Investigations on steel grades and tool materials for thixoforging
The thixoforging technology offers a large technical and economical potential for the production of highly stressed parts with complex shaped geometries. While thixoforming is an established process for aluminum parts, the efforts described below focus on the development of this technology for steel applications. Another focus is put on ceramic tool materials since these are required to enable such a high temperature process. Thixoforging of high-melting metals (e.g. forging steels) demands a sophisticated process control and adapted tool materials as well as suitable ingoing material. A large need for research is still required in this field. Possible products made by means of thixoforging are e.g. running and driving gear components which are produced by multiple conventional forging step operations today. In this paper first experiments concerning the use of ceramic tool materials for thixoforging of steel discussed. Therefore a hybrid die that consists of a steel shrink ring with a ceramic insert was developed and built up. Different monolithic and composite ceramics, based on nitrides, carbides, borides and oxides have been evaluated, with respect to the interfacial reactions between solids and melts by means of thermodynamical calculations and the method of sessile drop in combination with investigations by electron microscope (SEM, EDX). Due to the separation of the heating process of the slug from the tool system an encapsulated transport of the heated slug is needed to prevent unwanted cooling and high temperature oxidation (scale). This transfer is carried out inside high temperature resistant containers. The suitability (e.g. thermo shock resistance) of different materials for the use as transport containers was also subject of this research work. The forming experiments were carried out with the institute's hydraulic press (AP&T LPS 4000). The process logic controller (PLC, Type SINUMERIK 840D) of this press offers the appropriate flexibility and various control modes needed for a sophisticated and reproducible filling of the die. Furthermore, all relevant process parameters such as force, velocity and displacement of the stamp can be recorded by a built-in process computer.