CSMX4 Single Crystalline Superalloy Cannot be Welded, but Works Well with Additive Manufacturing

Metal additive manufacturing is both an art and a science, and an area of study that is constantly growing and evolving as new materials are developed. In a new research paper entitled “Microstructure and Mechanical Properties of CSMX-4 Single Crystals Prepared by Additive Manufacturing,” which you can access here, a group of scientists discuss CSMX-4, a second generation single crystalline superalloy.

CSMX-4 and other alloys like it are highly susceptible to cracking, and so they are considered to be hardly weldable or even non-weldable. Recently, though, materials experts have been looking into processing these types of alloys using additive manufacturing. The researchers who authored this recent article have discovered that CSMX-4 can be processed without cracks using Electron Beam Melting.

CSMX-4 is an emerging area in 3D printing so 3DPrint.com interviewed Ulf Ackelid, Senior Scientist at Freemelt to explain the material to us. Ulf has a long history in 3D printing, especially with EBM and deep materials experience in the domain, although he did council us that he was no expert on CSMX-4. He had this to say about single crystal nickel superalloys:

 “Such materials are used in turbine blades, due to their unique material properties. They have excellent creep deformation resistance due to the fact that they are free from grain boundaries where creep preferentially takes place. SX components in materials like CMSX-4 are traditionally fabricated by very special casting technologies. It is very promising that the feasibility of producing SX material with AM has been proven in recent years by the team at Erlangen University and their co-workers. You could even call it a breakthrough. It opens up new manufacturing possibilities in a long-term perspective.”

SX SEBM CMSX-4® sample with single crystalline core. (a) Vertical cross section as-built, (b) horizontal cross section and EBSD mapping, (c) vertical cross section after heat treatment

The researchers used an Arcam A2 EBM system to prepare 3D printed samples of the superalloy material. Some of the samples were heat-treated and compared to as-built samples as well as standard cast material and heat-treated cast material. Once the samples were printed, they were put through various tests: tensile, hardness, low cycle fatigue, creep, and microstructural investigations. The researchers found that strength and ductility increased with higher temperatures up to 800°C. The strength values of the heat-treated samples were comparable to those measured for conventional cast material.

The heat-treated EBM samples also yielded high elongation and contraction values. All in all, it appears that the heat-treated samples performed better than the as-built samples, and were comparable if not superior to the cast samples, heat-treated or not.

“Mechanical properties of SX CMSX-4^® produced by selective electron beam melting (SEBM) have been studied in the as-built and fully heat-treated conditions and compared with conventional cast and heat-treated material,” the researchers state. “The microstructures of the SEBM and cast material differ in dendrite size by about two orders of magnitude with much smaller dendrite spacing in the SEBM material and consequently much smaller interdendritic solidification pores. Further thermal treatment of the SEBM material results in a complete homogenization such that the dendrite structure vanishes and no chemical segregations remain.”

As-built microstructure and corresponding hardness: (a) SEM images of the microstructure as a function of the distance from the top layer. The cross section is perpendicular to the building direction. Dendritic and interdendritic areas are marked with DA and IA, respectively. (b) Hardness as a function of the distance to the top layer. (c) Magnification of the as-built γ/γ’ microstructure with coarse γ’ within the interdendritic area

There are several advantages to producing single crystalline superalloys via 3D printing, and many of them are similar to the advantages of additive manufacturing in general. According to Ackelid, these advantages include:

• Avoidance of gross casting defects
• The possibility of implementing in-situ process monitoring and control, layer by layer, from the bottom to the top of the component
• Low material waste and high recyclability
• High design freedom and flexibility
• Short lead times
• Low contamination risk due to the high purity of the environment in the additive manufacturing machine

Authors of the paper include C. Körner, M. Rampsberger, C. Meid, D. Bürger, P. Wollgram, M. Bartsch, and G. Eggeler.

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