Elsevier

Wear

Volume 248, Issues 1–2, March 2001, Pages 178-186
Wear

Dry and lubricated wear resistance of mechanically-alloyed aluminium-base sintered composites

https://doi.org/10.1016/S0043-1648(00)00553-6Get rights and content

Abstract

The friction and wear behaviour of three mechanically alloyed aluminium-base materials, consolidated by an alternative powder-metallurgy process, has been studied with a pin-on-disk tribometer, sliding against AISI 52100 steel pins. Their tribological properties have been compared, using simple sintered aluminium, as standard material. Volume loss, under dry wear conditions, is very dependent on the measurement method. The more regular wear tracks, achieved under lubrication, allows the track-width method to be used with better accuracy. In general, unreinforced Al shows the lowest wear resistance, while composites with the highest hardness and second-phase volume content are the most wear resistant, especially at high load. Unreinforced Al suffers a transition in wear mode, from mild to severe wear, at load 4.90 N, in lubricated tests. The wear mechanism is of the adhesive/abrasive type.

Introduction

Aluminium alloys get more and more importance as structural materials, but for many applications it is necessary to improve wear resistance. In particular, uses of aluminium alloys in automotive applications have been limited by their inferior strength, rigidity and wear resistance, compared with ferrous alloys [1]. Particulate-reinforced aluminium composites, nevertheless, offer reduced mass, high stiffness and strength, and improved wear resistance [2]. Specifically, the possibility of substituting iron-base materials for Al metal–matrix composites (MMCs), in automotive components, provides the potential for considerable weight reduction.

Significant developments have been achieved in the system SiC/Al MMCs. A typical composite may have a volume fraction of ceramic phase of 20 vol.% SiC and an average particle size of around 10 μm. The strength of SiC/Al composites is increased by increasing the volume percentage of ceramic phase, by increasing the strength of the Al matrix and by decreasing the size of the ceramic reinforcement; ductility, nevertheless, diminishes [2], [3]. The Al matrix can be strengthened by mechanical alloying (MA), a technique that can also refine ceramic powder particles. An additional advantage of MA aluminium material is their good hot-hardness behaviour [4].

Mechanical alloying is defined as a high-energy milling process for producing composite metal powders with a fine microstructure [5]. The general characteristics and mechanisms of mechanical alloying have been described [6]. Specifically, MA aluminium has been prepared by attrition milling in the presence of 1.5% of a powder wax, used as processing control agent (PCA) [7]. The addition of PCA is necessary for control of the fracture and welding of metallic particles during milling. By reaction of the PCA, and atmospheric oxygen, with Al (reaction milling) suitable dispersoids (Al4C3, Al2O3) are introduced into the metal–matrix during milling and the subsequent heat treatment [8]. Surface oxide films on Al particles are also incorporated into the interior of the milled powders as result of the repeated fracture and cold welding, which occur during MA.

MA Al powders are hard and homogeneous, and have a relatively equiaxial morphology. Oxide and hydroxide layers cover the surfaces of the MA Al particles. This raises special problems in consolidation processing. To achieve sufficiently strong contacts between particles, it is necessary to shear off the oxide layers. This requires a high deformation ratio. Therefore, powder consolidation is often performed by hot extrusion as the main stage in the process [9], [10].

The properties of MA powders are very sensitive to experimental milling conditions [7] (milling type, energy input, time, amount of PCA, atmosphere, etc.). On the other hand, properties of final pieces or parts depend on the consolidation method.

In general, particulate-reinforced aluminium–alloy matrix composites, produced by powder metallurgy (PM) routes, can show [11], [12] improved specific properties over monolithic alloys. Previous studies have been conducted on the wear resistance of aluminium matrix composites reinforced with ceramic or intermetallic particles, such as Ni3Al [13], Al2O3, Al3Ti [14], [15], Si3N4, SiO2, ZrO2, WC, graphite [16], [17], [18], [19] or SiC [20], [21], [22]. The authors have also previously reported [23] on the dry wear resistance of a family of MA aluminium matrix composites.

In the present investigation, the dry and lubricated wear resistance of three aluminium-base sintered composites is studied. The powders were consolidated by an alternative PM route [4]. The results are compared with those corresponding to simple sintered aluminium, consolidated by a similar process. The four consolidated powders were the just-mentioned simple sintered aluminium, designated Al-1, MA aluminium, designated Al-2, MA aluminium–5 wt.% titanium, designated Al-3, and MA aluminium–5 wt.% AlN, designated Al-4.

Section snippets

Powder milling and consolidation

Commercial atomised aluminium powder, 99.5% Al, with 0.15% Fe as main impurity, was used as basic starting material. The composite powders (Al-2, Al-3 and Al-4) were prepared by milling the as-received Al powder in a Szegvari attritor [24], for 10 h, in the presence of 1.5 wt.% of a powder wax, employed as PCA. A 5 wt.% Ti powder was added to the mill vessel for the Al-3 material, and a 5 wt.% AlN, in the case of Al-4. Details of attrition milling are given elsewhere [7]. The milling conditions are

Metallurgical tests

Properties relative to absolute density, porosity and hardness of the four studied materials are shown in Table 3. Data concerning volume content and mean size of reinforcing particles are also included. Size values of submicroscopic particles of Al2O3 and Al4C3 and Al2O3 content have been estimated from the literature [8], [28].

It can be seen in Table 3 that all materials have reached a very high sintered density, since porosity, excepting Al-1, is smaller than 1%. Hardness of materials and

Conclusions

The sliding wear behaviour of sintered aluminium and three MA aluminium-base sintered composites, with different second-phase volume content, has been investigated. The following conclusions have been drawn

  • 1.

    Volume loss, in dry wear tests, obtained from track-width and weight-loss measurements are very different. The fact that tests were carried out in air, the formation of alumina films of variable thickness, and the lateral plastic flow of material may account for such divergence. Nevertheless,

Acknowledgements

We are grateful to the Dirección General de Enseñanza Superior e Investigación Cientı́fica (PB97-1035), the Fundación Séneca (Exp. 01738/CV/98) and the CICYT (TRA99-0525) for financial support.

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