Effect of laser welding and laser wire filling on forming and properties of dissimilar steel welded joints

In order to study the effects of different welding methods and different butt gaps on the microstructure and properties of welded joints of steel with unequal thickness, the medium carbon steel of 2 mm 50CrV and low carbon steel of 6 mm SPHE were used as test materials, and the welding was carried out by single laser and laser wire filling welding. The results show that the laser wire filling welding can reduce the welding cracks, and with the increase of the butt gap, the melt height gradually decreases, and the weld morphology transitions from Y shape to H shape. With the increase of the butt gap, the weld defects form. The hardness of weld center decreases with the increase of butt gap. The average hardness of weld formed by laser welding alone is the highest, and the highest hardness of welded joint is located in the heat affected zone of 50CrV medium carbon steel. The tensile strength of welded joints first increases and then decreases with the increase of butt gaps. When the butt gap is 0.6 mm, the surface morphology is good and the tensile strength is the highest. The fracture position of the welding test is on the side of the 2 mm 50CrV medium carbon steel.


Introduction
As an efficient connection technology, laser welding has many advantages, such as large penetration depth, small heat-affected zone area, low residual stress, fast welding speed, and lower heat input coMPared with traditional arc [1][2][3].However, laser welding has higher requirements for workpiece assembly clearance.In general, the maximum butt clearance acceptable for laser welding should be less than 10% of the plate thickness and not more than 0.3 mm [4].When the butt gap is too large, it will cause the weld collapse, resulting in the need for secondary repair.In the actual production process, the workpiece assembly precision can be improved by machine adding, but the production efficiency is reduced.Laser wire filling welding can reduce the requirement of workpiece assembly accuracy to a certain extent [5][6][7].Laser wire filling welding is the use of laser heat source to melt the welding wire, and fill the welding wire to the specified welding area.The thermal action of laser acts on the welding wire and molten pool at the same time, melting the wire and the base metal fusion.The advantage of laser welding is that it exists in laser wire filling welding, and it can improve the weld performance, and thick plate connection can be realized through multi-layer and multi-channel.
The manufacture of automobile clutches requires two steel materials, 50CrV and SPHE.The welding of dissimilar steel not only meets the requirements of welding joints with complex working conditions, but also reduces the production cost.Prasad et al [8] used high speed imaging and streak images to study the influence of different process parameters such as clearance width, welding speed and power on the stability of laser welding.Shi Yan et al [9] studied sphericity in the mechanical properties of welding joints between ductile iron and mild steel.The pores caused by graphite can be removed by laser remelting.The tensile fracture strength of the welded joint with the graphite removed (laser welding) is 483 MPa, which is 98% of the strength of the QT450 base material.Wang et al [10] investigated a series of multicomponent alloys (CoCrFeNi, CoCrNi, and CoNiV) were laser welded with 304 stainless steel (304ss), and the microstructure and mechanical properties of the welded joints were analyzed.The addition of the V element enables the high strength-ductile synergy connection between CoNiV and 304ss.Khot Rahul S. et al [11] studied the process parameters of transformation-induced plasticity (TRIP) laser welding of steel sheets.The author studied angle of weld, power of laser, and welding speed on the strength of the joint.Naveed Iqbal et al [12] used a laser with blue wavelength to connect Nicoated copper to mild steel.The lower laser power results in insufficient fusion, which breaks at the interface.The tensile fracture location of the fully permeated sample is in the HAZ of Cu.Bo Cheng et al [13] study the effect of welding process on the welding of Nickel-based alloy and stainless steel, laser welding and argon arc welding were used for dissimilar metal welding.The tensile properties of laser welded joints are higher than those of argon arc welding.The corrosion properties of laser welding joints are lower than those of argon arc welding.Zhang et al [14] compared two methods, fiber laser welding and fiber laser welding with cold wire methods.laser wire filling welding reduces weld cooling rate and promotes heterogeneous nucleation.The microstructure of the material is finer, more uniform, and the mechanical properties are better.The forming and properties of heterogeneous welded joints with unequal thickness obtained by laser wire filling need further analysis.
In actual production, assembly gap is inevitable.However, the existence of assembly gap hinders the energy transfer, weakens the keyhole effect of laser deep fusion welding, and may also lead to defects such as non-fusion, virtual welding and welding leakage [15,16].It is of great significance to study the influence of butt gap on the morphology and mechanical properties of laser welded joints in heavy engineering industries such as automotive parts and shipbuilding industry.
In this experiment, low carbon steel of 6 mm SPHE and medium carbon steel of 2mm50CrV were used for butt welding.The microhardness and tensile strength of the welded joints were analyzed by studying different butt gaps.The test results can provide certain reference for the selection of welding methods of heterogeneous steel plates with different thickness in industrial production.

Methods
The welding system is composed of IPG LS-6000 fiber laser, ZFJ1500A semi-automatic welder and robot.The maximum output power of the laser is 6.0 kW, wavelength λ = 1.06 μm, fiber diameter is 200 μm, focal length f = 200 mm, spot diameter D = 0.3 mm.The laser beam direction is vertical incidence, the angle between the protection gas and the laser is 60°, and the Angle between the wire feeding Angle and the laser incidence Angle is 65°.The protection gas is 99.9%Ar, and the gas flow rate is 20 L min −1 .The schematic diagram of laser-wire filling welding system is shown in figure 1.
The test used 6 mm SPHE low carbon steel and 2 mm 50CrV medium carbon steel plate thickness, the length and width of the sample is 300 mm × 200 mm.The same laser power, welding speed and wire feed speed with defocus of +2 mm were applied to the test.The butt clearance is 0 mm, 0.2 mm, 0.4 mm, 0.6 mm and 0.8 mm respectively.The welding process parameters are shown in table 1.

Materials
The test materials were the medium carbon steel of 2 mm 50CrV and low carbon steel of 6 mm SPHE.The steel plates used in the test were from Shanghai Baoshan Iron and Steel Co., LTD.The steel plate is polished with sandpaper before welding.The surface of the steel plate should be smooth and rust free.Clean the weld surface with acetone before welding test.The wire is made of 304 stainless steel with a diameter of D = 0.6 mm.The chemical composition of welding test plate and welding wire is shown in table 2. After welding, section metallographic samples were prepared.The weld seam morphology was observed with an optical microscope (OM), the microstructure was observed with a scanning electron microscope (SEM), and the phase composition was identified through energy dispersive spectroscopy (EDS) and x-ray diffraction (XRD).In the case of a test load of 500gf and a residence time of 15 s, the position of the hardness test was 1mm below the upper edge face of the workpiece.

Influence of welding wire melting behavior on stability of welding process
The stability of welding depends on the droplet transition form of welding wire.It is a lower welding stability will worsen the weld formation and welding quality.The Laser wire welding is more complicated than laser welding.In order to avoid the occurrence of welding defects, it is necessary to conduct a lot of tests to obtain a good range of droplet transition process parameters.
When the laser power is 2000 W, the welding speed is 0.75 m min −1 , and the wire feeding speed is 3.6 m min −1 , the droplet transfer mode is liquid bridge transfer as shown in figure 2(a).The radiation from the laser and the molten pool formed by the laser irradiation on the workpiece surface and the high-temperature plasma act on the welding wire.At this time, most of the laser energy is used to form a stable molten pool on the workpiece surface, and the molten wire enters the molten pool smoothly through the liquid bridge.At this time, the welding process is stable.
When the laser power is 2500 W, the welding speed is 0.75 m min −1 , and the wire feeding speed is 3.6 m min −1 , the dripping transition mode is globular transfer, as shown in figure 2(b).The front end of the welding wire is melted by laser energy, the molten metal gathers in the front end of the wire and gradually grows, and the molten drops fall into the molten pool depending on gravity.
As the droplet grows up, its gravity changes constantly, and there is always an unbalanced relationship between the recoil force of metal vapor and the droplet gravity.The droplet has a high probability of irregular rotation and oscillation, which worsens the stability of welding process.When the large droplet falls off from the front end of the welding wire and flies to the welding pool, it has a strong impact on the welding pool and laser beam, which seriously interferes with the stability of the welding process.Explosive transfer is an unstable transition, the droplets will not only prevent the laser beam from entering the molten pool, but also cause obvious disturbance to the molten pool as shown in figure 2(c).According to the stability range of the droplet transition mode, the laser power is 2500 W, the welding speed is 0.75 m min −1 , the wire feeding speed is m•min −1 , and the defocus is +2 mm, and the welding test is carried out.there are pores and cracks in laser welding welds by color penetration testing.There are no pores in the welds of laser welding and laser wire filling welding, and the X-ray images are shown in figure 4.
The morphology of laser welded joints and the cross section morphology of laser wire-filled welded joints with different butt gaps are shown in figure 5.When the butt gap was 0 mm, the welded joint showed obvious characteristics of laser wire filling welding joint, with the upper part being the wire melting zone and the lower part being the laser zone.At this time, due to the insufficient laser power, the welding could not be penetrated.When the butt gap is 0.2 mm, the welded joint is better formed, forming a penetrating weld, and there is no porosity defect in the weld section.As the butt gap increases, the weld melt height decreases gradually.When the butt gap is 0.6 mm, the weld melt height is consistent with the height of the welding test plate.When the butt gap  is too large, the weld will collapse, as shown in figure 5(f).The width of the upper surface is consistent with that of the lower surface.
The variation rules of weld penetration depth, residual height and penetration width are shown in figure 6.The weld area formed by laser welding is much smaller than that of laser wire filling.The penetration depth of the weld increases with the increase of the butt gap.When the butt gap is 0 mm, the penetration depth of laser wire filling welding is 1.78 mm, and the sample is not penetrated.When the butt gap is greater than 0.2 mm, the weld is completely penetrated.When the butt gap is 0.8 mm, the weld surface defects are formed.The weld width has no obvious change due to the constant welding heat input.The residual height of the weld decreases with the increase of  the butt gap.When the butt gap is 0 mm, the maximum residual height of the weld is 0.85 mm.When the butt gap is increased to 0.6 mm, the weld pass is flush with the sample surface.When the butt gap is 0.8 mm, collapse is formed in the welding passage, and the butt gap is too large, and the laser wire welding fails to fill the welding passage.

Influence of butt gap on performance of wire filling welding joint
The morphology of laser welding is shown in figure 7(a).The microstructure of HAZ on one side of the laser weld and 50CrV steel is shown in figure 7(b).The dendrites in laser welded weld are fine and the structure of heat affected zone is martensite.The dendrite growth direction in the weld is perpendicular to the laser welding direction.When the butt gap is 0.6 mm, the morphology of weld is shown in figure 7(d).The microstructure     near the fusion line with 50CrV medium carbon steel is shown in figure 7(e).In the heat affected zone, a high hardness of plate-strip martensite was formed.The columnar dendrites inside the weld grow towards the tip of the weld.The dendrite growth direction in the weld is perpendicular to the laser welding direction as shown in figure 7(a).When the butt gap is 0.6 mm, the morphology of weld is shown in figure 7(e).The microstructure near the fusion line with 50CrV medium carbon steel is shown in figure 7(f).In the heat affected zone, a high hardness of plate-strip martensite was formed.The columnar dendrites inside the weld grow towards the tip of the weld as shown in figure 7(d).
It can be seen from figures 8(a) and (b) that the HAZ consists of massive and coarse proeutectoid ferrite and weistensite, which have larger grains than the recrystallization zone and are distributed in the weistensite structure, which in the figure is composed of acicular ferrite and perlite.The mechanism of eutectoid ferrite production is that before the eutectoid transformation (to produce perlitic), the carbon content of the material does not reach the carbon content of the eutectoid point, so it is necessary to precipitate part of the ferrite to increase the carbon content and provide conditions for the eutectoid transformation, and the ferrite produced by this process is the first eutectoid ferrite.By comparing figures 8(a) and (b), it can be seen that due to the fast welding speed of laser welding, the HAZ grains formed by smaller heat input are smaller.
Figure 9 shows the SEM images of laser welded joint and laser wire welded joint.The temperature gradient of laser welding varies greatly, resulting in martensite in the weld, and it can be seen in figure 9(a) that the grain is plate martensite.Laser wire filling welding regulates the microstructure in the weld through the welding wire element, and the microstructure in the weld is austenite.In figure 9(b), there is a higher Cr and Ni content at point 2, and austenite accumulates at the grain boundaries and first precipitates from the grain boundaries.
Figure 10 shows the XRD of laser welded joint and laser wire welded joint.The sampling position is the center of the weld.With the increase of butt clearance, the weld structure gradually transforms to complete austenite.More welding wire enters the weld pool due to increased butt clearance.The microstructure of laser welded joints is martensitic, which has a tendency of brittleness and hardness, which is not conducive to higher toughness reserve of welded joints.
When the laser power is P = 2.5kW, the welding speed is V = 0.75 m•min −1 , and the wire feeding speed is Vw = 3.6 m•min −1 , the tensile strength with different butt gaps is shown in figure 11.When the butt gap is 0.2 mm, 0.4 mm and 0.6 mm, the average tensile strength of the sample is 612.5 MPa, 595.6 MPa and 626.2 MPa, respectively, and the fracture position is all on one side of the thin plate.When the butt gap is 0 mm and 0.8 mm and laser welding is performed alone, the tensile strength joint formed shows low strength due to defects in the weld forming, and the fracture position is all at the weld center defect.There are welding cracks in laser welding, as shown in figure 12 The microhardness distribution of welded joints is shown in figure 13.With the increase of weld butt gap, the average hardness of weld decreases from 425.5 HV to 382.8 HV.This is due to the increase of butt gap, the weld residual height decreases, and more wire metal melts into the weld center.The highest hardness of laser wire filled welded joint appears in the heat affected zone of 50CrV carbon steel.When the butt gap is 0.8 mm, the maximum hardness of welded joint is 535.5 HV.The highest hardness of laser welded joint is 613 HV in the weld center.
When the butt gap is 0.6 mm, the fracture occurs on the 50CrV medium carbon steel side.The SEM image of its fracture is shown in figure 14.There is a significant neck contraction at the fracture.The surface fractures show good ductility and small dimples appear.The fracture is ductile fracture.The dimples are deep and the size of dimples is 2-5 μm, indicating high toughness.

Conclusion
In this experiment, low carbon steel of 6mmSPHE and medium carbon steel of 2 mm 50CrV were used for butt welding.The microhardness and tensile strength of the welded joints were analyzed by studying different butt gaps.The following conclusions are obtained: (1) Laser power P = 2500 W, welding speed V= 0.75 m•min-1 and wire feeding speed Vw= 3.6 m•min-1.
When the butt gap is 0.2-0.6 mm, a good welded joint can be formed by laser wire filling welding.
(2) When the butt gap is 0.6 mm, the maximum tensile strength of the sample is 612.5 MPa, and its fracture position is on the side of 50CrV medium carbon steel.(3) The hardness of weld center decreases with the increase of butt gap.The maximum hardness of laser wire filling weld is in the heat affected zone of 50CrV medium carbon steel, and the maximum hardness is 535.5 HV.The highest laser welding hardness is 613 HV in the weld center.

Figure 1 .
Figure 1.Schematic diagram of laser wire filling welding system.

3. 2 .
Influence of docking clearance on sample morphologyThe welding morphology of the upper surface of the unequal thickness plate formed by laser wire filling butt welding is shown in figure3.When laser welding alone, weld melt height is almost 0 mm as shown in figure3(a).When the butt gap is 0 mm, 0.2 mm, 0.4 mm, it can be seen that the surface of the weld is full and the remaining height is shown in figures 3(b)-(d).When the butt gap increases from 0 mm to 0.8 mm, the weld melt height gradually decreases with the increase of the butt gap.When the butt gap is 0.6 mm, the melt height is flush with the weld surface as shown in figure3(e).When the butt gap is 0.8 mm, the upper surface of the welded sample will collapse and the weld will not be full due to the large clearance as shown in figure 3(f).It can be found that

Figure 2 .
Figure 2. High-speed images of droplet transition during laser wire filling welding.(a) Liquid bridge transfer.(b) Globular transfer.(c) Explosive transfer.

Figure 4 .
Figure 4.The X-ray images of laser welding and laser fillet welding specimens.

Figure 6 .
Figure 6.Weld penetration, melt height and melt width with different butt gaps.

Figure 7 .
Figure 7. Microstructure of laser welded joint and laser wire welded joint.(a) The OM image of laser welded joint weld (b) Overall appearance of laser welded joint (c) The OM image of 50CrVsteel HAZ (d) The OM image of laser wire welding joint weld (e) Overall appearance of laser wire welding joint (f) The OM image of 50CrVsteel HAZ.

Figure 8 .
Figure 8. Microstructure of heat affected zone of laser welded joint and laser wire welded joint.(a) The OM image of HAZ for laser welded (SPHE side) (b) The OM image of HAZ for laser wire welded (SPHE side).

Figure 9 .
Figure 9.The SEM images of laser welded joint and laser wire welded joint.(a) The Microstructure image of laser welded joint weld (b) The Microstructure image of laser wire welded joint weld.

Figure 10 .
Figure 10.The XRD of laser welded joint and laser wire welded joint.

Figure 11 .
Figure 11.The influence of butt gap on tensile strength of weld.
(a).The weld is opened from the crack position, and the fracture morphology is shown in figure 12(b).It indicates an inter-granular fracture (IF).

Figure 12 .
Figure 12.Optical image of laser welding crack and SEM of tensile fracture.

Figure 13 .
Figure 13.The influence of butt gap on microhardness of weld.

Figure 14 .
Figure 14.The SEM image of tensile fracture.

Table 2 .
Test material chemical composition.