1. Preface

Low-alloy high-strength steel is characterized by high strength, high plasticity, and high toughness, and is widely used in various fields such as bridges, vehicles, ships, buildings, pressure vessels, and special equipment. Traditional welding methods typically employ CO2 gas shielded welding, which requires groove preparation for T-joint welding of thick plates, involving multi-pass welding. This method suffers from low welding efficiency, significant welding deformation, and poor welding joint quality. Laser-MAG hybrid welding combines laser and arc as heat sources, reducing the number of welds passes and minimizing heat distortion. It offers advantages such as high welding efficiency and deep weld penetration. Therefore, researching the laser-MAG hybrid welding process for T-joints is of significant importance for practical production.

This study employs a 20-kW fiber laser for laser-MAG hybrid welding, investigating the double-sided welding process of T-joints in Q355 low-alloy high-strength steel plates of medium thickness. It aims to determine the influence of hybrid welding parameters on weld formation.

2 Experimental Preparation

2.1 Testing Materials

The welding specimens consist of Q355 low-alloy high-strength steel ranging from 10mm to 50mm in thickness. The cross-sectional dimensions of the specimens are 300mm × 150mm, and the joint type is T-joint fillet weld without groove preparation. The welding wire used is of ER50-6 grade with a diameter of 1.2mm. Prior to welding, surface oxide layers and rust are removed, and the welding area is wiped with isopropanol to eliminate oil, water, and dust contaminants. The chemical composition and mechanical properties of the test materials are detailed in Table 1 and Table 2, respectively.

2.2 Testing equipment and methods

The experiment utilizes a 20kW fiber laser welding system with a maximum output power of 20kW, a wavelength of 1064nm, a collimator of 100mm, and a focus length of 300mm, resulting in a focused beam diameter of 0.45mm. The arc welding system employs a MAG welder with a maximum welding current of 500A, using a shielding gas mixture of 80% Ar and 20% CO2. The process involves laser-MAG hybrid welding. Initially, two test plates are positioned and welded to form a T-joint. Subsequently, they are clamped securely using a vise grip, and the plates are horizontally positioned for welding.

3 Testing results and analysis

3.1 Determination of welding angle and position 

In T-joint welding, since the laser primarily contributes to deep penetration through the plate, the laser incidence direction should ideally align with the welding end face. However, interference between the base plate and the welding joint may prevent the laser from horizontally welding the seam, resulting in a certain angle between the laser and the horizontal base plate, known as the laser’s incidence angle (see Picture 1). To ensure complete penetration of the weld seam, the laser penetrates from the backside of the weld seam; therefore, the laser welding position on the front side of the weld seam is located on the vertical plate of the T-joint.

Picture 1 Welding angle and position diagram

   During actual welding tests, the shape of the weld pool varies with the thickness of the plate. When the penetration depth is 8mm, there is no clear transition between the laser and arc welding pools, and the macroscopic metallographic structure of the weld seam appears triangular overall. When the penetration depth ranges from 8 to 12mm, there is a tendency for separation between the laser and arc welding pools, resulting in a “bell” shape in the macroscopic metallographic structure of the weld seam. When the penetration depth exceeds 12mm, the laser and arc welding pools are distinctly separated, with the arc welding pool concentrated on the surface of the weld seam and the laser pool positioned below the weld seam. The overall macroscopic metallographic structure resembles a “nail head.” As the plate thickness increases, the weld width gradually decreases from 6mm to 2mm.

When the laser incidence angle is too large and the welding position is biased upward, it can lead to welding misalignment and lack of fusion at the plate joint. To avoid interference between the welding joint and the base plate, a larger laser incidence angle is selected during actual welding. The welding position is biased towards the bottom of the weld seam. When welding thin plates, it can be observed that the weld pool can penetrate through the surface of the base plate from the backside of the weld seam.       

The optimal laser angle and welding position (distance from the base plate) under different thicknesses are summarized in Table 3, based on experimental findings.

When the plate thickness exceeds 12mm, the effectiveness of laser-MAG hybrid welding diminishes. The arc welding pool cannot penetrate deeply into the plate along with the laser, resulting in a narrow laser fusion width at the lower part of the weld seam and making it prone to welding misalignment and lack of fusion. Therefore, a significant adjustment of the laser incidence angle is necessary to ensure that the laser beam remains near the weld seam. When the plate thickness exceeds 18mm, the laser incidence angle has been reduced to a minimum angle of 7°. Adjusting the laser welding position closer to the base plate causes the weld seam to deviate towards the base plate, reducing effective penetration depth. Conversely, positioning the laser welding farther away from the base plate may result in welding misalignment defects at the transition point between the laser and arc welding, as well as incomplete fusion with the base plate.

3.2 Influence of Gap Size on Weld Formation 

When the welding plate thickness exceeds 12mm, the laser incidence angle significantly decreases, making the weld seam prone to defects such as misalignment and lack of fusion. The adaptability of laser hybrid welding decreases primarily because the effectiveness of laser-MAG hybrid welding diminishes, resulting in a too narrow fusion width. Under these conditions, the maximum welding thickness is 18mm. To weld plates thicker than 18mm, it is necessary to improve the shape of the weld pool and increase the width of the weld seam..

When the laser power is 8kW, welding current is 240A, arc voltage is 26V, welding speed is 0.9m/min, and the focus is set at -2mm, the macroscopic metallographic structure of the weld seam of a 16mm thick T-joint welded by laser-MAG hybrid welding with different gap sizes is shown in Picture 2.

According to Picture 2, when the gap in the weld joint is 0mm, the laser welding penetration depth is 9mm, and the laser fusion width is relatively narrow. When the gap is increased to 0.5mm, the weld penetration depth increases to 11mm, with little change in the laser fusion width. With a gap of 1mm, the weld penetration depth further increases to 13mm, and the fusion width noticeably increases. When the gap is 1.5mm, complete penetration of the 16mm plate is achieved, and the weld penetration depth and fusion width increase further. As the gap increases, both the weld penetration depth and fusion width increase. When the gap is within 0.5mm, the increase in penetration depth is more pronounced. With a gap of 0.5 to 1mm, there is an increase in penetration depth, while the fusion width increases slowly. When the gap is greater than 1mm, both the weld penetration depth and fusion width increase significantly, resulting in improved weld pool morphology and increased welding adaptability.

Picture 2 (Including Ing a,b,c,d)

Macroscopic Metallography of T-joint Weld Seam in Laser-MAG Hybrid Welding with Different Gap Sizes:

a)gap:0mm

b)gap:0.5mm

c)gap:1mm

d)gap:1.5mm

3.3 Influence of Laser Power and Defocus Amount on Welding Penetration Depth

When the welding current is 260A, arc voltage is 28.4V, defocus amount is 0mm, and the gap is 0.7mm, the influence of laser power on the penetration depth in laser-arc hybrid welding is depicted in Picture 3. The effect of laser power on the hybrid weld penetration depth varies with different defocus amounts.。

Picture 3  The Influence of Laser Power on Penetration Depth

 When the thickness is ≤8mm, the coupling effect between the laser and arc welding heat sources is very good, and the weld penetration depth is significantly greater than that of laser welding alone. When the thickness is >8mm, the arc welding penetration depth cannot effectively reach the bottom of the weld seam, and the penetration depth of the hybrid welding is similar to that of laser welding alone.

The defocus amount of the laser determines the energy density and stability of the keyhole during laser welding. The ratio of weld penetration depth at different focal points to the penetration depth at a defocus amount of 0mm was tested, as shown in Picture 4.

Picture 4  The Influence of Defocus Amount on Penetration Depth

From Figure 4, it can be observed that when the defocus amount of the laser is below -8mm, the penetration depth decreases rapidly. As it reaches -10mm, the rate of decrease slows down, and eventually, at -17mm, it reaches half of the penetration depth at 0mm defocus amount. When the defocus amount is -8mm, the penetration depth reaches its maximum, which is 1.4 times that at 0mm defocus amount. As the defocus amount varies from -8mm to +15mm, the laser penetration depth gradually decreases to 0mm, reaching half of the penetration depth at 0mm defocus amount.

 3.4 The ability for double-sided welding   

To test whether the 20kW laser-MAG hybrid welding can weld through both sides of a 50mm thick T-joint weld seam, single-sided welding penetration depth was tested on a 30mm thick T-joint weld seam. When the laser power is 20kW, it directly penetrates the 30mm thick T-joint. When the power is reduced to 17kW, the welding penetration depth is controlled to be 27mm, as depicted in the macroscopic metallography in Picture 5.。

Picture 5 Macroscopic Metallography of 30mm Thick T-Joint

When the conditions are as follows: laser incidence angle of 8°, laser power of 17kW, welding position of 1.5mm, defocus amount of -8mm, welding speed of 0.6m/min, welding current of 280A, and arc voltage of 29.5V, double-sided welding successfully penetrates through a 50mm thickness. The weld seam exhibits no significant internal defects, as depicted in the macroscopic metallography in Picture 6.

Picture  6 Macroscopic Metallography of Fully Penetrated Double-Sided Weld Seam in 50mm Thick T-Joint

  • Conclusion   

The laser incidence angle varies for different thicknesses of plates, with a smaller angle for thicker plates. When the laser incidence angle is less than 7°, it is prone to directly weld onto the vertical plate, leading to welding misalignment.   

An appropriate gap is beneficial for enhancing weld penetration depth, width, and improving the shape of the weld pool inside the seam. The best weld pool shape is observed when the gap is 1.2mm.   

 There is a linear relationship between laser power and penetration depth, with depth increasing as laser power increases. An appropriate defocus amount can significantly increase the penetration depth. In laser-MAG hybrid welding, the maximum penetration depth is achieved when the defocus amount is -8mm, which is 1.4 times the depth at 0mm defocus.

  With a gap of 1.2mm, laser power of 17kW, and defocus amount of -8mm, double-sided welding penetration through a 50mm thick T-joint weld seam can be achieved at a welding speed of 0.6m/min.

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