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.
An 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.
