Laser joining of dissimilar metals Copper and Aluminium allows for the precise delivery of laser energy into the material. The keyhole based process is very efficient for welding Al-Cu considering the high reflectivity and thermal conductivity of the materials being joined. For the Al-Cu system, the formation of detrimental intermetallic compounds is the main issue. Fusion welding with laser as heat source involves the melting of metals being joined. However, the high speed of the laser welding process is advantageous to terminate the excessive melting of Al and Cu, and eventually controlling the detrimental intermetallic phases. Therefore the information of melted material is an important criteria for joint strength.

In this paper, the photodiode signal measured during the laser welding of Cu to Al is investigated as a monitoring technique. In welding from Cu to Al, the melting of Al (bottom sheet) is very critical for joint performance. 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 rapid melting of Al. This study shows the correlation of the Al characteristic peak at the wavelength of 396 nm to the shear strength of the joint.