CHAPTER 7: MULTI-SCAN LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
7.4 Bending Mechanism: Effect of the Number of Scans
Figure 7.4. Absolute prediction error between numerical and experimental bend angle.
During experimental studies, it was noticed that for some of the process conditions such as P=500 W, V=1000 mm/min and D=3.87 mm, the work surface melts. Figure 7.3 (d) shows the bent specimen with a melted work surface. During melting, the applied energy is utilized for the phase transformation, which affects the plastic deformation of the worksheet. For an efficient bending operation, melting is not desired. Melting is generally governed by many random parameters, such as surface conditions, thickness of the coating, damage of the coating in preceding scans, and surrounding conditions. These parameters are difficult to control. Also, it is very difficult to model the effects of such random parameters in the numerical model.
However, the present numerical model considers the effect of melting by taking into account the latent heat (Section 4.2.2).
The validated numerical model was further used to investigate the multi-scan laser bending of magnesium alloy M1A. Simulations were carried out to study the effect of process parameters such as laser power, scan speed, beam diameter on the performance parameters, viz.
temperature, stress, strain, bend angle and edge effect during multi-scan laser bending process.
7.4.1 Temperature distribution
Figure 7.5 shows temperature history at Point A (top) and Point B (bottom) for multi-scan laser bending of magnesium alloy M1A for a typical process condition of P=300 W, V=1000 mm/min and D=3.87 mm. It can be observed that a steep temperature gradient occurs between top and bottom surfaces of the worksheet. In each successive scan, peak temperature at both top and bottom surfaces is higher than the previous laser scan as the worksheet is preheated.
The peak temperature and percentage change in the peak temperature at top and bottom surfaces after each laser scans are shown in Figure 7.6. It can be seen that the percentage change in temperature at the bottom surface is more than the top surface. It is due to the continuous heat flow from the top to the bottom surface as a result of temperature gradient. The peak temperature increases due to the preheating of worksheet in each successive scan, and exceeds the melting temperature (649 °C) in sixth laser scan. The similar observation was also made during the experimental study on multi-scan laser bending of magnesium alloy (see Figure 7.2 and Figure 7.3). This verifies the need for the consideration of the melting phenomenon in the numerical model, which makes the model more realistic.
Figure 7.5. Temperature history at top and bottom surfaces.
Figure 7.6. Change in temperature at top and bottom surfaces with number of scans.
7.4.2 Stress distribution
During laser bending, as the temperature increases, thermal expansion occurs in the heated region. The surrounding material at relatively low temperature resists the expansion of the heated region. This results in the generation of compressive stresses in the heated region and tensile stresses in the surrounding region.
Figure 7.7 shows x-direction stress history at Point A and Point B. It can be observed that as the laser beam reaches over Point A, compressive stresses are induced. It is because the temperature at Point A becomes higher compared to the surrounding material, and hence the expansion at Point A is resisted by the surrounding cooler materials. When the laser beam moves away, the temperature at Point A decreases, and therefore the thermal expansion at Point A is less as compared to the nearby heated region. This induces tensile stresses at Point A. In the worksheet the heat flows in the thickness direction quickly, which leads to the increase in temperature at Point B. This generates compressive stresses at Point B. These tensile and compressive stresses are induced during each laser scan (Figure 7.7). It can also be observed that the induced stresses decrease with the successive laser scans. It is because the worksheet temperature increases in each scan, and the heated material provides less restriction to the thermal expansion. This results in the decrease in thermal stresses after each scan.
Figure 7.7. x-direction stress history at top and bottom surfaces.
Figure 7.8. y-direction stress history at top and bottom surfaces.
Figure 7.9. Von-Mises stress history at top and bottom surfaces.
Figure 7.8 shows y-direction stress history at Point A and Point B. Stresses induced in y-direction result in the worksheet bending along the scanning path. It can be seen that the components of induced stresses decrease in each consecutive scan. It is due to the softening of material at the elevated worksheet temperature during each scan, which results in less resistance of material to the thermal expansion in the heated region. The decrease in the magnitude of induced thermal stresses components results in the decrease of von-Mises stresses as shown in Figure 7.9.
7.4.3 Plastic strains and distortions
During the laser bending operation, when the induced thermal stresses exceed the temperature dependent flow stress, the plastic deformation occurs in the worksheet. The x-direction plastic strain history at Point A and Point B is shown in Figure 7.10. It can be observed that the plastic deformation is compressive at both Points A and Point B. It may be attributed to the fact that the temperature at Point B is high enough to generate the plastic deformation. The plastic compressive deformation at Points A and Point B increases with increase in the number of scans. The difference between plastic deformation at Point A and Point B also increases with increase in the number of scans. This leads to the increase in bend angle after each scan. Thus, the worksheet bends mainly due to the difference between plastic deformations at the top and bottom surfaces. The history of worksheet bending during multi-scan laser bending process is shown in Figure 7.11. It shows that the bend angle linearly increases during the laser heating phase, and it is almost constant during the cooling phase. The bend angle increases with increase in the number of scans.
Figure 7.10. x-direction plastic strain history at
top and bottom surfaces. Figure 7.11. Worksheet bending history at middle of scanning path.