As-cast AZ91 Alloy Research Articles

Influence of Zinc Chloride Exposure on Microstructure and Mechanical Behavior of Age-Hardened AZ91 Magnesium Alloy.

The AZ91 magnesium alloy was subjected to a complex treatment involving age hardening (supersaturation and artificial aging) and simultaneous surface layer modification. The specimens were supersaturated in contact with a mixture containing varying concentrations of zinc chloride, followed by cooling either in air or water. After supersaturation, the specimens were subjected to artificial aging and then air-cooled. This process resulted in the formation of a surface layer made of zinc-rich phases. The thickness and microstructure of the surface layer were influenced by the process parameters, namely, the zinc chloride content in the mixture and the cooling rate during supersaturation. The treated specimens exhibited favorable tensile strength and greater elongation compared to the as-cast AZ91 alloy, with values comparable to those of the alloy subjected to standard T6 tempering. No cracking of the layer was observed under moderate deformation, though greater deformation resulted in the formation of cracks, primarily in the areas containing the Mg5Al2Zn2 intermetallic phase. The produced layer demonstrated strong metallurgical bonding to the AZ91 substrate.

Investigation of Microstructure and Wear Behaviors of AZ91 Alloy Under Different Heat Treatments

This article aims to experimentally investigate the effect of heat treatment applied to AZ91 magnesium alloys on wear behavior under different applied load conditions. Apart from as-cast AZ91 alloy, AZ91 alloy samples were subjected to both solid-solution treatment (400°C for 16 hours) and three different aging processes (216°C for 4, 8 and, 12 hours) under three different applied loads (10 N, 25 N and, 50 N). Microstructural, characteristic, hardness, friction coefficient, and wear rate behavior under various applied loads were investigated against each other.
 
 The results showed that the wear rate increased in all samples with the applied load, while the friction coefficient decreased. While the highest wear rate was observed only in the solid-solution treated sample, it was observed that the wear rate changed inversely with the aging time. Furthermore, the micro-hardness increased in direct proportion with the aging time. While it was observed that the sample, which was aged for 12 hours, wore 13.6% less under 10 N load than the sample that was only treated with solid-solution, the results showed that the same sample wore about 25% less under 50 N load. The increase in β-precipitates in the structure with the aging period increased the micro-hardness, while the increase in hardness decreased the wear.

Evaluation of AZ91-bioactive glass composites produced by the friction stir back extrusion technique

In the present study, the effects of bioactive glass (64SiO2-31CaO-5P2O5) reinforcement on the microstructure, mechanical properties, and in-vitro corrosion characteristics of AZ91-bioactive glass composite wire, produced via friction stir back extrusion (FSBE), were investigated. The findings reveal that the bioactive glass reinforcement phase, exhibiting a gradient distribution in the AZ91 matrix and the α-Mg and β-Mg17Al12 phases, predominantly appear in the extruded samples. Additionally, a higher rotational speed during FSBE reduces bioactive glass powder agglomeration within the AZ91 matrix. The optimal yield strength, ultimate tensile strength, and corrosion resistance were observed in the FSBE-processed composite containing 5 vol% bioactive glass. It was revealed that employing a rotational speed of 1200 rpm and an extrusion speed of 30 mm/min during FSBE on as-cast AZ91 alloy with 5 vol% bioactive glass led to an 86 % enhancement in corrosion resistance in simulated body fluid (SBF) compared to the as-cast alloy. Incorporating 5 vol% bioactive glass into the homogenized AZ91 alloy increased corrosion resistance by 98 %. Moreover, raising the rotational speed from 800 to 1200 rpm resulted in a 24 % increase in corrosion resistance, reaching a 0.0427 mm/year rate.

Friction extrusion of AZ91/bioactive glass gradient composite

The effect of the pre-friction extrusion microstructure of AZ91 billet on the microstructure, mechanical properties, and corrosion resistance of AZ91-bioactive glass surface composite rods was investigated. The results show that friction extrusion on AZ91 alloy billet with cast microstructure results in agglomerated bioactive glass particles in the composite rod microstructure. Compared to as-cast AZ91 alloy, friction extrusion on AZ91 alloy billet with cast microstructure results in 93% more corrosion resistance. Also, friction extrusion on AZ91 alloy billet with extruded microstructure leads to a gradient AZ91-bioactive glass composite rod with ultimate tensile strength, yield strength, and corrosion resistance of 9, 2, and 74% higher than AZ91 alloy rods, respectively.

Fabrication of new gradient AZ91-bioactive glass composite using friction stir back extrusion

This study investigated the microstructure, mechanical characteristics, and in vitro corrosion behavior of gradient AZ91-bioactive glass composite wire fabricated by friction stir back extrusion. The results showed that by decreasing the extrusion speed from 40 to 20 mm/min, the distribution factor of bioactive glass increased from 0.31 ± 0.04–0.45 ± 0.07, and more homogenous grain size was formed at the surface and center zone of the composite wire. Under the influence of heat and plastic deformation during friction stir back extrusion, despite the formation of the β-phase in separate islands, the α + β eutectic phase was also formed in the composite wire. In composite wire processed with a rotational speed of 800 rpm and extrusion speed of 20 mm/min, a gradient distribution of bioactive glass was formed in the cross section of the wire. Compared to an as-cast AZ91 alloy, friction stir back extrusion conducted at a rotational speed of 800 rpm and an extrusion speed of 20 mm/min resulted in a 44% increase in corrosion resistance in simulated body fluid (SBF).

Creep deformation study of squeeze cast AZ91 magnesium alloy

The AZ91 magnesium alloy has excellent mechanical properties; however, its application is limited by its poor creep resistance. In this study, AZ91 magnesium alloy prepared by squeeze casting technique has been tested for its creep performance at varying stresses and temperatures. The governing creep mechanism was examined in the wake of deformation. The microstructure for as-cast AZ91 alloy confirmed the massive dendritic β-Mg17Al12 phase. The creep study indicated that the governing creep mechanisms were dislocation creep and dislocation climb governed by lattice diffusion in the test regime of 50 MPa stress at 150oC to 175oC. The measured dislocation density values showed good agreement with the expected dislocation density in the power law creep mechanism.

Enhanced grain refinement and mechanical properties of AZ91 alloy by a novel Al-5.1 V-2.3B refiner containing VB2 and VB particles

Addition of specialized master alloys containing effective nucleation substrates has become the most common practice in the casting industries. In this study, a novel Al-5.1V-2.3B grain refiner containing VB2 and VB particles was designed. The growth mechanisms of VB2 and VB particles in grain refiner followed the principle of atomic adsorption and growing layer by layer based on the fundamental crystal growth theories. Grain refinement and mechanical properties of AZ91 alloy induced by the grain refiner and subsequent extrusion were investigated. The average grain size of the as-cast AZ91 alloy decreased from 217 μm to 78 μm with the optimal addition of 1 wt% Al-5.1V-2.3B grain refiner, and the UTS, YS and EL of the as-cast AZ91 alloy were improved by 27.6 %, 42.0 % and 43.0 %, respectively. Mechanical properties of the hot extruded samples were further enhanced due to the grain refinement, weakened texture and dislocation strengthening. The matching relationships between VB2, VB particles and α-Mg were characterized with the assistance of the high-resolution transmission electron microscope and edge-to-edge models. There were good matching relationships between VB, VB2 particles and α-Mg, as [001] / [1 2¯1 0] in (110) / (10 1¯0) and [0001] / [1 2¯1 0] in (10 1¯0) / (0002), which were the key for grain refinement.

Experimental investigations on corrosion performance of heat-treated AZ91 Mg alloy

Current examination stands rooted in corrosion rate estimation for AZ91 magnesium alloy. To evaluate corrosion rate, two different methods like weight loss analysis and electrochemical test were employed. It is important to acquire detail understanding of corrosion performance of magnesium alloy in aggressive biological environments. The bio-corrosion rates of AZ91 samples are estimated at different intervals (3, 15, 32, 48, 72, 86 hr) of exposure to the solution. From the experimental investigation on corrosion behaviour of AZ91 it is evident that the major issue associated with magnesium alloy is faster corrosion rate during initial phase of exposure. However, in the later phase of exposure corrosion rate remains relatively stable. The electrochemical corrosion analysis results showed further negative OCP develops for AZ91 alloy. It shows their lively performance perhaps oxidation of Mg2+ ion by varying stability potential further negative. Microstructure evaluation of the corrode surfaces is done through scanning electron microscope. SEM micrograph of corroded surface conform presence of oxides as a result of corrosion reaction in the solution. Immersion test and electrochemical corrosion study of the alloy was carried out at atmospheric conditions. EDS analysis of alloy sample depicts that throughout processing something like 79.87 % (wt.) of Mg is present in heat-treated AZ91 alloy. Additionally, throughout processing the average grain size of heat-treated AZ91 Mg alloy is mostly 90 µm as against 60 µm for as-cast AZ91 alloy.

Thermal cycles behavior and microstructure of AZ31/SiC composite prepared by stir casting

In the present work, the effect of thermal cycles on the physical and thermal properties of AZ31 alloy and AZ31/5wt%SiC and AZ31/10wt%SiC composites was investigated. Samples were prepared using the stir casting method and then subjected to precipitation hardening. Thermal cycles were done for as-cast and aged samples with V-shaped notch under 300, 600, and 900 heating and cooling cycles at 150 and 350 °C. The crack length (CL) was evaluated using optical microscope (OM), scanning electron microscope (SEM), and energy-dispersive scanning electron (EDS) analysis. Also, density, porosity, thermal expansion coefficient of the samples were evaluated. X-ray diffraction (XRD) analysis was employed to assess the phases present in the material. The results demonstrated that by increasing the number of thermal cycles up to 600 at 150 °C and 350 °C, the porosity and density of the as-cast and aged AZ31 alloy decreased and increased, respectively; however, the density and open porosity were remained constant for the composite samples. The crack's length enlarged with increasing the thermal cycles from 300 to 600 µm at 150 °C and 300 to 900 µm at 350 °C. It was found that the reinforcement and precipitates prevented the rapid growth of the crack in the magnesium matrix. All in All, composite and the aged samples demonstrated better thermal fatigue resistance compared with that of the unreinforced alloy and as-cast samples, respectively.

Dynamic recrystallization behavior of as-cast AZ91 magnesium alloy during hot compressive

This work systematically studies the dynamic recrystallization (DRX) behavior of as-cast AZ91 magnesium alloy under hot compression deformation at 350 °C. These results were also compared with the former results obtained at 300 °C in order to evaluate the effect of deformation temperature on DRX behavior of as-cast AZ91 alloy. The results show that the “{10–12} extension twin DRX” in original grains, “original grain boundary bulging (BLG) accompanied by progressive lattice misorientation” at the original grain boundaries and “particle-stimulated nucleation (PSN)” around massive eutectic β phases are observed during compression at 350 °C. The DRX mechanism of as-cast AZ91 alloy during hot compression at 350 °C is continuous DRX. Compared with the hot compression at 300 °C, the main twin type changes from 38° {10–11}–{10–12} double twin (300 °C) to 86° {10–12} extended twins (350 °C) at the beginning of hot compression (ε = 10%). And the 30° {10–11}–{10–12} double twin forms in the sample compressed at 350 °C with further compressive strain (ε = 20%). The DRX grain size and DRX volume fraction increase after hot compression at 350 °C compared with hot compression at 300 °C.

A study on the phase transformation of [formula omitted]-Al8Mn5 to LT-Al11Mn4 during solutionizing in AZ91 alloy

γ2-Al8Mn5 intermetallic particles are generally present in commercial Mg-Al-Zn alloys. The present study is concerned with the effect of solutionizing on the morphology and phase stability of the γ2-Al8Mn5 particles in an as-cast AZ91 alloy. Analysis using energy dispersive spectroscopy (EDS) and electron back scattered diffraction (EBSD) in a scanning electron microscope (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) suggests that solutionizing resulted in uphill diffusion of Al from the α-Mg matrix resulting in the phase transformation of γ2-Al8Mn5 to Low Temperature (LT) -Al11Mn4 phase. This leads to a shell type structure of LT-Al11Mn4 encapsulating γ2-Al8Mn5 and the formation of cracks in the LT-Al11Mn4 phase. These can propagate during external loading, affecting the mechanical properties. Additionally, the content of Fe and Si decreased in the LT-Al11Mn4, which could potentially decrease locally the corrosion resistance.

A novel Mg-5Al-2Zn-2Sn alloy with high strength-ductility synergy fabricated via simple hot rolling and annealing treatment

Abstract A novel Mg-5.34Al-2.23Zn-1.73Sn (wt%, AZT522 for short) alloy with high strength-ductility synergy was designed and fabricated via simple hot rolling and annealing treatment to compete with the commonly used Mg-9Al-1Zn-0.2Mn (wt%, AZ91) alloy. Different from the coarse microstructure of the as-cast AZ91 alloy, the as-cast AZT522 alloy has much finer microstructure, which consists of finer dendrites and smaller second phase particles. Hot rolling was carried out on the solution-treated alloys to effectively refine the microstructure and improve the mechanical properties. The results suggest that the AZT522 alloy shows better rolling formability. Then, the hot-rolled AZT522 alloy was annealed at the optimized parameter, i.e., 225 °C/1.5 h, leading to uniform fine grains with an average size of ~2.3 µm and numerous near-spherical precipitates (Mg17Al12 and Mg2Sn) with a mean diameter of ~90 nm. Benefiting from the fine annealing microstructure, the novel AZT522 alloy finally exhibited high tensile properties at room temperature, i.e., ultimate tensile strength (UTS) of ~343 MPa, yield strength (YS) ~271 MPa, and elongation to failure (δf) ~16%, which is a much better combination of strength and ductility than the annealed AZ91 alloy. This novel AZT522 alloy may provide a new insight to develop commercial high strength and ductility Mg alloys.

Synergistic effects of hybrid (SiC+TiC) nanoparticles and dynamic precipitates in the design of a high-strength magnesium matrix nanocomposite

Novel nanocomposites based on an AZ91 magnesium alloy matrix reinforced with a hybrid mixture of SiC and TiC nanoparticles with different nanoparticle contents (0.5% and 1%, by mass fraction) were fabricated by semi-solid stirring followed by ultrasonic vibration. Microstructural analysis indicated that the hybrid nanoparticles were uniformly distributed in the nanocomposite. The morphology of the Mg17Al12-based eutectic phase changed from plate-like, in the as-cast AZ91 alloy, to lamellar in the as-cast nanocomposites. The tensile strength and micro-hardness of the nanocomposites were improved by increasing the fraction of nanoparticles in the composite. Following extrusion, the matrix grains in the nanocomposite were significantly refined. The extent of the grain refinement increased at higher mass fractions of the nanoparticle. This refinement was not only due to dynamic recrystallization, but also the synergistic pinning effects arising from both the externally added nanoparticles as well as dynamically precipitated Mg17Al12 particles. The nanoparticle additions led to increases in yield strength (YS), ultimate tensile strength (UTS) and elongation to failure (EL). The increase in YS was mostly attributed to the effects of grain refinement, with additional contributions from the influence of Orowan strengthening and thermal expansion effects. The values of both the work hardening rate (θ) and strain hardening exponent (n) increased with increasing fraction of nanoparticles. The high θ value in stage III was attributed to both grain refinement and weakening of the basal plane texture, while the high n value was mainly related to the increase in resistance to dislocation movement caused by pinning effects of the nanoparticles.

2D and 3D characteristics of intermetallic particles and their role in fracture response of AZ91 magnesium alloy

The presence of intermetallic constituent particles strongly influences the deformation and fracture characteristics of as-cast magnesium alloys. The present study investigates the two-dimensional (2D) and three-dimensional (3D) microstructure of these intermetallic particles in a direct chill as-cast AZ91 alloy and their effect on the tensile deformation of the alloy. Electron backscattered diffraction (EBSD) was employed to characterize the non-equilibrium eutectic β-Mg17Al12 phase, which further assisted in hypothesizing the solidification process of the alloy. Transmission Kikuchi diffraction (TKD) indicated the small-sized spherical Mg17Al12 precipitates, formed adjacent to the non-equilibrium eutectic precipitate, did not exhibit any orientation relationship reported in the literature. Further, micron-sized AlMn inclusions were observed to be single phase γ2-Al8Mn5 particles exhibiting a cyclically twinned structure with 202¯1 habit plane. Additionally, optical microscopy (OM) in the dark-field (DF) and differential interference contrast (DIC) mode, in assistance with high resolution – transmission electron microscopy (HR-TEM) confirmed that the nano-sized γ2-Al8Mn5 particles resulted in a bimodal grain size distribution of the alloy. The three-dimensional (3D) spatial distribution of these intermetallic constituent particles along with fractography facilitated in understanding the damage process of the alloy in uniaxial tension.

Comparative study on the stress corrosion cracking susceptibility of AZ80 and AZ31 magnesium alloys

The stress corrosion cracking (SCC) susceptibility of an as-cast AZ80 magnesium alloy was evaluated using slow strain rate test (SSRT) in air and in 3.5% NaCl solution at various strain rates (10−5 s−1, 10−6 s−1, and 10−7 s−1). Furthermore, to elucidate the effect of aluminum content, the SCC susceptibility of an as-cast AZ31 alloy was also evaluated and compared with the as-cast AZ80 alloy. The SCC susceptibility was quantified using susceptibility indices corresponding to ultimate tensile strength (UTS), strain to failure and time to failure. The SCC susceptibility was found to increase with a decrease in strain rate for both the alloys. The higher SCC susceptibility of the as-cast AZ80 alloy as compared to the as-cast AZ31 alloy was attributed to the higher volume fraction of Mg17Al12 particles, and this was corroborated with the fractographic analysis using a combination of scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS).

Effects of minor additions of cerium, silicon and calcium on microstructure and mechanical properties of AZ91 magnesium alloy

Abstract The effects of minor Ce, Si and Ca additions on the microstructure and mechanical properties of the as-cast and T6 state AZ91 alloy were analyzed and compared in this paper by means of optical microscopy, scanning electronic microscopy, X-ray diffraction and tensile testing. Results showed that the addition of Ce and Ca could effectively modify the β-phase and the average grain size was refined from 239.7 ± 16.9 μm in AZ91 alloy to 57.8 ± 1.4 μm in AZ91-1.0Ce-0.5Ca alloy. Accordingly, the as-cast AZ91-1.0Ce-0.5Ca alloy showed optimum yield strength of 119 MPa and elongation of 10.7 %. Furthermore, AZ91-1.0Ce-0.5Ca alloy possesses a higher effect in the reduction of the discontinuous precipitates after aging and the yield strength increased to 164 MPa with elongation of 11.8 %.

Influence of trace As content on the microstructure and corrosion behavior of the AZ91 alloy in different metallurgical conditions

Abstract The influence of trace As content (0.04, 0.05, 0.06 wt%) on the microstructure and corrosion behavior of AZ91 alloy in different metallurgical conditions (as-cast, as-quenched, peak-aged) was firstly investigated. It is found that the corrosion resistance of AZ91 alloy in all conditions is significantly enhanced by As addition, mainly due to cathodic toxication, melt purification and incidence reduction of local corrosion. As alloying leads to the formation of Mg3As2 and makes the β-Mg17Al12 phase less continuous in the as-cast alloys. The alloy containing 0.06 wt% As obtains a higher corrosion rate than those of the alloy containing 0.05 wt% As owing to a more discontinuity of β phase. The β phase is dissolved into the matrix in the as-quenched alloys and reprecipitates along the grain boundaries after aging. The more continuous β phase distribution in the peak-aged alloys contributes to corrosion resistance. The corrosion rates are in the order of as-quenched>as-cast>peak-aged. The lowest corrosion rate (0.67 mm/y) in 3.5 wt% NaCl solution is obtained in the peak-aged AZ91-0.05As alloy, which is over 70% lower than that of as-cast AZ91 alloy (2.22 mm/y).

Grain Refinement of AZ91 Magnesium Alloy Induced by Al-V-B Master Alloy

It has long been recognized that grain refinement of Mg-Al alloys is difficult, although various methods have been tried. In the present paper, a novel grain refiner, Al-3.4V-1B master alloy, has been developed to refine the as-cast AZ91 alloy. A comparative study on grain refinement effects of Al-3.4V-1B, Al-5V, and Al-3Ti-1B master alloys was performed under the same solidification conditions. It is shown that Al-3.4V-1B master alloy not only has significant grain refinement ability, but also keeps stable anti-fading capacity with holding time up to 2 h. Based on the analysis of grain refinement, VB2 particles introduced by Al-3.4V-1B master alloy are the heterogeneous nuclei for AZ91 alloy.

Enhancement of Mechanical Properties and Rolling Formability in AZ91 Alloy by RD-ECAP Processing.

In this study, the influence of rotary-die equal channel angular pressing (RD-ECAP) processing on the mechanical properties and rolling formability of AZ91 alloys was investigated. The as-cast and pre-homogenized AZ91 alloys were pre-processed by RD-ECAP for 16 passes at 573 K and subjected to post-ECAP rolling at 573 K with a rolling speed of 10 m/min. The microstructure and deformation characteristics of the AZ91 alloys were characterized. Results demonstrated that fine-grained AZ91 alloys with improved strength and ductility were obtained via the high-pass RD-ECAP processing, indicating a good plastic formability. The ECAP-ed alloys were easily rolled at 573 K from 4.5 mm to 1.1 mm in thickness without edge cracking. After rolling, heterogeneous grain structures were observed with large numbers of twins and shear bands that created strong basal textures. The rolled AZ91 alloys exhibited higher tensile strength and appropriate elongation. The post-ECAP rolling was successfully used in the high productivity of AZ91 rolled plates with good mechanical properties.

Comparison of the short and long-term degradation behaviors of as-cast pure Mg, AZ91 and WE43 alloys

The corrosion behaviors of pure magnesium, AZ91, and WE43 alloys have been evaluated by weight loss, hydrogen evolution rate, pH change measurements and potentiodynamic polarization as well as electrochemical impedance spectroscopy (EIS) methods. Main corrosion product formed on the surface of Mg/Mg-alloys after immersion of 24 h was Mg(OH)2 on the other hand, at the end of the 20 days additional CaCO3 which was found to display a critical role in degradation characteristics of the samples, was found. Examination in the cross section of the polished surfaces revealed that protective layers became thicker and corrosion rate of the samples decreased possibly due to increased protective abilities of the surfaces. Intermetallics in AZ91 and WE43 alloys acted as cathodic centers and induced micro galvanic corrosion. Undermining of intermetallics in WE43 alloy intensified the corrosion rate. AZ91 alloy exhibited the lowest corrosion rate among the samples when tested in simulated body fluid (SBF).