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ORIGINAL: proptop

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ORIGINAL: TimC

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ORIGINAL: w8ye

The casting material used in the die castings of model engines has a high zinc content so the aluminum will fill out all the little intricate details of the casting design.
Aluminum castings with a high zinc content do not lend themselves to being the most weldable of metals. What the welder experiences is, that to get the metal hot enough to get any penetration, you will be within a hair breath of the whole thing melting into a puddle or at least a hole melting in it. The solution to this problem is to use a low melting point "solder type weld metal" which, as a result, does not have the strength of the original metal.
Most of our model engines are die castings. Some of the low production and one-off racing engines may have sand cast crankcases. These would typically be of a more weldable (356 prime) alloy with only 2.5% zinc and be much stronger with 2.5% copper.

I'm sure this will spark up some counter points from some metallurgical weld engineer and the discussion is welcome.

Jim, let me add my limited experience with welding cast aluminum. I would vee out the crack and make what looked like a good weld. As the weld cooled it would crack. My solution was to make the vee a larger angle than normal and weld. After it cracked, I would re-vee at a normal angle leaving weld metal to weld on. This time the weld wouldn't crack. I've welded aluminum engine cases but nothing as small as our airplane engines. I think this would pose a challange with a heli-arc rig. I also am hoping one of our members with experience with cast aluminum welding will post.

I think if you could find a way to keep it from cooling down too quickly, the chances of cracking would be lessened...
My dad was a welding engineer, and told me that Alum. (generally ) needs to be pre-heated, and the cool down after welding needs to be controlled.
If we were talking about a model engine case (or other parts ) for example, perhaps putting the case or parts in an oven before and after welding/alumabrazing, etc. would be a good idea?