Investigating the keyhole behavior by using x-ray and optical depth measurement techniques

Abstract

In deep-penetration laser welding processes, the shape of the keyhole determines both the incoupling efficiency of the laser radiation and the dynamics of the melt flow. The instability in its shape is responsible for the generation of defects including pores and melt ejections and is therefore closely correlated to the weld quality. The shape of the keyhole and its dynamic behavior is strongly influenced by welding parameters and material properties.In order to detect the shape of the keyhole and its stability during the welding process, online diagnostics are of great interest. A low coherence interferometry method known as Inline Coherent Imaging (ICI) was shown to be a suitable means to measure the optical path length in the keyhole with a temporal resolution better than 1 ms [1]. Such systems take benefit of the fact, that the keyhole is filled with vapor and therefore transparent for a wide range of wavelengths. Especially the welding depth of stable processes can be measured with good accuracy. In unstable processes such as welding of aluminum-magnesium alloys, we observe strong fluctuations in the indicated depth. To investigate the correlation of measured fluctuations with keyhole instabilities, the laser-welding process was observed with a high speed X-ray transmission imaging system during the optical depth measurement. It was seen that an unstable keyhole generates significantly more pores then a stable keyhole. With the X-ray system, the keyhole depth and width were analyzed with a frame-rate of 1 kHz. The ICI system was used at a sampling rate of more than 10 kHz.The depth measurement results of both systems were correlated to the generation of pores, which are clearly visible in the X-ray records. Different signal analyzing methods for the ICI signal will be presented and discussed. We show that ICI allows detection of keyhole instabilities and with it pore formation and melt pool ejections.In deep-penetration laser welding processes, the shape of the keyhole determines both the incoupling efficiency of the laser radiation and the dynamics of the melt flow. The instability in its shape is responsible for the generation of defects including pores and melt ejections and is therefore closely correlated to the weld quality. The shape of the keyhole and its dynamic behavior is strongly influenced by welding parameters and material properties.In order to detect the shape of the keyhole and its stability during the welding process, online diagnostics are of great interest. A low coherence interferometry method known as Inline Coherent Imaging (ICI) was shown to be a suitable means to measure the optical path length in the keyhole with a temporal resolution better than 1 ms [1]. Such systems take benefit of the fact, that the keyhole is filled with vapor and therefore transparent for a wide range of wavelengths. Especially the welding depth of stable processes can be measured with good accuracy. In ...