Da Tong Zhang

Microstructure and Mechanical Properties of a Fine-Grained AZ91 Magnesium Alloy Prepared by Multi-Pass Friction Stir Processing

Friction stir processing (FSP) is a novel severe plastic deformation technique developed in recent years to produce fine-grained structural materials. Through increasing the processing pass, further grain refinement can be achieved. In this paper, the microstructure and mechanical properties of AZ91 magnesium alloy prepared by the single-pass and two-pass FSP were studied. The results showed that the coarse, network-like eutectic β-Mg17Al12 phase was broken into particles and some of them dissolved into the magnesium matrix, and the α-Mg grains were remarkably refined after FSP. The average grain sizes of the single-pass and two-pass FSP alloys were 8.3 μm and 5.8 μm respectively. The ultimate tensile strengths of the specimens were 284.5 MPa and 319.7 MPa, and elongations were 13% and 14.5%, respectively. The improved mechanical properties of the two-pass FSP specimen were mainly attributed to the finer grain size and more homogenized microstructure.

Microstructures and Mechanical Properties of a Squeeze-Casted Al-Zn-Mg-Cu Alloy after Heat Treatment

The effects of heat treatment on the microstructures and mechanical properties of a squeeze-casted Al-6.8%Zn-2.7%Mg-2.0%Cu alloy were studied by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). It is found that squeeze casting can refine the microstructure of the alloy markedly accelerates the diffusion process of solute atoms during solution heat treatment. After solution heat treatment at 470°C for 10h and artificial aging at 130°C for 24h, the tensile strength and the elongation of the squeeze-casted alloy reach 590MPa and 5.0%, respectively, which are almost equal to those of the wrought alloy, and are significantly higher than those of the gravity-casted alloy (435MPa and 1.3%). Based on the experimental results, the mechanism of microstructural evolution and the effect of squeeze casting on the kinetics of solute diffusion and aging precipitation of the squeeze-casted Al-Zn-Mg-Cu alloy were discussed.

Deformation Behavior and Microstructure Evolution of 6063 Alloy during Hot Compression

Hot compression tests of homogenized 6063 Al alloy were carried out in the temperatures range from 390°C to 510°C and strain rates from 1s-1 to 20s-1 on a Gleeble-3500 thermal simulation machine. The results showed that the flow stress decreased with increasing deformation temperature or decreasing strain rate. The dynamic softening effect was more obvious when the alloy was deformed at strain rate of 20 s-1. The Arrhenius-type constitutive equation with strain compensation can accurately describe the flow stress of 6063 aluminum alloy during hot compression. Shear bands appeared in grains interior when the alloy deformed at high strain rates, corresponding to high Zenner-Hollomon (Z) parameters. When deformed under the conditions with low Z parameters, the dynamic recrystallization started occurred.

Hot Compression Deformation Behavior and Microstructural Evolution of Al-7.5Zn-1.5Mg-0.2Cu-0.2Zr Alloy

Hot compression tests of as-homogenized Al-7.5Zn-1.5Mg-0.2Cu-0.2Zr alloy were carried out on Gleeble-3500 thermal simulation machine at the temperature ranging from 350°C to 550°C and strain rate ranging from 0.001s-1 to 10s-1. Processing maps were established on the basis of dynamic material model, and the microstructure was studied using electron back scattered diffraction (EBSD) technique. The results showed that the peak stress and steady flow stress decrease with decreasing strain rate or increasing deformation temperature. There are one peak efficiency domain and one flow instability domain in the processing maps. The flow instability domain which exists in high-strain-rate region becomes larger with increasing strain. Shear bands occur at 45° toward the compression axis at grain interiors and meanwhile flow localization occurs. The optimum deformation temperature and strain rate ranges from 450°C to 500°C and 0.003s-1 to 0.1s-1, respectively, with high power dissipation efficiency of 34-39%.

Microstructure and Mechanical Properties of ZK60 Magnesium Alloy Prepared by Multi-Pass Friction Stir Processing

Friction stir processing (FSP) is a novel severe plastic deformation technique developed in recent years to produce fine-grained structural materials. In this paper, the microstructure and mechanical properties of ZK60 magnesium alloy prepared by the single-pass and two-pass FSP were studied. The first-pass was subjected with 800r/min-100mm/min and the second-pass was subjected with 600r/min-100mm/min. The results show that the coarse second phase was dissolved into magnesium matrix and the α-Mg grains were remarkably refined in stir zone after FSP. The average grain sizes of the single-pass and two-pass FSP alloys were 6.8μm and 6.0μm respectively. The microhardnesses of the specimens were 70HV and 65HV, the ultimate tensile strengths were 276MPa and 272MPa, and the elongations were 31.6% and 42.5%, respectively. Through increasing the processing pass, the microstructure became finer and the second phase is dissolved more thoroughly. The combined effect of grain refinement and second phase dissolved was responsible for the change of mechanical properties.

Microstructure Evolution and Mechanical Properties of AZ61 Mg Alloy Processed by Two-Pass Friction Stir Processing

Friction stir processing (FSP) has been considered as a promising technique for grain refinement. In this study, two-pass FSP was conducted under water to enhance the cooling rate during processing, and a fine-grained AZ61 magnesium alloy was prepared through this method. Compared with one-pass FSP, the microstructure of two-pass FSP alloy is more homogeneous and finer, with an average grain size of 1.8 μm. Both the tensile strength and ductility of the two-pass FSP sample are improved due to grain refinement. A tensile elongation of 31.8% was achieved in the two-pass FSP sample. The microstructure evolution mechanism during the processing was discussed based on the microstructure examination.

Microstructural Evolution of a Fine-Grained Mg-Y-Nd Alloy during Superplastic Deformation

Cast Mg-4.27Y-2.94Nd-0.51Zr (wt.%) alloy was subjected to submerged friction stir processing (SFSP) with at a rotation rate of 600 rpm and a traveling speed of 60 mm min-1. Superplastic behavior of specimens with an average grain size of ~1.3 μm were investigated in the temperature ranges of 683-758 K and the strain rate ranges from 1×10-1 to 4×10-4 s-1. Microstructure characteristics were investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the maximum elongation of 967% was obtained at 733 K and 3×10-3 s-1, the optimal HSRS of 900% achieved at 758 K and 2×10-2 s-1. Grains and second phase particles grew coarser with the increasing temperature or decreasing strain rate. Remarkable grain growth is the main reason that elongations are all significantly decreased when the strain rate decrease from 3×10-3 s-1 to 4×10-4 s-1. Grain boundary sliding is the main mechanism during superplastic deformation.

Effect of Enhanced Cooling on the Microstructure and Mechanical Properties of Friction Stir Spot-Welded AZ31 Magnesium Alloy

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.

Microstructure and Mechanical Properties of Submerged Friction Stir Processing Mg-Y-Nd Alloy

Mg-2.5wt%Y-4wt%Nd-0.5wt%Zr casting alloy was subjected to submerged friction stir processing (SFSP) with different rotation rates (ω) and travel speeds (υ). The influence of the ratio of ω/υ on the microstructure and mechanical properties of Mg-Y-Nd alloy was investigated in the present work by optical microscopy, scanning electron microscopy, transmission electron microscopy, tensile test and hardness measurement. The results showed that the average grain sizes of SFSP samples were significantly refined compared with as-cast sample, and the coarse net-shaped Mg12Nd phases which located at grain boundaries in as-cast sample were changed into small particles. The combined effect of grain refinement and uniform particles distribution was responsible for the enhancement of mechanical properties. The relative optimal parameter of 600 rpm/60 mm·min-1 in this research obtained the finest grain size and the best mechanical properties, which were 1.1 μm for average grain size, 305 MPa for ultimate tensile strength and 22% for elongation, respectively.

Corrosion Behavior of Tungsten Copper Alloy Produced by Infiltration Sintering

In this paper, W80CuNi0.3 alloy was prepared by cold isostatic pressing (CIP) and infiltration sintering, and its corrosion behavior was investigated by neutral salt spray accelerated test, immersion test and electrochemical measurement. It turned out that in the neutral salt spray and immersion test,the mass loss and corrosion rate of W80CuNi0.3 were far lower than those of 45 steel. The corrosion mainly occurs in the bonding phase of Cu, and the initial corrosion form of W80CuNi0.3 was pitting, the main reason of W80CuNi0.3 corrosion was Cl- erosion. The polarization curves analysis showed that compared with 45 steel, W80CuNi0.3 alloy had higher corrosion potential and lower corrosion current. Therefore, W80CuNi0.3 alloy are prone to stable passivation, so it has superior corrosion resistance.