Welding Aluminum: Theory and Practice
|Publication Date:||1 January 2002|
Superb strength-to-weight ratio, immunity to corrosion, and intrinsic recyclability render aluminum and its alloys desirable for many manufacturing applications. But to be competitive in the modern industrial world, a structural metal must be readily weldable.
The earliest welding techniques suitable for aluminum included oxy-fuel gas welding and resistance welding. Arc welding of aluminum was mainly restricted to the shielded metal arc process with a flux-coated electrode which created corrosion problems when the flux was inadequately removed.
The breakthrough for aluminum as a structural metal occurred with the introduction in the 1940s of the inert gas welding processes, such as Gas Metal Arc Welding (GMAW, also referred to as Metal Inert Gas or MIG) and Gas Tungsten Arc Welding (GTAW, also referred to as Tungsten Inert Gas or TIG). It became possible to make high strength welds without corrosive fluxes at high speeds and in all positions.
Today, aluminum and its alloys are readily weldable using a variety of techniques. This book will thoroughly explain the traditional joining methods for aluminum, such as GMAW and GTAW, and will introduce the latest technologies, such as laser welding. This book is dedicated to exposing North American industry to up-to-date knowledge of aluminum welding so that full advantage may be taken of the unique and beneficial properties of this useful metal.
Welded Aluminum Applications
Aluminum and its alloys are highly suitable for many manufacturing applications. Aluminum alloys have prevailed in the aerospace industry since its inception. Aluminum is used for a wide range of aerospace applications from rocket casings to space station modules.
Today the automotive industry, faced with increasing demands for fuel economy, has seriously begun to incorporate aluminum alloys into vehicles. Integration of aluminum alloys in closures, body structure, and frames provides desired light-weighting without compromising performance or safety. Highway truck bodies and some cabs are fabricated of welded aluminum, providing larger capacities without adding to gross weight. Fire engines are also built of aluminum to reduce weight and improve performance.
Overhead highway signs, lighting poles and protective railings are common on our highways. Aluminum for these applications offers good appearance, durability, and ease of handling during assembly.
A growing number of corrosion-resistant gondola rail cars is being built for service in the transportation of coal and other bulk commodities. Passenger vehicles for inter-city rail service, rapid transit, subway systems and for specialized people movers are commonly of welded aluminum.
The marine uses of aluminum include all types of work and pleasure craft as well as specialized military craft. In addition, there are many unusual applications such as helicopter recovery systems for marine vessels.
Welded aluminum pipelines are used where the corrosion resistance, weldability and other unique properties of aluminum are beneficial for transporting oil, gas, water and other chemical products.
The use of welded aluminum is also prevalent in other industries such as electronics, packaging, and architecture. There are numerous examples of the advantages of aluminum fabricated by welding. Just a few of these are temporary buildings, glass houses for plant nurseries, pedestrian bridges, specialized architectural components and storage tanks.
Contrary to popular belief, aluminum alloys are not difficult to weld. However, aluminum alloys have unique characteristics that must be considered for successful joining.
Aluminum alloys are highly susceptible to hydrogen porosity during fusion welding. Sources of hydrogen include water, die and rolling lubricants, and air. Hydrogen dissolves readily into the molten weld pool and produces gas pores upon solidification, as shown in Figure 1.1. Hydrogen porosity can be avoided by using proper gas shielding during welding, keeping the aluminum clean and dry, and avoiding temperature fluctuations that lead to condensation.
All aluminum alloys have a thin but tenacious surface oxide. This oxide layer can be removed mechanically or chemically, but it immediately reforms. The essentially instantaneous thickness is about 15Å, but the subsequent growth rate decreases so that on normal metal the oxide thickness may be 25-50Å. (An angstrom, Å, is about 4 billionths of an inch.)
While the oxide layer is extremely thin, it is sufficient to protect the metal against further oxidation as well as most corrosive attacks. For extra protection or for certain other benefits, such as decorative coatings, the surface may be anodized to produce much greater thicknesses (1000 times or more). Thermally treated metal also has a thicker oxide.
The oxide is very hard, being the hardest material after diamond. For this reason aluminum oxide is often used for the grit in grinding wheels. The melting point of the oxide is 3725°F (2052°C), which is about 3 times that of aluminum. The oxide is relatively stable and chemically inert; fluxes to remove it normally contain chloride and fluoride compounds and consequently pose a continuing corrosion hazard to the metal if not completely removed after joining.
The oxide is an electrical insulator. Normal oxide thicknesses are not sufficient to prevent initiation of an electric welding arc, but anodizing may produce a film thickness that prohibits arc welding
The surface of aluminum oxide is quite porous, and it can retain moisture or contaminants that may result in weld porosity. This is especially true of alloys containing magnesium because magnesium oxide or magnesium-aluminum oxide readily hydrates. Thus, it is good practice to keep aluminum clean and dry and to avoid temperature fluctuations that lead to condensation. Metal stored in the presence of high humidity and fluctuating temperatures may grow a thick oxide called a "water stain." This thick, contaminated oxide should be removed prior to welding since it can introduce hydrogen and other contaminants into the weld pool.
Aluminum is light; it has roughly one-third the weight of steel. Pure aluminum melts at 1220°F (660°C), which is less than one-half the melting point for steel. Thermal conductivity is about 6 times that of steel, which means that the heat to produce melting must be more intense for efficient welding. Thermal expansion is about twice that for steel and solidification shrinkage is 6% by volume, increasing both distortion and weld crater size.
Aluminum's electrical conductivity is high - about 65% that of pure copper. Aluminum does not change color as it is heated, which means care has to be taken not to touch what appears to be cold metal. Aluminum is non-magnetic. This eliminates problems of "arc blow," but it also means that magnetic lifting devices are not effective.
Pure aluminum is a weak but highly ductile metal. However, by alloying with one or more elements, the strength can be greatly increased while still retaining acceptable levels of ductility. The modulus of elasticity is approximately 10,000,000 psi (70,000 MPa), which is one-third that for steel. Thus, for the same applied force aluminum deflects three times more than steel, which gives it the ability to absorb greater energy under impact loadings.
Aluminum's toughness is not compromised at low temperatures. In fact, ductility increases as the temperature decreases, down to cryogenic temperatures.
Forms of Aluminum
Wrought aluminum is available in a wide range of forms, including sheet, plate, foil, rod, bar and wire. It can also be extruded into a variety of products such as structural shapes (angles, channels, T's, Z's, H-beams, I-beams) and into pipe and tubing. The metal can be forged for products requiring the unique properties afforded by forgings. Aluminum castings are also very common, whether sand cast, permanent mold cast or vacuum die cast, depending on the product requirements.
Sheet and plate can be clad with aluminum of different alloys to obtain superior corrosion resistance or for brazing filler metal. Almost any form that can be conceived is possible. Indeed, even aluminum joined to other metals such as steel, stainless steel and copper is available for special applications.