Weld seam tracking and lap weld penetration monitoring using the optical spectrum of the weld plume

Abstract

Joining of dissimilar materials is a long standing problem in manufacturing, with many tricks and special techniques developed to successfully join specific pairs of materials. Often, these special techniques impose stringent requirements on the process such as precise control of process parameters to achieve the desired joint characteristics.Laser welding is one of the techniques which has had some success in welding dissimilar metal alloys, and appears to be a viable process for these materials. Minimal heat input limits differential thermal expansion, and the small weld pool allows precise control of alloy mixing in the fusion zone. Obtaining optimal weld performance requires accurate monitoring and control of absorbed laser power and weld focus position.In order to monitor the laser welding process, we have used a small computer controlled optical spectrometer to observe the emission from the weld plume. Absorbed laser power can be related to the temperature of the weld pool surface and the plume above the weld. Focus position relative to the joint can easily be seen by the proportion of elements from each material existing in the plume.This monitor has been used to observe and optimize the performance of butt and lap welds between dissimilar alloys, where each alloy contains at least one element not found in the other alloy. Results will be presented for a copper-steel butt joint and a lap weld between stainless and low alloy steels.