Die-cast AM60B Magnesium Alloy Research Articles

Study on the Role of Casting Defects and Surface Finish on Fatigue Life in Die-cast AM60B Magnesium Alloy Using X-Ray Computed Tomography

Abstract Magnesium alloy has attracted considerable attention from the automotive industry because of its low density and high specific strength combined with excellent casting capabilities. However, the role of casting defects and surface finish on the fatigue life emerges as a major concern. To study the role of surface finish, the die-cast skin was removed from the selected test specimens by hand polishing. No major difference in surface roughness was observed between the die-cast and polished surface. To characterize the casting defects in each test specimen, prior to the fatigue test, X-ray computed tomography (XCT) scans were performed on a desktop micro-XCT scanner. The preliminary XCT scans revealed a large number of gas and shrinkage pores concentrated in the center sections of the test specimens and a few scattered close to the free surface. In addition to gas and shrinkage pores, in unpolished specimens, small microcracks and notch-like surface imperfections were also observed on the edge of the test specimens. Strain controlled fatigue tests were performed on polished and unpolished test specimens at a 0.3 % strain amplitude. After every 2,000 fatigue cycles, the test was interrupted and XCT was performed to study the crack initiation and propagation in the respective test specimens. The fatigue test exhibited no major difference in fatigue life between the unpolished and polished specimens. However, XCT scans revealed the dominant fatigue crack initiated from the surface imperfections in unpolished specimens and from casting pores closer to the free surface in polished specimens. The fatigue test results and spatial distribution of casting pores were compared with the specimens extracted from a die-cast shock-tower structure from a previous study. Although a difference in spatial distribution of the casting pores was observed, it had no major impact on the fatigue life between the two sets of specimens.

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

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.

Calibrating material parameters to model the thin-walled components made of die cast AM60B magnesium alloy

As a novel lightweight metal, die cast AM60B magnesium alloy continues to be considered as a potential replacement for steel in certain automotive components. However, proper numerical model representing this alloy has not yet been fully explored. Thus, an optimisation methodology was developed to calibrate the material parameters needed for four available material laws, namely, MAT_99, MAT_81, MAT_24 and MAT_107 in LS-DYNA explicit FEA package (LSTC, Livermore, CA). The basic idea was to combine computations with a set of optimisation procedures to systematically adjust various material parameters until the calculated mechanical responses optimally match those measured experimentally. The optimisation was based on uni-axial tensile coupon tests at different nominal strain rates, ranging from quasi-static to 800 s−1. The optimisation results were evaluated using component experimental data from both slow- and high-speed axial crushing and quasi-static four-point bending tests of a thin-walled top-hat structure, and then quantified using a gross correlation index (GCI). Calculated results indicate that using the material constants generated from this set of procedures, all of the four material models replicated experimentally obtained stress–strain curves well, and yielded similar values of GCI (The difference was less than 4%). But only MAT_99 could accurately capture the damage patterns as observed in structural component experimental tests, thus being considered the best among the four material types studied. It is also concluded that the damage behaviour of AM60B alloy is sensitive to the strain hardening model and failure criterion selected.

Scatter analysis of fatigue life and pore size data of die-cast AM60B magnesium alloy

Scatter behavior of fatigue life in a die-cast AM60B magnesium alloy was investigated. For comparison, those in a rolled AM60B alloy and a die-cast A365-T5 aluminum alloy were also studied. Scatter behavior of pore size was also investigated to discuss about dominant factors for fatigue life scatter in the die-cast materials. Three-parameter Weibull function was suitable to explain the scatter behavior of both fatigue life and pore size. The scatter of fatigue life in the die-cast AM60B alloy was almost comparable to that in the die-cast A365-T5 alloy, while it was significantly large compared to that in the rolled AM60B alloy. Scatter behavior of pore size observed on the specimen cross-section was comparable to that of fatigue life. Therefore, the scatter behavior of fatigue life can be estimated, if the scatter behavior of pore size on the cross section of specimen would be evaluated, as proposed in the present study.

Scatter Analysis of Fatigue Life and Pore Size Data of Die-Cast AM60B Magnesium Alloy

Abstract Scatter behavior of fatigue life in a die-cast AM60B magnesium alloy was investigated. For comparison, those in a rolled AM60B alloy and a die-cast A365-T5 aluminum alloy were also studied. Scatter behavior of pore size was also investigated to discuss about dominant factors for fatigue life scatter in the die-cast materials. Three-parameter Weibull function was suitable to explain the scatter behavior of both fatigue life and pore size. The scatter of fatigue life in the die-cast AM60B alloy was almost comparable to that in the die-cast A365-T5 alloy, while it was significantly large compared to that in the rolled AM60B alloy. Scatter behavior of pore size observed on the specimen cross-section was comparable to that of fatigue life. Therefore, the scatter behavior of fatigue life can be estimated, if the scatter behavior of pore size on the cross section of specimen would be evaluated, as proposed in the present study.

Monotonic and Cyclic Plasticity Response of Magnesium Alloy. Part I. Experimental Response of a High‐Pressure Die Cast AM60B

Abstract: The mechanical behaviours of a high‐pressure die cast AM60B magnesium alloy were studied. Monotonic and cyclic tests (with a strain ratio R = −1) were carried out at ambient temperature. The results of the mechanical testing were discussed with respect to the crystallographic texture and the microstructure of the alloy in terms of clusters of pores, grain size, second phase strengthening as well as the orientation‐dependent activation of different deformation mechanisms such as slip and twinning.

PS54 The Relationship between pore size and fatigue life distributions of AM60B magnesium alloy

The fractographic defect size distribution of die-cast AM60B magnesium alloy tested under four point bending fatigue test was investigated. The defect size distributions of AM60B were compared with that of die-cast A365 aluminum alloy. Porosity which was found to be the most hannful defect to fatigue life was quantified by SEM images of the fatigue test specimens. Defect size and fatigue life data were then statistically analyzed using Weibull distribution function. The scatter in pore size distribution indicated by Weibull modulus, m was in agreement with those obtained for fatigue life distribution. This confirms the inference that the scatter in fatigue life is attributed by the deviations in the size of pores.

Modeling fracture properties in a die-cast AM60B magnesium alloy II—The effects of the size and location of porosity determined using finite element simulations

This is the second of two papers that analyze the effects of porosity upon the fracture properties of the die-cast AM60B magnesium alloy. This paper investigates the effects of the size and location of porosity using finite element simulations. Failure models were developed for the cases of a pore centered in the cross-section, a volume of material intact in the casting, and samples removed from the casting for die-cast materials. The predicted fracture strains for a centered pore compared favourably with a previously developed analytical model. The fracture strains of samples removed from an AM60B casting and characterized using X-ray tomography were predicted with an average error percentage of 6.7%.

Variability of skin thickness in an AM60B magnesium alloy die-casting

The skin thickness is investigated in regions of different solidification conditions in a die-cast AM60B magnesium alloy component. The results of several different techniques for determining the skin thickness in six different samples are compared. The hardness profile, the elemental aluminum and oxygen contents, the change in grain sizes, the change in eutectic percentage, and the onset of large α-Mg dendrites through the casting thickness are used to determine a skin thickness. These different criteria for determining the skin thickness yield equivalent results. Regions of different solidification conditions result in different skin thickness measurements. The flow stress near the yield point is found to correlate well with the observed skin fractions determined from this study.