Hiroo Takenaka

Direct observation of keyhole behaviour during pulse modulated high-power Nd:YAG laser irradiation

When a high-power laser beam is irradiated on the surface of material, it is well known that a cavity, called a keyhole, is generated in the molten material. To examine the effect of laser beam pulse shape on the keyhole behaviour, we observed it directly with an in situ microfocused x-ray transmission imaging system. As a result, it was found that the keyhole began to be generated after about 0.6-0.7 ms and became deepest after about 1.5 ms from the laser irradiation initiation and collapsed about 1 ms after laser power declination when a pulse modulated laser beam of 1.1 ms rise time and 4.6 kW peak power was used. It was also understood that the generation and the collapse of the keyhole was repeated, synchronizing with laser beam irradiation of 100 Hz. The transition of the keyhole depth and the cross sectional area had good agreement with the transition of the irradiated beam shape. Moreover, it was observed that the porosity was frequently generated at the beam shape in which the laser power was decreased rapidly from a high peak power.

Keyhole behavior during all position welding with high power Nd:YAG laser

The keyhole behavior in flat, downward vertical, upward vertical and overhead welding was observed with X-ray transmission imaging system. The laser beam, where the pulsed beam of 2 ms width was superimposed to the modulated rectangular beam of 100 Hz frequency and 50 % duty at the delay time of 0 - 9 ms, was used in the welding. Under the formation conditions of sound beads without generation of sputters, the generation and collapse of a keyhole were repeated synchronizing with the modulated frequency of the laser beam in all welding positions.Moreover, in vertical welding, the molten metal was brought downward by gravity and blockaded the inlet of the keyhole. Then, bubbles often remained in the bottom of the molten pool, resulting in the formation of porosity. On the other hand, the penetration depth increased in the case of 1 - 2 ms of the delay time and in 4 ms of Td the penetration width increased without increasing the penetration depth in all welding positions. The penetration was the deepest in the overhead position welding and was the shallowest in flat position welding in all the ranges of the delay timeThe keyhole behavior in flat, downward vertical, upward vertical and overhead welding was observed with X-ray transmission imaging system. The laser beam, where the pulsed beam of 2 ms width was superimposed to the modulated rectangular beam of 100 Hz frequency and 50 % duty at the delay time of 0 - 9 ms, was used in the welding. Under the formation conditions of sound beads without generation of sputters, the generation and collapse of a keyhole were repeated synchronizing with the modulated frequency of the laser beam in all welding positions.Moreover, in vertical welding, the molten metal was brought downward by gravity and blockaded the inlet of the keyhole. Then, bubbles often remained in the bottom of the molten pool, resulting in the formation of porosity. On the other hand, the penetration depth increased in the case of 1 - 2 ms of the delay time and in 4 ms of Td the penetration width increased without increasing the penetration depth in all welding positions. The penetration was the deepest in t...

Welding and Cutting Car-body Metal Sheets with Fiber Delivered Output from the Continuously Pumped and Envelope-Modulated, High-Power Nd: YAG Lasers

Fiber delivery of CW-pumped, high power Nd:YAG laser is attractive for three-dimensional, flexible material processing. Maximum average output power exceeding 2kW has been obtained for CW-pumped Nd:YAG lasers. Some of the advantageous features of CW-pumped Nd:YAG lasers are, high overall efficiency exceeding 3.5%, long lamp life by utilizing Kr arc lamp, relatively compact and floor-space saving as compared with pulse pumped lasers, capability of peak-power enhancement by envelope modulation, etc.