An Investigation into Microstructures and Mechanical Properties of 1060 Pure Aluminum during Submerged Friction Stir Processing at a High Rotating Speed

Abstract

In this work, 1060 pure aluminum was subjected to high rotating speed submerged friction stir processing (HRS-SFSP). The heat cycle curve of the processing area was measured by K-type thermocouple and temperature recorder. The microstructure, grain size, texture, and tensile fracture of the processing area were analyzed by electron backscattered diffraction and scanning electron microscopy. The results show that the HRS-SFSP caused severe plastic deformation of 1060 aluminum and produced fine recrystallized grains. The minimum average grain size was 0.686 μm at the 2-pass. In addition, the dislocation density in the stirred region was greatly reduced and the high angle grain boundaries (HAGBs) were dominant. The texture strength of pure aluminum increased with the increase in processing passes. The maximum hardness of 66.3 HV and ultimate tensile strength of 95.2 MPa were obtained at 1-pass, which were 86% and 33.9% higher than those of the base material, respectively. The hardness and strength of the stirring zone (SZ) decreased with the increase in the number of processing passes. Therefore, HRS-SFSP pure aluminum can obtain high strength and hardness while maintaining good plasticity.

1. Introduction

Friction stir processing (FSP) is a new solid state processing technology, which was improved by R.S. Mishra et al. [1], as an improved technique based on friction stir welding (FSW). Its principle is similar to that of FSW, wherein a rotating tool with a pin and a shoulder acts on the surface of the material. The pin tool rotates at a high rotating speed and advances at a uniform rotating speed, resulting in severe plastic deformation in the nugget zone (NZ) as well as grain crushing and refinement, which greatly improves the performance of the base material. The NZ can be roughly divided into four areas [2,3]: the outermost unaffected base material (BM); the heat affected zone (HAZ), which is affected by heat but does not produce large plastic deformation; the stirring zone (SZ) of the stirring needle processing; and the thermo mechanical affected zone (TMAZ) between the HAZ and the SZ. At present, FSP and FSW have been used in the processing and welding of various aluminum alloys and copper alloys. Yadva and Bauri [4] conducted FSP on cast aluminum and found that the mechanical properties of cast aluminum were improved. Heinz et al. [5] reported that heat treatment after FSW could further improve the mechanical properties of 6013 aluminum alloy.

The rotating speed of the tool is always one of the main parameters that affect the performance of SZ. In the published literature, the stirring head speed of various materials in FSP is mostly based on a low rotating speed (200–2000 r/min), as shown in Figure 1 [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25], and there are few studies on FSP at higher rotating speeds (≥3000 r/min). Scholars [26] believe that heat input in FSW and FSP processes can be reduced at a low rotating speed, so as to obtain better weld quality. However, at a low rotating speed, the welding machine has to overcome larger resistance to work owing to insufficient flow plasticity, which exerts greater pressure on the spindle and limits the application of FSP. Therefore, Azimzadegan [27] proposed that increasing the rotating speed can soften the processing area, improve the flow plasticity, and then reduce the stress on the spindle. However, Li et al. [28] pointed out that a high rotating speed would produce high heat input, which would be detrimental to the performance of welded joints. Abolusoro et al. [29] found that, at 1850 r/min, the welding temperature reached 550 °C, which seriously affected the weld performance. External cooling is a method to improve the microstructure and properties during welding. Providing a water environment in the process can quickly reduce the temperature of the BM, effectively inhibit the growth of recrystallized grains, obtain a fine recrystallized structure, and improve the performance of the material. Su [30] found that providing a water environment in the process of FSP can take away the remaining heat in a timely manner and inhibit the recrystallization and growth of refined grains, while improving the strength and plasticity of materials.

Therefore, the concept of high rotating speed submerged friction stir processing (HRS-SFSP) is proposed in this paper. In order to understand the effect of HRS-SFSP on the microstructure and properties of an aluminum matrix, 1060 pure aluminum was used as the matrix material in this study.

2. Materials and Methods

The base material used in this study is 1060 industrial pure Al with 99.6% Al, Si, and Cu content of less than 0.4%. First, 1060 pure aluminum was rolled into a 3 mm thick sheet metal for 1, 2, and 3 passes of reciprocating FSP. Then, a 2 mm pin tool with a top diameter of 3.5 mm and a tip diameter of 2.5 mm with a shoulder diameter of 10 mm was used for the FSP. The rotating speed was kept constant at 3000 r/min, the travelling speed was fixed at 60 mm/min, and the downward pressure was 0.2 mm. The whole process was carried out under cold water, the water flow speed is about 300 mm/s, and the direction is opposite to the processing direction. A K-type thermocouple was inserted in a position about 1.5 mm away from each NZ, and the temperature recorder was used to record the temperature at two points per second to obtain the heat cycle curve. Before processing, the surface of the Al sheet was polished with sandpaper and cleaned with alcohol to remove oxides and impurities.

After the experiment, samples with a size of 20 mm × 10 mm × 3 mm were collected along the machining surface. The samples were mechanically ground and electrolytically polishing in −20 °C alcohol perchlorate solution (1:9) for 15 s at a voltage of 30 V. The microstructures were analyzed using electron backscattered diffraction (EBSD) technology with a scanning step of 0.05 μm. More than 700 grains were counted for each sample using Channel 5 software and the following standard definitions were adopted: θ_c represents the difference of orientation angle between neighboring grains and θ_c = 2° is defined as the boundary of sub-grains. When θ_c is greater than 2°, it is considered that the grain degenerates, and θ_c = 15° is used as the boundary between high angle grain boundaries (HAGBs) and low angle grain boundaries (LAGBs). Average grain size, poor orientation of adjacent grains, dislocation, and Schmidt factor were analyzed. The hardness was tested by (HCT-1000) micro-Vickers hardness tester. The applied load was 4 N, the pressure holding time was 15 s, the hardness was collected from the advancing side (AS) to the retreating side (RS) at a point every 0.5 mm downward along the cross section, and the hardness distribution in the NZ was obtained. Tensile test was carried out using Zwick/Z2.5 (Zwick, Ulm, Germany). Three groups were tested for each parameter, and the fracture morphology was observed by scanning electron microscopy (SEM). Tensile specimens were taken from the parallel processing direction, as shown in Figure 2a. Figure 2b shows the size of the tensile specimens. Figure 2c presents the FSP morphology, while Figure 2d shows the optical microstructure of the cross section.

3. Results and Discussion

3.1. Thermal Cycle Curve

Figure 3 shows the distribution curve of thermal cycle in the processing area under different passes. It can be seen that the peak temperature of the processing area reached 0.65 T_m–0.71 T_m (T_m is the melting point of the material) after 1–3 pass of FSP of pure aluminum. These temperatures are far lower than the melting point of aluminum (~660 °C) and higher than the recrystallization temperature of aluminum (0.4 T_m–0.5 T_m). This indicates that dynamic recrystallization has occurred in the SZ. The temperature of the processing zone can be expressed as follows [31]:

$$T=K{\left(\frac{{\omega }^2}{v\times {10}^4}\right)}^{\alpha }\times T_m$$

(1)

where ω is the rotating speed; v is the travelling speed; and both α and K are constant, ranging from 0.04 to 0.06 and 0.65 to 0.75, respectively. According to Equation (1), large heat input can be obtained at a high rotating speed. The temperature of 2 and 3 passes is higher than 1-pass, as show here. This is because the process is continuous. The heat generated did not completely disappear before the next processing pass, which caused heat accumulation. Consequently, the temperature continued to rise during the subsequent processing. The water cooling caused a rapid decline in the temperature of the processing area. Finally, the temperature distribution as shown in Figure 3 was formed.

3.2. Microstructure Characterization

Figure 4 shows the inverse pole figure (IPF) on the AS of 1–3 passes. It can be seen that small, recrystallized grains were obtained in SZ after HRS-SFSP. Combined with Figure 5, it can be seen that ultrafine-grains with an average grain size of 0.690 μm and 0.686 μm were obtained at 1 and 2 passes, respectively. However, with the increase in number of processing passes, the grains in SZ began to coarsen and reached 5.363 μm at 3-pass. The analysis shows that, on the one hand, multi-pass processing can further break the grains and help refine the grains again. On the other hand, temperature also significantly affects the recrystallization grain size in the SZ. Excessive heat input led to the continuous rise in the temperature in the SZ, as shown in Figure 3, which resulted in the growth and coarsening of the refined grains in the SZ. This explains why the average grain size of 1 and 2 passes was similar, and the average grain size of 3-pass was larger than that of 2-pass. The ultrafine-grains were still obtained at a high rotating speed owing to the cooling effect of water, which effectively inhibited the continuous growth of recrystallized grains. Hence, fine recrystallized grains were obtained.

Figure 6 shows the results of misorientation angle on the AS. It can be seen that the HRS-SFSP generated more HAGBs, which made crack propagation difficult, thus improving the toughness of the material. There was a high proportion (>85%) of HAGBs after 1 and 2 passes, as shown in Figure 5a,b, which is different from the results of other severe plastic deformation (SPD) and ordinary rotating speed FSP. SPD techniques such as equal channel angular extrusion (ECAE) and equal channel angular expansion extrusion with spherical cavity (ECAEE-SC) [32] produce mostly LAGBs, thus a large number of HAGBs should be formed by means of recovery recrystallization annealing to improve the toughness. More HAGBs can also be obtained by ordinary rotating speed FSP. Liu et al. [18] carried out FSP on 1050 pure aluminum at a speed of 1200 r/min and obtained 70–80% HAGBs on the forward side, which was lower than that obtained by HRS-SFSP. According to the analysis, a high rotating speed causes greater plastic deformation and strain rate. Under such conditions, more LAGBs were transformed into HAGBs by absorbing dislocation, that is, more complete “continuous dynamic recrystallization” [33] occurred, thus forming a large number of HAGBs. At the same time, as a result of water cooling, the growth of recrystallization was inhibited, which prevented the HAGBs from changing back to LAGBs. Finally, a large proportion of HAGBs were obtained. However, the 3-pass processing zone experienced a high thermal cycle, so static recrystallization occurred in the subsequent process, and some HAGBs were converted to LAGBs again, resulting in a significant decrease in the proportion of HAGBs. In general, SPD and low rotating speed FSP require multiple post-processing methods or require multiple passes to obtain a large number of HAGBs, while with HRS-SFSP, more HAGBs can be obtained in fewer passes.

In order to better understand the mechanical properties (hardness and tensile strength) of 1060 pure aluminum, it is necessary to consider grain size, dislocation density, and slip system. Among them, grain size and dislocation density are important factors affecting hardness and strength, while Schmid factor (SF) is an important parameter describing the slip system. Thus, the two parameters of geometrically necessary dislocation (GND) density and SF were used to quantitatively analyze strength and plasticity. Grain size information is given in Figure 3 and Figure 4. GND density information is obtained using the following formula [34]:

$${\rho }^{GND}=\frac{2\Delta {\theta }_i}{ub}$$

(2)

where Δθ_i means local misorientation kernel average misorientation, as shown in Figure 7; u is the unit point length (step size); and b is the burgers vector. For pure aluminum, the step size is 0.05 μm and b is 0.286 nm [35] (which was explained in the experimental part). The GND density can be obtained by substituting it into Equation (2), as shown in Figure 8. It can be seen that the geometric dislocation density (GND) of 1–3 passes must be much lower than BM, which was the result of recrystallization. The dislocation density of 3-pass was the minimum because the thermal cycle time of 3-pass was the longest and the grain grows, leading to the decrease in GND density.

SF can reflect the plasticity of the material, as shown in Figure 9. With the increase in number of processing passes, SF gradually increases. It can be considered that the increase in the number of processing passes cannot always improve the mechanical properties of the material. During HRS-SFSP, the plasticity of the processing area showed a large improvement at the time of 3-pass, which indicates that the plasticity of the processing area will improve at the time of 3-pass, which is unfavorable to the mechanical properties of the processing area.

3.3. Texture Characterization

Table 1. Main texture types and contents.

Texture Types	BM	1-Pass FSP	2-Pass FSP	3-Pass FSP
Cube {001}<100>	4.66%	6.48%	3.29%	7.26%
Rotating cubic {001}<110>	1.07%	3.28%	3.83%	2.28%
Copper {112}<111>	7.91%	5.02%	1.83%	0.39%
Brass {011}<211>	0.22%	5.93%	9.16%	1.22%
R {124}<211>	0.62%	5.42%	2.49%	<0.01%
Shearing {111}<110>	0.11%	7.48%	19.1%	30.8%
Goss {011}<100>	4.51%	2.95%	5.77%	2.73%

lists the main texture of 1060 aluminum BM and the AS of 1–3 passes processing. Copper texture, goss texture, and cube texture are mainly found in BM. However, after HRS-SFSP, the texture types and contents changed. Specifically, the content of copper texture decreased greatly, while the shearing texture and cube texture were the main textures, and their content showed an increasing trend. Combined with the polar graph of Figure 10, it can be seen that the texture strength at 1-pass was weak. With the increase in processing passes, the texture strength gradually increases, showing a new preferred orientation. At 3-pass, the relative content of shearing texture is up to 30.8%. According to the analysis, as aluminum is an FCC metal, the orientation factor in the <110> direction of {111} plane is the smallest during plastic deformation [36]. The high rotating speed stirring needle leads to large shear deformation, which leads to the dislocation in the grains to form {111} <110> orientation, that is, dynamic recrystallization. However, the temperature of HRS-SFSP was higher than the recrystallization temperature of aluminum. This resulted in the dynamic recovery of cube texture sub-grains in the SZ under the action of thermal cycling, and then the recrystallization growth occurred, namely, “discontinuous dynamic recrystallization” [37]. Finally, shearing texture and cube texture were formed.

3.4. Hardness

Hardness is one of the important mechanical properties to measure the ability of material to resist deformation caused by external force. The average Vickers hardness of 1060 pure aluminum used in this paper was 35.6 HV. Figure 11 shows the hardness distribution of 1–3 pass processing. At 1 and 2 passes, “M” type hardness distribution was observed, while “U” type hardness distribution was observed at 3-pass. Gan [10] et al. reported that the hardness distribution of pure aluminum presented a “U” type trend after low rotating speed FSP, and the hardness of SZ was lower than that of BM. However, the increase in hardness came from fine grains and high GND density after HRS-SFSP. However, with the increase in the number of processing passes, the recrystallized grains grew by consuming GND density, resulting in the decrease in GND density and growth of grain size, and finally the decrease in hardness. In conclusion, the highest average micro-hardness was obtained in SZ at 1-pass, reaching 66.3 HV, which was about 86% higher than that of the BM. In contrast, at 3-pass, the average hardness of SZ decreases to 32.6 HV, which was lower than that of the BM.

3.5. Ultimate Tensile Strength

Figure 12 shows the ultimate tensile strength and elongation under various parameters. It can be seen that the ultimate tensile strength showed a downward trend with the increase in the number of processing passes. The maximum ultimate tensile strength of 95.2 MPa was obtained at 1-pass, which can be explained by the classical hall–page relationship. The grain size at 1 and 2 passes was refined to submicron level, so the ultimate tensile strength increased greatly. Although the strength of 3-pass is still higher than that of the BM, the grain size of 3-pass was significantly larger than that of 1- and 2-passes, resulting in the reduction of ultimate tensile strength. Figure 13 shows the tensile fracture morphology of the BM and 1–3 passes. It can be seen that there were large dimples and cleavage surfaces in the fracture of BM, and mixed fracture occurred. In contrast, there were only dimples after 1–3 passes, which was a typical ductile fracture. Moreover, the dimples of 3-pass were smaller and deeper than those of 1- and 2-passes samples, indicating that the processed metal after 3-pass had better plasticity, which was also consistent with the variation trend of elongation reflected in Figure 5. The analysis shows that the long duration of multi-pass processing led to heat accumulation. Under the action of thermal cycle, static recrystallization of SZ occurred after processing. This causes the HAGBs to gradually change into LAGBs, resulting in the increase in texture strength. At the same time, the SF also increased accordingly, which improves the slip ability of SZ. Therefore, SZ shows good plasticity under the action of external force. These findings provide a theoretical basis for the future application of multi-pass HRS-SFSP in alloys and composites.

4. Conclusions

(1)

Fine and evenly distributed recrystallized grains of 1060 pure aluminum can be obtained after multi-pass HRS-SFSP. With the increase in processing passes, the grain size increases significantly.

(2)

HRS-SFSP of 1060 pure aluminum includes three processes: continuous dynamic recrystallization, discontinuous dynamic recrystallization, and static recrystallization. The proportion of HAGBs and LAGBs reflects the degree of recrystallization in each pass. The highest proportion of HAGBs was obtained after 1-pass processing, depending on the degree of dynamic recrystallization.

(3)

With the increase in the number of processing passes, the elongation increased gradually, while the strength and hardness decreased. After 3-pass, the hardness was even lower than that of the BM. These results show that, during HRS-SFSP, fewer passes are conducive to the improvement in strength and hardness, while more passes are conducive to the improvement in plasticity.