chapter 32: resistance and solid-state welding processes

Chapter 32:Resistance and Solid-State Welding Processes

DeGarmo’s Materials and Processes in Manufacturing

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32.1 Introduction

Electrical resistance heating to form the joint. Create joints without any melting of the workpiece

or filler material, know as solid –state welding process.

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32.2 Theory of Resistance Welding Both heat and pressure are used to induce

coalescence. Electrodes contact the material, and electrical

resistance heating is used to raise the temperature of the workpiece and the interface between them.

H=I2Rt

H = total heat input in joules

I = current in amperes R = electrical resistance in ohms t = time in seconds

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Basic Resistance Welding

FIGURE 32-1 The basic resistance welding circuit.

Total resistance:• The bulk resistance of the electrode and workpieces.• The contact resistance between the electrode and the workpieces.• The resistance between the surfaces to be joined, known as the faying surfaces.

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Resistance Welding Temperature Distribution

FIGURE 32-2 The desiredtemperature distribution acrossthe electrodes and workpiecesduring resistance welding.

Water cooling is usually used to keep the electrode temperature low and thereby extend their useful life.

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Current and Pressure for Resistance Welding

FIGURE 32-3 A typical current and pressure cycle for resistance welding. This cycle includes forging and postheating operations.

Applying pressure promotes a forging action, so resistance welds can be produced at lower temperatures than welds made by other processes.

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32.3 Resistance Welding Processes Resistance spot welding Resistance seam welding Projection welding

Mass production

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Schematic of Resistance Spot Welding (RSW)

FIGURE 32-4 Thearrangement of the electrodesand workpieces in resistancespot welding.

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Microstructure of a Resistance Weld

FIGURE 32-5 A spot-weld nugget between two sheets of 1.3-mm (0.05-in.) aluminum alloy. The nugget is not symmetrical because the radius of the upper electrode is greater than that of the lower electrode. (Courtesy Lockheed Martin Corporation, Bethesda, MD.)

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Tear Test

FIGURE 32-6 Tear test of a satisfactory spotweld, showing how failure occurs outside ofthe weld.

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Spot-Welding Equipment

FIGURE 32-7 Single-phase,air-operated, press-typeresistance welder withmicroprocessor control.(Courtesy Sciaky Inc., Chicago, IL.)

Rocker-arm machine: for light-production work where complex current-pressure cycles are not required.Press-type welder: for larger spot welds used at high production rates.

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Spot Welding Application

Monel: Alloy of Nickel, Copper, Iron and Manganese, with antacidity

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Resistance Seam Welding (RSEW)

FIGURE 32-8 Seam weldsmade with overlapping spotsof varied spacing. (CourtesyTaylor-Winfield Corporation,Brookfield, OH.)

• Gas- or liquid-tight welding

• Resistance butt welding

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Schematic of Seam Welding

FIGURE 32-9 Schematicrepresentation of theseam-welding process.those

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Tube Welding

FIGURE 32-10 Using high-Squeeze rollfrequency AC current to producea resistance seam weld in buttweldedtubing. Arrows from thecontacts indicate the path of thehigh-frequency current

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Projection Welding

FIGURE 32-11 Principle ofprojection welding (a) prior toapplication of current andpressure and (b) after formationof the welds.

For mass-production operation, not only one spot weld at a time.

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32.4 Advantages and Limitations of Resistance Welding Advantages

Rapid Fully automated Conserve material, no filler metal, shielding gases,

or flux Minimal distortion Skilled operation are not required Dissimilar metals can be easily joined High degree of reliability and reproducibility

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32.4 Advantages and Limitations of Resistance Welding (continued) Limitations

High initial cost Limitation to thickness of material (less than 6mm) Both sides of the joint require to apply the proper

electrode force or pressure Skilled maintenance on servicing the equipment Some materials need special preparation prior to

welding

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Process Summary for RW

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32.5 Solid-State Welding Processes Forge welding Forge-seam welding Cold welding Roll welding or roll bonding Friction welding and inertia welding Friction stir welding Ultrasonic welding Diffusion welding Explosive welding

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Forge Welding (FOW)

The most ancient of the welding process Using a charcoal forge, the blacksmith

heated the pieces to be welded to a practical forging temperature (by color) and then prepared the ends by hammering (hammer and anvil) so that they could be properly fitted together.

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Forge-Seam Welding

Still used in the manufacture of pipe – a heated strip of steel is first formed into a cylinder, and the edges are simply pressed together in either a lap or a butt configuration.

Welding is the result of pressure and deformation.

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Cold Welding

FIGURE 32-12 Small parts joined by cold welding. (Courtesy of Koldweld Corporation, Willoughby, OH.)

A variation of forge welding uses no heating but produces metallurgical bonds by means of room-temperature plastic deformation.

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Roll Welding or Roll-Bonding (ROW)

FIGURE 32-13 Examples ofroll-bonded refrigerator freezerevaporators. Note the raisedchannels that have beenformed between the roll-bondedsheets. (Courtesy Olin Brass,East Alton, IL.)

Two or more sheets or plates of metal are joined by passing them simultaneously through a rolling mill.

Perform either hot or cold and can be used to join either similar or dissimilar metals.

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Friction Welding (FRW)

FIGURE 32-14 Sequence for making a friction weld. (a) Components with square surfaces are inserted into a machine where one part is rotated and the other is held stationary. (b) The components are pushed together with a low axial pressure to clean and prepare the surfaces. (c) The pressure is increased, causing an increase in temperature, softening, and possibly some melting. (d) Rotation is stopped and the pressure is increased rapidly, creating a forged joint with external flash.

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Schematic for Friction Welding

FIGURE 32-15 Schematic diagram of the equipment used for friction welding. (Courtesy of Materials Engineering.)

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Inertia Welding

FIGURE 32-16 Schematicrepresentation of the varioussteps in inertia welding. Therotating part is now attachedto a large flywheel.

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Examples of Friction Welding

FIGURE 32-17 Some typicalfriction-welded parts. (Top)Impeller made by joining achrome–moly steel shaft to anickel–steel casting. (Center)Stud plate with two mild steelstuds joined to a square plate.(Bottom) Tube componentwhere a turned segment isjoined to medium-carbon steeltubing. (Courtesy of Newcor BayCity, Division of Newcor, Inc.,Royal Oak, MI.)

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Friction-Stir Welding (FSW)

First performed by the Welding Institute of Great Britain in 1991

A nonconsumable welding tool (shoulder + protruding cylindrical or tapered probe or pin) is rotated at several hundred revolutions per minute.

Most common application is the formation of butt welds, usually between plates of the lower-point metals (both wrought and cast alloys) or thermoplastic polymers.

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Schematic of Friction-Stir Welding

FIGURE 32-18 Schematic of the friction-stir welding process. The rotating probe generates frictional heat,while the shoulder provides additional friction heating and prevents expulsionof the softened material from the joint. (Note: To provide additional forgingaction and confine the softened material, the tool may be tilted so thetrailing edge is lower than the leading segment.)

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Example of Friction-Stir Welding

FIGURE 32-19 (a) Top surfaceof a friction-stir weld joining 1.5-mm- and 1.65-mm-thickaluminum sheets with 1500-rpmpin rotation. The welding toolhas traversed left-to-right andhas retracted at the right of thephoto. (b) Metallurgical crosssection through an alloy 356aluminum casting that has beenmodified by friction-stirprocessing.

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Features of Friction-Stir Welding

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Ultrasonic Welding (USW)

Coalescence is produced by the localized application of high-frequency (10,000 to 200,000 Hz) shear vibrations to surfaces that are held together under rather light normal pressure.

Some heating is existed at the faying surfaces, but the temperature at the interface is rarely exceeds one-half of the melting point of the material.

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Schematic of Ultrasonic Welding

FIGURE 32-20 Diagram of theequipment used in ultrasonicwelding

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Application of Ultrasonic Welding

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Diffusion Welding (DFW)

Joining surfaces are maintained in contact under sufficient pressure and time at elevated temperature.

Frequently used to join dissimilar metals and composite materials.

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Explosive Welding (EXW)

FIGURE 32-21 (Left) Schematic of the explosive welding process. (Right) Explosive weld between mild steel and stainless steel, showing the characteristic wavy interface.

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Reference Problems

Review Questions 4, 5, 9, 12, 21, 28, 32, 34, 37