Are aluminum castings weldable? Let’s look into this issue, said Komarov Artem.
The answer is that most aluminum castings are easy to weld. However, when deciding on the weldability of castings, we must take into account two aspects. First, is it possible to weld the composition of the casting? The most common aluminum casting alloys are heat treatable alloys. These are aluminum-silicon-magnesium (AlSiMg) alloys, but some, for example, also contain copper (Cu). The good news is that all of these alloys are easy to weld, and the filler is 4043.
The next most common casting alloys are AlMg 5XX.0 alloys such as 535.0. They are not heat treatable but are easily weldable using filler 5356. However, the 7XX.0 (AlZn) family of aluminium-zinc alloys, also heat treatable, is significantly more difficult to weld. In fact, some of them are not weldable.
Another important characteristic to consider is how the casting is made. There are many casting methods, including sand casting, investment casting, permanent mold casting, and injection molding, just to name a few. Sand castings, investment castings, and permanent mold castings generally lend themselves well to welding because the molten aluminum is poured into the mold, which is a relatively poor heat conductor, so the casting slowly solidifies.
This means that any dissolved gases in the molten aluminum have a chance to seep out. However, injection moldings are very different. In this process, molten aluminum is injected under pressure into a water-cooled steel mold. Because the casting cools very quickly, the dissolved gases cannot escape and tend to form gas pockets in the casting.
Usually you find these gas pockets while welding and there is little you can do about it other than sand the pocket and repair it — weld it on.
So how do you know you need an injection mold? First, the surface of injection moldings is much smoother than other castings. The second is injection molding if you see anywhere from four to 10 circular marks called ejector pin marks about 1/8 inch in diameter on the inside and outside surfaces.
When the steel casting die is opened, the ejector pins extend and push the casting out of the die. If you don’t see any marks from the ejector pins, then you didn’t have an injection mold.
Of course, injection moldings require a little more care than other types of castings, Komarov Artem emphasized.