Joint design and fit-up are both critical to achieving a strong bond upon weld completion. When picking a joint design, consider strength required, metal thickness, welding position and joint accessibility. Five basic types of joints exist--butt: in which two pieces of metal are joined simply by butting them together; corner: in which two metal pieces are joined at 90 degree angles to each other; edge: in which two parallel members are joined; lap: in which a piece of metal is extended; and tee: in which one piece of metal is welded to another to form a "T."
In addition to choosing the kind of joint, you must chose the type of welding to use. Shielded metal arc welding (SMAW) is used primarily on metals 1/8 inch or more in thickness, for small repair jobs. SMAW is the least-expensive way of welding aluminum, generating an electric arc between the coated electrode and base metal. The coated electrode provides shielding, but does create some inefficiency, in the form of slag coating and stub loss.
GTAW, one of the most popular welding methods, is easily performed on a variety of metals, including aluminum. In TIG, heat for welding is generated between a non-consumable tungsten electrode and the base metal. An inert shielding gas shields metals during the weld; argon is used for metal thicknesses up to 1/2 inch, while argon-helium mixtures or pure helium are used for greater thicknesses. Filler metal, with similar mechanical characteristics to those of the base metal, may be used, if necessary, to fill in the gouge created during welding.
GMAW can be used for both thinner and thinner cuts of metal, and, like GTAW, requires little post-weld cleanup. In GMAW, heat is produced by an arc between the electrode and the base metal. The two kinds of GMAW are short-circuit transfer, which is not used with aluminum, and spray transfer, which can be used with aluminum. In spray transfer welding, metal is transferred across the arc, creating a continuous spray of fine metal droplets onto the base metal.