Determination of energy absorption during laser welding by an iterative conduction method

Energy absorption is a key process in laser welding. While there is now good qualitative empirical knowledge of the dependence of the effective absorptivity on laser power level, detailed quantitative understanding is poor. In recent years there has been considerable interest in computational modeling of the heat transfer and fluid flow phenomena during laser welding, in order to have a better understanding of the physical processes involved in determining the final microstructure, and in order to explore process control strategies to optimize materials properties. The starting point of all such modeling efforts is the assumption of an effective absorptivity, or more precisely the total energy coupling efficiency. Unfortunately, the precision of our knowledge on coupling efficiency is considerably below the precision being pursued for thermo-fluid modeling. In view of this, an experimental program has been undertaken at the University of Michigan to obtain quantitative data on coupling efficiency under typical welding conditions to further the understanding of this important process. The study makes use of workpieces instrumented with a large number of imbedded thermocouples. The temperature histories of these thermocouples during and after laser irradiation and welding are analyzed by the use of nonlinear heat conduction computations to yield the time-dependent energy deposition of the laser. Data processing methodology and selected results will be presented. It is hoped that the results will be valuable in future thermo-fluid modeling efforts and in providing the database for the theoretical understanding of the mechanism of laser energy absorption by the workpiece.