Wrought Magnesium Alloy AZ31 Research Articles

Modeling of fatigue behavior in pre-corroded AZ31 magnesium alloy

In this paper, the mechanical and fatigue behavior of pre-corroded wrought AZ31 magnesium alloy was studied. For this purpose, the standard 3.5 wt.% NaCl corrosive solution was used. The samples were immersed for 3–24 h to characterize the effect of immersion time on the mechanical properties of AZ31 alloy. Standard specimens were also immersed for 1–3 h for the fatigue testing. Results of tensile tests showed that thorough the immersion of 0–24 h, the deviation of ultimate tensile stress and yield stress were less than 4 % and 6 %, respectively. Moreover, the deviation of elastic modulus was less than 20 %. Although, the elongation was deviated by 81 % through the immersion of 0–24 h. A drastic decrease was observed in the fatigue lifetime of pre-corroded alloy compared to the bare alloy. As the immersion time increased, the fatigue lifetime decreased. Maximum reduction in fatigue strength occurred when the immersion time was 3 h and the stress amplitude was 82.5 MPa. Fatigue results also showed that the Levenberg-Marquardt was a good method to find the materials' constants, as the maximum and average relative errors were 10.28 % and 2.78 %, respectively. The fatigue fracture surfaces of pre-corroded specimens indicated the brittle fracture. The Basquin model was used for fatigue lifetime prediction. A new model was proposed with a new parameter, initial virtual crack size, to relate the immersion time to the fatigue lifetime using the Paris equation. The fatigue lifetime of 1–3-h pre-corroded AZ31 magnesium alloy was estimated by the new model with acceptable relative errors.

Effect mechanism of texture orientation on mechanical properties and Hall-Petch relationship of CVCDE AZ31 magnesium alloy via crystal plastic finite element method (CPFEM)

Texture orientation explicitly influences the mechanical properties and deformation mechanism of AZ31 wrought magnesium alloy. Continuous variable channel direct extrusion can use its unique cavity structure to promote the formation of different texture distributions of magnesium alloy, which affects the material’s mechanical properties and deformation mechanism. Therefore, this paper selects the CVCDE AZ31 magnesium alloy rod and sheet as the research object and studies the mechanical properties of CVCDE AZ31 magnesium alloy under different process conditions with the help of crystal plastic finite element (CPFEM). The results show that the stress-strain curve shows a downward trend with the increase in loading angle. For CVCDE sheets, due to the influence of texture weakening and grain refinement, there is an anti- Hall-Petch relationship between yield strength and grain size. The relationship between yield strength and grain size of the CVCDE rod shows a strong linear correlation. Hall-Petch constant k first decreases and then increases in the tensile process of different loading directions (0°, 45°, and 90°). This clarifies the internal relationship between texture orientation and mechanical properties of CVCDE AZ31 magnesium alloy to a certain extent and reveals the strengthening law of CVCDE under different process conditions.

Temperature Dependency on the Microscopic Mechanism in the Normal Direction of Wrought AZ31 Sheet under Dynamic Compressive Behavior.

In order to analyze the competitive relationship of different deformation mechanisms in wrought AZ31 magnesium alloy, the dynamic compressive experiments were conducted by a Split Hopkinson Pressure Bar (SHPB) apparatus and a resistance-heated furnace in the range of temperature between 20 and 350 °C at the strain rate of 1000 s−1. With the help of Electron Backscattered Diffraction (EBSD) observation, theoretical calculated Schmid Factor (SF), Critical Resolved Shear Stress (CRSS), and critical equivalent stress (σ0.2), the dynamic compressive deformation behavior and corresponding mechanism of wrought AZ31 magnesium alloy along the normal direction (ND) were revealed in the current study. The results demonstrate that the c-axis of grains are gradually reoriented parallel to the normal direction of wrought AZ31-ND sheet with the temperature increasing, except the dynamic recrystallization (DRX) mechanism was activated or grains grew up. The non-basal slip and tension twinning are respectively the predominant deformation mechanisms at lower temperatures (≤250 °C) and higher temperatures (≥250 °C). The predominant type of DRX mechanism of wrought AZ31-ND sheet is rotational dynamic recrystallization (RDRX), which is regarded as an obstacle for the kernel misorientation concentration region enhancement.

Characterization and Corrosion Properties of Fluoride Conversion Coating Prepared on AZ31 Magnesium Alloy

Wrought AZ31 magnesium alloy was used as the experimental material for fluoride conversion coating preparation in Na[BF4] molten salt. Two coating temperatures, 430 °C and 450 °C, and three coating times, 0.5, 2, and 8 h, were used for the coating preparation. A scanning electron microscope and energy-dispersive X-ray spectroscopy were used for an investigation of the surface morphology and the cross-sections of the prepared coatings including chemical composition determination. The corrosion resistance of the prepared specimens was investigated in terms of the potentiodynamic tests, electrochemical impedance spectroscopy and immersion tests in the environment of simulated body fluids at 37 ± 2 °C. The increase in the coating temperature and coating time resulted in higher coatings thicknesses and better corrosion resistance. Higher coating temperature was accompanied by smaller defects uniformly distributed on the coating surface. The defects were most probably created due to the reaction of the AlxMny intermetallic phase with Na[BF4] molten salt and/or with the product of its decomposition, BF3 compound, resulting in the creation of soluble Na3[AlF6] and AlF3 compounds, which were removed from the coating during the removal of the secondary Na[MgF3] layer. The negative influence of the AlxMny intermetallic phase was correlated to the particle size and thus the size of created defects.

On the use of a fictitious notch radius equal to 0.3 mm to design against fatigue welded joints made of wrought magnesium alloy AZ31

In the present investigation, a large number of fatigue data generated by testing welded joints of magnesium alloy AZ31 were post-processed based on the reference radius concept. The local stresses used to perform this systematic re-analysis were determined by setting the fictitious notch radius, rf, invariably equal to 0.3 mm. According to this strategy, local stress-based S-N curves were then derived for load ratios, R, equal to −1, 0 and to 0.5. This post-processing exercise allowed us to demonstrate that a fictitious notch radius of 0.3 mm is successful in modelling the fatigue strength of weldments made of magnesium alloy AZ31. In particular, with rf=0.3 mm, this type of welded joints is recommended to be designed against fatigue by using an S-N curve having endurable local stress range, ΔσFAT, at N=2×106 cycles to failure equal to 40 MPa and a negative inverse slope, k, equal to 3.

A novel severe plastic deformation method for manufacturing AZ31 magnesium alloy tube

A new severe plastic deformation called TES (tube extrusion shearing) process, which combines direct extrusion with two-step shearing, is developed to manufacture tubes made of wrought AZ31 magnesium alloy. To explore the deformation mechanisms of TES process, both experiment and numerical simulation are carried out. Three-dimensional finite element modeling is used to investigate the plastic deformation behaviors of wrought magnesium alloy during TES process. Experiments show that by use of TES process the microstructures can be refined to 50% of the original grain size and with more uniform grain distribution. TES process could improve hardness obviously by comparing which fabricated by direct extrusion. Numerical results indicate TES increases the cumulative strains effectively by direct extrusion and additional shearing.

Microstructure, Texture, and Tensile Properties of Ultrafine/Nano-Grained Magnesium Alloy Processed by Accumulative Back Extrusion

An AZ31 wrought magnesium alloy was processed by employing multipass accumulative back extrusion process. The obtained microstructure, texture, and room temperature tensile properties were characterized and discussed. Ultrafine-grained microstructure including nano-grains were developed, where the obtained mean grain size was decreased from 8 to 0.5 μm by applying consecutive passes. The frequency of both low angle and high angle boundaries increased after processing. Strength of the experimental alloy was decreased after processing, which was attributed to the obtained texture involving the major component lying inclined to the deformation axis. Both the uniform and post-uniform elongations of the processed materials were increased after processing, where a total elongation of 68 pct was obtained after six-pass deformation. The contribution of different twinning and slip mechanisms was described by calculating corresponding Schmid factors. The operation of prismatic slip was considered as the major deformation contributor. The significant increase in post-uniform deformation of the processed material was discussed relying on the occurrence of grain boundary sliding associated with the operation of prismatic slip.

Characterization of Ni-P coating prepared via electroless deposition on AZ31 magnesium alloy

AbstractThis work deals with the characterization of Ni–P coating prepared via electroless deposition on wrought AZ31magnesium alloy. For the application of electroless deposition was proposed and optimized a suitable pretreatment process of magnesium alloy surface followed by Ni–P coating in the nickel bath. The chemical composition of Ni–P based coating was characterized using the scanning electron microscope with chemical composition analysis. Next, physico-chemical properties and mechanical characteristics of Ni–P coating were evaluated. The mechanism of corrosion degradation of the coating and the substrate was also studied in this work.

Corrosion behavior of wrought magnesium alloys AZ31 and AZ61 in Hank’s solution

Abstract Corrosion behavior of wrought magnesium alloys AZ31 and AZ61 was studied in Hank’s solution. Potentiodynamic curves measured after short-term of exposure showed higher corrosion resistance of AZ31 magnesium alloy in comparison with AZ61 magnesium alloy. On the contrary, long-term tests measured by electrochemical impedance spectroscopy showed higher corrosion resistance of AZ61 magnesium alloy in comparison with AZ31 magnesium alloy.

Cover Picture: Materials and Corrosion. 12/2016

Cover:SEM image of a spongelike halloysite‐reinforced Ce‐oxide layer on wrought magnesium alloy AZ31 produced by application of ultrasound (a), magnification (b), cross‐section (c) and TEM image of natural occurring halloysite (d). (Copyright DECHEMA‐Forschungsinstitut).

Wear behaviors and wear mechanisms of wrought magnesium alloy AZ31 fabricated by extrusion-shear

Abstract Extrusion-Shear (ES) process for magnesium alloy is a newly developed plastic deformation technique which combines direct extrusion (DE) with ECAP (equal-channel angular pressing). Grains of ES-processed magnesium alloys can be refined effectively and mechanical properties can be improved, but the tribological properties have not been studied. In this study, the wear behaviors and wear mechanisms of AZ31 magnesium alloy samples produced by direct extrusion (DE) and extrusion-shear (ES) technologies, respectively, have been evaluated by fiction coefficient and wear frequencies obtained from the dry sliding tests conducted at different loads and frequencies by using a high speed dry sliding test machine. SEM and EDS analyses technologies also have been used to analyze the wear surfaces and wear debris in different fiction conditions. Based on the results, the ES-processed samples show a better wear resistance than those prepared by DE process as the ES-process samples show a lower friction coefficient comparing to those fabricated by DE process at the same conditions of loads and wear frequencies. With the increase of normal loads and reciprocating frequencies, the wear mechanism change from mild wear (adhesion, abrasion and oxidation) to severe wear (delamination, plastic deformation and melting). Therefore, the extrusion ways have significant influences on the wear behaviors of the samples and that the wear resistance of AZ31 magnesium alloy can be improved by extrusion-shear.

Enhancing the age-hardening response of rolled AZ80 alloy by pre-twinning deformation

The age-hardening response of AZ80 alloy can greatly enhanced by pre-twinning deformation. A remarkably high strength is obtained for wrought magnesium alloy AZ80 by combined processes of twinning deformation and aging treatment. This is mainly attributed that twinning deformation promotes the continuous precipitation and inhibits the discontinuous precipitation. Moreover, promoting continuous precipitation is of great benefit to the improvement in ductility and yield asymmetry. Finally, the relevant mechanisms are investigated and discussed.

Crack Propagation Analysis of Magnesium Rod Processed by Extrusion-shear: Numerical Modeling and Experimental Verification

During extrusion-shear (ES) process, the surface cracks would be formed in the processed rod for AZ31 magnesium caused by nonhomogeneous metal flow and stress. To reduce the cracks defects, three-dimensional (3D) finite element method (FEM) of ES on a wrought AZ31 magnesium alloy have been performed. ES process experiments have been also conducted to verify the simulation results under the identical conditions. It can be found that the tendency to generate the dead zone is decreased by employing the ES die with the extrusion ratio of 28 comparing to the one with the extrusion ratio of 11.6. With the extrusion ratio 28, the surface stresses of the rod at the die exit are decreased greatly so that the surface cracks are avoided. The ES die with the extrusion ratio of 28 would increase temperature on the rod surface and subsequently result in the sacrifice of the tensile strength of the AZ31 rod. Damage model has been chosen and Cockcroft–Latham damage factor has been set and incorporated into DEFORM software. The simulation results show that the larger extrusion ratio would have the smaller crack propagation. Experimental validation shows that the results of experiments are in accordance with those from computer simulation. The study indicates that FEM can be used confidently for designing ES dies and process parameters in CAE environment for DEFORM software to improve the product quality and productivity by avoiding trail runs.

Cold formability of AZ31 wrought magnesium alloy undergoing semisolid spheroidization treatment

Abstract To investigate the effect of semisolid spheroidization treatment on the cold formability of magnesium alloys, reheating experiments and cold compression tests were conducted on AZ31 wrought magnesium alloy. Fine spheroidization of the microstructure was realized when AZ31 alloy was reheated to 630 °C and held isothermally for 1800s. Semisolid treatment increased the breaking strain of AZ31 from 0.163 to 0.214 and decreased the maximum stress from 545 to 307 MPa. The cold forming mechanism of semisolid-treated AZ31 alloy was investigated quantitatively by image and mathematical analyses. The improved cold formability of semisolid-treated AZ31 alloy was mainly attributed to the slipping and shear deformation that occurred in the eutectic mixture networks surrounding the spherical grains.

An investigation into the dissolution characteristics of γ precipitates in Mg-3Al-Zn alloy

In non-equilibrium conditions, the coring phenomena may occur in the α-Mg solid solution and γ-Mg17Al12 phase forms in the microstructure of AZ magnesium alloy series. This eutectic phase may introduce detrimental effect on workability of wrought alloys. In the present work, the dissolution characteristics of γ phase have been investigated in AZ31 wrought magnesium alloy. Considering the eutectic temperature of AZ31 alloy, the homogenization treatment was executed in temperature range of 300-437 ºC (i.e., somewhat below eutectic melting temperature of γ precipitate) and 437-500 ºC (i.e., higher eutectic melting temperature of γ precipitate) for different durations. A thorough microstructural investigation and also thermodynamic calculations were executed. The results indicated that the dissolution rate of γ phase is very low even at temperatures just below the eutectic point of the alloy, whereas it dramatically increases at temperatures higher than eutectic point. Moreover, it is suggested that partial melting of γ phase due to eutectic melting reaction (α+γ→L) may lead to reduced formability.

Multiaxial fatigue evaluation of laserbeam-welded magnesium joints according to IIW-fatigue design recommendations

Under non-proportional multiaxial loading, welded magnesium joints show a decrease of fatigue life compared to combined proportional loading as is also observed for welded ductile steels and aluminium alloys. The investigated laserbeam-welded overlapped tube-tube joints of the wrought magnesium alloys AZ31 and AZ61 were tested under combined in- and out-of-phase axial loading and torsion. The IIW-Recommendations for the evaluation of multiaxial stress states with constant and with changing principal stress directions describe, on the safe side, the results obtained for fully reversed loading R = −1 and the probability of survival P s = 50 %. The comparisons of the experimental results with the allowable values for P s = 97.7 % and R = 0.5 are quite conservative. However, as, at present, relatively few multiaxial fatigue investigations with magnesium alloys in the welded state are available, the suggested IIW procedure, the Gough-Pollard Equation with the multiaxial damage parameters D MA = 1.0 and 0.5, should be retained.

The effects of billet temperature on compound extrusion of magnesium alloy by three-dimensional finite element modeling and experiments

Extrusion–shearing of magnesium billets is associated with large deformations, high strain rates and high temperatures, which could result in many difficulties in process. Thermo-mechanically coupled three-dimensional finite element simulations of extrusion–shearing wrought magnesium alloy AZ31 into small rods at certain speed have been performed, and computed parameters including workpiece material characteristics and process conditions (billet preheated temperature, extrusion ratio, ram speed, friction factors and heat transfer coefficients) have been taken into consideration. The temperatures and stress and strain rates of extrudates with different billet preheated temperature have been calculated by finite element method software DEFORM-3D. A series of extrusion–shearing experiments and microstructure observations have been done to perform tests in order to validate the finite element method simulation results at different preheated temperatures of billets. Surface defects of rods for magnesium alloys extruded by the extrusion–shearing extrusion have been observed and analyzed. The influences of preheated temperature on the microstructures and the causes have been given. Finite element method software DEFORM-3D has been used successfully to simulate the temperatures and stress and strain rates under three different extrusion conditions. The results of these simulations helped to understand the formation of surface defects in the surface of extrusion–shearing rods. These results could assist in improving quality of extrusion–shearing extrusion.

Effect of Grain Refinement on Fracture Toughness and Fracture Mechanism in AZ31 Magnesium Alloy

The effect of grain size on fracture toughness was investigated for a wrought AZ31 magnesium alloy. The grain size was refined by equal channel angular pressing (ECAP) progress. Digital image correlation (DIC) was used to determine the fracture property. The fracture toughness was found to be improved by grain refinement. The fracture mechanism was examined through the progress of crack tip and the analyses of fracture surface. The fracture mechanism changed from dimple fracture to mixture of cleavage fracture and dimple fracture.

Effect of equal channel angular pressing on AZ31 wrought magnesium alloys

AZ31 wrought magnesium alloys are light weight materials which play an important role in order to reduces the environmental burdens in modern society because of its high strength to weight ratio, corrosion resistance, and stiffness and machinability. Applications of this material are mainly in structural component i.e., in constructions, automobile, aerospace, electronics and marine industries. In the present work, the microstructure characterization of the AZ31 alloys up to four ECAP passes at temperature of 573 K was observed for route Bc. Average grain size of the material was reduced from 31.8 μm to 8 μm after four ECAP passes. Mechanical properties of the alloy improved with increase in number of ECAP passes. Moreover, X-ray diffraction analysis was carried out for as received and ECAP processed material. Equal channel angular pressing (ECAP) technique is used to refine the grains into ultrafine grain size of the material. In the present work, AZ31 wrought Mg alloy is used as a material processed through ECAP up to 4 passes at the temperature of 573 K for route Bc. Initially, the Average grain size of the as received material was found to be 31.8 μm. The average reduced grain size after 4 ECAP passes was found to be 8 μm. Mechanical properties of the AZ31 alloy was studied for 4 ECAP passed specimen and an improvement was found to have occurred in comparison to as received material. X-ray diffraction analysis has been done for as received material, after homogenized specimen and after 4 ECAP processed specimen. • ECAP experiment carried out using die angle 120° and arc of curvature was 30°. • AZ31 material was deformed at a processing temperature of 573 K up to 4 pass. • Examine the grain refinement with increased ECAP passes for route Bc. • Microstructural Characterization was observed for AZ31 alloy using OM and SEM. • Evaluate the Mechanical properties and X-ray diffraction analysis of AZ31 alloy.

Characterization of ultrafine and nano grained magnesium alloy processed by severe plastic deformation

A wrought AZ31 magnesium alloy was severely deformed through a novel continuous severe plastic deformation technique, called accumulative back extrusion. The microstructure and texture obtained after applying multiple deformation passes at a temperature range of 80–380°C were characterized. After a single pass at 80°C an ultrafine/nano grained microstructure was obtained, while the initial texture was completely replaced by a new fiber basal texture, 40° inclined to transverse direction. As the processing temperature increased, the obtained texture intensities were strengthened, though the c-axis of crystals gradually rotated towards transverse direction and a <10–11> fiber texture parallel to normal direction was developed. Moreover, repetitive accumulative back extrusion was associated with tendency of basal plane to lie parallel to transverse direction, while the orientation of prismatic planes showed a random distribution around normal direction. After eight passes the most noticeable texture included fiber basal texture oriented almost parallel to the transverse direction, and <10–10> perpendicular to the extrusion direction and <10–11> parallel to the normal direction. The maximum texture intensity was decreased as the number of passes increased, which is attributed to strain path change involved during each consecutive accumulative back extrusion pass as well as promoted contribution of non-basal slip systems.