Tham khảo tài liệu 'laser welding part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 54 Laser Welding A 3x beam expander is used in combination with the mm triplet lens to obtain a minimum focus spot size of pm. Equation 1 shows how to calculate the minimum spot size. Lens Focal Length Spot size ---------- ---- ---------------- - Fiber Diameter Collimator Optics Focal Length Beam Expantion Factor mm 9 um um 25 mm 3 The laser beam is centered with respect to the beam expander and the laser head. The laser head contains the focusing triplet and can be adjusted using the outer ring. At the bottom of the cutting head there is a chamber that allows for shielding to flow out through the welding nozzle. This chamber is sealed by a special cover glass and a rubber gasket. The determination of the laser beam s focusing position was done by using a laser drilling technique. One of the fiber laser s particular characteristics is that when a laser pulse is released there is an approximately 1 500 W power spike that is output for about 1 ps before it drops to the steady-state power value of 300 W. By pulsing the laser for a very short time approximately 3 ps we can take advantage of this power spike and create a very high power density at the focus plane. This enables us to perform laser ablation to form a crater into a stainless steel plate. The focusing technique utilizes this process by creating spot welds or holes at different z-positions every 10 pm. A picture is taken of each group of welds holes and using special calibrated software the radii are measured and plotted versus the z-focus position. Fig. 2. Spot welds or holes at different focusing locations Since the laser spot size is very different at different focusing positions the pulsing will either create very small holes approximately on the order of the focused beam spot size or larger spot welds. When all the radii are plotted the minimum of the resulting curve shows the approximate location of the focusing plane. This is a relatively quick and effective way to find the .
