Time-dependent spectroscopy of plasma plume under laser welding conditions

Abstract

Momentary emission spectra of iron and argon lines were measured in a plasma plume induced during welding with a continuous wave CO2 laser. Time-dependent spectra were registered using a fast gate, lens coupled microchannel plate image intensifier placed between a spectrograph and a 1254 silicon intensified target detector connected to an optical multichannel analyser. The results, together with the analysis of the colour images from a fast camera, show that in the case when argon is the shielding gas, two plasmas exist: the argon plasma and the iron plasma. It has been found that during strong bursts the plasma plume over the keyhole consists mainly of metal vapour, not being diluted by the shielding gas. No apparent mixing of the metal vapour and the shielding gas has been observed. The space-averaged electron densities determined from the Stark broadening of the 7503.87, 7514.65 Å Ar I lines amounts to (0.75–1.05) × 1023 m−3 depending on the distance from the surface. Assuming that argon is not mixed with the metal vapour and is in local thermodynamic equilibrium these electron densities correspond to the temperatures of 12–13 kK. At the peaks of strong vapour bursts the space-averaged electron densities determined from the Stark broadening of the 5383.37 Å Fe I line are (0.6–1) × 1023 m−3. Numerical simulations showed that the maximum densities in the plasma centre are considerably higher and amount to ∼1.8 × 1023 m−3 and ∼2.45 × 1023 m−3 in the case of the argon and metal plasma, respectively. Consequently the absorption of the laser beam in the plasma plume amounts to ∼5% of the beam power in the case of argon and 10% in the case of metal plasma.

In-process sensing in selective laser melting (SLM) additive manufacturing

“…In SLM, a plasma plume is typically observed above the weld pool. This plasma consists of ionized gas from the build atmosphere as well as vaporized and/or ionized metal from the powder bed [34]. This plasma plume may absorb and/or refract laser radiation thus influencing energy absorption by the melt pool/powder bed [12,34,35].…”

“…This plasma consists of ionized gas from the build atmosphere as well as vaporized and/or ionized metal from the powder bed [34]. This plasma plume may absorb and/or refract laser radiation thus influencing energy absorption by the melt pool/powder bed [12,34,35]. The surface tension, and hence the shape of the melt pool, as well as Marangoni flow in the melt pool are also expected to be impacted by the build environment gas composition in SLM.…”

“…The high energy of the laser will also lead to the formation of a plasma plume above the melt pool, consisting of partially ionized gas from the build environment as well as metal vapor. Emission from this plasma plume adds to the electromagnetic signature of the melt pool [12,34,35].…”

Effect of active flux addition on laser welding of austenitic stainless steel

“…On the other hand, bluish radiation seen around the periphery of plasma plume is due to Fe metal plasma. 35 All the emission lines in the plasma spectrum suffered abrupt drop in intensity as a result of flux addition. ALBW did not introduce any new peaks in the plasma spectrum.…”

Spectral Analysis of Welding Plasma of Magnesium Alloy Using Flux Coated Wire

“…A great deal of research on spectra of welding plasma, including the acquisition of spectrum signal, 12) calculation of electron temperature and density of the plasma 13,14) and practical applications in monitoring the welding quality, 15) has been undertaken in recent years.…”