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The primary cracking mechanism in aluminum welds
There are a number of cracking mechanisms associated with the welding of metallic alloys. One of the most notorious is hydrogen cracking, also referred to as cold cracking. Hydrogen cracking is often a major concern when welding carbon steels and high strength low alloy steels. However, when welding aluminum alloys hydrogen cracking cannot occur.
Hot cracking is the cause of almost all cracking in aluminum weldments. Hot cracking is a high-temperature cracking mechanism and is mainly a function of how metal alloy systems solidify. This cracking mechanism is also known as hot shortness, hot fissuring, solidification cracking and liquation cracking.
There are three areas that can significantly influence the probability for hot cracking in an aluminum welded structure. They are susceptible base alloy chemistry, selection and use of the most appropriate filler alloy and choosing the most appropriate joint design.
The aluminum crack sensitivity curves (Fig 1) is a helpful tool in understanding 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 (Fig 1) 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. After studying the crack sensitivity curves, it is easy to recognize 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 these facts, it is clear that crack sensitivity of an aluminum base alloy is primarily dependent on its chemistry. Utilizing the same principals, it can be concluded that the crack sensitivity of an aluminum weld, which is generally comprised of both base alloy and filler alloy, is also dependent on its chemistry.
With the knowledge of the importance of chemistry on crack sensitivity of an aluminum weld, two fundamental principals apply that can reduce the incidence for hot cracking. First, when welding base alloys that have low crack sensitivity, always use a filler alloy of 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. When considering the welding of the more commonly used 5xxx series (Al-Mg) and the 6xxx series (Al-Mg-Si) aluminum base alloys, these principals are clearly illustrated.
The 5xxx Series Alloys (Al-Mg)
The majority of the 5xxx base alloys, which contain around 5% Mg, show low crack sensitivity. Often welded autogenously (without filler alloy), these alloys 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 exercise amounts of magnesium silicide can form in the weld and produce a joint with undesirable mechanical properties.
There are base alloys within this group, such as 5052, that have a Mg content that falls very close to the crack sensitivity peak. In the case of the 5052 base alloy with around 2.5% Mg, definitely avoid autogenous welding. The Mg base alloys with below 2.5% 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 below 2.5% Mg it is necessary to change the chemistry of the solidifying weld from the high crack peak level of the base alloy. We alter the chemistry of the weld by selecting a filler alloy with a much higher content of Mg, such as 5356 (5.0% Mg) or with the addition of silicon in the case of 4043.
The 6xxx Series Alloys (Al-Mg-Si)
The aluminum/magnesium/silicon base alloys (6xxx series) are highly crack sensitive because the majority of these alloys contain approximately 1.0% Magnesium Silicide (Mg2Si), which falls close to the peak of the solidification crack sensitivity curve. The Mg2Si content of these materials 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 (see fig 2). 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 TIG (GTAW) 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. One possible exception would be counteracting the cracking mechanism by maintaining a compressive force on the parts during the welding operation. This requires specialized fabrication techniques and considerations. For this reason, the method is seldom used.
The most appropriate and successful method used to prevent cracking in the 6xxx series base materials is to ensure adequate filler alloy is added during the welding operation.
Other considerations when welding this group of alloys (6xxx) are 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 frequently 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, concaved fillet welds that have reduced throat thickness and concaved root passes in butt welds may have a tendency to crack (see fig 3).
The crack sensitivity curves provides an excellent guide to the probability of hot cracking, however, there are other issues to consider in order to understand cracking in aluminum alloys. One of these issues is the effect of alloying elements other than the principal alloying elements addressed in the crack sensitivity curves. Most certainly, some aluminum base alloys can be difficult to weld and lead to cracking problems, especially without complete understanding of their properties and/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 (SCC).
The aluminum alloys that fall into this difficult-to-weld category can be divided into different groups. Always be aware of the small selection of aluminum alloys designed for machineability, not weldability. Such alloys are 2011 and 6262, which contain 0.20-0.6, Bi, 0.20-0.6 Pb and 0.40-0.7 Bi, 0.40-0.7 Pb, respectively. The addition of the elements (Bismuth and Lead) to these materials provides excellent chip formation in these free machining alloys. However, because of their low solidification temperatures, they can seriously reduce the ability to produce sound welds in these materials. In addition to the free machining alloys referenced above, many other aluminum alloys can be quite susceptible to hot cracking if arc welded. These alloys are usually heat treatable and are most commonly found in the 2xxx series (Al-Cu), and 7xxx series (Al-Zn) groups of materials,.
To understand why some of these alloys are unsuitable for arc welding, it is necessary to consider the reasons why some aluminum alloys can be more susceptible to hot cracking.
Hot cracking, or solidification cracking, occurs in aluminum welds when high levels of thermal stress and solidification shrinkage are present while the weld is undergoing various degrees of solidification. A combination of mechanical, thermal and metallurgical factors influence the hot cracking sensitivity of any aluminum alloy. By combining various alloying elements, many high-performance, heat treatable aluminum alloys have been developed to improve the materials’ mechanical properties. In some cases, the combination of the required alloying elements has produced materials with high hot cracking sensitivity.
Perhaps the most important factor affecting the hot crack sensitivity of aluminum welds is the temperature range of dendrite coherence and the type and amount of liquid available during the freezing process. Coherence occurs when the dendrites begin to inter-lock with one another so that the melted material begins to form a mushy stage.
The coherence range is the temperature between the formation of coherent interlocking dendrites and the solidus temperature. The wider the coherence range, the more likely hot cracking will occur due to the accumulating strain of solidification between the interlocking dendrites.
The 2xxx Series Alloys (Al-Cu)
Hot cracking sensitivity increases in the Al-Cu alloys when adding approximately 3% Cu; however, it then decreases to a relatively low level at 4.5% Cu and above. Alloy 2219 with 6.3% Cu shows good resistance to hot cracking because of its relatively narrow coherence range. Alloy 2024 contains approximately 4.5% Cu causing the perception of having relatively low crack sensitivity. However, alloy 2024 also contains a small amount of Magnesium (Mg). The small amount of Mg in this alloy depresses the solidus temperature, but it does not affect the coherence temperature; therefore, the coherence range extends and the hot cracking tendency increases. The problem when welding 2024 is that the heat of the welding operation will allow segregation of the alloying constituents at the grain boundaries, and the presence of Mg, as stated above, will depress the solidus temperature. Because these alloying constituents have lower melting phases, the stress of solidification may cause cracking at the grain boundaries and/or establish the condition within the material conducive to stress corrosion cracking later. High heat input during welding, repeated weld passes, and larger weld sizes can all increase the grain boundary segregation problem (segregation is a time-temperature relationship) and subsequent cracking tendency.
The 7xxx Series Alloys (Al-Zn)
The 7xxx series of alloys, when considering weldability, contain two separate groups: the Al-Zn-Mg and the Al-Zn-Mg-Cu types.
Al-Zn-Mg Alloys such as 7005 will resist hot cracking better and exhibit superior joint performance than the Al-Zn-Mg-Cu alloys such as 7075. The Mg content in this group (Al-Zn-Mg) of alloys would normally increase the cracking sensitivity. However, adding Zr to refine grain size effectively reduces the cracking tendency. This alloy group welds easily with the high magnesium filler alloys such as 5356, which ensures the weld contains sufficient magnesium to prevent cracking. The recommendation of silicon-based filler alloys, such as 4043, for these alloys is not desirable because the excess Si introduced by the filler alloy can result in the formation of excessive amounts of brittle Mg2Si particles in the weld.
Al-Zn-Mg-Cu Alloys such as 7075 have small amounts of Cu added. The small amounts of Cu, along with the Mg, extend the coherence range and, therefore, increase the crack sensitivity. A similar situation can occur with these materials as with the 2024 type alloys. The stress of solidification may cause cracking at the grain boundaries and/or establish the condition within the material conducive to stress corrosion cracking later.
The problem of higher susceptibility to hot cracking from increasing the coherence range is not only confined to the welding of these more susceptible base alloys, such as 2024 and 7075. Crack sensitivity can be substantially increased when welding incompatible dissimilar base alloys (which are normally easily welded to themselves) and/or through the selection of an incompatible filler alloy. For example, by joining a perfectly weldable 2xxx series base alloy to a perfectly weldable 5xxx series base alloy, or by using a 5xxx series filler alloy to weld a 2xxx series base alloy, or a 2xxx series filler alloy on a 5xxx series base alloy, we can create the same scenario. If we mix high Cu and high Mg during the welding operation, we can extend the coherence range and, therefore, increase the crack sensitivity.
Avoid hot cracking in aluminum alloys by applying one or more of the following appropriate principals:
Fig 1. This figure shows the effects of four different alloy additions on the crack sensitivity of aluminum
Fig 2 Here are two GTAW (TIG) welds side by side made on a 6061-T6 base plate.
1. The top weld was deposited with no filler alloy, and then subjected to liquid penetrant testing. The testing method revealed many fine linear indications (cracks) within the surface of the weld.
2. The lower weld conducted also without filler alloy, used a higher current and slower travel speed. The excessive heat input during the welding of this bead has developed far more stress across the weld, resulting in a much more obvious cracking situation. As can easily be seen, without liquid penetrant testing, a large longitudinal crack has developed down the center of the weld bead.
The conclusion is that hot cracking of one form or another can be expected if the 6xxx series base alloys are welded without the addition of filler material.
Fig 3. This picture shows two fillet weld terminations in the corner of a welded structure. Cracking is visible in both termination craters. Also cracking is visible along the center of both welds. The reason for this hot cracking is undesirable weld profile caused by poor welding technique. The reduction in throat thickness at the termination and for a portion of the fillet weld has allowed the stresses developed during welding to fracture the weld.