Figure 2 shows the macro morphologies of non-penetration lap welded joints of DC01-sus304 and DC01-Q235 steel sheets, with welding speed of 70 mm/s and laser power of 2.0 kW. The weld is well formed without any obvious macro defects. It must be pointed out that, even if the penetration depth of the lap weld is only about 0.1 mm, a steep peak is still formed on the back of the partially penetrated side of lap welded joints of DC01-SUS304 steel sheets, with a peak height of about 6µm, as shown by the red arrow in Fig. 2a. On the contrary, the bulging distortion of partially penetrated side of lap welded joints of DC01-Q235 steel sheets is gentle, as shown by the arrow in Fig. 2b, and there is a flat peak on the partially penetrated side, with a peak height of about 3µm.
3D model of the bulging profile on the partially penetrated side of lap welded joint of DC01 galvanized steel-SUS304 steel sheets, as obtained by the profilometer is shown in Fig. 3, in which different colors represent the relative height of different positions. Figure 3a shows a bulging profile model with the profilometer scanning width of 8 mm and laser power of 2.4 kW. It can be seen from Fig. 4a that the distortion exhibits obvious angular deformation, but the bulging profile is mainly in the range of 2 mm in width. From the visual inspection of decorative parts, visual distortion will reduce the subjective evaluation of the surface quality of the material. Generally, steep peaks will cause large height fluctuations on the surface of the parts, which will change the reflection path of the incident light and cause visual distortion. Therefore, the visually distorted area on the partially penetrated side of the SUS304 steel sheet should also be the bulging distortion zone within a width of 2 mm. In view of this, the following research focuses on this 2mm bulging distortion zone. Figure 3b, Fig. 3c, and Fig. 3d show the bulging profile models on the partially penetrated side of lap welded joints with a welding speed of 70mm/s, and a laser power of 0 kW, 2.0 kW, and 2.6 kW, respectively. In the range of 2mm in width, the maximum peak heights are 0µm, 6µm, and 27µm, respectively.
In order to describe the bulging distortion more directly, the position data of the 3D model of the bulging profile was derived from the profilometer, and the 2D profile of the bulging distortion in the vertical welding direction was drawn based on these data, as shown in Fig. 4. The profile of the bulging distortion zone of the lap welded joint of galvanized steel-SUS304 stainless steel sheets is shown in Fig. 4a. The region with the most obvious distortion, between two dotted lines, is at the center of the weld, and its width is about 2 mm. The profile of the bulging distortion is substantially a bilaterally symmetrical curve. When the laser power is lower than 2.0 kW, the curve is gentle, and the radius of curvature at the peak is large, and the slope of each point in the curve remains stable, indicating that the bulging distortion has the characteristic of angular distortion. As the laser power increases, the curve becomes steeper, the radius of curvature at the peak gradually decrease. When the laser power is increased to 2.2 kW or higher, the slope of the curve increases sharply, and the angular deformation feature of the bulging distortion zone disappears. Figure 4b shows the outline curve of the bulging distortion zone of the lap welded joint of galvanized steel-Q235 steel sheets. When the laser power is 2.0 to 2.8 kW, the partially penetrated side of Q235 steel sheet, does not show the sharp bulging distortion similar to that of SUS304 stainless steel sheet, but shows the feature of angular deformation.
By comparing Fig. 4a and Fig. 4b, it is found that the peak of the curve of the bulging distortion zone of Q235 steel sheet is lower than that of SUS304 steel sheet, and the slope of the curve is also lower than that of SUS304 steel sheet. For example, the lap laser power is 2.6 kW, the peak height of the contour curve of the bulging distortion zone of Q235 steel sheet is about 5µm, and the slope is about 0.005, while the peak height of the curve of SUS304 steel sheet is about 27µm, and the slope is about 0.024. Since the slope of the curve of the deformation zone of the Q235 carbon steel sheet is small, it has less influence on the reflected light, which will not cause visual distortion and deteriorate the surface quality of Q235 steel sheet.
Figure 5 shows the macroscopic morphology and microstructure of the SUS304 sheet on the partially penetrated side of the lap welded joint of DC01 galvanized steel-SUS304 stainless steel sheets. Figure 5a shows the morphology of the partially penetrated side of the lap welded joint. The welding heat-affected zone of the SUS304 steel sheet is in the shape of "V". Figure 5b shows the overheated zone near the fusion line, where the elongated original grains in the cold-rolled state, are completely recrystallized into coarse equiaxed grains. As shown in Fig. 5c, in the heat affected zone far from the fusion line, the recrystallized equiaxed grains become finer. Figure 5d shows, that even in the edge of the SUS304 stainless steel sheet, the original cold-rolled grains still disappear, and the elongated grains almost completely recrystallized into equiaxed grains. Figure 5e shows the EBSD image of the bulging distortion zone of the back surface of the SUS304 steel sheet, as shown by the arrow, there are a large number of sub-grains in the equiaxed grains, indicating that stress plastic distortion is accompanied by the formation of equiaxed grains.
Twins were observed in these equiaxed grains in the heat-affected zone of the SUS304 steel sheet, and the twins generated at different positions have different morphologies. As shown in Fig. 5b and Fig. 5c, these regions have higher temperatures due to being closer to the fusion line, resulting in coarser recrystallized equiaxed grains, and twins are also coarse and traverse the entire grains, which are typical annealing twins. The bulging distortion zone is located at the edge of SUS304 steel sheet, which has fine recrystallized equiaxed grains because it is far from the fusion line, as shown in Fig. 5d. A large number of densely-concentrated clusters of fine twins appear in the equiaxed grains, as shown by the red arrows, and the twin planes of these twins tend to be perpendicular to the cold rolling direction of the SUS304 steel sheet. These twins are lenticular, originate from the grain boundaries and end in the interior of the grains, which are typical deformation twins. Figure 6 shows the water-quenched microstructure of the cold-rolled SUS304 steel sheet held at 300°C to 700°C for 5 minutes. After being held at 300°C to 700°C for 5 minutes, the cold-rolled SUS304 sheet was in the recovery stage, and its grain morphology was still in the form of fibers elongated in the rolling direction, as shown in Fig. 6a to Fig. 6e. After being held at 850°C for 5 minutes, a small amount of annealed twins appeared in the cold-rolled SUS304 sheet, as shown in Fig. 6f. After being held at 900°C for 5 minutes, a small amount of recrystallized equiaxed grains appeared, as shown in Fig. 6g. After being held at 1000°C for 5 minutes, as shown in Fig. 6h, a large number of the recrystallized equiaxed grains and annealing twins appeared in the cold-rolled SUS304 steel sheet, which was similar to the microstructure of the welding heat-affected zone of the SUS304 steel sheet, as shown in Fig. 5. Therefore, the effect of heat treat at 1000°C *5 minutes on the microstructure of the cold-rolled SUS304 steel sheet was similar to that of the peak temperature of the thermal cycle of lap laser welding on the microstructure of the bulging distortion zone.
In order to analyze the type of stress that caused the deformation twins to be generated in the bulging distortion zone of the non-penetration lap welded joint, the cold-rolled SUS304 steel sheet was held at 1000°C for 5 minutes, and then immediately bent and quenched. Figure 7a shows the macroscopic shape of the bent cold-rolled SUS304 steel sheet. According to the stress distribution theory of plane bending structure, the outer arc of the bent SUS304 sheet is subjected to tensile stress σT, and the inner arc is subjected to compressive stress σC. Figure 7b shows the optical micrograph of the inner arc of the bent SUS304 sheet, where the recrystallized equiaxed grains are compressive into flat grains by compressive stress. Due to the preferential twins caused by compressive stress, the twin plane of annealed twins tends to be perpendicular to the compressive stress, that is, perpendicular to the rolling direction of the cold-rolled SUS304 sheet, as shown by the light blue arrows. In addition, the dense deformation twins occur in the inner arc of the bent sheet, as shown in Fig. 7c, and their direction is also perpendicular to the rolling direction of the cold-rolled SUS304 sheet. The recrystallized grains in the outer arc of the bent SUS304 sheet are elongated by tensile stress, as shown in Fig. 7d. The twin plane of annealed twins tends to be parallel to the tensile stress, that is, parallel to the rolling direction of the cold-rolled SUS304 sheet, as shown in Fig. 7e. Due to the absence of stress, the recrystallized equiaxed grains in the center of the bent SUS304 sheet still retain the polygonal shape, and the twin planes of the annealed twins have no obvious directionality, as shown by the yellow arrow in the Fig. 7f.
Comparing Fig. 5 and Fig. 7, the deformation twins formed in the bulging distortion zone of the SUS304 steel sheet of the lap welded joint are similar to the deformation twins formed in the inner arc of the bent SUS304 sheet. Therefore, it can be inferred that the type of stress that caused the bulging distortion of the partially penetrated side of SUS304 steel sheet is the same as the type of the stress in the inner arc of the bent SUS304 sheet, that is, compressive stress. However, non-penetration lap laser welding of thin plates did not always lead to steep bulging distortion. As shown in Fig. 4b, the curve of the distortion zone of the lap welded joint of the DC01 galvanized steel-Q235 carbon steel sheet was a flat slope, which may be due to the lower compressive stress on the partially penetrated side of the Q235 steel sheet.
In this research, the SUS304 and Q235 sheets are clamped by the rigid support of the clamp, with an opening for laser welding, as shown in Fig. 1, there is rigid restraint in the width direction of the non-penetration lap welded joints. During the laser welding process, the heat-affected zone (the "V" shaped area shown in Fig. 5a) on the partially penetrated side thermally expands, and due to the constraint from the lower temperature materials on both sides of this zone, compressive stress is generated. The thermal expansion coefficient of SUS304 stainless steel is 18.4×10− 6/℃, which is about 167% of that of Q235 steel (11×10− 6/℃) at the same temperature. The thermal conductivity of SUS304 stainless steel is 12 W.m− 1*K− 1, which is only about 23% of that of Q235 steel (51 W.m− 1*K− 1) [12, 13]. Therefore, the thermal expansion of the SUS304 sheet in the heat-affected zone is much higher than that of Q235 carbon steel, which causes the compressive stress generated on the partially penetrated side of the SUS304 sheet to be greater than that of the Q235 sheet. Due to the small thickness of the sheet, the heat-affected zone can easily extend to the back side of the partially penetrated side of lap welded joint. Comparing Fig. 5d and Fig. 6g, the instantaneous temperature of the welding thermal cycle in the bulging distortion zone of the SUS304 sheet is above 900°C, which causes the tensile strength of the SUS304 sheet to drop sharply below 90 MPa. Therefore, even if the compressive stress is low, the material in the heat-affected zone of SUS304 sheet will be plastically deformed.
According to the above analysis, the deformation mechanism of the bulging distortion on the non-penetration side of austenitic stainless steel sheet, can be described as follows: as shown in Fig. 8a, the expansion of the "V"-shaped heat-affected zone on the partially penetrated side of the SUS304 sheet, is constrained by the adjacent low temperature zone and the rigid of the clamp on the overlapped steel sheets, and generates compressive stress. Consequently, the metal in the heat-affected zone ("V" shaped HAZ) is “extruded” by the compressive stress. Due to the high welding thermal cycle temperature, the strength of the SUS304 austenitic stainless steel sheet on the partially penetrated side of the lap welded joint is lower than the compressive stress, which causes the metal in the heat-affected zone of the SUS304 sheet to be extruded out of the back plane, thereby forming the bulging distortion. Since the extrusion deformation is formed in the tiny area of the heat-affected zone on the back of the SUS304 sheet, the peak of the bulging distortion forms is steep. As the temperature decreases, the weld pool solidifies and shrinks gradually, which causes the transition from the compressive stress to the tensile stress in the "V" shaped HAZ, as shown in Fig. 8b. After the lap welded joint is cooled to room temperature and the clamps are released, the tensile stress causes the angular deformation of SUS304 stainless steel sheet, as shown in Fig. 8c, while the bulging distortion is still retained.