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Based on the characteristics of laser welded structural parts of auto bodies, the influence of variation in laser beam incidence angle on the lap welding performance of galvanized auto-body sheets was studied. Lap welding tests were carried out on the galvanized sheets for auto-body application at different laser beam incidence angles by using the optimal welding parameters obtained through orthogonal experiment. The effects of incidence angle variation on seam appearance, cross-sectional shape, joint mechanical properties and microstructure of weldments were analyzed. In addition, the main factors influencing the value of incidence angle were investigated. According to the results, the weld seams had a good appearance as well as a fine, and uniform microstructure when the laser beam incidence angle was smaller than the critical incidence angle, and thus they could withstand great tensile and shear loads. Moreover, all tensile–shear specimens were fractured in the base material zone. When the laser beam incidence angle was larger than the critical incidence angle, defects like shrinkage and collapse tended to emerge, thereby resulting in the deteriorated weldability of specimens. Meanwhile, factors like the type and thickness of sheet, weld width as well as inter-sheet gap all had a certain effect on the value of laser beam incidence angle. When the sheet thickness was small and the weld width was narrow, the laser beam incidence angle could be increased appropriately. At the same time, small changes in the inter-sheet gap could greatly impact the value of incidence angle. When the inter-sheet gap was small, the laser beam incidence angle should not be too large.

Dynamic behaviors of molten pools during active laser welding of thick 304 stainless steel plates are monitored in real time by utilizing a high-speed camera system. The effects of various active fluxes on the dynamic variations of molten pools are analyzed, and the results are compared with the uncoated specimens. The research reveals that all the molten pools present periodic variations. Among them, the specimen coated with NaF active flux exhibits the shortest cycle of molten pool change, with a single cycle duration of approximately 0.5 s. The specimen coated with TiO2 active flux exhibits the longest cycle of molten pool change, and the single cycle duration is around 1.1 s. Moreover, the molten pools also show varying widths. The maximum molten pool width is found with the specimen coated with CeO2 active flux, whose value is 3.277 mm. Meanwhile, the minimum molten pool width is found with the specimen coated with NaF active flux, whose value is 1.799 mm. In addition, the coating of active fluxes also produces certain effects on molten pool stability. The specimen coated with NaF active flux presents intense emission of metal vapor during welding, drastic oscillation of the molten pool, and severe spattering of metal particles. Contrastively, the molten pool of the specimen coated with Cr2O3 active flux is rather stable. In conclusion, by selecting an appropriate active flux, the cycle of molten pool variation can be prolonged, the pool stability can be enhanced, and the occurrence of spattering and other defects can be suppressed.

To study the feasvhibility of activating welding technique in increasing the width of low-power pulsed laser penetration weld joint and to improve the efficiency of low-power laser welding while reducing cost, theoretical analysis and experimental verification are conducted in this study to determine the effects of activating flux Cr2O3 on the welding performance of DC56D galvanized steel plate for a vehicle body. Research data on the variation in weld width, joint microstructure, mechanical properties, distribution of elements, and corrosion resistance are acquired. Results indicate that activating flux can effectively increase the YAG laser energy absorption of the steel plate and melting of parent metals, thus increasing the weld width. Although the joint microstructure is dominated by refined isometric crystals and columnar crystals, the heat-affected zone expands slightly because of the activating flux. No evident diffusion and segregation of active elements are observed in the weld joint. The involvement of activating flux does not change the relative contents and distribution of chemical components of the weld. The welded test piece with activating flux is superior to that without activating flux in terms of tensile strength, microhardness, and corrosion resistance, indicating that the addition of activating flux effectively improves the performance of low-power laser penetration welding test pieces.

In this study, the dynamic behaviors of metal vapor, plasma, keyhole, and weld pool during activating laser welding of stainless steel thick plates are monitored in real time by employing a novel "sandwich" method in combination with a high-speed camera system. Meanwhile, the welding quality of specimens is inspected and analyzed from the aspects like joint weld formation, mechanical properties, microstructure, and element distribution. The effects of Cr2O3 active agent on the welding stability and quality of specimens are explored. According to the results, adequate coating of active agent on the specimen surface can weaken and suppress the generation and drastic changes of metal vapor and plasma inside and outside the keyhole, increase the keyhole depth, improve the keyhole shape, weaken the oscillation amplitude of weld pool, and prolong the change cycles of keyhole and weld pool. Consequently, the stability of welding process is enhanced, the generation of porosity and splash defects is inhibited preferably, the quality of weld formation is improved, the weld penetration is increased, the grain structure in weld zone is refined, the width of heat affected zone is narrowed, and the joint strength and toughness are enhanced, without showing apparent change of the relative element contents in the welded joint. In conclusion, coating of Cr2O3 active agent on the specimen surface is effective in improving the welding properties of laser welded stainless steel plates.

For overlap joints with different galvanised sheet combinations, the inter-sheet gap size and range control under different conditions were experimentally studied, and the factors affecting the inter-sheet gap range were examined. The application of a δ control method to laser welding of car door parts was explored. The results indicate that the formation and strength of weld joints initially improve and then worsen as the inter-sheet gap increases. The inter-sheet gap for different types of sheet combinations has a specific range, the value of which is related to the sheet thickness, weld pool width, welding speed, and laser beam parameters. Further, the consistency of the size of δ along the entire welding bead can be maintained by using a subsection welding method. Application of the conditions identified here, in combination with optimal technological parameters, can facilitate the high-quality welding of auto-body parts.

Our previous study included a comparative analysis of the characteristics of CO 2 laser overlap welding and resistance spot welding in terms of the weld fundamentals, tensile–shear performance, microstructure, hardness, and corrosion resistance of welding joints. In this study, the processing properties of the two types of welding methods were further investigated. First, experiments were conducted on dissimilar auto body sheets using resistance spot welding and laser welding with different weld lengths. Next, the layout of the laser welding joints was analyzed. Finally, experiments were performed on an auto door sub-assembly using resistance spot welding and laser welding respectively. Experimental results showed that both the sheet thickness and weld pool width were correlated with the laser weld length (LWL). When the LWL was increased to 20 mm, the tensile–shear performance of the specimen was better than that of one resistance-spot welding point under various conditions. When performing weld joint layout using the “short-length and multi-segment” scheme, it was possible to produce laser welding specimens with a preferable joint morphology and mechanical properties. The laser welded auto door with welding joints having an optimized weld length and layout had better quality and performance, and the laser welding process had higher flexibility.

A systematic study on overlap welding properties of autobody galvanized steel is conducted by using the late-model fiber laser and CO 2 laser respectively. The welding joints surface formation, cross-sectional weld shape and mechanical properties of the two types of lasers are compared, and it is analyzed whether dissimilar laser sources would impact the welding quality, so as to control and optimize the craft quality of laser welding automobile body. The results show that under the test conditions, the weld fusion width presents wide at the top and narrow at the bottom and the cross-sections of overlap joints take a “Y” shape by using CO 2 laser welding, while those by using fiber laser welding have a shape like “I” which was mean the fusion widths of the upper and lower surface are almost the same. And the anti-shearing force of fiber laser welding joints was stronger due to its larger inter-sheet joint connection width. In addition, under certain conditions, the fiber laser welding could result in more uniform and finer joints, higher strength and hardness, and better mechanical properties.

In views of structure characteristics of the auto-body parts, the influences of the beam incident angle and joint gap on the performance of laser overlap welded joints were investigated. The experimental results indicate that there were the critical values of beam incident angle and joint gap during laser overlap welding of galvanized steel. The thickness of sheet and the width of joint had a certain influence on the critical beam incident angle and the limit joint gap. With regard to thicker sheet, the limit joint gap can increase appropriately, but the critical beam incident angle should not be too big. With narrow weld width, the laser beam incident angle can increase appropriately, but the joint gap should not be bigger. Additionally, the critical beam incident angle and the limit joint gap were varied with the thickness of the upper sheet. The tensile-shear tests show that the maximum tensile-shear strength of the joint can be obtained with an optimized beam incident angle and joint gap.

The CO 2 laser overlap welding and the resistance spot welding are respectively investigated on DC56D galvanized steel used for auto body. The characteristics of the two types of welding methods are systematically analyzed in terms of the weld molding, tensile-shear performance, microstructure, hardness, and corrosion resistance of welding joint. The results show that, the fusion widths of the upper and lower surface are almost the same for the resistance welding joint, and the weld nugget is surrounded by the heat-affected zone. While the laser welding joint belongs to deep penetration welding, the weld fusion width presents wide at the top and narrow at the bottom, and the heat-affected zone is situated on both sides of the weld pool. Compared with resistance spot welding joint, laser welding joints have much more ultrafine microstructures, much smaller heat-affected zones, as well as greater resistance to deformation and corrosion. In addition, the tensile-shear performance of laser weld joints is superior to that of resistance welding joints under certain conditions.

Based on the lumped parameter method, this study establishes an experimental scheme for YAG laser absorptance measurement. Laser absorptance is measured on artificial diamond and cubic boron nitride materials under different laser power levels at room temperature. This study presents an analysis of the influence of laser spot size, and number of repeated laser irradiation, heat-insulating material, etc. on absorptance. A theoretical parameter is provided for the process of laser truing and dressing of the corresponding abrasive wheel, and it is important for the analysis and modeling of laser-material interactions. Meanwhile, a reference for measuring the laser absorptance of material with the same physical performance is put forward.