Aluminum and Its Alloys: Properties, Welding, and Heat Treatment
Aluminum and its Alloys
Properties of Aluminum
Aluminum is a lightweight, silvery-white, and relatively soft metal. It’s known for its good corrosion resistance against air, oil, food, and many chemical agents. This resistance is due to a layer of aluminum oxide, called alumina, which prevents metal corrosion.
This alumina layer is refractory, meaning it has a very high melting temperature. It must be removed before or during welding to allow proper fusion of the base metal.
Aluminum is a very ductile metal (even at low temperatures) with low mechanical strength. However, aluminum alloys have higher mechanical strength than pure aluminum because alloying elements harden and improve the mechanical properties of aluminum.
Aluminum Alloys
Aluminum is primarily alloyed with copper, magnesium, silicon, and zinc. Small amounts of chromium, iron, nickel, and titanium are also often added. There are many aluminum alloys, each with characteristics superior to unalloyed aluminum.
There are two types of alloys:
- Heat-treatable alloys – also called solution heat-treated or hardenable by heat treatment.
- Non-heat-treatable alloys – also called non-solution heat-treated or not hardenable by heat treatment.
Heat treatment for solution heat-treated alloys involves heating at 500°C with rapid cooling. Then, depending on the alloy type, natural or artificial aging is conducted. Natural aging is maintenance at room temperature, and artificial aging is maintenance at about 200°C.
This treatment increases the hardness and tensile strength of solution heat-treated alloys. However, it doesn’t change the properties of non-solution heat-treated alloys.
Factors Influencing the Performance of Welded Joints
Melting of Aluminum
Pure aluminum melts at about 660°C, and aluminum alloys melt at about 560°C. These temperatures are extremely low compared to steel (1535°C) and copper (1082°C). However, aluminum alloys don’t change color during heating, so there’s a risk of puncturing the piece.
Thermal Conductivity
Aluminum alloys conduct heat three times faster than steel. This means more heat input is required for welding a piece of aluminum than steel, even if they have the same dimensions. Preheating is necessary for good fusion when the piece is thick.
Thermal Expansion
Aluminum alloys expand twice as much as steel when heated. This can cause large internal stresses and deformations in the parts during welding. It also increases the tendency for downward separation in the root of butt joints. Welding at low speeds and with a large amount of filler metal increases deformation and the tendency to rupture.
Aluminum Oxide
In contact with air, aluminum forms a layer known as alumina. It’s crucial to eliminate or remove this oxide layer using chemical stripping, flux, grinding, or the etching action of the arc.
In TIG and MIG welding (with NCDC or alternating current), the electric arc removes the oxide surface, and the inert gas shield prevents contamination of the weld pool.
Sometimes, due to poor cleaning, some oxide can be trapped in the melt, producing an imperfection in the weld. It’s essential to clean the parts before welding and brushing using stainless steel brushes. These brushes should only be used for cleaning aluminum.
Welding Processes
Aluminum and its alloys can be welded by most fusion welding processes, brazing, soldering, and solid-state welding. Fusion welding is usually performed with MIG, TIG, resistance, plasma, laser, and electron beam welding.
Shielded metal arc welding (SMAW) and gas welding are only used for repairs or when no other process is available. Submerged arc welding is not used due to excessive temperature.
Recommendations for Completing Welding
Geometry of the Junction
A special design can be used for TIG or MIG welding on one side and is essential for obtaining a smooth root. It can be used for thicknesses greater than 3mm in any position. Although good penetration is easily achieved, it requires a lot of filler metal, and deformations may be larger than those produced with conventional designs. It’s mainly used in pipes, particularly in a fixed position. For V-joints, bevel angles of less than 60° are not recommended.
Preheat
Preheating is usually necessary, especially for thick parts. Suitable preheating can decrease the heat input required to achieve fusion and penetration. While preheating during TIG welding may be normal, it’s not when using MIG.
Cleaning and Surface Preparation for Welding
Plasma cutting and beveling are typically used.
Cleaning before welding is crucial because any traces of grease, oil, or rust can deteriorate the weld quality. Alkaline solvents that don’t produce toxic fumes can be used. A common method is cleaning with a cloth soaked in a solvent like alcohol or acetone. Surfaces should be thoroughly dry before welding to prevent pores.
Oxide layers should be removed by scraping or, even better, by grinding. Any grinding stone, brush, or sandpaper used should be exclusively for aluminum. Steel and aluminum shouldn’t be worked on in the same area to prevent contamination of the aluminum parts.
Tack Welding
Care should be taken, and removing tack welds as welding progresses is recommended. If incorporated into the weld, cracks and defects in the tack welds should be repaired before continuing, and the start and end points should be prepared to allow good melting. For welds on both sides, the first pass should be repaired before welding the other side. As a general rule, each tack weld should be 10 times the thickness of the piece.
Filler Metals
Both rods and wires, as well as coated electrodes, can be used.
Post-Weld Heat Treatment
Sometimes, heat treatment is performed on welded pieces made of heat-treatable aluminum alloys.
In other cases, to reduce internal stresses, it can be beneficial to hammer the welds, which is only done on thick welds.