Optimisation of welding using two Nd–YAG laser beams combined at workpiece surface

Two Nd–YAG laser beams were combined at a certain point on the workpiece surface to increase weld penetration depth. One of the beams was a pulsed laser beam, and the other was a continuous wave laser beam or a modulated laser beam. Using this combination of laser beams, a wide range of welding conditions, such as average power, peak power, and power density, could be selected. A high peak power pulsed laser beam would play a significant role in forming a keyhole, but a severe spatter loss problem could be encountered under high peak power laser conditions, thus the conditions necessary to prevent spatter loss were investigated. The greatest penetration depth is obtained under the critical conditions for spatter loss. Critical conditions for spatter loss are controlled by the peak power of a pulsed laser beam, thus deeper weld penetration is obtained using a pulsed laser beam with higher average power, that is, of longer pulse width and/or a higher repetition rate within the limit of the oscillator output. Moreover, spatter loss is reduced under conditions providing large molten zones in the weld, thus a higher peak power pulsed laser beam can be employed under such conditions. Large molten zones are obtained using a modulated laser beam of a high average power and/or low welding speeds.