Composites of 2014 alloy made by dispersing 10 vol.% of fine (20–50 µm) SiC particles using vortex method ensuring uniform distribution of SiC particles in the matrix have shown uniform distribution of SiC particles. Mechanical properties of the composites have also registered an improvement over the alloy. In an attempt to further improve the properties, the composites were subjected to hot extrusion of cylindrical rods along with the alloys under similar experimental conditions. A temperature range of 300–350°C and an extrusion ratio of 10 : 1 were maintained during the process. The extruded samples were compared for their mechanical properties, and improvement was noted. The mechanism of material failure from fractographic studies showed difference in behaviour between the alloy and composite. Dry sliding wear studies carried out on extruded specimens exhibited improved wear behaviour in composites over alloys as measured by volume loss and wear rate. Wear mechanism was studied from the worn surface and correlated with the wear performance. It was observed that the presence of SiC particles reduces the tendency of delamination and thus material removal from the wear surface.
Worldwide, researchers have repeatedly demonstrated on a laboratory scale, attractive properties in aluminium-based metal matrix composites with SiC dispersoids. The property improvements relate to microstructural, mechanical properties such as specific modulus, strength, and wear resistance, in addition to a service temperature capability in selected aluminium-based composites with selected second phase dispersoids. It is an open knowledge for researchers in this field now as to the alloy systems, second phase’s nature, volume fraction, and fabrication routes that can exhibit improved performance. The properties attained are so attractive that these materials hold potential for applications in aerospace, automotive, electronic, sports, ballistic protection, and other general engineering fields [
Challenges of extruding Al-based MMCs have been overcome by the authors in the past and optimised conditions for extrusion established [
Aluminium based alloy conforming to 2014 system consisting of Al-4.4%Cu-0.5%Mg-0.8%Si-0.8%Mn was chosen for the present study. The alloys were cast into cylindrical billets of 75 mm diameter using the liquid metallurgy route. For preparing the composites, the alloy was remelted and 10 volume % SiC of size range 20–50
A hydraulic press with a capacity of 400 tonne fitted with an extrusion setup was used for the hot extrusion of cylindrical rod by the forward bar extrusion process under optimized extrusion conditions that is, pressing speed of 0.36 mm/sec and billet soaking temperature of 350°C for 2 hours [
Samples around 20 mm diameter cut from the cast and extruded specimens were metallographically polished and etched in Kellar’s reagent for metallographic studies. The microstructures were observed in the optical microscope and/or JEOL scanning electron microscope operating at 20 KV in the secondary mode of electron emission.
Hardness of the samples was measured on the polished surface using a Vickers’s Hardness Tester from K.B. Pruiftechnik, Model No. KB250BVRZ applying a load of 5 Kg. An average of at least seven readings was taken at different portions of the sample for comparison between cast and extruded specimens.
The mechanical properties, namely, Yield stress, UTS, and % elongation in the cast and extruded from both alloy and composite were determined using a Universal Testing Machine, of INSTRON make, Model No. 8801.
Dry sliding wear tests were carried out on samples in the form of pins of 10 mm diameter and 25 mm length with a small hole at its end to place the thermocouple, from extruded rods of the alloy and composite. The tests were carried out on a Magnum make wear testing machine as per ASTM G99-05, in which the pin was placed vertically. The pin was made to slide against a rotating stainless steel disc at predetermined speeds of rotation. Using a cantilever, the loads were adjusted. Wear takes place due to the sliding action in the absence of any lubricant. The polished sample was initially used which was slid against the disc before starting the test to make its surface matching with the disc as the running in period. The friction force generated and temperature rise were continuously noted. The weight of the specimen was measured after a fixed interval of time to measure the weight loss which was then converted to volume loss for comparison. The volume loss was plotted against the sliding distance (measured in meters) travelled, calculated as 2Π
The fractured and worn surfaces were observed in an FESEM (FEI make model No. Nova Nano SEM 430) to understand the nature of failure and mechanism of wear removal, respectively.
All the above properties were compared between the extruded alloy and composite to understand improvement on extrusion between alloy and composite.
The microstructure of the alloy and composites prior to and after extrusion is shown in Figures
Microstructure of alloy.
Microstructure of Composite.
The average hardness of the specimens is shown in Figure
Hardness under different conditions.
The stress-strain graph for the extruded alloy and composite is shown in Figure
Tensile properties of extruded specimens.
Sample | UTS, MPa | YS, MPa | Young’s Modulus, GPa | Elongation at UTS, % |
---|---|---|---|---|
Extruded alloy | 300.1 | 130 | 35.4 | 14.9 |
Extruded composite | 233.8 | 100 | 31.7 | 10.3 |
Stress-strain curves for extruded (a) alloy and (b) composites.
In general, it is found that making composites decreases the strength properties over the base alloy (Figure
UTS for alloy and composite under different conditions.
The fractured surface of the composite is shown in Figure
Fractographic representation of composite specimens.
The variation of volume loss as a function of different experimental factors for extruded composite is plotted in Figure
Variation of volume loss with sliding distance under different experimental conditions for extruded composite.
The maximum temperature rise and friction force during the duration of the test under different experimental conditions plotted in Figure
Variation of maximum temperature and friction force for composites under different experimental conditions.
It may also be mentioned that the maximum friction force and maximum temperature are reached quite early during the test and remains around the same value for most part of the duration of the test, exhibiting decrease at intervals when the specimen is unmounted for weight measurements (depicted as gaps in the plot). As an example, a typical plot of the variation of the temperature and friction force with time during a particular experimental set of conditions that at 700 rpm and 3 Kg load is plotted in Figure
Variation of temperature and friction force with time at sliding speed of 700 rpm and 3 Kg load.
As in the case of composite, the extruded alloys too exhibit a mixed response in the behaviour of volume loss with sliding distance with varying experimental conditions of speed and load (Figure
Variation of volume loss with sliding distance for extruded alloy under different experimental conditions.
A glance comparison of just the
Comparative volume loss of composite and alloy at different experimental conditions.
Improvement in the wear performance (inverse of volume loss) in composites over the base alloy is attributed to the resistance offered by the SiC dispersoids in the matrix. The load during wear is taken up by the hard SiC dispersoids saving the matrix from the exposure. This is in accordance with the often quoted improved performance of wear resistance of composites over its matrix alloy. However, in this case due to extrusion, the material has been further strengthened due to consolidation resulting in still better resistance to wear. Thus the improvement in wear performance in the extruded case is expected to be better than the cast conditions; this point however cannot be ascertained as it is not possible to compare the wear behaviour which is a relative phenomenon with past findings. The size of the SiC dispersoids in this case being very fine would further inhibit wear during sliding as larger particles have the tendency of breaking and falling off thus increases sudden and unpredicted wear/material losses. In this case, however, no breaking of dispersoids has been seen (Figure
Worn surface of composites under different experimental conditions.
The worn surface of the composites is characterized by wear marks throughout the tested region and it is scattered or continuous depending on the severity of wear; conditions which have exhibited high wear loss have deeper groves. All the above features can be seen on the worn surface (Figure
The worn surface in the case of alloys observed after completion of the tests is characterised by grooves and material removal between grooves; material overlapping is also observed in case of severe material damage (Figure
Worn surface depicting features commensurate with observed wear loss.
Al-based 2014 alloy with fine SiC dispersoids (size 20–50
Dry sliding wear of extruded composites shows a marked improvement over extruded alloys under all conditions of load and speed tested over the entire sliding distance tested. The material removal method in the case of alloys consists of first making groove marks on the surface, then as the conditions continue the grooves get deeper and the material forming a layer on the surface being worn, then this surface could form a layer somewhat elevated from the specimen and then gradually peels off. In the case of composites also material removal is through progressive delamination of worn surface layers. However, the difference in material removal mechanism between the alloy and composite is that in the former under severe conditions of material loss, material is scooped out from the grooves but in the case of composites, the complete layer peels off. This is probably due to the presence of the dispersoids that assist in delamination of the worn layer. Again inspite of the alloy recording much higher wear loss as compared to the composite, seizure is not observed in the former whereas composites tend to seize at even lower material loss if conditions are harsh. Seizure of the test specimen is characterized by machine stopping abruptly and depicted as material flow over layers and change in wear track as seen in the worn surface studies.