Oxygen contamination during sintering of Ti and Ti6Al4V HDH powders

Transcription

1 Oxygen contamination during sintering of Ti and Ti6Al4V HDH powders Stella Raynova 1, a, Brian Gabbitas 1,b, Barry Robinson 2,c 1 School of Engineering, Facility of Science, The University of Waikato, Hillcrest Rd, Hamilton 3240, New Zeland Waihoehoe Rd Drury 2113, New Zealand a stiliana@waikato.ac.nz, b briang@waikato.ac.nz, c barry@safegroup.co.nz Acknowledgments: This work was funded by the Ministry for Business, Innovation and Employment (MBIE), New Zealand.

2 Introduction It is well known that the limitations of widespread use of Ti and Ti alloys are because of their production and processing costs. The development of a powder metallurgy allows net shape manufacturing which in some cases could reduce the waste and machining costs by up to 70-80%. A disadvantage of Ti powder metallurgy derives from the high affinity of Ti to pick-up contaminants such as oxygen, nitrogen and carbon when powders are processed at temperatures over 400 C. This leads to a requirement for a protective environment such as a high vacuum or argon gas. Both of these protective environments increase the manufacturing cost and at the same time are not fully effective. There is still some oxygen, carbon and nitrogen pick up reported in the research done on high temperature consolidation of Ti and Ti alloys.

3 Motivation: To make Ti powder metallurgy more cost effective; And hopefully more attractive for industry, in particular to NZ small businesses; by: Using low cost Ti HDH powders; Using alternative sintering method Induction sintering (IS) instead High vacuum sintering (HVS); avoid using protective atmosphere;

4 Introduction- HVS Oxygen pick up is normally happened during the sintering process; Traditional sintering high vacuum furnace long processing time powder compacts are exposed to high temperatures 4h to 6h and more depending of processing parameters Table1 :Chemical analysis as a function of the processing parameters Table 2: Oxygen content of Ti32 alloy after HVS [2] for Ti 6Al 4V sintered samples in high vacuum furnace [1] ( vacuum 10 ⁵mbar, heating rate 5 /min) Starting powders wt% Table 3: Oxygen content of porous Ti product after 0.5h HVS [3] Starting powders 0.337wt% [1] L. Bolzoni, E.M.Ruiz-Navas, E. Gordo, Flexural properties, thermal conductivity and electrical resistivity of pre alloyed and master alloy addition powder metallurgy Ti64, Materials and design, 52 (2013) [2] Bolzoni, E.M.Ruiz-Navas, E. Gordo, Influence of sintering parameters on the properties of powder metallurgy Ti-3Al-2.5V alloy, Materials characterization, 84 (2013) [3] Q. Xu, B. Gabbitas, S. Matthews, D. Zhang, The development of porous Ti products using slip casting, J. of Mat. Process Technology, 213 (2013)

5 Introduction- IHP Recently more economically effective methods of powder consolidation have been researched using alternative source of heating by induction- inductive hot pressing [4], induction sintering [5], induction sintering followed by open die forging [6] The main advantage of these methods is the significantly shorter processing time which comes from: - faster heating rates up to 250 C/min - faster sintering effect: neck growth during IS grows much faster - cooling time is also much faster- prevents from grain growth - the whole sintering process could be completed in min 4] Bolzoni, E.M.Ruiz-Navas, E. Neubauer, E. Gordo, Inductiv hot pressing of Ti and Ti alloy powders, Materials Chemistry and Physics, 131(3)(012) Fig.1 Relative density results of the sintered materials processed by inductive hot-pressing for 15min at two temperatures[4]. Fig. 2 Oxygen content of the samples processed by hot-pressing compared with both the content of the starting powders and that of wrought materials [4].

6 stress,mpa Density before (blue) and after (red) IS,% Introduction- IS of Ti HDH powder compacts C no holding C 4min C no holding C-4min C no holding C 4min c) IS conditions Fig. 3 IS parameters and heating curves for Ti HDH powder compacts [5] Fig. 4 Increase of relative density due to short time IS up to 4min [6] 700 Ti-IS a) b) C,4min 1400C,4min 1400,no hold 1200C,no hold no hold strain,% Fig. 5 Tensile stress strain curves for IS TiHDH samples after IS [5] Fig. 6 Fracture surfaces of tensile testing specimens cut from Ti samples after induction heating to a) 1200 C no holding; and b) 1400 C 4min holding [5]. [5] S.Raynova, D. Zhang, D. Polo, L, Gonthier, W. Egea, V.N. Nadakaduru, Sintering behaviour of induction heated Ti and Ti6Al4V powder compacts, SMNZI Materials conference proceedings [6] S.Raynova, D. Zhang, D. Polo, L, Gonthier, W. Egea, V.N. Nadakaduru, Tensile properties of induction sintered Ti and Ti6Al4V powder compacts, Advanced Materials Research 275 (2011)

7 Introduction- application of IS for ODF of Ti64 HDH powder compacts[7] Table.4- Forging conditions [7] ( heating rates similar to previously mentioned studies [5] and [6] Forging condition F1 F2 F3 F4 F5 F6 F7 F8 F9 F10 Temperature, C Sintering min time, Argon was used as protective atmosphere powder compacts were directly ODFed into disks Oxygen pick up much less then these in previous study up to 0.072wt% Impressive tensile properties in some of the ODF conditions Table.5- Oxygen content after ODF [7] Ti6Al4V powders Oxygen, % F F F F F F F F F F [7] S. Raynova, D. Zhang, B. Gabbitas, Tensile properties of Ti64 discs produced by ODPDF of pre alloyed HDH powders, Key Engineering Materials 520 (2012) F1-1 F1-2 F1-3 F2-1 F2-2 F3-3 F3-1 F3-2 F3-3 F4-1 F4-2 F4-3 F5-1 F5-2 F5-3 F6-1 F6-2 F6-3 YS, MPa UTS, MPa ε, % Table.6- Tensile properties of material produced by ODF [7] F7-1 F7-2 F7-3 F8-1 F8-2 F8-3 F9-1 F9-2 F9-3 F10-1 F10-2 F10-3 YS, MPa UTS, MPa ε, %

8 The scope of this research: This study is a continuation of previous research where the sintering response and tensile properties of as-sintered Ti powder compacts were studied (5,6). The focus is on oxygen contamination during sintering and in particular induction sintering It consists of three investigation: A- The effect of green density on the oxygen contamination during induction sintering of pure Ti powder compacts; B- Oxygen contamination during IS without protective atmosphere coating applied directly on the powder compact was used instead; C- A Comparative study of oxygen pick up during sintering of Ti6Al4V powder compacts using two different sintering methods: HVS and IS;

9 oxygen content, wt% Results: Effect of green density on the oxygen contamination during induction sintering of pure Ti powder compacts: Table 7: Experimental parameters Powders used: Ti- HDH -200 mesh 0.35wt% oxygen Relative green density: 72% 85% Sintering parameters: 1200 C, 1300 C, 1400 C no holding at IS temperatures 4min holding at IS temperature flowing argon protective atmosphere -average heating rate 250 C/min The specimens with a higher green compact density pick up on average about 0.18% of oxygen, which is significantly less than an average of 0.37wt% for samples with a lower green density. The effect of the induction sintering temperature on the oxygen pick up is not very conclusive as received C 1200 C C 1300 C powder,4min 1400,4min C IS Temperature ( C)/ time conditions(min) 72% relative green density 82% relative green density Fig. 7: Oxygen content of Ti powder compacts induction sintered at different temperature time conditions. Prolonged holding for 4 minutes at the induction sintering temperature in some cases does not lead to any further increase in the level of oxygen contamination compare to the IS without holding at sintering temperature.

10 oxygen content, wt% Results: Oxygen contamination during IS of Ti6Al4V powder compacts without a protective environment: Table 8: Experimental parameters: Powders used: Relative green density: Sintering parameters: Ti powders: -200mesh 0.33wt% oxygen Al40V master alloy powders: -250mesh 0.6wt% oxygen % 1200 C; 1min holding at 1200 C; no protective atmosphere; protective coatings: - colloidal graphite suspension; - copper based; - boron nitride suspension; -average heating rate 250 C/min; original 1 none 2 graphite 3 copper 4 boron 5 powders based based nitride Figure 8: Degree of oxygen contamination. Table 9: Physical and mechanical properties after IS in air using various protective coatings: Coating: none graph ite less then 1mm from IS surface aprox 10mm from IS surface copp er based boron nitrid e Relative sintered density, %: Tensile properties after IS: YS, MPa X UTS, MPa Average elongation at break,% x significant decrease in oxygen contamination when coating is applied; copper based coating is the most effective; oxygen content close to the IS surface is only 0.16wt% for the copper, compared to 0.26wt% and 0.41wt% for graphite and boron nitride coatings; The oxygen contamination approximately 10mm away from the IS surface is much lower, 0.04wt%, 0.05% and 0.09% for a copper-based, graphite and boron nitride coatings, respectively; The tensile properties show promising results; The tensile strength reached over 1020MPa and the plastic deformation was 7.8% in the most ductile test-piece;

11 oxygen content, wt% Powders used: Ti powders: -200mesh 0.33wt% oxygen Al40V master alloy powders: -250mesh 0.6wt% oxygen Results: A comparative study on oxygen pick up during sintering of Ti6Al4V using two different sintering media: Table 9: Experimental parameters: Relative green density: 82% Sintering parameters: High vacuum sintering (HVS): 1300 C, 2h 3.7x10-4 Pa vacuum 10 C/min heating rate Induction sintering (IS): 1200 C, 4min flowing argon bellow 200ppm oxygen average heating rate 250 C/min The average sintered relative density was 96.6% for HVS samples and 95.5% for IS samples; Significantly less oxygen contamination during induction sintering compared with high vacuum sintering; The overall increase in oxygen was 0.18wt% for IS and 0.29% for HVS; blended 1 powders after 2 IS after 3 HVS Fig. 9: Oxygen content of Ti6Al4V as blended powders, after IS and HVS.

12 Conclusions and Summary: From the results found in the previous research and the presented one, there are few factors affecting the oxygen contamination during sintering: Powder particle size; sintering temperature and time; Sintering method; Green density of the powder compact; There is an alternative of use protective coating instead traditionally used vacuum or argon gas for protection from oxygen contamination: the use of a protective coating significantly decreases the oxidation rate from the surface to the center of an induction sintered sample, but it does not fully eliminate oxygen penetration; It needs more investigation on the requirements for the powder compact green density if protective coating is to be used to optimum effect; Overall the study shows the advantages of using induction sintering as alternative method for powder consolidation: this reduces processing time and offers a potential for reduced oxygen contamination;

13 . Thank you