Microstructure and tensile properties of the different thickness ASSAPH440 high strength steel/DC52D+ZF45 galvanized cold rolled sheet dissimilar metal welding joint by CO₂ laser welding for automobile manufacturing

The experimental materials are 2 mm thick ASSAPH440 high strength steel plate and 2.6 mm thickness DC52D+ZF45 galvanized cold rolled sheet. Its chemical composition and mechanical properties are shown in tables 1 and 2 respectively. The ASSAPH440 and DC52D+ZF45 are Chinese brands, which are manufactured by China BaoWu Steel Group Corporation Limited.

Large laser welding system produced by Shanghai Unity-Prima Laser Equipment Corporation Limited is adopted in this experiment, shown in the figure 1(a). The laser welding system is equipped with German Rofin DC045 CO₂ laser, Fagor8070 high performance PMC-1200 CNC system, HWIN high precision motion system, II-VI optical transmission system and multi-function reflective focus laser welding head. The main parameters of the CO₂ laser welding system show in the following:

• (1)  
  wave length: 10.6 um,
• (2)  
  beam stability: ± 2%,
• (3)  
  welding focus lens focal length: 200 mm,
• (4)  
  X, Y, Z axis stroke: 2500 mm × 2000 mm × 600 mm,
• (5)  
  X, Y, Z axis repeat positioning accuracy: ± 0.01 mm,
• (6)  
  maximum power: 4500 W,
• (7)  
  optical path length: 6000 mm.

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The self-developed semi-automatic pneumatic welding fixture is used to install and fix these workpieces. The welding fixture consists of two rows of cylinders mounted on the channel steel bracket. Sample workpieces are clamped or released by adjusting the air pressure to prevent the welding seam from being deformed or even cracked due to a large amount of heat input during CO₂ laser welding process welding, so as to ensure the dimensional accuracy of the welding seam. The number and size of cylinders are determined by the length and strength of these workpieces, which can be adjusted with increased or decrease. The workpiece fixture of laser welding is shown in figure 1(b).

The welding joints are cut by a DK7735 WEDM (Wire Electrical Discharged Machining) CNC wire cutter and the welding metallography sample was prepared according to standard procedures, which are shown in figure 2. The microstructure of base metal and welding seam are observed by ZEISS EVO 18 scanning electron microscope (SEM) and build-in energy depressive spectroscopy (EDS). The tensile property of the welding joints is tested by WDW-506 electronic universal testing machine according to Chinese National Standard GB/T228-2002 (eqv. ISO6892:1998). Because the thickness size of welding specimen is different (2 mm ASSAPH440 + 2.6 mm DC52D+ZF45, shown in figure 2(b)), so the minimum cross-sectional area is used to calculate tensile strength for different thickness dissimilar metal laser welding joints. The microhardness test is performed on the welding joint by an HV-1000 hardness tester with a pressing load of 9.8 N, a loading time of 10 s, and a dwell time of 10 s. The 2% solution of ethanol nitric acid is used as a metallurgical etchant to observe the microstructure of different thickness dissimilar metal welding joints.

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In this paper, the laser deep penetration welding process is used for experimental research. The theoretical diameter of the beam output by the Rofin DC045 laser is 20 mm, which will reach a 0.2 mm diameter after focusing by the optical system. The energy density of laser welding can reach 10⁵−10⁶ W cm⁻². Theoretically, when the laser power is 1000 W, the power density at the focal point can reach 3.18 × 10⁶ W cm⁻², which can achieve a deep penetration effect during sheet welding. Considering the energy output of the laser and the industrial production efficiency, the power range of laser welding in this study is set between 1600 W–2500 W, which is between 35% and 55% of the full power of the laser. Many research works show that the number of pores in the laser welding seam is closely related to the laser power. As laser power decreases, the number of pores in the welding seam is reduced [22]. Because the vaporization of the metal can be reduced as the laser power is reduced, which will suppress the pore formation in the welding seam. Therefore, the laser power is selected from 1600 W to 2500 W to ensure the quality requirements in this laser welding experiment.

Line energy is used to describe the laser radiation energy received by metal parts in the process of laser deep penetration welding, which is the definition of the energy absorbed by per unit length of the welding seam. As the welding speed increases, the line energy of the welding seam and the penetration are reduced. The welding speed should be selected according to the thermophysical properties of the metal, joint form and thickness of workpiece in the CO₂ laser welding, so that the workpiece can absorb the appropriate laser energy and achieve the desired penetration. Welding speed affects significantly penetration and width of the welding seam. In laser welding process, the penetration is generally an inverse relation to the welding speed. In this paper, the maximum penetration is 2.6 mm thick of DC52D+ZF45 galvanized cold rolled sheet, so the welding speed range is 20 mm s⁻¹∼50 mm s⁻¹ depending on the practical experience.

In the process of mass industrial production, negative defocus distance (namely clearance between the focal point and the plate surface) is used to ensure stable products quality. The defocus distance in CO₂ laser welding is generally 0.8–1.0 times of the workpiece thickness. Since it is so the minimum thickness of the ASSAPH440/DC52D+ZF45 welding workpiece is 2 mm during the laser tailor welding of different thickness dissimilar metal, the defocus distance is selected from −2 mm to 2 mm in this experiment.

Shielding gas is used to protect the welding region to ensure that the high temperature metal in the welding pool or welding seam is isolated from the outside air, which is easy to produce defects in CO₂ laser welding. The shielding gas also has an inhibitory effect on plasma cloud generated during the laser welding. Moreover, shielding gas also protects the focusing lens from metal vapor pollution and sputtering of molten splash droplets, and dissipate the heat of focusing laser. So flow of shielding gas is also one of important parameters to welding quality. On the other hand, the composition of shielding gas has also a certain influence on surface quality of welding seam, penetration formation and mechanical properties of the welding joint [18, 19]. The 99.9% purity argon shielding gas and 20 l min⁻¹ shielding gas flow are determined in these welding experiments. The gas nozzle is placed ahead of the welding direction. The diameter of the gas nozzle is 3 mm. The distance between the gas nozzle and the workpiece surface is 3 mm. The side-blown shielding gas method with a 60° angle between the workpiece surface and gas nozzle is adopted, which is shown in figure 3 during the different thickness dissimilar metal laser tailor welding.

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The welding parameters of the different thickness ASSAPH440/DC52D+ZF45 dissimilar metal in CO₂ laser welding are shown in table 3.

Our previous research works have shown that welding speed, laser power, defocusing distance is three key parameters for laser welding quality of different thickness dissimilar metal plates. Therefore, this case adopts the 4 factors and 5 levels (shown in table 3) of orthogonal experiment method to determine and optimize technological parameters for the LTWB. The effect of technological parameters to laser welding quality is analyzed. Welding joint specimens are shown in the figure 4. The orthogonal experimental results show the welding speed is 30 mm s⁻¹, the defocus distance is −1 mm, the shielding gas flow is 20 l min⁻¹ and the laser welding power is 2000 W, there has a good quality of welded joints. So the 30 mm s⁻¹ welding speed, −1 mm the defocus distance and 2000 W the laser welding power are taken as a research benchmark and optimized welding parameters in this paper.

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3.1.1. Relationship of the welding seam size with the laser welding power

In order to reduce the measurement error for penetration and width of welding seam, the average value method for multiple measurements is adopted to obtain all the relevant data. At the same time, so as to reflect the real influence rule that laser power change to welding formation, the quadratic polynomial fitting method is adopted to draw the relationship curve between laser power and penetration or width of welding joints. When the welding speed is 30 mm s⁻¹, the defocus distance is −1 mm, the shielding gas flow is 20 l min⁻¹ and the laser welding power is 1800 ∼ 2200 W, the relationship curve between laser power and penetration or width of welding joints is shown in figure 5 respectively. In the figure 5, as the laser power increases, the width and penetration of the laser welding seam are both increased owing to the increase of the heat input per unit volume of the material. The melting area of the welding joints also increases. If only for achieving full penetration of welding metal, the depth of welding seam needs 2 mm, equal to thickness of workpieces, so the laser power need more than 2200 W based on the figure 5. In this case, the laser power only uses 2000 W, because lots of researches show the pore defect of welding seam is positive correlation property to the laser power, and the evaporated zinc coating will reduce usually the joint quality in the DC52D+ZF45 galvanized cold rolled sheet CO₂ laser welding [22, 27]. Therefore, the 2000 W laser power is adopted modestly to ensure basic joining property and quality of the welding seam.

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3.1.2. Relationship of the welding seam size with the welding speed

The average value method and quadratic polynomial fitting method are also adopted in this chapter with a same previous reason. When the laser power is 2000 W, defocus distance is −1, the shielding gas flow is 20 l min⁻¹ and the welding speed is 25 mm s⁻¹, 30 mm s⁻¹, 35 mm s⁻¹, 40 mm s⁻¹, 45 mm s⁻¹, respectively, the relationships between the welding speed and width or penetration of welding seam are shown in figure 6. In the figure 6, as the welding speed increases, both the width and penetration of the laser welding seam are both decreased. As a result that the welding speed increases, the line energy decreases, which leads to the width and penetration decrease. It is in agreement with the numerical result by Vrtiel [23], and they found that under 2000 W laser power, a full penetration of 2 mm laser welded joint of high-strength TRIP steel cannot be ensured when the welding velocity is higher than 90 mm s⁻¹.

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3.1.3. Relationship of welding seam size with the defocus distance

When the laser power is 2000 W, the welding speed is 30 mm s⁻¹, the shielding gas flow is 20 l min⁻¹ and the defocus distance is −2 mm, −1 mm, 0 mm, 1 mm and 2 mm, respectively, the relationship between the defocus distance and width or penetration of welding seam is shown in figure 7. The average value method and quadratic polynomial fitting method are also adopted in this chapter with a same previous reason. In the figure 7, as the defocus distance increases, the width of the welding seam decreases, while the penetration has a small variation range with a trend of increasing first and then decreasing. When defocus distance is negative, the focal point is below the surface of the workpiece and its energy density is higher than the one with a focal point on the workpiece surface. Therefore, a negative defocus distance is more conducive to appropriate width of welding seam and energy transmission. The experimental results show that when the defocus distance is −2 mm, the width of welding seam is too big because the laser focal point is at the bottom of the workpiece, leading laser heating zone increase on the surface of welding seam. When the defocus distance is 2 mm, the width of welding seam is too small because the laser focal point is at the top of the workpiece. So the defocus distance is −1 mm, this has sufficient welding formation.

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Figure 8 shows the microstructure of the welding seam with 2000 W laser power, 30 mm s⁻¹ welding speed of and −1 mm defocus distance.

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Figure 8(a) shows the basic material of DC52D+ZF45 steel, which is mainly composed of equiaxed massive ferrite and pearlite with small grain size and uniform distribution. Figure 8(b) is the base material of ASSAPH440 steel, which is composed of massive ferrite and has a larger grain size than that of DC52D+ZF45 steel base metal. Therefore, its strength and hardness are relatively low.

Figures 8(c) and (d) are the interfaces of HAZ and base metal. Figure 8(c) is the interface of HAZ on the DC52D+ZF45 steel side and base metal. Figure 8(d) is the interface of HAZ on the ASSAPH440 steel side and base metal. In these figures, a boundary between HAZ and base metal can be obviously observed, which has a transition from the HAZ to the base metal with the ferrite content gradually increased and other components such as martensite gradually decreased.

Figure 8(e) shows the microstructure of the HAZ on the DC52D+ZF45 steel side from the partial enlargement of figure 8(c), which is mainly composed of granular bainite and ferrite. Figure 8(f) is the HAZ on the ASSAPH440 steel side from the partial enlargement of figure 8(d), which consists of granular bainite, acicular ferrite and martensite. The microstructure of the HAZ on the DC52D+ZF45 steel side has a smaller grain size and superior mechanical properties compared with the microstructure of the HAZ on the ASSAPH440 steel side. Figure 8(g) is the microstructure of the center of the welding seam, which consists of elongated lath martensite and a few small white pieces of ferrite. The lath martensite is formed mainly because the base metal is low carbon steel, therefore, after laser welding, the welding seam rapidly cools to form low carbon martensite in which a large number of dislocations which has been observed by Wang [28, 29] in the laser welded joint of TRIP steel and TWIP steel respectively. According to Wang's research, dense dislocations accumulated at the interface between lath martensite and lower bainite and formed the network structure. These dislocations are beneficial to hinder crack growth, increasing the fracture toughness of the welded joint. The formation of ferrite in the center of the welding seam is due to the slow cooling rate of the partial metal.

Figure 9 shows the microstructure of a cross section of the welding seam observed by SEM. The microstructure of the center of the welding seam is strip-shaped and staggered, which is lath martensite and consists of substantial dislocations. From HAZ to the base metal, there is a significant transition from the fine grains in the HAZ to the coarse grains in the base metal. ASSAPH440 steel base metal is mainly composed of ferrite with large grain size. DC52D+ZF45 steel base metal structure is mainly equiaxed ferrite but the second phase with tiny particles is distributed in the grain, which is island-pattern martensite structure.

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EDS technology is used to analyze the element content in the different thickness dissimilar ASSAPH440/DC25D+ZF45 laser welding seam, shown in the figure 10 and table 4.

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The result shows the content of main element C or Mn is higher than that of the DC25D+ZF45 respectively in tables 1 and 4, and it is similar to ASSAPH440. It is shown the high strength ASSAPH440 steel plays a leading role in the welding seam center formed by the laser fusion of different thickness dissimilar ASSAPH440/DC25D+ZF45 metal joint. The melting and solidification rates of the laser welding are very high, so these elements of ASSAPH440 and DC25D+ZF45 metal are too late to spread in the laser welding seam center.

CO₂ Laser welding has the characteristics of high energy density and fast cooling of the welding seam. According to the microstructure of welding seam, the hardened microstructure of lath martensite and granular bainite is formed in the center of the welding seam and the HAZ. The microhardness test was performed on the different thickness dissimilar ASSAPH440/DC25D+ZF45 laser welded joint, and the hardness distribution of the welded joint is shown in the figure 11 when the laser power is 2000 W, the welding speed is 30 mm s⁻¹ and the defocus distance is −1 mm.

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In the figure 11, the microhardness of welding seam center of different thickness dissimilar ASSAPH440/DC25D+ZF45 laser welding joint is the highest. Then the hardness decreases toward the base metal in both directions, and the microhardness of the DC52D+ZF45 steel base material is higher than that of the ASSAPH440 steel base material. As a result that welding seam is rapid and extremely cooled on the laser welding process, the hardened lath martensite is formed. The hardened lath martensite is gradually reduced as it transitions into both sides of the welding seam center. Moreover, the microstructure of DC52D+ZF45 steel is significantly smaller than that of ASSAPH440 steel and its microstructure is equiaxed and distributes uniformly, shown in the figure 12.

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The tensile strength of the welded joint can intuitively reflect the quality of the laser welded joint. When the laser power, welding speed and other relevant parameters are matched, a good quality laser welded joint can be obtained. As shown in figure 13, the fracture position of the welded joint sample is at the DC25D+ZF45 steel side instead of the welding seam, which indicates that the laser welded joint has high strength and good welding quality, when the laser power is 2100 W, the welding speed is 30 mm s⁻¹ and the defocus distance is −1 mm.

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The tensile (force-deformation) curve of the different thickness dissimilar metal ASSAPH440/DC25D+ZF45 laser welded joint sample is shown in figure 14, when the laser power is 1800 W, the welding speed is 30 mm s⁻¹ and the defocus distance is −1 mm. Owing to the good ductility of the material, the tensile curve of the ASSAPH440/DC25D+ZF45 laser welded joint is relatively smooth and has a long period of the planar transition stage. At the planar transition stage, a strong necking phenomenon appeared until the different thickness dissimilar metal laser welded joint breaks, which means that the overall strength and the toughness of the welded joint are high. The tensile results show the different thickness dissimilar metal ASSAPH440/ DC25D+ZF45 laser welding joint has enough plasticity and toughness for application, which can meet the requirement of strength and lightweight in the vehicle manufacturing process. CO₂ laser welding technology can be adopted to process the different thickness dissimilar metal ASSAPH440/DC25D+ZF45 joint. The tensile strength of ASSAPH440/DC25D+ZF45 also show the CO₂ laser welding can solve joining difficult to surface Zn galvanized layer of the DC25D+ZF45 steel by the high energy density gasification method.

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3.4.1. Effect of laser power on the mechanical properties of welding joint

The tensile test was performed on welded joint samples welded under four different laser power. The macro-morphology of the tensile fracture of the samples was shown in figure 15, when the welding speed is 30 mm s⁻¹ and the defocus distance is −1 mm.

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Sample 1#, Sample 2# are broken at the fusion center line of the welding seam, while Sample 3#, Samples 4# and 5# are broken at the DC25D+ZF45 steel base metal. Because Sample 1# or Sample 2# is welded on the laser power of 1800 W or 1900 W. This is a relatively low laser power, the metal material in welding seam is not sufficiently molten and penetration, which are consistent with the research results in figure 5. This leads to a smaller actual stressed area during tensile, therefore, the tensile strength of the welding joint is reduced.

While Sample 3#, Sample 4# and Sample 5# is welded on the condition of the laser power from 2000 W and 2200 W respectively, which achieves a full penetration and makes the two dissimilar metals form a close metallurgical bonding. As a result that the high energy density of laser welding, and the fast cooling rate of welding seam, welding joint contains substantial dislocations and high toughness non-equilibrium microstructure production, which can prevent crack growth and propagation during tensile. Therefore, on the laser power from 2000 W and 2200 W, these samples of welded joint have a higher tensile strength. After the tensile test, the fractures occurs at the DC25D+ZF45 steel base metal side which has a lower tensile strength. On the other hand, when the high energy density input [16, 19, 27], it is very easy to produce overburned and perforated phenomenon in the process of CO2 laser welding sheet, especially the DC25D+ZF45 is galvanized cold rolled sheet with surface coated the Zn layer. It is necessary to select an appropriate laser power and other parameters in order to prevent welding defects and achieve high tensile strength.

When the welding speed is 30 mm s⁻¹ and the defocus distance is −1 mm, the tensile strength and elongation of the welded joint under five laser powers (1800 W, 1900 W, 2000 W, 2100 W, 2200 W respectively) are shown in table 5, corresponding to the five laser powers, the sample number is from Sample 1# to 5#. The relationship curve between the laser power and the average tensile strength or average elongation is shown in figure 16 when the laser power changes from 1800 W to 2200 W. The macro-morphology of crack from Sample 1# to 5# is shown in figure 15.

Table 5.  Tensile properties of samples.

Sample number	${\sigma }_{s}$/Mpa	${\sigma }_{b}$/MPa	${\overline{\sigma }}_{b}$/MPa	${\rm{\Delta }}$/%	$\overline{{\rm{\Delta }}}$/%
1–1	416	589	592.7	10.0	10.4
1–2	431	597		11.2
1–3	405	592		10.1
2–1	401	603	602.3	10.4	10.7
2–2	420	608		11.6
2–3	417	596		10.2
3–1	407	641	644.3	23.8	23.9
3–2	402	643		23.6
3–3	412	649		24.2
4–1	431	645	641.0	24.6	23.9
4–2	436	641		23.8
4–3	422	637		23.2
5–1	474	646	643.3	22.0	22.7
5–2	409	646		22.0
5–3	409	638		24.0

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The figure 16 and table 5 show that when the laser power is 1800 W and 1900 W, the mechanical properties of the welded joints are poor. Because the laser power is too small, which leads to insufficient penetration so that the stress is concentrated at the welding joint during stretching, and the tensile sample broke on the welding seam zone finally. When the laser power changes from 2000 W to 2200 W, due to the full penetration of the welding seam in the thickness direction and the close combination between two plates, the welded joint has a good mechanical performance, nearly reaching the strength of ASSAPH440 steel base metal.

In this paper, when laser power is from 2000 W to 2200 W, there shows a certain positive correlation trend among laser power, tensile strength and elongation. This correlation had been reported in the research of laser welded joint of high-strength TRIP steel by Rahul [18], and they found that as the laser power increased, the positive correlation between elongation and tensile strength became more obvious. In this case, the highest average tensile strength and average elongation of ASSAPH440/DC25D+ZF45 laser welded joint is 644.3 MPa and 23.9% respectively on the condition of 2000 W laser power, which is calculated by the minimum cross-sectional area of the different thickness (2.0 mm + 2.6 mm) dissimilar metal welded joint.

While the strength of ASSAPH440 steel base metal is higher than that of DC52D+ZF45 steel base metal, showed in table 2. Therefore, when the high-quality welding sample is stretched, the crack produces on the DC52D+ZF45 steel side. On the condition of this study, when the laser power is chosen from 2000 W to 2200 W, the welding speed is 30 mm min⁻¹, the high quality of different thickness ASSAPH440/DC52D+ZF45 dissimilar metal welding joint can be ensured. If the CO₂ laser power is further increased [11, 12, 15], welding heat input is too large to produce overheating, softening, porosity and welding leakage phenomenon in the welding seam, which will damage the integrity of the welded joint and reduce the mechanical properties of the welded joint.

3.4.2. Effect of welding speed on the mechanical properties of welding joint

When the laser power is 2000 W and the defocus distance is −1 mm, the tensile tests were conducted on the sample of laser welded joint welded under five welding speeds (25 mm s⁻¹, 30 mm s⁻¹, 35 mm s⁻¹, 40 mm s⁻¹, 45 mm s⁻¹ respectively) are shown in table 6, corresponding to the five welding speeds, the sample number is Sample 6#, 3#, 7#, 8# and 9#. The relationship curve between the welding speed and the average tensile strength or average elongation is shown in figure 17 when the welding speed changes from 25 mm s⁻¹ to 45 mm s⁻¹.

Table 6.  Tensile properties of welded joint samples at different welding speeds.

Sample number	${\sigma }_{s}$/Mpa	${\sigma }_{b}$/MPa	${\overline{\sigma }}_{b}$/MPa	${\rm{\Delta }}$/%	$\overline{{\rm{\Delta }}}$/%
6–1	403	639	642.3	23.6	23.9
6–2	397	646		24.0
6–3	396	642		24.2
3–1	407	641	644.3	23.8	23.9
3–2	402	643		23.6
3–3	412	649		24.2
7–1	408	622	622.7	11.4	11.8
7–2	413	627		12.3
7–3	421	619		11.7
8–1	416	589	592.7	10.0	10.4
8–2	431	597		11.2
8–3	405	592		10.1
9–1	424	596	583.7	9.8	9.8
9–2	408	574		10.2
9–3	413	581		9.5

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Figure 17 shows that when the laser welding speed is 40 mm s⁻¹ or 45 mm s⁻¹, the material cannot be fully penetrated as a result of inadequate line energy input in CO₂ laser welding, which is consistent with the research results in figure 6. The tensile strength of these welded joints is not enough. When the welding speed is 25 mm s⁻¹ or 30 mm s⁻¹, the different thickness metal of welding seam can be fully penetrated so that the two dissimilar materials produce one close metallurgical bonding part. On the condition of this study, when the welding speed is chosen from 25 mm s⁻¹ to 30 mm s⁻¹, the laser power is 2000 W, the high quality of different thickness ASSAPH440/DC52D+ZF45 dissimilar metal welding joint can be ensured.

In addition, when the welding speed is 35 mm s⁻¹, laser power is 2000 W and −1 mm defocus distance, the sample #7 after tensile test is showed in figure 18(a). The result shows that the fracture of Sample 7# occurs at the fusion center line of the welding seam although the tensile strength of Sample 7# is above 620 MPa, but the average elongation of Sample 7# is only 11.8%. So the 35 mm s⁻¹ welding speed and 2000 W laser power is not optimized technological parameters, which make sure that welding seam is full penetration and produce an effective firm metallurgical bonding between different thickness ASSAPH440/ DC25D+ZF45 dissimilar metals.

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Figure 18(b) shows the surface of welding seam about Sample 7#. Compared with Sample 6# or Sample 3#, Sample 7# has a significant difference in the flatness and width of welding seam, which indicates that the welding speed has a reasonable influence to surface quality of welding joint. The surface quality of welding seam can reflect the mechanical properties of the welded joint within a certain range, so many researchers have proposed to monitor these parameters on the practical mass welding process by controlling the surface quality to ensure the high performance of welded joint [15–17]. On the other hand, The tensile results also show that Sample 6# and 3# are cracked on the DC52D+ZF45 steel base metal, while Samples 7#, Samples 8# and Samples 9# are cracked along the fusion line of the welding seam. The fracture position of sample is also consistent with tensile strength. Therefore, the cracks of good quality welded joint will appear at the base metal of DC52D+ZF45 steel side after the tensile test, 25 mm s⁻¹ or 30 mm s⁻¹ welding speed is satisfy to meet the reasonable matching on the condition of this laser power. When welding speed is very slow, the line energy increases quickly, welding joint may be overfired. The highest average tensile strength of ASSAPH440/DC25D+ZF45 laser welded joint is 644.3 MPa on the condition of 30 mm s⁻¹ welding speed, which is calculated by the minimum cross-sectional area of the different thickness (2.0 mm + 2.6 mm) dissimilar metal welded joint. Therefore, it is more appropriate to choose the 30 mm s⁻¹ welding speed.

In recent years, there are many researchers investigating the high-strength dissimilar metal laser welded joints with a different thicknesses. Gustavo [30] has studied the relationship between the process parameters and the geometry size of laser welded joint of high strength low alloy steel (HSLA). They found that as the laser power increases or welding speed decreases, the width and the penetration of the welded joints increases, which is consistent with our experiment results (show in figures 5–7). Jing [31] has investigated the microstructure of the laser welded joint of TRIP590/DP590 dissimilar high strength steel with different thicknesses. They found that the microstructure of the weld zone is martensite, and the microstructure is the mixture of martensite, bainite and ferrite on both sides of HAZ, which also has observed in this paper (show in figures 8–10). Sevc [32] has investigated the fiber laser welded TRIP/HLSA sheets with different thicknesses, they found that the highest microhardness is at the HAZ of TRIP steel side, while in this study, the higher microhardness is at the weld center, which also agrees with our research result (show in figure 11). Rahul [18] has investigated on the effect of power and velocity of laser beam welding on tensile strength of the TRIP steel butt weld joint, and Shu [33] has also studied the effect of process parameters on the tensile strength of the laser welded joint of HC550/DP780 high strength steel with different thicknesses. They regard that as the laser power increases, the tensile strength of the welded joint increases firstly and then slightly decreases, which are consistent with our research results (shown in figures 16 and 17). And as the welding speed increases, the tensile strength decreases continually, while in this paper the tensile strength does not decrease when the welding speed increase from 25 to 30 mm min⁻¹, which may attribute to the higher laser power (2 KW) used in this paper compared with Shu's case(0.7–1.7 KW).

In this case, the highest average tensile strength and average elongation of different thickness ASSAPH440/ DC25D+ZF45 dissimilar metal laser welded joint is 644.3 MPa and 23.9% respectively on the condition of 2000 W laser power and 30 mm s⁻¹ welding speed considering a low welding cost and high production efficiency.