J.P. Weiler

Process-Structure-Property Relationships for Magnesium Alloys

Gravity step-casting experiments were performed to investigate process-structure-property relationships in three different die-cast magnesium alloys – AM60, AZ91 and AE44. The step-cast mold was instrumented to capture temperature profiles of the solidification of molten magnesium. This paper investigates the structure-property relationships of these magnesium alloys, specifically the dependence of the fracture properties upon the porosity that forms during the casting process. Sixteen tensile specimens were cut from the step-casting perpendicular to the solidification front, for each alloy examined. Correlations from X-ray tomography data were used to estimate the maximum area fraction of porosity from the average volumetric porosity in the specimens, assuming a typical size and spatial distribution of porosity. This relationship can be used in the absence of more accurate measure of porosity (i.e. serial sectioning, computed x-ray tomography). A failure model for die-cast alloys – which depends upon the strain-hardening coefficient and the maximum area fraction of porosity in the specimen – was used to predict fracture strains for each specimen. The experimental tensile elongation of each specimen was compared with predicted values. The resulting mechanical properties determined from these cast magnesium alloys will be used to develop process-structure-property relationships.

Excimer Laser Processing of Al Containing Mg Alloys for Improved Corrosion Resistance

The implementation of Mg-based alloy components in Cl− containing environments is limited by the poor intrinsic corrosion resistance of the wrought alloys. Galvanic coupling of cathode secondary phase particles to the anodic Mg matrix combined with a low electrochemical potential result in an accelerated H2 evolution reaction, the controlling reaction of Mg corrosion. This behavior is closely followed by anodically induced cathodic behavior. To mitigate the accelerated corrosion rates, localized dissolution of secondary phases was performed by laser processing on three Al containing Mg alloys: AZ31B-H24, AM60B, and AZ91D. Mixed dissolution of the secondary phases was observed by electron and chemical microscopy. An order of magnitude increase in the electrochemical impedance estimated corrosion resistance was observed for the laser processed specimens immersed in a 0.6 M NaCl solution (18–60 h). The substantial increase in corrosion resistance stems from the reduced density of electrochemically noble secondary phase particles and localized enrichment of Al.

Effect of Process Variables on Microstructural Features during Solidification of AM60B Magnesium Alloy

In this study, commercial AM60B magnesium alloy was studied under different solidification conditions to understand the influence of cooling rate, thermal gradient, growth velocity, Niyama criterion, solidification time and mold dimensions on microstructural features such as secondary and tertiary dendrite arm spacing, grain size, porosity, pore shape and size, local morphological and phase variations. Porosity, grain size and dendrite arm spacing were measured as functions of the process variables. It was realized that the process of mold filling and solidification are simultaneous in nature and they significantly affect the microstructure development trends and its dependency on the process parameters. A significant effect, of the above mentioned, was found on the obtained porosity values and their variation along the casting. The results clearly indicate that rate of filling, nature of flow of liquid and shape of the mold greatly affect the solidification process and thereby the microstructure.

Strain-Rate Effects of Sand-Cast and Die-Cast Magnesium Alloys under Compressive Loading

The strain-rate effects of cast magnesium alloys were investigated with uniaxial compression and compressive impact testing. The compressive material response of specimens cut from sand cast AZ91, AE44, and AM60, and high-pressure die-cast AM60 was determined for strain-rates ranging from quasi-static levels to typical rates experienced during crash situations. Several different constitutive material models (Johnson-Cook, Cowper-Symonds, etc.) were used in an attempt to characterize the experimental results. These material models are typically available in commercial finite-element packages and can be used to model the resulting material response of die-cast automotive components produced with these alloys to more complex loading conditions. The resulting deformed microstructures and fracture surfaces of each alloy at different strain-rates were also analyzed.

Prediction of knit line formations in complex high pressure die cast magnesium alloy components

Abstract This work investigates the formation of as cast defects in knit line regions and their effects on local mechanical properties in a thin walled high pressure die cast AM60B magnesium alloy component. The defect distributions and the tensile and bending properties of specimens cut from knit line and other regions are compared. Numerical simulations of the mould filling process, computed X-ray tomography experiments and reports from the literature are used to formulate an initial explanation for the occurrence of metal front dispersion in knit line regions. This is an initial study for a future project investigating knit line formation and process–structure correlations in cast magnesium alloys.

Porosity, Damage Evolution and Fracture in Die-Cast Magnesium Alloy AM60B

A series of AM60B, die-cast magnesium alloy specimens have been examined using xray tomography at spatial resolutions from 1 – 20μm and further characterized in uniaxial tension. This paper first reports the utility of a critical strain model to link the local area fraction of porosity and the fracture strain. A discussion of the trade-off between predictive accuracy and the spatial resolution of the imaging technique is also presented in order to address the technological implications of these results. In the second half of the paper, serial tomographic imaging is used to illustrate the evolution of damage with increasing uniaxial strain. The implications are discussed with respect to numerical modeling of the performance of die-cast magnesium components.