As-cast AZ91 Research Articles

Enhanced corrosion resistance of AZ31 magnesium alloys through the use of high-purity raw magnesium

Poor corrosion resistance is a critical barrier to the widespread application of magnesium alloys. Statistically, the literature reported that approximately 70 % of as-cast AZ31 magnesium alloys exhibit corrosion rates exceeding 1 mm ·y−1 in 3.5 wt.% NaCl solution, which is unacceptable for industrial use. Furthermore, there is a considerable discrepancy in the corrosion rates reported by different studies (as-cast alloys ranging from 0.4 to 215 mm ·y−1). These phenomena may be attributed to the uncontrollable content of impurity elements in commercial magnesium alloys, which fluctuate widely between batches. In the present work, we prepared as-cast AZ31 magnesium alloys with different impurity contents using two different purities of raw magnesium (Mg-99.9% and Mg-99.99%). The impact of impurity contents on the corrosion resistance of AZ31 magnesium alloys was then analyzed. The AZ31 magnesium alloy prepared with 99.99% raw magnesium showed superior corrosion resistance compared with that prepared with 99.9% raw magnesium, with a reduction in corrosion rate by approximately 98 % and a decrease in the fluctuation range of corrosion rate by 91 %. Thus, enhancing the purity of raw magnesium is an effective method to improve both the corrosion resistance and consistency of magnesium alloys.

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.

Effect of Co-addition of Sn, Traces of RE and Ca on the Mechanical and Micro Structural Properties of As-cast AZ80 Alloy

In this study, the effect of co-addition of tin (0, 6 and 9 wt.%), cerium (0, 0.3 and 0.6 wt.%), gadolinium (0, 0.6 and 0.9 wt.%) and calcium (0, 0.6 and 0.9 wt.%) on the mechanical and microstructural properties of AZ80 magnesium alloy is investigated. The results indicate that the mechanical properties are improved for all the as-cast alloys when compared to base alloy except for one alloy which is due to the presence of Mg-Sn-Ca phase in high volume fraction. Also, it is found that the existence of Al2Ca, Mg2Ca, Al2RE, MgSnRE and Mg2Sn phases have a significant impact on the mechanical and microstructural characteristics of the as-cast AZ80 alloys. It is observed that without the addition of Sn content, the best room temperature tensile properties are obtained for the alloy containing 1.5 wt.% of RE and 0.9 wt.% of Ca. With the addition of Sn, it is found that the best room temperature tensile properties are obtained for the alloy containing 6 wt.% of Sn, 0.9 wt.% of RE and 0.5 wt.% of Ca.

In-situ EBSD study on twinning activity caused by deep cryogenic treatment (DCT) for an as-cast AZ31 Mg alloy

This work studied the activation of the twins in a commercial cast AZ31 Mg alloy by deep cryogenic treatment (DCT) using in-situ EBSD. The initial as-cast microstructure has a range of grain sizes. The DCT mainly activated twins in the areas of fine grain size, in which the twin area fraction increased with increasing DCT time. These twins were {10–12} extension twins, which were evoked and facilitated by the large tensile interior stress that was caused by the large temperature difference during the DCT. Furthermore, only {10–12} extension twins were activated. The activated {10–12} extension twins had the highest Schmid factor and conformed to Schmidt's law. One or two other kinds of tensile twinning variants of the six tensile twinning variants (V1–V6) were activated by the DCT. These activated variants had the highest Schmid factor and the lowest orientation difference (less than 10o), while those non-activated variants had an orientation difference of ∼60o. Theoretical evaluation indicated that these two kinds of tensile twinning variants were the para-position variant rather than the ortho-position variant or the meta-position variant.

Investigating the impact of heat treatment on the tribological behaviour of AZ80 magnesium alloy at high temperatures

The purpose of this research is to investigate the effect of heat treatment on the high-temperature wear characteristics of stir-cast AZ80 magnesium alloy. Dry sliding wear tests were performed on as-cast, T4, and T6 AZ80 using ASTM G99 - 17 at 25 °C, 150 °C, and 250 °C with a constant normal load of 10 N, a sliding speed of 1 m/s, and a sliding distance of 1000 m. Microstructures were analyzed with a Field-Emission Scanning Electron Microscope (FESEM) and an Electron Microprobe Analyzer (EMPA). X-ray diffraction (XRD) was used to examine the phases before and after the wear tests. In addition to α-Mg, the intermetallic phases Mg17Al12 and Mg2Al3 are also identified in AZ80. Compared to as-cast AZ80, T6 heat-treated alloy shows a maximum grain size of 72.64 ± 25 μm, with grain coarsening of 38.83 %. All test specimens, including T6 AZ80, exhibit indication of the Al-rich zone in the EPMA analysis, while T6 AZ80 shows evidence of the Mn-rich zone. Except for T4 AZ80 at 250 °C and T6 AZ80 at 150 °C and 250 °C, where the alloy experiences adhesive wear and oxidation, the abrasive wear and oxidation are common mechanisms observed in all test settings. At 150 °C, adhesive wear is responsible for 9.5 mg of the total wear in T6 AZ80. The alloy's wear behaviour is unaffected by presence of Zn. Intermetallic oxides (MgAl2O4) play a vital role in deciding the wear characteristics of the alloy.

Exploratory analysis and evolutionary computing coupled machine learning algorithms for modelling the wear characteristics of AZ31 alloy

The wear resistance of magnesium alloys is one of its key technological properties that could limit their practical application. In accordance with ASTM G99–95a standard, this study used a pin-on-disc method to analyze the wear behavior of ascast AZ31 magnesium alloy under dry-sliding conditions. With a track radius of 37.5 mm, varied sliding velocities of 0.25, 0.5, 1, and 1.5 m/s, and normal loads of 40, 60, and 90 N were employed to quantify wear rate over a fixed sliding distance of 600 m. The surface morphology of the alloy's corroded surface was investigated using a SEM/EDS. The effectiveness of two Evolutionary Computing integrated machine learning algorithms, Particle Swarm Optimization coupled Decision Tree (PSO-DT) and Particle Swarm Optimization coupled Gradient Boosting Regressor (PSO-GBR), is also compared in this study in predicting the particular wear rate of AZ31 magnesium alloy. The experimental observations of wear behavior at various sliding velocities and normal loads make up the dataset used in this study. The algorithms' prediction performance was assessed using the coefficient of determination (R2), mean squared error (MSE), and mean absolute error (MAE). The results show that when it comes to foretelling the precise wear rate of AZ31 magnesium alloy, the PSO-GBR algorithm works better than the PSO-DT algorithm resulting in the R2 value of 0.99970. The PSO-GBR algorithm's successful integration of Particle Swarm Optimization and the gradient-boosting regressor model is responsible for this higher performance. The PSO-GBR algorithm improved accuracy and better captured nonlinear patterns in the data by improving the algorithm's parameters and capturing complex wear mechanisms.

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.

Enhanced corrosion resistance of the as-cast AZ91 magnesium alloy by micro-addition of the strontium and rare earth elements

The synergetic effect of strontium (Sr) and rare earth elements (RE) on the corrosion properties of as-cast Mg–9Al–1Zn alloy were studied. Via the addition of 0.5 wt-% RE and 0.5 wt-% Sr (RE/Sr ratio of 1), a remarkable reduction in the corrosion rate of AZ91 was observed. The reason was found to be related to the decrement of the micro-galvanic corrosion by the modification of the β-Mg17Al12 phase (changing its morphology from semi-continuous to a more spheroid shape) and formation of proper amounts of Al11RE3 and Al4Sr intermetallics. However, the excess RE/Sr ratio impaired the corrosion resistance, which can be related to the micro-galvanic effect by excess amounts of Al11RE3 and Al4Sr intermetallics.

Corrosion behavior of as-cast AZ91 magnesium alloy with VN particle additions in NaCl solution

The microstructures and corrosion behavior of the as-cast AZ91−xVN (x=0, 0.25, 0.5 and 1 in wt.%) alloys were investigated using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. The results showed that VN can significantly refine and modify the microstructures of AZ91−xVN alloys. The corrosion resistance of the AZ91 alloy was first improved and then deteriorated with the addition of VN particles. The superior corrosion resistance of the as-cast AZ91−0.25VN alloy with the lowest corrosion rate (PW=(1.47±0.06) mm/a) was attributed to the additional grain boundaries. With the further increase of VN content, the amount of the lamellar β-Mg17Al12 precipitation phase increased, and thus the galvanic corrosion between β phase and the α-Mg matrix was promoted, resulting in the deteriorated corrosion resistance of the alloys.

Investigation of microstructure and corrosion behavior of AZ91/64SiO2-31CaO-5P2O5 composite wire fabricated by friction stir back extrusion

In this study, the AZ91-bioactive glass surface composite wires were created using a unique method called friction stir back extrusion (FSBE). The microstructure, mechanical properties, and corrosion resistance of FSBE-processed wires were studied. The results show that by applying the FSBE process, the amount of β-Mg17Al12 phase decreased compared to the as-cast AZ91 rod. Also, by increasing rotational speed during FSBE, the distribution of bioactive glass powder in the AZ91 matrix become more homogenous, and gradient AZ91 matrix composite is fabricated at a rotational speed of 1200 rev/min. By increasing temperature and plastic strain during FSBE at high rotational speed, the higher amount of discontinuous β-Mg17Al12 phase and α + β eutectic formed in the grain boundaries. By inserting bioactive glass powder using a hole drilling method and applying the FSBE process, a gradient AZ91-bioactive glass composite wire with ultimate tensile strength, yield strength, elongation, and corrosion resistance of 58, 64, 62, and 34 % higher than AZ91 as-cat rod can be achieved. Also, the corrosion resistance of gradient AZ91 matrix composite at simulated body fluid (SBF) increased 48 % compared to as-cast AZ91 base metal.

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.

Tensile and wear properties of as-cast and extruded AZ91-xCNT and AZ91-1B4C-1SiC-xCNT composites

Tensile and wear properties of as-cast and extruded AZ91-xCNTs and AZ91-1%B4C-1%SiC-xCNT (x:0.25,0.5,1wt.%) composites were studied. Compared to as-cast AZ91 matrix alloy, hot extruded AZ91-0.25%CNT showed a significant enhancement in UTS (385 MPa) and elongation (16%), with no substantial improvement in wear properties however, the AZ91-xCNT composites containing micron-sized 1%B4C and 1%SiC particles met the lower tensile properties. Interesting wear behaviour was obtained after adding 1wt.%B4C and 1wt.%SiC to AZ91-xCNT where improvement in wear results was considerable. Development in wear properties of the AZ91-1%B4C-1%SiC-xCNT composites can be attributed to the enhanced hardness and reduced coefficient of friction. Wear analyses of two composites exhibited various wear mechanisms, but the abrasion mechanism dominated after CNT addition and hot extrusion at 20 N load.

Microstructure and thermal stability of nanocrystalline AZ31 Mg alloys reinforced by ultrafine vanadium particles

In this paper, the microstructure evolution and thermal stability of nanocrystalline (NC) AZ31-2.5 wt%VP magnesium matrix composites were studied, while the hardness was analyzed. The results showed that the NC AZ31-2.5 wt%VP still kept NC scale after annealing at 300 °C and 350 °C. Even at 400 °C and 450 °C, the grain size of the annealed composite only grew to 113 nm and 132 nm, which was still in ultrafine grain scale, illustrating excellent thermal stability of NC AZ31-2.5 wt%VP. Furthermore, the V particles, which was uniformly distributed in the Mg matrix, had no obvious change in dimension after annealing, while it also had no new phase formed. The stabilization mechanism was expounded and found that the pinning effect of V particles effectively inhibited the grain growth of Mg matrix at high temperature. In addition, the grain growth kinetics equation (D6−D06=kt) was utilized to explain the grain growth behavior of NC AZ31-2.5 wt%VP, and the activation energy (Ea) of grain growth was calculated to be 130.9 kJ/mol, which was much higher than that of pure Mg (92 kJ/mol). Eventually, the hardness of NC AZ31-2.5 wt%VP was contrastive studied and the hardness evolution curves at different temperatures were obtained. After annealing treatment was performed at 450 °C for 180min, the hardness remained at 80 HV, which was higher than that of as-cast AZ31 (53 HV).

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.

Strain Hardening Exponent and Strain Rate Sensitivity Exponent of Cast AZ31B Magnesium Alloy

The flow curves of as-cast AZ31B magnesium alloy during high temperature deformation were obtained with a thermal compression test, and the effects of deformation amount, grain size, strain rate, and deformation temperature on the flow stress, strain rate sensitivity index, and strain hardening index were analyzed. The results showed that deformation and grain size were negatively correlated with both the strain rate sensitivity index and strain hardening index. The increase in strain rate increased the strain hardening index but made the strain rate sensitivity index show an opposite trend. Increasing temperature reduced the strain rate sensitivity index and strain hardening index but, when the temperature exceeded 700 K, the strain rate sensitivity index was no longer affected by temperature. Since the strain rate sensitivity index m and strain hardening index n are important parameters for measuring the plastic deformation of metal materials, this study has great significance for guiding the selection of process parameters in the plastic processing of as-cast AZ31 magnesium alloy.

Investigating the Synergic Effects of WS2 and ECAP on Degradation Behavior of AZ91 Magnesium Alloy

In this research, WS2/AZ91 metal matrix composites were manufactured using the stir casting method. The composites were severely deformed using equal channel angular pressing (ECAP). The degradation behavior of severely deformed and as-cast samples was investigated using the three-electrode system in a 3.5 wt% NaCl solution. The corrosion products and surface morphology of the corroded surfaces were investigated using an x-ray diffractometer and scanning electron microscopy. The results revealed that as-cast monolithic AZ91 exhibited the highest corrosion potential of (−1.553 mV) and a minimum degradation rate (4.099 m·a−1). The reduction ion grain size after severe plastic deformation increased the degradability of WS2/AZ91 composites. Severe plastic deformation reduced the grain size, which led to an increase in the corrosion rate. The synergic effects of (tungsten disulfide) WS2 and ECAP increased the degradation rate to (4.59 m·a−1) in two-pass 1 wt% WS2/AZ91. The increase in WS2 contents decreased the degradation rate (4.512 m·a−1) in homogenized 1 wt% WS2/AZ91 composites. The degradability of AZ91 increased in all conditions under the synergic effects of WS2 and severe plastic deformation. Magnesium and magnesium hydroxide were observed as corrosion products. The maximum surface roughness in two-pass 1 wt% WS2/AZ91 indicated the highest material removed by corrosion from the surface of the composites.