Papers by Keyword: Rheo-Casting

Abstract: The apparent viscosity model of the semi-solid A356 aluminum alloy, based on fitting of experimental data obtained by the Couette type viscometer, was developed in the paper. The commercial package CastSoft6.0 coupled with the model was used to simulate the mould filling of the semi-solid A356 aluminum alloy in the key-shaped component with iron cores. The simulation results showed that the position of the iron cores has an important effect on the filling of the semisolid slurry, and it is easy to obtain the completely filled key-shaped component when the iron cores were near to the inlet. The filling tests verified that the simulation results have good agreement with the experimental results. The fitting results indicated that the developed apparent viscosity model is practical and feasible and it can be used to simulate the mould filling process of the semisolid A356 aluminum alloy slurry. Also the parameters were optimized and the optimum parameters are as follows: the inject pressure is more than 15MPa, the inlet velocity is more than 1.73m/s and the forming temperature is over 585 °C.

Abstract: In the present work, a 2-pole linear electromagnetic stirrer (LEMS) is developed to study the effect of stirring during solidification of aluminium alloys. The stirrer design entails the placement of a stack of coils around the mold to generate a primary motion that recirculates along the longitudinal direction. The stirrer is first tested and validated by measuring the electromagnetic forces on solid aluminum cylinders of different diameters as a function of excitation current. The alloy to be stirred and solidified is placed in a cylindrical graphite mould located in the annulus of the LEMS. A suitable cooling arrangement is provided at the bottom of the mould to extract heat from the melt, in order to produce a rheocast billet inside the mould. Rheocasting experiments with A356 aluminium-silicon alloy are performed using a stirring current of 250A, in order to assess the effect of electromagnetic stirring on microstructure formation. The resulting microstructures and cooling curves with stirring are compared with those obtained without stirring.

Abstract: The needs for high-strength and light weight structural materials have increased in automotive and aerospace structure applications. The semi-solid processed light alloys inherently offer the opportunity to produce high integrity components for these requirements. Various processing methods exist for applying agitation to a molten metal during solidification to obtain metal slurries suitable for semi-solid metal processing. In this paper, a new technique (Cup-Cast method) to achieve semi-solid metal structure using agitation and direct spherical growth during solidification is reported. Cup-Cast method is the most quick and simple semi-solid processing route which semi-solid slurry would be prepared just by pouring molten metal into a metallic cup. In this study Cup-Cast method was introduced and effect of process parameters on micro-structural characterization of slurry prepared by this method was investigated.

Abstract: In order for SSM forming to produce homogeneous properties in a casting, it is important that there is a uniform distribution of the primary grains. Besides producing a sound casting free of porosity, the amount of liquid segregation must be minimized. The surface liquid segregation phenomenon was investigated on high pressure die cast (HPDC) A356 alloy. SSM slurries were prepared using the CSIR Rheocasting System and plates of 4mm × 80mm × 100mm were HPDC. The chemical composition depth profile from the surface was determined using optical emission spectroscopy (OES) and glow discharge optical emission spectroscopy (GDOES). It was found that a 0.5-1.0 mm eutectic rich layer existed on the surface of the alloy. The thickness of the segregation layer depended on the location on the casting. It was found that this layer was insignificant close to the gate of the casting but was relatively consistent over most of the plate. Although this segregation layer did not impact on the bulk mechanical properties, hardness tests did reveal that this region had significantly higher hardness values which may have a considerable impact on the fatigue properties.

Abstract: A new slurry making method for rheocasting termed “Melt Spreading and Mixing Technique” (MSMT), based on forced uniform solidification theory, has been proposed, by which the bulk metal is dispersed into thin liquid film that is cooled rapidly and uniformly to allow copious nucleation. With a model alloy Al-wt%6.5Si to conduct a series of experiments, feasibility of this method and influence of process parameters upon slurry microstructure were studied. The results show that semisolid slurry for mass production with uniformly fine nondendritic microstructures can be obtained consistently and continuously by optimizing process parameters.

Abstract: Rheo-casting of 4045 aluminum alloy bar was operated using an unequal diameter twin roll caster and a cooling slope. The semisolid slurry was made by the cooling slope. The thickness of the bar was 12 mm and width was 100mm. The casting speed was 1m/min. Quantity of the melt poured on the cooling slope was important to make sound bar. The primary crystal became spherical, and its size was about 40μm.

Abstract: Wrought aluminum alloys have wide variety applications in aerospace and automobile industries, due to their superior properties compared to casting aluminum alloys. Rheoforging is a modification of thixoforging, which starts directly from the liquid phase instead of reheating of a billet. In the present process, suitable semi-solid slurries of wrought aluminum alloys are prepared by the LSPSF (low superheat pouring with a shear field) rheocasting process within 25s. The effects of processing conditions on the degree of spherical grain refinement in 2024, 6082 and 7075 alloys are discussed. 2024 and 7075 alloys have been used in order to investigate rheoformability of high performance aluminum alloy. Experimental results show that rheoforging based on LSPSF process can produce relatively homogeneous microstructure throughout the cup-shaped component. However, high solid fraction of semi-solid slurry promotes metal flow and results in solid/liquid segregation. Subsequent optimized heat treatments raise significantly the mechanical properties. Future potentials and challenges to be solved are discussed.

Abstract: Novel In-mold rheocasting, which can satisfy all the development direction of rheocasting, has been successfully developed. In-mold rheocasting requires no additional processing equipment except diecasting machine, no grain refinement procedure and no additional cycling time to produce slurry-on-demand. Process concept of In-mold rheocasting is simply based on the paradigm shift from HOW to WHERE for producing slurry-on-demand and the shift from obtaining PERFECT solid globules to casting OPTIMUM products. Optimum external field can be generated in mold, that is, in die cavity exactly around biscuit. This paper will demonstrate the possibility to achieve slurry-on-demand by In-mold rheocasting and some commercial products.

Abstract: Rheo-die casting (RDC) based on LSPSF (low superheat pouring with a shear field) rheocasting process has been exploited. In case of secondary die casting aluminum alloy YL112, LSPSF allowed for preparation of sound semi-solid slurry in 15-20s that fully meet the production rate of HPDC, the primary α-Al exhibiting a mean equivalent diameter of 70 μm and shape factor of 0.93, without any entrapped eutectic. Compared to conventional HPDC, RDC improves microstructures in castings. Secondary solidification of semi-solid slurry takes place uniformly throughout the entire cavity, producing an extremely fine and uniform microstructure. The experimental results show the RDC 380 alloy has much improved integrity and mechanical properties, particularly elongation, and heat treatment can be used to enhance the mechanical properties.

Abstract: An improved and self-developed semisolid preparing and rheomoulding device — rotating barrel rheomoulding machine (RBRM) for light alloys in laboratory is introduced in this paper. It mainly consists of a melting furnace, a shearing system with two relative-rotating conical barrels, a central temperature control unit, gas protection system and a die-casting system. Microstructure-processing relationship of A357 aluminum alloy obtained by the RBRM process is investigated by different intensity of turbulence and different shear rate. The experimental results show that the improved RBRM is capable of eliminating coarse dendrites, and producing small and spherical solid particles uniformly distributed in a eutectic matrix. In addition, the process can eliminate entrapped gas and reduce fine shrinkage pores in the specimens as well. Compared with the original self-developed device in our laboratory, the improved equipment has the following advantages: accurate control of stirring temperature; small volume and convenient manipulation; fine and spherical solid particles, chemical and microstructural uniformity throughout the specimens and so on.