Papers by Keyword: Semi-Solid Processing

Abstract: Twin screw rheo extrusion (TSRE) is a continuous semisolid forming process. The process takes advantage of the thixotropic properties of semisolid metal slurry and the high shear, controllable forward driving and self-sweeping capabilities of a counter rotating twin screw mechanism, offering a one-step melt-to-product forming technology with significant savings on energy consumption and manufacturing cost. The present work was carried out to investigate the feasibility of the process for processing light alloys using an AZ91D magnesium alloy and to obtain optimized operation conditions for microstructure control. A range of processing parameters was tested for slurry making, feeding and extrusion and their effect on microstructure development was examined. Experimental results showed that slurry with non-dendritic primary phase particles of various volume fractions, typical of semisolid microstructure, was obtained prior to extrusion. The particles were further refined during extrusion due to dynamic shearing applied by the twin screw mechanism under controlled thermal schemes and the final microstructure obtained after extrusion was dominated by uniformly distributed equiaxed, non-dendritic spherical grains and the average grain size was in the range of 35-80 mm, depending on processing parameters. This paper presents the performance of the TSRE process, focussing on process optimization and microstructure control. Mechanisms of microstructural refinement and particle morphology evolution are discussed.

Abstract: The main alloys which have been semi-solid processed commercially are based on aluminium (particularly the cast compositions) and magnesium. There is a strong drive to broaden the range of alloys to the wrought compositions for aluminium, more creep-resistant magnesium recipes and to higher temperature alloys such as those based on copper, steels, stellites and cast irons. This paper will summarise the issues with such development including the scientific and practical issues for alloy design and the thermodynamic prediction of alloys suitable for semi-solid processing. After an initial introduction to semi-solid processing routes, the most important alloy systems for semi-solid processing from a development point of view (aluminium, magnesium, steels and composites- including nanocomposites) will be discussed. The key issues of alloy design specifically for semi-solid processing will be drawn out through the text.

Abstract: Metal matrix nanocomposites (MMNCs) are promising materials to produce engineering components for the automotive and aerospace industry. This study aims to determine the feasibility of Al/TiB2 nanocomposite fabrication by the combination of the ultrasonic method and flux-assisted particle incorporation for the production of thixoforming feedstock material. Flux assistance has been invoked to attempt to overcome challenges with the presence of oxide on the surface of the foil in the aluminium foil capsulate method. A356 alloy has been reinforced with 0.25 wt.% TiB2 nanoparticles using different methods; 1. Flux-assisted casting, 2. Flux-assisted casting with ultrasonic cavitation, and 3. The Al foil capsulate method for particle feeding with ultrasonic cavitation. The composite fabricated by the ultrasonic method with the use of flux agent provided a non-dendritic microstructure which is the requirement for thixoforming. It was found that the flux-assisted casting method is not appropriate for achieving nanoparticle entry into the melt in MMNC fabrication due to the buoyancy forces of nanoparticles, unlike micron-sized particles.

Abstract: In the Semi-Solid Processing of indirect squeeze casting, runners were needed for each part cavity. In our experiment, it was found that some of the runners were blocked, and some parts were not filling in full mold. Numerical simulation was applied to analyze this phenomenon, and the results show that it was the shell which emerged on the chamber surface that blocked the runner. Experimental results also confirmed our interpretation. Some measures have been used to correct it, such as runner design, two-layer coating, and process improvement which eliminated blocking of the runner.

Abstract: Ternary Mg-based thermoelectric materials are prepared by Semi-Solid Processing (SSP) and Spark Plasma Sintering (SPS). DTA, XRD and SEM analysis are applied to explore the processing conditions of Mg2Si1-xSnx (x=0, 0.2, 0.4, 0.6, 0.8, 1). The results show that raw-materials should be put into the furnace at 773 K, and kept 60 min in 1123 K. Then stirring 5 min under semi-solid state and air cooled finally. At temperature of 1023 K, pressure of 30 MPa, the sample is sintered by SPS. The XRD shows that the peaks of sample are sharp and complete, and move left with increasing Sn. The thermoelectric properties of Mg2Si1-xSnx (x=0.4, 0.6) are analyzed between 373 K to 673 K. Results indicate that electrical conductivity (σ) increases continually, Seebeck coefficient (α) increases then decreases, and thermal conduction (κ) decreases and increases with increasing temperature. Except that, electrical conductivity and Seebeck of Mg2Si0.6Sn0.4 are better than Mg2Si0.4Sn0.6 made by the same method. The properties of Mg2Si0.4Sn0.6 sintered at 1023 K, is better than the one at 973 K. The maximum zT of Mg2Si0.4Sn0.6 is 0.086 at 573 K.

Abstract: This paper deals with the simulation of two extrusion tests by thixoforming: a non stationary extrusion test and a double-cup extrusion test. The simulations are based on a thermo-mechanical one-phase constitutive law that has been presented in details in previous papers. A campaign of experimental extrusion testing has been conducted on a steel alloy and the comparison between the numerical and experimental results will validate the model under study. A new feature that has been added to the model is also discussed: the introduction of the phase change thermal effects such as the fusion latent heat and the contraction of the material.

Abstract: The production of nano-reinforced aluminium 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 aluminium melt makes it difficult to add appreciable numbers of particles to the melt, even when later de-agglomerated by techniques such as ultrasonic cavitation. The objective of this work was to develop a technique to incorporate particles using pressure applied while the alloy was in a semi-solid state. The composites produced could be used as a master alloy to inoculate large batches of metal for subsequent casting using any suitable technique. The results show excellent distribution of 50 nm alumina particles in 2014 material. The procedure appears to have broad applicability to a full range of aluminium alloys and particle reinforcements.

Abstract: Synchrotron radiography experiments are overviewed that directly image semi-solid deformation at the globule-scale. Globular Al-15Cu at 50-60% solid was deformed in direct-shear at 10-2 s-1. Deformation is shown to occur by globule rearrangement without discernible deformation of the individual globules. Globules were found to translate and rotate as quasi-discrete bodies in response to forces acting at globule-globule contacts, similar to liquid-saturated granular materials such as water-saturated sand. Rearrangement caused the globule packing-density (the solid fraction) to adjust by local compaction and local dilation of the globule assembly, and deformation is highly inhomogeneous. During shear, there was a net dilation and strain began to localize into a shear band of decreased solid fraction by the end of the experiments.

Abstract: In this study, the authors propose a new refining process different from the rheorefining (a screen method) or the melt zone method. This new refining process is based on the gradient of solidification speed of the molten metal poured into a container. The molten metal in the cylindrical container firstly solidifies to columnar crystals from the wall of the container and a liquid phase gradually changes to semisolid state and progresses to the central portion of the container. After an intended semisolid state is achieved, a backward extrusion is carried out to extract the liquid phase component. The liquid component is known to contain a lot of impurity substances. For the refining trials, three aluminium alloys were selected: wrought aluminium alloy A2011, cast aluminium alloy AC4C and aluminium beverage can scraps. Backward extrusion (extraction) was applied twice, and the amount of pure Al increased from 90.59 % in the initial ingot to 96.34 % in the refined material. This new refining process can easily achieve the purification level of the rheorefining or the melt zone method.

Abstract: The A357 Al-Si-Mg cast alloy is commonly used for cast production of automotive components in conventional HPDC and in semi-solid processes. The aim of this work is to study the application of A357 cast alloy in thixoextrusion processes. This technology offers new possibilities of application for cast alloys and several advantages in from of conventional processes of hot-extrusion: lower pressure, minor friction forces, higher material fluidity and longer tool life. The thixoextrusion was experimented in a pilot plant, at different speeds and temperatures, with an induction furnace and a 400 Tn press. With ingots of A357 as cast, a tool with a ratio of 8.5 and with temperatures of extrusion around 560 °C, it is possible to obtain extrusion speeds more than 3 m•s-1. In all cases, when the speed of extrusion increases, the press of extrusion decreases. The effect of heat treatments and extrusion speed in the microstructure of the extruded product is evaluated.