The welding technology that uses lasers as a heat source has advanced quickly in recent years as laser applications have become more popular. Laser welding has various advantages over traditional welding techniques, including minimal heat input, high efficiency, and good quality. Narrow gap laser (NGL) with filler wire will replace existing traditional welding methods as a new thick plate welding method. However, many challenges [hardening of the heat-affected zone (HAZ) and interlayer structure, lack of fusing of the sidewall and interlayer, porosity, and so on] hinder its further development. This study investigated the welding of shaft parts for real construction machinery to address the problem of high groove and machining accuracy requirements in NGL. The wobbling laser process was used to improve the groove gap tolerance and promote sidewall fusion. This study examined and discussed the dissimilar joint between Q355B steel and 35 steel tubes welded using NGL.
To weld the dissimilar joint of Q355B steel and 35 steel tubes with 22 mm thickness, 1 pass self-melting and 6 passes filling wire were used. According to relevant national standards, the tensile test, bending test, impact test, and microhardness measurement were conducted to evaluate the mechanical properties of joints. The cross-sections of the welded joints were ground, polished, and etched with 4% nitric acid alcohol solution. Weld seams microstructures were examined using an Olympus-GX53 optical microscope and a Quanta 250 field emission scanning electron microscope. The Wilson VH1102 semi-automatic tester was used to measure the microhardness distributions at the welded joints with a load of 0.5 N and a dwell time of 10 s. The YNS 1000 universal testing machine was used to perform tensile and bending tests. The impact test was performed using a Jb-500b pendulum impact testing machine.
There are no detects, such as penetration pores and cracks, lack of fusion for sidewall, and so on, inside the samples after non-destructive testing. The width of HAZ near Q355B has a wider breadth than that of the HAZ near 35 steel. Zone A is the fusing zone of the filling wire around 35 steel, as shown in Fig. 4. This region is mainly made up of a large number of fine acicular ferrite, proeutectoid ferrite, and tempered martensite. The interlayer area is designated as zone B. The top section of the previous weld bead is remelted, and the interlayer fusion line is not visible after cooling. The narrow area outside the fusion line is subjected to the additional heat treatment by next pass welding, which is equivalent to a normalizing treatment and results in grain refinement. Zone B has a lower heating degree than the upper welding wire-filled area, and the cooling rate is slow (Fig. 4).
The properties of tensile, impact, and bending specimens have superior qualities to those of basic metal.The microhardness difference of welded joint section is larger, and the hardness of the rear welding zone is greater than that of the weld zone.
The actual shaft parts were welded relying on the accumulation of previous experiments. The NGL with wire filling welding method was used to weld the shaft components without pausing by matching the welding parameters with the chuck speed (Fig. 10).
In this study, through the use of ER50-6 welding wire for NGL with filler wire butt welding of 35 steel and Q355B steel, the mechanical properties and microstructure can be analyzed, and the following conclusions can be drawn: (1) By optimizing the process parameters, the 35 steel and Q355B steel pipes were welded using the wobbling laser filler wire welding method, a total of 7 welding passes, the mechanical properties of the welded joints are great, and the tensile, bending and hardness properties are all up to the relevant national standards; (2) The wobble laser welding process is adopted to fully stir the molten pool throughout the welding process to promote and increase pore escape and sidewall fusion. Reduced linear heat input refines the heat-affected zone martensite and increases the content of acicular ferrite and granular bainite. The heat-affected zone structure in the middle layer undergoes a second thermal cycle, the coarse grains are refined, and the hardness is significantly reduced; (3) The narrow gap wobbling laser welding method successfully overcomes the shortcomings such as small groove tolerance and unstable processing, resulting in excellent sidewall and interlayer fusion of the welded joint, and a laser welding process for the narrow gap on the shaft components is proposed.
