Die-cast AZ91D Magnesium Alloy Research Articles

Effect of cyclic loading on mechanical properties and microstructure of die cast magnesium alloy AZ91D

Abstract Magnesium alloy materials are often affected by cyclic stress, resulting in fatigue fracture due to changes in mechanical properties. In this study, external cyclic loads were introduced to simulate the loading of AZ91D (Mg-9Al-0.5Zn) parts. Tensile tests under static and external circulation loads were performed on the same specimens, and the effect of loading conditions on the mechanical properties of the specimens (tensile strength, yield strength and elongation) were studied. The internal microstructure of the test specimens was analyzed, and the influence of cyclic loading on the microstructure change of the specimens was discussed. Compared with static load, under the condition of cyclic loading, the tensile strength and yield strength of the specimens are increased by more than 15%, and the elongation rate is increased by 3 times. In addition, the microstructure images showed that many twins and the second phase β-Mg17Al12 were generated inside the AZ91D microstructure under cyclic loading. The former results in specimens crack initiation and it is the root cause of fatigue fracture. And the latter accounts for the strengthening of the mechanical properties of specimens.

Coupled effect of ultrasonic vibration and post-weld remelting on the porosity and microstructure of die-cast magnesium alloy AZ91D

Ultrasonic vibration (UV), post-weld remelting (PWR), and UV coupled with PWR (UV + PWR) were used to reduce the fusion weld porosity of die-cast magnesium alloys. In addition, the microstructures of the resulting alloys were evaluated. Unlike conventional methods, this work adopted a novel approach to introducing UV. All three treatments reduced the porosity of the alloys. In particular, single and double PWR treatments reduced the porosity by 74.45 % and 69.92 %, respectively, whereas UV treatment reduced the porosity by 79.6 %. Furthermore, single and double UV + PWR treatments reduced the porosity by 92.23 % and 94.23 %, respectively. Further analysis suggests that the cavitation effect and the movement of the tungsten needle facilitated the uplifting of bubbles in the melt pool. The increase in the number of PWR applications increased the β-Mg17Al12 phase growth, aggregation, and grain size. However, multiple UV + PWR applications achieved finer and more diffused β-Mg17Al12 with smaller grain sizes, thereby demonstrating the effectivity of the novel ultrasonic introduction method in reducing porosity and refining microstructure. This work is expected to contribute in improving the welding performance of magnesium alloys.

On the deformation behavior of heterogeneous microstructure and its effect on the mechanical properties of die cast AZ91D magnesium alloy

Both a conventional flow distributer and an improved one with a flow buffer were applied respectively during the high pressure die casting (HPDC) process, and samples of AZ91D magnesium alloy with different microstructure mainly consisting of α-Mg grains, β-phase and porosities were obtained. According to the grain orientation analysis, the predominant deformation behavior in α-Mg grains was dislocation slip, supplemented by deformation twinning. Dislocation slip was more difficult to occur in the samples with the improved flow distributer on account of the fact that the size of α-Mg grains in the microstructure was finer and more uniform. During the in situ tensile deformation test, cracks were observed to initiate from gas-shrinkage pore and island-shrinkage, and two main crack propagation mechanisms, porosity growth and coalescence were found accordingly. When the crack was in contact with the β-phase, it would pass through and fracture the network β-phase, whereas bypass the island β-phase by detaching it from the surrounding α-Mg grains. Mechanical property tests showed that the samples with relatively more homogeneous microstructure would perform higher mechanical properties, which was the combined effect of matrix α-Mg grains, β-phase, and porosities.

Enhancement of the adhesion and anticorrosive properties of an epoxy coating by the application of a cerium based film

The formation of an epoxy coating on die-cast AZ91D magnesium alloy was studied. In order to improve the adhesion of the epoxy resin, a cerium-based film was previously synthesized on the alloy surface. The influence of the surface substrate on the adhesive and anticorrosive properties of the generated coatings was evaluated. The corrosion performance of the coated samples was studied by open circuit potential test, potentiodynamic polarization and electrochemical impedance spectroscopy in a chloride containing solution. The composition and morphology of all the samples were analyzed by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDX). In order to determine the surface free energy, contact angle measurements for the treated and untreated alloy surface were performed. An improvement in the adhesion and anticorrosive properties was obtained by applying epoxy coating onto a cerium-based film previously synthesized on Mg alloy.

Real-time in situ observation of the corrosion process of die-cast AZ91D magnesium alloy in NaCl solutions under galvanostatic polarization

In situ observations of the filiform corrosion process of die-cast AZ91D magnesium alloy were carried out in 0.1 and 0.01 M NaCl solutions (pH 8.0) through a microscale polarization technique. The changes in morphology were observed in real-time using an optical microscope. Filiform corrosion proceeded in lamellar α + β structures adjacent to β phases and sometimes in the α phase grains. The β phases remained intact after filiform corrosion.

Improved Corrosion Resistance of AZ91D Mg Alloy by Cerium-Based Films. Formation of a Duplex Coating with Polypyrrole

In order to improve the corrosion resistance of die-cast AZ91D magnesium alloy in simulated physiological solution, two cerium-based coatings were synthesized. The coatings were electrodeposited from solutions containing Ce(NO3)3, H2O2 and C6H8O7 or C4H4O6Na2. The influence of the presence of sodium tartrate and citric acid in the preparation solution on the morphology, composition and anticorrosive performance of the generated coatings was evaluated. The corrosion properties were examined in Ringer solution by polarization studies, open circuit measurements and faradaic impedance spectroscopy. Results showed that the incorporation of additives improves the anticorrosive properties of the films formed without sodium tartrate or citric acid. The coating formed in the presence of sodium tartrate showed the best anticorrosive performance. Subsequently, the possibility of forming duplex coatings employing the cerium-based films as inner layers and polypyrrole as top coating was evaluated. At this point, it was found that polypyrrole can be electropolymerized on top of the formed cerium-based film modified with sodium tartrate.

Corrosion Behavior of Diecast Mg–Al–Mn–Ca–Si Magnesium Alloy

The corrosion behavior of diecast Mg–Al–Mn–Ca–Si (AMXS6020) magnesium alloy with superior flame-retardance and heat-resistance was systematically investigated and compared to diecast AZ91D magnesium alloy and ADC12 aluminum alloy by salt spray test, cyclic corrosion test, electrochemical experiment, scanning electron microscopy and energy dispersive X-ray spectroscopy. The results showed that AMXS6020 alloy had a higher corrosion resistance than AZ91D and ADC12 alloys. The superior corrosion resistance of AMXS6020 alloy was attributed to netlike eutectic structure consisting of (Al, Mg)2Ca and α-Mg, which could hinder the corrosion growth. The barrier effect of netlike (Al, Mg)2Ca phase structure on corrosion growth was also confirmed by potentiodynamic polarization.

Effects of combined addition of Ca and Y on the corrosion behaviours of die-cast AZ91D magnesium alloy

The corrosion behaviours of commercial AZ91D alloy and Ca–Y-containing AZ91 alloy, fabricated by casting, were evaluated, and their microstructural changes and surface properties were analysed. The Ca–Y-containing AZ91 alloy showed superior corrosion resistance compared with the AZ91D alloy mainly because of the decrease in the Volta potential difference between the matrix and the secondary phases formed in the Ca–Y-containing alloy, such that micro-galvanic corrosion was significantly suppressed. Furthermore, passivity and stability of the surface film improved with the addition of Y to the AZ91 alloy. Thus, the overall corrosion resistance was significantly improved in the Ca–Y-containing AZ91 alloy.

Effect of Thickness on the Thermal Conductivity and Microstructure of Die-Cast AZ91D Magnesium Alloy

Magnesium alloys have many excellent properties and possess wide industrial application prospects. Sheets of AZ91D magnesium alloy with different thicknesses were produced by the die-cast process, and the cooling rates lay between 3.77 and 29.27 °C s−1 with the thickness ranging from 1.5 to 6 mm. With the increasing thickness, the grain size increased, and the concentration of Al solute atoms in Mg matrix decreased. The second phases transformed from homogeneous fine particles and short strips at 1.5 mm to a network at 6 mm. These changes can be attributed to the cooling rate. The thermal conductivity was found to increase with the increasing thickness at the same temperature, and to increase with the increasing temperature at the same thickness. The minimum thermal conductivity (45.19 W (m K)−1), and the maximum thermal conductivity (89.32 W (m K)−1) were obtained at a thickness of 1.5 mm at 25 °C and a thickness of 6 mm at 150 °C, respectively. The grain size increased, and the Al solute atoms in Mg matrix decreased with the increasing thickness, which reduced the lattice irregularity and scattering of electrons and phonons, and resulted in an increase in the thermal conductivity.

Microstructural Evolution of Vacuum Die-Cast AZ91D Magnesium Alloy during Solution Treatment

Microstructural Evolution of Vacuum Die-Cast AZ91D Magnesium Alloy during Solution Treatment

Correlation between Density and Microstructural Features in Vacuum Die Cast AZ91D Magnesium Alloy

Correlation between Density and Microstructural Features in Vacuum Die Cast AZ91D Magnesium Alloy

Effect of mischmetal on mechanical properties and microstructure of die-cast magnesium alloy AZ91D

Effects of the mischmetal addition in range of 0.4 wt.% to 1.7 wt.% on the microstructure and mechanical properties of die-cast magnesium AZ91D were investigated to improve the elevated temperature mechanical properties of the alloy by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and tensile tests. The results revealed that mechanical properties of die-cast magnesium alloy AZ91D-0.4%MM at 100 °C were near to those of die-cast magnesium alloy AZ91D. The ultimate tensile strength, 0.2% proof yield strength and elongation to failure of die-cast magnesium alloy AZ91D at 170 °C were 178, 129 MPa and 20%, respectively. In comparison, the ultimate tensile strength, 0.2% proof yield strength and elongation to failure of die-cast magnesium alloy AZ91D-0.4%MM at 170 °C reached to 206, 142 MPa and 26%, respectively increased by 15.7%, 10% and 30%. Proper addition of mischmetal could enhance the mechanical properties at an elevated temperature, which was attributed to the formation of Al-RE phases with high thermal stability. Hence sliding of grain boundaries and cracks could be effectively hindered by Al-RE phases.

Effect of multi-step slow shot speed on microstructure of vacuum die cast AZ91D magnesium alloy

Two multi-step (two-step and three-step) slow shot speeds were used in the vacuum die casting process of AZ91D magnesium alloy. The vacuum pressure variation in the die cavity before mold filling was monitored by using a pressure sensor. The microstructures of the produced castings were analyzed with optical microscope and image analysis software. The experimental results demonstrate that, the vacuum pressure in the die cavity at the beginning of mold filling is significantly reduced by using three-step slow shot speed, resulting in a low gas porosity level in the produced castings. At an appropriate multi-step slow shot speed, the dwell time of the liquid metal in the shot sleeve before mold filling can be reduced and the flow of the liquid metal in the shot sleeve at the later stage of the slow shot process can be restrained, which cause a low externally solidified crystal content in the produced castings.

Corrosion Performance of Anodized AZ91D Magnesium Alloy: Effect of the Anodizing Potential on the Film Structure and Corrosion Behavior

Die-cast AZ91D magnesium alloy samples have been submitted for anodizing at different potentials. Anodizing was conducted in an environmentally friendly solution which comprised 3 M KOH + 1 M Na2SiO3 at room temperature. The surface treatment was performed electrolytically at four different potentials: 3, 5, 8, and 10 V. The corrosion resistance was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization curves obtained after 7 days of immersion in a 3.5 wt.% NaCl solution at room temperature. The porosity of the anodic films was estimated by means of the linear polarization method. SEM images revealed that the surface oxide was thicker for the anodic films obtained at 3 and 5 V. The films obtained at these potentials were more porous than those formed at 8 and 10 V. The results showed that the thickness of the anodic film had a significant effect on the corrosion behavior of the AZ91D, whereas the influence of the initial porosity was not significant.

Effects of on-line solution and off-line heat treatment on microstructure and hardness of die-cast AZ91D alloy

The effects of on-line solution, off-line solution and aging heat treatment on the microstructure and hardness of the die-cast AZ91D alloys were investigated. Brinell hardness of die-cast AZ91D alloy increases through on-line solution and off-line aging treatment but decreases after off-line solution treatment. By X-ray diffractometry, optical microscopy, differential thermal analysis, scanning electron microscopy and X-ray energy dispersive spectroscopy, it is found that the microstructures of the die-cast AZ91D magnesium alloy before and after on-line solution and off-line aging are similar, consisting of α-Mg and β-Al12Mg17. The precipitation of Al element is prevented by on-line solution so that the effect of solid solution strengthening is enhanced. The β-Al12Mg17 phases precipitate from supersaturated Mg solid solution after off-line aging treatment, and lead to microstructure refinement of AZ91D alloy, so the effect of precipitation hardening is enhanced. The β-Al12Mg17 phases dissolve in the substructure after off-line solution treatment, which leads to that the grain boundary strengthening phase is reduced significantly and the hardness of die cast AZ91D is reduced.

Microstructure Characteristics of the Eutectics of Die Cast AM60B Magnesium Alloy

Under the cold-chamber high pressure die casting (HPDC) process, samples were produced with AM60B magnesium alloy to investigate the microstructure characteristics of the eutectics, especially focusing on the constitution, morphology and distribution of the eutectics over cross section of the castings. Attentions were also paid to study the efiect of heat treatment on the eutectics in the die castings. Based on experimental analysis using optical microscopy (OM), X-ray difiraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS), it was determined that fully divorced eutectics consisting of fi-Mg and fl-Mg17Al12 appeared at the grain boundary of the primary fi-Mg in the as-cast microstructure. Islands and networks of fl-Mg17Al12 phase were observed in the central region of the castings, while the fl-Mg17Al12 phase revealed a more dispersed and granular morphology on the surface layer. The two phases ratio fl/fi in the central region of the castings was approximately 10%, which was higher than that on the surface layer. Besides, the defect bands contained a higher percentage of the eutectics than the adjacent regions. After aging treatment (T6), only fi-Mg phase was detected by XRD in the AM60B magnesium alloy, though a small amount of precipitated fl-Mg17Al12 phase was observed at the grain boundary. In contrast to the microstructure of die cast AZ91D magnesium alloy under the same T6 heat treatment, no discontinuous precipitation of the fl-Mg17Al12 phase was observed in AM60B magnesium alloy die castings.

Effect of [Ca2+] and [[formula omitted]] levels on the formation of calcium phosphate conversion coatings on die-cast magnesium alloy AZ91D

The effect of varying [Ca2+]total and [PO43-]total concentrations (nine conditions in total) on the formation of calcium phosphate conversion coatings on magnesium alloy AZ91D was investigated. Corrosion performance was evaluated by measurement of polarisation behaviour and also by salt spray testing. A consistent trend was observed of improved corrosion performance with reduced ionic concentrations. Low [PO43-]total concentration was found to be especially important for the attainment of good corrosion resistance. Both direct measurement using X-ray photoelectron spectroscopy and chemical modelling using MEDUSA software indicated an increase in hydroxyapatite content of the coating accompanying the improved corrosion resistance.

Microstructure and mechanical properties of die-cast AZ91D magnesium alloy by Pr additions

Mg-9Al- xPr ( x=0.4, 0.8 and 1.2, mass fraction, %) magnesium alloys were prepared by high-pressure die-casting technique. The effects of Pr on the microstructures of die-cast Mg-9Al based alloy were investigated by XRD and SEM. Needle-like Al 11Pr 3 phase and polygon Al 6Mn 6Pr phase are found in the microstructure. With 0.4% Pr addition, fine needle-like Al 11Pr 3 phase and a small amount of polygon Al 6Mn 6Pr phase near the grain boundary are found in the microstructure. Increasing Pr addition to 0.8%, lots of coarse needle-like Al 11Pr 3 phase within grain and polygon Al 6Mn 6Pr phase on grain boundary are observed. Further increasing Pr addition, the size of needle-like Al 11Pr 3 phase decreases, while the size of polygon Al 6Mn 6Pr relatively increases. The mass fraction of Pr at around 0.8% is considered to be suitable to obtain the optimal mechanical properties. The optimal mechanical properties are mainly resulted from grain boundary strengthening obtained by precipitates and solid solution.

Influence of neodymium on high cycle fatigue behavior of die cast AZ91D magnesium alloy

High cycle fatigue behavior of die cast AZ91D magnesium alloy with different Nd contents was investigated. Axial mechanical fatigue tests were conducted at the stress ratio R=0.1 and the fatigue strength was evaluated using up-to-down load method on specimens of AZ91D with different Nd contents. The results showed that the grain of AZ91D alloy was refined, the size and amount of β-Mg 17Al 12 phase decreased and distributed uniformly with increasing Nd content. At the number of cycles to failure, N f =10 7, the fatigue strength of AZ91D evaluated by up-to-down load method increased from 75 MPa (0.5%Nd) up to 85 MPa (1.0%Nd) and then decreased to 79 MPa (1.5%Nd), respectively. The fatigue crack of AZ91D alloy initiated at the surface defect or porosities and inclusions which existed near the surface, and propagated along the grain boundary. The fatigue crack propagation region was composed of some small facets. The clear fatigue striations appeared at the alloy of containing 1.0%Nd. The fatigue fracture surfaces of test specimens showed the mixed-fracture characteristics of quasi-cleavage and dimple. The fatigue step, trans-granular and along-granular cracks existed in the local fracture surface.

Effect of excimer laser surface melting on the microstructure and corrosion performance of the die cast AZ91D magnesium alloy

Excimer laser surface melting (LSM) of the die cast AZ91D alloy has been investigated in terms of microstructure and corrosion behaviour. Excimer LSM of the alloy resulted in a highly homogeneous and refined melted microstructure, which improved the corrosion resistance of the alloy. The latter was associated with the large dissolution of intermetallic phases and the enrichment of aluminium within the melted layer. An increased number of laser pulses resulted in thicker melted layers, but also in enhanced porosity and the formation of micro-cracks at the overlapping area. Both factors diminished the corrosion resistance of the laser-treated alloy.