PROCEEDINGS OF THE 2003 INTERNATIONAL SYMPOSIUM ON LIQUID METALS JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 7175 – 7182 Effect of process parameters on grain structure formation during VAR of INCONEL alloy 718 A. KERMANPUR ∗ Department of Materials, Imperial College London, London SW7 2BP, UK D. G. EVANS Special Metals Corporation, New Hartford, NY 13413, USA R. J. SIDDALL Special Metals Wiggin Ltd., Hereford HR4 9SL, UK P. D. LEE † , M. M CLEAN Department of Materials, Imperial College London, SW7 2BP, UK E-mail: p.d.lee@imperial.ac.uk The development of grain structure during Vacuum Arc Remelting (VAR) of INCONEL 1 alloy 718, a nickel-based superalloy, is complex depending both on compositional variations and a range of process parameters. A multiscale model is presented which combines a macroscopic solution of the heat transfer, fluid flow and electromagnetism with a mesoscopic model of grain nucleation and growth. The model was used to investigate the influence of variations in process control parameters upon the macroscopic molten pool size and shape, together with the predicted grain structure. Simulations of structures produced for variations in melt rate, arc power and arc focus (both thermal and electrical) were compared with observations from instrumented and characterized plant-trials for steady state melting conditions; good agreement was achieved. C  2004 Kluwer Academic Publishers 1. Introduction The manufacture of aerospace gas-turbine discs from nickel-based superalloys, such as INCONEL alloy 718, involves solidification, hot deformation and heat treat- ment steps, each of which can influence the final disc homogeneity and microstructure. Vacuum arc remelt- ing (VAR) is the most widely used secondary remelt- ing process for the production of fully dense and homogeneous ingots of reactive and macrosegregation- sensitive alloys. The grain structure of the final wrought disc is dependent upon the initial as-cast ingot grain and segregation structure, together with the recrystalli- sation occurring during the subsequent deformation processes. The main objective of controlling the VAR process is to produce an ingot with a fine microstruc- ture, no macroscopic segregation and with no unac- ceptable melt-related defects such as freckles or white spots. The grain structure of the VAR ingot is critically dependent upon the temperature distribution and fluid motion within the molten pool, which in turn are deter- mined by the operational process control parameters. The principal process variables include current, melt ∗ On leave from Isfahan University of Technology, Isfahan, Iran. † Author to whom all correspondence should be addressed. 1 Trademark of Special Metals Corporation. rate, arc gap, and annular gap. However, the most im- portant process control parameters in VAR are thought to be current and drip short rate. Any fluctuation in these parameters may alter the heat and fluid flow within the molten pool and therefore generate melt-related de- fects. As an example, transient variations from the nom- inal melt rate, called melt rate excursions (MRE’s), may occur in the industrial VAR process. The magnitude of a MRE can vary from a few percent to hundreds of per- cents within a few seconds to tens of minutes. MRE’s can cause a range of solidification-related defects such as freckles, white spots, and tree rings. Bertram et al. recently investigated the effect of transient melt rate on white spot formation during VAR of alloy 718 in an industrial furnace [1]. Starting from nominal steady- state conditions, the melt rate was decreased in steps and then increased again. The ingot was then examined and they concluded that the formation of solidification white spot was activated once the melt rate was low enough, and then deactivated as soon as the melt rate increased to a sufficiently high level again. Changes in the grain structure as a function of melt rate was not reported. 0022–2461 C  2004 Kluwer Academic Publishers 7175