Laser Additive Manufacturing of Iron-Aluminum for Hybrid Steam Turbine Blades

Abstract

The optimization of steam turbine rotor blades is strongly restricted by centrifugal stresses. To reach higher rotational speed or to obtain larger airfoils it is desirable to realize blade designs with very light, but robust blade tips. Hence, the aim for a composite material design of a turbine blade is to investigate a new method of providing raw material for turbine blades which consists out of standard turbine steel around the root section and much lighter material on the outer diameter of the blade. Iron aluminide alloys are of increasing interest as a structural material for lightweight construction in hot or corrosive environments due in part to the good and economical availability of the main alloying elements. Currently, the feasibility is being tested of replacing component areas with FeAl through hybrid construction, depending on local load requirements, and thus achieving effective weight reduction. The additive process laser-based direct energy deposition (L-DED) of Fe-28Al is investigated to produce hybrid material consisting of 12% Cr turbine steel and Fe-Al. Parameters and build-up strategies are varied in order to produce a crack-free and low-stress connection of the material partners while complying with given thermal boundary conditions. Thermography is used to achieve homogeneous process conditions when scaling to component size. Microstructure, hardness and chemical composition of the hybrid material are investigated as well as mechanical strength. It is shown that crack-free machining of test specimens and a component blank is possible after heat treatment.