Welding & Metal Fabrication: Metals & Metallurgy

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Reprint of an article originally published in Welding Design and Fabrication magazine.

Gas shielding opened the door for aluminum, but didn’t slam it on other joining methods. They developed, too, adding to the versatility of, and demand for aluminum. Today, joining aluminum is mainly: Fusion: gas tungsten-arc, gas metal-arc, and electron beam - all fluxless methods. Fluxless welding gives the cleanest welds, inside and out. It entraps no slag, requires no cleaning between passes. Resistance welding - spot and seam. Excellent way to join sheets for autos, appliances, trucks, air planes, space ships. Bonding - explosive, diffusion, brazing, soldering, adhesive.

In the chemical, cryogenic, aerospace, and nuclear industries, aluminum and its alloys are widely used and frequently need to be joined by the application of welding. To fulfill this demand, the gas-shielded TIG and MIG processes, which are capable of producing high integrity welded joints, are used almost exclusively. However, before acceptable welds in aluminum such as those required within these industries can be produced, certain workshop practices and techniques have to be adhered to.

The use of some form of back-up to maintain and shape in position molten aluminum on full fusion butt joints welded from one side only are numerous and take many forms. Because of the high degree of expansion during welding, especially where it is impractical to use jigging, tack welding becomes necessary to maintain components in position. These tack welds must be made in the manner in which the main weld is to be deposited.

The specific demands made on welded constructions require the application of the most suitable alloy. Pure aluminum and the Al-Mn alloy are still in use for the construction of containers and chemical appliances owing to their favorable prices and good corrosion resistance, wherever the demands made on the mechanical resistance are not excessive. Typical alloys for the manufacture of vessels and containers, and indeed as welding materials, are the non-heat-treatable alloys Al-Mg or Al-Mg-Mn, which still have a medium resistance in the soft state.

Non-heat-treatable aluminum alloys constitute a group of alloys that rely solely upon cold work and solid solution strengthening for their strength properties. They differ from heat-treatable alloys in that they are incapable of forming second-phase precipitates for improved strength. Therefore, non-heat-treatable alloys cannot achieve the high strengths characteristics of precipitation-hardened alloys. The absence of precipitate-forming elements in these low- to moderate-strength non-heat-treatable alloys becomes a positive attribute when considering weldability, because many of the alloy additions needed for precipitation hardening (for example, copper plus magnesium, or magnesium plus silicon) can lead to liquation or hot cracking during welding.

The specific properties that affect welding are its oxide characteristics, its thermal, electrical, and nonmagnetic characteristics, lack of color change when heated, and wide range of mechanical properties and melting temperatures that result from alloying with other metals...