Alloy design of Co-based superalloys by additive manufacturing

Abstract

The blades of gas turbines used in transportation, defense, and plants are the parts exposed to the highest temperature and pressure inside the turbine and are made of Ni-based superalloys. The Ni-based superalloys have excellent mechanical strength, oxidation resistance, and creep resistance above 1000°C due to the γ/γ’ microstructures. Since the 1960s, cast alloys have been developed in the order of equiaxed grains, columnar grains, and single-crystal microstructures, increasing the operating temperature of Ni-based superalloys. However, the temperature at which the Ni-based superalloy properties are guaranteed almost reached the melting temperature of Ni. Therefore, a material for higher temperature application is needed. Since γ’ strengthened Co-Al-W superalloys were reported in 2006, a lot of studies have been actively conducted to replace the commercial Ni-based superalloys with Co-based or Co-Ni-based superalloys due to the higher melting temperature of Co than that of Ni. However, superalloys have many required properties as high-temperature materials, and to satisfy these properties, Ni-based superalloys contain more than 10 optimized alloying elements. Thus, a more efficient research method is needed to replace the Ni-based superalloys with Co-based superalloys that studied relatively shortly. The time required for alloy development can be greatly shortened by using additive manufacturing, which has recently been spotlighted. Additive manufacturing, one of the 3D printing methods, enables near-net shape fabrication using desired materials and can reduce the time, cost, weight of parts, and consumption of materials. In particular, it is possible to rapid alloy prototyping or screening with various compositions in that multi powders can be used and the powder feed rate is easy to control, thereby dramatically reducing the time required to develop a new alloy system. However, the steep temperature gradient of additive manufacturing causes high thermal stress to the alloys, resulting in hot-cracking. Unlike Ni-based superalloys, which have been reported to induce cracking due to high Al and Ti content, the cracking mechanism of Co-based superalloys has not been reported yet. This study focused that 1) solving the disadvantages and 2) reinforcing the strengths of additive manufacturing. The author aimed 1) the development of crack-free Co-based superalloys by revealing the cracking mechanism during the additive manufacturing process and 2) the development of a multi-component Co-based superalloy with excellent high-temperature properties by additive manufacturing. As the first research topic, Co-Al-W superalloy, a promising candidate alloy for high-temperature applications, was deposited. After microstructure analysis, it was revealed that the Al content and the resulting oxide formation had a great effect on the printability of Co-Al-W superalloys. To prevent hot-cracking, a novel Co-Al-W alloy was designed and fabricated by adjusting the Al content. The newly developed crack-free Co-Al-W alloy exhibited comparable creep resistance to conventional cast alloys. This study showed the possibility of applying additive manufacturing to high-temperature Co-based superalloys. As the second research topic, a new additive manufacturing method was applied to develop high-temperature Co-Ni-based superalloys. A metal 3D printer equipped with multiple powder feeder and single elemental powders were used to fabricate alloys with various compositions by adjusting the powder feed rate. Through this method, Co-Ni-Ti-Al-Mo superalloys of various compositions were deposited and an alloy screening process was performed based on high-temperature properties. The final selected alloy showed excellent thermal stability and yield stress while maintaining its γ/γ’ microstructures even after long aging heat treatment. The novel additive manufacturing method devised in this study was effective in manufacturing multi-component alloys with superior high-temperature properties.