Solid State Phenomena Vols. 141-143

Abstract: The birth of Semi-Solid Metal Forming (SSM) or as it has now come to be widely known, Thixoforming, is a typical case of development of a technological innovation. Serendipity, stroke of luck, call it what you may, the beginning of SSM is based on an almost accidental discovery by a student carrying out a series of meticulous experiments. On the one hand, some technological failures have contributed to the lack of success across the board for SSM technologies. On the other hand, the ‘long childhood’ of the resulting technology or the process of moving from ‘Innovative Idea to Market' has been largely the result of difficult and in hindsight sometimes wrong managerial decisions, occasional personality clashes, patent rights and at times unavoidable all out business 'warfare'. Of course, hindsight is beautiful but unfortunately it always comes after the event. However, if one looks carefully at some of the notable successes of SSM forming one can discern that the problems were more on the human scale; people failures rather than technology failures. This paper aims to bring out some of these points by outlining the historical development of Thixoforming.

Abstract: Fundamentals of semisolid metal molding, including the particulate feedstock, methods of its generation and features that make it useful for processing, are outlined. Melting characteristics of the feedstock under sole influence of heat are considered, covering a wide range of microstructural and microchemical factors, believed to be of importance at high temperatures. The generation of the thixotropic slurry within the injection molding system and its solidification behaviour are accompanied by detailed features of the molded structures and their correlation with properties of net-shape components. In addition to conventional techniques the novel processing concepts including near-liquidus molding, semisolid extrusion molding as well as the alloy and composite generation in a semisolid state are described. An update on commercialization progress is completed by a characterization of the modern equipment used for process implementation with broad references to metal die casting and plastics injection molding.

Abstract: Due to the distinct rheology of semisolid slurries the process has well established advantages over competing near-net-shape manufacturing technologies. Despite the obvious advantages of the process its adoption by the casting industry, however, has been slow. This is primarily due to lack of confidence of how these slurries flow in die cavities. The added cost associated with the specially prepared slurry has also affected the process commercial success. Nevertheless despite these problems attention to the semi-solid metal process has indeed increased over the years. The main focus of this review is the modeling of semisolid slurries. The objective here is to present an overview of relevant aspects of modeling by focusing on the physics of the slurry and by stressing consistent mathematical and analysis methods to determine the material constants.

Abstract: Since the first research works in the end of 1980s on the semi-solid forming of steel, this process has presented a great interest and a real industrial potential. Several research teams, all over the world, have shown the feasibility of such a process. Working on the parameters which have an influence on the process, they pointed out the "technical locks" which must be overcome to allow industrialization of the process. A first and perhaps most important difficulty is the reliability of the forming tools in an industrial production context. Much work has therefore been devoted to identify ways to increase tools life. A second important point is the possibility to obtain sound microstructure and satisfactory mechanical properties. This paper is a state of the art review on the subject of the thixoforming of steel, restricted to forming of semi-solid reheated steel. Semi-solid forming process carried out after partial solidification are therefore not covered. The reader interested in such processes may refer to the review recently published by Hirt et al. [1]. The present review considers, in turn, the different steps of an hypothetical production line and their particular challenges, from the raw material to the final product.

Abstract: At the Institute of Metal Forming (IBF) current investigations within the framework of the Collaborative Research Centre 289 are mainly concentrated on forming of semi-solid precursor material of the steel grades X210CrW12 and 100Cr6. One important task is the precise temperature for the composition of solid and liquid phase fraction in the preheated billet. Experimental measurements and numerical simulations show significant heat losses during transport of the billet and after its insertion into the die. These developing temperature gradients influence strongly the resulting temperature field in the formed component. In case of the forming tools the critical increase of the temperature depending on the tool material is shown. As fundamental research in the field of thixojoining the temperature development of the inserts is analysed and demonstrates the feasibility of joining higher and lower melting materials into the semi-solid matrix.

Abstract: Magnesium alloys had gained an increasing interest in recent years due to their promising property profile for light weight constructions. They offer drastic advantages in weight reductions in automotive industries compared to steel or even aluminium. Therefore they can be used to decrease the emission of green house gases as requested by the EU directive for the reduction of CO2 emissions and moreover due to their recyclability they also help to fulfill the requirements from the EU directive regarding the end of life of vehicles. But still there are some limitations with regard to strength, mostly at elevated temperatures above 130 °C. To overcome these limitations alloy development as well as process optimization has to be done for further enhancement of the range of magnesium applications. This paper will show and discuss the property profiles of the standard magnesium alloy AZ91D compared to the recently developed, heat resistant magnesium alloy MRI153. The alloys have been processed using normal high pressure die casting (HPDC), New Rheocasting (NRC) and Thixomolding® (TM). As methods of investigation tensile and creep tests have been applied. The creep properties have been determined in the temperature range of 135-150 °C and loads of 50-85 MPa. All these trials have been accompanied by metallographic observations (light optical metallography, SEM) and density measurements to investigate the influence of the processing routes on microstructure and the porosity of the materials. It will be shown that the differences in the property profile of the chosen alloys are dependent on their different chemical compositions as well as on different microstructures that are obtained by the different processing routes. While in the case of AZ91D, TM is showing advantages compared to HPDC for room temperature applications, the NRC in combination with the heat resistant alloy leads to an improvement of creep rates by two orders of magnitudes.

Abstract: A semi solid thin strip continuous casting process was used to obtain 50%wt Pb/50%wtSn strip by single and twin roll processing at speed of 15 m/min. A 50%wt Pb/50%wtSn plate ingot was also cast for rolling conventionally into strips of 1.4 mm thickness and 45 mm width for comparison with those achieved non-conventionally. This hypoeutectic alloy has a solidification interval and fusion temperature of approximately 31°C and 215°C respectively. The casting alloy temperature was around 280°C as measured by a type K immersion thermocouple prior to pouring into a tundish designed to maintain a constant melt flow on the cooling slope during semi solid material production. A nozzle with a weir ensures that the semi solid material is dragged smoothly by the lower roll, producing strip with minimum contamination of slag/oxide. The temperatures of the cooling slope and the lower roll were also monitored using K type thermocouples. The coiled semi solid strip, which has a thickness of 1.5 mm and 45 mm width, was rolled conventionally in order to obtain 1.2 mm thick strip. The coiled thixorolled strip had a thickness of 1.2 mm and achieved practically the same width as the conventional strips. Blanks of 40 mm diameter were cut from the strips in a mechanical press, ready for deep drawing and ironing for mechanical characterization. All the strips achieved from non-conventional processing had the same mechanical performance as those achieved conventionally. The limiting drawing ratio (LDR) achieved was approximately 2.0 for all strips. Microscopy examination was made in order to observe phase segregation during processing.