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Technical Background

Welding of medium and large thickness steel plates is mainly used in rail transit, shipbuilding, nuclear power equipment, and other industries. Requires extremely high welding technology, equipment, and welding quality. Welding technology is a key technology in ship manufacturing and the development of the shipbuilding industry. Its production efficiency and welding quality directly affect the production cycle, cost, and hull quality of ship manufacturing. Therefore, high-efficiency and high-quality ship welding technology is an important factor to realize the transformation and upgrading of my country’s shipbuilding industry, and it is also an important factor to improve the international competitiveness of my country’s shipbuilding enterprises.

As a new welding technology, laser-arc hybrid welding uses laser and arc as dual heat sources and acts on the same molten pool at the same time to form laser-guided and stable arcs. A welding method with bridging ability, which fully utilizes the advantages of laser welding and arc welding in the welding of medium and large thickness steel plates, and makes up for their respective shortcomings. The traditional welding method has disadvantages such as low joint strength, low efficiency, serious deformation, and large consumption of welding materials; and the use of single laser welding also has some shortcomings, such as high joint assembly process requirements, and welding capacity is greatly restricted by laser power, poor bridging ability, serious weld undercut, etc.

As a new type of welding method, laser-arc hybrid welding has the following three notable characteristics.

• Improve energy utilization, increase the primary penetration depth and welding speed of the weld;
• Reduce workpiece assembly requirements;
• To improve weld quality and improve weld formation.

Technical Characteristics

There are many forms of laser-arc hybrid welding, including Laser-MAG/MIG hybrid welding, Laser-TIG hybrid welding, laser-plasma arc hybrid welding, etc. Among them, the commonly used laser light source at this stage is fiber laser, as shown in Figure 1.

This paper takes Laser-MAG hybrid welding as an example to discuss, and its form is shown in Figure 2, which is different from the laser filler wire fusion welding shown in Figure 3.

The basic elements of the two welding methods are the same, which are composed of the laser beam, welding wire (welding gun), and workpiece to be welded. The protective gas is added according to actual needs and the appropriate type is selected. The main equipment involved is a wire feeder, welding machine, composite welding hard gun head (wire filling welding soft gun head), welding head, and high-power laser. There are essential differences between the two welding methods.

The Laser-MAG hybrid welding shown in Figure 2 has two welding heat sources, namely laser and MAG arc, which can form an effective molten pool when welding as a single heat source, but the characteristics of the molten pool are different: the characteristics of the laser welding molten pool It is “deep and narrow”, with the small opening area and large depth, which is not conducive to weld formation; MAG arc welding molten pool is characterized by “shallow and wide”, large opening area and small depth, which is conducive to weld formation and strong bridging ability.

In the laser-MAG hybrid welding process, two heat sources act on the base metal at the same time, there is a mutual influence between the two heat sources, and there is also mutual influence between the two molten pools, which will eventually form a new composite molten pool. The composite molten pool has both the “large depth” of the laser molten pool and the “large area” of the arc molten pool.

The composite molten pool has a large depth, good weld formation, and strong bridging ability. At the same time, MAG arc welding is filled with the welding wire, and the type of welding wire can be selected, suitable welding wire can be selected to add to the welding process according to the performance defects of the base metal itself, so as to improve the crack resistance of the weld at the micro-level. Purposeful improvement in fatigue resistance, corrosion resistance, wear resistance, etc.

As shown in Figure 3, laser welding with filler wire, laser as the only heat source in the welding process, not only needs the welding wire, but also needs to melt the base metal, and form the special hole effect of laser deep penetration welding on the base metal, forming a deeper in the molten pool, the components of the welding wire and the metal components of the base metal are fully mixed to form a new mixed molten pool. The element composition, proportion, and quality of the mixed molten pool are quite different from those of the welding wire and the base metal, so it is also possible to select the appropriate welding wire to add to the welding process according to the performance defects of the base metal itself, so as to improve the weld performance.

Applications

Aluminum Alloy Welding

Requirements: tailored welding of 5 mm and 6 mm aluminum alloys requires good weld formation and no undercut defects. Equipment: fiber core diameter 200 μm, unified welding machine (with its own arc voltage, current, wire feeding amount matching database), wire feeding machine, welding head (collimating lens focal length 100 mm, focusing lens focal length 300 mm). Process parameters: The welding process parameters are shown in Table 1.

Result: The forming is good, and the weld has no undercut defect, as shown in Figure 4.

Welding of Medium And Thick Ship Plates

Requirements: Tailored welding of AH36 marine steel plates with thicknesses of 6 mm and 12 mm requires that the weld formation and weld quality meet the requirements of the Chinese Shipbuilding Quality Standard (GB/T 34000-2016). Equipment: fiber core diameter 200 μm, unified welding machine (with its own arc voltage, current, wire feeding amount matching database), wire feeding machine, welding head (collimating lens focal length 150 mm, focusing lens focal length 300 mm). Process parameters: The main welding process parameters are shown in Table 2. Table 2 welding process parameters Result: The welding seam is relatively uniform. The welding seam is basically free of pores, as shown in Figure 5. The forming and welding seam quality and performance meet the requirements after testing.

Large Thickness Ship Plate Multi-Pass Welding

Requirements: 20 mm, 25 mm thick ultra-high-strength marine steel plates are tailor-welded, and the weld formation and weld quality are required to meet the requirements of the Chinese shipbuilding quality standard (GB/T 34000-2016). Equipment: fiber core diameter 200 μm, unified welding machine (with its own arc voltage, current, wire feeding amount matching database), wire feeding machine, welding head (collimating lens focal length 150 mm, focusing lens focal length 300 mm). Process parameters: The weld bead must be grooved, the groove size is shown in Figure 6, and other welding process parameters are shown in Table 3.

Result: The forming is good, there are no infusions, and the weld is basically free of pores. As shown in Figure 7, the forming, weld quality, and performance meet the requirements after testing. The welded sample is shown in Figure 8.

Equipment And Process Parameter Suggestion

At present, for the laser-arc hybrid welding technology, there are very few practical applications in China, and there is no systematic achievement for the time being. In addition, another key issue is that the bridging capacity of laser-arc hybrid welding is very large, which can weld joints with large gaps. From the point of view of weld formation, the gap tolerance can reach 1 mm, and the seam gap of the joint has a great influence on the process parameter window. It is relatively large, so here, we can only give two guiding suggestions for process data suggestions and equipment selection suggestions.

Based on foreign research results and self-test results, it is recommended that when selecting equipment:

• Ensure a certainly focused spot diameter, which is more suitable between 0.4-1.0 mm. If the spot is too large, the laser power demand will increase sharply. If the spot is too small, the weld seam is not very stable when welding thicker plates;
• The welding machine must be selected with pulse function, otherwise, the splash will be difficult to control.

As for the welding process parameters, it needs to be determined according to the specific situation. There are several points to pay attention to:

• It is more appropriate to control the welding speed within 3 m/min. Excessive welding speed will cause uneven weld formation and serious undercut;
• The angle between the welding torch and the welding seam is more suitable when it is greater than 40°, otherwise, it is easy to form unevenly;
• It is best to use mixed gas (Ar: CO₂=82%:18%) when welding carbon steel, and pure Ar when welding aluminum alloy;
• The filament spacing (the distance between the spot and the welding wire) is generally controlled at 2-4 mm. Too small spacing will bring about large splashes and unstable forming, and too large spacing will weaken two. The interaction between heat sources needs to be fully verified according to the characteristics of different materials in order to reflect the advantages of “1+1>2” laser-arc hybrid welding.

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