Temperature controlling process to eliminate metallurgical defects in spray forming with twin scanning atomiser

Abstract

Relations among pouring temperature, billet temperature distribution and deposit distance in spray forming procedures were analysed in this paper. Research results indicated that the top surface temperature of preforms would rise steadily even when the pouring temperature was kept steady. To illustrate this phenomenon by experiment, commercial software was used to simulate the temperature gradient in preforms during and after deposition. A hot spot was found forming in a superalloy billet that would shrink into hot cracking during the cooling process that usually forms at two-thirds of the billet’s total height. A novel process, called ‘thermal controlling’, was designed based on this finding to reduce metallurgical defects in large billets, including superalloys and high speed steel. The principle is that the top surface temperature of the preforms should be sustained in order to maintain an optimised ratio of liquid to solid. Several billets were produced at the Beijing Institute of Aeronautical Materials with this method, and no hot cracking was found. During the experiment on spray forming, the ratio of liquid to solid metal increases more than expected, which leads to a high risk of hot cracking. The hot spot is the main reason for the formation of metallurgical defects. The temperature controlling method is an effective way to reduce or eliminate defects in superalloy preforms. The thermal controlling method can reduce the microporosity that contributes to yielding high density materials.