Defect Analysis and Control of In Situ TiB/Ti Laser Cladding Layers on TC4 Titanium Alloy

TiB/Ti composite coatings have been extensively studied for surface strengthening of titanium alloys because TiB and Ti exhibit similar density and thermal expansion coefficients, and the TiB/Ti interface can be coherent or semi-coherent. Despite decades of research, however, the engineering application of laser-cladded TiB/Ti coatings is still severely limited by poor surface forming quality and cracking during rapid melting and solidification. In this study, TiB/Ti cladding layers were fabricated on TC4 titanium alloy by laser cladding using TiB₂ and Ti powders. The effects of laser power (1.5–2.5 kW), scanning speed (4–8 mm/s), and TiB₂:Ti molar ratio (1:1 and 1:3) on surface morphology, crack formation, and microstructure were systematically investigated. The results show that laser power and scanning speed improve surface quality through different mechanisms: increasing laser power prolongs the molten-pool lifetime and mainly suppresses ripples, whereas increasing scanning speed from 4 to 8 mm/s shortens the local interaction time at the track center and reduces the depression depth from 351–358 μm to 120–127 μm. Crack formation is strongly affected by the precursor molar ratio and the resulting reaction extent. In the stoichiometric 1:1 mixture, the short molten-pool lifetime leaves coarse residual dendritic TiB₂ as the dominant constituent, which obstructs liquid feeding and promotes local stress concentration, resulting in through-thickness cracking. In the Ti-rich 1:3 mixture, excess Ti promotes the TiB₂ + Ti → 2TiB reaction, reduces residual dendritic TiB₂, and increases the continuity of the Ti-rich liquid phase, thereby suppressing crack formation under the present conditions. This work provides a practical strategy for controlling surface forming quality and cracking in TiB/Ti laser-cladded layers by coordinating heat input and the TiB₂:Ti precursor ratio.