Investigation on Heat Transfer and Its Effect on Titanium Hot Forging Process

Abstract

Titanium forged components have been widely used in aircraft engine industry because of their superior specific strength to weight ratio at high temperature. Turbine disk is one of the most demanding forging parts. The flow stress of titanium alloy Ti-6Al-4V is strongly dependent on temperature and strain rate during hot forging. The cooling rate can be designed to manage the temperature profile of dies by distinct spray setup. The workpiece loses heat to die by contact when getting heat up for deformation. The study aims to assess the influence of the discrete cooling rate and interfacial contact heat transfer on the optimum plastic deformation and the optimum die life for a Ti-6Al-4V hot-die forging. A two-dimensional FEM model of titanium turbine disk is employed to study the mechanical and thermal interaction between the hot dies and the workpiece. After hundreds of runs of the forging cycles thermal-steady state is built up and the thermal-steady simulation is considered to reflect the actual production situation. The development of different microstructure and phase compositions in various regions of workpiece is the result of the high sensitivity of two-phase TI-6Al-4V to strain and temperature of plastic deformation. Proper selection of these parameters allows one to control its mechanical properties and avoid deformation failure. Providing the productivity and economic demands, thermal design is a more manageable way than strain rate control for hot die forging. Microstructure control and uniformity of deformation can be achieved through the selection of optimum processing conditions with the aid of processing maps. This research focuses on the effects of forging interfacial heat transfer coefficient and discrete cooling rate along tool cavity when other processes parameters such as strain rate and cycle time given fixed. The temperature control will help achieve a good balance among strength, ductility and fracture toughness. The flow stability, load and energy, die wear, and die tempering and chilling are investigated for a turbine disk hot-die forging using a two-dimensional FEM model.