Optimisation of an electric arc gun cluster design for the sprayform tooling process

The sprayform tooling process is a new method for the economic, rapid manufacture of hard steel production tooling. A robot-manipulated cluster of electric are spray guns is scanned over a disposable negative ceramic replica of the tool, depositing a molten steel droplet spray and forming a shell of several centimetres thickness. After spraying, the ceramic is removed and the remaining thick steel shell is trimmed, backed and then used in press forming and other production operations. The shell temperature during manufacture must be carefully controlled to produce shells of low or zero distortion. The shell temperature depends on the balance between the rate of heat input from the sprayed metal, the rate of heat output by convection to the N-2 atomising gas, and other heat transfer mechanisms. The sprayform process mass yield and the spray shell microstructure are further controlled by the angle at which the steel droplets deposit onto the growing shell surface. This paper describes the development of two new multiple arc wire gun cluster configurations, which are aimed to optimise the manufacture of: (i) relatively large shells and (ii) smaller shells but with severe topography. A mass deposition and subsequent heat flow model for the sprayform tool process have been combined with experimental measurements of critical process parameters, such as the shell surface heat transfer coefficient distribution, to investigate the effect of the electric arc spray gun cluster design on process yield and shell thermal distributions during the manufacture of thick sprayformed shells.