Laser welding is a process suitable for joining metals and thermoplastics. It has become particularly well established in highly automated production, for example in the automotive industry, because a laser operates with virtually no wear, is very fast and offers high precision. But until now, the quality of a weld seam could only be documented retrospectively, either by means of X-rays, magnetic analysis methods or by dissecting individual samples from production. Real-time monitoring of the weld quality would be a major advantage.
Stable or instable
While in conduction welding only the surface of the material is molten, in deep penetration welding the laser beam penetrates quickly and deeply into the material and produces a thin hole filled with metal and gas vapors, which is called “keyhole”. If the keyhole becomes too deep, the vapour pressure of the metal vapour decreases while the surface tension of the molten metal increases. The keyhole becomes unstable and can eventually collapse, leaving a pore in the weld seam – an unwanted fault in the material. It is therefore important for the quality of laser welding seams to detect the moment when the keyhole becomes unstable. This has not been possible to a sufficient degree until now. It was only possible to look into the keyhole from the top using optical methods.
A group of Empa researchers led by Kilian Wasmer has now succeeded in precisely detecting and documenting the moment of instability in laser deep penetration welding. To do this, they are using an inexpensive acoustic sensor on the one hand, and measuring the reflection of the laser beam on the metal surface on the other. The combined data are analyzed within only 70 milliseconds with the help of artificial intelligence (convolutional neural network). This allows the quality of the laser welding process to be monitored in real-time.
Proof at the ESRF synchrotron X-ray source
The Empa researchers recently demonstrated the accuracy of their monitoring method at the European Synchrotron ESRF in Grenoble. Using their laser, they melted a keyhole into a small aluminium plate, which was simultaneously scanned by hard X-ray radiation. The entire process, which takes less than a hundredth of a second, was recorded with a high-speed X-ray camera. The result: the individual phases of the welding process could be correctly identified with more than 90 percent certainty.
Once the laser beam hits the metal, the first phase of the heat conduction welding process begins – only the surface is being molten. Subsequently, a stable keyhole is formed, which “wobbles” (in-stable keyhole) with longer exposure times. Sometimes the keyhole spits out liquid metal, similar to a volcanic eruption (blowout). If it collapses in an uncontrolled manner, a pore is formed. All these phases can be detected in real-time with the help of Empa technology.