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Aluminum Storage & Preparation for Welding
One of the most frequently asked questions in the process of welding aluminum is: “Should the base metal be cleaned before welding?” To answer this question correctly, one must first determine the finished welded product requirements. If consistent, porosity free, high strength, high quality welds are desired, then the base metal must be thoroughly cleaned using a properly designed and executed procedure. Welding wire quality is a subject of constant concern among designers, engineers, and welders, however, base metal preparation and cleanliness is of equal or even greater importance and is often ignored. Producers of aluminum sheet, plate, rod, bar and other fabricated shapes generally ship their products with a protective coating of oil or other hydrocarbon to protect the surface. Depending on storage conditions and storage time, aluminum products are covered with oil, ink, grease, dirt, moisture, and a variable layer of hydrated oxide. These contaminants contain hydrogen and are broken down by the arc during welding, releasing atomic hydrogen which is absorbed by the molten aluminum in the weld puddle. During solidification, this hydrogen comes out of solution and coalesces into bubbles in the aluminum which we see as porosity.
The general melting temperature of aluminum alloys is around 1,2000F while the melting temperature of aluminum oxides is 3,7000F. Aluminum oxide is not melted during the welding process and if it is present to an excessive degree, it can easily cause lack of fusion and oxide inclusion type defects. With this in mind, we present the following guidelines for the proper storage, joint preparation, cleaning, and welding of aluminum:
STORAGE AND HANDLING:
· Position base metal vertically and space apart to provide for air circulation and minimize condensate contact points.
· Store inside, preferably in a heated room with as constant a temperature as possible. Humidity control is also desirable, if it can be achieved. (refer to discussion on "Atmospheric Conditions Affect Weld Quality")
· Store in a heated room with uniform temperature control and, if possible, with humidity control as well.
· Hold electrode in the welding area for 24 hours before unpacking to allow its temperature to equalize with that of the surrounding area.
· Store unpacked material in heated cabinet. Use dust covers on all welding equipment.
Oxyfuel Gas Cutting:
· Not recommended for aluminum because it leaves a large heat affected zone with harmful eutectic melting and heavy oxide films
Carbon Arc Cutting, Beveling, and Gouging:
· Not widely recommended or used for the same reasons as gas cutting. If it is used, it requires heavy mechanical surface removal before welding.
Plasma Arc Cutting, Beveling, and Gouging:
· This process has some limitations and must be carefully controlled. If it is used, it requires the power source to be set (DCEN) along with the use of small orifices to gain high velocity and concentrated heat. Heat affected zones will be crack prone particularly for 2XXX, 6XXX, and 7XXX series alloys and will require 1/8 inch or more of mechanical surface removal before welding. Series 1XXX, 3XXX, and 5XXX alloys are not as crack prone and can generally be welded as cut by this process.
Drilling, gouging, filing, milling, or router-type cutting produce the best surface for welding. Lubricants or coolants must not be used and tools should be sharp to avoid metal smearing.
Aluminum Storage & Preparation for Welding
· Blade speed:
Circular high-speed steel (8,000 fpm)
Circular carbide (12,000 fpm)
Band saw (5,000 fpm)
· Tooth shape and spacing:
Circular (std. spacing, high rake angle)
Band (3 to 4 teeth per inch)
· Lubricants or coolants must not be used and band saw surfaces should be removed by filing prior to welding.
· Wheel grinding is not recommended since it smears the surface of aluminum and can deposit organic binders from the wheel during grinding.
· Disc grinding can be used with grit size, 30 to 50 preferred, and speeds of 4,000 to 6,000 fpm. Only flexible discs should be used and grinding pressures should be moderate to prevent surface heating or smearing of the aluminum. Lubricants or coolants must not be used.
BASE METAL CLEANING
· Minute traces of moisture on aluminum can produce severe weld porosity. Both the welding filler metal and the base metal should be brought into the welding area 24 hours in advance to allow all material temperatures to equalize. A dew point test should be done prior to welding (See discussion on "Atmospheric Conditions Affect Weld Quality"). If preheating must be used, heat no higher than 1500F and remember that oxyfuel flames produce water as a byproduct of combustion.
· Before oxides can be removed from aluminum, the metal must be degreased. This is best done with a solvent. Toluene is the best general solvent for this purpose. Acetone is a poor solvent for oils and greases and is less effective than toluene. Chlorinated solvents are also good degreasers but are not recommended for this application because they present environmental problems and their vapors can decompose into toxic or poisonous gases in the presence of heat. Weld joints should be washed with solvent prior to assembly and wiped dry using clean cloth such as cheesecloth. Shop rags should not be used since they contain soaps and other organic compounds from the washing and conditioning processes used to treat them. Do not use compressed air to blow off or to dry solvent cleaned areas since it often contains moisture and oil.
· Wire Brushing:
Oxide removal must be done after degreasing and is best done with a stainless steel wire brush. Wire brushes must be frequently cleaned with the same solvent as the base metal. Wire brushing can bedone by hand or with a power brush. If power is used, keep rpm’s and pressures low to avoid heating and smearing the surface metal. Compressed-air power brushes should exhaust their air to the rear, not forward towards the brush where the compressed air can contaminate the base metal.
· Chemical Cleaning:
Chemical cleaning deoxidizes and etches the aluminum. These cleaners contain acids and can present problems in handling and disposal. If they are used, the base metal must be thoroughly rinsed and dried and should be milled or wire brushed prior to welding.
· Etch Cleaning:
This process uses a hot sodium hydroxide etch and nitric acid rinse. It effectively removes heavy oxides, rough machined, sawed, or smeared surfaces, and hydrocarbons. However, the process leaves a porous surface containing hydrated oxides that absorb moisture during storage faster than an as-fabricated mill surface. This surface should be milled or wire brushed prior to welding.