Getting the Base Metal Ready

Obviously, in order for a welding project to exist at all, there must be two or more pieces of metal that you wish to join together. Standard welding can only occur if both pieces of base metal are of the same metallic composition, and the filler metal used to add to the weld pool must likewise be of the same material. Typical welding techniques can only be used to bond steel to steel, aluminum to aluminum, and so forth.

This is true because of a very straightforward and basically unavoidable reason – all metals have different melting temperatures, while both pieces of base metal must become molten at the same time if the welding attempt is to have any effect. Trying to weld aluminum and steel in the standard manner, for example – using a MIG welder – would fail because the melting point of aluminum is 1,217˚ Fahrenheit, while high-carbon steel melts at 2,500˚ Fahrenheit.

Heating the aluminum to the melting point would leave the steel still solid, and no weld would be formed; heating the steel to the melting point would reduce the aluminum to a sagging, liquefied, hole-riddled ruin, and there would be nothing left for the molten steel to bond onto (even leaving out questions of chemical incompatibility).

The surface of the metal must be readied for welding on both items of base metal, if the weld is to be firm, clean, and successful. Surface contaminants need to be eliminated and, if the base metal is painted, the paint needs to be stripped completely away from the welding area both to ensure proper adhesion and to prevent toxic fumes from being released by the paint when the welding arc heats it and boils it away as vapor.

Steel is usually readied by washing it in phosphoric acid metal preparation solutions, which etches out impurities and gives the surface a bit of tooth in any case. A stainless steel brush can be used to aid this process. Steel and other metals can also be sanded with 80 grit sanding discs on a disc sander or an angle grinder (the latter being a useful tool for the welding workshop regardless).

Aluminum is a special case because it forms a thin, invisible layer of aluminum oxide over its surface within eight hours of the oxide being removed. This skin of aluminum oxide has a far higher melting point than the aluminum itself – aluminum oxide melts at 3,200˚ Fahrenheit, while the aluminum itself melts at only 1,217˚.

Attempting to weld with this layer in place can result in the heat of the welding arc being deflecting by the oxide, disrupting the welding process – or it can cause excessive heat to built up in the aluminum sandwiched between its layers, leading to sudden, unexpected holing and slumping of the metal.

It is therefore crucial to remove the aluminum oxide from the base metal’s surface – a process best carried out by washing with acetone, followed by a water rinse and scrubbing with a stainless steel brush. The brush should be used for aluminum projects only to avoid contaminating the welds – and the cleaning should be done immediately before the welding is carried out, or the aluminum oxide will form again and it will be necessary to start over.