Dry Sliding Wear Behavior of Aluminum 6063 Composites Reinforced with TiB 2 Particles

The influence of titanium diboride (TiB2) loading on the dry sliding wear characteristics of aluminum 6063 matrix alloy-titanium diboride (Al/TiB2) composite materials has been assessed using a pin-on disc wear tester at different loads. The composites with 5 and 10 wt% of fine TiB2 particles were fabricated using stir casting technique. For comparison, as-cast of the base alloy were made under the same processing applied for Al/TiB2 composites. The hardness of the composite materials was measured using Brinell hardness tester. Scanning electron microscopy (SEM) was used to analyze the wear surfaces of samples. The results indicate that fine TiB2 particles markedly improved the wear performance of the aluminum 6063 matrix alloy. The coefficient of friction decreases with increase in the amount of TiB2, but this effect was more pronounced in dry sliding. Hardness of composites increased with increasing TiB2 loading. The wear rates increase with increase in load and dependent upon TiB2 loading in the base alloy. Among the composites tested, Al/TiB2 composites containing 10 wt% TiB2 exhibited superior wear resistance over the base alloy and 5 wt% Al/TiB2 composites. These observations were correlated in terms of the TiB2 loading in base alloy which resulted in the variations of the hardness.


Introduction
Owing to the high strength-to-weight ratio, aluminium has been found wide application in areas where light weight is of primary considerations.Aluminium based particulate-reinforced metal matrix composites are well known for their higher specific modulus and strength as well as for their excellent wear resistance when compared to their monolithic counterparts [1].Metal matrix composites have emerged as an important class of high performance material for use in aerospace, automobile, chemical and transportation industries because of their improved strength, high elastic modulus and increased wear resistance over conventional base alloys.Aluminium based composites SiC, B 4 C, Al 2 O 3 , TiC [2] and graphite reinforce aluminium or its alloys have been the interest of research.Among these reinforcements, TiB 2 has emerged as a promising candidate for Albase composites; this is due to fact that the TiB 2 is stiff, hard and does not react to form the reaction products at the reinforcement interfaces [3] [4] [5].TiB 2 is a refractory compound that exhibits outstanding features such as high melting point (2790˚C), high hardness (86 HRA or 960 HV) and high modulus characteristics.Its resistance to plastic deformation even at high temperatures portrays it to be a good potential reinforcing candidate in an aluminum matrix.5 and 10 wt.% TiB 2 particles reinforced aluminum (Al6063) metal matrix composites produced by using master alloys of Al-Ti & B by stir casting process to obtain the material for the experiment [6] [7] [8] [9].Limited published work is only available on the sliding wear behavior of composites with TiB 2 as reinforcement material [10] [11] [12].Mandal et al. [10] in their study of sliding wear of composites stated that TiB 2 particles markedly improved the wear performance of Al-4Cu alloy.Lee et al. [11] showed that increased volume fraction of TiB 2 in the composite has not led to a parallel increase in wear resistance.This behavior is attributed to the presence of the unavoidable Al 3 Ti phase in the Al-Ti-B system.
Zhao Min et al. [12] showed TiB 2 /Al composites exhibit higher wear resistance than SiCp/Al composite.Severe plastic deformation and adhesive wear were found on the worn surfaces of SiCp/Al composite, but no such worn surfaces observed in the TiB 2 /Al composites.Roy et al. [13] have compared wear resistance of aluminium reinforced with TiC, TiB 2 , B4C, SiC.It was stated that TiB 2 showed better wear resistance than the other dispersiods.Kumar et al. [14] [15] noted that abrasive wear resistance improved by the addition of TiB 2 particle in the Al-4Cu alloy and it increases as the TiB 2 content in the composites increases.Ramesh et al. [16] observed that a decrease in wear rate with increase in the TiB 2 content in the composites.Maximum reduction is in the wear rate for the composites containing 10 wt% TiB 2 when compared with matrix alloy.Sivaprasad et al. [17] noted that as wt% of TiB 2 particles increases, volume loss decreases, and with increase in distance traversed, volume loss increases.
Natarajan et al. [18] showed that the dry sliding at room temperature increases the wear resistance of Al-6063 alloy by the reinforcement of TiB 2 particles.
Basavarajappa et al. [19] showed that wear rate decreases as the sliding speed increases when SiC particles reinforced into Al-2219 alloy, up to transition speed and load, due to work hardening of surface, formation of Iron oxide and crushing the SiC particles.The present study was attempted to study the dry sliding wear behavior of Al-6063 alloy reinforced with TiB 2 particles at different loads, sliding velocity and different wt% reinforcement.The worn surfaces are studied using SEM photographs.

Materials
Aluminium 6063 alloy was selected as the base line material as it possesses good formability, weldability, machinability and corrosion resistance, with medium strength compared to other grades of aluminium alloys.Its nominal chemical composition is shown in Table 1.The commercially available Al-6063 matrix alloy and master alloy are melted in an electric resistance furnace.The percentage weight of Al-Ti & B % was varied from 0 -10 wt% in steps of 5 wt%.The mixture of matrix alloy and master alloy were melted in an electric resistance furnace at a temperature of 800˚C and allowed to stand for duration of about 30 min to get melts.The melt was degassed using commercially available chlorine based tablets (Hexa-chloroethane) to remove the entrapped gases before stirring the melt using stirrer to get in-situ composites of TiB 2 in Al 6063 alloy.The melt is poured into the preheated metallic moulds.The different % composition (0, 5 and 10) of Al-6063-TiB 2 composites rods are prepared of size Ø 22 mm × 120 mm.

Microscopy, Density and Hardness Measurements
In order to know the dispersion of TiB 2 in Al6063, the samples for microscopic examinations were prepared based on the standard metallographic procedures, etched with Keller's agent and were analyzed by scanning electron microscope (SEM).The density of the composites was obtained by the Archimedes's principle of weighing small pieces cut from the composite disc first in air and then in water.Then, theoretical density of composite and its alloy was calculated from the chemical analysis data.The porosity of the composites was also determined.
The hardness of the composites and matrix alloy were measured after polishing to a 3 mm finish.The magnification of the images was 500×.Hardness of all samples was measured by Brinell hardness tester and mean of at least five readings was taken to represent the sample.

Dry Sliding Wear Test
A pin-on-disc machine shown in Figure 1 was used to investigate the dry sliding wear behavior of the aluminium alloy and TiB 2 /Al-6063 composites as per ASTM G99.Specimen of Ø 8 mm × 28 mm in length were cut out of rods of size Ø 22 mm × 120 mm, by the specimen cutter, machined, and then polished me-

Microstructure of TiB2-Al Composites
The properties of the metal matrix composites (MMCs) depend not only on the matrix, particle, and the volume fraction, but also on distribution of reinforcing particles and interface bonding between the particle and matrix.In practical way, to achieve a homogenous distribution is difficult.The photomicrographs of the aluminum composite reinforced with 5 and10 wt% of TiB 2 are shown in Figure 2(a) and Figure 2(b) respectively.The particles, with the average particle size of 25 µm, mainly formed in the surface showed a character of homogenous distribution within the matrix alloy.

Density and Hardness of TiB2-Al Composites
The variations of density and hardness of the composites are shown in Figure 3.
The density and hardness of the MMCs increased more or less linearly with the weight fraction of particles in the alloy matrix due to the increasing ceramic phase of the matrix alloy.A significant increase in both density and hardness

Specific Wear Rate
The specific wear rate of the matrix alloy and the composites are shown as a function of load and wt.% of TiB 2 in Figure 5(a) and Figure 5(b) respectively, defined as the volume of material worn per unit load.Figure 5(a) shows that the specific wear rate increases drastically at higher loads.Figure 5(b) shows specific wear rate decreases as the increase in wt.% TiB 2 particles in the composites and it decreases as the load increases due to work hardening.This significant improvement in the wear resistance of the TiB 2 /Al-6063 composites can be attributed to the following factor: 1) The increase in the hardness of the Al 6063 with increase in the loading of TiB 2 reinforcement.The wear rate decreases with increase in hardness.Various researchers reported that the severity of adhesive wear greatly depends on the material hardness.Further, there is an experimental support and practical evidence to suggest that the onset of adhesive process, such as scuffing and seizure are reduced by increasing the hardness of the parts in contact.
2) Also, the excellent bonding between reinforcement and matrix as evidenced by SEM picture shown in Figure 2.

Worn Surface Morphology
Dry sliding wear involves the transfer of material from one surface to another   For systematic analysis of the worn surfaces of neat alloy and its composites, selected photomicrographs at two different (low and high) loads and sliding velocities were examined using scanning electron microscopy.However, the same explanation holds good even for the other composites and its alloy with different sliding velocity and load.
The examination of the wear surfaces of the matrix alloy and composites re-   The worn surfaces of TiB 2 /Al-6063 composites exhibit a feather like structure.This worn surface is associated with plastic flow and adhesion effect, leading to a higher wear loss of composites slid at lower sliding velocity (0.5 m/s).However, the worn surface of these composites slid at higher velocity exhibits appearance of both smoother matrix region and rougher white patches.Some surface cracks are also evident from the photomicrographs as shown in Figures 10-13 for 5 and 10 wt% TiB 2 reinforced Al matrix alloy respectively.According to the well known Archard's law of sliding wear, the volumetric wear loss of the specimen is inversely proportional to its hardness.In present investigation, addition of 10 wt.% of TiB 2 particles reinforced alloy leads to an increase in the hardness value thereby improving its wear resistance significantly.This is also well reflected from the wear data obtained from Figures 4-6 and corroborates the worn surface features of composites.

Conclusions
Dry sliding wear tests using a pin-on-disc were conducted on TiB 2 /Al-6063 composites.The contribution of the reinforcement content and the applied load as well as the sliding velocity on the wear process and the wear rate has been investigated.The following conclusions can be drawn from this study: Micro-structural examination showed that the dispersion of TiB 2 particles is more or less uniform and lower interface porosity.1) Hardness of the aluminum alloy improved significantly by adding up of TiB 2 particles into Al-6063 alloy, while density of the composite also increased almost linearly with the weight fraction of particles.
2) The effect of the wt% of reinforcement found to be different for initial wear and the steady-state wear.
3) Sliding wear test results showed that wear rate increases as the increase in the applied load and increase in the sliding velocity.But as the TiB 2 reinforcement with the Al-6063 composite increased, the wear rate is decreased with increasing applied load and sliding velocity.
4) The specific wear rate of the composite decreases with the increase in both applied load and the TiB 2 particle reinforcement with the Al-6063 composite materials.
5) The wear resistance of Al-6063 alloy is improved by the addition of TiB 2 particle and it further increases as the addition TiB 2 particle in the composite material increases and it will be maximum for 10 wt% TiB 2 .
6) The grooves are deeper in the base matrix alloy due to the absence of TiB 2 particle and provide smooth surface compared to composites.7) Less damage of surface cracks observed in the higher reinforced composite compared to matrix alloy.8) 10 wt% TiB 2 reinforced Al-6063 composite material posses higher wear resistance and less wear rate, hence it is hard and strong materials compared with matrix and 5 wt% TiB 2 reinforced Al-6063 composite material.

Figure 1 .
Figure 1.Pin on disc wear testing machine.

Figure 4 .
Figure 4. (a) Variation of wear rate with load for TiB 2 /Al-6063 composites; (b) Variation of wear rate with content of TiB 2 reinforcement in Al-6063 matrix alloy.

Figure 5 .
Figure 5. (a) Variation of specific wear rate with load TiB 2 /Al-6063 composites; (b) Variation of specific wear rate with content of TiB 2 reinforcement in Al-6063 matrix alloy.

Figure 6 (
Figure 6(a) and Figure 6(b) shows the wear resistance, defined as the reciprocal of the wear rate, of the composites as a function of normal load and TiB 2 reinforcement, respectively.Figure 6(a) shows that the wear resistance decreases as the normal load increases and Figure 6(b) shows it increases as the wt% TiB 2 reinforcement in the composite increases.

Figure 6 .
Figure 6.(a) Variation of wear resistance with load TiB 2 /Al-6063 composites; (b) Variation of wear resistance with content of TiB 2 reinforcement in Al-6063 matrix alloy.

Figures 7 (
Figures 7(a)-(c) shows the microstructure of the TiB 2 reinforced Al-6063 alloy.From the figures, it is evident that TiB 2 particles are more or less disparesed uniformly in the Al-6063 matrix.The size of the TiB 2 particles ranges from 15 to 35 µm.It also seen from the figures that the TiB 2 reinforced Al-6063 composites are free from porosity and shrinkage cavity.