Aerospace or automobile industries need materials that have a combination of several features such as lightness, high strength, corrosion and wear resistance. With ceramic particulates reinforcement, the properties of aluminum alloys can be greatly improved. The aim of this study is to investigate the effect of precipitation age hardening and the mass percentage of SiC particles on hardness and wear resistance of the 6061 aluminum matrix composites produced by hot pressing. Composites were solution treated at 530 °C for 1.5 h and then artificially aged at 160 °C for 18 h. The ball-on-disc wear test was carried out under 2N load using an alumina ball as the counterpart. The density of the composites was calculated according to Archimedes principle. Porosity tended to increase with increasing SiC reinforcement. Hardness and wear resistance of composites were improved by SiC particles and aging. Maximum hardness and minimum wear loss was obtained heat treated sample that contains 20 wt.% SiC reinforcement.
Metal matrix composites (MMCs) are composites that comprising reinforcement of several one or more materials made of the metal structure. Metals are reinforced with hard particles such as SiC, B4C, TiC, and Al2O3 to enhance mechanical properties such as hardness, Young’s modulus, and yield strength. Today, aluminum metal matrix composites (AMMCs) are widely used in many areas such as aerospace, automobile, structural applications and electronic industries because of their lightness, corrosion resistance, low coefficient of thermal expansion, favorable machinability and enhanced mechanical properties (Laska and Kazior,
AMMCs can be manufactured by different techniques. For instance, Soltani
Karabacak
In case of using a heat treatable aluminum alloy, the matrix phase experiences solution treatment that involves the excess dissolution of the precipitates under hot pressing conditions (Wierszyłłowski
In this study, Alumix 321 coded pre-alloyed powders, from ECKA were used as a matrix material (
Chemical composition of Alumix 321 (6061 Al) powder
Element | Mg | Si | Cu | Fe | Bi | Sn | V | Al |
---|---|---|---|---|---|---|---|---|
wt.% | 1.31 | 0.5 | 0.32 | 0.10 | 0.01 | 0.03 | 0.01 | Balance |
The SiC particulates were dispersed in the matrix with 5, 10 and 20 % by weight. Powders were mixed with 3D shaker-mixer (Turbula T2F) for 30 min. Then mixed powders were loaded into a steel die and produced with a unidirectional hot press. Schematic illustration of the hot pressing unit is shown in the
Schematic illustration of hot pressing system.
The production steps of the Alumix321/SiC powder composites followed the order of cold pressing at room temperature under 100 MPa for 30 s and hot pressing at 500 °C under 300 MPa for 1 h. The produced Alumix321/SiC bulk composite was cut into 20×20×10 mm3 pieces. The aging process is applied to a portion of the hot pressed samples, while maintaining only a portion of hot-pressed state. A portion of hot pressed composites were solution treated at 530 °C for 1.5 h and then artificially aged at 160 °C for 18 h. Then all specimens were mechanically ground using abrasive SiC papers of increasing grade and finished with 1200 grit. They were polished with 6 and 3 µ diamond suspension and then cleaned with ethanol.
The density of the Alumix321/SiC composites was calculated using water displacement approach (Archimedean density, Buoyancy method). The theoretical density of Alumix321/SiC composites was calculated using the rule of mixtures. Samples were weighed in the air (Wa) then dropped in the water and weighed again (Ww) The actual densities were calculated according to ρa = [Wa / (Wa – Ww)] × ρw equation.
Where ρa is the actual density, Wa is the mass of the sample in the air, Ww is the mass of the sample in water and ρw is the density of water. The samples were weighed using a digital balance with an accuracy of 0.1 mg.
Optical, stereo microscopes and scanning electron microscope (SEM) were employed for microstructural examinations of Alumix321/SiC composites. Homogeneity of distribution of SiC particles was observed by optical microscope. Samples were investigated under a stereo microscope after the wear test.
The hardness values of the samples were determined by a Brinell Hardness Tester (Duravision 2000 EMCO Test) and the mean of at least ten measurements was recorded at a load of and 6.25 kgf with 2.5 mm steel ball. The test was carried out at room temperature and the measurements were observed at least 10 different locations to increase reliability.
Wear tests were carried out on the CSM instruments ball-on-disc wear-test unit (
Schematic model of ball-on-disc wear-test unit.
Microstructure of the composites consists of micro and nano scaled SiC particles embedded in a heat treatable Alumix321 matrix (
SEM image of Alumix 321+5 wt.% SiC: Micro and nano scaled SiC particles.
Microstructure of Alumix 321 + 20 wt.% SiC.
During the pressing process SiC particles tend to redistribute and gather together due to their low plastic deformation ability (Wang
Theoretical and hot pressed density values of the composites
Compositions | Theoretical Density (g·cm−3) | Hot pressed Density (g·cm−3) | Percentage of porosity (%) | Percentage of relative density (%) |
---|---|---|---|---|
Alumix321 + 0 wt.% SiC | 2.69 | 2.67 | 0.74 | 99.26 |
Alumix321 + 5 wt.% SiC | 2.72 | 2.66 | 2.20 | 97.80 |
Alumix321 + 10 wt.% SiC | 2.75 | 2.71 | 2.18 | 97.82 |
Alumix321 + 20 wt.% SiC | 2.81 | 2.74 | 2.49 | 97.51 |
Images of volume fraction calculations of composites are given in
Volume fraction of SiC particles
SiC (wt.%) | Theoretical Volume Fraction | Image Analysis |
---|---|---|
5% | 4.25% | 3.91 ± 0.5 % |
10% | 8.56% | 9.31 ± 1.1 % |
20% | 17.37% | 17.23 ± 0.9 % |
Volume fraction of SiC particles: a) 5 wt.% SiC, b) 10 wt.% SiC, and c) 20 wt.% SiC.
The change in the hardness values of the samples is shown in
Hardness values of composites before and after aging.
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The volume losses of the composites before and after the precipitation age hardening are shown in
Volume losses of the samples after the wear test.
Worn surface of: a) 10 wt.% SiC composite without aging, b) aged 10 wt.% SiC composite, c) aged and unreinforced sample, and d) 5 wt.% SiC composite without aging.
Worn surfaces of Alumix321/SiC composites involve scratches that indicate the abrasive wear (
Alumix321/SiC composites were produced successfully by hot pressing. The density of composite without SiC particles is 99.26 %. The effect of the ratio of the SiC reinforcement and aging treatment can be summarized as follows:
In the Alumix321 matrix, the SiC reinforcement exhibited a close distribution of homogeneity. The density of SiC reinforced composites varies between 97.82–97.51%. Increased fraction of SiC reinforcement diminished the compressibility and caused an increase of porosity in Alumix321/SiC composites.
Maximum hardness value was achieved at composites with 20 wt. % SiC reinforcement. The hardness of composites tended to increase with increasing SiC content.
As the amount of SiC wt.% in matrix was increased, the wear resistance of the samples increased. 20 wt.% SiC containing sample was showed the maximum wear resistance.
Aging treatment has improved the hardness of Alumix321/SiC composites by 21–25% in comparison with the non-heat treated condition. However, the amount of reinforcement has not exhibited a significant effect on wear rate in case of applying heat treatment. When the hardness and wear test results are analyzed for all samples, 5 wt.% SiC reinforcement is found to be sufficient for aged Alumix321/SiC composites for wear resistance.