Resistance Welding

Resistance Welding

Resistance Welding -
Introduction
Aluminium and aluminium/alloys whether they are cast or wrought, heat-treatable or non-heat-treatable, can be resistance welded, some more readily than others. Resistance welding processes are especially useful in joining the high strength heat-treatable alloys.
Resistance Weldability of some Al Alloys

Serial Number	% Composition of the alloy	Resistance Weldability
Non-heat-treatable wrought alloys
1	99.0 Aluminium (Min)	RW
2	Al-1.2 Mn	RW
3	Al-1.4 Mg	RW
4	Al-0.45 Mn-4 Mg-0.1 Cr	RW
5	99.6 Al (Min)	ST
Heat-treatable wrought alloys
6	Al-0.25 Cu-0.6 Si-1 Mg-0.2 Cr	RW
7	Al-0.5 Cu-0.55 Mn	RW
8	Al4.4 Cu0.8 Si-0.8 Mn-0.4 Mg	ST
9	Al-6.2 Cu-0.3 Mn	ST
Castings Alloys
10	Al-0.6 Cu-5 Si	RW
11	Al-4 Mg-1.8 Zn	ST
12	Al-12 Si	LW
13	Al-10 Mg	NR

N.B.
RW - Readily Weldable
ST - Weldable in most applications but may require special techniques for specific applications.
LW - Limited weldability and usually requires special techniques.
NR - Welding not recommended.

Characteristics of Al-alloys affecting Resistance Welding
1. High electrical and thermal conductivities of aluminium and aluminium alloys,
2. Low melting temperature range,
3. Low strengths at elevated temperatures,
4. Narrow plastic ranges,
5. High shrinkage during solidification,
6. Presence of oxide coating on the surface of aluminium and its alloys.

High electrical conductivity necessitates the use of high capacity welding machines which can supply high welding currents to generate enough heat (αI²-R) to melt the base metal to produce the weld. High thermal conductivity requires welding to be done rapidly so that heat does not dissipate into base metal. Since aluminium alloys soften more rapidly and at lower temperatures than does steel, low-inertia welding machine heads are needed. Energy input to weld must be controlled to bring aluminium up to, but not above, its plastic range because plastic range is narrow.
High strength heat treatable alloys exhibit considerable shlilkage during solidification which can result in cracking. To avoid porosity and cracking the electrodes must maintain proper pressure on the nugget until solidification is completed.
Presence of oxide coating on the surface of aluminium
(i) increases electrode pickup,
(ii) decreases electrode life,
(iii) Produces lower (shear) strength welds.

Surface Preparation before Resistance Welding
Welds free from porosity and cracks and having maximum strength, symmetry and consistency can be obtained only if the surfaces to be joined are thoroughly cleaned before welding. Dirt, oil, grease etc., is cleaned with commercial solvents by wiping, dipping, washing, spraying or vapour degreasing.
Mechanical cleaning action must cut through the hard oxide film. Mechanical methods are very suitable when welding is confined to a small area and oxide film is thin. Mechanical cleaning may be carried out with the help of stainless steel wool or a stainless steel wire brush.
Chemical cleaning action involves an acid dip after oxide removal. An alkaline etch before the acid dip is still better.

Alkaline etch
It involves immersion in an aqueous solution of 5% NaOH at 66 to 72°C for 20 to 50 seconds, followed by rinsing in cold water.

Acid dip:
A typical solution contains
12% (by volume) Conc. Nitric Acid
0.4% (by volume) Hydrofluoric Acid
0.2% (by weight) wetting agent
Temperature 21 to 27°C Immersion time 2 to 6 minutes.

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