Advanced Materials Research Vols. 391-392

Abstract: The effect of high electric current pulse on the microstructure and macroscopic mechanical behavior of X70 pipeline steel was studied. With the increasing of current density, the grains become refined and the tensile strength has an evident improvement without a decrease in elongation rate. The theoretical analysis shows that two kinds of refinement mechanisms, recrystallization and phase transformation, compete when the exerted current densities differ. Therefore, the ECP treatment should provide a promising method to refine materials and to improve their physical properties by using different current densities.

Abstract: In this study, the effect of welding parameter on the microstructure and properties of resistance spot welding joint for 400MPa ultra fine-grained steel is investigated. The results indicated that the microstructure of HAZ of ultra fine-grained steel joint consists of hardening organization: bainite, martensite, M-A constituent, i.e. while the fusion zone mainly consists of the coarse martensite. By applied forge-delay force during resistance spot welding will increase the nugget diameter, eliminate the shrinkage defect in nugget zone and improve the shearing strength of joints.

Abstract: Zinc ferrite (ZnFe₂O₄) crystalline was prepared via co-precipitation method, followed by calcinations at various temperatures from 400 to 600 . Poly (vinyl pyrrolidone) (PVP) was used as a stabilizer to prevent the particles from agglomeration. The variation of crystallite size has been investigated using X-ray powder diffraction (XRD), transmission electron microscopy(TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope(SEM), and a recipe has been developed for the preparation of nano ZnFe₂O₄ with 6.7nm size and complete crystallization.

Abstract: We have developed an ultrasound-assisted sol-gel approach for the synthesis of ultrafine nanocrystalline ZrO₂ powders. The ultrasound-assisted sol-gel synthesis is expected to be able to process continuously, and may lead to energy savings because of rapid heating to the needed temperature and increased kinetics of crystallization. The variation of crystallite size has been investigated using X-ray powder diffraction (XRD), transmission electron microscopy (TEM), TG–DTA analyze and photoluminescence. This method is very simple and can lead to powders with desirable characteristics such as very fine size, narrow size distribution, and good chemical homogeneity.

Abstract: Effect of heat treatment on microstructure of TRIP steel sheets containing aluminium was investigated on Gleeble 3500 thermal simulation testing machine. The microstructure evolution with variation of time and temperature was measured by means of optical metallography (OM) and X-ray stress analyzer. The tensile properties of TRIP steel at room temperature were also measured. It was shown that the maxium value of product of strength and ductility of 22858 MPa% was obtained by treatment of intercritical annealing temperature at 800 for 3 mins and bainitic holding temperature at 400 for 6 mins. The value of yield strength was mainly determinated by the volume fraction of bainite and the content of retained austenite was the key factor to result in optimum strength and ductility of TRIP steel. In addition, the properties of TRIP steel were markedly decreased because martensite, which was deleterious to the ductility, was present in microstructure of the steel soaking at bainitic temperature 350^。C for 6 mins.

Abstract: AZ31B magnesium alloys recycled by solid-state process from oxidized chip were extruded repeately. Microstructures and mechanical properties of recycled alloys for different extrusion times were studied. With the increasing extrusion times, the breaking degree and homogeneity of oxide increase and stream line feature of oxide become less obvious. Second extrusion make dynamic recrystallization microstructure of recycled alloy become more homogeneous and fine, but the microstructure is not refined furtherly after 3 and 4 times extrusion. The ultimate tensile strength increases with the increasing extrusion time, which resulting from the microstructure evolution during repeating extrusion and the enhanced bonding between oxide and magnesium alloy matrix. The elongation to failure of recycled alloy increases after second extrusion and continuously decreases after 3 and 4 time extrusion. This is determined not only by the variation of dynamic recrystallization microstructure and bonding strength between chips but also by the distribution status of oxide.

Abstract: This work mainly investigated the influence of element Nd on the microstructure and mechanical properties of the Zn-25Al-5Mg-2.5Si alloys with different Nd contents. The tensile and hardness tests were carried out at room and elevated temperatures. The results show that the addition of element Nd leads to the grain refining and the formation of Al2Nd and NdZn2 phases in the microstructure, and the mechanical properties of the alloys rise at first and then drop with the increasing of Nd content. The optimization of microstructure and performance especially the tensile strength at high temperature is obtained when Nd content is 0.8 wt.%. It can result in increase of tensile strength by 48.8% and micro-hardness by 67.4% at 180^。C.

Abstract: In order to differentiate the porcelain quality, the paper introduces the All Phase spectrum analysis technology and studies on analyzing porcelain acoustic emission (AE) signal. As for the energy leakage by traditional signal truncation method in processing the signal, the all phase truncation method somewhat reduce the leakage which affects the follow-up porcelain quality discrimination. All instances consisting sample point are considered and weighted average technology is introduced to make amplitude-frequency clearer. According to the simulation, the energy leakage based on all phase signal processing is weakened and the spectrum is able to be accurate. It is more beneficial to the follow-up porcelain quality discrimination.

Abstract: Porous reaction-bonded Si3N4-SiC composites were fabricated by using potassium chloride(KCl) as pore-forming agent. Green bodies with 20% SiC and 30% KCl in mass were subjected to presinter in Ar atmosphere at 1000~1200 and then reaction-sintered. The results indicated that Si₃N₄-SiC composites which with about 58.5% porosity could be prepared, and the main phase compositions were α-Si3N4 and SiC. After presintered, the porosity of composites had small decrease, but bending strength increased obviously and little amounts of Si₂N₂O had been found. The distribution of pores of porous ceramics was homogeneous and SiC grains were surrounded by Si3N4 grains and fibres. Lots of needle-like α-Si₃N₄, especially in the high porosity ceramics, could be observed.

Abstract: The information about the effect of material and pin surface on the vertical force in friction stir welding was collected and analyzed. The results shown that the vertical force experienced a steady process when tool was moving steadily in the workpiece; The vertical force in steady state was impacted by material, because the thermal and mechanical properties of different material were not same. The vertical force was the greatest when welding TC4 (about 8.40 KN), and the vertical force was greater when welding T2 (about 7.35 KN) than welding LY12 (about 3.10 KN). Pin surface also affected the vertical force; because threads generated forces when tool rotating in the workpiece. The vertical force was greater by using column pin with left thread (about 3.48 KN) than by using column pin with right thread (about 3.09 KN), and the vertical force was median (about 2.60 KN) by using column pin without thread.