Why do you get "crater cracks" in aluminum and how do you prevent them?
Crater cracks happen for three reasons:
1. High thermal rate of conductivity
2. The relatively large change in volume when aluminum solidifies
3. The concave shape of the crater
Aluminum cools so fast that it doesn't provide adequate time for the weld bead to flatten or the crater to fill. The deep depression of the crater quickly freezes in a concave shape, exerting high tensile stresses on the surrounding metal. It is in this area that a crack will propagate through the weld metal. As the weld cools, crater cracking is common if proper steps are not taken to minimize the problem.
Preventing Crater Cracks in Gas Tungsten Arc Welds (i.e., TIG)
It is usually fairly easy to fill the crater in a TIG weld. Just be sure not to leave the crater concave. It should be flat or convex. You do this by decaying the welding current smoothly. While doing that, add two or three small drops of filler metal to the weld until the crater is filled.
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