Malaysian Metallurgical Conference – MMC 2008 3-4 December 2008, UKM, Bangi, Malaysia
THE EFFECT OF SINTERING TEMPERATURE TO THE MICROSTRUCTURE OF PM F75 (Co-Cr-Mo) ALLOY M.I.M. Tajuddin, J.B. Shamsul, P. Noraziana School of Materials Engineering, Universiti Malaysia Perlis, Kompleks Pusat Pengajian Jejawi 2, 02600 Jejawi, Arau, Perlis. E-mail address:
[email protected]
ABSTRACT PM Co-Cr-Mo (F75) alloys are widely used in implants due to their mechanical properties, good wear resistance and as well as biocompatibility. Currently, they are fabricated by casting technique. In this present research, F75 was fabricated by powder metallurgy technique. The powder was mixed with 2 wt. % of stearic acid in order to form green body and compacted at 500 MPa. The effect of sintering temperature was investigated to observe its effect to the microstructure of F75 (Co-Cr-Mo). Samples were sintered for 2 hours at 2 different temperatures (12500C and 13000C) with 100C/min in argon atmosphere. Physical properties such as density and porosity were obtained by Archimedes principle. Microstructure was observed by using optical microscope Olympus BX41M. The results indicate that increasing the sintering temperature will influence the density and porosity, thus the microstructure itself. Keywords: Powder metallurgy; Sintering temperature; Physical properties; Microstructure.
INTRODUCTION Co-Cr-Mo alloys have been widely used due to their mechanical properties, good wear and corrosion resistances as well as biocompatibility [1]. The Co-Cr-Mo alloy powders (F75) were selected in this project because of their already proven performance and better response to powder metallurgy process. Powder metallurgy provides better compositional and microstructural control in forming multiphase or composite materials that superior to the casting method. Powder metallurgy offers interesting technology solutions in the range of obtaining new exploitative materials [2]. Sintering is the final step in powder metallurgy and most common technique for consolidating powders [3]. Sintering is a process of densification where powders are heated up usually between 70 to 90 % of the powder melting temperature, and with the aid of a driving force mechanism such as diffusion then the powder is consolidated [4]. It removes the pores between the starting particles, combined with the grain growth and developing strong bonding between particles [5]. The aim of the present investigation was to observe the effect of sintering temperature on the microstructure and physical properties of Co-Cr-Mo (F75).
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EXPERIMENTAL PROCEDURE Co-Cr-Mo (F75) alloy powders were purchased from Sandvik Osprey LTD., UK. The chemical composition and particle characteristics of the powders are given in Tables 1 and 2. Particle size distribution of the powders was determined using Malvern particle size analyzer. Figure 1 shows SEM micrograph of the powder particles, exhibiting nearly spherical shape of particles.
Table 1: Chemical composition (wt %) of Co-Cr-Mo (F75) powders Element Co Cr Mo Si Mn Weight Percent (wt)% 62.595 29.3 6.1 0.79 0.74
Ni 0.26
Table 2: Characteristics of Co-Cr-Mo powders Sample Particle size distribution (µm) D10 D50 Co-Cr-Mo (F75) 4.217 11.519
Fe 0.20
C 0.015
D90 23.851
Figure 1: SEM micrograph of Co-Cr-Mo (F75) powders.
Powder metallurgy method was used to fabricate the sample. The powder was mixed with 2 wt. % of stearic acid by using ball mill for 30 minutes in order to form green body. Ten grams of powder was poured into tool steel die assembly with 15 mm diameter and 500 MPa pressure was applied using hand press to flatten the powder layer. Samples were sintered for 120 minutes at 2 different temperatures (12500C and 13000C) with 100C/min in argon atmosphere. Bulk density and percentage apparent porosity values were obtained by water displacement (Archimedes) method. Microstructural studies were performed using optical microscope Olympus BX41M. The effect of different sintering temperatures was investigated to observe its effect to the microstructure of F75 (Co-Cr-Mo). Hardness for sintered sample Co-CrMo was determined by using Mitutoyo HM-114 Vickers Microhardness.
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RESULTS AND DISCUSSION Table 3 shows the sample characteristics for Co-Cr-Mo after sintered at 2 different temperatures (12500C and 13000C). The percentages of shrinkage of the samples sintered from different temperatures indicating not much different. This result may be due to the same diffusion mechanism occurs in the sample. The average grain sizes were defined by using line intercept method. The average grain size for sample sintered at 12500C is 44.19µm and as the sintering temperature increased to 13000C, the grain has grown and the average grain size has increased almost double around 80.81µm. Bulk density slightly increases and less porosity is found in the sample sintered at 13000C. The hardness of the sample sintered at 12500C temperature is higher than 13000C. This is because of the grain growth as shown in Figure 3.
Table 3: Sample characteristics of Co-Cr-Mo (F75) after sintered at different temperatures. Temperatures Sample Characteristics 12500C 13000C Grain Size (µm) 44.19 80.81 Shrinkage (%) 24.07 24.31 Bulk Density (g/cm3) 6.85 6.98 Apparent Porosity (%) 3.99 1.41 Hardness (HV) 251.80 211.35
In Figures 2 and 3, the microstructures of the sintered powders at two different temperatures in argon atmosphere are shown. Figure 2 (a) shows the microstructure of Co-Cr-Mo before etching, and figure 2 (b) shows the microstructure at high magnification of 200X, indicating several large pores with the size around 100 µm distributed in the sample. By increasing the sintering temperature to 1300ºC, a progressive change in the pore size is observed from large pores to small pores as shown in figure 2 (c).
(a) 1250ºC at magnification 100X
(b) 1250ºC at magnification 200X
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(c) 1300ºC at magnification 200X Figure 2: Optical micrographs show the microstructures of Co-Cr-Mo after sintering for 120 min in argon atmosphere at different temperatures (Before etching).
For Figure 3, optical micrographs show the microstructure of the sintered powder at different temperature after etching. The samples were etched with Marble regent (modified) for 15 seconds. Irregular pores within the grains or at the grain boundaries were observed after sintering at 1250ºC as shown in figure 3(a) at magnification of 200X. Samples that sintered at higher sintering temperature increased the density (less porosity) which is accompanied by grain growth as shown in figure 3 (c).
(a) 1250ºC at magnification 200X
(b) 1250ºC at magnification 100X
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(c) 1300ºC at magnification 100X Figure 3: Optical micrographs show the microstructures of Co-Cr-Mo after sintering for 120 min in argon atmosphere at different temperatures. (After etching)
CONCLUSIONS Powder metallurgy method was successfully used to fabricate Co-Cr-Mo alloy. Different sintering temperatures were used to examine the results of the physical properties of the samples. It shows that different sintering temperatures influenced the physical properties and the microstructure of the samples. Increasing of the sintering temperature will result in higher density, low porosity and low hardness of the Co-Cr-Mo alloy.
ACKNOWLEDGEMENT The authors thank to the MOSTI for providing sciencefund research grant.
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