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Modern Laser Welding Technology Special Topic — Laser-Arc Hybrid Welding

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Modern Laser Welding Technology Special Topic — Laser-Arc Hybrid Welding

Laser-Arc Hybrid Welding is an advanced modern welding technique that combines the advantages of laser welding and arc welding to improve welding efficiency and quality. Here are the key points about this technology:

Technology Principles and Advantages

Combination of Two Welding Processes:

Laser Welding: Utilizes a high-energy laser beam to locally heat and melt the weld seam, forming the weld joint.

Arc Welding: Uses an electric arc to heat and melt the welding material, similarly used for welding workpieces.

Advantages:

Rapid Heating: The laser provides rapid localized heating, speeding up welding rates and production efficiency.

Deep Penetration: Laser energy can deeply penetrate and melt materials, achieving greater weld penetration depth.

High Precision: Laser's high accuracy and focus control enable precise welding geometries and dimensions.

High-Quality Welds: Combines the stability of arc welding with the excellent performance of laser welding, producing uniform welds with low porosity.

Applications

Automotive Manufacturing: Used for welding car body components, chassis, and engine parts to enhance joint strength and quality.

Aerospace: Suitable for welding aerospace materials such as aluminum alloys and titanium alloys, ensuring high strength and lightweight structures.

Electronics Manufacturing: Provides high precision and minimal heat-affected zone in welding micro-components and electronic devices.

Energy Industry: Used in welding pipelines, pressure vessels, and turbine components to improve corrosion resistance and heat resistance.

Technological Challenges and Trends

Equipment Costs: High costs of high-power laser equipment and hybrid welding systems limit widespread application.

Process Control: Requires precise welding parameter control and system integration to ensure stable and repeatable welding quality.

Material Compatibility: Detailed research needed on welding characteristics and compatibility of different materials to meet diverse application demands of hybrid welding.

Automation and Intelligence: Moving towards intelligent welding systems and automated production lines to enhance productivity and reduce manual intervention.

1.Characteristics of Laser-Arc Hybrid Welding

Laser-Arc hybrid welding technology, proposed by British scholar W. Steen in the late 1970s, aims to effectively utilize arc energy to achieve greater welding penetration depth under lower laser power conditions. Simultaneously, it enhances the adaptability of laser welding to weld gap variations, thereby realizing efficient and high-quality welding processes. The diagram illustrates the basic principle of Laser-Arc hybrid welding and typical weld cross-sectional appearances. The combination allows both heat sources to fully utilize their respective advantages while compensating for each other's shortcomings, thereby forming an efficient heat source.

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Laser-Arc Hybrid Welding: Enhanced Adaptive Welding Method

Laser-Arc hybrid welding, formed by the interaction of laser and arc, represents an adaptive welding method that mitigates the shortcomings of single welding techniques. It offers numerous advantages such as increased energy efficiency, deeper penetration, stable welding processes, reduced assembly requirements, and capability to weld high-reflectivity materials. Key benefits include:

High Efficiency, Energy Savings, and Cost-Effectiveness

Increased Weld Penetration

Reduced Welding Defects and Improved Microstructure

Enhanced Weld Seam Formation

Improved Welding Adaptability

Minimized Welding Deformation


2.Methods of Laser-Arc Combination

Laser-Arc hybrid heat sources commonly utilize CO2 and Nd: YAG lasers. Based on the relative positioning of the laser and arc, there are distinctions between off-axis (eccentric) and coaxial (concentric) hybrid configurations. Off-axis hybrid welding involves the laser beam and arc acting at the same position on the workpiece at an angle. The laser can enter from either the front or rear of the arc. Coaxial hybrid welding, on the other hand, positions the laser and arc concentrically at the same location on the workpiece surface. Diagrams 6-10 and 6-11 illustrate principles of off-axis and coaxial laser-arc hybrid welding respectively.

Off-axis hybrid welding is easier to implement and can use TIG or MIG arcs. Coaxial hybrid welding, while more challenging and complex, often employs non-consumable TIG arcs or plasma arcs. In diagram 6-11(a), the arc is positioned between two laser beams. YAG laser beams exit through fiber optics, split into two beams, and are refocused by lenses. The electrode and arc are located beneath the lenses, aligning the laser focus point with the arc radiation point. Diagram 6-11(b) demonstrates a design where the laser passes through the center of the arc to achieve coaxial hybrid welding with TIG arc. Eight tungsten electrodes are evenly distributed at a 45° angle on a certain diameter ring. Each pair of electrodes operates in corresponding directions based on the welding gun's movement, generating front and rear heat sources. Additionally, hollow tungsten electrodes are designed to allow the laser beam to pass through the center of the circular arc, another common method for achieving coaxial hybrid welding. Coaxial hybrid welding resolves directional challenges associated with off-axis hybrid welding and is particularly suitable for welding three-dimensional structural components, although complex torch design remains a challenge.

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1. Laser-TIG Arc Hybrid

The study of laser-arc hybrid welding began with the off-axis combination of CO2 laser and TIG arc. TIG arc welding involves a non-consumable electrode, making the process relatively straightforward when combined with laser welding. The laser beam and arc can be arranged either coaxially or off-axis. Research indicates that the angle between the laser beam and arc, arc current intensity and output mode, laser power, arrangement direction, gap distance, arc height, and shielding gas flow rate are key factors influencing the hybrid welding process.

Under conditions of high-speed welding, the laser-TIG hybrid heat source can maintain a stable arc, resulting in aesthetically pleasing welds while effectively reducing welding defects such as porosity and undercutting. Particularly at low currents, high welding speeds, and extended arcs, the welding speed of the laser-TIG hybrid heat source can exceed twice that of conventional TIG welding, a feat not easily achieved with standard TIG welding.

The laser-TIG arc hybrid heat source is widely used for high-speed welding of thin plates and is also suitable for butt welding dissimilar thickness materials. When welding plates with larger gaps, filler metal can be used. The comparison of weld cross-sections obtained from laser-TIG hybrid heat source welding and laser welding alone is illustrated in the following diagram.

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2. Laser-Plasma Arc Hybrid Heat Source Welding

Plasma arc possesses several advantages such as good rigidity, high temperature, strong directionality, good arc ignition capability, narrow heating zone, and minimal sensitivity to external factors, making it highly suitable for hybrid heat source welding. Applications of laser-plasma arc hybrid welding include butt welding of thin plates, dissimilar thickness plate joints, overlap welding of galvanized plates, welding of aluminum alloys, cutting, and surface alloying, all of which have shown promising results. Similar to laser-TIG arc hybrid heat source welding, laser-plasma arc hybrid welding can be performed in both off-axis and coaxial configurations.


3. Laser-MIG Hybrid Heat Source Welding

Laser-MIG hybrid heat source welding is currently one of the most widely used methods in hybrid heat source welding. It finds applications in industries such as automotive manufacturing and shipbuilding. Consequently, many research institutions and companies specialize in the design and manufacture of laser-MIG hybrid heat source welding torches. The diagram illustrates two different types of laser-MIG hybrid heat source welding torches.


Laser-MIG hybrid heat source welding utilizes the advantages of MIG welding for wire feeding. It enhances welding penetration, adaptability, and improves the metallurgical properties and microstructure of welds. However, due to challenges such as wire feeding and droplet transition, laser-MIG hybrid welding is mostly performed in off-axis configuration. Compared to laser-TIG arc welding, laser-MIG hybrid welding exhibits greater versatility and can weld thicker plates. Especially due to MIG arc's strong directionality and cathode sputtering advantages, it is particularly suitable for welding difficult-to-weld metals such as thick plates and aluminum alloys.

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4. Laser-Dual Arc Hybrid Heat Source Welding

Laser-dual arc hybrid heat source welding is a welding process where laser is combined with two MIG arcs simultaneously. Two welding torches utilize independent power sources and wire feeding mechanisms, sharing the welding head through their respective supply line systems, as depicted in the diagram. Each welding torch can be adjusted relative to the other welding torch and the laser beam position.


Because three heat sources need to operate simultaneously within the same area, the arrangement of their positions relative to each other is crucial. To allow the welding heads to be repositioned vertically relative to the laser beam and other configurations, careful consideration is required during the research and design of experimental apparatuses to ensure the welding torches and laser beam focus sizes are precisely aligned.

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4. Advantages of Laser-Dual Arc Hybrid Heat Source Welding

In seamless joint welding, the welding speed of laser-dual arc hybrid heat source welding is 33% faster than conventional laser-MIG hybrid heat sources and 80% faster than submerged arc welding. The energy input per unit length is 25% lower than conventional laser-MIG hybrid heat sources and 83% lower than submerged arc welding. Moreover, the welding process is exceptionally stable, far surpassing the welding capability of conventional laser-MIG hybrid heat sources.


3.Interaction Between Laser and Arc

Due to the absence of droplet transition in TIG welding and its minimal impact on the weld pool, laser-TIG arc hybrid heat source welding exhibits excellent stability of the weld pool. During the welding process, the keyholes formed by the laser in the weld pool attract the arc, enhancing the stability of the welding process. Additionally, the presence of keyholes compresses the size of the arc root, significantly increasing the current density of the arc and effectively enhancing the utilization of arc energy. This effect is particularly pronounced with CO2 lasers, where the presence of photo-induced plasmas during the welding process has a more significant impact on the arc. Diagram 6-15 illustrates the morphological changes of the TIG arc before and after the action of CO2 laser.

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Similar to single laser welding, laser-TIG arc hybrid welding exhibits two welding mechanisms: hybrid deep penetration welding and hybrid heat conduction welding. The keyhole effect of the laser determines the transition between these two hybrid welding modes.


At medium to low currents, the arc plasma has a weaker shielding effect on the laser, and the arc stiffness is relatively low, allowing the laser keyhole to remain stable. This condition is favorable for compressing and attracting the arc root. The high energy density of the hybrid arc current enables greater welding depth, characteristic of hybrid deep penetration welding. As the current increases to a critical threshold, the arc temperature rises, and its stiffness increases. The arc plasma's shielding effect on the laser intensifies, causing significant energy loss as the laser passes through the arc. The laser energy reaching the workpiece surface becomes insufficient to sustain the keyhole, resulting in reduced compression of the arc root. The arc current density decreases, leading to a narrower and wider weld seam cross-section, characteristic of heat conduction welding, as shown in the diagram.


Moreover, higher laser power enhances its ability to attract and compress the arc, thereby raising the critical current threshold for the transition in hybrid welding mechanisms. Additionally, the transition of hybrid welding mechanisms is influenced by factors such as the configuration of the laser beam and arc (coaxial or off-axis), their interaction positions, the output mode of laser energy (pulse or continuous wave laser), and the wavelength of the laser.


To address the issues of laser energy loss and arc expansion in laser-TIG hybrid heat source welding, a potential future solution could be a novel hybrid welding method: laser-TIG pulse coordinated control hybrid welding.

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Since its inception in the late 1970s, laser-arc hybrid welding technology has demonstrated unique advantages and shown tremendous potential applications. It can be utilized in various fields such as high-speed welding and cutting of thick plates and difficult-to-weld metals, surface alloying, cladding, rapid prototyping, mold repair, and spot welding of precision parts. From a process perspective, laser-arc hybrid welding leverages the strengths of both technologies while compensating for their respective weaknesses, demonstrating excellent weldability and adaptability. From an energy standpoint, the significant increase in welding efficiency is the most notable feature of hybrid welding. In fact, the energy of the hybrid heat source far exceeds that achieved by simply combining the two heat sources.


4.1 Hybrid Deep Penetration Welding of Thick Plates

For decades, researchers have been exploring the use of laser welding for thick plates. However, strict assembly requirements, mechanical properties of weld metals, and the high cost of high-power lasers have limited the application of laser welding. Laser-arc hybrid welding not only allows for deep penetration welding of thick plates but also exhibits good adaptability to the edge preparation of weld seams, neutralization, and weld gap.


4.1.1 Hybrid Deep Penetration Welding of Thick Plates

The successful application of laser-arc hybrid welding in welding thick plates has benefited industries such as shipbuilding. To meet the increasingly stringent design requirements of navies and ensure structural stability, the United States Naval Surface Warfare Center has conducted comprehensive experimental studies on the efficiency, material characteristics, and weld deformation aspects of laser-MIG hybrid heat source welding, particularly focusing on HSLA thick steel plates used in reinforcing shipboard stiffeners. The goal is to apply this technology to the welding of typical ship structural materials for the U.S. Navy.


The consideration for applying hybrid heat source welding is driven by several factors:


Laser-arc hybrid welding enables the use of high-strength steel in its hot-rolled state within structural components, which is impractical under conventional welding conditions.

It increases welding speed, relaxes sensitivity to joint gaps, reduces welding deformation, and enhances welding quality.

In welding seams totaling 50% of the length in this structure, laser-arc hybrid welding results in deformation only 1/10th of that observed in dual-wire welding. Single-pass weld depths can reach 15mm, and double-pass weld depths can achieve 30mm. When welding a T-joint with a thickness of 6mm, welding speeds can reach 3m/min.

Figures 6-20 and 6-21 illustrate the experimental comparison results between conventional welding, laser welding, and hybrid heat source welding.

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4.2 Aluminum Alloy Laser-Arc Hybrid Heat Source Welding

Laser welding of aluminum alloys poses challenges such as high reflectivity, susceptibility to porosity and cracking, compositional changes, and the requirement for high laser power levels. Laser-arc hybrid heat source welding effectively addresses these issues. The reflectivity of the aluminum alloy liquid pool is lower than that of solid metal. The arc enhances the absorption of the laser beam directly onto the liquid pool surface, thereby increasing absorption and improving penetration depth. Alternating current TIG with direct current reverse polarity (DCEP) can be used to clean the oxide film before laser welding. Additionally, the large arc-generated molten pool moves ahead of the laser beam, enhancing wetting between the molten pool and solid metal and preventing undercut formation. This addition of the arc is generally not suitable for CO2 lasers used in welding aluminum alloys.


4.3 Lap Joint Laser-Arc Hybrid Heat Source Welding

Lap joints are widely used in automotive frame and floor structure welding. With increasing demands for automotive quality and environmental protection, lap joint welding of galvanized steel and aluminum materials is prevalent in automotive body welding. Applying hybrid heat source welding technology to lap joint welding on automotive floor panels not only reduces deformation and prevents defects such as sinking or undercutting but also further increases welding speed.


4.4 High-Speed Laser-Arc Hybrid Heat Source Welding

The main issue with laser high-speed welding of thin plates is discontinuous weld seam formation and surface bumps. Using plasma arc-assisted lasers on YAG and CO2 lasers for welding thin plates (0.16mm thick galvanized plates) effectively addresses discontinuous surface formation issues associated with laser high-speed welding, achieving approximately 100% higher welding speeds compared to standalone laser welding. Moreover, due to the interaction between the arc and the laser, the arc remains stable even at high welding speeds of up to 90m/min, resulting in wider weld seams and smoother weld seam surfaces.


For thick steel plates, high-speed welding can also be achieved using laser-MIG arc hybrid heat sources, as shown in Figure 6-25. During groove design, a guide channel is specifically provided for the laser, allowing the arc to reach deeper into the weld seam under the guidance of the laser. This approach results in greater penetration depth and welding speed than conventional laser welding, with the main melting of metal in gaps and grooves primarily relying on the arc.


These advancements highlight the versatility and efficiency of laser-arc hybrid heat source welding across various applications in different industrial sectors.


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