Tricks of the Trade: Here are some deeper insights into potential problems associated with cracking and the various characteristics of many different aluminum alloys.
My shop just received a job that involves welding aluminum, which we don’t really do a lot of. In the past, I have had problems with the welds cracking, but I can’t seem to isolate the specific problem so that I can correct it. How can I avoid cracking?
The majority of 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. In order to appreciate the potential for problems associated with cracking, it is necessary to understand the many different aluminum alloys and their various characteristics. Having this advance knowledge will help avoid cracking situations.
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 curve 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 below 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 shown in the diagram above 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.
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. 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.
Avoid hot cracking in aluminum alloys by applying one or more of the following appropriate principals:
- Avoid the extremely crack-sensitive base materials that are generally accepted as being non-weldable.
- Use a suitable filler alloy selection chart for selecting the most appropriate filler alloy for the specific base alloy, thereby avoiding the critical chemistry ranges (crack sensitivity ranges) in the weld.
- Select a filler alloy with a solidification point close to or below that of the base material.
- Select the most appropriate edge preparation and root gap to permit sufficient filler alloy material addition thus creating a weld metal chemistry outside the critical chemistry range.
- To counteract cracking problems, use reputable filler alloys that have grain refiners added, such as titanium or zirconium.
- Use the highest welding speed possible. The faster the weld is conducted, the faster the cooling rate and the less time the weld is in the hot cracking temperature range.
- Try to use welding and assembly sequences and techniques that minimize restraint, reduce residual stress and produce welds of acceptable profile.
- Apply a compressive force on the welded joint during welding to counteract the cracking mechanism.
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