LIST OF TABLES
CHAPTER 3: EXPERIMENTAL STUDIES ON LASER BENDING OF MAGNESIUM ALLOY M1A
3.3 Experiments on Laser Bending of Magnesium Alloy M1A
3.3.3 Bend angle measurement
The bend angle generated due to laser irradiation was measured at middle of the scanning path using ZeissTM make coordinate measuring machine (VISTA; 1620-14, DCC, BND, K) (Figure 3.6). The specifications of the machine are given in Appendix 3.3. The touch probe was moved along x, y and z axis to collect the data points on either side of the scan line. Two points were collected on each side of the laser scan line, which formed two lines on both sides of the scanning line. The bend angle was computed between these two lines. Figure 3.7 shows a schematic of data points located on the worksheet to obtain the bend angle as
1 1
tan d c tan a b
cd ab
u u u u
L L
, (3.1)
where, notations used in Equation 3.1 are shown in Figure 3.7.
Figure 3.6. Coordinate measuring machine used to measure the bend angle.
Scan line
a b
c d
ua
ub
u
cud
Lab
Lcd
b a d c
Lab
Lcd
Figure 3.7. Schematic of bend angle measurement.
The process (laser) parameters, viz. laser power (P), scan speed (V) and beam diameter (D) were varied to control the laser bending operation. Levels of various laser process parameters are shown in Table 3.1. Full factorial experiments were carried out for the laser
bending process. Each experiment was performed thrice to record the repeatability of the response, i.e. bend angle. The bend angles obtained for all sets of process conditions are reported in Appendix 3.4. The average of three trials was considered as the experimental value.
The coefficient of variation (CV) was calculated as the ratio of standard deviation to the mean of three trials. From the feasibility experiments, it was observed that a bend angle of about 1.4°
can be achieved by using single laser scan. The process condition with low heat flux density, for example P=300 W, V=3000 mm/min and D=7.74 mm, did not bend the worksheet. During the course of experiments, it was also observed that a combination of high laser power, fast scan speed and small beam diameter produced larger bend angle. The effects of various process parameters on the bend angle and edge effect are discussed in details in Chapter 5.
Table 3.1. The range of laser process parameters used in the experiments.
S.
No.
Laser Power (W)
Scan Speed (mm/min)
Stand-off Distance (mm)
Beam Diameter (mm)
1. 300 1000 20 3.87
2. 400 2000 30 5.81
3. 500 3000 40 7.74
Experimental studies on the laser bending of magnesium alloy M1A showed that the bend angle of about 1.4° can be achieved in single laser scan, which makes the process suitable for the micro-bending applications. However, many a times, a larger bend angle (of the order of 10º) needs to be generated in real practice. Larger bend angle can be generated by applying multiple scans instead of single laser scan. In the present work, the laser bending of magnesium alloy with multiple scans is also carried out to check feasibility of obtaining large bend angle using lasers in magnesium alloy sheets. Total ten number of scans were applied for the same set of process conditions used in the single scan laser bending process. All the ten scans were carried out in natural cooling conditions. Each laser scan was followed by a natural cooling of about 5 seconds. In this duration, the laser source was moved from the scan ‘end’ position to its ‘start’ position and prepared itself to start the next irradiation. The worksheet was cooled naturally during the complete scanning cycle of the process. The forced cooling was not applied at any stage of the experiments. Coating was applied only once before the first irradiation. The coating was ablated due to laser irradiation (see Figure 3.4), but it was not applied again before the start of next irradiation. After ten irradiations, the specimen was removed from the laser machine, and the bend angle was measured over the coordinate measuring machine (CMM).
Figure 3.8. Laser bent specimens with multi-scan laser bending process.
Figure 3.8 shows the magnesium alloy M1A worksheet specimens undergone multi- scan laser bending process. It is observed that a significantly large bend angle can successfully be produced with multiple irradiations. For various set of process conditions as mentioned in the Table 3.1, the bend angles obtained after the completion of ten laser scans for all sets of process condition are presented in Appendix 3.5. It can be seen that a significantly large bend angle of about 15° was achieved with ten laser scans. The smallest bend angle produced is about 0.89°. Thus, it can be concluded that the laser bending process is suitable for both micro as well as macro bending of magnesium alloys. During the course of experiments, it was also observed that a combination of high laser power, slow scan speed and large beam diameter produces small bend angle. It may be due to high temperature at the bottom surface and low temperature gradient along the thickness direction. The detailed analysis on bend angle in multi-scan laser bending process is presented in Chapter 7.
Laser bent specimens after ten laser scans are shown in Figure 3.8. During experimental study, it was observed that the melting occurs when laser power is high, scan speed is slow and beam diameter is small as shown in Figure 3.8 (a). In some cases, the melting zone was very small (Figure 3.8 (b)), while in some cases, a significant melting was occurred in the irradiated region (Figure 3.8 (a)). However, the melting was noted to be localized, and occurred in the irradiated region only. It can clearly be seen that the bend angle is less when melting occurs in the heated region.
Figure 3.9 (a) shows the bottom surface of the laser bent specimen. It can be observed that the bending radius is about 4.6 mm which is very small as compared with that obtained in the mechanically bent specimen shown in Figure 1.2 (Chapter 1). The laser bent specimen has a sharp edge on the bending line. The laser bent specimen was seen under the optical magnification using ZEISS made optical microscope. The magnified image of the bottom surface (away from laser irradiation) of the laser bent specimen is shown in Figure 3.9 (b). It can be observed that the cracks are not generated during multi-scan laser bending of the
magnesium alloy M1A worksheet. For the similar bend angle of about 17.52°, many cracks were observed in the mechanical bending of worksheet as shown in Figure 1.2. Thus, the experimental study revealed that the multi-scan laser bending is suitable for producing crack- free large bend angle in magnesium alloy worksheets.
Figure 3.9. Laser bending of magnesium alloys (a) bent specimen and (b) magnified image.