Papers by Keyword: AZ31 Magnesium Alloy

Abstract: AZ31 magnesium alloy is a representative magnesium alloy with well-balanced mechanical properties and castability. However, AZ31 magnesium alloy also has the disadvantage of poor corrosion resistance due to its low aluminium content. In previous research, it is known that the corrosion mechanism is such that filamentous corrosion is generated and then changes to full-scale corrosion. However, the relationship between corrosion and bending properties has not been revealed. In this study, AZ31 magnesium alloy was immersed in salt water with a concentration of 5%, and three-point bending tests were conducted to confirm changes in bending stress and strain due to strength loss caused by corrosion. Then, investigated which parameters Ra and Rz, which are indicators of surface roughness, are related to the maximum bending stress of the fully corroded AZ31 magnesium alloy. As a result, when evaluating the relationship between maximum bending stress and corrosion, it was found that it is better to evaluate by Ra of the corroded surface rather than by Rz.

Abstract: Magnesium alloys have the advantages being lightweight and high recyclability. On the other hands, it is thought that magnesium has the disadvantage of poor plastic workability at room temperature due to its crystal structure. Especially, in pipe materials, winkles occur on the compressed side during bending. We aim to improve the bending workability in magnesium alloy pipe by torsion and back-torsion. In this study, tensile and compressive tests using specimens of pipes processed by torsion and back-torsion showed reduction the difference of yield stress. Microstructural observation of processed pipes revealed reducing crystal grain size and forming deformation twinning. Vickers hardness tests shows increasing hardness by torsion and back-torsion. Moreover, bending tests showed decreasing flatting ratio by torsion and back-torsion. These results demonstrated that torsion and back-torsion have effect of improvement in bending workability for magnesium alloy pipes.

Abstract: The effect of the applied state-of-stress on the processing maps depicting the mechanisms for hot working of hot extruded Mg-3Al-1Zn alloy has been evaluated. Flow stresses at various temperatures in the range 300 – 500 °C and strain rates in the range 0.0003 – 1 s^-1 have been measured by deforming in compression and in tension. Processing maps have been developed from the respective flow stress data at a strain of 0.1. The maps are essentially similar irrespective of the mode of deformation – compression or tension, and exhibited two domains in the temperature and strain rate ranges: (1) 375 – 500 °C and 0.0003 – 0.01 s^-1, and (2) 450 – 500 °C and 0.1 – 1 s^-1. On the basis of slower strain rates, high tensile ductility, and the apparent activation energy (152 kJ/mole closer to that for self-diffusion), Domain #1 is interpreted in terms of the occurrence of climb controlled dynamic recrystallization. In Domain #2, which occurs at higher strain rates and has an apparent activation energy near to 165 kJ/mole, dynamic recrystallization occurs that involves second order pyramidal slip {11-22} <11-2-3> and recovery by cross-slip of screw dislocations. The state-of-stress imposed on the specimen (compression or tension) does not have any significant effect on the processing maps or the kinetics of hot deformation.

Abstract: Ellipsometry is a non-destructive and indirect technique able to characterize both optical and dielectric properties of thin films and bulks, besides determining the thickness of thin films. This characterization is performed by evaluating the change in the polarization state of the incident light when it interacts with the material of interest. In this work, the ellipsometry technique was used to characterize optical properties of AZ31 magnesium alloy samples. This alloy has several interesting properties such as low density, high thermal conductivity, good machinability, among others, which makes it suitable for use in automotive and aerospace components. However, when this alloy is exposed to ambient atmosphere, it undergoes natural oxidation, developing a surface film of oxides and/or hydroxides. This study aimed to establish an efficient methodology for accessing the optical and dielectric characteristics of the substrate (AZ31 alloy) as well as those for the surface film of oxide/hydroxides, and to obtain the thickness of this film. Four samples mechanically grinded and polished were investigated: One sample, namely P sample, was subjected to the ellipsometric measurements immediately after grinding and polishing and three samples, namely A10, A50 and A60, were exposed to the air at 150°C during 10, 50 e 60 minutes, respectively. From the results of the ellipsometric measurements for P sample, it was possible to determine the refractive index (n) and extinction coefficient (k) curves as a function of the wavelength (λ) for the substrate (AZ31 alloy). Besides, through appropriate modeling, it was possible to determine the thicknesses of the oxides/hydroxides films presented in A10, A50 and A60 samples. The thicknesses values obtained seem to be quite coherent when we analyze the surface roughness of these samples using the confocal microscope, validating the optical model constructed to represent the A10, A50 and A60 samples. The results achieved in this study can contribute to the study of both of oxides/hydroxides layers growth and protective coating films growth for AZ31 magnesium alloy.

Abstract: Hybrid superplastic forming (SPF) is a novel sheet metal forming technique that combines hot drawing with gas forming process. Compared with the conventional SPF process, the thickness distribution of AZ31B part formed by this hybrid SPF method has been significantly improved. Additionally, the microstructure evolution of AZ31 was examined by electron backscatter diffraction (EBSD). Many subgrains with low misorientation angle were observed in the coarse grains during SPF. Based on the tensile test results, parameters of hyperbolic sine creep law model was determined at 400 oC. The hybrid SPF behavior of non-superplastic grade AZ31B was predicted by ABAQUS using this material forming model. The FEM results of thickness distribution, thinning characteristics and forming height were compared with the experimental results and have shown reasonable agreement with each other.

Abstract: The mechanical properties such as stress-strain curves and anisotropic parameters at different elevated temperatures are obtained by the computerized screw universal testing machine. The friction testing machine is used to determine the friction coefficient between die and AZ31 sheets at different elevated temperatures. The finite element method is used to investigate the earing of the deep drawing process. In order to verify the prediction of FEM simulation of the earing in the cylindrical cup drawing process, the experimental parameters such as stress-strain curves, anisotropic parameters, fiction coefficient and blank holder force, are as the input data during analysis. The experimental cup height compared with the current simulation result of cylindrical deep drawing process at different elevated temperature.

Abstract: Metal forming of magnesium alloys often performed at elevated temperature, because magnesium alloys exhibit peculiar stress-strain relation and inferior ductility compared to conventional metals at room temperature. In the present study, deformation behavior and formability of cast and extruded AZ31 magnesium alloys under uniaxial and biaxial compressions at room temperature and at elevated temperatures were investigated. The results revealed that the compressive stress-strain relation of AZ31 magnesium alloy changed not only with the initial texture but also with the deformation temperature. The temperature dependency of flow stress of the cast alloy was smaller than that of the extruded alloy probably because of less influence of pre-deformation. In addition, the influence of compressive deformation pattern upon flow stress of the extruded alloy remained even at elevated temperature to 523 K. The temperature dependency of compressive fracture was also discussed and it was found that the equi-biaxial condition improved the compressive formability at elevated temperatures.

Abstract: A numerical simulation on the deformation behavior of AZ31 magnesium alloy during backward extrusion with counter pressure was investigated by FE software DEFORM. The results show that the steady load increases nonlinearly with the increment of counter pressure. The equivalent strain gradient decreases significantly results from the counter pressure and the unevenness in the top of wall disappeared approximately when the counter pressure is 10~15MPa. Furthermore, there are obviously shear band along the inner fillet to outer fillet of workpiece. Higher hydrostatic pressure generated with counter pressure compared that of no counter pressure leads to improve the plastic deformation limit. The damage factor is reduced significantly in the backward extrusion with counter pressure, which is beneficial to the improvement of crack.

Abstract: In this study, the deformation process of AZ31 magnesium alloy during equal channel angular pressing (ECAP) was simulated using the commercial software Deform-3D under different extrusion condition (passes and temperatures). To investigate the effect of temperature and deformation rate on grain refinement, the rules of flow and deformation homogeneity and also the extrusion load during ECAP was discussed. The simulation results indicate that the AZ31 magnesium alloy obtain homogenous and larger strain magnitude after 4 passes ECAP at 250°C~275°C. To verify the 3D finite element simulation results, the microstructure in the cross-section was observed. It shows that the grain of AZ31 magnesium alloy is homogenous refined by finite element method (FEM) results, thus the mechanical property is improved.

Abstract: AZ31 magnesium alloy was friction stir spot welded in air and cooling in water. The effect of the enhanced cooling rate on the microstructure and mechanical properties of the joint was analyzed. The results showed that flowing water had obvious cooling effect instantaneously, which significantly restrained the growth of dynamic recrystallized grains. The average grain size in stir zone was 1.3μm in cooling water condition, which is far smaller than that of the joint prepared in air cooling condition. Under the condition of enhanced cooling, the microhardness in stir zone significantly increased, the ultimate tensile load (~ 3.99kN) increased by 15.7%, and the tensile deformation value (~ 3.65 mm) increased by 62.2%. Dimples in SEM fracture morphologies indicated the better plastic deformation capacity of joints prepared by cooling water, which failed through a mixture mode of ductile and brittle fracture.