Materials Science Forum Vol. 1003

Abstract: The fatigue crack propagation of Al-Zn-Mg-Cu alloys could be influenced by the content of main alloying element. In the present work, two Al-Zn-Mg-Cu alloys with a same Zn/Mg ratio were treated by two stage over-aging aging treatment and typical T7651 states were extracted via mechanical properties. Fatigue crack propagation of the two alloys were tested and the related precipitation characteristics and fracture morphology were observed. The results showed that the alloy with higher Zn, Mg contents possessed a better fatigue crack propagation resistance compared with the alloy with lower Zn, Mg contents. The corresponding fracture morphology also showed the difference of fatigue striation, which provided an additional support. The precipitation observation demonstrated that the both alloys possessed GPII zone, η' phase and η phase while the alloy with higher Zn, Mg contents had a larger average precipitate size and a larger proportion of large size precipitates compared with the alloy with lower Zn, Mg contents. Cut and bypass mechanisms of dislocation-precipitate interactions were used to explain the difference of fatigue crack propagation between the two alloys.

Abstract: Effect of multi-section linear non-uniform heat transfer coefficient on quenching residual stress distribution in 27mm-thick Al-Zn-Mg-Cu aluminum alloy plate was simulation studied by using the finite element method, and the surface quenching residual stress distribution was measured by the X-ray diffraction method and hole-drilling method. The results show that the surface quenching residual stress represents the same distribution with non-uniform heat transfer coefficient in the transverse direction and the stress level maintains initial stress level of the heat transfer coefficient at each location. The distribution of the quenching residual stress in the center of the plate is approximately uniform and the stress level is approximately equal to average of maximum and minimum initial stress level. The measured surface quenching residual stress shows a wavy distribution in the transverse direction, which is similar to the simulated surface stress distribution without considering the stress level. The measurement results can be explained by the multi-section linear non-uniform quenching model.

Abstract: The hot deformation behavior and microstructure evolution of Al-3.5Cu-1.0Li-0.4Mg- 0.6Zn-0.3Ag aluminum lithium alloy were investigated by hot tensile tests on Gleeble-1500 thermal simulator at 480-510 °C and strain rates 0.0001-0.1 s^-1. The results show that obvious flow steady-state phenomena occur during hot stretching and the main mechanism changes from dynamic recovery to dynamic recrystallization with the increase of temperature and decrease of strain rate. The constitutive equation was calculated using the true stress-strain curve obtained by the hyperbolic sinusoidal pair of deformation activation energy Q and temperature T proposed by Sellars and Tegart. The deformation heat activation energy is 226.783 KJ/mol.

Abstract: Effects of different solution treatments on microstructure, texture and mechanical properties of 6A16 aluminum alloy sheet were investigated by electron back-scattered diffraction (EBSD) and tensile test. The results show that among the five solution treatments, with the increase of solution temperature and solution time, the average grain size increases and the yield strength of the alloy sheet gradually increases. The maximum elongation at break of the sheet is at 545 °C, and it increases with the increase of solution time. Therefore, the mechanical properties of the sheet are the best when the solution treatment is 545 °C for 5 min.

Abstract: Compared to conventional metal forming methods, processing by severe plastic deformation is mostly used to improve the mechanical properties and not for the shaping of a product. Processed material usually has an average crystal grain size of less than a micron and as a result, the material exhibits improvements in most of the mechanical properties, such as yield and ultimate tensile strength, microhardness, sufficiently high workability, good corrosion resistance, and implant biocompatibility and others. In this paper, a brief review of the processing by severe plastic deformation was presented, including the benefits, major methods, and the application. Additionally, a brief review of two methods made by authors was made.

Abstract: In this work, the microhardness of 7075 aluminum alloy friction stir welding (FSW) joint was measured by a micro vickers hardness tester, the microstructure of the joints was characterised by microscope, the precipitated phases among the welding nugget zone (WNZ), thermal mechanical affected zone (TMAZ), heat affected zone (HAZ) were affirmed by X-ray diffractometer (XRD) and the lattice fringe of transmission electron microscopy (TEM) high resolution image. Based on this, the precipition behavior of precipitated phases was studied. The results show that the microhardness distribution of the 7075 aluminium alloy FSW joints is heterogeneous in comparison with the base metal (BM). The precipitates in the joint mainly include MgZn rod shape and AlCuMg in elliptical shape. In the WNZ, the main precipitate is AlCuMg, and the fine grain strengthening effect is better, so the microhardness in this zone is relatively high. In the TMAZ, the quantity of AlCuMg decreased while the MgZn₂ increased relatively in comparison with the WNZ. At the same time, the effect of the fine grain strengthening was weakened, though the strain hardening increased. Therefore, the microhardness in the TMAZ still decreased. In the HAZ, the quantity of MgZn₂ increased furtherly, and there is no strain hardening and fine grain strengthening, so the microhardness of the HAZ was the lowest among the FSW joints. Besides, through comparative tests, the optimal process parameters of friction stir welding of 7075 aluminum alloy were obtained.

Abstract: A microstructure consisted of granular bainite, upper bainite, acicular ferrite, polygonal ferrite, and a little of pearlite was obtained by controlling rolling and cooling in marine steel. The grain size of the two steels was refined to 5-9 μm. The size of the MA island was about 3-6 μm. Both steels had higher tensile strength, yield strength, elongation and hardness, and the performance of steel B was better than that of steel A. Both steels exhibited large and deep dimples at -80 °C impact fracture. The longitudinal impact energy of the specimens was above 250J, and the steel B even exceeded 300J. Its longitudinal impact performance was better than lateral impact performance. Both steels had achieved good performance due to fine grain strengthening, MA island strengthening, precipitation strengthening and dislocation strengthening caused by controlled rolling and cooling. The alloying element nickel effectively improved the low temperature impact toughness of the experimental steel. Excellent impact energy of both steels was attributed to higher texture strength for RD specimens.

Abstract: In metastable beta Ti alloys, microstructural features can be varied over a wide range of length scales by changing different heat treatment parameters. Effect of cooling methods on microstructure and mechanical properties of Ti-3537 alloy after solution treatment was investigated. The result shows that with the decrease of cooling rate, the Vickers hardness of the alloy gradually increases. Among the three cooling methods of OQ, AC and FC, Ti-3573 alloy has the best shape and moderate yield strength, but tensile strength. The fractography of the β-substrate specimens showed that the fracture mode was ductile fracture. In the FC state, the α phase precipitates in a large amount in the Ti-3573 alloy, the yield strength and the tensile strength are greatly increased and the elongation is remarkably lowered. The tensile fracture shows a shallow fracture dip with low toughness.

Abstract: In this regard, two beta titanium alloys in the Ti-Al-Mo-V-Cr system, Ti-3Al-5Mo-7V-3Cr (Ti-3573) and Ti-3Al-8Mo-7V-3Cr (Ti-3873), have been designed. Comparison of the microstructure and mechanical properties of both alloys after solution treatment was conducted. The result shows that the β grains in Ti-3873 alloy are abnormally grown at WQ. The elongation of Ti-3573 alloy is higher than that of Ti-3873 alloy, it is related to the the smaller grain size. The Ti-3873 alloy has moderate plasticity but higher yield strength and tensile strength. Fine and deep dimples associated with ductile fracture were obtained for the Ti-3573 alloy. The fractography of the β-substrate specimens showed that the fracture mode was ductile fracture. The Ti-3873 alloy has a combination of slip and twinning during deformation. It is possible for the Ti-3573 alloy to undergo both twinning and TRIP effect upon deformation. Therefore, Ti-3573 alloy exhibited good plasticity and strength matching.