Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Master of Engineering Science

Program

Mechanical and Materials Engineering

Supervisor

Klassen, Robert J.

Abstract

This thesis aims to study the local mechanical properties of high pressure die cast (HPDC) AE44-2 and AE44-4 alloys at 25˚C and 200˚C and their microstructures. The chemical composition of the precipitates and grain size, the effect of cooling rate on the grains, the relationship between the grain size and mechanical properties, and the creep resistance of these two HPDC alloys were studied and discussed.

In this thesis, the spherical micro-indentation, constant Berkovich indentation tests, and tensile tests were performed on the specimens at 25˚C and 200˚C to probe their stress-strain response and creep behavior. This study used a new analytical technique to deduce the stress-strain curves from the spherical indentation tests.

This thesis suggests that the mechanical properties of these two alloys have a complicated dependence on the grain size. Although, with different RE additions, these two alloys have a similar indentation creep resistance at 25˚C and 200˚C.

Summary for Lay Audience

Magnesium (Mg) is one of the most abundant elements on earth. It was firstly discovered in 1808. Mg is located in the second group of the periodic table. It has a shiny silver-grey color. The density of Mg is only 1.728 g/cm3 making it be one of the lightest structure metals in the world. Relative to its low density, the strength of Mg is high. The properties of Mg can also be optimized by different alloying elements for various applications. Because of its light-weighted properties, Mg is essential for industry, especially for the transportation industry. For example, by applying the Mg alloys in a car, the weight of the car would be reduced, and the performance and fuel economy of the car would be improved.

However, the melting temperature of Mg is low, so some Mg alloys cannot be used at high temperatures. One solution for this problem is adding a certain amount of rare-earth (RE) elements (usually 2~4 wt%). The Mg-Al-RE alloys can maintain a good strength at high temperatures (125˚C~175˚C). This study aims to evaluate the mechanical strength of two types of Mg-Al-RE alloys named AE44-2 and AE44-4, respectively. The main difference between these two alloys is the RE addition. For AE44-2, the RE addition is cheaper than that of AE44-4.

The result of this study shows that these two alloys have a similar behavior at room temperature and 200˚C. This result proves that the AE44-2 is an economical replacement of the expensive AE44-4 alloys. This study contributes to the application of light-weighted Mg alloy components in the automobile industry and promotes the development of fuel-efficient cars.

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