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Common Defects in Laser Welding — Formation and Suppression Mechanism of Porosity and Cracks

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In the process of laser welding, porosity and cracks are two common defects that significantly impact the quality and performance of welded joints. Here is a discussion on the formation and suppression mechanism of porosity and cracks:


Formation Mechanism of Porosity:

Gas Solubility: During welding, gases (such as oxygen, nitrogen, etc.) dissolve in the liquid metal. If the welding speed is too fast or laser power is insufficient, the metal cools too quickly, trapping dissolved gases and leading to the formation of pores.

Surface Contamination and Preparation: Surface contaminants like oil, oxides, or coatings on the material can also trap gases within the weld seam, further promoting porosity.

Welding Environment: The quality and flow of surrounding gases (such as shielding gas) during welding can also affect the formation of pores.

Measures to Suppress Porosity:

Control Welding Parameters: Optimizing laser power, welding speed, and weld design ensures sufficient metal fusion and appropriate cooling rates, thereby reducing porosity.

Preheating and Post-processing: Preheating the base material, using suitable shielding gases, and applying post-weld heat treatments effectively reduce porosity.

Material Selection: Choosing materials with lower gas solubility or special alloys helps mitigate porosity formation.

Formation Mechanism of Cracks:

Thermal Stress: Rapid heating and cooling during welding induce thermal stresses in and around the weld area, potentially causing cracks.

Metal Microstructure Changes: Changes in metal microstructure due to welding, such as alterations in solubility and grain growth, contribute to crack formation.

Residual Welding Stress: Residual stresses remaining after welding make the weld area more susceptible to cracking.

Measures to Suppress Cracks:

Control Welding Process: Fine-tuning welding parameters, reducing heat input, and preheating workpieces help mitigate thermal stresses and reduce crack formation.

Post-weld Heat Treatment: Applying heat treatments or stress relieving to reduce residual stresses helps prevent crack propagation and formation.

Optimization in Design: Designing weld joints with reduced stress concentration helps minimize crack initiation.

Understanding the mechanisms of porosity and crack formation, along with effective suppression measures, enhances the quality and efficiency of laser welding, ensuring welded joints meet expected mechanical properties and aesthetic requirements.

Analysis of Mechanism and Suppression Solutions for Porosity Formation

In laser processing, there are primarily two types of porosity: metallurgical pores, mainly hydrogen pores, and process pores, primarily keyhole instability leading to pores filled with metal vapors and shielding gases.


1.1 Summary of Metallurgical Pores:

Hydrogen Pores: Hydrogen mainly originates from surface oxide films, oil contaminants, impurities carrying moisture, and hydrogen accumulation during metallurgical processes of the material. Water vapor decomposes at high temperatures, forming numerous regular circular pores in the weld seam.

Alloy Elements: Certain alloy elements with boiling points lower than that of the base material can segregate and accumulate, rapidly vaporizing to form clustered pores in the high-temperature environment of laser welding.

Porosity Characteristics: Dense, small, and regularly circular.

Verification by Spectrometer: Conduct elemental testing to validate.

Preventive Measures: Primarily involve incoming material verification and surface cleanliness:

Incoming Material Confirmation: Verify incoming materials to ensure they meet cleanliness and quality standards.

Surface Cleaning: Thoroughly clean surfaces to remove oxide films, oil residues, and other contaminants that contribute to hydrogen and moisture content.

Understanding and addressing these mechanisms are critical for effectively controlling and minimizing porosity in laser welding processes, ensuring weld quality and reliability.

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1.2 Summary of Process-Induced Pores:

Process-induced pores primarily result from instability in the keyhole during deep penetration laser welding. From the perspective of simulating small holes in the liquid column of fiber laser deep penetration welding, based on the stability principle of the liquid column, instability occurs when the length of the liquid column exceeds its circumference. This instability manifests as periodic necking and expansion at the upper end of the liquid column. As the heat source moves, the necking and expansion areas of the liquid column collapse under the influence of surface tension, detaching and forming bubbles that remain trapped in the weld seam.

1.3 Analysis of Bubble Escaping Process:

Regardless of whether they are metallurgical pores or process-induced pores, bubbles form first and then are affected by tensions and buoyancy within the molten pool. When the forces acting on the bubbles exceed a certain threshold, they begin to rise and escape. If they do not escape before the molten pool solidifies, they become pores. Therefore, the condition for pore formation is that the upward velocity of the bubbles must be slower than the solidification rate of the molten pool.

Understanding these processes is crucial for implementing effective measures to prevent and minimize the formation of pores in laser welding, ensuring high-quality welds with minimal defects.

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Vf: Upward velocity; Vs: Solidification velocity. When Vf < Vs, pores form inside the weld seam. When Vf = Vs, pores form on the surface of the weld seam. When Vf > Vs, bubbles successfully escape the molten pool, and thus pores do not form.

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1.4 Inhibition of Bubble Formation Process:

1.4.1 Hydrogen Pores:

Moisture on the surface of the weldment or increased content of O2, H2, H2O in the welding material or wire can lead to an increase in hydrogen pores in the weld seam.

During welding, insufficient shielding gas or low flow rate of shielding gas can cause air to be entrained into the molten pool or small holes, increasing the content of O2, H2, H2O in the molten pool and promoting the formation of hydrogen pores.

Hydrogen is poorly soluble in liquid metals, and its solubility increases with temperature. As the molten pool enters the solidification stage, rapid temperature decrease causes hydrogen to easily precipitate, forming bubbles that remain in the weld seam and become pores.

Solution:

Incoming Material Control: Ensure uniform composition and hydrogen content specifications.

Environmental Control: Manage humidity; clean surfaces of weld seams for oil residues, oxides, and other contaminants.

1.4.2 Process-Induced Pores:

The core issue is maintaining the stability of the keyhole during welding, ensuring a dynamic balance with small fluctuations.

Instability in the keyhole leads to process-induced pores. Given the nature of deep penetration laser welding, stabilizing the keyhole instability is challenging but can be managed by minimizing dynamic fluctuations. Techniques include optimizing the keyhole opening direction, reducing keyhole oscillation, and considering composite laser welding (such as fiber-semiconductor composite, laser arc composite, dual laser, etc.). Other optimizations involve beam shaping, circular beam spot, tilting the laser beam by 5-15°, and optimizing gas shielding.

Maintain stability in external factors such as environment, equipment, and gas shielding, especially controlling mechanical vibrations and turbulence in shielding gas.

2. Cracking Mechanism and Analysis of Suppression Solutions:

2.1 Definition of Cracks:

During welding, under stress, the local metal atomic bonding in the weld joint is disrupted, forming new interfaces known as cracks. They have sharp notches and a large aspect ratio. Solidification cracks are the most common and widespread type.

Understanding and addressing these factors are crucial for optimizing welding processes to minimize defects and ensure high-quality welds.

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2.2 Process of Crack Formation

First Stage: Liquid Phase

The alloy is entirely in the liquid state, freely flowing.

Second Stage: Slurry Phase

The temperature of the liquid metal has dropped below the liquidus line, and dendrites of the solid phase begin to precipitate. They do not contact each other and can freely flow, releasing stresses and strains generated during solidification. If there are any pores or cracks present at this stage, they are typically filled and healed by the liquid metal, preventing defects.

Third Stage: Mushy Zone

As temperature continues to decrease and the proportion of solid phase increases, dendrites start to interlock, forming a solid phase skeleton. As the proportion of solid phase increases, the reflow and healing of the liquid metal become increasingly difficult. Under localized stress, interdendritic liquid films or bridges can rupture, forming cracks. The mushy zone is particularly sensitive to the formation of hot cracks.

Fourth Stage: Solid Phase

A dense solid phase skeleton is formed where solidification shrinkage and thermal strains find it difficult to cause cracking.

Understanding these stages is essential for controlling welding processes effectively to minimize the occurrence of cracks. Proper heat management and stress control during each phase are critical to achieving high-quality welds.

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2.3 Main Factors Influencing Crack Formation

Due to thermal expansion and contraction, during the later stages of cooling and solidification of the weld pool, a solid phase skeleton forms and begins to shrink. However, the weld pool does not uniformly contract but cools from the edges to the center, resulting in stress (tensile force) and deformation in the mushy zone. The diagram illustrates plastic deformation curves in purple and strain (internal material deformation) in red, representing the elongation due to internal forces. When deformation exceeds the alloy's deformation capability in the brittle temperature range, cracks can form along grain boundaries. If the liquid metal in other areas can promptly fill the voids formed, cracks may not occur. However, if the interdendritic liquid film is too narrow or discontinuous, the liquid phase struggles to fill the voids, resulting in solidification cracks.

Understanding these factors is crucial for managing welding processes effectively, ensuring proper heat management, stress control, and minimizing conditions that lead to crack formation.

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Solidification Cracks are caused by insufficient filling of liquid metal during the solidification process. Slowing down the solidification rate of the weld pool to enhance liquid phase backfilling is the primary approach to inhibit solidification cracks.

To avoid solidification cracks, increasing the liquid flow rate or reducing the length of brittle regions is necessary as the solidification rate increases.

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2.4 Directions to Address Cracks — Slowing Down Solidification Rate

Cooling Rate Impact: The cooling rate directly affects crack sensitivity; higher cooling rates increase susceptibility to cracks. Slowing down the cooling rate provides sufficient time for the liquid phase to fill the voids between solid phases, thereby preventing crack formation.

Methods to Reduce Cooling Rate: Several methods can be employed:

Increasing preheating temperature to achieve gradual cooling effects.

Using composite heat sources to lower the weld pool's cooling rate.

Specific to Laser Welding: Techniques such as altering laser waveforms, such as using trapezoidal waves that gradually reduce energy instead of rectangular waves, can decrease cooling rates. Circular beam spots can also significantly improve crack prevention.

Low-Frequency Oscillation Welding: Creating a ripple-shaped weld seam through low-frequency oscillation stirs the weld pool, refines grains, promotes liquid phase filling of voids, and alters the tensile stress components that cause cracks, thus inhibiting thermal crack formation.

Increasing Welding Heat Input: This reduces thermal stresses and increases the thickness of the liquid phase film, aiding stress relief. A stronger reflow capability of the liquid film reduces liquation cracks.

2.5 Directions to Address Cracks — Backfill Distance

The width of the weld seam affects the backfill distance because during the cooling process of each weld seam, heat continuously dissipates towards the edges. When the overall temperature within the weld pool drops to a certain level, the edges of the weld pool solidify completely, followed by rapid overall solidification. Therefore, during solidification, the maximum backfill distance is approximately equal to the weld bead radius, which is the width of the weld seam. Reducing the width of the weld seam effectively reduces the occurrence of cracks.

Continuous Laser Welding: Utilizing small defocus and high speed.

Pulsed Laser: Adjusting parameters such as reducing pulse duration and peak power to mitigate crack formation.

These approaches are essential in optimizing welding processes to minimize cracking and ensure the quality and integrity of welds.



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