Laser welding is a welding process suitable for joining metals and thermoplastics. In highly automated production, such as in the automotive industry, lasers have become particularly well-established because they operate with almost no wear, very quickly, and with high precision.

But until now, the quality of welds can only be documented retrospectively using X-rays, magnetic analysis methods or by isolating individual samples from the production process. If welding quality can be monitored in real time, this will be a technological advancement.

In conduction welding, only the surface of the material is molten, while in deep penetration welding, the laser beam can penetrate quickly and deeply into the material, creating a small hole filled with metal and gas vapor, which is called a "keyhole" ".

If the "keyhole" is too deep, the vapor pressure of the metal vapor will decrease as the surface tension of the molten metal increases. The "keyhole" will become increasingly unstable and eventually collapse, leaving a small hole in the weld - an undesirable defect in the material.

Therefore, detecting when the laser weld pinhole becomes unstable is of great significance to improve weld quality. So far, there hasn't been enough technology to do it to this extent, and the keyhole can only be viewed optically from the top.

A research team from the European Metal Processing Manufacturers Association (EMPA) led by Kilian Wasmer has now precisely detected and recorded unstable moments in laser deep penetration welding. To do this, they use cheap acoustic sensors on the one hand and measure the reflection of laser light off metal surfaces on the other.

Researchers say that with the help of artificial intelligence (convolutional neural networks), the combined data will be analyzed in 70 milliseconds. This allows the quality of the laser welding process to be monitored in real time.

The research team recently demonstrated the accuracy of their monitoring method at the European synchrotron ESRF. They used a laser to melt a keyhole in a small aluminum plate while simultaneously scanning the plate with hard X-rays. The entire process took less than one hundredth of a second and was recorded by a high-speed X-ray camera.

↑Image source: European Metal Processing Manufacturers Association

The first stage of the thermal conduction welding process begins once the laser beam hits the metal - only the surface is molten, and a stable keyhole is then formed. Sometimes, the keyhole will erupt with liquid metal, like a volcano erupting. If it collapses in an uncontrolled manner, pores can form. In experiments, the researchers successfully created small holes in the weld and then closed them again with a second laser pulse. The researchers reported an 87% success rate in creating the keyhole and a 73% success rate in removing it.

This error correction method has great application prospects in laser welding. The researchers say that before, pores in welds could only be detected after the work was completed, but now they are able to detect the pores during the welding process and post-processing with lasers can begin immediately.