Review of the processes for recycling titanium alloys machining chips into additive manufactured aeronautical-quality products

Titanium and its alloys are widely used in the aeronautic field for their unique chemical and mechanical properties. However, this sector generates high amounts of waste, in the form of chips and scraps, when machining high-precision parts. These high-quality chips and scraps have begun to be recycled at an industrial scale, but the processes used are based on remelting technologies, which have high energy and consumable consumption. Moreover, these processes produce ingots that are not suitable for additive manufacturing purposes. The interest in alternative methods for the production of aeronautic-quality feedstock materials that can be used in additive manufacturing is increasing for complex geometries. The aim of this paper is to review the treatments and processes that can be used to recycle titanium-based chips and scraps into valuable additive manufacturing feedstock materials, with their advantages and shortcomings. Energy and consumable consumption are also given when available. Updated titanium production data are provided. The pre-treatments of titanium chips and scraps are also discussed, as titanium is highly sensitive to oxidation, which remains a serious industrial issue. From this state-of-the-art review of titanium chip and scrap recycling, the identified promising technologies are continuous rotary extrusion (CRE) for wire production, and ball milling (BM) for powder production.