Solid State Phenomena Vol. 348

Abstract: GISS (Gas Induced Superheated Slurry) is one of the more popular processes for the production of commercial semi-solid castings. For the successful production of high-quality castings, it is necessary to understand the impact of processing parameters on the flow behavior of the semi-solid metal. This paper describes the results of laboratory studies to examine the effects of processing conditions on the development of semi-solid feed material during GISS processing for two aluminum casting alloys, ADC12 and A356. Two series of tests were performed. The first involved a simple pouring test along an inclined section of a v-channel, to determine if differences in flow behavior could be identified. The second series of trials examined the effect of processing temperature and time on the consistency and flow behavior of the aluminum alloys. Ladles of molten aluminum alloy were treated using the GISS process at different temperatures for between zero and 10 seconds. After the treatment, the alloy was allowed to further cool in the ladle into the semi-solid temperature range, at which time the flow behavior of was compared. Consistency more suited to semi-solid casting was obtained when the GISS treatment temperature was lower and treatment time longer.

Abstract: Semisolid Al–Mg alloy strips were cast using high-speed twin-roll casting under very low roll loads to investigate the effect of low roll loads on surface cracking and center segregation of Mg. In the conventional twin-roll caster for aluminum alloys, the roll speed is usually less than 2 m/min, and the roll load is typically greater than 1 kN/mm to solidify the aluminum alloy and reduce casting defects. In the vertical type high-speed twin-roll caster, the roll speed can range from 10 to 90 m/min, and strips can be cast at roll loads below 500 N/mm, down to loads as low as 2 N/mm. Strips cast at 2 N/mm in this study did not completely solidify when released from the rolls; this means that the strips were semisolid. Al–Mg strips can be continuously cast without breaking when they are semisolid. The surface cracking and center segregation of these strips were compared with those of strips cast at a higher load of 88 N/mm. The effect of the small load on the presence of Mg at cross sections of the strip was investigated using etching with Weck’s reagent.

Abstract: In the automotive industry, casting products produced by high pressure die casting are essential. Due to the higher mechanical demands on these castings, the technological requirements of the process are also increasing. Therefore, the control of the microstructure and the development of defects play a major role. High pressure die casting parts made of aluminium usually contain gas porosity due to gas compression during the filling process of the cavity and the intensification during solidification. The use of semi-solid casting thus opens new doors to fulfil promising future demands. In this study, the venting system was adapted to the Rheometal^TM process of aluminium and designed in the form of gaps, thus ensuring better venting. Subsequently, the results obtained were compared with casting process simulations to highlight possible differences.

Abstract: Magnesium alloy castings are mainly formed by the die casting method, but this method has the disadvantage that product properties such as strength vary due to many defects inherent in casting. In this study, AZX912 (a flame-retardant magnesium alloy with 2% calcium added to AZ91D alloy) was semi-solid forged using a servo press and die cushion system to stabilize product properties. Experiments were conducted to refine the α-phase and reduce the yield point, and the stirring speed was changed and its effect was investigated. Observation of the microstructure of the molded product using an optical microscope showed that the average grain size of the α-phase became smaller as the stirring speed increased. Tensile tests were conducted on specimens cut from the compacts, and the yield point increased as the stirring speed increased. This is thought to be due to compliance with Hall Petch's law. The microstructure of the molded product was observed under an optical microscope and showed a three-layer structure with a dendrite shape in the upper part (about 5%), a grain structure in the middle part (over 90%), and a chill layer in the lower part (less than 5%). Comparison of the thickness of the chill layer showed that the thickness of the chill layer decreased as the solidus ratio increased.

Abstract: Industrial, electric railway, and automotive applications of inverters and converters use case-type semiconductor packages called power modules. Power modules have multiple joint points, and among them, from the viewpoint of reliability and bonding strength, many studies on metal-ceramic substrate bonding have been reported. Alumina Al203 or aluminum nitride AIN are mainly used for ceramics, and copper or aluminum are mainly used for metals, although alumina and aluminum have a price advantage. The joining methods are generally brazing and AMB (Active Metal Brazing), but there is a need to improve the joint strength, heat dissipation characteristics, and productivity, i.e., a joining method that does not use brazing material. The purpose of this study is to achieve direct joining of alumina A1203 and aluminum alloy A7075, and semi-solid forging [1, 2, 3, 4, 5, 6] was used as the method. The A7075 used in this study is considered to be suitable for this application because of its high strength and thermal conductivity among aluminum alloys. In this study, solid phase ratio 30 % semi-solid forging was actually performed using a servo press and a die, and the results were evaluated by observing the cross section of the fabricated specimens. The punches and dies were circular in shape to ensure uniform spreading of the molten material, and carbon steel S50C with high thermal conductivity was used to quickly dissipate the heat of the semi-solid slurry during the pressing process. The thickness of 99.6 % Al₂O₃ alumina plate was 2.5 mm. The semi-automatic stirring device was modified to stir the molten metal by attaching a 1000 mm round bar to the rotating part of a tabletop drilling machine and attaching a stirring blade to the tip of the bar. Debonding between aluminum alloy and alumina plate was not confirmed. There was no gap between the aluminum alloy and the ceramic plate.

Abstract: Metal direct writing in semi-solid slurry is an innovative technology to realize low-cost printing of load-bearing parts in contrast to laser-based additive manufacturing. However, it is challenging to achieve near net-forming of 3D parts in the current stage because of the out of controlled microstructure and hence the unstable macro extrusion of the used semi-solid slurry. Here, mixed powder remelting (MPR) is introduced to actively design the characteristics of solid phases, i.e., solid fraction, shape factor, and size distribution. Specifically, high-melting-point pure Al powder served as the dispersed solid phases in the liquid phase that transformed from Al-Si alloy powder after remelting, leading to hypoeutectic Al-Si semi-solid slurry. The effectiveness of this approach was experimentally examined and kinetically modelled, to prepare semi-solid slurry with pre-set microstructure. The improved extrusion stability of semi-solid slurry can be anticipated, and it is universal for manufacturing of metal matrix composites slurry.

Abstract: Die casting of metallic materials is a highly economical manufacturing process for producing complex functionally integrated components close to the final shape. Thixomolding of magnesium alloys is a special process of die casting. In this process, the magnesium alloy granulate is brought to a semi solid state in a screw conveyor and then injected into the mould. The production of the casting material in the Thixomolding screw allows the temperature conditions to be set for thermally controlled solid content. These are produced by heating the Mg granulate above solidus temperature and have significantly lower casting temperature compared to conventional die casting.In this study, a flow length tool for semi solid AZ91 is designed and flow length tests are performed. Also the general use of a composite material cores in semi-solid magnesium Thixomolding and their influence on flow length are investigated. Cast-in cores allow the reproduction of complex internal hollow geometries of castings or the integration of special materials.

Abstract: The performance of bracket component is low with uneven micro-structure, internal porosity and shrinkage defects produced by traditional liquid die casting process which cannot be strengthened by T6 heat treatment. In this paper, the aluminum alloy bracket with high-performance and light weight was prepared by a novel semi-solid slurry preparation technique that included chilling rod stirring and funnel-shaped cylinder device, which rapidly reducing the liquid temperature to near liquidus and realizing explosive nucleation. The effects of pouring temperature and CRS process on the microstructure and mechanical property of bracket were studied by optical microscopy (OM) equipped with polarizing mode and bench failure test. The results show that the as-cast microstructure mainly consists of near-spherical α-Al. And the semi-solid micro-structure was affected by these two critical parameters, the pouring temperature and CRS process. Without the condition of stirring, the micro-structure is typical dendritic with a certain proportion of fine spherical crystals, which was mainly generated during the filling of die casting process. And with the addition of the chilling rod, the finer and more uniform spherical semi-solid micro-structure was obtained with the lower pouring temperature. Based on the above semi-solid process and flow controlling through the mould design and vertical die casting process optimization, the high-performance bracket component without shrinkage can be prepared. Additionally, compared with the liquid die casting process, the internal quality and bench testing force of bracket prepared by CRS vertical die casting process are significantly improved.

Abstract: Semisolid processing can provide an avenue to reduced rejection rates during casting and increased capability of thin-walled castings leading to improved resource efficiency and reduced climate impact. In the RheoMetal^TM process, the slurry is formed far from equilibrium. A consequence to the deviation from equilibrium is that conventional guidelines for process stability may not give the correct appreciation of the process window, nor on the correct solid fractions generated. The solid fraction provides the slurry properties and its dependence on temperature should in theory provide the stable process window. This is discussed using data from literature and an alternative approach to identify the process stability window is given.