Micro-Brazing of Stainless Steel Using Ni-P Alloy Plating

applied sciences

Article Micro-Brazing of Stainless Steel Using Ni-P Alloy Plating

Shubin Liu 1,* , Ikuo Shohji 1, Makoto Iioka 2, Anna Hashimoto 2, Junichiro Hirohashi 3, Tsunehito Wake 3 and Susumu Arai 4

1 Graduate School of Science and Technology, Gunma University, 1-5-1, Tenjin-cho, Kiryu 376-8515, Japan; [email protected] 2 Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515, Japan; [email protected] (M.I.); [email protected] (A.H.) 3 Waki Factory Inc., 6-760 Higashi-sayamagaoka, Tokorozawa 359-1106, Japan; [email protected] (J.H.); [email protected] (T.W.) 4 Faculty of Engineering, Shinshu University, 4-17-1 Wakasato, Nagano 380-8553, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-277-30-1544

Received: 27 December 2018; Accepted: 11 March 2019; Published: 15 March 2019

Abstract: A Ni-P plated layer of 20 µm thickness containing 11 wt.% P was formed on the surface of a stainless steel (SUS304) plate by electroplating. The microstructure and joint strength of the brazed joint with the electroplated Ni-11P layer were investigated. The results indicated that the filler metal was homogeneously distributed between the SUS304 plates and no voids or flaws formed in the brazed filler zone. Fe-Ni-Cr solid solutions were formed at the brazed interface. Moreover, P was mainly concentrated in such brazed filler zone to form P-containing phases. The average shear strength of the brazed joints was determined to be 47.3 MPa. The results demonstrated that the brazing of SUS304 plates using the electroplated Ni-11P layer as the filler metal was successfully realized.

Keywords: electroplating; Ni-11 wt.% P layer; ﬁller metal; microstructure; shear strength

1. Introduction Stainless steel has often been used as a material for heat exchangers because of its excellent corrosion resistance and heat resistance. Nickel-based filler alloys are commonly used for brazing stainless steel, owing to their good corrosion resistance and oxidation resistance. To lower the melting point of the nickel-based filler metal, it generally contains a large amount of a melting point depressant, such as P, B, or Si. However, nickel-based alloys containing a melting point depressant easily form a eutectic structure that causes the brazed joint to be brittle [1,2]. When brazing the components with a small thickness, owing to the rapid diffusion of boron into the substrate and its low solubility in austenite, large amounts of brittle borides precipitate in the bonding-affected zone, increasing the brittleness of the bonding. Furthermore, the borides can decrease the corrosion resistance of the brazed joint, shortening the life span of the components. Compared with boron, it is difficult for phosphorus to diffuse into the substrate and generate metal compounds in the bonding-affected zone owing to its low diffusion and large atomic radius. In addition, it was found that nickel-based alloys containing phosphorus exhibit very good wettability and may form a ductile solid solution in a joint in the case of a very narrow gap [3]. Therefore, the Ni-P filler alloy is more suitable for the brazing of stainless steel with a small thickness. For a Ni-P alloy, some melting will occur at the lowest melting point (875 ◦C) of the Ni-P system, regardless of the phosphorus content, provided it is greater than 0.2%. Therefore, it is easy to set

Appl. Sci. 2019, 9, 1094; doi:10.3390/app9061094 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 1094 2 of 9 the brazing temperature using a eutectic Ni-P alloy in industrial applications. Currently, a eutectic Ni-(10-12)P (wt.%) alloy, code name BNi-6 (JIS Z 3265), is commercially used as the filler metal owing to its lowest melting temperature in the Ni-P system, extremely good fluidity and excellent brazability [4]. However, with the reduction in the weight of heat exchangers, especially multilayer plate fin heat exchangers, the stainless steel components of heat exchangers have become thinner and smaller. Therefore, a filler metal with a thickness of around 20 µm is required to join the stainless steel. Usually, BNi-6 filler metal is formed as a foil or paste because of its inherent hardness and brittleness, making it difficult to supply the correct quantity of the filler metal to the brazed area. An inappropriate quantity of the filler metal causes degradation and decreases the reliability of the brazed joint. In addition, the thickness of the filler metal is usually below 0.1 mm, and it is difficult to form a foil or paste with such a thickness [5]. It is widely known that Ni-P alloy layers can be produced by rapid quenching, ion implantation, vapor deposition, and chemical deposition [6–8]. However, for various reasons, these techniques do not lend themselves to industrial-scale applications. The chemical deposition method is more economically favorable than these techniques, and alloy layers of uniform thickness can be obtained by controlling the plating conditions. However, the plating solutions used in chemical deposition are laborious to control and produce large amounts of waste owing to their short life span. Furthermore, it is difficult to accurately control the phosphorus content of the plated layers because the properties of the plating solution change with time. Compared with the case of electrodeposition, the deposition rate of chemical deposition is low and a high operating temperature (about 80–90 ◦C) is required [8]. On the other hand, electrodeposition has been found useful at the industrial scale in preparing thin alloy layers, owing to its higher deposition rate and lower operating temperature than chemical deposition. Furthermore, variations in solution composition and operating parameters have resulted in a number of stable solutions capable of producing high-quality Ni-P plating layers [9–13]. Therefore, in this study, electroplating was used to form a thin Ni-11P (wt.%) alloy layer on the surface of a stainless steel plate as the filler metal. The brazing of the stainless steel plates was conducted using the electroplated Ni-11P alloy layer. Moreover, the microstructure was observed and the joint strength of the brazed joint was investigated.

2. Experimental Procedure A plating bath (Figure1) with a volume of 5 L was employed for the electrodeposition of the Ni-P alloy with a phosphorus acid-nickel salt system solution. The bath composition and concentration used for the Ni-P alloy electrodeposition are listed in Table1. The solution was prepared using analytic-grade chemicals and pure water. The plating conditions used in this study were a bath temperature of 50 ◦C, a current density (C.D.) of 1.0–4.0 A/dm2, a plating time of 40–180 min and an air pump used for stirring. In this study, no pH adjustments were made during the electrodeposition of the Ni-P alloy. A pure nickel plate was used as the anode. A stainless steel plate, which had an exposed surface area of 10 cm2 (3 × 3.3 cm2, shown in Figure1), was used as the cathode. Before plating, the stainless steel plate was mechanically polished with a #1000 emery paper, degreased with acetone, rinsed with tap water and dried with a dryer.

Table 1. Bath composition and concentration for Ni-P alloy electrodeposition.

Composition Concentration

NiSO4·6H2O 1.0 M NiCl2·6H2O 0.2 M H3BO3 0.5 M H3PO3 0.12–0.5 M C6H5Na3O7 0.5 M Appl. Sci. 2019, 9, 1094x 33 of of 9 Appl. Sci. 2019, 9, x 3 of 9

Figure 1. Schematic illustration of the plating equipment. Figure 1. Schematic illustration of the plating equipment. The chemicalchemical composition composition analysis analysis of the of Ni-11P the Ni-11P plated layerplated was layer performed was performed by X-ray ﬂuorescence by X-ray The chemical composition analysis of the Ni-11P plated layer was performed by X-ray (XRF,fluorescence Shimadzu (XRF, XRF-1700, Shimadzu Shimadzu, XRF-1700, Kyoto, Shimadzu, Japan) Kyoto, at a voltageJapan) at of a 40voltage kV and of 40 a current kV and ofa current 95 mA fluorescence (XRF, Shimadzu XRF-1700, Shimadzu, Kyoto, Japan) at a voltage of 40 kV and a current atof a95 scan mA rateat a scan of 8◦ rate/min of with8°/min an with aperture an aperture of 20 mm. of 20 The mm. structure The structure of the of Ni-11P the Ni-11P plated plated layer layer was of 95 mA at a scan rate of 8°/min with an aperture of 20 mm. The structure of the Ni-11P plated layer investigatedwas investigated by X-ray by X-ray diffraction diffraction (XRD, (XRD, Rigaku Rigaku RINT RINT 2200 VF,2200 Rigaku, VF, Rigaku, Tokyo, Tokyo, Japan) Japan) analysis analysis at 40 kV at was investigated by X-ray diffraction (XRD, Rigaku RINT 2200 VF, Rigaku, Tokyo, Japan) analysis at and40 kV 20 and mA 20 with mA Cuwith K αCuradiation Kα radiation at a scan at a scan rate ofrate 0.02 of◦ 0.02°/s/s in the in the 2θ range 2θ range of 30–70 of 30–70°.◦. The The thickness thickness of 40 kV and 20 mA with Cu Kα radiation at a scan rate of 0.02°/s in the 2θ range of 30–70°. The thickness theof the plated plated layer layer was was measured measured with with a a micrometer. micrometer The. The surface surface morphology morphology of of thethe platedplated layerlayer was of the plated layer was measured with a micrometer. The surface morphology of the plated layer was investigated by a scanningscanning electron microscopymicroscopy (FE-SEM,(FE-SEM, HITACHI S-4300SE/N,S-4300SE/N, HITACHI, HITACHI, Tokyo, Tokyo, investigated by a scanning electron microscopy (FE-SEM, HITACHI S-4300SE/N, HITACHI, Tokyo, Japan) at an acceleration voltagevoltage ofof 1010 kVkV andand aa probeprobe currentcurrent ofof approximatelyapproximately 1010 µμA.A. Japan) at an acceleration voltage of 10 kV and a probe current of approximately 10 μA. The brazing of the stainless steel plate with thethe electroplated Ni-11P alloy layer was conducted The brazing of the stainless steel plate with the electroplated Ni-11P alloy layer was conducted using a hydrogenhydrogen reductionreduction furnacefurnace withwith aa dewdew pointpoint ofof −−50 ◦°C.C. The brazing temperature and the using a hydrogen reduction furnace with a dew point of −50 °C. The brazing temperature and the holding time at the brazing temperature were set to 1020 ◦°CC and 10 min, respectively. Figure2 2 shows shows holding time at the brazing temperature were set to 1020 °C and 10 min, respectively. Figure 2 shows the geometrygeometry and and dimensions dimensions of the of brazed the brazed joint specimen. joint specimen. Specimens Specimens for cross-sectional for cross-sectional investigation the geometry and dimensions of the brazed joint specimen. Specimens for cross-sectional wereinvestigation cut with were a cutter, cut with embedded a cutter, in embedded epoxy resin, in andepoxy polished resin, and with polished a 1 µm with Al2O a3 1suspension. μm Al2O3 investigation were cut with a cutter, embedded in epoxy resin, and polished with a 1 μm Al2O3 Thesuspension. microstructure The microstructure of the brazed of joint the wasbrazed analyzed joint was by ananalyzed electron by probe an electron X-ray microanalyzer probe X-ray suspension. The microstructure of the brazed joint was analyzed by an electron probe X-ray (EPMA,microanalyzer Shimadzu (EPMA, EPMA-1600, Shimadzu Shimadzu, EPMA-1600, Kyoto, Shimadzu, Japan) at Kyoto, an acceleration Japan) at voltagean acceleration of 15 kV, voltage a sample of microanalyzer (EPMA, Shimadzu EPMA-1600, Shimadzu, Kyoto, Japan) at an acceleration voltage of current15 kV, a of sample 34 nA current for backscattered of 34 nA for electrons backscattered (BSE) image electrons observation (BSE) image and aobservation current of 50–100and a current nA for 15 kV, a sample current of 34 nA for backscattered electrons (BSE) image observation and a current mappingof 50–100 analysis.nA for mapping analysis. of 50–100 nA for mapping analysis. 40 mm 40 mm 5 mm (Side view) 5 mm (Side view) 1.5 mm

Brazed joint 1.5 mm (ClearanceBrazed joint before brazing: 40 μm) (Clearance before brazing: 40 μm)

(Top view) (Top view) 10 mm 10 mm

Figure 2. Geometry and dimensions of brazed joint specimen. Figure 2. Geometry and dimensions of brazed joint specimen. Shear tests were performed at room temperature with an Instron5567 universal testing machine (Instron,Shear Grove,Grove, tests PA,wereOK, USA) USA) performed to to evaluate evaluate at room the the sheartemperature shear strength strength with of of the an the brazed Instron5567 brazed joints. joints. universal The The cross-head cross-head testing speed machine speed for thefor(Instron, shearthe shear testsGrove, tests was OK, was set USA) to set 10 to mm/min.to 10 evaluate mm/min. After the After shear the shear the strength shear test, the oftest, the fracture the brazed fracture mode joints. observationmode The observation cross-head of the brazed ofspeed the jointbrazedfor the was jointshear performed was tests performed was by theset to SEMby 10 the andmm/min. SEM an and optical After an opti microscopy thecal shear microscopy test, (OM, the VK-X150, (OM, fracture VK-X150, KEYENCE, mode KEYENCE,observation Osaka, Osaka, Japan). of the Japan).brazed joint was performed by the SEM and an optical microscopy (OM, VK-X150, KEYENCE, Osaka, Japan). 3. Results and Discussion 3. Results and Discussion 3.1. Formation of Ni-11P Alloy Plated Layer 3.1. Formation of Ni-11P Alloy Plated Layer Appl. Sci. 2019, 9, 1094 4 of 9

3. Results and Discussion

3.1. Formation of Ni-11P Alloy Plated Layer Appl. Sci. 2019, 9, x 4 of 9 Figure3 shows the effects of the H 3PO3 concentration on the P content and thickness of the Ni-PFigure plated 3 layers. shows Withthe effects increasing of the HH33PO3 concentrationconcentration on in the the P bath, content the and P content thickness inthe of the plated Ni- layerP plated increases, layers. whereasWith increasing the thickness H3PO3 ofconcentration the plated layerin the decreases. bath, the P The content mechanism in the plated of the layer Ni-P electrodepositionincreases, whereas process the thickness involves theof followingthe plated reactions layer decreases. [14,15]: The mechanism of the Ni-P electrodeposition process involves the following reactions [14,15]: Ni2+ + 2e− → Ni, (1) Ni 2e →Ni, (1) + − 2H + 2e → H , (2) 2H 2e →H2, (2) + ++ −→ + H3HPOPO3 6H6H 6e6e →PHPH3 3H3H2O,O, (3) (3) 2+ + 2PH2PH3 + 3Ni3Ni →→3Ni2P6H3Ni + 2P + 6H , ,(4) (4) where reactions (1)–(3) are electrochemical reactions andand reaction (4)(4) is a chemical reaction. Increasing the H3POPO33 concentrationconcentration in in the the bath bath clearly clearly accelerates accelerates the the reactions reactions (3) and (4), which increases the P content in thethe platedplated layers.layers. InIn addition,addition, increasingincreasing thethe HH33PO3 concentration in the bath solution reduces the pH, thus accelerating the reaction (2). Consequently, the reaction (1) was signiﬁcantlysignificantly inhibited, decreasing the thicknessthickness ofof thethe platedplated layer.layer. The chemicalchemical compositioncomposition data data obtained obtained by by XRF XRF analysis analysis are are plotted plotted in Figure in Figure3. The 3. results The results show thatshow a that Ni-P a plated Ni-P plated layer oflayer 20 µ ofm 20 thickness μm thickness containing containing 11 wt.% 11 P wt.% can be P obtainedcan be obtained when the when H3PO the3 concentrationH3PO3 concentration in the bath in the is approximatelybath is approximately 0.4 M under 0.4 M the under plating the conditions plating conditions of a current of densitya current of 2.0density A/dm of2 2.0and A/dm a plating2 and timea plating of 90 time min. of 90 min.

Figure 3. Effect of H3PO3 concentration on P content and thickness of Ni-P plated layer (C.D.: Figure 3. Effect of H3PO3 concentration on P content and thickness of Ni-P plated layer (C.D.: 2.0 2.0 A/dm2, plating time: 90 min). A/dm2, plating time: 90 min). The XRD analysis result for the Ni-11P plated layer is presented in Figure4. The broad peak in The XRD analysis result for the Ni-11P plated layer is presented in Figure 4. The broad peak in the XRD spectra of the plated layer at 2θ of 44.44◦ suggests that the electroplated Ni-11P alloy layer the XRD spectra of the plated layer at 2θ of 44.44° suggests that the electroplated Ni-11P alloy layer has an amorphous structure. The incorporation of phosphorus in the nickel generally increases the has an amorphous structure. The incorporation of phosphorus in the nickel generally increases the number of defects in the crystalline lattice of the Ni-P plating layer, thereby transforming the plating number of defects in the crystalline lattice of the Ni-P plating layer, thereby transforming the plating layer from crystalline to amorphous [16]. No peaks from Ni3P or other Ni-P compounds were detected layer from crystalline to amorphous [16]. No peaks from Ni3P or other Ni-P compounds were in the electroplated Ni-11P layer. detected in the electroplated Ni-11P layer.

3.2. Effect of Current Density on Surface Morphology of Ni-P Alloy Plated Layer Figure 5 shows the SEM micrographs of the surface morphology of the Ni-P plated layers under different current densities. When the current density was increased from 1 A/dm2 to 2 A/dm2, the size of the spherical nodules on the surface of the plated layer decreased. Next, the nodules became elongated when the current density was increased to 4 A/dm2. In addition, with the increase in the Appl. Sci. 2019, 9, x 5 of 9

current density, the plated layer exhibited a higher porosity (shown in Figure 5c). The porous microstructure was ascribed to the enhanced hydrogen evolution reaction, in consequence, a series Appl.of Sci. hydrogen2019, 9, 1094 evolution channels formed in the plated layer. The higher current density resulted in a 5 of 9 loose plated layer. Therefore, the optimal current density is considered to be 2.0 A/dm2.

700 Ni 600 P 500

Appl. Sci. 2019, 9, x 400 5 of 9

current density, 300the plated layer exhibited a higher porosity (shown in Figure 5c). The porous microstructure was ascribed to the enhanced hydrogen evolution reaction, in consequence, a series of hydrogen evolution200 channels formed in the plated layer. The higher current density resulted in a 2 loose plated layer.unit) Intensity (arbitrary 100 Therefore, the optimal current density is considered to be 2.0 A/dm . 700 0 30 40 50 60Ni 70 600 P 2θ (degree) 500 FigureFigure 4. 4.XRD XRD diffractiondiffraction patterns patterns of of Ni-11P Ni-11P plated plated layer. layer. 400 3.2. Effect of Current Density on Surface Morphology of Ni-P Alloy Plated Layer 300 Figure5 shows the SEM micrographs of the surface morphology of the Ni-P plated layers under different current densities.200 When the current density was increased from 1 A/dm2 to 2 A/dm2, the

size of the sphericalunit) Intensity (arbitrary nodules on the surface of the plated layer decreased. Next, the nodules became 100 elongated when the current density was increased to 4 A/dm2. In addition, with the increase in the current density, the0 plated layer exhibited a higher porosity (shown in Figure5c). The porous microstructure was ascribed30 to the enhanced 40 hydrogen 50 evolution reaction, 60 in consequence, 70 a series of hydrogen evolution channels formed in the plated2 layer.θ (degree) The higher current density resulted in a loose 2 plated layer.Figure Therefore, 5. SEM micrographs theFigure optimal of 4. surface XRD current diffraction morphology density patterns of is plated considered of Ni-11P layers. plated The to beplated layer. 2.0 la A/dmyers were. obtained

in the bath containing 0.5 M H3PO3, with: (a) C.D.: 1.0 A/dm2, plating time: 180 min and thickness: 18 μm; (b) C.D.: 2.0 A/dm2, plating time: 90 min and thickness: 15 μm; (c) C.D.: 4.0 A/dm2, plating time: 40 min and thickness: 16 μm.

3.3. Microstructure Observation of Brazed Joint Figure 6 shows the EPMA mapping analysis results for the brazed joint using the Ni-11P plated layer as the filler metal. The Ni and P elements are homogeneously distributed between the SUS304 plates. Also, the filler metal shows good bondability without voids or flaws in the brazed zone. The bright gray phases (A), which exist at the brazed interfaces of the SUS304 plates, were formed by the dissolution of the component element of the SUS304 plates into the molten filler metal [3,17]. The chemical composition of these phases was determined by EPMA and the result is given in Table 2. It FigureFigure 5. SEM 5. SEM micrographs micrographs of of surface surface morphology morphology of plated plated layers. layers. The The plated plated layers layers were were obtained obtained was found that Ni and Fe are the main components. In addition,2 2 a small amount of Cr is contained in thein bath the bath containing containing 0.5 0.5 M M H 3HPO3PO33,, with:with: ( (aa)) C.D.: C.D.: 1.0 1.0 A/dm A/dm, plating, plating time: time:180 min 180 and min thickness: and thickness: 18 in those phases. Figure 7 shows2 the isothermal section of the Fe-Ni-Cr ternary phase2 diagram at 25 18 µm;μm; (b ()b C.D.:) C.D.: 2.0 2.0 A/dmA/dm ,2 plating, plating time: time: 90 min 90 min and andthickness: thickness: 15 μm; 15 (cµ) C.D.:m; (c )4.0 C.D.: A/dm 4.0, plating A/dm time:2, plating °C, which was calculated by Thermo-Clac 2017a (using SSOL6: SGTE Alloy Solutions Database v6.0). time:40 40 min min and and thickness: thickness: 16 μ 16m.µ m. In the figure, the chemical composition of point A shown in Table 2 is also plotted. Since the 3.3. Microstructurecomposition3.3. Microstructure of P Observation is Observation2.0 mol%, of it ofBrazed is Brazed ignored Joint Joint and the compositions of only Fe, Ni, and Cr are expressed in percentage in the plot. From Figure 7, the bright gray phases shown in the BSE image in Figure 6 Figure 6 shows the EPMA mapping analysis results for the brazed joint using the Ni-11P plated wereFigure identified6 shows as the Fe-Ni-Cr EPMA solid mapping solution. analysis results for the brazed joint using the Ni-11P plated layer as the filler metal. The Ni and P elements are homogeneously distributed between the SUS304 layer as the filler metal. The Ni and P elements are homogeneously distributed between the SUS304 plates. Also, the filler metal shows good bondability without voids or flaws in the brazed zone. The plates. Also, the filler metal shows good bondability without voids or flaws in the brazed zone. The bright gray phases (A), which exist at the brazed interfaces of the SUS304 plates, were formed by the brightdissolution gray phases of the (A), component which exist element at the of brazed the SUS304 interfaces plates of into the the SUS304 molten plates, filler metal were [3,17]. formed The by the dissolutionchemical of composition the component of these element phases was of the determined SUS304 platesby EPMA into and the the molten result is filler given metal in Table [3, 172. It]. The chemicalwas found composition that Ni and of these Fe are phases the main was components. determined In byaddition, EPMA a and small the amount result of is Cr given is contained in Table 2. It was foundin those that phases. Ni and Figure Fe are7 shows the mainthe isothermal components. section In of addition, the Fe-Ni-Cr a small ternary amount phase ofdiagram Cr iscontained at 25 in those°C, which phases. was Figure calculated7 shows by Thermo-Clac the isothermal 2017a (usi sectionng SSOL6: of the SGTE Fe-Ni-Cr Alloy Solutions ternary phaseDatabase diagram v6.0). at 25 ◦C,Inwhich the figure, was calculatedthe chemical by composition Thermo-Clac of point 2017a A (using shown SSOL6: in Table SGTE 2 is also Alloy plotted. Solutions Since Database the v6.0).composition In the figure, of P the is 2.0 chemical mol%, it compositionis ignored and ofthe point compositions A shown of inonly Table Fe, Ni,2 is and also Cr plotted. are expressed Since the in percentage in the plot. From Figure 7, the bright gray phases shown in the BSE image in Figure 6 were identified as Fe-Ni-Cr solid solution. Appl. Sci. 2019, 9, x 6 of 9 Appl. Sci. 2019, 9, x 6 of 9 Appl. Sci. 2019, 9, 1094 6 of 9 Table 2. Chemical composition of point A in brazed joint. Table 2. Chemical composition of point A in brazed joint. Element Composition (mol%) composition of P isPoint 2.0 mol%, Element it is ignored Composition and the (mol%) compositions Possible of only Fe,Phase Ni, and Cr are expressed Point Ni P Fe Cr Possible Phase Ni P Fe Cr in percentage in theA plot. From 41.3 Figure2.07 , the48.1 bright gray8.6 phasesFe-Ni-Cr shown solid in thesolution BSE image in Figure6 were identified as Fe-Ni-CrA solid 41.3 solution.2.0 48.1 8.6 Fe-Ni-Cr solid solution

Figure 6. EPMA mapping analysis results for brazed joint with Ni-11P plated layer. The plated layers FigureFigure 6.6.EPMA EPMA mappingmapping analysisanalysis resultsresults forfor brazedbrazed jointjoint withwith Ni-11PNi-11P platedplated layer.layer.The Theplated platedlayers layers used for brazing tests were obtained in the bath containing 0.4 M H3PO3 in the case of a current of 2.0 usedused forfor brazingbrazing teststests were obtained in the the bath bath containing containing 0.4 0.4 M M H H3POPO3 inin the the case case of ofa current a current of 2.0 of A/dm2 and a plating time of 90 min. 3 3 2.0A/dm A/dm2 and2 and a plating a plating time time of 90 of min. 90 min.

◦ Figure 7. Phase diagram of Fe-Ni-Cr at 25 °C.C. Figure 7. Phase diagram of Fe-Ni-Cr at 25 °C. Appl. Sci. 2019, 9, 1094 7 of 9

Table 2. Chemical composition of point A in brazed joint.

Element Composition (mol%) Point Possible Phase Ni P Fe Cr Appl. Sci. 2019, 9, x 7 of 9 A 41.3 2.0 48.1 8.6 Fe-Ni-Cr solid solution Moreover, there are two types of phases in the brazed filler zone, dark gray and the bright gray phases.Moreover, The BSE there image are shows two types that these of phases two intypes the brazedof phases filler mix zone, together dark in gray the and brazed the brightfiller zone. gray Inphases. this study, The BSE both image phases shows are too that small these to two perform types ofa chemical phases mix composition together in analysis. the brazed Nevertheless, filler zone. theIn this elemental study, both distribution phases are shows too small that P to does perform not aconcentrate chemical composition in the regions analysis. where Nevertheless, the bright gray the phaseselemental exist. distribution Moreover, shows Fe and that Ni P are does highly not concentrateconcentrated in in the the regions same whereregions. the This bright means gray that phases the structureexist. Moreover, of these Fe bright and Ni gray are phas highlyes concentratedappears to be in the the same same as regions. that of This the meansphase corresponding that the structure to pointof these A. brightIt has graybeen phasesreported appears that P tomainly be the concen same astrates that in of the the brazed phase correspondingfiller zone owing to pointto its A.low It diffusionhas been reportedand low thatsolubility P mainly into concentratesthe matrix metal in the [3]. brazed Thus, filler the dark zone gray owing phases to its loware inferred diffusion to and be thelow P-containing solubility into phases. the matrix To obtain metal a [ 3deeper]. Thus, understa the darknding gray of phases these arephases, inferred further to be study, the P-containing such as the XRDphases. analysis To obtain of the a deeperbrazed understandingjoint, is required. of these phases, further study, such as the XRD analysis of the brazed joint, is required. 3.4. Shear Test 3.4. Shear Test The shear strength is an important criterion for the quality of a brazed joint. Four brazed joint specimensThe shear were strength prepared is for an the important shear test. criterion The aver forage the shear quality strength of a brazed of the joint. brazed Four joints brazed obtained joint wasspecimens determined were preparedto be 47.3 for MPa. the shear The test. standard The average deviation shear of strength the shear of thestrength brazed was joints 4.35 obtained MPa, indicatingwas determined that the tobe shear 47.3 MPa.strength The of standard the brazed deviation joint ofwith the the shear electroplated strength was Ni-11P 4.35 MPa, filler indicating alloy is relativelythat the shear stable. strength Takayama of the et brazed al. [18] joint studied with the electroplatedjoint strength Ni-11P of SUS304 filler brazed alloy is with relatively BNi-2 stable. filler metalTakayama for use et al.in [a18 heat] studied exchanger. the joint The strength shear strength of SUS304 of brazedthe brazed with joint BNi-2 was filler approximately metal for use 100 in a MPa heat ◦ atexchanger. 1040 °C. TheWu shearet al. [19] strength reported of the that brazed the shear joint was strength approximately of the brazed 100 MPajoint atwith 1040 NiCrPC. Wu filler et al.metal [19] increasedreportedthat from the 36 shear MPa strengthat 980 °C of to the 137 brazed MPa jointat 1040 with °C, NiCrP i.e., the filler shear metal strength increased increases from 36 with MPa the at ◦ ◦ brazing980 C to temperature. 137 MPa at 1040 AlthoughC, i.e., the the shear shear strength strength ob increasestained in with this the study brazing is low, temperature. the brazedAlthough joint has excellentthe shear quality strength in obtainedterms of the in this absence study of is voids low, theand brazed flaws. The joint shear has excellent strength qualityobtained in in terms this study of the seemsabsence to ofcorrespond voids and to flaws. that of The the shear Ni-11P strength alloy and obtained it has a in meaningful this study guiding seems to role correspond in practical to thatterms. of the Ni-11PThe fracture alloy andmorphology it has a meaningful of the brazed guiding joint was role observed in practical by terms.SEM. The result is shown in Figure 8a. TheThe bright fracture gray morphology phases and of the the brazed brazed filler joint waszone observedwere observed by SEM. on Thethe resultfractured is shown surface, in indicatingFigure8a. Thethatbright failure gray occurred phases in and the thebrazed brazed filler filler zone. zone Figure were 8b observed shows an on optical the fractured micrograph surface, of theindicating cross section that failure of the occurredfractured in joint. the brazedA simila fillerr fracture zone. mode Figure was8b shows observed. an optical As mentioned micrograph above, of the P-containing cross section ofphases the fractured were mainly joint. concentrated A similar fracture in the modebrazed was filler observed. zone. These As mentionedphases are above,brittle compoundsthe P-containing and thus phases they were appear mainly to reduce concentrated the strength in the of brazed the brazed filler joints. zone. These phases are brittle compounds and thus they appear to reduce the strength of the brazed joints.

(a) SEM image of fractured surface (b) OM image of cross section of fractured joint

Figure 8. FractureFracture mode observation of brazed joint.

4. Conclusions In this study, a Ni-P alloy plated layer was fabricated on the surface of a SUS304 plate to replace a conventional foil or paste filler metal. Moreover, the microstructure and joint strength of the brazed joint with the electroplated Ni-P layer were investigated. The obtained results are as follows: 1. A Ni-11 wt.% P electroplated layer of 20 μm thickness was formed on the surface of the SUS304 plate in the case of 0.4 M H3PO3 in the bath, a current density of 2.0 A/dm2 and a plating time of 90 min. Appl. Sci. 2019, 9, 1094 8 of 9

4. Conclusions In this study, a Ni-P alloy plated layer was fabricated on the surface of a SUS304 plate to replace a conventional foil or paste ﬁller metal. Moreover, the microstructure and joint strength of the brazed joint with the electroplated Ni-P layer were investigated. The obtained results are as follows:

1. A Ni-11 wt.% P electroplated layer of 20 µm thickness was formed on the surface of the SUS304 2 plate in the case of 0.4 M H3PO3 in the bath, a current density of 2.0 A/dm and a plating time of 90 min. 2. The average shear strength of the brazed joints was determined to be 47.3 MPa. 3. Fracture occurred in the brazed filler zone where the brittle P-containing compounds existed. 4. The brazing filler metal formed by electroplating was homogeneously distributed between the SUS304 plates after brazing. This means that the electroplated Ni-P layer can be used as a brazing filler metal for SUS304.

Author Contributions: Writing—original draft, S.L.; writing—review & editing, S.L. and I.S.; conceptualization, I.S., J.H., T.W. and S.A.; formal analysis, S.L., A.H. and M.I.; investigation, M.I. and A.H.; methodology, I.S. and S.A.; project administration, I.S.; resources, I.S., J.H. and T.W.; supervision, I.S. Funding: This research was funded by the Japan Science and Technology Agency (JST) A-STEP program. Acknowledgments: This study was partially supported by the A-STEP Stage I industrial needs response type program of JST (Japan Science and Technology Agency). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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