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Considerations when repairing aluminum structures
Q: I work in a large job shop and I have noticed an increasing amount of aluminum components that require repair by arc welding. I have worked with the fabrication of new welded aluminum structures, so I am familiar with the basic principals of arc welding aluminum. I would like to know if there are any special considerations when repairing aluminum structures and can the repair of aluminum be conducted successfully?
A: The use of aluminum, often as a replacement for steel, is becoming more frequent within the welding fabrication industry. As a result, there has been an increase in the repair of aluminum components. Repair work on aluminum structures is conducted successfully on a regular basis. Items such as truck bodies and boat hulls are repaired after damage from collision or after wear and tear during severe service conditions. In order to produce successful repairs, you must be familiar with some of the areas that can influence the repair process of aluminum alloys.
Identification Of The Base Alloy
Probably the most important, and usually the first step in the repair operation, is identifying the aluminum base alloy being repaired. If the base alloy type is unknown, I would recommend contacting the original manufacturer of the component to establish the aluminum base material type. If the base material type is not available through a reliable source, it is impossible to select a suitable welding procedure. There are many different types of aluminum alloys, some have very good weldability and others have extremely poor weldability. Some can be welded with one type of filler alloy, and others may react unfavorably if welded with that same filler alloy. Unfortunately, if the base material type is not known, or unavailable, chemical analysis is the only one reliable way of establishing the exact type of aluminum alloy. 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.
Cleaning and Material Preparation Prior To Welding
Even when welding on new components made from new material, it is necessary to consider the cleanliness of the part to be welded. Molten aluminum has a great affinity to hydrogen and hydrogen’s presence in the weld area is often due to the cleanliness of the plate being welded. You must be extremely aware of the potential problems associated with a used component that may have been subjected to contamination through exposure to oil, paint, grease or lubricants. If not removed prior to welding, these types of contaminants may introduce hydrocarbons, which can cause porosity in the weld. Moisture, another source of hydrogen, must also be considered and is often introduced through the presence of hydrated aluminum oxide. With prolonged exposure to moisture, the aluminum oxide on the material’s surface can become hydrated (absorb moisture), grow in thickness and become a serious problem if not removed before welding. Excessive hydrated oxide is sometimes visible as a milky white stain on the surface of the aluminum. If not completely removed prior to welding, this type of contamination may cause oxide inclusions and excessive porosity.
It is very important to clean the repair area completely prior to performing the weld repair. This is typically achieved using a degreasing solvent to remove hydrocarbons followed by stainless steel wire brush to remove any hydrated aluminum oxide. More aggressive chemical cleaning may be required for some applications. In situations where it is necessary to remove existing weld or base material, in order to conduct the repair, you need to consider the methods available to perform this operation as well as their effect on the finished weld. If you need to remove a crack in the surface of a weld prior to re-welding, you must use a method that will not contaminate the base material to be welded. Care should be taken when using grinding discs as some have been found to contaminate the base material by depositing particles into the surface of the aluminum. Routing and chipping with carbide tools is often found to be a successful method of material removal.
Base Material Strength Reduction After Welding
There may be considerations relating to the effect of the heating of the base material during the repair welding process. Aluminum alloys are divided into two groups: the “heat treatable” and the “non-heat treatable” alloys. Typically, the non-heat treatable alloys are used in a strain-hardened condition and the heat treatable alloys are usually used in one heat-treated form or another. During the welding process, the heat introduced to the aluminum base will generally reduce the strength of the base material in the heat-affected zone. The amount of reduction in strength and the size of the area affected is dependent on the original condition of the base material prior to welding and the temperature and time at temperature of the base material during welding. The as-welded strength, as opposed to the original base material strength, may need to be considered after welding.
The Repair Of Some High Performance Aluminum Alloys
Another consideration associated with the repair of a small group of aluminum structures is the temptation to repair high performance, typically high replacement price components, made from specialty aluminum alloys. These materials are often found on aircraft, hand gliders, sporting equipment and other types of high-performance, safety-critical equipment, and are not usually welded on the original component. There are a small number of high-performance aluminum alloys that are generally recognized as being unweldable. Probably the two most commonly found aluminum alloys within this category are 2024, which is an aluminum, copper, magnesium alloy and 7075, an aluminum, zinc, copper, magnesium alloy. Both these materials can become susceptible to stress corrosion cracking after welding. Changes that occur within the base material adjacent to the weld during the welding process can produce a metallurgical condition that can result in intergranular micro cracking, which may be susceptible to propagation and eventual failure of the welded component. It is not recommended to perform repair work by welding on these alloys and then return them to service.
There are many considerations associated with the successful repair of aluminum alloys. Most important is to understand the many different aluminum alloys and that they all require individual consideration. The majority of the base materials used for general structural applications can be readily repaired using the correct welding procedure. The majority of welded aluminum structures are designed to be used in the as-welded condition and, therefore, with the correct consideration, repair work of previously welded components can be conducted satisfactorily.