Numerical Heat Transfer Modelling in Spray Formed IN718 Billets

A numerical finite difference model has been developed to describe the transient heat flow inside Ni superalloy IN718 billets manufactured by spray forming. This model described the progressive build-up and solidification of the billets, and accounted for the latent heat of solidification, convective and radiation heat loss, and heat removal through the substrate, coupled with transient heat transfer in the substrate. The model has been used to predict critical values of important process parameters: average droplet arrival temperature, deposition rate, and billet surface convective heat transfer coefficient, in terms of key microstructural features by correlation of quantitative predictions with qualitative microstructural investigations of the as-sprayed billets. On a micro scale, repeated re-heating/remelting of deposited layers because of subsequent deposition was predicted and has been suggested to be beneficial in reducing porosity and microsegregation, as well as in playing an important role in the formation of the characteristic equiaxed spray formed microstructure.