Physical laws governing the interaction of pulse-periodic CO 2 laser radiation with metals

A linear correlation between the depth and the length of the weld pool is found in laser beam welding experiments with varied laser beam power and constant welding speed. On the other hand, the weld pool length changes only slightly with increased welding speed and constant laser beam power. The existing analytical and numerical models fail to explain these dependences. The observed effects are essentially conditioned by the fluid flow in the weld pool caused by the thermocapillary effect, by the friction forces of the metal vapour passing through the capillary and by the convexity of weld pool and fusion zone caused by thermal expansion of the weld pool and the joined workpieces. In order to predict the weld pool length more accurately the model developed by Sudnik et al in 1996 is enlarged by the heat transport produced by the recirculating flow in radial sections of the weld pool. Verification of the model for 16MnCr5 steel with sheet thicknesses of 2 and 6 mm shows that it is suitable for predicting the weld pool geometry and for analysing the thermodynamics of the process. In order to gain a better understanding of the structure of heat transport in the weld pool, the different modes of transport are compared in respect of their contribution to the depth-to-length ratio of the weld pool. A calculation of the weld pool length for welding speeds of 1-8 m min −1 with a laser beam power of 2.5 kW shows that the relative contributions of the transport modes are as follows. Approximately 50-90% of the weld pool length (increasing with welding speed) results from conductive and translatory heat transport (with the fusion zone convexity contributing approximately 20-30%). The remaining 50-10% of the weld pool length (decreasing with welding speed) result from convective heat transport. The model predicts the shoulder in the weld pool trough. It also explains the change in the weld pool length by the effect of the gap width, by the transition from through welding to penetration welding and by improvements in beam quality.

Section: 43mentioning

confidence: 99%

Numerical simulation of weld pool geometry in laser beam welding

Sudnik

Radaj

Breitschwerdt

et al. 2000

“…Welding with the use of cw CO 2 lasers is a well established method of material processing applied in contemporary technology [1][2][3][4]. Nevertheless, laser seams are not always perfect and one can observe ripples and other defects at the surface of the joint.…”

Section: Introductionmentioning

confidence: 99%

“…A keyhole is a capillary which can be created by the laser beam in a molten material of a workpiece if the radiation intensity is above the threshold value [2,7]. The regime when the keyhole is created is called deep penetration welding.…”

Section: Introductionmentioning

confidence: 99%

Searching for chaos in fluctuations of a plasma induced during cw- laser welding

Kurzyna

1998

Fluctuations of cold ( eV) and dense plasmas which burn above metallic surfaces during welding with a cw laser are registered in monochromatic radiation. The aim of the present work is to check whether low-dimensional deterministic chaos can be an explanation for the irregular, random-like oscillations of the surface plasma induced during laser welding. The standard procedures of nonlinear time series analysis like the embedding technique, noise reduction by projection on the local manifold, calculations of the correlation dimension using the G-P algorithm, and looking for the largest Lyapunov exponent are applied. After cleaning original time series from the noise, 3D phase portraits displaying more or less regular structures of attractors were reconstructed. The values of the estimated fractal dimensions of these attractors lay in the range , for various cleaning conditions. The plots of the average orbits' `divergence' calculated for the estimation of the largest Lyapunov exponent display behaviour which is typically observed in chaotic systems. The value of the largest Lyapunov exponent obtained from the slope of the orbits' divergence curves is for our welding conditions, about and the average `period' of the plasma oscillation is estimated to be s. This means that the system is highly unpredictable. These results indicate that fluctuations of laser induced plasmas represent deterministic chaos and a nonlinear dynamical system consisting of several ordinary differential equations can be used for modelling laser welding.

“…Melt pool velocities predicted by the model can be further validated by comparing them with the result from equation ( 9) proposed by Vedenov [29,30]…”

Section: Resultsmentioning

confidence: 93%

Numerical analysis of the effects of non-conventional laser beam geometries during laser melting of metallic materials

Safdar¹,

Li²,

Sheikh³

2007

Laser melting is an important industrial activity encountered in a variety of laser manufacturing processes, e.g. selective laser melting, welding, brazing, soldering, glazing, surface alloying, cladding etc. The majority of these processes are carried out by using either circular or rectangular beams. At present, the melt pool characteristics such as melt pool geometry, thermal gradients and cooling rate are controlled by the variation of laser power, spot size or scanning speed. However, the variations in these parameters are often limited by other processing conditions. Although different laser beam modes and intensity distributions have been studied to improve the process, no other laser beam geometries have been investigated. The effect of laser beam geometry on the laser melting process has received very little attention. This paper presents an investigation of the effects of different beam geometries including circular, rectangular and diamond shapes on laser melting of metallic materials. The finite volume method has been used to simulate the transient effects of a moving beam for laser melting of mild steel (EN-43A) taking into account Marangoni and buoyancy convection. The temperature distribution, melt pool geometry, fluid flow velocities and heating/cooling rates have been calculated. Some of the results have been compared with the experimental data.