Materials Science Forum Vol. 850

Abstract: To improve the formability of aluminum alloy, a new forming technique-the preform annealing process is proposed. Based on the mechanical properties of the original aluminum alloy AA5182-O with two experienced preforming annealing treatment, and preforming annealing process of a complex geometric shape aluminum door inner panel was numerical simulated by using LS-DYNA software. As a result, the effects of annealing time at 365°C on stamping height of the panel and the optimum annealing time were obtained. The results showed that the preform annealing process was feasible in the door inner panel. At 365°C, the smooth forming of the door inner panel was achieved after annealing for 20s with the pre-deformation amount 97mm, Which are the most suitable parameters.

Abstract: The melting of a 201 austenitic stainless steel was conducted in mushy zone for 0, 5, 10, 15 min under nitrogen atmosphere. The effect of the holding time on nitrogen content was investigated. The results showed that with the increase of holding time, the nitrogen content was significantly increased. It was found that the holding in mushy zone could avoid ferrite region during solidification, and facilitate the diffusion of nitrogen from liquid into austenite. Thermodynamic analysis demonstrates that the difference between the chemical potential of nitrogen in liquid and austenite provides the driving force for the nitrogen diffusion. It is concluded that melt holding in mushy zone is an effective way for increasing the nitrogen content in stainless steel, and thus provides a new method for the production of high nitrogen stainless steel.

Abstract: The influence of different annealing cooling rates on microstructure,precipitation of the second phase and mechanical properties of IF steel was studied. The recrystallizing behavior of IF steels was analyzed by batch annealing process. After confirming the recrystallizing temperature of IF steel, the simulations of different continuous annealing cooling rates (5°C/s, 20°C/s, 50°C/s, 200°C/s 500°C/s and 1000°C/s ) were performed. At the same time, combined with TEM and XRD, the precipitating behavior and microstructure texture were analyzed. The results showed that the recrystallization temperature was speculated between 540°C and 560°C. As the cooling rate increased, the grain sizes became smaller and the yield strength increased significantly. And the tensile strength and elongation were more or less static. The tensile strength remained 240MPa and the elongation kept around 50%. With 1000°C/s cooling rate, the sample performed best with the r-value of 3.03. TiN precipitation was observed. The annealed texture is mostly composed of {111} texture.

Abstract: The principle of phase transformation in ultra-high strength steel for cone crusher was studied by DIL805 thermal mechanical simulate, and the critical temperature was determined. The Austenite continuous cooling transformation (CCT) diagram of the steel was confirmed by thermal expansion curve, dilatometry and metallography. The phase transformation model was established and offered a theory for deciding parameters of heat treatment process. The results proved that: when the cooling rate was under 0.5 °C/s, the structure was mainly Pearlite and Bainite. With the increase of cooling rate, the content of lower Bainite increased. When it came to 1°C/s , Martensite start to transform from Austenite. When the cooling rate is 5°C/s, Pearlite disappears, Bainite and Martensite were in the majority. Meanwhile, the mathematical equations of phase transformation have high degree to fit the experimental results, and the phase transformation model is feasible.

Abstract: Compared with traditional liquid and solid processing methods, semi-solid die casting process can apparently overcome shrinkage cavity and porosity defects in castings and high deformation resistance and high residual stress shortcomings in forging parts. Semi-solid die casting process with advantages such as high efficiency and low cost, will become the optimal process for high quality automobile parts. In this study, using the clamp as an example, the author introduced product structure optimization and die design for semi-solid die-casting process of aluminum alloy in a new product development.The Computer Aided Engineering technology was applied to the product structure optimization according to the stress analysis. The optimal mold structure, including cavity layout, gating system, overflow and vent systems, were confirmed based on the die design criteria for traditional die casting, combining with the characteristics of semi-solid forming and the simulation results. The semi-solid aluminum alloy clamp parts with excellent performances were finally developed successfully by means of product structure optimization, die design, parameters optimization of die casting process, and the mechanical properties test of products.The existing parts were optimized to make them more suitable for semi-solid die casting processing. In addition, a reasonable die design specially for semi solid processing was an important guarantee for a successful semi solid product applied in industry. Computer numerical simulation was applied in product structure design for semi-solid die casting, die design, die-casting process optimization and other aspects, to shorten the development cycle of new product, reduce cost and improve efficiency.

Abstract: The coating morphology of a hot press forming steel galvanealed by HDPS (hot dipping process simulator) in laboratory was studied in the paper. The results show that, firstly from thermodynamic calculations, the intermetallics increased the melting point of Zn, and it is possible to avoid the melting and volatilization of zinc coating for hot press forming steel strip, when the coating mainly consists of δ phase (~10% Fe) in galvanealing process, and lower re-austenization temperature (below 782°C) in hot press forming process. Secondly, according to the SEM morphology in the paper, the zinc coating formed an alloy predominantly composed of delta (δ) phase, a low iron zeta (ζ) phase formed on the surface, and a high iron gamma (Γ) phase formed at the interface with the steel substrate. The hard and brittle Γ phase failed to grow up on the interface of coating and matrix which results in high shear stress and good coating adherence.

Abstract: Electromagnetic stirring (EMS) process, as an advanced melt treatment technology, is increasingly applied in metallurgy industry. Direct chill (DC) casting process by annular electromagnetic stirring (A-EMS) was applied in this study to investigate the solidification behavior of a modified 7N01 alloy, and the effects of DC casting process without EMS, and A-EMS casting process on microstructure, composition segregation and mechanical properties of continuously cast billets were compared. The research results showed that the billets with higher performance, lower macro-segregation, more uniformly fine solidification structure were obtained in case of A-EMS.

Abstract: The microstructure and mechanical properties of a medium manganese quenching and partitioning (Q&P) steel for automobile were investigated by optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and mechanical property test. The grain size and recovery degree were greatly affected by annealing temperature normally. The result shows that the medium manganese steel after quenching and partitioning (Q&P) heat treatment exhibited good mechanical properties. The maximum tensile strength and yield strength was 1280MPa and 1421MPa at 600°C, respectively. Additionally, the product of strength and plasticity could reached to 40472MPa×% at 640°C. Annealing temperature also had a great influence on the volume of retained austenite which increases linearly with the rise of annealing temperature as well.

Abstract: As the grains of 7000 series Al alloy prepared by traditional casting method are usually quite coarse, and there exists serious segregation of composition and structure in the alloy, a new forming process for high-alloying alloy is in urgent need. Here we demonstrate a new method called tiny-molten-pool solidification forming to improve the above mentioned problems. In detail, the alloy powder was melt by the plasma and then deposits on the base plate. 7075 Al alloy bulk specimens with the size of about 200 mm × 8 mm × 4 mm were prepared and the effects of homogenizing treatment, cold rolling, T6 heat treatment for the alloy were studied. The results show that there were no oxide film inclusions in the single-pass multi-layers specimen prepared by AC tiny-molten-pool solidification forming, and the microstructure was homogeneous; The grains were homogeneous and mainly equiaxed grains with a uniform size of about 20 μm, which were much smaller than that of traditional casting 7075 Al alloy; After the homogenization treatment (470 °C × 48 h), most of the net-shape eutectic structure in the alloy is re-dissolved. After cold rolling with a total deformation of 60%, the alloy became compact as the stomata were compressed. The hardness of the alloy was HV183.51 after T6 heat treatment (480 °C × 2 h + 120 °C× 15 h). The micro-molten-pool solidification forming is feasible for the forming of bulk 7000 series Al alloy.

Abstract: The inoculation and fading behavior of Sr-modified aluminum alloy A356. 2 were studied for air bag bracket produced by squeeze casting. The effects of Sr, P, B contents and casting temperature on the microstructure and eutectic silicon morphology in different periods of inoculation were investigated by SEM and direct-reading Spectrometer. The influence of inoculation fading rate and addition of Sr on the casting mechanical properties and hydrogen absorption was studied. The experimental results showed that the inoculation process was completed in 1 h, and the eutectic silicon morphology can be maintained in almost subsequent 40 h after the addition of Sr. The fading rate decreased appreciably with the increase of casing temperature, P and B contents. The deleterious effect of the inoculation fading of Sr on the casting mechanical property can be compensated by the squeeze casting.