Solidification behaviour and phase formation during pre-placed laser cladding of Ti45Nb on mild steel

Scanning electron microscopy, Energy Dispersive Spectroscopy (EDS), X-ray diffraction and microhardness analysis were employed to study the solidification behavior and phase transformations occurring during the laser cladding a Ti45Nb coating on mild steel. A premixed powder of 55 wt.% Ti and 45 wt.% Nb was used and the processing parameters were set to ensure a laser power density of ∼ 970 W/mm2 and an interaction time range of 0.022–0.080 s between the laser beam and the substrate. It was shown that at a constant power density, the interaction time controls the solidification behaviour during the process. For extremely short interaction (ESI) time range, 0.024 ≤ τ ≤ 0.027 s, crack and pore-free coatings of excellent hardness (up to ∼ 1000 HV0.05) containing β and α″ Ti–Nb based solid solutions and a β + Fe–Ti solid solution eutectic matrix can be made. However, processing at higher interaction times, i.e. at short interaction (SI) time range 0.030 ≤ τ ≤ 0.048 s, results in the formation of additional brittle intermetallics, including an FeNb μ compound and an Fe2(Nb,Ti) ε compound, while only ε forms at the highest interaction times, i.e. long interaction (LI) time range 0.060 ≤ τ ≤ 0.080 s.