High Pressure Die Cast AM50 Research Articles

Corrosion Behavior of Gravity Cast and High-Pressure Die-Cast AM60 Mg Alloys with Ca and Y Addition

In this study, the microstructure and related corrosion behavior of AM60 alloys with/without Ca and Y addition upon gravity casting (GC) and high-pressure die-casting (HPDC) are investigated by means of SEM/EDS characterization, immersion/salt spray test, hydrogen releasing, as well as electrochemistry examination. When utilizing GC, AM60 alloy with Ca and Y addition (named AZXW6000 alloy) has better corrosion resistance compared with AM60 alloy. Thanks to grain refinement and second phase networks introduced in HPDC, the anti-corrosion properties of the AM alloy seem much better than those of the GC counterpart. The corrosion mechanism of both GCed and HPDCed AM60-based alloys is also investigated in detail. The inspiration from present work can provide more thinking for developing high corrosion-resistant Mg alloys using different casting methods.

Analysis of local stress/strain fields in an HPDC AM60 plate containing pores with various characteristics

Casting pores are regarded as the main factor causing stress/strain concentrations, facilitating the initiation of microcracks and shortening the plasticity of a cast component. The influence of pore characteristics (i.e., size, morphology, and position) on local stress concentration in a High Pressure Die Cast (HPDC) AM60 sample is studied utilizing X-ray tomography (XRT) and image-based finite element (FE) techniques. Heterogeneous porosity distribution is identified in terms of volume fraction and projected area ratio per segment. Statistically results demonstrate that small pores (volume < 0.01 mm3) have higher number frequency and sphericity. In contrast, large pores (volume > 0.01 mm3) account for a very low number frequency (0.3%), and complex shape. The present study addresses the local stress/strain behavior in a cast-pore containing HPDC AM60 sample during tensile loading investigated using 3D image-based models which consider the actual size, morphology, and spatial distribution of pores based on XRT images and consider the actual material properties of the AM60 alloy. Simulation results show that the local stress/strain around a pore evolves progressively with deformation. The strain at a certain pore location may be amplified or reduced in the presence of other pores, depending on the specific location of the concerning pores. Also, the ratio between the projected area of a pore and its shortest distance to the free surface (PA/SD) could give a good indication of the max local stress concentration around the pore surface. The weakening of Ktpl values of pores found in defect bands are principally due to the pore orientation concerning the load direction. These findings may be extended to provide additional insight into the damage mechanisms in material containing casting pores and to enhance understanding of the ductility variation in terms of local deformation fields around pores and their interactions.

Effect of Thermal Exposure Time on the Relaxation of Residual Stress in High Pressure Die Cast AM60

Effect of Thermal Exposure Time on the Relaxation of Residual Stress in High Pressure Die Cast AM60

Predicting the Influence of Microporosity on the Mechanical Properties and Fracture Behavior of High-Pressure Die-Cast AM50 Magnesium Alloy

In this paper, experimental and finite element modeling methods were adopted to investigate the effects of microporosity on the tensile properties and fracture behavior of high-pressure die-casting (HPDC) AM50 alloy. By specimen-to-specimen fractographic analysis, the variability in tensile properties could be quantitatively correlated with the areal fraction of the porosity presented in the corresponding fracture surfaces by using a simple power law equation. Numerical models of synthetic microstructures with different pore sizes, areal fractions of pores and pore distributions were established. Based on the experimental and numerical simulation results, it could be concluded that the fracture will initially occur in the region where has the highest intensity of equivalent stress field (i.e., contains the most highly localized cluster of pores and shrinkage), and then, fracture crack will fast propagate through the adjacent regions which have the relatively high intensity of stress field.

Influence of Ce addition on microstructure and mechanical properties of high pressure die cast AM50 magnesium alloy

The influence of Ce addition on the microstructure and mechanical properties of AM50 magnesium alloy was investigated to improve its mechanical properties. The results show that the addition of Ce to AM50 alloy results in the grain refinement and the mechanical properties of the Ce-modified AM50 at room and elevated temperatures are remarkably improved. AM50 magnesium alloy containing 1% Ce (mass fraction) shows better refinement and mechanical properties compared with the AM50 magnesium alloy with 0.5% Ce and even AM50 alloy without any Ce.

Influence of cerium additions on the corrosion behaviour of high pressure die cast AM50 alloy

The influence of Ce additions to AM50 magnesium alloy prepared by high pressure die casting on the microstructure and corrosion behaviour has been investigated. The corrosion behaviour was evaluated by hydrogen evolution, salt spray, potentiodynamic polarization and electrochemical impedance experiments. The results show that Ce addition promotes the formation of Al11Ce3 phase and reduces the amount of β-phase (Mg17Al12). The corrosion resistance is improved with increasing Ce content which can be attributed to the formation of Al11Ce3 phase, reduction of β-phase (reduced galvanic effects) and higher purity of AME50X alloys (melt cleaning effects).

Insights into microstructure based corrosion mechanism of high pressure die cast AM50 alloy

Corrosion mechanism of high pressure die cast (HPDC) AM50 magnesium alloy from a planar microstructure context is discussed. The progress of corrosion is mapped by means of electrochemical impedance spectroscopy, microscopy, and by elemental composition distribution analysis. Corrosion resistance of the alloy surface increased on prolonged exposure to the electrolyte (1.6wt.% NaCl solution), which is attributed to the selective dissolution of Mg-rich alpha phase, and to the evolution of Al-rich surface comprising of Mg17Al12 (beta) and Al-containing eutectic phases. A comparative study between HPDC and sand cast AM50 alloys revealed superior corrosion performance of die cast alloy.

Microstructure and Corrosion Characterization of Squeeze Cast AM50 Magnesium Alloys

Squeeze casting of magnesium alloys potentially can be used in lightweight chassis components such as control arms and knuckles. This study documents the microstructural analysis and corrosion behavior of AM50 alloys squeeze cast at different pressures between 40 and 120 MPa and compares them with high-pressure die cast (HPDC) AM50 alloy castings and an AM50 squeeze cast prototype control arm. Although the corrosion rates of the squeeze cast samples are slightly higher than those observed for the HPDC AM50 alloy, the former does produce virtually porosity-free castings that are required for structural applications like control arms and wheels. This outcome is extremely encouraging as it provides an opportunity for additional alloy and process development by squeeze casting that has remained relatively unexplored for magnesium alloys compared with aluminum. Among the microstructural parameters analyzed, it seems that the β-phase interfacial area, indicating a greater degree of β network, leads to a lower corrosion rate. Weight loss was the better method for determining corrosion behavior in these alloys that contain a large fraction of second phase, which can cause perturbations to an overall uniform surface corrosion behavior.

The correlation between microstructure and mechanical properties of high-pressure die-cast AM50 alloy

Scanning acoustic microscopy was used to characterize the micro-voids distribution for specimens in non-destructive mode. In addition, the in-situ scanning electron microscopy observation was performed during tensile deformation of high-pressure die-cast (HPDC) of AM50 alloy to obtain the mechanism of fracture induced by micro-voids. The effects of micro-voids on the mechanical properties were discussed. The results obtained from the examination suggest that fracture tends to occur at bigger micro-voids or in the cluster micro-voids area.

Tensile properties of high-pressure die-cast AM60 and AZ91 magnesium alloys on microporosity variation

The effect of microporosity on the variability in the tensile properties of high-pressure die-cast AM60 and AZ91 alloys was investigated, together with a theoretical prediction based on a constitutive model. The strain rate sensitivity of both alloys was measured through an incremental strain rate change test at room temperature, and the microporosity was measured through quantitative fractography analyses on the fractured surface. The variability in the tensile strength and elongation of both alloys can be empirically described as a power law relationship in terms of the microporosity variation. The defect susceptibility of the UTS and elongation to the microporosity variation in the AZ91 alloy is slightly higher than that in the AM60 alloy. The constitutive prediction on the tensile properties of the AM60 and AZ91 alloys is in good agreement with the experimental results, and it suggests that the defect susceptibility of the tensile properties to the microporosity variation is significantly decreased with the increase of strain hardening exponent.

Microstructure and Mechanical Properties of Low Pressure Die Cast AM50 Magnesium Alloy

Compact AM50 alloy components were cast by Low Pressure Die Casting (LPDC) process. The microstructure and mechanical properties of cast components were investigated under as-cast and heat treated states. It was found that the microstructure of LPDC AM50 is composed of α-Mg and second phases - Mg17Al12 and Al8Mn5. Compared with Gravity die casting, LPDC AM50 alloy had much coarser grains and higher density, with smaller sizes and less content of second phases. The density of AM50 alloy by LPDC process was ρ=1.7836g/cm3, with increase of 0.45% based on Gravity die casting and much more increase compared with high pressure die casting. The as-cast mechanical properties by LPDC process were: σ0.2=57.8Mpa, σb=192.3Mpa, δ=8.7%. These of Gravity die casting were: σ0.2=53Mpa, σb=173.4Mpa, δ=8.1%. UTS in LPDC increased about 20MPa, with better YTS and Elongation. Compared with that of high pressure die cast AM50, the YTS of LPDC was much lower, with comparable UTS and Elongation. The mechanical properties of the heat treated AM50 alloy were still in the same level of as-cast state. AM50 alloy by LPDC process is not necessary subjected to tempering treatment.

Identification and modeling of fatigue crack growth mechanisms in a die-cast AM50 magnesium alloy

We experimentally identified the fatigue crack growth micromechanisms operating near the limit plasticity regime in a commercial high-pressure die-cast AM50 alloy with a critical review of the available literature. An existing multistage fatigue model was modified to subsequently recognize these micromechanisms in a threefold fatigue crack growth regime. The formation of the main fatigue crack occurred almost exclusively at shrinkage pores and to a lesser extent at large Mn-rich particles. At shrinkage pores, several adjacent cracks typically incubated at the edge of flat interdendritic pores, propagated along the α-Mg dendrite cells/Al-rich eutectic interface, and rapidly coalesced into a main physically small fatigue crack that advanced through the Al-rich eutectic. In the long crack regime, the crack advanced in a mixed transdendritic–interdendritic mode along persistent slip bands spreading over several tens of dendrite cells. The model predicts well the fatigue life compared to the experimental data when these observed mechanisms are accounted for.

Reconstruction and visualization of complex 3D pore morphologies in a high-pressure die-cast magnesium alloy

Visualization and representation of three-dimensional (3D) pore morphologies in the high-pressure die-cast Mg alloys are of significant interest for understanding and modeling processing–microstructure–properties relationships. In this contribution, an efficient and unbiased montage serial sectioning technique is applied for reconstruction of large volume (∼1.24 × 10 9 μm 3) high-resolution (∼1 μm) 3D microstructure of a high-pressure die-cast AM50 Mg alloy containing gas (air) and shrinkage pores. The reconstruction of 3D microstructure from montage serial sections enables visualization and characterization of detailed 3D morphologies and spatial correlations of gas (air) and shrinkage pores. The reconstructed 3D microstructure is implemented in a 3D finite elements (FE) computational framework to illustrate how realistic complex microstructural feature morphologies can be incorporated in the simulations of micromechanical response of these high-pressure die-cast cast microstructures.

Effect of process parameters on porosity distributions in high-pressure die-cast AM50 Mg-alloy

High-pressure die-casting is the preferred manufacturing process for cast Mg-alloy components used for numerous applications. High-pressure die-cast components usually contain micro-porosity that adversely affects their mechanical properties. In this contribution, the effects of three important process parameters, gate velocity, intensification pressure, and melt temperature on the micro-porosity distributions in high-pressure die-cast AM50 Mg-alloy are quantitatively characterized. The amounts of total porosity, gas porosity, shrinkage porosity, and pore size distributions are experimentally measured using novel digital image analysis techniques that permit quantification of both gas and shrinkage pores in an unbiased manner. The experimental data lead to the following conclusions: (1) Application of intensification pressure significantly reduces the total amount of porosity primarily via reduction in the gas porosity. The intensification pressure significantly reduces the number density and area fraction of the gas pores larger than 100 μm diameter. (2) A decrease in the gate velocity decreases the total amount of porosity predominantly via a decrease in the gas porosity and a small extent of decrease in the shrinkage porosity. The lower gate velocity uniformly decreases the number density and area fraction of gas pores of all sizes (small and large). (3) A decrease in the melt temperature also reduces the total amount of porosity primarily via reduction in the gas porosity. The lower melt temperature reduces the number density and area fraction of the gas pores larger than 30 μm.

The Effect of Inverse Macro-Segregation on Fatigue Behavior of Die-Cast AM60 Magnesium Alloy

The normal and inverse solute macro-segregation are known to occur in Al and other nonferrous alloy castings and have been well studied and documented. However, these phenomena have not been investigated in the high-pressure die-cast Mg-alloys. Consequently, the effects of macro-segregation on the mechanical properties of cast Mg-alloys have not been characterized. The objective of this contribution is to investigate the effects of inverse macro-segregation and porosity on the fatigue behavior of high-pressure die-cast AM60 alloy. It is observed that the inverse macro-segregation of eutectic phase at the cast surfaces adversely affects the fatigue behavior: the fatigue resistance decreases substantially due to the presence of the surface segregation.

Variability in the tensile ductility of high-pressure die-cast AM50 Mg-alloy

Abstract Variability in the tensile ductility of high-pressure die-cast AM50 Mg-alloy is examined at five different test temperatures. The fracture path preferentially goes through the regions of clusters of pores. The percent ductility is correlated to area fraction of porosity in the fracture surfaces using a power law equation.

Characterization of the effects of process parameters on macrosegregation in a high-pressure die-cast Magnesium alloy

Normal and inverse solute segregations have been reported to occur in several Al-alloys as well as in some other nonferrous alloys. Nonetheless, these phenomena have not been studied in high-pressure die-cast (HPDC) Mg-alloys. This contribution reports an experimental metallographic evidence of inverse surface macrosegregation in a HPDC AM60 Mg-alloy and the effects of process parameters on the extent of inverse segregation. It is observed that low gate velocity, low liquid metal temperature before casting, and application of intensification pressure lead to high extent of inverse macrosegregation at the cast surfaces of HPDC AM60 Mg-alloy. Unfortunately, this combination of HPDC process parameters is also often utilized to achieve low micro-porosity levels. Consequently, in such cases, the beneficial effects of low levels of micro-porosity on the fracture related mechanical properties might be nullified by the adverse effects of enhanced macrosegregation.

Inverse surface macro-segregation in high-pressure die-cast AM60 magnesium alloy and its effects on fatigue behavior

Abstract Metallographic evidence is presented for existence of inverse surface macro-segregation in a HPDC AM60 Mg-alloy. The process conditions that facilitate the inverse surface macro-segregation are identified, and the effects of macro-segregation on the low and high cycle fatigue behavior of the HPDC alloy are studied.

ROLE OF MECHANICAL TWINNING IN TENSILE COMPRESSIVE YIELD ASYMMETRY OF DIE CAST MG ALLOYS

Significant tensile-compressive yield asymmetry was observed in both low and high pressure die cast polycrystalline magnesium alloys AM50, AE44 and AJ62A between room temperature and 125 °C. Yield strengths measured under tension were higher than those under compression with the average yield asymmetry ratio of 0.85 except for the aged high pressure die cast AM50 samples which revealed nearly identical tensile and compressive strengths. To study the mechanisms for the yield asymmetry, samples of some alloys/conditions were strained to 1% for microstructural examination. The observations indicate that the difference in the contribution of mechanical twinning to deformation is the cause for the different strengths under tension and compression. In the coarse grained low pressure die cast AM50 samples, a twinning softening effect was suggested, while for the fine grained high pressure AM50, AE44 and AJ62A samples, the observations indicated a twinning hardening effect. In the aged high pressure AM50 samples, mechanical twinning was suppressed by precipitates and no yield asymmetry was observed. Transmission electron microscopy (TEM) examination of the low pressure die cast AM50 alloy strained to 1% did not show noticeable differences in tensile and compressive samples other than the extent of twinning.On a observé une asymétrie distincte de la limite d’élasticité en tension–compression des alliages polycristallins de magnésium, AM50, AE44 et AJ62A, moulés tant à basse qu’à haute pression, entre la température de la pièce et 125 °C. Les limites d’élasticité mesurées en tension étaient plus élevées que celles mesurées en compression, avec le rapport moyen d’asymétrie de la limite d’élasticité de 0.85, sauf pour les échantillons vieillis de AM50, moulés à haute pression, qui ont révélé des valeurs presque identiques en tension et en compression. Pour étudier les mécanismes de l’asymétrie de la limite d’élasticité, on a sollicité jusqu’à 1% des échantillons de certains alliages/conditions pour examen de la microstructure. Les observations indiquent que la différence dans la contribution à la déformation du maclage mécanique est la cause des différentes valeurs en tension et en compression. Pour les échantillons de AM50 à gros grains, moulés à basse pression, on suggère un effet d’adoucissement du maclage, alors que pour les échantillons de AM50, AE44 et AJ62A, à grains fins et moulés à haute pression, les observations indiquent un effet de durcissement du maclage. Pour les échantillons vieillis de AM50, moulés à haute pression, le maclage mécanique était supprimé par les précipités et l’on n’a pas observé d’asymétrie de la limite d’élasticité. L’examen au microscope électronique à transmission (TEM) de l’alliage AM50, moulé à basse pression et sollicité jusqu’à 1%, n’a pas montré de différences notables dans les échantillons en tension et en compression autres que le degré de maclage.

Utility of micro-indentation technique for characterization of the constitutive behavior of skin and interior microstructures of die-cast magnesium alloys

There has been increasing thrust lately on the development of lightweight cast magnesium alloy components for structural automotive and other applications. The microstructure of the high-pressure die-cast Mg alloys usually contains a fine-grained “skin” having a microstructure significantly different from that of the bulk material. Characterization of the local constitutive behavior of the skin microstructure is of interest as it can affect the overall mechanical response of the component. However, the standard mechanical tests on the macro-specimens are not useful for characterization of the local stress–strain response of the skin microstructure. In this contribution, we present a novel methodology based on a combination of micro-indentation experiments and three-dimensional (3D) finite elements based simulations that permits computation of the local stress–strain (constitutive) behavior of the skin and the interior microstructures at the length scales of 100 μm in a cast high-pressure die-cast AM60 Mg-alloy. The methodology involves development of a numerical solution to the inverse problem. The computed constitutive equations are then utilized to simulate the effect of skin thickness on the overall global mechanical response of the alloy under uniaxial compression.