P. Kapranos

Semi-solid processing of alloys

EVOLUTION AND DESIGN OF MICROSTRUCTURE IN SEMISOLID ALLOYS.- Fundamental Aspects.- Characterization of Microstructure in Semisolid Slurries.- Evolution of Microstructure in Semisolid Alloys During Isothermal Holding (Soaking).- Recent Developments in Slurry Formation.- RHEOLOGY AND MODELING.- and Definitions for Rheology and Modeling.- Experimental Determination of Rheological Behavior.- Modeling of Semisolid Processing.- General Conclusions on Rheology and Modeling.- INDUSTRIAL APPLICATIONS OF SEMISOLID PROCESSING.- to Industrial Applications of Semisolid Processing.- Raw Material.- Process Control in Die Filling and Die Design.- Component Design Rules.- Practical Applications in Use Today.- The Future.

Thixoforming of an automotive part in A390 hypereutectic Al-Si alloy

Abstract Hypereutectic aluminium–silicon alloys offer the possibility of an in situ natural composite (the silicon acting as the reinforcing phase) with properties that make them attractive for a number of automotive applications. However, conventional casting techniques result in excessive growth of the silicon particles in the melt, which adversely affect the mechanical properties. Thixoforming allows hypereutectic Al–Si alloys containing 40–50% fraction liquid to be shaped into complex near net shape components, whilst keeping the silicon particle size quite fine. This paper describes the development of a series of hypereutectic alloys based on the A390 composition (17%Si, 5%Cu, 0.5%Mg), their thixoforming, their resulting microstructures and mechanical properties. Finally the thixoforming of an automotive component using an A390 alloy is also described.

Thermodynamic predictions of wrought alloy compositions amenable to semi-solid processing

Abstract Semi-Solid Metal (SSM) Processing or Thixoforming is now a commercially successful manufacturing route producing millions of near net-shape parts per annum for the automotive industry. Currently, effort is directed towards developing wrought performance alloys that are amenable to the thixoforming process. Here, the 7000 series Al–Zn–Mg–Cu system has been studied by means of thermodynamic modelling to identify those compositions likely to be most suitable. The non-equilibrium solidification has been determined by using the Scheil equation and the liquid fraction/temperature relationship obtained. The results show the effect of chemical composition and Zn/Mg ratio on the slope of the liquid fraction/temperature curves. As the Zn+Mg+Cu content increases and the Zn/Mg ratio decreases, the slopes of these curves become less steep. The effect of chemical composition, Zn/Mg ratio and temperature on equilibrium phases is also considered. The phases in equilibrium at a given temperature are strongly affected by the Zn/Mg ratio and Cu content.

Effect of heat treatment on properties of thixoformed high performance 2014 and 201 aluminium alloys

Tempering treatments T4, T5 and T6 have been applied after thixoforming to investigate the effects of heat treatment on microstructure and mechanical properties of thixoformed wrought aluminium alloy 2014, a ‘modified’ 2014 and high strength casting aluminium alloy 201. The ‘modified’ 2014 was a simplified version of 2014 with composition (wt%) Cu 3.87, Mg 0.6 and Si 0.14. The results showed that standard solution time for conventional wrought alloy 2014 (1 h at 500°C) gave incomplete homogenisation of thixoformed material. Longer times (5 and 17 h) at 500°C are required for complete solutionisation of CuAl2. Low hardness values in the as-thixoformed 2014 were increased by subsequent heat treatment. After 18 h ageing at 160°C, the hardness of material solutionised for 5 and 17 h is higher than that for the standard 1 h. The tensile properties of the thixoformed 2014 were improved and close to those of forged 2014 using optimised post-forming heat treatment (solution treatment for 17 h at 500°C followed by water quenching and then ageing for 24 h at 160°C). Thixoformed alloy 201 in the standard T6 condition (2 h at 513°C followed by 17 h at 527°C and water quenching and then ageing 20 h at 153°C) showed a significant improvement over permanent mould casting. Good elongation to fracture (19.6%) was found for thixoformed ‘modified’ 2014 in the T6 condition.

The Use of ADINA Software to Simulate Thixocasting Processes

The main purpose of work was to develop a methodology of physical and numerical simulation of the thixocasting processes. For the purpose of the studies an experiment was conducted using a GLEEBLE 3800 simulator. In this work, the GLEEBLE 3800 system was adapted for physical simulation of the processes of light metal alloy forming in the solid-liquid state. The physical simulations of thixoforming processes and characterization of thixotropic materials were supported by computer simulations using ADINA software. The numerical model of thixoforming processes was developed in order to estimate values of the rheological model parameters.

Thixoforming spray-formed aluminium/silicon carbide metal matrix composites

Abstract A particulate metal-matrix composite (LM25/SiC) has been thixoformed. The SiC particles flow with the semi-solid alloy so that the particulate volume percentage is maintained throughout the shaped material. Die fill and surface finish improved with increased softness of the semi-solid material prior to forming.

Thixo-Extrusion of 5182 Aluminium Alloy

Extrusion is a well established technology for the production of complex sections of aluminium alloys. Thixo-extrusion in comparison to traditional hot-extrusion offers several advantages such as lower extrusion and friction forces, higher material fluidity, longer tool life etc. Aluminium alloy 5182 is an important commercial alloy characterized by high strength and ductility, high corrosion resistance and good formability; it is commonly used for the production of wrought automotive components and it is also suitable for semi-solid applications thanks to its wide solidification range. The aim of this paper is to attempt the shaping of 5182 Al-Mg alloy through the thixo-extrusion process using a ceramic tool and evaluating the effect of different routes of making the feedstock on the semisolid microstructure. Particularly, two different methods were investigated: Near-solidus casting and Roll-casting using a cooling slope. All the samples produced were characterized by metallographic analysis in order to measure globule size and shape factor, as the main criteria used for assessing thixo-formability.

Rheological characterization of A201 aluminum alloy

The thermodynamic characterization as well as the rheological characterization of the A201 alloy were conducted. Thermodynamic simulations (CALPHAD method) and calorimetric experiments were performed to determine the solidus and liquidus temperatures, the melting range and the sensitivity of the solid fraction at the thixoforming temperatures. The rheology of aluminium alloy A201 was examined using a high temperature Searle rheometer. The flow behaviour was analyzed with concentric cylinders of graphite to avoid chemical interactions with the liquid or semi-solid aluminium. The rotational body was grooved to prevent a phenomenon called wall slippage. Continuous cooling experiment was used to observe the shear rate effects on the flow behaviour. It can be seen that the viscosity level decreases at higher shear rates. Shear rate jump experiment was carried out to evaluate the steady state flow curve within the analyzed shear rate range from 60 s −1 to 260 s −1 . It is found that the power law indexes are −1.35 and −1.49 for 35% and 45% solid fraction, respectively. Finally, some mechanical property data of as-cast and as-thixoformed A201 alloy are included indicating the potential for high strength applications.

Advanced Casting Methodologies: Inert Environment Vacuum Casting and Solidification, Die Casting, Compocasting, and Roll Casting

The chapter deals with vacuum casting, a process that refers to the casting and solidification of alloys within low pressure atmospheres which are often purged with inert gas. Pressures set at 0.1 Pa and below are commonly used with vacuum casting technology. This technology is employed where high cleanliness and purity are required during liquid metal processing. Vacuum casting helps to avoid contamination from reaction of the liquid metal with either the environment gases or crucible and mold container materials. In addition, recent advances in the high pressure die casting process, a process where a hydraulically powered plunger injects liquid metal alloys at high speed into a reusable steel mold are described for die castings produced from aluminum alloys, magnesium alloys, and zinc alloys. A review follows of the fabrication of composites by casting techniques, a promising route for manufacturing near net shape parts at relatively low cost and looks at several liquid fabrication techniques available to manufacture composite materials, classified according to the type of reinforcement. Fabrication techniques can vary considerably and include stir casting, liquid metal infiltration, squeeze casting, spray co-deposition, and compocasting. The chapter concludes with a review of both single roller and twin roller roll casting, processes that can cast strip directly from the melt in a one step, promising savings in effort and energy.

Viscosity - shear rate relationship during the thixoforming of HP9/4/30 steel

Thixoforming involves shaping metal components in the semi-solid state. Work on the thixoforming of high temperature materials, such as steel, is still at its initial stage; this is mainly due to the high processing temperatures involved and the potential for oxidation. For thixoforming to be possible, it is preferable for an alloy to have an appreciable melting range and before forming the microstructure must ideally consist of solid metal spheroids in a liquid matrix. This paper discusses the thixoforming load versus displacement curves of HP9/4/30 steel semi-solid slugs under compression. The data from the corresponding load-displacement curves is converted into viscosity against shear rate adopting Stefan’s equation for flow between two parallel planes. The viscosity at processing conditions, i.e. at processing temperatures in the range of 1470 to 1480°C and zero to two minutes soak times, showed a rapid decrease initially, which corresponds to a rapid breakdown in the structure, followed by a steady decrease to a near constant value with increasing rate. The work shows pseudoplasticity (or shear-thinning) behaviour of the HP9/4/30 semi-solid slurries. This data would be required for modeling the die fill with these slurries.