Electronic Thesis and Dissertation Repository

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Professor.J.T.Wood

Abstract

The primary goal of this study is the characterization of high pressure die cast magnesium alloys AM60 so that the failure model which is proposed by J.Weiler can be validated for two new castings. The input variables of failure model can be determined by characterization of different regions within the castings. Based on the location of gate system, the desired regions are identified across both castings and tensile specimens are extracted from various regions. Therefore the local mechanical properties are determined, and fracture strain of samples ranges from 1.2 to 9.5%. The area fraction of porosities and skin fraction of each region are obtained from microscopic studies, and skin fraction ranges from 0.27 to 5.5% for the samples which are removed from the knit line region. The area fraction of porosities ranges from 0.4 to 2.85% within the knit line region, while high fraction of shrinkage porosities are observed within the fracture surface of samples removed from last-to-fill regions. By introduction of skin fraction as ligament factor of failure model, the fracture strain can be estimated with high accuracy. The deviation between predicted and experimental values was less than 90% for most locations. The premature failure happens to the samples containing porosities within the skin region, and actual positions of pores are considered for prediction of fracture strain. The microscopic studies of sections along the flow path also indicate that the thick defect bands are formed within the flange gates, and higher fraction of the gas pores are detected within the region closer to the overflows, and average area fraction of porosities reaches 2.8%. To evaluate the yielding behavior of samples, the elasto-plastic transition point of the samples containing different fractions of large dendrites are determined by analysis of strain hardening rate. The fully plastic behavior begins at 107MPa for the samples containing accumulated large dendrites within the core region, while it ranges from 121 to 132MPa for the samples with fine microstructure.


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