A fundamental study of hot tearing

The relationship between the response to Imposed strain of a solidifying metal alloy and the formation of hot tears Is explored. In this investigation coarse-grain Al-4.0% S! and coarse- and fine-grain Al-4.0% Cu were partially solidified to fractions solid between 0 and 0.65. The liquid-soild mixtures were then strained 30% in shear at rates between 1 X 10(-4) in./sec and 0.5 in./sec. Strength In these alloys is shown to be dependent on the solidification structure at the time of test but not directly on composition, In coarse-grain castings the development of strength and hot tearing susceptibility coincides with the formation of a dendrite network about 0.25 fraction solid. Below this level dendrites are essentially independent of each other and there is no effective resistanct to strain. When resistance does develop deformation occurs through rupture of Interparticle bonds and localized rearrangement of grains. As fraction solid Increases, localized rearrangement of grains becomes more difficult. In shear tests above about 0.50 fraction solid such rearrangement no longer operates. In castings sheared at very low strain rates (10(-3) in./sec), strain accommodation through sliding of dendrites past one another is possible. At higher strain rates Interaction between dendrites results in the buildup of a plastically deformed structure in the casting which resists further strain. Tear formation is the result of progressive separation of dendrites to accommodate strain. In castings composed of small (.03 in. dia) equiazed grains grain rearrangement is localized in much smaller regions than in coarse-grain castings. As a result, shear strength does not develop in these grain-refined alloys until approximately 0.40 fraction solid and visible hot tearing is much less extensive than in coarse-grain alloys. Some conclusions of practical importance are summarized. One of these is that grain refinement improves resistance to hot tearing, not by making a casting "stronger" but by making it better able to accommodate local strains without formation of defects. A second is that it should be feasible to develop casting processes which form liquid-solid mixtures containing quite high fractions of solid.