Insufficient melting of the base metal at the start and end crater at the end of the weld
Due to the high heat conductivity of Aluminium it is difficult to create enough heat to melt the base metal right after arc ignition. Afterwards the heatflow in the work piece is faster than the welding speed and the conditions at the end of the weld are unfavourable for a proper end-crater fill. Current-control programs have been integrated in the welding power sources by its manufacturers by which higher arc-power is provided at start and lower power at the end of the weld. This definitely leads to an improvement, however, lack-of-fusion, porosity and end-crater-cracks are not prevented completely. Higher arc-power is always linked with a higher wire quantity (wire-feed-speed) and lower wire quantity at the end-crater with MIG-welding. Exactly the opposite would be required.
If possible the following steps should be taken:
- Use of run-in and run-out plates
- Beginning and end of the weld on the base metal
- Well-cooled welding fixtures