Study of Energy Density and Substrate Roughness for In Situ Ni–Ti Alloy Formation by Laser Powder Bed Fusion

NiTi alloys are widely recognized for their superelasticity and shape memory effect, making them attractive for advanced engineering applications. In laser powder bed fusion (LPBF), the formation of Ni–Ti alloys from elemental powders is strongly influenced by processing conditions, powder characteristics, and substrate properties. This study investigates the combined influence of laser energy density and substrate roughness on the early stages of in situ Ni–Ti alloy formation using elemental nickel and hydride–dehydride (HDH) titanium powders.

A fundamental single-track approach was adopted to isolate the effect of laser energy input on melt pool behavior and track formation. Laser power was varied from 100 to 300 W, while scan speed (10 mm/s) and powder layer thickness (400 µm) were kept constant. Linear, planar, and volumetric energy densities were calculated to establish correlations between processing conditions and track morphology. This one-factor experimental design was intentionally selected to identify a baseline processing window for elemental Ni–Ti feedstocks prior to the application of statistical optimization methods, such as Design of Experiments (DOE) or Machine Learning (ML).

Single-track experiments were conducted on textured titanium substrates whose surface roughness was characterized by three-dimensional optical interferometry and expressed as root mean square (RMS) values. The deposited tracks were evaluated by macroscopic examination and scanning electron microscopy (SEM) to assess melt pool stability, track continuity, and material distribution. The results provide insight into the coupled influence of laser energy input and substrate texture on track morphology and the initial stages of alloy formation, contributing to the understanding of process–structure relationships in LPBF processing of elemental Ni and HDH Ti powders.