Microstructural formation and mechanical performance of friction stir double-riveting welded Al-Cu joints

Shude JI, Zhiqing ZHANG, Peng GONG, Hua LIU, Xiao CUI, Yewei ZHANG

College of Aerospace Engineering, Shenyang Aerospace University, Shenyang 110136, China

KEYWORDS Dissimilar Al/Cu metals;Friction stir double-riveting welding;Mechanical interlocking;Mechanical properties;Metallurgical bonding

Abstract A novel friction stir double-riveting welding (FSDRW) technology was proposed in order to realize the high-quality joining of upper aluminum (Al) and lower copper (Cu) plates,and this technology employed a Cu column as a rivet and a specially designed welding tool with a large concave-angle shoulder.The formations, interfacial characteristics, mechanical properties and fracture features of Al/Cu FSDRW joints under different rotational velocities and dwell times were investigated.The results showed that the well-formed FSDRW joint was successfully obtained.The cylindrical Cu column was transformed into a double riveting heads structure with a Cu anchor at the top and an Al anchor at the bottom,thereby providing an excellent mechanical interlocking.The defect-free Cu/Cu interface was formed at the lap interface due to the sufficient metallurgical bonding between the Cu column and the Cu plate,thereby effectively inhibiting the propagation of crack from the intermetallic compound layer at the lap interface between the Al and Cu plates.The tensile shear load of joint was increased first and then decreased when the rotational velocity and dwell time of welding tool increased,and the maximum value was 5.52 kN.The FSDRW joint presented a mixed mode of ductile and brittle fractures.

1.Introduction

In order to meet the increasingly complex conditions of industrial application, high and comprehensive properties are required for the metal part.Dissimilar metals joining can integrate the advantages of different kinds of metals, and has a great potency to improve the comprehensive performances of metal part.1Aluminum(Al)and copper(Cu)alloys have excellent properties such as high electrical conductivity and great plasticity, and their hybrid structure has been widely used as a core component in Lithium-Ion batteries which is an important part of some equipments such as the electric-powered aircraft in the aerospace field.The joining of Al and Cu has gradually become a hot topic in recent years.2–4.

At present, mechanical connection and welding are the common processes for the joining of Al and Cu alloys.Different from the mechanical connection methods such as screw connection5and riveting,6the welding is an integrated manufacturing process and is beneficial to achieving better comprehensive performances of dissimilar metals joint.However, the welding between dissimilar metals is difficult due to the great difference in physical and chemical properties.7Fusion welding with high temperature always produces not only amounts of hard-brittle intermetallic compounds (IMCs)at the interface between the Al and Cu alloys(the Al/Cu interface)but also the cracks and large deformation,bringing about the poor mechanical properties of the joint.8It is known that the solid-state welding process is beneficial to reducing the hard-brittle IMCs at the joining interface of dissimilar metals.9Friction stir welding (FSW) is an excellent solid-state welding technology, and has the advantages of no cracks, small deformation and high strength, which can largely relieve and even avoid the above-mentioned defects of Al and Cu dissimilar metals joint by traditional fusion welding.10,11.

Spot welding techniques such as friction stir spot welding(FSSW)12and refill friction stir spot welding (RFSSW)13,14are the derived technologies of conventional FSW which is a seam welding, and have been proved to fabricate Al and Cu dissimilar metals joint (simplified as Al/Cu joint) with relatively high strength.4The Al/Cu spot welding techniques can be divided into two types.One type is that the tool does not plunge into the lower plate, and the corresponding joint strength is mainly related to the metallurgical bonding based on diffusion.14The other type is that the tool plunges into the lower plate, and the bonding quality of Al/Cu joint is related to not only the metallurgical bonding but also the mechanical interlocking.15In addition, the addition of interlayer is an effective way to reinforce the metallurgical bonding of joint under these two types of Al/Cu spot welding techniques.Boucherit et al.16added the Zinc (Zn) at the Al/Cu lap interface and found that the Zn addition significantly reduced the thickness of Al2Cu layer from 10 to 2 μm, and favored the formation of Al4.2Cu3.2Zn0.7precipitate, and then hindered the formation of more detrimental Al4Cu9compound, thereby significantly increasing the tensile shear strength of joint.

In fact, the mechanical interlocking between dissimilar Al/Cu metals is the key factor on heightening the loading capacity of lap joint.17In order to improve the joint strength under the condition of the tool not plunging into the lower plate, some researchers have adopted rational methods such as optimizing the tool plunging depth and proposing new process.Cardillo and Shen14joined the upper Al plate with 2 mm thickness and the lower Cu plate by RFSSW under different plunging depths of 1.6 mm, 1.8 mm and 2.0 mm.They found that increasing the plunging depth to 2.0 mm made the Al/Cu lap interface slightly concave and then form a mechanical interlocking, thereby increasing the joint lap-shear strength.Liu et al.18proposed friction stir spot riveting (FSSR) to reduce the generation of IMCs by using a pinless tool and a Cu rod, and stated that a Cu anchor from the deformation of Cu rod was obtained at the upper part of joint, which could achieve the mechanical interlocking of Al/Cu dissimilar metals.Certainly, when the tool pin plunges into the lower plate, the relatively strong mechanical interlocking is more easily formed no matter whether the rotational welding tool is remained in the joint.Zhou et al.17found that the materials of the lower plate around the bottom of tool pin flowed upward to form a Cu hook embedding in the upper Al plate, which deeply improved the degree of mechanical interlocking and then the tensile shear strength of Al/Cu joint.Bothiraj and Saravanan19employed friction stir blind riveting (FSBR) to join the Al/Cu dissimilar metals, during which a rotating rivet served as the welding tool to produce frictional heat.Their results showed that the effective mechanical interlocking was formed in joint,but the deformable rivet made in Al material inevitably produced insufficient heat input.

For the above-mentioned reported techniques based on FSW, the larger mechanical interlocking between the Al and Cu metals is always accompanied by a larger amount of IMCs at the Al/Cu lap interface.Therefore, how to obtain not only the large mechanical interlocking but also the lap interface with few IMCs needs the researchers to investigate.In this study, a novel friction stir double-riveting welding (FSDRW)method was proposed to join Al/Cu dissimilar metals, during which a Cu column serving as the controllably-deformable rivet and a specially designed welding tool with a large concave angle shoulder were employed.For this novel FSDRW, a controllably-strong mechanical interlocking and a no-IMC lap interface between the Cu column and the lower Cu plate(the Cu/Cu interface) can be achieved in the joint.In this study,how the rotational velocity and dwell time of tool influenced the formations, interfacial features and mechanical properties of Al/Cu FSDRW joints were studied in detail.

2.Experimental procedures

2.1.FSDRW process

The schematic of FSDRW process is shown in Fig.1.For the Al and Cu dissimilar metals lap joint fabricated by FSSW,there are two different lap configurations: one is that the Cu plate is placed on the Al plate,and the other is that the Al plate is placed on the Cu plate.1 For the Cu/Al lap joint, the sufficient heat input and then excellent diffusion bonding can be obtained for manufacturing the high-quality joint.So far, lots of researchers have investigated the upper Cu and lower Al dissimilar metals joint by FSSW.11,12,20For the FSDRW process,although the Al plate is placed on the Cu plate, the sufficient heat input can be obtained because the tool pin plunges into the Cu column in the Al plate, as displayed in Fig.1(a)-1(b).

The FSDRW process is divided into four stages including drilling, plunging, dwelling and retracting stages.The welding tool consists of a tool shoulder and a tool pin,which owns the non-deformable feature during the FSDRW process of dissimilar Al and Cu metals.At the drilling stage(Fig.1(a)),a circular through hole is fabricated on the Al plate,and a Cu column with a height larger than that of the through hole is filled into this through hole.Similar to the radial-additive friction stir repairing(R-AFSR)of exceeded tolerance hole,21the FSDRW process uses a filler material (FM) to fill a per-fabricated hole in the base material (BM).Song et al.21reported that the grinding phenomenon easily happened between the FM and the BM during R-AFSR process and negatively influenced the strength of repaired region.Certainly, this grinding phenomenon can be partly relieved and even solved by the tight fitting between the FM and the BM, which is the reason why the diameter of FM during R-AFSR was designed to be equal to that of the per-drilled hole.Therefore, the diameter of Cu column used in FSDRW is equal to that of the through hole.At the plunging stage (Fig.1(b)), the welding tool with highspeed rotation plunges into the Cu column, violently stirs the Cu materials and generates a large amount of heat.Notably,the tool shoulder used in FSDRW has a larger concave angle compared to that of the traditional tool shoulder, and this non-deformable welding tool ensures sufficient heat input and material flow to form effective mechanical interlockings between the Cu column, the Al plate and the Cu plate.Therefore, both the geometry of tool shoulder and the plunging depth of tool need to be specially designed.At the dwelling stage (Fig.1(c)), the welding tool dwells for a few seconds to form a reliable joint after the tool arrives at the designed plunging depth.At the retracting stage(Fig.1(d)),the welding tool is retracted from the joint, and an integral Al/Cu joint containing the double riveting heads structure is produced.Thereinto, one riveting head is located at the upper part of Al plate due to the tool shoulder with a large concave angle,the other riveting head is formed above the lap interface because of the combined effects of the deformation of Cu column and the flow of Al materials.

Fig.1 Schematic of FSDRW.

2.2.Materials and methods

In this study, Al-Mg alloy and T2 Cu plates were selected as the research objects,and the chemical compositions are shown in Table 1.The dimensions of Al-Mg alloy and T2 Cu plates were 2 mm thick, 30 mm wide and 100 mm long, and the lap width of plates was 30 mm.The diameters of through hole(D1) and the Cu column (D2) were both 8 mm, and the height of Cu column (H1) was 3.5 mm.The welding tool used in this study was composed of a shoulder with a concave angle of 14°and a threaded conical pin (Fig.1(a)-1(b)), whose sizes are shown in Table 2.Similar to the tool used in the reported literature,12,16,20the tool used in FSDRW was made of H13 steel.All FSDRW experiments were carried out by the FSW machine (FSW-3LM-4012), and the welding parametersincluding rotational velocity (ω), dwell time (T), plunging speed (V) and plunging depth (Pd) are shown in Table 3.

Table 1 Chemical compositions of Al-Mg alloy and T2 Cu.

Table 2 Designed dimensions of welding tool.

The metallographic specimen was etched by the solution of 25 mL HCl + 2.5 g FeCl3+ 50 mL H2O and then observed by the optical microscope(OM,OLYMPUS GX51).The scanning electron microscope (SEM, SU3500) equipped with an energy dispersive X-ray spectrometer(EDS)was used to analyze the metallographic specimen and the fracture surface.X-ray diffraction (XRD, Ultima IV) was used for phase component characterization.The tensile shear test was performed using a universal testing machine at room temperature with a loading speed of 2 mm/min, and the average value of three samples was taken as the final result.The microhardness of joint was measured by the microhardness tester(HVS-1000)with a load of 200 g and a dwell time of 10 s.Fig.2 shows the positions of two lines for the microhardness test.Line A was 0.5 mm away from the upper surface of Al plate while line B was just located at the lap interface between the Al and Cu plates.

2.3.Finite element model of FSDRW

In this study, the thermal process during FSDRW was analyzed by experimental and numerical methods,which was used to explain the IMC formation mechanism.

Table 3 Process parameters during FSDRW.

Fig.3 Finite element model and temperature measurement conditions.

Fig.2 Schematic of measured positions for microhardness test.

The three-dimensional finite element model of FSDRW was established by the MSC.Marc software.The dimensions of the plate used in the simulated model were the same as those of the experiment plate, and the eight-node hexahedral element(HEX-8) was used.A non-uniform mesh was used to divide the model, which meant that the smaller mesh was used near the welded region and the larger mesh was used far away from the welded region(Fig.3(a)).In this study,the model in Fig.3(a)had 3580 nodes and 2569 elements.The initial temperature of the model was set as 20 ℃.The heat convection coefficient of the plate surface in contact with air was set as 40 W/(m2?℃),and this coefficient of the plate surface in contact with the fixture and the backing plate was set as 200 W/(m2?℃).In the simulation of FSDRW, the frictional heat was mainly generated by the shoulder and the pin.The heat produced by shoulder (Qs) is expressed as:22

where L is the length of the pin.

The heat sources were applied to the finite element model through the flux subroutine written in the fortran language,and the temperature field was obtained after nonlinear calculation.The Birth-death element was used to simulate the movement process of metal materials during welding.The K-type thermocouples were used to measure the temperature, and the measured positions are displayed in Fig.3(b).Points P1,P2and P3were selected for temperature measurement.P1and P2were 2 mm and 3 mm away from the edge of the shoulder at the upper surface of Cu plate,and P3was located at the Al/Cu interface and 3 mm away from the edge of the shoulder.The actual measured results and the simulation results were compared to verify the accuracy of the finite element model in this study.The FSDRW process lasted 320 s including 105 s for plunging, 15 s for dwelling and 200 s for cooling.

3.Results and discussion

3.1.Surface morphologies

Fig.4 shows the surface morphologies of Al/Cu FSDRW joints at different rotational velocities and dwell times.The keyhole is left in the joint after the retreat of welding tool because the tool pin is plunged into the Cu column during welding, and this phenomenon about keyhole is similar to the traditional FSSW described by Boucherit et al.16The flash is formed on the top surface because the materials are squeezed out of the welding spot by the welding tool.16The tool shoulder with a large concave angle can accommodate plasticized materials and then reduce material loss, so the flashes in Fig.4 are all relatively small.At the plunging stage of FSDRW process, the edge of deformed Cu column embeds into the Al plate under the forging force by the welding tool and part of Al materials cover the edge of deformed Cu column, so a Cu ring formed on the top surface of joint owns a diameter smaller than that of the tool shoulder.Increasing rotational velocity heightens not only the material flow velocity due to the increased linear speed of contacting point between the tool and the material, but also reduced the material flow stress due to the elevated temperature,23which leads to the decrease of diameter of Cu ring (Fig.4(a)-4(c)).Certainly, increasing the dwell time during welding is beneficial to improving the material flow behavior due to the elevated temperature and the longer heating time, thereby leading to the reduced material flow stress and then the decreased diameter of Cu ring(Fig.4(b), 4(d) and 4(e)).

3.2.Thermal analysis

Fig.5 shows the temperature curves of observation points in numerical simulation and experiment results at ω of 1200 r/min and T of 15 s under FSDRW.Notably, the temperature firstly increases at the plunging and dwelling stages and then decreases at the retracting stage.The peak temperatures of points P1, P2and P3obtained by numerical simulation are 337.3 ℃,267.4 ℃and 264.8 ℃(Fig.5(a)),and the corresponding values by experiment are 332.9 ℃, 261.8 ℃and 257.9 ℃(Fig.5(b)).The relative errors of peak temperatures for points P1, P2and P3between simulation and experiment results are about 1.32 %,2.12 % and 2.69 %.These relative error values are all within 5%,demonstrating that the finite element model established in this study is reasonable.

Fig.6 shows the temperature fields of the top surface and cross section of FSDRW joint at ω of 1200 r/min and T of 15 s.The temperature field is asymmetrical because the asymmetrical placement of welded plates at both sides of the welding spot.The high temperature areas are circular on the top surface (Fig.6(a) and 6(b)).For the cross section, the high temperature areas show a funnel-shaped (Fig.6(c)) at the plunging stage and a basin-shape (Fig.6(d)) distribution at the dwelling stage.The temperature gradient near the rotational tool is higher than that far away from the tool.Besides,the peak temperature of joint is 466 ℃on the edge of contacting region of welded plate and welding tool at the end of dwelling stage (Fig.6(b) and 6(d)).

Fig.4 Surface morphologies of FSDRW joints at different rotational velocities and dwell times.

Fig.5 Temperature cycles of measurement points.

Fig.6 Temperature fields under FSDRW.

Fig.7 Temperature cycles of observation points at Al/Cu interfaces under FSDRW.

The temperature cycles of points M1, M2and M3at ω of 1200 r/min and T of 15 s are displayed in Fig.7, and these three points are respectively located at the Cu anchor,vertical Al/Cu interface and the lap Al/Cu interface (Fig.7(a)).The plunging and dwelling stages are the stages for the heat input during welding.The main heat input is generated by the friction between the welding tool and metal material,24so the corresponding larger friction region causes higher heat generation.Therefore,the heating rate of each stage is not consistent due to the difference in heat generation rate and the highest heating rate happens at the dwelling stage due to the maximum friction region during welding.

The peak temperatures of points M1,M2and M3are 416.7℃, 389.9 ℃and 378.6 ℃, respectively (Fig.7(b)).For the Al and Cu dissimilar metals FSDRW process, the temperature has a decisional effect on not only the formation and evolution of IMCs but also the tool wear, which is discussed in the following part.

3.3.Tool wear analysis

So far, many researchers have used the H13 steel tool to friction stir weld the similar and dissimilar materials joint,11,12,25when Cu alloy is chosen as the research object and the tool inserts into the Cu alloy.As is well known, the H13 steel tool wear is not needed to consider when the tool is used to weld the aluminum alloys.Compared with aluminum alloy, the Cu alloy owns much higher melting point.Thus, the tool wear is worthy of consideration, when the H13 steel tool inserts into the Cu alloy during FSW.Sahlot et al.25investigated the H13 steel tool wear during FSW of CuCrZr alloys, and stated that higher tool rotational speed led to greater tool wear due to enhanced relative surface velocities.They also found that the tool shoulder was upset, the tool pin diameter was reduced and the tool pin was shortened when the traversed distance of tool was only 300 mm under the 1000 r/min rotational speed of tool(Fig.8(a)).Fig.8(b)shows the hardness values of H13 steel and CuCrZr alloy under different temperatures,25and the main reason for the severe tool wear is that the difference of hardness between the H13 steel and the Cu alloy is significantly decreased when the temperature is higher than 800 K (Fig.8(b)), and the peak temperature is higher than 900 K during FSW.Fig.8(c) shows the tool image after the FSDRW joint under the 1200 r/min rotational velocity was fabricated 100 times.It is seen that the Al(gray color)and Cu(orange color)alloys are adhered on the tool surface.As shown in Fig.8(d),the dimensions of welding tool were measured after welding 100 times.For the welding tool,comparing the dimension data in Table 2 and Fig.8(d), it is known that only the diameter of pin tip is very slightly shortened after welding 100 times, and other dimensions of tool are almost not changed.Thus, the H13 steel tool wear during FSDRW is not severe and can be ignored, which results from the lower peak welding temperature of 466 ℃(739 K)during FSDRW and the lower hardness of T2 Cu (75 HV at room temperature).

3.4.Microstructures

Fig.9 shows the cross sections of FSDRW joints under different rotational velocities and dwell times.During FSDRW, the Cu column experiences the thermal–mechanical effect by the welding tool,and its geometry changes from a regular column shape to a complex shape named as double riveting heads structure in this study.This double riveting heads structure is mainly composed of three parts including two riveting heads at the top and bottom and an upset column in the middle.The top riveting head is formed under the effect of tool shoulder with a large concave angle,and it is named as Cu anchor.This Cu anchor is similar to the rivet cap which provides a fastening effect for the upper Al and lower Cu plates.The bottom riveting head is named as Al anchor,which is located above the lap interface of joint.The middle upset column presents a conelike shape, which has different diameters along the direction perpendicular to the lap interface of joint.

The Cu and Al anchors both play the mechanical interlocking effect, so some key morphological parameters are used to evaluate the mechanical interlocking effect, which is similar to the quantification of mechanical interlocking degree for the Cu hook in the study of Zhou et al.17For the Cu anchor,the key parameters are the height of Cu anchor (HC) and the diameter of Cu anchor (DC).For the Al anchor, the key parameters are the height of Al anchor(HA)and the diameter of Al anchor (DA).The HC represents the distance between the lowest and highest points of Cu anchor, and the HA represents the distance between the lap interface and the point of the maximum diameter of upset column.The DC and DA values are decided by the maximum diameter of Cu anchor and the maximum diameter of upset column, respectively.The morphological parameters of Cu and Al anchors are listed in Table 4, and these values are closely related to the material flow of Cu and Al metals, which is discussed in the following part.

Fig.8 Tool wear research of Sahlot et al.25 and this study.

Fig.9 Cross sections of FSDRW joints under different rotational velocities and dwell times.

Table 4 Morphological parameters of double riveting heads structure at different rotational velocities and dwell times.

The Cu materials in the joint are divided into stir zone(SZ),thermo-mechanically affected zone (TMAZ), heat affected zone (HAZ) and BM.The BM is composed of twin crystal grains (Fig.10(a)).Compared with the grains in BM, the grains in HAZ are obviously coarsened (Fig.10(b)).The recrystallization happens in TMAZ under the thermal–mechanical effect of the welding tool and the corresponding grain size is smaller than that in BM.Moreover,the grains in TMAZ are slightly elongated (Fig.10(c)) due to the mechanical effect of the welding tool.26,27Fig.10(d)-10(f)shows the microstructures at different locations of the SZ marked in Fig.10(b).The original grains of SZ are broken and recrystallized due to the thermal–mechanical effect of welding tool, and then the fine equiaxed grains are formed.28,29The grains in SZ1 (Fig.10(d)) are smaller than those in SZ2 (Fig.10(e)) and SZ3(Fig.10(f)),which is caused by the stronger stirring effect produced by the welding tool.

3.5.Formation mechanism of double riveting heads structure

The formation process of double riveting heads structure at the plunging stage is schematically shown in Fig.11.The material flow caused by the welding tool directly affects the formation of double riveting heads structure.

Before the Cu materials contact the concave part of tool shoulder, the tool pin plunging into the Cu column pushes the Cu materials to flow around.Because the material temperature of Cu column decreases with increasing the distance away from the tool pin, the Cu materials adjacent to the tool pin own higher flow ability than those at the edge of Cu column.Therefore, the Cu materials adjacent to the tool pin mainly flow upward and then out of the Cu column according to the law of minimum resistance,and then the flash-like structure is formed,as displayed in Fig.11(a).After the Cu materials contact the concave part of tool shoulder,the continuouslyplunging shoulder exerts the forging force on the plasticized Cu column, and the upper part of Cu column is finally transformed into the Cu anchor at the upper surface of Al plate,as displayed in Fig.11(b).Moreover, the edge of Cu anchor embeds into the Al material (Figs.9 and 11(b)), because the microhardness and melting point of Cu material are both higher than those of Al material.

Before the tool shoulder contacts the upper Al plate, the materials around the counterclockwise-rotating pin are driven to move downward along the right-thread groove on the pin,21and the Cu materials below the pin tip are also pushed downward.All the above-mentioned downward-flowing Cu materials accumulate below the pin tip and push the Cu materials around the pin tip to flow outwards.This outward-flowing behavior of Cu materials is constrained and its flowing direction is mainly upward because the Cu plate under the Cu column and the Al plate around the Cu column are both nearly moveless.Therefore, the edge of Cu column bottom at the lap interface,which is not subjected to the direct forging force by tool pin, moves upward according to the law of minimum resistance, thereby forming an upward warp between the edge of Cu column bottom and the upper surface of Cu plate(Fig.11(a)).After the tool shoulder contacts the Al plate,the plasticized Al materials near the Cu column constrained by the surrounding hard BM only flow downward under the pressing effect by the tool shoulder,and part of them are filled into the warp,thereby forming the Al anchor above the Al/Cu lap interface (Fig.11(b)).

In addition,when the tool shoulder contacts the upper surface of Cu column, the Cu column is upset(Figs.9 and 11(b))and its final geometry directly decides the morphological parameters of Al and Cu anchors (Table 4).With increasing rotational velocity and dwell time, the thermo-mechanical effect of the welding tool also increases.Under this condition,the flow ability of Cu and Al materials is enhanced,so the HC and DC are increased and the corresponding values respectively increase from 1.23 mm to 1.42 mm and from 10.73 mm to 13.93 mm when the rotational velocity varies from 1000 r/min to 1400 r/min, and the corresponding values respectively increase from 1.25 mm to 1.76 mm and from 10.04 mm to 12.26 mm when the dwell time varies from 5 s to 30 s.Similarly, the HA and DA are also increased and the corresponding values respectively increase from 0.36 mm to 0.53 mm and from 9.32 mm to 9.54 mm when the rotational velocity varies from 1000 r/min to 1400 r/min, and the corresponding values respectively increase from 0.39 mm to 0.62 mm and from 9.18 mm to 9.80 mm when the dwell time varies from 5 s to 30 s.

3.6.Interfacial characteristics

The magnified views of typical interfaces marked in Fig.9 are displayed in Fig.12.The Al/Cu interfaces of FSDRW joint include the double riveting heads structure interface and the lap interface between the Al and Cu plates,and the double riveting heads structure interface is further divided into the Cu anchor interface (Fig.12(a), 12(d), 12(g), 12(j), and 12(m)),the vertical Al/Cu interface (Fig.12(b), 12(e), 12(h), 12(k)and 12(n)) and the Al anchor interface (Fig.12(c), 12(f), 12(i), 12(l) and 12(o)).

Fig.12 Enlarged views of typical interfaces under different process parameter combinations.

Under the thermal–mechanical effect of welding tool, the vertical Al/Cu interface is slightly inclined because the Cu column is upset(Fig.12(b),12(e),12(h),12(k)and 12(n)),and the edge of Cu anchor is embedded into the upper part of Al plate (Fig.12(a), 12(d), 12(g), 12(j), and 12(m)), which is similar to the study of Liu et al.18Besides,some Cu fragments are peeled off from the Cu column and disperses in the joint due to the strong mechanical effect of the welding tool and the material flow around the Cu anchor in the Al plate.Song et al.21stated that insufficient heat input caused the poor material flow and then led to the formation of interfacial defects.The micro-cavity occurs at the Al anchor tip of joints at ω of 1000 r/min and T of 15 s and ω of 1200 r/min and T of 5 s due to insufficient heat input and material flow (Fig.12(c)and 12(l)).Increasing rotational velocity and dwell time produces more heat input and sufficient material flow,so the cavity disappears in the joint (Fig.12(f), 12(i) and 12(o)).

Besides,no obvious Cu/Cu interface is observed near the Al anchor (Fig.12(c), 12(f), 12(i), 12(l) and 12(o)), which reveals that an excellent metallurgical bonding by diffusion occurs and the defect-free Cu/Cu interface without IMCs is obtained.In addition, the Al/Cu lap interfaces have no obvious deformation because these interfaces do not undergo the direct stirring of welding tool.

3.7.Phase analysis

The scanning results of areas marked in Fig.9 at the Cu anchor interface and vertical Al/Cu interface are shown in Fig.13.The Al and Cu diffuse to each other through the bonding interface, and the IMC layers are generated.Due to the strong stirring effect of the welding tool, some IMC particles exist in the Al substrate around the Cu anchor interface(Fig.13(a)).However, no IMC particle appears in the Al substrate near the vertical Al/Cu interface (Fig.13(b)), which is because the stirring effect of the welding tool on the vertical Al/Cu interface is much weaker compared to the area near the Cu anchor interface.The XRD was applied to identify the existent phases on the cross sections of the joints, and the results show that the compounds of Al2Cu and AlCu are formed at the Al/Cu interface (Fig.13(c) and 13(d)).In fact,several metastable intermetallic phases are possible according to the Al-Cu binary system,17and the researchers have used the effective heat of formation (EHF) model to predict Al-Cu IMCs.17,30Guo et al.30reported that the EHF values of Al2Cu,AlCu,Al3Cu4,Al2Cu3and Al4Cu9were respectively-6.76 kJ/mol,-6.68 kJ/mol,-6.29 kJ/mol,-5.84 kJ/mol and-5.61 kJ/mol.Therefore, Al2Cu and AlCu IMCs are formed before other above-mentioned Al-Cu IMCs.Zhou et al.17stated that as for the hook interface back to the exit hole, a laminated layer consisting of Al2Cu and AlCu IMCs was formed under the temperature higher than 475.2 ℃.According to the temperature results in Figs.6 and 7,it is known that the peak welding temperature at all the Al/Cu interfaces of the joints (Fig.13) is lower than 416.7 ℃, and this relatively low temperature causes the formation of Al2Cu and AlCu IMCs rather than other Al-Cu IMCs owning the larger EHF value.Thus, the XRD results (Fig.13(c) and 13(d)) in this study can be verified to be right according to the temperature results in this study and the results reported by Zhou et al.17and Guo et al.30.

Fig.13 Area scanning results and XRD patterns of Al/Cu interfaces.

Fig.14 Magnified views of areas at the Al/Cu interface and line scanning results.

Table 5 Scanning results of points 1–11 marked in Figs.13-14.

Fig.14 shows the magnified views of areas at the Al/Cu interface and corresponding line scanning results under different rotational velocities and dwell times.Marstatt et al.31reported that the thickness of IMCs increased with the increase of heat input.The heat input increases with increasing the rotational velocity and dwell time of welding tool.The Al/Cu interfaces in Fig.14 were obtained under three different heat inputs of ω of 1200 r/min and T of 15 s, ω of 1400 r/min and T of 15 s and ω of 1200 r/min and T of 30 s.Under the heat input of ω of 1200 r/min and T of 15 s, the IMC thicknesses in Fig.14(a)-14(c) are respectively 2.51 μm, 4.77 μm and 2.58 μm.The IMC thicknesses of Al/Cu interface under the higher heat input of ω of 1400 r/min and T of 15 s respectively increase to 2.96 μm, 5.93 μm and 3.43 μm, as displayed in Fig.14(d)-14(f).Similarly, when the dwell time increases from 15 s to 30 s, the IMC thicknesses in Fig.14(g)-14(i)respectively increase to 3.36 μm, 5.81 μm and 3.81 μm.Garg and Bhattacharya20found that the thickness of the IMC layer reached 147.7 μm at the Al/Cu lap interface below the tool shoulder edge and the value was 6.56 μm at the weld center of FSSW joint.Compared with the FSSW joint reported by Garg and Bhattacharya,20the thickness of IMC layer at the Al/Cu lap interface of FSDRW joint is greatly reduced.In fact, the thickness of IMC layer greatly influences the loading capacity of Al/Cu joint.Yu et al.32concluded that the thickness of interface increased under higher heat input due to the enhanced intermetallic reaction and atomic diffusion, and a reasonable thickness of IMCs could improve the loading capacity of joint.Xue et al.33optimized the thickness of the IMC to about 1 μm during the Al/Cu friction stir butt welding,and stated that a thin IMC layer at Al/Cu interface was beneficial to joint strength.Boucherit et al.16summarized that the IMC layer with the thickness larger than 4 μm negatively affected the strength of Al/Cu joint.According to these above-mentioned literatures and the results in Fig.14, it is known that controlling the IMC thickness to a rational small value is beneficial to enhancing the joint loading capacity,and a relatively small IMC thickness (Fig.14(a), 14(d) and 14(g)) can more easily be obtained at the joint lap interface.

In this study,the IMC layer is divided into two layers, and the typical points marked in Figs.13 and 14 are chosen to perform the EDS analysis.According to the atomic percent shown in Table 5, it is concluded that the IMC layers at the Al/Cu interfaces all consist of two sub-layers, and the sublayer 1 is the AlCu layer while the sub-layer 2 is the Al2Cu layer.Zhou et al.17reported that AlCu nucleated on the surface of Al2Cu layer and gradually grew into a layer at the Cu side,which agrees with the results in Table 5.According to the above-mentioned analysis, the phase compositions of the IMC layers at the Al/Cu interface are schematically presented in Fig.15.

Fig.15 Schematic of phase composition of IMC layers at different Al/Cu interfaces.

Fig.16 Microhardness distributions of joints at different rotational velocities and dwell times.

The micro-cracks defects appear at the Al/Cu lap interface of joint at ω of 1400 r/min and T of 15 s and ω of 1200 r/min and T of 30 s(Fig.14(d)and 14(g)),and the specific location of micro-cracks is the interface between the sub-layers of IMCs.This phenomenon is because the materials at the lap interface shrink and then the stress concentration is generated due to the large thermal-physical difference of materials in the cooling process after welding.34When the bonding strength between the AlCu and Al2Cu layers is lower than the stress induced by material shrinking, the cracks are produced (Fig.14(d)and 14(g)).

3.8.Mechanical properties

Fig.16 shows the microhardness distributions of Al/Cu FSDRW joints under different rotational velocities and dwell times.The finer grain is associated with higher microhardness according to the Hall-Petch relation.35,36The fine grains(Fig.10(d)-(f)) are produced in the Cu column, so the microhardness of material in the Cu column is relatively high, and the value decreases with increasing the distance far away from the wall of keyhole.With the increase of rotational velocity,the microhardness of Cu column adjacent to the keyhole increases in lines A and B, which reveals that the mechanical effect is larger than the thermal effect of the welding tool.In this study, the highest values in lines A and B under different rotational velocities are respectively 127.8 HV at ω of 1400 r/min and T of 15 s and 114.6 HV at ω of 1200 r/min and T of 15 s.

The microhardness values at the Al/Cu lap interface in Fig.16(b)are smaller than those at the vertical Al/Cu interface in Fig.16(a)due to a smaller thickness of IMC layer(Fig.14).Because the IMC thickness increases with increasing the dwell time due to the higher heat input, the microhardness value at Al/Cu interface at ω of 1200 r/min and T of 30 s is the highest when the dwell time varies from 5 s to 30 s.Similarly, the microhardness values at the Al/Cu lap interface of ω of 1200 r/min and T of 30 s and ω of 1400 r/min and T of 15 s are larger than other process parameter combinations.

The grains in HAZ (Fig.10(b)) are larger than those in SZ and TMAZ, so the microhardness values of HAZ are lower than the values of SZ and TMAZ.Besides, the microhardness values of Al materials in Fig.16(a) have no obvious change,which is because the thermal effect of welding tool exerting on the Al materials is very weak.

Fig.17 shows the tensile shear loads (TSLs) of Al and Cu dissimilar metals joint by solid-state spot welding technologies without the assisted process.With the increase of the rotational velocity from 1000 r/min to 1400 r/min and the dwell time from 5 s to 30 s, the TSL of the joint by FSDRW increases first and then decreases.The maximum TSL of the joint is 5.52 kN at ω of 1200 r/min and T of 15 s.The reason why the joint at ω of 1200 r/min and T of 15 s by FSDRW owns the highest loading capacity is discussed in the following section.

In Fig.17,the solid-state spot welding technologies include FSSW, RFSSW, FSSR, FSBR and FSDRW proposed in this study.Therein, Zuo et al.13employed the response surface method to optimize welding process parameters of RFSSW and gained relatively high tensile shear strength of joint.It is obvious that the FSDRW has a superb superiority in joining the Al and Cu dissimilar metals,which results from three main advantages as follows:

Firstly,for other Al/Cu joining techniques such as FSSW17and RFSSW,14the metallurgical bonding of lap interface between the Al and Cu plates inevitably makes the formation of Al-Cu IMCs, and the IMCs exist in the whole effective bonding region.Zhou et al.15stated that the cracks usually generated and then propagated along the lap interface with IMCs.For the FSDRW technology proposed in this study,the lap interface contains the Al/Cu interface and the Cu/Cu interface,and this Cu/Cu interface without kissing bond defect under rational heat input can effectively hinder the propagation of crack from the IMC layer of Al/Cu interface.

Fig.17 TSLs of Al/Cu dissimilar metals joint by solid-state spot welding.

Secondly, compared with the single Cu anchor with little size at the upper part of joint by FSSR,18the double riveting heads of Cu and Al anchors with larger size are formed in the joint by FSDRW.These Cu and Al anchors can prevent the Al plate stripping from the Cu plate when the joint is subjected to load force, thereby deeply improving the strength of joint.Moreover, the Al and Cu anchors can not only expand the propagation path of crack but also reduce the propagation speed of crack, thereby further heightening the loading capacity of joint.

Thirdly,compared with the rotating Al rivet in the FSBR,19the Cu column as the rivet has a high melting point and thermal conductivity and the non-deformable tool shoulder is specially designed, which is beneficial to making the interface experience sufficient heat input and enough forging force.Under this condition, the metallurgical bonding of the interface during FSDRW is ensured.

3.9.Fracture analysis

In this study, the FSDRW joints under different process parameter combinations in Table 3 all fracture along the lap interface, and the typical fractured joint is shown in Fig.18.Besides, when the joint eventually fractures, the Cu column is still bonded with the Al plate, which means the great metallurgical bonding and mechanical interlocking exist between the Cu column and the Al plate.Compared with joints at other process parameter combination, the joint at ω of 1200 r/min and T of 15 s avoids the micro-cavity at the Al anchor tip of joint (Fig.12(c)) and the crack in the IMC layer of Al/Cu lap interface (Fig.14(d)), and owns higher microhardness at the Cu/Cu interface (Fig.16), thereby having the highest tensile shear load.Moreover, the IMC layer thickness values at the lap interface (Fig.14(a), 14(d) and 14(g)) are all smaller than 4 μm under different process parameter combinations and positively influences the joint loading capacity,16which reveals that the IMC layer thickness by FSDRW is well controlled since the FSDRW joint presents the shear fracture rather than tensile fracture.

Fig.18 Fractured joint after tensile shear test.

Fig.19 Macro-fracture and micro-fracture morphologies of fractured joint.

Fig.19(a) is the macro-fracture morphologies of the failed Al and Cu plates at ω of 1200 r/min and T of 15 s.The micro-fracture morphologies of regions C-H marked in Fig.19(a) are displayed in Fig.19(c)-19(h).Regions C-E are located at the Cu/Cu interface and regions F-H are located at the Al/Cu interface.Regions C-E present a large number of dimples, indicating good metallurgical bonding and ductile fracture characteristics (Fig.19(c)-19(e)).Besides, region D is far away from the keyhole and the dimples are larger than those in region C.In addition,the layered tear edges and elongated dimples in region E have a certain directionality because the interface mainly burdens the stripping force in the tensile shear test (Fig.19(e)), which is similar to the description by Liu et al.17.

At the Al/Cu interface, region F shows big granular fracture surfaces due to the formation of hard-brittle IMC layer,showing the characteristics of intergranular fracture which belongs to the brittle fracture (Fig.19(f)).Region G in Fig.19(g) presents layered tear edges due to the interface tear in the tensile test.For region H at the edge of tool shoulder,a small amount of Al materials adhere to the Cu plate and the small cleavage planes and micro-cracks are observed (Fig.19(h)), which means the adhesion-shear fracture.These morphologies in Fig.19(h) are similar to the results described by Garg and Bhattacharya.12In summary, the FSDRW joint in this study presents a mixed mode of ductile and brittle fractures.

4.Conclusions

(1) The sound joint with superb external and internal formations was obtained by the FSDRW technique.The double riveting heads structure which was transformed from the Cu column included a Cu anchor at the top, an upset column in the middle and an Al anchor at the bottom, which greatly enhanced the mechanical interlocking of joint.

(2)The interfaces influencing the joint strength included the Cu/Cu interface and the Al/Cu interface.The IMC layers generated at the Al/Cu interface,which consisted of the AlCu sublayer near the Cu side and the Al2Cu sub-layer near the Al side.The Cu/Cu interface with the excellent metallurgical bonding could effectively hinder the propagation of crack from the IMC layer of Al/Cu lap interface.

(3) The TSL of FSDRW joint increased firstly and then decreased with increasing the rotational velocity and the dwell time of welding tool.The joints were fractured at the lap interface under different process parameter combinations, and the fracture morphologies showed a mixed mode of ductile and brittle fractures.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Nos.51874201 and 52074184).