Laser-Material Interaction Focus: Common Defects in Laser Welding (② Mechanisms and Suppression of Spatter and Weld Bead Formation)

Laser-Material Interaction Focus: Common Defects in Laser Welding (② Mechanisms and Suppression of Spatter and Weld Bead Formation)

Spatter Formation Mechanism and Suppression:

Spatter refers to small molten droplets that are ejected from the weld pool during welding, potentially causing pores or surface irregularities on the weld joint. The main formation mechanisms include:

Unstable Weld Pool Surface: After concentrated heating by laser energy, the weld pool surface can become unstable due to uneven heating or gas interference, leading to spatter formation.

Gas Jet Effect: Protective gases such as argon used during welding may form bubbles around the weld pool. When these bubbles burst, they can induce spatter formation.

Surface Tension and Inertia of Droplets: Surface tension and inertia play significant roles in spatter formation, particularly evident in high-energy density laser welding.

Strategies to Suppress Spatter:

Optimize Welding Parameters: Control laser power, focal length, welding speed, and other parameters to stabilize the weld pool surface and reduce spatter tendencies.

Improve Weld Pool Environment: Ensure a well-maintained shielding gas environment, control gas flow rates and distribution to prevent bubble formation and rupture.

Use Spatter Prevention Devices: Employ spatter shields or gas covers to effectively capture and minimize spatter generation.

Weld Bead Formation Mechanism and Suppression:

Weld bead refers to protrusions or elevations formed when molten droplets do not fully integrate into the weld pool surface during welding. Key formation mechanisms include:

Unstable Welding Process: Insufficient integration of molten droplets into the weld pool surface due to excessive welding speed or improper welding parameter settings leads to accumulation and bead formation.

Poor Fluidity of Welding Material and Weld Pool: Some welding materials or specific alloys exhibit poor surface tension and fluidity in the liquid state, making it difficult for molten droplets to blend into the weld pool surface.

Strategies to Suppress Weld Bead Formation:

Optimize Welding Parameters: Control welding speed and power to ensure molten droplets fully integrate into the weld pool surface and prevent bead accumulation.

Adjust Surface Tension: Use surfactants or appropriate welding materials to enhance weld pool surface tension and fluidity.

Monitor Welding Process: Real-time monitoring of molten droplet accumulation during welding allows for timely adjustment of welding parameters and operations.

By understanding and applying these suppression mechanisms and strategies, spatter and weld bead formation during laser welding can be effectively reduced, thereby enhancing welding quality and efficiency.

1.1 Spatter Formation and Suppression Analysis

1.1.1 Spatter Mechanism - Metal Shear Force

Metal vapor jets upwards from the keyhole generate shear forces that carry molten droplets causing spatter formation. The shear force from metal vapor is influenced by the internal space of the keyhole. Any variation in keyhole size (collapse) or depth changes can lead to periodic fluctuations in shear force. Therefore, stabilizing the frequency and magnitude of keyhole fluctuations is crucial.

1.2 Spatter Mechanism - Keyhole Collapse

Directly monitoring the dynamic behavior of the keyhole reflects fluctuations caused by changes in energy, which affect welding stability. Monitoring the depth of penetration indicates the stability of the weld pool.

1.3 Spatter Mechanism - LDD Data Verification

Based on OCT (Optical Coherence Tomography) monitoring, LDD (Laser Deep Penetration Depth) collects melt depth data at very high frequencies. Any fluctuation in melt depth data indicates potential keyhole collapse, which significantly increases the likelihood of spatter formation.

The stability of the keyhole is closely related to keyhole collapse, which refers to the fluctuation within the keyhole itself. This stability can be characterized by the degree of fluctuation in melt depth. The fluctuation in melt depth indirectly evaluates the stability of parameters involved in the welding process.

1.4 Spatter Mechanism - Simulation Video

1.5 Potential Inhibition Strategies for Spatter Formation

1.5.1 Force Analysis

Reduce the Impact of Shear Forces

Reducing shear forces: By maintaining stable keyhole morphology (reducing speed, compound, circular, beam shaping, etc.), ensure that the keyhole remains in a vertically stable state. With stable surface tension, minimized shear forces do not carry away molten metal.

Reducing the impact of shear forces: Preventing shear forces from acting on the molten pool, such as by ensuring that shear forces do not touch molten metal (expanding the keyhole opening).

Increase Surface Tension

Increase the size of the molten pool; apply external force on the surface of the molten pool; reduce molten pool fluctuations.

Reduce the Frequency of Keyhole Periodic Collapse

Expand the keyhole opening; reduce plasma impact; improve stability of light source power.

1.5.2 Solution for Spatter Formation

The latest Adjustable Mode Beam (AMB) effectively enlarges the keyhole opening, enlarges the molten pool, and simultaneously applies recoil pressure generated by the evaporation of molten metal caused by the outer ring laser to the molten pool around the keyhole. This stabilizes the keyhole opening and molten pool fluctuations, greatly improving the stability of laser welding and reducing the incidence of spattering.

Outer ring stabilizes the molten pool, reduces temperature gradients, enlarges the keyhole opening size and duration, reduces spattering, and enhances surface formation and process stability.

II. Welding Spatter Formation and Inhibition Analysis

2.1 Definition of Welding Spatter

Welding spatter refers to tiny nodules formed during the welding process of metal materials, where localized high-temperature melting occurs due to electric current. When the liquid metal solidifies, these nodules are formed by the flow of metal under its own weight.

2.2 Causes of Welding Bead Defects

The causes of welding bead defects mainly include the following points:

Unreasonable Welding Parameters:

Welding parameters are one of the key factors affecting welding bead defects. Typically, these include welding current, welding voltage, welding speed, and electrode distance. When the welding current is too high, voltage is too high, speed is too slow, or electrode distance is too short, the temperature of the molten pool becomes excessively high. This can lead to easy detachment of droplets and increased likelihood of arc short-circuiting, resulting in significant spattering and welding bead defects. Conversely, when the welding current is too low, voltage is too low, speed is too fast, or electrode distance is too large, the molten pool temperature is too low, resulting in less droplet detachment and unstable arc, which leads to fewer instances of spattering and welding bead defects.

Inappropriate Welding Materials:

Inappropriate welding materials are also a factor influencing welding bead defects. Factors such as thermal conductivity, coefficient of thermal expansion, and specific heat capacity of the base metal affect welding bead defects. When a low thermal conductivity base metal contacts a high thermal conductivity welding wire, the welding wire generates excessive heat, causing high-temperature areas to spread deeper, resulting in larger welding bead defects. Conversely, when a low specific heat capacity base metal contacts a high specific heat capacity welding wire, the welding wire generates insufficient heat, causing low-temperature areas to spread to the surface, resulting in smaller welding bead defects.

Unregulated Welding Techniques:

Unregulated welding techniques also contribute to welding bead defects. Lack of experience or improper operation by the operator can result in excessive or insufficient welding bead defects. For example, improper equipment parameter settings during welding, such as excessive welding current or too slow welding speed, can lead to excessive welding bead defects, causing deformations, cracks, and other defects. Improper welding positions, such as excessively large or small welding gun angles, too fast or too slow gun oscillation, can result in insufficient welding bead defects, leading to incomplete penetration, porosity, and other defects.

2.3 Prevention Measures for Welding Bead Defects

Prevention measures for welding bead defects mainly include the following:

Selecting Appropriate Welding Standards:

Proper selection of welding standards is an effective method to prevent welding bead defects. By selecting appropriate welding parameters such as welding current, welding voltage, welding speed, and electrode distance, the temperature and flow of the molten pool can be controlled, thereby reducing the occurrence of welding bead defects. Generally, welding current should be moderate, neither too high nor too low, to ensure stable molten pool and normal droplet detachment. Welding voltage should be moderate to ensure stable arc and appropriate arc length. Welding speed should be moderate to ensure uniform molten pool and sufficient filling. Electrode distance should be moderate to ensure concentrated arc and even heat distribution.

Mastering Correct Welding Techniques:

Mastering correct welding techniques is another effective method to prevent welding bead defects. By mastering correct welding techniques, control over the angle and distance of the electrode or welding wire relative to the weld joint can be achieved, thereby controlling the occurrence of welding bead defects. Generally, the angle between the electrode or welding wire and the weld joint should be appropriate, neither too large nor too small, to ensure penetration and forming of the weld bead. The distance between the electrode or welding wire and the weld joint should be appropriate, neither too far nor too close, to ensure stable arc and concentrated heat. The oscillation of the electrode or welding wire should be appropriate, neither too fast nor too slow, to ensure uniformity of the weld and sufficient filling.

Flexibly Adjusting Electrode Angles:

Flexibly adjusting electrode angles is an effective method to prevent welding bead defects. By flexibly adjusting electrode angles, control over the relative position of the electrode or welding wire to the weld joint can be achieved, thereby controlling the occurrence of welding bead defects. Generally, the relative position of the electrode or welding wire to the weld joint should be appropriate, neither too forward nor too backward, to ensure penetration and forming of the weld bead. The relative position of the electrode or welding wire to the weld joint should be adjusted appropriately according to different welding positions to ensure formation of the weld and prevent loss of molten pool. For example, in flat welding, the electrode or welding wire should be slightly tilted backward to increase the melt width