Effects of Multi-Pass Friction Stir Processing on Microstructures And

metals

Article Eﬀects of Multi-Pass Friction Stir Processing on Microstructures and Mechanical Properties of the 1060Al/Q235 Composite Plate

Jian Wang 1,* , Yun Cheng 1,2, Bo Li 3 and Cheng Chen 1

1 School of Mechanical and Power Engineering, Nanjing Tech University, Nanjing 211816, China; [email protected] (Y.C.); [email protected] (C.C.) 2 School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang 212003, China 3 Additive Manufacturing and Intelligent Equipment Research Institute, School of Mechanical and Power Engineering, East China University of Science and Technology, Shanghai 200237, China; [email protected] * Correspondence: [email protected]; Tel.: +86-255-813-9352

Received: 28 January 2020; Accepted: 21 February 2020; Published: 25 February 2020

Abstract: Steel cuttings, holes and cracks always exist at the interfaces in the composite plate. Multi-pass friction stir processing (M-FSP) is proposed in this research to optimize the interface microstructure and the interface connection for the 1060Al/Q235 composite plate. Results show that the microstructures of 1060Al after M-FSP are ﬁne and uniform owing to the strong stirring eﬀect and recrystallization. Micro-defects formed by the welding can be repaired by the M-FSP. However, tunnel defects can also be formed in the matrix of aluminum by M-FSP, especially when the passes are one and two. The melting block and the melting lump in the composite plates are easy to become the source of crack. The shear strengths and the bending properties for the 1060Al/Q235 composite plate after M-FSP are the best when the passes are three, with the tool rotation speed of 1200 rpm and the forward speed of 60 mm/min. The optimized interfaces for the composite plate after M-FSP are mainly by the metallurgical bondings, with a certain thickness and discontinuous mechanical connections. Therefore, the crack extension stress is the largest and the mechanical properties are the best.

Keywords: friction stir processing; aluminum/steel composite plate; multi-pass; bonding interface; mechanical properties

1. Introduction Aluminum/steel composite plate is well known as a kind of common lightweight composite material and has been widely used in automobile, ship, aerospace and pressure vessel fields [1–3]. Compared with the single metallic materials, the aluminum/steel composite plate not only has superior properties but also greatly reduces the weight of equipment. However, the different physical and chemical properties of aluminum and steel make them unable to form effective and strong joint by fusion welding. Explosive welding is one of the joining methods belonging to the solid state welding process. The metal plates are joined together under a very high pressure and there is local plastic deformation at the interface. The interfaces after explosive welding are almost metallurgical bondings and even stronger than the base metals. Similar and dissimilar materials can be joined by the explosive welding [4]. However, steel cuttings, holes, cracks and other defects caused by the high temperature and the excessive explosion pressure restrict the application of composite plates [5–7].

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Friction stir processing (FSP) developed from friction stir welding (FSW) has been invested in improving the performance of materials [8–10]. Gupta et al. found that FSP can refine grain structures and eliminate defects like porosity in the matrix of AZ31 magnesium alloy [11]. Barmouz found that FSP can refine the second particle’s size and enhance the dispersion distribution of the second particles for Cu/SiC composites [12]. Nakata found that FSP can improve the hardness and tensile strength for ADC12 aluminum die casting alloy by grain refinement and disappearance of cold flake [13]. Pourali et al. investigated the different tool rotations and weld speeds for lap weld joint of 1100Al/St37 steel composite plate. They found that the tensile strength of composite with high tool rotation and low weld speed is superior since the interface of this joint formed a properly thick bonding layer compared with other parameters [14]. However, FSP can also reduce the mechanical properties when the parameters are inappropriate. Gupta et al. also found that tunnel defects are both formed in AZ31 magnesium alloy whether in the truncated conical tool or the cylindrical tool [11]. Kima et al. found that tunnel defects are easily formed in ADC12 aluminum die casting alloy during welding [15]. In order to improve the mechanical properties of composite plate, obtaining the appropriate interface is the main method since the interface of composite plate is the main carrying structure during service. Thus, there are two main factors that affecting the microstructures and mechanical properties of composite plate after FSP with different passes, which include the interface connections and the tunnel defects. In the first part, the bonding strength of composite plate is related to the interface connection mechanisms, which include the mechanical connection and the metallurgical bonding. Pourali et al. found that mechanical connections are the main connection methods of aluminum/steel composite plate under the condition of low welding speed [14]. During explosive welding, the energies of explosion flow cause some of the steel and aluminum soften or even melt, mix and finally form the morphologies of hooks or vortexes at the interface [16,17]. Meanwhile, defects such as steel cuttings and microcracks caused by explosion weaken the strength of composite. Since FSP can refines the second particles size and reduces the cold flake of aluminum die casting alloy [12,13], the steel cuttings and microcracks could also be repaired by strong stirring of aluminum. Therefore, FSP is proposed to repair the defects formed by explosive welding. The metallurgical bondings mainly include intermetallic compounds (IMCs) and barely have few solid solutions. In the process of welding, the formation mechanisms of interface layer are diffusion and metallurgical reaction in the result of temperature and pressure [18,19]. Since the mutual solubility of aluminum and steel is few, the mechanical properties are poor even if forming the solid solution. Therefore, IMCs are interface connections basis for the composite plate. Fe3Al, FeAl, Fe2Al5, FeAl2 and FeAl3 (Fe4Al13) are the dominating forming IMCs during welding process. According to the content ratio of aluminum to steel, IMCs can be divided into two categories—IMCs rich in Fe which are rigid and IMCs rich in Al which are toughness [20–24]. In addition, Bozzi et al. found that when the thickness of IMCs is 8 µm, mechanical properties of the composite plate are the best. While the thickness increases to 42 µm, mechanical properties of the composite plate are the worst [25]. In other words, the thickness of IMCs for the composite plate is a key factor influencing its mechanical properties. When the thickness of IMCs is lower than a certain value, the mechanical properties of composite plate improve with the IMCs thickness increasing [26,27]. With the FSP passes increasing, the accumulated heat input increases and finally IMCs thickness is also getting thicker. However, too thick IMCs decrease the bonding strength of interface. Thus, it is necessary to use M-FSP for obtaining the appropriate thickness of IMCs. In the second part, tunnel defects are usually found in the composite plate after the single-pass FSP [11,15,16,28]. Kima investigated different tool plunge downforces of FSW for ADC12 aluminum die casting alloy. They found that tunnel defects are caused by the insufficient heat input [15]. Javad et al. presented a mathematical model for the heat input generation during friction stir welding of 1060 aluminum alloy. They also found that the insufficient heat input can form tunnel defects Metals 2020, 10, 298 3 of 17 in the matrix of aluminum during welding [29]. Due to the insufficient heat input, the insufficient flow of material stirred causes the tunnel defects. Therefore, the heat input is increased by increasing the number of stirring passes to eliminate the tunnel defects. Meanwhile, the plastic flow of aluminum can also increase with the increase of passes. Thus, M-FSP is used to repair the tunnel defects in the composite plate. In order to find the relationships between microstructures and mechanical properties of aluminum/steel composite plate after FSP with different passes, the 1060Al/Q235 composite plates are chosen as the objects to study on account of its superior plasticity and extensive application. Meanwhile, compared to other alloys, the 1060Al and the Q235 have fewer alloying components than other materials, which makes it easy to study the interface fracture models. The optimized pass of FSP is obtained by microstructures observation and mechanical properties investigation. The above research provided a good data basis in the production of aluminum/steel composite plate in the future.

2. Experimental Materials and Methods The initial material for the sample used in this experiment was the 1060Al/Q235 composite plate and its chemical compositions (wt.%) were measured by ICP direct reading spectrometer, as shown in Table1. It conformed to 1060Al and Q235 standard speciﬁcation. The thickness of aluminum /steel composite plate was 6 mm and the thickness of aluminum and steel was both 3 mm.

Table 1. Chemical compositions for 1060Al/Q235 composite plate (unit: wt.%).

Ele. Fe Al Si Mn Ti Zn C S P 1060Al 0.19 Bal. 0.15 0.03 0.017 0.012 - - - Q235 Bal. 0.029 0.12 0.32 - - 0.13 0.009 0.022

M-FSP was performed using a FSW machine modified from a CNC Vertical milling machine (XK7132, Goldentop Company, Nanjing, China) with a tool rotation speed of 1200 rpm and forward speed of 60 mm/min according to the previous study. The axial force during M-FSP process was 10 kN. The FSW tool was made of W-Re alloy and the FSW tool geometry consisted of a threaded conical pin, with three flats (7 mm diameter) and a spiral (scroll) shoulder with a diameter of 25 mm. The tilt angle for the FSW tool during welding was 2◦. The pin tool plunge depth was 2.5 mm. The stirring head was inserted in the aluminum. The rotational orientation of the stirring head was counterclockwise. After each pass, the composite plate was cooled down about 7 min to avoid the influence residual heat from past passes and then the subsequent pass was performed. The overlap rate was 100%. The sample was polished until the surface was smooth and scratch-free. Then, the microstructures of 1060Al after M-FSP were etched with Keller’s reagent to imaging. The microstructures of Q235 were etched with 4% nitric acid alcohol prior to imaging. The microstructures were analyzed by an optical microscope (OM, DM2700, Leica Company, Wetzlar, Germany). A scanning electron microscope (SEM, Quanta 450, FEI Company, Hillsboro, USA) was used to observe the fracture surface of shear specimens. A transmission electron microscope (TEM, Tecnai T12, FEI Company, Hillsboro, USA) was used to observe structures of the interface. Based on the GB/T2651-2008 and the GB/T232-2010 standards, the shear specimens and the bending specimens were designed, as shown in Figure1. Three shear testing samples and two bending testing samples were tested for each group in order to give a better reliability for the testing datum. Metals 2020, 10, 298 4 of 17 Metals 2020, 10, 298 4 of 17 Metals 2020, 10, 298 4 of 17

Figure 1. Dimensions of mechanical samples: (a) bending specimen and (b) shear specimen (unit: Figuremm). 1.1. Dimensions ofof mechanicalmechanical samples: samples: ( a(a)) bending bending specimen specimen and and (b ()b shear) shear specimen specimen (unit: (unit: mm). mm). The test waswas carriedcarried outout usingusing anan electronicelectronic universaluniversal testingtesting machinemachine CSS-44100,CSS-44100, at anan initialinitial −4 4 strainThe rate test with with was a a carriedloading loading out speed speed using of of an1.2 1.2 electronic× 1010/s.− /Thes. universalThe microhardness microhardness testing machine tests tests were CSS-44100, were carried carried outat an on out initial HX- on × HX-1000TM1000TM/LCDstrain rate with/LCD semi-automatic a semi-automatic loading speed micro-indentation micro-indentationof 1.2 × 10−4/s. Thehardness hardness microhardness testing testing system system tests accordingwere according carried to to the theout nationalon HX- standard1000TM/LCD GBGB/T/T semi-automatic 229-2007.229-2007. micro-indentation hardness testing system according to the national standard GB/T 229-2007. 3. Results and Discussion 3. Results and Discussion 3.1. Macro- & Micro- Structures 3.1. Macro-Figure 2& 2 shows Micro-shows theStructuresthe 1060Al1060Al/Q235 /Q235 compositecomposite plate after M-FSP with didifferentfferent passes.passes. The repair zone for the 1060Al/Q235 composite plate after M-FSP with different passes are well-formed with zone Figurefor the 21060Al/Q235 shows the 1060Al/Q235 composite plate composite after M-FS plateP withafter differentM-FSP with passes different are well-formed passes. The with repair no no defects in it. However, with the increase of pass, especially for the passes are four, there shows defectszone for in the it. 1060Al/Q235 However, with composite the increase plate after of pa M-FSss, especiallyP with different for the passes passes are are well-formed four, there with shows no obvious toe flashes after friction stir processing, as shown in Figure2e with the red arrows. Beside obviousdefects in toe it. flashes However, after with friction the stirincrease processing, of pa ss,as shownespecially in Figurefor the 2e passes with theare redfour, arrows. there Besideshows this, according to the continuous axial force resulting in the toe flashes after M-FSP, the thickness of this,obvious according toe flashes to the after continuous friction stiraxial processing, force resultin as showng in the in toe Figure flashes 2e after with M-FSP, the red the arrows. thickness Beside of the aluminum plate becomes thinner gradually. Therefore, only four passes friction stir processing thethis, aluminum according plateto the becomes continuous thinner axial gradually. force resultin Therefore,g in the onlytoe flashes four passes after M-FSP, friction the stir thickness processing of have been applied in this study. havethe aluminum been applied plate in becomes this study. thinner gradually. Therefore, only four passes friction stir processing have been applied in this study.

Figure 2. 1060Al/Q2351060Al/Q235 composite plate after multi-pass fricti frictionon stir processing (M-FSP) with different di ﬀerent passes:Figure 2.( a 1060Al/Q235) 1060Al1060Al/Q235/Q235 composite composite plate plate after after multi-pass M-FSP; ( b fricti) single on stir pass; processing ( c) two passes; (M-FSP) ( d )with three different passes; ((passes:ee)) fourfour ( passes.passes.a) 1060Al/Q235 composite plate after M-FSP; (b) single pass; (c) two passes; (d) three passes; (e) four passes. Figure3 3 shows shows low-magniﬁcationlow-magnification OMOM imagesimages ofof thethe repairrepair zonezone forfor thethe composite composite plateplate afterafter M-FSP.Figure HolesHoles 3 canshowscan bebe observedobservedlow-magnification inin thethe repairrepair OM zone.zone. images TheThe of repairrepair the repairzonezone appearsappears zone for aa shapeshapethe composite ofof basins.basins. plate The rightafter placeM-FSP. of Holes the stirring can be zone observed (SZ) is in the the advancing repair zone. side Th (AS) e repair while zone the appears left place place a isshape is the the retreating retreatingof basins. sideThe side right(RS). (RS). Theplace AS of presentsthe stirring an zone abrupt (SZ) “zigzag is the advancing line” boundary side (AS) distinctly while theand left the place RS is is inthe a retreatingshape of aside gradual (RS). The AS presents an abrupt “zigzag line” boundary distinctly and the RS is in a shape of a gradual

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Thechange. AS presents This is because an abrupt the “zigzag temperature line” boundary and flow distinctly fields on andthe theAS RSare is different in a shape from of a those gradual on change.the RS This[30−33]. is because the temperature and flow fields on the AS are different from those on the RS [30–33].

Figure 3.3. MicrostructuresMicrostructures of of the the repair repair zone zone for 1060Alfor 1060Al/Q235/Q235 composite composite plate after plate M-FSP after withM-FSP diﬀ erentwith passes:different (a passes:) single ( pass;a) single (b) twopass; passes; (b) two (c passes;) three passes;(c) three (d passes;) four passes. (d) four passes.

Meanwhile,Table 2. Quantities holes and are area clearly of holes seen in in the Figure repair3. zone For thefor 1060Al/Q235 single-pass FSPcomposite (Figure plate3a), after the areaFSP of the largewith holedifferent is 9.8 passes.10 3 µm2 and the smaller one is 3.4 103 µm2. However, just one single hole × × is found in Figure3b with the area of 12 103 µm2. Although the area of the single hole increases, × 2 the total areasPass of the holes decrease. It Quantitiesis confirmed of thatholes the numbers and Total areas area/ of theμm holes have been repaired by1 the plastic flow of aluminum after2 M-FSP. The hole disappears13.2 by × the103influence of repair for the plastic2 flow in aluminum when the 1 passes of M-FSP are three, as 12 shown × 103 in Figure3c. However, when the passes of M-FSP are four, the hole appears again in the AS area (Figure3d). Since 3 0 0 the distances of these holes from the interface are nearly the same, these holes are the tunnel defects 3 resulted after FSP.4 The areas of tunnel defects in repair1 zone with different passes1.1 are × shown 10 in Table2. Therefore, with the increase of passes, the tunnel defects decrease. It can be concluded that M-FSP can also repairMeanwhile, tunnel holes defects are created clearly byseen single-pass in Figure M-FSP.3. For the single-pass FSP (Figure 3a), the area of the large hole is 9.8 × 103 μm2 and the smaller one is 3.4 × 103 μm2. However, just one single hole is found in FigureTable 3b 2. Quantitieswith the andarea area of 12 of holes× 103 inμ them2. repairAlthough zone forthe 1060Al area /ofQ235 the composite single hole plate increases, after FSP withthe total areasdi offf erentthe holes passes. decrease. It is confirmed that the numbers and areas of the holes have been repaired by the plastic flow of aluminum after M-FSP. The hole disappears by the influence of repair for the Pass Quantities of Holes Total Area/µm2 plastic flow in aluminum when the passes of M-FSP are three, as shown in Figure 3c. However, when 1 2 13.2 103 the passes of M-FSP are four, the hole appears again in the AS area (Figure× 3d). Since the distances of 2 1 12 103 these holes from the interface are nearly the same, these holes are the× tunnel defects resulted after 3 0 0 FSP. The areas of tunnel defects4 in repair zone with 1 different passes 1.1 are shown103 in Table 2. Therefore, with the increase of passes, the tunnel defects decrease. It can be concluded× that M-FSP can also repair tunnel defects created by single-pass M-FSP. Figure4 shows microstructures of the 1060Al /Q235 composite plate after FSP with different passes. Figure 4 shows microstructures of the 1060Al/Q235 composite plate after FSP with different The microstructures of base metal (BM) for the 1060Al are α-Al (Figure4b). The microstructures of BM passes. The microstructures of base metal (BM) for the 1060Al are α-Al (Figure 4b). The for Q235 are ferrites and pearlites (Figure4c). Compared with BM for 1060Al, the grains of repair zone microstructures of BM for Q235 are ferrites and pearlites (Figure 4c). Compared with BM for 1060Al, for 1060Al are refined by the stirring effects of M-FSP (Figure4d). The grain size of 1060Al after M-FSP the grains of repair zone for 1060Al are refined by the stirring effects of M-FSP (Figure 4d). The grain is about 3.6 µm, much smaller than that before M-FSP (22.6 µm). Meanwhile, with the influence of size of 1060Al after M-FSP is about 3.6 μm, much smaller than that before M-FSP (22.6 μm). temperature and stir, the microstructures of thermo-mechanical affected zone (TMAZ) are stretched Meanwhile, with the influence of temperature and stir, the microstructures of thermo-mechanical (Figure4e). Figure4f shows the interface for the 1060Al /Q235 composite plate after FSP and also affected zone (TMAZ) are stretched (Figure 4e). Figure 4f shows the interface for the 1060Al/Q235 exhibits the microstructure of heat affected zone (HAZ) in the Q235 side. Moreover, Figure4g shows composite plate after FSP and also exhibits the microstructure of heat affected zone (HAZ) in the TEM images of interface after M-FSP. The grain size of Q235 is about 600 nm. Even though it only stirs Q235 side. Moreover, Figure 4g shows TEM images of interface after M-FSP. The grain size of Q235

Metals 2020, 10, 298 6 of 17 Metals 2020, 10, 298 6 of 17 isin about the aluminum 600 nm. Even size, though the grains it only for Q235stirs in have the al beenuminum reﬁned size, by the M-FSP, grains which for Q235 improves have been the bonding refined byproperties M-FSP, ofwhich interface improves signiﬁcantly the bonding properties of interface significantly

Figure 4. 4. MicrostructuresMicrostructures of composite of composite plate plate after afterfriction friction stir processing stir processing (FSP) with (FSP) different with di passes:fferent (passes:a) repair (a zone;) repair (b) zone;base metal (b) base (BM) metal of aluminum; (BM) of aluminum; (c) BM of steel; (c) BM (d) ofstirring steel; zone (d) stirring (SZ); (e zone) thermo- (SZ); mechanical(e) thermo-mechanical affected zone aff (TMAZ)ected zone in the (TMAZ) aluminum in the side; aluminum (f) heat side;affected (f) heatzone a (HAZ)ffected in zone the (HAZ)steel side in andthe steel(g) transmission side and (g) electron transmission microscopy electron (TEM) microscopy image (TEM)near the image interface. near the interface.

3.2. Interface Interface Characterization Characterization Figure5 5 shows shows comparisons comparisons between between the the unrepaired unrepair interfaceed interface and and the repairedthe repaired interface. interface. The steel The steelcuttings cuttings are clearly are clearly seen atseen the at interface the interface of composite of composite plate plate without without FSP (FigureFSP (Figure5a). The 5a). sizes The ofsizes steel of steelcuttings cuttings are about are about 45 µ 45m μ (Figurem (Figure5b). 5b). Such Such a large a large steel steel cutting cutting has has a a severely severely disadvantage disadvantage eeffectﬀect on the strength of the interface.interface. Non-uniform an andd instantaneous temperatures caused by explosive welding areare responsible responsible for for these these steel steel cuttings. cuttings. However, However, these these steel cuttingssteel cuttings disappear disappear in the interface in the interfacewith FSP with (Figure FSP5c). (Figure Through 5c). theThrough temperature the temper and plasticature and ﬂows plastic of aluminum flows of aluminum stirred after stirred FSP, these after FSP,residual these steel residual cuttings steel react cuttings with react aluminum with alumin and otherum and materials other materials to form new to form IMCs new at the IMCs interface. at the interface.Figure5d,e Figure show 5d,e the SEM show and the Energy SEM and Dispersive Energy SpectroscopyDispersive Spectroscopy (EDS) analysis (EDS) of the analysis interface of forthe interfacethe BM. The for the black BM. particles The black are particles the oxides are of the aluminum oxides of and aluminum iron by meltingand iron and by melting reacting and with reacting oxygen within the oxygen air during in the explosive air during welding. explosive They welding. are aThey kind are of a brittle kind of and brittle hard and phase hard which phase will which reduce will reducethe bonding the bonding strength strength for the composite for the composite plate. While plate. in While Figure in5f,g, Figure there 5f,g, are nothere steel are cuttings no steel and cuttings black andparticles black at particles the interface. at the Theinterface. IMCs The at the IMCs interface at the are interface a kind ofare Al-rich a kind phaseof Al-rich [21]. phase This is [21]. to say This that is toFSP say can that repair FSP the can defects repairremained the defects by remained explosive by welding explosive and optimizewelding microstructuresand optimize microstructures and properties andof composite properties plate. of composite plate.

Metals 2020, 10, 298 7 of 17 Metals 2020, 10, 298 7 of 17

Figure 5. ComparisonsComparisons between between the the unrepaired unrepaired inte interfacerface and the repaired interface: ( (aa)) and and ( (b)) unrepaired interface;interface; ( c()c repaired) repaired interface; interface; (d) ( andd) and (e) Scanning (e) Scanning Electron Electron Microscope Microscope (SEM) and(SEM) Energy and DispersiveEnergy Dispersive Spectroscopy Spectroscopy (EDS) analysis (EDS) anal of theysis unrepaired of the unrepaired interface; interface; (f) and ( g(f)) SEM and ( andg) SEM EDS and analysis EDS ofanalysis the repaired of the repaired interface. interface.

The interface connections mechanism of composite plate is shown in Figure6 6.. TheThe mechanicalmechanical connections consistconsist of of hook hook connections connections and and vortex vortex connections connections formed formed by multiple by multiple hooks hooks (Figure (Figure6a,b). These6a,b). These mechanical mechanical connections connections are the are main the connections main connections of interface. of interface. Meanwhile, Meanwhile, the IMCs the formed IMCs byformed reaction by reaction of aluminum of aluminum and steel and are steel the main are th componentse main components of interface of interface during explosive during explosive welding. Thewelding. IMCs The on theIMCs interface on the are interface the basis are for the the basis metallurgical for the metallurgical bondings, as bondings, shown in as Figure shown6c. in Actually, Figure metallurgical6c. Actually, metallurgical bondings are madebondings up of are IMCs made layers. up of However, IMCs layers. these IMCsHowever, separated these from IMCs the separated interface arefrom disadvantageous the interface are tothe disadvantageous properties of composite to the pr plate.operties IMCs of show composite the appearance plate. IMCs of melting show block the ifappearance the areas ofof IMCs melting are block small. if Conversely, the areas of IMCs IMCs show are small. the appearance Conversely, of meltingIMCs show lump the if appearance the areas of IMCsof melting are large. lump Bothif the theareas melting of IMCs block are andlarge. the Both melting the melting lump are block easy and to becomethe melting originals lump ofare crack. easy Theto become IMCs layer originals with of a certain crack. thicknessThe IMCs has layer a superior with a ecertainﬀect on thickness the properties has a ofsuperior composite effect plate on [22the]. Sinceproperties explosive of composite welding plate is an [22]. instantaneous Since explosive welding welding process, is an the instantaneous composite plate welding could process, not obtain the enoughcomposite thickness plate could IMCs not layer. obtain Therefore, enough M-FSPthickness is usedIMCs to layer. repair Therefore, the bonding M-FSP interface is used and to obtainrepair athe proper bonding thickness interface IMCs and layer. obtain With a theproper increase thickne of passes,ss IMCs the layer. proportions With the of metallurgicalincrease of passes, bondings the increaseproportions in the of interfacemetallurgical ((Figure bondings6d). The increase increase in ofthe passes interface brings ((Figure the increase 6d). The of heatincrease input, of whichpasses willbrings aggravate the increase the di ofﬀ usionheat input, and metallurgical which will aggravate reaction at the the diffusion interface. and The metallurgical IMCs layer becomes reaction thicker at the dueinterface. to this. The Ultimately, IMCs layer the becomes thick IMCs thicker layer due leads to tothis. the Ultimately, increase of the metallurgical thick IMCs bondings. layer leads to the increase of metallurgical bondings.

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Figure 6. InterfaceInterface connection connection mechanism mechanism of of composite composite plate: ( (aa)) hook hook connections; ( (bb)) vortex connections; ( cc)) metallurgical bondings; ( d)) Fractions for mechanical connections and metallurgical bondings to the interface of composite plate after FSP with didifferentﬀerent passes.passes.

Figure 77 showsshows thethe interfaceinterface ofof compositecomposite plateplate afterafter FSPFSP withwith didifferentfferent passes. With With the the increase increase of passes, the thickness of IMCs layers grows. The interface is flat flat and straight with just single-pass FSP (Figure7 7a).a). In In other other words, words, FSP FSP cannot cannot repair repair the the interface interface of compositeof composite plate plate with with the single-passthe single- passFSP eFSPffectively. effectively. Compared Compared with single-pass with single-pass FSP, there FSP, is there a little is morea little thickness more thickness of IMCs of in theIMCs interface in the interfacewith two with passes. two passes. However, However, the eff ectthe ofeffect repair of repair with twowith passestwo passes is still is still insu insufficientfficient in contrastin contrast to tothe the whole whole interface. interface. That That is is to to say, say, the the e ffeffectect of of repair repair is is not not obvious obvious (Figure (Figure7 b).7b). The Thethickness thickness ofof IMCs clearly increases with three passes. There There is is a a superior effect effect of repair with the discontinuous mechanical connections (Figure7 7c).c). TheThe interfaceinterface isis soso thickthick thatthat thethe bondingbonding strengthstrength ofof thethe interfaceinterface is low. Too many passes make the heat input too large, whichwhich willwill makemake thethe interfaceinterface tootoo thick.thick. Due to the existence of too thick interface, the originaloriginal discontinuous mechanical connections have been connected together.together. TheThe propertiesproperties of of composite composite plate plat muste must be be aff ectedaffected with with the the too too thick thick interface interface and andcontinuous continuous connections connections (Figure (Figure7d). 7d). It is It clear is clear that that too too many many passes passes will will cause cause the the deterioration deterioration of ofproperties properties for for the the interface. interface.

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Figure 7.7. Interfaces of composite plate after FSP with didifferentﬀerent passes: ( a) single pass; ( b) two passes; ((cc)) threethree passes;passes; ((dd)) fourfour passes.passes.

InIn summary,summary, FSP FSP can can reduce reduce or even or even eliminate eliminate steel cuttings,steel cuttings, holes and holes cracks and caused cracks by caused explosive by weldingexplosive through welding the through plastic flowthe plastic of aluminum. flow of aluminum. However, the However, repair of the single-pass repair of FSPsingle-pass is nothing FSP like is enough.nothing Thelike M-FSPenough. can The eff ectivelyM-FSP can increase effectively the heat in inputcrease and the makeheat theinput IMCs and layer make grow the andIMCs thicken layer egrowffectively. and thicken Meanwhile, effectively. too many Meanwhile, passes also too deteriorate many passes the performances also deteriorate of composite the performances plate. of composite plate. 3.3. Mechanical Properties 3.3. MechanicalFigure8 shows Properties the fracture morphologies and mechanical property curves for the shear specimens and theFigure shear 8 strengthsshows the are fracture shown inmorphologies Table3. The fractureand mechanical mode of shear property specimens curves exhibits for the typical shear brittlespecimens fracture and (Figurethe shear8a). strengths The curve are of single-passshown in Table shows 3. theThe phenomenon fracture mode of yieldingof shear causedspecimens by theexhibits tunnel typical defects brittle under fracture the load. (Figure While 8a). the curveThe curve of two of passes single-pass and three shows passes the exhibitphenomenon the brittle of fractureyielding morecaused obviously by the tunnel (Figure defects8b). under Compared the load. with While the single-passthe curve of FSP,two passes M-FSP and has three a uniform passes refinementexhibit the ebrittleffect on fracture the aluminum more obviously and finally (Figur improvese 8b). Compared the strength with of aluminum.the single-pass When FSP, subjected M-FSP tohas load, a uniform cracks refinement preferentially effect appear on the at aluminum the interface and and finally the samplesimproves break the strength due to the of aluminum. fracture of theWhen IMCs subjected layer. Theto load, shear cracks strengths preferentially of composite appe platear at after the interface FSP with and diff erentthe samples passes arebreak shown due into Tablethe fracture3. Due of to the the IMCs yield layer. strength The of shear 1060Al strengths is about of 35composite MPa and plate the after shear FSP strengths with different of specimens passes rangeare shown from in 28.87 Table MPa 3. Due to 33.37 to the MPa, yield which strength show of 1060Al that the is shear about properties 35 MPa and of compositethe shear strengths plate after of FSPspecimens are in linerange with from requirements. 28.87 MPa to From 33.37 the MPa, shear whic strengthh show and that curves the shear of di propertiesfferent passes, of composite it can be seenplate thatafter when FSP are the in passes line with are three requirements. and four, theFrom shear the strengthshear strength is higher and than curves those of aredifferent one and passes, two. Althoughit can be seen the strengththat when increase the passes is not are obvious, three and the shearfour, propertiesthe shear strength of composite is higher plate than are stillthose improved are one byand M-FSP. two. Although the strength increase is not obvious, the shear properties of composite plate are still improved by M-FSP.

Metals 2020, 10, 298 10 of 17 MetalsMetals 2020 2020, ,10 10, ,298 298 1010 of of 17 17

FigureFigureFigure 8. 8. Fracture8. Fracture Fracture morphologies morphologies morphologies andand and strengthstrength strength of of shear shear specimens: specimens:specimens: ( a( (a)a ))fracture fracture fracture morphologies morphologies morphologies of of ofshear shear shear specimens;specimens;specimens; (b ( )b(b mechanical) )mechanical mechanical property property property curves curves curves ofof of shearshear shear specimens. specimens.

TableTableTable 3. 3. 3.Shear Shear Shear strengthstrength strength ofof of samples samples after afterafter FSP FSPFSP with with different different diﬀerent passes. passes. passes. 2 τ PassPass Pass SS (mm (mm2S)2) (mm )ForceForce Force (N) (N) (N) (MPa)ττ (MPa) (MPa) 11 125.1025.10 25.10 793.6 793.6 793.6 31.62 31.62 31.62 2 25.05 723.4 28.87 22 325.0525.05 24.90 723.4 723.4 829.5 33.31 28.87 28.87 33 424.9024.90 25.13 829.5 829.5 838.6 33.37 33.31 33.31 44 25.1325.13 838.6 838.6 33.37 33.37 The SEM and EDS analysis of shear specimens are shown in Figure9. Strippings are found in The SEM and EDS analysis of shear specimens are shown in Figure 9. Strippings are found in the fractureThe SEM surfaces, and EDS as shownanalysis in of Figure shear9 specimensa–c. Analysis are shown shows in that Figure these 9. strippingsStrippings areare formedfound in by thethe fracture fracture surfaces, surfaces, as as shown shown in in Figure Figure 9a–c. 9a–c. Analysis Analysis shows shows that that these these strippings strippings are are formed formed by by the failure of the original mechanical connections at the interface under the load. The second phase thethe failure failure of of the the original original mechanical mechanical connections connections at at the the interface interface under under the the load. load. The The second second phase phase particles are also obviously seen in the SEM. The EDS result shows that Si element is contained in these particlesparticles are are also also obviously obviously seen seen in in the the SEM. SEM. The The EDS EDS result result shows shows that that Si Si element element is is contained contained in in particles, which indicates that the second phase particles are Al-Si compounds (Figure9d). A small thesethese particles, particles, which which indicates indicates that that the the second second phase phase particles particles are are Al-Si Al-Si compounds compounds (Figure (Figure 9d). 9d). A A amount of dispersed Al-Si compounds can improve mechanical properties of composite plate. smallsmall amount amount of of dispersed dispersed Al-Si Al-Si compounds compounds can can impr improveove mechanical mechanical properties properties of of composite composite plate. plate.

FigureFigureFigure 9. 9. Scanning9. Scanning Scanning electron electron electron microscopemicroscope microscope (SEM)(SEM) (SEM) analysis analysis of ofof shear shearshear specimens: specimens: ( a(a ()a )single )single single pass; pass; pass; ( b(b) ( )btwo two) two passes; (c) three passes; (d) and (e) SEM and Energy Dispersive Spectroscopy (EDS) analysis of second passes;passes; (c )(c three) three passes; passes; (d (d)) and and ( (ee)) SEMSEM andand Energy Dispersive Dispersive Spectroscopy Spectroscopy (EDS) (EDS) analysis analysis of ofsecond second phase particles. phasephase particles. particles.

Metals 2020, 10, 298 11 of 17

Metals 2020, 10, 298 11 of 17 The shear mechanical properties of BM are also carried out, as shown in Figure 10. The results Metals 2020, 10, 298 11 of 17 show that the phenomenon of yield occurs on the aluminum side of BM. The maximum failure tensile force Theis 1777 shear N. mechanicalHowever, the properties failure strength of BM are is alsonot the carriedcarried shear out, strength, as shownshown while inin FigureisFigure the tensile 1010.. The strength results ofshow 1060Al. that theThe phenomenon failure strength of yield is 118.47 occurs MPa, on thethwhice aluminum h is conformed side of toBM. 1060Al The maximum standard failure specification. tensile Theforce fracture isis 17771777 position N.N. However,However, of BM thethe without failurefailure FSP strengthstrength is in the isis notno aluminumt thethe shearshear side strength,strength, rather whilethanwhile at isis the thethe interface. tensiletensile strengthstrength This is dueof 1060Al. to the refiningThe failure effect strength of FSP is in 118.47 aluminum, MPa, whicwhich h makesis conformed the aluminum to 1060Al obtain standard the grain specification. specification. refining strengthThe fracture after position FSP. Therefore, of BMBM withoutwithout FSP has FSPFSP an obvious isis inin thethe re aluminumalpairinguminum effect sideside on ratherrather the interface thanthan atat thethe of interface.compositeinterface. This plate. is due to the refiningrefining eeffectffect ofof FSPFSP inin aluminum,aluminum, whichwhich makesmakes thethe aluminumaluminum obtain the grain refiningrefining strength after FSP. Therefore,Therefore, FSPFSP hashas anan obviousobvious repairingrepairing eeffectffect onon thethe interfaceinterface ofof compositecomposite plate.plate.

Figure 10. Shear specimens of BM.

The results of bending test are shown in Table 4. The typical bending specimens are shown in Figure 10. Shear specimens of BM. Figure 11. Whether repaired or not, the bending strength is superior. Mainly due to the excellent plasticityThe resultsof 1060Al, of bending there are test almost are shownshown no cracks inin TableTable during4 4.. ThetheThe typicalprocesstypical bendingofbending bending. specimensspecimens However, areare cracks shownshown can inin beFigure seen 1111.in. aluminum Whether repairedafter FSP or with not, four the passes, bending as strengthstshownrength in isis Figure superior.superior. 12. The Mainly failure due angle to thethe of excellentexcellentbending isplasticity 137°. Excessive ofof 1060Al,1060Al, axial therethere force are are almost almost of the no no shoulder cracks cracks during du leringads the theto process processthe thinning of of bending. bending. of BM However, However, with the cracks cracks cracking can can be phenomenonseenbe seen in aluminumin aluminum caused after byafter FSPthe FSP withreduction with four four passes,of passes,the bearing as shownas shown area in Figureinin Figurebending. 12 .12. The Therefore,The failure failure angle it angle should of of bending bendingbe well is controlled in the M-FSP to ensure the properties of bending. 137is 137°.◦. Excessive Excessive axial axial force force of the of shoulder the shoulder leads to le theads thinning to the ofthinning BM with of the BM cracking with phenomenonthe cracking causedphenomenon by the caused reduction by the of the reduction bearing of area the inbearing bending. area Therefore, in bending. it shouldTherefore, be wellit should controlled be well in thecontrolled M-FSP in to the ensure M-FSP the propertiesto Tableensure 4. the Results of bending.properties of bending of bending. test of composite plate. Crack in steel Crack in aluminum Crack in steel Crack in aluminum Pass Table 4.4. Results of bending test of composite plate.plate. (tensile face) (compressed face) (compressed face) (tensile face) Crack in Steel Crack in Aluminum Crack in Steel Crack in Aluminum Pass Crack in steel Crack in aluminum Crack in steel Crack in aluminum Pass0 (TensileNo Face) (Compressed No Face) (Compressed No Face) (Tensile No Face) (tensile face) (compressed face) (compressed face) (tensile face) 1 0No No No No No No No No 20 1NoNo No No No No No No No No No 2 No No No No 1 No No No No 3 3No No No No No No No No 42 4NoNo No No No No No No Yes Yes No 3 No No No No 4 No No No Yes

Figure 11. 11. BendingBending specimens specimens of of composite composite plate plate after after FSP FSP with with two two passes: passes: (a) compressed (a) compressed face; face;(b) tensile(b) tensile face face Figure 11. Bending specimens of composite plate after FSP with two passes: (a) compressed face; (b) tensile face

Metals 2020, 10, 298 12 of 17 Metals 2020,, 1010,, 298298 12 of 17

Figure 12.12. BendingBending specimens specimens of of composite composite plate plate after after FSP withFSP fourwith passes: four passes: (a) tensile (a)) face;tensiletensile (b ) face;face; bending ((b)) bendingdiagram diagram of angle measurement.of angle measurement.

The microhardness of composite plates after FSP isis shownshown inin FigureFigure 1313.13.. The The microhardnessmicrohardness is is measured in the positive direction alongalong thethe aluminumaluminum withwith thethe interfaceinterface asas thethe origin.origin. The hardness on the interface near near the the alumin aluminumum side side without without FSP FSP is is 44.54HV 44.54HV0.050.050.05.. WhileWhile. While thethe the maximummaximum maximum hardnesshardness hardness ofof theofthe the interfaceinterface interface afterafter after FSPFSP cancan FSP bebe can achievedachieved be achieved toto 53.83HV53.83HV to 53.83HV0.050.05.. ItIt isis believedbelieved0.05. It that isthat believed FSPFSP cancan thatenhanceenhance FSP performancesperformances can enhance performancesof composite ofplate composite and has plate an obvious and has anrepairing obvious effect. repairing Meanwhile, effect. Meanwhile, FSP also has FSP an also effect has anof improvementeimprovementffect of improvement onon thethe propertiesproperties on the properties ofof metalmetal of nearnear metal thethe near interface.interface. the interface. TheThe hardnesshardness The hardness ofof Q235Q235 of withoutwithout Q235 without FSPFSP isis 198.5HVFSP is 198.5HV0.050.05 withwith0.05 thethewith distancedistance the distance ofof 3030μ ofm 30awayµm awayfrom fromthe interface, the interface, while while the thehardness hardness can can reach reach to 270.9HVto 270.9HV0.050.05 0.05withwithwith thethethe samesame same distancedistance distance afterafter after FSP.FSP. FSP. AlthoughAlthough Although justjust just repairingrepairing repairing inin in aluminum,aluminum, aluminum, FSPFSP FSP cancan still strengthen steelsteel throughthrough thethe plasticplastic flowsflows ofof aluminum.aluminum.

Figure 13. Microhardness of composite plate after FSP with different passes: (a) hardness of whole Figure 13. Microhardness of composite plate after FSP with different passes: (a)) hardnesshardness ofof wholewhole specimens; (b) hardness for the samples near the interface (partial enlargement of Figure 13a). specimens; (b)) hardnesshardness forfor thethe samplessamples nearnear thethe interfacinterface (partial enlargement of Figure 13a). 4. Discussion 4. Discussion The relationships between microstructures and mechanical properties for the aluminum/steel compositeThe relationships plate with di ffbetweenerent passes microstructures are analyzed and above. mechanical The results properties show that for M-FSP the aluminum/steel not only repair compositethe interface plate but with also repairdifferent the passes tunnel are defects analyzed in the above.above. aluminum TheThe resultsresults side. However,showshow thatthat too M-FSPM-FSP many notnot passes onlyonly repairalso have the negativeinterfacee butffects also on therepair interface the tunnel and metaldefects stirred, in the such aluminum as metal side. thinning However, and toe too flashes. many Therefore,passes also the have fracture negative mechanism effects on of the interface interface for and the metal 1060Al stirred,/Q235 such composite as metal plate thinning is obtained and toe by flashes.studyingflashes. Therefore,Therefore, the relationship thethe fracturefracture between mechanismmechanism structures andofof interfaceinterface properties forfor of thethe aluminum 1060Al/Q2351060Al/Q235/steel composite compositecomposite plate plateplate after isis obtainedFSP with dibyff erentstudying passes. the The relationship failure mechanism between can structures be divided and into properties the following of threealuminum/steel categories: compositeFirstly, plate when after the interfaceFSP with of different aluminum passes./steel composite TheThe failurefailure plate mechanismmechanism is thin and cancan flat, bebe the divideddivided crack extension intointo thethe followingstressfollowing of interface threethree categories:categories: is small due to the flat interface. Meanwhile, the aluminum is so soft that it is easy to occur the phenomenon of yield. Several voids will preferentially be formed in the aluminum side. Cracks are formed by the accumulation and growth of voids with the gradually increasing load.

Metals 2020, 10, 298 13 of 17

Eventually, cracks extend to the aluminum and lead to the failure of composite plate, as shown in Figure 14. This failure mostly occurs in the aluminum/steel composite plate without FSP. It is basically agreed with the yield fracture of BM in the shear test. Metals 2020, 10, 298 13 of 17

Figure 14. Failure mechanism of thin and ﬂatflat interface: ( a) physical diagram; ( b) failure diagram of flatﬂat interface.

Firstly,Secondly, when when the theinterface interface of aluminum/steel of aluminum/ steelcomp compositeosite plate is plate thin hasand aflat, certain the crack thickness extension and stressdiscontinuous of interface mechanical is small due connections, to the flat theinterface. crack Meanwhile, extension stress the aluminum of the interface is so soft is large. that it Several is easy tomicro-cracks occur the phenomenon will preferentially of yield. be formed Several at voids the interface will preferentially under the load. be formed If the composite in the aluminum plate has side. been Cracksinadequately are formed repaired by bythe less accumulation passes, there and could growth be some of defectsvoids with at the the interface. gradually The increasing phenomenon load. of Eventually,stress concentration cracks extend caused to by the defects aluminum would and accelerate lead to thethedevelopment failure of compos of micro-cracks.ite plate, as Therefore,shown in Figurethose micro-cracks 14. This failure would mostly coalesce occurs each otherin the rapidly aluminum/steel and ultimately composite lead to plate the fracture without of theFSP. whole It is basicallyinterface. agreed However, with if the compositeyield fracture plate of has BM been in the adequately shear test. repaired by enough passes, the interface would have a certain thickness. The propagation of micro-cracks needs a path. In addition to this, those discontinuous mechanical connections improve the bonding strength of interface, which make the cracks propagate slowly. Thus, a larger load is needed to make the composite plate break. This failure occurs mostly in the aluminum/steel composite plate after repairing by FSP, which is basically consistent with the case of single, two and three passes, as shown in Figure 15.

Metals 2020, 10, 298 14 of 17 Metals 2020, 10, 298 14 of 17

Figure 15. Failure mechanism of thick and discontinuousdiscontinuous interface. ( a) physical diagram; (b) failure diagram of ﬂatflat interface.

Thirdly,Secondly, when when the the interface interface is too of thick,aluminum/steel although there composite are discontinuous plate has a mechanical certain thickness connections and atdiscontinuous the interface, mechanical the overly thickconnections, interface the connects crack theextension discontinuous stress of mechanical the interface connections is large. Several to each other,micro-cracks which resultswill preferentially in a bending be and formed continuous at the interface.interface under The crack the load. extension If the stress composite of the plate interface has isbeen also inadequately large due to therepaired bending by interface. less passes, Micro-cracks there could will preferentiallybe some defects be formedat the atinterface. the interface The underphenomenon the load. of Duestress to concentration the thicker interface, caused by those defects micro-cracks would accelerate could easily the development bypass the mechanical of micro- connections.cracks. Therefore, Therefore, those the micro-cracks micro-cracks would grow coalesce rapidly witheachthe other increasing rapidly load.and ultimately Finally, the lead composite to the platefracture will of break the whole due to interface. the fracture However, of the interface. if the composite This failure plate occurs has been more adequately often in the repaired composite by plateenough after passes, FSP with the interface too many would passes. have The a fracture certain thickness. of the aluminum The propagation/steel composite of micro-cracks plate is basically needs thea path. same In asaddition that of to four this, passes, those asdiscontinuous shown in Figure mechanical 16. connections improve the bonding strength of interface, which make the cracks propagate slowly. Thus, a larger load is needed to make the composite plate break. This failure occurs mostly in the aluminum/steel composite plate after repairing by FSP, which is basically consistent with the case of single, two and three passes, as shown in Figure 15.

Metals 2020, 10, 298 15 of 17 Metals 2020, 10, 298 15 of 17

FigureFigure 16.16. FailureFailure mechanismmechanism ofof tootoo thickthick andand continuouscontinuous interface:interface: ((aa)physical)physical diagram;diagram; ((bb)) failurefailure diagramdiagram ofof ﬂatflat interface.interface.

5. ConclusionsThirdly, when the interface is too thick, although there are discontinuous mechanical connectionsIn this paper,at the microstructures interface, the overly and properties thick interface for the interfaceconnects ofthe the discontinuous 1060Al/Q235 mechanical composite plateconnections are studied. to each By other, analyzing which the results internal in a relationships bending and betweencontinuous microstructures interface. The and crack mechanical extension propertiesstress of the of theinterface interface, is also the fracturelarge due mechanisms to the bend ofing aluminum interface./steel Micro-cracks composite platewill preferentially are summarized. be Theformed main at conclusions the interface are under concluded: the load. Due to the thicker interface, those micro-cracks could easily bypass1. Defectsthe mechanical at the interface connections. for the aluminumTherefore, /steelthe micro-cracks composite plate grow could rapidl bey repaired with the by increasing the FSP. load.2. Finally, M-FSP the can composite increase the plate thickness will break of interface due to forthe thefracture composite of the plate.interface. Meanwhile, This failure it can occurs also repairmore often the tunnel in the defectscomposite remained plate after by the FSP single-pass with too many FSP. passes. The fracture of the aluminum/steel composite3. The plate melting is basically block and th thee same melting as that lump of infour the passes, composite as shown plates in are Figure easy to 16. become originals of crack. Therefore, when the interfaces of composite plate mainly consist of the metallurgical bondings, with5. Conclusion a certain thickness and discontinuous mechanical connections, its bonding strength is superior. In this paper, microstructures and properties for the interface of the 1060Al/Q235 composite Author Contributions: Conceptualization, J.W. and B.L.; methodology, C.C.; validation, J.W. and B.L.; investigation,plate are studied. J.W. and By Y.C.;analyzing data curation, the internal Y.C.; rela writing—originaltionships between draft preparation,microstructures J.W.; and writing—review mechanical andproperties editing, C.C.;of the supervision, interface, J.W.; the project fracture administration. mechanisms All authorsof aluminum/st have read andeel agreedcomposite to the publishedplate are versionsummarized. of the manuscript. The main conclusions are concluded: Funding:1. DefectsThis work at the was interface funded by for the the National aluminum/steel Natural Science composite Foundation plate of Chinacould (No. be repaired 51505293), by the the Natural FSP. Science2. FoundationM-FSP can ofincrease Jiangsu Provincethe thickness (No. BK20190684),of interface for the the Natural composite Science plate. Research Meanwhile, of the Jiangsu it can Higher also Education Institutions of China (No. 18KJB460016) and the Introduce Talent Special Funding for Scientific Research atrepair Nanjing the Tech tunnel University defects (No. remained 39802124). by the single-pass FSP. 3. The melting block and the melting lump in the composite plates are easy to become originals Conflicts of Interest: The authors declare no conflict of interest. of crack. Therefore, when the interfaces of composite plate mainly consist of the metallurgical

Metals 2020, 10, 298 16 of 17

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