Behaviour of Hard-to-Form Material in Friction Stir Incremental Forming Process

The application of light metal alloys such as magnesium, aluminium and titanium alloys are expanding to reduce weight of various industrial products. As a result of a combination of attributes like corrosion resistance, high strength-to-weight ratio and biocompatibility, these materials are employed in diverse engineering applications. But it is difficult to form these materials at room temperature due to its high specific strength. Friction stir incremental sheet forming is a promising method for forming metal sheets, by which parts can be manufactured without the use of dedicated dies. In this work, the behaviour of titanium (hard-to-form material) formability is investigated using friction stir incremental forming process, in which forming is done under localized friction heating caused by high tool rotation. Series of experiments with cranial plate-forming process is carried out in a vertical machining centre with grade 5-Ti-6Al-4V alloy sheets of size 300 mm × 300 mm × 1.2 mm with hemispherical end forming tool. To determine the formability of the material, Erichsen cupping test is performed using cupping testing machine and the Erichsen cupping index is found. The effect of key process parameters, namely the rotation speed, the feed rate and the step depth is investigated. The mechanism for the enhanced mechanical properties during friction stir increment forming is addressed. The interactive effect of process parameters on mechanical properties of formed parts is studied because mechanical properties are key factors for evaluating the product performance before industrial applications. This work addressed the key issues encountered by researchers worldwide on friction stir incremental forming and it will provide useful guidance in improving the product quality formed by friction stir incremental sheet forming.