Studies On The Dynamic Process And Shape Characteristic During Laser Lap Welding T-joints For High Strength Steel

High power laser welding is a quick and flexible method for manufacturing metal sandwich plate and is becoming the best choice of processing technology of metal sandwich plate. I-core metal sandwich plate is the most representative of sandwich structure with simple structure and easily manufactured core plate, while T-joint is the basic welding unit for I-core metal sandwich. Weld shape, process behavior and stability in laser lap welding T-joints were studied, which was of important realistic significance to provide the requisite technical support guidance for the application of laser welding technique in manufacturing sandwich plate and the solution on formulating laser welding metal sandwich plate.For the stability of welding process and the quality control of welding seam in laser welding web core sandwich plate, process experiment platform of laser lap welding T-joints was established in this paper, and the effect of processing parameters, assembly parameters and plate thickness on the weld bead geometry was investigated. High speed camera and spectroscopic monitoring system was built, and plasma plume, keyhole and molten pool dynamics and interaction between dynamics were studied during laser lap welding T-joints. Keyhole and molten pool dynamic behavior by x-ray transmission observation system were observed, and the formation mechanism of bubble and porosity and its prevention during laser lap welding for T-joints were discussed. The following are the main results:The major characteristic of laser lap welding T-joints is the process behavior change of plasma, molten pool and keyhole in different gap. First, plasma is divided into plama escaped from gap(gap plasma) and plasma located above the sample(upper plasma).The plasma escapes from the collapsed front keyhole wall in the gap, and results in the process behavior change of plasma. As increasing gap, the front keyhole wall presents periodic collapse and close, and plasma escapes from gap. The height and inclination angle of upper plasma decrease with a quadratic function approximately. The area of upper plasma linearly decreases. The area and intensity peak frequency of upper plasma decreases with a cubic function. The escaped time from the gap increases with a quadratic function. The size and dynamic behavior of molten pool and keyhole opening vary with gap. The length and area of molten pool surface decrease with a quadratic function approximately. The keyhole opening area linearly increases. Laminar flow of molten pool is translated into vortex and the flow speed quickens. The welding process behavior can be divided into quasi-stationary with small gap, instability with medium gap and false stability with large gap. The gap in laser lap welding can be real-time monitored and quantitative forecasted what is based on the qualitative and quantitative relationship of the process behavior characteristics and gap.Porosity or cavities forms the bottom and middle of weld bead of laser spot welded T-joints with small gap, while the porosity disappears with large gap. Porosity in laser lap continuous welded T-joints is process porosity. Plenty of porosity forms with 0.4-0.8 mm gap and the porosity rate are highest with 0.8mm gap. The porosity presents two parallel at the gap and bottom of weld seam. The porosity at the gap is round, while porosity at the bottom is irregular.The bubble and porosity in laser lap continuous welding T-joints are less with small gap, however, a lot of bubbles induced the violent cyclical swings of the tip of keyhole forms. Meanwhile, molten pool is divided into three parts that are upper molten pool, lower molten pool and molten pool at the gap. Large amount of bubbles are difficult to escape from the bottom of molten pool and the gap due to the crystalline frontier of liquid metals in the bottom of molten pool and poor fluidity of molten pool at the gap, and lots of porosity forms at the bottom of weld and the gap.The formation model of bubble in laser lap welding T-joints has been suggested based experimental observation and analysis.The formation mechanism of bubble is as follows: the stress states of keyhole wall occur to vary due to the disruption and close of front keyhole wall, moreover, internal and external plasma of keyhole change with increasing gap, which results in the disproportionate distribution of laser energy in keyhole. The tip and wall of keyhole presents unstable periodic fluctuation. The formation of bubble is the necessary condition of the formation of porosity, while the variation of molten pool flow with gap is the sufficient condition of the formation of porosity. Keeping small gap or using high welding speed avail to maintain the stability of keyhole and molten pool, and less bubbles and porosity form in laser lap welding T-joints.There are three kinds of welds that are Y-type, V-type and bowl-type weld in different heat input for laser lap welded T-joints. Y-type and V-type can be obtained in low and high welding speed, respectively, while bowl-type can be obtained when the molten pool width of the back of face plate is bigger than the thickness of core plate. Bead width at the interface is narrower with more than 3mm thick face plate, and it is difficult to be increased by optimizing process parameters. The weld bead easily offsets with less than 3mm thick core plate, and results in depression and bad fusion. Bead width at the interface depends on the face plate thickness and welding speed, and the big bead width at the interface can be achieved when the junction of core plate and face plate locates in above the inflexion of Y-type weld.The regression models on bead width at the interface and penetration were established upon process tests. The results show that plate thickness and laser power significantly affects bead width at the interface and penetration, respectively. The gap less affects weld profile geometry and melting efficiency, but a large amount of porosity forms with large gap. The excellent laser lap welded T-joints can be obtained under using low welding speed and controlling little gap and offset size with less than 3mm thick face plate and more than 3mm thick core plate, and welding efficiency and energy efficiency are higher.Microstructure of HAZ and fusion zone(FZ) of laser lap welded T-joints for high strength steel is composed of lath martensite and bainitic ferrite. The microstructure of part phase change zone presents strip-like characteristics and small block martensite distributes along the grain boundary, and the continuity of microstructure was cut off. The soft zone locates in transitional region between tempering zone and mixed grained zone. Microhardness of soft zone is 310 HV and drops 18% compared to base material. The width of soft zone and HAZ decreases by using low heat input, while the microhardness of HAZ increases.