Abstract
Friction stir processing (FSP) has been applied by several researchers to produce surface modification of metallic materials. The feasibility to produce TiC and B4C particle reinforced aluminum (AA6360) hybrid matrix composites (AHMCs) were studied in this paper. The measured content of TiC and B4C powders were compacted into a groove of 0.5 mm X 5 mm. Double pass FSP was carried out using a FSW tool at rotational speed of 1600 rpm, processing speed of 60 mm/min and axial force of 8 kN. A tool made of HCHCr steel; oil hardened to 62 HRC, having cylindrical threaded profile was used in this study. Optical micrographs revealed a defect free FSP zone. TiC and B4C particles were uniformly distributed and well bonded with the matrix alloy. The hardness of the FSW zone increased to 25% higher than that of the matrix alloy.
Key words: Surface composite, Friction stir processing, TiC, B4C.
1. Introduction
Aluminum matrix composites (AMCs) reinforced with ceramic particles exhibit high strength, high elastic modulus, and improved resistance to wear, creep and fatigue compared with unreinforced metals which make them promising structural materials for aerospace and automobile industries. However, these composites also suffer from a great loss in ductility and toughness due to the incorporation of non-deformable ceramic reinforcements, which limits their wide applications to a certain extent [1]. The surface properties dictate the life span of components in several applications. A combination of high surface wear resistance and high toughness of the interior bulk material is required to prolong the life span. This balance of properties is not achievable in many cases in monolithic or homogeneous materials. Therefore it is desirable that only the surface layer of components is reinforced with ceramic particles while its bulk retains the original composition and structure with higher toughness.
Several surface modification techniques, such as high energy laser beam, plasma spraying, cast sinter and electron beam irradiation have been developed over the last two decades to fabricate surface metal matrix composites (SMMC) [2-5]. Those techniques are based on liquid phase processing at high temperature. It is hard to avoid the excessive interfacial reactions of reinforcement with metal matrix and resulting in the formation of some detrimental phases. A critical control of processing parameters is necessary to obtain ideal solidified microstructure in surface layer. Those limitations can be overcome if processing of surface composite is carried out in the solid state.
Friction stir processing (FSP) is an emerging novel processing technique to fabricate surface composites which is based on the basic principles of friction stir welding (FSW) [6]. The distinct advantages of FSP are microstructural refinement, densification, homogeneity, accurate control and variable depth of the processed zone. FSP is a green and energy-efficient technique without deleterious gas and does not change the shape and size of the processed components.
Mishra et al [7] first demonstrated the application of FSP technology to fabricate AA5083/SiC SMMC. The incorporation of SiC particles on the surface was successful and bonded well with the matrix. Subsequently FSP evolved as a promising candidate to fabricate SMMC. Recently some investigators used FSP to fabricate SMMC [8, 9].
In the present work, an attempt is made to fabricate AA6360/8vol. % (TiC+B4C) hybrid surface composite to a depth of 5.8 mm on a 10 mm thick plate and study the microstructure and hardness distribution.
2. Experimental procedure Aluminium alloy AA6360 with a nominal composition of 0.31wt.% Mg, 0.70wt.% Si, 0.36wt.% Fe, 0.03wt.% Mn, 0.03wt.% Cu, 0.01wt.% Cr, 0.06wt.% Zn, 0.02wt.% Ni, 0.02wt.% Ti and balance Al was used in this study. A groove of 0.5 mm width, 5 mm depth, was made on the plate of 10 mm thickness and 100 mm long. The groove is compacted with equal quantity of TiC (~ 2 µm) and B4C (~ 13 µm) ceramic powders. The volume fraction of total powder was eight. FSP was carried out automatically in an indigenously built FSW machine (M/s RV Machine Tools, Coimbatore, INDIA). A pinless tool was initially employed to cover the top of the groove after filling with particles to prevent the particles from scattering during FSP. A tool made of HCHCr steel; oil hardened to 62 HRC, having cylindrical threaded profile was used. A shoulder diameter of 18 mm, pin diameter of 6 mm and pin length of 5.8 mm was used to carry out FSP at a rotational speed of 1600 rpm and traverse speed of 60 mm/min. A downward force of 8 KN was applied on the tool. Two passes were applied in opposite directions. FSP procedure is schematically shown in Fig.1. FSP of base alloy AA6360 was also carried out to estimate the effect of ceramic particles. A specimen of width 50 mm was obtained from the FSPed plate by cutting at the centre of the plate perpendicular to FSP direction. Both side of the specimen were polished as per standard metallographic procedure and etched with modified Keller reagent followed by Wecks reagent [10]. The digital image of the macrostructure of the etched specimen was captured using a digital optical scanner. The microstructure was observed using an optical microscope (OLYMPUS-BX51M). The microhardness was measured using a microhardness tester (MITUTOYO-MVK-H1) at 500 g load applied for 15 seconds along the cross section of specimen.
3. Results and discussions Fig. 2 shows the macrograph of the cross section of FSP zone. A defect free FSP zone is observed. Typical FSW defects (tunnel, pin hole, piping and worm hole) are absent. It is evident from the macrograph that the groove is completely bonded on all sides. The pin length is 0.8 mm higher than that of the groove depth which is adequate to produce full penetration. Hence, defects do not arise at the bottom side of the groove. The rubbing of the tool on the substrate generates frictional heat which plasticizes the alloy reaching semi solid state [11]. The vigorous stirring action of the tool distributes the packed TiC and B4C particles into the plasticized alloy. The translation of the tool moves this plasticized composite from advancing side to retreading side and forges at the back of tool. Thus SMMC is produced by FSP. The FSP zone is typically about the size of the rotating pin, namely width 6mm and depth 5.8 mm. Fig. 3 shows the optical photomicrograph of the interface zone at the retreading side between surface composite and aluminum alloy substrate. The surface composite layer appears to be very well bonded to the aluminum alloy substrate and no defects are visible at the interface. A narrow thermomechanically affected zone (TMAZ) is observed. The frictional heat generated by the rotating tool and application of high stresses during FSP lead to stretching of TiC and B4C particles along the shear stress directions. Fig. 4 shows the optical photomicrograph of FSP zone. TiC and B4C particles are homogenously distributed in the FSP zone. The grain size of the aluminum alloy is obviously refined by FSP. A homogenous distribution of ceramic particles is essential to attain higher mechanical properties in SMMC. Stirring causes higher plastic strain which results in rearrangement of TiC and B4C particles. FSW can be considered as a hot-working process in which severe plastic deformation is imported to the work piece through the rotating pin and shoulder. The size of TiC and B4C particles is not uniform throughout the FSP zone. The stirring action of the tool results in fragmentation of ceramic particles due to severe plastic deformation [12].The nucleation sites are increased with the presence of the reinforcement particles which lead to the reduction of aluminum matrix grain size. Fig. 5 shows microhardness distribution in the base alloy and composite. TiC and B4C particles enhanced the hardness of aluminum alloy. The average hardness of FSP zone is 25 % higher than that of the friction stir processed aluminum alloy. The hardness drops during FSP of aluminum alloy due dissolution of precipitates. The possible strengthening mechanisms which may operate in SMMC are; (i) Orowan strengthening; (ii) Grain and substructure strengthening; (iii) Quench hardening resulting from the dislocations generated to accommodate the differential thermal contraction between the reinforcing particles and the matrix; (iv) Work hardening due to the strain misfit between the elastic reinforcing particles and the plastic matrix [13].
According to the characteristics of the microstructure, the major contributions to the hardness of the surface composite layers fabricated by FSP are (1) the fine grain size of the Al matrix, and (2) the Orowan strengthening due to the fine dispersion of TiC particles.The peak hardness is observed away from the center at the advancing and retreating sides. The plasticized aluminum has to flow into the groove to fill it to yield a defect free continuous stir zone. Therefore, friction stir welding and processing take place at the center while friction stir processing alone takes place away from the center. The center may experience less deformation compared to the sides which may be responsible for the drop in hardness at the center.
4. Conclusion The following conclusions are obtained from the investigation carried out:
a) AA6360/8vol. % (TiC+B4C) hybrid surface composite was fabricated successfully using FSP.
b) The fabricated composite layer was well bonded to the aluminum substrate.
c) TiC and B4C particles were distributed homogeneously in the FSP zone.
d) The hardness of the FSW zone increased to 25 % higher than that of the matrix alloy.
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Fig. 2: Photomacrograph of FSP zone. Fig. 3: Photomicrograph of interface zone. Fig. 4: Photomicrograph of surface composite. Fig. 5: Microhardness distribution.