Eliminating cracks in aluminum welds
Because they are strong, yet lightweight, aluminum alloys are becoming a popular replacement for steel in applications such as automobiles, where reduced weight can translate to increased fuel efficiency. However, these alloys—especially the strongest ones—can be difficult to weld.
Now, Professor Sindo Kou and graduate student Guoping Cao have demonstrated a method to predict and eliminate one of the biggest problems: cracking around the weld area.
Called liquation cracking, these cracks can appear small but will over time propagate and cause the welded area to separate from the base metal (the workpiece). Seemingly paradoxically, the stronger an alloy is, the more likely it is to crack. Cracking is of particular concern to automobile manufacturers, since their parts are used in conditions of vibration, heat and stress that may further weaken cracked areas, says Kou.
Aluminum alloy plates typically are welded with a melted filler metal. As the liquid metal within the weld cools and solidifies, it contracts, pulling on the softened base metal around it, called the partially melted zone, or PMZ. Based on this knowledge, Kou and Cao developed a simple criterion for determining cracking susceptibility: If the weld metal solidifies earlier than the PMZ, it will pull apart the grains in the PMZ, causing cracks.
Applying the criterion is as straightforward as the concept itself. “The fraction of solid of a semisolid metal, which is its resistance to cracking, can be calculated easily,” says Kou. “All we have to do is compare.”
Thanks to commercial solidification software developed by Professor Emeritus Austin Chang, Kou can compare solid fractions of the weld metal and the PMZ, then determine if there is a point at which the weld metal will be more solid than the PMZ. If so, the weld is likely to crack.
“It’s a very reliable, very simple criterion,” says Kou.
Using a database of aluminum alloy properties, welders now can match a filler metal to any aluminum alloy they want to weld. In addition, engineers can find or develop a filler metal to meet the criterion where none is commercially available—even for alloys so prone to cracking, they previously were considered unweldable, says Kou.
Next, Kou hopes to apply the same principles to additional groups of problematic metals. “My dream is to extend it to other materials, like stainless steel or nickel-based alloys, which are very susceptible to cracking,” says Kou. “My hope is that I can extend what I’ve learned and apply it to those kinds of material systems.”
Kou and Cao published their finding in the Welding Journal. The American Welding Society awarded them the William Spraragen Best Paper Award for their research. They accepted the honor at a ceremony in November.