Date of Award

9-9-2019

Publication Type

Master Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

AZ91, Mehcnical Properties, PEO Coating, Section Thickness

Supervisor

Hu, H.

Rights

info:eu-repo/semantics/openAccess

Abstract

Magnesium alloy AZ91 is high pressure die cast (HPDC) into rectangular coupons with the section thicknesses of 2, 6 and 10 mm. The first group of samples is employed to investigate the effect of section thicknesses on tensile properties, strain-hardening and fracture behaviours of the die cast AZ91. The results of tensile testing indicate that the ultimate tensile strength (UTS), yield strength (YS), elongation (ef), modulus(E), toughness(Ut) and resilience(Ur) decrease to 129.17, 110.59 MPa, 0.37%, 25.9 GPa, 0.89 MJ/m3, and 236.10 kJ/m3 from 245.54, 169.26 MPa, 4.07%, 37.8 GPa, 8.34 MJ/m3, and 378.95 kJ/m3 with increasing the section thicknesses of die cast AZ91 to 10 mm from 2 mm, respectively. The analysis of true stress vs. strain curves shows that the straining hardening rates increases with decreasing the section thickness to 2 mm from 10 m. The analyses of scanning electron microscopy (SEM) reveal that the high tensile properties of the HPDC Mg alloy AZ91 with the thin section thickness should be attributed to the low porosity level, fine dendrite structure, high eutectic content, and relatively thick skin. The observation via SEM fractography illustrates that the fracture behaviour of die cast AZ91 is influenced by section thicknesses. As the section thickness increases, the fracture of AZ91 tends to transit from ductile to brittle mode due to arising porosity content and coarsening microstructure. The second group of samples is employed to study the effect of plasma electrolytic oxidation (PEO) coating on tensile properties of the HPDC Mg alloy AZ91. The tensile test results show that the PEO coating reduces the UTS and YS of the 2 and 6 mm samples, but slightly enhance the tensile properties of 10 mm sample. The SEM analysis reveals that the differences in the size and content of pores between the substrate and the ceramic coating should be responsible for the change in the tensile properties.

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