Aluminum Welding and Dissimilar-Metal Joining
Aluminum joining succeeds or fails on cleanliness, gas choice, and heat control. The exam tests those three plus the dissimilar-metal rule that keeps steel and aluminum apart.
Alloys and filler
Body aluminum is mostly 5xxx (non-heat-treatable) and 6xxx (heat-treatable, such as 6061-T6). ER5356 filler is the general choice for 5xxx and 6xxx structural work; ER4043 is common on castings and some 6xxx. Shielding gas is 100% argon - never CO2 or an argon/CO2 blend.
Equipment and technique
Aluminum wire is soft, so use a spool gun or push-pull gun, U-groove drive rolls, and a Teflon liner; a steel liner contaminates the wire and causes bird-nesting. Remove oxide with a stainless brush used only on aluminum. Weld with a push angle, fast travel, and low heat input; the oxide melts far above the base metal, so cleaning is part of the process, not just prep.
Strength and dissimilar metals
Heat-treatable alloys lose strength in the HAZ - 6061-T6 drops substantially - and welding never restores temper, so many OEMs specify rivet-bonding or adhesive for 6xxx closures. Steel and aluminum in direct contact create galvanic corrosion; OEMs separate them with structural adhesive, rivets, or isolation tape, never a fusion weld in body repair.
Worked example
An OEM procedure for an aluminum door skin specifies adhesive and rivets. A tech proposes GMAW with 5356. Reasoning: welding a 6xxx skin over-ages the HAZ, distorts a visible outer panel, and departs from a procedure written to preserve strength - the adhesive/rivet method is the compliant one.
Traps
Using the steel gun and liner; CO2 shielding gas; brushing with a disc used on steel, which leaves iron contamination; assuming a welded 6061 part regains T6 properties.