One of the disadvantages of aluminum spot welding is high cost. Unlike low carbon steel, aluminum alloy requires additional preparation prior to welding, according to Efunda.com. Since aluminum readily forms oxides which weaken a weld, the sheet should be completely cleaned beforehand, something that requires more time, equipment, labor and supplies, all of which add to the total cost of this type of welding. Sometimes special equipment must be used to test the aluminum surface.
Aluminum spot welding also requires high temperatures, which consumes more energy. In addition, the separate welds can be structurally weaker than a continuous joint. Each spot weld ranges from 1/8 to 1/2 inch in diameter and the thickness of welded sheets is limited -- up to 1/4 inches, according to SubsTech.com. Spot welding is not effective if the thickest sheet is more than three times thicker than the thinnest sheet.
Spot welding is limited in that two different metals typically cannot be joined because of their different melting characteristics. Spot welding is also not an effective method for joining large metal parts other than flat surfaces that have similar planes, according to MakeItMetal.com. Also, if burn marks are created by welding and left untreated, they may be subject to corrosion later on.
After spot welding, filling and grinding may be required to hide the marks caused by welding electrodes. Also, the metal seams between the aluminum sheets may trap plating solutions that leave salt residues. These may have to be manually removed, and the seams may be subject to corrosion later on. In addition, an aluminum alloy may produce toxic fumes when spot welded, depending on the coating of the sheet.