This paper describes the features and the application of three monitoring methods that we selected the optical emission, the acoustic emission and the plasma generating potential in laser welding.
In laser welding, various in-process monitoring methods have been studied. Some of them have been applied in actual manufacturing process, such as automobile industries, canmaking industries etc., for quality assurance.
However, it seems that features of them have not been clarified yet. This means it is difficult to clarify the relation between the welding phenomena and the sources of the monitoring signals.
We looked into the behaviour of laser induced plasma by monitoring the spectrum emission, acoustic emission and the plasma generating potential, as well as by observing plasma and molten pool by CCD video.
In the test, we performed a bead on stainless steel plate specimen by using 5.5kw CO2 laser. We selected the Fe atom spectrum for the optical emission. We detected the acoustic emission signal in the range from 10Hz to 40kHz. And we detected the plasma generating potential between the laser nozzle and a specimen.
Analyzing average, amplitude and the figure of signals from three monitoring method, the correlation among them and the relation between each of them and the welding characteristics were studied.
Main conclusions are as follows;
The plasma behaviour and monitored signals change strongly depending on selection of the shielding gas. In case of Ar gas shielding, amplitude value of the monitoring signals rather than that of average shows the closer relation with the welding characteristics.
The combination of the signals from the above three monitoring methods gives more definite information on the welding abnormality, such as lack of penetration, etc.
A model on the mechanism of laser welding phenomena is proposed from analyses of the monitoring signals and the CCD video pictures.
This study was supported by NEDO(New Energy and Industrial Technology Development Organization).