| Keyhole effects are the essential characteristic in deep penetration laser welding. In thisarticle, the development and Present Situation of research on laser welding technique at homeand abroad are introduced at first. And then, the domestic and abroad research on keyholeeffects is analyzed through keyhole profile, Fresnel absorption and inverse bremsstrahlungabsorption. Because the mechanism of keyhole effects is very complicated, recent studies athome and abroad are mainly concentrated either on the qualitative analysis or on simplequantitative calculation with highly simplified model. Considering the main problems ofpresent studies of keyhole effects, on the basis of the keyhole profile and the spectra emittedfrom keyhole plasma, both of which are observed directly in the experiment, thecomprehensive effects of Fresnel absorption and inverse bremsstrahlung absorption arestudied during deep penetration laser welding in the article. The distribution of laser intensityabsorbed by the keyhole wall through multiple reflections and Fresnel absorption, isquantitively calculated. And then, the mechanism of keyhole effects is studied deeply andsystematically.Firstly, with a specially-designed equipment which is used to observe keyhole profile,the keyhole profile is obtained by High-speed photography method during deep penetrationCO2laser welding of aluminum alloy6016. And then, by means of software Photoshop andMatlab, the keyhole profile is transformed into mathematical model. Under the assumption ofcircular cross section of the keyhole, a3D keyhole is reconstructed, which lays the foundationfor the theoretical study of keyhole effects.Secondly, with a specially-designed equipment which is used to observe spectra, bymeans of spectrograph and CCD, the spectra emitted from keyhole plasma is obtained directlyduring deep penetration CO2laser welding of aluminum alloy6016, including the spectraalong keyhole radius direction and depth direction. Through Abel reverse transform andrelative light intensity method, electron temperature of keyhole plasma is calculated both inkeyhole radius direction and depth direction. And then, with the spectra and average electrontemperature in the keyhole, combining with full width at half maximum of spectrum, thedistribution of electron density in the keyhole is calculated. In addition, the changingregularity of electron temperature and electron density of keyhole plasma is analyzed.At last, according to geometrical optics theory, that how laser beam propagates in thekeyhole is traced, and the laser intensity absorbed by keyhole wall through Fresnel absorption and inverse bremsstrahlung absorption of keyhole plasma is calculated. Results indicate that,during deep penetration laser welding of aluminum alloy6016, inverse bremsstrahlungabsorption of keyhole plasma plays a dominant role in absorbing laser power compared withFresnel absorption. The laser intensity absorbed through the former is some orders higher thanthat through the latter. |
