In arc welding, the heat required to melt the metals in the joint is generated by the arc between the electrode and the parts to be joined. Two types of electrodes are used: (1) a consumable stick or wire electrode, which not only conducts electricity but also melts and introduces filler material into the joint, and (2) a non-consumable stick electrode, which merely conducts current in the weld area.
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The arc generates a temperature of approximately 3500°C at the tip of the electrode and creates a pool of molten metal in
% of the weld zone. As the puddle solidifies behind the electrode as it moves away from the joint, a metallurgical bond is formed between the adjacent parts. In order to avoid a chemical reaction between the liquid metal and the oxygen or nitrogen in the ambient air, the welding zone is protected by a supply of shielding gas or slag.
In the automotive industry, arc welding is used for both steel and aluminum. However, the arc welding processes for steel and aluminum differ due to different melting points, thermal conductivities and thermal expansion coefficients (see Table 8.3). Aluminum arc welding is also affected by the presence of an aluminum oxide layer on its surface. The melting point of the oxide layer is around 2035 °C, which is three times higher than that of aluminium.This oxide layer tends to absorb moisture from the air and, since moisture is a source of hydrogen, leads to porosity in aluminum welds. Hydrogen can also come from oil, lubricants, paint and various surface contaminants. Because hydrogen is soluble in liquid aluminum, it is dissolved in the liquid weld pool. However, as the temperature drops during cooling, the solubility of the hydrogen in the aluminum decreases and dissolved hydrogen is released upon solidification. As
cools rapidly, free hydrogen becomes trapped in the weld and causes porosity.Therefore, before arc welding, the aluminum oxide layer must be removed from the aluminum surface. Small oxide particles that detach from the oxide layer not only cause hydrogen porosity, but can become lodged in the weld and lead to reduced ductility, incomplete fusion and cracking.
Weld metal integrity is not generally an issue with low carbon steels. However, care must be taken when arc welding galvanized steel as zinc fumes can cause weld porosity in high speed welding processes. In general, low carbon steel arc welding is as strong as the base steel; In most cases, however, the arc weld in the aluminum alloy is weaker, often significantly, than in the base aluminum alloy.For non-heat treatable 5000 series alloys, the weld zone exhibits zero temperature annealing properties regardless of the initial cold work. For heat treated 6000 series alloys, the weld zone properties are much lower than in the T6 temper. Post-weld heat treatment can help restore weld zone properties in heat-treatable alloys.
Of the aluminum alloys used in automobile bodies, the 5000 series alloys have better weldability than the 6000 series alloys. The 5000 series alloys can be welded with or without the addition of filler metal, while the 6000 series alloys require filler material to avoid shrinkage cracks. during the solidification of the molten liquid. An aluminum alloy with a high Mg content, for example the alloy 5356 (Al-5% Mg), is usually used as the filler material for aluminum alloys. The second
filler material used in the 6000 series alloys is a high Si aluminum alloy such as alloy 4043 (Al-5% Si). Another problem with arc-welded aluminum alloys is thermal distortion, which can lead to serious problems in maintaining dimensional stability.
With the increasing use of high strength steels and AHSS, the possibility of arc welding had to be considered. Table 8.4 lists the weld strength values obtained from single-turn shear tests of a high-strength low-alloy steel (HSLA), a conventional high-strength steel, and four AHSS steels, i. H. two DP steels and two martensitic steels (M ). Joint performance, defined as the ratio of weld strength to base metal strength, is very high for HSLA and DP steels but much lower for martensitic steels. Poor sealing performance in martensitic steels is attributed to softening of the heat-affected zone (WCC) due to tempering during the cooling phase.Interestingly, the fatigue strength of these steels is not affected by the softened HAZ and appears to be insensitive to the static strength of the base material (Yan et al., 2005).
