Study on the evolution processes of keyhole and melt pool in different laser welding methods for dissimilar materials based on a novel numerical model

The stake welded T-joints which are joined by face plate and web plate perpendicularly are as the typical structures with the lightweight and high-strength advantages, which has been extensively used in ships, bridges, rocket and other fields [[1], [2], [3], [4]]. To meet the diversified requirements, the stake welded T-joints with different performances can be customized by choosing dissimilar materials as face plate and web plate. As a green and advanced joining technology, laser welding is an ideal joining method to achieve the joining of face plate and web plate in the stake welded T-joints due to high energy density, high penetration capability and low distortion [5,6]. However, solute segregation, pores, spatters and other welding defects are easy to be generated in the welding of dissimilar materials stake welded T-joints owing to welding process instability and dissimilar materials property differences, which will affect the welding quality of stake welded T-joints greatly.

Dissimilar materials laser welding as a challenge has been focused in the welding field recently. Oliveira et al. [7] conducted CoCrFeMnNi high entropy alloy and 316L stainless steel laser welding. They found that the as-cast substrate hardness in the fusion zone was increased, which was probably caused by the solid solution strengthening due to dissimilar materials mixing and carbon incorporation from 316L stainless steel. Rossini et al. [8] carried out the laser welding experiments with four kinds of high strength steels. It was found that the segregation of Mn element might weaken the mechanical properties of dissimilar materials butt joints and lead to the failure in the fusion zone due to the intergranular fracture along the prior austenite grain boundaries. Wu et al. [9] obtained the SA553-SUS304 dissimilar materials joints by laser welding. The extremely inadequate mixture of dissimilar materials in the partial melting zone near SA553 interface was found under the condition of high cooling rate, which resulted in the formation of martensite with different morphologies and compositions. Yan et al. [10] investigated Fe/Al laser welding and observed that Fe₂Al₅, FeAl₃, FeAl and other Fe-Al intermetallic compounds were formed at Fe/Al interface due to composition segregation, which led to the decrease in plastic and ductility of welds. Chen et al. [11] obtained the 316L/GH909 dissimilar materials welded joints under the process conditions with the same linear energy. It was found that smaller temperature gradient at edge of melt pool was formed under low laser power and low welding speed, which resulted in more uniform distributions of elements and microstructure and better welded joint tensile property. Esfahani et al. [12] studied melt pool dynamic behaviors in low carbon steel and stainless steel laser welding by numerical calculation. They found that joint properties were influenced by alloy element concentration and weld homogeneity. The melt pool dynamic behaviors and alloy mixture in fusion zone were affected by temperature gradient and surface tension. Chatterjee et al. [13] analyzed microstructure in the Ti/Ni laser welding and found that weld macroscopic morphology asymmetry was caused by dissimilar materials thermophysical property differences. The macro-segregation was formed since molten metal flow behaviors induced by the density difference of material components. Chen et al. [14] analyzed the effect of heat source location on welded joint mechanical properties in dissimilar materials hybrid laser-TIG welding. Their results showed that the heat source location had great influence on weld morphology, microstructure and macro-segregation and welded joint mechanical properties. Halim et al. [15] discussed the transport phenomena in Al/Mg laser welding and found that molten metal dynamic behaviors which were influenced by temperature gradient variation and inter-diffusion between dissimilar materials had great effects on the molten metal convection and melt pool morphology.

As a new welding method with the potential to improve welding quality, oscillating laser welding (OLW) has received extensive attention. Ai et al. [16] discussed the effects of oscillation parameters on energy distribution and dynamic behaviors in circular shaped OLW. They found that increasing the oscillation amplitude or frequency could improve energy distribution uniformity. The stability of keyhole, the distributions of temperature and flow velocity as well as the morphology of melt pool could be improved by adjusting the oscillation parameters. Li et al. [17] explored the effect of oscillation path on melt pool dynamic behaviors. The melt pool with the lowest temperature gradient and the most complicated vortices and the stablest keyhole were obtained in circular shaped OLW, which helped reduce the pore defect in the weld. Liu et al. [18] revealed the mechanism of pore reduction in sinusoidal shaped OLW of 7075 aluminum alloy. They found that the keyhole was more stable and the pores in the weld were fewer in OLW than that in conventional laser welding (CLW), which was caused by increasing keyhole diameter and weakening surface tension. Xia et al. [19] obtained the weld with fewer pores by OLW. It was because oscillation process could improve energy distribution, reduce peak temperature of melt pool and weaken the effect of molten metal flow behaviors on keyhole. Zhou et al. [20] studied Ti/Al OLW and found that pore and crack defects were reduced, weld width at interface was increased and Al to Ti diffusion was weakened, which could improve the weld quality. Liu et al. [21] discussed the effect of laser beam oscillation on pore defect suppression in medium-thick high‐magnesium aluminum alloy laser welding. They found that pores were reduced due to keyhole opening expansion and suppression of keyhole collapse in infinity shaped OLW. Han et al. [22] proposed a novel welding strategy to produce ultra-fine equiaxed grains in Al-Li OLW. The OLW could reduce pore defect and facilitate Zr element diffusion to improve the uniformity of microstructure distribution. Chen et al. [23] carried out D406A ultra-high strength steel OLW. Their observed results indicated that the small bubbles were formed after the large bubbles hit by laser beam and then small bubbles were probably captured by keyhole and escaped from melt pool with metallic vapor.

From the previous researches, it can be found that the OLW under the suitable process conditions has the potential to improve the uniformities of element, microstructure and temperature distributions and reduce pore defect. Therefore, the circular shaped OLW is employed for the Q235 low carbon steel and 316L stainless steel laser welding. A novel numerical model based on a combined heat source and an improved recoil pressure formula which can achieve the high calculation accuracy is developed to explore the evolution processes of keyhole and melt pool in CLW and OLW, which is of great importance for improving dissimilar materials stake welded T-joint welding quality.