Repairing Aluminum Structures: Filler Alloy Selection

What Filler Alloy Should You Choose for Repairing Aluminum Structures

- I work in a small welding repair shop and am often asked to perform repair welding on aluminum structures. Sometimes, the type of base alloy is known, and on other occasions, it is not.

There are two questions relating to the repair of aluminum.

- First, do we have a filler alloy that can weld all types of aluminum alloys? And secondly, from time to time, I come into contact with two aluminum alloys of which I have found difficulty in obtaining information about arc welding. These alloys are 2024 and 7075. Can you provide information on how to weld these alloys in the GMAW or GTAW process?

Exploring the Types of Aluminum Alloys

No filler alloy is suitable for welding all types of aluminum alloys. We need to understand that there are over 400 wrought aluminum alloys and over 200 aluminum alloys as castings and ingots registered with the Aluminum Association. The alloy chemical composition limits for all of these registered alloys are contained in the Aluminum Association’s Teal Book entitled “International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys” and in their Pink Book entitled “Designations and Chemical Composition Limits for Aluminum Alloys in the Form of Castings and Ingot.

Aluminum alloys can be categorized into several groups based on the particular material’s characteristics, such as its ability to respond to thermal and mechanical treatment and the primary alloying element added to the aluminum alloy.

The most important consideration encountered during the repair operation of aluminum is the identification of the base alloy type. If the base material type is not available through a reliable source, it can be difficult to select a suitable welding procedure.

There are some guides as to the most probable type of aluminum used in different applications, such as: most extruded aluminum is typically 6xxx series (AL-Mg-Si). Air-conditioning systems and heat exchangers, within the automotive industry, are typically made from 3003 sheet and 6061 tubing. Car wheels are often made from 5454, which because of its controlled magnesium (less than 3% Mg), is suitable for temperature applications. Ship hulls are often manufactured from 5083 (5%Mg), which is recognized as a marine material. Unfortunately, there is no guarantee.

If the base material type is not known, or unavailable, there is only one reliable way of establishing the exact type of aluminum alloy, and that is through chemical analysis. A small sample of the base material must be sent to a reliable aluminum testing laboratory, and a chemical analysis must be performed. Generally, the chemistry can then be evaluated and a determination as to the most suitable filler alloy and welding procedure can be made. It is very important to be aware that incorrect assumptions as to the chemistry of an aluminum alloy can result in very serious effects on the welding results.

Welding 2024 and 7075 Aluminium Alloys

ANSWER: In response to the second question, the reason you are having difficulty finding information on welding 2024 and 7075 is that both of these materials belong to a small group of aluminum alloys that are generally considered as being unweldable by the arc welding process. These materials are often found on aircraft, sporting equipment and other types of high-performance, safety-critical equipment and are not usually arc welded on the original component. 

Stress corrosion cracking in 2024 and 7075

Probably, the two most commonly found aluminum alloys within this category are 2024, which is an aluminum, copper, magnesium alloy, and 7075, which is an aluminum, zinc, copper, and magnesium alloy. Both of these materials can become susceptible to stress corrosion cracking after welding. This phenomenon is particularly dangerous because it is not detectable immediately after welding, and usually develops at a later date when the component is in service. The completed weld joint can appear to be of excellent quality immediately after welding.

However, changes that occur within the base material adjacent to the weld during the welding process, can produce a metallurgical condition within these materials which can result in intergranular microcracking, which may be susceptible to propagation and eventual failure of the welded component. The probability of failure can be high, and the time to failure is generally unpredictable and dependent on variables such as tensile stress applied to the joint, environmental conditions, and the period of time which the component is subjected to these variables.

It is strongly recommended that great care be taken when considering the repair of components made from these materials. It must be stressed that if there is any possibility of a weld failure becoming the cause of damage or injury to a person or property, do not perform repair work by arc welding on these alloys and then return them to service.