Papers by Author: Wolfgang Püttgen

Abstract: The process of thixoforming incorporates a series of forming processes in the semi-solid state, which can be categorized between the conventional processes of forging and casting and combines the advantages of these processes. Thixoforming of steels in the semi-solid state, requires round, solid particles (globulites) in a liquid matrix which is deformed with low forming forces. In order to achieve laminar material flow and to produce segregation-free components, the material must fulfil diverse criteria. First, the melting interval should be as large as possible for an easy temperature regulation. Next, low solidus and liquidus temperatures are advantageous regarding tool loading. Additionally, thixoformable steels should show a melting behaviour that is finegrained and globular. Furthermore, these steels should possess low contents of intraglobular liquid phase fractions. This paper gives a survey of the current state of steel Thixoforming and deals with the development of adaptive heat treatment strategies. Regarding the structure formation and the development of suitable heat treatment strategies, the once semi-solid state yields new structures that can be applied in ways not previously possible with conventional hardening processes. New microstructures and up to date unknown better mechanical properties can be adjusted with an optimised heat treatment strategy. By this, new fields of application for thixo-materials can be entered and also advanced procedures for special applications can be established. For example the steel X210CrW12 leads to a very hard material with high wear-resistance, which can be used at higher temperatures than the conventional hardened material. In general, new generic microstructures after thixoforming results in unexpected favourable mechanical properties. Problems arise with respect to segregation and pores which resulting in inhomogeneous property distributions.

Abstract: The Thixoforming of metals is a new forming technology, which combines the advantages of casting and forging. This technology focuses on structural components with complex shapes, which can not be manufactured using conventional processing methods. Different components made of aluminum and magnesium are already industrially produced. Apart from this, the use of steels in the Thixoforming processes is still being researched and developed. During the last years it became obvious that the Thixoforming of steels may not be a complete substitution for highly automated die-casting or forging processes, but rather a new process route to produce special components with high quality and geometrical demands. Special interest lies in the development and production of filigree components, parts with voids or undercuts, integrated elements such as tubes or tube-systems and joining pieces or products made of different materials. Beside the design of suitable tool materials and tool concepts, the development of homogeneous heating strategies and the development of thixoformable steels is at the center of current research. Therefore, this paper focuses on material requirements which are needed for the Thixoforming of steels and on the production of components and occurring problems. By use of a failure catalogue for Thixoforming of steels, reasons for characteristic failures and suggestions are given to avoid them, so that the quality of thixoformed parts can be improved.

Abstract: This paper deals with the examination of the steel grade X210CrW12 and its suitability for Thixoforming. In the semi-solid state, the microstructure of this steel consists of a homogeneous distribution of globular particles surrounded by liquid. Additionally, carbides are stabile up to fraction liquid contents of approximately 40%. After quenching, this steel reveals a globular austenitic microstructure with eutectic (former liquid phase) at the grain boundaries. Compression tests permit to determine the influence of different process parameters on the properties and the material behaviour in the semi-solid state. Their influence on the microstructure was investigated but not yet highlighted. Further investigations on this steel allow a fine-tuning to establish optimal process windows of the different thixoforming- and rheoforming- routes.

Abstract: The bearing steel 100Cr6 in the forged and hardened condition is of great importance in industrial use. Escaping the geometry restrictions of conventional forging, the application of semi-solid metalworking (SSM) offers significantly increased design freedom. Using conventionally available rolled feedstock material with carbide banding, however, results in a higher segregation tendency during thixoforging, and thus special attention was paid to the feedstock’s “quality”. To achieve a fine-grained, globular microstructure in the semi-solid state, castings with and without the addition of 100 ppm titanium were compared with the hot rolled material. With its inherent nitrogen Ti forms TiN particles, which reduce grain-growth in austenite. The results indicate that TiN precipitates strongly affect grain growth during solid state processing, but the grain size in the semi-solid state can only be influenced for short process times. Generally the cast feedstock materials possess smaller globulites in the semi-solid state compared to forgings, so that a reduction of the sponge effect and a minimization of the segregation in produced components are expected. Since the cast material already showed a fine-grained, globulitic microstructure, the use of TiN is not recommended because of the possible negative influence of TiN on the dynamic mechanical properties.

Abstract: The semi solid forming of steel has the potential of an innovative technology, which could open a new possibility of metal components production. Earlier research works in the 1990s have already shown the principal feasibility of the process but also revealed the various kinds of technical problems. Since then, research works on steel processing have been continuously made in several locations in Japan, USA and Europe. Currently, the focus of these research activities is the development of suitable tool material and the achievement of high part quality, which is the key point for the success of the process. This paper gives an overview of the previous research works and the current state of the research with an outlook on future tasks for this challenge.