CHAPTER 5: SINGLE SCAN STRAIGHT LINE LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
5.4 Effect of Process Parameters on Bend Angle
In this section, the effects of energy (laser) parameters such as laser power, scan speed and beam diameter on bend angle are presented. The bend angle is measured at the middle of the scanning path. Based on the numerical and experimental analysis, the bend angle are also compared.
5.4.1 Effect of laser power
Laser power directly controls the input energy into the worksheet. The input energy is more at a higher laser power. Figure 5.14 to Figure 5.16 show the effect of laser power on bend angle.
It can be observed that at a scan speed of 2000 mm/min and 3000 mm/min, the bend angle increases with the increase in laser power. It is because the heat energy absorbed by the worksheet is more at higher laser power. This results in the increase in peak temperatures, which leads to the higher plastic deformation and larger bend angle.
Figure 5.14. Effect of laser power on bend angle at small beam diameter.
Figure 5.15. Effect of laser power on bend angle at medium beam diameter.
Figure 5.16. Effect of laser power on bend angle at large beam diameter.
At a slow scan speed of 1000 mm/min, the effect of laser power interacts with the beam diameter. At large beam diameter, the bend angle increases with the increase in laser power as a result of high energy input. At small beam diameter, the bend angle increases with the increase in laser power, attains a peak, and then decreases with further increase in the laser
power as shown in Figure 5.14. It is because the peak temperature at the bottom surface is high enough to generate the plastic deformation, which reduces the difference between plastic deformations at the top and bottom surfaces. The increase in compressive deformation at the bottom surface results in the decrease of bend angle at higher laser power.
Figure 5.17. Plastic strains at Point A and Point B at different power levels.
Figure 5.17 shows the comparison between plastic deformation at top (Point A) and bottom (Point B) surfaces for two different levels of laser power. The process conditions used are:
1. High power: P=500 W, V=1000 mm/min, D=3.87 mm 2. Medium power: P=400 W, V=1000 mm/min, D=3.87 mm
It can be seen that the x-component of plastic strain at the top and bottom surfaces are -0.03189 and -0.01527 respectively for the process condition with medium laser power, while it is -0.04887 and -0.0261 respectively for the high laser power. Due to the increase in laser power, the percentage increase in plastic deformation at the top and bottom surfaces is about 53.2% and 70.9%, respectively. Therefore, the percentage increase in plastic deformation is more at the bottom surface as compare with that of the top surface. This leads to the decrease in bend angle with increase in the laser power at slow scan speed and small beam diameter.
5.4.2 Effect of scan speed
Scan speed controls the input energy into the worksheet by changing heat input per unit length.
Figure 5.18 to Figure 5.20 show the effect of scan speed on bend angle. It can be seen that in general the bend angle reduces with the increase in scan speed. This is mainly due to the fact that the contact time between laser beam and worksheet surface decreases with the increase in scan speed. Thus, the energy absorption per unit length decreases at faster scan speed. This
results in the decrease in peak temperature, which leads to the reduction in plastic deformation in the heated region.
Figure 5.18. Effect of scan speed on bend angle at small beam diameter.
Figure 5.19. Effect of scan speed on bend angle at medium beam diameter.
Figure 5.20. Effect of scan speed on bend angle at large beam diameter.
At small beam diameter and high laser power, the bend angle first increases with scan speed, attains a peak, and then decreases with further increase in the scan speed as shown in Figure 5.18 and Figure 5.19. It is because the peak temperature and the resulting plastic deformation at the bottom surface is high enough to reduce the bend angle at high laser power and small beam diameter. The higher plastic deformation at the bottom surface reduces the bend angle at slower scan speed, when worksheet is irradiated with higher laser power. In that case, the temperature gradient between top and bottom surfaces increases with the increase in scan speed. The magnesium alloys have high thermal conductivity, and significant temperature gradient cannot be maintained at slow scan speed. The increase in scan speed leads to a lower
peak temperature at the bottom surface, which may increase the bend angle when plastic deformation at the bottom surface is significantly high.
5.4.3 Effect of beam diameter
Figure 5.21 to Figure 5.23 show that the bend angle decreases with the increase in beam diameter. The beam diameter controls the heat flux density. The heat flux density decreases with the increase in beam diameter. It results in the decrease in peak temperature and temperature gradient between top and bottom surfaces as shown in Figure 5.5. This leads to the decrease in bend angle. It can be observed that the rate of decrease in bend angle is more, when laser power is less. It is because the peak temperature at lower laser power is too less to create a significant plastic deformation. Comparing the Figure 5.21 to Figure 5.23, it can also be observed that the rate of decrease in bend angle with increase in beam diameter is more at faster scan speed.
Figure 5.21. Effect of beam diameter on bend angle at slow scan speed.
Figure 5.22. Effect of beam diameter on bend angle at medium scan speed.
Figure 5.23. Effect of beam diameter on bend angle at fast scan speed.