Solidification Cracking in Welds of Aluminum Alloys

非技术摘要铝合金越来越多地用于减轻车辆重量，而电弧焊是焊接它们的最通用工艺。然而，铝合金在凝固过程中容易产生裂纹，焊接时称为凝固裂纹，铸造时称为热裂。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，以及它是如何受到用于焊接的填充金属的影响。该指数可以很容易地计算使用商业软件包和数据库的铝合金。该指数可为选择现有的焊接填充金属或寻找新的填充金属以避免铝合金焊缝凝固裂纹提供有用的指导。该指数也可应用于铝铸件，其中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一个学习焊接、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存在，在凝固的终末阶段固体晶粒被薄液膜分隔开。在焊接或铸造过程中产生的张力作用下，半固态材料会沿着晶界开裂。在凝固过程中，半固态的固相分数fs如何随着温度T的降低而增加可以显著地影响裂纹敏感性，并且它在合金与合金之间显著地不同。与以前的研究不同，本项目考虑了发生裂纹的晶界。 它认为：1.使相邻晶粒分离而引起裂纹的张力，2.相邻晶粒朝向彼此生长以结合在一起以抵抗开裂，以及3.该液体进料以填充相邻晶粒之间的边界以抵抗开裂。基于裂纹发生在晶界的准则，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接填充金属在降低裂纹敏感性的有效性。该指数可以从T与fs的平方根的曲线的陡度确定，其可以使用商业软件包和数据库容易地计算。本文提出了一种新的评定工业铝合金焊接相对裂纹敏感性的试验方法，并对新的判据和指标进行了验证。为了在测试期间引起开裂，仅以类似于焊接期间自然发生的缓慢速率拉动焊接的半固态区域，而不是在焊接期间突然弯曲工件，这在最广泛使用的测试中进行并且在半固态区域中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.