Modeling for detecting micro-gap weld based on magneto-optical imaging

• A model based on magnetic charge theory can explain micro gap weld detection law of magneto-optical (MO) imaging.
• Transitional zone in a MO image corresponds to the weld position.
• Transitional zone in a MO image will become wide as the distance between MO sensor and weld increases.
• Transitional zone in a MO image will become narrow as the weld gap width increases.

A new method based on Faraday magneto-optical effect to detect micro-gap weld whose width is less than 0.1 mm in a butt joint laser welding is developed. The characteristics of magneto-optical images and magnetic field distribution reflecting the weld position are analyzed. An electromagnet exciter is adopted to magnetize the carbon steel weldment and establish a leakage magnetic field at the welding joint. Magneto-optical sensor is set over the magnetized weldment to obtain the magneto-optical images. A model based on the magnetic charge theory is proposed to investigate the magnetic field distribution over the welding joint and it is demonstrated by computer simulations and experiments, which show that the midline of welding joint is corresponding to the transitional zone midline in a magnetic-optical image. This model is evaluated by seam tracking experiment and shows that it can effectively and accurately track the micro-gap weld.