Materials Science Forum Vol. 749

Abstract: In this study, Strontium (Sr) was added as modifier during the casting of Al-11.6Si-0.5Mg alloy, and the effect of T6 heat treatment on microstructure and performance of alloy was also investigated. The results showed that the 0.3% Al-8%Sr master alloy can refine effectively the α-Al dendrite and eutectic structure; the best economic process of T6 heat treatment is solution at 535°C for 6 hrs., and water cooling at 50~60°C,aging at 160°C for 6 hrs , then air cooling. After Sr modification and T6 heat treatment, the mechanical properties of alloy are improved remarkably, i.e., the tensile strength increased to 348MPa from 183MPa before modification and the elongation raises from 3.0% to 6.5%. So this alloy is applied to the strain clamp products in electric power fitting industry.

Abstract: As stabilization elements added into ferritic stainless steels, various kinds of precipitates of niobium and titanium will form and have great effect on their microstructure, which has great effect on the mechanical and corrosion properties of the final products. Combined with thermodynamic calculation by FactSage software, microstructure and precipitates of ferritic stainless steels containing different niobium and titanium were investigated by optical microscope, scanning electron microscope, transmission electron microscope and energy dispersive spectrometer. The results show that titanium mainly exists in form of TiN but niobium exists mainly in form of NbC. Moreover, a certain amount of NbN particles precipitate when there is not enough titanium to react with nitrogen. TiN particles with size of 2μm~8μm promote the recrystallization but Nb-rich precipitates with size of less than 500nm suppress the recrystallization in the process of annealing.

Abstract: Parallel flow microchannel heat exchangers have been increasingly applied in HVAC (Heating, Ventilation and Air Conditioning) field due to their characteristics, such as high efficient heat transfer performance, compact form and low cost. Production rationalization of the heat exchanger is required for realization of a competitive, light-weight and easy-to-assemble product. The heat exchanger is usually bent to L-shape so as to decrease the volume of air conditioner. To correspond with design changes for multi-generation use and to achieve good forming quality, a dieless forming technique is developed to offer flexible manufacturing. Using this method, heat exchangers can be formed with variable bending radius conveniently, which can reduce the mold cost and obtain high productivity. Meanwhile, product defects such as fin damage can also be reduced because there is no physical contact against tools in the bending portion. In the present study, the forming principle of dieless bending process is first stated by theoretical analysis. Then to verify the proposed forming mechanism, a NC dieless bending machine is manufactured to implement the bending experiment. Finally, based on the verified FE model, the bending process for the 2-piece heat exchanger has been improved. As a result, the dieless bending process has been successfully developed and can be applied to produce square-shape heat exchanger with variable bending radius.

Abstract: In this paper, tungsten-copper composite powder was prepared on the particle size of 6 ~ 10μm tungsten powder surface by electroless copper plating. The orthogonal experimental results show that the primary and secondary order of factors affecting the deposition rate followwing the sequence: copper sulfate solution concentration > pH value> solution temperature> formaldehyde concentration > complexing agent concentration. The process of the electroless copper plating on the tungsten powder surface was investigated, and the best electroless copper plating solution composition and operation conditions were obtained as follows: plating temperature 323 K, stirring speed 30 r/min, PH =13, loadage 8g/L, CuSO4 5H₂O 0.032 mol/L, HCHO 0.274 mol/L, TEA 0.1208 mol/L, 2, 2 'league pyridine 12 mg/L.

Abstract: Effect of coil temperature on the texture and microstructure evolution during cold rolling and annealing were investigated by OM, ODF and EBSD. The results indicate that the {223}<110> and {114}<110> textures appeared after cold rolling are inherited followed by annealing, whose intensities would be higher as the coil temperature decreased. Lower coil temperature could improve the uniformity of grain size and increase the fraction of low-angel grain boundaries as well as coincidence site lattice boundaries. And the {111}//ND texture is improved in the lower coil temperature cold and annealing steel sheet, which is beneficial for the deep-drawability.

Abstract: For producing higher density PM parts a new method, High-Velocity Compaction process with additional upper relaxation assist (URA) device is presented in the paper. Using zinc stearate as a die wall lubricant, iron powder was pressed with and without upper relaxation assist device of mass 0.14 Kg focusing to investigate the density and mechanical properties. To explain a compaction process in loading stage a conservation of momentum principle has been introduced during collision of hammer, upper piston and upper relaxation assist device in die compaction. To observe the morphological characteristics and mechanical properties, a scanning electron microscopy (SEM) and computer controlled universal testing machine were used. The experimental results showed that the samples compacted with URA device had an improved green density and mechanical properties compared to the samples compacted in the absence of URA device.

Abstract: The microstructure of 5182 alloys, standard and modified by Zr, before/after homogenizing heat treatment were investigated by optical microscope (OM), scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and transmission electron microscope (TEM). The microstructures on optimum condition for homogenizing heat treatment and the effect of Zr were discussed. It is found that the intermetallic compounds formed on the grain boundaries are less in Zr modified 5182 than 5182 alloys for the as-cast structure. After the two alloys homogenized at 475 for 24h, the dendritic structure disappear and intermetallic compounds become discontinuous, Al₂Mg₃ phases are decomposed, and there are some precipitates of Al₆(Mn,Fe) and Al₆Mn in the grain interior. In addition, precipitates of Al₃Zr distributed in the matrix of Zr modified 5182 alloys.

Abstract: A high strength Al-Cu-Mg alloy was prepared by squeeze casting. The effects of squeeze casting and heat treatment on the microstructures and mechanical properties of the alloy were studied. It was found that squeeze casting refined the microstructure and reduced the micro-segregation markedly, and also accelerated the diffusion process of solute atoms during solution heat treatment. Tensile strength and elongation of squeeze casting alloy were much higher than those of gravity casting alloy under both the as-cast and heat-treated conditions. In addition, the Al-Cu-Mg alloy prepared by squeeze casting showed good natural aging response, and the naturally-aged alloy possessed a slightly lower tensile strength but better elongation compared to full artificial aging. After solution heat treatment at 495 for 9h and further natural aging for 48h or artificial aging at 190 for 6h, the tensile strength of squeeze casting alloy reached to 472MPa and 475MPa, respectively, and the elongation was 18.9% and 12.7% accordingly. Based on the experimental results, the mechanism of microstructural evolution of squeeze casting Al-Cu-Mg alloy during heat treatment was discussed, and the effect of squeeze casting on the kinetics of solute diffusion and aging precipitation was studied.

Abstract: Metal/ceramic composite materials with micro-cellular structure which is Al₉₀Mn₉Ce₁ micron powders coated by 10 wt% ZrO₂ nanosized powder is prepared by spark plasma sintering. Effects of the SPS pulse current on the sintering behavior and the composite interface structure is investigated via SEM, XRD, TEM and EDS. The experimental results show that the pulse current is the key factor to the formation of micro-cellular structure. When the Al₉₀Mn₉Ce₁/ZrO₂ composites are sintered by a large pulse current, the hot focus on ZrO₂ nanopowder among Al₉₀Mn₉Ce₁ alloy particles as well as Al₉₀Mn₉Ce₁ alloy particle surface, because of the effect of spark plasma and the concentration of the skin effect in short-term. Dividing temperature field on the interface of micro-cellular structure is benefit for the local high-temperature sintering of the ceramic cell walls layer, and also for the diffusion and interfacial bonding between Al₉₀Mn₉Ce₁ alloy cell body and the ZrO₂ nanosintered cell wall, so as to achieve good sintering.