Solid State Phenomena Vols. 217-218

Abstract: Semisolid process can be used as grain refining method to replace conventional chemical grain refining approach. Typical semisolid slurry making techniques were introduced in the paper as potential semisolid grain refining methods. The temperature and morphology control for grain refining through semisolid process were enhanced to achieve fine particle size and large particle number rather than spheroidal particle shape or low viscosity of the slurry comparing with conventional rheo-forming process. The grain refinement effect was demonstrated by a 40 mm cylinder casting of AZ31 alloy.

Abstract: the rheo-squeeze casting (Rheo-SQC) combining the rheocasting and the SQC was developed, in which semi-solid slurry was produced by the low superheat pouring with a shearing field (LSPSF) process. The three dimensional morphology of the primary α-Al phase and the rest spacing of slurry prepared by LSPSF process have been reconstructed and visualized, and the microstructures of squeeze cast A356 alloy have been obtained. Based on the three dimensional microstructure reconstructed, their three dimensional characterizations such as solid volume fraction, equivalent diameter of the extracted primary α-Al phase were measured and calculated, and the microstructures of cross section of squeeze cast product were investigated.

Abstract: The processing window is important for the semisolid processability of alloys. This study focusses on the kinetics of diffusion. It compares prediction of fraction liquid versus temperature taking into account both thermodynamic and kinetics, with experimental results from Differential Scanning Calorimetry (DSC) and Single Pan Scanning Calorimetry (SPSC). SPSC is a novel technique with an order of magnitude higher accuracy than DSC. A range of Al-Si binary alloys has been investigated. The studies reveal that the simulation results predicted by DICTRA (DIffusion-Controlled TRAnsformations) show the same pattern with experimental results in the relationship of fraction liquid-temperature. However, the SPSC results are closer to the prediction results than DSC curves even with the relatively large sample size associated with SPSC. This is potentially a significant result as conventionally one of the difficulties is predicting the liquid fraction versus temperature for the heating of a billet for semi-solid processing. DSC results are known to be unrepresentative because the heating rates which can be achieved in DSC are much lower than those in induction heating. In addition, the DSC results are dependent on sample size and heating rate. The long term aim is to gain confidence in prediction with software packages which will reduce trial and error.

Abstract: A356 alloy was widely used in automobile industry due to its excellent castability and comprehensive mechanical properties. But, with increasing demands of strictly safety of components, fatigue life of A356 alloy became the key properties which were considered seriously. To evaluate the time-consuming properties, elongation, as a replaced property, was employed for its easy testing and the relationship which was proportional to the fatigue life of materials. Semi-solid processing was proved that it can improve the elongation of materials while the mechanical properties still kept at the same level as original alloy. Presently, many semi-solid techniques were developed to produce various products, in which the additional equipment was necessary to form the semi-solid microstructure. Therefore, this work is aimed to development an easily technique to obtain the semi-solid microstructure. In present study, A356 alloy with typical semi-solid microstructure was obtained by addition of RE elements during melt processing. In addition, the melting and pouring process was kept the same as the normal gravity casting of A356 alloy. After the treatment, the elongation was 19.5% for A356 alloy with RE addition, which was much higher than that of 13% for normal A356 alloy. Microstructure observation showed that the morphology of Si was changed significantly, and the shape of spheroid was dominantly appeared other than short rod shape. The improvement of elongation was attributed to the morphology change of α-Al and eutectic Si.

Abstract: Taking AZ91D magnesium alloy as experimental material, the rheo-diecasting process was implemented by combining the self-developed forced convection rheomoulding device with high pressure die casting machine. Microstructural characteristics of the RDC tensile samples were discussed. Microstructure of semisolid slurry at different rotational speed prepared by FCR device was investigated. Tensile properties and fracture morphology of rheo-diecasting process in different conditions were studied as well. The results indicate that the excellent semisolid microstructure of AZ91D tensile samples can be obtained by forced convection rheo-diecasting process. With the increase of rotational speed, the mean size of the primary particles diminishes and the shape factor increases. The tensile properties of as-cast RDC samples are obviously better than conventional diecasting samples. T6 heat treatment can increase tensile strength and elongation of RDC tensile samples to 286MPa and 6.1%, respectively. The yield strength and elongation of T4 heat treated RDC samples are 100MPa and 12.6%.

Abstract: In this paper, the wear property of a rheo-squeeze casting Al-22Si-2Fe-1.4Mn alloy tube was investigated. The semi-solid slurry of Al-22Si-2Fe-1.4Mn alloy was prepared using segmented cooling plates. The results showed that the volume fraction and equivalent diameter of the hard particles (primary silicon and blocky Fe-rich phases) increased gradually from the outside to the inside of the tube wall. The microstructures of the tube were examined with an optical microscope, scanning electron microscope, X-ray diffraction. The micro-hardness of hard particles and the macro-hardness from the outside to the inside of the wall were tested, respectively. Wear resistance tests of the outside and the inside of the wall showed wear resistance, which depends on the distribution and the volume fraction of the hard particles, increased gradually from the outside to the inside of the tube wall, changed in gradient and agreeing with the distribution law of macro-hardness.

Abstract: This paper concerns with the optimisation of the innovative rheocasting process to produce a new generation of brake callipers, characterised by very high reliability and strength. The attained very promising properties favoured their use on a very high performance car and the presented technique can be further extended for other important challenging applications. The prototype components are produced using T6 heat treated A357 alloy. Results on the samples machined directly from the produced callipers are in detail described and analysed. Pieces exhibiting some small defects, individuated by non-destructive tests, as well as defectless pieces have been underlined to severe industrial tests, e.g. high pressure tight, as well as severe bench tests, and it has been observed that the proposed technological process assure the fulfilment of the requirements contained in standards.

Abstract: Semi solid metal processing has numerous technical and economic advantages, such as viscous, non-turbulent flow (thus no air entrapment during casting), ability to fill ultra-thin sections (thus reduced part weight), little solidification shrinkage in the die (thus little or no porosity), minimum heat imparted to tooling (thus long tool life) and good response to T-5 aging (thus reduced heat treating costs). Still, SSM has never achieved a prominent position in the field of light metals casting Why Perhaps the reason was largely the down economy of recent years and SSM will yet emerge with the prominence once expected of it.

Abstract: The goal of this paper is to examine industrial applications for semi-solid castings, and to develop strategies necessary for the wider commercialization of the semi-solid casting process. The performance and production techniques of semi-solid castings are reviewed, with the goal of identifying commercial niches where semi-solid castings can provide clear benefits over other casting process. A comparison of mechanical properties between semi-solid castings and other casting processes is presented. In addition, this paper provides an evaluation of the features of the optimal semi-solid casting processes, examines the characteristics of components that would benefit for production by semi-solid casting and describe the types of quality systems that casters need to have in place to make these types of castings. Cost analyses are presented suggesting that rheocasting can complete well with other casting processes.