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The Repair of Aluminum Structures
Author:BY ROCCO…    Source:BY ROCCO IASCONNE AND CRAIG C. MENZEMER    Update Time:2009-11-26 3:28:26

The Repair of Aluminum Structures


 

The Repair of Aluminum Structures

Aluminum alloys require individual consideration when performing repair work on previously welded components


Fig. 1 - The repair welding of a large aluminum-fabricated structure using a high strength 5% Mg filler alloy.


Without a doubt, aluminum is increasingly being used within the welding fabrication industry. There has been a major increase in usage by the automotive industry, and by industries such as furniture, recreation and sporting equipment, shipbuilding, transportation and containers, military, and aerospace. Developments in aluminum usage continue with a view toward using it as a replacement for steel.

As more aluminum components are produced, the need for reliable repair work on aluminum weldments has also increased. Repair work to aluminum structures is regularly carried out on such items as truck bodies and boat hulls needing repair after damage from collision or after wear and tear resulting from severe service conditions - Fig. 1.

This article will examine some common considerations associated with the repair of aluminum alloys that can help prevent problems associated with such work, thus ensuring successful repairs.

Identification of Alloy Type

Probably the most important consideration encountered during a repair operation is identification of the aluminum base alloy. If the base material type of the component requiring repair is not available through a reliable source, it can be difficult to select a suitable welding procedure. There are guides that list the most probable type of aluminum used in different applications, such as most extruded aluminum (typically 6-series Al-Mg-Si). Air-conditioning systems and heat exchangers manufactured by the automotive industry are typically made from 3003 or 5052 plate and 6061 tubing - Fig. 2. Car wheels are often made from 5454 and are suitable for temperature applications because of its controlled magnesium (less than 3% Mg). Ship hulls are often made with 5083 (5% Mg).

However, if the base material type is not known or is unavailable, chemical analysis is the only reliable way of establishing the exact type of aluminum alloy present. A small sample of the base material must be sent to a reliable aluminum testing laboratory, and a chemical analysis must be performed. The substance's chemistry can then be evaluated and a determination made as to the most suitable filler alloy and welding procedure to use. This is an important step to undertake because incorrect assumptions about the chemistry of an aluminum alloy can result in very serious effects on the welding results.

There are seven major types of aluminum alloys with a wide range of mechanical properties and, consequently, a wide range of performance and applications. Some have very good weldability while others do not and are unsuitable, if welded, for structural applications. Some can be properly welded with one type of filler alloy while others cannot, making welds with poor mechanical properties. Filler alloy and base alloy chemistry mixture are main considerations relating to welded joint suitability, crack sensitivity, and joint performance. Without knowing the base material type, it is difficult to assess the correct filler and base alloy mixture.

If an aluminum component is to be repair welded and later used in a structural application, welding should not commence until a thorough understanding of the alloy type and the correct welding procedure to be followed is established, particularly if a later weld failure could result in property damage and/or injury.

High-Performance Aluminum Alloy Repair

Another problem associated with the repair of small aluminum structures is the temptation to repair high-performance, high replacement price components made from exotic aluminum alloys. These materials are usually not welded onto original components and are often found on aircraft, hang gliders, sporting equipment, and other types of high-performance, safety-critical equipment.


Fig. 2 - The repair welding of an aluminum automotive air conditioner component using a 12% Si filler alloy.


There are a small number of high-performance aluminum alloys that are unweldable. Performing welding on such components and returning them to service can be dangerous. Probably the two most commonly found aluminum alloys within this category are 2024, an aluminum, copper, magnesium alloy and 7075, an aluminum, zinc, copper, and magnesium alloy.

Both materials can be susceptible to stress corrosion cracking after welding, which is particularly dangerous because it is generally a type of delayed failure, not detectable immediately after welding, that develops after the component has been in service for some time. The completed weld joint may appear to be of excellent quality immediately after welding. X-rays and ultrasonic inspection shortly after welding typically will find no indication of a welding problem. However, changes that occur within the base material adjacent to the weld during the welding process can produce a metallurgical condition within these materials that can result in intergranular microcracking that may be susceptible to propagation and eventual failure of the welded component. The probability of failure can be high and the time to failure unpredictable and dependent on variables such as tensile stress applied to the joint, environmental conditions, and the period of time that the component is subjected to these variables.

Care should be taken when considering the repair of components made from these materials. As stated before, if there is any possibility of a weld failure becoming the cause of damage to property or injury to persons, repair work by welding on these alloys should not be performed and the component should not be returned to service.

Base Material Strength Reduction after Repair Welding

There are considerations relating to the effect heating has on base material during the repair-welding process. Aluminum alloys are divided into heat-treatable and nonheat- treatable alloys. Each has a different effect on the repair process.

Since they do not respond to heat treatment, nonheat-treatable alloys are used in a strain-hardened condition to improve the alloy's mechanical properties. During the welding process, heat introduced to the aluminum base will generally return the base material, adjacent to the weld, to its annealed condition. This will typically produce a localized reduction in strength within this area and may or may not be of any design/performance significance.

Heat-treatable alloys are almost always used in one heat-treated form or another. Often, they are used in the T4 or T6 condition (solution heat-treated and naturally aged or solution heat-treated and artificially aged). Base materials in these heat-treated tempers are in optimum mechanical condition. Heat introduced to these base materials during the repair welding process can change mechanical properties within the repair area. Unlike nonheat-treatable alloys, which are annealed and returned to this condition when subjected briefly to a specific temperature, heat-treatable alloys are affected by time and temperature.

The effect from heating during welding repair on a heat-treatable alloy is generally a partial anneal and an overaging effect. Because the amount of reduction in strength is largely determined by overall heat input during the welding process, there are gridlines as to how this reduction can be minimized. Generally, minimum amounts of preheating and low interpass temperatures should be used to control this effect. However, even with the best-designed welding procedures, considerable loss in tensile strength is always experienced within the heat-affected zone when arc welding these types of materials.

Unfortunately, it is usually either cost restrictive or, more often, impractical to perform postweld-solution heat treatment because of the high temperatures required and distortion associated with the process.

Cleaning and Material Preparation Prior to Welding

Even when welding on new components made from new material, cleanliness of the part to be welded is important. Aluminum has a great attraction for hydrogen; hydrogen's presence in the weld area is often related to the cleanliness of the plate being welded. When working with this type of material, one must be aware of the potential problems associated with used components that may have been subjected to contamination through exposure to oil, paint, grease, or lubricants. Such contaminants can provide hydrocarbons that can cause porosity in the weld during the welding operation.

The other source of hydrogen that should be considered is moisture, often introduced through the presence of hydrated aluminum oxide. For these reasons, it is important to completely clean the area to be repaired prior to welding. This is typically achieved by using a degreasing solvent to remove hydrocarbons followed by stainless steel wire brushing to remove any hydrated aluminum oxide. More aggressive chemical cleaning may be required for certain applications.

In cases that require removal of an existing weld or base material to conduct a repair, one should consider the methods available to perform this operation and the affect it will have on the finished weld. If a crack in the surface of a weld prior to rewelding needs to be removed, a method that will not contaminate the base material to be welded should be used. Care should be taken when using grinding discs; some have been found to contaminate the base material by depositing particles onto the surface of the aluminum. Routing and chipping with carbide tools is often the most successful method for material removal. Care must be exercised if using plasma arc cutting or gouging, particularly on heat-treatable aluminum alloys. This can produce microcracking of the material surface after cutting, which is typically removed mechanically prior to welding.

Conclusion

There are many considerations associated with the repair of aluminum alloys. Perhaps the most important point to understand is there are many different aluminum alloys that 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 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 and should be conducted satisfactorily.


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ArticleInputer:hanns    Editor:hanns 
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