EXPLORING POWDER INJECTION MOLDING OF NIOBIUM

Transcription

1 EXPLORING POWDER INJECTION MOLDING OF NIOBIUM Gaurav Aggarwal, Ivi Smid and Randall M. German Center for Innovative Sintered Products The Pennsylvania State University University Park PA USA ABSTRACT Niobium and niobium-based alloys are used in a variety of high temperature applications ranging from light bulbs to rocket engines. Less than 2% of the global niobium consumption is in the form of niobiumbase alloys and pure niobium metal. This paper illustrates the problems and potential of applying powder injection molding to high temperature, high performance niobium alloys. Test geometries have been processed using powder injection molding of various niobium powders. The flow behavior, carbon content and densification after sintering were assessed versus particle characteristics. INTRODUCTION Powder injection molding (PIM) is a net-shaping powder metallurgy forming process, which combines a small quantity of polymer with an inorganic powder to form a feedstock that can be molded [1]. Its success versus established processes like die-compaction, machining and investment casting lies in producing intricate shapes at high production rates. PIM presents substantial cost advantages for the production of refractory metal components with complex shapes, since there is no powder cost penalty because the same powder can be used in PIM as used in alternative processes [2]. Material use is nearly 100% due to recycling of feedstock used in sprues and runners. Among refractory metals, niobium has a lower density, good ductility at low temperatures and is relatively inexpensive. Approximately 75% of all niobium metal is used as alloy additions in lowalloy steels [3]. Another 20-25% is used as additive in nickel-base superalloys and heat-resisting steels. Only 1-2% is used in the form of pure niobium and niobium-base alloys, which find their use in lighting, electronics, nuclear, and various other high temperature applications. With an aim of establishing fundamental knowledge of each step of the process and their interaction, this article summarizes the possibility of processing angular-shaped niobium powder via injection molding. Rheological aspects, carbon pickup during processing and sintering of niobium powders with two different particle sizes will be discussed. Part 8 298

2 EXPERIMENTAL PROCEDURES Materials Two niobium powders, Nb7 (D 50 ~7ìm) and Nb11 (D 50 ~11ìm) were used in the initial experiments. Both powders were acquired from Cabot Corporation. The characteristics of both powders are listed in Table 1. Figure 1a and 1b shows the morphology of Nb7 and Nb11 respectively. The internal microstructures are shown in Figure 2a and 2b. The binder system consisted of Dussek Campbell paraffin wax, Polyvisions ProFlow 3000 (polypropylene), DuPont Fusabond (polyethylene), and Fisher stearic acid as surfactant. The content and properties of each component are listed in Table 2. Powder Particle size, D 50 ìm Slope Parameter, S w Table 1: Characteristics of niobium powders Tap density, g/cm 3 Pycnometer density, g/cm 3 Carbon content, wt% Oxygen content, wt% Nb Nb Hydrogen content, wt% Table 2: Binder composition and properties of the components Component Content (wt%) Melting Point ( o C) Density (g/cm 3 ) Dussek Campbell Paraffin wax Polyvisions ProFlow DuPont Fusabond Fisher Stearic acid Figure 1a: Scanning Electron Micrograph of Nb7 Figure 1b: Scanning Electron Micrograph of Nb11 Part 8 299

3 Figure 2a: Internal microstructure of Nb7 Figure 2b: Internal microstructure of Nb11 The powder and binder components were weighed in required amounts and small batches of feedstock were compounded with high-shear mixing blades in a torque rheometer for simultaneous determination of critical solids loading. Torque Rheometer Test Torque rheometry is used to determine the critical solids loading of a powder. Using a load cell of high resolution to monitor applied torque, powder and binder are mixed with high-shear mixing blades. Once the feedstock is plasticized and is in thermal equilibrium, additional powder is added in weighed amounts to bring the solids loading up by one volume percent at a time. With each addition of powder, the mixing torque increases significantly, then levels out as thermal equilibrium and full dispersion is achieved. In a classical torque behavior, at the critical loading point, the spaces between the powders are just filled with binder and the mixing torque increases significantly and becomes erratic due to interparticle friction. A Haake Rheocord torque rheometer was used to determine critical solids loading. A starting batch of 50 vol% solids loading for both powders was used and mixed at 150 o C in the rheometer. Additional doses of powder were added, incrementing the solids loading by one percent each time. Figure 3a and 3b show the variation of mixing torque with time for Nb7 and Nb11 respectively, as solids loading Part 8 300

4 is incrementally increased. In Nb7 the powder additions were made upto 61% solids loading and in Nb11 upto 57% vol% 58 vol% 57 vol% 56 vol% 59 vol% Time (min) 60 vol% 61 vol% Torque (Nm) vol% 54vol% 53vol% 52vol% 57vol% 56vol% Time (min) Figure 3b: Solids loading determination of Nb11 feedstock Capillary Rheometer Test Capillary rheometry measures the pressure drop associated with a forced flow rate of a molten feedstock through a small capillary tube. Under the conditions of steady flow, pressure drop or volumetric flow rate through a capillary is measured, and associated shear stress and shear rate can be determined. Using this information viscosity can be calculated [4]. Viscosity variation of feedstock with time is an indication of non-homogeneity, the larger the variation in viscosity with time, the more significant the powder-binder separation. PIM feedstock is generally considered to be a pseudoplastic fluid [5]. By measuring the viscosity with increasing shear rate, gives the amount of shear sensitivity of feedstock [6]. Part 8 301

5 A Kayness Galaxy V capillary rheometer was used to measure viscosity of the feedstock. The capillary die had a diameter of 2 mm and a length of 30 mm, giving a length-to-diameter ratio of 15. The viscosity of each feedstock at different shear rates and at three temperatures (150, 160, and 170 o C) was measured. Binder Burnout and Sintering Removing the binder without disrupting the particles is a delicate process that is best achieved in multiple steps [1]. By employing suitable powder metallurgy technique fine-grained sintered niobium can be produced [7]. When the binder is removed, the component becomes very fragile until sintered, though it may be strong enough to retain its shape. Final debinding occurs as a part of the heating process before the sintering temperature is reached or pre-sintering is done as a part of final debinding, since handling a fully debound but unsintered structure is difficult. Sintering of niobium must be done in vacuum at high temperatures to degas oxygen and nitrogen by well-known mechanisms [8]. To demonstrate a debinding cycle, solvent debinding was performed by immersion of extruded capillary rheometry noodles in a heptane bath for 2 hours at 60 o C. Thermal debinding was done with subsequent pre-sintering in high purity argon at 1000 o C. The as-debound samples were sintered in a vacuum furnace at 1800 o C with a vacuum atmosphere of 0.1 Pa. RESULTS Rheological behavior of feedstock The viscosity data in Table 3 indicate the flowability of both feedstocks (57% solids loading) made from the two powders. The lower the value of viscosity, the easier it is for a feedstock to flow. Table 3: Viscosity of two feedstocks in Pa.s at six different shear rates and three temperatures Shear strain rate Feedstock Temp. ( o C) 77.64s s s s s s -1 Nb Nb It can be seen from Table 3 that at all temperatures the viscosity of the feedstock containing Nb11 is lower at every given shear rate as compared to feedstock with Nb7. The viscosity data in Table 3 also indicate that viscosity decreases with increases in shear rate. For a pseudoplastic fluid, ô = kã n, where ô is shear stress, ã is shear rate, k is constant and n is the exponent used to characterize the fluid. The value of n indicates the degree of shear sensitivity. The lower the value of n, the more quickly the viscosity of the feedstock changes with shear rate. It is desirable that the viscosity of the feedstock should decrease quickly with increasing shear rate during injection. This high shear sensitivity aids in producing complex geometries. The value of n was determined by plotting natural logarithm of the shear stress against natural logarithm of the shear rate for the temperature of 150 o C shown in Figure 4. The calculated values were values n Nb7 = and n Nb11 = 0.389, respectively. Part 8 302

6 11.8 ln ô (Pa) Ö= 57 vol% Nb Nb ln ã (1/s) Figure 4: Correlation of shear stress and shear rate at 150 o C The dependence of viscosity on temperature can be expressed by an Arrhenius equation [1], ç(t) = ç o exp(-e/rt), where E is the flow activation energy, R is the gas constant, T is absolute temperature, ç o is reference viscosity. The value of E expresses the influence of temperature on the viscosity of the feedstock. If the value of E is large, it shows a high sensitivity. That means a small fluctuation of temperature during injection molding will result in sudden viscosity change which often results in density gradients, a cause of stress concentration in the molded parts. By plotting ln viscosity against the reciprocal of temperature (Figure5a and 5b), the values of E Nb7 = 26.7 kj mol -1 and E Nb11 = 16.7 kj mol -1 were determined ln viscosity (Pa s) C 150 C Ö= 57 vol% C /T (1/K) Figure5a: Correlation of viscosity and temperature for Nb7 feedstock Part 8 303

7 C ln Viscosity (Pa-s) C Ö= 57 vol% C /T (1/K) Figure 5b: Correlation of viscosity and temperature for Nb11 feedstock Carbon control Carbon control is one of the important issues during processing of niobium and niobium-base alloys by PIM. Contamination by interstitial carbon impairs room temperature ductility of niobium due to formation of niobium carbide at grain boundaries and within grains. Generally carbon is kept low (below 0.072%) to avoid poor ductility. Table 4 gives the carbon content (wt %) for both powders from the asreceived condition through sintering at 1800 o C. Table 4: Carbon content during PIM processing steps Carbon content (wt %) Powders As-received After thermal debinding After solvent and Sintering thermal debinding Nb Nb Microstructure and density The optical micrographs of both the powders after sintering at 1800 o C for 1 hour are shown in Figure 6a and 6b. There is more grain growth in Nb7 (D 50 ~ 5ìm) as compared to Nb11 (D 50 ~8.5ìm). Part 8 304

8 Figure 6a: Sintered microstructure of Nb7 at 1800 o C Figure 6b: Sintered microstructure of Nb7 at 1800 o C The Archimedes densities of Nb7 and Nb11 were measured about 95% and 80% of theoretical (8.57 g/cm 3 ) after sintering at 1800 o C for 1 hour. The density of Nb11 increases up to 92% when it is sintered at 2000 o C for 1 hour (Figure 7) % Theoretical Density Ö= 57 vol% Temperature ( o C) Figure 7: Variation of density with sintering temperature for Nb11 CONCLUSION There was a non-classical torque rheometer behavior for both the powders due to the angular shape of the particles. Therefore, it was difficult to determine the optimal solids loading. With a powder loading of 57% the finer particle size powder shows higher shear sensitivity and temperature sensitivity, which in turn influence viscosity. An unexpected difference in flow activation energy values for two powders was observed with same binder system and solids loading. The combination of solvent and thermal debinding gave a better carbon control for both the powders versus just thermal debinding. The smaller size powder sinters well at lower temperature, but higher temperature sintering is required to achieve comparable density in coarser size powder. Part 8 305

9 Future efforts are focused on explaining the anomalies in rheological behavior and the purity issues of thermal processing of niobium by varying the binder system, debinding and sintering parameters. ACKNOWLEDGEMENTS Authors greatly acknowledge the funding for this research from Ben Franklin Technology Development Authority through the Center for Innovative Sintered Products at The Pennsylvania State University. REFERENCES 1. R. M. German and A. Bose, Injection Molding of Metals and Ceramics, Metal Powder Industries Federation, Princeton, R. M. German, Metal Powder Report, vol. 53, n2, pp. 40, C. Craig Wojcik, Mat. Res. Soc. Symp. Proc., vol. 322, pp , P. L. Shah, Handbook of plastic materials and technology, Wiley, New York, J. Shah and R. E. Nunn, Powder Metallurgy International, vol. 19, pp Y. Li, B. Huang and X. Qu, Powder Metallurgy, vol. 42, n1, pp , H. R. Z. Sandim and A.F. Padilha, International Journal of Powder Metallurgy, vol. 34, n2, p K. Schulze, H. A. Jehn and G. Horz, Journal of Metals, vol. 40, n10, p. 25, Part 8 306