Synthesis and characterisation of hot extruded aluminium-based MMC developed by powder metallurgy route
© The Author(s). 2017
Received: 11 May 2016
Accepted: 12 December 2016
Published: 11 January 2017
Improvement of the mechanical and tribological properties due to extrusion can be attributed to the improved density and excellent bond strength due to high compressive stress.
To avoid the product defects mathematically contoured cosine profiled die was used for the thermomechanical treatment. Improvement of mechanical characterization like density, hardness, compression test and three-point-bend test was inquired. Two body dry sliding wear behaviour of the prepared AMCs before and after extrusion were investigated by using a pin-on-disc wear testing method by varying the variable parameters like load (N), track diameter (mm) and RPM of the counter disc.
The effect of hot extrusion on mechanical and tribological characteristics of aluminium matrix composites (AMCs) developed by powder metallurgy route followed by double axial cold compaction and controlled atmospheric sintering was studied.
Shearing of the fine distributed graphite particles at the tribosurface acts as a solid lubricant and decreases wear rate. At higher loading and sliding velocity condition a mixed type of wear mechanism was observed.
Aluminium metal matrix composites are the most versatile replacement of other alloys in the sector of automotive, aerospace, defence and sports because of its high strength to weight ratio, ease and prevalence of processing techniques, excellent thermal and electrical conductivity and the ability to sustain in uncertain thermal and mechanical loading environment. This emerging area has influenced the researchers to tailor the mechanical, thermal and tribological properties of the composite using different types of reinforcements with various percentages with types of manufacturing process (Yigezu et al. 2013). The endless process of pursuance of mankind needs the material to behave substantially in critical environments.
Over the last few decades, there has been considerable attention to the evolution of Al-based MMCs developed by powder metallurgy (P/M) route of manufacturing. The main advantage of this kind of manufacturing process is the good distribution of reinforcing particles, low processing temperature and the ability to produce near net shape products with intricate designs (Min et al. 2005; Torralba et al. 2003). A number of studies have been conducted to study the effects of reinforcement of very hard metals as well as ceramics in different grades of aluminium series of powder matrix (El-Kady and Fathy 2014; Jabbari Taleghani et al. 2014; Abdollahi et al. 2014). Layers of oxide formation take place in the P/M specimen during sintering, as aluminium is highly prone to oxide formation. During thermo-mechanical treatments, the covered oxide layer breaks due to highly induced shear stress, leading to a strongly bonded microstructure and improved mechanical properties which eliminate the main drawback of AMCs (Schatt et al. 1997). The use of traditional shear-faced die in extrusion causes product defects owing to the existence of higher velocity relative difference at die exit (Zhang et al. 2012a; 2012b). The use of mathematically contoured die (preferably zero, die entry and exit angle) for the MMC extrusion is highly recommendable.
Ravindran et al. (2012; 2013) investigated the effect of graphite addition, applied load, relative velocity and sliding distance on the wear behaviour of aluminium-based P/M composite. Fine graphite reinforcement acts as a solid lubricant and prevents metal to metal contact so it improves wear resistance compromising with hardness and flexural strength or fracture toughness (Baradeswaran and Perumal 2014; Suresha and Sridhara 2010). The addition of zinc with aluminium improves hot extrudability but decreases the high-temperature performances. Considering these factors, the weight percentage of reinforcements kept less in this work. Improved amount of TiC causes a marginal increase in wear rate, whereas applied load and wear rate varies linearly (Gopalakrishnan and Murugan 2012). Anilkumar et al. (2011) investigated the mechanical properties of the fly ash reinforced aluminium alloy. Improvement of mechanical properties can be achieved by adding more amount of reinforcement by compromising with ductility.
The present investigation focuses on both mechanical and tribological properties of extruded AMC synthesised by powder metallurgy route. Two types of metal reinforcements like Zn and Ti in Al + Mg + Gr matrix has been added for the comparative study. The properties of AMCs before and after thermo-mechanical treatment (extrusion) through a mathematically contoured cosine die were inquired.
Al (92.33) + Mg (4.26) + Gr (0.85) + 1.70 Zn
Al (92.33) + Mg (4.26) + Gr (0.85) + 1.70 Ti
The mixture was allowed for uniform blending in a centrifugal blender. The weight ratio of stainless steel ball to powder was maintained 10:1. At an RPM of 200 for 10 h, the mixture was allowed for blending. The flow property of the blended powders was checked by measuring apparent density and tap density of the blended compositions.
The powder was subjected to dual axial compression for the preparation of green pellets. During compaction, the powder inside the container remains in floating condition in between both of the punches. The powder was subjected to a pressure of 275 MPa with a very slow rate of rise and with a dwell period of 10 min. Green density of the prepared 10-mm-diameter pellets was measured. Green pellets were subjected to sintering in a controlled atmospheric tubular furnace in an argon atmosphere. The ramp rate of 5 °C/min was set for all the temperature rises. Dwell period of 20 min at 110 °C to remove water vapour, 30 min at 450 °C to remove lubricant (zinc stearate) and 90 min at 590 °C to form the metallic bond was set for the process.
Mechanical and tribological characteristics
ρ i is the density of individual element and m i is the mass fraction of the individual element.
where W A is the mass of the sample taken at atmospheric air, W fluid is the mass of the considered fluid and ρ fluid is the density of the considered fluid.
Vickers micro-hardness of the sintered MMC composite was determined by dividing the applied load to the impressed area. 50 g of load were applied through a diamond pyramid having the face angle of 136° for a dwell period of 15 s to avoid spring back effects.
Three-point bend test has been performed to check the transverse rupture strength (TRS) of the sintered solid cylindrical specimen. The test was executed in UTM (universal testing machine, Instron -5979). A span of 30 mm with a compression rate of 2 mm/min at atmospheric temperature was maintained at the time of operation.
Variable parameters selected for experimentation
Wear track dia (D), (mm)
Normal load (L), (N)
RPM of counter disc (N)
Results and discussion
Physical characteristics of powders
Average size (μm)
Spherical and sub-rounded
Rounded and flakey
Spherical and sub-rounded
Very angular and irregular
Percentage improvement in densification (after extrusion)
Porosity before extrusion (%)
Porosity after extrusion (%)
Compression test of sintered specimen
3-point bend test
where P = the maximum load (N)
l = length of the sample (mm)
D = diameter of the sintered specimen (mm)
d = depth = width of the extruded square specimen (mm)
The average TRS for all sintered and extruded sample is presented in Fig. 6b. For the case of sample type 1, presence of zinc having a melting point of 420 °C caused liquid phase sintering at a temperature of 600 °C. In the case of Ti-reinforced sample, there exists a high-stress concentration at the boundary zone of the reinforcements and causes the initiation of fracture so comparatively lesser than TRS.
L9 orthogonal array followed for wear analysis
Track dia (mm)
RPM of counter disc
where w a and w b are the mass of the sample before and after the test, respectively.
Several researchers have reported the direct proportionality relation of hardness with wear resistance. The bond strength between the matrix and reinforcement material improves after extrusion which improves wear resistance, and it also avoids three-body abrasive wear (Ramesh et al. 1992).
At higher loading conditions of 40 and 60 N with higher RPM of 600 grooving and scratching plays a predominant role over abrasion. The images showing a large amount of white particles present at the tribosurface which can be attributed to the oxidation of the surface due to frictional heating as the aluminium surface is oxide-prone.
At very high loading and high-velocity condition delamination and combination of abrasion, delamination and adhesion mechanism of wear came into the picture. Due to frequent repetitive sliding behaviour, subsurface crack has been induced due to the fatigue failure of the pin. These subsurface cracks grow with increasing travel distance, and eventually, shear deformation occurs to the surface. Moreover, at the adverse conditions, melting, thermal softening and adhesion take the predominant role to cause plastic deformation. In the case of AMCs, the mechanism of wear is less severe than the base metal alloys. Metal/graphite composite forms a lubricating layer on the tribosurface due to the shearing of graphite particles which prevents the metal to metal contact, causing the reduction of friction and wear.
Mechanical properties of both types of AMCs are improved due to the improved bond strength after extruding it through mathematically contoured cosine profiled die.
It was found very less amount of surface defects (few cracks at the corner zone) in the extruded product and supports improvement of flexural strength and tribological properties. Wear rate of the extruded specimen is lesser compared to that of the sintered specimen for each run.
The addition of zinc causes liquid phase sintering because of its low melting temperature that leads higher bond strength and density and gives a comparative better performance for the composition. The addition of more amount of fine titanium may improve the properties manifold by compromising with thermal conductivity.
Shearing of the well-distributed graphite particles at the tribosurface acts as a lubricant. So, the addition of graphite particle improves the wear resistance by compromising the little amount of hardness.
At higher loading and sliding velocity condition, a mixed type of wear mechanism (oxidative, delamination, adhesive and abrasion) takes place. But oxidative and delamination are the predominating wear mechanism found on the surface.
All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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