Journal of Alloys and Compounds 640 (2015) 1–7

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Novel method of fluxless soldering with self-abrasion for fabricatingaluminum foam sandwich

http://dx.doi.org/10.1016/j.jallcom.2015.04.0050925-8388/Crown Copyright � 2015 Published by Elsevier B.V. All rights reserved.

⇑ Corresponding author.

Long Wan, Yongxian Huang ⇑, Tifang Huang, Shixiong Lv, Jicai FengState Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People’s Republic of China

a r t i c l e i n f o

Article history:Received 6 May 2014Received in revised form 23 March 2015Accepted 2 April 2015Available online 8 April 2015

Keywords:Aluminum foam sandwichFluxless solderingSelf-abrasionInterfacial characterizationThree-point bending

a b s t r a c t

The aluminum foam sandwich (AFS) with metallic bonding was fabricated by means of fluxless solderingwith self-abrasion. The effects of abrasion forms on interfacial feature, the deformation and failurebehavior of AFS are compared through microstructural observation and static three-point bending tests.The good wetting process was realized assisting with the abrasion. The self-abrasion of aluminum foamproduced mechanical stirring to make the microstructure more uniform and reduce the defects of thesoldering seam. The load–deflection curve of AFS with metallic bonding exhibited three distinct regions.Three failure modes with indentation, core shear and plastic hinge were identified during the bendingtests. An excellent bonding has been informed between aluminum alloy face sheets and aluminum foamcore due to the retained cohesion between them. Assisting with self-abrasion, fluxless soldering using azinc-based alloy has been proven to be suitable for fabricating AFS.

Crown Copyright � 2015 Published by Elsevier B.V. All rights reserved.

1. Introduction

Aluminum foam is a type of metallic foams which have manyattractive properties because of their light weight and cellstructure. Recently, aluminum foams have received considerableattention because of their great potential for a number of structuraland functional applications [1–7]. Aluminum foams as structuralapplications are normally used in combination with conventionaldense structures such as sheets, columns and the like, to obtainthe optimized mechanical properties in a given loading situation[8]. Sandwich constructions comprising light metallic cores andthin stiff face sheets have found applications in various fields, suchas the automotive industry, aerospace and civil engineering [9].Aluminum foam sandwich (AFS) is a typical example. The thin stiffface-sheets provide AFS with bending and stretching capacity,while the aluminum foam core offers the ability to undergo largedeformation at nearly constant stress [10–16].

A straightforward way to fabricate the AFS is adhesive bondingwhich bonds the aluminum foam core to the dense metallic sheets[17]. It is considered that the AFS fabricated using an adhesive can-not be used under high temperature conditions and has limitedusage owing to the difficulty in recycling and considerable environ-mental concerns [17]. To overcome these limitations, metallurgicalbonding between aluminum foam and dense sheets is performed. A

preferable way tends to roll-clad face sheets to a sheet of foamableprecursor material [18–20]. However, this technique is limitedwhen it comes to obtain large and complicated structures. A varietyof welding methods are also used to fabricate the AFS. Born et al.[21] investigated the effect of ultrasonic torsion welding onmanufacturing AFS. This method is apt to cause the deformationof aluminum foams, particularly when the porosity of aluminumfoam is high, and only superplastic metal sheets can be joined tothe aluminum foam core. Other pressure welding methods includ-ing diffusion bonding encounters the same problems as that of theultrasonic torsion welding [21,22]. Recently, fluxless soldering withsurface abrasion assisted by vibrations has been developed by Wanet al. for producing AFS [23]. The average bending strength canreach 571% of that of aluminum foam when AFS is made withassisted ultrasonic vibration. Fluxless soldering with surface abra-sion assisted by vibrations has been proven to be suitable forfabricating AFS [23]. The friction stir processing (FSP) has also beendeveloped to fabricate aluminum foam/dense metallic sheet com-posites by mixing the blowing agent powder into aluminum sheetsand the metallurgical bonding between the foamable precursor anddense metallic sheet can be conducted [24,25]. AFS can also beattained by means of fusion welding. Large amounts of fillermaterials have to be used in case for the collapse of the cellularstructures during fusion welding, however, this will increase theconsiderable weight of AFS structure. Moreover, fusion welding islikely to cause defects such as gas pores, cracks and other defects,due to the dissimilar properties between aluminum foams core

2 L. Wan et al. / Journal of Alloys and Compounds 640 (2015) 1–7

and sheets [26,27]. Consequently, it is necessary to develop moreavailable ways for the manufacture of AFS.

In this paper, a novel method of fluxless soldering with self-abrasion has been developed for fabricating AFS. The microstruc-tural observation was conducted to investigate the soldering seam.The deformation and failure behavior of AFS were analyzed bythree-point bending test. Effects of surface abrasion modes onmechanical properties and interface feature were discussed.

Fig. 3. Macroscopic overview of the AFS fabricated by fluxless soldering with self-abrasion and microscopic overview of typical soldering seam.

2. Experimental details

Fig. 1 shows a schematic illustration of the process of manufacturing the AFS byfluxless soldering with self-abrasion. Prior to soldering, the bonding faces of the Alplates were abraded using SiC abrasive paper up to 400-grit to remove the oxidefilm. The cell edges of the aluminum foam were uneven, and the specific solderingprocess was designed utilizing such kind of property. The detail steps and relatedparameters are summarized as follow. Before soldering, the bonding faces of alu-minum foam were not abraded, then manual oxygen–propane torch solderingwas performed in air condition, and the heat treatment temperature was approxi-mately 420 �C. Once the solder alloy was melted, it was homogeneously coated onthe bonding faces of the aluminum foam and of the Al plate while exposed to atmo-sphere, as shown in Fig. 1a. The aluminum foam was hold by the fixture which wasmovable to make it back and forth at the average speed of 10 mm/s on the Al platewhich was fixed on the fixed fixture under the pressure of 2–3 kPa for 2 min (that isself-abrasion of aluminum foam), as sketched in Fig. 1b. The length of the back andforth was about 30 mm. As shown in Fig. 1c, the movement was stopped and thebutt soldering was performed on aluminum foam and Al plate, keeping heatingwith the pressure of 2–3 kPa for 2 min. After cooling a soldering seam was formed.The 1.0-mm-thick 5056 aluminum alloy plate was used as face material withdimensions of 60 mm � 60 mm. The closed-cell aluminum foam is manufacturedby means of powder metallurgical process. The process starts from a powder mix-ture of aluminum alloy and blowing agent of TiH2 powder. Then, the compactedmaterial is heat treated near melting point of aluminum alloy. The TiH2 particlesdecompose to release hydrogen gas at high temperature and the gas is used to pro-duce a foamed aluminum ingot. The selected foam block is cut to dimensions of60 mm � 60 mm � 20 mm by electric discharge machining (EDM), and the plateletsare then rinsed in acetone. The chemical composition of the foam wasAl–1.2 wt.%Ca–1.1 wt.%Ti–0.3 wt.%Zn, and the mean cell size was approximately2–3 mm in sheets 20 mm thick. The density variation was 0.22–0.35 g/cm3 and

Fig. 1. Schematic illustration of the process of manufacturing the AFS by fluxless solderin– movable fixture, 5 – fixed fixture.

Fig. 2. Theoretical and actual model for the three bending tests. (a) Schematic of AFS undtest under three-point bending.

the porosity was approximately 90%. To form a possible joint between a foamand a dense part, a solder alloy of 6.2 wt.% Al, 4.3 wt.% Cu, 1.2 wt.% Mg, 0.8 wt.%Mn, 0.5 wt.% Ag and balance Zn was chosen. Its melting point was between396 �C and 405 �C.

In this study, the aluminum foam specimens used to be joined was classifiedto three groups. The specimens of the first group were abraded with SiC abrasivepaper up to 400-grit before soldering. The method of self-abrasion soldering wasapplied to the second group. Neither the abrasion prior to soldering nor the self-abrasion was applied to the specimens of the third group. Then the butt solderingwas directly performed for 4 min (the same time as that of heating of joining thesecond group to Al plates) for the first and the third group. Except for that, othersteps and parameters were the same for the three groups. For convenience, thejoints whose aluminum foam was abraded by SiC abrasive paper are donated ‘case

g with self-abrasion. 1 – Aluminum foam, 2 – molten solder alloy, 3 – 5056 Al plate, 4

er three-point bending showing geometrical and material parameters and (b) actual

Fig. 4. SEM cross-sectional images of AFS in the case 1.

L. Wan et al. / Journal of Alloys and Compounds 640 (2015) 1–7 3

1’ and the joints obtained by self-abrasion assistant soldering are denoted ‘case 2’.The joints whose core materials were got from the third group are denoted ‘case3’.

The microstructure and chemical composition of the joints were analyzedusing a scanning electron microscope (SEM, HITACHI S-3500N) equipped withan energy dispersive spectrometer (EDS). The joints were sectioned across theinterface zone for microstructural characterization. To better analyze themechanical properties of the joints, the static three-point bending tests wereperformed using a universal mechanic testing machine (Instron UTM, model5569) at room temperature. A sketch of sandwich beam loaded in three-pointbending was shown in Fig. 2. The values of L, t, c and b are 50, 1, 20 and20 mm, respectively. For these tests, the density of the foams was not deliberatelyselected to avoid variations.

Fig. 5. The line-scanning analysis of the chemical composition of the AFS for case 1. (a) Thplate, (c) line-scanning analysis of the interface of the solder alloy/aluminum foam and

3. Results and discussion

3.1. Macro and microstructures of joints

Fig. 3 shows the macroscopic overview of the AFS fabricated byfluxless soldering with self-abrasion and microscopic overview oftypical soldering seam. During the self-abrasion soldering process,the pressure put on the aluminum foam was about 2–3 kPa, whichwas lower than the compressive strength of aluminum foam. Onthe other hand, the cell edges of the aluminum foam were unevenso that the friction coefficient and horizontal friction force betweenaluminum foam and the 5056 Al plate was high, resulting in themechanical scratching effect. No visible macroscopic deformationwas observed, and the collapse of foam aluminum structure wasnot existed. The effect of the filler metal on increasing the weightof the AFS was minor, and the original excellent properties of thealuminum foam, which significantly determined by their porosityand morphometric parameters, were persevered. The visual obser-vation showed no macroscopic cracks in the soldering seams. Sincethe wetting and spreading were the decisive prerequisite for form-ing a continuous seam, such macroscopic overview suggested thatgood wetting and spreading behaviors occurred during the solder-ing process.

Fig. 4 is the SEM cross-sectional images of case 1. The dis-tribution of the microstructure was no-uniform. There seemed tobe two lines which divided the soldering seam to three parts.The parts of solder alloy/Al plate and aluminum foam/solder alloywere mainly consisted of Al–Zn solution phase while the other partwas Zn–Al solutions, which can be seen in Fig. 5. Additionally,micro-cracks and gas pores were concentrated along the aluminum

e interface of the solder alloy/aluminum foam, (b) the interface of the solder alloy/Al(d) line-scanning analysis of the interface of the solder alloy/Al plate.

4 L. Wan et al. / Journal of Alloys and Compounds 640 (2015) 1–7

foam/solder alloy interface. These results can be explained by twomain reasons. First, the contact between the solder alloy and thealuminum foam was equal to the contact between face and point,while the solder alloy and the Al plate equal to the contact betweenface and face. It directly leaded to the different amount of Al atomsdissolved into the two sides of solder alloy/parent metals when thesolder alloy was coated on the parent metals. Once the butt solder-ing was performed to fabricate the AFS structure, the liquid phasecould not sufficiently mix and the chemical composition was notuniform. Then different solidification formed in the three partsrespectively. The difference of heat input (which was due tothe different thermal conductivity of dissimilar metals) alsocontributed to the formation of different solidification structures.As for the existence of gas pores and micro-cracks, the differentcoefficient of linear expansion of the dissimilar metals made thehigh residual tensile stress exist particularly within the center ofthe soldering seam.

Four types of phases were identified according to theirmicrostructural morphology and chemical composition. Each typeof phases was marked with A, B, C and D successively, as sketchedshown in Fig. 5. The EDS analysis revealed that the dark gray andlight gray phases were Al–Zn solid solution (74.61 wt.% Al,22.52 wt.% Zn and 60.24 wt.% Al, 39.23 wt.% Zn) and the whiteand little white phases were Zn–Al solid solution (79.27 wt.% Zn,2.69 wt.% Al and 64.62 wt.% Zn, 30.77 wt.% Al), respectively, asshown in Fig. 6. The line-scanning analysis was used to examinethe variation in element composition across the interfaces, asshown in Fig. 5c and d, respectively. According to the results ofthe line-scanning analysis, the Al and Zn elements were distributed

Fig. 6. The EDS analysis of phases existed in AFS in the case

continuously throughout the cross-sectioned joint, and the inter-diffusion contributed to the joining at the atomic scale to improvethe strength of the joints. Such interaction between the solder alloyand the parent metals directly confirmed the good wetting andspreading behaviors. Part of the oxide film of the parent metals,which was a block for wetting and spreading, was removed dueto the abrasion by SiC abrasive paper, and thus the parent metalswere wetted by molten solder alloy. Moreover, the melting pointwould decrease when the interdiffusion occurred and when theconcentration of the Al and Zn elements changed. The decreasedmelting point made the liquid phase form under the oxide film,the process that significantly helped to break the oxide film. Asound joining therefore formed between the solder alloy and theparent metals.

Fig. 7 shows the SEM cross-sectional images of AFS for the case2 taken from Fig. 3a and b. The microstructure distributed uni-formly and no defects were found within the whole soldering seamcompared with that of case 1. The different phenomenon to case 1is attributed to the multi-effect of the self-abrasion which occurredas the aluminum foam moved on the Al plate under pressure. Thefunction of the relative movement between the two joined parentmetals, resulting in horizontal friction force between aluminumfoam and the 5056 Al plate, was equal to stir the liquid phase.Such mechanical stirring led to the refinement of microstructure.Forced convection arose from the mechanical stirring, producingfragmentation of the dendritic front and subsequent transport ofthe fragments [28–30]. The fragments offered more opportunitiesfor nucleation [28]. Fluid flow in the liquid also increased the rateof heat transfer and the removal of liquid superheat, thus reducing

1. (a) Phase A, (b) phase B, (c) phase C and (d) phase D.

Fig. 7. SEM cross-sectional images of AFS for case 2. (a) The interface of the solder alloy/aluminum foam and (b) the interface of the solder alloy/Al plate.

L. Wan et al. / Journal of Alloys and Compounds 640 (2015) 1–7 5

the temperature gradient in the liquid ahead of the solidificationfront and decreasing the likelihood of the remelting of the solidfragments. Overall, the increasing nucleation opportunities, andthe enhanced speed of solidification due to the decreasing tem-perature gradient, had co-effect on promoting the refinement ofmicrostructure. Since the refinement of microstructure increasedthe grain boundaries and thus enhanced the resistance to cracks,the micro-cracks were rarely found within the soldering seam.Additionally, the mechanical stirring facilitated the liquid flow tomix sufficiently, homogenizing the chemical composition. Themacro and micro segregation was reduced and the uniformmicrostructure was formed. The mechanical stirring also con-tributed to the exclusion of the gas in the liquid [28]. No gas poreswere found in the soldering seam of case 2.

Despite of the different features, the same phases detected andthe interdiffusion between the molten solder alloy and the parentmetals as that of case 1, suggested that the self-abrasion of alu-minum foam effectively removed the oxide film in case 2. To fur-ther decide whether the self-abrasion of aluminum foamcontributed to wetting and spreading, the microstructure of case3 was investigated. Fig. 8 is the metallographic images of the inter-face of the solder alloy/aluminum foam of the case 3. The obviouscracks along the boundary of the parent metal reduced the joiningstrength of the solder alloy/parent metal. The indications of theinterdiffussion were not found. The self-abrasion of aluminumfoam did contribute to the wetting and spreading process of themolten solder alloy according to the comparison between case 2and 3.

Fig. 8. The metallographic images of the interface of the solder alloy/aluminumfoam of case 3. The inset is the enlarged interface.

3.2. The static three-point bending tests

Fig. 9 shows the typical load–deflection curves of comparisonsof cases 1, 2 and 3 under three-point bending (support span of50 mm). The inset is the typical load–deflection curve of aluminumfoam. The failure modes were found to be similar to foam’s modesunder uniaxial compressive loading [31–33]. Three distinct regionsare existed in the load–deflection curve of AFS. The short initial lin-ear-elastic behavior occurred; increasing the load on the foamcaused to buckling of cell edges in weak regions of the foam; adeformation band perpendicular to the loading direction develops,in which plastic collapse of the cells take place; this was accom-pany by the beginning of the plateau region in the stress–straincurve; with increasing strain, additional deformation bands areformed until most of the cells have collapsed and the densificationregion was reached. Long plateau of load for around 15 mm wasobserved, which represents good energy absorption capacity ofthese aluminum foam sandwich. The maximum yield stress ofthe aluminum foam is obviously lower than that of AFS fabricatedin case 1, 2 and 3. The face sheet can provide a significant flexuralstrength for the whole AFS. In addition, the metallic bonding inter-face between 5056 Al plate and aluminum foam improves thestructure stiff and absorbing energy capability. The force of plateauregion in case 3 is the lowest because of the relatively poor joiningquality, as shown in Fig. 8. However, one exception that the force ofplateau region in case 2 is lower than in case 1 also happened. Thismay be due to the whole quality of the base aluminum foam is notuniform.

Fig. 9. Typical load–deflection curves of the three cases under three-point bending.The inset is the typical load–deflection curve of aluminum foam.

Fig. 10. Failure modes of the AFS specimen with metallic bonding for three cases. (a) AFS specimen in case 2 before testing, (b) AFS specimen in case 2 after testing and (c) AFSspecimen in case 1 after testing, and AFS specimen in case 3 after testing.

6 L. Wan et al. / Journal of Alloys and Compounds 640 (2015) 1–7

Fig. 10a and b shows the typical deformation and failure modesfor AFS samples in case 2 under static three point bending. A mixedmode composed of three failure modes, including indentation (ID),core shear (CS) and plastic hinge (PH), can be observed for the pre-sent AFS with metallic bonding on interface. The most of cells onboth sides of the specimen undergo slight or no deformation, suchas cell A. It can be concluded that tension stress is introduced in thecells nearly perpendicular to the loading direction and causedcracks on the stretched cell face, such as cell B and E. As the strainincreases, significant plastic deformation for most cells adjacent tothe indentation is also observed, such as cell C, D and F. With theincrease of the strain, additional deformation bands were formeduntil most of the cells have collapsed, and the densification regionwas reached. Fig. 10c shows the typical deformation and failuremodes for AFS samples in case 1 under static three point bending.The above three failure modes (ID + CS + PH) can also be observeddue to the metallic bonding on interface. Fig. 10d shows the speci-men in case 3 after the three-point bending test performed as theabove method. The crack and core shear of aluminum foams arethe main failure modes. The whole sandwich structure failed oncethe soldering seam yielded. The cracks occurred in the aluminumfoam and expanded to the whole sandwich structure. The deforma-tion process absorbs a small amount of energy, indicating that theinterfacial structure in case 3 is instability and inefficient, whichcan also be seen in Fig. 8.

In fact, sandwich structure consisted of aluminum foam coreand aluminum face sheets, can fail in five modes: face yielding, facewrinkling, core shear, core indentation and delamination.However, the delamination of aluminum foam core and aluminumalloy face sheets has not been detected. The cracks only occurred inthe aluminum foam and their expansion was limited due to thecomplex cellular structure of aluminum foam. Moreover, the bot-tom face sheet show little indentation and plastic hinge exceptbending. The delamination is the main failure mode of AFS madewith adhesive bonding and the whole deformation process absorbsonly a small amount of energy, indicating that the adhesive struc-ture is instability and inefficient [34]. Delamination can be avoidedwhen having the perfect bonding between face sheets and core[35,36]. As a comparison, the delamination of aluminum foam coreand 5056 aluminum alloy plates did not occur during the bendingtests. These facts suggest that the sound joining was achieved incase 2. The present AFS with metallic bonding is stronger than thatwith adhesive bonding being commercial products. The inter-diffusion and the solid solutions formed between the solder alloyand parent metal, significantly enhanced the joining strength.The sound metallurgical joining was achieved.

4. Conclusions

Novel method of fluxless soldering with self-abrasion wasdeveloped for fabricating AFS. On the basis of present investigation,the following conclusions are reached:

(1) Excellent metallic bonding has been informed between alu-minum foam and the face sheets due to the good wettingand spreading process by means of fluxless soldering withself-abrasion. The mechanical stirring arose from self-abra-sion had multi-positive effect to make the microstructureuniform, reducing the defects of the soldering seam. Theself-abrasion could effectively contribute to remove theoxide film and to facilitate the wetting and spreading ofthe molten filler metal.

(2) The load–deflection curve of AFS exhibited three distinctregions: a linear elastic region, a plateau region and a densi-fication region. Three failure modes including indentation,core shear and plastic hinge were observed simultaneouslyduring the bending tests. The delamination of aluminumfoam core and 5056 aluminum alloy plates did not occurin the bending tests.

(3) Fluxless soldering with self-abrasion using a zinc-basedalloy has been proven to be suitable for fabricating AFS.

Acknowledgments

The work was jointly supported by the National Natural ScienceFoundation of China (No. 50904020) and the FundamentalResearch Funds for the Central Universities (No. HIT. NSRIF.2012007).

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