Résultats exploitables
The main result of the project is the technology for welding aluminium alloys with high power Nd-YAG lasers. The most efficient laser mode is CW laser with high beam quality in comparison to pulsed and pulse sustainer mode. The aluminium welding process developed has the following advantages:
high welding speed: with 3 kW CW laser power, up to 10 m/min for 2 mm thick sheets;
possibility of a fully automatic process even on 3D path using a robot;
high mechanical strength of the weld.
Knowledge has been gained regarding the definition of Nd-YAG laser quality parameters necessary to achieve the process parameters, the focusing heads and the connection with the automatic movement equipment along one or three dimension paths. This expertise has mainly been developed by the partners CRF, COMAU and Haas as laser constructor. A weld quality monitoring system integrated into an optical head has been developed by CRF using a commercial focusing head of Haas. The device is based on the measurement of the laser radiation reflected from the weld zone.
A new technology to weld aluminium alloys with (neodymium-yltrium aluminium garnet (Nd-YAG) laser has been developed. The objective was to remove the limitation of weld depth and of weld quality of 2 kW continuous or pulsed Nd-YAG lasers. The analysis of the weld quality with different laser mode has been done. The modes considered were: continuous, pulsed, pulsed-sustainer with a continuos mode superimposed to a pulsed mode, and the Q switched mode. The best quality was achieved with continuous mode using a high beam quality laser source. In fact, the beam spot on the material had to be so little to reach the threshold intensity of about 1.5E6 W cm{-2} even with power levels of 2 kW. In this condition better key-hole stability produced better weld quality. Successive pulses produced the opening and closing of the key-hole. With pulse-sustainer mode and Q switching mode the additional energy of the pulse created unstable conditions for the key-hole and the melt pool. Two ranges of process parameters have been developed experimentally. Two millimetre thick aluminium alloy sheets have been welded with 3 kW of continuous power at speeds up to 10 m/min. In order to weld thickness up to 5 mm, 5 kW of continuous power has been delivered to the workpiece using a new optical head with twin fibre system to sum two laser beams. Weld speeds up to 2.5 m/min have been reached. The other technological issues addressed by the project were the mathematical models of the welds, the monitoring on line the weld quality, and the movement of the optical head connected to the laser sources with fibres along a three dimension path using an anthropomorphic robot. Positive answers to these issues have also been given.
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