How to deal with Cracking issues in 6xxx Series Alloys

Understanding Solidification Crack Sensitivity in 6xxx Series Base Alloys

The following two questions are associated with a problem that unfortunately is relatively common.  They are related to a form of cracking that may occur when welding particular aluminum base alloys.  This problem is associated with solidification crack sensitivity, which in turn is directly associated with the actual chemistry in the weld pool.  To appreciate this problem, we need to understand that the addition of various alloying elements within aluminum can seriously affect the material’s crack sensitivity.  The specific alloying elements can be identified with the amount or range at which these elements increase solidification crack sensitivity.  This information can be obtained from solidification crack sensitivity curves (Fig1) and used during welding procedure development to prevent undesirable chemistry mixtures in the weld.

Q1 -  I am experiencing a weld cracking problem on our TIG (GTAW) production line where we weld thinner sections of 6063-T6 sheet material.  We are often required to perform outside corner welds where we sometimes use little or no R4043 filler material, dependent on the joint fit up.  Why do you think my welds are cracking? And why is it that not all of my welds, but only some of them are cracking?

A1 -   We should start by considering the crack sensitivity of the 6xxx series base material.  The aluminum/magnesium/silicon base alloys (6xxx series) are highly crack sensitive because they contain approximately 1.0% Magnesium Silicide (Mg2Si), which falls close to the peak of the solidification crack sensitivity curve (Fig 1 at Al-Mg2Si curve).  The Mg2Si content of these materials is the primary reason that there are no 6xxx series filler alloys made. The cracking tendency of these alloys is lowered to acceptable levels during arc welding by the dilution of the weld pool with excess magnesium (by use of the 5xxx series Al-Mg filler alloys) or excess silicon (by use of the 4xxx series Al-Si filler alloys).  

When we TIG (GTAW) weld on thin material, it is often possible to produce a weld, particularly on corner joints, by melting both edges of the base material together without adding filler material.  In most arc welding applications with this base material, we must add filler material if we want to have consistently crack-free welds.  A possible exception would be counteracting the cracking mechanism by maintaining a compressive force on the parts during the welding, which requires specialized fabrication techniques and considerations.  This method is seldom used.

I suspect that the welds in question that are not cracking are the ones that have filler material added during welding.  My advice would be to ensure that filler alloy is added to all welds during welding to reduce crack sensitivity.  Consideration should also be given when evaluating the cause of cracking to any differences in welds associated with weld size, and variations in tensile stresses introduced by shrinkage, joint expansion, or externally applied loads.

Fig 1

Q2 -  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.

A2 -  The answer to this question is also related to solidification crack sensitivity.  If we consider the alloying effect of magnesium (Mg) in aluminum, the weld crack sensitivity increases sharply with an increase in Mg content up to about 1.5% and then it decreases with further Mg additions (Fig 1 at Al-Mg curve).  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”, 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.
Fig 2