Peter Nerman

Abstract

This work is a part of a bigger project in the frame research program at Joining Technology Centre (JTC) at Swedish Institute for Metals Research (SIMR).

Lack of fusion (LOF) is a very serious type of weld defect that may occur under certain conditions with most common fusion welding methods (MMA, TIG, MIG/MAG, SAW etc.). However, it is said that MIG/MAG-welding has a higher risk for LOF than other methods. This has lead to fear using MIG/MAG in applications with stringent demands on the weld quality such as pressure vessels.

The LOF-problem in MIG/MAG-welding is regarded as frustrating since MIG/MAG is a very productive and economic welding method. MIG/MAG can either be used fully manually or easily be implemented in a robot cell. MIG/MAG is the most common fusion welding technique in the manufacturing industry today. With this in mind it is not strange that a lot of effort has been made to understand and find ways to prevent the LOF-problem. However, these efforts has only been partially successful and there are still a lot of work to be done in order to fully understand all aspects of the LOF-problem.

This work should be seen as an attempt to gain more basic understanding of the LOF-problem in MIG/MAG-welding. It is also a concluding study of earlier work performed at JTC-SIMR.

The main activities in this and the earlier project were focused on how to deliberately reproduce LOF in MIG/MAG-welds performed in an inclined welding position. The purpose with this was to try to understand the basic nature of LOF and thereby identify critical process factors. The welding trials were performed in an inclined position with angles from 0 to 30° using spray arc, pulsed arc and short arc both in carbon steel and stainless steel.

Some situations have been identified where the risk for LOF is high. Some actions to reduce the risk have been found. Also conditions where LOF can be completely avoided has been found.

It must also be noted that LOF-defects in the type of welding studied actually should be denominated LOF/Slag due to its nature.