Welding Signals and the Physics of the Weld Zone
The weld zone contains several physically distinct regions — weld pool, keyhole, plasma, spatter — each producing characteristic signals. Understanding what each region is and what it emits is the foundation for interpreting sensor data.
The laser weld zone is not a single homogeneous region. It contains several physically distinct phenomena, each of which produces characteristic signals that sensors capture and ML models learn from. Understanding the physics of each region is prerequisite to understanding what the sensor data means.
The Weld Zone Regions
Weld pool: the pool of molten material immediately around the laser interaction point. Its geometry (width, depth, shape) is directly related to weld quality. High-speed cameras and thermal sensors monitor it continuously.
Keyhole: when laser power is high enough, a deep narrow cavity forms in the melt, filled with metal vapour and molten plasma. The keyhole allows the laser to penetrate deep into the material. Its stability is critical: a collapsing keyhole traps gas and creates porosity defects.
Plasma: the ionised gas above and within the weld pool, produced by vaporised metal and shielding gas. Spectroscopic sensors capture the emission spectrum of the plasma, which carries information about process temperature and material composition.
Spatters: droplets of molten material ejected from the weld pool. Spatter events are detectable in camera images and acoustic signals. Excessive spatter indicates the process is outside its optimal operating window.
Welding arc / laser beam: the laser itself, which delivers energy to the material. Laser power, welding speed, and focus position are the three primary control parameters.
Process Parameters and Their Effects
Laser power controls the energy input to the material. Think of it as a heat or volume control. Too little power and the weld lacks fusion. Too much and the material vaporises and splatters. The keyhole becomes unstable and the weld degrades.
Welding speed controls how fast the laser moves across the joint. Higher speed means less heat input per unit length: less fusion but also less warping. Lower speed increases fusion but can cause distortion of the base material.
Focus position determines where in the material the laser beam is most concentrated. Shifting the focus above or below the surface changes the beam diameter at the workpiece and therefore the energy density.
Signals Produced
Every region produces signals that sensors can capture:
- Acoustic signals from the keyhole dynamics, spatter events, and stress waves in the solidifying metal
- Optical and spectroscopic signals from the plasma and weld pool emission
- Thermal signals from the temperature distribution around the melt
- Geometric signals (from cameras and laser scanners) showing pool shape and joint position
These signals are closely correlated with welding quality. The role of ML is to learn this correlation from labelled data and generalise it to new welds.