Functionally Gradient Materials via a Casting Process Involving Partial Solidification

Abstract

A novel technique for the production of functionally gradient materials, developed by the authors, is presented. The process is known as the Cast-Decant-Cast or CDC process, and involves a partial solidification step. This process takes expensive and time-consuming factors out of the production of functionally gradient materials by enabling utilisation of standard foundry equipment, and the process is carried out in a single multi-step casting operation. The CDC process involves simultaneous but separate melting of two alloys of different composition in order to produce a gradient in material properties. Details of the process are presented. It is shown that the outcome depends on partial local remelting and alloy mixing, which results in a gradual change or gradient between the first and second alloys in the as-cast condition, hence producing a functionally gradient material. The process has been adapted to conventional casting methods such as gravity casting and low pressure casting. It is the method of decanting the first material from the mould that differentiates the process variants. The decanting step for the low pressure method is controlled by pressure application and release on the molten alloys within their respective, but separate, sealed holding chambers. Decanting for the gravity casting method began as a physical inversion of the mould and is now at the point of autodecanting through careful design of an innovative gating system. The CDC process has been proven by means of metallographic study of microstructure to produce functionally gradient materials. Adapting the CDC process to conventional casting methods has made it a potentially commercially viable option for numerous applications. The results of recent research on the process are presented in summary form, including multi-alloy experiments, observations on the influence of timing and thermal control, and FGM manufacture using MMCs and joining dissimilar metals.