Elsevier

Wear

Volume 271, Issues 9–10, 29 July 2011, Pages 1868-1877
Wear

Effect of extrusion ratio on wear behaviour of hot extruded Al6061–SiCp (Ni–P coated) composites

https://doi.org/10.1016/j.wear.2010.12.078Get rights and content

Abstract

Metallic coated reinforcements are currently gaining popularity as regards processing of high quality metal matrix composites (MMCs) by liquid metallurgy route. The major reasons being improvement of wettability between reinforcement and molten alloy and to eliminate interfacial reactions. Both these factors lead to inferior mechanical and tribological properties of developed MMCs with non uniform distribution of reinforcement. In the light of the above, the present work focuses on development of Al6061–SiCp (Ni–P coated) composites by stir casting followed by hot extrusion using 200 T hydraulic press at a temperature of 550 °C under different extrusion ratios. Extrusion ratios of 1:4, 1:5, 1:10 and 1:15.5 were adopted. Microstructure studies microhardness, tensile strength tests were conducted on hot extruded composites. Pin-on-disk wear tests were carried out at different loads and sliding velocities for the extruded composites at different extrusion ratios. SEM of worn surfaces was also carried out. As the extrusion ratio increases, microhardness and tensile strength of the composites increased. Further, under all the wear tests conditions studied, the wear resistance of the hot extruded MMCs was superior for the highest extrusion ratio studied.

Introduction

Aluminium based metal matrix composites are the most sought after candidate materials in aerospace, space and automotive industries owing to their superior mechanical and tribological properties [1]. These popular composites are currently manufactured by powder metallurgy, liquid metallurgy, spray forming and diffusion bonding techniques. However, the most simplified approach to develop near net shaped aluminium based composites is by liquid metallurgy route as it is economical and can result in mass production [2]. Further, these cast composites can be subjected to secondary processing such as extrusion and forging to improvise upon the mechanical properties in particular strength coupled with practical ductility. However, since three decades, it has been reported that few challenges like inferior bond, interfacial reaction product need to be addressed to in composites developed by liquid metallurgy route. These problems do have a direct deteriorating effect on the mechanical and tribological properties of the composites making them unsuitable for industrial components [3]. These challenges have been addressed to by use of metallic coated reinforcement and the addition of reactive metals like magnesium by several researchers across the globe [4]. Davidson and Regener [5] have reported on the beneficial effect of copper coating on SiC particles reinforced in aluminium matrix composites. They have reported that metallic coating on reinforcement has improved the interfacial bonding which has resulted in large failure strain. Cocen and Onel [1] have reported the role of reinforcement volume fraction and hot extrusion process on the strength and ductility of aluminium alloy/SiC composite. It is reported that the forming applied to the metal matrix composites do affect the microstructure and thereby its mechanical properties [6]. It is reported that there is reduction in the reinforced particle size after extrusion of cast MMCs [7]. Smaller the reinforcement size for a given volume fraction higher will be the strength of the composites [8], [9]. Bauri and Surappa [10] have reported a decrease in 0.2% proof stress and compressive strength due to clustering of SiC particles and poor interfacial bonding in higher volume reinforcement of SiC in aluminium lithium alloys. Ramesh et al. [11] have reported the tribological behaviour of Ni–P coated Si3N4 reinforced Al6061 composites. Increased content of Si3N4 in the matrix alloy has resulted in lower wear rates of the composites. Moustafa et al. [12] have carried out a comparative study on tribological properties of uncoated and metallic coated graphite reinforced copper composites. They have reported that, metallic coated graphite reinforced composites exhibit lower wear rate and coefficient of friction when compared with uncoated ones. The transition in wear has been changed to higher loads with increased content of copper coated graphite composite when compared with uncoated ones. Ramesh and Safiulla [13] have reported the wear behaviour of hot extruded Al6061 based composites. Extruded composites possess higher microhardness and lower wear rates under all loads and sliding velocities studied when compared with cast composites. Zhan and Zhang [14] have studied the mechanical and wear properties of uncoated and nickel coated reinforced copper composites. They have observed that nickel coated SiCp–Cu composites exhibit better combination of flexural strength and ductility than the uncoated SiCp–Cu composites. They have also noticed that nickel coated SiCp particles reinforced composites possessed improved wear resistance.

In the light of the above, the present investigation focuses on wear behaviour of uncoated and Ni–P coated SiC reinforced Al6061 composites processed by liquid metallurgy route followed by hot extrusion. Further, influence of extrusion ratio on wear rates of developed composites has been investigated.

Section snippets

Composite preparation and hot extrusion

Al6061 alloy with the chemical composition given in Table 1 was used as the matrix material. Silicon carbide (SiC) in powder form having particle size of range 5–40 μm was used as reinforcement. Silicon carbide particles were subjected to electroless nickel coating. The detailed coating procedure is as described in our earlier works [15].

The composites were developed using stir cast method. Both Ni–P coated and uncoated silicon carbide was varied in proportions of 2–10 wt%. Dispersion was

Microstructure

Fig. 1 shows the optical microphotographs of extruded Al6061 alloy and Al6061–10 wt%SiC composites (both coated and uncoated) at different extrusion ratios. At all the extrusion ratios studied uncoated SiC reinforced Al6061 composites exhibit a microstructure indicating non homogeneous distribution of SiC particles throughout the matrix alloy and agglomeration along the extrusion direction in banded form. This may be attributed to poor wettability of uncoated SiC particles in molten aluminium

Conclusion

Ni–P coating of SiC has a significant effect on the wear behaviour of extruded composites. For a given extrusion ratio, Ni–P coated SiC reinforced extruded composites exhibited a decrease in the wear rate by 66% and 25% when compared with alloy and uncoated SiC reinforced composites, respectively. Higher the extrusion ratio lower are the wear rates of alloys and extruded composites for the given load and sliding velocity studied. For an increase in extrusion ratio from 1:4 to 1:15.5, a decrease

Acknowledgements

The authors wish to acknowledge their sincere thanks to Department of Science and Technology (DST), India, for sponsoring this research work (Project No.: SR/S3/ME/19/2004-SERC). The authors are grateful to Mr. G.N. Jaiprakash, R. D & E, Kennametal, Bangalore for extending co-operation in carrying out SEM studies.

The authors would like to express their deep sense of gratitude to Prof. D. Jawahar, CEO, PES Group of Institutions and Dr. K.N.B. Murthy, Principal and Director, PESIT, Bangalore.

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