Advanced simulations for hot forging: heat transfer model for use with the finite element method

In a companion paper, a friction model for isothermal forging is described. Extension to a general hot forging case requires consideration of heat transfer across the tooling/workpiece interface, since a chilling of the workpiece by the dies will have evolving material properties that can significantly affect the forging operation. This is a complicated problem, since the film thickness, surface roughness and hence fractional contact area can vary with respect to die location and time in a forging operation. Heat transfer across the tooling/workpiece interface is a complex combination of heat transfer through the asperity contacts and heat transfer across the lubricant film. Like friction, this evolves as deformation proceeds. This paper presents a thermal interface model for forging operations where a temperature difference exists between the workpiece and tooling, which is the most common situation in forging. The model considers heat flow through direct asperity contacts as well as heat flow through the lubricant film, and calculates an effective heat transfer coefficient based on lubricant, material and process parameters. The model is implemented as a user routine in a popular commercially available finite element code, DEFORM 2D. The model yields predictions that match previous experimental observations. Sample hot forging simulations are presented for upsetting, ring compression, spike test and embossing die configurations for Ti-6Al-4V and Rene-88, two popular aerospace alloys.