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THE REPAIR OF ALUMINUM STRUCTURES
Without a doubt, aluminum is being increasingly used within the welding fabrication industry. We are seeing a major increase in usage within the automotive industry, where the use of aluminum continues to develop. Also within other industries such as furniture, recreation and sporting equipment, shipbuilding, transportation and containers, military and aerospace we see continued developments with aluminum, often as a replacement for steel.
As more components are produced from aluminum, the need for reliable repair work on aluminum weldments is also increasing. Repair work to aluminum structures is conducted extremely successfully on a regular basis, such items as truck body’s and boat hulls are repaired after damage from collision or after wear and tear during severe service conditions. This article shall examine some of the more common considerations associated with the repair of aluminum alloys in an attempt to help prevent problems associated with repair work and also to help ensure consistently successful repairs.
Identification Of Alloy Type: Probably the most important consideration encountered during the repair operation is the identification of the aluminum base alloy type. If the base material type of the component requiring the repair 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, 5052, plate 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, 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.
There are 7 major types of aluminum alloys that have a wide range of mechanical properties and, consequently, a wide range of performance and applications. Some have very good weldability, and others are considered to have extremely poor weldability, and are unsuitable, if welded, for structural applications. Some can be welded with one type of filler alloy, and others will produce unacceptable, extremely poor mechanical properties if welded with that same filler alloy. Filler alloy and base alloy chemistry mixture is one of the main considerations relating to welded joint suitability, crack sensitivity, and joint performance. Consequently, without knowing the base material type, you are unable to assess the correct filler in order to prevent an unsuitable filler alloy, base alloy, mixture.
I must definitely recommend that, if an aluminum component is to be repair welded, and after this, used for any structural application, particularly, if a weld failure, can in any way damage property and / or create injury, do not weld it without understanding its alloy type, and being satisfied that the correct welding procedure is to be followed.
The Repair Of Some High Performance Aluminum Alloys: Another problem 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 exotic 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. It can be very dangerous to perform welding on these components and then return them to service. 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. This phenomenon (stress corrosion cracking) is particularly dangerous because it is generally a type of delayed failure, 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. X-rays and ultrasonic inspection shortly after welding will typically find no indication of a welding problem. However, changes, which 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 micro cracking, 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 that 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. Again, it must be stressed that if there is any possibility of a weld failure becoming the cause of damage or injury to person or property, do not perform repair work by welding on these alloys and then return them to service.
Base Material Strength Reduction After Repair Welding: There are 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. We should consider the differences between these two groups and the effect on each during the repair process. Typically, the non-heat treatable alloys are used in a strain-hardened condition. This being the method used to improve their mechanical properties, as they do not respond to heat treatment. During the welding process, the 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.
The heat treatable alloys are almost always used in one heat-treated form or another. Commonly 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 their optimum mechanical condition. The heat introduced to these base materials, during the repair welding process, can change their mechanical properties considerably within the repair area. Unlike the non-heat treatable alloys, which are annealed and returned to this condition when subjected briefly to a specific temperature, the heat-treatable alloys are affected by time and temperature. The effect from the heating during the welding repair on the heat-treatable alloy is generally a partial anneal and an over-aging effect. Because the amount of reduction in strength is determined largely by overall heat input during the welding process, there are gridlines as to how this reduction can be minimized. Generally, minimum amounts of pre-heating 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 post weld solution heat treatment because of the high temperatures required and the distortion associated with the process.
Cleaning And Material Preparation Prior To Welding: Even when welding on new components made from new material we need to consider the cleanliness of the part to be welded. Aluminum has a great attraction for hydrogen and hydrogen’s presence in the weld area is often related to the cleanliness of the plate being welded. We need to be extremely aware of the potential problems associated with a used component that may have been subjected to contamination through their exposure to oil, paint, grease, or lubricants. These types of contaminants can provide hydrocarbons that can cause porosity in the weld during the welding operation. The other source of hydrogen which we need to consider is moisture, often introduced through the presence of hydrated aluminum oxide. For these reasons it is important to completely clean the repair area to be welded prior to performing the weld repair. This is typically achieved through the use of 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 the case were we are required to remove existing weld or base material in order to conduct the repair. We need to consider the methods available to perform this operation and their effect on the finished weld. If we need to remove a crack in the surface of a weld prior to re-welding we must use a method which will not contaminate the base material to be welded. Care should be taken when using grinding discs, 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. Care must be exercised if using plasma arc cutting or gouging, particularly on the heat-treatable aluminum alloys. This can produce micro cracking of the material surface after cutting which is typically required to be removed mechanically prior to welding.
Conclusion: There are many considerations associated with the repair of aluminum alloys. Perhaps the most important is to understand that 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 is conducted satisfactorily.