Yong’an Zhang’s research while affiliated with General Research Institute for Nonferrous Metals (GRINM) and other places

In this paper, Al–11Zn–3Mg–2Cu–0.2Zr alloy was prepared by spray-forming, and then it was treated by defect reduction, hot worked and heat treatment. Microstructure and mechanical properties of the aluminum alloy were researched during the whole processing by metallographic microscope, scanning electron microscope (SEM), and tensile test. Average grain size of spray-formed aluminum alloy was about 10–20 μm due to the high cooling rate closing 1000 K/s, and shrinkage porosity and cavity was inevitable in spray-formed ingot because of nitrogen as carrier. Through hot isostatic pressing and hot extrusion, the relative density increased from 87– 90% to almost 99–100%, but it decreased again to about 98% after solution, which meant some porous defects could not be eliminated thoroughly. Comparing with casting, the original grain of spray-formed ingot was much smaller, which was unsuited for plastic processing with large deformation. A large plastic deformation caused lots of recrystallization, which weakened the mechanical properties. In the areas without defects, the best properties of the spray-formed alloy were 825 MPa in tensile strength, 808 MPa in yield strength, and 9% in elongation.

The effect of solution treatment temperature on mechanical properties and fracture behavior of 7N01/7050 aluminum alloy multilayer plate are investigated in this paper. With the increase in solution treatment temperature, the size of the second phase decreases, while the size of the sub-grains increases. Moreover, the tensile strength and impact toughness of the multilayer plate first rise and then decrease as the solution temperature increases. With solution treatment at high temperature, overheat structures are founded in 7050 layer, resulting in the deterioration of mechanical properties. An improved dissolution of the residual phase, a lower recrystallized grains fraction and smaller sub-grains can be acquired by proper solution treatment in both 7050 and 7N01 layers. This leads to a higher tensile and impact property of 7N01/7050 aluminum alloy multilayer plate.

6xxx series aluminum alloy is one of the ideal automotive lighting materials for its high strength, excellent formability, good corrosion resistance, and weldability. In this paper, the effect of pre-aging technology on microstructure and properties of 6111 aluminum alloy was investigated by using TEM, tensile, and Erichsen test. The results showed that better properties of 6111 alloy could be obtained through 140 °C/10 min pre-aging treatment within 30 min after solution treatment (named T4P treatment). The n, r, IE, and yield strength values of 6111-T4 alloy were 0.31, 0.62, 8.06 mm, and 149 MPa, respectively, while those of 6111-T4P alloy were 0.33, 0.76, 8.45 mm, and 133 MPa, respectively. After simulated paint baking at 170 °C/30 min, the yield strength of 6111-T4 and 6111-T4P alloys increased to 154 and 212 MPa, respectively. Compared with T4 treatment, the pre-aging treatment reduced precipitating temperature of β″ phase and promoted precipitation during simulated paint baking. Pre-aging treatment benefits press forming of automotive body sheet and enables strengthening of the materials after simulated paint baking.

The flow behavior of a high-alloying Al–Zn–Mg–Cu alloy was studied by compression tests with the temperature range of 300–440 °C and the strain rates range of 0.001–1 s⁻¹, and the corresponding microstructural evolution was observed. Results show flow stress curves exhibit the peak value at a critical strain, and the peak stress decreases with increasing of deformation temperatures. Numerous precipitated particles with a small size and high-density dislocations should be responsible for the high flow stress. Dynamic recovery is the main way of flow softening while dynamic coarsening of precipitated particles and dynamic recrystallization also play a role in flow softening under low-temperature and high-temperature conditions, respectively. The continuous dynamic recrystallization is the major mechanism for dynamic recrystallization behavior.

The accuracy of different residual stress measurement methods has always been the research focus from the beginning of research on residual stress. In this study, both conventional and newly-developed methods were applied to measure the residual stress in as-quenched 7055 aluminum plate. Methods such as hole drilling, X-ray diffraction based on sin²Ψ and cos α approaches, crack compliance method and neutron diffraction method were used. In the meanwhile, finite element simulation was used to obtain the residual stress distribution as a comparison. The results showed that among the methods studied, X-ray diffraction method has the greatest test error due to its shallow test depth. However, if the measurement condition was well controlled, the error could be acceptable. The absolute values of residual stress obtained by X-ray diffraction method were slightly greater than hole drilling method. If calculated with the reasonably chosen crack compliance function, the test result was similar to neutron diffraction method. Under different quenching conditions, all the studied methods showed that the greater the quenching cooling rate, the greater the absolute value of residual stress.

Isothermal compression tests of the Al–9.4Zn–1.9Mg–2.0Cu alloy were carried out at the temperature ranging from 300 to 460 °C and the strain rate from 0.001 to 10 s⁻¹, and the deformation degree was 70%. Flow stress curves show that the flow stress decreases with the increasing deformation temperature and the decreasing strain rate. The measured flow stress was corrected because of the effect of friction. The corresponding corrected stress values are lower than measured stress values. The effect of friction is far greater when hot-deformations occurred at lower temperatures or higher strain rates. A constitutive equation considering the effect of strain on material constants (i.e. α, n, Q and A) are established based on the Arrhenius-type equation. Compared with the experimental results, the flow stresses calculated by the constitutive equation have a high precision with the correlation coefficient of 0.95. Results show that higher deformation temperatures and lower strain rates are beneficial for hot deformation of the Al–9.39Zn–1.92Mg–1.98Cu alloy.

Homogenization treatment of an ultra-high strength Al-Zn-Mg-Cu alloy was studied by SEM, TEM, DSC, hardness and electrical conductivity test. The second phases in as-cast 7A56 alloy were AlZnMgCu, Al2Cu and Al7Cu2Fe. AlZnMgCu phase has a similar structure of MgZn2. After 380 °C homogenization treatment, transformation from AlZnMgCu to Al2CuMg occurred; after 470 °C homogenization treatment, AlZnMgCu were mainly directly dissolved into the matrix. That phenomenon was caused by the diffusion behavior of Zn and Cu at two different temperatures that Zn diffuses rapidly even at 380 °C while Cu shares a much lower diffusion rate. Much of the AlZnMgCu phase can be dissolved at 470 °C for a short time while at 380 °C it was seldom eliminated. After homogenized at 470 °C for 24 h, the dissolution process gradually fall into a balance with more than 95% had been eliminated and prolonging time imposes no evident influence on promoting the dissolution. Hardness and electrical conductivity are highly consistent with the vibration of AlZnMgCu phase. With the dissolution of which the hardness increases while electrical conductivity increases.

The effects of interrupted multi-step aging on the microstructure and properties of Al-Cu-Mg-Ag-Zr alloy were studied by tensile, hardness, electrical conductivity tests and transmission electron microscopy (TEM). Interrupted multi-step aging delayed the peak aging time compared to one-step aging and kept the same levels of hardness, electrical conductivity, ultimate tensile strength (UTS), yield strength (YS) and elongation as those of the T6 temper alloy while increased the fracture toughness notably. Ω phase and a little θ′ phase precipitated and grew simultaneously in the process of one-step aging at 160°C. During the second-step aging at 65°C of interrupted multi-step aging, no TEM characteristic of Ω precipitates could be found. During the third step of interrupted multi-step aging, Ω began to dominate the microstructure like what happened in the process of one-step aging. The difference of properties between the T6 temper and the interrupted multi-step aged alloys might be related to the different precipitation sequences in the process of the two heat treatment technologies. Keywordsaluminum copper alloys–heat treatment–microstructure–fracture toughness–aging

The elevated temperature performances of 2D70 Al alloy hot extrusion rods after two-stage homogenization and intensive deformation were studied by measuring the elevated temperature enduring strength and the creep ultimate strength. The fracture morphology of some selected samples after testing at different elevated temperatures was observed by scanning electron microscopy (SEM). The results indicate that, as the test temperature increases, the elevated temperature enduring strength of 2D70 Al alloy decreases gradually. In a comparison between 150°C and 240°C, the notch enduring strength drops from 375 to 185 MPa and the smooth enduring strength drops from 337 to 130 MPa. Enduring strength is not sensitive to the notch. The notch sensitivity ratio (NSR) coefficient is in the range of 1.119 to 1.423 from 150°C to 240°C. The creep test results show that, as the test temperature increases from 150°C to 240°C, the creep ultimate strength of 2D70 Al alloy rods drops gradually from 312 to 117 MPa. Keywordsaluminum alloys–high temperature properties–strength of materials–creep–notched bar tensile testing

The effects of the two-step ageing parameters (temperature and time) on the mechanical properties and electrical conductivity of 7B04 (Al-Zn-Mg-Cu) pre-stretched thick plates were studied. The results reveal that the initial T1 ageing contributes a major increase of the tensile strength, and the 0.2% proof stress value reaches 482 MPa after ageing for 7 h at 115°C. Behavioral differences in the tensile properties of the alloy after the two-step ageing treatment were less with the first-step ageing at 115°C for different time periods (7, 14, and 21 h). The effects of the second ageing parameters on the properties and microstructure of the 7B04 alloy were remarkable. TEM analysis of the samples aged at Temper I (7 h at 115°C + 12 h at 160°C) and Temper II (7 h at 115°C + 16 h at 165°C) indicates that two kinds of phases, i.e. η' and η phases, precipitate from the matrix and efficiently improve the tensile strength of the alloy, and the grain boundary precipitates are coarse and discrete. There are obvious precipitate free zones (PFZs) along the grain boundary in the microstructure of the alloy after the two-step ageing treatment.