Papers by Author: Ji Shan Zhang

Abstract: The diversities and relations of microstructures and properties between the center and surface layer in thick-plate 7150 alloy were investigated by means of OM, TEM, SEM and tensile tests. The results indicated that the average width of elongated grains with fewer substructures in the center of hot-rolling thick plate is about 2 times larger than that in the surface layer. The coarser second phases are massed up at the grain boundaries of the center layer while those are crushed into smaller particles in the surface layer. By aging, the strengthening precipitates in the center are mainly GP zone and η' phase, while in the surface layer η' and η phases play the main roles. Consequently, the strength is about 80Mpa (15%) higher in the center than in the surface, with lower elongations (26% lower).

Abstract: The microstructures and hardness of spray-formed and aged H13 steel were investigated by OM, SEM, XRD and hardness tester. Compared to the as-cast H13 steel, the microstructures of the spray-formed H13 steel is characterized by lower bainite, martensite and much more retained austenite, inducing lower hardness than the as-cast H13. The peak aging temperature for the spray-formed H13 steel is 50°C higher than the commercial H13 steel (500°C) for their different microstructures. After aged at 550°C for 50h, a much higher hardness (about 53 HRC) can be retained in the spray-formed H13 steel, showing an excellent thermal stability at 550°C.

Abstract: The M3 type high speed steel billets with / without Nb addition were prepared via spray forming. The effects of cooling rate and Nb on the microstructures and property of the M3 type high speed steel were investigated. The results show that the as-cast steels contain coarse primary dendrites and M₂C eutectic carbide, but the equiaxed grains and fine carbides are formed in the spray-formed steels. It can be seen that carbide precipitation and grain growth were suppressed by high cooling rate. The Nb additions can change the types of primary carbides. The solidified carbide microstructure was effectively changed from M₂C to M₆C type during the cooling of hot spray-formed billet. Niobium improves the microstructural thermostability of the M3 type high speed steel during heat treatments.

Abstract: The hypereutectic Al-Si alloys with different Cr additions have been prepared by spray forming and cast processes. With adding Cr into Al-25Si-5Fe-3Cu (wt.%, denoted as 3C) alloy, the long needle-like δ-Al4FeSi2 phases in the cast 3C alloy were almost substituted by skeletal α-Al(Fe,Cr)Si phase, which was refined into granular α-Al(Fe,Cr)Si phase (≤ 3-5 μm) in Cr-added deposits. By using hot extrusion almost full densification of the deposits can be made, but induces coarsening of Fe-bearing phases in as-deposited 3C and C10 (adding 1.0 wt.% Cr into 3C alloy) alloys. The size of the granular α-Al(Fe,Cr)Si phase is still less than 5 μm after hot extrusion of the as-deposited C20 alloy, while some plate-like β-Al5(Fe,Cr)Si phases (~20 μm) disappeared. The thermal stability, mechanical properties were also evaluated.

Abstract: A high Zn content Al-Zn-Mg-Cu alloy was prepared by spray forming process and the precipitate behavior and microstructure of the extruded alloy were also investigated. The precipitate sequence of the spray-formed alloy could be described as “α-solid solution → GPI zone → GPII zone (also called Metastable ′ )→ Stable  (MgZn2)” during artificial ageing treatment. In the early stage of artificial ageing treatment, the GPI zone was the main strengthening phase and kept coherent relationship with the matrix. With the increasing of ageing time, ′ phase dominate strengthening phase and kept semi-coherent relationship with the matrix. With the further increasing of ageing time,  phase took the place of ′ phase, and dominated the strengthening phase in the alloy. The grain size of the spray deposit is finer than that of cast alloys. The ultimate tensile strength of the alloy is over 810MPa in peak ageing condition.

Abstract: In this study, a high strength Al-Zn-Mg-Cu alloy was prepared by the spray deposition technique. The microstructures of spray-deposited and homogenized Al-Zn-Mg-Cu alloys were studied using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The results indicate that the microstructure of the spray-deposited alloy mainly is composed of the Al matrix and the Mg(ZnCu)2 compounds. The spray-deposited process has an obvious modification in size, morphology and distribution of the secondary phases in the microstructure as well as reduce of segregation. After homogenization, the coarsening of the grains in the microstructure is not obvious, a phase transformation of primary Mg(ZnCu)2 particles to Al2CuMg phase was found.

Abstract: Al-Zn-Mg-Cu alloy containing 2%Mn was made using spray forming method and hot extrusion. XRD, OM, and SEM were used to investigate the microstructure. Results showed that after being hot extruded and solid solution treated, the Al alloy matrix had uniformly fine recrystallized grain with an average size of 8μm. The particles of MnAl6 were formed during the spray forming process and distributed along the grain boundary. Deformation and spheroidization of MnAl6 particles were occurred during post hot extrusion and solid solution treament, which would be benefit to the improvement of toughness of Al alloy. When aged at the T6 conditions, the tensile strength of the Al alloy was increased up to 775MPa with the elongation of 4.3%, the fracture surface consisted mainly of dimples with a size lower than 500nm.

Abstract: In this study, cylindrical billets of Mg-3Al-1Zn (AZ31) alloy have been synthesized by spray forming technique. The alloy billets were hot-extruded into rods with proper processing parameters. The microstructures and mechanical properties of alloys were investigated. The results indicated that spray formed AZ31 alloys have a homogeneous and equiaxial grains with average grain size of 20μm. The further grain refinement with an average grain size of 5μm was attributed to the continuous dynamic recrystallization during the extrusion processing. The room temperature mechanical properties of the extruded rods are remarkably higher than those of as-cast AZ31 alloy. The average tensile ultimate and yield strength of the alloy are 321MPa and 237MPa, respectively, with an elongation of 15.2%.