Life prediction and failure analysis in laser powder bed fused TiC/Ti6Al4V titanium matrix composite under high cycle and very high cycle fatigue conditions

Abstract

This study investigates the failure behavior of a TiC-reinforced titanium matrix composite (TMC) manufactured by laser powder bed fusion (L-PBF) through fully reversed and pulsating tension fatigue tests in both high-cycle and very-high-cycle regimes. Inhomogeneous hardening zone (IHZ) and un-melted TiC zone (UMTZ) resulting from TiC agglomeration, along with pore and lack of fusion (LOF) generated during the L-PBF process, play a crucial role in inducing fatigue failure and influencing the variability of fatigue life. Advanced microscopy techniques, including optical microscopy, scanning electron microscopy, and electron backscatter diffraction, were employed to elucidate four failure modes: LOF-assisted IHZ cracking, IHZ cracking, pore cracking, and UMTZ cracking. IHZ cracking originates from stress concentration caused by the mechanical property mismatch between IHZ and Ti matrix, leading to brittle fracture of α'-grains within the IHZ. This constitutes the predominant failure mode. The presence of LOF near IHZ further exacerbates stress concentration, diminishing the material's load-carrying capacity and significantly reducing fatigue life. Considering the fatigue cracking mechanism, microstructural characteristics and the influence of defect location and stress concentration, a crack nucleation life prediction model is developed. The agreement between experimental and predicted data is satisfactory.