A Partitioning Method for Friction Stir Welded Joint of AA2219 Based on Tensile Test

metals

Article A Partitioning Method for Friction Stir Welded Joint of AA2219 Based on Tensile Test

Guanglong Cao 1,2, Mingfa Ren 1,2, Yahui Zhang 2, Weibin Peng 3 and Tong Li 2,*

1 State Key Laboratory of Structural Analysis for Industrial Equipment, Dalian University of Technology, Dalian 116024, China; [email protected] (G.C.); [email protected] (M.R.) 2 Department of Engineering Mechanics, Dalian University of Technology, Dalian 116024, China; [email protected] 3 Beijing Institute Astronautical System Engineering, Beijing 100076, China; [email protected] * Correspondence: [email protected]; Tel.: +86-411-84706036

Received: 3 December 2019; Accepted: 27 December 2019; Published: 1 January 2020

Abstract: The partition of aluminum alloy welded joint often depends on microscopic methods such as scanning electron microscopy before. This paper provides a novel partitioning method, which can obtain the material properties and partition results at the same time based on tensile test. The mechanical properties of every point on the whole welded joint are first obtained by the digital image correlation (DIC) method. Then, the mechanical property function of the weld joint along the weld center is established due to the changes of plastic property and strain hardening exponent at each point and the boundary between different areas is then determined. Metallographic detection technology and nano-mechanical testing techniques are employed to validate this partitioning scheme. The partition result of the strategy proposed in this paper is consistent with the partitioning result of the classical method. Compared to classical method, the proposed partitioning method is more practical and effective, as it can obtain mechanical properties and partition boundary through a single tensile test and reduce the cost of metallographic test.

Keywords: 2219 aluminum alloy; friction stir welded joint; partitioning; mechanical properties

1. Introduction 2219 aluminum alloy (AA2219) is widely used in aerospace engineering because of the outstanding formability, weldability, and high strength-to-weight ratio [1–3]. The major chemical composition in the heat-treatable AA2219 is copper except aluminum, which is required for precipitation strengthening. The strengthening of AA2219 can be further improved by refining the grain size to a few micrometers and lower in the alloy. This target can be achieved by friction stir processing, which is a solid-state processing technique developed from the principles of friction stir welding (FSW) and used for microstructural modification by grain refinement [4,5]. The stirring action of the rotating pin causes intense plastic deformation of the locally heated material. The combination of plastic deformation, mixing, and thermal exposure results in a modified microstructure in the weld zone (WZ), which is commonly characterized by fine and equiaxed grain structure with predominant high-angle boundaries. The modification is generally attributed to dynamic recrystallization and break-up of constituent particles [6]. Next to the WZ, a narrow transition region known as the thermo-mechanically affected zone (TMAZ) is formed, followed by the heat affected zone (HAZ), and finally the unaffected base metal (BM). In general, TMAZ is located on the dividing line between WZ and HAZ. Due to the variation of mechanical properties at different positions in the welded joints, it is necessary to partition the welded joint to different regions while analyzing the mechanical performance of the welded joints. At present, a lot of achievements have been obtained by partitioning the welded

Metals 2020, 10, 65; doi:10.3390/met10010065 www.mdpi.com/journal/metals Metals 2020, 10, 65 2 of 14 joints to discuss their material properties [7–10]. There are two main methods to partition the aluminum alloy welded joints. One of them is to establish a relationship between hardness and material properties by means of hardness measurement, thereby dividing the welded joint into WZ, HAZ, and BM. He [11] sliced the aluminum alloy welded joint into a small tensile specimen, and measured the properties of a welded joint by indentation technique. J. Rojek [12] combined the tensile test, microhardness test, and obtained the stress-strain relationship of the welded joint by numerical inversion method, then the mechanical properties of each part of welded joints are given. Yazdipour [13] studied the friction stir welded joints by microstructure observation and hardness experiment, and summarized the mechanical properties of the BM, HAZ, TMAZ, and WZ. Although these above methods can divide the aluminum alloy welded joints into different zones by the microstructure and mechanical properties, the location of the micro-hardness test and the nanoindentation test are generally based on empirical judgment. Due to the limited quantity of selected points, the hardness-based partitioning method has a limitation in accurately determining the partition boundary in the welded joint. Another method for partition aluminum alloy welded joint is based on the metallographic analysis. Microscopic observation is the main method to study the microstructure of materials. Microscopic observation can provide lots of material information, including the morphology of the phases present in the material, the grain size, the distribution of these phases and the non-metallic inclusions in the structure. The microstructure of various materials produced by different processing techniques can be determined, and the properties of the materials can be distinguished. Therefore, many studies have used metallographic microscopy to distinguish different regions for welded joints. Hector [14] and Lemmen [15] used metallographic analysis to partition the friction stir welded joints of aluminum alloys. Based on the experimental results, the different mechanical properties of aluminum alloy welded joints in different regions are evaluated. In addition, Liu [16] used metallographic analysis technology to characterized the welded joint of 2219-T6 aluminum alloy. Combined with tensile test, the effect of the direction of friction stir welding on the mechanical properties of both sides of welded joints is studied. These abovementioned methods face the challenges of identifying the boundaries between different zones and the evolution principle of material properties, i.e., modulus and strength is difficult to be identified in different zones. The boundary position is critical to the material performance as the mechanical/structural discontinuity at the boundary leads to stress concentration that is dangerous for the structure. Rao [17] showed that the existence of a weakened area is an important factor for the failure of welded joints. Therefore, it is important to accurately and efficiently determine the extent of the welded joints. Therefore, a detailed mapping of mechanical properties in the welded joint is valuable to the fine analysis of the mechanical performance of the whole welded joints. In this paper, standard tensile experiments of the AA2219 welded joint are conducted and the functional relationship that can express the mechanical properties of any position on the welded joint is proposed based on a fine partitioning scheme. A theory of the evolution of elastoplastic behavior of the AA2219 is proposed as the fundaments of further engineering calculation. The partitioning method presented in this article has two main advantages over the classical methods. One is that the mechanical properties of the welded joint can be obtained at the same time as the partition and reduce the cost of metallographic test. In addition, the distribution of mechanical properties in each region can also be obtained. The other is that the partition boundary can be determined accurately, which avoids the possible error caused by human eye observation. At last, experimental characterizations including scanning electron microscopy (SEM), metallographic microscopy, nanoindentation, and atomic force microscopy (AFM) are used to validate this partitioning scheme. Metals 2020, 10, 65 3 of 14

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2.1. Welded Joints

TheThe BM under investigation is an 8-mm-thick8-mm-thick AA2219 plate shownshown inin FigureFigure1 1.. TheThe chemicalchemical compositions of the BM are listedlisted inin TableTable1 .1. ButtButt weldweld isis mademade alongalong thethe longitudinallongitudinal directiondirection (perpendicular toto the the rolling rolling direction) direction) of theof weldingthe welding samples samples by using by anusing FSW an machine. FSW machine. The welding The toolwelding consisted tool consisted of a smooth of concavea smooth shoulder concave and shou a conicallder and right-hand a conical screwed right-hand pin. Thescrewed rotation pin. speed The androtation the weldingspeed and speed the welding are 600 rpm speed and are 200 600 mm rpm/min, and respectively. 200 mm/min, respectively.

Figure 1. Friction stir-welded plate.

Table 1. Chemical composition of 2219 aluminum alloy. Table 1. Chemical composition of 2219 aluminum alloy. Element Si Fe Cu Mn Zr Al Element Si Fe Cu Mn Zr Al Mass Fraction%Mass Fraction% 0.2 0.2 0.3 0.3 5.8–6.85.8–6.8 0.3 0.3 0.18 0.18Bal Bal

2.2. Tensile Test After welding,welding, thethe joints joints are are all all cross-sectioned cross-sectioned perpendicular perpendicular to the to welding the welding direction direction for tensile for tests.tensile The tests. tensile The specimenstensile specimens are conducted are conducted strictly following strictly following tensile test tensile standards, test andstandards, the dimensions and the ofdimensions the specimen of the are specimen shown in are Figure shown2. It shouldin Figure be noted2. It should that the be tensile noted specimen that the tensile retains specimen the weld residualretains the height. weld In residual order to height. measure In theorder mechanical to measure properties the mechanical of diﬀerent properties regions of thedifferent welded regions joints accurately,of the welded both electricaljoints accurately, (electrical both gauge) electrical and optical (electrical (digital imaginggauge) and correlation) optical measurements(digital imaging are usedcorrelation) to obtain measurements the overall variation are used ofto theobtain strain the historyoverall invariation the entire of welding-relatedthe strain history area, in the and entire the real-timewelding-related stress data area, are and read the by areal-time tensile tester stress (Material data are Test read System-MTS, by a tensile Eden tester Prairie, (Material MN, USA). Test TheSystem-MTS, equipment Eden can providePrairie, MN, a maximum USA). The load equipment of 100 kN. can The provide specimen a maximum is loaded by load displacement of 100 kN. andThe thespecimen loading is speedloaded is by set displacement as 0.5 mm/min. and the loading speed is set as 0.5 mm/min. Strain gauges are arranged at the positions of the WZ and the HAZ of the tensile specimen, measurement point 1 is located on the middle of one edge of the test piece, and measurement point 4 is on the other edge on the same cross section of the WZ with measurement point 1. Measurement points 2, 3, 5, and 6 are in the HAZ and are equidistant from the center of the weld. Figure3 is a schematic view showing the positions of these electrical measurement points of the tensile specimen.

Figure 2. Stretched sample shape size. (Unit: mm).

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2.1. Welded Joints

The BM under investigation is an 8-mm-thick AA2219 plate shown in Figure 1. The chemical compositions of the BM are listed in Table 1. Butt weld is made along the longitudinal direction (perpendicular to the rolling direction) of the welding samples by using an FSW machine. The welding tool consisted of a smooth concave shoulder and a conical right-hand screwed pin. The rotation speed and the welding speed are 600 rpm and 200 mm/min, respectively.

Figure 1. Friction stir-welded plate.

Table 1. Chemical composition of 2219 aluminum alloy.

Element Si Fe Cu Mn Zr Al Mass Fraction% 0.2 0.3 5.8–6.8 0.3 0.18 Bal

2.2. Tensile Test After welding, the joints are all cross-sectioned perpendicular to the welding direction for tensile tests. The tensile specimens are conducted strictly following tensile test standards, and the dimensions of the specimen are shown in Figure 2. It should be noted that the tensile specimen retains the weld residual height. In order to measure the mechanical properties of different regions of the welded joints accurately, both electrical (electrical gauge) and optical (digital imaging correlation) measurements are used to obtain the overall variation of the strain history in the entire welding-related area, and the real-time stress data are read by a tensile tester (Material Test

MetalsSystem-MTS,2020, 10, 65 Eden Prairie, MN, USA). The equipment can provide a maximum load of 100 kN.4 ofThe 14 specimen is loaded by displacement and the loading speed is set as 0.5 mm/min.

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Strain gauges are arranged at the positions of the WZ and the HAZ of the tensile specimen, measurement point 1 is located on the middle of one edge of the test piece, and measurement point 4 is on the other edge on the same cross section of the WZ with measurement point 1. Measurement points 2, 3, 5, and 6 are in the HAZ and are equidistant from the center of the weld. Figure 3 is a schematic view showing the positions of these electrical measurement points of the tensile specimen. FigureFigure 2. 2.Stretched Stretched sample sample shape shape size. size. (Unit:(Unit: mm).mm).

Figure 3.3. SchematicSchematic diagram diagram of theof the position position of the of electrical the electrical measurement measurement point of point the tensile of the specimen. tensile specimen. 2.3. Digital Imaging Correlation 2.3. DigitalIn recent Imaging years, Correlation with the improvement of digital imaging technology, the DIC method has been rapidly developed [18]. This method can accurately obtain the displacement field of the surface of the In recent years, with the improvement of digital imaging technology, the DIC method has been part and combine the computer analysis techniques to obtain the surface strain field. Compared to rapidly developed [18]. This method can accurately obtain the displacement field of the surface of traditional strain gauge [19], the measurement range of DIC is not affected by the deformation size, the part and combine the computer analysis techniques to obtain the surface strain field. Compared and the measurement points are not limited by the number of strain gauge. The effective measurement to traditional strain gauge [19], the measurement range of DIC is not affected by the deformation range of the conventional strain gauge used in this experiment is 4000 micro-strain, which is limited for size, and the measurement points are not limited by the number of strain gauge. The effective capturing the whole strain history of the tensile test. Many researchers worked on the DIC method to measurement range of the conventional strain gauge used in this experiment is 4000 micro-strain, obtain the local mechanical properties of welded joint. Reynolds and Duvall [20] first applied the DIC which is limited for capturing the whole strain history of the tensile test. Many researchers worked technique to determine the constitutive relations of friction stir-welded joints [21–23] and laser-welded on the DIC method to obtain the local mechanical properties of welded joint. Reynolds and Duvall joints [24,25]. [20] first applied the DIC technique to determine the constitutive relations of friction stir-welded DIC method is applied in this work to obtain the detailed strain information during mechanical joints [21–23] and laser-welded joints [24,25]. testing, the positions selected for DIC characterization are shown in Figure4a. Because the aim of this DIC method is applied in this work to obtain the detailed strain information during mechanical paper is to present a method to partition the welded joints, the iso-stress condition is assumed in this testing, the positions selected for DIC characterization are shown in Figure 4a. Because the aim of work, which ensures the material properties of all cross-sections of aluminum alloy welded joints are this paper is to present a method to partition the welded joints, the iso-stress condition is assumed in consistent during the test [26]. In order to get the variation trend of strain field, 9 points are selected this work, which ensures the material properties of all cross-sections of aluminum alloy welded along the length direction of the specimen to obtain the strain history during the tensile test for the joints are consistent during the test [26]. In order to get the variation trend of strain field, 9 points are later partitioning purpose. It is necessary to point out that optical measurement point 5 and electrical selected along the length direction of the specimen to obtain the strain history during the tensile test measurement point 1 are in symmetrical positions on the two sides of the specimen, which can be used for the later partitioning purpose. It is necessary to point out that optical measurement point 5 and electrical measurement point 1 are in symmetrical positions on the two sides of the specimen, which can be used for method validation of optical measurements. Figure 4b shows the test apparatus (including the loading machine and the imaging system).

Metals 2020, 10, 65 5 of 14 for method validation of optical measurements. Figure4b shows the test apparatus (including the loadingMetals 2020 machine, 10, x FOR and PEER the REVIEW imaging system). 5 of 14

Figure 4.4.Positions Positions selected selected for for DIC DIC and and Test apparatus:Test apparatus: (a) Positions (a) Positions selected selected for DIC, for (b) TestDIC, apparatus (b) Test includingapparatus theincluding loading the machine loading and machine the imaging and the system. imaging system. 2.4. Metallographic Microscopy 2.4. Metallographic Microscopy In this study, the test is conducted to determine the microstructure the aluminum alloy welded In this study, the test is conducted to determine the microstructure the aluminum alloy welded joints on the metallographic microscopy (MEF-4, Keyence, Osaka, Japan) as a proof for the partitioning joints on the metallographic microscopy (MEF-4, Keyence, Osaka, Japan) as a proof for the strategy proposed in this paper. This device can be applied to the use of reflective metal materials partitioning strategy proposed in this paper. This device can be applied to the use of reflective metal to determine their histological morphology and microstructure. The metallographic microscopy test materials to determine their histological morphology and microstructure. The metallographic intercepts the sample along the cross-section of the sample (rolling direction), then etches the polished microscopy test intercepts the sample along the cross-section of the sample (rolling direction), then sample with mixed acid (1.0%HF + 1.5%HCl + 2.5%HNO3 + 95.0%H2O) for 30 s, and observes the etches the polished sample with mixed acid (1.0%HF + 1.5%HCl + 2.5%HNO3 + 95.0%H2O) for 30 s, microstructure of the base metal and FSW welded joint. The size of the sample is 80 mm 10 mm and observes the microstructure of the base metal and FSW welded joint. The size of the sample× is 80× 4 mm (length width height), and the measuring points at each position are three. mm × 10 mm × 4 mm (length× × width × height), and the measuring points at each position are three. 2.5. SEM (EDS) 2.5. SEM (EDS) SEM imaging is conducted on the field emission scanning electron microscopy (SUPRA55, SEM imaging is conducted on the field emission scanning electron microscopy (SUPRA55, Germany). This device could provide magnification from 12 to 500,000, and it can be applied to Germany). This device could provide magnification from ×12× to ×500,000,× and it can be applied to the the use of reflective metal materials to determine their microstructure, and elements of specimens use of reflective metal materials to determine their microstructure, and elements of specimens can be can be analyzed quantitatively and qualitatively by energy dispersive spectroscopy (EDS) module. analyzed quantitatively and qualitatively by energy dispersive spectroscopy (EDS) module. The The SEM test intercepts the sample along the cross-section of the sample (rolling direction), similar SEM test intercepts the sample along the cross-section of the sample (rolling direction), similar to the to the metallographic microscopy test, then etches the polished sample with mixed acid (1.0%HF + metallographic microscopy test, then etches the polished sample with mixed acid (1.0%HF + 1.5%HCl + 2.5%HNO3 + 95.0%H2O) 90 s, then observes the microstructure of the FSW welded joint by 1.5%HCl + 2.5%HNO3 + 95.0%H2O) 90 s, then observes the microstructure of the FSW welded joint the field emission scanning electron microscopy. The size of the sample is 50 mm 10 mm 5 mm by the field emission scanning electron microscopy. The size of the sample is 50 mm× × 10 mm× × 5 mm (length width height), and the measuring points at each position are three. (length ×× width × height), and the measuring points at each position are three. 2.6. Nano-Indentation 2.6. Nano-Indentation Nanoindentation test is an effective method to identify the local property of materials in the welded joint.Nanoindentation In this study, the test nanoindentation is an effective test method is conducted to identify on thethe nano-indentationlocal property of testingmaterials machine in the (Hysitronwelded joint. TI-950, In USA)this study, to validate the nanoindentation the partitioning methodtest is conducted proposed inon this the paper. nano-indentationThe instrument testinghas amachine 500 nm (Hysitron resolution TI-950, microscopy USA) in to a validate top view the for partitioning precise positioning method of proposed test points. in Thethis indentationpaper. The testinstrument can record hasthe a 500 load-displacement nm resolution microscopy data during in the a top process view andfor precise fit the calculated positioning elastic of test modulus, points. hardnessThe indentation equivalent. test can In orderrecord to the determine load-displacemen the differencet data in during local mechanicalthe process propertiesand fit the ofcalculated welded joints,elastic amodulus, range of 20hardness mm from equivalent. the center In of theorder weld to isdetermine selected forthe measurement difference in in local this experiment.mechanical Theproperties size of theof samplewelded is joints, 50 mm a 10range mm of 520 mm mm (length from widththe centerheight), of the and weld the measuring is selected points for × × × × atmeasurement each position in arethis three. experiment. The size of the sample is 50 mm × 10 mm × 5 mm (length × width × height), and the measuring points at each position are three.

2.7. Atomic Force Microscopy In order to validate partition results more comprehensively, atomic force microscopy (AFM) test is conducted (Bruker Dimension Icon, Germany) to provide the surface morphology of materials at the nanoscale to show the variation of surface morphology between different zones. In the AFM

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Metalstest, the2020 observation, 10, 65 scope (5 μm × 5 μm) is taken. The size of the sample is 80 mm × 10 mm × 46 ofmm 14 (length × width × height), and the measuring points at each position are three.

2.7.3. Partitioning Atomic Force Strategy Microscopy In order to validate partition results more comprehensively, atomic force microscopy (AFM) test 3.1. Tensile Behavior of Welded Joints is conducted (Bruker Dimension Icon, Germany) to provide the surface morphology of materials at the nanoscaleBased on tothe show real-time the variation loading ofoutput surface and morphology the incompressibility between di offf erentplastic zones. deformation, In the AFM the test,true thestress observation during the scope tensile (5 µm process5 µm) of is taken.each measurement The size of the samplepoint can is 80 be mm calculated.10 mm A4 comparison mm (length × × × betweenwidth theheight), optical and measurement the measuring results points atand each the position electrical are three.measurement results at the same ×cross-section× is carried out to validate the reliability of DIC method for the welded joints. The results 3.showed Partitioning that the Strategy electrical measurement and the optical measurement results are in good agreement, as shown in Figure 5a. Compared to strain gauges, the DIC method can provide the displacement 3.1. Tensile Behavior of Welded Joints field and strain field on the sample, as shown in Figure 5b, enabling the stiffness mapping of the weldedBased joints on thebased real-time on the loadingassumption output that and the the various incompressibility weld regions of are plastic arranged deformation, in series theand true the stresscross-section during theat any tensile location process in of the each specim measurementen is homogeneous point can be [26]. calculated. In Figure A comparison 5b, “V[mm]” between is the thedisplacement optical measurement in the direction results of loading, and the electrical“eyy [1] La measurementgrange” is the results strain atin thethe samedirection cross-section of loading. is carriedThe out stress-strain to validate thecurves reliability at each of DICmeasuremen method fort point the welded are provided joints. The in resultsFigure showed5c and thatthe thecorresponding electrical measurement Young’s modulus and the and optical yield measurement strength are resultsshown are in inFigure good 5d. agreement, Here, as as the shown data inresults Figure are5a. symmetric Compared along to strain the weld gauges, center, the only DIC the method results can of provideone side theof the displacement weld center field are given and strainin Figure field 5d. on The the sample,Young’s as modulus shownin shows Figure no5b, variatio enablingn with the stirespectffness to mapping the distance of the from welded the center joints basedline (near on the 70 assumptionGPa), however that the the yield various strength weld regionsincreases are with arranged the value in series of distance and the (from cross-section 150 to 260 at anyMPa), location indicating in the that specimen the plastic is homogeneous properties of [26a material]. In Figure is the5b, major “ V[mm]” changing is the characteristic displacement due in the to directionthe heat treatment of loading, in “eyy the welding [1] Lagrange” operation. is the strain in the direction of loading.

FigureFigure 5.5.Tensile Tensile test test results results of 2219of 2219 aluminum aluminum alloy weldedalloy welded joints: (joints:a) Stress-strain (a) Stress-strain comparison comparison curve of electricalcurve of photometryelectrical photometry at the same at cross-section, the same cross-section, (b) optical results(b) optical diagram, results (c) diagram, photometric (c) photometric stress-strain curve,stress-strain (d) the curve, yield strength(d) the yield curve strength and the curve modulus and ofthe elasticity modulus curve. of elasticity curve.

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The stress-strain curves at each measurement point are provided in Figure5c and the corresponding Young’s modulus and yield strength are shown in Figure5d. Here, as the data results are symmetric along the weld center, only the results of one side of the weld center are given in Figure5d. The Young’s modulus shows no variation with respect to the distance from the center line (near 70 GPa), however the yield strength increases with the value of distance (from 150 to 260 MPa), indicating that the plastic properties of a material is the major changing characteristic due to the heat treatment in the weldingMetals 2020 operation., 10, x FOR PEER REVIEW 7 of 14

3.2. Plasticity Model in Welding Joints A newnew partitioningpartitioning strategy strategy is is proposed proposed in thisin this section section and theand ﬂowchart the flowchart of this of strategy this strategy is shown is inshown Figure in6 Figure. 6.

Figure 6. Flowchart of the partitioning strategy.

For elastoplastic materials,materials, aa stress-strainstress-strain constitutive constitutive relation relation proposed proposed by by Ramberg Ramberg et et al. al. [27 [27]] in thein the form form of aof power a power function function is often is often used, used, which which is shown is shown as Equation as Equation (1); the(1); twothe two parameters parametersH and H nandare n strength are strength coeﬃ coefficientcient and strainand strain hardening hardening exponent, exponent, respectively. respectively.

1 n1 =+σσσσ n εε= + (1)(1) EEHH In this equation, the the strain strain is is ex expressedpressed as as a a sum sum of of elastic elastic strain strain and and plastic plastic strain. strain. In Inaddition, addition, σ σis isstress, stress, ε isε isstrain, strain, E isE isthe the Young’s Young’s modulus, modulus, and and H andH and n aren are the the parameters parameters to be to bedetermined determined by bythe theleast-squares least-squares method. method. If we If we assume assume the the value value of of HH andand nn areare constants constants for for the the whole welding joint, these two parameters can be directly obtained by fittingfitting thethe experimentexperiment results and shouldshould be similar forfor didifferentfferent measurement measurement points points in in the the welded welded joint. joint. However, However, after after parameters parameters fitting, fitting, it can it becan found be found that thesethat these parameters parameters vary withvary respect with respect to the distanceto the distance to the center to the line, center which line, is shownwhich inis Tableshown2, provingin Table that 2, aproving simple constantthat a simple definition constant of the definition parameters ofH theand parametersn cannot accurately H and n describe cannot theaccurately evolution describe of plasticity the evolution in the welding of plasticity joint. in the welding joint.

Table 2.2. Fitting results of H and n values.

NumberNumber 11 22 33 44 55 66 7 8 9 9 H 457.899 524.807 583.647 579.322 553.554 609.958 611.871 531.496 458.543 H 457.899 524.807 583.647 579.322 553.554 609.958 611.871 531.496 458.543 nn 0.1040.104 0.1630.163 0.2260.226 0.2130.213 0.2240.224 0.234 0.221 0.173 0.173 0.108 0.108

Herein, a new mechanical-property-driven partitioning strategy is proposed to describe the uneven distribution of materials due to the welding operation. Based on the symmetry of welding joints, only the data on one side of the welded joint are selected for later analysis, and each data point is 1 mm apart from the next point, as shown in Figure 7a. A constitutive relationship includes the WZ and the HAZ is proposed based on the evolution of parameters (n and H) in the welding joints. The parameters are summarized as functions (H = f(s), 1/n = g(s)) of the distance from the center line of welding (s), and the constitutive relationship in Equation (1) can be revised as Equation (2). For AA2219, these functions can be simplified as polynomial functions, as shown in Equations (3) and (4).

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Herein, a new mechanical-property-driven partitioning strategy is proposed to describe the uneven distribution of materials due to the welding operation. Based on the symmetry of welding joints, only the data on one side of the welded joint are selected for later analysis, and each data point is 1 mm apart from the next point, as shown in Figure7a. A constitutive relationship includes the WZ and the HAZ is proposed based on the evolution of parameters (n and H) in the welding joints. The parameters are summarized as functions (H = f (s), 1/n = g(s)) of the distance from the center line of welding (s), and the constitutive relationship in Equation (1) can be revised as Equation (2). For AA2219, these functions can be simpliﬁed as polynomial functions, as shown in Equations (3) and (4).

!g(s) σ σ ε = + (2) E f (s)

For WZ: ( f (s) = as2 + bs + c (3) g(s) = d

For HAZ: ( f (s) = as + b (4) g(s) = cs2 ds + e Metals 2020, 10, x FOR PEER REVIEW − 9 of 14

Figure 7. Comparisons of test results and mathematical models: (a) Yield strength curve for the whole Figure 7. Comparisons of test results and mathematical models: (a) Yield strength curve for the area, (b) stress-strain curve in WZ, (c) stress-strain curve in heat aﬀected zone (HAZ). whole area, (b) stress-strain curve in WZ, (c) stress-strain curve in heat affected zone (HAZ).

In order to validate this partitioning result, two random points are selected in the WZ and the HAZ to verify the Equations (5) and (6), respectively. The distance from the weld center is 2.5 mm and 7.5 mm. The comparison between the mathematical model and the test results is shown in Figure 7b,c. The comparison between the mathematical model of the weld zone and the heat affected zone and the test results is good, and the maximum error is less than 8%, which indicated that Equations (5) and (6) can effectively describe the mechanical properties distribution of different joints in the AA2219 welded joints after friction stir welding.

4. Validation of the Partitioning Strategy

4.1. Metallographic Characteristics Figure 8 is the microstructure of 2219 aluminum alloy welded joint given by metallographic microscopy, which shows a distinct difference from the WZ to the BM. The microstructure variation from the WZ to the HAZ is more drastic than the HAZ to the BM. It can be seen that the BM has an obvious trace of rolling direction and the shape of the grains of BM is stretched towards the rolling direction. The weld nugget area is affected by the welding heat cycle and agitation, and the morphology changes significantly. During the FSW process, a large amount of heat is generated between the stirring needle and the workpiece and the shoulder of the stirring head to the workpiece, so that the surrounding metal is plasticized and plastic flow enough. The density of the dislocations increases with the stirring force, and when the stored energy is increased to a certain extent. The density of the dislocations increases with the stirring force. When the stored energy is

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According to the abovementioned rules, the welded joints can be partitioned based on the Equations (3) and (4). In this study, all the parameters can be determined by the data ﬁtting. At last, Equations (5) and (6) are obtained for WZ and HAZ in AA2219 welding joints, respectively. Therefore, the intersection of the two curves in Equations (5) and (6) can be determined as the boundary between WZ and HAZ. For this friction stirred welding joint, the position of the boundary line is determined to be 4.815 mm from the center of the weld, as shown in Figure7. For WZ: ( f (s) = 1.1s2 + 16.7s + 599.6 − (5) g(s) = 4.471

For HAZ: ( f (s) = 1.146s + 489.2 (6) g(s) = 0.0111s2 0.0113s + 5.583 − In order to validate this partitioning result, two random points are selected in the WZ and the HAZ to verify the Equations (5) and (6), respectively. The distance from the weld center is 2.5 mm and 7.5 mm. The comparison between the mathematical model and the test results is shown in Figure7b,c. The comparison between the mathematical model of the weld zone and the heat affected zone and the test results is good, and the maximum error is less than 8%, which indicated that Equations (5) and (6) can effectively describe the mechanical properties distribution of different joints in the AA2219 welded joints after friction stir welding.

4. Validation of the Partitioning Strategy

4.1. Metallographic Characteristics Figure8 is the microstructure of 2219 aluminum alloy welded joint given by metallographic microscopy, which shows a distinct difference from the WZ to the BM. The microstructure variation from the WZ to the HAZ is more drastic than the HAZ to the BM. It can be seen that the BM has an obvious trace of rolling direction and the shape of the grains of BM is stretched towards the rolling direction. The weld nugget area is affected by the welding heat cycle and agitation, and the morphology changes significantly. During the FSW process, a large amount of heat is generated between the stirring needle and the workpiece and the shoulder of the stirring head to the workpiece, so that the surrounding metal is plasticized and plastic flow enough. The density of the dislocations increases with the stirring force, and when the stored energy is increased to a certain extent. The density of the dislocations increases with the stirring force. When the stored energy is increased to a certain extent, the crystal nucleus begins to form continuously in the metal, and the structure undergoes dynamic re-crystallization. The lamella structure of the original base material is transformed into an equiaxed recrystallized structure, and the crystal grains are uniform, and the average size is much smaller than the grain size of BM (Figure8a). The TMAZ is a long narrow area, which is located at the boundary of WZ and HAZ in this work. This area is affected by the mixing needle and the shoulder and is subjected to a higher temperature welding thermal cycle, and the grain has a large bending deformation. As shown in Figure8b, the grain size is the smallest area in the welded joint. The microstructure of the HAZ is shown in Figure8c. The HAZ is not directly subjected to agitation during the welding process, is only subjected to thermal cycling during the welding process, and the grain sizes are not uniform, so a part of the grains retain the characteristics of BM and a part of the grains has a smaller size. It is necessary to point out that the separation line between HAZ and BM zones have been obtained by visual inspection. According to the difference of the metallographic characteristics of each area of the welded joint, the partition of the welded joint is determined. The proposed method in this paper is consistent with the observation result. Metals 2020, 10, x FOR PEER REVIEW 10 of 14 increased to a certain extent, the crystal nucleus begins to form continuously in the metal, and the structure undergoes dynamic re-crystallization. The lamella structure of the original base material is transformed into an equiaxed recrystallized structure, and the crystal grains are uniform, and the average size is much smaller than the grain size of BM (Figure 8a). The TMAZ is a long narrow area, which is located at the boundary of WZ and HAZ in this work. This area is affected by the mixing needle and the shoulder and is subjected to a higher temperature welding thermal cycle, and the grain has a large bending deformation. As shown in Figure 8b, the grain size is the smallest area in the welded joint. The microstructure of the HAZ is shown in Figure 8c. The HAZ is not directly subjected to agitation during the welding process, is only subjected to thermal cycling during the welding process, and the grain sizes are not uniform, so a part of the grains retain the characteristics of BM and a part of the grains has a smaller size. It is necessary to point out that the separation line between HAZ and BM zones have been obtained by visual inspection. According to the difference of theMetals metallographic2020, 10, 65 characteristics of each area of the welded joint, the partition of the welded joint10 of is 14 determined. The proposed method in this paper is consistent with the observation result.

FigureFigure 8.8. TheThe microstructure microstructure of 2219of 2219 aluminum aluminum alloy weldedalloy welded joint given joint by metallographicgiven by metallographic microscopy. microscopy.(a) weld zone (a (WZ),) weld ( bzone) thermo-mechanically (WZ), (b) thermo-mechanically affected zone affected (TMAZ), zone (c) HAZ, (TMAZ), (d) base (c) HAZ, metal ( (BM).d) base metal (BM). 4.2. SEM Morphological Characteristics 4.2. SEMThe Morphological precipitated phase Characteristics of the joint is affected by the welding heating cycle and agitation, and there are significantThe precipitated differences phase in of the the distribution joint is affected patterns by ofthe the welding joints. Figureheating9 showscycle and scanning agitation, electron and theremicrographs are significant of FSW differences of 2219-T87 in aluminum the distribution alloy. Thepatterns 2219 of aluminum the joints. alloy Figure is based 9 shows on αscanning(Al) and electroncontains micrographs many small particles. of FSW Theof 2219-T87 white particles aluminum are analyzed alloy. The by electron2219 aluminum spectroscopy alloy (EDS is based analysis). on αThe(Al) results and contains are shown many in small Table 3particles., where SD The represents white particles standard are deviationanalyzed by value. electron It shows spectroscopy that the (EDScomponents analysis). of The the αresultsphase are in threeshown di infferent Table parts 3, where of WZ, SD TMAZ, represents and standard HAZ are deviation basically thevalue. same, It showsbut the that precipitated the components phase is quiteof the di αff erent.phase Thein three partition different result parts determined of WZ, by TMAZ, the proposed and HAZ method are agrees well with the SEM morphological observation.

Table 3. EDS results.

Position Al (SD%) Cu (SD%) Fe (SD%) Mn (SD%) α phase of WZ 97.17 (0.1) 2.83 (1.6) - - Precipitated phase of WZ 59.22 (0.6) 40.78 (0.7) - - α phase of TMAZ 97.26 (0.1) 2.74 (2.8) - - Precipitated phase of TMAZ 60.49 (0.5) 32.27 (0.8) 5.8 (1.2) 1.44 (2.2) α phase of HAZ 93.07 (0.1) 6.93 (1.7) - - Precipitated phase of HAZ 46.98 (0.7) 43.43 (0.6) 8.1 (2.0) 1.49 (3.1)

SEM results of diﬀerent magniﬁcation of the WZ are shown in Figure9a,d. In the welding process, WZ maintains a high temperature and has enough time to form precipitation phase in the grain, Metals 2020, 10, x FOR PEER REVIEW 11 of 14 basically the same, but the precipitated phase is quite different. The partition result determined by the proposed method agrees well with the SEM morphological observation.

Table 3. EDS results.

SEM results of different magnification of the WZ are shown in Figure 9a,d. In the welding process, WZ maintains a high temperature and has enough time to form precipitation phase in the grain, so it can be seen that there are precipitation phases in the grain and at the grain boundary. As a result of recrystallization, grains of uniform size are formed, and the radius of the grain size in WZ is about 25 μm. The results of different magnification of the TMAZ are shown in Figure 9b,e. The temperature in the affected area is not as high as the weld nugget zone, but the action time is longer. After stirring andMetals welding2020, 10, 65 thermal cycle, the local dynamic recrystallization of grains occurs, and 11some of 14 precipitated phase is located at the boundary of the grains. In the HAZ, the heating near the heat source is fast, the peak temperature is high, and the coolingso it can speed be seen is higher. that there Therefore, are precipitation the microstructu phasesre inin thethe grainHAZ also and has at the a large grain difference. boundary. Due As to a theresult high of peak recrystallization, temperature, grains in the of HAZ uniform near sizethe WZ are formed,, the grain and grows the radius significantly; of the grain much size larger in WZ than is theabout size 25 ofµ m.the grain in the base metal, and the shape is equiaxed, as shown in Figure 9c,f.

Figure 9.9. TheThe microstructure microstructure of 2219of 2219 aluminum aluminum alloy a weldedlloy welded joint measured joint measured by SEM. by (a) WZSEM. magnified (a) WZ magnified2000 times, 2000 (b) TMAZ times, magnified (b) TMAZ 2000 magnified times, (c) HAZ2000 magnifiedtimes, (c) 2000HAZ times, magnified (d) WZ 2000 magnified times,500 (d) times, WZ magnified(e) TMAZ magnified500 times, 500(e) TMAZ times, (magnifiedf) HAZ magnified 500 times, 500 (f) times. HAZ magnified 500 times.

4.3. Nano-MechanicalThe results of di Characteristicsfferent magnification of the TMAZ are shown in Figure9b,e. The temperature in the affected area is not as high as the weld nugget zone, but the action time is longer. After stirring In this study, nano-indentation is also conducted to validate the partitioning method proposed and welding thermal cycle, the local dynamic recrystallization of grains occurs, and some precipitated in this paper. The test result is shown in Figure 10, the hardness of WZ increases with the distance phase is located at the boundary of the grains. from the welding center, and there is a sudden change at the junction with HAZ. The maximum In the HAZ, the heating near the heat source is fast, the peak temperature is high, and the cooling hardness is found at the junction of WZ and HAZ. It has been reported by Cavaliere [28] and speed is higher. Therefore, the microstructure in the HAZ also has a large difference. Due to the high Yazdipour [13] that the grain size is smaller at the junction of WZ and HAZ, resulting in a higher peak temperature, in the HAZ near the WZ, the grain grows significantly; much larger than the size of hardness. This is because the material flow at the junction is more serious compared to the other the grain in the base metal, and the shape is equiaxed, as shown in Figure9c,f. areas, which produces finer microstructures at a higher rotation speed. Since then, the boundary of 4.3. Nano-Mechanical Characteristics

In this study, nano-indentation is also conducted to validate the partitioning method proposed in this paper. The test result is shown in Figure 10, the hardness of WZ increases with the distance from the welding center, and there is a sudden change at the junction with HAZ. The maximum hardness is found at the junction of WZ and HAZ. It has been reported by Cavaliere [28] and Yazdipour [13] that the grain size is smaller at the junction of WZ and HAZ, resulting in a higher hardness. This is because the material ﬂow at the junction is more serious compared to the other areas, which produces ﬁner microstructures at a higher rotation speed. Since then, the boundary of the WZ and HAZ lies between 4 mm and 5 mm from the weld center line, which agrees well with prediction results by the proposed method. Metals 2020, 10, x FOR PEER REVIEW 12 of 14 Metals 2020, 10, x FOR PEER REVIEW 12 of 14 the WZ and HAZ lies between 4 mm and 5 mm from the weld center line, which agrees well with the WZ and HAZ lies between 4 mm and 5 mm from the weld center line, which agrees well with predictionMetals 2020, 10 results, 65 by the proposed method. 12 of 14 prediction results by the proposed method.

Figure 10. Nanoindentation test hardness of the welded joint (red line denotes the boundary Figure 10.10. NanoindentationNanoindentation test test hardness hardness of theof weldedthe welded joint (redjoint line (red denotes line denotes the boundary the boundary obtained obtained by the partitioning method proposed in this paper). obtainedby the partitioning by the partitioning method proposed method proposed in this paper). in this paper). 4.4. Surface Surface Characteristics Characteristics 4.4. Surface Characteristics AFM test is conducted as an assistant proof of the partition result as it cancan givegive the variation of AFM test is conducted as an assistant proof of the partition result as it can give the variation of surface morphology between different diﬀerent areas. Within 10 mm from the centerline of the welding, three surface morphology between different areas. Within 10 mm from the centerline of the welding, three measurement pointspoints are are selected selected every every 1 mm. 1 Themm. surface The surface roughness roughness (Ra: Arithmetic (Ra: Arithmetic mean roughness) mean measurement points are selected every 1 mm. The surface roughness (Ra: Arithmetic mean roughness)of the material of the in dimaterialﬀerent in areas different are calculated areas are and calculated shown inand Figure shown 11 in (5 Figureµm 511µ m).(5 μ Them × results5 μm). roughness) of the material in different areas are calculated and shown in Figure× 11 (5 μm × 5 μm). Theshow results that the show average that the roughness average of roughness WZ is obviously of WZ is higher obviously than thathigher of HAZ,than that and of the HAZ, boundary and the of The results show that the average roughness of WZ is obviously higher than that of HAZ, and the WZboundary and HAZ of WZ has and the HAZ lowest has roughness the lowest value. roughness value. boundary of WZ and HAZ has the lowest roughness value.

Figure 11. SurfaceSurface morphology morphology results of the welded joint. ( (aa)) Surface Surface morphology morphology of WZ, ( b) surface surface Figure 11. Surface morphology results of the welded joint. (a) Surface morphology of WZ, (b) surface morphology of partition line, (c) surface morphology of HAZ. morphology of partition line, (c) surface morphology of HAZ. 5. Conclusions 5. Conclusions 5. ConclusionsDue to the development of the DIC method, the research on welded joints is no longer limited to a Due to the development of the DIC method, the research on welded joints is no longer limited specificDue mechanical to the development property of of each the area,DIC butmethod, can obtain the research the mechanical on welded property joints ofis everyno longer point limited on the to a specific mechanical property of each area, but can obtain the mechanical property of every point wholeto a specific welded mechanical joint. In this property paper, of based each onarea, the bu standardt can obtain tensile the test mechanical and the elastoplastic property of every mechanical point on the whole welded joint. In this paper, based on the standard tensile test and the elastoplastic onmodel, the whole an elastoplastic welded joint. constitutive In this paper, model isbased established on the tostandard express tensile the mechanical test and propertiesthe elastoplastic of any point of AA2219 welded joint. According to the elastoplastic constitutive model, the partition scheme Metals 2020, 10, 65 13 of 14 of the welded joint of AA2219 is given, and the boundary between the WZ and the HAZ of the AA2219 welded joint of friction stir welding is determined. Finally, a series of verification tests are carried out. The results show that the partitioning result of the strategy proposed in this paper is consistent with partitioning result of the classical method. The proposed method can not only accurately determine the partition boundary, but also obtain the mechanical properties of the welded joint at the same time effectively.

Author Contributions: Conceptualization, G.C. and M.R.; validation, Y.Z.; data curation, G.C.; writing—original draft preparation, G.C. and T.L.; writing—review and editing, G.C. and T.L.; project administration, W.P.; funding acquisition, M.R. All authors have read and agreed to the published version of the manuscript. Funding: This research was supported by the National Basic Research Program of China (2014CB046506, 2014CB049000), the National Natural Science Foundation of China (11372058, 11802053), and the Start-up funding from DUT (DUT18RC(3)031). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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