Analysis of the physical processes occurring during deep penetration laser welding under reduced pressure

Recent published experimental results obtained on deep penetration laser welding realized under reduced ambient pressure have shown very interesting results: resulting weld seams have geometrical characteristics that are similar to those obtained with electron beams. They show an increased penetration depth that can reach a factor two compared to atmospheric experiments, a larger aspect ratio with narrow and parallel sides of the weld seam. Also some humps around the rim of the keyhole appear. Of course, these modifications depend on the ambient pressure, but one also observes that these interesting improvements become independent of the ambient pressure below some critical pressure and also disappear at high welding speeds. Moreover, it is also observed that this critical pressure, below which these improvements do not vary, increases with the welding speed. In these previous publications, all these different characteristic results have not been explained. It is therefore the purpose of this paper to explain these different results. This has been obtained by using 3D numerical simulations of deep penetration laser welding and by studying the corresponding variation of physical parameters inside the keyhole (temperature, recoil pressure, and hydrodynamics of the vapor plume). An explanation for the evolution of these resulting weld seam geometries, as a function of the main operating parameters is proposed: ambient pressure, welding speed, and laser beam parameters (power and beam spot diameter). It is then possible to estimate this characteristic critical pressure, which is compared favorably with the corresponding previous experimental results. As a consequence, this analysis allows to define the optimum conditions for the improvement of the weld seam characteristics realized under reduced ambient pressure for an industrial environment

Explanation of penetration depth variation during laser welding under variable ambient pressure

It has been observed that the penetration depth during laser welding (LW) under vacuum or reduced ambient pressure could be significantly greater than that during welding under atmospheric pressure. Previous explanations of this phenomenon usually limit to specific wavelength laser welding and have difficulties in explaining why the variation will disappear, as the welding speed increases. Here, we propose that this variation is caused by the temperature difference of keyhole wall under variable ambient pressure based on a correct physical description of related processes. A new surface pressure model, dependent on ambient pressure, is proposed for describing the evaporation process during laser material interaction under variable ambient pressure. For laser welding of a 304 stainless steel with 2.0kW laser power and 3m/min welding speed, it is shown that the average keyhole wall temperature is around 2900K under atmospheric pressure, and only around 2300K under vacuum, which results in significant penetration depth variations. Interestingly, it is also shown that as the welding speed increases, the average temperature of the front keyhole wall gradually rises due to the reduction of the mean incident angle of laser, and the magnitude of this increase is larger in welding under vacuum than under atmospheric pressure. It allows us to explain why the penetration depth improvement decreases to zero with the increase of welding speed

Laser induced arc dynamics destabilization in laser-arc hybrid welding

The interaction between laser and arc plasma is a central issue in laser-arc hybrid welding. We report a new interaction phenomenon called laser destabilizing arc dynamics in kilowatt fiber laser-TIG hybrid welding of 316L stainless steel. We found the laser action significantly oscillates the arc tail with a 1–3 kHz high frequency. Direct numerical simulation demonstrates that the destabilization mechanism is due to the high-speed oscillated metal vapor ejecting from the mesoscopic keyhole. More interestingly, the high-speed metal vapor could contrict the arc plasma by physical shielding. This provides a fundamentally different explanation from the generally adopted metal vapor ionization theory for laser constrict arc plasma phenomenon. Also, the results substantiate that the arc plasma cannot easily enter into the keyhole because of the violent metal vapor

Modeling of the Plasma 3D Deposition of Wire Materials

The numerical modeling of the physical process of manufacturing parts using additive technologies is complex and needs to consider a variety of thermomechanical behavior. This is connected with the extensive use of the finite element computer simulation by means of specialized software packages that implement mathematical models of the processes. The algorithm of calculation of nonstationary temperature fields and stress-strain state of the structure during the process of 3D deposition of wire materials developed and implemented in ANSYS is considered in the paper. The verification of the developed numerical algorithm for solving three-dimensional problem of the production of metal products using arc 3D deposition of wire materials with the results of the experiment is carried out. The data obtained from calculations on the developed numerical model are in good agreement with the experiment

Distribution of Al Element of Ti–6Al–4V Joints by Fiber Laser Welding

In the process of laser welding, the uneven distribution of solute elements caused by element burning loss and flow of molten pool affects the quality of joints. In this paper, butt welding experiments were conducted on the 3 mm thick Ti–6Al–4V specimens with different preset ratios of Al and Si powders by using 4 kW fiber laser. The distribution of Al solute element and its influence on the microstructure and mechanical properties of the final weld joint were investigated. The results showed that the self-diffusion of Al element and the flow of molten pool affects the alloy elements distribution in laser welding. And the microhardness of the welded joint with Ti–6Al–4V and 90% Al + 10% Si powders was significantly higher than that with only Ti–6Al–4V, with the difference of about 130HV. At the same time, in the joint with 90% Al and 10% Si powders, the acicular α’ size was finer, and basketweave microstructure was present as well. This research is helpful to better understand the distribution of Al solute element and its influence on the joint quality during laser welding of Ti–6Al–4V alloy, which provides a certain reference for improving the weld or surface properties of Ti–6Al–4V alloy during laser processing