PRODUCTION AND CHARACTERIZATION OF ALUMINIUM MATRIX COMPOSITES VIA PRESSURELESS SINTERING

Hasan Erdem ÇAMURLU*, Feyzi Emrah BAŞAR**

*Akdeniz University, Engineering Faculty Mechanical Eng. Dept., Antalya-Turkeyerdemcamurlu@gmail.com

**Akdeniz University, Engineering Faculty Mechanical Eng. Dept., Antalya-Turkeyfebasar@akdeniz.edu.tr

1. ABSTRACT

In this thesis, non-pressurized sintering and characterization of TiB2 particulate reinforced

aluminum matrix composites were studied. As a matrix material 96% Al – 4% Cu alloy was

used. Al-TiB2 powder mixtures containing 10%, 15%, 20% and 30% TiB2 by volume were

prepared and were cold pressed uniaxially at 800 MPa. Non-pressurized sintering was

performed in a nitrogen gas atmosphere at 620 oC. Obtained composites were subjected to

density measurements, microstructure examinations, 3 point bending tests, hardness tests and

wear resistance tests. Same tests were repeated after age hardening of the composites.

After age hardening treatment, unreinforced sample presented a maximum 3 point bending

strength of 940 MPa, 28% strain and 100 HB10 hardness value. Wear rate was 12,16 ± 3,16 x

10-3 mm3 / N.m in the unreinforced sample. The least wear rate was seen in 20% TiB2

containing samples. This composite presented 867 MPa bending strength, 7,41% strain and

130 HB10 hardness.

Keywords: Powder Metallurgy, Sintering, Aluminum Matrix Composites, Metal Matrix Composites, TiB2

1. ÖZET

Bu çalışmada TiB2 parçacık takviyeli alüminyum matrisli kompozitlerin basınçsız

sinterleme ile üretimi ve karakterizasyonu gerçekleştirilmiştir. Matris malzemesi olarak

alüminyum-%4 bakır alaşımı kullanılmıştır. Hacimce %10, 15, 20 ve 30 oranlarında TiB2

içeren ve toz formunda olan başlangıç malzemeleri karıştırıldıktan sonra soğuk preste 800

MPa basınçta preslenmiştir. Basınçsız sinterleme işlemi nitrojen atmosferi altında 620 oC’de

gerçekleştirilmiştir. Elde edilen kompozitler yoğunluk ölçümlerine, mikro yapı

incelemelerine, 3 nokta eğme, sertlik ve aşınma testlerine tabi tutulmuştur. Çökelti

sertleştirmesi işleminden sonra aynı testler tekrarlanmıştır.

Katkısız numune ısıl işlem sonrasında en yüksek 940 MPa 3 nokta eğme mukavemetine, %

28 uzama ve 100 HB10 sertlik değerine ulaşmıştır. Aşınma oranı katkısız numunede 12,16 ±

3,16 x 10-3 mm3 / N.m olmuştur. En düşük aşınma oranına % 20 TiB2 içeren kompozitte

ulaşılmıştır. Bu kompozit 867 MPa mukavemet, %7,41 uzama ve 130 HB10 sertlik

göstermiştir.

Anahtar Kelimeler: Toz Metalurjisi, Sinterleme, Alüminyum Matrisli Kompozitler, Metal Matrisli Kompozitler, TiB2

2. INTRODUCTION

Composite material technologies are revolved around polymer matrix composites for some time and material scientists made great discoveries using advanced polymers in this field. While having some amazing attributes polymer matrix composites cannot withstand high temperatures due to low melting point of their matrix. Material scientists also found a way around this problem by substituting polymer matrix with a metal matrix. Regardless of a few exceptions, reinforcement material is usually fiber or particulate ceramic [1]. Metal matrix combined with ceramic reinforcement created a material with good attributes such as high wear resistance, high durability and resistance to thermal stress. However production of metal matrix composites is a difficult process since the process temperature and reactivity of the metals make it difficult and expensive to produce metal matrix composites. The chemical reactions between metallic matrix and the ceramic particles creates dozens of chemical compounds each of which effecting the overall health of the product. Also, many ceramic particles will precipitate on bottom when added to liquid metals due to difference in density of the materials. Extensive stirring is needed to dissipate the particles evenly in the liquid metal. Also usage of inert gases or vacuum atmosphere is an obligation to prevent severe oxidation and gas pollution inside the metal matrix composite [2]. To overcome this obstacle powder metallurgy techniques are used in bulk production of metal matrix composites. However, powder metallurgy techniques are very sensitive processes in which the process parameters must be controlled very carefully. Most of the academic work is done on aluminum matrix composites. This is because aluminums attributes: lightweight, can be alloyed, can be heat treated and can be flexibly manufactured [3].

3. MATERIALS AND METHODS

In this study, non-pressurized sintering and characterization of TiB2 particulate reinforced aluminum matrix composites were studied. As a matrix material 96% Al – 4% Cu alloy was used. Al-TiB2 powder mixtures containing 0%, 10%, 15%, 20% and 30% TiB2 by volume were prepared and were cold pressed uniaxially at 800 MPa. Non-pressurized sintering was performed in a nitrogen gas atmosphere at 620 oC. Obtained composites were subjected to density measurements, microstructure examinations, 3 point bending tests, hardness tests and wear resistance tests. Same tests were repeated after age hardening of the composites.

4. RESULTS AND DISCUSSION

1. Non-reinforced specimens

Non-Reinforced specimens can be thought as a control group for the experiments. The effects of reinforced materials can be compared to the control group. The relative density of the control group is above 98% which indicates a good sintering. As seen on Fig. 1 grain boundaries are free of oxides which indicate that the protective nitrogen atmosphere is effective. After sintering bending, hardness and wear resistance tests are made. Bending tests yielded a bending strength of 420 to 550 MPa and 20% to 28% elongation with a hardness value of 50 to 55 HB10. After the age hardening heat treatment, bending strength become 810 to 940 MPa, elongation become 16% to 24% and the hardness become 90 to 100 HB10. The wear rate of age hardened specimen is measured as 12,16 ±3,16 (x103) (mm3 / N.m).

Figure 1. Microstructure photograph of non-reinforced specimens (red line is 20µm long)

2. 10% Reinforced specimens

10% reinforced specimens are much harder and durable than the non-reinforced specimens. Addition of 10% TiB2 by vol. increased the brittleness and the hardness of the aluminum. As can be seen on Fig. 2 TiB2 particles are dispersed equally in the aluminum matrix. The black areas are thought to be pores or holes that are left by dislodged reinforcement material due to heavy grinding and polishing. After sintering bending, hardness and wear resistance tests are made. Bending tests yielded a bending strength of 670 to 690 MPa and 10% to 12% elongation with a hardness value of 84 to 86 HB10. After the age hardening heat treatment, bending strength become 790 to 908 MPa, elongation become 9% to 11% and the hardness become 90 to 100 HB10. The wear rate of age hardened specimen is measured as 1,69 ±3,16 (x103) (mm3 / N.m). Compared to non-reinforced specimens the 10% reinforced specimens are show a great increase in wear resistance.

Figure 2. Microstructure photograph of 10% reinforced specimens (red line is 20µm long)

3. 20% Reinforced specimens

In Fig. 3 20% reinforced specimens microstructure photograph can be seen. Inspection of the microstructure photograph shows that dispersion of ceramic particles is even and the density of the ceramic particles is increased. The overall density of the specimens is decreased due to the increase in ceramic content. This is expected as stated by Karakaş in 2007; the higher the ceramic content the more surface area liquid metal has to wet [4]. Higher sintering temperatures may help reducing the pores to increase the overall density. After sintering bending, hardness and wear resistance tests are made. Bending tests yielded a bending strength of 570 to 625 MPa and 7,4% to 6,4% elongation with a hardness value of 83 to 96 HB10. After the age hardening heat treatment, bending strength become 777 to 867 MPa, elongation become 5,9% to 6,7% and the hardness become 123 to 130 HB10. The wear rate of age hardened specimen is measured as 0,35 ±3,16 (x103) (mm3 / N.m). Compared to non-reinforced specimens the 20% reinforced specimens have nearly 40 times better wear resistance properties.

Figure 3. Microstructure photograph of 20% reinforced specimens (red line is 20µm long)

4. 30% Reinforced specimens

In Fig. 4 microstructure photograph of the 30% reinforced specimens can be seen. Upon inspection it can be seen that the amount of matrix between the ceramic particles are reduced and probably not enough to withstand physical stresses.

After sintering bending, hardness and wear resistance tests are made. Bending tests yielded a bending strength of 388 to 445 MPa and 3,93% to 2,28% elongation with a hardness value of 87 to 94 HB10. After the age hardening heat treatment, bending strength become 570 to 654 MPa, elongation become 3,75% to 4,49% and the hardness become 110 to 133 HB10. The wear rate of age hardened specimen is measured as 0,56±3,16 (x103) (mm3 / N.m). Compared to 10% or 20% reinforced specimens 30% reinforced specimens has an overall decrease in physical attributes due to the reduction of matrix phase.

Figure 4 Microstructure photograph of 30% reinforced specimens (red line is 20µm long)

5. Overall review of the experiments and discussion

Table 1a and 1b shows overall summary of the conducted experiments can be seen. First half of the table is non-age hardened specimens and the second half is age hardened specimens. Wear resistance tests are made on only the heat treated specimens which have relatively higher durability.

Table 1a Summary of the non-age hardened specimens

Reinforcement %

Specimen Number

Average Hardness (HRB10)

Standart Deviation of Hardness

Strength (Mpa)

Elongation %

Wear Rate (x10-3) (mm3/N.m)

0 39-1-2 49,26 0,66 435,9 20,76  0 39-2-1 52,6 3,53 418,1 20,36  0 54-1 56,68 0,91 556,7 28,03  0 54-2 55,4 1,72 542,4 27,86  10 50-1 84,76 5,76 673,3 12,86  10 50-2 82,2 3,71 668,1 14,18  10 52 86,28 6,8 690,3 10,44  15 45-1-2 79,84 3,43 494,3 7,86  20 38-2-2 86,8 3,17 571,3 7,41  20 41-3-1 90,72 1,84 572,3 7,38  20 60-1 93.80 2 621.60 6,39  20 60-2 91.66 1,62 624.60 6,32  

30 40-3-1 94,7 4,45 388,5 3,75  30 59-1 86,96 2,29 423,9 3,28  30 59-2 92,98 2,05 445,3 3,93  

Table 1b Summary of the age hardened specimens

Reinforcement %

Specimen Number

Average Hardness (HRB10)

Standart Deviation of Hardness

Strength (Mpa)

Elongation %

Wear Rate (x10-3) (mm3/N.m)

0 61-1 89,06 3,48 833,7 16,47  

0 61-2 98,38 1,42 942,8 24,6912,16 ± 3,16

0 61-3 99,92 1,96 809,9 21,35  10 37-3-2 111,8 4,94 789,6 9,19  10 37-4-2 111,88 3,62 735,8 9,11  10 62-1 109,48 2,88 852,5 10,04  

10 62-2 109,44 1,38 908,9 11,671,69 ± 0,13

15 45-3-1 123,12 5,48 757,7 8  20 41-1-2 123,68 3,26 789,6 6,37  20 41-2-1 130,2 0,76 777,4 6,78  20 57-1 128,6 4,6 858 5,93  

20 57-2 124,6 1,51 867 6,60,35 ± 0,06

30 40-1-1 110,52 17,53 570,6 4,49  30 40-2-1 133,27 8,49 654,5 4,43 0,56±0,09

30 40-2-2 112,92 9,76 579,8 4,21  30 58-1 122,12 1,92 612,3 3,75  30 58-2 119,48 0,71 596,6 4,26  

Following figures are graphs of the results of the experiments.

0 5 10 15 20 25 300

10

20

30

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140

Non-Age Hardened Age Hardened

TiB2% by vol.

Har

dnes

s(H

B 1

0)

Figure 5. A graph of hardness values (HB10)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 320

50

100

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Non-ReinforcedNon-Reinforced, Age Hardened10%10% Age Hardened20%20% Age Hardened30%30% Age Hardenedİ

Elongation %

Thre

e Po

int B

endi

ng S

tren

gth(

Mpa

)

Figure 6. Summary of three point bending strength tests

In Fig 5it can be seen that the hardness values of the composites are increased in co ordinance with the ceramic content up to 20% TiB2. After that point amount of matrix material is not enough to hold ceramic particles together which caused stagnancy in hardness values. Three point bending values for non-reinforced specimens are better than all of the reinforced specimens. This is expected as ceramic particles are foreign materials and can cause weak points in the matrix, which than reduces the overall durability of the material. The three point bending strength of the 10% and 20% specimens are acceptable but the 30% reinforced specimens are very brittle due to the nature of the ceramic content as can be seen on Fig 6. It is advised to use 20% reinforced materials in applications that do not require high durability but instead require high wear resistance and light weight. If needed 10% reinforced materials can be used to gain some durability by sacrificing some wear resistance properties.

REFERENCES

1. T.W. Clyne,”Metal Matrix Composites: Matrices and Processing”, Encyclopedia of

Materials: Science and Technology, 2001

2. J.U. Ejiofor and R.G. Reddy, “ Developments in the Processing of Particulate Al-Si

Composites”, Journal of Materials, 1997

3. V. Kevorkijan, “ Development of Al MMC Composites for Automotive Industry”,

Deformation and Structure of Metals and Alloys Symposium, Belgrade Yugoslavia, 2007

4. M.S. Karakaş, “Effect of aging on the mechanical properties of boron carbide particle

reinforced aluminum metal matrix composites”, METU Doctorate Thesis, 2007