Effect of carbonaceous reinforcements on the mechanical and tribological properties of friction stir processed Al6061 alloy
Graphical abstract
Introduction
Al6061alloy is extensively used in aerospace, automobile and marine areas due to its high-strength to weight ratio, but it shows inferior tribological properties [1]. Particulate reinforced Al metal-matrix composites (MMCs) are considered as promising alternative to Al alloys due to their superior wear resistance. It is reported that SiC and Al2O3 are commonly used reinforcement in order to improve the mechanical and tribological properties of Al6061 alloys [2], [3].
Among the various reinforcement, carbonaceous material such as carbon nanotubes (CNTs), graphene, and graphite are found suitable due to their high thermal conductivity, lower coefficient of thermal expansion, and self-lubricating properties [4]. Powder metallurgy (solid state) and casting (liquid state) are commonly used processing techniques for fabricating the Al MMCs [5]. Bastwros et al. investigated that 1 wt% graphene reinforced Al6061 composites prepared by powder metallurgy shows 47% enhancement in the flexural strength when compared to that of Al660 alloy [6]. A.M. Al-Qutub et al. studied the tribological properties of Al6061 alloy reinforced with 0.5, 0.75, 1 and 2 wt% CNT composites, prepared by spark plasma sintering technique (SPS) [7], and have reported that 1 wt% CNT reinforced composite showed lower coefficient of friction (COF) of ~ 0.35 when compared to that of unreinforced Al6061alloy (~ 0.45) because of the higher hardness imparted by the CNTs. Baradeswaran et al. reported the wear behavior of Al6061 reinforced with 5, 10 and 15 wt% graphite composited fabricated by casting process [8], and observed that 5 wt% graphite reinforcement resulted in lower wear rate (0.006–0.008 mm2/m) as compared to unreinforced Al6061alloy (0.008–0.012 mm2/m) due to its self-lubricating behavior. Though conventional techniques (powder metallurgy and casting) are very effective for MMC processing, but do not result in the homogeneous dispersion of reinforcement [9].
Since homogeneous dispersion of the reinforced is required for enhancing tribological resistance [1], therefore, friction stir processing (FSP) technique is utilized in resulting uniform distribution of reinforcement. FSP is a solid state technique, for microstructural modification, based on the friction stir welding (FSW) joining process that was initially applied for Al alloys [10]. Apart from the microstructural modification, FSP is an effective technique for MMC fabrication [11], [12]. During the FSP, a rotating tool is inserted into the material surface and is rotated along with providing a traverse feed [13]. Rotation of tool stirs the metallic surface and the heat generated by the friction between the tool and metallic surface results in the mixing of the surface material (such as coated powder) with the matrix. Aruri et al. have reported that uniform dispersion of 20 μm sized SiC particles occurred via FSP in the Al6061 matrix [14]. Prakash [15] and Aruri [1] et al. have studied the tribological behavior of FSP Al2O3-Graphite (traverse speed 60 mm/min at 700 rpm) and SiC-Gr reinforced Al6061 hybrid composite (traverse speed 60 mm/min at 900, 1120 and 1400 rpm). Prakash et al. have reported that Al6061reinforced with 6 wt% Al2O3–0.5 wt% Gr possessed higher hardness (165 BHN) and lower wear rate (0.812 g/s) when compared to that of unreinforced Al alloy (102 BHN and 1.35 g/s respectively) and the grain size was refined from 100 μm (parent metal) to 10 μm in the stirred zone. Aruri et al. have reported that Al6061 reinforced with 8 vol% SiC–4 vol% Gr shows minimum wear rate (2 mm3/m) at a slower rotational speed (900 rpm) when compared to that at faster rotational speed of 1400 rpm (6 mm3/m), whereas unreinforced Al alloy elicited highest wear rate of 7–12 mm3/m [14]. Liu et al. processed 1 and 3 wt% CNT reinforced Al2009 composited by FSP and observed that 1 wt% CNT showed higher UTS (477 MPa) when compared to that of unreinforced Al alloy (305 MPa) after four passes of FSP [16]. They proposed that CNTs dispersed along the grain boundaries encouraged the grain refinement from 3 μm (0 wt% CNT) to 1.8 μm (3 wt% CNT) [16]. Lui et al. have proposed that grain refinement due to dynamic recrystallization during FSP and strong interface between CNTs and Al matrix enhances the mechanical properties [17]. Jeon et al. have investigated the role of graphene (G) reinforcement (water colloid of 15 mg/ml) in Al5052 via FSP performed at constant traverse speed 70 mm/min and rotational speed of 700 rpm [9]. An increased elongation at failure of 28% is observed with graphite reinforcement in FSP Al5052 when compared with that of untreated Al5052 alloy (i.e. 18%) due to the dynamic recrystallization of grains in the stir zone [9].
Many researchers have studied the mechanical and tribological properties of isolated carbonaceous reinforcements (i.e. graphite, CNT, and graphene) Al based MMCs via FSP at various processing condition (rotational speed, travelling speed). Thus, the present work is aimed at comparative study of the mechanical and tribological properties of carbonaceous particle reinforcement (i.e. Graphite, CNT, and Graphene) in the Al6061alloy at constant processing condition.
Section snippets
Materials and friction stir processing
Starting material utilizes cast Al6061 (Si-0.7 wt%–Mg-1.0 wt%, Cu-0.3 wt%, Fe-0.2 wt%, Cr-0.17 wt%, Mn-0.04 wt% and Al-97.59 wt%) alloy for the friction stir processing (FSP). Additives with Al6061during FSP include graphite (Gr, 99% pure, particle size 16 μm, bulk density—0.8–1.2 g/cm3, purchased from M/S Lobachemi Pvt. Ltd, India), carbon nanotubes (CNTs, 95% pure, dia of 30–50 nm and length of 10–20 μm, purchased from Nanostructured & Amorphous Material, Inc. US) and graphene (G, particle size of 15 μm,
Phase analysis
The X-ray diffraction (XRD) pattern of untreated and FSP samples (Fig. 3) shows the peaks corresponding to Al and Al-Fe-Si precipitate. XRD pattern does not show formation of any new phase (such as Mg2Si or Al4C3) after FSP. A significant difference in the peak-intensity and peak position of FSP samples was observed due to the grain refinement and strain generated during the processing. Severe plastic deformation associated with FSP [18] has resulted in grain refinement (crystallite size
Conclusions
Al6061 alloy was friction stir processed (FSP) with carbonaceous particle reinforcements (i.e. graphite, CNT and graphene composites). A decrease in the crystallite size (to ~ 26 nm) was observed after FSP when compared to that of untreated condition (~ 41 nm). An increased orientation factor with carbonaceous reinforcement is also elucidated with an increased residual compressive stresses (> 1 GPa) in FSP Al6061 reinforced with Graphite (T-Gr), Carbon nanotube (T-CNT) and graphene (T-G) when
Acknowledgments
Authors acknowledge Advanced Centre for Material Science, IIT Kanpur for extending the nanoindentation and optical profilometer facility. Prof. Gouthama is acknowledged for assisting with TEM studies, and Prof. Kaustubh Kulkarni is acknowledged for providing Al6061 plates in kind for the study. KB acknowledges P.K. Kelkar Fellowship, IIT Kanpur.
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Have contributed equally as first author.