Quote:
Originally Posted by SLacker
... You also will have locally "drawn back" the heat treatment of the aluminum material in the area of the repair. A purist will say that you have to re-quench and temper, but a softer spot on the head may or may not be a problem. It is reasonable to take a chance on that considering the savings.
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Not so with aluminum. Aluminum is solution treated and aged, not quench and tempered. Welding would render the localized area overtempered (soft). This area cannot be "drawn back". The only way to reharden the area is to re-solution treat and age the whole head.
The damage looks familiar to me. I've seen it on heads I've studied in the lab where nucleant boiling of the coolant takes place. This occurs on heads where the coolant doesn't have a high enough boiling point. Nucleant boiling produces an corrosive/erosive effect on the area that sees boiling. Coolant that is at the suction side of the water pump enhances this effect.