Welding processes5 resistance welding

Welding processes5 resistance welding

Solid-State Welding Processes

Joining takes place without fusion at the

interface

No liquid or molten phase is present at the joint

Two surfaces brought together under pressure

For strong bond, both surfaces must be clean:

— No oxide films

— No residues

— No metalworking fluids

No adsorbed layers of gas

— No other contaminants

Solid State Bonding Involves one or more of:

¢ Diffusion: the transfer of atoms across an

interface

— Facilitated by heat

¢ Friction

* Electrical-resistance

« Induction

« Pressure:

— The higher the pressure, the stronger the interface

— May combine pressure & resistance heating

¢ Relative interfacial movements

— Create clean surfaces

— Even small amplitudes improve bond strength

Resistance Welding (RW)

The heat required for welding is produced by

means of electrical resistance across the two

joining components

Process does not require:

— Consumable electrodes

— Shielding gasses

— Flux

Bond strength depends on surface roughness &

cleanliness

Requires specialized machinery (generally non-

portable)

Many facilities now automated

Low operator skill level

Resistance Spot Welding

The simplest and most commonly used resistance-welding process

The tips of two opposing solid, cylindrical electrodes touch a lap joint of two sheet metals

Resistance heating produces a spot weld

A strong weld nugget is made by applying pressure until the current is turned off and the weld has solidified

FIGURE 31.6 (a) Sequence of events in resistance spot welding

(b) Cross section of a spot weld, showing the weld nugget and the indentation

of the electrode on the sheet surfaces This is one of the most commonly used processes in sheet-metal fabrication and in automotive body assembly

Electrodes Wield nugget

1 Force 2 Current 3 Current off, 4 Force

(a)

Electrode

Electrode tp hidentotion

Sheet

Weld nugget “Ss separation

£

s

= Hest-allected zone Electroie ——t- 1

`

——

(bi

Resistance Spot Welding

A variety of electrode shapes are used to spot-weld areas that are difficult to reach

FIGURE 31.7 Two electro

Workpiece

1

Workptece

de designs for easy access to the

components to be welded

Electrodes

(a)

(b)

Resistance Spot Welding

Widely used for fabricating sheet metal parts

Modern equipment is computer controlled for optimum timing of current and

pressure; spot welding guns are manipulated by programmable robots

FIGURE 31.8 Spot-welded (a) cookware and (b) muffler

(c) An automated spot-welding machine The welding tip can move in

three principal directions Sheets as large as 2.2 x 0.55 m (88 x 22 in.)

can be accommodated in this machine with proper workpiece supports

Source: Courtesy of Taylor—Winfield Corporation

Tests for weld nugget strength

(a) Tension shear tests are easy to perform and are inexpensive; therefore

commonly used in fabricating facilities (b) & (c) Both capable of revealing flaws, cracks, and porosity in the weld area (d) Peel test commonly used

for thin sheets

FIGURE 31.9 Test methods for spot welds: (a) tension-shear test, (b) cross-tension test, (c) twist test, (d) peel test (see also Fig 32.9)

GP sf

(a)

t

Hole left in part )

(ea indicates quality

(c) (d)

Resistance Seam Welding (RSEW)

FIGURE 31.10 (a) Seam-welding process in which rotating rolls act as

electrodes (b) Overlapping spots in a seam weld (c) Roll spot welds

and (d) Mash seam welding

Electrode wheels

Resistance Seam Welding (RSEW)

A modification of spot welding: the electrodes are replaced by rotating wheels or

Rollers Using a continuous AC power supply, the electrically conducting rollers produce a spot weld wherever the current reaches a sufficiently high level in the AC cycle

(b) With a high enough frequency or slow enough traverse speed, these spot welds

actually overlap into a continuous seam that is liquid and gas tight Process used to make the longitudinal (side) seam of cans, mufflers, gasoline

tanks, and other containers

High Frequency Resistance Welding (HFRW)

FIGURE 31.11 Two methods of high-frequency continuous butt

welding of tubes

Contacts

Vee

Squeeze roll

Tube travel

roll

High Frequency Resistance Welding (HFRW)

Similar to seam welding

Exception: employ high-frequency current (up to 450 kHz)

The heated edges are pressed together by passing the tube through a pair of

squeeze rolls

Any flash is then trimmed

Can fabricate structural sections (such as | beams) by welding the webs and

flanges made from long flat pieces

(b) High Frequency Induction Welding (HFIW)

Note the electrical coil

Resistance Projection Welding (RPW)

FIGURE 31.12 (a) Schematic illustration of resistance projection welding (b) A welded bracket (c) and (d) Projection welding of nuts or

threaded bosses and studs (e) Resistance projection welded grills

rr

Flat electrodes ———-f tee Weld

nu t

Workpiece

!

Weld nugget Bolt Weld nugget

-T tk 2t

Resistance Projection Welding (RPW)

High electrical resistance at the joint is developed by embossing one or more

projections (dimples) on one of the surfaces to be welded

The projections may be round or oval for design or strength purposes

High localized temperatures are generated at the projections (first contact)

Can weld nuts & bolts to sheets ((c) & (d))