Papers by Keyword: Rheo-Casting

Abstract: In semi-solid metal casting processes a slurry, obtained operating in the temperature window between the liquidus and the solidus curve of the phase diagram of the alloy (32.7 °C for AlSi8, 11.3 °C for AlSi11), is injected into the mould. Since the control over the process parameters becomes more complicated as the involved quantities increase, to apply these processes on an industrial scale it is necessary to widen as much as possible this narrow temperature window. This goal can be obtained by tailoring the concentration of elements constituting the alloy. A EN AC-46000 secondary alloy was selected for this study, because of its wide use in foundries. Using a CALPHAD approach, various Al-Si-based pseudo-binary phase diagrams containing main constituents of the alloy (Cu, Zn, Fe, Mg, Mn, Ni, Cr and Ti) were studied, keeping the Si content fixed from 8 to 11 wt.% and the Al content as a complement. The composition limits of acceptability of the commercial alloy EN AC-46000 were used as extremes of the simulation field. The study was then proceeded investigating the pseudo-ternary diagrams, keeping silicon and a second element fixed and a third element variable for all possible combinations. Further constituent elements of the alloy were progressively added, considering all main constituents of the alloy, and evaluating how the concentration of these elements can extend the temperature window. Mg, Fe, Mn and Ti are the most promising candidates for achieving this goal. The results of this study can provide foundries with a prediction tool to define more restrictive ranges of acceptability than those of the commercial alloy, allowing them to estimate through a simple chemical analysis whether scrap of variable composition entering the plant are suitable for the SSM process or they need preliminary corrections of composition.

Abstract: The microstructure and mechanical properties of 7050 alloy rheo-castings after treated by Annular Electromagnetic Stirring (A-EMS) melt treatment were investigated. The results revealed that, under A-EMS, the refinement and homogeneity of the solidification structure could be improved greatly and the slurry was suitable for the following rheo-casting; and also the hot-cracking defects in the casting process were significantly alleviated, meanwhile, the strength and ductility of the alloy casting were found to be comparable to those of conventionally forged 7000 series alloys.

Abstract: Generally nano-SiC particles are difficult to be added into molten aluminum metals because of poor wettability. Nano-SiC particles reinforced A356 aluminum alloy composites were prepared by a new process, i.e., a molten-metal process combined with mechanical stirring at semi-solid state and ultrasonic vibration method. The nano particles were β-SiC_p with an average diameter of 40 nm, and pre-oxidized at about 850°C to form an oxide layer with thickness of approximately 3.6 nm. The SEM analysis results show that nano-SiC particles are dispersed well in the matrix and no serious agglomeration is observed. The tensile strength and elongation of the 2wt.% nano-SiC_p/A356 composite in as-cast state are 259 MPa and 5.3%, and they are improved by 20% and 15% respectively compared with those of the A356 alloy.

Abstract: The present investigation aims to study the microstructure and mechanical properties of (A365/ SiC particulate) MMC produced by stir casting technique. The A356 matrix was reinforced by the addition of 3% SiC ceramic particulate having particle size ranging from 20-90 μm. The SiC particles were added to the molten in the semi-solid state during stirring at rotating speed of 700 r.p.m for duration of 1 minute. The composite was then poured in a preheated metallic mould, after which standard samples were prepared for microstructure and mechanical testing characterization. The results of the study revealed that the consolidation of the SiC and its distribution in the A356 matrix as investigated from the microstructure has increased; the tensile stress from 170 to 195 MPa, compressive yield limit from 156.5 to 190 MPa and ultimate compression stress at 35% strain increased from 350 to 460 MPa. Also macro-hardness of composite material has increased from 72.5 HV for unreinforced alloy to 80 HV, while the elongation% and impact toughness was reduced by 25% and 55% respectively. On the other hand, a dramatic increase in the toughness value for un-notched impact samples of A356 alloy when it cast in SSM (20 J) rather than that when it cast in fully liquid state (6 J).

Abstract: F357, a hypo-eutectic Al-7%Si-0.6%Mg alloy without beryllium, was processed with CSIR-Rheo technology to the Semi-Solid Metal (SSM) state and cast in plates by means of High Pressure Die Casting. The castings were either left in the as-cast (F) condition or subjected to T4 or T6 heat treatments prior to autogenous Nd:YAG laser welding. A welding operating window was established and within this window, the weld porosity and undercut were minimised. Butt welds complying with ISO 13919-2: 2001 could be produced by means of the optimum parameters of 3.8 kW laser power and a welding speed of 4 m/min with a twin spot laser beam configuration. The mechanical properties of age-hardenable Al-Si-Mg alloys are dependent on the rate at which the alloy is cooled after the solution heat treatment. The low heat input provided by the laser welding process resulted in high enough cooling rates to ensure that both the fusion zone and HAZ were in the T4 condition after welding. Tensile properties equivalent to the parent metal in T6 condition were obtained after subjecting welded T4 plates to conventional artificial ageing treatment.

Abstract: In this paper, a physical model of semisolid rheo-casting process by cooling sloping plate is established, and the features and parameters of this process are analyzed. The calculation results show that there exists lamellar flow and turbulent flow on the sloping plate surface commonly. The critical transfer distance from lamellar flow to turbulent flow decreases with the increment of the initial flow velocity gradually. The critical transfer distance decreases with the increment of the sloping angle slowly. The effect of the sloping angle on the critical transfer distance is relatively not obvious.

Abstract: The semisolid slurry of A356 Al alloy was prepared by low superheat pouring and weak traveling-wave electromagnetic stirring (LSPWES), and then the slurry was further soaked for proper time and finally rheo-cast into a rheo-casting sample in this paper. The experimental results show that if the melt of A356 Al alloy is poured at 630-650°C and meanwhile stirred by weak electromagnetic field for a short time, the most primary α-Al grains solidified in the slurry are spherical and only a few are rosette-like. The shape of primary α-Al becomes more spheroidal and more homogeneous distribution in the microstructure by the process of the uniform temperature fields. The experimental results also show that the slurry temperature, injection pressure and speed have a great influence on filling ability. The higher the slurry temperature, injection pressure and speed, the higher the slurry filling ability, the more easily the die cavity is filled. With the given rheo-casting sample, if only the slurry temperature should be equal to or more than 585°C, or the injection pressure is equal to or more than 20 MPa, or the injection speed should be equal to or more than 0.24m/s, the rheo-casting sample die cavity can be filled completely. The microstructure of the castings indicates that the shape, size and numbers of the primary α-Al grains in different parts of the castings are highly consistent. After being held at 535 °C for 5 h and then aged at 160 °C for 16 h, the tensile strength, elongation rate and hardness of the rheo-casting samples with the above process are respectively 321.5 MPa, 8.03% and 105.9HB.

Abstract: Based on the soft of FLOW3D, the numerical simulation was study the effect of runner-reduction ratio on the dispersing uniformity of added powders inside alloy melt. The parameters studied were gating shape and reduction ratio in cross section area. The results revealed that dispersing uniformity of powders, the flat gating system is better than the comb one, and the best dispersing uniformity was achieved when the reduction ratio was up to 50%.

Abstract: The heat treatment cycles that are currently applied to semi-solid processed components are mostly those that are in use for dendritic casting alloys. These heat treatments are not necessarily the optimum treatments for non-dendritic microstructures. For rheocast alloy A356, it is shown that natural aging prior to artificial aging causes the time-to-peak-hardness to be longer compared to the time when only artificial aging is used. Furthermore, a hardness plateau is maintained during artificial aging at 180oC between 1 and 5 hours without any prior natural aging. A natural aging period as short as 1 hour results in a hardness peak (rather than a plateau) to be reached during artificial aging after 4 hours at 180oC.

Abstract: An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheocasting. By study the evaluation law of microstructure analyzed with a quantitative metallographic analysis system for semi-solid alloys, the variation law of apparent viscosity with time has been obtained. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer, and the results of simulated experiment and theoretical calculation are in a good agreement.