The microstructural development of Mg-9 pct Al-1 pct Zn alloy during injection molding
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I. INTRODUCTION
ALTHOUGH there are a number of distinct technologies to process alloys and composites in a semisolid state, they fall into two main categories. The first one covers processing slurry that has been cooled from the fully liquid state and is represented by rheocasting or rheomolding.[1,2] The second group contains processes in which the alloy has been heated from a fully solid state, and here, thixocasting or thixoforming can be listed as examples.[3,4] An obvious benefit of the latter, i.e., the lower preheating temperature, is diminished by a requirement of a special billet that has to be converted into the thixotropic state in a separate operation. The preceding disadvantage was eliminated in recently commercialized THIXOMOLDING,* a hybrid technology, *THIXOMOLDING and THIXOMOLDED are trademarks of Thixomat, Inc., Ann Arbor, MI.
combining die casting and plastics-injection molding, where, in a single step, the as-cast feedstock is transformed into a thixotropic state, accumulated, and injected under high pressure into a mold cavity.[5] It is anticipated that because of thixotropic structure, the viscosity of the material is reduced, and the force required to fill even complex geometry is relatively low, compared to that needed for conventional processing. While for die casting and some semisolid technologies, a development of the microstructure is quite well described; for THIXOMOLDING, the role of processing parameters in microstructural evolution still remains unclear. In particular, the contribution of shear imposed by the injection screw and external heat is a matter of controversy. The uniqueness of THIXOMOLDING does not allow for directly adopting phenomena and mechanisms that are common for other semisolid techniques. First, as opposed to a majority of semisolid
F. CZERWINSKI, Design Specialist—Metallurgy and Materials, is with Index Development Engineering, Husky Injection Molding Systems Ltd., Bolton, ON, Canada L7E 5S5. Contact e-mail: [email protected] Manuscript submitted January 17, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
processes that rely on the formation of the thixotropic structure during solidification,[6] during THIXOMOLDING, the globular features are predominantly created at the stage of melting. Second, the feedstock used represents a specific form of an alloy whose microstructure was modified during conversion into small chips, either by mechanical fragmentation or rapid solidification.[7] The objective of this study is to assess the development of the microstructure during injection molding of an Mg-Al-Zn alloy. Ex-situ microstructural analysis is intended to provide an insight into processes that happen at high temperatures and cannot be observed directly to give us the power to predict the quality of molded parts. II. EXPERIMENTAL DETAILS The research material was a commercial magnesium alloy with a nominal composition of 8.5 pct Al, 0.75 pct Zn, 0.3 pct Mn, 0.01 pct Si, 0.01 pct Cu, 0.001 pct Ni, 0.001 pct Fe, and Mg-balance, which corresponds to the AZ91D gr
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