Objective Single-pass laser welding double-sided forming technology has the advantages of small welding deformation, high welding strength, large welding aspect ratio, and high welding efficiency. Under normal circumstances, the thickness limit of a sheet with double-sided welding in a single pass is 13 mm. When the sheet thickness reaches 15 mm or more, the biggest problem is to no longer optimize the process parameters to obtain a good weld seam; however, after the laser power exceeds the threshold and laser energy, steam, plasma, and other coupling behaviors in the keyhole are complex, the root molten pool drips and the weld is difficult to form. This paper proposes a method of using the upward Ampere force generated by a stable magnetic field and directional current to assist in improving the flow characteristics of the molten pool and suppressing the dripping of the root molten pool; improving stability of the welding process, quality of weld formation, and welding efficiency; and greatly improving the thickness limit of thick plates in single-pass laser welding.
Methods The experimental material used in this study is 316L stainless steel, and the specifications of the specimens are 300 mm×40 mm×16 mm and 300 mm×40 mm×30 mm. The magnetic field is generated by a permanent magnet attached to the vise, and direct current is generated by a large current generator. In this experiment, 16-mm 316L stainless steel was first used to study the root dripping defects, and then 30-mm 316L stainless steel was used to investigate the ultimate thickness of laser single-pass penetration. In the study of 16-mm 316L stainless steel to suppress root dripping defects, four sets of comparative tests were first carried out: laser welding, laser current welding, laser magnetic field welding, and electromagnetic assisted laser welding. In order to ensure accuracy and credibility of the test and avoid the influence of current or magnetic field on the welding process, the single-factor experiment method is used to change only the magnetic field or current and keep other process parameters unchanged. The question is whether the Ampere force is the real force when electromagnetic-assisted laser welding of thick plates is studied, and the flow behavior of the molten pool at the root is photographed by a high-speed camera. In the study of the thickness limit of the 30-mm 316L stainless steel single-pass laser penetration process, the Ampere force was changed by variable current and constant other process parameters. The influence of the Ampere force on the depth of the 30-mm thick plate welding pool and weld formation quality impact was studied.
Results and Discussions Among the results of the four sets of comparative tests (Fig. 3), only the test method of electromagnetic-assisted laser welding can achieve significantly better welding results than other sets of results. The weld seam is Y-shaped; weld formation and weld quality is good. Through the test results of electromagnetic-assisted laser welding of 16-mm 316L stainless steel plates (Fig. 4 and Fig. 5) and by adjusting the magnitude of the magnetic field or current, the weld morphology can be significantly changed. Therefore, in the experiments, it is the Ampere force rather than the current or magnetic field that inhibits the root molten pool dripping. With the presence of Ampere force, even if the dripping defect occurs, the root molten pool drips evenly and the root of the weld seam is flush, which can ensure formation of the weld without cutting. The high-speed camera shooting result (Fig.7) shows the flow behavior of the root molten pool during electromagnetic-assisted thick plate laser welding, indicating that the Ampere force cannot suppress root protrusion. However, the Ampere force suppresses dripping of the molten pool and the root molten pool protrusions after that. Under the combined action of the Ampere force and the surface tension of the molten pool, the molten pool flows back into the weld without other defects and a well-formed weld is obtained finally. Through the experiment results of electromagnetic-assisted laser welding of 30-mm 316L stainless steel plates (Fig. 6), it can be seen that the Ampere force affects the depth of the weld, but hardly affects the width of the weld. Only by selecting the appropriate Ampere force, the dropping in the molten pool can be suppressed thereby forming a good weld and ensuring that the material can be completely welded. By setting the appropriate process parameters, a single-pass laser welding can be achieved for a 30-mm stainless steel plate, which greatly improves the welding efficiency.
Conclusions In this paper, electromagnetic-assisted laser welding of thick plates is used to carry out root molten pool dripping defect suppression process experiments and molten pool flow behavior research on 16-mm and 30-mm 316L stainless steel. In the experiments of 16-mm stainless steel to suppress the dripping defect of the root molten pool, neither the applied current nor the magnetic field alone can effectively suppress the root molten pool dripping defects generated during the single-pass laser welding of thick plates. Only by applying a constant electromagnetic field at the same time, the steady-state Ampere force generated can inhibit the dripping of the molten pool. During the welding process, the Ampere force can effectively inhibit the dripping of the molten pool, ensure good formation of the weld seam, ensure good welding quality, and improve the welding efficiency. In the 30-mm stainless steel laser single-pass penetration process test, the electromagnetic-assisted laser welding process method at constant current and constant magnetic field is feasible, laying the foundation for the development of ultra-thick plate laser welding process. During the welding process, the Ampere force can not only inhibit the dripping of the molten pool, but also significantly change the depth of the molten pool. The attached Ampere force cannot prevent the formation of root protrusions during the welding process; however, together with surface tension, it inhibits the downward dripping of the molten pool and helps the protrusion molten pool reflow to the weld area, ensuring effective weld formation.
