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Most aluminum base alloys can be successfully arc welded without cracking-related problems. However, using the most appropriate filler alloy and conducting the welding operation with an appropriately developed and tested welding procedure is significant to success. To appreciate the potential for problems associated with cracking, a welder should understand the many different aluminum alloys and their various characteristics.
PRIMARY CRACKING MECHANISM IN ALUMINUM WELDS
Hot cracking is the cause of almost all cracking in aluminum welds. Hot cracking—a high-temperature cracking mechanism that is mainly a function of how metal alloy systems solidify—is also known as hot shortness, hot fissuring, solidification cracking and liquation cracking.
Three areas can significantly influence the probability for hot cracking in an aluminum welded structure: (1) the susceptible base-alloy chemistry, (2) selection and use of the most appropriate filler alloy and (3) choosing the most appropriate joint design.
The aluminum crack sensitivity curves (Figure 1) can help understand why aluminum welds crack and how the choice of filler alloy and joint design can influence crack sensitivity. The diagram shows the effects of four different alloy additions—silicon (Si), copper (Cu), magnesium (Mg) and magnesium silicide (Mg2Si)—on the crack sensitivity of aluminum. The crack-sensitivity curves reveal that with the addition of small amounts of alloying elements, the crack sensitivity becomes more severe, reaches a maximum and then falls off to relatively low levels. These curves reveal that most of the aluminum base alloys considered unweldable autogenously (without filler alloy addition) have chemistries at or near the peaks of crack sensitivity. Additionally, the figure shows alloys that display low cracking characteristics have chemistries well away from the crack-sensitivity peaks.
Based on this, it becomes clear that crack sensitivity of an aluminum base alloy is primarily dependent on its chemistry. By the same token, the crack sensitivity of an aluminum weld, which is generally comprised of both base alloy and filler alloy, is also dependent on its chemistry.
Two fundamental principles can reduce the incidence for hot cracking. First, when welding base alloys that have low crack sensitivity, always use a filler alloy of a similar chemistry. Second, when welding base alloys that have high crack sensitivity, use a filler alloy with a different chemistry than that of the base alloy to create a weld-metal chemistry that has low crack sensitivity.
These principles step to the footlights when considering two common welds, one using the 5xxx series alloys (Al-Mg) and the other using the 6xxx series (Al-Mg-Si).
THE 5XXX SERIES ALLOYS
Most 5xxx base alloys, which contain around 5 percent Mg, show low crack sensitivity. Often welded autogenously, they are easy to weld with a filler alloy that has slightly more Mg than the base alloy. This can provide a weld with excellent crack resistance and a solidification temperature a little lower than the base alloy. These alloys should not be welded with a 4xxx series filler alloy as excess amounts of magnesium silicide can form in the weld and produce a joint with undesirable mechanical properties.
Some base alloys within this group, such as 5052, have an Mg content that falls very close to the crack-sensitivity peak. For this reason, avoid autogenous welding 5052 base alloys with around 2.5 percent Mg. The Mg base alloys with less than 2.5 percent Mg, such as 5052, can be welded with both the 4xxx filler alloys, such as 4043 or 4047, and the 5xxx filler alloys, such as 5356.
When welding base alloys with less than 2.5 percent Mg, it is necessary to change the chemistry of the solidifying weld from the high-crack peak level of the base alloy. A welder can alter the weld's chemistry by selecting a filler alloy with a much higher content of Mg, such as 5356 (5.0 percent Mg) or with the addition of silicon in the case of 4043.
6XXX SERIES ALLOYS (AL-MG-SI)
The aluminum/magnesium/silicon base alloys (6xxx series) are highly crack sensitive because most contain approximately 1.0 percent magnesium silicide (Mg2Si), which falls close to the peak of the solidification crack-sensitivity curve. The Mg2Si content is the primary reason there are no 6xxx series filler alloys. Using a 6xxx series filler alloy or autogenously welding will invariably produce cracking problems. During arc welding, the cracking tendency of these alloys is adjusted to acceptable levels by the dilution of the base material 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).
Particular care is necessary when gas tungsten arc (TIG) welding on thin sections of this type of material. It is often possible to produce a weld, particularly on outside corner joints, without adding filler material by melting both edges of the base material together. However, in the majority of arc welding applications with this base material, the addition of filler material is required to create consistent crack-free welds. The most appropriate and successful method to prevent cracking in the 6xxx series base materials is to ensure adequate filler alloy is added during the welding operation.
Other factors consider the effect of joint design on base-alloy and filler-alloy dilution as well as the weld profile relating to susceptibility to cracking. Square groove welds in this material are extremely vulnerable to cracking because very little filler alloy mixes with the base material during welding. It is often necessary 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. In addition, concave fillet welds with reduced throat thickness and concave root passes in butt-joint welds may have a tendency to crack.
FURTHER CONSIDERATIONS
Although the crack-sensitivity curve provides an excellent guide, other issues must be considered to understand cracking in aluminum alloys. One involves the effect of alloying elements other than the principal alloying elements addressed in the crack-sensitivity curves.
Some aluminum base alloys can be difficult to weld and lead to cracking problems, especially without complete understanding of their properties, or if inappropriately handled. In fact, some aluminum base alloys are unsuitable for arc welding, and for this reason they are usually joined mechanically by riveting or bolting. These aluminum alloys can be difficult to arc weld without encountering problems during and/or after welding. These problems are usually associated with cracking, most often hot cracking and, on occasion, stress-corrosion cracking.