How does ultrasonic cutting affect the macroscopic deformation and microstructure evolution of fibre-reinforced titanium matrix composites?

Continuous silicon carbide (SiC) fibre-reinforced titanium (Ti) matrix composites (SiC_f/Ti) possess exceptional properties, making them promising for aerospace applications. Continuous SiC fibres significantly enhance the axial tensile strength of SiC_f/Ti compared to traditional Ti alloys. To utilise this material fully, its axial dimensions are fixed during manufacturing, but the outer Ti matrix layer must be thinned to meet structural accuracy requirements. Thinning often leads to interfacial cracking and fibre breakage owing to machining stress, which presents a major challenge in manufacturing. The deformation mechanism during thinning is unclear and the lack of low-stress thinning methods significantly limits the potential applications of SiC_f/Ti. This study investigates the macroscopic deformation and microstructural evolution of SiC_f/Ti under ultrasonic cutting (UC) through orthogonal experiments. Compared with conventional cutting (CC), UC reduces cutting force by 20 % and surface residual stress by 60 %, while increasing subsurface residual stress and nano-hardness. The acoustic softening effect in UC reduces cutting force and surface stress, while high-frequency stress waves elevate subsurface stress. Digital image correlation (DIC) analysis reveals that the combined effects of loading and unloading cycles during UC produce an elastic recovery strain, reducing the overall deformation in SiC_f/Ti during machining. Additionally, UC promotes grain refinement in the outer Ti layer of SiC_f/Ti and induces a stress concentration at the α-Ti and β-Ti interface, facilitating the transformation of α-Ti to β-Ti. The presence of SiC fibres amplifies the effects of the ultrasonic energy, accelerating dislocation diffusion and annihilation, promoting dynamic recrystallisation, and reducing the dislocation density between the fibres. Moreover, UC homogenises and realigns the stress field at the SiC_f/Ti interface, making the composition and structure of the interface more uniform and reducing interfacial damage. This study provides theoretical and practical insights into low-stress thinning, paving the way for broader applications of SiC_f/Ti in advanced structural components.