Purdue First to 3D Print Aluminum Alloy 7075 Without Cracking

Purdue University researchers have achieved what many thought impossible: reliable 3D printing of aluminum alloy 7075. The high-strength material has long resisted additive manufacturing because it cracks during cooling. A team led by adjunct professor I. Emre Gunduz found a way around the problem by depositing tiny molten droplets onto a heated stage.

The key is temperature control. Gunduz's group heats the build platform to 500 degrees Celsius. That keeps the deposited metal molten longer, letting it weld together without the thermal stresses that normally cause hot cracking. Droplets arrive at a rate of 400 per second, each less than half a millimeter wide. The computer scans the part every few layers and corrects errors on the fly.

After final heat treatment, the printed specimens matched or beat the properties of stock AA7075 by up to 10 percent. Nanoscale X-ray scans at Brookhaven National Laboratory confirmed the results. Samples printed on a 200-degree stage showed voids and cracks. The 500-degree samples came out dense and consistent.

AA7075 is prized in aerospace and defense for its steel-like strength at one-third the weight. It is also cheaper than titanium. The downside has always been manufacturability. Conventional powder-bed fusion melts the alloy unevenly, degrading its structure. Liquid metal jet printing avoids that by never fully melting the material in a crucible. Instead, it softens the wire feedstock just enough to deposit and bond.

The work opens the door to printing other difficult alloys. Gunduz believes the same cooling-rate thresholds could apply to nickel-based superalloys and other high-performance systems. The paper appears in Metallurgical and Materials Transactions A.