Abstract
The microstructure of nano-SiC enhancing friction stir spot welding (FSSW) joint with dwell time of 3 s was characterised by an onion ring structure which consisted of alternate SiC-free zones and SiC-rich zones where SiC particles refined the grains. However, onion ring structure disappeared and SiC particles dispersed homogeneously when dwell time was 5 s. The microhardness of stir zone (SZ) and tensile shear load of SiC enhancing FSSW joint were higher than those of conventional FSSW joints. After heat treatment at 200°C for an hour, grains of the SZ grew substantially and coursed reduction in mechanical properties of joints, while grain size of SZ and tensile shear load of SiC enhancing joint was invariant but the microhardness of SZ increased.
Introduction
Magnesium alloys attract global attention because of their advantageous attributes such as low density, high specific strength, good castability and good thermal conductivity.1–3 Therefore, they are regarded as the most promising candidates to substitute for the aluminium alloys and carbon steels in the automotive industry to reduce the weight of automobiles. However, the high chemical reactivity of magnesium alloys limits the application of conventional fusion welding techniques to fabricate magnesium alloy structural components. In recent years, friction stir welding (FSW) technology, invented by TWI, UK, was applied to weld the butted plates. As a solid-state welding technology, FSW achieves the joints of good quality by microstructural refinement.4–6 Based on the FSW, friction stir spot welding (FSSW) technique was drawn up and used to weld the overlapped plates. It is operated by plunging a rotating welding tool into the workpiece and then extracting tool after a dwell time. During FSSW, tool penetration and the dwell period essentially determine the heat generation, material plasticisation around the pin, weld geometry and therefore mechanical properties of the welded joint. 7 Compared with FSW, FSSW is transient welding process due to its relatively short welding cycle.8,9 Therefore, the heat input during FSSW is relatively lower, which reduces the deformability of magnesium alloys. Moreover, the effective thermal conductivity of magnesium alloys also decreases the heat input, which further deteriorates its deformability. Hence, the poor deformability of magnesium alloys inhibits an extensive application of FSSW in magnesium alloys joining. Recently, some literature had reported solving this crucial issue by increasing the downward force, extending the dwell time or adding external heat assistance.9–12 Tran et al. 13 pointed out that the strength increased with increasing tool dwell time in AA5754 and AA7075 friction stir spot welds. These methods have improved the deformability of magnesium alloys considerably. However, because the deformed magnesium alloys with high stored energy were highly susceptible to the heat input, the excessive heat input coarsened the grain microstructure of the welds, which weakened the static strength of the FSSW joints.
Therefore, in the present paper, nano-SiC particles with an average size of 40 nm were introduced to the FSSW AZ31 magnesium alloy joints to refine the grains of stir zone (SZ). The nano-SiC particle enhancing FSSW (namely, NPE-FSSW) process was carried out using a threaded welding tool to stir the nano-SiC particle powder embedded in the plates. The effects of nano-SiC particles on the microstructure and mechanical properties were investigated by microstructural examination, tensile shear and microhardness tests.
Experiments
Hot-extruded AZ31 magnesium alloy plates commercially available were used for FSSW experiment. Machined plates were dimensioned in 100 mm × 22 mm × 2 mm. All the specimens were welded in lap configuration a 40 mm × 22 mm overlap area. The FSSW process was applied in the centre of the overlap area. Prior to welding, the surfaces of all the specimens were polished slightly by diamond powders and then they were cleaned with acetone in order to eliminate impurities. Then conventional FSSW was carried out with no nano-SiC particles addition as shown in Fig. 1a. As to NPE-FSSW, a rectangular hole with a dimension of 1.4 mm × 1.5 mm was machined (seen in Fig. 1b) to prestore the nano-SiC particle (Fig. 2). All specimens were welded by a right-hand threaded H13 welding tool which possessed a flat shoulder of 10 mm in diameter and a pin of 4 mm in diameter and 2.5 mm in length. The plunge depth of tool penetration was 2.8 mm. The welding parameters were listed in Table 1. The rotating direction was counterclockwise.
Schematic illustration of FSSW and NPE-FSSW processes Nano-SiC particle powder and EDS analytical result of SiC Welding parameters used in FSSW and NPE-FSSW tests
After welding tests, both the FSSW and NPE-FSSW joints were heated in the electric furnace for an hour at 200°C. The specimens with a length of 22 mm and a width of 10 mm for metallographic study were cross-sectioned through the centreline of the spot welds by a numerically controlled linear cutting machine. Then these specimens were prepared by using a standard metallographic procedure. The lap-shear tensile tests (as showed in Fig. 3) on the room temperature using a tensile machine (SANA-XUA305C) with a tensile speed of 1 mm min−1. The microhardness tests were operated by a Vickers hardness tester (V-1000) with a period of 20 s, a load of 100 g and a step size of 0.25 mm. Three welded specimens under the same welding condition were used for above tests and the average values of them were adopted.
Schematic illustration of tensile test configuration (mm)
Results and discussion
Microstructures
Figure 4a displays a typical cross-section of the spot weld. The upper and lower sheets are compressed together during the plunging action of the pin resulted in a joint interface.
The macrostructure of FSSW welds at low magnification a, the basemetal/HAZ/TMAZ/stir zone b
This action also displaces a portion of the upper magnesium sheet decreased its thickness.
Figure 4b shows the optical images of basemetal/heat affected zone (HAZ)/thermal-mechanically affected zone (TMAZ) at high magnification.
Figure 5 shows the optical micrographs of the NPE-FSSW-1 joint, typical onion ring structure (a) and alternate layers (b) which is a magnified graph for onion ring structure. The onion ring structure consists of alternating layers which are composed of different microstructure and chemical constituent14,15 as shown in Fig. 5a. From Fig. 5b, it can be seen that the microstructure of alternate layers is non-uniform. The layers contain alternate fine grain zones and coarse grain zones. The average grain size in fine grain zones is apparently smaller than that in coarse grain zones.
Optical micrographs of NPE-FSSW-1 joint: typical onion ring structure a and alternate layers b
Figure 6 displays the SEM micrographs of alternate layers. Figure 7 is EDS analytical result for the white dot in Fig. 6. The result stated that the white particles were exactly the nano-SiC particles. In Fig. 6a, the SiC-rich zone (fine grain zone) contained plenty of SiC particles, corresponding to the fine grain zone. By contrary, the SiC-free zone (coarse grain zone) contained almost no particles. The SiC-free zone is composed of coarse equiaxed grains without SiC particles in it (as shown in Fig. 6b). Comparatively, Fig. 6c shows the grains in the SiC-rich zone were much finer and the SiC particles distributed non-uniformly along the grain boundaries and inside the grains.
SEM micrographs of alternate layers, alternate layers a, SiC-free zone b and SiC-rich zone c EDS analytical result for the white dot in Fig. 4c
Figure 8 shows the optical micrographs of NPE-FSSW-2 joint with the dwell time of 5 s. With the extension of dwell time, the onion ring structure was eliminated and the microstructure of the SZ was homogenised as seen in Fig. 8a. In Fig. 8b, it can be seen that the grain microstructure in the SZ was tiny and uniform. From Fig. 8c, it also can be seen in the uniform fine grain (0.5 μm) microstructure. Compared with NPE-FSSW-1 joint, the SiC particles in the SZ of NPE-FSSW-2 joint homogeneously distributed along the grain boundaries and inside the grains.
Optical micrographs of NPE-FSSW-2 joint: stir zone a and microstructure of stir zone b and the distribution of SiC particles in stir zone of NPE-FSSW-2
Figure 9 displays the comparison on microstructure of the base material (a), SZ of FSSW joint (b) and SZ of the NPE-FSSW-2 joint (c). The base material was characterised by equiaxed grains, while the SZ of FSSW joint featured finer and uniform grain microstructure. This is because dynamic recrystallisation occurring during welding process refined the grain microstructure. From Fig. 9c, it can be noted that the grains in the SZ of NPE-FSSW-2 joint are tiny and finest (the grain size is evaluated to approximately 3.6 ± 0.4 um). This is attributed to both dynamic recrystallisation and SiC particles in the SZ.
Comparison on microstructures of base material a, stir zone of FSSW joint b and stir zone of NPE-FSSW-2 joint c
Su et al. and Gerlich et al.14–16 had pointed out that the formation of onion ring structure was attributed to material flow movement in the SZ during welding process. When the welding tool was plunged into the upper plate, the SiC particles under the probe pin were mixed with magnesium alloy material to form the composite. In the case of the tool was penetrating into the bottom plate, the magnesium alloy material belong to the upper plate was moving downward to form a magnesium alloy lamella, which was corresponding to the SiC-free zone. And the bottom composite was moving upward and outward to form a composite lamella which was corresponding to the SiC-rich zone. Therefore, a ribbon of these two contiguous lamellas was developed. During the dwell time, the ribbon moved downward via the thread of the rotating pin. When the ribbon contacted the bottom, it was discharged from the thread, then moved outward and upward before moving back to the tool periphery and downward again, so a vertical helical rotational flow was generated. As the procedure was repeated the onion ring structure was constructed as shown in Fig. 5a. With the dwell time extended to 5 s, the particles distributed uniformly in the whole SZ and thus onion structure disappeared.
FSSW is known as a severe plastic deformation process. The dynamic recrystallisation frequently takes place during FSSW and hence the grains are refined.13,17 As for the materials containing particles, it was believed that the particles had three effects on the dynamic recrystallisation of the matrix metals. 18 First, the particles may produce an increase in the stored energy and the driving force for the recrystallisation. Second, the sites where the deformation is heterogeneous because of the particles are preferable places for the recrystallisation nucleation. This effect was named particle stimulated nucleation (PSN). 19 Third, particles have a pinning effect on the grain boundary and thus inhibit the growth of grain boundary. Some literature19–21 had reported that PSN occurred not only during static recrystallisation of alloys containing hard particles but also during dynamic recrystallisation. 21 In present research, the fine grains were supposed to be induced by the pinning effect of SiC particle and PSN. As indicated in Fig. 6c and Fig. 8c, the SiC particles inside the grains provided beneficial places for the recrystallisation nucleation and the SiC particles distributing along the grain boundaries played a role of barrier to the migration of grain boundary when the grains grew.
Figure 10 demonstrates the impacts of heat treatment on the microstructure. After heat treatment, the grain size of BM did not change, while the grains of SZ of FSSW joint grew significantly (as seen in Table 2). This is because a high energy stored in the SZ of FSSW joints stimulated the grains to grow abnormally. However, although a higher energy was stored in the SZ of the NPE-FSSW joints, the grains were kept invariant due to the pinning effect of SiC particles.
Optical micrographs after the heat treatment: BM a, stir zone of FSSW joint b and stir zone of NPE-FSSW (NPE-FSSW-2) joint Grain size in the SZ of the FSSW and NPE-FSSW welded joint with post-heat treatment
Mechanical properties
The tensile shear loads of FSSW and NPE-FSSW (NPE-FSSW-2) joints are plotted in Fig. 11a, respectively. The value of tensile shear load of the NPE-FSSW joint is 3117 N, larger than that of the FSSW joint (2562 N). In this study, the enhancement in the strength of the FSSW/NPE-FSSW weld was mainly attributed to the fine grain size (as seen in Table 2) and homogeneous distribution of the refined SiC particles. According to Habibnejad-Korayem et al.,22,23 the effect of the single particles on the strengthening was concluded by:
The comparison on the tensile shear strength between FSSW and NEP-FSSW joints a and the transverse microhardness distribution at the distance of 1.5 mm from the surface of upper plate b
induced by the load transfer from the matrix to the particles
24
; strengthening 
induced by the difference of coefficient of thermal expansion (CTE) of particles and matrix
25
; strengthening 
related to the generation of the dislocations because of the geometric requirements during deformation to the particles.
24
The contribution of grain refinement in magnesium alloys for strengthens were discussed based on the Hall–Petch formula:
is the grain sizes of α-Mg, 
is the parameter that describes the relative strengthening contributions of grain boundaries.
Figure 11b depicts the typical hardness profiles across the FSSW and NPE-FSSW joints 1.5 mm below the surface of the upper plate. Three characteristic zones SZ, TMAZ and HAZ were developed in sequence from the periphery of the keyhole to the base material. Obviously, the value of microhardness in the stir zone is the highest and that in the HAZ is the lowest. As for these two kinds of joints, the microhardness difference in HAZ and TMAZ is negligible. But in stir zone, the difference is very significant. These superior mechanical properties of the NPE-FSSW joint were due to two factors: one was the grain boundary strengthening which resulted from finer grains; another was Orowan strengthening induced by the interaction between SiC particle and magnesium matrix due to their incoherent interface.
After the heat treatment, it can be observed that both tensile shear load and microhardness of stir zone of FSSW joint were reduced remarkably. This is caused by the grain coarsening. Comparatively, the heat treatment had almost no evident impact on the tensile shear load of NPE-FSSW joint and the heat treatment increased the microhardness of stir zone of NPE-FSSW joint considerably. Bon et al. 26 and Zhang et al. 27 had reported that the high stored energy and intense deformation would promote the precipitation process during aging. Therefore, the abnormal phenomenon of increasing microhardness can be attributed to the precipitates.
Conclusions
In this study, nano-SiC particles were introduced into the FSSW welded joint. The conclusions were summarised as follows:
When the dwell time was 3 s, a typical onion ring structure was formed in the stir zone, which was composed of alternate SiC-free zones and SiC-rich zones. The grain microstructure in SiC-rich zones was finer than that of SiC-free zones due to the pinning effect and PSN of SiC particles. In case of the dwell time extended to 5 s, the onion ring structure was eliminated and the SiC particles distributed uniformly along the grain boundaries and inside the grains. Both tensile shear load and microhardness of stir zone of NPE-FSSW joints were higher than those of FSSW joints by grain boundary strengthening and Orowan strengthening. The grain coarsening of stir zone of FSSW joints caused by the heat treatment led to a decrease in the tensile shear load and microhardness. By contrast, there was no evident change in the grain size of the stir zone and tensile shear load of NPE-FSSW joints after the heat treatment, but the microhardness of stir zone increased abnormally for the precipitate.
Footnotes
Acknowledgements
This project is supported by a National Natural Science Foundation of China (CN) (Grant No. 51375511), a Fundamental Research Funds for the Central Universities of China P R China (Project No. CDJZR14130008) and a Chongqing Research Program of Basic Research and Frontier Technology (Project No. cstc2016jcyjA0167).