Development of Welding Cast Aluminum

Cast aluminum, like other cast products, was originally welded using oxy-fuel gas welding because that was the process that was available. Oxy-acetylene and oxy-hydrogen forms of welding were also used. To protect the aluminum weld during these processes, a flux was used and then removed to prevent corrosion of the weldment. Since the invention of GTAW, oxy-fuel welding is rarely used for welding cast aluminum; however, it may be found in your career for non-critical welds.

Today, GTAW and GMAW are used to weld cast aluminum similarly to their use for welding wrought aluminum.

Basics of Welding Cast Aluminum

Like many cast products, cast aluminum is welded for repairs at the foundry to fill in missing surfaces. It is also welded to repair cracks on in-service castings. Cleanliness is important for repairs of in-service equipment because they could be covered in or have soaked up oil, greases, fuels, and other hydrocarbons used to lubricate or fuel the machine parts. You may not be able to remove all of the impurities before welding, but you must do the best that you can.

When welding a repair of a casting such as a crack, it is very important to first identify the size and length of the crack. Start by cleaning the surface surrounding the crack by removing any paint, 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 you first thought.

The next step is to drill the ends of the cracks to prevent them from spreading any further; this step removes the sharp point of the crack and replaces it with a round hole. Structurally speaking, round holes do not allow a stress concentration to form and the stress flows around the hole instead. When you drill the ends of the hole, make sure to countersink it so you can inspect the metal below the surface to see if the crack extends below the surface.

Preheating aluminum is important due to the brittle nature of casts and the nature of its thermal conductivity (the ability of heat to travel through a material) being lower than for 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. Do this by slowly and evenly heating up castings to their required preheat temperature. Large castings and ones with differently sized parts sticking out from the main fabrication generally require more preheat than smaller castings. Check the base material’s temperature often and use additional preheat to maintain the temperature as needed.

As important as it is to preheat your aluminum, it is equally important to have all of your welding equipment and materials ready to go before beginning the process so that you are ready to weld as soon as it is complete and don’t need to spend 30 minutes looking for a tool while the casting cools off and aluminum oxide starts to form. Preheating costs money, and any additional preheating caused by delays means more money. However, if the preheating process reveals previously unobserved cracks in the casting, stop and address the new cracks as if they were originals. This will likely mean that the weldment has to cool back down to room temperature. This must be done slowly, as air cooling is generally too fast. If you used an oven to heat the cast, place the weldment back into the oven (turned off) and allow it to cool inside (the thermal insulation of the oven will slow the process). Welding blankets or burying the weldment in sand can also be used.

After the preheating and identifying all cracks that must be repaired, you are ready to weld. Note that if you are using GTAW, you will likely see a lot of junk and dirt floating to the surface of your weld pool while welding. This is normal and you will not be able to remove all of this dirt; 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 dirt looks like and how the welders in those videos handle it.

If you are interested in welding on cast aluminum, ask your welding instructor, as there may be cast aluminum in the shop or a way to get some. Also, 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, and you could be that “just in case” they had in mind in your journey to learn. You could also 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, and you may be able to score some scrap cast aluminum to weld on.

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, as it means a lesser vehicle weight and, therefore, 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 it takes their car to reach the finish line, and aluminum engine blocks—compared to iron or steel—are lighter and result in faster acceleration.

Attributions

1. 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