Air gap formation and hot tearing in solidification processing of Al- and Cu-base alloys

Sammanfattning: Shrinkage in a solidifying metal has been studiedexperimentally as well as theoretically. The main focus hasbeen to examine the mechanisms causing an air gap to formbetween a casting and a mould, and hot crack formation to occurin a solidifying metal.¨The formation of an air gap has been experimentallystudied during solidification of Al- and Cu-based alloys in acylindrical mould. The displacements of the casting and themould causing an air gap have been measured duringsolidification and cooling of the casting. The temperaturedistribution was measured simultaneously. Mathematicalmodelling has been performed to increase the understanding ofthe solidification process and the strains formed in thesolidifying metal contributing to the formation of an air gapbetween casting and mould. Most of the work was dedicated todevelop a new model to describe the strain duringsolidification, but traditional theory was used for themodelling work as well.The model suggested in this work includes non-equilibriumeffects on the solidification process and the shrinkage. Theformation and condensation of lattice defects formed in thesolid phase during solidification and its effect on thesolidification process as well as on the material shrinkageresulting in air gap formation was considered. The results fromthe modelling work show good agreement with the experimentalresults. The conclusion is that it is important to includethese non-equilibrium effects in modelling of shrinkage duringsolidification.The same conclusion was drawn from results of experimentalwork with high temperature tensile testing of in situsolidified samples and the development of a new theory for hotcrack formation. It was found that a super saturation oflattice defects formed during the solidification processenhances the nucleation and growth of hot cracks duringcooling.