# Effect of Heat Input on LMHMW Joint of Carbon Steel
## 1 Introduction  
With the characteristics of excellent mechanical properties, outstanding weldability and good hot workability, Q235 steel has widely and successfully been adopted in the high-voltage transmission tower, bridge, automotive, container and shipbuilding industries [1, 2, 3]. Q235 steel is a common carbon structural steel with moderate carbon content and a tensile strength that reaches 370-500 MPa. Fusion welding is the most common and effective welding method for structural steel connections [4, 5]. Typical arc welding processes such as gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) are found in abundance in the industry, but due to low power density, higher welding speeds are not possible. Hence, arc welding processes usually have a higher heat input which gives rise to a large number of distortions. On the contrary, laser welding, being a high power density heat source, can produce weld joints with larger depth to width ratios [6, 7, 8]. Additionally, high-quality joints can be obtained with a laser because of the short-acting time, higher welding speed, higher efficiency and minimal distortions [9, 10]. However, laser welding still has some shortcomings such as poor gap bridgeability, higher reflectivity (especially at larger wavelengths), high equipment costs and prone to produce cracks and pores in the welded joint [11]. Laser-arc hybrid welding combines laser and arc heat sources to make up for their individual shortcomings [12, 13, 14]. Thus, laser-arc hybrid welding has the merits of a large welding penetration depth, high process stability, high welding efficiency, lower heat input, a wide range of weldable materials and small welding deformation which is a potential development direction for Q235 plate welding [15, 16, 17]. [PERSON] [18] used a hybrid fiber arc laser welding (HFAL) process to study a new 8 mm thick hot-rolled Nb-Ti-Mo micro-alloyed steel. The results showed that with the increase of heat input, the martensite content decreased, but the granular bainite (GB) content increased for the microstructure components. [PERSON] and [PERSON] [19] studied the effects of laser and arc power, heat input and preheating on the macrostructure and microstructure of HY-80 steel composite welds. The results reveal that as the heat input increases, the ferrite microstructure increases, and the weld metal microhardness decreases. The research of [PERSON] [20] shows that the strength enhancement of the fusion zone and heat-affected zone of the LAHW joint are mainly attributed to grains refinement and precipitation strengthening. The arc causes most of the laser energy to be dissipated from the keyhole, so the heat input is reduced. Lower heat input makes the grains size refined, which weakens the over-aging and improves the tensile strength. [PERSON] [21] studied the joint microstructure and porosity distribution of 6.9 mm thick 5A06 aluminum alloy plates by laser metal inert gas arc (MIG) hybrid welding. The results show that metallurgical pores are densely populated in the upper area of the weld joint, resulting in obvious joint softening. The formation of pores has a much greater influence on joint softening than grain enlargement. [PERSON] [22] compared the MIG welding and laser-MIG hybrid multi-layer welding methods to get a more appropriate method to overcome the poor weldability of the invar36 alloy. The simulation and experimental results were compared, it was proven that the Gauss and cone combined heat source model can characterize the laser-MIG hybrid welding heat source well. The above discussion reveals that laser-arc hybrid welding technology has been recognized by the welding industry and has great development prospects. Hence, laser-MIG hybrid multi-layer welding (LMHMW) has been used by some large enterprises to replace traditional arc welding for structural steel connection. However, the work of most research groups has been focused on the exploratory process test of LMHMW, which broadens the welding range of hybrid welding but lacks parametric optimization.