Relationship between tensile and shear strengths of the mushy zone in solidifying aluminum alloys
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I. INTRODUCTION
THE rheological properties of fully molten and partially solidified metals[1] have received increasing attention during the past decade, particularly related to the development of semisolid processing. However, the rheology of solidifying alloys has also become of significant interest because the stability and strength of the mushy zone have a controlling impact on the behavior of that zone and many defects that can form during solidification. Stress-related defects that form during solidification include hot tearing, surface tearing, and banded defects (shear bands), which often form in pressurized casting processes.[2,3] Porosity formation can also be linked with the strength of the mushy zone through the action of the burst feeding mechanism.[4,5] The onset of air-gap formation has been shown to be directly related to the strength development characteristics of the mushy zone during solidification.[6] The ability to control dimensions and the shape of castings is therefore also related to the mushy zone rheology.[7,8] The rheological properties of solidifying alloys have been shown to display two characteristic transition points.[1,2] The first is the dendrite coherency point where the first measurable shear strength is developed because of impingement of the tips of the growing dendrites with their neighbors. The second characteristic point is the maximum packing point where strength starts to develop at a much higher rate because the grains have been interlocked in a three-dimensional network and little reorganization of the structure can occur. Many different techniques have been developed and used to measure strength of the mushy zone, in principle, testing A.K. DAHLE, Senior Lecturer, and T. SUMITOMO, Ph.D. Student, are with the Division of Materials Engineering, The University of Queensland, Brisbane, Qld 4072, Australia. Contact e-mail: [email protected] S. INSTONE, Casting Technology-Project Engineer, is with Research and Development, Hydro Aluminium Deutschland GmbH, 53117 Bonn, Germany. Manuscript submitted March 18, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
either in tension,[9–19] compression,[20] or shear.[21–25] Some techniques are based on partial remelting of the solid sample,[11,14–17,20] while other techniques perform measurements on the solidifying material.[9,10,13,18,19,21–25] Results have shown that mushy zone strength is strongly dependent on the solid fraction/temperature, microstructure size and morphology, and shear rate.[9–25] However, mushy zone strength measurements reported in the literature show a significant deviation in results between semisolid material tested in shear[22,23,25] and in uniaxial tension for the same alloy.[10,11,14–16] Furthermore, tensile strength results for the same alloy often differ many orders of magnitude at the same solid fraction.[10,11,14,15,16] This raises the question of whether different deformation mechanisms apply and which are important with respect to predicting the formation of various defects, such as hot tearing,
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