| With the development of10-kW level laser, the technology of high power laserwelding thick plate has been expanded and applied in manufacturing of kinds of largeequipments. However, the weld defects, such as spatter, underfilling, porosity and soon, can be easily generated in the welding process. Compared to the kilowatt laserwelding, during the10-kW level laser welding, owing to the large amount of inputlaser energy, much more metallic vapor and much deeper keyhole can be generated,and the behaviors of vapor and molten pool may be more complicate. The mechanisminside the deep keyhole needs further investigation before revealed completely. Sothat, this paper has been concentrated on the interaction coupling behavior betweenthe metal vapor and molten pool and its relationship with the stability of weldingprocess during10-kW lever high-power deep penetration laser welding. Thesystematicly research has been carried out. The interaction physical processingbetween laser beam and substrate metal and the varying pattern have been studieddeeply, in order to reveal the reason and rule for the unstable welding process and theimportant effect to the welding process. The research results will provide a theoreticalguidance for implementing a stable welding process and improving the weld quality.Firstly, the experiment of the inclined-laser irradiating on the metal surface wasdesigned and implemented. The laser-induced vapor and molten pool were observedduring the interaction between the high-power laser and the metal. And the multiplereflections of laser beam were investigated at the same time. The experimental resultsindicated that the ripples of fluctuating molten metal could be easily generated on themolten pool surface under the irradiation of laser. As the power density of irradiatedlaser increased, the generating frequence and moving velocity of ripples increased.The small deformation of molten metal surface could change the reflected laserenergy greatly, and result in the wide difference of absorbed laser energy on differentpartial metal surface. The results of multiple reflections and absorptions revealed thatthe considerable absorption of laser energy mainly happened during the directirradiation of laser beam. In addition, a special experiment was designed and carriedout to investigate the thrust of the ejecting metallic vapor flow duringdeep-penetration laser welding. The results indicated that the ejecting vapor flow hascaused a velocity increment of the falling metal ball. But the value of velocity increment was much small than the velocity of generated spatter. In conclusion, thevapor flow outside the keyhole had a small effect, while the vapor flow inside thekeyhole plaied a greater role to accelerate the molten metal.Secondly, the modified “sandwich” method was introduced to implement theexperiment of10-kW level deep-penetration laser welding. A stainless steel sheet anda GG17glass were clamped together to make a butt joint sample of dissimilarmaterials. The characteristics of keyhole plasma was observed and investigated duringthe experiments. Based on the measured optical signal of plasma, the electrontemperature, electron density, ionization degree and pressure of plasma werecalculated. The results indicated that the short wavelength fiber laser induced lessplasma, and the number of plasma spectral lines induced by the shorter wavelengthfiber laser was smaller than that induced by the longer wavelength CO2laser.Compared to the plasma outside of the keyhole, the keyhole plasma had a largernumber of spectral lines and a higher ionization degree. The calculated resultsindicated that: the electron temperature of plasma was4400-6000K, the electrondensity of plasma was2-16×1016cm-3, the ionization degree of plasma was lowerthan0.01, and the transient pressure of plasma was5-943×105Pa. The largefluctuation of transient pressure of plasma could cause a fluctuation of vapor flowinside the keyhole. After investigation and comparison, it was obtained that: themetallic vapor in the present experiment had a high density and a bigReynolds-number, the vapor flow could perform a turbulent characteristic, and themetallic vapor flow could cause an impat effect to the molten pool.Thirdly, based on the modified “sandwich” method, the keyhole shape and thepatterns of vapor flow and keyhole wall were observed and studied during10-kWlevel laser welding. Under different welding conditions, the change of keyhole shapewas investigated and the interaction coupling behavior between vapor flow andkeyhole wall was analysed deeply. The results revealed that the gauffers generated onthe front keyhole wall could change the distribution of laser energy inside the keyhole,which could cause the fluctuation of vapor flow inside the keyhole. The rear keyholewall could fluctuate easily, which could result in the waves on the keyhole wall.Based on the force analysis on the waves, the conclusion were drawn that the tractiveforce to deform the keyhole wall was the vapor pressure and the auxiliary force wasthe pressure of molten metal. During the full penetration laser welding, the movementof the molten metal on the rear keyhole wall was determined by the vapor flow. Thefluctuation of vapor flow could cause the wave on the rear keyhole wall. And, when the wave broke at the keyhole outlet, it was accompanied by swelling, spatter,columns, a decrease of the diameter of keyhole inlet, and a change in the direction ofejecting vapor plume. When the defocused distance was changed, the distribution oflaser energy inside the keyhole was changed, which caused the pattern change of rearkeyhole wall. So to summarize: the input energy distribution of laser beam inside thekeyhole has a great effect to change the behaviors of vapor and molten pool and thedynamic keyhole shape.Finally, during10kW-level laser welding, the high speed camera was used toobserve the flow states of molten pool at upper surface, and the X-ray transmissionimaging system was used to investigate the flow states of internal molten pool. Therelationships between the welding parameters, the fluid flow of internal molten pooland the special weld shape were studied. And the affecting factors to the flow states ofmolten pool were analyzed. The results showed that the molten metal gathered atupper surface and the internal of molten pool could result in nail-head cross section ofweld and the size increase at certain depth inside weld, respectively. The materialvaporization was more violent due to the increase of power or decrease welding speed,which caused more molten metal extruded out form the keyhole. The changes of focalposition could change the whole flow states of both molten metal on the upper surfaceand the internal molten pool. Three different conditions of focal position: Positivedefocus,0defocus and negative defocus, resulted in three different flow states ofmolten pool at upper surface. After the analysis, the conclusion was drawn that: thechange of focal positon of laser beam resulted in the change of the distribution oflaser energy inside the keyhole, which led to the change of vapor flow states insidethe keyhole and led to the change of flow states of molten pool finally. To sum up, thedistribution of laser energy and the stability of vapor flow in the keyhole were theimportant factors to influence the molten pool. |
PDF Full Text Request