Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 Aluminum Alloy Brazing

applied sciences

Article Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 Aluminum Alloy Brazing

Chengyin Peng 1,2, Dandan Zhu 3, Kaifeng Li 3, Xiang Du 1,2, Fei Zhao 1,2,*, Mingpan Wan 1,2 and Yuanbiao Tan 1,2

1 College of Materials and Metallurgy, Guizhou University, Guiyang 550025, China; [email protected] (C.P.); [email protected] (X.D.); [email protected] (M.W.); [email protected] (Y.T.) 2 Key Laboratory for Materials Structure and Strength of Guizhou province, Guiyang 550025, China 3 Guizhou Yonghong Heat Transfer & Cooling Technology Co., Ltd., Huishui 550600, China; [email protected] (D.Z.); [email protected] (K.L.) * Correspondence: [email protected]; Tel.: +86-139-8488-3791

Featured Application: Research on a Low-Melting-Point Solder for a 6063 Aluminum Alloy Cold Plate Radiator.

Abstract: A new type of low melting point Al-Si-Cu (Ni) filler metal for brazed 6063 aluminum alloy was designed, and the microstructure and properties of the filler metal were systematically studied. The results show that when the content of Cu in the Al-Si-Cu filler metal increased from 10 wt.% to 20 wt.%, the liquidus temperature of the filler metal decreased from 587.8 ◦C to 533.4 ◦C.

Its microstructures were mainly composed of the α-Al phase, a primary Si phase, and a θ(Al2Cu) phase. After a proper amount of Ni was added to the Al-Si-20Cu filler metal, its melting range was narrowed, the spreading wettability was improved, and the microstructure was refined. Its Citation: Peng, C.; Zhu, D.; Li, K.; microstructure mainly includes α-Al solid solution, Si particles, and θ(Al2Cu) and δ(Al3Ni2) inter- Du, X.; Zhao, F.; Wan, M.; Tan, Y. metallic compounds. The results of the shear strength test indicate that the shear strength of the Research on a Low Melting Point Al-Si-Cu (Ni) Filler Metal for 6063 brazed joint with Al-6.5Si-20Cu-2.0Ni filler metal was 150.4 MPa, which was 28.32% higher than that Aluminum Alloy Brazing. Appl. Sci. of the brazed joint with Al-6.5Si-20Cu filler metal. 2021, 11, 4296. https://doi.org/ 10.3390/app11094296 Keywords: 6063 aluminum alloy; Al-Si-Cu (Ni) filler metals; microstructure; shear strength

Academic Editor: Filippo Berto

Received: 21 April 2021 1. Introduction Accepted: 7 May 2021 Brazing holds the advantages of low deformation of the weldment, high dimensional Published: 10 May 2021 accuracy, and high production efficiency. Brazing is one of the commonly used connection methods for heat exchangers [1–3]. As a light alloy material with excellent weldability Publisher’s Note: MDPI stays neutral and heat conductivity, 6063 aluminum alloy can be used as the main material of the heat with regard to jurisdictional claims in exchanger [4–6]. Al-Si system filler metal is typically used in the brazing of aluminum and published maps and institutional affil- its alloys. The content of Si is 7~13 wt.%, and the liquidus temperature is approximately iations. 570 ◦C[7,8]. To obtain excellent brazed joints, the brazing temperature must be higher than 595 ◦C, which is close to the solid phase temperature of 6063 aluminum alloy (approximately 615 ◦C), which may cause base metal grain growth or over-burning [2,4,7]. In addition, an excessive brazing temperature not only enhances the manufacturing cost but Copyright: © 2021 by the authors. also causes dissolution of the base metal, and the quality of the brazed joint obtained is Licensee MDPI, Basel, Switzerland. poor [5,9]. Therefore, it is becoming increasingly important to develop a new type of low This article is an open access article melting point, aluminum-based filler metal for low-cost and high-quality aluminum alloy distributed under the terms and brazed joints. conditions of the Creative Commons To solve the problem of an excessively high brazing temperature, in recent years, Attribution (CC BY) license (https:// many researchers have developed a series of new low melting point, aluminum-based filler creativecommons.org/licenses/by/ 4.0/). metals by alloying methods with the theoretical support of eutectic components of Al-Si,

Appl. Sci. 2021, 11, 4296. https://doi.org/10.3390/app11094296 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 4296 2 of 14

Al-Cu, Al-Ge, Al-Zn, etc., and the corresponding properties have been studied [2,7,10–12]. The addition of Ge to the Al-Si filler metal can narrow the melting point of the Al-Si-Ge filler metal, but the melting range of the filler metal will widen with increasing Ge content, and the fluidity of the filler metal will decrease. In addition, Ge is costly; as a result, Al-Si-Ge filler metal is not suitable for large-scale applications [10]. Dai et al. [8,11] studied the Al-6.5Si-42Zn filler metal, the melting range of which was found to be 27.7 ◦C. When it was used to braze 6061 aluminum alloys, the joint tensile-strength was improved to 129 MPa compared with Al-Si filler metal. However, the content of Zn in Al-Si-Zn filler metal is too high, which easily causes the dissolution of the base metal and reduces the strength of the brazed joint. Moreover, the vapor pressure of Zn is too high, which may have adverse effects on the vacuum–brazing process [13]. Al-Si-Cu filler metal has the benefits of a low melting point and good wettability, which is a hot research direction of aluminum alloy filler metals [4,5,7]. Chang et al. [7] studied the brazing of 6061 aluminum alloy with Al-10.8Si-10Cu and Al-9.6Si-20Cu filler metals at 560 ◦C, and found that there are more θ(Al2Cu) brittle phases in the brazed joints, leading to a lower shear strength of the brazed joints (47 MPa and 67 MPa, respectively). The results indicate that adding an appropriate amount of Ni to Al-Si-Cu filler metal can replace part of the Cu and reduce the adverse effect of θ(Al2Cu) on joint properties [4,5,14]. Luo et al. [4] developed Al-10Si-15Cu-4Ni filler metal with a melting range of 24.9 ◦C and successfully brazed 6061 aluminum alloy at 570 ◦C. Meanwhile, the joint shear strength was as high as 144.4 MPa, which was higher than that of Al-Si-Cu brazing filler metal [2,7]. However, Al-Si-Cu filler metal still has the problems of a wide melting interval and a low shear strength of the brazed joint, which urgently needs to be solved. Based on the pseudoeutectic point composition of Al-Si-Cu ternaries [15], a new type of Al-Si-Cu (Ni) multicomponent aluminum-based filler metal with a low melting point and a narrow melting interval was developed. The optimal Al-Si-Cu (Ni) filler metal was obtained by a comprehensive analysis after studying the melting characteristics, the spreading wettability, the microstructure, and the brazeability of different Al-Si-Cu (Ni) filler metals. Its properties have been significantly improved compared with those of existing filler metals.

2. Materials and Methods The base material used in this experiment was 6063 aluminum alloy plates with a thick- ness of 1 mm. Its chemical composition is listed in Table1. The melting temperature of the base metal was approximately 617.8~657.7 ◦C, and the tensile strength at room temperature was 257.94 MPa. A series of Al-Si-Cu and Al-Si-Cu-Ni filler metals were prepared using the master alloys Al-50Cu (≥99.970 wt.%), Al-30Si (≥99.970 wt.%), and Al-10Ni (≥99.000 wt.%) and high-purity aluminum particles (≥99.900 wt.%) as raw materials, which were melted in a well-type resistance furnace covered with molten salt with a mass ratio of NaCl:KCl = 1:1. Hexachloroethane was used the refining agent. The nominal compositions of the investigated filler metal are listed in Table2. To ensure the compositional uniformity of the brazing alloys, each group was remelted twice, and the obtained castings were heat-retained at 450 ◦C for 12 h for the diffusion annealing treatment.

Table 1. Chemical composition (wt.%) of 6063 aluminum alloy.

Alloy Si Mg Fe Cu Mn Cr Zn Ti Al 6063 0.462 0.603 0.173 0.016 0.018 0.006 0.005 0.053 Bal.

The melting temperature of the filler metal was determined by differential thermal analysis (DTA, TG/DTA7300, Japan). The filler metal was heated from room temperature to 700 ◦C under an argon atmosphere at a heating rate of 10 ◦C/min. The fluidity and wettability of filler metals were assessed by wetting experiments. After the filler metals with a size of 3 mm × 3 mm × 3 mm were weighed, they were placed at the center of the Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 14

Appl. Sci. 2021, 11, 4296 3 of 14 1 6.5 10 0 Bal. 521.3 587.8 66.5 2 6.5 15 0 Bal. 521.6 539.7 18.1 base3 material 60636.5 aluminum20 alloy (400 mm × 40Bal. mm × 1521.4 mm) and completely533.4 covered12.0 ◦ with4 QJ201 ﬂux6.5 (with a melting20 range1.0 of 460~520Bal.C) (Figure519.81). The process528.4 parameter8.6 ◦ of the5 spreading6.5 test was 57020 ± 3 C1.5 for 5 min.Bal. The spread520.5 sample 528.2 was washed7.7 with ◦ warm6 water (50~606.5 C), photographed20 2.0 with a camera,Bal. and imported519.2 into526.8 Image-Pro Plus7.6 to measure the spreading area of the ﬁller metals. The results were averaged by the spreading indicates the solidus temperatures of the filler metals. indicates the liquidus temperatures areaof the per filler unit metals. mass. Δ indicates the melting ranges of the filler metals (Δ = −).

TableThe 2. Nominal melting chemical temperature compositions of the and filler thermal meta propertiesl was determined of the ﬁller by metals differential used in this thermal study. analysis (DTA, TG/DTA7300, Japan). The filler metal was heated from room temperature Chemical Compositions (wt.%) ◦ ◦ ◦ toSamples 700 °C under an argon atmosphere at a heating rate ofTS (10C) °C/min.TL( TheC) fluidity∆T( C) and wettability of fillerSi metals Cuwere assessed Ni by wetting Al experiments. After the filler metals with 1a size of 3 6.5mm × 3 mm 10 × 3 mm were 0 weighed, Bal. they were 521.3 placed 587.8at the center 66.5 of the base material2 6063 6.5 aluminum 15 alloy (40 0mm × 40 mm Bal. × 1 mm) 521.6 and completely 539.7 covered 18.1 with QJ2013 flux (with 6.5 a melting 20range of 460~520 0 °C) Bal.(Figure 1). 521.4 The process 533.4 parameter 12.0 of the 4 6.5 20 1.0 Bal. 519.8 528.4 8.6 spreading5 test was 6.5 570 ± 3 °C 20 for 5 min. 1.5 The spread Bal. sample 520.5was washed 528.2 with warm 7.7 water (50~606 °C), photographed 6.5 20with a camera, 2.0 and imported Bal. into 519.2 Image-Pro 526.8 Plus to measure 7.6

TtheS indicates spreading the solidus area of temperatures the filler ofmetals. the filler The metals. resultsTL indicates were averaged the liquidus by temperatures the spreading of the area filler metals.per unit∆T mass.indicates the melting ranges of the filler metals (∆T = TL − TS).

Figure 1. Schematic illustration of the spreading testing.

The brazing experiment was completed inin a box-type resistance furnace.furnace. AA schematic illustration of of the the brazed brazed joint joint is is shown shown in in Figure Figure2. The 2. The specification specification of the of base the base material material was 50was mm 50 ×mm20 × mm 20 mm× 1 mm,× 1 mm, the specificationthe specification of the of filler the metalfiller metal was 15 was mm 15× mm1 mm × 1× mm0.2 × mm 0.2, themm, lap the length lap length of the of jointthe joint was was 1.5 mm1.5 mm± 0.3 ± 0.3 mm, mm, the the gap gap was was 0.1 0.1± ±0.05 0.05 mm,mm, andand thethe fluxflux used for for brazing brazing was was QJ201. QJ201. Before Before brazin brazing,g, the the surface surface of the of base the base metal metal and the and filler the fillermetal metal was polished was polished with sandpaper. with sandpaper. After Afterthe acetone the acetone was degreased, was degreased, the surface the surface oxide oxidefilm was film removed was removed with awith 10% aNaOH 10% NaOH solution solution and then and neutralized then neutralized with a with10% aHNO 10%3 HNOsolution.3 solution. Finally, Finally, the samples the samples were cleaned were cleaned with ethanol, with ethanol, dried, dried, and assembled. and assembled. The braz- The brazinging technological technological parameters parameters were were as follows: as follows: raise raise the furnace the furnace temperature temperature to 570 to 570°C, put◦C, putthe prepared the prepared sample sample into intothe furnace the furnace after afterthe furnace the furnace temperature temperature becomes becomes stable, stable, keep ◦ keepthe temperature the temperature at 570 at °C 570 forC 10 for min, 10 min,stop stopheating, heating, open open the furnace the furnace door door to take to take out outthe ◦ thesample sample for air for cooling air cooling when when the furnace the furnace temp temperatureerature drops drops to approximately to approximately 500 °C, 500 andC, ◦ andfinally finally clean clean the sample the sample repeatedly repeatedly with hot with wa hotter (50~60 water (50~60°C) to removeC) to removethe flux theresidue. flux residue.Two step-aging Two step-aging treatments treatments were adopted were adopted to improve to improve the mechanical the mechanical properties properties of the of thebrazed brazed parts. parts. The Theheat heat treatment treatment system system was wasas follows: as follows: after after solid solid solution solution treatment treatment (530 ◦ (530°C × 3C h)× and3 h) androom room temperature temperature water water quench quenchinging of ofthe the brazed brazed parts, parts, the the brazed brazed parts ◦ were immediatelyimmediately subjected subjected to to primary primary aging aging (160 (160C °C× 1× h)1 h) and and water water cooling, cooling, Finally, Finally, the ◦ brazedthe brazed parts parts were were subjected subjected to secondary to secondary aging aging for 3for h at3 h 190 at 190C and °C and water water cooling. cooling. Appl. Sci. 2021, 11, 4296 4 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 14

FigureFigure 2. 2.Schematic Schematic illustrationillustration ofof the the spreading spreading testing. testing.

TheThe microstructuresmicrostructures ofof thethe fillerfiller metalsmetals werewere analyzedanalyzed byby anan X-ray X-ray diffractometer diffractometer (XRD,(XRD, Cu Cu K Kαα,, Empyrean, Empyrean, Holland). Holland). The The scanning scanning angle angle was was 10 10°◦ to to 90 90°,◦, the the scanning scanning rate rate waswas 0.1880.188°/s,◦/s, the the accelerating accelerating voltage voltage was was 45 45 kV, kV, and and the the working working current current was was 40 mA. 40 mA. The Theshear shear strength strength of a ofbrazed a brazed joint joint was was measur measureded on an on Instron an Instron 8501 8501 universal universal testing testing ma- machinechine at a at tensile a tensile speed speed of 0.5 of 0.5mm/min. mm/min. To ensu To ensurere the accuracy the accuracy of the of results the results on the on shear the shearstrength strength of the of brazed the brazed joints, joints, three three specimens specimens were were brazed brazed under under the same the same conditions conditions with witheach each filler filler metal, metal, and and thethe average average values values ofof the the test test results results were were calculated. calculated. The micro-micro- VickersVickers hardness hardness of of brazed brazed joints joints was was measured measured by by anan MHV-2.0MHV-2.0 automatic automatic microhardness microhardness tester,tester, three three points points were were measured measured in in each each area, area, and and the the average average value value of of the the results results was was taken.taken. ScanningScanning electronelectron microscopymicroscopy (SEM.(SEM. SUPRA SUPRA 40,40, Germany) Germany) and and energy energy dispersive dispersive spectrometryspectrometry (EDS) (EDS) were were used used to observeto observe the microstructurethe microstructure of the of filler the metals filler andmetals brazed and jointsbrazed after joints etched after in etched a mixture in a ofmixture 1vol.% of HF, 1vol.% 1.5 vol.% HF, 1.5 HCl, vol.% 2.5 vol.% HCl, HNO2.5 vol.%3 and HNO 95 vol.%3 and H O at room temperature for 20 s. 952 vol.% H2O at room temperature for 20 s. 3. Results and Discussion 3. Results and Discussion 3.1. Melting Characteristics of the Al-Si-Cu (Ni) Filler Metal 3.1. Melting Characteristics of the Al-Si-Cu (Ni) Filler Metal Figure3 shows the melting characteristics of filler metals with different Cu and Ni contents.Figure 3 Theshows melting the melting temperatures characteristics of the of filler filler metals metals in with this different study, determined Cu and Ni accordingcontents. The to the melting DTA temperatures curve in Figure of the3, arefiller summarized metals in this in study, Table 2determined. Combined according with Tableto the2 DTAand thecurve Figure in Figure3 analysis, 3, are whensummarized the Cu in content Table 2. was Combined reduced with from Table 10 wt.% 2 and to the 20Figure wt.%, 3 theanalysis, liquidus when temperatures the Cu content of Al-Si-Cu was reduced alloys from decreased 10 wt.% from to 20 587.8 wt.%,◦C tothe 533.4 liquidus◦C, thetemperatures solidus temperature of Al-Si-Cu had allo noys significantdecreased from effects, 587.8 and °C the to melting533.4 °C, temperature the solidus temper- ranges ofature Al-6.5Si-(10, had no significant 15, 20) Cueffects, filler and metals the melting gradually temperature became narrowerranges of Al-6.5Si-(10, (66.5 ◦C, 18.1 15,◦ 20)C, andCu filler 12.0 ◦metalsC, respectively). gradually Withbecame increasing narrower Cu (6 content,6.5 °C, 18.1 the °C, composition and 12.0 °C, of therespectively). Al-Si-Cu fillerWith metal increasing gradually Cu content, approached the composition the eutectic compositionof the Al-Si-Cu point filler of the metal Al-Si-Cu gradually ternary ap- alloy,proached noncongruent the eutectic melting composition gradually point changed of the Al-Si-Cu into congruent ternary melting, alloy, noncongruent thus reducing melt- the meltinging gradually temperature changed range into [ 15congruent]. It can bemeltin seeng, from thus Figure reducing3 that the the melting DTA curve temperature of the Al-6.5Si-10Curange [15]. It can filler be metal seen from was similarFigure 3 to that that the in DTA previous curve research of the Al-6.5Si-10Cu [4,7]; it was not filler a single metal endothermicwas similar to peak that butin previous a certain research number [4,7]; of endothermic it was not a peaks, single andendothermic the corresponding peak but a temperaturescertain number were of endothermic a: 528.5 ◦C, b:peaks, 552.4 and◦C, th ande corresponding c: 587.8 ◦C. According temperatures to the were Al-Si-Cu, a: 528.5 Al-Cu,°C, b: 552.4 and °C, Al-Si and phase c: 587.8 diagram °C. According [16–18], to it the was Al-Si-Cu, inferred Al-Cu, that endothermic and Al-Si phase peak diagram a was related[16–18], to it the was reverse inferred reaction that endothermic of the L→α (Al)peak + a Si was + θ related(Al2Cu) to reaction, the reverse and reaction endothermic of the peaksL→α(Al) b and + Si c + were θ(Al relatively2Cu) reaction, smooth, and endothermic which was presumed peaks b and to c be were related relatively to the smooth, Al-Cu eutecticwhich was and presumed Al-Si eutectic to be reactions. related to the Al-Cu eutectic and Al-Si eutectic reactions. WhenWhen 1.01.0 wt.%,wt.%, 1.51.5 wt.%,wt.%, andand 2.02.0 wt.%wt.% NiNi werewere addedadded toto thethe ternaryternary fillerfiller metalmetal Al-Al- 6.5Si-20Cu,6.5Si-20Cu, thethe solidussolidus temperaturestemperatures of of thethe fillerfiller metalsmetals diddid notnot changechange obviously,obviously, but but the the liquidusliquidus temperature temperature was was somewhat somewhat reduced. reduced. The The solidus solidus and and liquidus liquidus temperatures temperatures of the of ◦ ◦ ◦ Al-Si-Cu-Nithe Al-Si-Cu-Ni alloys alloys were were 519.8~528.4 519.8~528.4C, 520.8~528.2 °C, 520.8~528.2C, and °C, 519.2~526.8 and 519.2~526.8C, respectively. °C, respec- Becausetively. Because the addition the addition of Ni introducesof Ni introduces a novel a novel low melting low melting temperature temperature alloy alloy reaction, reac- thetion, reverse the reverse reaction reaction of L→ ofα (Al)L→α +(Al) Si was + Si inhibited,was inhibited, and theand liquidus the liquidus temperature temperature of the of brazingthe brazing alloy alloy was reducedwas reduced [18]. The[18]. composition The composition of the Al-6.5Si-20Cu-2.0Niof the Al-6.5Si-20Cu-2.0Ni filler metal filler wasmetal close was toclose the to eutectic the eutectic point point of the of the Al-Si-Cu-Ni Al-Si-Cu-Ni quaternary quaternary alloy, alloy, so so the the DTA DTA curve curve ◦ ofof thethe fillerfiller metal metal was was relatively relatively gentle gentle between between 526.8 526.8 and and 530.1 530.1 °C [4,15,19],C[4,15, 19and], the and melt- the meltinging temperature temperature range range of Al-Si-Cu-Ni of Al-Si-Cu-Ni filler filler metals metals was was narrower narrower than than that that of Al-Si-Cu. of Al-Si- ◦ Cu.Compared Compared with with previous previous scholars' scholars’ research research on Al-10Si-15Cu-4Ni( on Al-10Si-15Cu-4Ni(ΔT ∆= T24.9 = 24.9 °C) [4],C) [Al-4], Appl. Sci. 2021, 11, 4296 5 of 14 Appl.Appl. Sci.Sci. 20212021,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 55 ofof 1414

◦ ◦ Al-8.5Si-25Cu-1.5Ni-0.2Re(8.5Si-25Cu-1.5Ni-0.2Re(8.5Si-25Cu-1.5Ni-0.2Re(ΔΔTT ∆ ==T 18.018.0 = 18.0 °C)[5],°C)[5],C) Al-10.8Si-10Ge( [Al-10.8Si-10Ge(5], Al-10.8Si-10Ge(ΔΔTT ==∆ 61.061.0T = °C) 61.0°C) [10][10]C) andand [10] Al-6.5Si- andAl-6.5Si- Al- ◦ 6.5Si-42Zn-Sr42Zn-Sr42Zn-Sr ((ΔΔTT = (=∆ 28.128.1T = °C) 28.1°C) [11],[11],C) [thethe11], meltingmelting the melting temperaturtemperatur temperatureee rangesranges ranges ofof thethe of theAl-6.5Si-20Cu,Al-6.5Si-20Cu, Al-6.5Si-20Cu, andand and Al-Al- Al-6.5Si-20Cu-(1.0,6.5Si-20Cu-(1.0,6.5Si-20Cu-(1.0, 1.5,1.5, 1.5, 2.0)2.0) 2.0) NiNi Ni fillerfiller filler metalsmetals metals werewere were narrower,narrower, narrower, which which will will facilitatefacilitate thethethe filling fillingfilling of ofof thethethe joint jointjoint gaps gapsgaps with withwith the thethe liquid liquidliquid filler fillerfiller metals metalsmetals during duringduring the thethe brazing brazingbrazing process processprocess [ 4[4,20].[4,20].,20].

FigureFigureFigure 3. 3.3. DTA DTADTA curves curvescurves of ofof the thethe ( a ((a)a) Al-6.5Si-10Cu,) Al-6.5Si-10Cu,Al-6.5Si-10Cu, (b (()bb Al-6.5Si-15Cu,)) Al-6.5Si-15Cu,Al-6.5Si-15Cu, (c ) ((c Al-6.5Si-20Cu,c)) Al-6.5Si-20Cu,Al-6.5Si-20Cu, (d ) ((d Al-6.5Si-20Cu-d)) Al-6.5Si-Al-6.5Si- 1.0Ni,20Cu-1.0Ni,20Cu-1.0Ni, (e) Al-6.5Si-20Cu-1.5Ni, ((ee)) Al-6.5Si-20Cu-1.5Ni,Al-6.5Si-20Cu-1.5Ni, and (f) andand Al-6.5Si-20Cu-2.0Ni ((ff)) Al-6.5Si-20Cu-2.0NiAl-6.5Si-20Cu-2.0Ni ﬁller metals. fillerfiller metals.metals.

3.2.3.2.3.2. Effect EffectEffect of ofof Cu CuCu and andand Ni NiNi on onon the thethe Microstructure MicrostructureMicrostructure of ofof Al-Si-Cu Al-Si-CuAl-Si-Cu (Ni) (Ni)(Ni) Filler FillerFiller Metals MetalsMetals TheTheThe microstructures microstructuresmicrostructures of ofof the the the ﬁller filler filler metals metalsmetals were were were analyzed analyzed analyzed by byby SEM, SEM,SEM, XRD, XRD,XRD, and and and EDS. EDS. EDS.Figure FigureFigure4 shows44 showsshows the thethe XRD XRDXRD patterns patternspatterns of Al-Si-Cu ofof Al-Si-CuAl-Si-Cu alloys alloysalloys and Al-Si-Cu-Niandand Al-Si-Cu-NiAl-Si-Cu-Ni alloys. alloys.alloys. The XRD TheThe results XRDXRD resultsresults show thatshowshow the thatthat Al-6.5Si-(10, thethe Al-6.5Si-(10,Al-6.5Si-(10, 15, 20) 15,15, Cu 20)20) were CuCu mainly werewere mainlymainly composed composedcomposed of the ofofα -Al thethe phase,αα-Al-Al phase,phase, a Si phase, aa SiSi phase,phase, and a θ(Al Cu) phase. The Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni were mainly composed of the α-Al andand aa2 θθ(Al(Al22Cu)Cu) phase.phase. TheThe Al-6.5Si-20Cu-(1.0,Al-6.5Si-20Cu-(1.0, 1.5,1.5, 2.0)2.0) NiNi werewere mainlymainly composedcomposed ofof thethe phase, a Si phase, a θ(Al Cu) phase, and δ-Al CuNi (Al Ni ). αα-Al-Al phase,phase, aa SiSi phase,phase, a2a θθ(Al(Al22Cu)Cu) phase,phase, andand3 δδ-Al-Al33CuNiCuNi3 (Al(Al2 33NiNi22).).

FigureFigureFigure 4. 4.4. XRD XRDXRD patterns patternspatterns of ofof the thethe ( (a(aa))) Al-Si-Cu Al-Si-CuAl-Si-Cu and andand ( (b(bb))) Al-Si-Cu-Ni Al-Si-Cu-NiAl-Si-Cu-Ni ﬁller fillerfiller metals. metals.metals.

FigureFigureFigure5 5 5shows showsshows the thethe SEM SEMSEM imagesimages ofof the the fillerfiller filler metals.metals. metals. FigureFigure Figure 5a–c5a–c5a–c indicatesindicates indicates thatthat that thethe the Al-Al- Al-6.5Si-(10,6.5Si-(10,6.5Si-(10, 15,15, 15, 20)20) 20) CuCu Cu fillerfiller filler metalsmetals metals areare are composedcomposed composed ofof thethe of the αα-Al-Alα -Al solidsolid solid solutionsolution solution phase,phase, phase, bulkbulk bulk pri-pri- primarymarymary Si,Si, Si, andand and aa θθ a(Al(Alθ(Al22Cu)Cu)2Cu) intermetallicintermetallic intermetallic compound.compound. compound. WithWith With increasingincreasing increasing CuCu Cu content,content, content, thethe the numbernumber num- θ berofof θθ of(Al(Al(Al22Cu)Cu)2Cu) phasesphases phases graduallygradually gradually increases,increases, increases, andand and thethe the sizesize size graduallygradually increases. increases. AccordingAccording to toto FigureFigureFigure5 d–f,5d–f,5d–f, the thethe Al-6.5Si-20Cu-(1.0, Al-6.5Si-20Cu-(1.0,Al-6.5Si-20Cu-(1.0, 1.5, 1.5,1.5, 2.0) 2.0)2.0) Ni Ni fillerNi fillerfiller metals metalsmetals had hadhad reticular reticularreticular structure, structure,structure, and theandand reticular solid solution phase was the δ(Al Ni ) phase according to the EDS analysis and thethe reticularreticular solidsolid solutionsolution phasephase waswas thethe 3 δδ(Al(Al2 33NiNi22)) phasephase accordingaccording toto thethe EDSEDS analysisanalysis XRDandand XRD patternsXRD patternspatterns [21,22 [21,22].].[21,22]. With increasingWithWith increasingincreasing Ni content, NiNi content,content, the number thethe numbernumber of bulk of primaryof bulkbulk primaryprimary Si phases SiSi decreased,phasesphases decreased,decreased, part of thepartpart Si ofof phase thethe SiSi was phasephase refined, waswas refined,refined, showing showingshowing a fibrous aa fibrous structure,fibrous structure,structure, and the andand brittle thethe θ(Al2Cu) phase gradually transformed from a bulk to a lamellar-type structure, as shown brittlebrittle θθ(Al(Al22Cu)Cu) phasephase graduallygradually transformedtransformed fromfrom aa bulkbulk toto aa lamellar-typelamellar-type structure,structure, asas in Figure5d–e. shownshown inin FigureFigure 5d–e.5d–e. Appl. Sci. 2021, 11, 4296 6 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 14

Figure 5. Microstructures of the ( a) Al-6.5Si-10Cu, ( b) Al-6.5Si-15Cu, ( c) Al-6.5Si-20Cu, ( d) Al- Al-6.5Si- 6.5Si-20Cu-1.0Ni,20Cu-1.0Ni, (e) Al-6.5Si-20Cu-1.5Ni, (e) Al-6.5Si-20Cu-1.5Ni, and ( fand) Al-6.5Si-20Cu-2.0Ni (f) Al-6.5Si-20Cu-2.0Ni ﬁller metals.filler metals.

To further analyze the elemental distribution of the fillerfiller metals and the influenceinfluence of Ni on the microstructure of the Al-Si-Cu fillerfiller metals,metals, an EDSEDS surfacesurface scanningscanning analysisanalysis was carried out on the microstructures of the Al-6.5Si-20Cu and Al-6.5Si-20Cu-1.5Ni filler filler metals. The EDS elementelement mappingmapping are are shown shown in in Figure Figure6, 6, and and the the EDS EDS results results are are shown shown in inTable Table3. The 3. The EDS EDS analysis analysis showed showed that Athat and A G and were G mainlywere mainly Si phases, Si phases, C and EC were and mainlyE were mainlyα-Al phases, α-Al phases, B and D B wereand D mainly were Almainly2Cu phases,Al2Cu phases, and F wasand mainlyF was mainly a δ(Al3 Nia δ2(Al) phase.3Ni2) phase.The element The element distribution distribution diagram diagram indicates indica thattes part that of part the Siof phasethe Si phase was embedded was embedded in the α θ in-Al the phase, α-Al phase, and part and of part the Cuof the was Cu replaced was replaced by Ni in by the Ni (Alin the2Cu) θ(Al phase2Cu) region, phase causingregion, θ causingthe local the intermetallic local intermetallic compound compound(Al2Cu) θ(Al to2 changeCu) to change from elongated from elongated to discontinuous. to discontinuous.The blade The shape blade can shape effectively can effectively reduce thereduce adverse the adverse effect of effect the intermetallicof the intermetallic compound com- θ(Al Cu) on the solder alloy. pound2 θ(Al2Cu) on the solder alloy. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 14 , , 4296 7 of 14

FigureFigure 6. 6. EDSEDS element element mapping mapping of of microstructures microstructures of of the the (a (a) )Al-6.5Si-20Cu Al-6.5Si-20Cu and and (b (b) )Al-6.5Si-20Cu- Al-6.5Si-20Cu- 1.5Ni filler ﬁller metals.

Table 3. EDS results on the chemical composition (wt.%) of points marked in Figure 6. Table 3. EDS results on the chemical composition (wt.%) of points marked in Figure6. Testing Points Al Si Cu Ni Testing Points Al Si Cu Ni A 1.05 98.88 0.06 0 A 1.05 98.88 0.06 0 B 42.20 0.23 57.57 0 B 42.20 0.23 57.57 0 CC 96.41 96.41 0.21 0.21 3.38 3.38 0 DD 41.63 41.63 0.06 0.06 57.18 57.18 0.82 0.82 EE 95.87 95.87 0.47 0.47 3.34 3.34 0.31 0.31 FF 37.47 37.47 0.12 0.12 52.64 52.64 9.78 9.78 G 1.06 98.26 0.31 0.38 G 1.06 98.26 0.31 0.38

3.3. Wettability of Al-Si-Cu (Ni) Filler Metal on Substrates 3.3. Wettability of Al-Si-Cu (Ni) Filler Metal on Substrates The fluidity of the filler metal was one of the factors affecting the quality of brazed The fluidity of the filler metal was one of the factors affecting the quality of brazed joints, and the spreading area of the brazing alloy on the base material can well reflect the joints, and the spreading area of the brazing alloy on the base material can well reflect the wetting and flowing joint-filling ability of brazing filler metal. The effects of different Cu wetting and flowing joint-filling ability of brazing filler metal. The effects of different Cu and Ni contents on the spreading properties of the Al-Si-Cu and Ni filler metals are shown and Ni contents on the spreading properties of the Al-Si-Cu and Ni filler metals are shown in Table4. The spreading area per unit mass of Al-Si-(10, 15, 20) Cu filler metals increased in Table 4. The spreading area per unit mass of Al-Si-(10, 15, 20) Cu filler metals increased from 0.58 cm2 to 2.87 cm2. Compared with the Al-6.5Si-20Cu filler metal, the spreading from 0.58 cm2 to 2.87 cm2. Compared with the Al-6.5Si-20Cu filler metal, the spreading area per unit mass of Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni filler metals increased by 12.5%, 8.0%, area per unit mass of Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni filler metals increased by 12.5%, 8.0%, and 6.8%, respectively. When 2.0 wt.% Ni was added, the liquidus temperature of the filler and 6.8%, respectively. When 2.0 wt.% Ni was added, the liquidus temperature of the filler metal increased, the melting range became wider, and the spreading ability decreased, metal increased, the melting range became wider, and the spreading ability decreased, Appl.Appl. Sci. Sci.2021 202111, 11, x FOR PEER REVIEW 8 of 14 , , 4296 8 of 14

which may be due to the formation of more complex alloy compounds between Ni and which may be due to the formation of more complex alloy compounds between Ni and Al, Al, which affected the fluidity of the filler metal [21–23]. which affected the ﬂuidity of the ﬁller metal [21–23].

Table 4. Analysis of the spreading properties of the filler metals. Table 4. Analysis of the spreading properties of the ﬁller metals. Filler Metals Spread Area per Unit Mass (cm2/g) Filler Metals Spread Area per Unit Mass (cm2/g) Al-6.5Si-10Cu 0.58 Al-6.5Si-15CuAl-6.5Si-10Cu 1.71 0.58 Al-6.5Si-15Cu 1.71 Al-6.5Si-20CuAl-6.5Si-20Cu 2.87 2.87 Al-6.5Si-20Cu-1.0NiAl-6.5Si-20Cu-1.0Ni 3.28 3.28 Al-6.5Si-20Cu-1.5NiAl-6.5Si-20Cu-1.5Ni 3.12 3.12 Al-6.5Si-20Cu-2.0NiAl-6.5Si-20Cu-2.0Ni 3.08 3.08

FigureFigure7 shows7 shows the microstructuresthe microstructures of the spreadingof the spreading interface ofinterface the Al-6.5Si-20Cu/base of the Al-6.5Si- material20Cu/base and material the Al-6.5Si-20Cu-1.5Ni/base and the Al-6.5Si-20Cu-1.5Ni/base material. According material. toAccording Fick’s law to ofFick's diffusion, law of therediffusion, was athere large was concentration a large concentration difference diff betweenerence thebetween filler the metal filler and metal the and base the metal; base therefore,metal; therefore, Cu in the Cu filler in the metal filler diffusedmetal diff intoused the into base the metal base metal along along the grain the grain boundaries bound- andaries reacted and reacted with Al with in theAl in base the material base material to form toθ form(Al2Cu) θ(Al intermetallic2Cu) intermetallic compounds. compounds. The baseThe materialsbase materials interlaced interlaced with with each each other othe at ther at spreading the spreading interface, interface, forming forming a diffusion a diffu- layersion betweenlayer between the filler the metal filler and metal the baseand material.the base Thematerial. microstructures The microstructures of the spreading of the interfacespreading in interface Figure7c,d in wereFigure analyzed 7c,d were by EDS,analyzed and by the EDS, results and are the shown results in Tableare shown5 and in FigureTable8 5. and Table Figure5 shows 8. thatTable the 5 microstructure shows that the of microstructure the Al-6.5Si-20Cu of fillerthe Al-6.5Si-20Cu metal/Al spread- filler ingmetal/Al interface spreading was mainly interface composed was ofmain threely composed phases: the ofα –Althree phase, phases: a θ (Althe2 αCu)–Al phase, phase, and a θ a(Al Si phase.2Cu) phase, Figure and8 shows a Si phase. that NiFigure replaced 8 shows a certain that Ni amount replaced of Cu a certain in the microstructureamount of Cu in ofthe the microstructure Al-6.5Si-20Cu-1.5Ni of the Al-6.5Si-20Cu-1.5Ni filler metal/Al spreading filler metal/Al interface spreading such thatthe interface Cu and such Ni arethat interlaced,the Cu and which Ni are weakened interlaced, thewhich influence weakened of Cu the on influence the brittleness of Cu on of the the brittleness brazed alloys. of the Therefore,brazed alloys. the addition Therefore, of Ni the can addition not only of advance Ni can thenot interdiffusiononly advance ofthe elements interdiffusion between of theelements filler metal between and thethe basefiller metal, metal but and also the refine base themetal, grains but andalso improve refine the the grains performance and im- ofprove the filler the performance metal. of the filler metal.

FigureFigure 7. 7.Microstructures Microstructures of of the the spreading spreading interface interface of theof the Al-6.5Si-20Cu Al-6.5Si-20Cu (a,c )(a and,c) and Al-6.5Si-20Cu-1.5Ni Al-6.5Si-20Cu- 1.5Ni (b,d) filler metal. (b,d) ﬁller metal.

Table 5. Chemical composition (wt.%) of the points in Figure 7c.

Testing Points Al Si Cu A 90.27 1.70 8.03 Appl. Sci. 2021, 11, 4296 9 of 14

Table 5. Chemical composition (wt.%) of the points in Figure7c. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 14 Testing Points Al Si Cu A 90.27 1.70 8.03 B 40.36 1.15 58.49 BC 40.36 51.06 1.15 6.77 58.49 42.17 C 51.06 6.77 42.17

Figure 8. Line-EDS analysis analysis of of the the spreading spreading interface interface in in Figure Figure 7d:7d: ( (aa) )Cu, Cu, Ni Ni elements; elements; (b (b) Al,) Al, Si, Si, Mg elements.

3.4. Brazeability of 6063 Aluminum A Alloylloy with Al-Si-Cu (Ni) Filler Metal Figure 99 showsshows thethe microstructuremicrostructure ofof thethe aluminumaluminum alloyalloy brazedbrazed jointsjoints afterafter heatheat treatment strengthening. The The microstructures microstructures of of the the brazed brazed joints were compactcompact andand showed good metallurgical bonding. Table 66 shows shows thethe resultsresults ofof thethe EDSEDS analysisanalysis ofof thethe six points marked in Figure 99a,c.a,c. ItIt cancan bebe concludedconcluded thatthat thethe microstructuremicrostructure ofof thethe Al-Al- α θ 6.5Si-20Cu brazed joint was mainly composed of the α-Al phase, a coarse θ(Al2Cu) phase, and a Si phase. There was a small amount of Al2Cu intermetallic compound at the brazed joint interface, and and some Si particles were embedded in the matrix [[14,24].14,24]. Figure 99b–db–d show the microstructure diagrams of Al-6.5Si-20Cu-Ni brazed joints. The addition of Ni produces the δ(Al Ni ) phase, which makes the Al Cu at the brazing seams transition produces the δ(Al3Ni3 2)2 phase, which makes the Al2Cu2 at the brazing seams transition from afrom bulk a to bulk a lamellar-type to a lamellar-type structure, structure, and the and microstructure the microstructure distribution distribution of the joints of the is jointsmore monotonous.is more monotonous. Figure 10 Figure shows 10 the shows line scan the linedistribution scan distribution diagram of diagram the elements of the elementsin Figure 9a.in Figure In the9 brazinga . In the process, brazing Cu process, and Si were Cu and enriched Si were at enrichedthe interface at the between interface the between brazing the brazing seam and the base metal, and the post-welding heat treatment promoted the seam and the base metal, and the post-welding heat treatment promoted the interdiffu- interdiffusion between atoms, so the contents of Al and Mg at the interface of the brazing sion between atoms, so the contents of Al and Mg at the interface of the brazing seam were seam were significantly reduced [4,11]. Figure 11 shows a schematic diagram of the surface significantly reduced [4,11]. Figure 11 shows a schematic diagram of the surface scanning scanning consequences of the microstructure of Al-6.5Si-20Cu-1.5Ni-brazed 6063 aluminum consequences of the microstructure of Al-6.5Si-20Cu-1.5Ni-brazed 6063 aluminum alloy alloy joints. Cu and Ni were uniformly distributed in the central area of the brazing seams, joints. Cu and Ni were uniformly distributed in the central area of the brazing seams, the the segregation of Si was reduced, which was conducive to reducing the Si phase, and segregation of Si was reduced, which was conducive to reducing the Si phase, and the the θ(Al2Cu) phase influenced the brittleness of the brazed joints. The results of the EDS θ(Al2Cu) phase influenced the brittleness of the brazed joints. The results of the EDS anal- analysis (Table6) show that the Si and Cu contents at B and F were higher than those of ysis (Table 6) show that the Si and Cu contents at B and F were higher than those of the the base material at the joint interface, which indicates that during the brazing process base material at the joint interface, which indicates that during the brazing process of Al- of Al-Si-Cu (Ni) filler metals and 6063 aluminum alloy, the elements between the liquid Si-Cu (Ni) filler metals and 6063 aluminum alloy, the elements between the liquid solder solder and the base metal diffused, forming a diffusion layer. The post-weld heat treatment andenhanced the base the metal diffusion diffused, of elements forming and a diffus effectivelyion layer. promoted The post-weld the metallurgical heat treatment bonding en- hancedreaction the between diffusion the fillerof elements metal andand the effect baseively metal promoted [20]. the metallurgical bonding reaction between the filler metal and the base metal [20].

Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 14

B 40.36 1.15 58.49 C 51.06 6.77 42.17

Figure 8. Line-EDS analysis of the spreading interface in Figure 7d: (a) Cu, Ni elements; (b) Al, Si, Mg elements.

3.4. Brazeability of 6063 Aluminum Alloy with Al-Si-Cu (Ni) Filler Metal Figure 9 shows the microstructure of the aluminum alloy brazed joints after heat treatment strengthening. The microstructures of the brazed joints were compact and showed good metallurgical bonding. Table 6 shows the results of the EDS analysis of the six points marked in Figure 9a,c. It can be concluded that the microstructure of the Al- 6.5Si-20Cu brazed joint was mainly composed of the α-Al phase, a coarse θ(Al2Cu) phase, and a Si phase. There was a small amount of Al2Cu intermetallic compound at the brazed joint interface, and some Si particles were embedded in the matrix [14,24]. Figure 9b–d show the microstructure diagrams of Al-6.5Si-20Cu-Ni brazed joints. The addition of Ni produces the δ(Al3Ni2) phase, which makes the Al2Cu at the brazing seams transition from a bulk to a lamellar-type structure, and the microstructure distribution of the joints is more monotonous. Figure 10 shows the line scan distribution diagram of the elements in Figure 9a. In the brazing process, Cu and Si were enriched at the interface between the brazing seam and the base metal, and the post-welding heat treatment promoted the interdiffusion between atoms, so the contents of Al and Mg at the interface of the brazing seam were significantly reduced [4,11]. Figure 11 shows a schematic diagram of the surface scanning consequences of the microstructure of Al-6.5Si-20Cu-1.5Ni-brazed 6063 aluminum alloy joints. Cu and Ni were uniformly distributed in the central area of the brazing seams, the segregation of Si was reduced, which was conducive to reducing the Si phase, and the θ(Al2Cu) phase influenced the brittleness of the brazed joints. The results of the EDS analysis (Table 6) show that the Si and Cu contents at B and F were higher than those of the base material at the joint interface, which indicates that during the brazing process of Al- Si-Cu (Ni) filler metals and 6063 aluminum alloy, the elements between the liquid solder Appl. Sci. 2021, 11, 4296 and the base metal diffused, forming a diffusion layer. The post-weld heat treatment10 of en- 14 hanced the diffusion of elements and effectively promoted the metallurgical bonding reaction between the filler metal and the base metal [20].

Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14

Figure 9. Microstructures of the (a) Al-6.5Si-20Cu, (b) Al-6.5Si-20Cu-1.0Ni, (c) Al-6.5Si-20Cu-1.5Ni, FigureFigure 9. 9.Microstructures Microstructures of of the the ( a(a)) Al-6.5Si-20Cu, Al-6.5Si-20Cu, ( b(b)) Al-6.5Si-20Cu-1.0Ni, Al-6.5Si-20Cu-1.0Ni, ((cc)) Al-6.5Si-20Cu-1.5Ni,Al-6.5Si-20Cu-1.5Ni, and (d) Al-6.5Si-20Cu-2.0Ni joints after brazing at 570 °C for 10 min. andand ( d(d)) Al-6.5Si-20Cu-2.0Ni Al-6.5Si-20Cu-2.0Ni joints joints after after brazing brazing at at 570 570◦ C°C for for 10 10 min. min.

Table 6. Chemical composition (wt.%) of the points in Figure 9. TableTable 6. 6.Chemical Chemical composition composition (wt.%) (wt.%) of of the the points points in in Figure Figure9. 9. Testing Points Al Si Cu Ni Mg TestingTesting Points Points Al Al Si Si Cu Cu Ni Ni Mg Mg A 49.04 0.85 49.76 0 0.35 AA 49.0449.04 0.850.85 49.7649.76 00 0.350.35 B 96.46 0.62 2.61 0 0.30 BB 96.4696.46 0.620.62 2.612.61 00 0.300.30 C 34.12 61.89 3.99 0 0 CC 34.1234.12 61.89 61.89 3.993.99 00 0 0 DD 23.6723.67 74.83 74.83 1.071.07 0.290.29 0.150.15 D 23.67 74.83 1.07 0.29 0.15 EE 59.9559.950.85 0.85 29.8329.83 9.379.37 0 0 E 59.95 0.85 29.83 9.37 0 FF 90.5990.592.51 2.51 6.596.59 0.310.31 0 0 F 90.59 2.51 6.59 0.31 0

Figure 10. Elemental line scanning results for Figure 9a. Figure 10. Elemental line scanning results for Figure 9a. Figure 10. Elemental line scanning results for Figure9a.

Figure 12a shows that the shear strength of the Al-6.5Si-20Cu brazed joint was approximately 117.2 MPa, and the shear strength of the Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni brazed joints gradually increased with the increase in Ni content to 123.6 MPa, 136.3 MPa, and 150.4 MPa, compared with the Al-6.5Si-20Cu brazed joints whose shear strength increased by 5.46%, 16.30%, and 28.32%, respectively. Compared with the joint shear strength (62.6 MPa) obtained by brazing 6063 aluminum alloy with the Al-8.5Si-25Cu-1.5Ni-0.2Re ﬁller metal studied by Zhang et al. [5], the shear strength (136.3 MPa) of the joint brazed with the Al- 6.5Si-20Cu-1.5Ni ﬁller metal developed in this experiment increased by 117.73%. Figure 12b

Figure 11. Element mapping of the 6063 aluminum alloy joint brazed with Al-6.5Si-20Cu-1.5Ni Figure 11. Element mapping of the 6063 aluminum alloy joint brazed with Al-6.5Si-20Cu-1.5Ni filler metal at 570 °C for 10 min. filler metal at 570 °C for 10 min. Figure 12a shows that the shear strength of the Al-6.5Si-20Cu brazed joint was ap- Figure 12a shows that the shear strength of the Al-6.5Si-20Cu brazed joint was approximately 117.2 MPa, and the shear strength of the Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni proximately 117.2 MPa, and the shear strength of the Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 14

Figure 9. Microstructures of the (a) Al-6.5Si-20Cu, (b) Al-6.5Si-20Cu-1.0Ni, (c) Al-6.5Si-20Cu-1.5Ni, and (d) Al-6.5Si-20Cu-2.0Ni joints after brazing at 570 °C for 10 min.

Table 6. Chemical composition (wt.%) of the points in Figure 9.

Testing Points Al Si Cu Ni Mg A 49.04 0.85 49.76 0 0.35 B 96.46 0.62 2.61 0 0.30 C 34.12 61.89 3.99 0 0 D 23.67 74.83 1.07 0.29 0.15 E 59.95 0.85 29.83 9.37 0 F 90.59 2.51 6.59 0.31 0

Appl. Sci. 2021, 11, 4296 11 of 14

shows the microhardness curve of the brazed joints. There were Al2Cu intermetallic compounds and primary Si in the brazed seam center area of the weld [2,7,25], and the hardness of this zone was 281~310 HV. During the brazing process, the elements between the ﬁller metal and the base metal diffused to form a diffusion zone whose hardness was slightly higher than that of the base metal. The hardness of the interface area of the brazed seam was 107~216 HV, and the hardness of the base metal was 102~105 HV. Therefore, the microhardness of the brazing seam appears to decrease from the brazed seam center area Appl. Sci. 2021, 11, x FOR PEER REVIEWto the base metal. 11 of 14 Figure 10. Elemental line scanning results for Figure 9a.

brazed joints gradually increased with the increase in Ni content to 123.6 MPa, 136.3 MPa, and 150.4 MPa, compared with the Al-6.5Si-20Cu brazed joints whose shear strength increased by 5.46%, 16.30%, and 28.32%, respectively. Compared with the joint shear strength (62.6 MPa) obtained by brazing 6063 aluminum alloy with the Al-8.5Si-25Cu- 1.5Ni-0.2Re filler metal studied by Zhang et al. [5], the shear strength (136.3 MPa) of the joint brazed with the Al-6.5Si-20Cu-1.5Ni filler metal developed in this experiment increased by 117.73%. Figure 12b shows the microhardness curve of the brazed joints. There were Al2Cu intermetallic compounds and primary Si in the brazed seam center area of the weld [2,7,25], and the hardness of this zone was 281~310 HV. During the brazing process, the elements between the filler metal and the base metal diffused to form a diffusion zone whose hardness was slightly higher than that of the base metal. The hardness of the interface area of the brazed seam was 107~216 HV, and the hardness of the base metal was 102~105 HV. Therefore, the microhardness of the brazing seam appears to decrease from Figure 11. Elementthe mapping brazed seamof the 6063center aluminum area to theallo basey joint metal. brazed with Al-6.5Si-20Cu-1.5Ni Figure 11. Element mapping of the 6063 aluminum alloy joint brazed with Al-6.5Si-20Cu-1.5Ni filler metal at 570 ◦C for 10 min. filler metal at 570 °C for 10 min.

Figure 12a shows that the shear strength of the Al-6.5Si-20Cu brazed joint was approximately 117.2 MPa, and the shear strength of the Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni

Figure 12. (a) Shear Shear strength strength of of 6063 6063 aluminum aluminum alloy alloy joints joints after after brazing brazing with with Al Al-Si-Cu-Si-Cu (Ni) (Ni) filler filler ◦ metals atat 570570 °CC for 10 min. (b) Microhardness ofof thethe brazedbrazed joints.joints. Figure 13 shows the shear fracture morphology of 6063 aluminum alloy joints brazed Figure 13 shows the shear fracture morphology of 6063 aluminum alloy joints brazed with Al-Si-20Cu and Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni filler metals. The fracture morphology with Al-Si-20Cu and Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni filler metals. The fracture morphology of the brazed joints indicates the characteristics of brittle fracture. Table7 lists the results of of the brazed joints indicates the characteristics of brittle fracture. Table 7 lists the results the EDS analysis of the typical phases of the fracture morphology of Al-Si-Cu (Ni) brazing of the EDS analysis of the typical phases of the fracture morphology of Al-Si-Cu (Ni) braz- joints. The results show that A, B, and C were the primary Si phase, the θ(Al2Cu) phase, ing joints. The results show that A, B, and C were the primary Si phase, the θ(Al2Cu) and the θ(Al2Cu) + δ(Al3Ni2) phase, respectively. In the process of brazing, elements in the phase, and the θ(Al2Cu) + δ(Al3Ni2) phase, respectively. In the process of brazing, elements matrix and liquid filler metal inter-diffused, so that the filler metal and the base metal had in the matrix and liquid filler metal inter-diffused, so that the filler metal and the base a better metallurgical bond. A small amount of Mg was detected in the joint fracture, so metal had a better metallurgical bond. A small amount of Mg was detected in the joint it can be judged that the fracture location of the brazed joint was roughly in the interface fracture, so it can be judged that the fracture location of the brazed joint was roughly in area of the brazed seam. the interfaceBased on area the of analysis the brazed of the seam. microstructure, shear strength, and shear fracture mor- phologyBased of on the the joint, analysis when of Al-6.5Si-20Cu the microstructure, brazed shear the 6063 strength, aluminum and shear alloy, fracture there was mor- a phology of the joint, when Al-6.5Si-20Cu brazed the 6063 aluminum alloy, there was a large θ(Al2Cu) brittle phase in the brazing seam, but the large θ(Al2Cu) phase had poor compatibilitylarge θ(Al2Cu) with brittle the phase substrate in the deformation, brazing seam, the but strain the waslarge concentrated θ(Al2Cu) phase at the had phase poor compatibility with the substrate deformation, the strain was concentrated at the phase interface, the crack initiation near the θ(Al2Cu) phase was accelerated, and the fracture of the welded joint was accelerated, so the shear fracture of the brazed joint was a smooth and typical brittle fracture. When 6063 aluminum alloy was brazed with the Al-6.5Si- 20Cu-Ni filler metal, the existence of Ni not only made the brittle phase of the Al2Cu finer and more dispersed but also improved the deformation coordination of the θ(Al2Cu) phase and matrix and the shear strength of the brazed joint, reduced the strain concentration degree, and presented a mixed fracture mode. Appl. Sci. 2021, 11, 4296 12 of 14

interface, the crack initiation near the θ(Al2Cu) phase was accelerated, and the fracture of the welded joint was accelerated, so the shear fracture of the brazed joint was a smooth and typical brittle fracture. When 6063 aluminum alloy was brazed with the Al-6.5Si-20Cu-Ni ﬁller metal, the existence of Ni not only made the brittle phase of the Al2Cu ﬁner and more dispersed but also improved the deformation coordination of the θ(Al2Cu) phase and Appl. Sci. 2021, 11, x FOR PEER REVIEWmatrix and the shear strength of the brazed joint, reduced the strain concentration12 degree, of 14

and presented a mixed fracture mode.

FigureFigure 13. 13. ShearShear fractographs fractographs of of the the 6063 6063 aluminum aluminum alloy alloy joints joints brazed brazed with with ( (aa)) Al-Si-20Cu, Al-Si-20Cu, ( (bb)) Al- Al- Si-20Cu-1.0Ni,Si-20Cu-1.0Ni, ( (cc)) Al-Si-20Cu-1.5Ni, Al-Si-20Cu-1.5Ni, and and ( d) Al-Si-20Cu-2.0Ni filler ﬁller metals.

Table 7. Chemical composition (wt.%) of the points in Figure 13. Table 7. Chemical composition (wt.%) of the points in Figure 13. Points Al Si Cu Ni Mg Points Al Si Cu Ni Mg A 5.06 92.79 2.08 0 0.06 A 5.06 92.79 2.08 0 0.06 B 43.70 0.94 54.68 0 0.67 B 43.70 0.94 54.68 0 0.67 CC 40.66 40.66 1.42 1.42 55.12 55.12 1.74 1.74 1.05 1.05

4.4. Conclusions Conclusions InIn this this paper, a new low-melting-point Al-Si-Cu (Ni) filler filler metal was designed and prepared. TheThe meltingmelting characteristics, characteristics, microstructure, microstructure, and and spreading spreading wettability wettability of the of filler the fillermetal metal were were researched researched and the and microstructure the microstructure and mechanical and mechanical properties properties of the joint of the brazed joint brazedwith 6063 with aluminum 6063 aluminum alloy were alloy studied. were studied. The conclusions The conclusions can be summarized can be summarized as follows: as follows: (1) The Al-Si-Cu filler metal was mainly composed of α-Al solid solution, a θ(Al2Cu) intermetallic(1) The Al-Si-Cu compound, filler and metal bulk was primary mainly Si. composed With increasing of α-Al Cu, solid the meltingsolution, temperature a θ(Al2Cu) intermetallicof the Al-Si-Cu compound, filler metal and decreased, bulk primary the Si. melting With increasing interval became Cu, the narrower, melting tempera- and the turespreading of the performanceAl-Si-Cu filler improved. metal decreased, The melting the melting range of interval the Al-6.5Si-20Cu became narrower, filler metal andwas the spreading521.4~533.4 performance◦C, and the improved. spreading areaThe melting per unit range mass of was the 2.87 Al-6.5Si-20Cu cm2. filler metal was 2 521.4~533.4(2) The °C, Al-Si-Cu-Ni and the spreading filler metal area was per mainly unit composedmass was of2.87α-Al cm solid. solution, a θ(Al2Cu) intermetallic(2) The Al-Si-Cu-Ni compound, bulkfiller primarymetal was Si, andmainly a δ(Al composed3Ni2) intermetallic of α-Al solid compound. solution, The a θaddition(Al2Cu) ofintermetallic Ni not only compound, narrowed thebulk melting primary range Si, ofand the a Al-Si-Cuδ(Al3Ni2)filler intermetallic metal but com- also pound.transformed The addition the coarse, of Ni brittle not Alonly2Cu narrowed phase from the a melting bulk to arange lamellar-type of the Al-Si-Cu structure, filler and metal the butstructure also transformed was refined. the With coarse, increasing brittle Ni Al content,2Cu phase the from spreading a bulk abilityto a lamellar-type of the Al-Si-Cu-Ni structure,filler and metal the decreased. structure Thewas meltingrefined. rangesWith incr of theeasing Al-Si-Cu-Ni Ni content, solder the spreading were 8.6 ◦ C,ability 7.7 ◦ ofC, the Al-Si-Cu-Ni filler metal decreased. The melting ranges of the Al-Si-Cu-Ni solder were 8.6 °C, 7.7 °C, and 7.6 °C, and the spreading areas per unit mass were 3.28 cm2, 3.12 cm2, and 3.02 cm2, respectively. (3) When Al-6.5Si-20Cu and Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni filler metals were used to braze 6063 aluminum alloy at 570 °C for 10 min, brazing joints with shear strengths of 117.2 MPa, 123.6 MPa, 136.3 MPa, and 150.4 MPa were obtained. Therefore, the addition of Ni can effectively increase the shear strength of the brazed joints. Appl. Sci. 2021, 11, 4296 13 of 14

and 7.6 ◦C, and the spreading areas per unit mass were 3.28 cm2, 3.12 cm2, and 3.02 cm2, respectively. (3) When Al-6.5Si-20Cu and Al-6.5Si-20Cu-(1.0, 1.5, 2.0) Ni ﬁller metals were used to braze 6063 aluminum alloy at 570 ◦C for 10 min, brazing joints with shear strengths of 117.2 MPa, 123.6 MPa, 136.3 MPa, and 150.4 MPa were obtained. Therefore, the addition of Ni can effectively increase the shear strength of the brazed joints.

Author Contributions: Conceptualization, C.P. and F.Z.; methodology, C.P. and F.Z.; software, C.P. and F.Z.; validation, C.P., F.Z., and X.D.; formal analysis, C.P. and F.Z.; investigation, D.Z. and K.L.; resources, M.W. and Y.T.; data curation, C.P. and X.D.; writing—original draft preparation, C.P.; writing—review and editing, C.P.; visualization, X.D.; supervision, D.Z. and K.L.; project administration, M.W. and Y.T.; funding acquisition, F.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the key special project of “Science and Technology Boost Economy 2020” of the Ministry of Science and Technology of China and the Science and Technology Program of Guizhou Province with Grant Nos. 20194106 and 20165654, 2021109. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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