Solidification of aluminium spray-formed billets

A critical part of the billet spray-forming process is the successive intermittent deposition of thin layers of semi-solid aluminium alloy at different points on the top surface of the billet. Each thin layer is made up of a large number of impacted semi-solid spray droplets. As successive layers of alloy are deposited significant re-melting and re-freezing of underlying layers can occur. If the layers become too dry, high porosity will result; if they are too wet, fluid dynamic surface instabilities are possible. In extreme cases no billet will form. The process is essentially incremental, so that heat fluxes within the deposit very close to the top surface play a major role in determining the final deposit microstructure.In this paper transient heat transfer and solidification processes in the billet are analysed. First, some general features of billet heat transfer are discussed. The focus then narrows onto a thin layer of the deposit, lying very close to the billet surface. A boundary layer approximation is derived and computational results from this approximation are used to answer a number of questions of high practical value.