A Low Cost Method for Manufacturing of Aluminum/Alumina Composite by Anodizing and CRB Process R. Jamaati 1, M. R. Toroghinejad 1, E. Mohammadi Zahrani 2 1 Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran 2 Department of Materials Engineering, The University of British Columbia, 6350 Stores Road, Vancouver, B.C., Canada, V6T1Z4. Keywords: Metal Matrix Composite; Anodizing; Cold Roll Bonding; Microstructure; Mechanical Properties Abstract In this work, Al/Al 2 O 3 composite strips are manufactured by a low cost method consist cold roll bonding (CRB) process of anodized aluminum strips. This technique has the flexibility to control the volume fraction of metal matrix composites (MMCs) by varying the oxide layer thickness on the anodized aluminum strip. Meanwhile pre- and post-rolling annealing treatment was performed on some produced MMCs. Microstructure, hardness, tensile strength, and elongation of composite strips are investigated as a function of alumina quantity and the applied production method. It is found that higher quantities of alumina improve hardness and tensile strength, while the elongation value decreases negligibly. Furthermore, pre-rolling annealing was found to be as the best method for producing this composite via the anodizing and CRB processes. Finally, it was found that both monolithic aluminum and aluminum/alumina composite exhibited a ductile fracture, having dimples and shear zones. Introduction There has been a wide interest in developing metal matrix composites (MMCs) due to their unique mechanical properties such as light weight and high elastic modulus. The common fabrication routes of particulate reinforced MMCs include spray deposition, liquid metallurgy, and powder metallurgy [1, 2]. Since expensive equipment is required and the processing routes are usually complex, the high cost of producing MMCs by these methods has limited application of MMC materials. Among the current composite material technologies, cold roll bonding (CRB) for producing composite sheets and foils has experienced a rapid growth and development in recent years owing to its efficiency and economic considerations. Cold roll bonding is a solid phase welding process, in which bonding is established by joint plastic deformation of the metals to be bonded. Bonding is obtained when the surface expansion breaks the oxide layers and the roll pressure bonds the surfaces together causing the material to be extruded through cracks in the fractured oxides, if present [3 7]. CRB is also referred to by different authors as cold pressure welding by rolling [8], bonding by cold rolling [9], clad sheet by rolling [10], and cold roll bonding [3 7]. This process can be used with a large number of materials. In addition, materials that cannot be bonded by traditional fusion, often respond well to CRB. Comparison with other methods, CRB is low cost and simple and can be easily automated. To date, this method has been widely used for producing dissimilar layered composites including Al/Steel [11], Al/Zn [12], Al/Ti [13] and Al/Ni [14]. Also, the present authors have been 669
produced the metal matrix composites by CAR (continual annealing and roll bonding) [15, 16] and ARB (accumulative roll bonding) [17 19] processes. These MMCs exhibited excellent microstructure and mechanical properties but, the production methods were not low cost. However, there is no conclusive research on either the production of MMCs with different quantities of reinforcement by anodizing and CRB processes and on the effects of different annealing treatments (pre- and post-rolling) on their microstructure and mechanical properties. The aim of the present study is to manufacture the Al/Al 2 O 3 composite via the anodizing and CRB processes and to investigate the composite s microstructure and mechanical properties such as tensile strength, elongation and microhardness. Also, the effects of pre-rolling and post-rolling annealing treatments on microstructure and mechanical properties are examined. Experimental Procedures As-received commercial purity aluminum sheets were cut into 200 mm 50 mm 0.4 mm strips parallel to the sheet rolling direction. Also, some of the specimens were annealed at 643 K for 2 h (specifications are given in Table 1). Table 1. Specifications of commercial purity aluminum. Material Chemical composition (wt.%) Condition Tensile strength (MPa) Yield strength (MPa) Elongation (%) Microhardness (HV 0.1 ) Al 1100 99.11Al, 0.17Si, 0.49Fe, 0.12Cu, 0.02Mn, 0.09 others As-received 157.4 142.3 7.2 48 Annealed 84.5 39.3 37.8 19 Some of the as-received and annealed strips were anodized in 15 wt.% sulphuric acid under an applied voltage of 16 V for two different times (5 and 60 min) to generate two extra oxide film thicknesses. Prior to anodizing, the specimens were cleaned in NaOH and then in a HNO 3 bath. Chemical compositions of the baths are given in Table 2. To ensure a constant and homogeneous temperature throughout the solution, forced convection was provided by electrolyte stirring. These oxide layers were formed at a low electrolyte temperature (16 C) favoring rapid growth and reduced dissolution of the oxide layer. Then, strips were neutralized in ammonium acetate (Table 2) under an applied voltage of 16 V for 15 min, to enhance bonding in the CRB process. The thickness of the alumina layers obtained by the anodizing process was determined by scanning electron microscopy of the oxide cross-sections. Average and standard deviations of about 20 measurements were calculated. The alumina coating thicknesses on Al 1100 specimens anodized for 5 and 60 min were about 5.1±0.2 and 16±0.5 μm, respectively. Bath NaOH HNO 3 Ammonium acetate Table 2. Specifications of the baths used. Chemical composition 60 g/lit NaOH + 120 g/lit Al 3+ + 10 g/lit additives 50 wt.% HNO 3 + H 2 O 2 g/lit NH 4 CH 3 COO + H 2 O The schematic illustration of the CRB process is shown in Figure 1. First, the two annealed strips (non-anodized) were surface prepared. A number of authors have claimed degreasing followed by scratch brushing with a rotating steel brush to be the best method for surface preparation [6, 20]. Therefore, the preparation processes for some of the (non-anodized) strips in this study included degreasing in an acetone bath followed by scratch brushing using a stainless steel brush. 670
Then the anodized strip was laid between the prepared surfaces of strips. The strips were stacked over each other, fastened at both ends, and roll-bonded with a specific reduction percentage equal to 60%. The CRB experiments were carried out with no lubrication, using a laboratory rolling mill, with a loading capacity of 20 tons. A number of the samples were annealed at 643 K for 2 hr before or after the CRB process in order to investigate the effects of pre-rolling and postrolling annealing on mechanical properties. Figure 1. Schematic illustration of the principle of CRB for producing the composite The microstructures of the CRBed composite strips under various conditions were evaluated by optical microscopy (OM) and scanning electron microscopy (SEM) PHILIPS XL30. To evaluate alumina distribution in the matrix and the bonding conditions of the CRBed composite strips, optical examination of the strips was conducted. All optical microstructures were observed along the RD-ND plane of the strips. To evaluate the alumina layer thickness after anodizing and before CRB process and investigation of the fracture surfaces after tensile test, SEM examination of strips was conducted. Vickers microhardness of the samples was measured under a load of 100 g. Microhardness was measured randomly at ten different points on the strips for each sample, the maximum and minimum results were disregarded, and the mean microhardness value was calculated using the remaining eight values. The tensile test specimens were machined from the rolled strips according to the ASTM E8M tensile sample, oriented along the rolling. The tensile tests were conducted at ambient temperature on a Hounsfield H50KS testing machine at an initial strain rate of 1.67 10-4 s -1. Three tensile tests were performed with each sample. Results and Discussion Microstructure Observation Figure 2 demonstrates the SEM micrograph of the RD-ND plane of the strips anodized for 60 min before subjecting to the CRB process. It is clear that the alumina layer has formed on aluminum strip. The thickness of this layer throughout the strip is constant, which is one of the advantages of the anodizing process. As mentioned before, the alumina layer thicknesses on aluminum strips anodized for 5 and 60 min were about 5.1 and 16 μm, that are equal to 1.13 and 3.55 vol.%, respectively. Figure 3 illustrates the OM micrographs of the microstructures of the 3.55 vol.% composite strips produced by the CRB process for various production methods. For as-received and post- 671
rolling annealed samples, the alumina particles are larger and non-uniformly distributed in the matrix compared to the pre-rolling annealed sample. Figure 2. SEM micrograph of RD-ND plane of the 60 min anodized strip. Figure 3. OM micrographs of the composite strips with 3.55 vol.% produced by the CRB process for: (a) as-received, (b) post-rolling annealed and (c) pre-rolling annealed samples. During rolling process, the aluminum matrix plastically deforms and extends, but alumina layer is very brittle and can respond to stress by necking, fracturing and departing phenomena. Therefore, the alumina layer breaks up into particles or platelets and it is consequently uniformly distributed in the aluminum matrix. Due to the cracks that open up in the alumina layer, aluminum flows through the fractured alumina regions. The interface, therefore, is a combination of oxide fragments and bonded areas of extruded aluminum. Consequently, the cracking of the alumina layer allows metal-metal contact and roll-bonding to take place. The matrix of the prerolling annealed sample can open all the cracks in the alumina layer due to high plasticity which subsequently allows stronger bonding to take place when compared with the as-received and 672
post-rolling annealed samples. As mentioned before, during rolling, necking, fracturing and departing phenomena took place in the alumina layers. It is important and interesting that in asreceived composite strip, dominant phenomenon is necking, while for post-rolling annealed sample, both necking and fracturing are dominant phenomena. Finally, in pre-rolling annealed composite strip, all three phenomena (necking, fracturing and departing) take placed, suitably and therefore, a MMC with uniform particle distribution and finer particle size produced. Based on the above results, it can be concluded that pre-rolling annealing is good alternative method for producing the Al/Al 2 O 3 composite by the CRB process. Mechanical properties Figure 4 shows variations in microhardness versus quantity of alumina for different production methods. It should be noted that microhardness measurements was performed on 1/3 thickness of samples. Maximum and minimum values of microhardness were obtained for the as-received (95, 104, and 119 HV) and post-rolling annealed (34, 39, and 43 HV) strips as seen in Figure 4. In other words, the greatest value of microhardness was achieved when the strip was rolled without pre- or post-rolling annealing treatment. For post-rolling annealed strips, a remarkable decrease was achieved in microhardness, which was almost 3 times that of the strip before postrolling annealing treatment. This may be related to the significantly decreased amount of dislocations and dislocation debris after annealing treatment and the consequent decrease in work hardening. Figure 4. Variations in microhardness versus alumina quantity with different methods. From Figure 4, it is obvious that the microhardness value improved when the alumina quantity increased. For as-received and pre-rolling annealed samples, this is attributed to reinforcing role of alumina particles in the aluminum matrix which result in additional strain hardening in matrix and therefore, increasing the microhardness. For post-rolling annealed sample, improving the microhardness with increasing the alumina quantity is related to strain hardening as a result of the mismatch between the matrix and the particles in terms of their coefficients of thermal expansion (CTE). Thermal expansion coefficients for aluminum and alumina are 22.2 10-6 and 5.4 10-6 m/mk. Furthermore, the value of microhardness error bars increased by increasing alumina particles. In other words, the values of microhardness obtained for CRBed strips without alumina particles are very close. This can be related to the non-uniform distribution of particles with higher quantities of Al 2 O 3 particles. Figure 5 presents the stress-strain curves for the aluminum/alumina composites produced by the CRB process with various quantities of alumina using the pre-rolling annealing method, a monolithic aluminum (without alumina particles) produced by the same process, and an annealed 673
aluminum used as the raw material. According to Figure 5, tensile strength values for the aluminum/3.55 vol.% alumina composite and monolithic and annealed samples were equal to 256, 201, and 84 MPa, respectively. In fact, the composites have a higher tensile strength than the monolithic and the annealed aluminum strips so that the tensile strength of the composite (with 3.55 vol.% alumina particles) is 1.3 and 3.1 times higher than that obtained for the monolithic and annealed aluminum strips, respectively. This is while the elongation value decreases negligibly. Furthermore, it can be seen that tensile strength increases by increasing the quantity of alumina in the matrix. These results are related to the following two main effects of alumina particles: (a) during the tensile test, alumina particles act as a barrier to dislocation movement, causing enhancement of strength, and (b) presence of alumina particles in the soft aluminum matrix generates dislocation to pile up in their neighborhood. Therefore, dislocation density in the matrix near the aluminum alumina interfaces increases to enhance strength. In other words, pinning the dislocations and impeding their motion by alumina particles results in enhanced dislocation density, in dislocation dislocation interactions, and thereby, in improved strength. Figure 5. The stress-strain curves of the annealed and monolithic aluminum as well as the Al/Al 2 O 3 composites produced by pre-rolling annealing with 1.13 and 3.55 vol. % alumina. Fractography Fracture surfaces of monolithic aluminum and Al/3.55 vol.% Al 2 O 3 composite produced by prerolling annealing after tensile test are presented in Figure 6. Regarding Figure 5, the tensile elongation of composite strip is lower than that of monolithic aluminum. This is attributed to presence of Al/Al 2 O 3 interfaces which act as crack source and crack propagation during tensile test (Figure 6(b)). In fact, it has been reported [1] that the failure of composite materials is related to void formation in the matrix within reinforcement/matrix interface and therefore, the elongation of composites decreased compared to pure materials. In addition, higher volume fraction of alumina causes more void formation and strain hardening during plastic deformation and as a result the ductility comes down. Figure 6 also reveals that both monolithic aluminum and aluminum/alumina composite exhibited a ductile fracture, having dimples and shear zones. This kind of fracture occurs by formation and coalescence of microvoids ahead of the crack and very limited dislocation activity [16, 17]. It is important that for monolithic sample, the quantity and depth of dimples is higher compared to composite sample. This is attributed to additional strain hardening due to presence of alumina particles in aluminum matrix for composite sample. 674
Figure 6. The fracture surfaces after tensile test for: (a) monolithic aluminum and (b) composite with 3.55 vol. % alumina. Conclusions The present work investigated the effects of quantity of alumina particles produced by anodizing, and the production method of Al/Al 2 O 3 composite using the CRB process on the microstructure and mechanical properties of the product. For the as-received and post-rolling annealed samples, the alumina particles were larger and non-uniformly distributed in the matrix compared to the pre-rolling annealed specimen. Microhardness increased with increasing alumina. Furthermore, the highest values for microhardness were obtained in the as-received strips (without pre- and post-rolling annealing treatments). The tensile strength of the CRBed strips increased with increasing alumina. Also, elongation improved negligibly with decreasing alumina content. Prerolling annealing was identified as the best method for producing Al/Al 2 O 3 composite strips via the CRB process. Both monolithic aluminum and aluminum/alumina composite exhibited a ductile fracture, having dimples and shear zones. References 1. D.J. Lloyd, Particle reinforced aluminum and magnesium matrix composites, International Materials Reviews, 39(1994), 1 23. 2. P.K. Rohatgi, R. Ashthana, and S. Das, Solidification structures and properties of cast metal ceramic particle composites, International Materials Reviews, 31(1986), 115 139. 3. R. Jamaati, and M.R. Toroghinejad, Investigation of the parameters of the cold roll bonding (CRB) process, Materials Science and Engineering A, 527(2010), 2320 2326. 4. R. Jamaati, and M.R. Toroghinejad, Effect of friction, annealing conditions and hardness on the bond strength of Al/Al strips produced by cold roll bonding process, Materials and Design, 31(2010), 4508 4513. 5. R. Jamaati, and M.R. Toroghinejad, Effect of Al 2 O 3 nano-particles on the bond strength in CRB process, Materials Science and Engineering A, 527(2010), 4858 4863. 6. R. Jamaati, and M.R. Toroghinejad, The role of surface preparation parameters on cold roll bonding of aluminum strips, Journal of Materials Engineering and Performance, (2010), doi:10.1007/s11665-010-9664-7. 675
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