IE 337: Materials & Manufacturing Processes

Lecture 16: Introduction to Joining

Chapters 30, 31 & 32

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This Time

Considerations in Joining Joint & Weld Types Joining Processes Brazing and Soldering Processes HW 6 (Due next Tuesday)

Multiple Choice Quiz and Problems From Chapters 30, 31 & 32

Assembly Business

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A Matter of Scale

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Space Shuttle Assembly Molecular Motors

Examples

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Joining and Assembly Defined

Joining - welding, brazing, soldering, and adhesive bonding

These processes form a permanent joint between parts

Assembly - mechanical methods (usually) of fastening parts together

Some of these methods allow for easy disassembly, while others do not

Classification of Joining Processes

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Welding

Joining process in which two (or more) parts are coalesced at their contacting surfaces by application of heat and/or pressure Many welding processes are accomplished by heat

alone, with no pressure applied Others by a combination of heat and pressure Still others by pressure alone with no external heat

In some welding processes a filler material is added to facilitate coalescence

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Welding Pros & Cons

Most welding operations are performed manually and are expensive in terms of labor cost

Most welding processes utilize high energy and are inherently dangerous

Welded joints do not allow for convenient disassembly

Welded joints can have quality defects that are difficult to detect Warping Cracking Cavities / Porosity Inclusions Incomplete fusion Unacceptable contour

Provides a permanent joint Welded components become a

single entity Usually the most economical way

to join components in terms of material usage and fabrication costs Mechanical fastening usually

requires additional hardware components (e.g., screws and nuts) and geometric alterations of the parts assembled

Not restricted to a factory environment Welding can be accomplished

"in the field"

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Considerations in Joining

Coalescence Conditions of the Material: Heat Pressure Surface Conditions

Faying surfaces

Atmosphere Inert versus need for flux

Resulting Joint: Fusion Zone Heat Affected Zone Base Metal (Unaffected Zone)

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Heat (Power) Density

Power transferred to work per unit surface area (power density), W/mm2 (Btu/sec‑in2) If power density is too low, heat is conducted into work, so

melting never occurs High thermal conductivity in the material is a problem

If power density too high, localized temperatures vaporize metal in affected region

There is a practical range of values for heat density within which welding can be performed

Oxyfuel gas welding (OFW) develops large amounts of heat, but power density is relatively low because heat is spread over a large area

Oxyacetylene gas, the hottest of the OFW fuels, burns at a top temperature of around 3500C (6300F)

Arc welding produces high energy over a smaller area Local temperatures of 5500 to 6600C (10,000 to 12,000F) common

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Power Density

Power Density is the power entering a surface divided by the corresponding surface area:

where:

PD = power density, W/mm2 (Btu/sec‑in2);

P = power entering surface, W (Btu/sec); and

A = surface area over which energy is entering, mm2 (in2)

AP

PD

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Approximate Power Densities for Several Welding Processes

Welding process W/mm2 (Btu/sec-in2)

Oxyfuel 10 (6)

Arc 50 (30)

Resistance 1,000 (600)

Laser beam 9,000 (5,000)

Electron beam 10,000 (6,000)

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Unit Energy for Melting

Um is the quantity of heat required to melt a unit volume of metal

Sum of: Heat to raise temperature of solid metal to melting

point Depends on volumetric specific heat

Heat to transform metal from solid to liquid phase at melting point

Depends on heat of fusion

Depends on melting temperature of material

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Heat Available for Welding

Not all of the input energy is used to melt the weld metal. The net heat available for welding (Hw) is:

Hw = f1 f2 H

Where:

f1 = heat transfer efficiency - actual heat received by workpiece divided by total heat generated at source;

f2 = melting efficiency - proportion of heat received at work surface used for melting; the rest is conducted into work metal; and

H = total heat generated by welding process

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AWS Joint & Weld Types

Joint Types:A. Butt JointB. Corner JointC. Lap JointD. T-JointE. Edge Joint

Weld Types: Fillet Welds Groove Welds Plug/Slot Welds Spot Welds Flange Surfacing Welds

A B D EC

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Joining Processes

Welding Processes Fusion Welding

Oxyfuel Arc Resistance

Solid-State Welding Friction Diffusion Ultrasonic

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Factors Affecting Weldability

Filler metal Must be compatible with base metal(s) In general, elements mixed in liquid state that form a

solid solution upon solidification will not cause a problem

Surface conditions Moisture can result in porosity in fusion zone Oxides and other solid films on metal surfaces can

prevent adequate contact and fusion

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Oxyfuel Welding Processes

Heat provided by fuel gas and oxygen Flame environment affects the junction

material, controlled by oxygen to fuel ratio Carburizing/Reducing Flame Neutral Flame Oxydizing Flame

Characteristics: Low investment cost Portability High operator skill

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Flame Environment Illustrated

Maximum temperature reached at tip of inner cone Outer envelope spreads out and covers work

surfaces to shield from surrounding atmosphere

Figure 31.22 ‑ The neutral flame from an oxyacetylene torch indicating temperatures achieved

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OFW Process Illustrated

Figure 31.21 ‑ A typical oxyacetylene welding operation (OAW)

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Arc Welding Processes

Heating is accomplished by electric arc AC current equipment is less expensive to purchase and

operate, but generally restricted to ferrous metals DC current can be used on all metals and is generally noted

for better arc control Highly automatable Processes:

Shielded Metal Arc Welding Gas Metal Arc Welding Gas Tungsten Arc Welding Flux Cored Arc Welding Submerged Arc Welding Others (Plasma, Stud, …)

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Arc Welding Illustrated

Figure 31.1 ‑ The basic configuration and electrical circuit of an arc welding process

A pool of molten metal is formed near electrode tip As electrode is moved along joint, molten weld pool

solidifies in its wake

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Two Basic Types of AW Electrodes

Consumable – consumed during welding process Source of filler metal in arc welding Welding rods (also called sticks) are 9 to 18 inches and 3/8

inch or less in diameter and must be changed periodically Weld wire can be continuously fed from spools with long

lengths of wire, avoiding frequent interruptions

Nonconsumable – not consumed during process Electrode is tungsten, which resists melting Electrode is gradually depleted during welding (vaporization is

principal mechanism) Any filler metal must be supplied by a separate wire fed into

weld pool

Figure 31.3 Shielded metal arc welding (SMAW).

Shielded Metal Arc Welding

31.4 Gas metal arc welding (GMAW).

Gas Metal Arc Welding

GMAW Advantages over SMAW

Better arc time because of continuous wire electrode Sticks must be periodically changed in

SMAW Better use of electrode filler metal than SMAW

End of stick cannot be used in SMAW Higher deposition rates Eliminates problem of slag removal Can be readily automated

Figure 31.9 Gas tungsten arc welding.

Gas Tungsten Arc Welding

Advantages / Disadvantages of GTAW

Advantages: High quality welds for suitable applications No spatter because no filler metal through

arc Little or no post-weld cleaning because no

flux

Disadvantages: Generally slower and more costly than

consumable electrode AW processes

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Arc Welding Safety Issues

Arc gives off UV Light Eye safety concerns

Fuel combustion fumes, fuel stocks storage Storage, misconnection concerns

Electrical energy safety Guarding concerns

Flux has environmental concerns

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Resistance Welding Processes

Coalescence is achieved by using heat from the electrical resistance to the flow of current at the faying surfaces

Highly automatable (5 steps in process) Surface finish issues Processes:

Resistance Spot Welding Resistance Seam Welding Projection Welding Others (Flash, Upset, Percussion, …)

Figure 31.12 Resistance welding, showing the components in spot welding, the main process in the RW group.

Resistance Welding

Advantages / Drawbacks of RW

Advantages: No filler metal required High production rates possible Lends itself to mechanization and automation Lower operator skill level than for arc welding Good repeatability and reliability

Disadvantages: High initial equipment cost Limited to lap joints for most RW processes

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Resistance Welding Illustrated

Figure 31.13 ‑ (a) Spot welding cycle, (b) plot of squeezing force

& current in cycle: (1) parts inserted between

electrodes

(2) electrodes close, force applied

(3) current on, force maintained

(4) current off or reduced, force maintained

(5) electrodes opened, welded assembly removed

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Solid-State Welding Processes

Coalescence occurs due to pressure alone, or heat (below Tm) and pressure combined

Processes: Diffusion Welding

Surfaces held under pressure at elevated temperature, coalescence occurs by solid-state diffusion of atoms

Friction Welding Coalescence occurs by heat of friction between surfaces

Ultrasonic Welding Vibrational friction provides heat and moderate pressure

completes the coalescence

Others (Forge, Explosion, …)

Diffusion Welding (DFW)

SSW process uses heat and pressure, usually in a controlled atmosphere, with sufficient time for diffusion and coalescence to occur

Temperatures 0.5 Tm

Plastic deformation at surfaces is minimal Primary coalescence mechanism is solid state

diffusion Limitation: time required for diffusion can range

from seconds to hours

200 µm wide channels

Microchannel Process Technology

channel header

channels

Single Lamina

• Channels – 200 µm wide; 100 µm deep

– 300 µm pitch

• Lamina (24” long x 12” wide)– ~1000 µchannels/lamina

– 300 µm thickness

Patterning: • machining (e.g. laser …) • forming (e.g. stamping …)• micromolding

Microchannel Process Technology

• Device (12” stack)~ 1000 laminae= 1 x 106 reactor µchannels

• Laminae (24” long x 12” wide)– ~1000 µchannels/lamina

– 300 µm thickness Bonding: • diffusion bonding• solder paste reflow• laser welding …

Patterning: • machining (e.g. laser …) • forming (e.g. stamping …)• micromolding

24”

12”

12”

12”

24”Cross-section of

Microchannel Array

Microchannel Process Technology

Bonding: • diffusion bonding• solder paste reflow• laser welding …

Interconnect• welding• brazing• tapping

24”12”

12”

Microchannel Reactor

Bank of Microchannel Reactors(9 x 106 microchannels)

• Device (12” stack)~ 1000 laminae= 1 x 106 reactor µchannels

• Laminae (24” long x 12” wide)– ~1000 µchannels/lamina

– 300 µm thickness

Microlamination [Paul et al. 1999, Ehrfeld et al. 2000*]

*W. Ehrfeld, V. Hessel, H. Löwe, Microreactors: New Technology for Modern Chemistry, Wiley-VCH,

2000.

24”12”

12”

Microchannel Reactor

Microlamination of Reactor

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Friction Welding Illustrated

Figure 31.28 ‑ Friction welding (FRW): (1) rotating part, no contact; (2) parts brought into contact to generate friction heat; (3) rotation stopped and axial pressure applied; and (4) weld created

Weld Quality

Concerned with obtaining an acceptable weld joint that is strong and absent of defects, and the methods of inspecting and testing the joint to assure its quality

Topics: Residual stresses and distortion Welding defects Inspection and testing methods

Residual Stresses and Distortion

Rapid heating and cooling in localized regions during FW result in thermal expansion and contraction that cause residual stresses

These stresses, in turn, cause distortion and warpage

Situation in welding is complicated because: Heating is very localized Melting of base metals in these regions Location of heating and melting is in motion

(at least in AW)

Welding Defects

Cracks Cavities Solid inclusions Imperfect shape or unacceptable contour Incomplete fusion Miscellaneous defects

Figure 31.31 Various forms of welding cracks.

Welding Cracks

Cavities

Two defect types, similar to defects found in castings:

1. Porosity - small voids in weld metal formed by gases entrapped during solidification Caused by inclusion of atmospheric gases,

sulfur in weld metal, or surface contaminants

2. Shrinkage voids - cavities formed by shrinkage during solidification

Solid Inclusions

Solid inclusions - nonmetallic material entrapped in weld metal

Most common form is slag inclusions generated during AW processes that use flux Instead of floating to top of weld pool,

globules of slag become encased during solidification

Metallic oxides that form during welding of certain metals such as aluminum, which normally has a surface coating of Al2O3

Also known as lack of fusion, it is simply a weld bead in which fusion has not occurred throughout entire cross section of joint

Figure 31.32 Several forms of incomplete fusion.

Incomplete Fusion

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Weld Profile

Weld joint should have a certain desired profile to maximize strength and avoid incomplete fusion and lack of penetration

Figure 31.33 ‑ (a) Desired weld profile for single V‑groove weld joint

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Profile Defects

Figure 31.33 ‑ Same joint but with several weld defects:

(b) undercut, in which a portion of the base metal part is melted away;

(c) underfill, a depression in the weld below the level of the adjacent base metal surface; and

(d) overlap, in which the weld metal spills beyond the joint onto the surface of the base part but no fusion occurs

Brazing

Joining process in which a filler metal is melted and distributed by capillary action between faying surfaces of metal parts being joined

No melting of base metals occurs Only the filler melts

Filler metal Tm greater than 450C (840F) but less than Tm of base metal(s) to be joined

Brazing Compared to Welding

Any metals can be joined, including dissimilar metals

Can be performed quickly and consistently, permitting high production rates

Multiple joints can be brazed simultaneously Less heat and power required than FW Problems with HAZ in base metal are reduced Joint areas that are inaccessible by many

welding processes can be brazed; capillary action draws molten filler metal into joint

Disadvantages and Limitations of Brazing

Joint strength is generally less than a welded joint

Joint strength is likely to be less than the base metals

High service temperatures may weaken a brazed joint

Color of brazing metal may not match color of base metal parts, a possible aesthetic disadvantage

Figure 32.1 (a) Conventional butt joint, and adaptations of the butt joint for brazing: (b) scarf joint, (c) stepped butt joint, (d) increased cross‑section of the part at the joint.

Butt Joints for Brazing

Figure 32.2 (a) Conventional lap joint, and adaptations of the lap joint for brazing: (b) cylindrical parts, (c) sandwiched parts, and (d) use of sleeve to convert butt joint into lap joint.

Lap Joints for Brazing

Some Filler Metals for Brazing

Base metal(s) Filler metal(s)

Aluminum Aluminum and silicon

Nickel-copper alloy Copper

Copper Copper and phosphorous

Steel, cast iron Copper and zinc

Stainless steel Gold and silver

Figure 32.4 Several techniques for applying filler metal in brazing: (a) torch and filler rod. Sequence: (1) before, and (2) after.

Applying Filler Metal

Figure 32.4 Several techniques for applying filler metal in brazing: (b) ring of filler metal at entrance of gap. Sequence: (1) before, and (2) after.

Applying Filler Metal

Soldering

Joining process in which a filler metal with Tm less than or equal to 450C (840F) is melted and distributed by capillary action between faying surfaces of metal parts being joined

No melting of base metals, but filler metal wets and combines with base metal to form metallurgical bond

Soldering similar to brazing, and many of the same heating methods are used

Filler metal called solder Most closely associated with electrical and

electronics assembly (wire soldering)

Soldering Advantages / Disadvantages

Advantages: Lower energy than brazing or fusion welding Variety of heating methods available Good electrical and thermal conductivity in joint Easy repair and rework

Disadvantages: Low joint strength unless reinforced by

mechanically means Possible weakening or melting of joint in

elevated temperature service

Figure 32.8 Techniques for securing the joint by mechanical means prior to soldering in electrical connections: (a) crimped lead wire on PC board; (b) plated through‑hole on PC board to maximize solder contact surface; (c) hooked wire on flat terminal; and (d) twisted wires.

Mechanical Means to Secure Joint

Figure 32.9 Wave soldering, in which molten solder is delivered up through a narrow slot onto the underside of a printed circuit board to connect the component lead wires.

Wave Soldering

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You Should Have Learned

Considerations in Joining Joint & Weld Types Joining Processes Brazing and Soldering Processes

HW 6 (Due next Tuesday) Multiple Choice Quiz and Problems From Chapters 30, 31 &

32

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Next Time

Microfabrication and Nanofabrication

Chapter 26, 37 & 38