Preliminary investigation on joining performance of SiC p -reinforced aluminium metal matrix composite (Al/SiC p MMC) by vacuum brazing

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1 Composites: Part A 30 (1999) Preliminary investigation on joining performance of SiC p -reinforced aluminium metal matrix composite (Al/SiC p MMC) by vacuum brazing X.P. Zhang 1, *, G.F. Quan, W. Wei School of Mechanical Engineering, Xi an Jiaotong University, Xi an, Shaanxi 10049, People s Republic of China Received 2 June 1997; received in revised form 20 July 1998; accepted 15 October 1998 Abstract The present study has investigated the joining performance of SiC particulate reinforced aluminium metal matrix composite (Al/SiC p MMC) by vacuum brazing process. After the joints brazed with Al Si Mg foil brazing filler metal at different brazing batches, both the mechanical properties and the microstructures of brazed joints were estimated. Moreover, the influence of SiC p size, SiC p volume percentage and the brazing parameters on bonding quality of the joints have also been discussed in detail. The results have revealed that the bonding quality either in SiC p /Al interface or in SiC p /SiC p interface belongs to weak bonding, and the results also show that under the same brazing parameters condition, the strength of brazed joints decreases along with increasing the SiC p volume percentage. In addition, the results also indicate that for a constant SiC p volume percentage the strength of brazed joints decreases when SiC p size increases. These results are very useful for the joining design of discontinuously reinforced metal matrix composites and further for the optimum design of composition of composites Elsevier Science Ltd. All rights reserved. Keywords: A. Metal-matrix composites (MMCs); E. Joints/joining; B. Strength; Vacuum brazing 1. Introduction The discontinuously reinforced metal matrix composite is one of the new structural materials, and a rapid development can be seen in recent years because, this kind of composite has many excellent properties and wide application prospects in the near future. For several years research on fabrication methods and material property estimations for discontinuously reinforced metal matrix composites (especially aluminium metal matrix composites) has been one of the focuses in composite fields, and many excellent research results have been obtained. Along with the rapid development in research and the great potential needs in application, however, the consequent problems in discontinuously reinforced metal matrix composites both in academic and applied aspects concerned with its secondary processes (for example the machining and the joining and so forth) have been put forward. Being a new kind of structural material, as with any other engineering material, the widely successful application of this material is also directly dependent upon the research and development in the relevant fields, such as the joining * Corresponding author. 1 Present address: Department of Mechanical and Mechatronic Engineering, the University of Sydney, N.S.W. 2006, Australia. and tooling technologies. At present, the problems concerning the joining of discontinuously reinforced metal matrix composites have become more and more interesting for many researchers and customers. The research works in this field have started to draw more attention worldwide. There are still many problems on the joining of discontinuously reinforced aluminium matrix composite materials by the generally used fusion welding processes (e.g. laser welding and arc welding), such as [1 5]: 1. In the melting state, the liquid welding pool of composite had a great viscosity with poor flowability, and it is difficult for composite itself to mix with the filler materials (for example the Al Si alloy filler wire); Consequently, the formation of weld and the quality of welded joint are not satisfactory. 2. When the molten welding pool was cooled down, the reinforcement phases, such as the SiC particulates or whiskers (SiC p or SiC w etc.), was rejected by the solidification front, and the normal solidification processes of the welding pool had been broken down, and this would lead to microsegregation or inhomogeneous distribution of the reinforcement phases. Finally there would be many defects in the weld, and the properties of the welded joint would be very poor. 3. Under the welding conditions, the harmful reactions X/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S X(98) 转载

2 824 X.P. Zhang et al. / Composites: Part A 30 (1999) Table 1 The mechanical properties of test materials and the brazed joints Materials s 0.2 (MPa) s UTS (MPa) d 5 (%) E (GPa) Aluminium a Aluminium b 72 c Brazed joint / / a General industry aluminium (bar or plate). b Fabricated from aluminium powder by powder metallurgical method. In this work, it is regarded as a special kind of aluminium composite material with reinforcement volume percentage of zero. c This value is the average of 5 specimens, the same for following joint specimens. between aluminium and SiC were particularly problematic as aluminium carbide disintegrates in air. Moreover, there also exist some other problems unsolved regarding fusion welding processes, such as the influence of welding process on properties of base composite material itself due to the high temperature, and the harmful chemical reactions between the molten aluminium and solid reinforcement (for example SiC p or SiC w etc.) in weld and base materials [2,6]. Besides the fusion welding processes, there would be some other selections for the joining of Al/SiC p MMC, for example: 1. The few existing research results show that most aluminium matrix composites can be joined by friction welding processes [2,7 9]. But it can be known clearly that for most commercially available equipments friction welding process has many limitations, and it is not suitable for components having a complicated shape or structure (such as the asymmetrical cross-section of workpiece and very thin wall components and so forth). 2. Moreover, there were also some results published in Fig. 2. Brazed joint specimen for mechanical property test. which the investigations on joining the carbon fibre reinforced aluminium composites were carried out by brazing and diffusion bonding processes [10 12]. Until now, however, it can be said that we are still far from a better understanding on the joining characteristics of discontinuously reinforced metal matrix composites. There are still many fundamental research works that should be done to investigate the joining performance of this kind of composite materials in detail, especially Al/SiC p MMC. In the present work, a preliminary research has been carried out to investigate the joining performance of SiC particulate reinforced aluminium metal matrix composites by means of vacuum brazing process. The influence of SiC p reinforcement, SiC p size, SiC p volume percentage and the brazing parameters on bonding quality of brazed joints have been investigated in detail. 2. Experimental procedure 2.1. Material preparation The materials used in this work were SiC particulate reinforced aluminium metal matrix composites which were fabricated by powder metallurgical method. The raw materials were pure aluminium powder and SiC ceramic powder (commercial abrasive products, size: 5, 10 and 20 mm). Here, it should be indicated that the pure aluminium matrix was chosen to simplify the research primarily, and other practical aluminium alloy MMCs would be based on aluminium matrixes. In fabrication processes, the powders were mixed mechanically or wet mixed in organic solute, dried and then cold compacted, hot pressed or hot extruded. The fabricated composite materials take the crosssection shape of rectangular or round, and the density of the materials is larger than 97%. The mechanical properties of the industry aluminium and the fabricated pure aluminium hot pressed material (no SiC p inside) are shown in Table 1. It can be seen that the ultimate tensile strength of the fabricated pure aluminium hot pressed material is a bit higher than that of industry aluminium. The results of mechanical properties of the fabricated Al/SiC p MMCs changing with SiC p volume percentage are shown in Fig Specimen preparation and vacuum brazing Fig. 1. The mechanical properties of Al/SiC p MMC versus SiC p volume percentage. The joint specimen used for mechanical property estimation is a butt joint with the brazement in the middle of the

3 X.P. Zhang et al. / Composites: Part A 30 (1999) Table 2 The brazing parameters used in present work Batch Temperature( C) Brazing holding time (min) Heating rate ( C/h) No No No specimen, as shown in Fig. 2. The brazing filler metal used in this test is a Al Si Mg foil with the thickness of 0.10 mm and the composition of 12.5%Si, 1.5%Mg and 86%Al. The melting temperature of the brazing filler is C. Before brazing, Al Si Mg foil, all specimens to be brazed were cleaned carefully (acid cleaning, and then ultrasonic cleaning in acetone), and were finally fixed in a specific fixture. The brazing tests were carried out in a commercial vacuum furnace (Type: ZQL-100), and the brazing parameters are shown in Table 2. Just before brazing, several pure magnesium particles were put into the vacuum furnace near the specimens. The vapourization of the magnesium particles during the brazing process would be of benefit for improving the brazing quality, because it is normally regarded that magnesium has the effect on deoxidising the oxides. The vacuums at ambient temperature and brazing temperature were Pa and Pa respectively. 3. Experimental results and discussion 3.1. Influence of SiC p reinforcement on the joining characteristics of Al/SiC p MMC From the experimental results obtained, it is known that the average strength of the brazed joints is 78.2 MPa, where the brazing variable batch is No.2 (i.e. 590 C/10min) and SiC p volume percentage is 10% (SiC p size: 10 mm), and the typical microstructure of this kind of brazed joint is shown in Fig. 3. For comparison, the brazed joints of pure aluminium hot pressed material have also been prepared. The mean Fig. 3. The microstructure of brazed joint (10% SiC p,10mm). ultimate tensile strength of this kind of brazed joints is 104 MPa, while the pure aluminium hot pressed material (without brazed joints) possesses the tensile strength of 120 MPa, as shown in Table 1. The former is a bit lower than the later. In addition, it can also be seen that the strength of the brazed joints made up of 10%SiC p Al/ SiC p MMC is 25% lower than that of the brazed joints of pure aluminium hot pressed material (104 MPa). From Fig. 3., it can be seen that for the joining of Al/ SiC p MMCs there exists three kinds of interfaces, that is, Al/Al, SiC p /Al and SiC p /SiC p. From the above comparison, it can be known that the first interface is very easy to be joined by many conventionally used methods, and in the brazed joints of pure aluminium hot pressed material all the interfaces belong to this kind, and the joint strength is satisfactory. While for the brazed joints of 10%SiC p reinforced Al/SiC p MMC, there exist three interfaces, the second and third interfaces are relatively difficult to be joined except for the first interface, or it can be said that the brazed joints containing many of the second and third kinds of interfaces belong to the weak bonding. Therefore, the reinforcements can be regarded as the obstacle for obtaining the ideal joining of composite materials. Although we have known that the reinforcements are not beneficial to the joining and it is also very difficult to estimate accurately the bonding strength of SiC p /Al and SiC p /SiC p interfaces, we can still estimate the influence of the quantity of SiC p /Al and SiC p /SiC p interfaces on bonding quality of the brazed joints. Therefore, we still need to take some detailed investigation concerning the influences of reinforcement quantity and reinforcement size on bonding quality of Al/SiC p MMC brazed joints Influence of SiC p volume percentage on vacuum brazing performance of Al/SiC p MMC With the joints brazed under the brazing batch No.2 shown in Table 2, the experimental results of the influence of SiC p volume percentage on brazed joint strength is shown in Fig. 4, where the mean size of SiC particulates is 10 mm, and SiC p volume percentages changed by 0%, 5%, 10% and 20% respectively. On comparing Fig. 4 and Fig. 1, it can be seen that the strength of Al/SiC p MMC base material increases on adding SiC p volume percentage, and the plasticity proportionally. While for the vacuum brazed joints of Al/SiC p MMC, the joint strength decreases evidently with increasing SiC p volume percentage. With using vacuum brazing method, the joining process

4 826 X.P. Zhang et al. / Composites: Part A 30 (1999) Fig. 4. The influence of SiC p volume percentage on strength of joints. Fig. 6. The influence of brazing temperature on strength of brazed joints. of Al/SiC p MMCs mainly occurs in the surface region of the materials, and SiC p reinforcements can be regarded as taking an arbitrary and uniform distribution both in volume and surface of Al/SiC p MMC. Therefore, the larger the SiC p volume percentage, the more the SiC p reinforcements existing in the surface region of brazed joints. Consequently the more weak bonding interfaces of SiC p /Al and SiC p /SiC p in the joining surface. Finally the lower the strength of brazed joints. This conclusion may be very useful for the composition design of particulate reinforced aluminium composite materials which require joining work in the latter processing steps, and additionally it will also be useful to the optimal consideration of the joining work Influence of SiC p size on vacuum brazing performance of Al/SiC p MMC Fig. 5 shows the result of influence of SiC p size on the tensile strength of vacuum brazed joints where the brazed joints were obtained under the brazing variable of 590 C/ 10min. In Fig. 5, all Al/SiC p MMC joint specimens have the same SiC p volume percentage of 10%. SiC p size changes according to 0, 5, 10 and 20 mm respectively. From Fig. 5, it can be seen clearly that the strength of brazed joints decreases with increasing SiC p size. In brazed joints, both the SiC p /SiC p and SiC p /Al interfaces belong to weak bonding interface, and SiC p reinforcements in Al/SiC p MMC may be regarded as the high strength inclusion phases. The larger the SiC p size and the more the SiC p amount, it is more harmful to the bonding quality and the brazed joint strength. This conclusion is similar to that in conventionally used fusion welding processes where the larger the defect size and the more the defect number existing in a weld, the poorer is its joint properties. Moreover, the results of the friction welding on a SiC p /2618Al composite material shown in Ref. [9] can also be useful to help us understand our results above. In Strandwood s work [9], it had been found that, during the friction process of the welding, the SiC particulates in the bonding interface of joints had been broken up as a result of the strongly repeated impact and hit of the particulates, and the size of the particulates became smaller. Further, the fine SiC particulates may be of benefit for improving the bonding quality of joints in the consequent processes of upsetting and welding Influence of vacuum brazing variable on joining performance of Al/SiC p MMC Fig. 5. The influence of SiC p size on tensile strength of brazed joints. Fig. 6 shows the result of influence of brazing temperature on the tensile strength of brazed joints. It can be seen clearly that for the present brazing parameters condition the batch No.2(590 C/10 min) corresponds to the maximum strength of brazed joints. For the Al Si Mg foil brazing filler metal used in the present work, at the brazing temperature of 580 C the filler metal melts because its melting

5 X.P. Zhang et al. / Composites: Part A 30 (1999) region is C. And at the brazing temperature of 580 C, both the flowability and the activity of the liquid filler metal are poor, thus the formation of the brazement is not very satisfactory and the quality of the brazed joints is also not fine. While at the brazing temperature of 600 C, although the liquid brazing filler metal has good flowability and activity, the strength of the brazed joints is still lower than that of the joints brazed by brazing variable match No.2(590 C/10 min). Evidently, the effect of brazing temperature on properties of brazed joints is very complicated, and there is still much work to be carried on. Additionally, it should be indicated that, the brazing variables not only have an important influence on the metallurgical processes of composite in brazing regions, but also have the obvious effect on the properties of parent material of composites. Therefore, these problems should be investigated in detail in future, especially for the strengthened Al/SiC p MMC that has undergone a certain heat-treatment. In the end, it should be indicated that one of the main aim of this work was to fundamentally investigate the joining performances of the discontinuously reinforced Al/SiC p MMC, and the relevant research works concerned with using diffusion bonding processes has also been performed. 4. Conclusions 1. For the joining of Al/SiC p MMC, there exist three kinds of interfaces, these are: Al/Al (base material/base material), SiC p /Al and SiC p /SiC p. The properties of the brazed joints depend upon the bonding quality of each of these joining interfaces. 2. The bonding quality of SiC p /Al and SiC p /SiC p interfaces in brazed joints of Al/SiC p MMC may be regarded as the weak bonding, and the brazed joints containing more of these two kinds of interfaces would possess comparatively low joint strength. 3. Under the same brazing parameter conditions, the strength of the brazed joints of Al/SiC p MMC decreases on adding SiC p volume percentage, and for a certain SiC p volume percentage the strength of the brazed joints decreases with increasing SiC p size. These results may be very useful for the joining design of discontinuously reinforced metal matrix composites and further for the optimum design of composition of metal matrix composite materials. Acknowledgements The authors greatly acknowledge the support of Science Research Foundation of Xi an Jiaotong University (K525) and the Research Foundation of State Key Laboratory for Mechanical Behaviour of Materials. The author (XPZ) is also grateful to Professor Lutz Dorn (Welding Institute, Technical University Berlin) and Alexander von Humboldt Foundation for providing working conditions in TU-Berlin to finish this paper. References [1] Yue TM, Xu JH, Man HC. Pulsed Nd-YAG laser welding of a SiC particulate reinforced aluminium alloy composite, Proceedings of the 10th International Conference on Composite Materials, Vol. 2: Metal matrix composite, Whistler, B.C., Canada, August, 1995, p [2] Ellis MBD, Gittos MF, Threadgill PL. Progress in joining aluminium based metal matrix composites, Proceedings of the Pre-assembly Symposium of 47th Annual Assembly of IIW, Dalian, China, 1 2 September, 1994, p [3] Kivineva EI, Olson DL, Matlock DK. Particulate-reinforced metal matrix composite as a weld deposit. Welding Journal 1995;74(5):83s 92s. [4] Hu C, Baker TN. AA6061Al SiC p surface MMC s produced by laser processing-a comparison of the preplacement and injection techniques for introducing SiC p, Proceedings of the 10th International Conference on Composite Materials, Vol. 2: Metal matrix composite, Whistler, B.C., Canada, August, 1995, p [5] Wang MZ, Xu ZC, Du XC, Pan LX. Experimental studies on laser welding of SiCw/6061Al composite materials. Metal Science and Technology 1991;10(4): [6] Lienert T, et al. Composition of laser and EB welds of SiC reinforced Al A356 A356 MMCs. Scripta Metallurgica 1993;28: [7] Midling OT, Grong F. A process model for friction welding of Al Mg Si alloys and Al SiC metal matrix composite-i. Acta Metallurgical Materials 1994;42(5): [8] Midling OT, Grong F. A process model for friction welding of Al Mg Si alloys and Al SiC metal matrix composites-ii. HAZ microstructure and strength evolution. Acta Metallurgical Materials 1994;42(5): [9] Strangwood M, et al. Development of bonding line microstructures in solid state Al SiC joints, Proceedings of Microscopy of Composite Materials II, Oxford, UK, April, [10] Monomo T. Diffusion bonding of SiC fibre reinforced aluminium matrix composite material with aluminium. Light Metals 1989;2: [11] Partridge PG, et al. The role of interlayers in diffusion bonded joints in a MMC. Journal of Materials Science 1991;26: [12] Guo Y, Liu P, He ZJ. The brazing joining on the mini-structure of carbon fibre reinforced aluminium composites. Transactions of the China Welding Institution. 1993;14(4):