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Weld crackling problems using 6061-T6 base material

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QUESTION: I am having cracking problems with my aluminum groove weld procedures. I am MIG (GMAW) welding a 6061-T6 base material, 3/8 inch thick, with a square edge preparation. The weld is cracking immediately after welding, with the crack in the center of the weld running along the weld’s length. I am using ER5356 filler material.

ANSWER: The answer to this question is also related to solidification crack sensitivity. If we consider the alloying effect of magnesium (Mg) in aluminum, we see that weld crack sensitivity is shown to increase sharply with an increased Mg content up to about 1.5% and then decrease with further Mg additions (Fig 1 at Al-Mg curve).

Fig 1

With this problem, we need to consider the effect of joint design on base alloy and filler alloy dilution. Square groove welds in this material can be particularly susceptible to cracking because very little filler alloy is mixed with the base material during welding. If we examine Fig 2, “Dilution Effect on Weld Composition”,

Fig 2

we can see the difference in the amount of Mg in each of the joint designs. The square groove showing dilution of 20% of the 5%Mg found in the 5356 filler material plus 80% of the 1%Mg found in the 6061 base alloy, provides a total Mg content of around 1.8% in the weld. In comparison, the single bevel groove weld configuration has 60% of the 5%Mg in the filler alloy and 40% of the 1%Mg found in the base alloy and provides a much higher Mg content of around 3.2% in the weld. If we look again at Fig 1, we can see at the Al-Mg curve that there is a considerable difference in crack sensitivity between a weld with 1.8%Mg and one with 3.2%Mg. The 1.8%Mg is marginally past peak crack sensitivity, and the 3.2% is well beyond that point.

My recommendation is to evaluate the use of a v-groove weld preparation, which will introduce more filler alloy to the weld metal mixture and lower the crack sensitivity.