The laser bending mechanism is categorized into different mechanism based on the direction of bending. The workpiece geometries i.e. thickness, length and width of the workpiece play the key role to decide the direction of bending.
Similarly laser parameters (laser scan speed, power and laser beam spot diameter) and properties of the material also have significant effect on laser bending. It is possible to achieve the required bend direction and bending by proper selection of workpiece geometries, laser parameter and material properties (Vollertsen and Rodle, 1994).
The three fundamental mechanism for laser bending process was reported by Geiger and Vollertsen (1993); Shen and Vollertsen (2009) and Vollertsen and Roodle (1994). The workpiece and laser process parameters have significant role over these mechanisms. The three mechanisms are
1. Temperature gradient mechanism (TGM), 2. Buckling mechanism (BM),
3. Upsetting mechanism (UM).
Guan et al. (2005) mentioned as a result of temperature difference between upper and lower face of the workpiece across the thickness direction the bending of the sheet material occurs. The temperature difference induces stress owing to the variable expansion of neighbouring layers.
2.2.1 Temperature gradient mechanism (TGM)
TGM is the most frequently appeared mechanism in most of the cases of sheet metal bending. Hu et al. (2002), Kant and Joshi (2012a), Merklein et al.
(2001), and Vollertsen and Rodle (1994) reported that the direction of bending is influenced by the laser beam diameter, scan speed and thickness of the workpiece.
TGM occurs when workpiece thickness is of the order of laser beam diameter. The scan speed should be fast enough to create a sharp temperature gradient (Li and Yao, 2001a).
TGM operates in two stages, heating of the workpiece followed by natural cooling. During heating, reverse side bending occurs because of thermal expansion of heated area. The thermal gradient produces expansion which is resisted by neighbouring materials causing compressive stresses in the heated region. On achieving flow stress, near the top surface plastic deformation is produced.
However, in the vicinity of bottom surface due to low temperature plastic deformation is not produced. The bending direction is towards the laser beam during cooling, as a result of contraction and shortening of the material at the upper layers.
Hence the final bend occurred towards laser beam.
Lawrence et al. (2001) reported that for single pass laser bending, bend angle in the range of 0.1º to 3º was achieved. Wang et al. (2016) studied thickening behaviour of stainless steel/carbon steel laminated plate (SCLP) in laser bending zone. SCLP comprised the matrix layer of carbon steel cladded on both side by stainless steel plates. Researchers concluded that due to both thermal and plastic deformation effect, thickening initiated.
2.2.2 Buckling mechanism (BM)
BM is generated when laser beam diameter is higher than the thickness of the sheet, in the range of 10 times of the thickness of the sheet (Hu et al. 2002). It occurs due to generation of minor temperature gradient through thickness (Li and Yao 2001b; Hu et al. 2002 and Shi et al. 2006b). Also for a thin sheet with greater thermal conductivity, it may occur when the sheet is scanned using a larger laser spot diameter and gentle scan speed. This results a higher strain (thermo-elastic) (Vollertsen et al., 1995 and Dearden and Edwardson, 2003). For deforming thin metal sheets, BM is used .The bend angle produced in BM lies in the range of 1°–
15° for single laser pass and it significantly higher than in TGM (Lawrence et al., 2001).
The bending procedure in BM operates in two stages, heating stage and cooling stage. In heating stage due to laser application, thermal expansion of the material occurs and compressive stresses got generated in the heated region. The sheet develops swelling with increase in compressive stresses and decrease in flow stress of the heated region. Further reduction in flow stress, the sheet bends plastically in the locality. In cooling stage due to reduction in temperature relative increase in the flow stress in heated region occurs. Hence elastic bending of the sheet happens.
The bend direction in BM is not well acquainted. It can produce the bending in both direction (towards and away) from laser source. Number of factors influence bending direction in BM (Edwardson et al. 2001; Hennige et al. 1997; Shi et al.
2006a and Jamil et al. 2011a). The key factors are workpiece geometry, laser parameters, internal and external stresses including gravitation forces. Pre-bending orientation of the sheet and residual stresses are responsible for the bending direction. Nevertheless, by using BM, it is possible to bend a sheet metal in a predefined way suitably.
Chakraborty et al. (2016) accomplished experimental investigation and simulations (Finite Element) to form a surface having bowl shape using a stationary laser beam. The irradiation was done at the centre of flat circular sheet whose diameter was 25 with 1 mm thick and the material was AISI 304 stainless steel sheet. They used a 2 kW Yb fibre laser. Irradiation time was taken from 1 to 4 s,
with variable laser spot diameter from 6 to 12 mm and power as 300 W. It was reported that with constant laser beam diameter and laser power, for higher irradiation time more bending occurred. BM was produced while using a beam diameter around 10 times of sheet thickness.
2.2.3 Upsetting mechanism (UM)
Upsetting mechanism (UM) initiates for laser beam diameter significantly smaller than the thickness of sheet. Furthermore, high thermal conductivity of the workpiece material assists UM (Pretorius, 2009). In UM, the workpiece gets shortened in length and it becomes thicker in thickness direction near to laser irradiation. Therefore, it is also termed as shortening mechanism (Shen 2008). Shi et al. (2012) reported that the UM appears due to irradiation on thick sheet accompanied by gentle scanning speed of laser and smaller laser spot diameter. The process parameters in UM are similar to BM with the exception that heated area is considerably smaller as compared to sheet thickness; thus the buckling is not permitted by the workpiece. The slow speed produces almost consistent heating of the sheet across the thickness. There is a decrease in flow stress and approaching of thermal strain toward elastic strain at yield stress in the heated area. Further heating results a plastic compression on heated material due to restriction for free expansion by surrounding bulk material. Consequently, the bulky thermal expansion gets converted into plastic compression. It results compression of the sheet with an almost constant strain along the thickness. Finally it leads to a decrease in length with increase in thickness for the sheet (Li and Yao 2001a; Lawrence et al. 2001; Hu et al. 2002 and Shi et al. 2006b). The mechanisms for laser bending are shown in Fig. 2.1.
(a) (b) (c)
Figure 2.1 Schematic representation of laser forming: (a) temperature gradient mechanism, (b) buckling Mechanism and (c) upsetting mechanism