The optical coherence tomography (OCT) is a measuring technology which is well-established in medical engineering since the early 1990s. More recently, the technology found its way into laser materials processing where it is used for seam tracking and inspection and also for monitoring and control of deep-penetration laser welding. In this work, deep-penetration laser welding of aluminum and steel using an OCT-system for in-process monitoring of the weld depth was investigated. It is shown that statistical data processing is mandatory to extract the actual keyhole depth. Therefore, two different measures, percentile filtering and considering the frequency distribution of the OCT-data, were considered. Thereby, it is demonstrated that the frequency distribution of the OCT-data has a specific pattern with a local maximum which correlates with the keyhole depth. Moreover, this feature is more significant for welding aluminum and therefore enables one to detect the weld depth more independently and accurately compared to currently applied measures.

Background. Spatter formation at melt pool swellings at the keyhole rear wall is a major issue for laser deep penetration welding at speeds beyond 8 m/min. A gas nozzle directed towards the keyhole, that supplies shielding gas locally, is advantageous in reducing spatter formation due to its simple utilization. However, the relationship between local gas flow, laser spot size, and the resulting effects on spatter formation at high welding speeds up to 16 m/min are not yet fully understood. Methods. The high-alloy steel AISI 304 (1.4301/X5CrNi18-10) was welded with laser spot sizes of 300 μm and 600 μm while using a specially designed gas nozzle directed to the keyhole. Constant welding depth was ensured by Optical Coherence Tomography (OCT). Spatter formation was evaluated by precision weighing of samples. Subsequent processing of high-speed images was used to evaluate spatter quantity, size, and velocity. The keyhole oscillation was determined by Fast Fourier Transform (FFT) analysis. Tracking the formation of melt pool swellings at the keyhole rear wall provided information on the upward melt flow velocity. Results. The local gas flow enabled a significant reduction in the number of spatters and loss of mass for both laser spot sizes and indicated an effect on surface tension by shielding the processing zone from the ambient atmosphere. The laser spot size affected the upward melt flow velocity and spatter velocity.

Laser sources with wavelengths in the visible blue spectrum are suitable for heat conduction mode welding of materials like copper and nickel due to the significantly increased Fresnel absorption compared to infrared laser radiation. Recently, blue laser sources with 445 nm wavelength have become available with sufficient power and beam parameters to exceed the intensity threshold for laser deep penetration welding. In laser beam deep penetration welding, the total absorption is significantly increased due to the multiple reflections in the keyhole compared to heat conduction mode welding. However, since the absorbed energy per reflection inside the keyhole is wavelength-dependent, it can be hypothesized that the choice of laser wavelength causes changes in the local energy distribution inside the keyhole, changing the keyhole dynamics. To investigate this, laser beam deep penetration welding experiments were carried out on 2.4068 pure nickel using an infrared laser source and a blue laser source with comparable beam properties. The experiments were monitored and compared by a multi-sensor setup and metallographic analyses. This setup included measurements of airborne acoustic emissions and two high-speed video cameras for spatter tracking and tracking of the keyhole area. The use of a blue laser beam led to a lower spatter quantity, an increase of porosity and a significant change of acoustic emissions, proving the hypothesis for pure nickel.

In the case of “hidden” T-joints, the nonvisible web sheet is welded through the face sheet. Positioning tolerances and movement due to the distortion of the web sheet represent the main challenges to maintain on track during laser beam welding. This study investigated the assumption that keyhole monitoring based on optical coherence tomography (OCT) could be utilized for seam tracking by detecting critical misalignments between the laser beam and the web sheet position. It hereby analyzed whether an emerging misalignment between the laser beam and the web sheet could be discerned in the OCT data before the laser beam misses the web sheet and penetrated only the face sheet. This prediction represents a key requirement for the industrial use of this approach. To this end, this study investigated the welding of hidden T-joints with angular and parallel offsets of the laser path relative to the web sheet centerline while recording with OCT. The interface zone between the web and face sheets was recorded by a high-speed camera to detect the transmission of laser radiation through a lower keyhole opening. It was shown that a lower keyhole opening, in combination with transmitted radiation, was advantageous for the reliable detection of an emerging misalignment. This lower keyhole opening can occur lateral to the web sheet prior to a misalignment, therefore enabling a prediction of spot misalignments via OCT. The welding parameter dependent maximum distance of the laser spot edge to the web sheet edge at detection was 0.1 mm in this study.

In the automotive industry, the seam edge quality of laser brazed seams is a crucial factor for the required high optical quality standards of customer-visible connections of zinc-coated car body parts. The type of zinc coating is decisive for the resulting geometry of the seam edges, which are aspired to be straight. Especially brazing of hot-dip galvanised steel with conventional brazing setup is not resulting in seam edges which fulfil the required optical quality. In this study, the wetting of copper-based filler wire on electrogalvanised and hot-dip galvanised steel sheets is analysed to increase the understanding of the influence of the wetting fronts on the seam edge quality. For this, the wetting fronts are determined in high-speed camera images and the resulting seam edges are captured by microscopic pictures. The results show that two major wetting regimes occur on both types of zinc coatings but with a different occurrence probability: one with a steep wetting front and zinc evaporation in front of wetting (“steep-angle-wetting”) and one where liquid zinc connects with the wetting front and creates a flat wetting angle (“flat-angle-wetting”). It is found that the resulting seam edge quality is mainly depending on the present wetting regime. However, for flat-angle-wetting, in the regime with insufficient seam edge quality, a self-stabilising effect is identified, which is responsible for a missing process window for brazing of hot-dip galvanised steel with sufficient seam edge quality.

In deep penetration laser welding, the quality of the weld seam is strongly correlated with the process dynamics. Pronounced dynamics of the melt pool and the keyhole cause defects like pores and blow-outs which negatively affect the mechanical properties of the seam. The process dynamics are primarily governed by the fluctuations of the keyhole shape. Therefore, in order to avoid the occurrence of defects in the weld seam, it is necessary to gain control over the keyhole stability by understanding the underlying physical processes that contribute to its dynamics. Besides the intensity distribution of the laser beam on the keyhole walls, the keyhole wall temperature is a crucial influencing factor when it comes to the assessment of the local metal evaporation rate and the induced local recoil pressure acting on the keyhole wall. Using small high-melting tantalum probes as measuring channels, temperatures in the vicinity of the Knudsen layer at the keyhole front wall were measured at different depths below the sample surface by means of high-speed pyrometry. For this purpose, bead on plate welding was conducted at low process velocities in the Rosenthal regime using pure aluminium (EN AW-1050A) as substrate material. It is shown for different laser powers and process velocities that the melt temperature in the vicinity of the keyhole wall lies in the range of 3000 K ± 400 K, therefore intermittently exceeding the vaporisation temperature of the substrate material. Significant correlations between the keyhole wall temperature and the laser power as well as the measuring depth could not be identified.