Abstract

Magnesium (Mg) alloys present a promising alternative to aluminum (Al) alloys in lightweight applications. However, relative to Al alloys, Mg alloys have poor castability. Castability is influenced significantly by the dendrite coherency point (DCP), which represents the temperature, time, and solid fraction at which an interlocking solid network forms during solidification. An increase in the solid fraction at coherency may improve the castability of the alloy and reduce casting defects such as porosity, hot tears and misruns. A successful method for increasing the solid fraction at the DCP in Al alloys involves the use of grain refiners such as titanium (Ti). However, the influence of Ti refiners on the DCP in Mg alloys has not been thoroughly investigated. The objective of this research was to study the effect of Al-5Ti-1B refiner on the dendrite growth mechanism, DCP and porosity of AZ91E magnesium alloy. This thesis is a pioneering effort in relating the grain refinement effect of Ti on the DCP, coherency solid fraction, and porosity development during the solidification of Mg alloy, AZ91E. It represents an important step in improving the castability of Mg alloys. Varying levels of Al-5Ti1B grain refiner (0.005, 0.05, 0.1, 0.2, and 0.3 wt.% Ti) were added to AZ91E. The effect of Al-5Ti-1B grain refiner on the microstructure and dendrite growth mechanism of AZ91E was investigated. Quench experiments were performed to observe transformations in the dendritic morphology that resulted from the refiner additions. The growth rate and DCP were determined using the rheological method. The changes in porosity levels were determined for the grain refiner additions.

Highlights

  • Magnesium (Mg) alloys have the lowest density of all structural metals [1, 2]

  • The rheological data are examined in the second section to determine the impact of the TIBOR refiner on the dendrite growth rate and dendrite coherency point (DCP)

  • The final section presents the data from the porosity experiments and analyzes the relation between the DCP, microstructure, and porosity of the casting

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Summary

Introduction

Magnesium (Mg) alloys have the lowest density of all structural metals (approximately 35% less dense than aluminum and 80% less dense than steel) [1, 2]. For this reason, Mg alloys have gained popularity in the automotive and aerospace industry. Volkswagen used magnesium for the crankcase and transmission housing of the Beetle These components weighed a total of 17 kg, which was said to represent a savings of 50 kg when compared with using cast iron components. This saving of weight played a critical role in improving the stability of the rear-engine vehicle [1]. Of the Mg-Al alloys, AZ91 alloy is among the most common due to its high corrosion resistance and good castability in comparison to the other Mg alloys [1]

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