AZ31 Magnesium Alloy Sheets Research Articles

Development of liquid-nitrogen-cooling friction stir spot welding for AZ31 magnesium alloy joints

A liquid-nitrogen-cooling friction stir spot welding (C-FSSW) technology was developed for welding AZ31 magnesium alloy sheets. The liquid-nitrogen cooling degraded the deformability of the welded materials such that the width of interfacial cracks increased with increasing cooling time. The grain size of the stirred zone (SZ) and the heat-affected zone (HAZ) of the C-FSSW-welded joints decreased, whereas that of the thermomechanically affected zone (TMAZ) increased with increasing cooling time. The maximum tensile shear load of the C-FSSW-welded joints welded with a cooling time of 5 or 7 s was larger than that of the friction stir spot welding (FSSW)-welded joint, and the tensile shear load decreased with increasing cooling time. The microhardness of the C-FSSW-welded joints was greater than that of the FSSW-welded joint. Moreover, the microhardness of the SZ and the HAZ of the C-FSSW-welded joints increased, whereas that of the TMAZ decreased, with increasing cooling time.

Microstructure and Properties of Rolled Magnesium Alloy Thick Plate at Different Positions

The microstructure and properties of rolled AZ31 magnesium alloy sheet with the change of normal position were investigated by electronic universal testing machine, ZEISS optical microscopy and scanning electron microscopy. The tensile test results showed that the mechanical properties of sample selected at the depth of 8mm, which is 2/5 of the whole thickness counting from rolling plane are closest to the properties for whole sheet. Microstructure characterization indicated that a large number of tiny grains are distributed around the large grains. The majority of these grains are equiaxed grains, and the few twins could be observeded. The fracture mechanism was ascribed to mixed ductile and brittle fractures.

Mechanical properties and deformation mechanism of Mg-Al-Zn alloy with gradient microstructure in grain size and orientation

The surface mechanical attrition treatment was taken to fabricate the gradient structure in AZ31 magnesium alloy sheet. Microstructural investigations demonstrate the formation of dual gradients with respect to grain size and orientation, where the microstructural sizes decreased from several microns to about 200nm from center area to treated surface, while the c-axis gradually inclined from being vertical to treated plane towards parallel with it. According to tensile results, the gradient structured sample has yield strength of 305MPa in average, which is increased by about 4 times when compared with its coarse-grained counterpart. Meanwhile, contrary to quickly failure after necking in most traditional magnesium alloys, the failure process of gradient structure appears more gently, which makes it has 6.5% uniform elongation but 11.5% total elongation. The further comparative tensile tests for separated gradient layers and corresponding cores demonstrate that the gradient structured sample has higher elongation either in uniform or in post-uniform stages. In order to elucidate the relationship between mechanical properties and deformation mechanisms for this dual gradient structure, the repeated stress relaxation tests and pole figure examinations via X-ray diffraction were conducted in constituent gradient layer and corresponding core, as well as gradient structured sample. The results show that the pyramidal dislocations in dual gradient structure are activated through the whole thickness of sample. Together with the contribution of grain-size gradient, more dislocations are activated in dual gradient structure under tensile loading, which results in stronger strain hardening and hence higher tensile ductility.

Stress intensity factor solutions for welds in lap-shear specimens of dissimilar sheet materials with and without kinked cracks

Global stress intensity factor solutions for welds with various widths in lap-shear specimens of magnesium alloy AZ31 and high-strength low-alloy steel sheets are developed using finite element analyses. Results are compared to and validated with the analytical solutions based on the beam bending theory and semi-infinite planes with connection. Analyses are also conducted using modified elastic constants with a bimaterial constant of zero for specimens with small kinked cracks. Results indicate that the stress intensity factor solutions for a vanishing kink length can be approximated by the available analytical solutions for the zero bimaterial constant for fatigue cracks growth models. The results of the global stress intensity factor solutions and the local stress intensity factor solutions for kinked cracks with the experimentally observed kink angle as functions of the kink length can be adopted for fatigue life estimations of ultrasonic welds in lap-shear specimens of magnesium and steel sheets.

Ameliorating the mechanical properties of magnesium alloy: Role of texture

AZ31 magnesium alloy sheet was fabricated by two different approaches, namely conventional extrusion (CE) and differential speed extrusion (DSE), under the same process conditions and extrusion ratio. The microstructures, textures and mechanical properties of AZ31é magnesium alloy sheets processed by the CE and DSE processes were investigated. It was found that with the DSE process more uniform and finer microstructures were obtained. The DSE sheet exhibited the lower yield strength and yield ratio, but the larger n value combined with the lower r value, which made the DSE sheet achieve an excellent formability. Those results were mainly attributed to a tilted weak texture resulted from the differential flow speed of alloy at the severe deformation zone.

Improvement of formability of AZ31magnesium alloy by repeated roll bending process

Abstract Dispersion of the basal texture is effective to improve the formability. A repeated roll bending process (7 work rolls, work roll radius of 32 mm, roll pitch of 40 mm) was applied to AZ31 magnesium alloy sheets by activating the tensile twins. The larger applied bending strain per pass was effective to increase hardness at the surface of the sheet. The Erichsen value was slightly increased from 3.0 to 3.5 mm by subjected 2 passes repeated roll bending and subsequent at 180 °C for 12 hours annealing without decreasing the hardness of the sheet before bending (Hv=60). The Erichsen value of 3.3 and over hardness of Hv 65 through thickness were obtained by 2 passes bending and 160°C for 12 hours annealing. The tensile twins and texture randomizing were observed around center of thickness by electron backscatter diffraction (EBSD) analysis. It was possible to increase formability of AZ31 magnesium alloy by the repeated roll bending and subsequent low temperature annealing.

Joining of AZ31 Magnesium Alloy and Steel Sheet under Four Different Coating Conditions Based on Gas Metal Arc Weld-Brazing

Joining of AZ31 Magnesium Alloy and Steel Sheet under Four Different Coating Conditions Based on Gas Metal Arc Weld-Brazing

Study on Magnesium Alloy Thin Sheet by Symmetric Rolling and Asymmetric Rolling Process

Magnesium alloy is honored as green engineering material for its low density, high specific strength, high specific rigidity, well cutting processing property, well electromagnetic shielding property, heat conduction and easy to recycle. In this paper, AZ31 Magnesium alloy sheet at difference thickness were prepared by symmetric and asymmetric rolling employed with six-roller mill. Microstructure of the two kinds of rolling magnesium alloy thin sheets at 0.5mm thickness were investigated. The grain distribution of AZ31magnesium alloy sheets made by asymmetry rolling at room temperature are more uniform than those made by symmetry process. The grains made by asymmetry are more tiny and the tensile strength and elongation increased obviously and the mechanical properties got better. At room temperature, value of n increased. Large value of n benefit to stamping forming. At room temperature, the value of LDR of asymmetry rolling sheets is 1.26, which was higher than symmetry rolling. So asymmetry rolling benefits to stamping forming.

Effect of As-Rolled Microstructure on Static Recrystallization Characteristics and Texture Evolution during Annealing

Magnesium AZ31 alloy sheets were rolled at 100 °C at a high rolling speed of 1000 m/min. After 30% reduction, the microstructure was heavily twinned and shear banded, while a partially dynamically recrystallized and twinned microstructure was seen at the reduction of 49%. The as-rolled specimens were then annealed at 500 °C for increasing times. Microstructure and texture were characterized by optical microscopy, electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). Texture weakening was found during annealing of the specimens at both reductions. However, the texture weakening was more effective in the fully twinned and shear banded specimen than the partially DRXed and twinned specimen. Effects of as-rolled microstructure on static recrystallization characteristics and texture evolution during annealing were studied.

Effect of dimple patterning conditions of Periodical Straining Rolling on microstructures and mechanical properties of AZ31 sheets

Magnesium alloys are well-known for the attractive physical properties such as the lowest density among the structural metallic materials, high specific strength, high vibration absorption and so on. However, the strong (0001) basal texture of wrought magnesium alloys causes the poor plastic formability at room temperature in spite of high Lankford value. Texture and microstructure control of magnesium alloys are quite necessary to improve the cold sheet formability in order to prevail the wider application of wrought magnesium alloy products. The authors have already proposed the novel rolling process called “Periodical Straining Rolling (PSR)” using the small grooved roll to introduce periodically localized plastic strain in rolled sheet and confirmed that its effectiveness of the proposed rolling can realize the micro texture and microstructure control. This paper is aimed to discuss the effects of control of spatial strain distribution on microstructure, micro texture and mechanical properties by using various dimple patterning conditions of PSR process. The microstructure and micro texture observations of AZ31 magnesium alloy sheets PSR processed under the several dimple patterning conditions (two-sided PSR processing conditions) evince that the weak crystallographic orientations are largely prominent by changing dimple patterning conditions. As a result, the higher Erichsen values can be effectively achieved by changes in dimple patterning as compared to the conventional flat rolling process.

Theoretical prediction of forming limit diagram of AZ31 magnesium alloy sheet at warm temperatures

A theoretical prediction on forming limit diagram (FLD) of AZ31 magnesium alloy sheet was developed at warm temperatures based on the M–K theory. Two different yield criteria of von Mises and Hill’48 were applied in this model. Mechanical properties of AZ31 magnesium alloy used in the prediction were obtained by uniaxial tensile tests and the Fields–Backofen equation was incorporated in the analysis. In addition, experimental FLDs of AZ31 were acquired by conducting rigid die swell test at different temperatures to verify the prediction. It is demonstrated from a comparison between the predicted and the experimental FLDs at 473 K and 523 K that the predicted results are influenced by the type of yield criterion used in the calculation, especially at lower temperatures. Furthermore, a better agreement between the predicted results and experimental data for AZ31 magnesium alloy sheet at warm temperatures was obtained when Hill’48 yield criterion was applied.

Microstructure and formability evolutions of AZ31 magnesium alloy sheets undergoing continuous bending process

Continuous bending (CB) process along rolling direction was performed to improve the formability of AZ31 magnesium alloy sheets. The microstructure and texture evolutions were characterized by optical microscopy (OM) and electronic backscatter diffraction (EBSD). The results reveal that the basal texture intensity of continuously bent and annealed (CBA) sample is drastically weakened. A large number of twins are induced on the concave surface by the 1st pass bending and the density of twins obviously declines during the 2nd pass bending owing to the occurrence of detwinning. Due to the asymmetric tension–compression strain states between the outer and inner regions during V-bending, twinning and detwinning are generated alternatively during the CB process. The Erichsen value is 5.2 mm which increases by 41% compared with that of as-received sample. This obvious improvement of formability can be attributed to the weakened basal texture, which leads to a smaller plastic strain ratio (r-value) together with a larger strain-hardening exponent (n-value).

Tailoring the textures and mechanical properties of AZ31 alloy sheets using asymmetric composite extrusion

In the present study, an effective asymmetric composite extrusion (ACE) was used to produce AZ31 magnesium alloy sheets and compared with the composite extrusion (CE). The microstructures, textures and mechanical properties of AZ31 magnesium alloy sheets processed by CE and ACE processing at room temperature were investigated. The results demonstrated that the grains of the sheet fabricated by ACE processing were refined and the anisotropy could be improved in the tensile direction of ED, 45° and TD compared with the sheet fabricated by CE processing. The yield strength of the sheet processed by ACE was lower than that of the sheet processed by CE with the decrease of about 35MPa in the TD tensile direction, and its ductility was increased from 26.1% to 31.4% in the 45° tensile direction at room temperature. The higher n-values and lower r-values were obtained for the sheet processed by ACE processing in three tensile directions. These results led to a good formability of the sheet fabricated by ACE at room temperature. The improved mechanical properties of AZ31 Mg alloy sheet were mainly attributed to texture weakening resulting from asymmetric shear strain during ACE processing.

Correlation of static recrystallization and texture weakening of AZ31 magnesium alloy sheets subjected to high speed rolling

MagnesiumAZ31alloy sheets were rolled at a high speed of 1000m/min(HSR) and a low speed of 15m/min(LSR) at 100°C to a reduction of 30%. The as-rolled microstructures were heavily twinned and shear banded at both speeds. Annealing was performed on the as-rolled sheets at three different temperatures (200, 350 and 500°C) for increasing time intervals. Texture weakening was achieved after annealing of both HSRed and LSRed sheets due to static recrystallization (SRX) on twins and shear bands. This, in turn, greatly improved the ductility of the sheets. It was found that the intensities of the basal texture of the HSRed specimens were lower than those of the LSRed specimens at as-rolled and all annealed conditions. This is related to the higher amount of contraction twins and more <c+a> slips activated during HSR. Correlation of the maximum intensity of the basal texture with softening/recrystallization fraction was found to be involved in two weakening stages. During the first stage, nucleation on twins and shear bands was the dominant mechanism for texture weakening, while growth of the non-basal grains was responsible for the second stage of texture weakening. Interestingly, it was found that the weakest texture was attained after annealing at 500°C for 5min in the HSRed specimen, which can be related to thermal activation of more random nucleation sites and preferential grain growth of the non-basal oriented grains.

Experimental study for rubber pad forming process of AZ31 magnesium alloy sheets at warm temperature

In this paper, forming behavior of AZ31 sheet in rubber pad forming with warm temperature was investigated by experimental method. Rubber pad is always considered as it is temperature resistant. However, some types of rubbers can endure warm temperature, such as polyurethane for 120 °C and silica gel for 250 °C. Thus, forming for coupling with effects of temperature and flexible of rubber pad is proposed in this paper. The forming process of AZ31 sheet experienced bulging first and then gradually contacting with die, which was different from conventional forming. Fracture presented flat at room temperature and fracture became dentation obviously at 100 °C. When the temperature elevated to 200 °C, the fracture does not occur but the polyurethane was melted. The AZ31 sheet could fill the die successfully with silica gel as rubber pad at 200 °C. The effects of different load velocity for forming of AZ31 sheet were investigated. Thickness increased at bottom of deformed part, due to pressure stress acted from surrounding area. The thickness deceased at the bottom corner. Microstructure of deformed AZ31 sheet was also analyzed. It was indicated that dynamic recrystallization and twinning deformation mechanics were activated which had positive influence for improving formability of AZ31 sheet.

A cross-shear deformation for optimizing the strength and ductility of AZ31 magnesium alloys.

Magnesium alloys have recently attracted great interest due their lightweight and high specific strength. However, because of their hexagonal close-packed structure, they have few active slip systems, resulting in poor ductility and high mechanical anisotropy at room temperature. In the present work, we used a cross-shear deformation imposed by a differential speed rolling (DSR) technique to improve the room temperature strength and ductility of AZ31 magnesium alloy sheets. To introduce the cross-shear deformation, the sheets were rotated 180° around their longitudinal axis between the adjacent passes of DSR. The sheets of the AZ31 alloy subjected to the cross-shear deformation showed a uniform fine microstructure (1.2 ± 0.1 μm) with weak basal textures. The fabricated sheets showed a simultaneous high ultimate tensile strength and elongation-to-failure, i.e., ~333 MPa and ~21%, respectively. These were explained based on the structural features evolved due to the cross-shear deformation by DSR. The high strength was attributed to the uniform fine microstructure, whereas the high ductility was explained based on the basal texture weakening.

Microstructure and mechanical properties of cryorolled AZ31 magnesium alloy sheets with different initial textures

AZ31 magnesium alloy sheets with different strong textures were cryorolled at the liquid-nitrogen temperature to the strain of 4% and 8%. The microstructure and texture of the rolled sheets were investigated via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). The mechanical properties of the sheets were tested through in-plane uniaxial tensile tests at ambient temperature. The tensile stress was exerted in the rolling direction (RD) and transverse directions (TD). The microstructural and textural evolutions of the alloy during cryorolling were investigated. Due to active twining during rolling, the initial texture significantly influenced the microstructural and textural evolutions of the rolled sheets. A \(\left\{ {10\;\bar 12} \right\}\) extension twin was found as the dominated twin-type in the cryorolled samples. After cryogenic rolling, the ductility of the samples decreased while the strength increased. Twinning also played an important role in explaining the mechanical differences between the rolled samples with different initial textures. The samples were significantly strengthened by the high stored energy accumulated from cryorolling.

Cold Deep-drawing of AZ31 Magnesium Alloy Sheet under Controlled Forming Motion Using Servo Press Machine

The deep drawing of an AZ31 magnesium alloy sheet with a commonly used metal mold by three kinds of drawing motions (crank, vibration, and step-by-step motion) was performed, and the effect of drawing motions on the forming performances of the alloy sheet was investigated. For all the forming motions, the limited drawing ratio (LDR) reached was 1.25, and crack formation was limited at the flange part with the rolling direction. The vibration motion, which has a bigger shoulder region, cause greater lattice distortion of the side wall than other motions and the lattice strain for vibration motion increased from the bottom to the side wall of the sheet after drawing, whereas the heteromorphic histories for other motions were different. The crystallite size was largest at the bottom of the sheet for all the forming motions. However, it decreased at the shoulder part of the sheet and also at the side wall. The crystalline grains structure was not affected by drawing motions. It was found that the drawing motions affected the state of stress-relaxation after drawing.

Incorporation of SiC particles in FS welded zone of AZ31 Mg alloy to improve the mechanical properties and corrosion resistance

Abstract The solid-state nature of the friction stir welding (FSW) process leads to several advantages over fusion welding methods as problems associated with cooling from the liquid phase are avoided. This process eliminates many of the defects associated with fusion welding techniques such as shrinkage, solidification cracking and porosity. In general, FSW has been found to produce a low number of defects and is very tolerant of variations in parameters and materials. Nevertheless, physical and chemical properties of the weld zone developed during FSW are a primary concern. In this study, the mechanical properties and corrosion behavior of the weld zone resulting after FSW are investigated and improved by addition of SiC particles. AZ31 magnesium alloy sheets were FS welded and simultaneously processed by incorporation of SiC particles. Mechanical properties and corrosion behavior of FS welded specimens were compared with those of FS welded and processed specimens. The results showed that the addition of SiC particles during FSW considerably improved the mechanical properties and corrosion resistance of the weld zone. Based on the obtained results, incorporation of SiC hard particles in the microstructure of weld zone produced by FSW is highly recommended.

Influence of Extrusion Joint on Microstructural Evolution and Properties of Mg Alloy Sheet

The mechanical behaviour of AZ31 magnesium alloy sheet with the extrusion joint (EJ) was evaluated. Extruded joint of AZ31 alloy sheets was obtained by the hot extrusion process. Tensile tests were carried out along the extrusion direction at room temperature, and both the non-uniform plastic deformation and the fracture behaviour were studied. It is found that the samples with EJ present significantly deteriorated mechanical properties compared with the EJ-free counterpart. Inhomogeneous microstructure distribution around EJ zone brings in the uncoordinated deformation due to the high density of \(\{ 10\bar{1}2\}\) twins which were readily activated during plastic deformation.