Elsevier

Wear

Volume 262, Issues 11–12, 10 May 2007, Pages 1450-1462
Wear

Effect of reinforcement of flyash on sliding wear, slurry erosive wear and corrosive behavior of aluminium matrix composite

https://doi.org/10.1016/j.wear.2007.01.026Get rights and content

Abstract

In the present experimental investigation, Al (12 wt% Si) as matrix material and up to 15 wt% of flyash particulate composite was fabricated using the liquid metallurgy route. The wear and friction characteristics of the composite in the as-cast conditions were studied by conducting sliding wear test, slurry erosive wear test and fog corrosion test. The sliding wear behavior of the MMCs were investigated by varying parameters like normal load, percentage flyash, and track velocity. Pin-on-disc wear testing machine was used for investigating sliding wear behavior. In slurry erosive wear studies, percentage flyash and pH value of the slurry were used as variables. Corrosion studies were carried out using fog corrosion test. The specimens were exposed to a fog of NaCl. The worn surfaces were analyzed using optical microscope and scanning electron microscope. The results indicate that the wear resistance of the flyash reinforced material increased with increase in flyash content, but decreases with increase in normal load, and track velocity. The microscopic examination of the worn surfaces, wear debris and subsurface shows that the base alloy wears primarily because of micro cutting. But the MMCs wear because of delamination, micro cutting, oxidation and thermal softening. Corrosion has increased with increase in flyash content.

Introduction

In the recent past, another class of material, namely the metal matrix composites (MMCs), is becoming increasingly important and drawing attention of engineers. This class of material has been widely studied by numerous investigators with respect to friction and wear behavior.

Discontinuously reinforced aluminium matrix composites have emerged from the need for light weight, high stiffness materials which are desirable in many applications such as high speed reciprocating machinery. Reinforcement usually comprises particles or whiskers of a ceramic such as silicon carbide, alumina, graphite, etc. Significant increase in stiffness and strength can be conferred with even small reinforcement volume fractions. Many of the applications for which MMCs are desirable also require enhanced tribological performance. There exists a large amount of literature concerning the wear performance of such materials. Much of it showing the composites in a good light compared to the alloys in the unreinforced state. The wear resistance of composites has received much attention in the literature but direct comparison between findings is often difficult due to specific differences in the wear testing procedure. Work concerning the unlubricated sliding wear behavior of such materials has examined a number of variables [1] such as the contact pressure [2], [3], [4], sliding velocity [5], [6], temperature [7], [8], particle volume fraction [9], [10], particle size [11], matrix type [12] and heat treatment [13], [14]. A number of mechanisms have been proposed to explain the sliding wear behavior of these composites, many of which are discussed in an review of the subject [15]. While certain experiments show the converse, it is generally demonstrated that reinforcement of aluminium alloys with particulate materials does result in reduced sliding wear rates and increases the critical contact pressure at which transitions from mild to severe wear occur.

Aluminium is potentially an important material for tribological applications because of its low density and high thermal conductivity. However, aluminium by itself exhibits poor tribological properties. Therefore, the study of the tribological behavior of aluminium based materials is becoming increasingly important. Al–Si alloys, for example, have been extensively studied in this regard [16].

Deuis et al. [17] showed that aluminium–silicon alloys and aluminium-based metal–matrix composites (MMCs) containing hard particles offer superior operating performance and resistance to wear and in industrial processes where abrasive slurries are transported by rotating paddles or impellers, elements fabricated from MMC materials provide higher abrasive resistance and therefore a longer service life compared to those made from iron or nickel-based alloys. They also showed that composites characterized by a hardness greater than the abrasive particles and a reinforcement phase of high fracture toughness and low mean free path, compared to the abrasive grit dimension, exhibit high abrasive wear resistance. Studies related to abrasive wear of Al–Si alloys and aluminium-based MMCs that contain discontinuous reinforcement phases were reviewed.

Kwok and Lim [18] in their paper documents their investigation into the various mechanisms of wear which are observed to operate under this same range of sliding conditions when the four composites, two fabricated in-house by a powder metallurgy route incorporating mechanical alloying and two sourced commercially, were tested. Five dominant mechanisms have been found, viz. (1) abrasive and delamination wear; (2) a combination of abrasion, delamination, adhesion and melting; (3) melt wear; (4) severe adhesion and (5) severe melting. The regions of dominance of the various mechanisms are also presented in terms of the applied load and sliding speed used. It was found that the size of the particulate SiC reinforcement phase controls the high-speed wear resistance of the composites tested; massive wear will occur if the particles are smaller than a threshold value. They suggested Al/SiCp composites with small SiC particles are more suitable for lower-speed applications.

Rohatgi et al. [19] reported that a low-pressure infiltration casting technique was used to produce Al–50 vol% graphite particle composites. The tribological behavior of the composites worn surfaces was investigated. They found that the wear rate for the composites was lower than the base alloy. They noticed that the reduced wear rates of the composites in comparison to the base alloy are due to the formation of a solid at the surface.

Yýlmaza and Buytozb [20] studied the effects of volume fraction, Al2O3 particle size and effects of porosity in the composites on the abrasive wear resistance of compo-casting Al alloy MMCs for different abrasive conditions. It was seen that porosity in the composites is proportional to particle content. In addition, process variables like the stirring speed and the position and diameter of the stirrer affect the porosity content in a way similar to that observed for particle content. In addition, the abrasive wear rates of composites decreased more rapidly with increase in Al2O3 volume fraction in tests performed over 80 grade SiC abrasive paper than in tests conducted over 220 grade SiC abrasive paper. Furthermore, the wear rates decreased with increase in Al2O3 size for the composites containing the same amount of Al2O3. Hence, it was deduced that aluminium alloy composites reinforced with larger Al2O3 particles are more effective against abrasive wear than those reinforced with smaller Al2O3 particles. At the same time the results showed that the beneficial effects of hard Al2O3 particles on wear resistance far surpassed that of the sintered porosity in the compocasting metal–matrix composites (MMCs).

In a study by de Mello [21], the three-body abrasion of aluminium matrix composites reinforced with silicon carbide particles SiC has been investigated. The metal matrix composites MMCs were fabricated by a powder metallurgy route involving a final hot extrusion step, with Al 1100 matrix and a-SiC reinforcement with mean sizes of 10, 27 and 43 mm, in the proportions of 5, 10 and 20 vol.%. Using a wet monolayer tester, three-body abrasive wear tests were conducted under a constant load against silicon carbide and alumina abrasives with four different grits of 320, 400, 600 and 1000. The microstructural characterizations were performed using light microscopy. The dominant wear mechanisms were identified using scanning electron microscopy. The influence of type of the abrasive particles on wear rate and dominating wear mechanism is reported. Relationships between size and volume fraction of the SiC reinforcement and wear rate is discussed. It is shown that SiC reinforcement increases the abrasion resistance against all the abrasives used. This increase is generally higher against alumina than silicon carbide abrasives.

In the present investigation, aluminium based metal matrix composite containing up to 15 wt% of flyash particulates were successfully synthesized using vortex method. Dry sliding wear behavior of the MMCs was investigated under parameters like varying load, % flyash, and track velocity. The worn surfaces were analyzed using Nickon optical microscope with digital camera attachment.

Section snippets

Matrix material

The matrix material used in the experimental investigation was an aluminium alloy (Si 12.2%) whose chemical composition (in wt%) is listed in Table 1. This alloy has a composition very close to the Al–Si eutectic. It therefore has a low melting point (577 °C). Aluminium and silicon have no solid solubility below the eutectic and the microstructure solidifies as silicon particles in an aluminium matrix. Aluminium–silicon alloy in its unmodified state is extensively used in sand casting and

Experimental procedure

The synthesis of the metal matrix composite used in the present study was carried out by using stir casting method. Al–12% Si alloy in the form of ingots were used for the trials. The cleaned metal ingots were melted to the desired super heating temperature of 800 °C in graphite crucibles under a cover of flux in order to minimize the oxidation of molten metal. Three-phase electrical resistance furnace with temperature controlling device was used for melting. For each melting 3–4 kg of alloy was

Results and discussion

In this study flyash was added in weight percentage of 5, 10, and 15% in Al metallic matrix using vortex method. The optical micrograph of 10 and 5% flyash reinforced MMCs are shown in Fig. 2. The micrograph showed near uniform distribution of particulates. The properties like bulk hardness, microhardness, % elongation, density, corrosion resistance and slurry erosive wear resistance were investigated are given in Table 3 [23]. The increase in the reinforcement wt% increased MMCs hardness and

Conclusions

  • 1.

    The effect of increased reinforcement on the wear behavior of the MMCs is to increase the wear resistance and reduce the coefficient of friction. The MMCs exhibited better wear resistance (20–30% improvement) due to its superior load-bearing capacity.

  • 2.

    The initial wear regime time is found to be very small and transition to steady state has taken place within 3 min of commencement of the test.

  • 3.

    Increased normal load and sliding velocity increases magnitude of wear and frictional force.

  • 4.

    Four different

Acknowledgement

The authors would like to thank the authorities of AICTE (All India council for technical education) New Delhi, India for financial support provided during the course of this investigation.

References (31)

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