DENSIFICATION STUDY OF TITANIUM POWDER COMPACTS

Transcription

1 DENSIFICATION STUDY OF TITANIUM POWDER COMPACTS Márcia Cristina Carneiro Ueta (1), Carlos Augusto Fracote (1), Vinicius André Rodrigues Henriques (2), Mario Lima Alencastro Graça (2), Carlos Alberto Alves Cairo (2) (1)Universidade do Vale do Paraíba UNIVAP (2) Centro Técnico Aeroespacial, AMR/IAE/CTA Key-words: Powder Metallurgy, titanium powders, Hydride-Dehydride Process (HDH), densification. Abstract: Powder compaction characteristics is a very important parameter to control in order to obtain products with best mechanical properties made by P/M techniques. This work presents a study on the densification of titanium powders trying to optimize the particle size distribution for the best packing and the maximum densification by pressure compaction. The powders used were made from titanium sponge obtained by the Kroll process. The powders were embrittled by mean of the Hydride-Dehydride process (HDH) and milled in a rotative ball-mill under vacuum. Powders with different particles sizes distributions were mixed in several proportions according to 's and s Theory. The samples were compacted by uniaxial and isostatic pressing and sintered under vacuum. The evaluation of the densification was made following the Standard method of test for density of glass by Buoyancy (ASTM C693-74) and by scanning electron microscopy (SEM). The samples made with powder milled during 36 hours and 12 hours presented better densification than the ones milled during shorter time and the ones with distributions combinations. Introduction Traditional metallurgic processes to obtain titanium alloys are very expensive, due to the high reactivity of the titanium. Powder metallurgy technology enables to produce high quality metallic components with complex parts and low tolerances (near net shape), with lower costs. However, the disadvantage of this process is that the metallic components present high porosity after sintering, and consequently lower mechanic strength when compared with other producing methods. The utilization of powder with optimized size particle distribution which allow better packing during the pressing before sintering is a way to solve this problem. The principle of particle packing is based on selecting particles in such sizes and fractions that produces compacts with controlled density [1]. Particle size distribution that enable to obtain dense compacts shows some advantages like minimizing dimensional changes during drying or firing and improving the compound s properties. Alloys without macropores and with higher mechanic strength is also favored by using this optimized particle distribution [2]. Dense packing of particles is based on selecting particles in such sizes and fractions that voids between larger particles are occupied by successively smaller particles. The remaining porosity is then composed of interstices created by the non-existence of smallest particles in the

2 distribution. Particle size distribution, particle shape, shape factor, surface roughness are some factors that determine final properties of the consolidated powder. Numerous approaches have been proposed to optimize particle packing density in compacts. Among these, can be cited the s model, which considers continuous distribution of particle size, and s model, which is a mathematical review of s and Furna s models [3]. s model is based on the similarity between large particles and small particles distributed around. This similarity condition necessarily leads to a power law equation form which is the form of the equation proposed by for particle packing systems: D p CPTF = 100 (1) DL where: CPFT = cumulative percent of particle finer than a diameter D p ; D p = particle diameter; D L = largest particle in the distribution; q = distribution modulus. s model assumed that all particle sizes exist, including even infinitely small particles and these very small particles doesn t have effect on the CPFT versus particles size plot. Studies performed by Funk and Dinger [3] showed, by computing simulations, that the exponents that give the maximum packing density is Later, Funk and Dinger [3] verified that the system without infinitely small particles could cause significant deviation in particle packing. Inserting a minimum particle size, that is a real systems characteristic, in the s model they developed the s model: Where: Ds is the smaller particle diameter in the system q q q = D p DS CPFT 100 (2) q D q L DS Equation (2) is a improvement from s similarity condition and it include Furnas concept of a finite smallest particle size (Ds). s equation is currently the most suitable model for real systems packing. In the present work, both models are used to study the titanium powder densification with different particle size proportions, with a distribution modulus equal to The compacts were obtained by uniaxial and isostatic pressing, to intend the best composition and the maximum densification alloys produced by powder metallurgy. Materials and Methods Titanium s powder densification with different particle size distribution was prepared and analyzed by the following steps: The influence of particle size distribution on compacts densification was based in s model and s model. In these studies was also considered that particles have a non-spherical shape. This non-spherical shape is not in accordance with the both theory.

3 Titanium powder was obtained by Hydride-Dehydride process (HDH), starting from titanium sponge. The powder was milled in a rotative ball mill under vacuum, during 30 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 12 hours, and 36 hours, to obtain various particle size ranges. A sample of each milled powder was took and analyzed by laser particle analyzer (Cilas 1064). The morphological analysis was performed by scanning electron microscopy (SEM), LEO 440. The data were compiled into software specially developed to model particle size distribution with the desired characteristics. The software combines raw materials in proportions designed to fit a target particle size distribution in this case, and with a distribution modulus equal to Three different particle compositions, obtained from mixtures of three different powder distributions were selected based on the software calculations. These selected compositions was the one that theoretically should have the best packing during compaction, before sintering. After have been defined these best compositions, 50 grams of each composition were prepared. It was compacted 5 samples of each milled powder (single distribution) and of each composition ( mixture of three distributions). The samples were compacted by isostatic pressing at 200 MPa. The samples were sintered under vacuum 10-6 torr, at 1400 ºC with 1 h soaking. The investigation of the apparent density was made following the Standard method of test for density of glass by Buoyancy (ASTM C693-74). The internal and external morphology of the samples were characterized by scanning electron microscopy (SEM). Results and Discussion The titanium powder morphological analysis performed by scanning electron microscopy (SEM) showed that the grains size decreased with the increasing in the milling time. The SEM also showed that the particles don t have a spherical shape. Figure 1 shows the morphology of powders milled for 30 min.; 12 h; and 36 h. These powders were used to prepare the compositions. Figure 2 shows a comparison between theoretical size distribution for maximum densification and experimental particle size for accumulative and discrete distribution curves for the powders milled. After the theoretical accumulative curves of s model and s model with q=0.37 have been analyzed, it was possible to determine which powder combinations that best fit the theoretical curves. The powders proportions are showing at table 1. Figure 3 shows the size particle distribution curves of the samples that best fit to theoretical size particle accumulative distribution model and Table 2 presents apparent density values and relative density for each sample. Table 1: Proportion of milled powders to obtain samples that best fit with s and s curves. Milling time Amount of powder, % Sample F1 Sample F2 Sample F3 30 minutes hours hours

4 A B C Figure 1: Scanning electron micrographs of titanium powder, milled for 30 minutes (A), 12 hours (B), and 36 hours (C) A 30 min C 12 hours E 36 hours Discrete fraction / % CPTF / % 1 0, B 30 min D 12 hours F 36 hours Equivalente spherical diameter / µm Figure 2: Comparison between theoretical and experimental particle size cumulative and discrete distribution curves for the powders milled for 30 minutes (A and B); 12 hours (C and D); and 36 hours (E and F).

5 Discrete fraction / % CPFT / % ,1 6 4 A B 61% 30 min 39% 36 hours 61% 30 min 39% 36 hours C D 60% 30 min 30% 36 hours 60% 30 min 30% 36 hours E F 70% 30 min 20% 36 hours 70% 30 min 20% 36 hours Equivalent spherical diameter / µm Figure 3: Size particle distribution curves of the samples F1 (A and B), F2 (C and D), and F3 (E and F). Table 2: Apparent relative density values, packing factor and standard deviation. Sample Apparent Density Relative Density 30 minutes 4,36 ± 0,04 96,7 ± 0,9 12 hours 4,49 ± 0,01 99,7 ± 0,1 36 hours 4,50 ± 0,01 99,8 ± 0,2 F1 4,40 ± 0,01 97,5 ± 0,1 F2 4,44 ± 0,01 98,4 ± 0,1 F3 4,46 ± 0,01 98,9 ± 0,1 Figure 4 shows the scanning electron micrographs of the surface of Ti sample, milled for 36 hours, after sintering at 1400 ºC and Figure 5 shows the scanning electron micrographs of the cross section of Ti samples, milled for 12 and 36 hours, after sintering at 1400 ºC. Although the particle size distribution of powders obtained by the mixtures that best fitted with the theoretical curve, according to s and s models, the final densification for these powder was lower than that obtained to the single powders milled for 24 and 36 h, that presents a fine particles percent higher that predict by the models. The best densification results after sintering (99,8% mean relative density), was obtained for powder milled for 36 h, that presents a trimodal distribution with higher frequencies for sizes 15 µm, 5 µm and 0,8 µm. The differences between theoretical and practical results was attributed to the irregular form of the titanium particles, that produce an irregular array of voids in the compacted and enhances packing of powder with trimodal distribution.

6 Figure 4: Scanning electron micrographs of the surface of Ti sample, milled for 36 hours, after sintering at 1400 ºC. 12 h 36 h Figure 5: Scanning electron micrographs of the cross sections of Ti samples, milled for 12 and 36 hours, after sintering at 1400 ºC, dark points are porous. Conclusion The particle size distribution of titanium powder, with irregular shapes, to maximum densification do not follow the distribution predict by theoretical models of Andreassen and. The best apparent density after sintering was obtained using a milled powder with a continuous distribution with trimodal characteristic of particle size 15 µm, 5µm and 0,8 µm. In this powder the fine fraction is superior to one obtained using the mathematical approach. The irregular shape of particles produce an irregular array of voids with the small particles filling voids between the great ones enhancing compaction. REFERENCES [1] R.K.Mc Geary, Mechanical packing of Spheroidal Particles. J. Am. Ceram. Soc.,44 (1961) [2] J.E. Funk. D. R. Dinger, Particle packing, part I fundamentals of particle packing monodisperse spheres. Interceram, 41, 1 (1992) [3] FUNK, J. E., DINGER, D. R. Particle packing, part VI: Applications of Particle Size Distribution Concepts. Interceram, 43, 5, (1994) 350-3