Solid State Phenomena Vols. 141-143

Abstract: Semi-solid Continuous roll-casting process applied to produce the AZ91D magnesium strips and its microstructure was studied in this paper. In order to examine further process ability and forming property of the roll-casting strips, hot and cold rolling and punching experiments were investigated. It was clarified that it is significant to combine the semi-solid process techniques with roll-casting techniques, through which we can get high quality magnesium alloy strips with nondendritic structure and improve the overall properties of the products. The largest deformation by cold rolling and hot rolling can reach 18% and 21% respectively with one-pass roller.

Abstract: The aim of this work is to study by X-Ray microtomography carried out at ESRF Grenoble the microstructure of an Al-4wt%Cu alloy which was previously cold rolled to obtain globules of the solid phase upon heating in the semi-solid range. Since this process produced entrapped liquid in the globules, 3D quantification of this liquid was performed. Moreover, the influence of the addition to the alloy of a small amount of Ba, which has been shown to decrease the contiguity between the solid globules as a consequence of the decrease of the solid-liquid interfacial energy σsl, was investigated. It is in particular shown that the amount of entrapped liquid is much larger in the Ba-containing alloy in agreement with the reduction of σsl, whereas the size of the liquid pockets is similar. In addition characterization of the interglobular liquid shows that the interface area between this liquid and the solid per unit volume is larger for the alloys containing Ba in agreement with previous observations carried out on 2D sections. The influence of strain during cold rolling is also reported but it is shown to have a quite limited influence on the previous parameters.

Abstract: Semisolid metallic alloys are commercially produced by means of mechanical or electromagnetic stirring. Among the mechanical devices, the rotating pin immersed in a solidifying alloy seems to be easier to manage in industrial practice although it can induce some porosity, depending on the shape of the pin. As known, ultrasounds are mechanical waves which, when applied to liquid metals, increase the number of solidification nuclei, so that the cast products show superior mechanical performances, as a consequence of the finer grain structure. In this paper the use of ultrasound waves applied to different alloys during solidification was studied in order to obtain feedstock for semisolid die-casting application. A dedicated ultrasound power unit, together with a proper sonicator pin, was designed and manufactured by the authors and, subsequently, the effect of the ultrasonic treatment on the microstructure of A356 aluminum alloy and ZA27 zinc alloy was investigated. All the produced samples were characterized by metallographic analyses to measure the globule size and shape factor, which are main criteria for thixo-microstructure assessment. The results were compared to those obtained with traditional mechanical stirring, showing the higher capability of ultrasound treatment in producing better semisolid microstructure. An optimized combination of process parameters seems to be necessary to get a reasonable thixotropic structure in treating ZA27 alloy. Less severe production conditions are needed in the case of aluminum alloy, revealing the potentiality of ultrasounds as an alternative treatment to traditional mechanical stirring, with the further advantage of alloy degassing and grain refinement, without the use of expensive addictions (TiB2). Trials were finally performed on a continuous casting pilot plant in combination with electromagnetic stirring to produce semisolid billets.

Abstract: Ever since copious nucleation was shown to be an efficient, cost effective method for producing semi-solid slurry, many processes have been developed to take advantage of the cost savings inherent in this method of slurry production. Despite great advances in various aspects of semi-solid processing, the cost competitive nature of the industry, most noticeably the auto industry, has prevented a wider adoption of semi-solid casting technology. This research aims to realize a more industrial appealing process by combining the synergistic benefits of semi-solid casting technology with metal matrix nanocomposite (MMNC) technology, thus creating higher value products with superior properties cost-effectively. To do this, a process that produces a semi-solid slurry though the nucleation catalysis induced by nanoparticle additions as small as 1 wt. % to alloys is proposed and the results are presented in this paper. Examination of the potential for nano-scale inoculants to catalyze nucleation of solidification showed that despite their small sizes, inoculants on the scale of tens of nanometers are capable of catalyzing nucleation in the zinc and aluminum alloys studied. Employing the differential scanning calorimetry (DSC), differential thermal analysis (DTA), and droplet emulsion techniques with nanocomposite samples showed a significant reduction in undercooling owing to the homogeneous distribution of nanoparticles by ultrasonic mixing and the potency of those nanoparticles to catalyze nucleation. Comparison of undercoolings between different types of nanoparticles, such as silicon carbide (SiC), gamma and alpha alumina (Al2O3), and titanium carbide (TiC), to relative potencies predicted by minimum lattice disregistry showed a strong correlation. Results were also examined in light of free growth and nucleation controlled grain initiation. For nanoparticles predicted to be potent nucleation catalysts by lattice disregistry, the undercoolings observed fell into the free growth controlled grain initiation regime.

Abstract: Microstructural evolution of the spray atomized and powder thixoformed hyper-eutectic A390 aluminum alloy was investigated. The spray atomized powder revealed homogeneous and very fine silicon particles distribution, due to the rapid solidification of the alloy. The semi-solid powders were extruded into a closed die cavity through a hole for the plastic deformation of the powder particles. A drop forge of 45kg weight at different heights was used in this investigation. Remarkable rearrangement and growth of the silicon rich phase was revealed in the final stage.

Abstract: The effect of reheating to the semisolid state (soaking treatment) on the microstructure evolution of the A356 aluminum alloy prepared by ultrasonic melt treatment was studied in this paper. The results showed that in general the longer the soaking process the larger and the more round the grains obtained. Higher roundness occurs at shorter soaking times in the fine-grained ascast samples, and at longer times in the inhomogeneous or the coarser-grained as-cast structures. The optimum thixotropic condition (high roundness, 0.72, and small globule sizes < 90 μm) are achieved after 5 min. soaking in the samples treated by UST at 623 and 620oC, which is the typical soaking time dictated by the industrial practice in SSM. The amount of entrapped eutectic as observed after soaking treatments is uniquely very small, suggesting that the UST-treated ingots will have better formability in the semisolid state. The growth rate constants are substantially low: in the order of 479-748 μm3/s. These growth rate constants are much lower than those reported for MHDcast A356 ingots. The growth rates of the samples produced by UST in the liquid state (i.e., 626, 623 and 620oC. Note that liquidus temperature is 619oC) are lower than those of the samples treated in the semi-solid temperatures, i.e., 617 and 614oC. The Ostwald ripening is most likely the dominant growth mechanism in the UST-treated samples during the soaking treatments. These results reveal the feasibility and competence of UST as a potential route for thixotropic feedstock production.

Abstract: In the process of metal plastic forming severe plastic deformations occur to the workpiece. As a result the constitutive relations between material and geometry are nonlinear. One of the most common tools to solve such problems is the finite element method. In thixoforming processes of semi-solid metal many factors affect thixoforming fluidity. Therefore it is important to simplify the problem and predict the metal flow properties. For that reason the thixoforming process of semi-solid AZ91D magnesium alloy was modeled with the theory of rigid viscoplastic finite element method using the commercial finite element software DEFORM-3DTM. The fluid and effective stress-strain fields in the thixoforming process were obtained and the relationships among stress, strain rate and temperature were analyzed.

Abstract: The semi-solid forming (SSF) of aluminum alloys offers many advantages over conventional casting processes. Nevertheless, the semi-solid forming is still far from its full commercial potential and mainly used in specific niche markets. The market positioning requires simple, low cost, and versatile SSF processes that are capable of processing a wide range of alloy composition including wrought alloy compositions. However, wrought alloys must be adapted to obtain the desired semi-solid processing ability and proper mechanical properties. The processing of these attractive alloys with the SEED process offers the possibility to better target specific applications and customers’ needs. In the present paper, the alloy development of AA-6061 aiming to minimize the hot tearing propensity during forming process is reported. An overview of the industrial SEED process used to produce the semi-solid AA-6061 feedstock is presented. The mechanical properties of the cast parts subjected to a specific heat treatment were evaluated. As part of the joint effort between Alcan International Limited and the National Research Council of Canada (NRC), the fatigue results obtained from the semi-solid AA-6061 die cast parts will be also reported.

Abstract: The semisolid processing technology is not widely used due to the high cost of raw material and the equipment it requires. New low-cost raw materials and processes could be the key to expand the use of this technology. This paper describes an initial effort to develop new Al-Si-Mg in terms of raw material production and processing. The morphological evolution of all the alloys produced was characterized during their reheating to the semisolid state at 45 and 60% solid fraction, as well as the semisolid behaviour in terms of viscosity versus shear rate. The adaptation of the semisolid technology to the thixoforming process via eccentric press was tested using an equipment up to 25 tons. This type of equipment is not commonly employed in this kind of processing. Results indicate that alloys with low silicon content, e.g., 2 or 4wt%Si, behave similarly to alloys with 7wt%Si, which are normally used in the thixoforming process, with a viscosity of about 2 * 105 Pa.s. The semisolid behaviour of low silicon alloys indicates the potential expansion of the range of raw materials for this application. Thixoforming of semisolid materials in an eccentric press appears to be a very promising technology, yielding parts that, despite their simplicity and restricted shape, display a very good final mechanical behaviour.

Abstract: This paper deals with the examination of the influence of alloying elements on the thixoformability of a Cr steel. It focuses on the liquid fraction curves of different Cr steels with modifications of composition. The effect of composition modifications was observed. The solid fraction versus temperature has been obtained by a thermodynamics software (MTData) and by Differential Scanning Calorimetry (DSC), limited to low heating rates. MTData permits to obtain information on the influence of alloying elements, with the advantage of the possibility to change the chemical composition very easily. The DSC tests determine the influence of the heating rate on these curves.