Joining of AL-6016 to Al-foam using Zn-based joining materials

Accepted Manuscript Joining of AL-6016 to Al-foam using Zn-based joining materials Graziano Ubertalli, Monica Ferraris, Muhammad Kashif Bangash PII: DOI: Reference:

S1359-835X(17)30067-2 http://dx.doi.org/10.1016/j.compositesa.2017.02.019 JCOMA 4580

To appear in:

Composites: Part A

Received Date: Revised Date: Accepted Date:

7 November 2016 8 February 2017 13 February 2017

Please cite this article as: Ubertalli, G., Ferraris, M., Kashif Bangash, M., Joining of AL-6016 to Al-foam using Znbased joining materials, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa.2017.02.019

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

JOINING OF AL-6016 TO AL-FOAM USING ZN-BASED JOINING MATERIALS

Graziano Ubertalli, Monica Ferraris, Muhammad Kashif Bangash Politecnico di Torino Department of Applied Science and Technology, Corso Duca degli Abruzzi, 24, 10129, Torino, Italy. e-mail: [email protected] , [email protected], [email protected]; web page: http://polito.it, www.composites.polito.it

Key words: Al-Foam Sandwich (AFS); Soldering; Automotive; Aerospace

Abstract: To obtain an Aluminum Foam Sandwich (AFS), Al-6016 sheets were successfully joined to a 9 mm thick Aluminium (Al) foam, by using Zinc (Zn) based joining materials (pure Zn and Zn alloy with 2% Al) at 430 °C in argon atmosphere. The microstructure of the joints was analysed by Optical Microscope (OM), Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS). Moreover, three-point bending tests were carried out to evaluate the flexural properties of the AFS components. Current experimental work is focused on optimization of the AFS joining process

and

the

mechanical

properties

of

AFS

components.

1. Introduction Aluminium (Al) foam is a lightweight, non-flammable and porous material with high energy absorption, better sound absorptivity and electromagnetic pulse shielding and lower conductivity in comparison to bulk Al. Foamed metals have been intensely studied in the recent past and are potential materials for a number of engineering applications due to the aforementioned properties [1–3]. Al-foam can be used in automotive and aerospace applications for improved impact

1

resistance, noise and weight reduction [4,5]. Al-6016 is a light Al alloy and is commonly used as a structural material in the automotive and aerospace industries. An Aluminium Foam Sandwich (AFS) is a special class of sandwich composites materials. It is fabricated by sandwiching a thick, low density Al-foam core between two thin but stiff skins of Al to merge the strength properties of the skin and the core. An AFS component provides good dimensional stability, specific strength, improved damping and acoustic insulation properties [6,7]. The core materials play a vital role in the overall mechanical performance of sandwich composites. These materials are being widely used in the automobile and aerospace industries due to their multi-functionality and unique performances [8,9]. The final AFS component has low density, high specific flexural strength and stiffness, acoustic and thermal insulation properties and improved damping properties together with high efficiency in energy dissipation [10,11]. To improve the performance of sandwich composites, several core configurations have been investigated and compared [12–15]. In order to produce AFS composites it is necessary to join the skins with the core. Though commercial adhesives are being used to produce AFS components, their temperature resistance is limited to 220°C [16,17]. For aerospace applications, the thermal expansion, moisture absorption and low elastic modulus of adhesives are also a drawback [18]. Several other techniques for fabricating AFS components have been proposed and investigated, such as flux-less soldering with surface abrasion [19], laser foaming to produce the Al foam cores inside a hollow profile [20], the friction stir incremental forming technique to transform a surface layer into a massive skin [21], pressing-bonding, rolling bonding and the powder metallurgy foaming process and Self-propagating High-temperature Synthesis (SHS) [11]. Brazing processes are also proposed: for instance, Al86SiMg alloy was adopted as a filler material for the joining of Al-foam to Al-sheet [22]. These joining techniques have made AFS components an interesting solution for a number of practical applications, such as low cost lightweight structures with high mechanical strength and enhanced capacity of energy dissipation under impacts [11,23].

2

Another work suggested that reliable joining properties can be achieved through typical diffusion brazing if the brittle intermetallic formation and low temperature eutectic constituents are avoided [24]. Zn is a typical alloying element for Al alloys and its solubility is the highest among the most used alloying elements [25]. Al and Zn do not form intermetallic phases, due to a weak interaction between Al and Zn atoms. The eutectic point for Al-Zn system is at about 380°C and, at this temperature, the a solid solution is the stable phase in alloys containing Al percentage higher than 30%. The small difference between the electrode potential of Al and Zn reduces the possibility of galvanic corrosion phenomena. Moreover, the joining of Al components is feasible in an inert atmosphere because of the high reactivity of Al towards Oxygen (O) [26–28]. The current research aims to develop a new soldering method to obtain AFS components by joining Al-6016 sheets to Al-foams. In this work, Zn based fillers were selected to satisfy the abovementioned requirements. As a joining material in the production of AFS components, Zn based fillers, resolved the issues such as high temperature resistance and moisture absorption which affect commercial adhesives.

2. Experimental Procedures 3

Aluminium Alloy (AA)-6016 (Al 98.75 % + Mg 0.25% + Si 1%) plate (density 2.7 g/cm ), 1.2 mm thick, was selected as the skin material for the AFS. It is one of the most common Al-alloy used for the Body In White (BIW) parts in the automobile industry. An ultralight, 100% non-flammable ecofriendly Al-foam plate with closed cells, 9 mm thick (average density 0.28 g/cm 3), produced by Foamtech, South Korea, and supplied by Vaber, Italy, was used as core material. The foam plate is obtained by cutting a large foam block. The resulting surface morphology is sponge-like with variation of the size and distribution of pores, Fig. 1. The micrographs of Al-foam surface were analysed using a free software, Image J and the average pore size and distribution was calculated. The average pore diameter calculated was 2.87 ± 1.5 mm, and the distribution fits into a bell shape curve. The pore cell walls, which constitute the potential joining area, accounted for around 12.8 % of the total Al-foam plate area. The soldering materials composed of pure Zn-foil (250 microns

3

thick) and Zn+2%Al strip (350 microns thick) were provided by Lucas Milhaupt, USA. The Al-6 flux (working temperature 420-470 oC, supplied by Stella srl-Italy, was applied on the joining surfaces to facilitate the chemical bonding and avoid oxidation. (Fig. 1, Fig. 2) The Al-foam, soldering material and Al plate were cut off in the form of sheets. The joining surfaces of the Al-sheets and Al-foam were firstly abraded with 120-360 mesh SiC paper in order to clean the surface from Al oxide and facilitate the joining process. The grinded surfaces were then o

cleaned with alcohol in an ultrasonic bath for 10 minutes at 60 C and activated by using 12% nitric acid solution for 30 minutes as also reported in [11]. AFS components were stacked in the configuration shown in Fig. 2, to undergo the soldering process in a tubular furnace (Bicasa, Italy), at 420-430 oC for 1-5 min at a heating rate of 1000 °C/h in argon atmosphere to avoid oxidation. The specimens were then slow cooled in flowing argon, at a cooling rate of 100 °C/h. The slow cooling ensures no residual stresses and allows phases closer to the stable equilibrium state to be achieved [28]. In fact, during the soldering process, most of the joining melt formed at soldering temperature enters the foam and very little is left for the contact areas between the skin and foam. The nonhomogeneous distribution of the joining melt sometimes results in localized melting of the Al skin. Therefore, several different combinations of time/temperature values were tried in order to establish the optimal soldering parameters and achieve good joints. The optimal soldering parameters were 5 minutes at 430 oC when pure Zn foil was used as a o

joining material and 1 minute at 420 C when Zn with 2% Al was used. In these conditions, eight specimens were produced (four for each soldering material) for microscopic analyses and mechanical characterisation. The suggested joining technique was carried out in the laboratory by a batch process. However, with the proposed soldering conditions, this process can easily be industrialised in a continuous cycle for mass production.

4

The AFS samples produced for mechanical test were 60 mm long, 20 mm wide with a total thickness of about 11 mm with an average density equal to1.1 g/cm 3. Three-point bending tests (MTS Model: 661.23B-01, Fig. 3) were carried out (at room temperature) on AFS samples, Al-6016 sheets and Al-foam plate to investigate the mechanical properties and evaluate the failure modes. In the bending test, crosshead velocity was set at 2 mm/min. The three-point bending test assembly consisted of three cylindrical steel rollers with 20 mm diameter and 47 mm span length. When 8 mm cross head displacement was reached, loading was stopped. (Fig. 3)

3. Results and Discussion 3.1. Microstructure

Transverse metallographic sections of the sound AFS specimens were observed with an optical microscope and a Scanning Electron Microscope (SEM) equipped with Energy Dispersive Spectroscopy (EDS).

Fig. 4 represents the macrographs of polished AFS specimen cross-sections prepared with pure Zn (Fig. 4a) and Zn+2% Al soldering alloys (Fig. 4b). The variation in pore size and distribution determines the extent of the joining area between the foam and the skin. The smaller the pore size, larger will be the potential joining area due to the increased number of cell walls which represent the joining surface of the Al foam in contact with the soldering material. However, at the same time the foam density will also increase. According to the Al-Zn phase diagram, Zn melts and reacts with solid Al (sheet and foam) producing a liquid solution (joining melt) at the soldering temperature. As the joining melt is enriched with Al, β’ solid solution forms and the melting temperature increases [25]. Therefore, the proposed soldering system is self-adjusting and avoids the localised melting of Al sheet by uniform distribution of the joining melt in the foam and skin interface. Since the Al-foam is produced from liquid aluminum, the solid part of it has a dendritic microstructure, as evident in the etched cross

5

section of Al foam, Fig. 5. Moreover, small pores are visible in the Al-foam cell walls. These pores are developed during the solidification of the Al melt during the Al foam production process. (Fig. 4, Fig. 5) The metallographic cross sections of the AFS joints soldered with Zn+2%Al and with a pure Zn foil are shown in Fig. 6 and Fig. 7, respectively. It is evident that, at soldering temperature, the joining melt (Zn based soldering alloys) provoked the diffusion effect and subsequent melting of the Al foam and sheets. After cooling, the new liquid solution solidifies, producing a microstructure that sometimes contains a certain amount of porosity. The porosity at the joining interface is mainly due to the shrinkage phenomenon during the liquid to solid transformation. After etching, the joining interface shows dendrites and eutectic morphology (Fig. 6). In particular, the reaction of the liquid (Zn+Al) with α Al phase produces a certain amount of Zn-rich β (fcc) phase, which during solidification (or cooling) becomes unstable supersaturated Zn-rich β’s (fcc) and decomposes into Al-rich α phase (fcc) and Zn-rich η phases (hcp). (Fig. 6, Fig. 7) The presence of porosity in the joints is generally an undesirable condition because it affects the localized mechanical properties. The interface is an intermediate zone of progressive change from the massive skin to a core material where porosity is highly desired. In this case, the porosity in the joining interface is acceptable, because the core material itself is porous. The amount and type of soldering material affects the volume quantity of the joining melt, and therefore, the joining thickness. In fact, at the same soldering temperature, the diffusion effect was more evident when soldering was carried out using Zn+2% Al soldering alloy than it was with pure Zn. This is due to the difference in melting point and the higher thickness of Zn+2% Al strips (350 micron) compared to pure Zn ones (250 micron). The AFS cross sections observed with the SEM show the localized melted pool on the Al sheet and Al foam interface, Fig. 8b and Fig. 9b. The dendritic microstructure is evident with a primary arm columnar growth starting from the wall pool. A reduced amount of interdendritic morphologies are

6

also noticed together with shrinkage porosities. At the soldering temperature, the Silicon (Si) solves in the joining melt and, due to its very low solubility in Al rich and Zn rich phases respectively, induces a sudden crystalline Si nucleation and growth. Some polygonal or rounded Si particles, bigger than those observed in the Al sheets and foam, are evident in the joint interface, (Fig. 8c, EDS 3). (Fig. 8) (Fig. 9) At higher magnifications during SEM analysis, the interdendritic microstructure is seem to have a physical mixture of two phases (α and η) with a relative distribution which is sometimes lamellar,

Fig. 9. The EDS analysis was carried out to obtain the localised elemental composition of apparent dendrite and interdendrite regions respectively (Table 1), show higher amount of Zn in the interdendritic zone, which is also confirmed by the η phase precipitation observed in metallographic microstructure. (Table 1) The depth of the joining interface along of the Al-sheets was non-homogeneous in the entire surface but was thicker at the points where the foam cell walls touch the Al sheets. In these zones, the joining depth ranged from around 200 µm, in case of pure Zn soldering alloy and 800 µm when Zn+2% Al soldering alloy was used. 3.2. Mechanical Characterization To analyse the effect of the soldering cycle on the base materials, four samples of Al-6016 sheet and Al-Foam each were subjected to the three-point bending test before and after the thermal treatment similar to one adopted for soldering the AFS component. Four samples from each the two families of the AFS component were also subjected to the same bending test to observes the mechanical properties. The average bending strength values are reported in Table 2. The results reported in Fig. 10 show the average and representative behaviour of the Al-6016 sheet and Al-foam plate subjected to three-point bending. The curves of load-cross head

7

displacement for Al sheets almost overlap while those for Al-foam showed a certain scattered behavior due to the non-homogeneity of pore size and distribution. A decrease of around 60% in bending strength was observed for Al-6016 sheets after the thermal treatment similar to the soldering conditions adopted for AFS joining, while the mechanical behavior of Al-foams was observed unaffected. No prominent changes were observed in Al-sheet from the microstructural point of view after the thermal treatment and the decrease in bending strength is related to the over aging phenomenon. The base materials for AFS production show elastic behavior until load of 30 N (post heat treatment) and 40 N for Al-sheet and for Al-foam respectively and displacements of around 0.4 mm; beyond these loads, the specimens started to deform plastically but with a different behavior . The Al-sheet display strain hardening behavior at increased deformation until the test stopped. The Al-foam showed a remarkable strain hardening until 50 N and 1.5 mm of displacement, when it needed the maximum load required for deformation. From this point, an evident local plastic deformation in the foam at the load contact point was observed which resulted in the reduction of required bending strength to around 30 N at the displacement of 7 mm, followed by bulking effect. (Fig. 10,Fig. 11) Fig. 11 shows the behaviour of AFS components produced by using pure Zn foils and Zn+2%Al strips as soldering material, subjected to three-point bending. Both the curves, representative of the two categories, have an initial linear elastic behavior with values high enough to satisfy the structural integrity. This is then followed by a plateau region with around constant or slightly decreasing load at increased cross head displacement. This displacement reached around 8 mm for both sets, when the tests were immediately stopped because the specimen touched the machine fixture. The plateau region shows that the AFS components absorbs considerable workload for an extended strain, confirming the high-energy absorption property of the AFS, mainly due to the Alfoam.

8

The AFS components produced using Zn+2% Al as joining material showed slightly higher stiffness properties as compare to AFS components produced using pure Zn foil as soldering material. The trend of bending test curves for AFS components in Fig. 11 are similar to those reported in other papers [29–32]. The failure modes of the AFS were investigated during the three-point bending tests with a camera to record step-by-step deformation. The collapse of the AFS initiated with localized deformation of the skin in contact with the central head cylinder, followed by foam compression and shear displacement, until cracks appeared mainly in the middle of the core, sometimes producing a consequent face skin delamination or partial face skin delamination, Fig. 13. However, it was observed that the specimens with a relatively higher number of connecting points per area between the surface skins and foam did not exhibit face skin delamination. The cracks formed in the middle of the core were attributed to excessive core shear displacement (Fig. 12). The propagation of cracks and/or delamination is clearly related to non-uniform pores size and the distribution of porosity in the aluminum foam. (Fig. 12) (Fig. 13) The AFS component

collapse behavior involves localized indentation at loading point, core

compression, core shear and sometimes cracks in the Al-foam, along with face buckling (top skin) or face skin delamination (top or bottom skin) after a certain amount of deflection as also reported in [16,33,34]. The deformation response of the only Al-foam can generally be divided into three stages. The first stage involves the localized plastic straining at cell nodes; the second stage involves plastic buckling, elastically constrained by surrounding cells [35] and the third stage involves the consequent plastic collapse of cells with the increase of strain. Adopting different test parameters such as span length [36,37], cross-head velocity [38,39], core and skin thickness [40], to analyse the mechanical properties of AFS components, results in dissimilar values, which makes the comparison of results very difficult [16]. However, the joining technique and test parameters used in [40,41] are comparable to those used in the current study

9

and the maximum bending loads observed for the AFS produced are compared in Error! Reference source not found.Table 2.

4. Conclusion The Joining of Al skins to Al foam (AFS) was successfully achieved using Zn or Zn+2%Al soldering alloys in argon atmosphere. In the joining area, the presence of Al rich and Zn rich phases confirms the diffusion, ensuring strong metallic bonding. However, by controlling the diffusion of the soldering alloy in the Al skin can avoid the localised melting. AFS specimens produced using Zn+2%Al as the soldering material show higher stiffness in comparison to AFS components produced using pure Zn as the soldering material. The smaller the pore size (higher the density) of the Al-foam, the higher will be the number of joining points, so reducing AFS skin delamination. In this work, AFS composites were produced in a batch soldering furnace, but the joining process proposed, favoured by diffusion, can easily be automated in a continuous furnace, guaranteeing high productivity, reproducibility and cheap industrial costs.

Acknowledgements The financial support of Higher Education Commission (HEC) Pakistan in the form of PhD fellowship is greatly acknowledged. The role of Prof. Paolo Matteis, Dr. Valentina Casalegno and Antonio Favero for providing the mechanical characterisation facility and support is highly appreciated.

References [1]

Norbert I, Májlinger K. Materia ls and Design Description of the compressive response of metal matrix syntactic foams. Mater Des 2013;49:1–9. doi:10.1016/j.matdes.2013.02.007.

[2]

Rabiei A, Vendra LJ. A comparison of composite metal foams properties and other comparable metal foams. Mater Lett 2009;63:533–6. doi:10.1016/j.matlet.2008.11.002.

[3]

Dass M, Peroni M, Solomos G, Pada D, Matsagar VA, Gupta AK, et al. Dynamic compression behavior of cenosphere aluminum alloy syntactic foam. Mater Des

10

2014;42:418–23. doi:10.1016/j.matdes.2012.06.013. [4]

Dass M, Matsagar VA, Gupta AK. International Journal of Impact Engineering Blast resistance of stiffened sandwich panels with aluminum cenosphere syntactic foam. Int J Impact Eng 2015;77:134–46. doi:10.1016/j.ijimpeng.2014.11.017.

[5]

Garcia-avila M, Portanova M, Rabiei A. Ballistic performance of a composite metal foamceramic armor system. Procedia Mater Sci 2014;4:151–6. doi:10.1016/j.mspro.2014.07.571.

[6]

Huang Z, Qin Z, Chu F. Damping Mechanism of Elastic-Viscoelastic-Elastic Sandwich Structures. Compos Struct 2016. doi:10.1016/j.compstruct.2016.05.105.

[7]

Yu GC, Feng LJ, Wu LZ. Thermal and mechanical properties of a multifunctional composite square honeycomb sandwich structure. Mater Des 2016;102:238–46. doi:10.1016/j.matdes.2016.04.050.

[8]

Biancolini ME. Evaluation of equivalent stiffness properties of corrugated board. Compos Struct 2005;69:322–8. doi:10.1016/j.compstruct.2004.07.014.

[9]

Herrmann AS, Zahlen PC, Zuardy I. Sandwich Structures Technology in Commerical Aviation: Present Applications and Future Trends. Sandw Struct Adv with Sandw Struct Mater 2005;7:13–26. doi:10.1007/1-4020-3848-8_2.

[10]

Duarte I, Banhart J. A study of aluminium foam formation—kinetics and microstructure. Acta Mater 2000;48:2349–62. doi:10.1016/S1359-6454(00)00020-3.

[11]

Qingxian H, Sawei Q, Yuebo H. Development on Preparation Technology of Aluminum Foam Sandwich Panels. Rare Met Mater Eng 2015;44:548–52. doi:10.1016/S18755372(15)30036-9.

[12]

Velea MN, Schneider C, Lache S. Second order hierarchical sandwich structure made of self-reinforced polymers by means of a continuous folding process. Mater Des 2016;102:313–20. doi:10.1016/j.matdes.2016.04.049.

[13]

Yu B, Han B, Su PB, Ni CY, Zhang QC, Lu TJ. Graded square honeycomb as sandwich core for enhanced mechanical performance. Mater Des 2016;89:642–52.

11

doi:10.1016/j.matdes.2015.09.154. [14]

Sun Z, Shi S, Guo X, Hu X, Chen H. On compressive properties of composite sandwich structures with grid reinforced honeycomb core. Compos Part B Eng 2016;94:245–52. doi:10.1016/j.compositesb.2016.03.054.

[15]

Yokozeki T, Takeda S ichi, Ogasawara T, Ishikawa T. Mechanical properties of corrugated composites for candidate materials of flexible wing structures. Compos Part A Appl Sci Manuf 2006;37:1578–86. doi:10.1016/j.compositesa.2005.10.015.

[16]

Li Z, Zheng Z, Yu J, Qian C, Lu F. Deformation and failure mechanisms of sandwich beams under three-point bending at elevated temperatures. Compos Struct 2014;111:285–90. doi:10.1016/j.compstruct.2014.01.005.

[17]

Hexcel. Bonding technique. J Chem Inf Model 2013;53:1689–99. doi:10.1017/CBO9781107415324.004.

[18]

Pantelakis S, Tserpes KI. Adhesive bonding of composite aircraft structures: Challenges and recent developments. Sci China Physics, Mech Astron 2014;57:2–11. doi:10.1007/s11433-013-5274-3.

[19]

Huang Y, Gong J, Lv S, Leng J, Li Y. Fluxless soldering with surface abrasion for joining metal foams. Mater Sci Eng A 2012;552:283–7. doi:10.1016/j.msea.2012.05.041.

[20]

Campana G, Ascari A, Fortunato A. Laser foaming for joining aluminum foam cores inside a hollow profile. Opt Laser Technol 2013;48:331–6. doi:10.1016/j.optlastec.2012.11.005.

[21]

Matsumoto R, Tsuruoka H, Otsu M, Utsunomiya H. Fabrication of skin layer on aluminum foam surface by friction stir incremental forming and its mechanical properties. J Mater Process Technol 2015;218:23–31. doi:10.1016/j.jmatprotec.2014.11.030.

[22]

Nannan C, Yi F, Jie C, Bin L, Fanyan C, Jingsong Z. Vacuum Brazing Processes of Aluminum Foam. Rare Met Mater Eng 2013;42:1118–22. doi:10.1016/S18755372(13)60072-7.

[23]

Schwingel D, Seeliger HW, Vecchionacci C, Alwes D, Dittrich J. Aluminium foam sandwich

12

structures for space applications. Acta Astronaut 2007;61:326–30. doi:10.1016/j.actaastro.2007.01.022. [24]

David M. Jacobson GH. Principles of brazing, Ohio: ASM International; 2005, p. 207–16.

[25]

Zhu YH. General Rule of Phase Decomposition in Zn-Al Based Alloys (II)-On Effects of External Stresses on Phase Transformation-. Mater Trans 2004;45:3083–97. doi:10.2320/matertrans.45.3083.

[26]

Popović Stanko GB. Precipitation and Dissolution Phenomena in Al-Zn Alloys *. Croat Chem Acta 1999;72:621–43.

[27]

Finch N. Soft soldering alunlinum and its alloys. vol. 9. 1985.

[28]

Skoko Ž, Popović S, Štefanić G. Microstructure of Al-Zn and Zn-Al Alloys. Croat Chem Acta 2009;82:405–20.

[29]

Hipke T, Hohlfeld J, Rybandt S. Functionally Aluminum Foam Composites for Building Industry. Procedia Mater Sci 2014;4:127–32. doi:10.1016/j.mspro.2014.07.550.

[30]

Crupi V, Montanini R. Aluminium foam sandwiches collapse modes under static and dynamic three-point bending. Int J Impact Eng 2007;34:509–21. doi:10.1016/j.ijimpeng.2005.10.001.

[31]

Duarte I, Vesenjak M, Krstulovi Opara, Lovre I, Ferreira. Manufacturing and bending behaviour of in situ foam-filled aluminium alloy tubes. Mater Des 2015;66:532–44. doi:10.1016/j.matdes.2014.04.082.

[32]

Yan LL, Han B, Yu B, Chen CQ, Zhang QC, Lu TJ. Three-point bending of sandwich beams with aluminum foam-filled corrugated cores. Mater Des 2014;60:510–9. doi:10.1016/j.matdes.2014.04.014.

[33]

Bastawros A. Experimental analysis of deformation mechanisms in a closed-cell aluminum alloy foam. J Mech Phys Solids 2000;48:301–22. doi:10.1016/S0022-5096(99)00035-6.

[34]

Pagalank P, April S, Francisco S, Schwartz DS, Evans AG. Porous and Cellular Materials

13

for Structural Applications. vol. 521. 1998. [35]

Borovinšek M, Mathier V, Fiedler T. Mechanics of Materials Dynamic compression of aluminium foam derived from infiltration casting of salt dough 2016;93:96–108. doi:10.1016/j.mechmat.2015.10.012.

[36]

Styles M, Compston P, Kalyanasundaram S. The effect of core thickness on the flexural behaviour of aluminium foam sandwich structures. Compos Struct 2007;80:532–8. doi:10.1016/j.compstruct.2006.07.002.

[37]

Yu JL, Wang E, Li J, Zheng Z. Static and low-velocity impact behavior of sandwich beams with closed-cell aluminum-foam core in three-point bending. Int J Impact Eng 2008;35:885– 94. doi:10.1016/j.ijimpeng.2008.01.006.

[38]

Rabiei A, Garcia-avila M. Materials Science & Engineering A Effect of various parameters on properties of composite steel foams under variety of loading rates. Mater Sci Eng A 2013;564:539–47. doi:10.1016/j.msea.2012.11.108.

[39]

Duarte I, Teixeira-Dias F, Graça A, Ferreira AJM. Failure Modes and Influence of the Quasistatic Deformation Rate on the Mechanical Behavior of Sandwich Panels with Aluminum Foam Cores. Mech Adv Mater Struct 2010;17:335–42. doi:10.1080/15376494.2010.488563.

[40]

Kabir K, Vodenitcharova T, Hoffman M. Response of aluminium foam-cored sandwich panels to bending load. Compos Part B Eng 2014;64:24–32. doi:10.1016/j.compositesb.2014.04.003.

[41]

Wan L, Huang Y, Lv S, Feng J. Fabrication and interfacial characterization of aluminum foam sandwich via fluxless soldering with surface abrasion. Compos Struct 2015;123:366– 73. doi:10.1016/j.compstruct.2014.12.055.

14

List of figures: Fig. 1. Surface appearance of the Al-foam plate. Fig. 2. AFS components stacking configuration. Fig. 3. Three-Point bending test assembly. Fig. 4. Macrograph of polished AFS cross-section a) Brazed using Pure Zn foil, b) brazed using Zn+2%Al joining material. Fig. 5. Dendrites and porosities observed in etched Al-foam cell wall. Fig. 6. After etching, dendrites appeared at Al-sheet/Al-foam joint. Fig. 7. Voids appeared in Al-sheet/Al-foam joint formed during solidification of the joint. Fig. 8. a) Macro, b) and c) higher magnification images of AFS component brazed with pure Zn foil as soldering material. Fig. 9. a) Macro, b) and c) higher magnification micrographs of AFS component brazed with Zn+2% Al soldering material. Fig. 10. Three-Point bending test results for base materials (Al-6016 skin and Al-foam core). Fig. 11. Three-Point bending test representative results for both of the two AFS components families. Fig. 12. Cross section of AFS component brazed using pure Zn foil as joining material a) before and b) after mechanical test. Fig. 13. Cross section of AFS component brazed using Zn+2%Al as joining material a) before, b) after mechanical test.

15

Tables: Table 1 EDS analyses results effected on the defined points. Fig. 8c

Composition (wt.%)

Fig. 9c

Composition (wt.%)

EDS 1

23%Al - 77%Zn

EDS 1

22%Al - 78%Zn

EDS 2

60%Al - 40%Zn

EDS 2

62%Al - 38%Zn

16

Table 2 Results of three-point bending test (Current work) and its comparison to other studies. References

Skin Material

Current Work

AL-Foam

-

Specimen Dimensions, 3 [mm ] * 60x20x9

Current Work

Al-6016 (Pre-heat treatment)

-

60x20x1.2

70±3

Current Work

Al-6016 (Post heat treatment)

-

60x 20x1.2

30±3

Current Work

Al-6016

Al-foam

Pure Zn foils

60x20x11.4

Soldering

785±50

Current Work

Al-6016

Al-foam

Zn2Al alloy strips

60x20x11.4

Soldering

904±60

[40]

Al 1100-0

Al-foam

Epoxy

150x35x12

Adhesive

450

Al 3104H19

Al-foam

Epoxy

150x35x12

Adhesive

650

Al-5056

Al-foam

Zn6.2Al4.3Cu1.2Mg0.8Mn0.5A g alloy

60x15x17.4

Soldering (without vibration)

930

[41]

Core Material

Joining Material

*Specimen Dimension: Length(l) x Width(b) x Thickness (t).

17

Joining Method

Bending Strength, [N] 40±10

Figures: Fig. 1. Surface appearance of the Al-foam plate.

18

Fig. 2. AFS components stacking configuration.

19

Fig. 3. Three-Point bending test assembly.

20

Fig. 4. Macrograph of polished AFS cross-section a) Brazed using Pure Zn foil, b) brazed using Zn+2%Al joining material.

21

Fig. 5. Dendrites and porosities observed in etched Al-foam cell wall.

22

Fig. 6. After etching, dendrites appeared at Al-sheet/Al-foam joint.

23

Fig. 7. Voids appeared in Al-sheet/Al-foam joint formed during solidification of the joint.

24

Fig. 8. a) Macro, b) and c) higher magnification images of AFS component brazed with pure Zn foil as soldering material.

25

Fig. 9. a) Macro, b) and c) higher magnification micrographs of AFS component brazed with Zn+2% Al soldering material.

26

Fig. 10. Three-Point bending test results for base materials (Al-6016 skin and Al-foam core).

27

Fig. 11. Three-Point bending test representative results for both of the two AFS components families.

28

Fig. 12. Cross section of AFS component brazed using pure Zn foil as joining material a) before and b) after mechanical test.

29

Fig. 13. Cross section of AFS component brazed using Zn+2%Al as joining material a) before, b) after mechanical test.

30