Study On The Technology And Plasma Behaviour During The Fiber Laser And Laser Hybrid Welding Of Aluminum Alloy

It will be widely applied in the modern industrial fields due to prominent advantages oflaser and laser-arc hybrid welding technology. High power fiber laser is becoming moreand more attractive in recent years due to its excellent characteristics, but the relatedresearch was far less than CO2laser and YAG laser because of it appearing in a relativelyshort time. Considering the domestic and foreign research situation, although the appliedscope of laser and laser-arc hybrid welding technology can be expanded with the help ofprocess investigation, it is greatly restricted in industrial practical applications on accountof the lack of understanding mechanisms.In this dissertation, systematic process tests of fiber laser welding and its hybridwelding in aluminum alloy were carried out using the self-built test platform. Plasmabehaviors were observed by high-speed video and spectrometer, and the mechanisms werestudied, such as the influence of plasma on the laser energy transmission, the interactionbetween laser induced plasma/vapor and arc plasma, and so on. Aiming at practicalapplications, the formation mechanism and preventive measures of the weld defectsoccurring in fiber laser lap welding of aluminum alloy also were also studied. Thefollowing are the main results:The plasma/vapor induced during fiber laser welding of aluminum alloy can be roughlydivided into two parts: the keyhole plasma and the floating plasma. The keyhole plasma isfound close to the keyhole and always exists, but its size and brightness changeperiodically. As its size increases, its brightness also increases. The floating plasma staysabove the weld pool. Its size and brightness also change periodically, and it can be blownaway by the gas flow. The periodic change of the floating plasma synchronizes closelywith that of the keyhole plasma. The laser induced plasma/vapor oscillates periodically atcycles of450to600s. The use of a shielding gas has little effect on the oscillation cycle.A new interpretation about the periodical oscillation of plasma/vapor induced during fiberlaser welding was developed-the periodic oscillation of the plasma/vapor can be largely attributed to the oscillation of the keyhole opening and the molten pool.With the increase in the laser power and the decrease in welding speed during fiber laserwelding of aluminum alloy, the size of plasma/vapor increase, and they have the sameeffects on the penetration depth. Under a certain condition, the size of plasma/vapor canreflect the penetration depth. Plasma/vapor has little influence on the laser transmission;the method had a noteless effect on the penetration depth by blowing the plasma/vaporthrough the side shielding gas.It can be found in the process research on5754aluminum alloy of2mm thickness infiber laser lap welding: weld joints with good surface appearance can be obtained underthe appropriate process parameters using the flat nozzle with the extended form; thoughplasma/vapor having less effect on the laser transmission, spatters are formed because ofpart of the liquid metal flying out molten pool under the action of plasma/vapor, and thespatters increase with the increase of laser power density; only when welding speed wasvery low, hot cracks would be produced due to less tendency to produce hot cracks, andthe critical welding speed of appearing hot cracks decrease with the decrease of the laserpower; the weld joints with tiny grains and little burning loss of low boiling point alloyelement Mg have good tensile shear performance and have no obvious joint softened zone,micro-hardness of which is slightly lower than the base metal.A new kind of cavity was proposed-bi-film cavity, many of this kind of cavity withirregular shape and large size have acute angle and contain black oxide material interiorly.The formation of this kind of cavity can be attributed to the refractory oxide film betweenthe upper and lower sheets in the lap welding. This kind of unique cavity forming due tothe particularity of the lap welding had a prodigious influence on mechanical properties,which can be effectively decreased by means of chemical cleaning or mechanicalprocessing methods to remove the oxide film, the mechanical properties of weld jointsincrease greatly.A new hybrid plasma model was developed, by which the mechanism of the interactionbetween laser induced plasma/vapor and arc plasma has been expounded. The hybridplasma forms a channel between the welding wire and the keyhole during the fiberlaser-MIG hybrid welding process. The channel is composed of two parts: region A and region B. Region A is generated by the laser induced plasma/vapor, the most significantchange is that its size and brightness are enlarged. Region B comes from the core of theelectrical arc, the most significant change is that its direction is altered.