Evaluating the Characteristics of Cast AZ91 Magnesium Alloy for Upset Protrusion Joining Method

Increasing pressures on automobile weight reduction, driven primarily by the need to reduce CO₂ emissions, are driving increased use of light metal alloys, especially magnesium and aluminum. For the automotive industry, it is imperative to develop a process that provides practical, robust, and cost-effective joining of magnesium alloys (especially castings) to each other and wrought materials and dissimilar materials at high process rates. The work presented in this article explores a new hot joining process referred to as Upset Protrusion Joining (or UPJ) to mechanically join cast AZ91 alloy (with a protrusion) to aluminum sheet material that has a hole to fit the protrusion. The protrusion gets heated by resistance heating, followed by its rapid compression. The protrusion expands circumferentially to fill the hole and the region above the hole. The sheet metal gets entrapped between the cast plate and the deformed protrusion (in a mushroom shape). The effect of the factors such as maximum applied current, current duration, compression rate, and compression distance as process variables is studied to produce visibly (defect-free) good quality joints. The UPJ process demonstrated repeatability in flow behavior and joint head formation at given processing conditions. Under uniaxial tension, sufficient joint strengths were obtained for the joints produced under optimum processing conditions when tested in lap-shear mode.