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 Alfred's and Andreasen’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

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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 Andreasen’s model, which considers continuous distribution of particle size, and Alfred’s model, which is a mathematical review of Andreasen’s and Furna’s models [3].

Andreasen’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 byAndreasen for particle packing systems:

100×

=

q

L

p

DD

CPTF )

where: CPFT = cumulative percent of particle finer than a diameter Dp; Dp = particle diameter; DL = largest particle in the distribution; q = distribution modulus.

Andreasen’s model assumed that all particle sizes exist, including even in

particles and these very small particles doesn’t have effect on the CPFT versus particStudies performed by Funk and Dinger [3] showed, by computing simulations, that tthat give the maximum packing density is 0.37.

Later, Funk and Dinger [3] verified that the system without infinitely small pcause significant deviation in particle packing. Inserting a minimum particle size, systems characteristic, in the Andreasen’s model they developed the Alfred’s model:

100×

−

−= q

SqL

qS

qp

DDDD

CPFT

Where: Ds is the smaller particle diameter in the system

Equation (2) is a improvement from Andreasen’s similarity condition and it inconcept of a finite smallest particle size (Ds). Alfred’s equation is currently the most sfor real systems packing.

In the present work, both models are used to study the titanium powder densdifferent particle size proportions, with a distribution modulus equal to 0.37. The coobtained by uniaxial and isostatic pressing, to intend the best composition and tdensification alloys produced by powder metallurgy. Materials and Methods

Titanium’s powder densification with different particle size distribution was analyzed by the following steps:

The influence of particle size distribution on compacts densification wAndreasen’s model and Alfred’s model. In these studies was also considered that panon-spherical shape. This non-spherical shape is not in accordance with the both theo

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(1

finitely small les size plot. he exponents

articles could that is a real

(2)

clude Furnas’ uitable model

ification with mpacts were

he maximum

prepared and

as based in rticles have a ry.

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, Alfred and Andreasen with a distribution modulus equal to 0.37. 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 Alfred’s model and Andreasen’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 Alfred’s and Andreasen’s

curves.

Amount of powder, % Milling time Sample F1 Sample F2 Sample F3

30 minutes 61 60 70 12 hours - 10 10 36 hours 39 30 20

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A B

C

Figure 1: Scanning electron micrographs of titanium powder, milled for 30 minutes (A), 12 hours

(B), and 36 hours (C).

0,1

1

10

100

A

Alfred Andreasen 30 min

0 10 20 30 40 50 60 70 80

0

2

4

6

B

Alfred Andreasen 30 min

C Alfred Andreasen 12 hours

0 10 20 30 40 50 60 70 80

D

Alfred Andreasen 12 hours

E

Dis

cret

e fra

ctio

n / %

CPT

F / %

Equivalente spherical diameter / µm

Alfred Andreasen 36 hours

0 10 20 30 40 50 60 70 80

F

Alfred Andreasen 36 hours

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).

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0,1

1

10

100

A

Alfred Andreasen 61% 30 min

39% 36 hours

0 10 20 30 40 50 60 70 80

0

2

4

6B

Alfred Andreasen 61% 30 min

39% 36 hours

E

Alfred Andreasen 70% 30 min

10% 12 hours 20% 36 hours

0 10 20 30 40 50 60 70 80

Dis

cret

e fra

ctio

n / %

CPF

T / %

F

Alfred Andreasen 70% 30 min

10% 12 hours 20% 36 hours

C Alfred Andreasen 60% 30 min

10% 12 hours 30% 36 hours

0 10 20 30 40 50 60 70 80

D

Equivalent spherical diameter / µm

Alfred Andreasen 60% 30 min

10% 12 hours 30% 36 hours

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 Alfred’s and Andreasen’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.

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Figure 4: Scanning electron micrographs of the surface of Ti sample, milled for 36 hours, after

sintering at 1400 ºC.

Figure 5

Conclus The pardo not fapparenwith trimis superproduceenhanci REFERE [1] R.K.

522[2] J.E.

mon[3] FUN

Dis

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12 h

: Scanning electron micrographs of the cross

hours, after sintering at 1400 ºC, dark points

ion

ticle size distribution of titanium powder, withollow the distribution predict by theoretical mt density after sintering was obtained using a m

odal characteristic of particle size 15 µm, 5µmior to one obtained using the mathematical an irregular array of voids with the small pang compaction.

NCES

Mc Geary, Mechanical packing of Spheroidal . Funk. D. R. Dinger, Particle packing, podisperse spheres. Interceram, 41, 1 (1992) 10K, J. E., DINGER, D. R. Particle packin

tribution Concepts. Interceram, 43, 5, (1994) 3

36 h

sections of Ti samples, milled for 12 and 36 are porous.

irregular shapes, to maximum densification odels of Andreassen and Alfred. The best illed powder with a continuous distribution and 0,8 µm. In this powder the fine fraction approach. The irregular shape of particles rticles filling voids between the great ones

Particles. J. Am. Ceram. Soc.,44 (1961) 513-

art I – fundamentals of particle packing -14. g, part VI: Applications of Particle Size

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