3.1 Surface property
The key process parameters of AFSD are rotational speed, feed rate, and tool in-plane velocity, these process parameters determine the amount of heat input, the cooling rate and plastic deformation rate. The as-deposited samples manufactured by different parameters are shown in Table 2. Figure 6 shows the appearance morphology of all specimens deposited with different processing parameters. All specimens form a circular curved area at the trailing end, and the edges of the retreating side of the deposited layer are smoother than the edges of the advancing side which is consistent with the results of Mejpa et al. [13]. It can be found that the morphology of the as-deposited layer varied with the bar feed rate. When the feed rate was 100 mm/min (NO.3), the surface was smooth and dense without obvious curved lines and no flash at the initial position. When the feed rate is 50 mm/min and 75 mm/min (NO.1 and NO.2), unevenly distributed curved lines have been found. Also, some flash and hole defects have been spotted at the initial position.
Additionally, the smoothness of the edges of the retreating side and the advancing side of the deposited layer differs significantly, with the smooth edge of the retreating side and the rough serrated edge shape of the advancing side. The reason for the difference in morphology is that the low feed rate of the bar leads to low plasticization of the material and insufficient flow of the viscoplastic material which forms uneven curved lines in the forward direction driven by the rotating tool. The flow direction of the viscoplastic material is from the retreating side to the advancing side. The low degree of plasticization of the material exacerbates the irregularity of the advancing side edge, resulting in forming an irregular serrated edge.
The influence of bar rotational speed on deposited layer appearance morphology is less profound. The appearance of specimens manufactured at different rotational speeds (e.g. NO.5 and NO.6) are nearly identical except for the rough surface at the starting end. At high rotational speed, the starting side of the deposited layer is more prone to the formation of porosity and flash defects. Hartley et al. [20] also found that microscopic defects are formed at high rotational speeds (e.g. 900 rpm) due to excessive heat input. The unconstrained material flow can cause surface defects. In this study, the surface morphology of the NO.4 specimens is best, which indicates with the parameters (the speed and feed rate) are optimal for the required heat input and the degree of plastic deformation.
Specimen
|
Ration speed ω1 (r/min)
|
Feed rate F (mm/min)
|
Traverse velocity V (mm/min)
|
Table 2
Summary of the AFSD processing conditions for depositing 6061-T6 in this work.
NO.1
|
700
|
50
|
100
|
NO.2
|
700
|
75
|
150
|
NO.3
|
700
|
100
|
200
|
NO.4
|
600
|
75
|
150
|
NO.5
|
800
|
75
|
150
|
NO.6
|
900
|
75
|
150
|
3.2 Microstructure
Figure 7 shows the evolution microstructure of feedstock material and AFSD specimen under different feed rates including grain size and grain boundary misorientation distribution. Low angle grain boundaries (LAGB) are classified as boundaries with misorientation angles less than 15°, and high angle grain boundaries (HAGB) are classified as boundaries with misorientation greater than 15°. As shown in Fig. 7(a) and Fig. 7(b), the feedstock is a slender type of grain (not shown in full) with an average grain size of 69.77um. During the AFSD process, the feedstock undergoes plastic deformation under the action of frictional heat and axial force, and the grains are transformed into equiaxed grains with grain size of 3.79-4.79um. The obvious grain refinement indicates that dynamic restoration and dynamic recrystallization occur during the AFSD process.
Dynamic recrystallization can be divided into continuous dynamic recrystallization and discontinuous dynamic recrystallization. Perry et al. [21] concluded that the difference between continuous and discontinuous dynamic recrystallization is that discontinuous recrystallization is characterized by dislocation-free grain "nucleation" and growth, which usually results in the formation of a non-uniform microstructure before recrystallization is completed. However, continuous dynamic recrystallization (CDRX) does not include dislocation nucleation and grain growth. It is partly characterized by the gradual accumulation, annihilation, and reorganization of dislocations due to the simultaneous presence of deformation and dynamic restoration resulting in the formation of subgrains. With further strain development, the grain orientation difference increases, and the subgrain boundaries transform to LAGB and eventually to HAGB. As shown in Fig. 7, the fraction of LAGB in the feedstock is 98%, which decreases to 50.5%, 50.5% and 52.9% during AFSD respectively. The transformation of LAGB to HAGB indicates that the recrystallization mechanism in the AFSD process is continuous dynamic recrystallization.
The average grain size was 3.76um,3.79um,4.79um under different feed rates, and the grain size tended to increase with increasing feed rate. Bandar et al [22] showed that the average grain size of the additive manufactured parts produced by Die cast A356 alloy at feed rates of 3, 4, and 5 mm/min. The average grain size of Die cast A356 alloy parts produced at feed rates of 3, 4, and 5 mm/min were 0.62 ± 0.1, 1.54 ± 0.2, and 2.40 ± 0.15 µm respectively, and the grain size increased with increasing feed rate. The reason for the increase in grain size can be attributed to the increase in feed rate resulting in more metal being plastically deformed per unit time, thus causing an increase in heat production from plastic deformation. After the completion of dynamic recrystallization, some of the larger grains swallow the smaller grains and merge into larger grains under the effect of additional heat of plastic deformation thus resulting in grain growth. The heat of plastic deformation is smaller compared with the frictional heat input so the grain size increase caused by increasing the feed rate is smaller. Dynamic restoration and continuous dynamic recrystallization occur with characteristics of the formation of subgrains and an increase in the LAGB ratio. As the strain increases, the LAGB is transformed into HAGB, and a larger number fraction of HAGB indicates a higher degree of recrystallization. As shown in Fig. 7, the HAGB number fraction of N0.1 and NO.2 is larger than that of NO.3, which indicates that the recrystallization degree of NO.1 and NO.2 is larger than that of NO.3.
Fig.7 shows the evolution microstructure of AFSD specimen under different rotational speeds including grain size and grain boundary misorientation distribution. As shown in Fig.8(b), Fig.8(e), Fig.8(h), the average grain size is 3.50um, 3.63um, and 5.83um at 600r/min, 800r/min, and 900r/min respectively. The plastic deformation process of the AFSD material is similar to that of the friction surfacing (FS), which occurs under the combined effect of frictional heat and plastic deformation heat[23]. The frictional heat increases with the increase of the rotational speed provided that the feed rate is kept constant. Grain growth is caused by migration through grain boundaries, and heat affects atomic diffusion; the higher the heat diffusion coefficient, the easier the grain boundaries are to migrate. Therefore, with the increase of rotational speed, the heat input increases, and the grain boundary migration ability is stronger causing grain growth. In addition, the heat input of frictional heat is much larger than the heat of plastic deformation, and the increase in rotational speed leads to larger grain size. As shown in Fig. 8(h) where the maximum grain size is 22 um. In addition, as shown in Fig. 8(c), Fig. 8(f), Fig. 8(l), the HAGB fraction was 57.6%, 50%, and 42.7% at 600r/min, 800r/min, and 900r/min respectively. As such the degree of crystallization decreased with the increase in rotational speed.
3.3 Mechanical property
The tensile test results of the specimens prepared at different feed rates are shown in Fig. 9(a), and each curve is the average of the results of the three tests. The tensile force that the AFSD material can withstand is significantly smaller than that of the 6061-T6 material. 5243N is the maximum tensile force of the 6061-T6 aluminum alloy, while the maximum tensile force of the AFSD material is 3252N, which can only reach 62% of that of the 6061-T6 material. 6061 aluminum alloy is heat-treatable, and its hardness and strength are greatly increased after solid solution strengthening and artificial aging. Its main strengthening mode is precipitation strengthening, and the β'' phase (Mg5Si6) and β'(Mg9Si5) phase precipitated during aging are the main strengthening phases. In the AFSD process, the bar rotating at high speed generates a large amount of frictional heat with the substrate under the action of axial force. The frictional heat accumulates as the AFSD process proceeds, and the temperature can reach approximately 550°C when measured with a thermal imager. 6061-T6 is annealed by holding it at 415°C for 2–3 h [12]. The temperature generated by friction is greater than the annealing temperature resulting in the strengthening phase dissolution occurring and the strength and hardness decreasing significantly. However, the plasticity of the deposited material increased significantly, and it can be observed from Fig. 9(b) that the elongation of the deposited material was significantly greater than that of 6061-T6 which indicates the increased plasticity of the deposited material.
The maximum tensile forces that can be achieved in the deposited state vary less when the feed rates are different. the maximum tensile forces of NO.1, NO.2, and NO.3 are 3172N, 3211N, and 3252N respectively, and the tensile forces increase with increasing feed rates. the elongation of NO.1 and NO.2 are approximately identical, and the elongation of NO.3 is greater, which indicates that increasing the feed rate can improve the strength of the material. The strengthening effect is not significant. When the feed rate increases, the elongation decreases and reduces the plasticity of the material, and the elongation of NO.1 is the largest. Figure 9(b) compares the ultimate tensile strength(UTS) and elongation at break(EAB) of the materials with different feed rates. The UTS increases with increasing feed rate, and the EAB decreases with increasing feed rate. The UTS of 6061-T6 is 320 MPa and the EAB is 30%, which is less plastic than the deposited material. In Fig. 9(c), both 6061-T6 and the deposited material showed significant shrinkage after tensile testing, but the deposited material showed greater cross-sectional shrinkage, 35% for 6061-T6 and 50% for the deposited material (NO.1), thus demonstrating the superior plasticity of the deposited material.
For the aluminum alloy material, the tensile strength is proportional to the microhardness. Figure 9(d) shows the microhardness of the first deposited layer at different feed rates. it is obvious that the microhardness of NO.3 is higher than that of NO.2 and NO.1, but the difference between the microhardness of NO.2 and NO.3 is smaller. the average value of microhardness of NO.3 is 55 HV, while the average values of microhardness of NO.2 and NO.1 are 52 Hv and 51 Hv, respectively. When the feed rates are different, the microhardness of NO.3 is proportional to the feed rate.
It is known that frictional heat is the main input heat in the AFSD process, and the rotational speed is positively correlated with frictional heat when the axial force applied on the bar is constant. Figure 10(a) plots the tensile force curve of the deposited state material at different rotational speeds. The purpose of this test was to investigate the variation of the material strength at different rotational speeds. Although the variation of tensile force is small at different rotational speeds, the tensile force still shows a subtle variation with the increase in rotational speed. As shown in Fig. 10(a), the maximum tensile force is 3231N,3250N, and 3318N for NO.4, NO.5, and NO.6, respectively. Therefore, the maximum tensile force increases with the increase of rotational speed. The maximum temperature of the deposited material at different rotational speeds was measured using a thermal imaging camera and was 404.5°C, 444.5°C, and 471.5°C for 600 r/min (N0.4), 800 r/min (NO.5), and 900 r/min (NO.6), respectively as shown in Fig. 11. As mentioned above, the main reason for the decrease in strength of the deposited material is the dissolution of the reinforcing phase back into the matrix due to frictional heat, which decreases the tensile strength of the deposited material compared to the 6061-T6 material. On the contrary, we found that the higher the frictional heat, the higher the tensile force of the deposited material.
Figure 10(b) plots the UTS and EAB of the deposited material at different rotational speeds. The UTS are 204 MPa, 206 MPa, and 210 MPa for NO.4, NO.5, and NO.6, respectively, and the UTS increases with the increase of rotational speed. The EAB was 60%, 56%, and 47% for NO.4, NO.5, and NO.6, respectively. This indicates that the plastic deformation capacity of the deposited material decreases as the rotational speed increases. We then tested the microhardness of the first deposited layer at different rotational speeds. Figure 10(c) shows the microhardness of NO.4, NO.5, and NO.6. However, the maximum values of microhardness of NO.4, NO.5, and NO.6 were 55Hv, 55Hv, and56 Hv, respectively, which indicated that the increase of rotational speed did not cause a significant increase of microhardness. Although the increase in rotational speed did not cause a significant increase in microhardness caused a local increase in hardness. It can be speculated that the microhardness will increase with the increase of rotational speed when the gradient of rotational speed changes more.
3.4 Fractography
The 6061-T6 feedstock was compared with a randomly selected NO.3 specimen (6061-T6 AFSD) from all specimens for SEM analysis of the fracture surface. Macroscopic images of the fracture surface show significant shrinkage in both 6061-T6 AFSD and 6061-T6 feedstock material tensile specimens indicating plastic deformation of the specimens during the tensile process. As shown in Fig. 12, a large number of equiaxed dimples can be observed on the fracture surfaces of both 6061-T6 AFSD and 6061-T6 feedstock。Hence, the main fracture mechanism is a ductile fracture caused by micro-void coalescence. In addition, weak bonding defects were observed in Fig. 12(a), which was mainly since the AFSD tool used in this test was a flat tool (no projection on the shoulder surface), so the first and second layers were not sufficiently mixed and the interlayer bonding was weak. The number of equiaxed dimples per unit area of 6061-T6 AFSD is greater than that of 6061-T6 feedstock (Fig. 12(b) and (e)). Zhu et al. [24] showed that the equiaxed dimples density is positively correlated with plasticity, so the plasticity of 6061-T6 AFSD is greater than that of 6061-T6 feedstock, which also verifies that in section 3.3, The EAB of 6061-T6 AFSD is greater than that of 6061-T6. In Fig. 12(c) and (f), it is observed that the equiaxed dimples size of 6061-T6 AFSD is greater than that of 6061-T6 feedstock.