Formation Mechanism And Mitigation Measures of Solidification Cracks in Laser Welding

01 Introduction
Laser Beam Welding (LBW), with its high speed, high precision, and non-contact nature, is widely used in the automotive, aerospace, and electronics industries. It demonstrates unique advantages in joining dissimilar materials. However, solidification cracking—cracks that occur during the end stages of fusion zone solidification—remains a key defect limiting industrial application. These cracks are triggered by the combined effects of thermal stresses, solidification shrinkage, and liquid films along grain boundaries, significantly degrading joint mechanical properties and fatigue life.

02 Formation Mechanisms
The core mechanism of solidification cracking lies in the liquid films retained along grain boundaries at the final stage of solidification. During solidification, the weld pool contains three zones: the unfettered liquid zone, the constrained liquid zone, and the solid zone (see Figure 1). In the constrained zone, liquid flow is restricted, preventing compensation for shrinkage-induced strain and causing grain boundary separation. The stability of the liquid film depends on the ratio of grain boundary energy (γgb) to solid–liquid interface energy (γsl): if γgb<2γsl, the film is unstable and grains coalesce; otherwise, a stable film promotes cracking.

Figure 1, Schematic diagram of laser welding (left) Molten pool area during solidification (right).

Material metallurgical properties—such as solidification range, shrinkage rate, and alloy composition—also influence cracking susceptibility. Materials with extensive low-melting-point eutectic phases are particularly prone to forming continuous liquid films, exacerbating crack formation.

Laser welding parameters (power, speed, beam size) affect solidification crack formation by altering heat input and temperature gradients. Higher power with slower speed yields higher heat input and slower cooling rates, promoting columnar grain growth and increased cracking sensitivity. In contrast, lower power and faster speed encourage equiaxed grain formation and reduced crack susceptibility.

03 Suppression Strategies
To effectively suppress solidification cracks in LBW, researchers have proposed strategies focusing on grain structure control, process parameter optimization, and material property enhancement:

• Grain Refinement via Beam Oscillation:
  Laser beam oscillation disrupts columnar grain growth, inducing equiaxed grains. It increases melt flow, widens the weld pool, lowers thermal gradients, and prolongs solidification time, allowing liquid films to backfill grain boundaries (see Figure 3).

  
  

  Figure 2, Liquid film distribution at grain boundaries under different molten pool shapes.

  
  

  Figure 3, Schematic diagram of welding molten pool, a, b) no oscillation, c, d) transverse oscillation, e, f) longitudinal oscillation, g, h) circular oscillation.

• Dual-Laser Source Strategy:
  Combining a primary CO₂ laser with a pulsed Nd:YAG secondary source improves thermal cycling, promoting equiaxed grain formation and reducing crack sensitivity (see Figure 4).

  
  

  Figure 4, a) CO2 laser welding, b) solidification crack along columnar crystal, c) dual light source welding, d) no crack in weld.
  

• Welding Parameter Optimization:
  Adjusting laser power, speed, and beam size controls heat input and gradients. Preheating further slows cooling rate, encouraging equiaxed grain microstructure (see Figure 5). Pulsed lasers and higher welding speeds also favor equiaxed grains, lowering crack risk.

  
  

  Figure 5, a) No preheating, b) Equiaxed grains preheated at 300°C.
  

• Dissimilar Material Measures:
  For copper–aluminum joints, controlling laser offset and welding speed limits intermetallic compound formation and reduces cracking sensitivity. Using filler materials can also improve joint toughness by adjusting composition and microstructure.

04 Conclusion
Solidification cracking is a common defect in laser welding, resulting from complex interactions of thermal, mechanical, and metallurgical factors. Understanding these mechanisms provides theoretical guidance for crack suppression. Strategies such as grain refinement, process optimization, and material enhancement can effectively reduce crack sensitivity and enhance weld quality. However, due to LBW’s complexity and variability across materials and conditions, further research is needed to refine suppression mechanisms across diverse welding scenarios.

**--Cite the article published by 高能束加工技术 on February 10, 2025, in the WeChat public account "High-Energy Beam Processing Technology and Applications."