Solid State Phenomena Vols. 141-143

Abstract: In this work Al-7wt%Si-10wt%SiCp composites were produced by injection of the reinforcement in the form of SiCp particles or a specially made particulate composite powder of aluminum and SiCp into the melt of proper composition at a temperature corresponding to 10% solid fraction. This paper presents the results of the investigation on the effects of reinforcement addition form, reinforcement addition temperature, stirring speed and magnesium addition on the incorporation and distribution of the reinforcement particles. The results showed that incorporation of SiCp particles was considerably improved by their injection in the form of milled Al/SiCp composite powder. Better particle wetting, improved particle dispersion and reduced particles size were achieved by injection of milled Al/SiCp composite powder instead of SiCp powder. Magnesium addition and high temperature injection were necessary for achieving good incorporation. Reinforcement incorporation was improved by increasing the stirring speed up to 500 rpm, after which the incorporation decreased slowly.

Abstract: Novel In-mold rheocasting, which can satisfy all the development direction of rheocasting, has been successfully developed. In-mold rheocasting requires no additional processing equipment except diecasting machine, no grain refinement procedure and no additional cycling time to produce slurry-on-demand. Process concept of In-mold rheocasting is simply based on the paradigm shift from HOW to WHERE for producing slurry-on-demand and the shift from obtaining PERFECT solid globules to casting OPTIMUM products. Optimum external field can be generated in mold, that is, in die cavity exactly around biscuit. This paper will demonstrate the possibility to achieve slurry-on-demand by In-mold rheocasting and some commercial products.

Abstract: An advanced rheo-diecasting technique, based on a modified multi-electromagnetic stirring continuous preparation (MSCP) process was developed for manufacturing near-net shape Al-alloys components with high integrity. The modified MSCP process innovatively combines noncontact electromagnetic stirring and an annular chamber with specially designed profiles to in situ make high quality semisolid slurry, and intensive forced shearing and self-wiping action can be achieved under high shear rate and high intensity of turbulence inside the annular chamber. Three Al-Si alloys with hypoeutectic, eutectic and hypereutectic compositions were tested in this study. The results indicate that, compared with those produced by conventional high-pressure diecasting (HPDC) process, rheo-diecasting samples based on the modified MSCP process have finer and more uniform microstructure.

Abstract: Novel hot chamber rheo-diecasting process has been successfully developed. It requires no additional processing equipment except commonly used hot chamber diecasting machine, no molten metal control like grain refinement procedure and no additional cycling time to produce slurry-on-demand. Additionally, it could lower molten metal temperature in a furnace. Process concept of novel hot chamber rheo-diecasting is simply based on the paradigm shift from HOW to WHERE for producing slurry-on-demand. This paper will demonstrate the possibility to achieve slurry-on-demand by novel hot chamber rheo-diecasting and the advantages for 3Cs applications.

Abstract: Recently, BCAST at Brunel University has developed a MCAST (melt conditioning by advanced shear technology) process for conditioning liquid metal at temperature either above or bellow the alloy liquidus using a high shear twin-screw mechanism. The MCAST process has now been combined with the twin roll casting (TRC) process to form an innovative technology, namely, the melt conditioned twin roll casting (MC-TRC) process for casting Al-alloy and Mg-alloy strips. During the MC-TRC process, liquid alloy with a specified temperature is continuously fed into the MCAST machine. By intensive shearing under the high shear rate and high intensity of turbulence, the liquid is transformed into conditioned melt with uniform temperature and composition throughout the whole volume. The conditioned melt is then fed continuously into the twin-roll caster for strip production. The experimental results show that the AZ91D MC-TRC strips with different thicknesses have fine and uniform microstructure. The strip consists of equiaxed grains with a mean size of 60-70μm. The strip displays extremely uniform grain size and composition throughout the whole cross-section. Investigation also shows that both TRC and MC-TRC processes with reduced deformation are effective to reduce the formation of defects, particularly the formation of the central line segregations.

Abstract: For hypereutectic Al/Si alloys, one of the advantages of thixoforming in comparison with casting routes is the relatively short processing times at high temperatures and hence limited coarsening of the Si phase. Coarse silicon particles give poor mechanical properties. Here two hypereutectic Al/Si alloys (magneto-hydro-dynamically (MHD) stirred A390 from Pechiney and an extruded A390 alloy from Showa in Japan) have been thixoformed to form pistons. Opening up the die entrance to the full width of the crown made the flow into the die more uniform and helped to reduce the tendency for large pores to form due to swirling of the slurry. Die heating reduced cold shuts in the skirt (thin section) of the piston. Placing inserts into the die to make holes for the piston pins (i.e. having an obstacle in the thicker regions) evened up the flow between the thick and the thin regions in the die. Massive pores experienced in earlier shots were then eliminated. Placing ceramic material in the die entrance considerably reduced the shrinkage porosity in the crown. The use of the Showa alloy, where the globular semisolid microstructure is achieved by a solid state deformation route rather than MHD, gave reduced shrinkage porosity and eliminated macrosegregation of the eutectic and the silicon. Computer modeling has aided optimization of the die.

Abstract: The development of the thixoforming process made great progress since its beginning. But whereas thixoforming of aluminium and other low melting point alloys is now an industrial reality, thixoforming of high melting point alloys, as steel, is still at the research level. High temperature issues, die wearing and production rate are problems that must be solved and are more and more investigated. This work is a study of the robustness of a complete thixoforming unit dealing with steel. This flexible unit allows parts of various shapes be formed in conditions close to industrial ones. The study focus on the heating strategy, heat exchanges between the slug and the tool, the feeding and the cycle time. In this work, modelling has been used to simulate heating and feeding. In parallel, experiments have been made to obtain the needed parameters, to validate the modelling and to evaluate the capacities of such a line.

Abstract: The deformation process of steel in the semi-solid state, or thixoforming is an alternative solution to the classical industrial processes that are the casting or the hot forging. Thixoformed steel parts are of better quality and may substitute advantageously parts realized in cast iron leading to an improvement of the rigidity, for example, but also parts in aluminium alloys obtained by the main deformation processes: for lower volume, thixoformed steel parts, keeping the same functionality, present superior mechanical properties than the parts realized in aluminium alloys. The industrial development of the steel thixoforming must be accompanied by the knowledge and the various parameters control of the process: steel grades definition and elaboration, the homogeneous high-temperature heating of the slug before deformation, conception or re-conception of the part to adapt it to the process and the practice properties, the equipment definitions which are used for the deformation operation at the semi-solid state. The handling system is also important in the thixoforming process; it is going to allow the slug transfer, at the semi-solid state, between the heating zone and the tooling or to evacuate the thixoformed part towards the mean assuring the quality heat treatment of the part. The means choice which are industrially going to ensure the process have a big importance to obtain successful production line and so, a cost reduction by “finished” part competitive with regard to processes used classically today. After the description of the applicable key points to the main parameters and the solutions used to obtain a successful product: high mechanical characteristics, near net shape or net shape geometry, no flash and limited machining operation, ... are described , The investigations finished with an approach comparing the part cost realised by thixoforming process with regard to classical processes.

Abstract: This work is a contribution to improve the knowledge of components behavior produced by semi-solid processes particularly the Sub-Liquidus-Casting process. Die design was supported by using the Procast simulation program. The effect of the different variables of the process in structural integrity of the product has been described and analyzed. The components were produced using A356 alloy formed on a pilot plant with a 400 Ton THT press. The components study was made by RX, metallographic analysis and mechanical tests. The effects of T5 and T6 heat treatments were also studied.

Abstract: A technique to achieve the globular weld structure using stirring the localized semisolid zone during butt-joining of zinc AG40A (Zamak-3) die cast alloy is reported. Since the semisolid temperature range of this alloy is very narrow, the accurate controlling of weld pool temperature must be considered. By presented process, globular microstructure of the weld zone can be achieved. Moreover the near weld zone would have the globular structure due to semisolid holding. A gas heating system was designed to heat up the nitrogen gas to desired temperature accurately. A heating element was embedded in a castable ceramic block while a stream of gas could pass closely around the hot element length. Hot nitrogen gas flow through a precise ceramic nozzle was used to create a localized semisolid pool. At this stage a fine stirrer was introduced into the weld seam in order to mix the two sides into a single uniform joint. Local mechanical properties of different zones show a good strength in the weld metal zone and heat affected zone by results of the shear punch tests and hardness tests. Air entrapment in the pool decreases by controlling the speed of substrate movement and stirrer rotation under the nozzle. The strengths of each zone show relative correspondence to the variation of the hardness values.