I used to TIG aluminum castings every day. We’re an aluminum foundry and, on certain jobs, were allowed by the customer to weld certain defects in certain areas of the casting. You need to know if the casting was made using one of the sand casting processes. If so, it is going to be full of trash. As the casting is heated, all of the sand, process coatings, and such that are in the porosity are goi no to float to the top, the molten area. When our guys were welding, you could see them flicking the filler rod. They were flipping the trash off as it floated to the surface. If the casting was made in China, Korea, or India, this problem will be even worse.
That's the key, you have to burn the impurities out and the success depends on the material.
