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The present study systematically investigated the effects of a laser welding speed and a defocusing amount on the microstructure and mechanical properties of low alloy high strength Q345 steel joints. The macro-morphology, microstructure, Vickers microhardness, and tensile properties of as-welded joints were analyzed. The results indicated a significant correlation between the laser welding speed and the size of individual zones within the welded joint. As the laser welding speed increased, the size of each zone decreased noticeably. The microstructure of the fusion zone was composed of proeutectoid ferrite, ferrite side-plate, and acicular ferrite. An increase in welding speed and defocusing amount facilitated microstructural changes towards formation of ferrite side-plate and acicular ferrite. Maximum tensile strength and maximum elongation of as welded joints were measured to be 669 MPa and 21.8%, respectively. The joints with maximum tensile strength tended to fracture within the fusion zone. The increase in welding speed and defocusing amount caused a significant decrease in normalized enthalpy and volumetric energy density. [ABSTRACT FROM AUTHOR]

• The effect of different FRM laser powers on weld morphology has been investigated. • The difference in the suppression of welding spatter for different laser combination modes of FRM laser welding has been studied. • The relation between microstructure and mechanical properties of the FRM welded joints has been revealed. Flexible Ring Mode (FRM) laser welding is a relatively new composite laser welding technology. The present study focused on the FRM laser welded joints produced from 4.5 mm-thick 304 stainless steel. The effects of various laser modes on the macroscopic morphology, microstructure, microhardness, and tensile properties of the produced welded joints were systematically investigated. The results showed that, when the ring laser power was set at 1000 W and the center laser power at 3000 W, a maximum weld area of 6.23 mm2 was obtained. Employing a composite laser with a center laser power greater than the ring laser power resulted in the fewest number of welding spatter particles on both sides of the weld bead, indicating an effective inhibition of welding spatter by the ring laser. Conversely, using a composite laser with a center laser power equal to the ring laser power still led to the presence of small welding spatter particles, indicating differences in the laser combination modes in suppressing welding spatter for the welding of 304 stainless steel. Increased cooling and a large presence of small turbulence contributed to the grain refinement in the fusion zone of the welded joints produced using a composite laser with a central laser power greater than the ring laser power. Consequently, this welded joint exhibited the highest maximum tensile strength among all tensile test specimens reaching 95.2 % of the base metal's maximum tensile strength. However, the fractured surfaces of the tensile test specimens produced using a composite laser with a center laser power equal to the ring laser power exhibited the highest number density and largest size of oxide particles, resulting in a joint elongation of only 77.3 % of the base metal's elongation. This study emphasizes the potential application of this FRM laser and provides theoretical guidance for selecting the optimal laser combination mode for FRM laser welding of stainless steel. [ABSTRACT FROM AUTHOR]

Here, ultrasonic–wave–assisted gas tungsten pulsed arc (U–GTPA) welding is proposed as a new alternative welding process to gas tungsten arc (GTA) welding. To better understand the advantages of this new process, in this paper, the microstructure, mechanical properties and corrosion behavior of GTA- and U–GTPA-welded joints of 316L stainless steel are systematically compared. These results show that the weld zone (WZ) depth-to-width ratio of the U–GTPA-welded joint increased, and the area of the equiaxed grain zone was larger than that of the GTA-welded joint. This results in finally increasing the strength and hardness for U–GTPA-welded joints, and the ultimate tensile strength and elongation of the U–GTPA-welded joints were 7.1% and 26.2% greater than those of the GTA-welded joint, respectively. For the U–GTPA-welded joint, under the action of the pulsed arc, the grain distribution with high-angle boundaries (HABs) was different from that of the GTA-welded joint. The minimum of the HAB fraction corresponded to the fracture position for both joints in tensile tests. It shows that a large number of HABs were beneficial in improving joint tensile properties. However, for electrochemical corrosion experiments of two WZs in 3.5% NaCl solution, despite this GTA WZ having a higher HAB fraction, the corrosion current density and corrosion potential of U–GTA WZ were lower and higher than those of the GTA WZ, respectively. The corrosion rate and corrosion sensitivity of U–GTPA WZ indicated good corrosion resistance. [ABSTRACT FROM AUTHOR]

The microstructural characteristics, mechanical properties, and corrosion behavior were compared for Al–Zn–Mg alloy (thickness: 8 mm) joints obtained via Gas Metal Arc (GMA) and Ultrasonic-wave-assisted GMA (U-GMA) welding. The results reveal that, compared with the GMA welded joint obtained under the same heat input condition, the U-GMA welded joint consists of a larger weld zone (WZ) and narrower heat affected zone (HAZ). The WZ trends of the joints may be summarized as follows: The Fe-rich phases are smaller and considerably fewer in U-GMA welded joint, resulting in higher fracture toughness than that of the GMA welded joint. Ultimate tensile strength (UTS) and elongation determined from tensile testing of the joints are 342 MPa, 7.4 % for U-GMA welded joint and 305.9 MPa, 5.4 % for GMA welded joint. Electrochemical corrosion experiments in a 3.5 wt.% NaCl solution indicate that the corrosion resistance of the WZs in both joints is affected by the distribution of intergranular Fe-rich phases. Each specimen suffered from intergranular corrosion. However, the WZ of the U-GMA welded joint exhibits stronger corrosion resistance than the WZ of the GMA welded joint. [ABSTRACT FROM AUTHOR]