Materials Science Forum Vols. 675-677

Abstract: In order to obtain the non-dendritic feedstock for the semisolid forming process, a cooling slope processing was used. In this work, the effects of the angle, length of cooling slope and pouring temperature on the microstructure of A356 aluminum alloy were investigated. It showed that these parameters affect the size and morphology of α-Al phase to some extent. The results indicate that a pouring temperature of 650°C and a cooling slope with 45° in angle and 50 cm in length are the optimum parameters for preparing fine and globular grain structures. To eliminate the solidification shell formed in the surface of cooling slope, a nitride dope was coated on the surface of the cooling slope.

Abstract: A fine grain size generally leads to improved structural uniformity of magnesium alloys. AlN has been identified as a potential grain refiner and electromagnetic stirring may have great effects on microstructure and grain growth. This study will be focused on the effects of AlN particles and electromagnetic stirring on the as-cast structure of AZ31 alloys.The grain refinement mechanism of both methods on magnesium alloy and their interaction effects are also discussed.

Abstract: Aluminum base composites with particles reinforced have high thermal conductivity, low thermal expansion coefficient, low density, and excellent working ability. On one hand the Sip/Al composites are prepared by pressure-less infiltration in experiments and take advantage of the finite element analysis to establish the models of Sip/Al composites. On the other hand, the Sip/Al composites morphology, the particles size, the volume fraction and the ratio between different Si particles have been discussed in the experiments. In experiments how they affect the thermal conductivity of the composites also has been explained. The thermal conductivity of Sip/Al composites is around 100 W/(m.k) at 50°C. When the Si volume fraction is definite, the thermal conductivity of the composites is little affected by the morphology, the size and the ratio between different Si particles. The study also showed that with the element Si content increasing, the thermal conductivity will decrease.

Abstract: Chemical liquid-vapor deposition (CLVD) process is a new style of fast densification, which combines the advantages of PIP process and CVI process. 2D C/SiC composites were prepared at 800~1200°C for 3~4 hours with liquid polycarbosilane and carbon fiber cloth by CLVD process with induction heating, and had the density of 1.7 g/cm3, the flexural strength of 84.6MPa, and the flexural modulus of 20GPa. XRD pattern of the sample proved that the matrix was β-SiC. It was found that SiC deposited mainly around single fiber instead of among fiber bundles and layers.

Abstract: In situ synthesized Fe-Cr-C-TiC high-chromium Fe-based ceramic composite coating was fabricated on substrate of Q235 steel by plasma transferred arc (PTA) weld-surfacing process using the mixture of ferrotitanium, ferrochromium, ferroboron and ferrosilicium powders. The microstructure and wear properties of the composite coating were investigated by XRD, SEM, EDS, microhardness tester and wear tester. Results show that the coating consists of TiC, (Cr,Fe)7C3 and austenite. The coating is metallurgically bonded to the Q235 steel substrate. TiC particles formed by PTA weld-surfacing process present cubic, dendrite and flower-like shape. The wear resistance of the composite coating is approximately 11 times higher than that of the base body Q235. As the load increases, the wear mass loses slowly, which demonstrates the composite coating has excellent load character.

Abstract: Fe-Cr-C ceramic composite coating was fabricated on substrate of Q235 steel by submerged-arc welding process added alloy powder using the material of chromium powder, iron powder, colloid graphite powder and H08 welding wire. Microstructure and wear properties of the composite coating were investigated by means of X-ray diffraction (XRD), scanning electron micrograph (SEM), energy dispersive X-ray analysis (EDS) and microhardness tester. wearresisting property of the coating was tested at room temperature and dry sliding wear conditions. Results show that the composite coating consists of (Cr,Fe)7C3 eutectic phase and austenite. There are many dendrite crystal structure in the coating, and most of the dendrite crystal are parallel, pointing to the surface of the coating. The wear mass loss of the base body Q235 steel is 10 times higher as that of the composite coating. The coating has excellent wear-resisting property because there are many eutectic carbide (Cr,Fe)7C3. It can increase wear resisting property of the coating that the austenite could occurr strain and induced martensite in the coating during wear.

Abstract: Compared with the conventional low carbon steels, the expulsion is more prone to occur in resistance spot welding (RSW) of dual phase (DP) steels. Especially when the initial gap exists between the steel sheets, the weld expulsion would reduce the weld quality of the joint. It is important to decrease or inhibit the occurrence of the weld expulsion in auto-body assembly line in order to guarantee the joint quality. In this study, the experiments of RSW with different initial gaps have been done to study the effect of the electrode force on the expulsion using different thicknesses of dual phase steels. The results show that the increment of the electrode fore would enable to decrease the occurrence of expulsion with the increase of the sheet thickness and gap spacing in resistance spot welding of DP steel sheets.

Abstract: Petroleum coke desulfuration residue, from combustion desulfuration of high sulfur petroleum coke, presents drying powdery and a favorable cementing performance. Lime and gypsum are replaced by petroleum coke desulfuration residue to produce aerated concrete in this research. The experimental results indicate that products made of high-sulfur petroleum coke desulfuration residue satisfy the requirements in national code of autoclaved aerated concrete [1] (GB11968-2006) and has better performance than traditional lime aerated concrete products. Consequently, the application of high-sulfur petroleum coke desulfuration residue in aerated concrete broadens the resourcelized utilization of petroleum coke desulfuration residue, and solves the scarcity of building materials resources. It ought to have expansive application prospect because of remarkable economic, social, and environmental benefits.

Abstract: Functionally gradient materials(FGMs) were successfully fabricated with compositions gradually changed from 100% stainless steel 316L to 100% Ni25 alloy using laser engineered net shaping(LENS) technology. The microstructure characterization, composition analysis and microhardness along the gradient direction are investigated. Meanwhile, the defects in the sample such as cracks and pores are analyzed. The results indicated that the microstructure evolves from columnar dendrite to cellular-crystal or equiaxed dendrite.

Abstract: The three layers clad strip was fabricated by a tandem positioned vertical type twin roll casters. The effect of the casting conditions of the overlay strip on the interface between the base strip and the overlay strip was investigated. The temperature of the molten metal, roll-load and solidification length were chosen as the casting conditions. Base strip was AA8079 and overlay strip was AA6022. The proper melt temperature of the overlay strip was above the liquidus line of the base strip. The proper roll-load, which was lower than rolling, was existed. There was not clearance at the interface when the solidification length was appropriate.