Weld Cracking in 6XXX Series Aluminum

By Bruce James, Gooderham Centre for Industrial Learning

When welding 6XXX series aluminum alloys, there are certain metallurgical factors that need to be considered because the composition of the parent material can produce sensitivity to cracking. The heat affected zone adjacent to the weld is prone to micro-fissuring and the weld deposit chemistry must be modified from the parent composition to prevent cracking. For these reasons 6061 is not often autogenously welded. The mechanism of cracking and role of the filler alloy to produce a crack-free weld will be discussed in this article.

Heat input from welding lowers the strength of aluminum alloys dramatically. Also the weld-bead strength generally does not greatly overmatch the strength of the heat-affected-zone. With all of this, how do you weld that material? Figure 1 shows the results of two alternatives.

Figure 1. Gas tungsten arc weld of 6061 extrusion.
Left side welded with filler metal addition – no cracking.
Right side welded without filler metal – cracking evident.

The cracking of 6XXX series aluminum alloys is primarily due to "hot shortness" of the alloy and is dependent upon its chemistry and the shrinkage forces acting on the weld as it solidifies. Weldable aluminum alloys do not harden by quenching (as a steel could) therefore weld zone cracking is not related to loss of ductility associated with increased hardness. Hot shortness is that characteristic of a metal that results in complete loss of plasticity while the alloy is between the liquidus and solidus temperatures; commonly called the "freezing range" of an alloy. The liquidus temperature is the temperature when the alloy is completely liquid. The solidus temperature is the temperature when the alloy is completely solid. Between these two temperatures, the metal exists as a mixture of a liquid and solid phase. The 6XXX series aluminum alloys have different freezing ranges dependant upon their composition. In general, the wider the freezing range the greater the sensitivity to hot cracking.

Figure 2. Crack sensitivity vs. composition

Figure 2 shows that the peak crack sensitivity of the Al-Mg₂Si system is at about one percent Mg₂Si. This is the composition of the commonly welded extruded structural alloy AA-6061. It is for this reason that AA-6061 requires filler-alloy addition to change the chemistry of the weld metal. It is possible to produce a more weldable 6061 and this can be accomplished by adding more magnesium to the parent material.

The work hardening effects of magnesium in aluminum make it unpopular for extrusion companies. As the magnesium content increases, the extrusion speed must be slowed — reducing productivity. Therefore, 6XXX ingots purchased for extrusion into structural shapes will commonly be specified with only minimum magnesium content. This reduces the weldability of the alloy and, as a result, 6061 plate with typically higher magnesium content will be more weldable than extrusion.

Therefore before attempting to weld 6061 it is important to know the chemistry of the base material, and the manufacturing method. Welding without addition of the filler metal required special precautions to prevent cracking.

Information on aluminum and its’ welding can be found in Gooderham Center For Industrial Learning Modules 24 to 29.