Solidification Cracking in Welds of Aluminum Alloys

非技术铝合金越来越多地被用于汽车轻量化，而弧焊是最通用的焊接工艺。然而，铝合金在凝固过程中容易产生裂纹，在焊接中称为凝固裂纹，在铸造中称为热裂。在本项目中，提出了一个简单的新指标来预测铝合金的相对裂纹敏感性以及焊接用填充金属对相对裂纹敏感性的影响。该指数可以使用商用的铝合金软件包和数据库很容易地计算出来。该指标对于选择现有的焊料或寻找新的焊料以避免铝焊缝的凝固开裂具有重要的指导意义。该指标也适用于铝铸件，铝铸件的热裂是一个严重的问题。该项目旨在为西班牙裔研究生提供学习焊接、铝合金和焊接部件失效分析的机会。技术摘要合金在凝固过程中以弱半固态状态存在，在凝固末期固体颗粒被薄液膜隔开。在焊接或铸造过程中产生的拉力作用下，半固态会沿晶界开裂。在凝固过程中，半固态的分数f随温度T的降低而增加的程度对裂纹敏感性有显著的影响，不同的合金有很大的不同。与以前的研究不同，本项目考虑了晶界，也就是开裂发生的地方。它认为：1.分离相邻颗粒引起开裂的张力；2.相邻颗粒相互生长以粘结在一起以防止开裂；3.液体补缩以填充相邻颗粒之间的边界以抵抗开裂。根据晶界裂纹产生的判据，推导出一个简单的新指标来预测铝合金的相对裂纹敏感性和焊料对降低裂纹敏感性的有效性。该指数可以根据T-f的平方根曲线的陡度来确定，这可以很容易地使用商业软件包和数据库来计算。开发了一种简单的新试验来评估工业铝合金焊接过程中的相对裂纹敏感性，并验证了新的准则和指标。为了在测试过程中导致裂纹，单独的焊缝半固态区域以与焊接过程中自然发生的速度相似的缓慢速度拉伸，而不是在焊接过程中突然弯曲工件，这是在最广泛使用的测试中进行的，这会在半固态区域引起不切实际的快速和高张力。

Non-Technical Abstract Aluminum alloys are increasingly used for vehicle weight reduction, and arc welding is the most versatile process for welding them. However, aluminum alloys are susceptible to cracking during solidification, which is called solidification cracking in welding and hot tearing in casting. In the present project a simple new index is presented to predict the relative crack susceptibility of aluminum alloys and how it is affected by the filler metals used for welding. The index can be calculated easily using commercially available software packages and databases for aluminum alloys. The index can be a useful guide to selecting existing welding filler metals or searching for new ones to avoid solidification cracking in aluminum welds. The index can also be applied to aluminum casting, where hot tearing is a serious problem. The project seeks to provide an opportunity for a Hispanic graduate student to learn welding, aluminum alloys, and failure analysis of welded components.Technical AbstractAn alloy exists in the weak semisolid state during solidification, with solid grains separated by thin liquid films during the terminal stage of solidification. The semisolid can crack along grain boundaries under the tension induced during welding or casting. How the fraction solid fs of the semisolid increases as temperature T drops during solidification can affect the crack susceptibility significantly, and it differs significantly from alloy to alloy. Unlike previous studies, the present project considers the grain boundary, where cracking occurs. It considers: 1. the tension to separate neighboring grains to cause cracking, 2. the growth of neighboring grains toward each other to bond together to resist cracking, and 3. the liquid feeding to fill the boundary between neighboring grains to resist cracking. Based on the criterion required for cracking to occur at the grain boundary, a simple new index is deduced to predict the relative crack susceptibility of aluminum alloys and the effectiveness of welding filler metals in reducing the crack susceptibility. The index can be determined from the steepness of the curve of T vs. the square root of fs, which can be calculated easily using commercial software packages and databases. A simple new test is developed to assess the relative crack susceptibility of commercial aluminum alloys in welding and to verify the new criterion and index. To cause cracking during the test, the semisolid region of the weld alone is pulled at a slow rate similar to that occurring naturally during welding, instead of bending the workpiece suddenly during welding, which is done in the most widely used test and which induces unrealistically fast and high tension in the semisolid region.