Once the proper wire has been selected, focus must shift to the proper technique for mold alloy to be welded. This is the largest contributor to a successful weld repair. A proper technique involves more than the actual welding; it must also take into account ambient conditions and preheating of the material, which will remove condensation and increase its weldability.

Typically, alternating current (AC) is used for welding aluminum, however direct current (DC) can produce better results in repairing mold alloys. AC and DC refer to the polarity of the electrical current as it runs through the electrode. Selecting an electrode with the appropriate polarity will have a significant impact on the strength and quality of a weld.

As mentioned earlier, when aluminum is exposed to air, a thin layer of aluminum oxide forms. If this oxide layer is not removed, proper weld fusion and fluidity will be negatively affected. To remove this layer requires a much higher temperature than the temperature required to melt the base aluminum. Alternating current flows in one direction for half the time and in the other direction for the second half. Because AC changes polarity so often (120 times per second with a 60-Hz current), the oxide layer is removed first, enabling the base metal to then melt and fuse more quickly. For best results, an equal and balanced current in both directions is needed.

Direct current flows in only one direction, creating constant polarity. A DC pulse weld, in which the arc pulses between a high peak and low background current, will narrow the HAZ and the heat applied to the parent material, preserving the metal’s original properties. Using an AC pulse weld exposes the welded area to greater porosity as well as poor color match. Porosity occurs when gases become entrapped by welding turbulence when too low of a welding current is used. On aluminum, welding should be performed at a quick rate in order to prevent heat from penetrating into the parent block. If it is performed too slowly, the risk of excessive burn-through increases.

As the use of aluminum tooling increases, piece-part costs will decrease, and this in turn will make the use of aluminum tooling even more common. Selecting optimal welding wires and techniques will make potentially difficult processes easier and provide superior welding results.