Vacuum Brazing of 55 Vol.% Sicp/ZL102 Composites Using Micro-Nano Brazing Filler Metal Fabricated by Melt-Spinning

metals

Article Vacuum Brazing of 55 vol.% SiCp/ZL102 Composites Using Micro-Nano Brazing Filler Metal Fabricated by Melt-Spinning

Dechao Qiu 1, Zeng Gao 1,*, Xianli Ba 1, Zhenjiang Wang 1 and Jitai Niu 1,2,3 1 School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454003, China; [email protected] (D.Q.); [email protected] (X.B.); [email protected] (Z.W.); [email protected] (J.N.) 2 State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, China 3 Henan Jingtai High-Novel Materials Ltd. of Science and Technology, Jiaozuo 454003, China * Correspondence: [email protected] or [email protected]; Tel.: +86-391-396-6901

Received: 17 September 2020; Accepted: 27 October 2020; Published: 4 November 2020

Abstract: The joining methods of Aluminum matrix composites reinforced with SiC particles (SiCp/Al MMCs) are a challenge during the manufacturing process due to the significant differences between SiC particles and base aluminum in terms of both physical and chemical properties. Micro-nano brazing filler metal Al-17.0Cu-8.0Mg fabricated by melt-spinning technology was employed to deal with the joining problem of 55 vol.% SiCp/ZL102 composites in this work. The result indicated that the foil-like brazing filler metal contained uniformed cellular nano grains, with a size less than 200 nm. The solidus and liquidus temperatures of the foil-like brazing filler metal decreased by 4 ◦C and 7 ◦C in comparison with the values of the as-cast brazing filler metal due to the nanometer size effect. The maximum joint shear strength of 98.17 MPa achieved with a brazing temperature of 580 ◦C and holding time of 30 min was applied in vacuum brazing process. The width of the brazing seam became narrower and narrower with increasing brazing temperature owning to the strong interaction between the micro-nano brazing filler metal and 55 vol.% SiCp/ZL102 composites. The fracture morphology of the joint made at a brazing temperature of 580 ◦C was characterized by quasi-cleavage fracture. After brazing, the chemical concentration gradient between the brazing filler metal and base material disappeared.

Keywords: SiCp/ZL102 composites; micro-nano brazing ﬁller metal; vacuum brazing; microstructure; shear strength

1. Introduction

Aluminum matrix composites reinforced with SiC particles (SiCp/Al MMCs) have attracted more and more attention from researchers around the world because of their excellent performances such as high specific stiffness and specific strength, low thermal expansion coefficient, good radiation resistance, and so on [1–4]. In recent years, SiCp/Al MMCs have been widely used in aerospace, weaponry, and electronics industries [5–8]. SiCp/Al MMCs are also favored in the field of electronic packaging due to their high dimensional stability and thermal conductivity. However, the joining methods of SiCp/Al MMCs are still a challenge for researchers during the manufacturing process due to the significant differences between SiC particles and base aluminum in terms of both physical and chemical properties, which severely limit the promotion and application of SiCp/Al MMCs [9–12]. At present, joining methods of SiCp/Al MMCs mainly focus on fusion welding [13–15], diffusion welding [16,17], and brazing [18–20]. However, it is difficult to obtain high-quality welded

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joints by those conventional joining methods. For example, when fusion welding was applied to SiCp/Al MMCs, the segregation of SiC particles appeared in a molten pool due to the poor liquidity. In addition, a mass of brittle phases such as Al4C3 would be generated at the weld seam [21,22]. Wang X.H. [23] et al. conducted TIG welding on 15 vol.% SiCp/6061Al composites and found that the brittle phase of Al4C3 was formed when SiC particles reacted with base Al at high temperature. In order to reduce the heat input, Pichumani S. [24] et al. used pulse current TIG welding during the welding of 8 vol.% SiCp/Al composites. However, the Al4C3 phase was still produced during the welding, which significantly affected the performance of the welded joint. When diffusion welding was used to weld high-volume fraction SiCp/Al MMCs, the aluminum oxide film produced during the welding process was difficult to remove, which damaged the continuity of the SiC particle-reinforced phase. More importantly, diffusion welding requires a stricter welding sample size and shape. As a consequence, it is hard to employ diffusion welding to satisfy the complex engineering applications of SiCp/Al MMCs [25,26]. Brazing has the characteristics of a low welding temperature, uniform heating of the welding sample, and less thermal deformation. Therefore, it is considered to be one of the most effective joining methods for welding high-volume fraction SiCp/Al MMCs. To braze SiCp/Al composites, Wu M. [27] et al. reduced the melting point of brazing filler metal by adding elements Ni and Cu into Al-Si eutectic brazing filler metal, which made the brazing temperature lower and increased the joint performance. Wang P. [28] et al. used active brazing filler metal Al-12Si-1.5Mg-4Ti to braze nickel-plated SiCp/Al MMCs and found that the active brazing filler metal had strong wettability to SiCp/Al MMCs. Bangash, M.K. [29] et al. used Al-Cu-Mg and Al-Si-Mg-Ti two aluminum-based brazing filler metals to braze Al-6016 aluminum alloy plates and aluminum alloy foams. It was found that the Al-Cu-Mg amorphous brazing filler metal had higher diffusion into the alloy than the Al-Si-Mg-Ti amorphous bonding alloy. Gao Z. [30] et al. added Ni to Al-Cu-Mg brazing filler metal for brazing SiCp/Al MMCs. A brazed joint with better performance, better wettability, and fluidity was obtained. It is very important to improve the wettability between the brazing filler metal and SiCp/Al MMCs by selecting the appropriate alloying elements. It is well known that SiCp/Al MMCs oxidize easily in the atmosphere environment due the strong affinity between aluminum and oxygen; the resulting oxide film limits the wetting and spreading of the brazing filler metal, which makes it difficult to form high-quality brazed joints. In view of this, a self-developed micro-nano foil-like brazing filler metal was utilized in this research to braze 55 vol.% SiCp/ZL102 composites in a vacuum environment. The thermodynamic properties, microstructural characteristics and phase composition of the brazing filler metal produced by melt-spinning technology were studied primarily. Different brazing temperatures were applied in this research to evaluate the influence of a micro-nano sized brazing filler metal on the microstructure evolution, element distribution, and mechanical property of the joints. The fracture morphology of the brazed joint at the optimal brazing temperature was investigated as well. This research provides a new possible means for the joining of high-volume fraction SiCp/Al MMCs.

2. Materials and Methods

In this experiment, 55 vol.% SiCp/ZL102 composites made by pressureless infiltration technology were used as brazing specimen. The microstructure is shown in Figure1 and was composed of SiC particles with 55% volume and a ZL102 aluminum alloy matrix. The chemical composition of the ZL102 aluminum alloy is shown in Table1. Brazing specimens with the size of 20 mm 10 mm 2 mm × × were cut by a wire cut electric discharge machine (DK7745, Suzhou Baojun CNC Equipment Co., Ltd., Suzhou, China). To prepare the micro-nano brazing filler metal, pure Al and Cu, as well as an Al50-Mg master alloy with a purity of 99.99% were melted in a vacuum induction furnace and cast in an iron mold three times to remove impurities in the raw materials. After smelting, the as-cast brazing filler metal alloy with the chemical composition of Al-17.0Cu-8.0Mg was obtained. The metallographic structure of the as-cast brazing filler metal is shown in Figure2. Then, the as-cast brazing filler metal alloy was moved into the quartz glass tube in a melt-spinning machine. The melt-spinning was also Metals 2020, 10, x FOR PEER REVIEW 3 of 13 shownMetals 2020 in, Figure10, 1470x FOR 2. PEER Then, REVIEW the as-cast brazing filler metal alloy was moved into the quartz glass3 tube of 13 in a melt-spinning machine. The melt-spinning was also carried out in a vacuum environment with shown in Figure 2. Then, the as-cast−2 brazing filler metal alloy was moved into the quartz glass tube acarried vacuum out degree in a vacuum of 2.0 environment× 10 Pa. In withorder a to vacuum obtain degreedefect-free of 2.0 and10 micro-nano2 Pa. In order sized to foil-like obtain in a melt-spinning machine. The melt-spinning was also carried out in× a vacuum− environment with brazingdefect-free filler and metal, micro-nano the rotational sized foil-like speed brazingof the copper filler metal, roller the was rotational set to 1400 speed r/min, of the and copper the argon roller aMetals vacuum 2020, 10 degree, x FOR PEERof 2.0 REVIEW × 10 −2 Pa. In order to obtain defect-free and micro-nano sized foil-like3 of 13 wasinjection set to pressure 1400 r/min, in the and quartz the argon glass injectiontube was pressure set to 0.02 in theMPa. quartz The prepared glass tube foil-like was set brazing to 0.02 MPa.filler brazing filler metal, the rotational speed of the copper roller was set to 1400 r/min, and the argon Themetalshown prepared with in Figure thickness foil-like 2. Then, of brazing 70 the μm as-cast and filler width metalbrazing of with 10 filler mm thickness metal is shown alloy of 70in was µFigurem moved and 3. width into ofthe 10 quartz mm is glass shown tube in injection pressure in the quartz glass tube was set to 0.02 MPa. The prepared foil-like brazing filler inFigure a melt-spinning3. machine. The melt-spinning was also carried out in a vacuum environment with metal with thickness of 70 μm and width of 10 mm is shown in Figure 3. a vacuum degree of 2.0 × 10−2 Pa. In order to obtain defect-free and micro-nano sized foil-like brazing filler metal, the rotational speed of the copper roller was set to 1400 r/min, and the argon injection pressure in the quartz glass tube was set to 0.02 MPa. The prepared foil-like brazing filler metal with thickness of 70 μm and width of 10 mm is shown in Figure 3.

Figure 1. Microstructure of 55 vol.% SiCp/ZL102 composites.

Figure 1. Microstructure of 55 vol.% SiCp/ZL102 composites. Figure 1. Microstructure of 55 vol.% SiCp/ZL102 composites. Table 1. Chemical composition of ZL102 (in wt.%).

Element Si Fe Cu Mn Mg Zn Ti Al wt.% 10.0–13.0Figure 0.0–0.71. Microstructure0.30 of 550.50 vol.% SiCp0.10/ZL102 composites.0.10 0.20 Balance ≤ ≤ ≤ ≤ ≤

Figure 2. Metallographic structure of the as-cast brazing filler metal.

Figure 2.Table Metallographic 1. Chemical structure composition of the of as-cast ZL102 brazing (in wt.%). filler metal.

Element Si Table 1. Fe Chemical Cu composition Mn of ZL102Mg (in wt.%).Zn Ti Al wt.% 10.0–13.0 0.0–0.7 ≤0.30 ≤0.50 ≤0.10 ≤0.10 ≤0.20 Balance Figure 2. Metallographic structure of the as-cast brazing ﬁller metal. Element FigureSi 2. Metallographic Fe structureCu ofMn the as-castMg brazing fillerZn metal.Ti Al wt.% 10.0–13.0 0.0–0.7 ≤0.30 ≤0.50 ≤0.10 ≤0.10 ≤0.20 Balance Table 1. Chemical composition of ZL102 (in wt.%).

Element Si Fe Cu Mn Mg Zn Ti Al wt.% 10.0–13.0 0.0–0.7 ≤0.30 ≤0.50 ≤0.10 ≤0.10 ≤0.20 Balance

Figure 3. Foil-like brazing material fabricated by melt-spinning technology. Figure 3. Foil-like brazing material fabricated by melt-spinning technology. The 55 vol.% SiC /ZL102 composites should be pretreated before vacuum brazing due to the The 55 vol.%Figure SiC p3./ZL102 Foil-like composites brazing material should fabricated be pretreat by melt-spinninged before vacuum technology. brazing due to the presence of oxidation filmfilm and oil stains on the surface of composites.composites. The The process was was as follows: firstfirst ofTheof all,all, 55 the thevol.% joining joining SiC surfacesp /ZL102surfaces of composites theof specimenthe specimen should were were polishedbe pretreat polished byed 800# beforeby sandpaper. 800# vacuum sandpaper. Secondly, brazing Secondly, the due polished to the polishedpresencespecimens specimensof were oxidation put were into film aput bathand into oil filled stainsa bath with on filled a the 1:1 wi mixturesurfaceth a 1:1 of of mixturecomposites. alcohol-acetone of alcohol-acetone The andprocess then was the and as specimens thenfollows: the Figure 3. Foil-like brazing material fabricated by melt-spinning technology. werespecimensfirst of cleaned all, werethe ultrasonically joining cleaned surfaces ultrasonically for 10 of min. the specimenSubsequently,for 10 mi weren. Subsequently, the polished cleaned by brazing the800# cleaned sandpaper. specimens brazing wereSecondly, specimens corroded the in 7% NaOH solution for 30 s, and then placed in 5% HNO solution for 60 s. Finally, the brazing werepolished corroded specimens in 7% were NaOH put solution into a bathfor 30 filled s, and wi thenth a placed1:1 mixture3 in 5% of HNO alcohol-acetone3 solution for and60 s. then Finally, the The 55 vol.% SiCp/ZL102 composites should be pretreated before vacuum brazing due to the specimens werewere washedcleaned with ultrasonically ultrasonic waterfor 10 formi 10n. min.Subsequently, After pretreatment, the cleaned the brazing assembled specimens overlap thepresence brazing of oxidationspecimens film were and washed oil stains with on theultras surfaceonic waterof composites. for 10 min. The Afterprocess pretreatment, was as follows: the werejoints corroded with the in brazing 7% NaOH filler solution metal werefor 30 joined s, and inthen a vacuumplaced in furnace 5% HNO (ZHS-60,3 solution China for 60 Electronics s. Finally, first of all, the joining surfaces of the specimen were polished by 800# sandpaper. Secondly, the the brazing specimens were washed with ultrasonic water for 10 min. After pretreatment, the polished specimens were put into a bath filled with a 1:1 mixture of alcohol-acetone and then the specimens were cleaned ultrasonically for 10 min. Subsequently, the cleaned brazing specimens were corroded in 7% NaOH solution for 30 s, and then placed in 5% HNO3 solution for 60 s. Finally, the brazing specimens were washed with ultrasonic water for 10 min. After pretreatment, the

Metals 2020, 10, 1470x FOR PEER REVIEW 4 of 13 assembled overlap joints with the brazing filler metal were joined in a vacuum furnace (ZHS-60, TechnologyChina Electronics Group Technology Corporation, Group Taiyuan, Corporation, China). Figure Taiyuan,4 shows China). the schematic Figure 4 diagram shows ofthe the schematic brazing joint.diagram During of the the brazing brazing, joint. a pressure During of 0.01the brazin MPa wasg, a applied pressure to theof 0.01 sandwich MPa specimen.was applied Based to the on thermodynamicsandwich specimen. properties Based of theon brazingthermodynamic ﬁller metal properties and characteristic of the brazing of SiCp/ ZL102filler composites,metal and thecharacteristic brazing temperature of SiCp/ZL102 was composites, set to be within the brazing the range temperatur of 560 ◦eC was to 600set ◦toC, be the within brazing the timerange was of set560 to°C 30 to min,600 °C, and the the brazing heating time rate was was set 10 to◦C 30/min. min, Heating and the startedheating whenrate was the vacuum10 °C/min. was Heating lower thanstarted5 when10 3 Pathe, andvacuum cooling was started lower afterthan brazing.5 × 10−3 Pa, The and cooling cooling rate started was 3 afterC/min, brazing. and the The sandwich cooling × − ◦ specimenrate was 3 was°C/min, taken and out the when sandwich cooled specimen to below 180was◦ C.taken out when cooled to below 180 °C.

Figure 4. Schematic diagram of the brazing joint.

Before the the vacuum vacuum brazing, brazing, thermodynamic thermodynamic properti propertieses of the of theas-cast as-cast and andfoil-like foil-like brazing brazing filler fillermetal metalwere analyzed were analyzed by differential by differential scanning scanning calorimetry(DSC) calorimetry used (DSC) a thermal used a analyzer thermal (Setaram analyzer (SetaramEvolution Evolution2400, Global 2400, (Hong Global Kong) (Hong Technology Kong) Co., Technology Ltd., Hong Co., Kong, Ltd., China). Hong The Kong, temperature China). Thewas temperatureraised to 600 was °C raisedat a rate to 600of 10◦C °C/min. at a rate The of 10 phase◦C/min. composition The phase of composition the foil-like ofbrazing the foil-like filler brazingmetal was filler analyzed metal was by analyzed using bya usingD8 ADVANCE a D8 ADVANCE X-ray X-ray diffractometer diffractometer (Bruker, (Bruker, Karlsruhe, Karlsruhe, α Baden-Wurttemberg, Germany).Germany). DuringDuring XRD XRD measurements, measurements, Cu-K Cu-Kradiationα radiation was was utilized utilized with with the followingthe following test test conditions: conditions: tube tube voltage voltage of 40of kV,40 kV, tube tube current current of 150 of 150 mA, mA, scanning scanning angle angle of 10–90 of 10–◦, step90 °, sizestep of size 0.02 of◦, 0.02°, and scanning and scanning speed ofspeed 10◦/ min.of 10 After°/min. the After brazing the process,brazing process, the joints the were joints polished were bypolished 400~1000# by 400~1000# sandpaper. sandpaper. The metallographic The metallographic structure ofstructure the brazed of the joint brazed was observedjoint was byobserved an optical by microscopean optical microscope (OLYMPUS-CK40M, (OLYMPUS-CK40M, Shanghai BajiuShanghai Industrial Bajiu Co.,Industrial Ltd., Shanghai,Co., Ltd., China).Shanghai, The China). shear strengthThe shear of thestrength brazed jointof the was brazed tested onjoint a universal was test electroniced on a testing universal machine electronic (CMT5205, testing MTS machine Systems (China)(CMT5205, Co., MTS Ltd., Systems Shenzhen, (China) China), Co., and Ltd., the shearShenzh rateen, wasChina), set to and 0.1 mmthe /shearmin. Observationsrate was set ofto the0.1 microstructuremm/min. Observations and fracture of the morphology microstructure of brazedand fracture joints morphology were carried of out brazed by a joints scanning were electron carried microscopeout by a scanning (Carl Zeiss electron NTS microscope GmbH, Merlin (Carl Compact, Zeiss NTS Jena, GmbH, Thuringia, Merlin Germany), Compact, secondary Jena, Thuringia electron , imagingGermany), (SEM) secondary and the electron composition imaging and (SEM)and diffusion ofthe the composition elements in and brazed diffusion joints of were the analyzedelements byin energy-dispersivebrazed joints were X-rayanalyzed spectroscopy by energy-dispersive (EDS). X-ray spectroscopy (EDS). 3. Results and Discussion 3. Results and Discussion 3.1. Thermodynamic Property and Microstructural Characteristics of the Brazing Filler Metal 3.1. Thermodynamic Property and Microstructural Characteristics of the Brazing Filler Metal The DSC curve of the brazing filler metal before and after the melt-spinning process is shown in FigureThe 5DSC. Figure curve5a of is athe DSC brazing curve filler of the metal as-cast before brazing and fillerafter metalthe melt-spinning made by the process high-frequency is shown inductionin Figure 5. melting Figure furnace,5a is a DSC and curve Figure of5 bthe shows as-cast the brazing DSC curve filler ofmetal the foil-likemade by brazing the high-frequency filler metal madeinduction by melt-spinning melting furnace, technique and Figure characterized 5b shows with thean DSC extremely curve of rapid the foil-like cooling rate.brazing It can filler be foundmetal frommade Figure by melt-spinning5 that the solidus technique and liquidus characterized temperatures with ofan theextremely foil-like brazingrapid cooling filler metal rate. decreasedIt can be found from Figure 5 that the solidus and liquidus temperatures of the foil-like brazing filler metal by 4 ◦C and 7 ◦C in comparison with the values of the as-cast brazing filler metal. The corresponding decreased by 4 °C and 7 °C in comparison with the values of the as-cast brazing filler metal. The molten temperature region decreased by 3 ◦C. The decrease of the thermodynamic temperature was relatedcorresponding to the grain molten size. temperature The melt-spinning region techniquedecreased canby refine3 °C. The the graindecrease size of significantly the thermodynamic due to the rapidtemperature cooling was rate. related to the grain size. The melt-spinning technique can refine the grain size significantly due to the rapid cooling rate.

Metals 2020, 10, 1470 5 of 13 Metals 2020, 10, x FOR PEER REVIEW 5 of 13 Metals 2020, 10, x FOR PEER REVIEW 5 of 13

(b) 513.28°C 530.64°C (a)(a) (b) 513.28°C 530.64°C 0 517.22°C 537.64°C 0 0 517.22°C 537.64°C 0

-20 -20

DSC/(mW/mg) -20

DSC/(mW/mg) -20 DSC/(mW/mg) DSC/(mW/mg) 521.45°C -40 521.45°C -40 529.43°C 529.43°C

0 200 400 600 -40 0 200 400 600 -40 0 200 400 600 Temp/°C 0 200Temp/°C 400 600 Temp/°C Temp/°C FigureFigure 5. 5. DSCDSC curvecurvecurve of ofof the thethe brazing brazingbrazing filler fillerfiller metal metalmetal alloy: alloy:alloy: (a) as-cast ((aa)) as-castas-cast brazing brazingbrazing filler metal; fillerfiller metal; (metal;b) foil-like ((bb)) foil-likefoil-like brazing brazingbrazingfiller metal. filler filler metal. metal.

FigureFigure 6 6 showsshows the thethe microstructure microstructuremicrostructure of of the the traditio traditio traditionalnalnal as-cast as-cast as-cast brazing brazing brazing filler filler filler metal metal metal viewed viewed viewed under under under a a scanningscanninga scanning electronelectron electron microscope.microscope. microscope. ItIt cancan It can bebe seen beseen seen thatthat that therethere there werewere were manymany many cellcell cell crystalscrystals crystals andand and dendritesdendrites dendrites inin thethe in as-castas-castthe as-cast brazingbrazing brazing fillerfiller filler metal,metal, metal, andand and somesome some areasareas areas hadhad had defectsdefects defects suchsuch such asas as pores,pores, pores, andand and thethe the graingrain grain sizesize size in in the the as-castas-cast brazingbrazingbrazing filler fillerfiller metal metalmetal was large.waswas Thelarge.large. melt-spinning TheThe melt-spinningmelt-spinning technology technology cantechnology produce can thecan microstructuresproduceproduce thethe microstructureswhichmicrostructures can vary greatly whichwhich canfromcan vary vary those greatlygreatly formed fromfrom in traditional thosethose formedformed slow-cooled inin traditionaltraditional bulk material.slow-cooled slow-cooled Figure bulkbulk7 shows material.material. the FigureFiguremicrostructure 77 showsshows of thethe the microstructuremicrostructure foil-like brazing ofof fillerthethe foil-likfoil-lik metalee made brazingbrazing by thefillerfiller melt-spinning metalmetal mademade technique.byby thethe melt-spinningmelt-spinning The side of technique.technique.the foil-like TheThe brazing sideside filler ofof thethe metal foil-likefoil-like contacting brazingbrazing the fillerfiller copper metalmetal roller contactingcontacting was called thethe the coppercopper near-roller rollerroller surface waswas calledcalled while thethe near-rollernear-rollerother side was surfacesurface called whilewhile the free thethe surface. otherother Figure sideside 7waswasa displays callcalleded the thethe microstructure freefree surface.surface. foil-like FigureFigure brazing 7a7a displaysdisplays filler metal thethe microstructuremicrostructureat the free surface; foil-likefoil-like the average brazingbrazing grain fillerfiller size metalmetal is about atat thethe 159.09 freefree surface;surface; nm, and the the Figure averageaverage7b displays graingrain sizesize the is microstructureis aboutabout 159.09159.09 nm,nm,at the andand near-roller FigureFigure 7b7b surface; displaysdisplays the thethe average microstructuremicrostructure grain size isatat about thethe near-rollernear-roller 129.63 nm. surface;surface; Figure 7 the theindicated averageaverage that graingrain there sizesize was isis aboutaboutno microdefect 129.63129.63 nm.nm. such Figure Figure as micro-voids 7 7 indicated indicated andthat that elementthere there was was segregation no no microdefect microdefect in the foil-likesuchsuch as as micro-voids brazingmicro-voids filler and and metal. element element It can segregationsegregationbe known from in in the the the foil-like foil-like microstructure brazing brazing filler thatfiller foil-likemetal. metal. It It brazing can can be be fillerknownknown metal from from mainly the the microstructure microstructure contained uniform that that foil-like foil-like cellular brazingbrazingnano grains, fillerfiller and metalmetal the mainly sizesmainly of contained thecontained cellular uniformuniform grains were cellularcellular less than nanonano 200 grains,grains, nm, which andand thethe had sizessizes the characteristics ofof thethe cellularcellular of grainsgrainsnanomaterials. were were less less Asthan than shown 200 200 nm, nm, in Figure which which7 ,had had the grainthethe characteristics characteristics size at the near-roller of of nanomaterials. nanomaterials. surface was As As smallershown shown in thanin Figure Figure that at7,7, thethe grain freegrain surface sizesize atat due thethe to near-rollernear-roller the faster coolingsurfacesurface rate.waswas Thesmallersmaller results thanthan showed thatthat atat that thethe owing freefree surfacesurface to its small duedue sizetoto thethe and fasterfaster high coolingcoolingsurface freerate.rate. energy, TheThe resultsresults the chemical showedshowed potential thatthat owingowing of the toto foil-like ititss smallsmall brazing sizesize andand filler highhigh metal susurface containingrface freefree energy,energy, micro-nano thethe chemicalchemicalgrains was potential potential much higher of of the the thanfoil-like foil-like that brazing brazing of the bulk filler filler material. me metaltal containing containing As a result, micro-nano micro-nano the melting grains grains point was ofwas the much much micro-nano higher higher thanthansized thatthat brazing ofof thethe filler bulkbulk metal material.material. was depressed AsAs aa result,result, incomparison thethe memeltinglting to pointpoint that of ofof the thethe bulk micro-nanomicro-nano crystals. sized Comparedsized brazingbrazing with fillerfiller the metalmetalbrazing waswas filler depresseddepressed metal prepared inin comparisoncomparison by conventional toto thatthat ofof methods,thethe bulkbulk crystals. therecrystals. was ComparedCompared a lower melting withwith thethe point brazingbrazing and better fillerfiller metalmetaldiffusibility. preparedprepared High-quality byby conventionalconventional brazing methods,methods, joints can therethere be obtained waswas aa lowerlower when meltingmelting micro-nano pointpoint sized andand better brazingbetter diffusibility.diffusibility. filler metal High-qualitywasHigh-quality utilized inbrazing brazing experiment. joints joints can can be be obtained obtained when when micro-nano micro-nano sized sized brazing brazing filler filler metal metal was was utilized utilized inin experiment. experiment.

Figure 6. Microstructure of the as-cast brazing ﬁller metal. FigureFigure 6. 6. Microstructure Microstructure of of the the as-cast as-cast brazing brazing filler filler metal. metal.

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Figure 7. Microstructure of the foil-like brazing filler metal: (a) microstructure of the free surface; FigureFigure 7.7. MicrostructureMicrostructure ofof thethe foil-likefoil-like brazingbrazing fillerfiller metal:metal: ((aa)) microstructuremicrostructure ofof thethe freefree surface;surface; ((bb)) (b) microstructure of the near-roller surface. microstructuremicrostructure ofof thethe near-rollernear-roller surface.surface. Figure8 is the X-ray di ffraction analysis (XRD) curve of the foil-like brazing filler metal, which FigureFigure 88 isis thethe X-rayX-ray diffractiondiffraction analysisanalysis (XRD)(XRD) curvecurve ofof thethe foil-likefoil-like brazingbrazing fillerfiller metal,metal, whichwhich indicates that there are mainly five kinds of phases: α-Al, Al MgCu, AlCu, AlCu and Cu Mg, indicates that there are mainly five kinds of phases: α-Al, Al2MgCu,2 AlCu, AlCu3 and3 Cu2Mg,2 and indicates that there are mainly five kinds of phases: α-Al, Al2MgCu, AlCu, AlCu3 and Cu2Mg, and and the content of the Al MgCu phase is relatively high. Due to the high chemical activity at elevated the content of the Al2MgCu2 phase is relatively high. Due to the high chemical activity at elevated the content of the Al2MgCu phase is relatively high. Due to the high chemical activity at elevated temperature, alloying elements Mg and Cu in filler metal were mainly in the form of intermetallic temperature,temperature, alloyingalloying elementselements MgMg andand CuCu inin fillerfiller metalmetal werewere mainlymainly inin thethe formform ofof intermetallicintermetallic compounds. Combined with the analysis in Figure7, we can infer that the sizes of intermetallic compounds.compounds. CombinedCombined withwith thethe analysisanalysis inin FigureFigure 7,7, wewe cancan inferinfer thatthat thethe sizessizes ofof intermetallicintermetallic compounds were limited to the nanoscale. Those intermetallic compounds distributed in α-Al were compoundscompounds werewere limitedlimited toto thethe nanoscale.nanoscale. ThThoseose intermetallicintermetallic compoundscompounds distributeddistributed inin αα-Al-Al werewere very uniform due to the rapid cooling rate during manufacturing. veryvery uniformuniform duedue toto thethe rapidrapid coolingcooling raterate duringduring manufacturing.manufacturing.

♥—Al( #85-1327) ♥—Al( #85-1327) ♠—Al2MgCu( #28-0014) ♠—Al2MgCu( #28-0014) ♦ ♣—AlCu( #88-1713) ♦ ♣—AlCu( #88-1713) ♣ ♦—AlCu3( #28-0005) ♣ ♦—AlCu3( #28-0005) ♥ ∇ ∇—Cu2Mg( #77-1178) ♥ ∇ ∇—Cu2Mg( #77-1178) ♦ ♣♦ ♠♣ ♠ ♥ ♥ Intensity / a.u. Intensity / a.u. ♣ ∇ ♣ ∇ ♦ ♠ ♦ ♣ ♦ ♠ ♠♦ ♠ ♠ ♦ ♣ ♣ ♦ ♦ ♠♠ ♠ ♠ ♣ ♥ ♥ ♦ ♠ ♠♠ ♠ ♠♠ ♠ ♠ ♠ ♥ ♥ ♥ ♠ ♠♠ ♠ ♠ ♠ ♥

10 20 30 40 50 60 70 80 90 10 20 30 40 50 60 70 80 90 2-Theta / degree 2-Theta / degree Figure 8. X-ray diffraction analysis curve of the foil-like brazing filler metal. FigureFigure 8.8. X-rayX-ray diffractiondiffraction analysisanalysis curvecurve ofof thethe foil-likefoil-like brazingbrazing fillerfiller metal.metal. 3.2. Microstructure and Element Distribution Analysis of Brazed Joints 3.2.3.2. MicrostructureMicrostructure andand ElementElement DistDistributionribution AnalysisAnalysis ofof BrazedBrazed JointsJoints Figure9 shows the optical microstructure evolution of 55 vol.% SiC p/ZL102 composites joints Figure 9 shows the optical microstructure evolution of 55 vol.% SiCp/ZL102 composites joints madeFigure in a brazed 9 shows temperature the optical range microstructure of 560 C to evolution 600 C by of using 55 vol.% micro-nano SiCp/ZL102 foil-like composites brazing joints filler made in a brazed temperature range of 560 ◦°C to 600 ◦°C by using micro-nano foil-like brazing filler metalmade Al-17.0Cu-8.0Mg.in a brazed temperature As shown range in of Figure 560 °C9a, to some 600 °C small by using defects micro-nano such as an foil-like unwetted brazing area andfiller metal Al-17.0Cu-8.0Mg. As shown in Figure 9a, some small defects such as an unwetted area and micro-voidmetal Al-17.0Cu-8.0Mg. between the fillerAs shown metal in and Figure SiC particle9a, some existed small atdefects the interface, such as an which unwetted had significant area and micro-void between the filler metal and SiC particle existed at the interface, which had significant emicro-voidffect on the between joint mechanical the filler property. metal and The SiC reason particle for that existed was theat the wettability interface, and which interaction had significant between effect on the joint mechanical property. The reason for that was the wettability and interaction theeffect filler on metal the joint and compositesmechanical was property. very weak The at re aason lower for brazing that was temperature the wettability of 560 Cand on interaction account of between the filler metal and composites was very weak at a lower brazing temperature◦ of 560 °C on thebetween lower the atomic filler activity. metal and Figure composites9 indicated was that very brazed weak jointsat a lower were brazing continuous temperature and tightly of 560 bonded, °C on account of the lower atomic activity. Figure 9 indicated that brazed joints were continuous and withoutaccount physicalof the lower defects atomic such activity. as micro-voids Figure and9 indicated cracks whenthat brazed the brazing joints temperaturewere continuous exceeded and tightly bonded, without physical defects such as micro-voids and cracks when the brazing 570tightlyC. Thebonded, interaction without between physical the filler defects metal such and compositesas micro-voids was enhanced and cracks under when the conditionthe brazing of a temperature◦ exceeded 570 °C. The interaction between the filler metal and composites was enhanced relativelytemperature high exceeded brazing 570 temperature. °C. The interaction It can be betwee seen inn Figure the filler9b–e metal that and a gray composites needle-like was or enhanced massive under the condition of a relatively high brazing temperature. It can be seen in Figure 9b to Figure 9e eutecticunder the Si structurecondition appeared of a relatively in the high center brazing of the brazingtemperature. seam. It As can shown be seen in Figurein Figure9e, the9b to quantity Figure of9e that a gray needle-like or massive eutectic Si structure appeared in the center of the brazing seam. As eutecticthat a gray Si in needle-like the brazing or seam massive was eutectic almost as Si muchstructure as in appeared composites. in the This center was dueof the to brazing the large seam. number As shown in Figure 9e, the quantity of eutectic Si in the brazing seam was almost as much as in ofshown Si elements in Figure in the 9e, base the material, quantity which of eutectic diffused Si into in thethe brazingbrazing joint seam during was thealmost brazing as processmuch as and in composites. This was due to the large number of Si elements in the base material, which diffused formedcomposites. the eutectic This was structure due to withthe large Al elements number inof the Si elements brazing joint in the [31 base]. This material, indicated which that adiffused strong intointo thethe brazingbrazing jointjoint duringduring thethe brazingbrazing processprocess anandd formedformed thethe eutecticeutectic structurestructure withwith AlAl elementselements inin thethe brazingbrazing jointjoint [31].[31]. ThisThis indicatedindicated thatthat aa strongstrong interactioninteraction betweenbetween thethe micro-nanomicro-nano brazingbrazing

Metals 2020, 10, 1470 7 of 13

Metals 2020, 10, x FOR PEER REVIEW 7 of 13 interaction between the micro-nano brazing ﬁller metal and 55 vol.% SiCp/ZL102 composites occurred. fillerConsequently, metal and the 55 widthvol.% ofSiC thep/ZL102 brazing composites seam became occurred. narrower Conseq and narroweruently, the with width the of increase the brazing in the seambrazing became temperature. narrower and narrower with the increase in the brazing temperature.

Figure 9. OpticalOptical microstructure microstructure of of joints joints br brazedazed at atdifferent diﬀerent temperatures: temperatures: (a) (560a) 560 °C;◦ (C;b) (570b) 570°C; ◦(cC;) 580(c) 580 °C; ◦(C;d) (590d) 590°C; ◦(C;e) 600 (e) 600°C. ◦C.

SEM image of aa typicaltypical regionregion atat the the interface interface of of the the 55 55 vol.% vol.% SiC SiCp/pZL102/ZL102 composite composite joint joint made made at at580 580◦C °C and and corresponding corresponding energy energy dispersive dispersive X-ray X-ra analysisy analysis are presentedare presented in Figure in Figure 10. Figure10. Figure 10 was 10 wastaken taken in secondary in secondary electron electron mode. mode. It can It be can clearly be observedclearly observed that there that were there two were kinds two of interfaces: kinds of interfaces:the interface the between interface the between brazing the filler brazing metal filler and metal aluminum and aluminum matrix, as matrix, well as as the well other as the interface other interfacebetween thebetween brazing the filler brazing metal and filler SiC particle.metal and Figure SiC 10 particle.a indicated Figure that the 10a combination indicated ofthat the twothe combinationtype of interfaces of the was two quite type compact. of interfaces The chemicalwas quite composition compact. The of chemical the joint wascomposition analyzed of using the joint EDS, wasas shown analyzed in Figure using 10 EDS,a. Some as shown white block in Figure phases 10a. appeared Some white near the block brazing phases seam. appeared Testing near points the A brazingand B were seam. located Testing at the points center A of and these B whitewere blocklocated phases. at the At center point of A, these the chemical white block composition phases. was At pointmeasured A, the as chemical 48.4Al-50.6Cu-0.9Si-0.1Mg composition was (wt.%).measured At as point 48.4Al-50.6Cu-0.9Si-0.1Mg B, the chemical composition (wt.%). was At measured point B, theas 29.8Al-37.4Cu-21.0C-8.7Si-3.0Mg chemical composition was measured (wt.%). as 29.8Al-37.4Cu-21.0C-8.7Si-3.0Mg Elements of Al and Cu were the primary(wt.%). Elements composites of Al in andthese Cu white were regions. the primary At point composites B, a small in amountthese white of Mg regions. was included At point whichB, a small may amount form the of AlMgCuMg was includedintermetallic. which Moreover, may form these the white AlMgCu block intermetalli phases werec. Moreover, mainly distributed these white around block the phases SiC particle, were mainlywhich meant distributed that the around interface the between SiC particle, the SiC which particle meant and that aluminum the interface was the between main di theffusion SiC pathparticle for andCu. Thealuminum white blockwas the phase main can diffusion be found path at the for top Cu. left The corner white in block Figure phase 10a. can This be suggested found at thatthe top the leftdiff usioncorner distance in Figure of 10a. element This Cusuggested exceeded that at th leaste diffusion 100 µm underdistance the of brazing element temperature Cu exceeded of at 580 least◦C, 100which μm can under reduce the thebrazing effects temperature of brittle intermetallic of 580 °C, which compounds can reduce such asthe Al effects2Cu and of AlCubrittle in intermetallic the brazing compoundsseam. At point such C, as the Al2 chemicalCu and AlCu composition in the brazin wasg measured seam. At aspoint 95.1Al-4.0Cu-0.9Mg C, the chemical composition (wt.%). Al was measured as 95.1Al-4.0Cu-0.9Mg (wt.%). Al was the primary composite in the brazing seam. The content of Cu and Mg in this area was much lower than that in the original brazing filler metal (Al-17.0Cu-8.0Mg). Figure 10b to Figure 10e display the individual elemental line scanning profile

Metals 2020, 10, 1470 8 of 13 the primary composite in the brazing seam. The content of Cu and Mg in this area was much lower Metalsthan that2020, in10, thex FOR original PEER REVIEW brazing filler metal (Al-17.0Cu-8.0Mg). Figure 10b–e display the individual8 of 13 elemental line scanning profile of Al, Mg, Cu, and Si, respectively. As can be seen, the distributions of ofelements Al, Mg, Al, Cu, Mg, and Cu, Si, and respectively. Si were quite As homogeneouscan be seen, the except distributions the areas ofofthe elements white blockAl, Mg, phase Cu, and and SiC Si wereparticle. quite After homogeneous brazing, the except chemical the concentrationareas of the wh gradientite block between phase theand brazing SiC particle. filler metalAfter andbrazing, base thematerial chemical disappeared. concentration The gradient line scanning between profile the alsobrazing indicated filler metal that the and inter-di base materialffusion phenomenondisappeared. The line scanning profile also indicated that the inter-diffusion phenomenon between the brazing between the brazing filler metal and 55 vol.% SiCp/ZL102 composites occurred intensively. filler metal and 55 vol.% SiCp/ZL102 composites occurred intensively.

Figure 10. SEM image of a typical region at the interface of the 55 vol.% SiCp/ZL102 composite joint Figure 10. SEM image of a typical region at the interface of the 55 vol.% SiCp/ZL102 composite joint made at 580 C and corresponding energy dispersive X-ray analysis: (a) SEM image; (b–e) individual made at 580◦ °C and corresponding energy dispersive X-ray analysis: (a) SEM image; (b–e) elemental line scanning proﬁle of Al, Mg, Cu, and Si, respectively. individual elemental line scanning profile of Al, Mg, Cu, and Si, respectively. Under the condition of high temperature, Mg was more active than Al, which made it have the Under the condition of high temperature, Mg was more active than Al, which made it have the possibility to react with alumina on the surface of SiCp/Al MMCs [32], as shown in Equation (1). possibility to react with alumina on the surface of SiCp/Al MMCs [32], as shown in Equation (1). 3Mg + Al O = 3MgO + 2Al (1) 3Mg Al2O3 =3MgO2Al (1)

That reaction cancan eliminateeliminate some some part part of of dense dense alumina, alumina, which which will will improve improve the the wettability wettability of the of thebrazing brazing filler filler metal. metal. Through Through the energy the energy spectrum spectrum analysis analysis in Figure in Figure 10a, it can10a, beit seencan be that seen the that content the contentof Mg in of the Mg joint in the was joint much was lower much than lower that than in the that original in the brazing original filler brazing metal. filler As metal. is known, As is Mg known, has a Mgquite has high a quite saturated high vapor saturated pressure vapor compared pressure to comp the otherared elements,to the other such elements, as Al, Cu, such Fe, etc.as Al, During Cu, theFe, etc.course During of vacuum the course brazing, of the vacuum volatile Mgbrazing, was graduallythe volatile vaporized Mg was and accumulated.gradually vaporized A part of and the accumulated.accumulated MgA part vapor of brokethe accumulated the aluminum Mg oxide vapor film broke covering the aluminum on the surface oxide of film SiC coveringp/Al MMCs on andthe surfacethen was of discharged.SiCp/Al MMCs As and a consequence, then was discharged. the liquid As brazing a consequence, filler metal the could liquid di brazingffuse into filler the metal base couldmaterial diffuse along theinto broken the base location material of the along aluminum the broken oxide film.location Therefore, of the the aluminum chemical compositionoxide film. Therefore,balance between the chemical the brazing composition seam and balance base material between was the achieved. brazing seam and base material was achieved.

Metals 2020, 10, x FOR PEER REVIEW 9 of 13

3.3. Mechanical Properties of Brazed Joints To find out the optimal process parameters, the effect of brazing temperature on the mechanical property of the joint was investigated, as displayed in Figure 11. Five specimens were tested for each brazing condition to obtain the average shearing data. The joint shear strength increased with the increasing in brazing temperature from 560 °C to 580 °C. At a brazing

Metalstemperature2020, 10, 1470of 560 °C, the joint shear strength was only 57.43 MPa due to the insufficient diffusion9 of 13 between the brazing filler metal and the matrix, shown from the analysis in Figure 9a. The maximum shear strength of the joint was 98.17 MPa at the brazing temperature of 580 °C. For vacuum brazing, 3.3.mutual Mechanical diffusion Properties has to ofabide Brazed by Joints the Arrhenius equation expressed in Equation (2), which is the diffusionTo find coefficient out the optimalof the filler process materials. parameters, As can the be eseen,ffect ofthe brazing diffusion temperature coefficient on D(T) the is mechanical a function propertyof temperature of the joint(T), which was investigated, is a process asparameter displayed and in can Figure be controlled11. Five specimens during the were experiment. tested for each 𝑄 brazing condition to obtain the average𝐷(𝑇) shearing =𝐷 exp data.(− The) joint shear strength increased with the(2) 𝑅𝑇 increasing in brazing temperature from 560 ◦C to 580 ◦C. At a brazing temperature of 560 ◦C, the joint where T is the diffusion brazing temperature, Q the activation energy, R the gas constant, and D0 shear strength was only 57.43 MPa due to the insufficient diffusion between the brazing filler metal the diffusion constant. and the matrix, shown from the analysis in Figure9a. The maximum shear strength of the joint was With the temperature increase, the diffusion coefficient of elements in the brazing system will 98.17 MPa at the brazing temperature of 580 ◦C. For vacuum brazing, mutual diffusion has to abide increase significantly. However, a further increase in the brazing temperature to 590 °C and 600 °C by the Arrhenius equation expressed in Equation (2), which is the diffusion coefficient of the filler caused a decrease of shear strength to 80.25 MPa and 60.36 MPa, respectively. The change in shear materials. As can be seen, the diffusion coefficient D(T) is a function of temperature (T), which is a strength at that high temperature was due to the excessive interaction of elements in the joint and process parameter and can be controlled during the experiment. matrix, which caused the generation and growth of brittle intermetallics such as Al2Cu and AlCu having a remarkable influence on the shear strength. OnQ the other hand, a small part of liquid D(T) = D0 exp ( ) (2) aluminum effused from the 55 vol.% SiCp/ZL102 composites−RT during the course of vacuum brazing when the brazing temperature of 600 °C was utilized. This phenomenon had a considerable negative where T is the diffusion brazing temperature, Q the activation energy, R the gas constant, and D0 the effect on the joint shear strength and mechanical property of 55 vol.% SiCp/ZL102 composites. diffusion constant.

98.17 100 78.83 80.25 80

57.43 60.36 60

Shear Strengh/MPa Shear 20

0 560 570 580 590 600 Temperature/°C Figure 11. Shear strength of brazed joints at didifferentﬀerent temperatures.temperatures.

3.4. FractureWith the Analysis temperature of Brazed increase, Joints the diffusion coefficient of elements in the brazing system will increase significantly. However, a further increase in the brazing temperature to 590 ◦C and 600 ◦C causedFigure a decrease 12 displays of shear the strength typical toscanning 80.25 MPa fracture and 60.36 morphology MPa, respectively. and EDS analysis The change of the in shearjoint strengthmade using at that micro-nano high temperature brazing wasfiller due metal to theAl-17.0Cu-8.0Mg excessive interaction at a temperature of elements inof the580 joint °C andand holding time of 30 min after room temperature shear testing. As can be seen in Figure 12, both sides matrix, which caused the generation and growth of brittle intermetallics such as Al2Cu and AlCu havingof the joint a remarkable fracture were influence characterized on the shear by the strength. quasi-cleavage On the otherfracture. hand, No a SiC small particles part of can liquid be found in fracture surface, which meant that the fracture occurred in the brazing seam rather than aluminum effused from the 55 vol.% SiCp/ZL102 composites during the course of vacuum brazing the interface between the brazing filler metal and 55 vol.% SiCp/ZL102 composites. Using EDS when the brazing temperature of 600 ◦C was utilized. This phenomenon had a considerable negative analysis, the chemical composition of point A was measured to be 97.5Al-2.5Cu (wt.%). It indicated effect on the joint shear strength and mechanical property of 55 vol.% SiCp/ZL102 composites. that the scratching area in the fracture was mainly composed of aluminum solid solution, having 3.4.good Fracture plasticity. Analysis The ofchemical Brazed Jointscomposition in the cleavage fracture area, such as point B, C, and D was composed of elements of Al, Si, Cu, and Mg, which were 65.2Al-32.2Si-2.1Cu-0.4Mg (wt.%), Figure 12 displays the typical scanning fracture morphology and EDS analysis of the joint made 29.7Al-27.6Si-22.2Mg-17.4Cu-3.1O (wt.%), and 37.8Al-22.7Si-21.8Mg-16.5Cu-1.3O (wt.%), using micro-nano brazing filler metal Al-17.0Cu-8.0Mg at a temperature of 580 ◦C and holding time of 30 min after room temperature shear testing. As can be seen in Figure 12, both sides of the joint fracture were characterized by the quasi-cleavage fracture. No SiC particles can be found in fracture surface, which meant that the fracture occurred in the brazing seam rather than the interface between the brazing filler metal and 55 vol.% SiCp/ZL102 composites. Using EDS analysis, the chemical composition of point A was measured to be 97.5Al-2.5Cu (wt.%). It indicated that the scratching area in the fracture was mainly composed of aluminum solid solution, having good plasticity. The chemical Metals 2020, 10, 1470 10 of 13

Metals 2020, 10, x FOR PEER REVIEW 10 of 13 Metalscomposition 2020, 10, x in FOR the PEER cleavage REVIEW fracture area, such as point B, C, and D was composed of elements of10 Al, of 13 Si, Cu, and Mg, which were 65.2Al-32.2Si-2.1Cu-0.4Mg (wt.%), 29.7Al-27.6Si-22.2Mg-17.4Cu-3.1O (wt.%), respectively. The only source of element Si was 55 vol.% SiCp/ZL102 composites. As can be seen in respectively.and 37.8Al-22.7Si-21.8Mg-16.5Cu-1.3O The only source of element (wt.%), Si was respectively. 55 vol.% SiC Thep/ZL102 only source composites. of element As Sican was be 55seen vol.% in Figure 12, the Si content at points B, C, and D exceeded 22.7 wt.%, which signified that FigureSiCp/ZL102 12, the composites. Si content As at can points be seen B, in C, Figure and 12D, theexceeded Si content 22.7 at wt.%, points which B, C, and signified D exceeded that interdiffusion between the micro-nano brazing filler metal and 55 vol.% SiCp/ZL102 composites interdiffusion22.7 wt.%, which between signified the that micro-nano interdiffusion brazing between filler the metal micro-nano and 55 brazingvol.% SiC fillerp/ZL102 metal andcomposites 55 vol.% took place sufficiently during the course of vacuum brazing. The intermetallic compound phases tookSiCp /placeZL102 sufficiently composites during took place the sucoursefficiently of vacuum during the brazing. course The of vacuum intermetallic brazing. compound The intermetallic phases (IMC) such as Mg2Si, Al2Cu, and Cu2Mg would be formed due to the high activity of the (IMC)compound such phases as Mg (IMC)2Si, Al such2Cu, as and Mg Si,Cu Al2MgCu, would and Cu beMg formed would due be formedto the duehigh to theactivity high activityof the micro-nano brazing filler metal at 2high temperature.2 2 During vacuum brazing, the formation of micro-nanoof the micro-nano brazing brazing filler metal filler metalat high at hightemperature. temperature. During During vacuum vacuum brazing, brazing, the theformation formation of brittle IMC would cause stress concentration around the IMC interface. In the following shearing brittleof brittle IMC IMC would would cause cause stress stress concentration concentration around around the the IMC IMC interface. interface. In In the the following following shearing shearing test, the joint was easily cracked in this area of stress concentration. However, the fracture test,test, thethe jointjoint was was easily easily cracked cracked in this in areathis ofarea stress of concentration. stress concentration. However, However, the fracture the appearance fracture appearance demonstrated that there was no large blocked IMC in the joint, and this would be very appearancedemonstrated demonstrated that there was that nothere large was blocked no large IMC blocked in the IMC joint, in and the thisjoint, would and this be would very beneficial be very beneficial in terms of increasing the joint mechanical property. To better analyze the fracture beneficialin terms ofin increasing terms of theincreasing joint mechanical the joint property.mechanical To property. better analyze To better the fracture analyze mechanism the fracture of mechanism of brazed joints, Figure 13 shows the shear curve with temperature of 580 °C and mechanismbrazed joints, of Figurebrazed 13joints, shows Figure the shear13 shows curve the with shear temperature curve with of 580temperatureC and holdingof 580 °C time and of holding time of 30 min. The force-displacement curve was relatively smooth◦ and there was no yield holding30 min. time The of orce-displacement 30 min. The force-displacement curve was relatively curve smoothwas relatively and there smooth was and no yieldthere phenomenon.was no yield phenomenon. Therefore, the brazing joint is a brittle fracture. phenomenon.Therefore, the Therefore, brazing joint the is brazing a brittle joint fracture. is a brittle fracture.

Figure 12. Scanning fracture morphology and EDS analysis of the joint made using micro-nano FigureFigure 12. ScanningScanning fracturefracture morphology morphology and and EDS EDS analysis analys ofis the of joint the madejoint usingmade micro-nanousing micro-nano brazing brazing filler metal Al-17.0Cu-8.0Mg at a temperature of 580 °C and holding time of 30 min: (a) one brazingﬁller metal filler Al-17.0Cu-8.0Mg metal Al-17.0Cu-8.0Mg at a temperature at a temperat of 580ure◦C of and 580 holding °C and time holding of 30 time min: of (a )30 one min: side (a of) one the side of the fracture; (b) the other side of the fracture. sidefracture; of the (b fracture;) the other (b) side the ofother the side fracture. of the fracture.

Figure 13. Shear curve at a temperature of 580 C and holding time of 30 min. Figure 13. Shear curve at a temperature of 580 °C◦ and holding time of 30 min. Figure 13. Shear curve at a temperature of 580 °C and holding time of 30 min. 4. Conclusions 4. Conclusions 4. Conclusions In this work, 55 vol.% SiCp/ZL102 composites were successfully bonded through the brazing In this work, 55 vol.% SiCp/ZL102 composites were successfully bonded through the brazing processIn this under work, a vacuum 55 vol.% condition. SiCp/ZL102 Micro-nano composites brazing were ﬁllersuccess metalfully Al-17.0Cu-8.0Mg bonded through fabricated the brazing by process under a vacuum condition. Micro-nano brazing filler metal Al-17.0Cu-8.0Mg fabricated by processmelt-spinning under technologya vacuum condition. was utilized Micro-nano for brazing brazing in a temperature filler metal range Al-17.0Cu-8.0Mg of 560–600 ◦C, withfabricated a holding by melt-spinning technology was utilized for brazing in a temperature range of 560–600 °C, with a melt-spinningtime of 30 min. technology This new was micro-nano utilized brazingfor brazing ﬁller in metal a temperature and corresponding range of brazing560–600 process°C, with can a holding time of 30 min. This new micro-nano brazing filler metal and corresponding brazing process holdingprovide time a technical of 30 min. reference This new and micro-nano theoretical brazing value for filler dealing metal with and corresponding the joining problem brazing of process SiCp/Al can provide a technical reference and theoretical value for dealing with the joining problem of canMMCs. provide The maina technical conclusions reference can beand summarized theoretical asvalue following: for dealing with the joining problem of SiCp/Al MMCs. The main conclusions can be summarized as following: SiCp/Al MMCs. The main conclusions can be summarized as following: (1) Using melt-spinning technology, the foil-like brazing filler metal Al-17.0Cu-8.0Mg was (1) Using melt-spinning technology, the foil-like brazing filler metal Al-17.0Cu-8.0Mg was obtained. The microstructure analysis indicated that the foil-like brazing filler metal mainly obtained. The microstructure analysis indicated that the foil-like brazing filler metal mainly

Metals 2020, 10, 1470 11 of 13

(1) Using melt-spinning technology, the foil-like brazing filler metal Al-17.0Cu-8.0Mg was obtained. The microstructure analysis indicated that the foil-like brazing filler metal mainly contained uniformed cellular nano grains, with a size less than 200 nm. The solidus and liquidus temperatures of the foil-like brazing filler metal decreased by 4 ◦C and 7 ◦C in comparison with the values of the as-cast brazing filler metal due to the nanometer size effect. Compared with the brazing filler metal prepared by conventional methods, the micro-nano brazing filler metal had higher toughness, lower melting point, and better diffusibility, which were more conducive to the wetting and spreading behavior between the brazing filler metal and base material. (2) The microstructure observation indicated that brazed joints were continuous and tightly bonded, without physical defects such as micro-voids and cracks when the brazing temperature exceeded 570 ◦C. Moreover, the width of the brazing seam became narrower and narrower with increasing brazing temperature owning to the strong interaction between the micro-nano brazing filler metal and 55 vol.% SiCp/ZL102 composites. Some white block phases mainly containing elements Al and Cu appeared near the brazing seam. The main diffusion path for Cu was the interface between the SiC particle and aluminum. After brazing, the chemical concentration gradient between the brazing filler metal and base material disappeared.

(3) The maximum joint shear strength was achieved at 98.17 MPa at the brazing temperature of 580 ◦C. The joint shear strength increased in the temperature range of 560 ◦C to 580 ◦C. However, a further increase in the brazing temperature to 590 ◦C and 600 ◦C caused a decrease in shear strength to 80.25 MPa and 60.36 MPa, respectively. The excessive interaction in the joint and matrix caused the generation and growth of brittle intermetallics such as Al2Cu and AlCu having a remarkable inﬂuence on the shear strength.

(4) The fracture morphology of the joint made at a brazing temperature of 580 ◦C was characterized by quasi-cleavage fracture. Fractures occurred in the brazing seam rather than the interface between the brazing ﬁller metal and 55 vol.% SiCp/ZL102 composites since no SiC particles could be found in the fracture surface. There was no large blocked intermetallic compound in the joint, and this would be very beneﬁcial in terms of increasing the joint mechanical properties.

Author Contributions: Investigation, D.Q., Z.G., and X.B.; resources, Z.W.; supervision, J.N.; writing—original draft preparation, D.Q.; writing—review and editing, Z.G. and J.N. All authors have read and agreed to the published version of the manuscript. Funding: The research was financially supported by the National Natural Science Foundation of China (No. 51245008), the Science and Technology Project of Henan Province, China (No. 202102210036), and the Fundamental Research Funds for the Universities of Henan Province, China (No. NSFRF180405). Conflicts of Interest: The authors declare no conflict of interest.

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