SELECTIVE ELECTRON BEAM MELTING OF THE SINGLE CRYSTALLINE NICKEL-BASE SUPERALLOY CMSX-4® : FROM COLUMNAR GRAINS TO A SINGLE CRYSTAL

Selective electron beam melting (SEBM) is a powder-bed-based additive manufacturing technology. SEBM offers a great potential to process high performance alloys like intermetallics, e.g. TiAl or FeAl, and Ni-based superalloys, which are hard or non weldable materials with high susceptibility to crack-formation. Especially the high process temperatures during SEBM are beneficial for reducing stresses and warpage. In addition, SEBM works under vacuum which guarantees a perfect protection against contamination. In this contribution, SEBM of the single crystalline nickel -base superalloy CMSX-4 (R)(1) is investigated. It is shown that crack formation can be avoided by appropriate processing conditions. Regarding the microstructure, SEBM opens the possibility to tailor the microstructure. Whether a columnar grained or a single crystalline grain microstructure is produced depends on the process strategy used. For the first time it is shown, that it is even possible to produce a single crystalline material by an additive powder bed manufacturing process without any grain selector or a seed crystal. The inherent high cooling rates during SEBM lead to a strong reduction of the length scale of the well-known strong segregations in CMSX-4 (R) to some microns. Therefore, long-time and expensive heat treatments are reduced to minutes for a complete homogenization of the single crystal specimen, even for Re. The specimens produced were investigated by using classical optical-metallography techniques and SEM imaging. Complementary microprobe element mappings and EBSD measurements were performed for a detailed analysis of the microstructure.