Electroplastic effect in AZ31B magnesium alloy sheet through uniaxial tensile tests

The electroplastic effect in AZ31B magnesium alloy sheet was investigated through uniaxial tensile tests. In order to show the athermal effect of the electrical pulses, two types of uniaxial tensile tests at the same testing temperature were carried out： uniaxial tension in environmental cabinet and uniaxial tension with electrical pulses. In addition, the distribution of temperature field in the cross-section area during uniaxial tension with electrical pulses was simulated. The results show that the distribution of temperature field along the cross-section area is homogeneous. By comparing the true stress？true strain curves of AZ31B alloy under uniaxial tensile tests, the athermal effect with electrical pulses was confirmed. The microstructure evolution after the uniaxial tension was studied by optical microscopy. The results indicate that the electrical pulses induced dynamic recrystallization plays an important role in the decrease of flow stress. Finally, a flow stress model of AZ31B sheet taking the influence of electroplastic effect into account was proposed and validated. The results demonstrate that the calculated data fit the experimental data well.

Influence of strain rate on microstructure and formability of AZ31B magnesium alloy sheets

The effects of strain rate on microstructure and formability of AZ31B magnesium alloy sheets were investigated through uniaxial tensile tests and hemispherical punch tests with strain rates of 10^-4, 10^-3, 10^-2, 10^-1 s^-1 at 200℃. The results show that the volume fraction of dynamic recrystallization grains increases and the original grains are gradually replaced by recrystallization grains with the strain rate decreasing. A larger elongation and a smaller r-value are obtained at a lower strain rate, moreover the erichsen values become larger with the strain rate reducing, so the formability improves. This problem arises in part from the enhanced softening and the coordination of recrystallization grains during deformation.

Analysis of Hot Rolling Routes of AZ31B Magnesium Alloy and Prediction of Tensile Property of Hot-rolled Sheets

At the initial rolling temperature of 250 to 400 ℃, AZ31B magnesium alloy sheets were hot rolled by four different rolling routes. Microstructures and mechanical properties of the hot-rolled magnesium alloy sheets were analyzed by optical microscope and tensile tests respectively. Based on the Hall-Petch relation, considering the average grain size and grain size distribution, the nonlinear fitting analysis between the tensile strength and average grain size was carried on, and then the prediction model of tensile strength of hot-rolled AZ31B magnesium alloy sheet was established. The results indicate that, by rolling with multi-pass cross rolling, uniform, fine and equiaxial grain microstructures can be produced, the anisotropy of hot-rolled magnesium sheet can also be effectively weakened. Strong correlation was observed between the average grain size and tensile property of the hot-rolled magnesium alloy sheet. Grain size distribution coefficient d（CV） was introduced to reflect the dispersion degree about a set of grain size data, and then the Hall-Petch relation was perfected. Ultimately, the prediction accuracy of tensile strength of multi-pass hot-rolled AZ31B magnesium alloy was improved, and the prediction of tensile property can be performed by the model.

Effects of cold and warm cross-rolling on microstructure and texture evolution of AZ31B magnesium alloy sheet

Mg alloys conventionally rolled often present strong basal textures that affect negatively further deformations,limiting their applications.The present research found that cross-rolling experiences in adequate conditions can weaken those intense basal textures as a result of the interaction of deformation mechanisms and dynamic recrystallization.The effects of rolling temperature and strain rate on the microstructure and texture of an AZ31B magnesium alloy sheet generated heterogeneous microstructure where the initial basal texture was strengthened during cold cross-rolling and it was gradually weakening by the rolling reduction and the rolling temperature increases in such a way that a rather weak basal fiber was produced applying reductions higher than 15%at temperatures higher than 200℃.Their ODF functions supported the texture weakening,exhibiting a combination of two crystallographic orientations represented by{0001}<211^(-)0>and{0001}<101^(-)0>.

Temperature evolution and fatigue life evaluation of AZ31B magnesium alloy based on infrared thermography

The surface temperature of extruded AZ31B alloy plate was measured by infrared thermograph in air during tension and high-cycle fatigue tests. The mechanism of heat production was discussed and the value of critical fatigue damage temperature was calculated according to the P—ΔT curve. Results show that the variation trend of temperature is different between tension and fatigue tests. The temperature evolution in tension test consists of four stages： linear decrease, reverse linear increase, abrupt increase, and final drop. The initial decrease of temperature is caused by thermal elastic effect, which is corresponding to the elastic deformation in tension progress. When cyclic loading is above the fatigue limit, the temperature evolution mainly undergoes five stages： initial increase, steep reduction, steady state, abrupt increase, and final drop. The peak temperature in fatigue test is caused by strain hardening that can be used to evaluate the fatigue life of magnesium alloy. The critical temperature variation that causes the fatigue failure is 3.63 K. When ΔT≤3.63 K, the material is safe under cyclic loading. When ΔT3.63 K, the fatigue life is determined by cycle index and peak temperature.

Fatigue fracture mechanism of AZ31B magnesium alloy and its welded joint

Fatigue test was carried out on AZ31B magnesium alloy. Under 2×10 6 cycle times, the fatigue strengths of base metal （BM）, butt joint （BJ）, transverse cross joint （TJ）, lateral connection joint （LJ） are 66.72, 39.00, 24.38 and 24.40 MPa, respectively. The crack propagation behavior of the alloy was analyzed by optical microscopy. The AZ31B magnesium alloy base metal has a smooth crack propagation macroscopic path. However, the microscopic path is twisted and some cracks have two forks, and the crack propagation is transgranular. The crack initiates in the weld toe and the crack propagates along the HAZ for the BJ and TJ; for the LJ crack initiates in the fillet weld leg. The fatigue fracture mechanisms were analyzed by SEM. The fatigue fracture surface consists of quasi-cleavage patterns or cleavage step and a brittle fracture occurs. Numerous secondary cracks are observed; some fatigue striations exist in butt joint and its size is about 5 μm.

Welding characteristics of AZ31B magnesium alloy using DC-PMIG welding

Direct current pulsed metal inert-gas （DC-PMIG） welding was carried out on AZ31B magnesium alloy to obtain continuous welding joints of 3 mm and 8 mm thickness with 1.6 mm diameter of filler wire. The behavior and mechanism of metal transfer, the parameter ranges of stable welding process, the microstructure and mechanical properties of magnesium alloy were investigated. The results show that the metal transfer form of magnesium alloy using DC-PMIG welding is realized in modes of globular transfer, projected transfer and spray transfer. Welding spatter with a large size will be produced in the mode of globular transfer, and high-quality joints with few spatters can be obtained in the mode of projected transfer and spray transfer when the linear energy of filler wire is 242-27l J/cm, droplet diameter is 1.6-0.9 mm tensile strength of weld beads is 94.2% of that of base metals. and transition frequency is 30-69 Hz. The average ultimate

Sliding wear behaviour of AZ31B magnesium alloy and nano-composite

AZ31B magnesium alloy and nano-composite were manufactured by hybrid casting process and hot extruded at 350 °C. The sliding wear behaviour of alloy and nano-composite was estimated at room temperature using the standard pin-on-disc wear test equipment. The tests were conducted under a normal load of 10 N at different sliding speeds ranging from 0.60 to 1.2 m/s for distance up to 2000 m. The wear mechanisms of the worn out surface were studied using SEM analysis. The influence of test parameters on wear rate of the pins was established using a linear regression model statistically. Compared with the AZ31B magnesium alloy, the nano-composite shows lower wear rates due to higher hardness improvement caused by the reinforcement. The wear mechanism appears to be a mix-up of ploughing, rows of furrows, delamination and oxidation.

Microstructure and mechanical properties of AZ31B magnesium alloy prepared by solid-state recycling process from chips

AZ31B magnesium alloy chips were recycled by three solid-state recycling processes including cold-pressing,hot-pressing followed by hot extrusion and double extrusion.Microstructure and mechanical properties of the recycled specimens and reference specimens were compared.For the recycled specimen by cold-pressing,the grains are refined to a large extent during hot extrusion due to the presence of twins and high density dislocation.The recycled specimens by hot-pressing and double extrusion do not exhibit finer grain than that the recycled specimen by cold-pressing.Consequently,higher ultimate tensile strength of the recycled specimen by hot-pressing and double extrusion is not achieved.For hot pressing process,more compact billet lowers the porosity in recycled material,so elongation to failure of the recycled specimen increases.The recycled specimen fabricated by double extrusion process shows slightly higher elongation than the reference specimen.The second extrusion makes the oxides further crush and distribute more dispersedly,and minimizes porosity,which is responsible for the improved ductility.

Effect of laser power on microstructure and mechanical properties of laser heat conduction lap welded joint between AZ31B magnesium alloy and DP780 galvanized steel

In this work, laser heat conduction lap welding(LHCLW) of AZ31B magnesium alloy sheet and DP780galvanized steel sheet was carried out by the defocused laser beam. The effects of laser power on the microstructure and mechanical properties of the joint were studied. The pros and cons of the joint were identified and evaluated by measuring the tensile shear strength, microhardness and microstructure observation. The formation mechanism of various phases at the Mg/steel interface was analyzed. The results indicated that the galvanized layer could promote the metallurgical bonding between magnesium alloy and steel by improving the diffusion ability of molten magnesium alloy at the steel interface and reacting with Mg, so as to enhance the strength of the joint. A continuous dense layered eutectic structure(α-Mg+MgZn) was formed at the interface of the joint, while MgZn_(2)and MgZn phase was formed at the weld edge zone and heat affective zone(HAZ), whereas no reaction layer was generated between the uncoated steel and magnesium alloy. A sound joint could be obtained at 2.5 kW, and the corresponding tensile shear strength reached the maximum value of 42.9 N/mm. The strength was slightly reduced at 2.6 kW due to the existence of microcracks in the eutectic reaction layer.

As-extruded AZ31B Magnesium Alloy Fatigue Crack Propagation Behavior

The fatigue crack propagation rate of as-extruded AZ31B magnesium alloy was studied. Compact tension [C（T）] of the notch direction parallel （T-L）, vertical （L-T）, and inclined at 45o to the extrusion direction was investigated. The experimental results indicate that the crack propagation direction is parallel to the extrusion direction for T-L and L-T specimens, whereas the specimen inclined at 45o has an angular deflection of 9° to 11° toward the extrusion direction. The T-L specimen has the fastest fatigue crack propagation rate, and the L-T specimen has the slowest rate, the fatigue crack propagation rate of the specimen inclined at 45o is between the two directions. The crack tip propagates by both transgranular and intergranular fractures. Fatigue fractures consist of cleavage plane or quasi-cleavage and are brittle fractures. The fatigue striation occurs for specimens inclined at 45o and its size is 3-15 μm.

Investigation on Two Electrically-Assisted Forming Processes of AZ31B Magnesium Alloy Sheets

Two new types of electrically-assisted forming(EAF) processes of AZ31B magnesium alloy sheets,i.e., electrically-assisted blanking(EAB) and electrically-assisted sheet upset-extruding(EASUE), are proposed in this paper. During EAB, pulsed current(PC) decreases blanking load and improves blanked surface quality,and higher frequency has a more effective improvement;when the frequency reaches 600 Hz, the whole punched hole fracture is improved to smooth zone. Moreover, EAB does not influence the microstructure and does not induce obvious annealing, and this process is advantageous to the materials that cannot change microstructure.During EASUE, PC can reduce extrusion load and enhance extrusion height, and higher frequency also has a more effective improvement. Both processes indicate that introducing PC can effectively improve the part quality and have an application prospection.

Experimental Exploration of Electrical Pulse-Assisted Roller Hemming of AZ31B Magnesium Alloy Sheet

The purpose of this research is to explore the feasibility of roller hemming of AZ31 B magnesium alloy sheet. During the test, a special experimental setup considering the current flow and insulation was designed and employed. For the sake of simplicity, the flat surface-straight edge hemming style was selected. Electrical pulses were introduced in the pre-hemming and final hemming processes. The results show that when hemming at 448 K with hemming speed of 30 mm/min, AZ31 B magnesium alloy sheet was successfully hemmed with excellent perceived quality. No recoil/warp defect, cracking or springback was observed. Electrical pulses induced dynamic recrystallized grains were observed along the grain boundaries in the severely deformed area, which is of great importance to the improvement of the ductility of the material. The hardness was slightly increased after hemming with electrical pulses, and this fact could improve the friction resistance performance of the hemmed part.

Research on Thermal Deep-drawing Technology of Magnesium Alloy (AZ31B) Sheets

Forming technology of Mg alloy (AZ31B) sheets can be investigated by thermal deep drawing experiments. In the experiments, the blank holder and die contacting with the blank were heated to the same temperature as the blank by using the heating facility. The circular blank heated in an oven is formed at a temperature range of 100~400 ℃ to obtain the optimum forming temperature range and the effects of major technical parameters on the workpiece quality. It is found that the blank is brittle at temperatures lower than 200℃. Temperatures higher than 400℃ are not suitable for forming of the sheets because of severe oxidation and wrinkling. AZ31B shows an excellent formability at temperatures from 300 to 350℃ and can be formed into a workpiece with good quality. When the blank holder force is 9 kN, extruded sheets with a thickness of 1 mm can be formed into cups without wrinkling. Workpieces show strong anisotropic deformation behavior on the flanges.

Microstructure and properties of alloying coating on AZ31B magnesium alloy

Vacuum thermal diffusion technique was applied to preparing alloying coating on AZ31 B magnesium alloy. The microstructure and phase composition of the coatings prepared at different holding time were investigated in detail using optical microscopy（OM）, scanning electron microscopy（SEM）, energy dispersive spectrometer（EDS） and X-ray diffraction（XRD）, and so on. The microhardness tester and electrochemical workstation（PS-168a） were used to measure the microhardness and corrosion resistance of the alloying coating. The results showed that the alloying coatings gradually generated with the extension of holding time under constant temperature. And the obvious bonding interface between the coating and substrate was observed, and the bonding interface was changed from smooth to zigzag. EDS and XRD analyses showed that the microstructure of alloying coating mainly consisted of eutectic α-Mg phase and continuous network β-Al（12）Mg（17） phase. The average microhardness of the coatings increased by 113% in comparison to the substrate, and the self-corrosion potential increased from-1.389 to-1.268 V at the same time.

Deformation Mechanism of AZ31B Magnesium Alloy Under Different Loading Paths

Deformation and texture evolution of AZ31 B magnesium（Mg） alloy sheet under uniaxial tension, compression, and reverse loading after different pre-strain（compression and tension） were investigated experimentally. The results indicate that the pre-compressive strain remarkably affects the reverse tensile yield stress and the width of the detwinning-dominant stage during inverse tension process. Similar to stress–strain curve of the uniaxial compression, the curve of reverse tensile yield value also has ‘S＇ shape, and its minimum value is only 38 MPa. The relationship between pre-compressive strain and the width of detwinning-dominant stage presents a linear growth, and the greater the precompressive strain is, the smaller the strain hardening rate of the detwinning-slip-dominant stage is. Compared with the reverse tension under pre-compression, the influence of the pre-tension deformation on the deformation mechanism of subsequent compression is relatively simple. With the increase in pre-tension strain, the yield stress of the reverse loading is rising.

Finite Element Analysis and Experiment on Viscous Warm Pressure Bulging of AZ31B Magnesium Alloy

Aiming at overcoming the low plasticity of magnesium alloy at room temperature, we researched viscous warm pressure bulging（VWPB） of AZ31B magnesium alloy based on the excellent thermal stability of viscous medium under the warm forming condition. The potential improvements of plastic deformation ability and forming quality of AZ31B magnesium alloy are expected with the aid of thermal characteristics of viscous medium. During bulging process the velocity field variation and pressure stress field distribution of viscous medium are observed at different temperatures through which the effect of temperature on the mechanical property of viscous medium and AZ31B magnesium alloy are analyzed. The results show that the formability of AZ31B magnesium alloy increases first and then decreases as the temperature increases and it is the best at 200 ℃. On the other hand, the viscous medium which can build non-uniform pressure stress field also exhibits a good flow property at elevated temperature, and it is helpful to improving the formability of AZ31B magnesium alloy.

Optimization of pulsed current gas tungsten arc welding process parameters to attain maximum tensile strength in AZ31B magnesium alloy

An empirical relationship to predict tensile strength of pulsed current gas tungsten arc welded AZ31B magnesium alloy was developed. Incorporating process parameters such as peak current, base current, pulse frequency and pulse on time were studied. The experiments were conducted based on a four-factor, five-level, central composite design matrix. The developed empirical relationship can be effectively used to predict the tensile strength of pulsed current gas tungsten arc welded AZ31B magnesium alloy joints at 95% confidence level. The results indicate that pulse frequency has the greatest influence on tensile strength, followed by peak current, pulse on time and base current.

Effect of welding speed on microstructural characteristics and tensile properties of GTA welded AZ31B magnesium alloy

The effect of welding speed on tensile and microstructural characteristics of pulsed current gas tungsten arc welded（PCGTAW） AZ31 B magnesium alloy joints was studied. Five joints were fabricated using different levels of welding speeds（105-145 mm/min）. It was found that the joints fabricated using a welding speed of 135 mm/min yielded superior tensile properties compared to other joints. The formation of fine grains and uniformly distributed precipitates in the fusion zone are the main reasons for the superior tensile properties of these joints.

Wrinkling behavior of magnesium alloy tube in warm hydroforming

In tube hydroforming with axial feeding,under the effect of coupled internal pressure and axial stress,wrinkles often occur and affect the forming results.Wrinkling behavior of an AZ31B magnesium alloy tube was experimentally investigated with different loading paths at different temperatures.Features of wrinkles,including shape,radius and width,were acquired from the experiments,as well as the thickness distribution.Numerical simulations were carried out to reveal the stress state during warm hydroforming,and then the strain history of material at the top and bottom of the wrinkles were analyzed according to the stress tracks and yielding ellipse.Finally,effects of loading paths on expansion ratio limit of warm hydroforming were analyzed.It is verified that at a certain temperature,expansion ratio limit can be increased obviously by applying a proper loading path and realizing enough axial feeding.