High-power laser welding of contaminated steel sheets

Abstract

A deep penetration laser welding process differs from a heat conduction laser welding process in the generation of a steam capillary through the material. The so called keyhole is formed if high energy densities of the laser beam vaporize the base material, leading to a vaporfilled channel which is surrounded by a liquid weld pool. The shape and the oscillations of the keyhole are affected by various physical parameters of the welding process. Under constant process conditions a balance of pressure keeps the capillary open. If the material is contaminated by a layer with physical properties differing from the bulk material (e.g. oil), the equilibrium is disturbed in a negative way. The coating evaporates during the melting of the base material, resulting in vapor locks at the capillary and the weld pool, leading to pores and failures along the weld seam. In order to understand and avoid this effect, it is the aim to analyze and describe the failure generating mechanisms. Additionally, a control method of the laser power is investigated which influences the keyhole's oscillations and could suppress an unwanted collapse.

Observing molten pool surface oscillations during keyhole processing in laser powder bed fusion as a novel method to estimate the penetration depth

“…The use of transmitted electromagnetic waves through the large powder bed in an industrial system still requires fundamental scaling up and safety issues to be considered. An indirect approach for detecting the melt pool depth can be developed by analysing the light emitted from the melt pool [38][39][40][41][42][43]. Alternatively, 7 surface wave oscillation detection by means of probe light reflections can be employed as an indirect penetration depth measurement method and has been conducted in both Gas Tungsten Arc welding processes and laser based processes [44][45][46][47][48][49].…”

Signal overlap in the monitoring of laser welding

“…Previous workers [8] have generally concentrated on the fluctuations of the signal curves which are in the tens or hundreds of Hertz. In this paper an analysis of the correlation between the signals at higher frequencies was carried out which revealed an interesting link between the T and P signals.…”

An Experimental Approach for the Direct Measurement of Temperatures in the Vicinity of the Keyhole Front Wall during Deep-Penetration Laser Welding

“…The exposition to the flow velocity of the prerunning melt could cause a tilting of the temperature screen, which could account for the strong and abrupt temperature changes on time scales too long, as they could be ascribed to vibrations. The latter ones, as they are labeled in Figure 12, exhibit frequencies of f vib = 2 kHz to 5 kHz and, therefore, possibly emerge from the keyhole oscillations, according to [14]. A comparison of the two temperature measurements obtained during the weld processes executed under equal conditions and with the same set of process parameters is given in Figure 13.…”