Influence of Laser Scan Strategy on the Microstructure and Structural Integrity of SLM Fabricated CM247LC

The use of the laser powder-bed method to manufacture Ni-base superalloy components has been identified as a potentially economic approach for low production run components that operate at a high-temperatures/stress environment. The Concept Laser M2 SLM system has been utilised in the fabrication of CM247LC samples using the default ‘island’ scan strategy. Microstructural examination (Optical and SEM) has identified the presence of a distinctive patterned bimodal grain structure as a result of this laser scan strategy. EBSD mapping has been used to further characterise this bimodal structure in terms of texture/grain‑boundary misorientation and an explanation based on the relative cooling rates due to the island strategy has been suggested to account for these observations. Furthermore, MicroCT analysis has been employed to visualise the crack distribution in the as-fabricated samples and can be directly related to the bimodal grain structure supporting the ductility‑dip cracking (DDC) mechanism suggested as their cause.

In a further investigation, CM247LC samples were fabricated using a simplified scan strategy and have shown very different cracking behaviour. These samples have been compared to those utilising the island scan strategy and highlight the need for greater understanding of the heat transfer during SLM to produce homogeneous microstructures, but also potentially the production of tailored microstructures through the control of the laser scan strategy.