Effects of prior compression on ductility and yielding behaviour in extruded magnesium alloy AZ31

Abstract

In the present paper, the effects of precompression along extrusion direction (ED) on subsequent compression perpendicular to ED were investigated in an extruded magnesium alloy AZ31. The results showed that the yield stress under compression perpendicular to ED increased if there was precompression along ED. The evolution of deformation mechanism was responsible for increase in yield stress because plastic deformation was dominated by both basal slip and {10–12} twinning under compression perpendicular to ED in samples without prestrain, but basal slip was difficult to be activated and {10–12} twinning dominated deformation in samples with precompression. However, because basal slip had no obvious contribution to plastic deformation, the ductility decreased if there was precompression along ED.     

Development of ternary Mg based alloy for soldering of AZ31B magnesium alloy

Abstract

Two kinds of ternary Mg based alloys were designed to join the AZ31B magnesium alloy plates by high frequency induction soldering with argon shielding gas. The microstructures and properties of the filler metals and joints were investigated by SEM, X-ray diffraction, differential scanning calorimetry, spreading test and tensile test. The results have shown that the microstructures of Mg–31·5Al–10Sn filler metal mainly consist of Mg₁₇Al₁₂, Mg₂Sn and a trace amount of α-Mg phases, while the microstructures of Mg–29·5Zn–1Sn filler metal include α-Mg phase and Mg₇Zn₃ with a trace of α-Mg and Mg₂Sn phases. Both of the filler metals have narrow melting zones; however, the spreading area of the Mg–31·5Al–10Sn filler metal is much larger than that of the Mg–29·5Zn–1Sn filler metal on the AZ31B base metal. The average tensile strength of solder joints with Mg–31·5Al–10Sn filler metal is a little higher than that of the latter solder joints with Mg–29·5Zn–1Sn filler metal.     

Cyclic hardening/softening behaviour in AZ31B magnesium alloy based on infrared thermography

The work-hardening/softening behaviour of AZ31B magnesium alloy during high cycle fatigue was investigated. The superficial temperature evolution during fatigue tests was used as a criterion for the different levels of work-hardening/softening. The microstructures under different cycles were observed by transmission electron microscope. Tensile test (with post-fatigue) was conducted to quantify the work-hardening/softening behaviour which showed that high dislocation density after cyclic loading lead to high tensile strength. The temperature evolution of the specimens with different levels of work-hardening/softening during tensile tests is related to the microstructures; the results indicated that the temperature rise of the specimen with high density dislocation was lower. Microstructures after tensile tests showed that high dislocation density after cyclic loading would lead to high twinning density.     

Pulsed MIG welding of AZ31B magnesium alloy

Abstract

Pulsed metal inert gas welding of AZ31B magnesium alloy is carried out, and continuous butt joints of high quality are obtained at optimised parameters. The effects of parameters on weld formation and welding stability are studied. The microstructure, mechanical properties and fracture of weld beads with different filler wires are investigated. The results show that it is a stable drop transition process with optimised parameters, which belongs to globular transfer. The precipitates in fusion zone and heat affected zone (HAZ) are uniform, dispersive and almost granular. The grain size in fusion zone is fine, and the grain size does not grow too large in HAZ compared with the base metal. The ultimate tensile strength of weld beads can be 94% of base metal, and the average elongation is 11%. Dimples and coarse tearing ridges can be observed on the fracture of the weld bead.     

Corrosion fatigue behavior of extruded magnesium alloy AZ31 in sodium chloride solution

In the present study, corrosion fatigue experiments were done using the extruded magnesium alloy AZ31 in the 3% sodium chloride solution to clarify the corrosion fatigue characteristics of the material. Corrosion fatigue lives greatly decreased as compared with those in laboratory air. It was also clarified that most of the corrosion fatigue life (70–80%) at the lower stress amplitude is occupied with the period of the corrosion pit growth. Corrosion fatigue lives were evaluated quantitatively by dividing the corrosion fatigue process into the following two periods, i.e. (1) the corrosion pit growth period preceding the crack initiation from the pit and (2) the crack growth period before the specimen failure. In the analysis, the law of the corrosion pit growth proposed by authors was used to deal with the above first period. The evaluated results corresponded well to the experimental results.     

Microstructures and tensile properties of AZ31 magnesium alloy by continuous extrusion forming process

The objective of this study is to investigate the possibility of continuous extrusion forming (Conform process) of AZ31 magnesium alloy. The results indicate that continuous extrusion forming can refine the structure, improve the degree of the structure homogeneity and change the crystal orientation of basal plane and hence enhance the ductility but decrease tensile strength at room temperature. The fracture mechanisms of the material prepared by Conform process change from the mixture of ductile and brittle to the full dimpled rupture compared with the conventional extrudate.     

Dynamic behaviors of a Ca-P coated AZ31B magnesium alloy during in vitro and in vivo degradations

Surface modification can be an effective way to control the biodegradation behavior of magnesium alloys and even improve their biological properties. Much attention has been paid to the initial protection ability and biological properties of magnesium alloys coating. In this work, the dynamic behaviors of a Ca-P coated AZ31B magnesium alloy during the degradations in vitro and in vivo, including hemolysis, mechanical loading capability and implantation in animals, were investigated. The hemolytic rates of the alloy with and without coating were all declined to be lower than 5% after more than 20 days immersion in PBS, though an increase happened to the alloy at the early immersion of 3-7 days. Reduction of the mechanical loading capacity was gradually evolved for the coated alloy and the peak load retention of 85% was still maintained after 120 days degradation. The in vivo implantation indicated that the Ca-P coated AZ31B alloy showed a more suitable time dependent degradation behavior which was favorable for growth of the new tissue and the healing dynamics of bones, making it a promising choice for medical application.     

Growth and corrosion of aluminum PVD-coating on AZ31 magnesium alloy

Magnetron sputtering was applied to prepare aluminum coating on a mechanically polished AZ31 magnesium alloy. A loose oxide film was spontaneously formed on the surface of AZ31 magnesium alloy during polishing process. The aluminum coating, which was subsequently deposited on this oxide layer, presented a developed columnar microstructure. Attributed to the barrier effect of Al coating, the Al coated AZ31 showed a higher corrosion resistance than bare AZ31 in corrosion tests. Generally, Al coating is cathodically protected by magnesium alloy substrate. But it is interesting in this study that Al coating still suffered from severe corrosion due to the occurrence of the alkalization effect.     

Microstructural features of spray-formed AZ31 magnesium alloy

Abstract

Mg alloy AZ31 was spray-formed using an indigenously developed spray atomisation and deposition unit under protective atmosphere and various processing parameters were optimised. The microstructural features of the bell shaped AZ31 spray-formed deposit were characterised using optical microscope, scanning electron microscope/energy dispersive spectrometer, X-ray diffraction and high resolution transmission electron microscope. It was observed that the microstructural features are critically dependent on location in the spray-formed deposits. Under optimised processing conditions, the central region of the bell shaped deposit exhibited minimal porosity and a uniform fine grained equiaxed microstructure with fine Mg₁₇Al₁₂ intermetallics preferably located at the grain boundaries. However, the peripheral regions of the spray-formed deposit indicate higher porosity with distinct microstructural characteristics different from those in the central region. These microstructural features, observed at different locations in the spray-formed deposit, have been analysed and their evolution is discussed in the light of variations in thermal and solidification conditions of the droplets in flight, during impingement as well as those of the deposition surface.     

Fracture mechanism of AZ31 magnesium alloy processed by equal channel angular pressing comparing three point bending test and tensile test

A commercial magnesium alloy, AZ31 in hot-rolled condition, has been processed by equal channel angular pressing (ECAP) to get microstructure modified. Uniaxial tensile tests were conducted along the rolling/extrusion direction for as-received AZ31 alloy and ECAPed AZ31 alloy. Then, three point bending fracture tests were conducted for specimens with a pre-crack perpendicular to the extruded direction. Digital image correlation (DIC) technique was adopted to determine the deformation field around the crack tip. The fracture surfaces of the failed specimens after tensile tests and fracture tests were observed by Scanning Electron Microscope (SEM). To explore the deformation mechanism, the microstructure and texture of different regions on the deformed specimens were examined through electron backscatter diffraction (EBSD). The results show ECAP process improves both the tensile elongation and fracture toughness of AZ31 alloy. Different from the slip dominated deformation mechanism in the tensile test, deformation twinning presents in the deformation zone adjacent to the crack tip in the three point bending fracture tests. The fracture surface is characterized by co-occurrence of dimple and cleavage features.     

Temperature evolution analysis of AZ31B magnesium alloy during quasi-static fracture

Fracture toughness of AZ31B magnesium alloy subjected to quasi-static loading was investigated by infrared thermography. The results showed that temperature evolution around the crack propagation path during fracture underwent three stages: initial steady stage, monotonic increase stage and final steady stage. The temperature increase at the beginning of stage II is nearly corresponding to the initiation of unstable crack propagation. And based on this phenomenon, a method applying infrared thermography to estimate fracture toughness of AZ31B magnesium alloy was proposed. Fracture toughness was calculated through infrared thermography, which was in good agreement with the result determined by traditional standard method. Finally, the fracture mechanism was investigated.     

Experimental study on tensile property of AZ31B magnesium alloy at different high strain rates and temperatures

As the lightest metal material, magnesium alloy is widely used in the automobile and aviation industries. Due to the crashing of the automobile is a process of complicated and highly nonlinear deformation. The material deformation behavior has changed significantly compared with quasi-static, so the deformation characteristic of magnesium alloy material under the high strain rate has great significance in the automobile industry. In this paper, the tensile deformation behavior of AZ31B magnesium alloy is studied over a large range of the strain rates, from 700 s⁻¹ to 3 × 10³ s⁻¹ and at different temperatures from 20 to 250 °C through a Split-Hopkinson Tensile Bar (SHTB) with heating equipment. Compared with the quasi-static tension, the tensile strength and fracture elongation under high strain rates is larger at room temperature, but when at the high strain rates, fracture elongation reduces with the increasing of the strain rate at room temperature, the adiabatic temperature rising can enhance the material plasticity. The morphology of fracture surfaces over wide range of strain rates and temperatures are observed by Scanning Electron Microscopy (SEM). The fracture appearance analysis indicates that the fracture pattern of AZ31B in the quasi-static tensile tests at room temperature is mainly quasi-cleavage pattern. However, the fracture morphology of AZ31B under high strain rates and high temperatures is mainly composed of the dimple pattern, which indicates ductile fracture pattern. The fracture mode is a transition from quasi-cleavage fracture to ductile fracture with the increasing of temperature, the reason for this phenomenon might be the softening effect under the high strain rates.     

Dissimilar material laser welding between magnesium alloy AZ31B and aluminum alloy A5052-O

Joining technology of lightweight dissimilar metals between magnesium and aluminum alloys is essential for realizing hybrid structure cars and other engineering applications. In the present study, the normal center-line welding of lap joint was carried out by laser welding. It was found that the intermetallic layer formed near interface between two metals significantly degraded the joining strength. FEM heat transfer analysis was carried out to find out an available method to control penetration depth and width of molten metal, which contributes to control thickness of intermetallic compound layer. Based on the results of FEM analysis, the edge-line welding of lap joint was carried out, which could easily control the thickness of intermetallic layer and successfully obtained high joining strength.     

Modeling high-temperature tensile deformation behavior of AZ31B magnesium alloy considering strain effects

The hot tensile deformation behaviors of AZ31B magnesium alloy are investigated over wide ranges of forming temperature and strain rate. Considering the effects of strain on material constants, a comprehensive constitutive model is applied to describe the relationships of flow stress, strain rate and forming temperature for AZ31B magnesium alloy. The results show that: (1) The effects of forming temperature and strain rate on the flow behaviors of AZ31B magnesium alloy are significant. The true stress–true strain curves exhibit a peak stress at small strains, after which the flow stress decreases until large strain, showing an obvious dynamic softening behavior. A considerable strain hardening stage with a uniform macroscopic deformation appears under the temperatures of 523 and 573 K. The strain hardening exponent (n) increases with the increase of strain rate or the decrease of forming temperature. There are not obvious strain-hardening stages when the forming temperature is relatively high, which indicates that the dynamic recrystallization (DRX) occurs under the high forming temperature, and the balance of strain hardening and DRX softening is easy to obtain. (2) The predicted stress–strain values by the established model well agree with experimental results, which confirm that the established constitutive equation can give an accurate and precise estimate of the flow stress for AZ31B magnesium alloy.     

Assessment of the effect of existing corrosion on the tensile behaviour of magnesium alloy AZ31 using neural networks

A concept has been devised to assess the effect of existing corrosion damage on the residual tensile properties of structural alloys and applied for the magnesium alloy AZ31. The concept based on the use of a radial basis function neural network. An extensive experimental investigation, including metallographic corrosion characterization and mechanical testing of pre-corroded AZ31 magnesium alloy specimens, was carried out to derive the necessary data for the training and the prediction module of the developed neural network model. The proposed concept was exploited to successfully predict: the gradual tensile property degradation of the alloy AZ31 to the results of gradually increasing corrosion damage with increasing corrosion exposure.     

Anisotropic and tensile flow behaviors of Mg alloy AZ31B thin sheet in H24 condition at elevated temperatures

In this work, a series of experiments was performed to explore the effects of anisotropy, strain rate, and temperature on microstructure change and associated mechanical response of a rolled AZ31B-H24 Mg alloy sheet under tension. Tensile tests were carried out on specimens in the 0, 45, and 90° to the rolling direction, using initial strain rates in the range of 4 × 10⁻³ to 1 × 10⁻¹ s⁻¹ at temperatures of 250 and 370 °C. Results showed that variations in flow behavior under tension could be related to the changes in microstructure resulting from applied tensile conditions. Resultant microstructures, such as degree of dynamic recrystallization, grain growth, and shape of the grain, were associated with temperature, strain rate, and tensile loading direction. The initial texture influenced the variations in changes in microstructure and mechanical properties upon testing in different directions. The specimens upon testing in the 45° to the rolling direction yielded higher m-value, lower strength, and greater elongation to failure under all test conditions.     

Creep deformation behaviour of AZ31 magnesium alloy over wide range of temperature and stress

Abstract

The creep deformation behaviour of coarse grained AZ31 magnesium alloy was examined in the temperature range from 423 to 673 K (0·46–0·73T_m) under various constant stresses covering low strain rate range from 4×10^?9 to 2×10^?2 s^?1. Most shape of the creep curve was typical of class II behaviour. However, only at low stress and low temperature, the shape of the creep curve was typical of class I behaviour. At very low stress at 673 K, the stress exponent for the secondary creep rate was ～2. At low stress level, the stress exponent was ～3 and the present results were in good agreement with the prediction of Takeuchi and Argon model. At high stress level, the stress exponent was ～5 and the present results were in good agreement with the prediction of Weertman model. The transition of deformation mechanism from solute drag creep to dislocation climb creep could be explained in terms of solute atmosphere breakaway concept.     

Texture modification and mechanical properties of AZ31 magnesium alloy sheet subjected to equal channel angular bending

Analysis of the recently proposed equal channel angular bending(ECAB)process is provided on thin hotrolled AZ31 magnesium alloy sheets.In particular,effects of deformation temperature and strain path on the texture evolution and mechanical properties are systematically investigated under single pass ECAB at various temperatures and multi-pass ECAB process that involves changes in strain paths.It is found that simultaneous activation of multiple twinning types is successfully introduced during ECAB,which results in obvious tilted component of basal texture.Attributed to the domination of extension twins,weaker basal textures are detected after both single pass ECAB at 150℃and three cross passes ECAB at 200℃.After annealing,the basal texture is further weakened via twin-related recrystallization and the annealed microstructure is featured with mixture of basal and non-basal orientated grains.Additionally,the effect of grain orientation on the mode of plastic deformation and the roles of grain orientation and grain boundary on the local strain accommodation are coherently studied.This study reveals that over 60%increase of uniform elongation with marginal reduction of tensile strength less than 5%can be achieved for single pass ECAB at 150℃and three cross passes ECAB at 200℃,which is the result of larger fraction of grains favored with extension twinning and better local strain accommodation.     

Exceptional superplasticity in an AZ61 magnesium alloy processed by extrusion and ECAP

Experiments were conducted on a commercial AZ61 alloy to evaluate the potential for achieving an ultrafine grain size and superplastic ductilities through the use of the EX-ECAP two-step processing procedure of extrusion plus equal-channel angular pressing. The results show that EX-ECAP gives excellent grain refinement with grain sizes of 0.6 and 1.3 μm after pressing at 473 and 523 K, respectively. The alloy processed by EX-ECAP exhibits exceptional superplastic properties including a maximum elongation of 1320% after pressing through four passes when testing at 473 K with an initial strain rate of 3.3 × 10⁻⁴ s⁻¹. This result compares with an elongation of 70% achieved in the extruded condition without ECAP under similar testing conditions.