The oxide is also heavier than aluminum and, when melted, tends to sink or be trapped in the molten aluminum. Aluminum is an excellent conductor of heat. It requires large heat inputs when welding is begun, since much heat is lost in heating the surrounding base metal. After welding has progressed a while, much of this heat has moved ahead of the arc and pre-heated the base metal to a temperature requiring less welding current than the original cold plate. If the weld is continued farther on to the end of the two plates where there is nowhere for this pre-heat to go, it can pile up to such a degree as to make welding difficult unless the current is decreased.
Some aluminum alloys exhibit hot short tendencies and are crack sensitive.
The proper choice of filler metal and welding procedures along with smaller beads can help eliminate many problems of this kind. Some experts recommend backstepping the first inch or so of each aluminum weld before finishing in the normal direction. The metal produced in the weld pool is a combination of filler and parent metals that must have the strength, ductility, freedom from cracking, and the corrosion resistance required by the application. See table below for recommended filler metals for various aluminum alloys. Maximum rate of deposition is obtained with filler wire or rod of the largest practical diameter while welding at the maximum practical welding current.