Abstract: Semi-solid CuSn10P1 alloy slurry was fabricated by a novel enclosed cooling slope channel (for short ECSC). The effect of filling velocity on microstructure and properties by squeeze casting was studied. The results showed that primary α-Cu phase gradually formed from dendrites evolved into worm-like or equiaxed crystals by ECSC. As the filling velocity increases, the ultimate tensile strengths and elongations of the shaft sleeve increase first and then decrease. The ultimate tensile strength and elongation of semi-solid squeeze casting CuSn10P1 alloy reached a maximum of 417.6MPa and 12.6% when the forming pressure is 100MPa and filling velocity is 21mm/s, which were improved by 22% and 93%, respectively, as compared to that of liquid squeeze casting.
Abstract: An internal cooling agent is used in rapid slurry forming (RSF) process to produce a high solid fraction slurry for a short period of time. In the process used in this research, the swarf which is known to be a low enthalpy material was added to the melt as the internal cooling agent. During the process, the swarf started to melt and a semi-solid slurry with a relatively high solid fraction was formed. This slurry was formed by exchanging the enthalpies between the low and high enthalpy materials. A commercial Al-Si-Cu alloy, i.e. AS9U3 Aluminum alloy, was used in this investigation. The microscopic examination showed that the Al-Si eutectic colonies start to melt during the melting process of swarf material resulting in the formation of globular Alpha-Al grains due to the multiplication of secondary dendrites arms. The fracture of dendrites arms and the subsequent spheroidization were suggested to be the origin of non-dendritic globular grains in the final microstructure. The amount of primary globular Alpha-phase was measured by the image analysis software. The results showed that during high pressure die-casting of AS9U3 Aluminum alloy using 4 mm thick samples, around 35 percent solid has been formed at the temperature of 580 oC.
Abstract: The 355 aluminium alloy is known to have excellent thermodynamic characteristics that render it suitable as a raw material for rheocasting and thixoforming. However, besides the controllable transition from solid to liquid phase, the refined microstructure required in the semisolid range is one of the key factors with a strong influence on the rheology of the material. This paper intends to analyse the in situ behaviour of the microstructure, in terms of morphological change, using high-temperature laser scanning confocal microscopy. The 355 alloy was prepared via conventional casting, and refined with a 30-s exposition via ultrasonic melt treatment (UST - 20 Hz, 2 kW). The material was reheated up to the thixoforming target temperature of 595 °C at which it was maintained for 0, 30, 60, 90, and 120 s, after which all the samples were cooled in water. The samples subjected to UST prior to the heat treatment were more refined in terms of microstructural evolution; they exhibited reduction in grain size (~107 ± 16 μm), smallest primary phase particle size (~81 ± 7 μm), and high circularity shape factor (~0.59 ± 0.19 μm). In situ observation methods were employed to analyse evolution mechanisms such as Ostwald ripening and coalescence.
Abstract: The commercial application of wrought aluminum alloys to semi-solid casting would be extremely beneficial, as wrought alloys often exhibit better strength-ductility combinations than cast aluminum alloys. Semi-solid casting typically reduces the hot tearing tendency, as it requires a globular microstructure and produces grain refinement, but hot tearing often still occurs during the semi-solid die casting of complex-shaped components produced from wrought alloys. This study examined the impact of intensification pressure and grain refinement on the hot tearing tendency of an Al-Zn-Mg-Cu alloy. Semi-solid slurries were produced using the SEED (Swirled Equilibrium Enthalpy Device) process. A specially designed constrained rod mold was used to evaluate hot tearing. Results showed the tendency for hot tearing decreased with increasing of intensification pressure. Grain refinement (with 0.06Ti) was also found to be beneficial to the elimination of hot tearing.
Abstract: A three phase electromagnetic stirrer was used to agitate aluminum A356 slurry and a dry and oxygen free argon gas was introduced in to the slurry by a porous graphite core at a same time. The prepared semi-solid slurry was then transferred into a metallic mold and was compacted by a drop weight. Results demonstrated a favorable increase in shape factor, decrease in aspect ratio and average diameter size at different intensities of stirring. The intensity of stirring was changed by altering the current passed through the magnetic coil and also bubbling intensity via the porous graphite diffuser. Different time intervals for electromagnetic stirring and gas induction were applied. Agitating the slurry for 90 Sec. separately by electromagnetic stirrer and GISS method, gave better results in terms of shape factor, decrease in average diameter of the globules and aspect ratio.
Abstract: This research investigates the effects of particle size and weight percent on the hardness of A356 matrix composite reinforced with SiC particulates produced by semi-solid stir casting. The particle sizes of the reinforcing phase were in the form of 37 and 100 μm SiC powders with loading content of 5, 10, 15 and 20 percent by weight. A356 alloy was melted and SiC powder was then gradually added into the continuously stirred slurry to promote uniform distribution of the particulates. It has been found that in the as cast condition, the average hardness values of A356 reinforced with different amounts of 37μm SiC, increase as expected with increasing amounts of particulates. A356 reinforced with 100 μm 10 wt% SiC, followed by T6 treatment, exhibited the highest percentage of hardness increment.
Abstract: Thixoforming or Semi-Solid Metal (SSM) forming is a manufacturing route that relies on a non-dendritic microstructure, a structure that is retained after processing resulting in components of fine and uniform microstructures with enhanced mechanical properties as compared to conventional cast products. In this project, an investigation is conducted on the microstructural development of Aluminium A201 alloy in the semi-solid range, through a combination of variable time and temperature conditions. Using equilibrium and non-equilibrium (Scheil) curves, the solidus and the liquidus temperatures as well as the melting behaviour of this alloy are estimated. The temperature range for the thixoforming process typically lies within the 40-60% fraction liquid and this is found to be between 620oC to 637oC for this particular alloy. The aim is to heat treat specimens of this alloy within the semi-solid range (the process window) and observe the resulting microstructures to establish if they comprise of near-spheroidal solid phase particles surrounded by a liquid phase. This microstructure gives the material its thixotropic properties; i.e. the material flows like a liquid when sheared but thickens and behaves like a solid when it is allowed to stand. Three different routes for obtaining the non-dendritic microstructure necessary for thixoforming are investigated in this work: Magneto-Hydrodynamic (MHD) stirring, Strain Induced Melt Activated (SIMA) and Semi-Solid Rheocasting via Cooling Slope (SSR+CS) route. These routes are compared and their potential for future thixoforming applications studied.
Abstract: A201 alloy is the strongest cast aluminum alloy, but it is considered one of the most difficult aluminum alloys to cast due to its susceptibility to hot tearing during solidification. Semi-solid casting, which characterizes fine near-globular or non-dendritic grains and relatively narrow solidification range, is potential to reduce hot cracking tendency of alloys. In this present work, semi-solid slurries of A201 alloy were prepared using Swirled Enthalpy Equilibrium Device (SEED) technique and then injected into a self-designed high pressure hot tearing mold. The microstructures of A201 semi-solid slurries with different pouring temperatures were examined. Effects of different casting pressures on the hot tearing sensitivity of A201 have been investigated. This study finds that SEED is capable of producing satisfying A201 semi-solid slurries. Lower pouring temperatures produce A201 semi-solid slurries with finer and rounder grains as well as more uniform microstructure distribution. Increasing the intensification pressure significantly decreases the hot treating tendency of A201 alloy. When the pressure reaches to 90 MPa and the mold temperature of about 250 °C, the hot tearing susceptibility (HTS) index value is nearly zero, which means almost no surface cracks are found in the semi-solid A201 die casting parts.
Abstract: A bar drawing process of an aluminium alloy in semisolid state is presented in the work. The drawing process depends on various parameters such as temperature, die-angle, shear rate etc., accordingly a study is considered. The work involves development of a model to investigate the drawing process of A356 alloy in semisolid range. The rheology of the alloy in semisolid state shows a distinct behaviour and reduces energy requirement during the drawing process. In the context, a model suitably represents the rheology of the alloy is considered to perform a study of the process in details. An analytical and a numerical solutions are combined together to solve the governing equations. Finally, in the work, the distribution of velocity, viscosity variation and drawing power of the semisolid alloy under shear are predicted in the domain. It is found that the energy requirement is reasonably less in case of semisolid bar drawing process compare to a conventional bar drawing process. Finally, the drawing power required to deform a conventional solid A356 alloy is compared with that of the semisolid A356 alloy.
Abstract: The 7075/6061 bimetallic composite ingots were prepared by semi-solid/liquid method. The microstructure characteristics of composite ingots were studied, and the diffusion law of interface components was analyzed. The results show that the semi-solid/liquid composite of 7075 and 6061 can be realized under the conditions of casting temperature of 983 K, semi-solid heating temperature of 918 K, holding time of 1h, and rapid water cooling. The obtained composite ingot has a layered distribution characteristic of "semi-solid microstructure/dendritic structure". The interface is clear, continuous, and free of impurities. From the interface to the semi-solid side, it shows such a pattern of change: “Dendritic structure” →”Semidendritic and hemispherical” → “Globularization”. Through the observation of the microstructure and the analysis of the components, the semi-solid/liquid composite mechanism is discovered to be a combination of fusion bonding and diffusion bonding.