Effect of Hydrostatic Pressure on the 3D Porosity Distribution and Mechanical Behavior of a High Pressure Die Cast Mg AZ
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REDUCING the weight of transportation vehicles is a world trend aiming to diminish the emissions of greenhouse gases and the amount of required fuel. Magnesium (Mg) alloys are key light-weighting materials,[1–4] as their specific strength is one of the highest among all structural metals. Despite the intense research efforts devoted over the last decade to understand the physical metallurgy of Mg alloys, the design of optimized processing routes that give rise to stable microstructures with predictable mechanical properties under service conditions still requires further advances in this area.
FEDERICO SKET, is with the IMDEA Materials Institute, C/ Eric Kandel 2, 28906, Getafe, Madrid, Spain. Contact e-mail: federico.sket @imdea.org ANA FERNA´NDEZ, Postdoctoral Researcher, formerly with the Physical Metallurgy Department, IMDEA Materials Institute, is now with the Laboratory of Mechanical Metallurgy, Institute of Materials at the EPFL, Lausanne, Switzerland. ANTOINE JE´RUSALEM, Associate Professor, formerly with the IMDEA Materials Institute, is now with the Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK. JON M. MOLINA-ALDAREGUI´A, Senior Researcher, is with the Micro- and Nano-Mechanics Department, IMDEA Materials Institute. MARI´A TERESA PE´REZ-PRADO, Senior Researcher, is with the Physical Metallurgy Department, IMDEA Materials Institute. Manuscript submitted April 20, 2015. Article published online July 1, 2015 4056—VOLUME 46A, SEPTEMBER 2015
High pressure die casting (HPDC) is the leading processing technology for Mg components in the automotive industry as it allows forming parts with complex geometries in one single operation with a limited cost. In HPDC, the molten metal is injected into the die at a high speed until the cavity is completely filled. In order to reduce the volume fraction of pores and casting defects, a pressure of approximately 40 to 50 MPa (400 to 500 bar) is subsequently applied and withdrawn only when the solidification process is complete. The heat released by the part is mostly eliminated through the casting-die interface.[5] Finally, the part is rapidly removed from the die. The most common Mg die cast alloys belong to the magnesium-aluminum series AM60 (Mg-6wt pctAl0.5wt pctMn) and AZ91 (Mg-9wt pctAl-1wt pctZn). Even though HPDC was invented more than a 100 years ago, relatively few studies relating the casting parameters to the resulting microstructures exist to date, e.g., References 6 through 9. This is motivated by the large cost of setting up and maintaining HPDC facilities in a research laboratory. A large number of studies have tried to relate HPDC microstructures to their mechanical behavior.[10–22] In general, it is agreed that porosity is the microstructural feature that has the most deleterious effect on the mechanical properties. In particular, the presence of pores gives rise to fracture at smaller strains than in their wrought counterparts. Furthermore, the current impossibility to reproduce three-dimensional (3D) pore distributi
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