Papers by Keyword: Semi-Solid Processing

Abstract: An application of semisolid processing to magnesium alloys is discussed emphasizing both the fundamental and applied research activities aimed at better understanding the microstructure-property relationship. The reduced temperature of semisolid processing, providing common benefits of longer tool life, tighter dimensional tolerances and better process consistency is of special importance for magnesium alloys due to their high affinity to oxygen, requiring an expensive protection and leading otherwise to ignition and burning. However, the reduced temperature resulting in higher part integrity does not create beneficial microstructural characteristics converting to substantially improved mechanical properties. Major microstructural factors controlling properties of magnesium alloys after semisolid processing are discussed.

Abstract: Semi-solid processing of steels is typically studied using high-alloy steels with higher carbon levels, as those offer a long freezing range which is favourable for conducting the process. The drawback to their application is their microstructure which typically consists of austenite grains embedded in ledeburitic network. This type of microstructure typically fails in brittle manner by fracturing along the interface of the hard network and ductile austenite grains. This is why a way was sought to altering or even inverting the configuration of the microstructure. Eventually, suitable steel chemistries were found which allow the inverted microstructure to be obtained. With regard to the high content of alloy additions, these steels have to be made by powder metallurgy methods. Five different steels of this kind were selected for the experimental programme. All contained high amounts of alloying elements and a large fraction of carbides. Their carbon content was taken into account as well, ranging from 0.55 to 3.4 %. Differences between the steels consisted in the levels of major alloying elements, namely chromium, vanadium, molybdenum, tungsten and cobalt. After suitable process parameters were found, semi-solid processing was used to prepare demonstration products. The transition through semi-solid state transformed the ferritic matrix to austenitic-martensitic one, in which the high-stability carbides were retained. The resulting microstructures were of unconventional nature where carbide particles were embedded in tough metal matrix. Their configuration was thus inverted in contrast to the ones typically obtained by semi-solid processing of tool steels.

Abstract: Aluminium Rheinfelden has improved the competitiveness of semi solid processing by developing a casting process & alloy combination that is capable of ultra thin, ultra light parts having attractive engineering properties without need for full heat treatment. The THINCASTTM rheocasting process, together with Rheinfeldens Magsimal-59 alloy has been demonstrated capable of producing moderate sized castings with only 1-2 mm wall sections, thus providing a 30-50% reduction in traditional-process part weight. THINCASTTM can be adapted to a variety of die casting machines and will enable competitively improving the quality of conventional high pressure die castings as well as creating entirely new applications.

Abstract: The production of nanoreinforced aluminum alloys in volume and quality suitable for subsequent shape casting has been problematic. Large specific surface area and high interfacial energy of the particles combined with high surface tension of the aluminum melt makes it difficult to add appreciable numbers of particles to the melt, even when later de-agglomerated by techniques such as ultrasonic cavitation. Previous work by the authors used semi-solid squeeze casting to produce master alloys for dilution into the casting alloy. While the technique was shown to be effective, the master alloy proved difficult to remelt and was expensive to produce. The objective of this new work was to use an extrusion process that could be more readily scaled to produce nanoreinforced aluminum master alloys. This paper describes the process developed for incorporation of nanoparticles using semi-solid extrusion and the results of alloys produced using that process.

Abstract: It was reported that micrometer-sized ductile crystalline phases can improve the ductility of Zr-based bulk metallic glasses. The present authors reported the synthesis of bulk metallic glass composites by a process combining cooling slope casting and suction casting for Zr_66.4Nb_6.4Cu_10.5Ni_8.7Al₈ alloys. This study aims to investigate basics of Zr-based metallic glass matrix composites with semisolid structure. As a result, it was found that the Zr-based metallic glass matrix composites with very uniform semisolid structure can be produced by heat treatment process at semisolid region. Moreover, it was found that experimental results can be explained by the pseudo-binary phase diagram.

Abstract: The cooling slope technique has been developed in recent years, which controls the nucleation and growth of the primary grains during solidification to achieve fine and non-dendritic microstructures. In this study, A356 Al alloys were processed through a modified cooling slope technique to obtain fine, non-dendritic microstructures, in which the cooling rate of the cast crucible was controlled. Three process parameters, namely pouring temperature, inclined slope angle, and the cooling rate of the cast crucible, were varied during the processing. The cooling slope was water-cooled with a constant water flow rate. The solid fraction and the size distributions of the primary grains along the vertical and horizontal positions of the cast ingots were measured individually. The macro-segregation was examined in terms of the distribution of the solid fraction. The yields of the ingots were calculated for studying the efficiency of the cooling slope technique. The effects of the three process parameters on the microstructures, macro-segregation, and yields were studied by the Taguchi method.

Abstract: Thixo-forming is an unconventional semi-solid forming process, by which complex-shaped products can be manufactured using a single forming operation. It can even be applied to difficult-to-form materials, including those which are impossible to process by conventional methods. Today, commercial semi-solid processing is used for low-melting materials, primarily aluminium and magnesium alloys. Due to its technological complexity, thixo-forming of high-melting alloys is still under development. For this reason, the present experimental programme was focused on the tool steel CPM REX 121 with a melting point above 1200 °C produced by powder metallurgy. The total content of alloying elements in this steel is 37.5 %. Owing to the high levels of alloying elements, namely Co, Mo, W, V and Cr, this material cannot be formed by conventional methods. The purpose of the present experiment was to explore its potential for forming in semi-solid state and to find suitable processing parameters. Experimental forming took place in a mini-thixoforming die, a tool specially-developed for this thixo-forming variant intended for producing very small parts. The resulting microstructures were examined by means of optical and electron microscopy. It was found that semi-solid processing leads to the development of microstructure with austenitic grains, martensite, chromium and V-W-Mo complex carbides and also a eutectic formed by partial melting of carbides.

Abstract: The effect of semi-solid processing on microstructures and mechanical properties of rheo-sulrries A356 sand cast produced by Gas Induce Semi-Solid (GISS) process was investigated. Step plate sand molds were used in this study. The results found that the initial solid fraction of rheo-slurries did not influence on the non dendritic α-Al phase size of the as-cast alloy, while no initial solid fraction of low temperature conventional casting specimens obtained dendritic α-Al phase microstructure. In addition, both α-Al phase size and dendrite arm spacing increased with increasing the casting modulus. Tensile strengths and elongations of rheo-slurry casting specimens are higher than that of conventional casting specimens. This study indicates that low initial solid fraction slurry is suitable for gravity sand casting due to higher feeding time of this alloy and obtain the highest quality index value of 400 (tensile strength of 281 MPa and elongation of 6.1%).

Abstract: The semi-solid microstructure evolution of hypereutectic Al-20Si-3Fe-1Mn-4Cu-1Mg alloy was studied by unidirectional compression deformation experiment, at a range of deformation temperature of 833K~873K and a range of strain rate of 0.1s^-1~0.001s^-1. The results showed that the microstructure of the reheating alloy was more spherical and fine than the microstructure of as-cast, the alloy was a positive strain rate sensitive material that the flow stress was decreased with increasing in deformation temperature and it was increased with increasing in strain rate. The mechanical properties of the alloy were hardly improved when the deformation temperature was too high to fracture the thick phase of the alloy.The lower strain rate was not only reducing the productivity but also reducing the plastic deformation. The microstructure of the alloy which the thick phase was broken fundamentally and the grain became further refinement can be obtained at 833K~853K, and at 0.1s^-1~0.01s^-1.It can be done that reducing the plastic deformation resistance and strengthening the fabrication procedure of the alloy.

Abstract: 50Si-50Al alloys were synthesized by the Spray Forming process, and were further deformed in the semi-solid state. The spray-formed microstructure consists of small equiaxed primary Si in 3-5 μm scale. For semi-solid deformation at 650°C at solid fraction (fs) of 0.4, the stress increases initially to reach the peak stress, then decreases to a plateau level, followed by last stage of stress climbing, and the effects of the deformation speeds on the stress level are significant, but not large. For deformation at 750°Cat fs of 0.31, the stress increases initially, then increases at a slower rate, followed by last stage of fast climbing, and the lower the deformation speed is, the higher the deformation stress. The average deformation stress needed at 650 °C was much higher that that needed at 750 °C. The deformation was inhomogeneous at 650°C to become pieces of connected semi-solid clusters, while the deformation was much more homogeneous at 750°C.