Tianjiao Luo

Effect of Bi Addition on Microstructures and Mechanical Properties of AZ80 Magnesium Alloy

The effects of Bi addition on the microstructures and mechanical properties of as-cast AZ80 alloy were investigated. The results show that with the addition of Bi, the coarse eutectic phases are refined and become discontinuous; some flaky and granular Mg(3)Bi(2) phases with a hexagonal structure of D5(2) are observed along the grain boundaries and between dendrites. The tensile strength and elongation increase first, and then decrease with increasing Bi content. AZ80-0.5% Bi alloy has optimum combination mechanical properties. When the content of Bi is above 1.0% ( mass fraction), the amount of flaky Mg(3)Bi(2) phase increases markedly, which splits the matrix and deteriorates the tensile strength and elongation.

High cycle fatigue properties of die-cast magnesium alloy AZ91D-1%MM

The high cycle fatigue properties of the die-cast magnesium alloy AZ91D containing 1% mischmetal (mass fraction) at a fatigue ratio of 0.1 were investigated. The difference in the microstructure between the skin and core region of the die-cast magnesium alloy was analyzed by optical microscopy. The mechanical property tests indicate that the values of the tensile strength, elongation and hardness are 185 MPa, 1.5% and HBS 70 +/- 3 at room temperature, respectively. The p-S-N curve (p=50%) of the die-cast magnesium alloy AZ91D-1%MM is determined and the mean fatigue strength corresponding to 3.8 X 10(5) cycles is 70 MPa. A linear relation between S and N(p) in log scale between 10(3) and 10(6) cycles is written with a equation. The mechanical properties are influenced by the casting defects. The fatigue life of the samples with minor defects is near to the upper limit of the fatigue life data. The fatigue fracture surface of the samples with minor defects possesses the mixed characteristics of quasi-cleavage, lacerated ridge and dimple and it is brittle fracture mode as a whole.

Hot deformation behavior and processing maps of as-cast Mg–8Zn–1Al–0.5Cu–0.5Mn alloy

Abstract The hot deformation behavior of as-cast Mg–8Zn–1Al–0.5Cu–0.5Mn alloy was studied by hot compression tests at temperatures of 200–350 °C and strain rates of 0.001–1 s −1 . The results show that the flow stress increases significantly with increasing strain rate, and decreases as the temperature increases. The flow stress model based on the regression analysis was developed to predict the flow behavior of Mg–8Zn–1Al–0.5Cu–0.5Mn alloy during the hot compression, and the model shows a good agreement with experimental results. Meanwhile, the processing maps were established according to the dynamic materials model. The processing maps show that the increase of strain enlarges the instability domains, and the alloy shows good hot workability at high temperatures and low strain rates.

Effects of trace element and purification on properties of AZ80 magnesium alloy

Abstract The effects of trace element Fe on the corrosion behavior of AZ80 magnesium alloy were investigated by salt spray test and electrochemical measurements. The results show that the corrosion rate decreases with decreasing the trace element Fe content in an approximately linear relation even though the amount of trace element Fe reduces to 0.000 2% (mass fraction). The electrochemical measurements show that the corrosion potential (φ corr ) of the alloy with lower trace element Fe content shifts to less negative value. It is suggested that the control trace element by purification is an effective way to enhance the corrosion resistance of AZ80 magnesium alloy.

Hot compression behavior and deformation microstructure of Mg-6Zn-1Al-0.3Mn magnesium alloy

Abstract The hot compression behavior of a wrought Mg-6Zn-1Al-0.3Mn magnesium alloy was investigated using Gleeble test at 200–400 °C with strain rates ranging from 0.01 to 7 s−1. The true stress-strain curves show that the hot deformation behavior significantly depends on the deformation temperature and strain rate. The calculated hot deformation activation energy Q is 166 kJ/mol with a stress exponent n=5.99, and the constitutive equation is deduced to be ɛ ˙ =3.16×1013[sinh(0.010σ)]5.99 exp [−1.66×105/(RT)]. Deformation microstructure shows that the incompletely dynamically recrystallized grains can be found at grain boundaries and twins with the strain rates ranging from 0.01 to 1 s−1 at 250 °C, and completely dynamic recrystallization occurs when the temperature is 350 °C or above during hot compression, the size of recrystallized grains decreases with the increment of the strain rate at the same temperature. The relatively suitable deformation condition is considered temperature 330–400 °C and strain rate of 0.01–0.03 s−1, and temperature of 350 °C and strain rate of 1 s−1.

Fatigue behavior of friction stir spot welded AZ31 Mg alloy sheet joints

Abstract The fatigue behavior of friction stir spot welded (FSSW) AZ31 magnesium alloy sheet joints was investigated by tension–compression of fatigue test. The results suggest that all the fatigue failures occur at the stir zone of the FSSW AZ31 sheet joints, and all cracks initiate at the stir zone outer edge between the upper and lower sheet. When the cycle force equals 1 kN, the crack propagates along the interface of heat-affected zone and thermo-mechanical zone, simultaneously across the direction of force; while the cycle force equals 3 kN, the crack propagates along the diameter of stir zone and shear failure occurs finally. Moreover, the transverse microsections indicate that there is a tongue-like region at the outer edge of stir zone between the two AZ31 sheets, and the direction of tongue-like region is toward outside of the stirred zone and all fatigue cracks initiate at the tongue-like region.

As-cast structure and tensile properties of AZ80 magnesium alloy DC cast with low-voltage pulsed magnetic field

The effects of a low-voltage pulsed magnetic field on the solidified structure and mechanical properties of DC casting AZ80 magnesium alloy were investigated. The results showed that the solidified structure of the DC casting AZ80 magnesium alloy was refined obviously by the low-voltage pulsed magnetic field and significant grain refinement in the DC casting ingot of AZ80 magnesium alloy was achieved. Meanwhile, the morphology of the dentritic in the DC casting ingot was transformed from coarse dentritic to fine rosette with the application of low-voltage pulsed magnetic field. The ability of deformation of the ingot was enhanced and especially the plasticity of the ingot center after upsetting was improved greatly by more than 80% after deformation.

Effects of extrusion and heat treatments on microstructure and mechanical properties of Mg–8Zn–1Al–0.5Cu–0.5Mn alloy

Abstract The effects of extrusion and heat treatments on the microstructure and mechanical properties of Mg–8Zn–1Al–0.5Cu– 0.5Mn magnesium alloy were investigated. Bimodal microstructure is formed in this alloy when it is extruded at 230 and 260 °C, and complete DRX occurs at the extruding temperature of 290 °C. The basal texture of as-extruded alloys is reduced gradually with increasing extrusion temperature due to the larger volume fraction of recrystallized structure at higher temperatures. For the alloy extruded at 290 °C, four different heat treatments routes were investigated. After solution + aging treatments, the grains sizes become larger. Finer and far more densely dispersed precipitates are found in the alloy with solution + double-aging treatments compared with alloy with solution + single-aging treatment. Tensile properties are enhanced remarkably by solution + double-aging treatment with the yield strength, tensile strength and elongation being 298 MPa, 348 MPa and 18%, respectively. This is attributed to the combined effects of fine dynamically recrystallized grains and the uniformly distributed finer precipitates.

Microstructure and mechanical properties of as-cast Mg-4Zn-0.5Zr-0.2Cu-0.2Ce alloy

ABSTRACT In this study, an approach is proposed to improve the microstructure and mechanical properties of Mg-4Zn-0.5Zr alloy by combining trace Cu and rare earth Ce addition. The results showed that Cu and Ce additions led to obvious grain refinement and the formation of Mg-Zn-Cu and Mg-Zn-Ce phases. The Mg-Zn-Ce phase was identified to have an orthorhombic structure. The length of the [0001]α rods in the Cu-containing alloys remarkably decreased. The yield strength increased slightly after Cu and Ce co-addition, which was attributed to grain refinement and precipitation strengthening. The coarse Mg-Zn-Ce phase distributed at the grain boundaries would reduce the ductility by promoting crack propagation during tensile processes.

Direct chill casting of magnesium alloy under pulsed magnetic field

To refine the grain size and improve the hot workability, the billets of AZ80 magnesium alloy were cast by a new process of the pulsed magnetic field-direct chill casting. The effect of pulsed magnetic field on the grain size, segregation and mechanical properties of cast billet were investigated by experiments. The results indicate that the grain size of cast billet with pulsed magnetic field was greatly refined, and the homogeneity of microstructure was improved significantly. Meanwhile, the macrosegregation and microsegregation of main alloying elements in the cast billet with pulsed magnetic field were suppressed. Compared with the conventional cast billet, the yield strength, ultimate tensile strength and elongation of the cast billet with pulsed magnetic field were increased obviously.