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, and any additional preheating caused by delays means more money. As discussed with wrought aluminum, aluminum oxide forms quickly and you want to start welding shortly after cleaning and preheating. With all of the equipment for welding in place, you can begin preheating.
Due to the thermal stresses discussed above, it is very important to slowly and evenly heat up castings to their required preheat temperature. Larger casting generally requires more preheat than smaller ones, and castings with different sized parts sticking out from the main fabrication require more preheat also. Check the base material’s temperature often and use additional preheat to maintain the temperature as needed.
Now that the base materials have been prepped, welding should start immediately. If the heat of welding reveals new cracks in the casting, stop welding and address the crack as if it were an original crack. This will likely mean that the weldment will have to cool back down to room temperature. This must be done slowly. Air cooling is generally too fast. If you used an oven to heat the cast, place the weldment back into the turned off oven and allow it to cool inside. The thermal insulation of the oven will slow the cooling process. Welding blankets or burying the weldment in sand can also be used to slow the cooling process.
If using GTAW as the welding process, you will likely see a lot of junk and dirt floating to the surface of your weld pool. This is normal and you will not be able to remove all of this junk, however you can add more weld metal in an effort to coax the dirt out and replace it with sound weld metal. There are many good videos online that show what this junk looks like and how the welders in those videos handle it.
If you are interested in welding on cast aluminum, look for some old cast aluminum parts such as broken wheels or junk engine blocks in a shop. Many shops save materials “just in case” they are needed, you could be that “just in case” they had in mind in your journey to learn. Check online for manufacturers of cast aluminum and ask them for a tour of their facilities. This is a great way to network and get used to speaking with people in the manufacturing industry. You may be able to score some scrap cast aluminum to weld on. As always, also ask your welding instructor, there may be cast aluminum in the shop or a way to get some.
Uses of cast aluminum in industry today
Cast aluminum is used in industry in many applications where lightweight yet strong fabrications are needed. An aluminum engine block is a great example where a lighter engine block means less weight of the vehicle and improved fuel economy.
Drag racing cars are less concerned with fuel economy when racing a quarter mile, but they are concerned with shortening the time to the finish line, and aluminum engine blocks compared to iron or steel are lighter and result in faster acceleration. Figure 21.9 above has an image of an aluminum engine block.
Attributions
- Figure 21.9: A cylinder head made of lightweight, high-temperature ACMZ, a new suite of aluminum-based alloys developed by researchers at ORNL. © Jason Richards, Oak Ridge National Laboratory Used with permission. Courtesy of Oak Ridge National Laboratory, U.S. Dept. of Energy. ORNL Security & Privacy Notice
A piece of metal that has been formed by pouring liquid metal into a mold. Casting can also refer to the mold used itself.