AZ31 Alloy Research Articles

Stress Evaluation Method by Neutron Diffraction for HCP-Structured Magnesium Alloy

Tensile deformation in situ neutron diffraction of an extruded AZ31 alloy was performed to validate conventional procedures and to develop new procedures for stress evaluation from lattice strains by diffraction measurements of HCP-structured magnesium alloys. Increases in the lattice strains with respect to the applied true stress after yielding largely vary among [hk.l] grains. Some [hk.l] grains have little or no increase in lattice strain, making it difficult to use the conventional procedures to determine the average phase strain by using lattice constants or by averaging several lattice strains. The newly proposed procedure of stress evaluation from the lattice strains shows very high accuracy and reliability by weighting the volume fraction of [hk.l] grains and evaluating them in many [hk.l] orientations in addition to multiplication by the diffraction elastic constant. When multiple hk.l peaks cannot be obtained simultaneously, we recommend to use the 12.1 peak for stress evaluation. The lattice strain value evaluated from the 12.1 peak shows a good linear relationship with the applied true stress for the whole deformation region.

Influence of Gd and Li additions on the microstructural evolution and mechanical properties of hot-rolled AZ31 alloy

Thermomechanical processing and alloying are two common approaches to enhance the mechanical properties of Mg. Thus, the effects of Gd and Li elements, as two elements with various properties, on the microstructure and mechanical properties of a hot-rolled and consequently annealed AZ31 alloy were analyzed. The microstructure and texture was analyzed by scanning electron microscopy and electron back scattered diffraction. Accordingly, the mean grain size of 46 μm in the AZ31 alloy was reduced to 41 and 26 μm in the AZ31–2.9Gd and AZ31–2.3Li alloys, respectively. Grain refinement occurred due to continuous dynamic recrystallization and twinning-induced recrystallization in all of the alloys. Textural evolution showed that neither Gd nor Li could inhibit the occurrence of basal texture. Room temperature mechanical properties were assessed by using the shear punch test (SPT) method. The results exhibited that the strength of the AZ31 alloy was improved from 116 MPa to 139 MPa and 141 MPa by respective additions of Gd and Li. Strength enhancement was mainly attributed to the grain boundary strengthening and second-phase particles hardening. One promising observation was the simultaneous enhancement of shear strength and ductility by the addition of Gd and Li. This observation was ascribed to the more uniform distribution of second-phase particles and lower volume fraction of twins as potential sites for crack initiation. As long as mechanical strength is concerned, results favor the use of Li as alloying element in Mg, especially due to the more noticeable lightweighting potential compared to Gd.

Fabrication of Mg/Ti composite with excellent strength and ductility by hot rolling

In this study, commercial pure Ti is employed to strengthen AZ31 alloy by fabricating Mg/Ti laminated composite via hot rolling bonding. Strong interfacial bonding between the AZ31 and Ti can be realized by hot rolling with a reduction of 50 % and above. In addition, with the same thermo-mechanical treatment conditions, the peeling strength between the AZ31 layer and the Ti layer in the Mg/Ti laminated composite is higher than that between the AZ31 and AZ31 in the Mg/Mg sheet. Compared with the individual AZ31, the Ti/Mg/Ti composite presents better mechanical properties including strength and ductility. Extra strength above the calculated yield strength based on the rule of mixture exists in the Ti/Mg/Ti composite, and the uniform elongations of the Ti/Mg/Ti composites are higher than those of all component layers. Digital image correlation technology and slip trace method were employed to reveal the deformation behavior of Ti/Mg/Ti composite. Gradient strain near the interface is formed and more non-basal slips are observed in the AZ31 layer during the tension. The strengthening and toughening mechanisms in the Ti/Mg/Ti composite are discussed. This study can provide a reference for enhancing the mechanical properties of magnesium alloys by fabricating Mg based laminated composite.

Corrosion Behaviour of High-Pressure Die-Cast AZ91 Alloy in NaCl Solution: Effects of Friction Stir Process at High Rotational Speed.

The AZ series of Mg alloys have become promising in several industrial fields thanks to its potential microstructure refinement and the β-Mg17Al12 eutectic that controls the mechanical behaviour. Simultaneously, the rapid degradation characterizing Mg alloys makes the investigation of their corrosion behaviour necessary. The present work considers high-pressure die cast (HPDC) AZ91 alloy to evaluate its corrosion behaviour in 1M NaCl solution and investigates how different friction stir process parameters can affect the corrosion responses. No studies analyse the effects induced by the friction stir processed zone, reached using high rotational speeds (>2000 rpm), on the unprocessed HPDC AZ91 alloy. In addition, the morphological analysis of the corroded surfaces having a friction stir processed zone, in which the grain refinement was not obtained, is not present in the literature yet. Microstructural features were investigated by optical microscope and X-ray diffraction analysis before and after the friction stir process. These were subsequently correlated to the corrosion responses after the immersion tests. The results show that HPDC samples with a very smooth surface have the best corrosion resistance with a corrosion rate lower than 3 mm/year, evaluated through the weight loss, compared to the rougher ones. Both the amount of β-Mg17Al12 eutectic and the wt.% Al in the α-Mg matrix, as well as the surface roughness, influence the corrosion behaviour of friction stir processed samples. The best corrosion resistance was obtained with an HPDC alloy processed at 2500 rpm and 50 mm/min.

Combined PEO and Spray Pyrolysis Coatings of Phosphate and ZnO for Enhancing Corrosion Resistance in AZ31 Mg Alloy

Oxide films produced from plasma electrolytic oxidation are porous in structure. While they have some passivating effect in Mg alloys, the pores still lead to corrosion over long periods of exposure. In this study, spray pyrolysis was used to seal the porous oxide layer developed through the plasma electrolytic oxidation method on Mg alloy AZ31. The PEO coating acted as a good base for the application of spray pyrolysis due to its morphology. Three different kinds of coatings were obtained using different precursors: zinc acetate for ZnO, phosphoric acid for phosphate (P), and a mixture of zinc acetate and sodium phosphate for ZnO+P. The corrosion performance of all three coatings was studied by performing electrochemical impedance and polarization tests on the samples. Mass loss over a duration of 1 week was measured in 3% NaCl solution using immersion gravimetry. The coating with only phosphate (P) was found to be most corrosion-resistant with 52 times lower rate of corrosion and 50 times more polarization potential. The chemical composition of the corrosion products was studied using XRD and SEM-EDS analysis. Mass loss in ZnO+P was the highest, at up to 1.4 and 5.1 times higher than ZnO and P, respectively.

Multisine impedimetric monitoring with an in-depth distribution of relaxation times analysis of WE43 and AZ31 magnesium alloys corrosion

This research aims to develop and utilize an impedance-based tool for monitoring non-stationary electrochemical processes, coupling the multisinusoidal perturbation signal approach and distribution of relaxation times (DRT) analysis for the first time. The approach was used to distinguish independent processes occurring at the surface of AZ31 and WE43 Mg alloys undergoing corrosion in Hank’s Balanced Salt Solution at 37 °C. We highlighted two common processes related to corrosion product layer formation. Detailed DEIS-DRT analysis was capable of real-time identification of a unique third process for the WE43 alloy, resulting from a spatially localized filiform corrosion attack in the vicinity of intermetallic particles. The proposed tool proved to be highly efficient in terms of studying non-stationary processes, while the DRT analysis allowed for an in-depth and precise localization of the number and the kinetics of the ongoing processes. The corrosion mechanism description was supported by numerous microscopic and spectroscopic tools.

A polyphenol‐induced hydroxyapatite coating modulates corrosion resistance and biocompatibility of magnesium alloys

AbstractIn order to solve the problem of excessive degradation rate and insufficient biocompatibility of magnesium‐based bone implants, a polyphenol (EGCG) induced hydroxyapatite (HA) coating was prepared on the surface of AZ31 alloy. The physical and chemical properties and corrosion resistance of the coating were analysed in depth, and its biocompatibility was preliminarily explored in vitro. The results showed that the polyphenol (EGCG) conversion coating constructed on the AZ31 could successfully induce the formation of HA by complexing the phenolic hydroxyl group with calcium ions. The electrochemical and long‐term immersion experiments showed that the corrosion resistance of EGCG/HA composite coating was significantly improved. The self‐corrosion current density, hydrogen evolution and the increase of pH value of AZ31‐EGCG/HA were significantly lower than those of AZ31. On the basis of inhibiting the excessive corrosion of the substrate, the composite coating significantly improves the compatibility of pre‐osteoblasts, supports the adhesion and spreading and effectively reduces the haemolysis rate to less than 5%. The preparation method of the coating is simple, low cost and suitable for complex shape surfaces, which can significantly improve the corrosion resistance and biocompatibility of the AZ31 substrate. It is expected to provide a solution for the surface modification of magnesium‐based bone implants.

Fatigue and Fracture Behaviors of Short Carbon Fiber Reinforced Squeeze Cast AZ91 at 20 °C and 250 °C

AZ91 is one of the most broadly used Mg alloys because of its good castability and reasonable mechanical properties. Strengthening AZ91 with carbon short fibers aims to increase tensile and fatigue strength, creep, and wear resistance. One of the proposed applications of reinforced AZ91 is the production of pistons for trucks. Such reciprocating parts are subjected to alternating fatigue loads which can lead to fatigue failure. In this respect, studying the tensile and fatigue behavior of materials subjected to such loading conditions is of great interest. The alternating low-cycle fatigue (LCF) and high-cycle fatigue (HCF) of unreinforced AZ91 and carbon fiber-reinforced AZ91 (AZ91-C) were investigated at 20 °C and 250 °C. Tensile tests were carried out at the same testing temperature to find the appropriate fatigue testing stress and strain for stress-controlled and strain-controlled tests, respectively. The fatigue curves of stress against the number of cycles (S–N) revealed that the composite AZ91-C’s fatigue strength was 55 MPa under HCF, while that of the matrix alloy AZ91 was only 37 MPa at 250 °C. Fracture investigations were conducted on the broken test samples. The fracture approach in the matrix material (AZ91) is mixed ductile/brittle containing fatigue serration, fiber fracture, and separation in the reinforced material (AZ91-C).

Addition of Coconut Shell Ash Particle with AZ31 Magnesium Alloy - Assessment of Tribological and Mechanical Behaviour

Desires of materials with lesser weight made wide spread use of alloys based on magnesium in automobiles and aerospace applications in the view of overcoming lower strength of these materials when compared to aluminium, these materials are added with some hard materials like ceramics, metal oxides or natural remaining as a reinforcing phase with a proof for improvement in properties. In the present work AZ31 alloy is strengthened by adding Coconut Shell Ash particles (CSAp) by means of powder metallurgy and the examinations like microstructure, micro hardness, compressive strength and wear are conducted. The ash particles content as reinforcement has not imparted larger variation in the strength and hardness, the wear resistance is increased with the addition of ash particles and the addition of CSAp exceeding 3% leads to crack.

Influence of Cathodic Polarization on Plasma Electrolytic Oxidation of Magnesium and AZ31 and AZ91 Magnesium Alloys

In this study, the influence of cathodic polarization on the plasma electrolytic oxidation (PEO) behaviors of pure magnesium and AZ31 and AZ91 magnesium alloys with varied Al alloying contents was systematically examined in a dilute alkaline silicate electrolyte by adjusting the cathodic-to-anodic current density ratio (R = jc/ja) from 0 to 3.2. The results show that moderate cathodic polarization (R = 0.6) led to the thickest coatings on the Mg and Mg alloys, and the coatings grew in an outward-and-inward mode compared with the inward growth at R = 0. Excessive cathodic polarization (high R ratios) differently influenced the PEO behaviors of the magnesium alloys. For the pure magnesium and AZ31 alloy, the coatings blistered or peeled off when the R ≥ 0.9. However, the tolerance to cathodic polarization was significantly improved for the AZ91 Mg alloy. The coatings were undamaged even with the highest R ratio of 3.2, and their compactness was further improved as the R ratio increased to 0.9 and 1.2. An increase in cathodic polarization led to a reduction in the anodic potential and spark softening but did not result in an improvement in the coating quality. Optical emission spectroscopy identified two spectral lines at 559.79 and 570.11 nm, which are assigned to the Mg species but not found in databases or the literature. The corrosion and wear resistance of the PEO coatings were also investigated. The coating formed on the AZ91 magnesium alloy at R = 1.2 displayed the narrowest wear track due to its high compactness.

Effect of Ti particles on microstructure and mechanical properties of Mg–9Al–1Zn based composite sheets

It is a great challenge to simultaneously improve the strength and ductility of magnesium matrix composites (MMCs). In this work, AZ91 alloys reinforced with 1 wt%, 2 wt%, and 3 wt% Ti particles were produced using stir casting combined with ball milling process. The microstructure of the extruded composite sheets was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), electron backscatter diffraction (EBSD). Transmission electron microscopy (TEM) was utilized to observe the structure of the Ti/Mg interface. The mechanical properties of the composite sheets were also investigated. The results show that the adding Ti particles, the texture strength decreased, and the growth of recrystallization grains was also restrained. Due to the interfacial reaction between Ti and Al, the formation of β-phase (Mg17Al12) was inhibited. The tensile test demonstrated that the Ti particles could significantly improve the mechanical properties of the AZ91 alloy sheets. Compared to AZ91 sheet, 2Ti/AZ91 composite sheet along extrusion direction (ED) showed the best comprehensive mechanical properties. The improvement in mechanical properties was mainly attributed to grain refinement, texture weakening, mismatched coefficient of thermal expansion (CTE), and work hardening.

Superior strength–toughness synergy of heterogeneous TiB2p/AZ91 composites containing hard/soft zones via tailoring bimodal grain structure

Magnesium matrix composites show great prospects due to lightweight and high specific strength, but strength-toughness tradeoff has limited their engineering applications. Here, a novel strategy for strengthening and toughening TiB2p/AZ91 (TiB2p: TiB2 particles) Mg matrix composites via tailoring heterogeneous configuration containing hard/soft zones and bimodal grain structure is proposed. The results show that hot extrusion deformation can achieve heterogeneous structure of hard and soft zones resulting from alternating arrangement TiB2p, which induces the formation of coarse and fine grain zones. With the decrease of extrusion temperature, both the hard zone spacing and the number of coarse grains have a great decrease. In particular, TiB2p/AZ91 composites extruded at 400 °C and 350 °C exhibit obvious bimodal grain structure, while almost uniform fine grains are obtained in the composites extruded at 300 °C. This contributes to the excellent comprehensive mechanical properties of TiB2p/AZ91 composites at 400 °C and 350 °C, with UTS of 345 MPa, 388 MPa, and elongation of 12.1%, 10.1%, respectively. Compared with AZ91 alloy, the strength of composites extruded at 400 °C and 350 °C is significantly improved, while the ductility is not sacrificed, indicating that superior strength–toughness synergy is achieved. The improved toughness is ascribed to the increased strain hardening capacity, suppressing and deflecting effects of the hard zones on the crack propagation. Lower extrusion (300 °C), however, leads to the reduced toughness of the composites, which is due to the premature initiation of cracks caused by smaller hard zone spacing and large amount of precipitated phase.

Influence of Ca and Ce Additions on Microstructure and Microhardness of Squeeze-Cast AZ91 Mg Alloy

In this study, AZ91 alloy was used as the base material and calcium and cerium were added as alloying elements. Microstructural analysis through optical microscope (OM) and field emission scanning electron microscope (FESEM) revealed that AZ91 base alloy contains α-Mg matrix and β-Mg17Al12 interdendritic network. The inclusion of individual calcium and cerium resulted in a more homogeneous distribution of the interdendritic network in the AZ91-1wt.% Ca and AZ91-1wt.% Ce alloy. The secondary phase (Mg17Al12) was refined in the microstructure as a result of Ca and Ce addition where Ce addition forms a new rod-like phase that is recognized as Al11Ce3 and Ca addition forms a skeleton like structure of Mg17Al12 and Al2Ca. Due to the formation of new Al2Ca and Al11Ce3 intermetallics, the volume fraction of β-Mg17Al12 was more suppressed with Ca and Ce alloy additions. The grain size determined from Electron Backscatter Diffraction (EBSD) maps indicate the reduction in average grain size with individual Ca and Ce additions. The addition of these elements was found to improve the hardness of AZ91 alloy. Overall, the results of this study demonstrate the potential for using Calcium and Cerium as alloying elements in AZ91 alloy to improve its mechanical properties by modifying its microstructure.

Strength-ductility synergy in a wrought AZ80 magnesium alloy by microstructure engineering

The present investigation is conducted on microstructure engineering of a Mg–8Al-0.5Zn (AZ80) alloy by different processing routes including conventional aging, cold rolling followed by aging, and aging followed by warm and hot rolling in order to find a strength and ductility synergy. Aging at 175 °C followed by hot rolling at 200 °C is suggested to obtain remarkable mechanical properties with a strength of ∼450 MPa together with an acceptable total elongation of ∼8%. This procedure decorates a bimodal microstructure containing a low-volume fraction of discontinuous precipitates at the grain boundaries and a high-volume fraction of fine continuous precipitates in grain interiors. Microstructural analyses and a microstructure-based strength calculation confirm that the recommended thermomechanical treatment leads to bimodal structure and grain boundaries strengthening together with increasing precipitation strengthening significantly by reducing the size of CP and increasing CP/DP. Achieving superior mechanical properties without adding alloying elements for strengthening is the main advantage of the proposed method. It is suggested that aging followed by hot rolling plays a pivotal role in engineering the microstructure to improve the mechanical properties of AZ80 alloy which is applicable to other age-hardenable magnesium alloys.

Bio-Resorption Control of Magnesium Alloy AZ31 Coated with High and Low Molecular Weight Polyethylene Oxide (PEO) Hydrogels.

Magnesium AZ31 alloy has been chosen as bio-resorbable temporary prosthetic implants to investigate the degradation processes in a simulating body fluid (SBF) of the bare metal and the ones coated with low and high-molecular-weight PEO hydrogels. Hydrogel coatings are proposed to control the bioresorption rate of AZ31 alloy. The alloy was preliminary hydrothermally treated to form a magnesium hydroxide layer. 2 mm discs were used in bioresorption tests. Scanning electron microscopy was used to characterize the surface morphology of the hydrothermally treated and PEO-coated magnesium alloy surfaces. The variation of pH and the mass of Mg2+ ions present in the SBF corroding medium have been monitored for 15 days. Corrosion current densities (Icorr) and corrosion potentials (Ecorr) were evaluated from potentiodynamic polarisation tests on the samples exposed to the SBF solution. Kinetics of cumulative Mg ions mass released in the corroding solution have been evaluated regarding cations diffusion and mass transport parameters. The initial corrosion rates for the H- and L-Mw PEO-coated specimens were similar (0.95 ± 0.12 and 1.82 ± 0.52 mg/cm2day, respectively) and almost 4 to 5 times slower than that of the uncoated system (6.08 mg/cm2day). Results showed that the highly swollen PEO hydrogel coatings may extend into the bulk solution, protecting the coated metal and efficiently controlling the degradation rate of magnesium alloys. These findings focus more research effort on investigating such systems as tunable bioresorbable prosthetic materials providing idoneous environments to support cells and bone tissue repair.

The role of aluminium content on the corrosion initiated mechanical failure of AZ series magnesium alloys

In AZ series magnesium (Mg) alloys, the percentage of aluminium (Al) influences the quantity and the phase distribution within the magnesium matrix. AZ31 and AZ91 are the widely known alloys in the AZ series which contain 3% and 9% of aluminum (Al) by weight, respectively. Due to the presence of more secondary phase and eutectic regions, AZ91 exhibits higher strength and hardness compared with AZ31 Mg alloy. These secondary phases made of Mg and Al also influence the corrosion behavior in AZ series Mg alloys. Mechanical failure of Mg alloys exposed to corroding environment is influenced by several factors in which the time of exposure is one important element. Therefore, in the current work, both the AZ31 and AZ91 Mg alloys were exposed to corroding environment for different intervals of time (1, 3 and 7 days) and then their mechanical performance has been evaluated. Without corrosion attack, AZ91 has shown superior strength compared with AZ31 Mg alloy. Interestingly, when exposed to corroding environment, AZ91 sample has failed at lower stress values compared with AZ31 sample. The results demonstrated the negative role of Al containing secondary phases on the corrosion initiated mechanical failure of AZ series Mg alloys. It is suggested that for environments with moisture or corroding solvents, AZ91 Mg alloy is more susceptible to corrosion compared with AZ31 Mg alloy.

High-Modulus Laminated SiC/AZ91 Material with Adjustable Microstructure and Mechanical Properties Based on the Adjustment of the Densities of the Ceramic Layers.

To address the issue of inadequate strength and plasticity in magnesium matrix composites, SiC preforms were prepared using the freeze-casting process. The effects of sintering temperature on the microstructure, mechanical properties, and fracture behavior of SiCp/AZ91 magnesium matrix composites were studied by controlling the density of SiC preforms through low-temperature sintering. The results indicate that as the sintering temperature decreases, the reaction products in the SiC layer decrease, resulting in lower SiC preform density and increased content of AZ91 alloy filling in the layer. The increased alloy content in the ceramic layer not only inhibits crack initiation but also hinders crack propagation, thereby endowing the SiCp/AZ91 laminated material with excellent compressive strength and compressive strain. At the sintering temperature of 900 °C, the SiCp/AZ91 laminated material exhibits impressive compressive strength and strain values of 623 MPa and 8.77%, respectively, which demonstrates an excellent combination of strength and toughness.

Investigation of Zn alloy coating with submicron phases on magnesium alloy by the novel hot-dip process

A Zn alloy coating with submicron phases was fabricated on magnesium alloys by a novel hot-dip process, exhibiting excellent corrosion resistance. The microstructure and phases of coatings were characterized and their correlation with the corrosion performance was investigated. The results indicated that the coating immersed in Zn-3Mg molten bath for 4 s primarily consisted of submicron dot-like Zn phases with well-ordered distribution, while the coating immersed in Zn-3Mg molten bath for 7 s mainly comprised rod-like compact MgZn2 phases with a scattered distribution of Zn phase. The presence of these fine phases of coatings effectively impeded ion permeation in corrosive environments, leading to a significant enhancement in corrosion resistance. Notably, the exceptional corrosion resistance of the coating immersed for 4 s can be attributed to the submicron Zn phase with a grain size of 0.51 μm, resulting in a 62-fold increase in low-frequency impedance value compared to the AZ31 alloy, and a 22-fold decrease in corrosion rate compared to the AZ31 alloy.

Numerical Material Testing Method for Hexagonal Close-Packed Metals Based on a Strain-Rate-Independent Finite Element Polycrystal Model

The purpose of this study was to develop a numerical material testing method applicable to hexagonal close-packed (hcp) materials that can predict complex material behavior such as biaxial test results from relatively easy-to-perform uniaxial tests. The proposed numerical material testing method consists of a physical model that represents the macroscopic behavior of the material and a means of determining the included crystallographic parameters using macroscopic experimental data. First, as the physical model, the finite element polycrystal model (FEPM) previously applied by the authors for face-centered cubic (fcc) materials was applied and modified for hcp materials. A unique feature of the FEPM is that it avoids the use of strain-rate-dependent coefficients, whose physical meaning is ambiguous, because the deformation analysis can be performed while automatically determining the activity of all slip systems. The applicability of FEPM to numerical material testing methods was verified in hcp materials through this study. Then, a material parameter optimization process was developed using a genetic algorithm (GA). The proposed method was validated using literature values of magnesium alloy AZ31. First, the proposed optimization process was performed on cast AZ31 using uniaxial tensile and compressive stress—strain curves as teaching data to confirm that the stress—strain curves for the biaxial state could be predicted. Then, the proposed method was applied to rolled sheet AZ31, where the pseudo-anisotropic crystal orientations generated by numerical rolling were used as initial values. The prediction of unknown material data showed that, even in the case of sheets, the crystallographic parameters could be reasonably determined by the proposed optimization process.

Ultrasonic atomization of magnesium alloy AZ61 based on the TIG melting method

Ultrasonic atomization of magnesium alloy AZ61 based on the TIG melting method