List of Tables
Chapter 5 End forming behaviour of FSPed Al 6063-T6 tubes at different tool
5.1 Methodology
5.2.1 Tensile behaviour and hardness distribution
The tensile behaviour of processed zone at different traverse speeds has been shown in Fig. 5.2. It has been observed that with the increase of traverse speed, strength, ductility and strain hardening exponent of the processed zone increases. With the increase of traverse speed, the heat input to the processed zone decreases. As a result, a strong processed zone having larger flow stress is obtained at a higher traverse speed. When compared with base material, the strength (UTS) of the FSPed tube has decreased in all cases, while formability has improved in case of traverse speed 125 mm/min.
The hardness distribution across the processed zone has been shown in Fig. 5.3(a- c). The hardness distribution suggests that material softening has occurred, because of heat experienced by the processed zone during FSP. The hardness in different regions of processed zone, i.e., stirred zone (SZ), heat affected zone (HAZ) and thermomechanically affected zone (TMAZ) varies between 55‒70 VHN after FSP for different cases. A reduction in hardness after FSP in the processed zone is responsible for a lower flow strength of processed zones made at different traverse speeds. The hardness has improved after end forming operations. Although a hardness variation of 10-20 VHN has been observed at different regions of processed zones for any case (before forming or after forming), an average hardness has been calculated for the whole processed zone of width 16-18 mm, and has been presented in Table 5.5.
Chapter 5
Fig. 5.2 Engineering stress-strain data for FSPed zone specimen at different traverse speeds
(standard deviation in Flow stress: Base metal:13 MPa, for 75 mm/min: 15 MPa, for 100 mm/min: 9 MPa, for 125 mm/min: 38 MPa)
Table 5.4 Mechanical properties of base metal and processed zone made at different traverse speeds
Traverse speed (mm/min)
/Base metal
Yield strength
(MPa)
Ultimate tensile strength
(MPa)
Uniform elongation (%), at 12.5
mm gauge length
Total elongation (%), at 12.5
mm gauge length
Strain hardening exponent,
n
Strength coefficient,
K (MPa)
75 88.62±6 131.46±10 11±1 24±1 0.15±0.01 207.35±30
100 89.16±10 158.54±11 22±0.2 24±0.2 0.22±0.02 276.83±9
125 93.75±22 176.48±40 22±2 35±8.2 0.23 313.37±70
Base metal
152±11 258±12 20±0.6 31±0.02 0.17±0.01 407±13
The hardness of the FSPed zone decreases by about 32.98 %, 33.59 % and 32.65
% in case of traverse speeds 75 mm/min, 100 mm/min and 125 mm/min respectively (Table 5.5). Moreira et al. (2008) has reported a similar decrement in processed zone hardness after FSP for AA6063-T6. In their work AA 6063-T6 has been friction stir welded and a hardness decrement of about 27 % and 32 % has been observed in the upper surface and in the side surface of the FSW region. Sato et al. (1999) FSWed 6 mm thick AA6063-T5 sheet and found that hardness decreases drastically in the weld zone. It was observed that all the precipitates have dissolved in the softened region because of a larger temperature observed in the precipitate free region (402°C) as compared to 201°C in in the base metal region which is quite high and responsible for dissolution of all precipitates in the softened region. This phenomenon ultimately causes a decrease in hardness in the weld region. This is a characteristic of precipitation based hardened aluminium alloy.
Chapter 5
Fig. 5.3 Hardness plot across the processed zone before forming and after forming in case of, (a) tube expansion, (b) tube reduction, (c) tube beading
(average standard deviation for hardness in weld zone in case of expansion before forming is ±6 and after forming is ±8, in case of reduction before forming is ±4 and after forming is ±4, in case of beading before forming is ±2 and after forming is ±4)
Chapter 5 Table 5.5 Hardness of the processed zone before forming and after forming (Base metal
hardness: 93.76 ± 13 VHN)
Traverse speed (mm/min)
Hardness (VHN)
Expansion Reduction Beading
Before forming *,
Hi
After forming,
Hf
Before forming*,
Hi
After forming,
Hf
Before forming*,
Hi
After forming,
Hf
75 62.61±5 90.08±9 62.18±3 89.92±3 63.72±3 85.86±5
100 61.07±4 80.67±8 62.05±3 93.16±9 63.67±3 81.23±10
125 62.56±3 90.02±7 63.05±5 87.10±2 63.75±2 85.42±3
*Before forming is equivalent to after FSP
Hardening of FSPed zone after end forming is primarily due to strain hardening during end forming. Lee et al. (2002) applied accumulative roll bonding (ARB) process to a 6061 aluminum alloy and observed a hardness increase through the thickness of the specimen. This is explained as work hardening due to larger redundant strains induced between roll surface and specimen. Similar work hardening has been observed in the present work in FSPed zone.
Microstructural images of different regions of processed zone has been shown in Fig. 5.4. The main purpose of the microstructural examination is to observe the grain size changes in the processed zone. Three distinct regions, i.e., SZ, TMAZ and HAZ, based on grain size, have been identified at each traverse speed in the processed zone. The stir zone shows smaller grain size at all traverse speeds, while HAZ shows larger grain size. Due to severe plastic deformation and recrystallization taking place at high temperature, the grain size is smaller in SZ when compared with other regions of the processed zone. A coarser grain size approaching to that of the base metal grain size has been noticed in HAZ. This region is only affected by the thermal cycle and deformation is almost absent in this region. A combined effect of deformation and thermal effect results in elongated equi- axed grains in TMAZ. However, recrystallization is usually absent in this region because of lesser heat produced. A combination of varying thermal effect and plastic deformation is responsible for grain size variation across processed zone. However, no direct correlation is seen between grain size and hardness after FSP. Table 5.6 compares the average grain size at different regions of FSP zone.
Chapter 5
Fig. 5.4 Microstructural images of different regions of FSPed zone at varied traverse speeds: (a) SZ at 75 mm/min, 100 mm/min and 125 mm/min, (b) TMAZ at 75 mm/min, 100 mm/min and 125 mm/min, (c) HAZ at 75 mm/min, 100 mm/min, and 125 mm/min, and (d) Base metal (BM)
Table 5.6 Average grain size of different regions of FSPed zone at different traverse speeds (Base metal grain size: 91.15±22)
Traverse speed (mm/min)
Grain size (μm)
SZ TMAZ HAZ
75 24.6±3 29.25±4 58.45±7
100 17.4±2 24.6±3 76.65±18
125 24.6±3 24.62±3 64.50±15
Chapter 5 Hardness index, H (%), based on initial hardness and change in hardness, during
end forming operations has been proposed as follows:
( ) (5.2)
where Hi is the initial hardness (after FSP) and Hf is the final hardness (after forming). It is concluded from Fig. 5.5 that the hardness change is more sensitive to tube expansion and tube reduction as compared to tube beading. However, all the end forming operations show considerable variation in H, at all the traverse speeds. Tube reduction shows considerable change in hardness (or H) in almost all cases causing severe strain hardening as compared to the other end forming operations.
Fig. 5.5 H % versus end forming operations at varying traverse speeds 5.2.2 Dislocation density changes
Fig. 5.6a shows the directly measured X-ray diffraction peak data. The indices of each (h k l) peak are presented for the FCC Al alloy. Fig. 5.6b shows the (2 2 0), (3 1 1) and (2 2 2) peak profiles in the SZ for three different tool traverse speeds. The peak broadening is slightly different for different traverse speeds which is related to different dislocation densities.
Chapter 5
Fig. 5.6 (a) X-ray diffraction peak profile pattern in the HAZ region of FSPed zone made at 75 mm/min, (b) peak profiles measured using X-ray diffraction in the SZ for 75 mm/min, 100 mm/min and 125 mm/min tool traverse speeds
Modified Williamson-Hall plot for different zones and traverse speeds has been shown in Fig. 5.7. In case of SZ (Fig. 5.7a), the base metal shows the highest slope between successive peaks as compared to different traverse speeds which means that base metal has higher dislocation density as compared to stir zone. As traverse speed increases, slope decreases indicating a decrease in dislocation density. The dislocation density decrease indicates improvement in strain hardening exponent with increase in traverse speed (refer Table 5.4).
The TMAZ made at different traverse speeds exhibit larger slope (Fig. 5.7b) as compared to the base metal except 125 mm/min. In this case, the slope is closer to the base metal. It means that TMAZ for different traverse speeds possesses either a larger or at par dislocation density as compared to base metal. Moreover, as the traverse speed increases, the slope decreases indicating a decrease in dislocation density. In case of 75 mm/min, the slope observed is largest among cases indicating a larger dislocation density in this case.
In HAZ region, the slopes of base metal and traverse speeds are comparable (Fig.
5.7c). It suggests that dislocation density in base metal and HAZ region are almost same for different traverse speeds. Similar results have been shown by Woo et al. (2008) for FSWed 6061-T6 alloy. They explained that the low dislocation density in the DXZ can be due to recrystallization at the elevated temperature. The higher dislocation density
Chapter 5 observed near the boundary between TMAZ and HAZ could be explained by the network
structure of many dislocations in sub-grains. HAZ region has got nearly same dislocation density as that of base metal in Woo et al. (2008) work as well.
Fig. 5.7 Peak broadening analysis using the modified Williamson-Hall plot for (a) SZ, (b) TMAZ, (c) HAZ, of FSP zone. The FWHM (ΔK) in each (hkl) peak is presented as a function of KC1/2 (K=2Sinθ/λ) and C is the dislocation contrast factor
Dislocation densities at different processing conditions and for different zones has been calculated with the help of equations 4.1 and 4.2 as described in section 4.1.3 in Chapter 4 and are summarized in Table 5.7. SZ shows lower dislocation density as compared to base metal, while TMAZ exhibit higher dislocation densities at 75 and 100 mm/min traverse speeds as compared to base metal. A larger value of dislocation density in the case of 75 mm/min and hence a reduced ductility are acceptable. For a traverse speed of 125 mm/min, a lower dislocation density has been obtained in TMAZ. A considerable variation in dislocation density in TMAZ has been observed for different traverse speeds. The dislocation density obtained in HAZ region is comparable with that of average dislocation density in base metal.
Chapter 5 Table 5.7 Dislocation density for base metal and for different zones
Traverse speed (mm/min)
Dislocation density (× 1017 m−2) Average dislocation density (×
1017 m−2) in FSPed zone
SZ TMAZ HAZ
75 1.60 513.68 5.29 173.52±294
100 8.12 27.79 7.37 14.42±11.57
125 6.40 2.14 6.39 4.97±2.45
*Base metal dislocation density: 8.6±0.5 (× 1017 m−2)
Hardening capacity (Hc) for base metal and processed zone at different traverse speeds and their respective n values has been shown in Table 5.8. For 75 mm/min traverse speed, Hc and n are lesser as compared to base metal. On the other hand, for 100 mm/min and 125 mm/min, Hc and n are larger as compared to base metal. Moreover, both Hc and n increases with increase in traverse speed. The same is depicted in Fig. 5.2 with 125 mm/min case showing higher ductility and almost same plastic stress slope with respect to the base metal. High strain hardening exponent n at higher traverse speed is because of low dislocation density at higher traverse speed. Williamson Hall plot (Fig.
5.6) and Table 5.8 confirm the same result.
Table 5.8 Hc and n for base metal and FSPed zones at different traverse speeds
Base metal/FSPed zone Hc n
Base metal 0.69 0.17
75 mm/min. 0.48 0.15
100 mm/min. 0.77 0.22
125 mm/min. 0.88 0.23