Welding The Unweldable

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AA7075 aluminum alloy is as fantastic as it is frustrating. Originally developed in secret by Sumitomo Metal during World War 2 for airframe construction, it’s unique properties make it as strong as steel at one-third the weight. The downside is you can’t weld two pieces of AA7075 together. Titanium carbide nanoparticles to the rescue.

The unique properties that give AA7075 it’s combination of strength and light weight are the same that have made it unweldable. The unique composition includes 5.6–6.1% zinc, 2.1–2.5% magnesium, 1.2–1.6% copper, and less than a half percent of silicon, iron, manganese, titanium, chromium, and other metals. These elements flow in an uneven way when welded and the metal fractures along the length of the weld. An Engineering team at the UCLA Samueli School of Engineering has made a breakthrough in solving the weldability issue with AA7075. The solution involves fabricating rods of AA7075 infused with titanium carbide particles. The titanium-infused rods were then placed in between the two pieces of AA7075 acting as a buffer. When the arc welding process begins, the tiny particles of titanium carbide solve the problem of uneven flow and eliminated the cracks.

UCLA professor Xiaochun Li explained the process; “Nanoparticles make the elements in the liquid metal solidify together more uniformly, thus preventing phase segregation. Phase segregation normally will block the liquid metal flow during cooling, thus inducing shrinkage and cracks without liquid filling. Since the metal solidifies more uniformly after adding nanoparticles, the liquid metal flows better during solidification, thus no cracking due to segregation.”

The alloy has been in use for over 50 years, but it typically needed to be either riveted or bolted together. Professor Li and his team hope this new process will open the door to new applications for the alloy. “The new technique is just a simple twist, but it could allow widespread use of this high-strength aluminum alloy in mass-produced products like cars or bicycles, where parts are often assembled together,” said Li.