Resistance Welding

Based on principle: 1. elect (AC) current passed through metal decrease in Amps increase in volts 2. resistance of metal to electric flow 3. heats metal at contact until molten 4. pressure 5. cooling

Rougher metal = more resistance

Adv: fast little warpage accurately controlled well suited for production

10 Processes

(1) Spot - most common

two pieces fused with small nugget

lapping metal together clamp between two electrodes - current passes with pressure - solidifies can make up to 2000 spot welds on mild steel before redressing electrode

5 variables 1. time - Squeeze, Weld, Hold, Off 2. current 3. force - mechanical, elect, pneumatic, hydraulic

4. machine selection

rocker

press

portable

5. electrode contact area - weld size should equal tip diameter - proper current

Defects expulsion - molten metal squirts out from weld - current too high

blow holes - caused by excessive heat or too little pressure

large indentions - caused by improper electrode alignment - excessive pressure

(2) Projection

form projections with dies

pieces are pressed together

current passes through two pieces and is concentrated at bumps

forms a spot weld

adv: several spots welded at once and welds are concentrated at bumps

disadv: not cost effective for short runs

(3) Seam

two wheeled electrodes travel over metal - current passes through them

can produce continuous seam

can produce intermittent seam of overlapping spots for leak proofing

Seam welding I-Beams

(4) Butt Seam another form of seam

Weld longitudinal pipe seam elect current heats pipe to molten state before passing through rollers