Correlation of optical signal during laser fusion welding of copper to aluminum

Laser joining of dissimilar metals, copper and aluminum, allows for the precise delivery of laser energy and high process speed. The keyhole-based process is very efficient for welding Al–Cu, considering the high reflectivity and thermal conductivity of the materials joined. For the Al–Cu system, the formation of detrimental intermetallic compounds is the main issue. Fusion welding with laser as a heat source involves the melting of metals. However, the high speed of the laser welding process is advantageous for terminating the excessive melting of Al and Cu and eventually controlling the detrimental intermetallic phases. Therefore, information pertaining to melted material is an important criterion for achieving joint strength. In this paper, the photodiode signal measured during the laser welding of Cu to Al is investigated as a process monitoring technique. In welding from Cu to Al, the melting of Al (bottom sheet) is very critical for joint strength. The amount of Al material that can be intermixed in Cu depends on the solubility limit of the Al–Cu system. The plasma plume emission during laser welding is used to extract the rapid melting of Al. This study shows the correlation of the Al characteristic peak at a wavelength of 396 nm with the shear strength of the joint.