Dynamics of penetration depth during laser welding

The results of an experimental investigation of weld depth dynamics during spot welding of stainless steel 304 with a CO₂ laser are reported. High speed photography was used to investigate the melt penetration rate for two values of laser power, 1.42 kW and 2.70 kW, during the first millisecond of interaction. The CO₂ laser beam was focussed on the interface between steel and glass targets. The interaction zone was illuminated with an Argon-ion laser beam, and photographs of the weld zone were taken through the glass with a temporal resolution of 20 μs and frame frequency 10⁴ Hz. In the range from 2 ms to 500 ms, the dependence of penetration depth on time was determined by varying CO₂ laser pulse duration and measuring the weld depth corresponding to a given pulse duration. During the stage of keyhole growth, penetration velocity v^* weakly depends on laser power P(v^* ∼ P^1/4). At this stage the penetration velocity in steel for incident laser power in the kilowatt range is on the order of 0.1 - 1 m/s. As the keyhole reaches its maximum length, penetration velocity decreases by two orders of magnitude and is determined by heat diffusion. The near-surface plasma exhibits a strong influence on the observed dynamics.