Fundamental Understanding of the Dissolution of Oxide Film on Ti Powder and the Unique Scavenging Feature by LaB 6
- PDF / 2,686,867 Bytes
- 6 Pages / 593.972 x 792 pts Page_size
- 51 Downloads / 172 Views
er metallurgy (PM) offers the potential for both cost-effective near-net shape fabrication of titanium (Ti) components and novel Ti alloy development.[1] The oxygen (O) content of titanium powder is a key issue that affects both the cost of manufacturing and the resulting mechanical properties of titanium alloys fabricated from titanium powder by various powder metallurgy (PM) processes.[1,2] To ensure adequate ductility for structural applications, the O content of PM Ti alloys must be kept at a sufficiently low level. For instance, the critical O content for as-sintered Ti-6Al-4V has been determined to be around 0.32 wt pct beyond which the tensile elongation decreases from more than 10 pct to less than 5 pct with 0.35 wt pct O and further to less 2 pct with about 0.45 wt pct O.[2] However, reducing the O content of Ti powder is challenging due to the high chemical affinity of Ti for O.
Y.F. YANG is with the State Key Laboratory of Multiphase Complex Systems, Chinese Academy of Sciences, No.1 Zhongguancun North St, Beijing 100190, China and also with the Centre for Additive Manufacturing, School of Engineering, RMIT University, Melbourne, VIC 3000, Australia. Contact e-mail: [email protected] M. QIAN is with the Centre for Additive Manufacturing, School of Engineering, RMIT University and also with the School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, QLD 4072, Australia. Contact email: [email protected] Manuscript submitted June 13, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
More serious issue is that the surface oxide film on Ti powder could dissolve into the underlying Ti metal at the elevated temperature, owing to the tremendous solubility of O in Ti up to 14 wt pct.[1] Once the oxygen in the surface oxide film has gone into the Ti solid solution, it will become even more challenging to remove the O from Ti. Although rare earth (RE) can scavenge O from beta Ti solid solutions, the oxygen-scavenging process by RE is controlled by the diffusion of oxygen and is therefore difficult to complete.[3,4] This is evidenced by the resulting oxygen-deficit RE oxides after even 120 minutes of scavenging at 1623 K (1350 C).[4] A critical fundamental question is thus: what is the temperature corresponding to the active dissolution of oxide film into the underlying Ti metal and can the surface titanium oxide films on Ti powder be scavenged prior to their dissolution? The binary Ti-O phase diagram indicates that under equilibrium heating conditions titanium oxide will disappear at 873 K (600 C), corresponding to the maximum solubility limit of O in a-Ti.[5] For non-equilibrium heating, early work suggested that the oxide film on titanium powder might disappear around 823 K (550 C).[5] This suggestion is now inconsistent with the current well-established Ti-O phase diagram. A subsequent study reported that it took about 60 minutes for the surface oxide film to disappear on loose Ti powder at 1273 K (1000 C).[6] This estimation appears to be too conservative as experiments have shown that an isoth
Data Loading...
