Casing the extrusion joint

Senior Editor

The capacity to form complex shapes with relatively simple tooling — in a single operation — makes aluminum extrusions an attractive alternative to machined and stamped components. Throw in a variety of joining methods and the design possibilities grow geometrically. For instance, assemblies of smaller extrusions can replace single, larger units, without compromising structural integrity. Smaller sections are also easier to handle, can be made to tighter tolerances, and may lower die costs in many cases. But choose a joining method when spec'ing dies because sections are often made specifically for it.

A JOINT DECISION
The elasticity of aluminum makes possible clipped joints, a method of snapping together multiple extrusions that cannot otherwise be made in one piece. Joints that are repeatedly joined and separated may eventually lose their shape, however. In such cases use plastic clips or steel springs for a joint's bending portion.

Another fastenerless technique called milling (not to be confused with the removal of material with a cutter) uses overlapping, locking sections to form a permanent joint. One extruded component contains a raised portion and a slot while the mating part engages the slot. Bending over the raised portion locks the two parts together.

Integral hinges are yet another design option. Rotal hinges from Hydro Aluminum North America, for example, bring into mesh two extruded axially aligned gear sets. A third extrusion locks the sections together. The resulting joint is able to swing through 90 and requires no secondary machining.

But many designs still require conventional joining methods, and extrusions can ease the assembly process. Galvanized or stainless-steel bolts make sense should it be necessary to dismantle a product. Nuts can be separate or made part of the extrusion itself. Blind nuts or inserts work when it is impractical to thread thin-walled extrusions or when joints are frequently disassembled. Here, extruded channels prevent unwanted turning of nuts or bolts.

When using steel fasteners, paint aluminum contact surfaces with zinc chromate and sealing compound to help prevent corrosion. Physically insulate steel bolts from aluminum surfaces in strongly corrosive conditions. Both nylon and neoprene are good choices for insulation, though neoprene must not contain carbon as an additive.

Holes for fasteners can be drilled and reamed or just drilled, depending on the application. For low-stress joints, bolt-to-hole clearances up to 0.040 in. are acceptable and reaming isn't necessary. Heavily loaded joints should have holes reamed for a maximum 0.006-in. bolt-to-hole clearance. For hot-dip galvanized fasteners that number is 0.012 in. based on fastener diameter before galvanizing. In any case, only the unthreaded portion of bolts should bear on edges of reamed holes.

Extrusions may also be riveted or slid together longitudinally in tracks or snapped together and then locked with screws or cylindrical plugs, or with a fitted cover. The latter technique eases assembly and lowers die costs by replacinga relatively expensive hollow extrusion die with a simpler, solid one.

ADHESIVES
An advantage of adhesives is the abilityto permanently join different material types. Adhesives more evenly distribute load than fasteners, dampen vibration, and stop galvanic corrosion. They also seal as well as bond surfaces to exclude contaminants. Basic joint designs include butt, angle or corner, T, and lap. But joints are only as good as the pretreatment step, regardless of joint geometry and adhesive type.

Pretreatment typically involves cleaning and degreasing parts with an acidic, alkaline, or solvent-based degreaser. Additional steps may include grinding, blasting, chemical etching, conversion coating, anodizing, or priming. Obviously pretreatment adds manufacturing time and is a downside to adhesives. Exposure to high temperatures may also weaken adhesive bonds, another negative.

WELDING
Welding doesn't suffer the temperature limitations of adhesives and is cost competitive with them and other joining methods. Welding is also the simplest way to make airtight and watertight joints. The most common fusion welding methods are metal-inert gas (MIG) and tungsten-inert gas (TIG).

MIG excels at robotics-based production because filler wire doubles as the electrode and automatically feeds to the work surface from large spools. MIG works for materials 0.120-in. thick and up, though special equipment can weld 0.028-in.-thick materials. Speed and good penetration are its strong points. Welding speeds of 15 to 30 ipm for 0.160 to 0.800-in.-thick materials are possible, about twice as fast as TIG.

TIG uses a nonconsumable tungsten electrode to generate an arc and separately feeds filler rod when needed. TIG works on all aluminum alloys and produces high-quality welds on a wide range of material thickness, but excels at delicate work on thin sections (0.028 to 0.040 in.).

But aluminum welding can be tricky. Aluminum exposed to ambient air for even a short time forms a surface layer of aluminum oxide. Heavy layers of aluminum oxide act as an insulator, resulting in an erratic welding arc. Aluminum oxide melts at about 3,750°F or roughly three times the melting point of aluminum alloy itself. Failure to remove a heavy oxide layer before welding can result in inclusions in the solidified weld zone. An aluminum-oxide surface is porous and tends to trap moisture and other surface contaminates that encourage weld porosity. Hydrogen bubbles forming in the solidifying weld pool are mostly responsible for weld porosity. Hydrogen contamination typically comes from moisture or oil on the surface being welded. Parts to be welded must be thoroughly cleaned with detergent and dried beforehand.

Interestingly, aluminum needs more heat to weld than steel, despite a low melting point of about 1,100°F versus 2,730°F for steel. This is because the heat capacity of aluminum is about double that of steel while thermal conductivity is roughly four to six times greater. Stress from uneven heating during welding tends to form solidification cracks, especially when parts are physically constrained and can't thermally grow. Cracks may form at weld joints, at the interface between welds and joined materials, or in the material itself. Long cooling times encourage crack formation-while rapid cooling appears to have the opposite affect.

A more recent technique called friction stir welding (FSW) gets around the problems associated with high welding temperatures. (See "Causing a Stir in Welding," MD, March 21, 2002.) FSW does away with the electric arc and instead presses a translating, rotating tool onto a joint surface. The resulting friction locally raises temperatures to about 200 to 300°F below the 1,100°F melting point of aluminum, mixing the surfaces together in the plastic state. The method requires no filler or protective atmosphere and makes welds that are virtually free of heat distortion. The method can join aluminum extrusions between 0.080 and 0.320 in. thick at speeds to 3 ft/min.

MAKE CONTACT
Hydro Aluminum North America,
Linthicum, Md., (410) 487-4500,
www.hydroaluminumna.com