Numerical process design for hot forging of steel encased titanium workpieces

This paper presents the numerical design of a hot forging process for titanium Ti-6Al-4V. A steel-encasing made of AISI 316L is used to enclose the titanium and protect it from the surrounding gases and excessive cooling. Oxidation by the ambient gases can lead to a reduction in the mechanical properties of the titanium in the near surface regions. At the same time, costs and resources can be saved, as time-consuming reworking of the titanium and therefore the amount of reworking required can be reduced. Finite element simulation was carried out for efficient process design. For a realistic depiction of the process the coefficient of thermal expansion as well as the flow curves were determined in experimental tests. A numerical parameter study was carried out by varying process boundary conditions such as the starting temperature of the billet, the ram velocity and the thickness of the steel-encasing. The numerical results indicate that a steel-encasing of two millimetres at the top area of the billet, compared to four millimetres, is not recommended for the process due to a fracture in the steel-encasing. Overall, it was revealed that an increasing billet temperature results in a higher final temperature in the titanium and greater local thinning of the steel. The same tendency was also determined with increasing ram velocity. For the variants investigated here, the combination of a billet temperature of 1,100 °C and a medium ram velocity of 20-40 mm/s with a thickness of four millimetres of the steel-encasing at the top is recommended.