Development of the thin shell casting technology for downwards directional solidification

The directional/single-crystal solidification (DS/SC) of turbine blades made of superalloys is a key technology on a high scientific and technical level. Industrial DS/SC turbine blades are traditionally produced by using the expensive Bridgman process, which is characterized by a relatively inefficient heat dissipation and a low thermal gradient. In this thesis a Thin Shell Casting (TSC) process for downwards directional solidification is developed. This novel technique is a combination of the Czochralski, dynamical baffle and GCC technologies, which is characterized by a thin shell mould (~1 mm), a complete isolation between the heating and cooling zone as well as a gas cooling. By pulling up the shell mould through the floating baffle from the melt bath, a directional solidification is performed downwards. Application of gas cooling enables a significantly increased heat extraction. Thus the specific disadvantage of the Bridgman process, i.e. the low temperature gradient and the consequent casting defects could be generally overcome by this new technique. Using a Cu alloy in the first work phase, the feasibility of this novel process was successfully confirmed. With simplified shell moulds, DS/SC components were cast. The second work phase dealt with the directional solidification of Al alloys, in order to fundamentally investigate the solidification condition and to systematically analyze the resultant microstructure in the TSC process. Thereafter DS/SC turbine blades of Al alloys were also successfully cast. In the third part, some DS rods of the Ni-base superalloy CMSX-6 were cast under inert gas. According to the metallographic examination, an extremely high temperature gradient G of about 24 K/mm was evaluated, which is a considerable increase by an order of magnitude over that of the conventional Bridgman process. As a consequence, the primary dendrite arm spacing lambda1 is reduced by a factor of three. The reduction of the size and volume fraction of gamma/gamma'-eutectic is even more pronounced. Due to the strongly shortened interdendritic feeding path, the interdendritic porosity is almost completely eliminated. The result is a significant improvement in the mechanical properties of DS/SC components