A comprehensive review on solid-state recycling of titanium for sustainable engineering applications

Titanium (Ti) and its alloys are increasingly vital in high-performance sectors such as biomedical, energy, and automotive engineering due to their exceptional strength-to-weight ratio, corrosion resistance, and biocompatibility. However, the extensive machining involved in processing Ti components generates significant scrap, raising both economic and environmental concerns. This review presents a comprehensive and critical examination of the solid-state recycling (SSR) of Ti and its alloys, an emerging, energy-efficient alternative to conventional melting-based recycling methods. The review highlights the effects of pre-processing conditions, including machining-induced twinning, oxide layer development, and chip boundary, on the microstructure and mechanical performance of recycled titanium. Processing parameters, including temperature, strain, and pressure during SSR processing are also discussed in relation to their role in enhancing bonding and refining microstructure. By comparing SSR techniques with traditional melting routes, the review highlights the potential of SSR to preserve alloy composition, reduce energy consumption, and refine microstructures, leading to improved mechanical properties. This review uniquely emphasizes the critical relationship between chip features, bonding behavior, and processing conditions, and their combined impact on the quality of recycled titanium. The paper also identifies current challenges, research gaps, and future directions necessary for industrial-scale implementation. Overall, this review offers critical insights into optimizing solid-state recycling strategies for titanium, paving the way for more sustainable, cost-effective, and high-performance material recovery solutions.