Solid State Phenomena Vols. 192-193

Abstract: The A357 Al-Si-Mg cast alloy is commonly used for cast production of automotive components in conventional HPDC and in semi-solid processes. The aim of this work is to study the application of A357 cast alloy in thixoextrusion processes. This technology offers new possibilities of application for cast alloys and several advantages in from of conventional processes of hot-extrusion: lower pressure, minor friction forces, higher material fluidity and longer tool life. The thixoextrusion was experimented in a pilot plant, at different speeds and temperatures, with an induction furnace and a 400 Tn press. With ingots of A357 as cast, a tool with a ratio of 8.5 and with temperatures of extrusion around 560 °C, it is possible to obtain extrusion speeds more than 3 m•s-1. In all cases, when the speed of extrusion increases, the press of extrusion decreases. The effect of heat treatments and extrusion speed in the microstructure of the extruded product is evaluated.

Abstract: The constitutive equation of semi-solid 6061 alloy was established according to the thermal simulation compression tests of semi-solid billet of 6061 alloy prepared by near-liquids semi-continuous casting method. Parameters of Bingham model, such as apparent viscosity, had been investigated based on the constitutive equation of semi-solid 6061 alloy. The software ANYCASTING was applied to simulate and analyze the semi-solid die-casting processes of semi-solid 6061 alloy. A well-designed runner and gating system of automobile aluminum alloy part were presented and the casting defect were investigated here utilizing these parameters by simulation. The results show that a well-designed runner and gating system is very important to secure good quality die castings through providing a homogenous mould filling pattern. Most defects of semi-solid casting products can be founded in the final solidification of the parts of casting.

Abstract: Annular electromagnetic stirring (A-EMS) process is verified an advanced metal melt treatment technology. Application research was performed to investigate the solidification behavior of Al-Zn-Mg-Cu alloy in this study. The results indicated that uniformly fine microstructure, less composition macro-segregation and improved mechanical properties were achieved.

Abstract: The process of a novel rheo-extrusion was investigated by the combination of theoretical analysis and experimental verification based on a prototype counter rotating twin screw extruder. The investigation on the viscosity variation of AZ91D slurry and heat exchange of the system during extrusion shows both extrusion temperature and speed can affect the temperature and viscosity of the slurry. Thus, a continuous mass flow can be established by setting proper processing parameters to prevent bridging phenomenon in the extrusion according to the analysis. A thermal strategy for continuous output was setup at the highest extrusion speed of the extruder after investigation. The feasibility of the process and thermal strategy was then demonstrated by extruding an AZ91D rod using rheo-extrusion method.

Abstract: Adopting the rheoforming technology, namely rheo-diecasting (RDC) and rheo-twin-roll casting (RTRC) processes were investigated for preparing of AZ91D and AZ31 magnesium alloy products, and their structure characterization and property evaluation were presented. The trials instruct that both semi-solid and melt-intensive-homogenized rheoforming processes, in which a metal-slurry is treated near liquidus by subjecting a forced homogenization are applicable in fabrication of quality magnesium alloys. The experimental results show that the rheoformed alloys obtain more finely and homogeneously dispersed structures, and superior mechanical properties. The average grain size of the α-Mg was 20-40 μm, and the tensile strength was 250 MPa for RDC AZ91D magnesium alloy sample; The RTRC strips consist of equiaxed grains with a mean size of 50–80 μm, correspond to the tensile strength of 213 MPa and 215 MPa plus the elongation of 5.5% and 8.0% for AZ91D and AZ31 alloy strips respectively. Finally, the AZ31 magnesium alloy sheets with thickness of 1.6 mm were prepared by hot and cold rolling from the initial thickness of 6.5mm RTRC strips.

Abstract: The RheoMetal process, previously also called the Rapid Slurry Forming process (“Rapid S”) or RSF process, is a novel method to produce high quality, cost effective, semi-solid magnesium slurries for component casting. The RheoMetal process uses an Enthalpy Exchange Material (EEM) as internal cooling to produce the slurry. Typical critical process parameters in the RheoMetal process are alloy content, stirring speed, superheat and EEM to melt ratio. In this study the effects of EEM to melt ratio and superheat on the slurry characteristics was examined for binary Mg-Al alloys in the range 5.8 - 11.2 wt % Al. Samples were quenched after slurry preparation and the microstructure was studied with respect to solid fraction and -Al grain diameter. The solid fraction increased with an increasing EEM to melt ratio for all three Al-contents investigated. Further, it was found that the solid fraction as well as the grain diameter decreased with increasing aluminium content (at constant EEM to weight ratio).

Abstract: AA5182 alloy was cast using the single roll caster equipped with the scraper. Improvement of flatness of the free solidified surface of the strip cast by a single roll caster was tried by scribing of semisolid layer using a scraper. The semisolid layer could be scribed and be made flat by the scraper. The scribing force was controlled by a dead weight. Defects which occurred on the scribed surface were investigated. The property of the free solidified surface (scribed surface) was compared with roll contact surface by the deep drawing test. The property of the scribed surface was almost as same as that of the roll contact surface. Mechanical property was investigated by the tension test.

Abstract: In this study, the authors propose a new refining process different from the rheorefining (a screen method) or the melt zone method. This new refining process is based on the gradient of solidification speed of the molten metal poured into a container. The molten metal in the cylindrical container firstly solidifies to columnar crystals from the wall of the container and a liquid phase gradually changes to semisolid state and progresses to the central portion of the container. After an intended semisolid state is achieved, a backward extrusion is carried out to extract the liquid phase component. The liquid component is known to contain a lot of impurity substances. For the refining trials, three aluminium alloys were selected: wrought aluminium alloy A2011, cast aluminium alloy AC4C and aluminium beverage can scraps. Backward extrusion (extraction) was applied twice, and the amount of pure Al increased from 90.59 % in the initial ingot to 96.34 % in the refined material. This new refining process can easily achieve the purification level of the rheorefining or the melt zone method.

Abstract: Semi-solid processing is complicated by various inherent technical problems. However, once these problems are solved, thixoforming allows intricately shaped components to be manufactured very effectively – often with microstructures that cannot be produced by any other techniques. The recently introduced mini-thixoforming method is an example of such a novel technique for semi-solid processing of steel. The wall thicknesses of resulting parts are about 1 mm. Microstructures of semi-solid-processed steels typically consist of a high proportion of globular particles of metastable austenite embedded in a carbide network, the latter being much harder and more brittle. This paper illustrates that mini-thixoforming allows inverting that microstructural configuration. As an experimental material, powder steel with increased content of vanadium and chromium was used. The post-thixoforming microstructure consisted of a dispersion of carbides and high-vanadium and high-chromium eutectic in an austenitic matrix. Applying optimised processing parameters, complex-shaped parts could be produced. According to the high hardness of resulting microstructural components, the new materials are likely to exhibit extraordinary strength and wear resistance.

Abstract: The CDC (Cast-Decant-Cast) Process is novel semi-solid processing method for joining two alloys, allowing the production of components from two or more alloys. The process produces a functionally gradient material (FGM) which, instead of having a sharp interface between the two alloys, features a transition region exhibiting a smooth gradient in concentration, microstructure and properties. Functionally gradient materials provide engineers with the flexibility to design components with different material properties at different locations. The details of the CDC Process are described. This paper, for the first time, provides information regarding the fabrication of functionally gradient castings from steels, cast irons and cobalt alloys. The first example involves the production of functionally gradient castings from a Ni-Hard cast iron + mild steel. The second example describes the production of functionally gradient castings from Stellite + mild steel. Processing parameters are documented and microstructures of the castings described.