List of Tables
Chapter 4 End forming behaviour of FSPed Al 6063-T6 tubes at different tool
4.2 Results and discussion
4.2.3 End forming behaviour
4.2.3.2 Thickness evolution
Chapter 4 Only slight deviation in maximum load is found for all the cases of FSPed tubes (Fig.
4.15). The hardness index (H %) during beading of tubes at rotational speed of 1200 rpm, 1350 rpm and 1500 rpm is 22, 36.59 and 27.31 respectively. Though the maximum load is not changing much with rotational speed, the load requirement is larger in case of 1200 rpm and least for 1500 rpm, up to a displacement of 2 mm.
Fig. 4.15 Tube beading: (a) load-displacement behaviour at different tool rotational speeds, (b) maximum load at different tool rotational speeds (Data show the maximum load)
During beading of tubes, fine cracks are observed in base metal region towards the end of deformation, at a displacement of 8-9 mm. It has been observed during thickness measurement of beading of tubes that the thickness of the base metal region in the peak bulged height and the circumferential stretching of the bulged portion of the tube increases in the outward radial direction of the tube and hence cracks are observed along the length direction of the tube in the peak bulged region.
Chapter 4 show significant changes in thickness during deformation, but the base metal show more
thickness increment as compared to processed zone. Also it is observed that thickness increment of unprocessed tube is less as compared to thickness increment of base metal zone in processed tubes. In summary, tube expansion witnesses thickness reduction, while thickening occurs in the case of tube reduction and beading.
The percentage thinning of processed zone and base metal in processed tube has been evaluated at three different rotational speeds. The final thinning at the last stage of end forming is presented in Fig. 4.17(a, b). The thinning severity (%) is described by, ( ) (4.7)
where T(%) is the thinning severity (%), „ti‟ is the initial tube thickness and „tf‟ is the final tube thickness.
It is observed that FSPed zone in FSPed tube is more sensitive to thickness change during expansion and is least sensitive during beading. For the same rotational speed, thinning in tube expansion and thickening in tube reduction are severe for FSPed zone of the processed tube as compared to the base material in the same processed tube ((Fig.
4.17(a, b)). For example, at 1200 rpm, about 24 % thinning is seen in the FSPed zone during tube expansion, while it is just about 2 % in base metal. At 1350 rpm, about 44 % thinning of processed zone is seen during tube expansion, while it is less than 1 % in base metal. This is true for 1500 rpm as well. Similar situation exists in the case of tube reduction, except that thickening happens in this case. The FSPed zone shows larger thickening as compared to base metal in tube reduction. But the situation is opposite in tube beading. The base metal shows larger thickening ability than the FSPed zone in all the rotational speeds ((Fig. 4.17(a, b)).
Chapter 4
Fig. 4.16 Thickness evolution in FSPed zone and base metal (both in FSPed and unprocessed tube) in case of (a) tube expansion, (b) tube reduction, (c) tube beading
Chapter 4
Fig. 4.17 % thickness variation (thinning or thickening) at different rotational speeds (a) in processed zone, and (b) in base metal, of FSPed tube
The strength (or hardness) difference between the FSPed zone and base metal governs the thickness evolution in the case of tube expansion and reduction, as both the operations are either in circumferential tension or circumferential compression modes of deformation. Since the FSPed zone is soft, thinning and thickening is significant in the FSPed zone in comparison to the base material of the same FSPed tube. The strain hardening exponent of the FSPed zone governs the tube thickening phenomenon as compared to base material in tube beading, as bi-axial stretching is expected in the bead forming region. Since the strain hardening exponent of the base material is better than FSPed zone, it shows larger thickening as compared to FSPed zone.
The load-displacement behaviour during end forming operation depends also on the thickness of the FSPed zone after processing. Table 4.10 gives the FSPed zone thickness after FSP for all the cases. The thickness of the processed zone varies between 1.5 and 2.3 mm and depends on the rotational speed, but randomly. The thickness of the processed zone also contributes to the load bearing ability of the FSP tube. FSP tube made at 1350 rpm exhibit lowest initial thickness as compared to 1200 and 1500 rpm is the case of tube expansion and reduction. This in turn has affected the load evolution during tube expansion and reduction, as seen in Fig. 4.13 and Fig. 4.14. But in the case of tube beading, the case of 1500 rpm shows lesser load bearing ability and has got lesser thickness as well (Table 4.10) after FSP, as compared to 1200 and 1350 rpm.
Chapter 4 Table 4.10 Thickness of the FSPed zone with respect to end forming behaviour and
rotational speed
End forming operations Initial thickness (average) of the weld zone in welded tube (mm)
1200 rpm 1350 rpm 1500 rpm
Tube expansion 2.31 1.674 2.058
Tube reduction 1.984 1.618 1.716
Tube beading 2.134 2.038 1.506
Average thickness deviation: ±0.27 mm
The following are some important observation on the instabilities developed during end forming operations of FSPed and parent tubes.
1) In the case of tube expansion, failure of FSPed tubes occurred at the middle of the processed zone. This is mainly due to the reduction in hardness of the processed zone as compared to the base material of the tube. The softer processed zone is stretched perpendicularly in the circumferential direction during tube expansion and failure is seen at the middle of the weld zone (Fig. 4.18a), where minimum hardness value is witnessed.
2) In tube reduction, since compressive stress acts along the circumferential direction in the FSPed zone, it is pushed away from the die surface in the inward direction. FSPed zone is softer, and as a consequence, wrinkling is seen in the FSPed zone (Fig. 4.18b).
It is found that wrinkling starts at a displacement of about 4 mm. Later in the reduction process the wrinkle overlaps. Because of this, measuring thickness in the FSPed zone becomes difficult and hence not measured after a particular stage. The overlapping of wrinkle has been observed in 1200, 1350 and 1500 rpm rotation speeds at a displacement of 18 mm, 10 mm and 12 mm respectively. H (%), in 1350 rpm is minimum and hence overlapping occurs easily and at an early stage of 10 mm. In this case, H (%) is maximum at 1200 rpm and hence overlapping occurs at a later stage in this case. No wrinkling is observed in the unprocessed tubes.
In the case of tube beading experiments, due to biaxial stretching in the deforming region, cracks are witnessed in the base material part of FSPed tubes and in the unprocessed tube as well. Cracks are located at the bead region of the tube and oriented along the length direction of the tube (Fig. 4.18c). The ductility of the FSPed zone is better than that of base metal and hence cracks are not seen on the FSPed zone. The position of cracks in the base metal of the FSPed tube is random and has no relationship with FSPed zone. Fig.
Chapter 4 4.18 describes the mechanism of instabilities developed during the three end forming
operations of FSPed tubes.
A comparison between end forming behaviour of unprocessed and FSPed tubes is shown in Fig. 4.19(a-c). The failure in case of processed tube for tube expansion is at the middle of the processed zone and the fracture propagation is almost straight, while in case of unprocessed tube the fracture line is slightly inclined Fig. 4.19a. During reduction, Fig.
4.19b, a uniform reduction without wrinkling is observed in unprocessed tube, while wrinkling is observed in case of FSPed tube. During beading of tubes, larger cracks are observed on the peak bulged height of the base metal in unprocessed tube, while the size of crack is relatively small in base metal part of the FSPed tube Fig. 4.19c.
Fig. 4.18 Mechanism of instabilities developed during end forming operations, (a) elemental region stretched circumferentially during tube expansion, (b) elemental region compressed circumferentially during tube reduction, (c) elemental region undergoing biaxial stretching during tube beading.
Chapter 4
Raw tube FSPed tube Raw tube FSPed tube
Raw tube FSPed tube
(a) (b)
(c)
Failure
Wrinkle
Crack
Fig. 4.19 Comparison of end forming behaviour of, (a) tube expansion, (b) tube reduction, (c) tube beading, between unprocessed (left) and FSPed tube (right)