Papers by Author: Chang Shu He

Abstract: The intergranular corrosion behavior of friction-stir-welded 7N01-T5 aluminum alloy joints was investigated by using sliced samples extract from the top, middle and bottom layers of the joint, with the sliced direction parallel to the welding direction. The relationship between microstructure and local corrosion property of the welded joints was analyzed. The results show that the heat-affected zone (HAZ) of the weld exhibit the highest susceptibility to intergranular corrosion, and the corrosion degree of the top and bottom surface layers were more serious than the central layer in the HAZ, continuously dispersed precipitates on grain boundaries and the grains size may be the main factor which caused the different corrosion severity of HAZ. The thermo-mechanically affected zone (TMAZ) is characterized by a highly deformed structure, and the temperature reached in this local zone during welding is high enough for the strengthening precipitates, especially the precipitates on the grain boundary to be partially re-dissolved. The temperature gradient caused resistance to intergranular corrosion of TMAZ reduced from top to bottom surface. The stirred zone (SZ) experienced heavily plastic deformation and temperature cycling, the sensibility of intergranular corrosion was low, which is attributed to the sufficient re-dissolve of the strengthening precipitates during welding.

Abstract: A heat treatable A7N01 (Al-Zn-Mg) aluminum alloy mainly used to fabricate high-speed train body structures was double-sided welded by MIG welding. The natural aging behavior of the joint was systematically studied by local and global mechanical property testing method. Based on TEM observation and EPMA analysis, the key factors that lead to different natural aging behavior among various regions of the joint were discussed. The global tensile strength of the joints increased obviously after natural aging. All the joint samples fractured in the weld zone, which demonstrated the weld zone was the weakest zone of the joints. And the strength of the global welds were depended on the microstructure and natural aging ability of the weld zone. Moreover, the increase of the tensile property of the three local regions in the weld zone after post natural aging decreased as Middle>lower>upper. The above phenomenon was also further confirmed by microhardness measurement. The age hardening behavior is greatly influenced by the concentration of strengthening precipitates forming element Zn. The Zn content in the weld zone determined by EPMA showed that element Zn was dispersed unevenly, which is attributed to the double V-groove design used in this work and Zn vaporization in the molten pool during welding. The Zn content in the middle layer metals is higher than that of the upper and lower layers, resulting in the remarkable hardening response for the middle layer of the weld zone during natural aging process. Additionally, the solution zone containing the same Zn content as the base metal and undergoing sufficient solid solution during welding was found to possess the high hardening ability after natural aging process.

Abstract: Mechanical spectra of an Al-12.7Si-0.7Mg alloy under extrusion and the solid solution treatment followed artificial aging (T6) states were measured at different frequencies. It has been found that their spectra present differences. There was an obvious internal friction peak near to 200°C with increasing the temperature. The value of internal friction presented increase started from 300°Cand height of the peak was low, there existed a peak near to 270°C during the cooling processes in the spectrum for the extrusion state. For the T6 state alloy, no peaks could be observed during the heating processes. With decreasing the temperature, the internal friction peaks were again observed near 200°C and 270°C.

Abstract: An Al-5.4Si-0.5Mg filler wire was developed and produced by direct chill (DC) casting, extrusion, and roll-die and hole-die drawing for the welding of newly developed Mg-containing high-Si aluminum alloys. The Al-12.7Si-0.7Mg alloy plates were butt-welded by tungsten inert gas arc welding (TIG) method using the as-produced welding wire. The microstructures and mechanical properties of these welded joints in different solid solution and artificial aging treatment conditions were studied to evaluate the weld qualities of the novel welding wire. The results showed that the Al-5.4Si-0.5Mg welding wire was an ideal special welding wire of the Al-12.7Si-0.7Mg alloy and the filler metal proved to be heat-treatable. The welded joints exhibited much higher mechanical properties after the post-weld heat treatment (PWHT). All of this provides the possibility of wide application of the novel Al-5.4Si-0.5Mg welding wire and the Al-12.7Si-0.7Mg alloy weldments.

Abstract: A novel Al-12.7Si-0.7Mg welding wire was successfully produced via DC casting, extrusion, roll die drawing and hole die drawing. An investigation was performed on the microstructure and mechanical properties of the welded joints using the as-produced special filler wire in different solid solution and artificial aging treatment conditions. The results showed that the as-produced Al-12.7Si-0.7Mg welding wire was an ideal special wire of the Al-12.7Si-0.7Mg wrought alloy and the homogeneous welded joints proved to be heat-treatable. Solid solution treatment and artificial aging treatment had significant influence on the microstructure, microhardness and tensile properties of the as-welded joints. A transformation of microstructures from a coarse structure to a fine structure occurred in the welded joints after the heat treatment. The microhardness and the strength of the joints increased remarkably as the solution temperature and aging time increased.

Abstract: 3-mm thick Al-12.7Si-0.7Mg alloy plates were cut from the hot extrusion profiles. A butt-welding joint was made by gas metal arc welding (GMAW). The microstructures and mechanical properties of welded joint were studied by scanning electron microscope and tensile test methods. The results show that weld bead with good appearance and internal quality was obtained under the optimized welding parameters. The ultimate tensile strength for base material and welded joints of hot extrusion Al-12.7Si-0.7Mg alloy are much higher than that of 6063 alloy in T4 condition.

Abstract: The texture evolution of the pure iron sample after the surface mechanical attrition treatment (SMAT) was investigated by means of electron backscattering and X-ray diffraction (XRD) analysis. Experimental observations indicated that four sections along depth were formed in the pure iron sample during the SMAT, i.e., nanostructured regime in the surface layer, submicro-sized, micro-sized and plastic deformed regime. Compared to the microstructure of sample, texture analysis was performed. It can be found that the <110>//ND fiber texture is the prominent texture. A strong orientation of (110)[11] along the <110>//ND fiber was formed in the plastic deformed regime, and as the depth from the top surface decreases, <110> //ND fiber texture intensity increases. The maximum intensity was reached in the micro-sized regime, and then it start to reduce. In the nanostructured regime, <110> //ND fiber texture nearly disappear.

Abstract: Specimens cut from a cold-rolled IF steel sheet of 1 mm thickness were respectively annealed at 750°C for 20min under a range of DC electric fields (1kV/cm~4kV/cm). The Effect of electric field strength on recrystallization texture of IF steel sheet was studied by mean of X-ray diffraction ODF analysis. It was found that γ-fiber textures were notably enhanced as electric field strength increased. The strength of γ-fiber textures got their peak values as the applied electric field reached to 4kV/cm. The possible reason for such phenomena was discussed in the viewpoint of interaction between the applied electric field and the orientation-dependent stored-energy in deformed metals which is known as the driving force for recrystallization during annealing.

Abstract: For off-stoichiometric Ni₂MnGa ferromagnetic shape memory alloys, a large shape change could be induced through the rearrangement of martensitic variants under an external magnetic field. Insight into the orientation relationships of martensitic variants and the characteristics of variant boundaries is thus essential for understanding the magnetic shape memory performance. In this paper, a thorough crystallographic investigation was made on the incommensurate 7M modulated martensite in one polycrystalline Ni₅₀Mn₃₀Ga₂₀ alloy by means of X-ray diffraction and SEM electron backscattered diffraction (EBSD). Locally, there are four differently-oriented martensitic variants, being twin related to one another. The twin interface planes are coherent and they are in coincidence with the respective twinning planes (K₁). A primary exploration was performed to improve the microstructure by repeated magnetic field training during phase transition. The present investigation could offer useful guidance to develop specific technique for microstructure optimization.

Abstract: The present studies are to investigate the microstructure features during transformation from austenite to ferrite without and with magnetic field on Fe-0.76%C alloy. It is found that the area fraction and numbers of proeutectoid ferrite grain as well as the lamellar spacing of pearlite in Fe-0.76%C alloy increased considerably with the increase of magnetic field intensity. The reason is that, the magnetic field increases the driving force of proeutectoid ferrite nuclei and shifts the eutectoid point to the side of high carbon content and high temperature, which increases the starting-temperature of the transformation from austenite to ferrite. The proeutectoid ferrite grains are elongated along the magnetic field direction, which can be explained as follows: the proeutectoid ferrite becomes the magnetic dipolar under high magnetic field, and then the polarized austenite atoms are much easier to diffuse into ferrite grains along the magnetic field direction. Key words: high magnetic field; Fe-0.76%C alloy; microstructure