Inline weld depth measurement for high brilliance laser beam sources using optical coherence tomography

Abstract

As a result of the rapidly growing importance of applications in electro mobility that require a precisely defined laser weld depth, the demand for inline process monitoring and control is increasing. To overcome the challenges in process data acquisition, this paper proposes the application of a novel sensor concept for deep penetration laser beam welding with high brilliance laser sources. The experiments show that optical coherence tomography (OCT) can be used to measure the weld depth by comparing the distance to the material surface with the distance to the keyhole bottom measured by the sensor. Within the presented work, the measuring principle was used for the first time to observe a welding process with a highly focused laser beam source. First, a preliminary experimental study was carried out to evaluate the influence of the angle of incidence, the material, and the weld joint geometry on the quality of the sensor signal. When using a multimode fiber laser with a focus diameter of 320 μm, the measurements showed a distinct behavior for aluminum and copper. The findings about the measurement signal properties were then applied to laser beam welding with a single-mode fiber laser with a spot diameter of only 55 μm. The spot diameter of the OCT measuring beam was about 50 μm and thus only slightly smaller than that of the single-mode processing beam. A wide variety of tests were carried out to determine the limits of the measurement procedure. The results show that the application of OCT allows inline monitoring of the weld depth using both a multimode and a highly focused single-mode laser beam. In addition, various influences on the signal were identified, e.g., the material-specific melt pool dynamics as well as several characteristic reflection and absorption properties.

Laser Metal Deposition with Coaxial Wire Feeding for the Automated and Reliable Build-Up of Solid Metal Parts

“…To enable a highly accurate measurement of the layer height ℎ 𝑝 in real-time, optical coherence tomography (OCT) represents a highly promising alternative to the existing approaches described above. This sensor technology based on low-coherence interferometry (LCI) is already established in several laser material processing applications [44,45]. By splitting a coherent light beam, the relative distance between two reflecting surfaces can be determined based on the interference pattern of the reflected light beams [46].…”

A Novel Approach to the Holistic 3D Characterization of Weld Seams – Paving the Way for Deep Learning-Based Process Monitoring

“…Low-coherence interferometry also allows for a direct weld depth measurement during laser beam welding. It could be demonstrated that the interferometric measuring principle, applied coaxially to the laser beam, enables a measurement of the capillary depth during deep penetration welding [21]. In addition to the determination of the penetration depth, the quality of the weld seam surface concerning the occurrence of irregularities in the topography could be determined based on the measured capillary depth [22].…”

In-process range-resolved interferometric (RRI) 3D layer height measurements for wire + arc additive manufacturing (WAAM)

“…Interferometric techniques (including RRI) offer illumination and detection from an instrument mounted at a single location and are hence more easily integrated into mounting structures. Optical coherence tomography (OCT), an interferometric technique conceptually similar to RRI, has been widely applied to welding research [14][15][16][17], although in these implementations the ability of OCT to image structure within turbid materials [10] such as human skin is not utilised as the light cannot penetrate the metal surface. OCT can in principle achieve 0.01 mm but typically has a working range of only a few millimetres (compared to up to 10 s of cm for RRI).…”