Grain Refinement and Improvement of Solidification Defects in Direct-Chill Cast Billets of A4032 Alloy by Melt Conditioning

Abstract

Melt conditioned direct-chill (MC-DC) casting is an emerging technology to manipulate 11 the solidification process by melt conditioning via intensive shearing in the sump during 12 DC casting to tailor the solidification microstructure and defect formation. Using 13 MC-DC casting technology in an industrial scale DC cast billet of an A4032 aluminum 14 alloy, significant grain refinement and uniform microstructure can be achievedin the 15 primary -Al phase with fine secondary dendritic arm spacing (SDAS). Improved16 macrosegregation is quantitatively characterized and correlated with the suppression of 17 channel segregation. The mechanisms for the prevention of channel segregation are 18 attributed to the increased local cooling rate in the liquid-solid phase region in the sump 19 and the formation of fine equiaxed dendritic grains under intensive melt shearing during 20 MC-DC casting. A critical cooling rate has been identified to be around 0.5~1 K/s (oC/s) 21 for the channel segregation to happen in the investigated alloy, based on quantitative 22 metallographicresults of SDAS. Reduction and refinement of microporosity is 23 attributed to the improved permeability in the liquid-solid phase region estimated bythe24 Kozeny-Carman relationship. The potential improvement in the mechanical properties 25 achievablein MC-DC cast billets is indicated by the finer and more uniform forging 26 streamline in the forgings of MC-DC cast billet.