Optimization of investment casting of Ti6Al4V HIP prostheses by numerical and experimental methods

Hip prosthesis presents a cylindrical shape with high length to diameter ratio. This medical device has to withstand many years of cyclical loading equal to at least 3 to 5 times the body weight. This intensive use requires that the product must have very good performance [1]. Ti6Al4V is a biocompatible alloy that is widely applied for manufacturing hard-tissue replacement systems such as total hip prostheses. Investment casting as a near net shape forming method is a costless and good way to produce this type of orthopaedic implants. Nonetheless, this casting process is not a straight forward task and it will easily cause centre line shrinkage. Also, Ti6Al4V is a highly reactive alloy in molten state, which will react with the ceramic shells used in investment casting. Casting defects like shrinkage and residual stress may lead to the premature fatigue failure of the prosthesis as this device is subjected to approximately one million of cyclic loads per year. Thus, during the investment casting of Ti6Al4V it is crucial to develop methodologies for eliminate or minimize the surface oxidization and central line shrinkage [2]. The face coat material has a significant effect on the surface property of this kind alloy. Moreover, the wrap technique influences the solidification sequence of the alloy. In this study, an optimization method for the investment casting of hip prostheses was explored. Different face coat materials of shell were also investigated throughout this work. Furthermore, the solidification sequence of this long cylinder shaped part was studied. Herein, experimental and numerical methods were applied. The experimental results matched well with the outcomes of the simulation runs, which validates and corroborates the proposed optimization methodology.