21.3 Welding Cast Aluminum
David Colameco, M.Ed.
Development of welding cast aluminum
Cast aluminum like other cast products was originally welded using oxyfuel gas welding because that was the process that was available in the early days. Oxyacetylene and oxyhydrogen forms of OAW were used. To protect the aluminum weld a flux was used with OAW that was then removed to prevent corrosion of the weldment. With the invention of GTAW, OAW is rarely used for welding cast aluminum however it may be found in your career for non critical welds.
GTAW and GMAW are used today to weld cast aluminum similarly to their use for welding wrought aluminum.
Basics of welding cast aluminum
Cast aluminum like other cast products is welded for repairs at the foundry to fill in missing surfaces. It is also welded to repair cracks on castings that have been in service. Just like other castings, cleanliness is important because many casts are repairs of equipment that were used in service and could be covered in or soaked up oil, greases, fuels, and other hydrocarbons used to lubricate the machine parts or fuel the parts if it is an engine block, for example. You may not be able to remove all of the impurities but you must do the best that you can.
If you are repairing a casting, it is very important to identify the size of the repair. If you are repairing a crack in a casting you must determine how long the crack is. First clean the surface of the casting in the area of the crack by removing any paint but also being careful not to smear metal over the crack through mechanical removal of the paint. You can use dye penetrant tests if doing a repair at home and follow the instructions for timing of the application. The crack is likely longer than you can see with the naked eye, and the dye penetrant might show you a crack that is a few inches longer than first thought.
The next step is to end drill the cracks to prevent them from spreading any further. Drilling the ends of the cracks removes the sharp point of the crack and replaces it with a round hole. Structurally round holes do not allow a stress concentration to form and the stress flows around the hole. When you end drill the hole, make sure to countersink it so you can inspect the metal below the surface to see if the crack is extending below the surface.
Preheating aluminum is important due to the brittle nature of casts. The thermal conductivity, the ability of heat to travel through a material, is lower than other materials. Check to see what preheat is needed for your cast aluminum so thermal stresses can be reduced as much as possible in an effort to reduce the possibility of your cast repair failing due to cracking.
It is equally important to have all of your welding equipment and materials ready to go before preheating begins. That way once preheating is completed you are ready to start welding, and are not off spending 30 minutes looking for something while the part cools off. Preheating of casts costs money