Thixoforming of steel offers the advantages of casting technology in combination with
high mechanical strength that can only be achieved by forging. The progress in establishing this
technology in industry depends on the success in the development of suitable steel grades.
Recent investigations dealt with the development of steel grades that are especially adapted to the
thixoforming process. For this, alloys were developed with a lower solidus temperature and a wider
process temperature range compared to classic forging steels. In consequence, the inductive heating
process is more tolerant to inaccuracies and for a given liquid fraction the process temperature
window is easier to handle.
It is desired to obtain great degrees of deformation at rather low forming forces as these parameters
determine the size of the needed presses. This behaviour is affected by the present liquid fraction in
the slug and the heat transfer between work piece and die. It was detected that variations of the
forming force have a direct influence on the quality of the thixoformed parts.
In order to make the thixoforming technology of steels competitive versus other forming
technologies, the parts must show a favourable microstructure and thus, good in-use properties. In this
paper various solutions are compared.
The main results obtained in the optimization research, namely, the steel grades adapted to semi-solid
forming, the resulting process parameters and the mechanical properties of thixoforming parts will be
presented for two exemplary steel grades. By producing a real automotive part, thixoforging of steels
with regard to the adapted materials and to the ongoing industrial implementation of this process is
proved.
Today thixoforming processes are mainly established in the production of parts made of aluminium alloys. Compared to high melting alloys the process temperature is low and thus the process is easier to handle. Because of process temperatures up to 1470°C thixoforming of steels demands a sophisticated process control and adapted tool materials. In this field there is still a large need for research. This paper deals with experiences made at the IFUM concerning the thixoforging of steel. The use of ceramic tool materials for thixoforging of steel is presented. For this purpose hybrid dies where a ceramic insert is prestressed with a hot working steel shrink ring as well as different stamp geometries were developed and built up. Different ceramic materials from Si s N 4 to ZrSi0 4 were tested and evaluated.To prevent an unwanted cooling and oxidation of the slug during its transfer, the transfer is carried out encapsulated in high temperature resistant crucibles that can provide a protective atmosphere. The suitability (e.g. thermo shock resistance) of different materials for the use as transport crucibles was also subject of this research work. The forming experiments were carried out with the institute's hydraulic press. The process logic controller of this press offers the appropriate fleXibility and various control modes needed for a sophisticated and reproducible filling of the die. In this way the die filling process of different steels can be tested.
In recent years several attempts were made to transfer the thixoforming technology of steel parts
into industrial applicable processes. This paper gives an overview about the progress of a European
consortium that established a fully automated thixoforming process for the series production of
automotive steel parts. Due to the multi-faceted nature of this technology, problems concerning the
development of suitable steel grades and tool materials as well as the development and application
of an inductive heating system, a handling unit and of a complex forming tool had to be solved.
Besides the development of adapted steel grades and the inductive heating, the handling of the semi
solid billets plays a special role because during the manipulation of the parts from the heating station
into the tool a loss of heat is unavoidable. Furthermore, scaling of the parts must be prevented.
By means of a fully automated process line existing constraints were reduced and the forming process
is kept reproducible.
Improved silicon nitride composites have been developed as a tool material, which show good mechanical
properties in combination with an acceptable chemical stability at the occurring process
temperatures as well as in contact with semi solid steel.
Basing on the practical experience a comparison of the thixoforming technology to existing processes
and an outlook for the future are given.
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