Spatter formation and keyhole observation with high speed cameras - Better understanding of the keyhole formation
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Abstract:
Pores and spatters are a severe issue in high-brightness laser keyhole welding. An improved understanding of the process is necessary to develop means to prevent these effects.A key to understand the spatter behavior and the keyhole formation is to observe the welding process with high speed cameras around the workpiece. Different views of the process, from above (in different angle), from below and also from the side through a glass in to the keyhole, were combined. Results of such experiments are presented in the present talk.Keywords:
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Quantitative evaluation of keyhole stability during laser welding using optical coherence tomography
Online monitoring of keyhole during laser welding is of great importance to evaluate the stability of the live process and the final quality of the weld. Indirect methods based on visual imaging and thermal radiation are lack of strong correlations with keyhole behavior, resulting in a low accuracy. Optical coherence tomography (OCT) shoots beam coaxially with the processing beam into the bottom of the keyhole, enabling direct monitoring of keyhole behavior. However, current OCT keyhole monitoring systems suffer from the harsh noise and are limited to only evaluating the keyhole depth through statistical analysis. In this paper, an enhanced OCT system was built with a capacity of capturing keyhole open/collapse behavior, opening a way of quantitatively evaluating the keyhole behavior. First, the signal was denoised and filled given the nature of fluid dynamics in time domain. Then an algorithm was developed to automatically detect the keyhole open/collapse behavior, based on which the weld status (heat conduction mode and keyhole mode) is determined, and the metrics of interests are characterized quantitatively. The proposed method provides a solid method to quantitatively evaluate the keyhole stability, which is well suited for developing new laser welding process and monitoring laser involved high volume manufacturing.
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The width-to-length ratios of keyhole edges and the distances among the centers of keyhole edges and that of laser spot were obtained by processing the coaxial images of keyhole,which could indicate the keyhole shape in radial and depth direction.So they were used as keyhole shape parameters to describe the characteristics of keyhole shape.Variations of keyhole shape with welding parameters were studied in this paper.The experimental results indicated that increasing laser power makes keyhole rounder in radcal and much more perpendicular in depth.For lower welding speed,the keyhole shape varies little,and the keyhole will be longer in welding direction and much more inclined in depth for mid-high welding speed.Defocusing distance has few influence on keyhole shape in some range of defocusing distance.
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Coaxial
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Keyhole
Plasma arc welding
Weld pool
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ABSTRACT In this study, we studied the keyhole imaging technique to 3D Phase‐contrast magnetic resonance angiography (PC MRA) to improve its temporal resolution. Previously, our research group has already studied the 2D PC MRA combined with keyhole technique, and evaluated the applicability. For keyhole‐3D PC MRA, the keyhole factor was used from 12.5% to 50% of the full k ‐space. With keyhole factors above 50%, the images were similar to the original image and the vessels in the brain were well observed. We believe the keyhole‐3D PC MRA will give some advantages for improving the temporal resolution of MR systems.
Keyhole
Magnetic resonance angiography
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Pores and spatters are a severe issue in high-brightness laser keyhole welding. An improved understanding of the process is necessary to develop means to prevent these effects.A key to understand the spatter behavior and the keyhole formation is to observe the welding process with high speed cameras around the workpiece. Different views of the process, from above (in different angle), from below and also from the side through a glass in to the keyhole, were combined. Results of such experiments are presented in the present talk.
Keyhole
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Bar (unit)
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Keyhole
Plasma arc welding
Torch
Weld pool
Horizontal position representation
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The keyhole status and its dimensions are critical information determining both the process quality and weld quality in plasma arc welding (PAW). It is of great significance to measure the keyhole shape and size and to correlate them with the main process parameters. In this study, a low-cost vision system is developed to visualize the keyhole at the backside of the test-pieces in PAW. Three stages of keyhole evolution, i.e. initial blind stage (non-penetrated keyhole), unstable stage with momentarily disappeared keyhole and quasi-steady open keyhole stage (fully-penetrated keyhole), are measured in real-time during the PAW tests on stainless steel test-pieces of thickness 8 mm. Based on the captured images of keyhole under different welding conditions, the correlations of the main welding process parameters (welding current, welding speed, plasma gas flow rate) with the keyhole length, width and area are visualized through vision measurement. It lays a solid foundation for implementing keyhole stability control and process optimization in keyhole PAW.
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Plasma arc welding
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It is of great significance to develop a mathematical model of keyhole shape and dimension in order to widen the process parameter window and improve the process stability in keyhole plasma arc welding (PAW). In this study, a keyhole model was developed according to the force-balance conditions on the keyhole wall. The establishing process of quasi-steady state keyhole was numerically simulated for stainless steel plates of 6 mm thickness, and the keyhole shapes and dimensions were obtained under different welding process parameters. The transformation mechanism of the keyhole from blind (partial) to open (complete) states in PAW process was analyzed based on the calculated action forces on the keyhole wall. The values of action forces at different locations on the keyhole wall were calculated. With increasing of welding current, the keyhole depth rised in a nonlinear way. There existed a critical value of welding current, i.e., if welding current was a little bit higher than this value, the keyhole inside the weld pool would suddenly transform from partial state (blind keyhole) into complete state (open keyhole). The fast centralization of the plasma arc force at the keyhole bottom region resulted in the sudden transformation from a partial keyhole to an open keyhole. The keyhole PAW experiments were conducted to validate the numerical analysis results.
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Plasma arc welding
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