Optimisation of LPBF process parameters and residual stress analyses of Invar-10wt% TiC and Invar-10wt% TiN metal matrix composites

Abstract To enhance the mechanical properties of Invar (Fe–36Ni) for a broader range of applications, reinforcement with Titanium Carbide (TiC) and Titanium Nitride (TiN) was investigated. Laser powder bed fusion was used to manufacture the Invar metal matrix composites with TiC and TiN respective additions. Optimization of the process parameters was conducted using response surface methodology. The optimal parameters for Invar-TiC are 180 W laser power with a scanning speed of 450 mm/s, while for Invar-TiN, the optimal parameters are 190 W laser power with a scanning speed of 400 mm/s. High densities (> 99%) and significant improvements in hardness were achieved. X-ray diffraction and scanning electron microscopy analyses confirmed the uptake of TiC and TiN into the Invar matrix, leading to the enhanced properties. Residual stress evaluation through non-destructive neutron diffraction (ND) measurements and inherent strain modelling (ISM) simulations was done. The addition of TiC and TiN to the Invar matrix influenced the stress distribution, with Invar-TiC showing higher tensile stresses due to its thermal conductivity properties, and higher compressive stresses due to grain refinement. Close agreement was found between the ISM simulation and ND-measured results, indicating predominantly compressive stresses in the interior and tensile stresses on the sample surfaces. These findings demonstrate the potential for developing Invar-based MMCs with enhanced mechanical properties through LPBF. Due to the enhancement in hardness and, thus, wear resistance, the investigated compositions offer applications in parts and tools used in rough and demanding conditions, such as mouthpieces for extrusion or turbine blades in water turbines.