Analysis and Countermeasures of Common Problems in Laser Welding of BIW (2)

3. Porosity

Porosity is a common defect in the laser fusion welding process, and its causes include the following:

  • Instability in the shape and volume of the weld pool during laser fusion welding, resulting in the continuous formation and closure of small holes in the base material.
  • Inadequate escape time for zinc vapor from the metal surface.
  • Unclean weld seam surface with oil stains, moisture, or metal oxide layers.

Control measures include:

  • Coordinating laser power and welding speed to ensure the stability of the weld pool as much as possible, and allowing sufficient time for zinc vapor to escape.
  • Ensuring proper cleaning of the weld seam surface.

If the porosity is small or non-penetrating and does not affect the mechanical properties, there is no need for temporary rework. However, if the porosity is a through-hole and relatively large, MIG welding can generally be employed for rework, followed by grinding.

4. Frontal Weld Seam Depression

The cause of frontal weld seam depression is excessive spattering or significant loss of alloying elements due to severe burning. This is related to laser power and welding speed. Control measures involve reducing the laser power and increasing the wire feed rate. When the depth of the weld seam depression is less than 30% of the base material thickness, rework is not necessary. However, if the depression depth exceeds 30% of the base material thickness, the welded piece should be scrapped.

5. Welding Cracks

Cracks in laser welds can be categorized into two types: hot cracks and cold cracks. Hot cracks occur due to the rapid cooling of the welding pool. The crystalline region experiences excessive shrinkage stress during solidification, leading to cracking along grain boundaries. Hot cracks are not commonly observed in laser fusion welding and can often be mitigated by optimizing welding process parameters.

Cold cracks, on the other hand, are primarily caused by high hydrogen content in the weld, resulting in cracking along grain boundaries in stress concentration zones. Proper cleaning of the weld area before welding and the use of inert gas shielding during the welding process can help control cold cracks. In general, short cracks and non-penetrating cracks within the weld do not significantly impact weld strength and do not require rework. However, the presence of long cracks or internal penetrating cracks would necessitate scrapping the welded component.

The primary goal of laser welding research is to identify parameters that affect the stability and reproducibility of laser welding and to develop methods for controlling these parameters. With the development of technology and the application of intelligent and neural network-based monitoring systems, laser welding quality has become more stable and reliable, leading to wider prospects for its application.

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