] Osawa Y, Sato A. Grain refinement of solidified structures by ultrasonic

vibration. J. JFS, 2000, 72: 733-738.

[5] Yoshiki Tsunekawa, Masahiro Okumiya, and Takahiro Motomuraon

“Semisolid casting with ultrasonically melt-treated billets of Al-7mass%Si

alloys”

[6] Ma Qian and A. Ramirez on “An approach to assessing ultrasonic

attenuation in molten magnesium alloys”

[7] Xiaoda Liu, Shian Jia, Laurentiu Nastac on “ultrasonic Cavitation-

assisted molten metal processing of cast a356-nanocomposites.

[8] Mula, S., Padhi, P., Panigrahi, S.C., Pabi, S.K.,Ghosh, S., “On Structure

and Mechanical Properties of Ultrasonically Cast Al-2% A1,03

nanocomposite,”Mater. Res. Bull., vol. 44, pp. 1154-60 (2009)” [9]T. V. Atamanenko*1, D. G. Eskin2 and L. Katgerman, “Temperature

effects in aluminium melts due to cavitation induced by high power

ultrasound.

[10] N. U¨ nal, H. E. C¸ amurlu*, S. Koc¸ak and G. Du¨ztepe, “Effect of external untrasonic treatment on hyoereutectic cast aluminum –silicon alloy