So far, weld joints have mainly been characterised using elaborate cross-section preparation to obtain information about their quality, especially their size. However, this method is time-consuming, destructive, and not suited for inline qualification. In recent years, non-destructive characterisation of weld joints using Scanning Acoustic Microscopy (SAM) and X-ray absorption have emerged as promising alternatives.

Via laser micro-welding, we interconnect an 18650-battery cell case and a cell con-nector, both made of Nickel-plated steel with a material thickness of 300 µm. With our industrial laser bonder (F&K Delvotec) we weld a circular ring seam with a diam-eter of 1500 µm, at a speed of 100 mm/s, with a wobble amplitude of 200 µm and a wobble rate of 500 Hz. We reduce the laser power to 75 W to intentionally create defects in the weld seam, specifically insufficient penetration depth.

Our non-destructive method involves using SAM and X-ray imaging to analyse the actual interconnection area and position of the weld joints on the battery cells. This approach preserves the integrity of the sample and allows for further analysis.

Comparing the X-ray image with SAM and digital microscopic images, we observe that the material thickness in the weld seam area does not correspond to the actual contact area between the metal parts. However, irregularities in the welding pro-cess can be identified through X-ray absorption.

To evaluate the interconnected area, we use a specific gate (gate 7) in the SAM measurement, which has a length of 7 ns. The sound velocity of steel is used to determine the depth of information, which is approximately 288 µm from the sur-face. Grey value segmentation of the SAM image yields an interconnected area of approximately 1.5 mm². However, interpreting SAM images is challenging due to the height profile of the circular ring weld seam.

To validate the results obtained by SAM, we determine the interconnected area at the interface of the two Nickel-plated steel parts through step-by-step metallo-graphic cross-sections. Due to manual grinding and polishing procedure, the steps are not equidistant. We assume that the missing weld seam area forms part of a circular ring segment. According to the cross-sections, the interconnected area is also approximately 1.5 mm².

In conclusion, our non-destructive evaluation method shows promise for assessing the size of laser micro-welded joints in battery cells and has the potential for in-line qualification of welding processes. Our metallographic cross-sections confirm the accurate determination of the actual contact area using SAM. However, SAM measurements of domed surfaces are challenging for exact interpretation.