Forming Behaviour of Al-TiC In-situ Composites

Transcription

1 Materials Science Forum Vol. 765 (2013) pp (2013) Trans Tech Publications, Switzerland doi: / Forming Behaviour of Al-TiC In-situ Composites Ram Naresh Rai 1,a, A.K. Prasada Rao 2,b, G.L. Dutta 3,c and M. Chakraborty 4,d 1 National Institute of Technology, Agartala, India 2 BCAST, Brunel University, West London, UK, UB8 3PH 3 K.L. University, Guntur, India 4 Indian Institute of Technology- Bhubaneswar, India a nareshray@yahoo.co.in, b akprasada@yahoo.com, c chancellor@kluniversity.in, d madhu@metal.iitkgp.ernet.in Keywords: Metal-matrix composites, Intermetallics, Mechanical properties, Forging, Forming Abstract. The forming behaviour of in-situ Al-TiC composites was investigated by comparing microstructure and mechanical properties of as-cast, forged and rolled specimens. The microstructures of forged and rolled specimens reveal uniform distribution of the TiC particles, which are responsible for the enhancement of the tensile strength of the composite. The formed samples were found to be crack free. This feature is very likely to be due to good interface bonding of uniformly dispersed sub-micron size TiC particles with the Al matrix. Introduction During the past two decades, in-situ metal matrix composites have drawn the attention of many researchers due to their low density, excellent wear resistance, high specific strength and high specific modulus [1-3]. Particle reinforced metal matrix composites are likely to find high volumes of commercial applications due to their isotropic properties, ease of fabrication and improved properties [1-5]. The metal matrix composites derive good demand for their use in automobile and aerospace applications [6]. However, the application of these materials is often limited by their poor ductility which is generally associated with inhomogeneous size and distribution of the reinforcing particles [7]. It is well known that a metal takes a desired shape and size during forming by improving the strength of the components, due to plastic deformation [8]. Few reports prove that the distribution of the reinforcing particles in an as-cast composite can be improved by mechanical working (rolling, forging, extrusion etc), resulting in enhanced tensile strength [7-10]. Ozdemir et al. [9] reported that forging improves the distribution of the re-inforcing particles in a Al-Si alloy leading to enhanced mechanical properties. In another study [10], a significant change was reported in the tensile strength of a cold-rolled SiC/Al composite after 30 %, 50 % and 70 % reduction in thickness. In their independent studies on AA2814/ 20 vol % Al 2 O 3 ex-situ composites, Cavaliere et al. [11] and Ceschini et al. [12] had shown that forging leads to decohesion of the Al 2 O 3 particles from the matrix. There are a few reasons for such behaviour of ex-situ composites. First one is that, the wettability of the particles with matrix is somewhat limited in the case of ex-situ composites. Second is the particle size which is much coarser with non-uniform distribution. These features of the fabricated composite, would lead to fragmentation of the particles and decohesion of the Al 2 O 3 particles from the matrix during deformation. However, in-situ composites overcome some of the above limitations of ex-situ composites. Although several reports have been found dealing with various aspects of Al based in-situ composites, unfortunately, none was found to demonstrate the forming behaviour of in-situ Al-TiC composites. Hence, the present work was taken-up to focus on the understanding of the effect of forming on the microstructure and mechanical properties of the in-situ Al-TiC composites. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID: /05/13,10:55:04)

2 Materials Science Forum Vol Experimental Details Al-TiC in-situ composites were prepared by the reaction of molten Al with K 2 TiF 6 and graphite powder at 1200 C in a medium frequency induction furnace. As-cast composite ingot rods were thereafter subjected to the forming operations such as rolling and forging. A two-high strip rolling mill was used for rolling the Al-TiC composite specimens. Detailed rolling experiments involving hot rolling at 400 C and % reduction (20, 40, 60 and 80 %) as well as cold rolling at room temperature and % reduction (20, 40, 60, and 80 %) were carried out. A 25T hydraulic press was used for carrying out the cold and hot forging experiments. Al-TiC composite was cold forged in an open die by applying an incremental load of 2, 4, 6 18, 20T. Another forging experiment was also conducted by applying a 20T load directly. The deeply-etched microstructures of the forged and rolled Al-TiC composites were characterized using Scanning Electron Microscope (SEM) (Note: the central portion of the forged samples was used for microstructural examination). Further, tensile tests were performed (using an Instron tensile testing machine, while five specimens were tested for each condition) for the composites in as-rolled and as-forged condition. Subsequently the respective microstructures were correlated with their tensile strengths and the results are discussed in the following section. Results and Discussion Microstructure and Phase Identification in As-cast Al-TiC Composites. The microstructure of the as-cast specimen was studied using SEM. It is evident from the microstructure shown in Fig. 1a, that a number of particle clusters are found to be along the α-al grain boundaries. However, SEM studies alone do not confirm the phases. Hence XRD studies were also conducted on a solid Al-TiC as-cast specimen and also on the particles extracted from the as-cast specimen (which was done by dissolving the matrix in NaOH solution). The XRD patterns obtained both from the solid as-cast specimen and the extracted particles are shown in Fig. 1b. From the XRD patterns, the formation of the TiC particles is confirmed in the Al-matrix. Influence of Rolling and Forging on Microstructure of In-situ Al-TiC Composites. Fig. 2a-b represents the microstructure of the cold rolled composite after 20 % reduction. Similarly Fig. 2c and Fig. 2d-e depict the microstructures of cold rolled specimen after 40 % and 60 % reduction. Microstructures (Fig. 2a-e), indicate no effect on cracking or damage of TiC particles after cold rolling. However, the clusters of TiC particles seen in as-cast condition (Fig. 1a) are affected by cold-rolling (Fig. 2a-e). This is due to the fact that the cohesion of the clustered TiC particles gets weakened and particles are redistributed in the matrix, which is much pronouncedly seen up to about 40 % reduction in the cross section of the rolled composite. Interestingly, when the percentage reduction is increased to say 60 %, it has been found that besides de-clusturization, the particles are redistributed and embedded into the soft Al matrix. This may further increase the matrix/tic interfacial strength. On the other hand, hot rolling (Fig. 3a-f) also shows similar effect as cold rolling on the TiC particle clusters. During hot rolling, it has been observed that distribution of the TiC particles is somewhat better than cold rolling. Thus both cold as well as hot rolling have more or less similar effect on microstructure of Al-TiC composites. Hence it can be understood that rolling has significant effect on the distribution of TiC particles in the Al matrix. SEM photomicrographs of cold and hot forged Al-TiC composites are shown in Fig. 4a and Fig. 4b respectively. The effect of de-clusturization is much pronounced during hot forging (Fig. 4b) than in the case of cold forging (Fig. 4a). The most apparent difference in microstructural features between as-cast (Fig. 1a) and forged composites (Fig. 4a-b) is in terms of the TiC clusters. It has been found that the clusters initially present in larger size in the as-cast composites, are further reduced in size after forging with an increased uniform distribution of TiC particles. It is also seen that in the ascast condition, the TiC particle clusters are present in the grain boundaries. As and when forging load was applied, the de-clusterization occurs and the particles are redistributed in the matrix. In contrast, in the case of ex-situ composites with relatively coarser reinforcing particles with less uniform distribution, the particles tend to break during forming, in addition to the decohession of the interface between the particles and the matrix, as reported earlier [11-12].

3 420 Light Metals Technology 2013 Tensile Strength. Tensile tests were conducted on the cold and hot rolled Al-3TiC, Al-5TiC and Al-10TiC composites subjected to different reduction levels of 20 %, 40 %, 60 % and 80 %. The results presented in Fig. 5 indicate that an increase in percentage reduction in cross sectional area of Al-3TiC, Al-5TiC and Al-10TiC composites leads to an increase in the ultimate tensile strength (UTS). It has been found that the increase is high at low reduction levels and low at high reduction levels. The increase in strength with increase in % reduction of cross sectional area can be attributed to the combined strengthening effect of work hardening and the re-distribution of TiC particles in the Al matrix during rolling. The increase in strength is more in the cold rolled samples than in the hot rolled samples from the as-cast condition. Both yield strength and UTS increase with increase in the percentage of TiC content in the composites (Fig. 5). It is understood that strain hardening tendency is higher in the case of cold rolling than that in hot rolling, which can be attributed relaxation of the plastic strains in hot rolling. Hence, cold rolled composites exhibit higher strength than their hot rolled counterparts. Fig. 1. (a) SEM micrograph and (b) XRD pattern of Al-10TiC in-situ composite in as-cast condition. Fig. 2. SEM microphotographs of Al-10TiC composites at (a-b) 20, (c) 40, and (d-e) 60 % of reduction (cold rolled).

4 Materials Science Forum Vol Fig. 3. SEM micrographs of hot rolled Al-10TiC composites at various % reductions (a) 20 %, (b) 40 %, (c) 60 %, and (d) 80 %. Fig. 4. SEM micrographs of (a) cold forged and (b) hot forged Al-10TiC composites at 20T load. Tensile tests results of cold and hot forged samples made of Al-3TiC, Al-5TiC and Al-10TiC composites, subjected to an incremental applied load of 2, 4, 18, 20T in steps or 20T forging load in one step, are presented in Fig. 6. It has been found that the strengths of Al-3TiC, Al-5TiC and Al-10TiC composites increase after forging. It is also observed that Al-TiC composites forged in steps have higher strength than the composites forged in a single step Fig. 5. Effect of % reduction and wt % of TiC particles, with application of the same final forging on the UTS of (a) cold rolled (CR) and (b) hot rolled load. E.g., Al-3TiC composites forged in (HR) Al-TiC composites. steps and forged in a single step have tensile strengths (UTS) of 173 and 160 MPa respectively. A similar trend has been observed in the case of Al-5TiC and Al-10TiC composites also. This can be attributed to the work hardening and dislocation pile-up during forging, causing an increase in dislocation density due to thermal mismatch between matrix and reinforcement, which restricts the deformation of the composites. All these structural changes contribute to increase the yield strength (YS) and tensile strength (UTS) of the composites. The influence of forging on the observed increase in yield strength and tensile strength is attributed to the collective effect of work hardening and the uniform redistribution of the TiC particles. (Note: Detailed work has been reported elsewhere [13].)

5 422 Light Metals Technology 2013 Conclusions Rolling and forging de-clusterize and redistribute the TiC particles in Al-TiC composite. Size of the TiC clusters decreases continuously with the reduction in cross-section during rolling. In the case of forging, cluster size decreases with increase in forging load through damage/disintegration of TiC clusters. Rolling and forging improve the yield and tensile strengths of Al-TiC composites through the strain hardening mechanism. However in the case of hot rolling and hot forging, the strengthening effect is caused by recrystallization phenomenon. Forging in steps makes the composites stronger than if forged in one step, for the same final load. Fig. 6. Effect of the forging conditions and the wt% of TiC particles, on the UTS and YS of Al-TiC composites. References [1] J. Jiang, B. Dodd, Workability of Aluminium- based metal- matrix composites in cold compression, Composites 26 (1995) [2] D.J. Lloyd, Particle reinforced aluminium and magnesium matrix composites, Int. Mater. Rev. 39 (1994) [3] S. Jerome, B. Ravisankar, K.M. Pranab S. Natarajan, Synthesis and evaluation of mechanical and high temperature tribological properties of in-situ Al-TiC composites, Tribology International 43 (2010) [4] T.W. Clyne, P.J. Withers, An introduction to metal matrix composites, Cambridge University Press, U.K., [5] R.K. Everest, R.J. Arsenault, Metal Matrix Composites: Processing and interfaces, Academic Press, U.S.A., [6] K.K. Chawla, Composites Materials: Science and Engineering, Springer-Verlag, New York, [7] A.R. Kennedy, S.M. Wyatt, Characterizing particle-matrix interfacial bonding in particulate Al-TiC MMCs produced by different methods, Composite Part A: Appl. Sci. and Manuf. 32 (2001) [8] I.M. Osman, J.J. Lewandowski, W.H Hunt, Fabrication of particulate reinforced composites, Materials Park, OH: ASM International (1990) [9] Ozdemir, U. Cocen, K. Onel, The effect of forging on the properties of particulate-sicreinforced aluminium-alloy composites, Comp. Sci. Technol. 60 (2000) [10] W. Zhang, M. Zhang, M. Gu, D. Wang, Z. Yao, Effect of cold-rolling on tensile strength of SiC/Al composite, Rare Metals 22 (2003) [11] P. Cavaliere, E. Evangelista, Isothermal forging of metal matrix composites: Recrystallization behaviour by means of deformation efficiency, Comp. Sci. Technol. 66 (2006) [12] L.Ceschini, G. Minak, A Morri, Forging of the AA2618/20 vol.% Al 2 O 3 composite: Effects on microstructure and tensile properties, Comp. Sci. Technol. 69 (2009) [13] R. Naresh Rai, Al-TiC in-situ metal matrix composites, PhD Thesis, IIT Kharagpur, India, 2007.