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metals Article

Study of the Microstructure Evolution and Properties Response of a Friction-Stir-Welded Copper-Chromium-Zirconium Alloy Ruilin Lai 1 , Diqiu He 1,2 , Guoai He 3, *, Junyuan Lin 1 and Youqing Sun 1 1

2 3

*

State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083, China; [email protected] (R.L.); [email protected] (D.H.); [email protected] (J.L.); [email protected] (Y.S.) Light Alloy Research Institute, Central South University, Changsha 410083, China State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China Correspondence: [email protected]; Tel.: +86-0731-8883-0938

Received: 17 August 2017; Accepted: 11 September 2017; Published: 19 September 2017

Abstract: In this article, the copper-chromium-zirconium (CuCrZr) alloys plates with 21 mm in thickness were butt joined together by means of FSW (friction stir welding). The properties of the FSW joints are studied. The microstructure variations during the process of FSW were investigated by optical microscopy (OM), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM). The results show that the grains size in the nugget zone (NZ) are significantly refined, which can be attributed to the dynamic recrystallization (DRX). The microstructure distribution in the NZ is inhomogeneous and the size of equiaxed grains are decreased gradually along the thickness direction from the top to bottom area of the welds. Meanwhile, it is found that the micro-hardness and tensile strength of the welds are slightly increased along the thickness direction from the top to the bottom area of the welds. All the nano-strengthening precipitates in the BM are dissolved into the Cu matrix in the NZ. Therefore, the decreases in hardness, tensile strength, and electrical conductivity can be attributed to the comprehensive effect of dissolution of nano-strengthening precipitates into the supersaturation matrix and severe DRX in the welded NZ. Keywords: copper-chromium-zirconium alloy; friction stir welded; microstructure; precipitates; micro-hardness; tensile properties; electrical conductivity

1. Introduction The copper-chromium-zirconium (CuCrZr) alloy is a new functional structure material and widely applied in industry owing to its excellent combinations of electrical conductivity, vacuum compatibility, and outstanding mechanical properties at elevated temperature [1–4]. However, due to the extreme high thermal diffusivity (i.e., approximately 10 to 100 times higher than that in some steels and nickel alloys [5–7]), copper alloys are generally classified as non-weldable alloys which cannot be fabricated by conventional techniques, such as fusion welding. Durocher et al. [8,9], Drezet et al. [10], and Gogari [11] found that hot cracking frequently occurs during electron beam welding of the CuCrZr alloys. Feng et al. [12] observed the appearance of residual stress and impurities introduced by the flash butt welding process, which thus caused the decrease of the electrical conductivity. Kanigalpula et al. [13] revealed the presence of voids and cracks in the microstructure of the investigated alloys after electron beam welding. Therefore, the fusion welding, such as the flash butt welding and the electron beam welding, are considered as an unattractive method for the welding of the CuCrZr alloy. In comparison with fusion welding, friction stir welding (FSW), invented by The Welding Institute (TWI) of the UK in 1991 [14], is a relatively new solid-state joining process. The FSW process is proved

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to be a perfect choice for copper alloys [7,15–18], and it has been successfully implemented in industrial applications [19,20]. However, little research has been carried out on the CuCrZr alloy fabricated by FSW. Metals Moreover, limited research related to the microstructure evolution of the CuCrZr alloy in the 2017, 7, 381 2 of 13 FSW process has been reported so far. In this study, the FSW introduced to successfully the 21 mm thickness of the CuCrZr process is proved to be was a perfect choice for copper alloysweld [7,15–18], and it has been successfully in industrial [19,20]. However, research been carried out on theof the plate.implemented The properties includingapplications the micro-hardness, tensile little strength, andhas electrical conductivity CuCrZr fabricated by FSW. Moreover, limited research to the microstructure evolution by CuCrZr alloyalloy joints are studied. The microstructures aroundrelated the welding joints are investigated of the CuCrZr alloy in the FSW process has been reported so far. OM, TEM, and EBSD. In this study, the FSW was introduced to successfully weld the 21 mm thickness of the CuCrZr

plate. The properties including the micro-hardness, tensile strength, and electrical conductivity of 2. Materials and Methods the CuCrZr alloy joints are studied. The microstructures around the welding joints are investigated

The chemical compositions of the CuCrZr alloy in this work are listed in Table 1. The plates by OM, TEM, and EBSD. with 21 mm in thickness were longitudinally butt-welded using a computer-controlled FSW machine 2. Materials and the weldingand toolMethods with a shoulder of 36 mm in diameter and a conical pin of 20 mm in length, as shownThe in Figure 1. The plunging and plunging speed toolinwere as 1 mm chemical compositions of depth the CuCrZr alloy in this workof arethe listed Tableselected 1. The plates and 0.5 mm/s, respectively. The pinlongitudinally rotation speed and travel using speedawere chosen as 1500 FSW rpm and with 21 mm in thickness were butt-welded computer-controlled machine and the welding The tool microstructures with a shoulder ofof 36the mmFSW in diameter a conical pin of mm in 150 mm/min, respectively. sampleand perpendicular to20the welding length, as shown in Figure 1. The plunging depth and plunging speed of the tool were selected as 1 direction are analyzed by OM with a confocal laser scanning microscope (CLSM, ZEISS, Oberkochen, mm and 0.5 mm/s, respectively. The pin rotation speed and travel speed were chosen as 1500 rpm Germany), TEM, and EBSD. For the CLSM observation, the specimens were polished by diamond 150 mm/min, respectively. The microstructures of the FSW sample perpendicular to the welding paste,and etched in a solution of 40 mL distilled water, 10 mL hydrochloric acid, and 2 g iron (III) chloride. direction are analyzed by OM with a confocal laser scanning microscope (CLSM, ZEISS, Intermetallic particles found in micrograph were further analyzed by EDS for determination of the Oberkochen, Germany), TEM, and EBSD. For the CLSM observation, the specimens were polished elemental composition. For the TEM study, some thin foils of 0.5 mm thinness were cut out from by diamond paste, etched in a solution of 40 mL distilled water, 10 mL hydrochloric acid, and 2 g different zones. Then, Intermetallic the foils were grinded intoinone of a thickness of 70 µm~80 µmbyand iron (III) chloride. particles found micrograph were further analyzed EDSpunched for out several (Φ3 mm) discs for electro twin polishing in a study, solution of thin 75%foils methanol and 25% nitric determination of the elemental composition. For the TEM some of 0.5 mm thinness ◦ C~−20 ◦ C, and at a voltage of 10 V. acid under theout parameters of the temperature −30into were cut from different zones. Then, the ranging foils werefrom grinded one of a thickness of 70 μm~80 μm and punched out several (Φ3 mm) discs for electro twin polishing in a solution of 75% methanol TEM experiments were conducted on the Tecnai G2 F20 (FEI Corporation, Hillsboro, OR, USA) with and 25% nitric acid under the parameters of the temperature ranging from −30 °C~−20 °C, at a in an acceleration voltage of 120 keV. For the EBSD analysis, the corresponding sampling and positions voltage of 10 V. TEM experiments were conducted on the Tecnai G2 F20 (FEI Corporation, Hillsboro, the NZ were shown in Figure 2. Also, the data was collected using a FEI Quanta 650 FEG scanning OR, USA) with an acceleration voltage of 120 keV. For the EBSD analysis, the corresponding electron microscopy (FEI Corporation, Hillsboro, OR, USA). sampling positions in the NZ were shown in Figure 2. Also, the data was collected using a FEI Quanta 650 FEG scanning electron microscopy (FEI Corporation, Hillsboro, OR, USA). Table 1. The chemical compositions of the copper-chromium-zirconium alloy plate. Table 1. The chemical compositions of the copper-chromium-zirconium alloy plate.

Cu

Cu Bal. Bal.

Cr Cr 0.8 0.8

Zr Zr 0.30.3

Al Al 0.25 0.25

Mg Mg 0.1 0.1

Fe

Fe 0.090.09

Si

Pb Si 0.04 0.04 0.02

Pb 0.02

Figure 1. Schematic presentation of friction stir welding technique and the tool geometry used for this work.

Figure 1. Schematic presentation of friction stir welding technique and the tool geometry used for this work. In order to investigate the mechanical properties of the welded joints, the block was further cut

along the plate thickness direction into three layers (top, middle and bottom) with a thickness of 5 mm, as seen in Figure 2. The Vickers micro-hardness measurements were conducted on the cross section for each layer using a Vickers indenter, wherein a 100 g load was imposed and held for 10 s.

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investigate the mechanical properties of the welded joints, the block was further 3 ofcut 13 along the plate thickness direction into three layers (top, middle and bottom) with a thickness of 5 mm, To zone-related Vickers micro-hardness, points 2 mm apart other as further seen in explore Figure 2.the The Vickers micro-hardness measurements werewith conducted on thefrom crosseach section for 7, 381measurements. of 13 were selected for The wherein tensile properties at room temperature thefor welded joints for eachMetals layer2017, using a Vickers indenter, a 100 g load was imposed and of held 10 s. 3To further each layer also examined an Instron-8032 (Grove City,other PA, USA) with a explore thewere zone-related Vickersusing micro-hardness, pointstesting with 2machine mm apart from each were selected To further explore the zone-related Vickers micro-hardness, points with 2 mm apart from each other crosshead speed of The 2 mm/min. The facture surfaces were analyzed on a field scanning for measurements. tensile properties at room temperature of the welded jointsemission for each layer were were selected for measurements. The tensile properties at room temperature of the welded joints for electron microscopy (SEM Quanta 650). also each examined using an Instron-8032 testing machine (Grove City, PA, USA) with a crosshead speed layer were also examined using an Instron-8032 testing machine (Grove City, PA, USA) with a study the effects the microstructure evolution electrical conductivities, tests on both the of 2 To mm/min. The facture surfaces were analyzed onwere a on field emission scanning electron microscopy crosshead speed of 2of mm/min. The facture surfaces analyzed on a field emission scanning parent material and the(SEM welded alloy were carried out at each layer by employing a double bridge (SEMelectron Quanta 650). microscopy Quanta 650). circuitTo and currents (intensity up to 50 A) under a ventilated condition at a constant study thethe effects ofofthe evolution onelectrical electrical conductivities, on the both the Todc study effects themicrostructure microstructure evolution on conductivities, teststests on temperature both parent material and the welded alloy were carried out at each layer by employing a double bridge of 25 °C. parent material and the welded alloy were carried out at each layer by employing a double bridge circuit and dc currents (intensity upunder to 50 A) under a ventilated condition at a constant temperature and circuit dc currents (intensity up to 50 A) a ventilated condition at a constant temperature of 25 ◦ C. of 25 °C.

Figure 2. Schematic of three layers for tensile strength and and vickershardness, hardness, and electrical test and Figure 2. Schematic threelayers layersfor for tensile tensile strength and and electrical test and Figure 2. Schematic of of three strength andvickers vickers hardness, electrical test and threethree locations (a–c) for the electron back-scattered diffraction (EBSD) test. locations (a–c) the electronback-scattered back-scattered diffraction test. three locations (a–c) forfor the electron diffraction(EBSD) (EBSD) test.

3. Results and and Discussions 3. Results Discussions 3. Results and Discussions 3.1. Microstructure of FSW Joints 3.1. Microstructure of FSW Joints 3.1. Microstructure of FSW Joints picture FSW joint theCuCrZr CuCrZr alloy alloy is Clearly, there are four The The picture FSW joint of ofthe is shown shownininFigure Figure3. 3. Clearly, there are four The picture FSW thejoint CuCrZr alloy in Figure 3.theClearly, therezone are four distinguishable zonesjoint of theof FSW including: (a)istheshown base metal (BM); (b) heat affected distinguishable zones of the FSW joint including: (a) the base metal (BM); (b) the heat affected zone distinguishable of the FSW joint including: (a) theand base (BM); (b)(NZ); the heat (HAZ); (c) thezones thermo-mechanically-affected zone (TMAZ); (d) metal the nugget zone these affected all (HAZ); (c) the thermo-mechanically-affected zone (TMAZ); and (d) the nugget zone (NZ); these all during the process of FSW. zoneoccur (HAZ); (c) the thermo-mechanically-affected zone (TMAZ); and (d) the nugget zone (NZ); these occur during the process of FSW. all occur during the process of FSW.

Figure 3. Picture of the cross sections of the friction stir-welded CuCrZr joints, showing the different weld zone regions from base metal (BM) to nugget zone (NZ).

Figure Figure3.3.Picture Pictureofofthe thecross crosssections sectionsofofthe thefriction frictionstir-welded stir-weldedCuCrZr CuCrZrjoints, joints,showing showingthe thedifferent different Figureregions 4 shows thebase OM picture microstructures corresponding to locations “a”, “b”, “c”, and weld weldzone zone regionsfrom from basemetal metal(BM) (BM)to tonugget nuggetzone zone(NZ). (NZ). “d”, which are indicted in Figure 3. In the BM, larger lath and coarse grains are apparently observed and considerable numbers of irregularly shaped particles can be found distributed randomly in the Figure 4 shows the OM picture microstructures corresponding to locations “a”, “b”, “c”, and Cu matrix, as shown in Figure 4a. To further reveal the composition of these particles, the EDS “d”, which are indicted in Figure 3. In the BM, larger lath and coarse grains are apparently observed mapping analysis is performed and the results are presented in Figure 5. The results show that the

and considerable numbers of irregularly shaped particles can be found distributed randomly in the Cu matrix, as shown in Figure 4a. To further reveal the composition of these particles, the EDS mapping analysis is performed and the results are presented in Figure 5. The results show that the

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Figure 4 shows the OM picture microstructures corresponding to locations “a”, “b”, “c”, and “d”, which are indicted in Figure 3. In the BM, larger lath and coarse grains are apparently observed and considerable Metals 2017, 7, 381 numbers of irregularly shaped particles can be found distributed randomly in4the of 13Cu matrix, as shown in Figure 4a. To further reveal the composition of these particles, the EDS mapping analysis and the results areparticles, presented in Figure 5. The results that locations the element element Crisisperformed found to concentrate in the while element Cu is absentshow in these butCr is found to concentrate in the particles, while element Cu of is absent indetected these locations butthe distributed distributed uniformly in the alloy matrix. The concentration Zr is not based on results uniformly in the alloy matrix. The concentration of Zr is not detected based on the results of EDS, as of EDS, as demonstrated in Figure 5d. These findings are consistent with previous reports [21,22]. demonstrated 5d.Figure These4b, findings are consistent previous reports [21,22]. In the HAZ,inasFigure seen in the grain structures with are similar to that of the base metal, but In theformed HAZ, as seen around in Figure the grains grain structures arestatic similar to that of the base metal, but some newly grains the4b, large driven from recrystallization are detected. some newly formed grains around the large grains driven from static recrystallization are detected. Within this HAZ, as seen in Figure 4c, a unique metallurgical zone in FSW experiencing both high Within heating this HAZ, as seen inand Figure 4c, deformation a unique metallurgical in as FSW experiencing both frictional temperatures plastic is generally zone defined TMAZ [23–25]. The high frictional heating temperatures and plastic deformation is generally defined as TMAZ [23–25]. microstructure of TMAZ is characterised by a highly deformed structure with an upward flowing The microstructure of TMAZ a highly deformed structure with an severe upwardplastic flowing pattern next to the NZ owingistocharacterised the rotated by deformation. Meanwhile, in the NZ, pattern next toathe NZ owing the rotated deformation. Meanwhile, in the NZ, severe plastic deformation and significant rise to in temperature make the dynamic recrystallization (DRX) become deformation and a significant rise in temperature make the dynamic recrystallization (DRX) become apparent [23,24,26–38], resulting in the existence of near fully equiaxed grains. In addition, the apparent [23,24,26–38], resulting the are existence ofand neardistribute fully equiaxed grains.inInthe addition, particles particles that exist randomly in theinBM refined uniformly NZ, as the shown in that exist Figure 4d. randomly in the BM are refined and distribute uniformly in the NZ, as shown in Figure 4d.

Figure Microstructures friction stir-welded CuCrZr alloy joints. Base metal (BM), heat Figure 4. 4. Microstructures of of friction stir-welded CuCrZr alloy joints. (a) (a) Base metal (BM), (b)(b) heat affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and nugget zone (NZ). affected zone (HAZ), (c)(c) thermo-mechanically affected zone (TMAZ), and (d)(d) nugget zone (NZ).

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Figure 5.5.Scanning Scanning electron microscopy (SEM-EDX) mapping ofalloy. CuCrZr alloy. (a) Figure electron microscopy (SEM-EDX) mapping analysisanalysis of CuCrZr (a) Micrograph Micrograph of the surface revealing the presence of intermetallic (b)(d)Copper, (c) of the surface revealing the presence of intermetallic particles, (b) Copper, (c)particles, Chromium, Zirconium. Chromium, (d) Zirconium.

To further uncover the microstructure details of nugget zone (NZ), the technique of EBSD was To further uncover the microstructure details of nugget zone (NZ), the technique of EBSD was employed. Figure 6 shows the inverse pole figure (IPF) corresponding to three different locations along employed. Figure 6 shows the inverse pole figure (IPF) corresponding to three different locations the thickness direction of NZ. As seen in Figure 6a, nearly equiaxed grains appear at the top of NZ. along the thickness direction of NZ. As seen in Figure 6a, nearly equiaxed grains appear at the top As distance away from the top increases, the grain size was found to decrease gradually (Figure 6b,c). of NZ. As distance away from the top increases, the grain size was found to decrease gradually During the FSW process, a large amount of heat that arose from severe deformation was formed. (Figure 6b,c). During the FSW process, a large amount of heat that arose from severe deformation The deformation heat then dissipated toward to the surroundings and the base steel that was attached was formed. The deformation heat then dissipated toward to the surroundings and the base steel to the welded CuCrZr alloy. The high thermal conductivity of the base steel makes the larger amount that was attached to the welded CuCrZr alloy. The high thermal conductivity of the base steel of dissipated heat at the bottom. In addition, the heat produced at the top is higher than that at the makes the larger amount of dissipated heat at the bottom. In addition, the heat produced at the top bottom, leading to temperature gradients and thus a diverse microstructure. is higher than that at the bottom, leading to temperature gradients and thus a diverse The TEM micrographs of BM and NZ are shown in Figure 7. Figure 7a,b shows a bright field microstructure. TEM micrograph and the corresponding selected area electron diffraction pattern (SADP) of the Cu The TEM micrographs of BM and NZ are shown in Figure 7. Figure 7a,b shows a bright field matrix in the BM alloy. As shown in Figure 7a, all the strengthening precipitates are presented as an TEM micrograph and the corresponding selected area electron diffraction pattern (SADP) of the Cu equiaxed shape with a range of 2–5 nm in diameter, decorating uniformly inside the grains of the Cu matrix in the BM alloy. As shown in Figure 7a, all the strengthening precipitates are presented as an matrix. Moiré fringes in two directions, nearly perpendicular to each other, were clearly observed in equiaxed shape with a range of 2–5 nm in diameter, decorating uniformly inside the grains of the some of the precipitates in Figure 7a. It was identified that most of the Moiré fringes were parallel Cu matrix. Moiré fringes in two directions, nearly perpendicular to each other, were clearly to the planes of the Cu matrix [31]. In the Cu directions, no Moiré fringes can be found in the observed in some of the precipitates in Figure 7a. It was identified that most of the Moiré fringes precipitates. A contrast associated with the strain fields surrounding the precipitates was observed, were parallel to the planes of the Cu matrix [31]. In the Cu directions, no Moiré fringes can be suggesting that the precipitates were coherent with the matrix. The SADP of the Cu matrix did not found in the precipitates. A contrast associated with the strain fields surrounding the precipitates reveal any diffraction spots from the fine precipitates. was observed, suggesting that the precipitates were coherent with the matrix. The SADP of the Cu matrix did not reveal any diffraction spots from the fine precipitates.

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Figure of NZ; NZ; (b) (b) Middle Middle of of NZ; NZ; (c) (c) Bottom Bottom of of NZ. NZ. Figure 6. 6. EBSD EBSD reconstructed reconstructed microstructure: microstructure: (a) (a) Top Top of

TEM micrographs of BM and NZ are shown in Figure 7. Figure 7a,b shows a bright field TEM TEM micrographs of BM and NZ are shown in Figure 7. Figure 7a,b shows a bright field TEM micrograph and the corresponding selected area electron diffraction pattern (SADP) of the Cu micrograph and the corresponding selected area electron diffraction pattern (SADP) of the Cu matrix matrix in the BM alloy. As shown in Figure 7a, all the strengthening precipitates are presented as in the BM alloy. As shown in Figure 7a, all the strengthening precipitates are presented as equiaxed equiaxed shape with a range of 2 nm–5 nm in diameter, decorating uniformly inside the grains of shape with a range of 2 nm–5 nm in diameter, decorating uniformly inside the grains of the Cu matrix. the Cu matrix. Moiré fringes in two directions, nearly perpendicular to each other, were clearly Moiré fringes in two directions, nearly perpendicular to each other, were clearly observed in some of observed in some of the precipitates in Figure 7a. It was identified that most of the Moiré fringes the precipitates in Figure 7a. It was identified that most of the Moiré fringes were parallel to the planes were parallel to the planes of the Cu matrix [31]. In the Cu directions, no Moiré fringes can be of the Cu matrix [31]. In the Cu directions, no Moiré fringes can be found in the precipitates. found in the precipitates. A contrast associated with the strain fields surrounding the precipitates A contrast associated with the strain fields surrounding the precipitates was observed, suggesting was observed, suggesting that the precipitates were coherent with the matrix. The SADP of the Cu that the precipitates were coherent with the matrix. The SADP of the Cu matrix did not reveal any matrix did not reveal any diffraction spots from the fine precipitates. diffraction spots from the fine precipitates. In the NZ, sub-grains can be obviously observed around the small recrystallized grains, while In the NZ, sub-grains can be obviously observed around the small recrystallized grains, while nano-strengthening precipitates are absent (Figure 7c). The results of SADP on these regions did not nano-strengthening precipitates are absent (Figure 7c). The results of SADP on these regions did not reveal any streaks, as can be seen in Figure 7d. Källgren [19] found that the friction heat can raise reveal any streaks, as can be seen in Figure 7d. Källgren [19] found that the friction heat can raise the temperature up to over 800 ◦°C between the rotating shoulder, the pin, and the workpiece during the temperature up to over 800 C between the rotating shoulder, the pin, and the workpiece during the FSW process of the Cu alloy. For the high strength CuCrZr alloy, the peak welding temperature the FSW process of the Cu alloy. For the high strength CuCrZr alloy, the peak welding temperature should be higher than for pure copper. High welding temperatures and thermal cycling during should be higher than for pure copper. High welding temperatures and thermal cycling during FSW FSW can result in the dissolution of nano-strengthening precipitates into the supersaturating matrix can result in the dissolution of nano-strengthening precipitates into the supersaturating matrix in NZ. in NZ. Similar results can also be found in Heinz [30], Rhodes et al. [25], Sato et al. [39], Liu et al. Similar results can also be found in Heinz [30], Rhodes et al. [25], Sato et al. [39], Liu et al. [40], and [40], and Jata et al. [41]. Jata et al. [41].

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Figure 7. TEM micrographs of (a) the BM showing precipitates and exhibiting Moiré fringes; (b) the corresponding SAED pattern of BM; (c) shows that the small recrystallized grains of the NZ contain sub-grains; and precipitates showing in fine grains of NZofand corresponding SAED pattern. and(d) (d)nono precipitates showing in fine grains NZtheand the corresponding SAED pattern.

3.2. Properties 3.2. Properties 3.2.1. Micro-Hardness 3.2.1. Micro-Hardness Figure 8 shows the micro-hardness of three distinct zones: (A) NZ; (B) the transition zone, TMAZ, and HAZ; (C) thethe BMmicro-hardness corresponding of to different layers. As shown in Figure the NZ had the Figureand 8 shows three distinct zones: (A) NZ; (B) the8, transition zone, lowest with the(C) average value of 64 HV in the top layer,layers. 70 HVAs in the middle layer, and 80 NZ HV TMAZ,hardness and HAZ; and the BM corresponding to different shown in Figure 8, the in thethe bottom layer, respectively. It average is found value that the micro-hardness from to the layer, edge had lowest hardness with the ofvalue 64 HVofin the top layer, 70 HVthe incentre the middle increases with and the highest the base metal. From and 80 HV in increasing the bottomdistance, layer, respectively. It ismicro-hardness found that theappears value ofinmicro-hardness from the demonstration of average hardness values, the tendency variation be summarized as followed: centre to the edge increases with increasing distance, andofthe highestcan micro-hardness appears in the top middleFrom > bottom, and BM > HAZ TMAZ hardness > NZ. It’s values, widely the accepted that of thevariation hardnesscan of the base>metal. the demonstration of >average tendency be CuCrZr alloy as is dependent on the grain size and theand distribution of >the nano-strengthening precipitates. summarized followed: top > middle > bottom, BM > HAZ THAZ > NZ. It’s widely accepted In NZ, the grain size CuCrZr is small alloy but the hardness ison low; indicates that grain size of plays thatthe the hardness of the is dependent thethis grain size and thethe distribution the anano-strengthening secondary role in determining In this case, precipitates precipitates.the In hardness. the NZ, the grain sizethe is nano-strengthening small but the hardness is low; play this aindicates major role in the affecting hardness.role As indicated by Figure the nano-strengthening that grain the sizevalue playsofathe secondary in determining the 7d, hardness. In this case, the precipitates in the NZprecipitates are nearly absent to the temperature that arose the FSW nano-strengthening play owing a major roleelevated in affecting the value of theduring hardness. As indicated by Figure 7d, the nano-strengthening precipitates in the NZ are nearly absent owing to the elevated temperature that arose during the FSW process. It is be reasonably concluded that the

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process. It is be reasonably concluded that the remarkable decrease of hardness in the NZ is mainly remarkable decrease of hardness in the NZ is mainly attributed to the dissolution of attributed to the dissolution of nano-strengthening precipitates (Figure 7d). nano-strengthening precipitates (Figure remarkable decrease of hardness in 7d). the NZ is mainly attributed to the dissolution of Generally, the temperature the layer be the thehighest highestone one owing to the Generally, the temperature in thetop top layer should should be owing to the heatheat that that nano-strengthening precipitatesin(Figure 7d). was released from the bottom layer to the platform. In this case, the gradient microstructures was Generally, released from bottom layer to the In this the gradient microstructures the the temperature in the topplatform. layer should becase, the highest one owing to the heatoccur, thatoccur, wherein a fraction of the nano-strengthening precipitates will remain in the bottom layer alloy wherein a fraction of the nano-strengthening precipitates in the microstructures bottom layer alloy but but was released from the bottom layer to the platform. In thiswill case,remain the gradient occur, near-totally dissolve the matrix.Compared Compared bottom, the lowest hardness in the of wherein a fraction of into the precipitates remain in the bottom layer alloy but near-totally dissolve into thenano-strengthening CuCu matrix. to the thewill bottom, the lowest hardness in top the top of the weld NZ could be tomore theCompared more severe DRX, with larger grain sizes caused near-totally dissolve intoattributed the Cu to the bottom, the lowest hardness in the top of the weld NZ could be attributed to matrix. the severe DRX, with larger grain sizes caused by by thethe higher higher welding temperature topthe of weld the weld NZ could be attributed to more [42]. severe DRX, with larger grain sizes caused by the welding temperature at the top at of the weld [42]. higher welding temperature at the top of weld [42].

Figure Typicalhardness hardness profiles profiles across across the welding (FSW) Figure 8. 8.Typical the weld weld ofofananfriction frictionstirstir welding (FSW) copper–chromium–zirconium alloy. Nugget zone (NZ); zone: thermo-mechanically Figure 8. Typical hardness profiles acrosszone the (NZ); weld B: an friction stir welding (FSW) copper–chromium–zirconium alloy. A:A:Nugget B:oftransition transition zone: thermo-mechanically affected zone (TMAZ) and Heat affected zone (HAZ); and C:B:base metal (BM). copper–chromium–zirconium alloy. A: Nugget zone (NZ); transition zone: thermo-mechanically affected zone (TMAZ) and Heat affected zone (HAZ); and C: base metal (BM). affected zone (TMAZ) and Heat affected zone (HAZ); and C: base metal (BM).

3.2.2. Tensile Strength 3.2.2. 3.2.2. Tensile Strength Tensile Strength In order to study the tensile strength of the welded joints of the CuCrZr alloy along the joint In order todirection, study thetensile tensile strength of theofwelded jointsjoints of the alloyalloy along the thickness thickness tests were performed specimens with dimensions of 120along mmjoint ×the 4 mm × In order to study the tensile strength theon welded ofCuCrZr the CuCrZr joint 4 mm which have been signed according to their slice positions inof the welding as×shown thickness direction, tensile tests were on specimens with dimensions ofjoint, mm ×44mm mmin × direction, tensile tests were performed onperformed specimens with dimensions 120 mm × 4120 mm which 2. Theaccording fracture of the joints are in Figure It shown iswelding obvious that as all2.shown the 4Figure mmsigned which have beenlocations signed according to their slice positions in9.the in have been to their slice positions in shown the welding joint, as in joint, Figure Theweld fracture jointsofare severely necking fractured in joints the Figure 2. fracture locations the shown in Figure 9. It is obvious thatseverely all the weld locations theThe joints are shown in of Figure 9. NZ. It are is obvious that all the weld joints are necking joints are severely necking fractured in the NZ.

fractured in the NZ.

Figure 9. Typical fracture location of the FSW CuCrZr alloy joints. Figure 9. Typical fracture location of the FSW CuCrZr alloy joints.

Figure 9. Typical fracture location of the FSW CuCrZr alloy joints.

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As As illustrated illustrated in in Figure Figure 10, 10, the the average average tensile tensile strength strength of of the the welded welded alloys alloys is is determined determined to to be be As illustrated in Figure 10, the average tensile strength of the welded alloys is determined to be 269 MPa (equivalent to 60% of the base metal strength) and the average elongation is 18.3% (much 269 MPa (equivalent to 60% of the base metal strength) and the average elongation is 18.3% (much 269 MPa (equivalent to 60% of the base metal strength) and the average elongation is 18.3% (much higher the base metal). the thickness direction, it can be found higher than than that that of of the the base base metal). metal). Along Along found that that the the tensile tensile higher than that of Along the the thickness thickness direction, direction, it it can can be be found that the tensile strength of the welds is slightly increased from the top to the bottom area of the welds. However, strength of the welds is slightly increased from the top to the bottom area of the welds. However, strength of the welds is slightly increased from the top to the bottom area of the welds. However, an an opposite opposite tendency tendency is is found found for for the the elongation. elongation. This This phenomenon phenomenon can can be be attributed attributed to to the the two two an opposite tendency is found for the elongation. This phenomenon can be attributed to the two microstructure-related aspects of nano-strengthening precipitates and the size of grains at microstructure-related aspects of nano-strengthening precipitates and the size of grains at the the microstructure-related aspects of nano-strengthening precipitates and the size of grains at the fracture fracture fracture location, location, which which have have been been discussed discussed in in the the previous previous sections. sections. location, which have been discussed in the previous sections. To To further further investigate investigate the the fracture fracture mechanisms, mechanisms, the the fracture fracture appearance appearance is is examined examined and and To further investigate the fracture mechanisms, the fracture appearance is examined and shown shown in Figure 11. From the fracture morphology, it is found that the major fracture mechanism shown in Figure 11. From the fracture morphology, it is found that the major fracture mechanism of of in Figure 11. From the fracture morphology, it is found that the major fracture mechanism of the the the FSW FSW is is the the joints joints behaviors behaviors ductile. ductile. The The ductile ductile fracture fracture is is apparently apparently presented presented in in both both the the FSW is the joints behaviors ductile. The ductile fracture is apparently presented in both the fracture fracture fracture morphology morphology of of the the base base metal metal and and the the welded welded alloy. alloy. However, However, for for case case of of the the welded welded joint, joint, morphology of the base metal and the welded alloy. However, for case of the welded joint, larger voids larger voids were observed than for that of base metal, as shown in Figure 11a,b. larger voids were observed than for that of base metal, as shown in Figure 11a,b. were observed than for that of base metal, as shown in Figure 11a,b.

Figure 10. 10. Transverse Transverse tensile tensile properties Figure Figure 10. Transverse tensile properties of of weld weld nugget nugget in in FSW FSW CuCrZr CuCrZr alloy alloy at at room-temperature. room-temperature.

Figure SEM images Figure 11. 11. SEM SEM images of of the the fracture fracture appearance: appearance: (a) (a) base base metal metal (BM); (BM); and and (b) (b) as-welded as-welded alloy. alloy.

3.2.3. Electrical Conductivity Conductivity 3.2.3. Electrical The conductivity of of the the CuCrZr CuCrZr alloys alloys is is one one of of the the reasons that they are widely The high high electrical electrical conductivity reasons that they are widely be used in industry. Hence, in this study, the electrical conductivity of the FSW CuCrZr alloy joints study, the the electrical electrical conductivity conductivity of the FSW CuCrZr alloy joints be used in industry. Hence, in this study,

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were measured. The specimen sampling is parallel to the direction of the weld, with a dimension of were measured. The specimen sampling is parallel to the direction of the weld, with a dimension of 2 mm thickness × 2 mm width ×120 mm length for the electrical conductivity test. 2 mm thickness × 2 mm width ×120 mm length for the electrical conductivity test. Figure 12 shows the variations of electrical conductivity corresponding to three distinct zones: Figure 12 shows the variations of electrical conductivity corresponding to three distinct zones: (A) NZ; (B) Transition zone, TMAZ and HAZ; and (C) BM, all at different height layers. (A) NZ; (B) Transition zone, TMAZ and HAZ; and (C) BM, all at different height layers.

Figure 12. 12. Electrical conductivity of of FSW CuCrZr joints: A: A: nugget zone (NZ); B: B: transition zone: Figure Electrical conductivity FSW CuCrZr joints: nugget zone (NZ); transition zone: Thermo-mechanically affected zone (TMAZ) andand heatheat affected zone (HAZ); andand C: base metal (BM). Thermo-mechanically affected zone (TMAZ) affected zone (HAZ); C: base metal (BM).

seen in Figure 12, variation the variation the electrical conductivity of the sample in is the NZ is AsAs seen in Figure 12, the of theofelectrical conductivity of the sample in the NZ slight keeps constant (~40%atIACS) at adistance certain distance the center welded(~8 center (~8 mm),then which butslight keepsbut constant (~40% IACS) a certain from thefrom welded mm), which thentostarts to increase to 50~60% in the transition zoneand (TMAZ and HAZ). Thevalue highest value starts increase to 50~60% IACS in IACS the transition zone (TMAZ HAZ). The highest (~65% (~65% IACS) of electrical conductivity was in obtained inmetal. the base metal. IACS) of electrical conductivity was obtained the base is well documented that the electrical conductivity mainlybyaffected by the electron It isIt well documented that the electrical conductivity is mainlyisaffected the electron scattering scattering of the solid solution atoms. model A theoretical model describe the relationships between of the solid solution atoms. A theoretical to describe the to relationships between temperature, temperature, impurity and the electrical resistance a metal is given by impurity content, and thecontent, electrical resistance of a metal solid of solution is solid givensolution by Matthiessen’s Matthiessen’s ρi(T) ρ=ρ i + (T), where ρi is the resistivity ρm(T)metal. is the rule [43]: ρi (T) = ρrule (T), where theρmresidual resistivity and residual ρm (T) is the resistivityand of pure i + ρm[43]: i is resistivity metal. The of ρconcentration i is determined by The impurity [44].aThe ρi was The value of ρofi ispure determined by value impurity [44]. ρi wasconcentration reported to play leading reported play a leading role lower of temperatures anincrease increaseof ofthe ρi will result in role at lowertotemperatures and an at increase ρi will resultand in an resistivity of an theincrease alloy. of Based the resistivity of theanalysis, alloy. the contribution of a high welding temperature and thermal cycling on the above Basedresulted on theinabove analysis, of thethecontribution of a high welding into temperature and thermal during FSW the dissolution nano-strengthening precipitates the supersaturation cycling during FSW resulted in the dissolution of the nano-strengthening precipitates intotothe matrix in the weld zone (Figure 7d). The increase of the degree of supersaturation would lead matrix in of thethe weld zoneand (Figure increase degree of supersaturation thesupersaturation higher lattice distortion matrix thus7d). the The scattering of of thethe electrons would increase. would lead to the of higher lattice distortion of the matrix and thus the scattering theCrCuZr electrons Therefore, the value ρi will increase correspondingly. During the FSW process ofofthe would increase. Therefore, the value of ρ i will increase correspondingly. During the FSW process alloy, the gradual re-dissolution of solute atoms to the matrix leads to an increase of ρi , and then the of the CrCuZr alloy, the gradual with re-dissolution of solute atoms to the matrix leads to the an increase of ρi, electrical conductivity decreases the increasing welding temperature. Moreover, significant and thenofthe decreases withlead the to increasing welding temperature. occurrence theelectrical recoveryconductivity and recrystallization would the continuous occurrence of theMoreover, crystal the distortion significantand occurrence of thethe recovery recrystallization would lead the lattice thus accelerate decreaseand of the electrical conductivity in thetoNZ of continuous the FSW occurrence CuCrZr alloy. of the crystal lattice distortion and thus accelerate the decrease of the electrical conductivity in the NZ of the FSW CuCrZr alloy. 4. Conclusions The microstructure and properties of a friction stir welded CuCrZr alloy are investigated. Some important conclusions can be drawn as follows:

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4. Conclusions The microstructure and properties of a friction stir welded CuCrZr alloy are investigated. Some important conclusions can be drawn as follows: 1.

2.

3.

4.

The weld zone that is defect-free is formed in the condition of 1500 rpm and 150 mm/min. The microstructure near the weld zone is very different from that of the base metal. HAZ is characterized by some newly formed grains around the large grains. TMAZ is characterized by a highly deformed structure with an upward flowing pattern, and the fine and equaxed grain structure is apparently found in the NZ. The strengthening precipitates with a range of 2 nm–5 nm in diameter are coherently exhibited with the Cu matrix in the BM. After FSW, however, all the nano-strengthening precipitates are dissolved into the Cu matrix in the NZ. Also, the grains’ size is significantly refined owing to the occurrence of DRX. The coarse chromium-rich particles are crushed into finer ones, which distribute more uniformly in the NZ. Along the thickness direction, the microstructure distribution is inhomogeneous and the size of the equiaxed grains in the NZ is decreased gradually from the top to the bottom area of the welds because of the distinctive heat production and the heat dissipation on the welding joint. That led to the micro-hardness and tensile strength of the welds, which are slightly increased from the top to the bottom area of the welds. Decreases in hardness, tensile strength, and electrical conductivity are detected in the welded NZ, which result from the comprehensive effect of the dissolution of the nano-strengthening precipitates into the supersaturation matrix and severe DRX.

Acknowledgments: This work was supported by the National Basic Research Program of China (“973 Program”, 2014CB046605) and Nonferrous Metal Oriented Advanced Structural Materials and Manufacturing Cooperative Innovation Center (“2011 Program”). The authors would like to thank the co-workers at the State Key Laboratory of High Performance Complex Manufacturing of Central South University for their invaluable suggestions and discussions on the current research. Author Contributions: Diqiu He was the principle investigator of the research. Ruilin Lai and Junyuan Lin carried out the welding tests. Guoai He and Youqing Sun characterized the microstructure of the welded samples. Ruilin Lai performed mechanical tastings, hardness, fractography, and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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