Partially-Melted-Zone Cracking in Aluminum Welds

铝合金在熔合区外的部分熔化区（PMZ）容易发生热裂，焊接时晶界液化发生在该区域。对6000种合金（Al与Mg、Si和一些Cu、Cr和Fe）的研究表明，焊缝金属的成分对防止PMZ开裂至关重要。有人提出，如果焊缝金属成分达到TWS（焊缝金属固相温度）TBS（母材固相温度），就会发生开裂。然而，由于不准确的TWS和TBS和/或非平衡凝固，已经报道了与TWS TBS开裂标准相矛盾的地方。威斯康星大学麦迪逊分校和美国铝业公司对铝焊接中的PMZ裂纹进行了合作研究，重点研究了裂纹的机理和焊缝金属成分的影响。首先，对几种铝合金的PMZ组织进行观察，以证明凝固收缩的焊缝金属对液化的PMZ产生拉扯和撕裂作用。其次，将已知TWS和TBS的二元和三元合金进行制备和焊接，测试TWS TBS的开裂准则。将Al-4.5 cu合金与纯Al （TWS TBS）和Al-6.3 cu （TWS TBS）填料焊接，并检查焊接后的焊缝是否存在PMZ裂纹。同样，Al- 1mg -0.6 si合金（接近目前用于车架的流行合金6061）将与纯Al (TWS TBS), Al-5.2 si （TWS TBS）和Al- 5mg （TWS TBS）填料焊接并进行检查。第三，用非平衡T（温度）vs. fs（固体分数）曲线评估PMZ开裂敏感性的可行性将通过二元和三元合金的焊缝进行验证。TWS和TBS反映的是平衡凝固，但在焊接中普遍存在非平衡凝固。在任何T处，比焊缝金属高fs的PMZ可能比焊缝金属更强，从而抵抗其拉拔和撕裂。T vs. fs将使用威斯康星大学麦迪逊分校基于热力学和动力学模型开发的计算机代码进行计算，并使用铝合金进行广泛测试。随着汽车工业转向全铝汽车，更好地了解和控制铝焊缝缺陷是至关重要的。美国铝业公司对铝合金的热裂有浓厚的兴趣，并将为威斯康星大学麦迪逊分校铸造二元和三元合金。研究生将前往美国铝业公司学习如何铸造用于焊接的特殊合金，并复制美国铝业公司在威斯康星大学麦迪逊分校的设施。＊＊＊

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. ***