Prediction of electrical resistance of laser-welded copper pin-pairs with surface topographical information from inline post-process observation by optical coherence tomography

Abstract

Laser welding of copper hairpins is required to produce a conductive connection in electric stators. Past manufacturing processes introduce misalignments that lead to poor weld connections with increased electrical resistance. In this work, we discuss correlations between the electrical resistance of the weld connection and possible misalignment types. Misalignments lead to a deformed surface topography of the weld. We correlate inline measurements of the weld topography by optical coherence tomography (OCT) with misalignment types and hence erroneous weld connections. We identify a connection between surface topographical weld features with the electrical resistance of the weld. As a result, a quantified separation of process results is possible with a surface topographical feature of the hairpins that allows for concluding the electrical resistance of the pin-pair connection. Correlation coefficient is identified as the most relevant feature indicating a linear trend in the height profile. Reference measurements with a symmetrical weld pearl show a correlation coefficient of around 0, whereas misalignments with a skewed surface topography show increased absolute correlation coefficient values up to 0.75. The identified correlation between the electrical resistance and different misalignment types can be depicted with the correlation coefficient for the given boundary conditions. Defective weld results with electrical resistances above 6 µΩ can be identified with feature values above 0.5, whereas reference welds with an electrical resistance below 5 µΩ can be identified with an absolute correlation coefficient below 0.2.