Open access peer-reviewed chapter

Study on Microstructure Evolution and Mechanical Properties of Al5083 Joint Obtained from Friction Stir Spot Welding: Effect of Vibration and Plunge Depth

Written By

Behrouz Bagheri, Mahmoud Abbasi and Farzaneh Sharifi

Submitted: 15 November 2021 Reviewed: 19 December 2021 Published: 23 June 2022

DOI: 10.5772/intechopen.102082

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Engineering Principles - Welding and Residual Stresses

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Abstract

In this investigation, the vibration of the workpiece is accompanied by the rotating movement of the tool during friction stir spot welding. The method is entitled to friction stir spot vibration welding (FSSVW). Al5083 alloy samples are joined by two welding methods, friction stir spot welding (FSSW), and FSSVW under different plunge depths (DP). The microstructures and fracture surface of the welded zones were analyzed by optical microscopy (OM) and scanning electron microscopy (SEM), respectively. Analyzing the microstructures and mechanical properties of welded samples in both methods revealed that weld region grain size reduced and its hardness increased as the mechanical vibration and high plunge depth have been applied. In addition, the strength and ductility values of FSSV welded specimens with high plunge depth were higher than those produced by FSSW under low plunge depth. Furthermore, it was concluded that the effect of vibration on microstructure and mechanical properties of welded specimens increase as vibration frequency is increased.

Keywords

  • friction stir spot welding
  • mechanical vibration
  • mechanical properties
  • microstructure
  • grain size

1. Introduction

The welding process is a significant issue in metal industrial applications such as automotive, aerospace, electronics, and medical functions. Friction stir spot welding (FSSW) is known as a solid-state metal joining process that is applied to join two dissimilar metal plates. It is most often utilized when the combination of metals through fusion welding is not applicable. In this process, a welding tool, including a shoulder and a pin, is rotated, and then contact with workpieces is going to be joined. This process includes three main steps: plunging, stirring, and drawing out [1, 2, 3]. In the plunging step, the rotating tool with the pin plunges and penetrates the joining workpieces until the shoulder contacts the surface of the up workpiece and reaches the desired depth. Due to friction between the tool and workpiece heat is generated. The heated and softened material around the pin deforms plastically in the stirring step and the two workpieces are mixed. In the drawn-out step, the solid-state bonding between the upper and lower workpieces is achieved. The most important parameters of FSSW include rotational speed (RS) of the tool, dwell time, plunging rate, and plunge depth. These parameters are the key parameters for the size of the stirred zone during welding and weld outcome.

Several welding procedures and several applications for FSSW have been reported [4, 5, 6, 7, 8]. Czechowski [9] studied the effect of corrosion cracking on various aluminum alloys during the friction stir welding process. The heat generation analysis in the FSSW process of aluminum alloys was carried out by Awang and Muncino [10]. The results showed that most of the heat (around 96.84%) was produced from friction at the interface of the tool and workpiece. Matori et al. [11] investigated the corrosion properties of AA6061-T6 joint obtained using FSSW. The effect of pin length on the FSSW process for dissimilar aluminum and steel joints was studied by Piccini and Svoboda [12]. It was found that the maximum tool load value increases as tool penetration depth increases and the pin length decreases. Mechanical properties of the weld produced between AA6082-T6 sheets via FSSW were investigated by Aydin et al. [13]. The results showed that the tensile shear load increased almost linearly with increasing plunge depth. Jedrasiak et al. [14] determined the thermal modeling of FSSW for Al-Al and Al-steel joints. It was reported that the results gave a noticeable quantitative prediction of the radial diversity of the thickness of the intermetallic layer. FSSW for automotive applications was studied by Capar et al. [15].

Chevan and Shete [16] studied the optimization of FSSW parameters by using of Artificial Neural Network (ANN). It was found that the FSSW provided the maximum lap shear strength of 3.749 N/mm2 on the tool rotation speed of 900 rpm and dwell time of 30 sec for the taper cylindrical pin. Astarita et al. [17] studied the stress corrosion behavior of joints made by FSSW between aluminum alloys for aeronautic applications. It was reported that the weld area has lower resistance to intergranular and pitting corrosion compared to other areas. Shekhawat and Nadakuduru [18] analyzed the bonding zones during the FSSW in underwater and normal conditions for Al6061-T6 alloys.

Rostamiyan et al. [19] mixed two welding methods, including FSSW, and ultrasonic welding to improve the weld quality. In their research, FSSW was performed by the ultrasonic vibration of the tool. The impact of process parameters namely vibration, tool rotary speed, tool plunge depth, and dwell time on mechanical properties such as lap-shear force and hardness were examined. It was reported that introduce of vibration enhanced the lap shear force and hardness. Ji et al. [20] introduced the “ultrasonic-assisted friction stir spot welding” (UAFSSW) technique. This process was employed to join dissimilar AZ31and AA6061alloy sheets. It was shown that ultrasonic vibration was significant for the upward flow of the bottom plate and to get a flawless joint. It was found also that the existence of ultrasonic vibration improved the stir zone width and led to finer grains in the stir zone.

In this research, a new method to increase the efficiency of FSSW is presented. The workpiece is vibrated normally to the tool plunge path while the tool is rotated. This new method is entitled friction stir spot vibration welding (FSSVW). Mechanical properties of friction stir spot vibration (FSSV) welded samples such as hardness, fracture surface, and shear strength are compared with those from friction stir spot (FSS) welded samples.

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2. Materials and process

Workpieces with 25 mm width and 100 mm length are prepared from joining sheets. The chemical composition and mechanical properties of the studied sheet are presented in Tables 1 and 2, respectively. The workpieces in a lap position are installed on the machine, designed for doing the FSSW and FSSVW, while the thin one is on top. The schematic view of this machine is presented in Figure 1. The machine is fixed on the milling machine table.

SiMnCrCuMgFeAl
0.50.40.250.14.80.4Bal

Table 1.

Chemical composition of base metal (wt%).

Ys (MPa)UTS (MPa)Elongation %E (GPa)Hardness (V)
184307157194

Table 2.

Mechanical properties of base metal.

Figure 1.

Schematic view of the machine designed and manufactured for FSSVW.

According to Figure 1, the workpiece vibration is applied through a fixture. The motor shaft rotation is transformed into the linear and reciprocating movement of the fixture by a camshaft mechanism. The vibration amplitude is adjusted to be 0.5 mm. An AC motor with 0.5 kW power is used for FSSVW. Motor shaft speed which controls the vibration frequency is adjusted using a driver. A non-consumable tool (Figure 2), consisting of the pin, from carbide tungsten, and shoulder, from M2 heat-treated steel, is used for welding processes. The welding tool was rotated in the clockwise direction during the welding process.

Figure 2.

a) the geometry and, b) the design of pin and shoulder utilized for FSSW and FSSVW processes.

During the FSSVW process, different vibration frequencies are implemented to investigate the effect of vibration frequency on microstructure and mechanical characteristics. Table 3 shows the welding condition for different welded samples.

ParameterVibrationVibration frequency (cycles/s)Rotational speed (rpm)plunge depth (mm)
Vibration+ (state 1)
− (state 1)
+ (state 2)
− (state 2)
42

42
-
1500
1500
1500
1500
2
2
2.5
2.5
Frequency+
+
+
25
42
55
1150
1150
1150
2.5
2.5
2.5

Table 3.

Welding conditions for different welding trials (+ donates FSSVW and − donates FSSW).

Metallography techniques based on ASTM are used to reveal the microstructure of weld regions. Mounted samples were ground by rotating discs of abrasive paper of silicon carbide and then were polished and etched. The linear intercept method (ASTM-E112-13) is applied to measure the grain size. Shear tensile test according to ANSI/AWS/SAE/D8.9-97 is used to obtain lap shear strength-displacement curves of welded specimens. For each welding condition, four specimens are tried. Vickers micro-hardness method based on ASTM-E384 is applied to assess the hardness. For hardness tests, the load is 300 gf and the dwell time is 10 s. For each welding condition, 4 data are measured.

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3. Discussion and results

3.1 Microstructure

Figure 3 shows the microstructures of welded samples relating to different welding conditions, according to Table 3. According to Figure 3, different weld region zones, namely, stir zone (SZ), thermo-mechanically affected zone (TMAZ) and heat-affected zone (HAZ) are also observed for FSSV welded specimens, as well as FSS welded specimens. The microstructures of the stir zone for FSS and FSSV welded specimens are shown in Figure 4. It is obvious that the presence of vibration during welding reduces the grain size of the stir zone for both rotational speeds; additionally, grain sizes of samples welded samples with high plunge depth are lower than those welded by low plunge depth. These can be related to the effect of plastic deformation on dislocation production in metals.

Figure 3.

Microstructures of TMAZ, HAZ, and SZ of different samples, (a, c) FSSVW and (b, d) FSSW (a and b relate to the welding situation 1 of Table 3, c and d relate to the welding situation 2 of Table 3).

Figure 4.

SZ microstructure of FSS (a) and FSSV (b) welded samples (welding situations 2 of Table 3).

High plunge depth or presence of vibration increases the plastic deformation. Studies have noted that dislocation density increases as plastic deformation increases. As dynamic recrystallization (DRX) is the main reason for grain refinement during FSW [21, 22], an increase of dislocation density leads to enhanced DRX and correspondingly, finer grains are developed.

Figure 5 shows the stir zone grain size values for different welding conditions. It is observed that the stir zone grain size for all welded specimens is lower than the base metal grain size. Additionally, Figure 5 shows that FSSV welded specimens have lower grain sizes for FSS welded specimens. Based on to Kaibyshev [23], the microstructure modification during severe plastic deformation includes two consecutive processes: (i) the formation of three-dimensional arrays of low angle boundaries (LABs) and (ii) the gradual transformation of LABs into high angle boundaries (HABs) (≥15°). LABs with low misorientation (~1°) are constantly formed in pure aluminum and its alloys by dynamic recovery during deformation by rearrangement of accumulating lattice dislocations (Figure 6a). At high strain values, mobile dislocations migrate across sub-grains and are trapped by sub-boundaries increasing their misorientation. Extensive rotation of sub-grains leads to increasing misorientation of LABs with strain within sub-grains. These processes result in the formation of individual segments of HABs, and this can be considered as proof for the occurrence of dynamic recrystallization (Figure 6b). The recrystallized grains persistently replace sub-grains evolved at small strains through the continuous transformation of their boundaries, and accordingly, grain size refinement occurs [24].

Figure 5.

Stir zone grain size results for different samples welded by different welding conditions (welding factors values were based on Table 3; (−) and (+) signs indicate non-presence and presence of vibration, respectively).

Figure 6.

Schematic illustration of dynamic recrystallization: A dynamic recovery and formation of LABs and b grain size refinement due to gradual transformation of LABs into HABs [23].

3.2 Mechanical characteristics

Shear strength curves of different welded specimens are presented in Figure 7. According to Figure 7, samples welded using the FSSVW method have higher strength compared to samples welded using the FSSW method, and additionally, maximum shear load increases as plunge depth increases. It was observed (Figure 3) that the presence of vibration, decreases the grain size. As grain size decreases, the volume fraction of grain boundaries increases, and the movement of dislocations decreases. According to the Hall–Petch equation (σ = σ0 + kD−1/2) [25], strength (σ) increases as grain size (d) decreases. Additionally, as plunge depth increases, more volume fraction of material enters within the stir zone and more mixing of up and down workpieces is carried out in the weld area and this leads to more strength of the weld.

Figure 7.

Lap-shear strength curves of FSS and FSSV welded specimens.

Fracture surfaces of FSS and FSSV welded specimens, after the shear test, are seen in Figure 8. According to Figure 8, fracture surfaces of all specimens show dimples. The presence of dimples is characteristic of ductile fracture surfaces [26]. It is known that during the straining of ductile materials, voids form within the microstructure, and as straining proceeds, voids coalescence and grow. These voids are responsible for the constitution of dimples [27, 28]. It is observed in Figure 8 that dimples for FSSV welded specimens are smaller than those observed in FSS welded specimens and dimples for specimens welded under high plunge depth are smaller than those constituted in specimens welded under low plunge depth.

Figure 8.

SEM fracture surface of a) FSV welded sample with PD: 2 mm, b) FSVS welded sample with PD: 2 mm, c) FSS welded sample with PD: 2.5 mm and d) FSV welded sample with PD: 2.5 mm.

Generally, less ductile metals show dimples with larger sizes and fracture occurs at lower values of strain [26]. Correspondingly, more ductility is predicted for FSSV welded specimens compared to FSS welded specimens. Additionally, more ductility is anticipated for specimens welded with higher plunge depth compared to those welded with lower plunge depth. These predictions are in agreement with the results presented in Figure 7. It is obvious in Figure 7 that displacement at maximum load for FSSV welded specimens are higher than that for FSS welded specimens and this variable increases as plunge depth increases.

Figure 9 shows the hardness values of different weld zones of FSS and FSSV welded specimens. Although, the average grain size in the SZ is smaller than the BM, the microhardness values of the SZ are lower than the BM. It can be explained by the existence of two competing phenomena. First, the reduction in the average grain size induced by DRX results from severe plastic deformation which contributes to the increase in the microhardness. Second, the dissolution of the iron-rich phases and the precipitates resulting from intense mixing under severe plastic deformation and the high temperature contributes to the softening of the material. These two competing mechanisms have a strong influence on the final mechanical properties of the different zones and the entire weld. Based on Figure 9, hardness values of SZ and TMAZ regions for FSSV welded specimens are higher than those relating to FSS welded specimens. Additionally, Figure 9 shows that the hardness value increases as plunge depth increases. These can be related to the effect of grain size refinement as vibration is applied and plunge depth increases. It was observed (Figure 3) that the presence of vibration and increase of plunge depth both result in more grain refinement. As grain size decreases, the impediment to dislocations movement enhances, and strength and hardness increase. Grain size refinement is known as a strengthening mechanism [29].

Figure 9.

Micro-hardness values of various zones of FSSW and FSSVW samples: a) SZ, b) TMAZ, and c) HAZ (− and + sign indicate without and with vibration, respectively).

3.3 Effect of vibration frequency

Figure 10 shows the shear strength curves of various FSSV welded specimens and Figure 11 shows the SZ hardness values of these specimens. For all of these specimens, the welding conditions are the same but the vibration frequency is different. According to Figure 10, the maximum shear strength increases as vibration frequency increases. It should be mentioned that DRX is the main mechanism for grain refinement during FSSW [30]. As vibration frequency increases, more strain is applied to the material within the stir zone. It has been known that dislocation density increases as straining increases [31, 32, 33]. Higher dislocation density leads to more DRX and correspondingly, finer grains are developed and higher strength and hardness are obtained.

Figure 10.

Shear strength curves of FSSV welded samples with various vibration frequencies (welding factor values were based on Table 3).

Figure 11.

Stir zone micro-hardness values of FSSV welded samples with various vibration frequencies (welding factor values were based on Table 3).

The fracture surfaces of the base metal and FSSV welded specimens with various vibration frequencies are presented in Figure 12. Dimples, which are characteristics of ductile fracture surfaces, are seen in fracture surfaces of all specimens. Figure 12 shows that dimples for base material are the largest and dimple size decreases as vibration frequency increases. This can be related to the effect of vibration frequency on grain size refinement. Barooni et al. [34] found that SZ grain size decreases as vibration frequency increases. Voids, which are responsible for the constitution of dimples in the fracture surface of ductile materials, form around the second phase particles and inclusions as well as dislocation locks. As deformation proceeds, the voids grow and coalescence of them form large voids. Grain boundaries act as barriers to the growth of voids. As grain size decreases, more nuclei for void formation are constituted and on the other hand, the voids cannot grow large and the voids are smaller. In Figure 12, the smallest voids are seen for specimens welded with the highest vibration frequency.

Figure 12.

SEM fracture surface of FSSV welded specimens with different vibration frequencies a) 55 Hz, b) 42 Hz, c) 25 Hz, and d) BM.

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4. Conclusion

In this study, the microstructure and mechanical properties of the Al5083 joint fabricated by FSSW and FSSVW methods were compared. The results showed that the presence of vibration affected the microstructure and led to more grain refinement. The results also indicated that:

  1. The grain size of the stir zone and thermo-mechanically affected zone decreased as the vibration was applied during FSSW.

  2. The shear strength and ductility of FSSV welded specimens were higher than those relating to FSS welded specimens.

  3. Fracture surfaces of all specimens showed that dimples for FSSV welded specimens with high plunge depth are smaller than those observed in FSS welded specimens with low plunge depth.

  4. Shear strength and hardness values of the FSSV welded specimen increased as vibration frequency increased.

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Written By

Behrouz Bagheri, Mahmoud Abbasi and Farzaneh Sharifi

Submitted: 15 November 2021 Reviewed: 19 December 2021 Published: 23 June 2022