Solid State Phenomena Vols. 116-117

Abstract: The essential first step in semi-solid forming is to obtain a high grain density during the initial stages of solidification of an alloy melt. This is usually done by a combination of cooling and convection. If the grain density is too low, the resulting structure is one of conventional coarse dendrites, unsuitable for semi-solid forming. At higher grain densities, fine dendrites grow that can be coarsened to a spheroidal structure in a short enough time to be of practical interest for semi-solid forming. If the initial grain density is still higher, the grains grow in a fully spheroidal manner. Thus there are two distinctly different paths to formation of the desired structure: 1) coarsening of fine dendrites, and 2) direct spheroidal growth. We are beginning to understand quantitatively the conditions necessary to form spheroids in these two different ways.

Abstract: The numerical modeling of semi-solid processing requires data on the rheological properties of materials. This data is often obtained by rheometry but there are difficulties with characterizing high solid fractions, where the torque which can be exerted with the rheometer is insufficient. A number of other methods for measuring the flow parameters, including compression between platens, have been utilized. The various methods will be reviewed in this paper. Computational fluid dynamics modelers have often used data from steady state experiments but it is the behaviour during rapid transients which is more relevant to the actual semi-solid processing route.

Abstract: The globular microstructure required for semi solid metal (SSM) processing was initially developed by agitating melt to fragment dendrites during alpha-phase solidification. More recently, it was learned that a suitable structure was also achievable through copious nucleation of fine grains at a temperature close to the liquidus coupled with controlled cooling to prevent dendrite growth; a technique that can be applied to liquid casting processes but that is especially useful for semi solid Rheocasting (the slurry version of SSM). Rheocasting experiments were conducted to test the interactions of microstructure developed by the copious nucleation route, fractions of solid up to 0.5 during casting and slurry flow velocities up to 5 meters per second. Their combined effects on casting defects and tensile properties will be presented and discussed.

Abstract: The semi solid forming of steel has the potential of an innovative technology, which could open a new possibility of metal components production. Earlier research works in the 1990s have already shown the principal feasibility of the process but also revealed the various kinds of technical problems. Since then, research works on steel processing have been continuously made in several locations in Japan, USA and Europe. Currently, the focus of these research activities is the development of suitable tool material and the achievement of high part quality, which is the key point for the success of the process. This paper gives an overview of the previous research works and the current state of the research with an outlook on future tasks for this challenge.

Abstract: Among semisolid forming processes, increasing attention has now been focused on the commercialization of casting processes based on rheocasting in aluminum automotive casting fields because of its advantages compared to thixoforming, in regards to the efficiency in cost and productivity, and mechanical properties. In this paper, an advanced rheocasting process is introduced, which has been developed recently at Yonsei University and is now on the way to practical uses in Korea and Japan. In this method, the semisolid slurry production is based on the nucleation method, in which the nuclei of primary
phase formed in the melt are prevented to grow into dendrites, instead they grow into fine globular particles to become semisolid slurry having rheological behavior. By increasing the number of nuclei, very fine and uniform globular microstructures can be obtained, resulting in an increase of mechanical properties. Recently, commercial rheo-diecasting machines based on the present method have been developed, and now are being used in manufacturing aluminum automotive casting parts. Furthermore, the present method is now being tested to investigate its possibility of application to various alloy systems.

Abstract: This paper focuses on the liquid fraction curves of several steels and the correlation between liquid fraction, temperature and heating rate. The work has been performed along two main axes. First, the solid fraction versus temperature has been obtained experimentally by differential scanning calorimetry (DSC), limited to low heating rates. Then, a shift of the liquid fraction curves has been noticed at high industrial heating rates. The quantification of this effect could not be carried out by DSC and required the elaboration of another experimental device.

Abstract: Semi-solid processing is nowadays a powerful technology for the realization of structural components; in addition to the increase in their mechanical properties, due to the globular structure instead of the dendritic one, further developments are most likely to be expected from alloy chemical composition adjustments in order to achieve higher performances compared with the industrially used A356 and A357. Aim of this research is to try to set up new aluminium alloys for semisolid foundry applications, starting from the standard Al-Si system, at the base of all known casting processes. Different chemical compositions, based on either foundry or wrought Al alloys, have been investigated by means of computational thermodynamics (Pandat®), producing quantitative data about solidus-liquidus interval, solid fraction as a function of temperature, phase diagrams i.e. potential for age hardening, etc.. Some selected alloys, fitting the needs of good castability, proper concentration of hardening elements in the alpha phase and, obviously, easy production of feedstock material have been mechanically stirred by means of an experimental apparatus designed and self-constructed in the foundry laboratory of the university; the effect of different stirring tool configurations on the semi-solid state obtainment has also been assessed. Subsequently, the manufactured alloys have been reheated and cast into a simple die, properly designed, for the production of small samples. Microstructural investigations have been done on the stirred alloy (before and after re-heating), on the as cast and the heat treated samples to evaluate the efficiency of the designed system and of the defined alloys. Experimental tests on the processed alloys have been carried out by means of an instrumented crucible in order to verify the predicted thermodynamic properties supplied by simulation study (i.e. fs-temperature curve).

Abstract: The continuous rheoconversion process (CRPTM) is a novel slurry-on-demand process that was developed at MPI/WPI in 2002. The process is based on a passive liquid mixing technique in which the nucleation and growth of the primary phase are controlled using a specially designed “reactor”. The reactor provides heat extraction, copious nucleation, and forced convection during the initial stage of solidification, thus leading to the formation of globular structures. This paper presents our recent work on the scale-up of the CRPTM for industrial applications. In Part II of this paper, we present salient results on alloy optimization via thermodynamic simulations, as well as validation results obtained from industrial Beta trials.

Abstract: Mg-8%Al-1%Ca was selected as a base alloy composition and small amounts of minor alloying elements, RE or Sr, were added. Microstructure of as-cast Mg-8%Al-1%Ca base alloys consists of dendritic primary Mg, Mg-Al-Ca and Mg17Al12 phases. Mg-Al-RE-(Ca) phase in the RE-added alloys and Mg-Al-Sr-(Ca) phase in the Sr-added alloys were additionally found. The creep resistance of Mg-8%Al based alloy was significantly improved by the minor alloying elements additions with 1%Ca. Comparatively globular primary phase could be obtained after just 30min. through an isothermal heating of as-cast Mg-Al-Ca alloys.