The Effect of Interlayer Materials on the Joint Properties of Diffusion-Bonded Aluminium and Magnesium

Article The Effect of Interlayer Materials on the Joint Properties of Diffusion-Bonded Aluminium and Magnesium

Stefan Habisch 1,*, Marcus Böhme 2 ID , Siegfried Peter 3, Thomas Grund 2 and Peter Mayr 1 ID

1 Institute of Joining and Assembly, Chemnitz University of Technology, D-09107 Chemnitz, Germany; [email protected] 2 Institute of Materials Science and Engineering, Chemnitz University of Technology, D-09107 Chemnitz, Germany; [email protected] (M.B.); [email protected] (T.G.) 3 Institute of Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-371-53132336

Received: 15 December 2017; Accepted: 12 February 2018; Published: 17 February 2018

Abstract: Diffusion bonding is a well-known technology for a wide range of advanced joining applications, due to the possibility of bonding different materials within a defined temperature-time-contact pressure regime in solid state. For this study, aluminium alloys AA 6060, AA 6082, AA 7020, AA 7075 and magnesium alloy AZ 31 B are used to produce dissimilar metal joints. Titanium and silver were investigated as interlayer materials. SEM and EDXS-analysis, micro-hardness measurements and tensile testing were carried out to examine the influence of the interlayers on the diffusion zone microstructures and to characterize the joint properties. The results showed that the highest joint strength of 48 N/mm2 was reached using an aluminium alloy of the 6000 series with a titanium interlayer. For both interlayer materials, intermetallic Al-Mg compounds were still formed, but the width and the level of hardness across the diffusion zone was significantly reduced compared to Al-Mg joints without interlayer.

Keywords: diffusion bonding; aluminum; magnesium; interlayer; titanium; silver; PVD-coating; microstructure; tensile testing; fracture surface analysis

1. Introduction Diffusion bonding is a widely used technology for creating similar and dissimilar joints from challenging materials. These materials are usually joined at elevated temperatures, between 0.5 and 0.8 of the absolute melting point, using a defined contact pressure with a joining time ranging from a few minutes to a few hours. However, the joining process and weldability are highly influenced by the tendency to form oxide layers, e.g., for aluminium and magnesium [1,2]. To improve the limited weldability of aluminium and magnesium, surface treatment is necessary to remove the stable and tenacious oxide layer and limit the formation of brittle intermetallic Al-Mg compounds [3,4]. Diffusion bonding of aluminium and magnesium with interlayer materials has been extensively investigated to improve the joint properties and promote atomic diffusion processes across the bond line [1,3–10]. For this purpose, a large group of interlayer metals is available to influence the properties of the diffusion zone. Nickel with a thickness of 6 µm was used as interlayer for diffusion bonding of pure aluminium and pure magnesium. After mechanical surface treatment by grinding, both materials were bonded at 440 ◦C, 1 N/mm2 contact pressure and 60 min bonding time. The results showed that the formation of the intermetallic Al-Mg compound can be avoided, but at the interfaces of both aluminium/nickel

Metals 2018, 8, 138; doi:10.3390/met8020138 www.mdpi.com/journal/metals Metals 2018, 8, 138 2 of 12 and nickel/magnesium, brittle intermetallic compounds were formed. Therefore, a relatively low joint strength of 26 N/mm2 was achieved [5]. Good results were achieved for a diffusion bond between AA 6061 and AZ 31 B with zinc or a zinc-aluminium interlayer with a thickness of 35 µm. A shear strength of 86 N/mm2 was reached, due to the eutectic formation of Al-Zn compounds along the bond line. No formation of intermetallic Al-Mg compounds was observed, but intermetallic Mg-Zn compounds still limited the joint strength [6,7]. Silver as interlayer material was examined for diffusion bonds of pure aluminium to pure magnesium. The maximum joint strength of 14.5 N/mm2 was reached at 390 ◦C bonding temperature, 30 min holding time and 5 N/mm2 contact pressure with a silver layer thickness of 25 µm. The silver diffused completely into both base materials, and brittle intermetallic compounds of Al-Mg and Mg-Ag were formed. Furthermore, the hardness measurement across the diffusion zone revealed a hardness peak on the magnesium side of the diffusion zone, which consisted of different Mg-Ag phases and weakened the joint [8,9]. The aim of this investigation was to systematically study the effect of titanium and silver as interlayer materials on microstructure and mechanical properties for different Al-Mg diffusion bonds.

2. Experimental Section

2.1. Materials, Equipment and Process Parameters For this study, the aluminium alloys AA 6060, AA 6082, AA 7020 and AA 7075 and the magnesium alloy AZ 31 B were investigated. The chemical composition of the materials is summarized in Table1. All aluminium alloys were used in the T6 heat-treated condition except for AA 6060, which was used in condition T66.

Table 1. Chemical composition of the base materials according to manufacturer’s speciﬁcations [11,12].

Chemical Composition in wt % Materials Al Mg Zn Si Cu Mn Fe Ti Cr AA 6060 bal. 0.35–0.6 0.15 0.3–0.6 0.1 0.1 0.1–0.3 0.1 0.05 AA 6082 bal. 0.6–1.2 0.2 0.7–1.3 0.1 0.4–1.0 0.5 0.1 0.25 AA 7020 bal. 1.0–1.4 4.0–5.0 0.35 0.2 0.05–0.5 0.4 - 0.1–0.35 AA 7075 bal. 2.1–2.9 5.1–6.1 0.4 1.2–2.0 0.3 0.5 0.2 0.18–0.28 AZ 31 B 2.8–3.0 bal. 1.0 ------

Cylindrical specimens 20 mm in diameter and 15 mm in length for the aluminium alloys and 10 mm in length for the magnesium alloy were machined. Aluminium samples were produced by turning. The magnesium samples were cut out of sheet metal using a water jet. The joining surfaces were therefore equivalent to the rolled surfaces of the sheets. All surfaces were characterized by tactile surface measurement (Hommel Etamic T8000, Jenoptic, Jena, Germany). An area of 1.5 mm by 1.5 mm was measured with a tip radius of 5 µm and 90◦ angle (measuring sensor: Hommel Etamic TKL 100, Jenoptic, Jena, Germany) and a measuring speed of 0.15 mm/s. Joining was carried out using a PVA TePla COV 323 HP diffusion bonding machine (PVA TePla AG, Wettenberg, Germany) directly coupled to a glovebox. Details of the setup can be seen in Figure1 and in [13]. Bonding temperatures of max. 1100 ◦C and loads of max. 100 N/mm2 for an area of 300 mm by 300 mm. Diffusion bonding experiments were carried out under argon with a pressure of 700 hPa. A bonding temperature of 415 ◦C, bonding time of 60 min and a contact pressure of 8.5 N/mm2 was used, based on preliminary experiments on aluminium and magnesium diffusion bonding. All bonding parameters, such as temperature, time and pressure, were automatically controlled throughout the whole bonding process. For each conﬁguration, six specimens were bonded. One specimen was Metals 2018, 8, 138 3 of 12

Metals 2018, 8, 138 3 of 11 subjectedMetals 2018, 8 to, 138 a detailed metallographic investigation and micro hardness measurements and3 of ﬁve 11 specimensmeasurements were and used five for specimens tensile testing. were used The for bonded tensile samples testing. wereThe bonded machined samples to top were hat machined samples measurements and five specimens were used for tensile testing. The bonded samples were machined (Figureto top 2hata), samples in accordance (Figure with 2a), in the accordance ram tensile with test the [ ram14]. tensile The tensile test [14]. strength The tensile was determinedstrength was to top hat samples (Figure 2a), in accordance with the ram tensile test [14]. The tensile strength was usingdetermined a universal using test a universal machine test and machine a special and punch-die-equipment a special punch-die-equipment (Figure2b). (Figure Fracture 2b). surfaces Fracture determined using a universal test machine and a special punch-die-equipment (Figure 2b). Fracture ofsurfaces tensile specimensof tensile andspecimens the microstructural and the microstructu characteristicsral characteristics of the diffusion of zone the were diffusion examined zone using were surfaces of tensile specimens and the microstructural characteristics of the diffusion zone were aexamined SEM (Zeiss using LEO1455VP, a SEM (Zeiss Carl LEO1455VP, Zeiss AG, Oberkochen, Carl Zeiss AG, Germany) Oberkochen, and EDXS Germany) analysis and (EDAX EDXS Genesis, analysis examined using a SEM (Zeiss LEO1455VP, Carl Zeiss AG, Oberkochen, Germany) and EDXS analysis AMETEK,(EDAX Genesis, Inc., Berwyn, AMETEK, PA, USA).Inc., Berwyn, PA, USA). (EDAX Genesis, AMETEK, Inc., Berwyn, PA, USA).

Figure 1. Sectional view of the technology for in-line surface treatment and diffusion bonding. Figure 1. Sectional view of the technology for in-lin in-linee surface treatment andand diffusion bonding.

(a) (b) (a) (b) Figure 2. Tensile testing of diffusion bonded joints (a) characteristic geometrical dimensions of a top Figure 2. Tensile testing of diffusion bonded joints (a) characteristic geometrical dimensions of a top hat sampleTensile and testing(b) schematic of diffusion testing bonded setup. joints ( ) characteristic geometrical dimensions of a top hat sample and (b) schematic testing setup.setup. 2.2. Surface Preparation 2.2. Surface Preparation All aluminium samples were machined to a pre-defined surface roughness. Subsequently, the All aluminium samples were machined to a pre-defined surface roughness. Subsequently, the joiningAll surfaces aluminium were samples cleaned wereusingmachined an electron to beam a pre-deﬁned and immediately surface coated roughness. with titanium Subsequently, or silver joining surfaces were cleaned using an electron beam and immediately coated with titanium or silver theusing joining physical surfaces vapor were deposition cleaned (PVD). using anThe electron layer thickness beam and for immediately all layers was coated 2 µm. with Therefore, titanium the using physical vapor deposition (PVD). The layer thickness for all layers was 2 µm. Therefore, the ornew silver formation using physical of oxide vapor layers deposition on the (PVD). joining The surfaces layer thickness was inhibited. for all layers Titanium was 2 µwasm. Therefore,chosen as new formation of oxide layers on the joining surfaces was inhibited. Titanium was chosen as theinterlayer new formation because ofthe oxide interlayer layers will on form the joininga diffusion surfaces barrier was for inhibited. the intermetallic Titanium Al-Mg was compounds chosen as interlayer because the interlayer will form a diffusion barrier for the intermetallic Al-Mg compounds interlayerduring diffusion because bonding. the interlayer However, will form the interdiffusion a diffusion barrier of Al-Ti for and the Mg-Ti intermetallic will form Al-Mg the joint compounds between during diffusion bonding. However, the interdiffusion of Al-Ti and Mg-Ti will form the joint between duringthe aluminum diffusion and bonding. magnesium However, materials. the interdiffusion In contrast ofto Al-Tititanium, and Mg-Tisilver willshows form a good the joint diffusivity between of the aluminum and magnesium materials. In contrast to titanium, silver shows a good diffusivity of Al-Ag and Mg-Ag. The diffusion of silver into the bas materials can influence the interface properties Al-Ag and Mg-Ag. The diffusion of silver into the bas materials can influence the interface properties most-likely. Furthermore, the soft silver interlayer will improve the forming process of the joining most-likely. Furthermore, the soft silver interlayer will improve the forming process of the joining surfaces to each other. surfaces to each other.

Metals 2018, 8, 138 4 of 12

the aluminum and magnesium materials. In contrast to titanium, silver shows a good diffusivity of Al-Ag and Mg-Ag. The diffusion of silver into the bas materials can inﬂuence the interface properties Metalsmost-likely. 2018, 8, 138 Furthermore, the soft silver interlayer will improve the forming process of the joining4 of 11 surfaces to each other. The magnesium surfaces were chemically treated in the glovebox directly before joining. The The magnesium surfaces were chemically treated in the glovebox directly before joining. The oxide oxide layer was removed by immersing the specimens in HNO3 with a chemical concentration of layer was removed by immersing the specimens in HNO with a chemical concentration of 10 wt % 10 wt. % for 30 s. Both the titanium and silver coatings 3and the chemical surface treatment were for 30 s. Both the titanium and silver coatings and the chemical surface treatment were carried out so carried out so that the roughness of the joining surfaces was not significantly affected. that the roughness of the joining surfaces was not signiﬁcantly affected. 2.3. Diffusion Bonding Procedure 2.3. Diffusion Bonding Procedure After the chemical surface treatment, the aluminium and magnesium samples were assembled After the chemical surface treatment, the aluminium and magnesium samples were assembled on top of each other and transferred from the glovebox into the diffusion bonding chamber. A small on top of each other and transferred from the glovebox into the diffusion bonding chamber. A small contact pressure of ca. 3 N/mm2 was applied during heating up to 350 °C (heating rate of 3–5 K min−1), contact pressure of ca. 3 N/mm2 was applied during heating up to 350 ◦C (heating rate of 3–5 K min−1), to reduce the macroscopic deformation of the magnesium sample. When a temperature of 350 °C was to reduce the macroscopic deformation of the magnesium sample. When a temperature of 350 ◦C was reached, the contact pressure was increased to 8.5 N/mm2 to reach a near full-faced contact. At 415 °C, reached, the contact pressure was increased to 8.5 N/mm2 to reach a near full-faced contact. At 415 ◦C, the pre-determined bonding time of 1 h was held. At the end of the diffusion bonding procedure, the pre-determined bonding time of 1 h was held. At the end of the diffusion bonding procedure, specimens were cooled to room temperature in the chamber at 2–10 K min−1. specimens were cooled to room temperature in the chamber at 2–10 K min−1.

3.3. Results Results and and Discussion Discussion

3.1.3.1. Joining Joining Surfaces Surfaces TheThe surface surface roughness roughness SzSz afterafter turning turning of of the the aluminum aluminum specimens specimens was was SzSz == 5.2 5.2 µmµm ±± 0.30.3 µm.µm. CoatingCoating only only slightly slightly changed changed the the joining surfaces to Sz == 5.1 µµmm ±± 0.2 µµm.m. The The macroscopic macroscopic characteristiccharacteristic tracestraces ofof the the turning turning process process were were still visiblestill visible (Figure (Figure3a). The 3a). chemical The chemical surface treatment surface treatmentof AZ 31 Bof changed AZ 31 B the changed surface the roughness surface roughness from Sz = from 21.6 µ Szm = to 21.6Sz = µm 20.4 toµ Szm, = due 20.4 to µm, the removaldue to the of removalmaterial of (Figure material3b). (Figure 3b).

(a) (b)

FigureFigure 3. 3. TactileTactile measured measured surface surface topographies topographies of of the the joining joining surfaces surfaces after after surface surface treatment treatment with with ((aa)) Sz == 5.25.2 µµmm of of AA AA6082 6082 coated coated with with titanium titanium and and (b) Sz (b=) 20.4Sz =µ m20.4 of AZµm 31of BAZ after 31 chemical B after treatment.chemical treatment.

The rolled magnesium surfaces exhibit a higher surface roughness compared to the aluminium The rolled magnesium surfaces exhibit a higher surface roughness◦ compared to the aluminium surfaces,surfaces, but but through through the the enhanced enhanced formability formability of of AZ AZ 31 31 B B above above 230 230 °C,C, a a full-faced full-faced contact contact can can be be assumed.assumed. Furthermore, Furthermore, through through a a higher higher surface surface defo deformation,rmation, the the amount amount of of lattice lattice defects defects increases, increases, andand thus thus diffusivity diffusivity is is enhanced. enhanced.

3.2. Joint Properties with Titanium Interlayer All aluminium alloys showed a similar morphology in the diffusion zone. Figure 4 depicts the diffusion zone of an AA 7020-AZ 31 B bond. No diffusion of titanium into the aluminium was detected. The interlayer showed similar thickness to the as-coated condition with the exception that

Metals 2018, 8, 138 5 of 12

Metals 2018, 8, 138 5 of 11 3.2. Joint Properties with Titanium Interlayer a few discontinuitiesAll aluminium due alloysto fracture showed of a the similar layer morphology were observed. in the diffusion The EDXS zone. line Figure scan4 depicts confirmed the interdiffusionthe diffusion of Al zoneand ofMg an for AA all 7020-AZ joints. 31 Mg, B bond. in particular, No diffusion diffused of titanium from into the the AZ aluminium 31 B into was the base materialsdetected. of the 6000 The interlayer series aluminum showed similar alloys, thickness and toAl-Mg the as-coated compounds condition were with formed. the exception The thataluminium a few discontinuities due to fracture of the layer were observed. The EDXS line scan confirmed the alloys of the 7000 series showed a higher concentration of Zn in the diffusion zone and intermetallic interdiffusion of Al and Mg for all joints. Mg, in particular, diffused from the AZ 31 B into the base Al-Mg compounds,materials of the respectively 6000 series aluminum (Figure alloys, 5). Characterization and Al-Mg compounds confirmed were formed. that Thethe aluminiumformation of the intermetallicalloys Al-Mg of the 7000 compound series showed was a not higher avoided concentration by the of titanium Zn in the interlayer. diffusion zone The and discontinuous intermetallic layer structureAl-Mg and compounds,the relatively respectively thin layer (Figure thickness5). Characterization most likely confirmed were that not the formationsufficient of to the suppress interdiffusion.intermetallic The thickness Al-Mg compound of ca. 40–50 was not µm avoided of the by intermetallic the titanium interlayer. Al-Mg compound The discontinuous along layerthe interface structure and the relatively thin layer thickness most likely were not sufficient to suppress interdiffusion. is similar to the results of joining experiments without interlayer material for the same bonding The thickness of ca. 40–50 µm of the intermetallic Al-Mg compound along the interface is similar to parameters,the results e.g., ofcompared joining experiments to the results without of interlayer Dietrich material et al. for[15]. the However, same bonding no parameters, joints with e.g., titanium interlayercompared exhibited tothe cracks results or of voids Dietrich along et al. the [15]. bond However, interface, no joints indicating with titanium that interlayer full metallic exhibited continuity had beencracks reached. or voids along the bond interface, indicating that full metallic continuity had been reached.

Figure 4. SEM image of the diffusion zone of Al-Mg joint consisting of (1) Al3Mg2; (2) Al12Mg17 with Figure 4. SEM image of the diffusion zone of Al-Mg joint consisting of (1) Al3Mg2;(2) Al12Mg17 with ca. 3 at-%ca. Zn, 3 at-% (3) Zn;Al12 (3Mg) Al1712 withMg17 withca. 2 ca. at-% 2 at-% Zn Zn; and and (4 ()4 )Al Al1212MgMg1717..

Figure 6Figure shows6 shows the results the results of ofVickers Vickers micro-hardness micro-hardness measurements measurements across across the interface the interface of the of the Al-Mg joints.Al-Mg A joints. significant A signiﬁcant increase increase in hardness in hardness can can be be seen seen for for all all diffusion diffusion zones zones of of the the bonds. bonds. The magnesiumThe magnesiumside of the side 7000 of series the 7000 aluminium series aluminium joints joints show show a different a different behavior behavior comparedcompared to theto the 6000 6000 series. The correlation of Figures5 and6 conﬁrms that a higher concentration of zinc on the series. Themagnesium correlation side leads of Figures to higher micro-hardness.5 and 6 confirms that a higher concentration of zinc on the magnesium side leads to higher micro-hardness.

Figure 5. Elemental distribution across the interface of the Al-Mg joint of with titanium interlayer (Figure 4) measured by EDXS line scan.

Metals 2018, 8, 138 5 of 11 a few discontinuities due to fracture of the layer were observed. The EDXS line scan confirmed the interdiffusion of Al and Mg for all joints. Mg, in particular, diffused from the AZ 31 B into the base materials of the 6000 series aluminum alloys, and Al-Mg compounds were formed. The aluminium alloys of the 7000 series showed a higher concentration of Zn in the diffusion zone and intermetallic Al-Mg compounds, respectively (Figure 5). Characterization confirmed that the formation of the intermetallic Al-Mg compound was not avoided by the titanium interlayer. The discontinuous layer structure and the relatively thin layer thickness most likely were not sufficient to suppress interdiffusion. The thickness of ca. 40–50 µm of the intermetallic Al-Mg compound along the interface is similar to the results of joining experiments without interlayer material for the same bonding parameters, e.g., compared to the results of Dietrich et al. [15]. However, no joints with titanium interlayer exhibited cracks or voids along the bond interface, indicating that full metallic continuity had been reached.

Figure 4. SEM image of the diffusion zone of Al-Mg joint consisting of (1) Al3Mg2; (2) Al12Mg17 with

ca. 3 at-% Zn, (3) Al12Mg17 with ca. 2 at-% Zn and (4) Al12Mg17.

Figure 6 shows the results of Vickers micro-hardness measurements across the interface of the Al-Mg joints. A significant increase in hardness can be seen for all diffusion zones of the bonds. The magnesium side of the 7000 series aluminium joints show a different behavior compared to the 6000 series. The correlation of Figures 5 and 6 confirms that a higher concentration of zinc on the Metals 2018, 8, 138 6 of 12 magnesium side leads to higher micro-hardness.

FigureFigure 5. Elemental 5. Elemental distribution distribution across across th thee interface interface ofof thethe Al-Mg Al-Mg joint joint of withof with titanium titanium interlayer interlayer (Figure(Figure 4) measured4) measured by byEDXS EDXS line line scan. scan. Metals 2018, 8, 138 6 of 11

FigureFigure 6. Hardness profiles profiles across across the the interface interface of the of diffusion-bonded the diffusion-bonde Al-Mgd jointsAl-Mg with joints titanium with interlayer. titanium interlayer. 3.3. Joint Properties with Silver Interlayer 3.3. Joint Properties with Silver Interlayer In contrast to the results with titanium interlayer, a reaction of the silver coating with the base materialsIn contrast during to the joining results was with observed. titanium The silverinterlayer diffused, a reaction completely of intothe silver the base coating materials, with and the an base materialsintermetallic during Al-Mgjoining compound was observed. was formed. The silver This zonediffused also containedcompletely silver. into The the white base dottedmaterials, regions and an intermetallicin zone 3 Al-Mg of Figure compound7 are slightly was enriched formed. in This silver zone (Figure also8 contained). Kirkendall silver. voids The were white formed dotted on the regions magnesium side, due to the higher coefficient of diffusion of magnesium atoms in aluminium. This is in zone 3 of Figure 7 are slightly enriched in silver (Figure 8). Kirkendall voids were formed on the in accordance with other researchers, such as Jafarian et al. [10], who observed similar phenomena for magnesium side, due to the higher coefficient of diffusion of magnesium atoms in aluminium. This diffusion bonding of aluminium and magnesium with or without interlayer materials. An increase in is in accordancethe contact pressure with other will avoidresearchers, the formation such as of Jafarian Kirkendall et voids;al. [10], however, who observed macroscopic similar deformation phenomena for diffusionwill increase. bonding The hardnessof aluminium profiles and of magnesium the Al-Mg joints with with or without silver interlayer interlayer exhibit materials. an increase An increase in in thehardness contact at thepressure interface will of the avoid intermetallic the formatio compoundn of Al Kirkendall12Mg17 and thevoids; magnesium however, base macroscopic material. deformationThis is pronounced will increase. for the The aluminum hardness alloys profiles of the of 7000 the seriesAl-Mg (Figure joints9). with The chemicalsilver interlayer composition exhibit of an increasethe areasin hardness of high hardnessat the interface again showed of the anintermetallic increase of zinccompound in this area. Al12 TheMg enrichment17 and the magnesium of zinc in the base material.magnesium This is can pronounced form a metallurgical for the aluminum notch in this allo areays away of the from 7000 the bondseries line. (Figure This notch 9). The effect chemical is a composition of the areas of high hardness again showed an increase of zinc in this area. The enrichment of zinc in the magnesium can form a metallurgical notch in this area away from the bond line. This notch effect is a typical cause of failure for similar and dissimilar joints. For aluminium and magnesium joints without interlayer, the metallurgical notch is located at the interface of the aluminium base material and the Al3Mg2 intermetallic compound. These effects were also observed by Liu et al. [7,10].

Figure 7. SEM image of the diffusion zone of an Al-Mg joint revealing (1) impurities of Al, Mg, Si and O2, (2) the Al-rich intermetallic phase Al3Mg2 with ca. 1.5 at-% Ag and (3) the Mg-rich intermetallic

phase Al12Mg17 with ca. 3 at-% Ag.

Metals 2018, 8, 138 6 of 11

Figure 6. Hardness profiles across the interface of the diffusion-bonded Al-Mg joints with titanium interlayer.

3.3. Joint Properties with Silver Interlayer In contrast to the results with titanium interlayer, a reaction of the silver coating with the base materials during joining was observed. The silver diffused completely into the base materials, and an intermetallic Al-Mg compound was formed. This zone also contained silver. The white dotted regions in zone 3 of Figure 7 are slightly enriched in silver (Figure 8). Kirkendall voids were formed on the magnesium side, due to the higher coefficient of diffusion of magnesium atoms in aluminium. This is in accordance with other researchers, such as Jafarian et al. [10], who observed similar phenomena for diffusion bonding of aluminium and magnesium with or without interlayer materials. An increase in the contact pressure will avoid the formation of Kirkendall voids; however, macroscopic deformation will increase. The hardness profiles of the Al-Mg joints with silver interlayer exhibit an increase in hardness at the interface of the intermetallic compound Al12Mg17 and the magnesium base material. This is pronounced for the aluminum alloys of the 7000 series (Figure 9). The chemical composition of the areas of high hardness again showed an increase of zinc in this area. The enrichmentMetals 2018of zinc, 8, 138 in the magnesium can form a metallurgical notch in this area away from7 of the 12 bond line. This notch effect is a typical cause of failure for similar and dissimilar joints. For aluminium and magnesiumtypical causejoints of without failure for interlayer, similar and dissimilarthe metallur joints.gical For aluminium notch is andlocated magnesium at the joints interface without of the aluminiuminterlayer, base the material metallurgical and the notch Al3 isMg located2 intermetallic at the interface compound. of the aluminium These effects base material were also and theobserved by LiuAl et3 Mgal. 2[7,10].intermetallic compound. These effects were also observed by Liu et al. [7,10].

FigureFigure 7. SEM 7. imageSEM image of the of thediffusion diffusion zone zone of of an an Al-Mg Al-Mg jointjoint revealing revealing (1) ( impurities1) impurities of Al, of Mg, Al, Si Mg, and Si and

O2, (2) Othe2,( 2Al-rich) the Al-rich intermetallic intermetallic phase phase Al Al3Mg3Mg2 2withwith ca. 1.51.5at-% at-% Ag Ag and and (3) the(3) Mg-richthe Mg-rich intermetallic intermetallic Metals 2018, 8, 138 7 of 11 phase phaseAl12Mg Al1712 withMg17 ca.with 3 ca.at-% 3 at-% Ag. Ag.

Figure Figure8. Elemental 8. Elemental distribution distribution across across the the interface interface of of the Al-MgAl-Mg joint joint with with silver silver interlayer interlayer (Figure (Figure6) 6) measuredmeasured by EDXS by EDXS line linescan. scan.

Figure 9. Hardness profiles across the interface of the diffusion-bonded Al-Mg joints with silver interlayer.

3.4. Tensile Testing and Fractography Figure 10 summarizes the results of the tensile tests with the top hat specimens. Joints with titanium interlayer exhibit the highest strength levels, especially for the aluminium alloys of the 6000 series. The hardness measurements of these show the lowest increase in hardness across the diffusion zone. The hardness profiles of the 7000 series aluminium alloys show an increased hardness by 75 HV 0.01. The fracture surface of an AA 6060 and AZ 31 B joint shows a dense layer of the titanium coating with only few areas of intermetallic Al-Mg compounds (Figure 11). For both the AA 6060 and the AA 6082 aluminium alloys, the fracture surfaces exhibited a dense titanium coating that corresponds to the high rupture strength of 48.3 N/mm2. However, lower joint strengths were observed for the 7000 series joints with titanium interlayer. The fracture surfaces of the AA 7020 reveals a higher amount of intermetallic Al-Mg compounds (Figure 12). These intermetallic compounds exhibit very brittle behavior, especially when exposed to a tensile load. As a consequence, the base material strength of magnesium of 260 N/mm2 was not nearly reached. Furthermore, a partial delamination of the titanium coating was observed on the fracture surfaces (Figure 12, No. 3). Significant lower joint strengths were achieved using silver as interlayer material. The hardness profiles (Figure 8) already showed both a higher averaged hardness level in the diffusion zones and a hardness peak on the magnesium side of the 7000 series aluminium alloys. The fracture surface of

Metals 2018, 8, 138 7 of 11

Figure 8. Elemental distribution across the interface of the Al-Mg joint with silver interlayer (Figure 6) Metals 2018, 8, 138 8 of 12 measured by EDXS line scan.

FigureFigure 9. 9. HardnessHardness profilesprofiles across across the th interfacee interface of the of diffusion-bonded the diffusion-bon Al-Mgded joints Al-Mg with joints silver interlayer.with silver interlayer. 3.4. Tensile Testing and Fractography 3.4. Tensile Testing and Fractography Figure 10 summarizes the results of the tensile tests with the top hat specimens. Joints with titaniumFigure 10 interlayer summarizes exhibit the the results highest of strength the tensile levels, tests especially with the for top the aluminiumhat specimens. alloys Joints of the with titanium6000 interlayer series. The exhibit hardness the measurements highest strength of these levels, show especially the lowest for increasethe aluminium in hardness alloys across of the the 6000 series.diffusion The hardness zone. The measurements hardness profiles of these of the show 7000 series the lowest aluminium increase alloys in show hardness an increased across the hardness diffusion zone.by The 75 HVhardness 0.01. The profiles fracture of surfacethe 7000 of anseries AA 6060aluminium and AZ alloys 31 B joint show shows an increased a dense layer hardness of the by 75 HVtitanium 0.01. The coating fracture with surface only few of areasan AA of 6060 intermetallic and AZ Al-Mg31 B joint compounds shows a (Figuredense layer11). For of the both titanium the AA 6060 and the AA 6082 aluminium alloys, the fracture surfaces exhibited a dense titanium coating coating with only few areas of intermetallic Al-Mg compounds (Figure 11). For both the AA 6060 and that corresponds to the high rupture strength of 48.3 N/mm2. However, lower joint strengths were the AAobserved 6082 for aluminium the 7000 series alloys, joints withthe fracture titanium interlayer.surfaces Theexhibited fracture surfacesa dense of titanium the AA 7020 coating reveals that 2 correspondsa higher amountto the high of intermetallic rupture strength Al-Mg compounds of 48.3 N/mm (Figure. However,12). These intermetalliclower joint compoundsstrengths were observedexhibit for very the brittle 7000 behavior,series joints especially with whentitanium exposed interlayer. to a tensile The load. fracture As a surfaces consequence, of the the AA base 7020 revealsmaterial a higher strength amount of magnesium of intermetallic of 260 N/mm Al-Mg2 was compounds not nearly reached.(Figure Furthermore,12). These intermetallic a partial compoundsdelamination exhibit of very the titaniumbrittle behavior, coating wasespecially observed when on exposed the fracture to a surfaces tensile load. (Figure As 12 a ,consequence, No. 3). the baseSignificant material lower strength joint of strengths magnesium were of achieved 260 N/mm using2 was silver not as nearly interlayer reached. material. Furthermore, The hardness a partial delaminationprofiles (Figure of the8) alreadytitanium showed coating both was a higher observed averaged on hardness the fracture level in surfaces the diffusion (Figure zones 12, and No. a 3). Significanthardness lower peak joint on the strengths magnesium were side achieved of the 7000 using series silver aluminium as interlayer alloys. The material. fracture The surface hardness of profilesthe (Figure aluminium 8) already side, shown showed in Figure both 13a ,higher exhibits av aeraged rough surface,hardness which level is in characteristic the diffusion of zones brittle and failures of intermetallic compounds. The EDXS analysis of the fracture surfaces showed that Al Mg a hardness peak on the magnesium side of the 7000 series aluminium alloys. The fracture surface3 2 of and Al12Mg17 intermetallic phases are present. Despite high hardness on the magnesium side and the confirmed metallurgical notch in this position, the joint finally failed in-between the intermetallic compounds. As a result, the rupture strength for AA 7020 was only 27.7 N/mm2. In contrast to the 6000 series aluminium alloys (Figure 13), the fracture surface of AA 7020 (Figure 14) appears smoother, but still contains rough areas. The fracture location was probably between the aluminium base material and the intermetallic Al-Mg compound, which contained traces of silver and zinc. Similar fractures as for AA 6060 were observed for AA 6082 and AA 7075. Metals 2018, 8, 138 8 of 11 Metals 2018, 8, 138 8 of 11 the aluminium side, shown in Figure 13, exhibits a rough surface, which is characteristic of brittle failuresthe aluminium of intermetallic side, shown compounds. in Figure The 13, EDXS exhibits an alysisa rough of thesurface, fracture which surfaces is characteristic showed that of Al brittle3Mg2 andfailures Al12 Mgof intermetallic17 intermetallic compounds. phases are The present. EDXS Despite analysis high of thehardness fracture on surfaces the magnesium showed sidethat andAl3Mg the2 confirmedand Al12Mg metallurgical17 intermetallic notch phases in arethis present. position, Despite the joint high finally hardness failed on in-between the magnesium the intermetallic side and the compounds.confirmed metallurgical As a result, notchthe rupture in this strength position, for the AA joint 7020 finally was onlyfailed 27.7 in-between N/mm2. Inthe contrast intermetallic to the 6000compounds. series aluminium As a result, alloys the rupture (Figure strength 13), the for fracture AA 7020 surface was only of AA27.7 7020N/mm (Figure2. In contrast 14) appears to the smoother,6000 series but aluminium still contains alloys rough (Figure areas. 13),The thefracture fracture location surface was ofprobably AA 7020 between (Figure the 14) aluminium appears basesmoother, material but andstill containsthe intermetallic rough areas. Al-Mg The compound, fracture location which was contained probably traces between of silver the aluminium and zinc. SimilarMetalsbase material2018 fractures, 8, 138 and as thefor AAintermetallic 6060 were Al-Mg observed compound, for AA 6082 which and contained AA 7075. traces of silver and 9zinc. of 12 Similar fractures as for AA 6060 were observed for AA 6082 and AA 7075.

Figure 10. Joint strength of diffusion-bonded aluminum and magnesium with titanium or silver Figure 10. Joint strength of diffusion-bonded aluminum and magnesium with titanium or silver interlayers. interlayers.Figure 10. Joint strength of diffusion-bonded aluminum and magnesium with titanium or silver interlayers.

Figure 11. SEM image of a fracture surface of an AA 6060 and AZ 31B joint with titanium interlayer Figure 11. SEM image of a fracture surface of an AA 6060 and AZ 31B joint with titanium interlayer onFigure the aluminium11. SEM image side of revealing a fracture (1 )surface the surface of an ofAA the 6060 titanium and AZ coat 31Bing joint with with only titanium a few areas interlayer of (2) on the aluminium side revealing (1) the surface of the titanium coating with only a few areas of intermetallicon the aluminium Al-Mg side compounds. revealing (1) the surface of the titanium coating with only a few areas of (2) (intermetallic2) intermetallic Al-Mg Al-Mg compounds. compounds.

Metals 2018, 8, 138 10 of 12 Metals 2018, 8, 138 9 of 11 Metals 2018, 8, 138 9 of 11

Figure 12. SEM image of a fracture surface of a AA 7020 and AZ 31B joint with titanium interlayer on Figure 12. SEM image of a fracture surface of a AA 7020 and AZ 31B joint with titanium interlayer on Figurethe aluminium 12. SEM imageside and of a(1 fracture) the surface surfac ofe ofthe a AAtitanium 7020 andcoating, AZ 31B (2) intermetallicjoint with titanium Al-Mg interlayer compounds on the aluminium side and (1) the surface of the titanium coating, (2) intermetallic Al-Mg compounds and theand aluminium (3) a delamination side and of ( 1the) the coating surface from of thethe titaniumaluminium coating, surface. (2 ) intermetallic Al-Mg compounds (3) a delamination of the coating from the aluminium surface. and (3) a delamination of the coating from the aluminium surface.

Figure 13. SEM image of a fracture surface of a AA 6060 and AZ 31B joint with silver interlayer on the Figure 13. SEM image of a fracture surface of a AA 6060 and AZ 31B joint with silver interlayer on the Figurealuminum 13. SEMside image(1) presumably of a fracture Al surface3Mg2 containing of a AA6060 traces and of AZ silver 31B and joint ( with2) presumably silver interlayer Al12Mg on17 aluminumcontaining tracesside ( 1of) silver.presumably Al3Mg2 containing traces of silver and (2) presumably Al12Mg17 the aluminum side (1) presumably Al3Mg2 containing traces of silver and (2) presumably Al12Mg17 containing traces of silver. containing traces of silver.

Metals 2018, 8, 138 11 of 12 Metals 2018, 8, 138 10 of 11

Figure 14. SEM image of a fracture surface of a AA 7020 and AZ 31B joint with silver interlayer on the Figure 14. SEM image of a fracture surface of a AA 7020 and AZ 31B joint with silver interlayer on the aluminium side (1) presumably Al3Mg2 containing traces of silver and zinc and (2) aluminium base aluminium side (1) presumably Al3Mg2 containing traces of silver and zinc and (2) aluminium base material with traces of magnesium. material with traces of magnesium.

4. Conclusions 4. Conclusions In the present study, the effect of titanium and silver as interlayer materials on diffusion-bonded In the present study, the effect of titanium and silver as interlayer materials on diffusion-bonded AA 6060, AA 6082, AA 7020, AA 7075 and AZ 31 B dissimilar joints was examined. The following AA 6060, AA 6082, AA 7020, AA 7075 and AZ 31 B dissimilar joints was examined. The following conclusions can be derived: conclusions can be derived: Titanium and silver as interlayers, deposited by a PVD-process on the oxide-free aluminum Titanium and silver as interlayers, deposited by a PVD-process on the oxide-free aluminum surface, made redundant a further surface treatment after exposure of specimens to air. surface, made redundant a further surface treatment after exposure of specimens to air. A solid and almost dense interlayer was formed by titanium on the surface. However, A solid and almost dense interlayer was formed by titanium on the surface. However, intermetallic intermetallic Al-Mg compounds were still formed during diffusion bonding. Nevertheless, the Al-Mg compounds were still formed during diffusion bonding. Nevertheless, the hardness hardness measurements across the interface showed a lower hardness level for the diffusion zone measurements across the interface showed a lower hardness level for the diffusion zone compared to compared to Al-Mg joints without interlayers. As a consequence, a joint strength of up to 48 N/mm2 Al-Mg joints without interlayers. As a consequence, a joint strength of up to 48 N/mm2 was reached. was reached. The 2 µm thick silver interlayer completely diffused into both base materials and formed a brittle The 2 µm thick silver interlayer completely diffused into both base materials and formed a brittle diffusion zone. Kirkendall voids were observed on the magnesium side and a joint strength of only diffusion zone. Kirkendall voids were observed on the magnesium side and a joint strength of only 28 N/mm2 was achieved. 28 N/mm2 was achieved. Fracture of the joints with titanium interlayer occurred at the interface of the intermetallic Al-Mg Fracture of the joints with titanium interlayer occurred at the interface of the intermetallic Al- compound and the titanium. As the silver diffused into the base materials, the joints failed at the Mg compound and the titanium. As the silver diffused into the base materials, the joints failed at the interface of Al3Mg2 and Al12Mg17. interface of Al3Mg2 and Al12Mg17. Acknowledgments: The authors gratefully acknowledge the funding by the German Research Foundation (DeutscheAcknowledgments: Forschungsgemeinschaft, The authors gratefully DFG) within ackn theowledge framework the funding of the by Collaborative the German Research Research Centre Foundation 692 (SFB(Deutsche HALS 692). Forschungsgemeinschaft, DFG) within the framework of the Collaborative Research Centre 692 (SFB AuthorHALS Contributions: 692). Stefan Habisch carried out all joining experiments and is the main author of this manuscript.Author Contributions: Marcus Böhme Stefan supported Habisch the carried project without all microstructural joining experiments characterization and is the using main SEM author and EDS.of this Siegfried Peter supported the project with the PVD coatings of titanium and silver. Thomas Grund and Peter Mayr aremanuscript. senior scientists, Marcus developed Böhme supported and supervised the project the project with “Joiningmicrostructural concepts characteri for bulk andzation sheet using metal SEM structures and EDS. ofSiegfried high-strength Peter light-weight supported the materials” project withinwith the SFB PVD HALS coat 692.ings of titanium and silver. Thomas Grund and Peter Mayr are senior scientists, developed and supervised the project “Joining concepts for bulk and sheet metal Conﬂicts of Interest: The authors declare no conﬂict of interest. structures of high-strength light-weight materials” within SFB HALS 692.

Conflicts of Interest: The authors declare no conflict of interest.

Metals 2018, 8, 138 12 of 12

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