Solid State Phenomena Vols. 217-218

Abstract: The work explores the production of Al/SiC and Al/SiC/NiAl₂O₄ composites by thixoforming compacted mixtures of AA7075 machining chips and reinforcing particles. It is analyzed the influence of processing parameters in the various processing steps, such as mixing, compacting, heating and thixoforming, in the final quality of products. Results showed the general viability of producing composites by the proposed technique; the utilization of recycling material is particularly important in the high demanding energy processing sector, as the Al industry. NiAl₂O₄ particles and, in less extent, also SiC particles, can penetrate into the liquid phase present in the thixotropic microstructure within the chip, promoting their disaggregation; reinforcement distribution in the composite depends on appropriate choose of processing parameters.

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: Abstract. Bubble or blowhole is one of the most common defects in the workpiece produced by a semisolid alloy process. Except some bubbles are discharged out of the melt, many of the bubbles remained in the semisolid slurry will be deformed, enlarged or merged in the storage and transport process of the semisolid slurry, and be compressed, burst, flattened into crack which is called as gas induced crack in the further semisolid process. How to control and reduce the bubble defects is a key problem to give full play to the advantages of semisolid processing technology in industrial applications. In this paper, the behaviors of growing, floating, escaping and changing of the bubbles in semisolid alloys were theoretically explored during the smelting, filling and forming, and the mathematical models for predicting bubble dimensions and remained bubble ratio in the semisolid slurry were derived based on the theory of twophase flow and the principle of rheology. Moreover, the mechanism and critical conditions for forming the bubbles defects and gasinduced cracks defects in a workpiece were discussed by mechanics analysis. Finally, the relationships between blowhole defects, cracks defects and process parameters were built through kinetic analysis for the rheology behaviors of the semisolid slurry and bubble growing process. These mathematical models will provide a reference for controlling and preventing defects of the blowhole and cracks in the semisolid process.

Abstract: It became clear that semisolid Al-25%Si has excellent flow ability and suitable for thin product die casting thinner than 1mm. Semisolid die casting of thin fin was tried using Al-25%Si. The rheocasting was selected. The pouring temperature of melt was 1103K, and the casting temperature was as same as the liquidus line 1033K. The fine, which thickness was 0.5mm, height was 25mm and draft angle was 0.5 degree, could be cast at the plunger speed of 0.8 m/s. The primary Si was smaller than 0.2mm. The α-aluminum existed by the super cooling.

Abstract: The visual appearance and surface roughness were experimentally investigated in industrial scale for a low silicon containing aluminium alloy cast in semisolid state integrated with HPDC machine. A visual comparative technique and surface roughness in the form of the Rq value were used to evaluate the surface appearance and the surface roughness respectively. The results were investigated statistically to find significant models. It was shown that high quality appearance, following a comparative scale, was possible using high die temperature and high injection speed. It was also found that improving the surface roughness will deteriorate the visual appearance.

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: The spheroidal grains in billets used for semi-solid casting are generally manufactured by electromagnetic stirring (EMS) during the casting process. This method however, is not economically applicable for small quantities of the thixo billets. Swirled Enthalpy Equilibration Device (SEED) has been developed as a rheocasting process, and the SEED process is of interest for developing new thixo alloys, as well as for optimizing the thixocasting processes for high quality components. The objective of this paper is to compare the microstructure and mechanical properties of aluminum alloy 319s billets and castings produced using EMS and SEED feed materials. The experimental results show that for as-cast billets made from SEED process, a well-developed spheroidal grain structure is distributed throughout the cross-section of the billet, while for as-cast EMS billets, the grain structure is inhomogeneous, i.e., a dendritic structure was present adjacent to the surface of the billet, while a uniform, spheroidal structure was present at the centre. After the thixocasting process, however, the both SEED and EMS billets have well-developed, spheroidal grain structures. Mechanical properties of thixocast and T61 heat treated components are comparable for the both SEED and EMS billets.

Abstract: The refining effects of La-Ce binary alloy on the primary α phase in semisolid A356 alloy prepared by electromagnetically stirred were researched, in which the refinement mechanism of the binary alloy on primary α phase were emphatically approached. The different amount of La-Ce was added into A356 melt, the melt was poured at 620°C and slightly electromagnetically stirred for 15s with an electromagnetic stirrer which was controlled by frequency conversion, then it was isothermally held at 615°C for 10min. The shape and size of primary α phase were measured, and the distribution of La and Ce was researched by SEM and EDS. The results indicated that the RE elements were easy to uniformly distribute around the primary α phase under electromagnetic stirring, and had the obvious refining effect on primary α phase in semisolid A356 alloy, in which there was the finer primary α phase with particle-like in the melt. The refinement mechanism of La-Ce binary alloy under electromagnetic stirring was concerned with the eutectic reaction between RE elements and Al in the melt, in which the α-Al phase resulted from the Al-RE eutectic reaction could serve as the nucleating site of the primary α phase during solidification of A356 alloy. The other reactant from the Al-RE eutectic reaction, Al₁₁Ce₃, was also directly used as nucleation of α phase during the solidification.

Abstract: Due to the current trend in prices of raw material and their sources, near net shaping of mechanical components will become a key factor for the companies to get the desired competitiveness. Semisolid metal (SSM) forming is one of those near net shape forming techniques revealing a high potential to reduce material as well as energy consumption compared to conventional process technologies. Thus, the aim of this research work is to demonstrate the above by manufacturing a steel commercial automotive spindle by thixo-lateral forming from three different steel grades. The starting material, the microstructure and mechanical properties are analysed along the article. Material savings of 20% have been reported together with a substantial decrease of the forming forces. In addition, great mechanical properties have been achieved which brings the process closer to the desired final industrial application.

Abstract: Cold-work tool steel is considered to be a non-weldable metal due to its high percentage content of carbon and alloying elements. To address this problem the application of a new process of semisolid joining using a direct partial remelting method was developedto achieve a spherical join structure between two parts of AISI D2 cold-work tool steel. Since the surface oxidation of this metalis very high, the control of the atmosphere during joining had to be considered. Samples were heated in an argon atmosphere at two different temperatures of 1250°C and 1275°C for 10 minutes. Metallographic analyses along the joint interface showed that an increase in temperature promoted the final joining properties and also that at a liquid fraction of 15% joining was not fully practicable. However, a20% liquid fraction can produce a very good joint and microstructure as compared to the other experimental liquid fraction. Metallographic analyses along the joint interface showed a smooth transition from one to the other and neither oxides nor microcracking was observed. The current work confirmed that avoidance of a dendritic microstructure in the semisolid joined zone and high bonding quality components can be achieved without the need for force or complex equipment when compared to conventional welding processes.