How much preheat should I use when welding aluminum?
aluminiumsheet > 12-17-2019, 06:38 AM
While a little preheat is good, too much preheat can degrade the mechanical properties of the aluminum.
As was discussed earlier, the last heat treatment for heat treatable alloys is 400° F so if the operator preheats the aluminum to 350° F and holds the temperature in that range while welding, the aluminum's mechanical properties are changed.
For the non-heat treatable alloys such as the 5000 series, if the operator holds the temperature even in the 200° F range - he or she can sensitize the material to stress corrosion cracking. In most cases, some preheat is acceptable to dry the moisture away from the piece, but preheat should be limited.
Many inexperienced aluminum welders use preheat as a crutch. Since equipment for welding aluminum needs to operate at higher capacities, many feel that preheat helps eliminate equipment limitations, but this is not the case. Aluminum has a low melting point -- 1200° F compared to 2600° F to 2700° F for steel. Because of this low melting point, many operators think they only need light duty equipment to weld the aluminum. But, the thermal conductivity of aluminum is five times that of steel, which means that the heat dissipates very quickly. Therefore, welding currents and voltages for welding aluminum are higher than they are for steel so operators actually need heavier duty equipment for aluminum.