Research On Characteristics Of Deep Penetration Welding Of Thick Q235 Steel Plates With 30kW Level Laser

Ultra high power and high quality fiber lasers have been put into the market and gradually began to be applied to high power laser welding of thick plates.The laser beam power density can reach 107~108W/cm2,which means that the penetration depth of a weld can be even deeper,so that a new solution can be obtained for the high efficiency and low deformation welding of thick plate large scale components.This paper systematically studied the welding characteristics of Q235 steel plate with 40 mm thickness welded by 30 kW laser deep penetration welding.The study found that there are welding defects such as spatter,undercut and crack on the welds.Only through changing the welding speed,defocus,the type of protective gas and other welding parameters are difficult to obtain deep and well formed welds.Further,high speed camera were used to observe the surface characteristics of the molten pool and the unique metal vapor/plasma morphology for revealing the formation process of the spatter in the ultra high power laser welding.When 30 kW laser welding,the molten pool fluctuates violently.The amount of the metal solution that flows out from the small hole continuously increases,and finally spatters escape.At the same time,the metal vapor/plasma generated during the deep penetration welding of 30 kW laser is far more than the conventional laser welding.The height of metal vapor/plasma reaches 50 mm.Metal vapor/plasma consists of a cyan plasma and gold metal vapor.Its shape is variable,and its volume changes periodically with time.The spectral characteristics of 30 kW ultra high power laser induced plasma were measured using spectrometer.Spectrums of Ar,Fe,Mn,and C elements can be detected in the spectrum.The appearance of C element in the spectrum is a special phenomenon of ultra-high power laser welding of carbon steel.The reason for the formation of welding defects in the welding process was revealed.On the one hand,The formation of ultra-high power laser welding defects such as spatter is due to the violent evaporation of metal materials caused by ultra-high power lasers.And on the other hand,because the metal vapor/plasma generated during the ultra-high power laser welding is too large,it has a great influence on the amount of energy input to the workpiece and the stability of the molten pool,resulting in limited weld penetration and surface undercut.Based on the causes of welding defects,the control methods of welding defects were proposed: The laser beam leans backward by 15° in the welding direction,and the shielding gas is blown against the center of the opening of the keyhole.Welding defects such as splashing were suppressed,and welds with good surface formation and deep penetration were obtained.The maximum penetration depth of the optimized 30 kW laser welding reaches 27.4mm,which is significantly larger than the maximum penetration depth of 19.8mm before optimization.The microstructure of Q235 steel plate welded by 30 kW laser deep penetration welding is obviously nonuniform.The columnar grain size in the upper part of the weld and in the middle of the weld is larger than the lower part of the weld.The upper and middle parts of the weld bead are pre-eutectoid ferrite and acicular ferrite,and the main parts of the weld are martensite and bainite.All weld tensile specimens in different locations are fractured in the parent metal far from the weld.The maximum tensile stress fluctuates within the range of 472.7~487.4MPa.Fracture surface is composed of small and shallow dimples,indicating that the specimen fails in a ductile manner under the action of tensile loading.