Process characteristics of Q890D joint using hybrid fiber laser-arc hybrid welding

This article conducted a study on the process characteristics, structure, and properties of the Q890D joint. The results indicated that three-layer welding methods can obtain a joint free of pores and defects. The base metal was composed of a large amount of polygonal ferrite and dispersed granular carbides, while the weld metal consisted of lath martensite. The results also revealed that the average hardness of WM was 417.24 HV (20.7% higher than BM). The tensile performance of welded joints was better than that of BM. There were no obvious cracks on the surface of the bending specimens. Compared with BM, the impact strength and toughness of WM had been improved, since the impact toughness of HAZ was highly dispersed.


Introduction
Hitherto, high-strength steel has been widely used in the shipbuilding industry [1][2][3], and other engineering steel structures.However, when welding thick plate high strength steel, traditional welding efficiency is greatly reduced owing to the low penetration of arc welding.Recently, people have begun to apply laser-arc hybrid welding technology to industrial, with high energy density [4], high welding efficiency [5], deep penetration [6], and good welding quality [7].
Welding process research mainly focuses on the influence of various welding parameters on weld formation.The parameters of laser arc hybrid welding mainly include laser power, defocusing amount, welding speed, arc current, arc voltage, and wire feeding speed.

Test details
The materials used in this study were 12 mm thick Q890D.The heat treatment state was adjusted.The chemical composition of Q890D and wire are listed in Tables 1 and 2.
Table 1 The metallurgical specimens were etched with 5% nitric acid alcohol solution and were observed by OLYMPUS BX-53 optical microscope.An HV-1000 hardness tester was used to measure the hardness of three layers.Tensile, bending, and impact performance tests were performed on an MTS electronic universal testing machine.The size and notch characteristics of tensile specimen, bending specimen and impact specimen are shown in Figure .1.

Welding process
Since the thickness of the steel selected in this welding is 12 mm, it is decided to use three welds to achieve material connection.When welding, a high-power laser is used in principle to obtain as much laser energy as possible on the surface of the workpiece and to achieve deep penetration.
The initial selection of test parameters for the current 205 A, welding speed 1.6 m/min.Figure 2 indicates that the laser power in 5000 W and 5400 W can achieve full penetration.The top of the weld is well-shaped without obvious defects, while the bottom of the weld is uniform residual height.At 5400 W, the top weld is rougher and has obvious spatter.At the same time, the residual height of the bottom weld is not uniform.It is easily seen from Figure 3 that the weld can achieve full penetration at all currents.When the arc current is low, a large spatter is produced and the weld is discontinuous.As the current increases, the arc ionization increases, and the preheating effect on the steel is increasing, which helps to improve the absorption of laser energy on the surface of the steel.Meanwhile, the increase in current makes the weld width wider.When the arc current is 205 A, the weld has the best morphology, and a continuous and stable full-penetration weld can be obtained.A. The welding process of the cover layer was chosen to use similar process parameters as those of the filler layer, and a low-power laser of 800 W was selected to make the weld surface more continuous and stable.Under the 240 A arc current, the weld seam is uniform (Figure 5(c)).Figure 6 shows that the front of the weld joint is uniform, without obvious undercuts or collapses.The back is transparent, and the excess height is small (Figure 6(a)).The X-ray non-destructive testing (Figure 6(d)) of the weld joint indicates that there are no internal cracks or air pores.The joint is in the shape of a wine cup.The bottom layer is thin and long, which presents the characteristics of laser welding.Meanwhile, the filling layer and cover layer are short and wide, which presents the characteristics of arc welding.It is worth noting that the high-power laser in the bottom layer results in a greater depth-width ratio than the other two layers.The widths of the bottom layer, filling layer, and cover layer are 1.240 mm, 6.196 mm, and 9.384 mm.The average penetration of the bottom layer is similar to the other two layers, while the width of the bottom layer is only 1/5, 1/8 of that of the other two layers.

Microstructure
Figure 7 shows the microstructure of welded joints.WM is composed of martensite.The partially transformed region, fine-grained region, and coarse-grained zone are composed of granular bainite and ferrite.Besides, the base metal consists of a large amount of massive ferrite and carbides.Table 3 shows the impact properties of WM and HAZ.The reason why the impact toughness of HAZ is higher than that of the weld is that HAZ contains a portion of ferrite and diffuse carbide composition, and its toughness is higher than that of martensite.The impact absorption energy and impact toughness of the weld are only one-third of those of HAZ, but the numerical distribution is relatively uniform, and the numerical dispersion of HAZ is large, which is also related to the greater heterogeneity of the HAZ. Figure 10(d)The impact fracture exhibits typical ductile fracture characteristics.

Conclusion
(1) The joint is in the shape of a wine cup.The bottom layer is thin and long, while the filling layer and cover layer are short and wide.The average penetration of the bottom layer was similar to that of the filler layer and cover layer.
(2) High-power lasers were suitable for bottom welding.Current affects the shape of the weld indication.
(3) The hardness of the weld metal was higher than BM.There was a softening zone with a width of about 1 mm at HAZ.
(4) The tensile specimens were fractured at BM.The weld has no obvious cracks on the bending specimens.The average values of -20℃ impact toughness absorption work in WM and HAZ of the welded joints are 54.6 J and 155.4J.

Figure 2 .
Figure 2. Appearance of weld: (a) top of the weld (5000 w); (b) laser power (5400 w); (c) back of the bottom layer (5000 w); (d) back of the bottom layer (5400 W).It is easily seen from Figure3that the weld can achieve full penetration at all currents.When the arc current is low, a large spatter is produced and the weld is discontinuous.As the current increases, the arc ionization increases, and the preheating effect on the steel is increasing, which helps to improve the absorption of laser energy on the surface of the steel.Meanwhile, the increase in current makes the weld width wider.When the arc current is 205 A, the weld has the best morphology, and a continuous and stable full-penetration weld can be obtained.

Figure 3 .
Figure 3. Weld morphology under different arc currents: (a) 182 A; (b) 182 A weld back; (c) 186 A; (d) 186 A weld back; (e) 189 A; (f) 189 A weld back; (g) 205 A; (h) 205 A weld back.The filling layer has to use a higher arc current to melt more wire.Figure4demonstrates that low arc current (290 A) sidewalls are not completely fused.Excessive arc current will cause spatter, biting edge, and other welding defects.It can be seen that the most appropriate filling layer arc current should be 300 A.

Figure 4 .
Figure 4. Weld morphology of filling layer under different arc currents: (a) 290 A; (b) 300 A; (c) 330A.The welding process of the cover layer was chosen to use similar process parameters as those of the filler layer, and a low-power laser of 800 W was selected to make the weld surface more continuous and stable.Under the 240 A arc current, the weld seam is uniform (Figure5(c)).

Figure 8 Figure 8 .
Figure 8. Hardness distribution curve.It can be known from Figure9that the tensile strength is 993.67 MPa and the elongation is 22.5%.Besides, cracks sprout from the BM.The tensile strength has increased by nearly 200 MPa and the average elongation has increased by 1.5% than Q890D.Observation under high-power electron microscopy shows that the fracture surface is composed of pores and deep dimples, which indicates that the material has good toughness.In this experiment, the narrow width of the softening zone does not affect the overall tensile properties of the joint.

Figure 9 .
Figure 9. Tensile test results: (a) stress-strain curve; (b) microfracture morphology of tensile specimen.Conduct front and back bending tests on welded joints.No surface cracks were found on both the front and back sides (Figure 10(a)(b)), indicating that the joint has good ductility and sufficient bending strength.Table3shows the impact properties of WM and HAZ.The reason why the impact toughness of HAZ is higher than that of the weld is that HAZ contains a portion of ferrite and diffuse carbide composition, and its toughness is higher than that of martensite.The impact absorption energy and impact toughness of the weld are only one-third of those of HAZ, but the numerical distribution is relatively uniform, and the numerical dispersion of HAZ is large, which is also related to the greater heterogeneity of the HAZ.Figure10(d)The impact fracture exhibits typical ductile fracture characteristics.

Figure 10 .
Figure 10.Bending and impact specimens: (a) macroscopic morphology of test pieces; (b) macroscopic morphology of bending specimens; (c) fracture morphology of impact specimens; (d) microscopic fracture morphology of impact specimens.

Table 3 .
Impact performance of weld metal and heat affected zone.