A Study of Weld Cracking of Aluminum and its Alloys

Abstract

A modified FISCO type cracking test was used to study the cracking properties of inert-gas metalarc welds in commercially pure aluminium (2S) and aluminium-magnesium alloys (52S, 54S and 56S).
An automatic welding head was employed to make intermittent welds in FISCO type specimens and the cracks were detected by X-ray radiographic and fluorescent penetrant methods.
Weld metal cracks were studied by classifying into four types, namely, longitudinal crack at the start of a bead, longitudinal crack in the intermediate portion of a bead, crater crack and longitudinal crack throughout the bead.
The results obtained in the experiment were as follows.
1. It is possible to compare weld cracking tendencies of Al and Al-Mg alloys by the modified FISCO type cracking test. Weld cracking tendency was not so serious for a plate thickness of 12 mm, but greatly increased for greater thickness of 20 and 30 mm.
2. Longitudinal cracks at start or in intermediate portion were increased with increase of welding speed, root opening or plate thickness.
3. Crater cracks were made greater with increase of welding current and decreased with welding speed, since the size of crater changed itself with them.
5. Variation in arc voltage scarcely affected the weld metal cracking.
5. Vee-grooved specimens were more crack-sensitive than I-grooved ones.
6. Among several combinations of welding wires and plates, the most crack-sensitive one was 2S wire-51S plate or 52S wire-2S plate. The combination of 52S wire and 52S plate came next. The three combinations, 2S wire-2S plate, 54S wire-52S plate and 56S wire-52S plate, were equally less crack-sensitive than the above three ones.
In other words, the weld metals with their magnesium contents between 1 and 2 percentages showed e greatest crack sensitivity, while both the lower and higher magnesium content regions gave far better cracking resistance.
7. For the prevention of weld cracking, welding should be done with lower welding speed and a suitable value of welding current, an adequate choise of welding wire and a suitable welding procedure at the start of bead are also necessary. At the end of a weld the bead should run off the groove onto the plate surface and the crater should be finished with a crater filler method.