Titanium, the ninth-most abundant element in the Earth's crust, has been used in the aerospace industry since 1948. Since then, Ti and its alloys have been widely used in various applications such as in ships, chemical plants, automobiles, and even medical implants. They possess several attractive properties as structural materials for use in modern industries, including low density, high specific strength, good corrosion resistance, and high workability. Hence, many researchers and engineers have investigated the processing of Ti and its resulting microstructures and properties. However, the production of Ti still faces numerous challenges. The global production volume of Ti is substantially lower than those of Fe and Al. The cost of processing a thousand cubic feet of Ti, which includes melting and refining, is five times higher than that of Al. In particular, the cost of processing Ti ingots, which includes fabricating plates, billets, and rods by rolling or forging, is ten times higher than that of Al. In addition, the poor forming ability of Ti leads to large amounts of Ti wastes and a low rate of material usage. Therefore, the development of methods for recycling Ti wastes such as Ti scraps is a topic worth researching. Ti scraps are inevitable in the processing of final product shapes from mill products during machining. Contamination by oxygen during processing and cutting is also unavoidable. Owing to Ti's high affinity toward oxygen, a large amount of interstitial oxygen exists in Ti scraps and degrades the mechanical and physical properties such as ductility, fracture toughness, and corrosion resistance.
Thus, the present study focuses on recycling Ti scrap via the electromagnetic cold crucible (EMCC) process coupled with Ca treatment. An EMCC process, which comprises an electromagnetic cold crucible with a feeder system, is a promising method for melting Ti scraps and producing high-purity Ti continuously and productively. Ti scraps are melted in a water-cooled copper crucible, and various amounts of Ca are added to the molten Ti. The composition of the refined Ti ingot is characterized by elemental analysis and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The microstructure of the refined Ti ingot is investigated via optical microscopy and field emission-scanning electron microscopy (FE-SEM). The mechanical properties are examined using hardness and tensile strength tests.
Typical microstructures of (A) as-melted Ti and Ti after Ca treatment with (B) 0.1% Ca, (C) 0.3% Ca, and (D) 0.5% Ca.