Die-cast AM50 Magnesium Alloy Research Articles

Fatigue Analysis of Magnesium Alloys Components for Car Industry

Abstract The use of magnesium alloys in the automotive industry increased in the last decade because of their low weight and relative good mechanical properties. However, the variable loading conditions require a good fatigue behavior. This paper summaries the fatigue properties of magnesium alloys and presents new fatigue curve results for die cast AM50 magnesium alloy.

Formation of Calcareous Deposition Layers on AM50 Magnesium Alloy in Presence of Ca2+ in De-Icing Salt Solutions: Immersion Vs. Salt Spray Test

The influence of the presence and absence of de-icing salt ingredient Ca2+ (CaCl2) on the corrosion behavior and morphology of calcareous deposition products on die cast AM50 magnesium alloy was investigated by using an electrolyte close to the de-icing salt composition (NaCl, MgCl2) with and without Ca2+ addition. The aim of the work is to point out the difference of corrosion mechanisms between the presence or absence of Ca2+ and between immersion and salt spray conditions. Electrochemical measurements (OCP, EIS, potentiostatic and potentiodynamic polarization) and several surface analysis techniques (FTIR, XRD, XPS, SEM / EDX) were used. During immersion experiments in the presence of Ca2+, a grey-white, voluminous and open-porous deposition layer has been formed on the surface of magnesium samples, which covered the whole surface after 12 days ­ with layer thicknesses above a tenth of a millimeter. XRD was used to identify Calcite (CaCO3) as a major component in the deposition layer. It has been shown previously by the authors [1, 2] that Ca2+ has an inhibiting effect on Mg alloy corrosion during short time immersion experiments (up to 4.5 h). The inhibiting effect of the Ca2+ in the first 4.5 h cannot be attributed to the incorporation of Ca2+ into the surface layer, which was shown by XPS. Further, an increase of the thickness of the corrosion product layer in presence of Ca2+ seems to be responsible for the inhibiting effect, shown by XPS depth profiling. After several days of immersion in the presence of Ca2+, the element Ca could be detected on the surface by EDX. XRD and FTIR proved the presence of calcite with layer thicknesses up to 155 µm, shown by SEM cross section investigations. The inhibiting effect of Ca2+is not present anymore as Calcite occurs in the surface layers at larger immersion times (> 4,5 h). Furthermore, the influence of the used testing method was investigated. In addition to immersion experiments, salt spray testing was performed up to 13 weeks which resulted in a different morphology of corrosion and deposition products (see Figure 1). Mass gain and mass loss measurements were carried out and showed a 5-times higher mass loss under immersion conditions in comparison to salt spray testing. The presence of a higher amount of carbonates after salt spray testing could be proven. Calcite is present in the surface layers in significant amounts after both testing methods. The increasing OH- concentration during Mg corrosion triggers the deposition of CaCO3 under conditions of low CO2 partial pressure, whilst the carbonate anion derives from the CO2 in the atmosphere.

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 Ca2+in Deicing Salt on the Corrosion Behavior of AM50 Magnesium Alloy

To lower the freezing point of deicing salt, calcium chloride (CaCl2) is commonly used. The influence of the presence and absence of the deicing salt ingredient Ca2+ on the corrosion behavior of die-cast AM50 magnesium alloy was investigated using an electrolyte close to the deicing salt composition with and without Ca2+ addition. The goal of the work was to point out the differences in the corrosion mechanism with and with-out Ca2+ addition by electrochemical investigations, H2 evolution, and weight-loss characterization. Under polarizing conditions the resulting current densities are significantly lower in the presence of Ca2+. Electrochemical impedance spectroscopy (EIS) measurements up to 4.5 h showed an increasing polarization resistance, RP, with time, and after 4.5 h, a breakdown of RP values in the presence of Ca2+ combined with the appearance of an inductive loop. Mass-gain, mass-loss, and hydrogen evolution measurements confirmed the inhibiting behavior in the presence of Ca2+ up to 1 day. Durin...

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.

Dislocation movements during creep in a die-cast AM50 magnesium alloy

Tensile creep tests were combined with detailed transmission electron microscopy in order to characterize the dislocation movements during creep of the die-cast AM50 magnesium alloy. TEM observations indicate that dislocations are introduced within the primary α-Mg grain interior in the die-casting process, which consist of both the basal and non-basal segments. The non-basal segments of dislocations, having smoother curvature in as die-cast state, partially exhibit steps parallel to the basal plane during high temperature exposure. The basal segments of dislocations are able to bow out and glide on the basal planes under the influence of a stress, and the non-basal segments and/or jogs follow the basal segments with the help of climb during creep.

INFLUENCE OF ELECTROLYTIC PLASMA OXIDATION COATING ON TENSILE BEHAVIOR OF DIE-CAST AM50 ALLOY SUBJECTED TO SALT CORROSION

Three different thickness ceramic coatings were deposited on die-cast AM50 magnesium alloy in KOH and NaAlO 2 solution using electrolytic plasma oxidation (EPO) technology for corrosion prevention. Immersion corrosion tests were carried out in 3.5% NaCl solution for 336 hours to investigate the effect of coating thicknesses on tensile and fracture behaviors of the coated AM50 alloys. The results show that the yield strength (YS) and ultimate tensile strength (UTS) of the coated AM50 alloy subjected to immersion corrosion increase with an increase in coating thicknesses. Further analyses on stress and strain curves indicate that the coating enhances the strain-hardening rates of the corroded alloy during its plastic deformation. SEM examination on the fractured surface manifests that the substrate AM50 alloys exhibit characteristics of ductile deformation with deep dimples. However, brittle features prevail on the fractured surface of the mixed layer of coating plus oxidation corrosion product. Micro cracks were observed between the mixed layer and the AM50 alloy substrate induced by corrosion and within the mixed layer induced by EPO process, which could be responsible for the brittle fracture.

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.

Internal Stress during Creep in a Die-Cast AM50 Magnesium Alloy at 473 K

The internal stress during high temperature creep was investigated for the die-cast AM50 magnesium alloy at 473 K through the strain-transient dip test technique. The microstructure of the alloy is characterized by fine α-Mg grains together with the β-Mg 17 Al 12 particles located on the grain boundaries. The internal stress is proportional to the applied stress, when the applied stress ranges below the yield stress. The ratio of the internal stress to the applied stress is 0.36 for the alloy, which is about half of those for conventional pure metals such as Cu, α-Fe and Ni. The small internal stress is a characteristic feature in the creep for the alloy.

Creep characteristics of a diecast AM50 magnesium alloy

Tensile creep tests were conducted to examine the creep behavior of a diecast AM50 magnesium alloy in the temperature range from 423 to 498 K. A normal transient creep stage is followed by a minimum creep rate stage and finally by an accelerating stage at each creep condition. The stress exponent of the minimum creep rate, n, increases from ∼5 at lower stresses to ∼10 at higher stresses at each temperature, and the value of n changes at the yield stress of the alloy. The activation energies for the creep, Q c , are determined to be 121±10 and 162 kJ/mol, at lower and higher stresses, respectively. It is concluded that the creep of the diecast AM50 alloy is controlled by the high-temperature climb of dislocations, whereas the instantaneous plastic strain introduced by the higher stress of the creep test is assumed to cause the increased values of n and Q c .

A thousandfold creep strengthening by Ca addition in die-cast AM50 magnesium alloy

The effect of calcium addition on the microstructure and creep strength of the die-cast AM50 magnesium alloy was investigated. The α-Mg grains with the diameter of 4.9 µm are surrounded by the eutectic phases for the AM50-1.72 mass pct Ca alloy, while the β(Mg17Al12) particles are located mainly on the grain boundaries of the α grains for the AM50 alloy. The minimum creep rates of the AM50-1.72 mass pct Ca alloy are three orders of magnitude lower than those of the AM50 alloy at 423 K typically below 120 MPa. The thousandfold creep strengthening by the Ca addition is ascribed to the thermally stable eutectic phases appearing in the AM50-1.72 mass pct Ca alloy, which is expected to yield effective grain boundary strengthening or to resist the plastic flow of the α-Mg grains.

An investigation on the microstructure of an AM50 magnesium alloy

The microstructure and the dislocation arrangement in the die cast AM50 magnesium alloy as well as in the stressed states have been investigated using conventional transmission electron microscopy (TEM), high-resolution TEM and energy dispersive X-ray analysis. The microstructure of the die cast AM50 alloy is found to mainly consist of α-Mg, β-Mg 17Al 12 and Al 8Mn 5 phases. Two kinds of β-Mg 17Al 12 particles with different sizes have been found in the stressed AM50 magnesium alloy. Besides the normal β-Mg 17Al 12 particles with size around several micrometers as in the die-cast AM50 magnesium alloy, some finer β-Mg 17Al 12 particles in the stressed states of the AM50 alloy have also been found. The finer β-Mg 17Al 12 particles are found to be only about tens of nanometers with oval or rod-like morphology, which may be formed during deformation. Dislocation pile-ups have been found in the stressed AM50 alloy for the first time. The spacing between each parallel dislocation in the pile-ups is only several nanometers. The dislocations are confined in the slip planes and piled up against grain boundaries. Dislocations in the networks are found to increase with deformation of the alloy. Also, dislocation networks have been found in the β-Mg 17Al 12 and Al 8Mn 5 phases as well as in the matrix in the deformed AM50 magnesium alloy. However, the dislocation pile-ups are found to be almost identical from 1.3% deformation to rupture, which explains the stable tensile yield strength of the AM50 magnesium alloy during the deformation.