Laser welding of dissimilar metals is an important welding method, which is widely used in aerospace, automobile manufacturing, electronics, battery energy, and other industries. In laser welding, the active element oxygen in air is inevitably absorbed by the weld pool, but in some welding technologies, specific quality adjustments can be made by adding oxygen to wires. Some scholars have studied the effects of oxygen on the weld pool flow, weld pool size, and laser welding properties. However, the effects of oxygen on laser welding of dissimilar metals are reported, especially the effects of oxygen on the weld morphology, microstructure, and properties of dissimilar metal with large different thermal-physical parameters, chemical compositions, and mechanical performances. Here, 304 stainless steel (304SS) and nickel are selected for the experiments of laser welding of dissimilar metals. The effects of oxygen content in the shielding gas on weld pool morphology and dimension, solidified microstructure, alloy element dilution, and mechanical properties are analyzed. This research provides useful references for the utilization and protective effect of active element oxygen in laser welding of dissimilar metals and the performance regulation of welded joints.

The experiments are conducted on a five-axis numerical control machining robot, using a continuous fiber laser with a wavelength of 1060 nm. A protective gas mixing device is designed to realize the quantitative mixing of oxygen and argon. The 304SS and pure nickel plates (40 mm×30 mm×1.3 mm) are used as the experimental materials. The welding experiments are first conducted under the mixed shielding gas of oxygen and argon, and the volume fraction of oxygen changes from 0, 8%, 16% to 21% (volume fraction) in the parametric study. Then, the weld pool morphology, solidified microstructure, and alloy element distribution of the obtained metallographic samples are observed by optical microscopy, scanning electron microscopy (SEM), and energy dispersive spectrometer (EDS), respectively. Finally, the material testing machine is used to test the tensile strength.

The cross-sectional morphology of weld has the obvious geometric asymmetry, in which the melted area and melted depth at the 304SS side are large. With the increase of oxygen content, the weld width decreases, but the weld depth and melted area increase (Fig. 5). Besides, the weld surface is oxidized seriously due to the increased oxygen content (Fig. 4). When oxygen is mixed into the shielding gas, the surface tension coefficient changes from negative to positive, resulting in the change of the flow mode of the weld pool. As a result, the energy absorbed by the liquid metal from the laser is transported to the bottom of the weld pool, which reduces the width and increases the depth of the weld pool. The smaller thermal conductivity and liquidus temperature of 304SS lead to a deeper and larger weld pool as well as a larger melted area at the 304SS side. The value of the morphological parameter G/R [the ratio of temperature gradient (G) to solidification rate (R)] at the bottom of the weld pool is larger, and the columnar dendrites exist in the cases of 21% oxygen and pure argon. From the bottom to the top of the weld pool, the G/R decreases slowly in the case of pure argon, and the top is the mixed dendrites of columnar dendrites and equiaxed dendrites. In the case of 21% oxygen, G/R decreases rapidly, and the top is the equiaxed dendrites (Fig. 6). The cooling rate GR in the case of 21% oxygen is also large, and the microstructure is fine. Due to the smaller scale of microstructure, the microhardness (Fig. 8) and tensile strength (Fig. 9) are high in the case of 21% oxygen. Additionally, the presence of oxygen might produce metal oxides to further improve the microhardness of the weld. Due to the concentration gradient, the alloy elements diffuse from the matrix at both sides to the middle of the weld pool. In the case of 21% oxygen, the convection flows from the fusion boundary to the center of the weld pool, which is consistent with that for the element diffusion, and promotes the dilution of different elements to the center. Thus, the element mixing in the case of 21% oxygen tends to be more uniform. In the case of pure argon, the results are opposite (Fig. 7).

In present study, laser welding experiments are conducted on the 304SS and nickel plates under mixed protective atmosphere with different oxygen contents and the effects of oxygen content on weld morphology, microstructure, and properties are analyzed. Because of the active element oxygen, the direction of Marangoni convection changes to be opposite, the shape of weld pool also changes from shallow and wide to deep and narrow, and the area of weld pool extends. In the case of 21% oxygen, the distributions of alloy elements are more uniform. The solidified microstructure is mainly the equiaxed dendrites on the top of weld pool and the columnar dendrites at the bottom. In the case of pure argon, the microstructure in the top area is the mixed crystal of equiaxed dendrites and columnar dendrites, in contrast, and that at the bottom is columnar dendrites. Due to the fine microstructure of the weld, the microhardness in the case of 21% oxygen is higher than that in the case of pure argon. However, with the increase of oxygen content, the weld surface quality becomes worse. Therefore, the welding quality can be controlled to a certain extent by adjusting the oxygen content in the shielding gas. With the increase of oxygen content in the shielding gas, the driving force and flow mode of liquid metal convection in the weld pool change, which increases the depth and melted area of weld pool and makes the asymmetry of weld pool at both sides more obvious. Furthermore, when the oxygen content increases, the metal elements are mixed more uniformly, and the microstructural morphology is almost unchanged. However, the microstructural size decreases, and the microhardness and tensile strength increase.