AE42 Alloy Research Articles

Weld porosity in fibre laser weld of thixomolded heat resistant Mg alloys

Porosity in fibre laser welds of two thixomolded heat resistant magnesium alloys AE42 and AS41 was investigated in detail, and porosity formation mechanism was discussed in terms of gas compositions in porosity. It is found that the area percentage of porosity in welds decreases with increasing welding speed, and can be correlated to width of weld metal. Microstructure observation and gas composition analysis in porosity show that the porosity in welds is mainly attributed to the micropores pre-existing in base metals during melting of AE42 and AS41 alloys by fibre laser welding, which are formed due to air entrapment during thixomolding process. Hydrogen rejection and Ar shielding gas entrapment are also the possible reasons for the porosity formation; however, their contribution is much smaller than that of pores in base metals. Furthermore, the addition of rare earth element may probably decrease porosity amount in the thixomolded Mg alloys and their welds.

Dry sliding wear behaviour of magnesium alloy based hybrid composites in the longitudinal direction

In the present investigation, the wear behaviour of a creep-resistant AE42 magnesium alloy and its composites reinforced with Saffil short fibres and SiC particles in various combinations is examined in the longitudinal direction i.e., the plane containing random fibre orientation is perpendicular to the steel counter-face. Wear tests are conducted on a pin-on-disc set-up under dry sliding condition having a constant sliding velocity of 0.837 m/s for a constant sliding distance of 2.5 km in the load range of 10–40 N. It is observed that the wear rate increases with increase in load for the alloy and the composites, as expected. Wear rate of the composites is lower than the alloy and the hybrid composites exhibit a lower wear rate than the Saffil short fibres reinforced composite at all the loads. Therefore, the partial replacement of Saffil short fibres by an equal volume fraction of SiC particles not only reduces the cost but also improves the wear resistance of the composite. Microstructural investigation of the surface and subsurface of the worn pin and wear debris is carried out to explain the observed results and to understand the wear mechanisms. It is concluded that the presence of SiC particles in the hybrid composites improves the wear resistance because these particles remain intact and retain their load bearing capacity even at the highest load employed, they promote the formation of iron-rich transfer layer and they also delay the fracture of Saffil short fibres to higher loads. Under the experimental conditions used in the present investigation, the dominant wear mechanism is found to be abrasion for the AE42 alloy and its composites. It is accompanied by severe plastic deformation of surface layers in case of alloy and by the fracture of Saffil short fibres as well as the formation of iron-rich transfer layer in case of composites.

Onset of Hot Tearing in AE42 Magnesium Alloy

Magnesium alloy AE42 has long been recognized as a superior high temperature magnesium alloy for aerospace and automotive components. The elevated temperature strength of this alloy is attributed to the Mg-Alx-REy intermetallics on the grain boundaries preventing grain boundary sliding. However, these intermetallics also hinder interdendritic liquid feeding during casting solidification and contribute to the alloy's high susceptibility to hot tearing.In this research, the conditions associated with the onset of hot tearing in the AE42 alloy were identified. Thermal analysis suggested that a casting with a hot tear experienced long vulnerable interval, when interdendritic feeding was minimal and the alloy was susceptible to hot tearing. Microscopic analysis revealed the presence of interdendritic shrinkage pores with Al-RE intermetallics at hot-tear nucleation sites. Further, the elastic residual strain measured by neutron diffraction indicates that tensile strain resulting from contraction of the casting during solidification was responsible for opening and propagation of hot tears in the AE42 alloy.

Onset of Hot Tearing in AE42 Magnesium Alloy

Magnesium alloy AE42 has long been recognized as a superior high temperature magnesium alloy for aerospace and automotive components. The elevated temperature strength of this alloy is attributed to the Mg-Alx-REy intermetallics on the grain boundaries preventing grain boundary sliding. However, these intermetallics also hinder interdendritic liquid feeding during casting solidification and contribute to the alloy's high susceptibility to hot tearing.In this research, the conditions associated with the onset of hot tearing in the AE42 alloy were identified. Thermal analysis suggested that a casting with a hot tear experienced long vulnerable interval, when interdendritic feeding was minimal and the alloy was susceptible to hot tearing. Microscopic analysis revealed the presence of interdendritic shrinkage pores with Al-RE intermetallics at hot-tear nucleation sites. Further, the elastic residual strain measured by neutron diffraction indicates that tensile strain resulting from contraction of the casting during solidification was responsible for opening and propagation of hot tears in the AE42 alloy.

Microstructural modification and mechanical property improvement in friction stir zone of thixo-molded AE42 Mg alloy

Thixo-molded AE42 Mg alloy was friction stir welded, and the soundness of joints was evaluated, together with the microstructure evolution and mechanical properties in friction stir zones. According to X-ray radiography, the optimum FSW condition range of AE42 alloy exists between AZ61 and AZ31 alloys, and it seems that the optimum welding condition range increases with decreasing Al content in the Mg alloys. There are mainly two kinds of compounds in the thixo-molded AE42 alloy, and FSW has little influence on the grainy Al 10RE 2Mn 7 compound, but it has great influence on Al 11RE 3 phase, which is changed from lamellar eutectic to small particles after welding. Furthermore, the average diameter of Al 11RE 3 particles in SZ decreases with increasing the traveling speed at constant rotation speed due to less heat input. The hardness in SZ is higher than that in BM, and tensile strength and elongation are both improved after welding because the stirring refines and uniforms the microstructure and intermetallic compounds.

Microstructures and Mechanical Properties in Friction Stir Zone of Thixo-Molded AS41 Mg Alloy

The friction stir weldability of thixo-molded AS41 Mg alloy was investigated, and the microstructures and mechanical properties in friction stir zones were examined and evaluated. The non-destructive inspection by means of X-ray radiography shows that the optimum FSW condition range of AS41 alloy exists between AZ61 and AE42 alloys, and it seems that the optimum welding condition range increases with decreasing Al content in the Mg alloys. There are mainly three kinds of compounds, i.e. Mg2Si, MnSi and Al12Mg17, in the thixo-molded AS41 base metal, while only two kinds of compounds are found in SZ except Al12Mg17 phase. This is because of the decomposition of Al12Mg17 phase at the higher temperature caused by the FSW heat input. Furthermore, the average diameter of compounds in SZ decreases with increasing the traveling speed at constant rotation speed due to less heat input. The hardness in SZ is higher than that in BM, and tensile strength and elongation are both improved after welding because the stirring refines and uniforms the microstructure and intermetallic compounds. [doi:10.2320/matertrans.M2009122]

Investigation of minimum creep rates and stress exponents calculated from tensile and compressive creep data of magnesium alloy AE42

Creep specimens prepared of magnesium alloy AE42 were investigated under constant load in compressive and in tensile creep. Material was cast via the squeeze casting process in order to obtain a dense microstructure without pores. Creep tests were performed at constant temperatures between 150 °C and 240 °C and constant applied stresses between 40 MPa and 120 MPa until a minimum creep rate was reached. It could be seen that the minimum creep rates in compressive creep tests were smaller than in tensile creep tests, and the difference increased with increasing applied stress. Stress exponents were determined according to the Norton equation and it was found that a threshold stress had to be introduced into the analysis. The threshold stress is based on strengthening by aluminum-rare earths precipitates. Calculating the true stress exponent, deformation mechanisms during creep could be clarified.

Microstructural analysis of the creep resistance of die-cast Mg–4Al–2RE alloy

The microstructure and microstructural stability of die-cast AE42 (Mg–4Al–2RE) alloy were investigated by transmission electron microscopy. It is shown that the formation of Mg17Al12 after ageing at 200 °C is not due to the decomposition of A111RE3 as reported in the literature, but, rather, is associated with the supersaturation of Al solute in the α-Mg matrix. The level of Al solute retained in the α-Mg matrix after die-casting is suggested to be an important factor in influencing creep resistance.

Evolution of microstructure and hardness of AE42 alloy after heat treatments

The AE42 magnesium alloy was developed for high pressure die casting (HPDC) from low-aluminum magnesium alloys. In this alloy the rare earth (RE) elements were shown to increase creep resistance by forming AlxREy intermetallics along the grain boundaries. The present work investigates the microstructure of squeeze cast AE42 magnesium alloy and evaluates its hardness before and after heat treatments. The change in hardness is discussed based on the microstructural observations. Some suggestions are given concerning future design of alloy compositions in order to improve high temperature creep properties even further. It is shown that the microstructure of the squeeze-cast AE42 alloy is stable at high temperature 450 °C. The subsequent solution and ageing treatments have a limited effect on the hardness. The weak age-hardening is attributed to the precipitation of small amount of Mg17Al12-phase with the use of about 0.7 wt.% aluminum. The heat treatment to achieve a maximum increase in the hardness is: solution treatment at 450 °C for 5–10 h followed by an ageing treatment at 190–220 °C for about 5 h.

Effect of Microstructural Inhomogeneity on Creep Response of Mg-Sn Alloys

The development of new creep resistant magnesium alloys has become a major issue in recent years. The alloys investigated in the present work are based on the binary system Mg-Sn. Sn as major alloying element was chosen due to its high solid solubility over a wide temperature range and due to the possible formation of Mg2Sn intermetallic precipitates with a high melting temperature of about 770°C. These characteristics suggest that a fairly large volume fraction of thermally stable Mg2Sn particles can be formed during solidification. This makes it possible that the Mg-Sn alloys can be developed as creep resistant magnesium alloys. In fact, previous investigations indicate that the Mg-Sn alloys have a comparable or even better creep property than AE42 alloy. The present work investigates the microstructure of Mg-Sn alloys with and without creep deformation using SEM and TEM technique. The effects of microstructural inhomogeneity on the creep response are presented. Based on the microstructural analysis, the mechanism responsible for improving the creep resistance will be discussed. It is shown that the grain boundary sliding is a dominant creep mechanism for the Mg-Sn binary alloy.

Creep Behaviour and Microstructure of Magnesium Die Cast Alloys AZ91 and AE42

Tensile creep behaviour of the die-cast magnesium alloys AZ91 and AE42 has been studied at temperatures between 85oC and 200oC and at stresses in the range from 30 to 100 MPa. Microstructural investigations, mainly by TEM, have been performed on the selected crept samples to characterise the microstructural change during the plastic deformation, which reveals several phenomena related to the creep process including formation of dislocation sources, denuded zones around grain boundaries and microvoids, and changes in the nature of intermetallic phases. The active creep mechanisms have been discussed on the basis of the creep data in combination with the microstructural change during creep.

Tensile and indentation creep behavior of Mg–5% Sn and Mg–5% Sn–2% Di alloys

The tensile properties and the indentation creep resistance of as-cast Mg–5 wt% Sn and Mg–5 wt% Sn–2 wt% Di alloys were studied in this paper. It has been shown that the tensile properties of Mg–5% Sn–2% Di alloy are comparative with that of AE42 alloy at room temperature and at 150 °C and 175 °C. The indentation creep experiments were conducted at 150 °C and 175 °C, suggesting that Mg–5% Sn–2% Di alloy has significantly better indentation creep resistance than AE42. The microstructure of Mg–5% Sn alloy is characterized by the presence of thermally stable Mg 2Sn particles mainly along grain boundaries, and addition of 2 wt% Di to this alloy results in the appearance of new Sn–Di phase, which further significantly enhance the strength and creep resistance.

Effect of heat treatment on the superplasticity of magnesium alloys

A necessary microstructural condition for the manifestation of the effect of superplasticity in alloys is a small grain size (d 100 μm. We have studied the regimes of heat treatment of such materials in AZ91, AE42, QE22, and ZRE1 alloys with a purpose of obtaining a fine-grained structure. The optimum temperature of overaging of quenched magnesium alloys lies between 300 and 350°C. After hot pressing of heat-treated alloys, the average grain size is 6.4 (AZ91), 6.2 (AE42), 1.2 (ZRE1), and 0.7 (QE22) μm. The best characteristics of superplasticity are manifested by the ZRE1 and QE22 alloys with a relative elongation of 750% and strain-rate sensitivity m = 0.75 at T = 420°C and strain rate $$\dot \varepsilon $$ = 3 × 10−4 s−1. Under these conditions, the AZ91 and AE42 alloys have δ ≤ 260% and m = 0.45.

Quasi-static behavior of Mg-alloys with and without short-fiber reinforcement

Magnesium alloys AE42 and AZ91 reinforced with 23 vol.% carbon short fibers ( D f ≈ 7 μm, L f ≈ 100 μm) were tested under quasi-static loading. The carbon fibers were quasi-isotropically distributed in the horizontal plane (reinforced plane) of the casting. Compression and tensile tests were carried out on both the matrix alloys and the composites at temperatures between 20 °C and 300 °C. Specimens were machined to be loaded either parallel or normal to the reinforced plane. Due to the reinforcement, the compression yield stress of the composite AE42-C increased to a value approximately three-fold greater than the yield strength of the matrix; for composite AZ91-C this parameter was approximately 2.5-fold greater than that of the AZ91 matrix. The improvement in tensile strength was less than that in compression, which could be related to early tensile fracture through decohesion at the matrix–fiber interface, as detected by SEM investigations conducted on failed tensile specimens. Flow curves for the matrix alloys at different temperatures were described by a modified Kocks–Mecking material law. An idealization of a 2-D mesomodel was used for finite-element simulation of the mechanical behavior of the composites. The fibers were first considered as elastic bodies and the behavior of the matrix material was set according to the material law determined from the flow curves for the matrix alloys. Other calculations were carried out by considering elasto-plastic behavior of the fibers for application of a failure initiation technique to simulate the behavior of the composite materials beyond the ultimate stress.

Visualization of three-dimensional pore morphologies in a high-pressure die-cast Mg–Al–RE alloy

Reconstruction and visualization of three-dimensional (3-D) pores are of interest for understanding microstructure–properties relationships in cast Mg-alloys. In this contribution, a montage serial sectioning technique is applied to reconstruct a large volume of the 3-D microstructure of a new high-pressure die-cast AE44 Mg-alloy at a high resolution. The 3-D reconstruction reveals that the shapes and morphologies of the gas pores in the AE44 alloy are substantially different from those in other important cast Mg-alloys, which may partly account for the AE44 alloy’s superior properties.

Thermal cycling behaviour of the magnesium alloy based hybrid composites in the transverse direction

The thermal cycling behaviour of a creep-resistant AE42 alloy reinforced with 20 vol% saffil short fibre as well as various volume fractions of saffil short fibre and SiC particulate has been examined in the transverse direction in the temperature range of 30–350 °C. For comparison, the thermal cycling behaviour of the unreinforced AE42 alloy is also examined. It is found that the linear instantaneous CTE curves of composites show a positive deviation from linearity above 215–230 °C, which is attributed to the release of residual compressive strain arising from the squeeze casting process. The experimentally obtained CTE of the composites is compared with the theoretically calculated values. The CTE of the hybrid composite is calculated by a linear subtraction of the expected decrease in CTE with the addition of SiC particulates from the CTE calculated for composite containing short fibres alone assuming a rule of mixture. However, it is observed that the fibres and particulates interact in a more complex manner and the decrease in CTE with the addition of particulates in a hybrid composite is greater than that obtained by the simple linear subtraction assuming a rule of mixture.

Effect of overageing temperature on the superplastic behaviour in magnesium alloys

An especially fine-grained structure d < 10 μm is necessary for superplastic forming. Conventional magnesium alloys in cast condition show only a very coarse-grained structure with an average grain size of d > 100 μm. Yet through overageing and successive warm extrusion moulding a sufficient number of recrystallization nuclei are formed in the magnesium alloys so that during the final recrystallization treatment a very fine microstructure results. It has been shown that the optimal temperature for overageing lies in the range from 300 to 350 °C for the magnesium alloys AE42, AZ91, ZRE1 and QE22. Using this temperature range we obtained an average grain size of 6.2 μm in the AE42 alloy and 6.4 μm in the AZ91 alloy. In the ZRE1 magnesium alloy a grain size of 1.2 μm and in the QE22 a grain size of 0.7 μm were obtained. The heat-treated magnesium alloys QE22 and ZRE1 investigated here elongate up to 750% and have an m value of 0.75 under a constant strain rate of 3 × 10 −4 s −1 and a deformation temperature of 420 °C. Under the same conditions AZ91 und AE42 elongate up to 260% and have m-values of 0.45.

Investigation of HVOF spraying on magnesium alloys

Magnesium alloys are promising alternatives to other lightweight alloys such as aluminum alloys due to their high specific strength and stiffness. However, the use of magnesium alloys is limited by their poor wear behaviour and corrosion performance. Recent studies have shown an enormous potential of thermal spray techniques for the surface modification of Mg alloys. The high particle velocities and moderate temperatures achieved by the High Velocity Oxy-Fuel (HVOF) flame spray process lead to very dense coatings with outstanding wear behaviour and superior bond strengths in comparison to other thermal spray processes. In this study, two Mg alloys AZ91 and AE42 were coated using the HVOF spray process. The substrates were compared in terms of the measured bond strength and the observed adhesion mechanisms of the coating. Furthermore, the coatings were characterized concerning their corrosion performance on AZ91 substrates. It was found that dense WC–Co coatings could be applied on Mg alloy substrates using the HVOF spray process. The high kinetic energy of the WC–Co particles led to a “self roughening” effect on the substrate, enabling the deposition on polished Mg alloy substrates. The coatings showed a very good adhesion to the substrates. The corrosion tests showed that the unsealed WC–Co coatings could not improve the corrosion performance of Mg alloys. In contrast, the duplex coating system with an Al bond coat improved significantly the corrosion resistance of Mg alloys. The sealed coatings showed a very good corrosion behaviour.

Modeling the creep behavior of Mg alloys with and without short-fiber reinforcement

Compressive creep curves for Mg alloys AE42 and AZ91 at 150, 200 and 250°C were mathematically described and the parameters were determined. Modeling was carried out using three relationships describing separately the three stages (primary, secondary and tertiary) of the creep curve. The combination of these three relationships provides a model that describes well the whole creep curve. The creep behavior of a Mg alloy reinforced with short fibers was numerically determined using simple idealization of the composite material. The material law parameters of the matrix alloy were applied, while the fibers were considered as elastic bodies. Finite element simulation showed satisfactory agreement with the experimental results.

Investigations on thermal fatigue of aluminum- and magnesium-alloy based composites

Both the KS1275® piston and AE42 alloys and their composites have realistic and/or potential applications as engine components in the automotive industry. Used as engine components, the dimensional stability is of great concern. Thermal cycling experiments can simulate the service conditions of the materials and give an evaluation how the dimension changes during their service in the changing temperature environments. The present paper investigates the thermal fatigue of the short fiber reinforced KS1275® piston and AE42 alloys, with an emphasis on the changes in the strain and hardness before and after thermal cycling. The effects of fiber orientation and composition, and subsequent heat treatment, on the thermal strain were discussed. It is shown that the thermal strain was affected by experimental condition of the thermal cycling and the strength of matrix. After thermal cycling, the hardness decreases due to the occurrence of the matrix overageing and recovery.