The behaviour of material components under laser bending process were influenced by combinations of laser process parameters, geometry, material parameters and shape of heat source. In this work, the effect of process parameters, such as laser power, beam diameter, type of heat source and workpiece geometry on the bending mechanism, bend angle and edge effect are presented.
4.3.1 Effect of laser power
Laser power is an important parameter that directly controls the heat flux density of the laser beam and the energy input into the workpiece surface. The heat flux density and energy input both increase with the laser power. The effects of laser power on the bending of 25 mm 20 mm 2 mmworkpiece size was observed for stationary and moving heat sources. The simulation and experimental results revealed that the bend angle was critically dependent on the laser power. It was observed that,
the numerical results were in good agreement with the experimental results. The trend of variation of bend angle predicted by the developed numerical model was similar to those obtained in the experiment.
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Figure 4.2 Effect of laser power on the bend angle for stationary and moving heat sources on 25 mm 20 mm 2 mm workpiece for (a) 8 mm beam diameter and (b) 4 mm beam diameter
It is observed that laser bending is not possible at laser power below certain level. It is because of the reversible elastic effect that requires a threshold energy. In present work, it is observed that for a typical case i.e., D = 4 mm and scan speed = 20 mm/s, there is no bending of the workpiece up to 200 W. With further increase in laser power, the bend angle of the workpiece increases.
In case of stationary heat source with 8 mm laser beam diameter, the bend direction was negative (away from laser source) for the range of laser power of 100200 W. Beyond 200 W, the workpiece started to bend towards positive direction (towards laser source) as shown in Fig. 4.2 (a). However, for moving heat source, the bending direction was negative for the range of 100250 W. The bend angle obtained in case of moving heat source was too small as compared to the stationary heat source.
In case of stationary heat source with 4 mm beam diameter, the bending direction was positive for the range of laser power of 150250 W, while it was negative for the laser power of 100 W as shown in Fig. 4.2 (b). In the case of moving heat source, the bend angle was negative for the range of 100200 W. After 200 W, it started to bend toward positive direction.
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Figure 4.3 Effect of laser power on the bend angle for stationary and moving heat sources on 15 mm 10 mm 2 mm workpiece for (a) 8 mm laser diameter and (b) 4 mm laser diameter
The similar study was carried out for 15 mm 10 mm 2 mmworkpieces.
Here, for 8 mm laser beam diameter, the bend angle was very small and the bend was away from the heat source (negative) for both stationary and moving heat source as shown in Fig. 4.3 (a). The experimental results revealed that there was no bending of the workpiece from 100 W150 W laser power. However, with further increase in laser power the workpiece bent towards negative direction (Fig. 4.3 a). The trend was different for 4 mm laser beam diameter as shown in Fig. 4.3 (b). The bend direction was negative in the range 100 W150 W of laser power, but the sheet started to bend toward positive direction after 150 W for stationary heat source. In the case of moving heat source, the bend direction was negative for 100 W250 W laser power.
4.3.2 Effect of laser beam diameter
Figure 4.2 and 4.3 show the effect of laser beam diameter on bend angle after applying moving and stationary heat sources, respectively. From Fig 4.2 (a) and (b), it can be seen that the bend angle increased with the decrease in laser beam diameter.
This was due to increase in the heat flux density and temperature gradient along the thickness direction with the decrease in beam diameter. The same behaviour is seen in Fig 4.3.
4.3.3 Edge effect
In all the cases, the bend angle was greater in the case of stationary heat source.
However, in the case of stationary heat source, the bend angle was not uniform across the width as shown in Fig. 4.4. At laser power of 100 W, the bend angle along scan direction was almost uniform. The variation in the bend angle along width direction increased with increase in the laser power. The bend angle was higher at the middle of the workpiece along the scan direction.
Figure 4.4 The effects of stationary heat source on bend angle along width direction by using 4 mm laser diameter on 25 mm 20 mm 2 mm workpiece
In the case of 25 mm 20 mm 2 mm, the edge effect for both type of heat sources was studied. For 250 W laser power, the bend angle deviation between stationary and moving heat source was approximately 2° and the bend angle was the maximum at the middle of the bent workpiece in both experimental and simulation results (Fig. 4.5 a). However, for 100 W moving heat source, the edge effect was very small (Fig. 4.5 b). For 100 W laser power, the bend angle was larger at the middle of the workpiece in both experimental and simulation results. However, the bend angle direction along scanning path was opposite to that for 250 W of laser power.
The edge effect in the workpiece occurs due to uneven temperature distribution along the scan length. In case of stationary heat source the temperature is the maximum at the heating point and reduces at the points away from it, whereby the bend angle is the maximum at the heating point. However, in moving heat source the temperature is the maximum at nearby end point of the scan line which reduced the temperature gradient along the thickness direction and therefore the bending was only slightly reduced at the end.
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Figure 4.5 Effect of laser power on bend angle for 25 mm 20 mm 2 mm workpiece for (a) 250 W laser power and (b) 100 W laser power