Partially-Melted-Zone Cracking in Aluminum Welds

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0098776KouAluminum alloys are susceptible to hot cracking in the region right outside the fusion zone called the partially melted zone (PMZ), where grain boundary liquation occurs during welding. Studies on 6000 alloys (Al with Mg, Si and some Cu, Cr and Fe) have demonstrated that the composition of the weld metal is critical in preventing PMZ cracking. It has been proposed that cracking occurs if the weld metal composition is such that TWS (the weld metal solidus temperature) TBS (the base metal solidus temperature). However, contradictions to this TWS TBS criterion for cracking have been reported possibly because of inaccurate TWS and TBS and/or non-equilibrium solidification. This collaborated study between UW-Madison and ALCOA on PMZ cracking in aluminum welds focuses on the mechanism of cracking and the effect of the weld metal composition. First, the PMZ microstructure of several aluminum alloys will be observed to prove that the solidifying and shrinking weld metal pulls and tears the liquated PMZ to cause cracking. Second, binary and ternary alloys of known TWS and TBS will be prepared and welded to test the TWS TBS criterion for cracking. Al-4.5Cu alloy will be welded with pure Al (TWS TBS) and Al-6.3Cu (TWS TBS) fillers, and the resultant welds will be checked for PMZ cracking. Likewise, Al-1Mg-0.6Si alloy (close to the popular alloy 6061, which is being used for car frames) will be welded with pure Al (TWS TBS), Al-5.2Si (TWS TBS) and Al-5Mg (TWS TBS) fillers and checked. Third, the feasibility of assessing the PMZ cracking susceptibility with non-equilibrium T (temperature) vs. fs (fraction of solid) curves will be checked with the welds of the binary and ternary alloys. TWS and TBS reflect equilibrium solidification but non-equilibrium solidification prevails in welding. A PMZ higher in fs than the weld metal at any T is likely to be stronger than the weld metal, resisting its pulling and tearing. T vs. fs will be calculated using a computer code developed at UW-Madison based on thermodynamic and kinetic models and tested extensively with aluminum alloys. %%%As the auto industry is moving to all-aluminum cars, it is essential to better understand and control defects in aluminum welds. ALCOA has a strong interest in hot cracking in aluminum alloys and will cast the binary and ternary alloys for UW-Madison. Graduate students will go to ALCOA to learn how to cast special alloys for welding and duplicate the ALCOA facility at UW-Madison. ***