Repairing of exit-hole in friction-stir-spot welded joints for 2024-T4 aluminum alloy by resistance welding

1.   Introduction

Precipitation-hardened aluminum alloys, 2xxx series alloys, are used extensively in the transportation applications in the realms of aircraft, automobiles, and high-speed trains [1]. However, it should be noted that the joining of aluminum alloys with fusion welding remains a great challenge because of solidification defects such as porosity, oxidization, and hot cracks, especially for the precipitation-hardened aluminum alloys [2].

Different from the traditional fusion welding process, such as laser welding and resistance spot welding, friction stir spot welding (FSSW) is developed from friction stir welding (FSW) and has been applied to the spot joining in light alloys [3]. During FSSW process, a rotating tool plunges into two overlapped sheets at a specified position and draws out after a setting dwell time, then a stir zone with an exit-hole is formed when the rotating tool pulls out [4]. In order to eliminate the inherent exit-hole, probeless FSSW (P-FSSW) [5–6] and refill FSSW (RFSSW) [7–9] were proposed. Li et al. [10] systematically investigated the effect of the stir zone width and the stir zone edge angle on the mechanical strength of the P-FSSWed joint. Experiments and simulations were performed to reveal the plastic materials flow behaviors of the P-FSSWed Al–Li alloy by Chu et al [11]. In the case of P-FSSW, the flow area of plastic metal is limited as its pinless, which is suitable for the joining of thinner sheet whose thickness is less than 2 mm. de Castro et al. [12] reported that the lap shear strength of the joint was decreased with the increase of tool wear after 2350 welds. Zou et al. [13] conducted RFSSW to weld aluminum alloy plates with different thicknesses and found that the thickness of the lower plate had significant effect on the temperature distribution, microstructure, mechanical properties, and fracture behavior of RFSSWed joints. The tensile/shear strength of RFSSWed joints was improved as the exit-hole was eliminated, while its equipment was extremely complex [6]. Therefore, under the consideration of the limitations of P-FSSW and RFSSW, traditional FSSW still has its advantages. However, the exit-hole must be repaired.

In order to eliminate the exit-hole in the traditional FSSW joint, large numbers of efforts have been made to refill the exit-hole in FSSWed joints, and encouraged results have been obtained and the related techniques have been proposed [14–16]. Sajed [17] conducted the P-FSSW technology to refill the conventional FSSWed 1100 aluminum with 2 mm thickness and improved the tensile/shear strength of the joint when the pinless tool with 14 mm diameter were used. Mehta et al. creatively performed probeless tool to successfully repair the exit-hole in dissimilar FSWed joints, such as, AZ31B-to-6061 [18] and AA6061-to-Cu (electrolytic tough pitch copper) [19]. Friction plug welding technology could be also used to repair the exit-hole [20–22]. Wang et al. [23] carried out tungsten inert gas welding (TIG) followed friction stir processing (FSP) to repair the exit-hole.

The filling exit-hole based on resistance welding (FEBRW) technique, an efficient repairing method, was firstly proposed by our group to completely fill or repair the exit-hole in friction stir spot welding. It should be noted that the tensile shear (TS) strength of the repaired joints was much higher than that of the traditional FSSWed joints [24].

During the FEBRW process, a plug with similar or dissimilar material to that of base metal was pressed into the exit-hole by a resistance welding machine with an electrode pressure monitoring platform, and then the resistance welding was carried out. Simple and high efficiency are the advantages of the FEBRW technology. However, the repairing stages and mechanisms of FEBRW are still unclear. The purpose of this work is to clarify the repairing mechanism by dividing the repair process into several stages based on their microstructure evolution. The interface between plug and exit-hole wall was characterized in detail by using optical microscope (OM) and electron backscatter diffraction (EBSD). A finite element model was constructed to describe the temperature distribution in the initial stage because of its nor for visualization.

2.   Experimental

2.1   Experiment equipment

A 300 kVA three-phase secondary rectifier resistance spot welding machine was used to achieve the FEBRW process. Based on the resistance spot welding machine, an experimental platform was built. The schematic diagram of the machine is shown in Fig. 1, and a pressure sensor was added. The force information was recorded during the repairing process, which can be utilized to analyze the dynamic behavior of the repairing pressure.

Fig. 1.  Schematic diagram of the resistant spot welding machine: (a) pressure transducer; (b) upper electrode; (c) specimens with exit-hole & plug; (d) lower electrode.

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2.2   Experiment materials

A 1.5-mm thick 2024-T4 aluminum alloy was friction stir spot welded in the lap configuration. The geometries of exit-hole are as shown in Fig. 2(a). The depth of the exit-hole is 2.5 mm, the diameters of the upper part and bottom part are 3.5 mm and 2.0 mm, respectively. The geometries of the plug are displayed in Fig. 2(b). The plug with the diameter of 3.4 mm and length of 4.0 mm was cut from a 2024 aluminum alloy bar. The rotating trace of welding tool on the inner wall of the exit-hole helped to increase the contact resistance between the plug and the exit-hole and disperse the current flowing through both of them.

Fig. 2.  Geometries of (a) exit-hole and (b) plug (unit: mm).

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2.3   Repairing parameters

The process parameters of additive repair mainly include the repair current, repair pressure, and repair time. Thirteen specimens with repaired holes for each welding condition with different currents (31 kA, 34 kA, 37 kA, 40 kA, 43 kA), pressures (8 kN, 10 kN, 12 kN, 14 kN, 16 kN), and times (160 ms, 180 ms, 200 ms, 220 ms, 240 ms) are listed in Table 1.

Table  1.  Parameters of repairing exit-holes by FEBRW

Sample No.	Current / kA	Time / ms	Pressure / kN
1	31	200	12
2	34	200	12
3	37	200	12
4	40	200	12
5	43	200	12
6	37	160	12
7	37	180	12
8	37	220	12
9	37	240	12
10	37	200	8
11	37	200	10
12	37	200	14
13	37	200	16

 | Show Table

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2.4   TS tests and macro/microstructure characterizations

TS tests were conducted at a crosshead speed of 1 mm/min at room temperature. OM was used to observe the macro and microstructures of the cross-section of the repaired joint. EBSD was conducted to characterize the microstructures of typical zones of the repaired joint. Scanning electron microscope (SEM) was utilized to observe the fracture morphologies of the repaired joint.

3.   Results and discussion

3.1   Tensile shear load of the repaired joints and fracture mechanism

The TS load of the repaired joints at various repairing parameters is displayed in Fig. 3. It can be observed that the TS loads of the repaired joints firstly rise and then decline with the increase of repairing current (Fig. 3(a)), repairing time (Fig. 3(b)), and repairing pressure (Fig. 3(c)). When the repairing current of 37 kA, the repairing time of 200 ms, and the repairing pressure of 12 kN were carried out, the highest TS load, 7.43 kN, was obtained, which was 34.7% higher than that of the as-welded joints. Weak fusion would be appeared when the repair current was smaller and the repair time was shorter. Inversely, spatter could be occurred. Moreover, spatter and weak fusion happen under the lower and higher pressures, respectively. Therefore, a reasonable matching relationship must be selected to obtain a high-quality repaired joint. Fig. 4 shows the TS loads of the joints with similar thickness fabricated by friction-based-spot-welding technologies. The sheet thickness, shoulder diameter, materials are illustrated in Fig. 4. It can be seen that the TS load in this work is in the higher level when compared with the other investigations, which indicates the high feasibility of FEBRW.

Fig. 3.  TS loads of repaired joints at various repairing parameters: (a) repairing current; (b) repairing time; (c) repairing pressure.

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Fig. 4.  Tensile shear strength of joints fabricated by different welding technologies [7,12–13,16–17,25–31].

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Fig. 5 presents the fracture morphologies of upper, middle, and bottom zones of the FEBRW joint under the optimized repairing parameters. Lots of dimples can be observed in these three zones, especially in the middle zone, indicating ductile fracture occurs through micro void coalescence. Particles also could be found in the dimples, which plays a positive role in the TS load improvement. Moreover, more tearing edges without dimples could be identified in the upper and bottom zones, which may be related to the interfaces between plug and exit-hole wall.

Fig. 5.  Fracture morphologies of different zones of the FEBRW joint under the optimized repairing parameters: (a) upper zone; (b) middle zone; (c) bottom zone; (d) magnified region in (a); (e) magnified region in (b); (f) magnified region in (c).

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3.2   Joint forming and microstructural evolution

Fig. 6 presents the forming morphologies of the as-welded and repaired joints. Fig. 6(a) and (b) shows the surface forming of the as-welded joint with exit-hole and the repaired joints, respectively. It is seen that the exit-hole was completely repaired. It could be inferred from Fig. 6(c) that sufficient heat was conducted on the whole thickness of the joint. No defects could be found in the macrostructure of the repaired joint as illustrated in Fig. 6(d). Thus, a sound repaired joint was obtained by the FEBRW. Various zones can be distinguished based on their appearance as shown in Fig. 6(d). It is obviously observed that a nugget (marked by the red line in Fig. 6(d)) appears in the middle part of the plug (marked by the dotted line). The size of nugget zone in the horizontal direction is larger than that of the exit-hole. That is because the interface between the plug and the thermal-mechanical affected zone (TMAZ) of exit-hole was broken, melted, and then disappeared under the resistant heat.

Fig. 6.  Photos of the unfilled and filled joints: (a) the as-welded joint with exit-hole; (b) surface of the repaired joint; (c) bottom of the repaired joint; (d) cross sectional macrostructure of the repaired joint.

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Fig. 7 illustrates the microstructures at different zones in Fig. 6(d). Fig. 7(a) presents the microstructure of the location 1 in Fig. 6(d), which experienced melt and solidification during the FEBRW process due to the occurrence of equiaxed dendrite grains, indicating a small temperature gradient and fast cooling rate in the middle part of the plug. It is convinced that the typical solidification microstructures are dominant in the nugget zone, which is similar to that of the core in a FSSWed aluminum alloy joint. Thus, the center of plug is termed as the fusion welding zone (FWZ). The formation of the FWZ should be attributed to the higher temperature and lower cooling rate within the center of the plug, where most welding current primarily flows through between the upper and lower electrodes because of its shortest distance. The FWZ with a relatively smaller area is limited by the exit-hole. At the early stage of charging for the repairing process, a fusion joining occurs immediately, the contact resistance disappears instantly, and the boundary between the plug and the exit-hole undergoes metallurgical bonding, which forms a good movement channel of the current and heat. By adjusting the processing parameters and the shape of the plug, the formation time of the FWZ could be precisely controlled, which is the significant factor in performing exit-hole refill using the resistance heat. An obvious interface can be observed in Fig. 7(b) (location 2 in Fig. 6(d)) and Fig. 7(c) (location 3 in Fig. 6(d)), and the black zone is characterized by the fine equiaxed grains, which is derived from the initial exit-hole microstructure, while the grey zone is the initial plug where no fusion occurrs during the FEBRW process. It should be noted that no defects can be found at this interface. Therefore, it could be inferred that a robust diffusion bonding exists at this interface under the coupling effect of resistance heat and pressure during the repairing process. The microstructure of the nugget zone edge (location 4 in Fig. 6(d)), similar to that of fusion zone, is shown in Fig. 7(d). Partial recrystallized fine grains in the exit-hole were retained after the repairing process.

Fig. 7.  OM microstructures of different zones in Fig. 6(d): (a) location 1; (b) location 2; (c) location 3; (d) location 4.

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The EBSD orientation maps of the enlarged location 1, location 2, location 3, and location 4 in Fig. 6(d) are shown in Fig. 8. Seen from Fig. 8(a), the grain size of the nugget zone is highly inhomogeneous due to the solidification process. Fig. 8(b) and (c) illustrates the grain morphology of the interface between the plug and exit-hole wall. The grain size of the plug is much larger than that of the exit-hole wall. Fig. 8(d) presents the microstructure of the interface between the nugget zone and exit-hole wall, which is consist of columnar crystal and fine recrystallized grains. The direction of the columnar crystal is parallel to the cooling direction.

Fig. 8.  EBSD orientation maps of different zones in Fig. 6(d): (a) enlarged location 1; (b) enlarged location 2; (c) enlarged location 3; (d) enlarged location 4. TD: transverse direction; WD: welding direction; ND: normal direction.

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Fig. 9 shows the recrystallized (blue), substructured (yellow), and deformed grain (red) fractions of the enlarged location 1, location 2, location 3, and location 4 in Fig. 6(d). The fraction values are listed in Table 2. The recrystallized fraction of location 3 is much lower than those of the other three locations. Different thermal mechanical coupling extents are conducted on different zones of the repaired joint. For location 3, far from the nugget zone, mechanical force is more than heat, resulting in large numbers of substructures in this zone compared with location 2 (Fig. 9(b)). In the case of location 4, at the interface between the nugget zone and exit-hole, more heat is introduced compared with the other three locations, inducing lower substructures.

Fig. 9.  EBSD recrystallization fraction maps of different zones in Fig. 6(d): (a) enlarged location 1; (b) enlarged location 2; (c) enlarged location 3; (d) enlarged location 4.

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Table  2.  Recrystallization fraction values of different enlarged zones in Fig. 6(d)

Location	Recrystallized / %	Substructured / %	Deformed / %
1	11.8	86.3	1.9
2	10.8	85.5	3.7
3	3.0	93.8	3.2
4	34.9	64.4	0.7

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Fig. 10 presents the Kernel average misorientation (KAM) or local misorientation maps of the enlarged location 1, location 2, location 3, and location 4, which can be used to evaluate the average misorientation between a specified point and its nearest neighboring points in the same grain [23–25]. The local misorientation values of location 4 are much lower than those of location 2 and location 3, which should be attributed to the higher heat conducted during the FEBRW process. It is noted that the local misorientation values of location 1 are also higher than that of location 4, which is induced by the shrinkage effect during solidification of the nugget zone. The local misorientation/grain orientation spread (GOS) value distributions of these mentioned 4 locations are illustrated in Fig. 11. The most local misorientation values are in the range of 0.3°–1.2°. There are no obvious differences for location 1 to location 3. In the case of location 4, most of the local misorientation values are nearly 0.4°, which are much lower than those of the other cases.

Fig. 10.  Grain orientation spread maps of different zones in Fig. 6(d): (a) enlarged location 1; (b) enlarged location 2; (c) enlarged location 3; (d) enlarged location 4.

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Fig. 11.  GOS value distributions of different enlarged zones in Fig. 6(d).

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3.3   Dynamic behavior of the repairing process

The resistance welding heat can be expressed as Eq. (1):

where ${Q}_{\mathrm{F}}$ is the effective heat of the joint, which is one of the main factors determined by the connection effect between plug and exit-hole wall; ${Q}_{\mathrm{L}}$ is the heat losing during the repairing process, which is intimately related to the repairing pressure; I is the repairing current; t is the repairing time. ${r}_{\mathrm{B}}$ is the body resistance of the plug to the exit-hole wall in the path of the repairing current. Before the plug and exit-hole fully pressed, the value of ${r}_{\mathrm{B}}$ is basically constant. ${r}_{\mathrm{C}}$ is the contact resistance between the plug and the exit-hole wall. The value of ${r}_{\mathrm{C}}$ would be changed with the change of the pressing state between the plug and exit-hole unless the plug is fully pressed into the exit-hole. Therefore, the repairing pressure simultaneously affects the heat dissipation efficiency and the pressing state between the plug and the exit-hole. The repairing pressure becomes a dynamic variation due to the influence of the inertia force of the upper electrode and its attached pressurizing mechanism.

The dynamic behavior of the force is shown in Fig. 12. An obvious damping and rising oscillation phenomenon occurs. This indicates that the upper electrode acts on the upper surface of the electrode and presses down together with the plug, which makes the plug soften and deformed under the action of the contact resistance heat. It is inferred that the softened extent and plug pressing amount gradually decrease due to the higher current density of the contact part between plug and exit-hole wall. Then, the upper electrode loses pressure as the inertia force in the pressing down stage. After the plug softening portion completely squeezed into the exit-hole, the unconscious portion hinders the plug to be further pressed into the exit-hole. Thus, the upper electrode pressure gradually recovers, and the heat conducted in the plug is gradually accumulated resulting in the plug soften. The contact state between the plug and exit-hole wall becomes better, leading to the further deformation in the plug. As the repairing process goes on, the heat dissipation effect is gradually enhanced, while the heat accumulation rate is slowed down, reducing the soften amount of the plug. Therefore, the amplitude of the pressure curve is gradually reduced and the cycle is lengthened until a metallurgical bonding appears at the interface between plug and the exit-hole. With the increase of conductive and heat dissipation area in the joint, a dynamic balance stage occurs because of the balance between heat generation and dissipation. Then, the pressure tends to be gradually stable. At this time, most of the contact resistance between the plug and the exit-hole disappears, and the body resistance acts as the main heat source. In the end, a fusion weld and diffusion bonding are constructed in the middle and the surrounding area of the repaired joint, respectively.

Fig. 12.  Dynamic curve of repairing force.

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The metallurgical bonding of the area between the plug and the side wall appears in the last stage of the repairing process, which can basically reflect the dynamic behavior of the repairing pressure at the interface. The dynamic behavior of filling pressure is the macroscopic manifestation of the pressing process of plug. In contrast, the exit-hole wall heat dissipation is obvious. Because of low temperature, plug seriously gives out heat, leading to the poor heat dissipation and the rising of temperature. Therefore, in the process of press, the exit-hole wall has shear force on the contact area. High temperature plastic metal slips or forms, and plug is pressed into the gap of interface, which is the microscopic manifestation of the pressing process of the plug. With the continuous pressing of the plug, the bonding surface with the exit-hole wall and the current density increase, the contact resistance, the amount of extruded plastic metal, and pressing decrease. This is the reason for the oscillation behavior of the pressure curve.

Therefore, the mechanisms of FEBRW are to refill the pressure and pushed the plug into the exit-hole. Under the combined actions of contact resistance heat, body resistance heat, electrode pressure and heat dissipation of electrodes and workpieces, and the contact regions between the plug and the exit-hole wall are continuously softened-pressed, down-softened-pressed, and down-melted completely pressed to the static heating state. Finally, fully metallurgical bonding with the exit-hole wall completely repaired the exit-hole.

3.4   FEBRW process analysis

The model analysis and macrostructure evolution of the exit-hole repairing process based on FEBRW technology is presented in Fig. 13. The process of repairing exit-hole based on resistance welding could be divided into three stages, including stage 1 (plug dynamic pressing stage), stage 2 (plug press in stationary stage), and stage 3 (metallurgical bonding stage of plug and exit-hole).

Fig. 13.  Model of filling process based on FEBRW: (a) schematic diagram and (b) simulated temperature distribution of stage 1; (c) schematic diagram and (d) cross-sectional macrostructure of stage 2; (e) schematic diagram and (f) cross-sectional macrostructure of stage 3. HAZ: heat affected zone; FJZ: fusion joint zone.

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3.4.1   Plug dynamic pressing stage

In the initial stage 1, the plug is dynamically pressed into the exit-hole. The inner wall of the exit-hole has traces of threads left after FSSW and some turbulent metal welding bumps, leading to the plug cannot be smoothly pressed into the exit-hole, so the bottom circumference of the plug firstly contacts with the inner wall of the exit-hole, forming the initial contact resistance R_i (R_i is the contact resistance), which is also the main channel of the current line. Under the action of resistance welding heat, the material is rapidly melted at the beginning and softened the surrounding solid-state metal. Under the push pressure, the plug moves down and penetrates into the exit-hole, and the air at the bottom is escaped through the melting part and carries the liquid metal into the upper gap.

3.4.2   Plug pressing in stationary stage

In the stage 2, the plug is pressed into the static stage. After the initial process, the plug is completely pressed into the exit-hole, and all air is extruded side by side. The current line passes through the upper electrode, plug, interface, and the lower electrode, and accumulates a lot of resistance heat at the position of the plug body resistance and contact resistance. In the meanwhile, the middle region of the plug and the interface is far away from the upper and lower electrodes, and the heat accumulation is the most significant. Therefore, large area melting occurs in both of them, forming liquid phase similar to that of the three-dimensional structure of resistance spot welding (RSW) fusion core, namely, the interface between the plug and the exit-hole wall in this region firstly disappears. Plug upper and lower (via a thin metal layer at the bottom of the exit-hole) contact with the upper and lower electrode, respectively. As the heat loss is relatively serious, the interface is hard to melt by the accumulated heat. However, under the combined action of heat and force, the micro convex part of the rough interface contacts and deforms, the surface adsorption layer is immediately opened, the oxide film is crushed, and each micro convex is flatly extruded due to plastic deformation, so as to achieve close contact and form bonding. Under the continuous action of electrode pressure and resistance heat, the contact area between the plug and the exit-hole continues to deform and gradually expands, forming the intergranular connection. At the same time, due to the influence of dislocation and other factors, the local contact surface produces new clean convex, which is a favorable condition for the continuous formation of metal bonding at the whole interface.

3.4.3   Metallurgical bonding stage

In the stage 3, under the condition of termination of current and continuous pressure, many grains at the solid–liquid interface around the middle vacuole are in a semi-molten state, and the liquid phase in the vacuole could well wet the surface of semi-molten grains with different orientations. The temperature of the vacuole rapidly decreases, the composition undercooling is large, and a high temperature gradient is formed in front of the solidification boundary, creating conditions for the formation of the organizational structure as shown in Fig. 7(a), and finally forming the repairing core structure of solder-like structure in the middle region of the plug and exit-hole. In the area near the electrode above and below the plug and exit-hole, the temperature sharply drops to room temperature after the current terminated. The diffusion rate rapidly decreases. Moreover, the current action time is relatively short, and the atomic diffusion is not sufficient. Once the electrode pressure is cancelled, the diffusion process almost stops. Under the conditions of appropriate filling process parameters, metallurgical bonding could be formed at the bonding interface between the plug and the exit-hole, but the migration process is absent diffusing from interface to the depth direction, making a large difference in grain sizes on both sides of the bonding surface as shown in Fig. 7(d). Therefore, it is defined as the shallow diffusion junction zone (DJZ).

4.   Conclusions

Based on the macro and microstructure evolution, dynamic force behaviors, and repairing stages division of the repaired joints by FEBRW, the following main conclusions could be drawn:

(1) In the FSSWed joint with exit-hole repaired by FEBRW, fusion bonding and shallow diffusion bonding are the repairing mechanisms. The fusion bonding area is in the middle of the joint, and the shallow diffusion connection area is in the upper and bottom of the repaired joint.

(2) The technical mechanism of FEBRW is that the repairing pressure pushes the plug into the exit-hole. The plug undergoes the repeated process of softening and pressing down, finally fully metallurgical bonding with the exit-hole wall, and the exit-hole is completely repaired by the plug.

(3) The pressure monitoring device based on the piezoelectric effect of quartz crystal could effectively reflect the dynamic behaviors of the plug pressed into the exit-hole.

(4) Under the conditions of repairing current of 37 kA, repairing pressure of 12 kN, and repairing time of 200 ms, the contact surface between the plug and the exit-hole wall disappeared and achieved completely metallurgical bonding. The highest tensile shear strength, 7.43 kN, were obtained.

(5) The whole repairing process could be divided into three stages, which are the plug dynamic pressing stage, the plug pressing in stationary stage, and the metallurgical bonding stage.

Acknowledgement

This work was financially supported by the National Natural Science Foundation of China (No. 51874179).

Conflict of Interest

The authors declare no competing interest.