Laser micro-spot welding of copper by real-time process monitoring

A new laser micro-spot welding procedure for copper has been developed in order to eliminate the shortcomings of existing techniques based on laser pulse shaping or in-situ temperature control. This procedure consists in dividing the welding process in four distinct temporal phases. Each phase is characterized by its set of control parameters and strategies: During the analysis phase the initial reflectivity of the work-piece surface is measured in order to estimate the absorptivity. During the heating/melting phase the temperature in the focal zone is increased at a programmed rate based on the reflectivity measurement in phase I. When the temperature reaches the melting point, the onset of melting is detected by measuring the drop of reflected laser power. The decrease in reflectivity is related to the size of the molten material. This allows to monitor the molten pool in real-time. The diameter and the depth of the molten pool grow steadily during the melt spreading/welding phase. Here, a fixed pulse shape is used for achieving and maintaining surface temperature stability. Finally, the cooling phase must guarantee a slow cooling rate during re-solidification in order to minimize the formation of defects like cracks. This novel technique allows producing highly reliable spot welds on copper. Only one active control based on the reflected laser power is needed during the most critical steps of the process. A simple passive control scheme based on a fixed laser power shape can be used for controlling the critical parameters throughout the remaining time of the welding cycle. The new technique is compared to other "state of the art" welding techniques: the weld quality can be largely improved with respect to simpler methods such as, e.g., pulse shaping. On the other hand, weld qualities comparable to those obtained by a more complex method, the real-time temperature control, are achieved. However, the new procedure has a much higher potential for further miniaturizing the welding process.