Effects of External Heating on Additive Friction Stir Deposition of Aluminum 7075

Additive friction stir deposition (AFSD) is an emerging solid-state additive manufacturing process where metal bars are rotated and compressed by a non-consumable tool. This process plasticizes the material, allowing it to flow and form into distinct beads while remaining below the melting temperature. The solid-state nature of the process lends itself to materials that are susceptible to solidification cracking, such as 7xxx series aluminum. In this study, a heated fixture plate was utilized to maintain the base of several builds at given setpoint temperatures. A total of four single track oval builds with envelopes of 275 mm x 150 mm x 120 mm were deposited in aluminum 7075: one without external heating and the remaining with external heating setpoints of 200, 250, and 300 °C. Temperature based process control was utilized during the builds where spindle speed was varied to maintain a tool temperature setpoint of 360 °C. Deposition rate, traverse speed, and layer height were held constant to isolate the effects of external heating. Following deposition, the material was rough machined to remove flash and underwent a solution heat treatment and aging procedure to bring the deposits to a T6 temper. The deposits were then characterized through in-situ measurements, optical microscopy, tensile testing, fractography, conductivity, ultrasonic inspection, and x-ray computed tomography to understand how the addition of external heating affects the process-structure-properties relationships of aluminum 7075 produced through AFSD. Results indicate that higher external heating temperatures lead to lower process forces, limited grain growth post-heat treatment, reduced flaw indications via ultrasonic inspection, and improved tensile response in both the build and traverse directions. While traverse direction tensile results exceed traditional forging specification minimums, further work is required to improve build direction ductility and ultimate tensile strength.