Study of weld depth control in electron beam welding using an on-line imaging technique

Some welding applications require a welding condition where the joint is welded through thickness, but the beam through current and impingement on backing material is minimised. Such a weld can be produced in a single pass by a keyhole that is open at the cap only, with a conduction limited weld root. This is termed a full penetrating closed keyhole (FPCK) weld.

The FPCK welding condition offers a number of advantages, such as elimination of spatter at the root and negligible through current, and thus minimum impingement. However, the window of electron beam (EB) welding parameters (i.e. focus current and beam current) that allows generating this type of weld is small. In this work, an optical based on-line monitoring technique was used to control the depth of the EB keyhole and thus reliably reproduce the FPCK condition. The weld depth measurements collected on-line with the optical system were compared to the depth measurements from the transversally and longitudinally sectioned samples. The optical sensor was integrated in a feedback loop with the EB welder, which allowed automatic adjustment and control of the weld depth during the welding process by tuning the beam current.