When 3:00 PM - 5:00 PM Dec 12, 2016
Where GM Conference. Room, LEC Building
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The Effect of Aluminum Content and Processing on the Tensile Behavior of High Pressure Die Cast Mg Alloys


Erin Deda
Thesis Defense

Due to their high specific strength and good castability, magnesium alloys are desirable for use in weight reduction strategies in automotive applications. However, the mechanical properties of high pressure die cast (HPDC) magnesium can be highly variable and dependent on location in the casting. To better understand the relationship between microstructure and tensile properties, the influence of alloying and section thickness on the microstructural features and tensile properties of binary Mg-Al and ternary Mg-Al-Mn alloys is quantified. This investigation provides experimental input to modeling activities for the development of an Integrated Computational Materials Engineering (ICME) capability, to assess and quantify the impact of microstructure on the tensile behavior of HPDC Mg AM series (magnesium-aluminum-manganese) alloys. It is found that with increasing aluminum content, the yield strength increases and the ductility decreases. Increasing the plate thickness results in a decrease in both the yield strength and ductility. HPDC components have varying microstructural features through the plate thickness, due to the development of a “skin” and “core”. The grain size, β-Mg17Al12 phase, and solute content are all quantified through the thickness of the plates. The skin shows an increased hardness, due to a fine grain structure. Based on the microstructure quantification, a physics-based model has been developed for predicting alloy and plate thickness effects on yield strength. The primary factors affecting strengthening in these alloys, are accounted for using a linear superposition model of solid solution, grain size, and dispersion hardening. This model takes into account through-thickness microstructure gradients that exist in high pressure die cast components by using a composite model to incorporate the skin and core changes. The yield strength in these alloys is dominated by grain size strengthening and solute hardening effects. In order to isolate the effects of β-phase, shrinkage porosity and oxide films, HPDC plates were solution treated and processed by hot isostatic pressing. Solution treatment dissolves the β- phase, quantifying the effects of beta phase on the yield strength and ductility. HIP reduces the β-phase and heals the shrinkage porosity. It is found that there is a hierarchical effect on ductility associated with these features - oxide bi-films have the most deleterious effect, followed by shrinkage porosity and finally β-phase. By identifying the critical microstructural features, we can better predict and design for the desired properties during the manufacturing process. This allows for improved utilization of magnesium in industrial applications, and improved fuel economy.

 

DISSERTATION COMMITTEE:

Co-Chair: Prof. John Allison , MSE

Members:

Prof. J. Wayne Jones, MSE

Prof. Amit Misra, MSE

Cognate:

Assoc. Prof. Veera Sundararaghavan, Aerospace