New developments in advanced welding

Advances in laser welding processes

6.4.1 Remote laser welding

Remote or scanner laser welding is a highly efficient bonding process and CO2 lasers up to 6kW power are available with focal lengths up to 1.5 m.16,17 A system using a CO2 laser of extremely high beam quality has been developed to weld car parts, where a beam with long focal length is deflected by scanner optics, positioned and moved over the workpiece at high speeds. This welding can be completed in a much shorter time than can processes involving general resistance spot welding and laser seam welding.16 Attention should be paid to such items as complex clamping devices and accessibility, the effect of laser beam inclination angle to the workpiece on penetration and the gas shielding situation.17 It is anticipated that new concepts including robot guided scanner welding or flexible remote welding will allow flexible manufacturing for a wide range of applications, as shown in Fig. 6.23.18 Fiber and disk lasers of high beam quality are heat source candidates for robot-coupled systems.

6.4.2 On-line or in-process monitoring during laser welding and adaptive control

On-line or in-process monitoring and feedback or adaptive control are necessary and have been intensively investigated to produce high quality welds.74-86 In the monitoring process a reflected laser beam, light emission from the plasma/ plume and/or molten pool, etc., sound from the plasma or keyhole inlet, ultrasonic or acoustic sound from the metal inside, the plasma potential between the plate and the nozzle or laser-induced plume and other phenomena are investigated as signals in conjunction with penetration or welding defects. Monitoring/adaptive control systems are available; they utilize imaging observation of a keyhole and a molten pool and the reflection light of another laser beam such as LD and He-Ne laser from the butt-joint edge to be
welded or from the weld bead surface profile after welding. These are shown in Figs. 6.24 and 6.25.74 Coaxial imaging observation can judge a gap and also full or partial penetration in sheets and plates. Adaptive or feedback control systems are applied by varying either laser power or the welding speed on the basis of the data after detecting a butt-joint or a lap-joint gap during continuous or stitch welding with a high power laser. According to recent research involving aluminum alloys, it is interpreted that large spatters increase the heat radiation signal, and the melt-removed surface after spattering increases the beam reflection in the upward and subsequent inclined direction.81 The correlation between the reflected beam intensity and the heat radiation from the molten pool can be used to interpret full and partial penetration in the sheet. 81

Nd:YAG laser optics

6.25 Schematic drawing of different sensor set-ups for CO2 and Nd:YAG laser systems.

Underfilling, which degrades strength and formability, is also detected by the laser line interference method or plume light intensity.11, 83 Some on-line and off-line systems are used in the production lines of tailored blanks or aircraft panels, as shown in Fig. 6.24 and 6.26,77 respectively.

Moreover, in YAG laser spot welding of thin sheets, the lap-welded areas sometimes vary with samples and a through-hole defect is easily formed in the upper thin sheet during laser irradiation in some samples. Examples of the formation of a normal weld and non-bonded weld with a through-hole are illustrated schematically in Fig. 6.27.85 To produce sound laser partial - penetration lap-joint welds consistently, a new procedure of in-process monitoring and adaptive control has been developed for laser micro-spot lap

welding of A3003 aluminum alloy sheets of 0.1 mm and 1 mm in thickness. The system is shown in Fig. 6.28.85 The reflected laser beam and the radiated heat from the welding area are revealed to be effective as in-process monitoring signals in detecting melting, keyhole generation and through-hole formation in the upper sheet during laser irradiation. Laser pulse duration and peak power can be controlled at every 0.15 ms interval during the laser spot welding on the basis of the heat radiation signal. Sound partially penetrated spot welds are produced in all samples subjected to laser lap welding under the two proposed in-process monitoring and adaptive control methods. An in­process repairing technique has also been developed, during which the laser power is increased so as to melt further the lower sheet for bonding the two sheets after the detection of through-hole defect formation during spot welding, as shown in Figs 6.29 and 6.30.85 Such in-process monitoring and adaptive control systems have been developed to produce consistently sound partially and fully penetrated lap spot welds.85,86