Synthesis of Ti -TiC nano composite from waste Ti machining chips by planetary milling followed by spark plasma sintering

Abstract

Titanium is one of the fastest growing materials used in aerospace applications due to light weight. Most production processes like machining and forging generate large amount of titanium and titanium alloy scrap in the form of chips. From the economic, strategic and environmental point of view, these scrap should be reused and recycled. Powder metallurgy provides a very suitable and economical way for the utilization of waste titanium scraps. In the present investigation, an attempt has been made to prepare Ti-TiC nano composite from Ti chips and graphite powder by planetary milling. The Ti-TiC nano composite has also been consolidated by spark plasma sintering. Planetary milling is carried out in a high energy dual-drive planetary mill (DDPM) using stainless steel jar of 1 L volume and stainless steel ball of 10 mm diameter. Jar volume of 30 % is filled with cleaned Ti waste chips, graphite powder and steel ball. Ti chips and graphite powder are taken in 50:50 atomic ratio and ball to powder/chips weight ratio of 10:1 is maintained. Milling is carried out in dry condition and argon gas is purged before milling. Milling is carried out for a total period of 3 hours at the intervals of 30 minutes. At each interval, argon gas is purged without opening steel jar. Powder is characterized by X-Ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The powders obtained are irregular in shape with a crystallite size of 5-10 nm. Islands of TiC and Ti2C are observed in a matrix of Ti. Finally, 3 hours milled powder is sintered by spark plasma sintering (SPS). SPS is carried out at 900 and 1000°C for 5 minutes at a pressure of 50 MPa. It has been found that sample sintered at 1000°C exhibits higher density and hardness than sintered at 900°C. There is a considerable increase in hardness from 378 to 614 HV on increasing the temperature from 900 to 1000°C.