lect 5 resistance welding

Lecture 5Resistance Welding

Welding and Joining TechnologyEDPT 701

Dr. Mohamed Harraz12 Oct 2010 1

Resistance Welding

Resistance weldingDr. Mohamed Harraz

12 Oct 2010 2

Electric resistance welding (ERW) refers to a group of welding processes such as

spot and seam welding that produce coalescence of faying surfaces where heat to

form the weld is generated by the resistance of the welding current through the

workpieces. Some factors influencing heat or welding temperatures are the

proportions of the workpieces, the electrode materials, electrode geometry, electrode

pressing force, weld current and weld time. Small pools of molten metal are formed

at the point of most electrical resistance (the connecting surfaces) as a high current

(100–100,000 A) is passed through the metal. In general, resistance welding

methods are efficient and cause little pollution, but their applications are limited to

relatively thin materials.

Resistance Spot Welding (RSW), Resistance Seam Welding (RSEW), and

Projection Welding (PW) are commonly used resistance welding

processes. Resistance welding uses the application of electric current and

mechanical pressure to create a weld between two pieces of metal. Weld electrodes

conduct the electric current to the two pieces of metal as they are forged together.

The welding cycle must first develop sufficient heat to raise a small volume of metal

to the molten state. This metal then cools while under pressure until it has adequate

strength to hold the parts together. The current density and pressure must be

sufficient to produce a weld nugget, but not so high as to expel molten metal from

the weld zone.

http://en.wiktionary.org/wiki/coalesce

http://en.wikipedia.org/wiki/Faying_surface




12 Oct 2010 3

Resistance Spot Welding

In the electric resistance spot welding

process shown in the Fig, electrodes 1

and 7 press against workpieces 3 and

5. A current is then passed through

these components. Because of the

electrical contact resistance, heat will

be generated at electrode/work-piece

interfaces 2 and 6 and faying surface 4.

The heat at the faying face melts the

workpieces to form a nugget, 4.

To prevent melting at the electrode/

work-piece interface, water is circulated

in the cooling chamber of the

electrodes


12 Oct 2010 4

Processing and EquipmentSpot welding involves three stages; the first of which

involves the electrodes being brought to the surface of

the metal and applying a slight amount of pressure. The

current from the electrodes is then applied briefly after

which the current is removed but the electrodes remain

in place in order for the material to cool. Weld times

range from 0.01 sec to 0.63 sec depending on the

thickness of the metal, the electrode force and the

diameter of the electrodes themselves.

The equipment used in the spot welding process

consists of tool holders and electrodes. The tool holders

function as a mechanism to hold the electrodes firmly in

place and also support optional water hoses which cool

the electrodes during welding. Tool holding methods

include a paddle-type (foot switch), light duty, universal,

and regular offset. The electrodes generally are made

of a low resistance alloy, usually copper, and are

designed in many different shapes and sizes depending

on the application needed.

After the current is removed from the work-piece, it is

cooled via the coolant holes in the center of the

electrodes. Both water and a brine solution may be

used as coolants in spot welding mechanisms.


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Spot Welding Temperature distribution

A steady-state heat transfer analysis produced this

temperature distribution for the spot welding model,

which simulates internal heat generation based on

current distribution from the electrostatic current and

voltage analysis.


12 Oct 2010 6

Resistance Welding Benefits and Limitations

Benefits:•High speed welding

•Easily automated

•Suitable for high rate production

•Economical

Limitations

•Initial equipment costs

•Lower tensile and fatigue strengths

•Lap joints add weight and material


12 Oct 2010 7

Resistance Seam Welding

Resistance seam welding is a process that produces a weld at the faying surfaces of two similar

metals. The seam may be a butt joint or an overlap joint and is usually an automated process. It differs

from butt welding in that butt welding typically welds the entire joint at once and seam welding forms the

weld progressively, starting at one end. Like spot welding, seam welding relies on two electrodes,

usually made from copper, to apply pressure and current. The electrodes are disc shaped and rotate as

the material passes between them. This allows the electrodes to stay in constant contact with the

material to make long continuous welds. The electrodes may also move or assist the movement of the

material.

A transformer supplies energy to the weld joint in the form of low voltage, high current AC power. The

joint of the work piece has high electrical resistance relative to the rest of the circuit and is heated to its

melting point by the current. The semi-molten surfaces are pressed together by the welding pressure

that creates a fusion bond, resulting in a uniformly welded structure. Most seam welders use water

cooling through the electrode, transformer and controller assemblies due to the heat generated. Seam

welding produces an extremely durable weld because the joint is forged due to the heat and pressure

applied. A properly welded joint formed by resistance welding is typically stronger than the material from

which it is formed.

A common use of seam welding is during the manufacture of round or rectangular steel tubing. Seam

welding has been used to manufacture steel beverage cans but is no longer used for this as modern

beverage cans are seamless aluminum.


http://en.wikipedia.org/wiki/Butt_welding


12 Oct 2010 8

Resistance Seam Welding


12 Oct 2010 9

Project welding for fasteners

A locating electrode has an insulated hole

in the center that protects the fastener's

threads from current during the weld.

Projection electrodes carry more current and weld thicker

material than spot welding electrodes. For this reason, the

projection process begins to show its advantages at about 1

mm and thicker (though it certainly can weld thinner metal).

During the weld, the extra metal within each projection heats

up into a liquid nugget, which in turn helps control and focus

heat to initiate fusion. Proper fusion happens when both the

weld fastener and the base metal reach the welding

temperature simultaneously.

Projection welds use two flat electrodes large enough to

cover the entire face of the fastener, and the projections

themselves help absorb heat, thereby producing a clean

weld without discoloration. The result: The process produces

strong welds that are close to being cosmetically perfect.

The large tooling has another benefit: The more bearing

surface, the longer the electrode life. Projection welding

electrodes also can have a harder copper facing that

extends their life even more. And unlike spot welding

electrodes, projection electrodes can weld a wide range of

sheet thicknesses, reducing changeouts between different

runs.

Resistance weldingDr. Mohamed Harraz12 Oct 2010 10

Project welding for fasteners

A weld screw fused with no

insulation inside the locating

electrode causes distortion in the

threads.

The process requires even contact and pressure

between the base metal and projections on the

fastener. Excessive electrode pressure actually

reduces the resistance between the fastener and

material being welded, causing weak, incomplete

welds or no weld at all. Insufficient pressure can

cause flashing, burning, and discoloration.

The positions of fastener projections can vary, and

each has its pros and cons. Some fasteners have

projections located at the center of the head or, for

nuts or screws, the midpoint of the flange. This

helps concentrate heat in the center of the fastener

itself or its flange, preventing excessive heat at the

edges and eliminating spatter. Other designs place

projections near the edge of the fastener. For

fasteners with external threads, projections near

the edge concentrate heat farther away from those

threads, reducing distortion. However, because

those projections are near the flange edge, spatter

can be an issue.


Resistance Welding Problems and Discontinuities

• Cracks

• Electrode deposit on work

• Porosity or cavities

• Pin holes

• Deep electrode indentation

• Improper weld penetration

• Surface appearance

• Weld size

• Irregular shaped welds

If the operation produces poor, weak, or no welds at all, try one of the

following:

•Increase weld time

•Increase heat (secondary voltage)tap

•Reduce electrode pressure

•Check for dirty or scaly metal


Safety

It is common for a spray of molten metal droplets (sparks) to be ejected from

the area of the weld during the process.

While spot welding does not generate UV light as intensely as arc welding,

eye protection is still required. Welding goggles with a 5.0 shade are

recommended

lect 5 resistance welding

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