Numerical Simulation of Thermal History in a Novel Spray Forming Process

Abstract

A novel spray forming process was developed to produce large billets, wide plates or thick tubes with excellent microstructures and high cooling rates. Its uniqueness lies in a combination of the wide-range reciprocating movement and the swing scan of a gas atomizer, and the externally forced cooling of substrate during this spray deposition procedure. Its basic concept is that both good sticking and rapid solidification can be achieved if droplets with high liquid fractions impact a cold substrate, spread fully and then deposit on the surface. In order to control and optimize this new process, the thermal histories of droplets and deposits for spray forming of aluminum alloy billets were simulated with a set of new numerical models. Through shortening spray distance and raising melt superheat properly, the liquid fraction of droplets before deposition will increase and their spread on the deposit surface can improve for good sticking. Simulation results show that the optimal liquid fraction of droplets for deposition is about 0.2 higher than that in the conventional Osprey process. Its optimum spray distance is about 0.25m, which is nearly half as that in the Osprey process. In addition, this new process increases the mushy layer area and the specific surface area of heat extraction during deposition. Together with the forced cooling of substrate, it results in higher cooling rates. A high-quality large billet can be obtained by controlling the atomizer movement, the droplet liquid fraction and the deposit surface temperature properly in this new process.