Solid State Phenomena Vols. 116-117

Abstract: The structural changes of the wrought magnesium alloy AZ31B in semisolid state were clarified using optical microscopy and hot-stage microscopy. The influence of heat treatment variables was assessed. Compression tests covering a range from room temperature to 673 K were carried out for mechanical property assessment; flow stress and breaking strain were determined. The following are the results: (1) The grain growth of the hot-extruded AZ31B without preprocessing sensitively reacted at temperature and retention time. (2) The hot-extruded AZ31B with 30 % preprocessing showed an almost perfectly spheroidized structure in a semisolid state under certain conditions. (3) Heating velocity markedly affected the spheroidizing rate of grains. (4) From the direct observation of the hot-extruded alloy AZ31B by hot-stage microscopy, spheroidization was observed in some crystal populations. (5) Spheroided materials in the semisolid temperature range had a lower flow stress and a larger breaking strain than nonspheroidized materials. These results indicate the possibility of manufacturing wrought magnesium alloy by cold working.

Abstract: In this paper, SiC nanoparticles reinforced AM60 alloys were fabricated by semisolid process with pretreatment of SiC nanoparticles. The microstructure, hardness and mechanical properties of the alloys have been investigated. The X-ray photoelectron spectroscopy (XPS) was also used for analyzing elements chemical status of the nanoparticles and the alloys. The microstructure, hardness and mechanical properties of SiC nanoparticles reinforced magnesium alloys are better than those of AM60 alloys without SiC nanoparticles addition. The results show that semisolid process was suitable for fabricating SiC nanoparticles reinforced magnesium alloys.

Abstract: Numerical simulation of pressure influence on the relative densities of both solid and hollow cylindrical workpieces during Thixoforging was carried out by using ANSYS software. The results showed that there was a big difference in relative density between different points within the workpiece during Thixoforging with the use of a single punch. Therefore, partial feeding with the help of assembly punches was proposed to improve workpiece density uniformity and properties, and the effectiveness of this approach was well-demonstrated by numerical simulation.

Abstract: Samples of aluminium alloy A356 were manufactured by Semi Solid Metals HPDC technology, developed recently in CSIR-Pretoria. They were butt welded in as cast conditions using an Nd:YAG laser. The base metal and weld microstructure were presented. The effect of different heat treatments on microstructure and mechanical properties of the welds were investigated. It was found that the fine dendrite structure of the weld metal contributed for equalizing the mechanical properties of the joint.

Abstract: The bearing steel 100Cr6 in the forged and hardened condition is of great importance in industrial use. Escaping the geometry restrictions of conventional forging, the application of semi-solid metalworking (SSM) offers significantly increased design freedom. Using conventionally available rolled feedstock material with carbide banding, however, results in a higher segregation tendency during thixoforging, and thus special attention was paid to the feedstock’s “quality”. To achieve a fine-grained, globular microstructure in the semi-solid state, castings with and without the addition of 100 ppm titanium were compared with the hot rolled material. With its inherent nitrogen Ti forms TiN particles, which reduce grain-growth in austenite. The results indicate that TiN precipitates strongly affect grain growth during solid state processing, but the grain size in the semi-solid state can only be influenced for short process times. Generally the cast feedstock materials possess smaller globulites in the semi-solid state compared to forgings, so that a reduction of the sponge effect and a minimization of the segregation in produced components are expected. Since the cast material already showed a fine-grained, globulitic microstructure, the use of TiN is not recommended because of the possible negative influence of TiN on the dynamic mechanical properties.

Abstract: The present work studies the effect the solution heat treatment, during artificial ageing and re-aging, has on the mechanical properties of an A357 aluminium alloy component formed by New Rheocasting. The effect that the evolution of silicon, during the solution treatment at various times, has on the mechanical properties was also examined. The mechanical properties were evaluated performing tensile tests, fractographical analysis and hardness tests. The microstructural characterization was made using optical and electronic microscopy.

Abstract: The coarsening behavior of spray-formed A-35wt%Si and A-45wt%Si alloys in the semisolid state were studied. The coarsening kinetic and Si size distributions after coarsening at various temperatures and times in the semi-solid state were investigated. The coarsening exponents, n, the rate constants, K, in the LSW theory, and the modified rate constants were determined from previous works. The coarsening activation energies of the two spray-formed alloys were calculated from the slopes of ln(KT) vs. 1/T plots. The Si size distribution functions after holding at various temperatures and times in the semisolid state were determined for the two spray-formed alloys. The effects of the stokes-motion in addition to pure diffusion mechanism on the variations of coarsened Si size distribution were also investigated.

Abstract: This paper describes the basic investigation on material behaviour of cold working steel X210CrW12 in semi-solid state concerning the semi-solid forging. It includes a study of flow behaviour and microstructure by means of a high temperature isothermal upsetting test applying a special cylinder sample with a shell, which is designed to avoid collapsing of samples during the compression. The flow curve of the semi-solid steel is determined by an inverse modelling via FEM using the obtained load curve of the samples and the separately obtained flow stress of the shell material. An extensive microstructure study of the compressed samples provides a comprehensive understanding on the flow mechanism of the semi-solid steel. At the end of the paper some general restrictions and remarks for the semi-solid forging process are given based on the gained results.

Abstract: Vibrating the slope during cooling slope casting of a metallic alloy has shown helpful for causing the microstructure more spherical and thinner in a previous research by the present authors. This paper gives results from a direct observation of the SSM microstructure formation during cooling slope casting of an NH4Cl-H2O alloy with vibrating the slope. It is found that the vibration exerted to the slope can break down the solidification shell formed at the surface of the slope through rapid chilling and produce effectively fine globular grains in the microstructures. The main factors affecting the fine grain formation include the chilling extent, frequency and amplitude of the vibration of the slope, and the superheat of the alloy melt, respectively. The higher vibration frequency and amplitude, the lower superheat of the alloy melt, and the lower chilling temperature will help the formation of finer globular grains.

Abstract: The effect of microstructural features of A356 alloys on plastic deformation behavior was studied in the present study. To authors' knowledge, the microstructural effect on mechanical properties of A356 alloy has not been well understood even though this alloy system is one of the most widely used alloys for the industrial purpose. Specially, quantitative relationship between properties like ductility and fracture toughness with microstructural features is lacking. In the present study, four different processing methods were used to investigate how the size and distribution of primary alpha and eutectic phases influence the plastic deformation characteristics. The prcessing routes for A356 control arm include low pressure casting, squeeze casting, rheo-casting and cast-forging processes. Special focus was given to understand which microstructural features do the beneficial or detrimental effect and how they work.