###### Abstract  
The weld penetration variation in laser-MIG hybrid welding under sensitive laser power range was investigated by welding experiments and CFD (computational fluid dynamics) simulation. During this investigation, joints of AH36 sheetsthe plate deformation was small, the melt pool depth was large, and the thick steel plate could be welded through in a single pass. [PERSON] et al. [8] used pulsed laser welding of stainless-steel heterogeneous joints; they found that inputting a low energy pulse facilitated the production of fine grains and good mechanical properties. [PERSON] et al. [9] used laser welding of 316 L and EH36 heterogeneous joints; they found reducing the welding speed was effective in reducing the degree of tissue segregation in the joints. The laser-arc hybrid welding process couples the two heat sources with each other, fully integrating the advantages of laser and arc welding. During welding, the arc stability is good, and the laser keyhole is stable. Laser-arc hybrid welding can produce high-quality products with high efficiency and has been widely used to weld thick plates of 15 mm or more [10; 11; 12; 13]. [PERSON] et al. [14] studied the melting drop transition form of laser-MIG hybrid welding; they found the arc energy determines the melting drop transfer mode, and the laser energy has an important effect on the melt drop transition. [PERSON] et al. [15] researched the laser-arc hybrid welding process for thick plates; they found that arc can effectively reduce the requirements for laser power and optimize energy input. In laser-MIG hybrid welding, metal forms a hybrid molten pool by absorbing laser and arc energy. Under arc irradiation with low energy density, the molten pool is mainly affected by the Marangoni effect caused by surface tension [16; 17], so the resulting molten pool is large in width and small in depth. Under the action of a high-energy-density laser, the molten pool is mainly affected by the steam recoil pressure. In addition, the generated keyhole is more likely to transfer the energy to the melt pool's bottom and increase the penetration depth. The laser-arc hybrid weld forms a typical \"nail\" shape with the coupling of heat sources. Furthermore, the melt flow affects the forming quality of the weld. The study of melt flow is important to analyze the morphology and mechanical properties of welded joints. A three-dimensional transient mathematical model using CFD can help to study the melt flow inside the melt pool. [PERSON] et al. [18] established a simple 3D heat transfer model, taking into account the influence of phase transition, and found the relationship between heat transfer, mass transfer, and temperature gradient in the molten pool. [PERSON] et al. [19] established a 3D heat transfer and fluid flow model, using a laser-arc hybrid heat source and considering the influence of electromagnetic force, recoil pressure, and surface tension. They also researched the influence of welding speed, current, laser power, and other parameters on melt flow and weld morphology of AA6082 aluminum alloy. It was found that the keyhole behavior plays a leading role in the penetration depth. [PERSON] et al. [20] established a mathematical model to study laser-arc hybrid welding of stainless steel. They found that the welding process parameters can be optimized by simulating keyhole variations in the melt pool. [PERSON] et al. [21] researched the effect of droplet transition on melt flow in laser-MIG hybrid welding; a 3D mathematical model of droplet transition was established, which further considered the effects of surface tension, electromagnetic force, and buoyancy on melt flow. It was found that the penetration of the welded joint is larger and the width is smaller when the droplet impact is considered. [PERSON] et al. [22] established a 3D mathematical model for laser-arc composite welding of high strength steel to simulate the deformation and residual stress of a steel plate after welding.