Two batches of pre-alloyed Hastelloy-X powder with different Si, Mn and C contents were used to produce specimens by Selective Laser Melting (SLM). Cracks with various morphologies were found in some of the parts. Two major reasons that control crack formation and propagation were considered: (i) internal strain accumulation due to the thermal cycling that is characteristic to SLM processing; (ii) crack formation and propagation during solidification. This phenomenon, known as hot tearing, is frequently found in conventional casting and is dependent on chemical composition. Using thermodynamic software simulation, the temperature vs fraction of solid curves was used to determine hot tearing sensitivity as a function of Si, Mn and C content. It was found that low Si and C contents help in avoiding crack formation whereas cracking propensity was relatively independent of Mn concentration. Hence, the cracking mechanism during SLM is believed to be as follows: crack initiation is mainly induced during solidification and is dependent on the content of minor alloying elements such as Si and C, whereas crack propagation predominantly occurs during thermal cycling. If microstructures free of micro-cracks after solidification can be generated with optimised SLM parameters, these manufactured parts can sustain the internal strain level and, thus, crack formation and propagation can be avoided.
- Nickel superalloys
- Selective laser melting