Aluminum alloys are susceptible to liquation cracking in the partially melted zone (PMZ). In the present study, four different cases of the weld metal T-fS (temperature vs. fraction solid) curve relative to the PMZ T-fS curve of full-penetration aluminum welds are presented and related to their liquation cracking susceptibility. In case one, the weld-metal fS < PMZ fS throughout PMZ solidification and there is no liquation cracking, for instance, alloy 6061 welded with filler metal 4043. In case two, the weld-metal fS > PMZ fS throughout PMZ solidification and there is severe liquation cracking, for instance, alloy 2219 welded to alloy 1100 with filler metal 1100. In case three, the weld-metal fS > PMZ fS during PMZ terminal solidification and there is liquation cracking, for instance, alloy 6061 welded with filler metal 5356. In case four, the weld-metal fS > PMZ fS except during PMZ terminal solidification and there is liquation cracking, for instance, alloy A357 welded with filler metal 5356. Based on these four cases, it is proposed that if the weld-metal fS > PMZ fS after the PMZ has begun to develop some strength, say, after fS > 0.3, liquation cracking is likely to occur in full-penetration aluminum welds.
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