Aluminium Fabrication

14.5 Fabrication

Cutting and forming

Cutting and machining

Thin material can be like steel, but more readily. For material cold-sawing is used, with either a circular or saw. Aluminium (except in the softest tempers) can be faster than steel, especially if suitable coarse-toothed saw used.
Aluminium is also more readily machined than steel, and it is unusual in design to employ extrusions incorporating flanges which are machined away over the greater of the member. Stiffened panels in aircraft are often out of the solid.)
Ordinary flame-cutting is unsuitable for aluminium, because the ragged edge produced. Instead, one can employ
, an adaptation of the tungsten inert gas (TIG) process.

Bending

The heat-treatable alloys in the full strength T6 condition are less easily manipulated than steel. They will only accept a small deformation when bent cold, due to their lower ductility.
Heating, on the other hand, disturbs the heat treatment and causes severe softening. One solution is to form the material in the more ductile T4 condition and then bring it up to the full T6 strength by subsequent artificial ageing in a low-temperature furnace.
With the nonheat-treatable alloys the practice for forming is more as for steel. Cold-bending is employed when possible, the
temper of the material being selected to suit the severity of the bend. Springback is more than for steel. For severe manipulations it is possible to apply local heating with a gas flame, the necessary temperature being 450 to 50O0C. Great care is necessary to avoid overheating the aluminium, since there is no color change at this temperature. Temperature-sensitive crayons may be used; alternatively one can rub a pine stick on the heated area and see if it leaves a mark.

Mechanical joints

Riveting

For many years, riveting was the normal means of making shop joints in aluminium. More recently there has been a wholesale move to welding, even for structures in the 6000-group alloys which are severely affected by HAZ softening. Riveting is little used and rivets have become hard to get. One wonders if the swing to welding has not been overdone.
Small solid rivets would usually be in 5154A alloy with the small ‘pan’ head driven cold. Squeeze riveting is preferred to hammering. Larger rivets can be driven hot. Alternatively, one can use 6082-T4 rivets (or equivalent) which have been held in a refrigerator since quenching, to suppress natural ageing. These are readily driven cold, after which they age-harden in position to attain their proper T4 strength.
Proprietary fasteners such as ‘Pop’ and ‘Chobert’ rivets are available for joints in sheet-metal work. These are suitable for blind riveting, i.e. from one side, and are quick to use

Bolting

Aluminium structures can be assembled using either ordinary bolting (dowel action) or high-strength friction-grip (HSFG) bolting (friction action).
Ordinary bolding is used with clearance or close-fitting reamered holes as appropriate, possible bolt materials being: 6082-T6 aluminium (or equivalent), steel (suitably coated) or stainless steel (316Sl 6 or 304Sl 5). Aluminium bolts are none too good in tension, especially in fatigue. On the other hand it may be difficult to get steel bolts with a coating of sufficient durability to match that of the aluminium, unless they are painted. The ideal answer is stainless steel, which is usually worth paying for.
In recent years it has become acceptable to employ HSFG bolting for aluminium, taking care with the protection of the steel bolts. Bolt material (high yield steel) and torqueing procedures follow HSFG practice in steel. Proper attention must, of course, be paid to the condition of the contact surfaces, which should be grit-blasted. The slip resistance can be improved by applying epoxy resin (HSFG bolting is not recommended for use on plates having a 0.2% proof stress under 230 N/mm2).

Screwing

Tapped holes in aluminium tend to be unsatisfactory.
Patentstainless-steel thread-inserts are available, which give good service on parts that have to be screwed and unscrewed repeatedly.screwed .

Welding

Welding processes


Alloys in all groups except 2000 are readily welded. Unfortunately, welding is accompanied by local HAZ softening. This occurs to a greater or lesser degree depending on the parent alloy (see section 14.7.4), except with annealed material.
The standard arc-welding process is manual inert gas (MIG), using d.c. current. This is similar to CO2 welding of steel except that the shielding gas is argon (or helium in North America). It is easy to operate and ideal for positional welds. It can be used on thicknesses down to about 2mm. With the MIG process, aluminium can be welded as easily as steel, after an initial training period. Current settings are higher and deposit areas tend to be greater.
For thin work the TIG process is used instead pf MIG. In this the arc is struck from a nonexpendable tungsten electrode, the filler wire being held in the left hand. This is an a.c. process which needs more skill than MIG. It is slower and causes more distortion.
Aluminium can be spot-welded, but with .higher energy inputs than for steel.

Filler wire

Simplified recommendations for selection of arc welding filler wire material are shown in Table 14.3. For further information refer to BS 3019 or 3571.3 Table 14.3

Adhesive bonding

Aluminium is eminently suitable for glued joints using epoxy resin, a technique successfully used for lamp posts and other components. The epoxies are attractive because of their ability to tolerate poor fit-up. Shear strengths up to 15 N/mm2 can be developed, but it is essential to guard against premature failure due to peeling from the end of a connection. An extruded tongue-and-groove feature is often a good way of preventing this.
The resin can be used cold or, alternatively, can be hot-cured to give improved strength. In the latter case the curing temperature is the same as that needed for artificial ageing. Thus, with heat-treatable alloys it is economic to order the material in the T4 condition, and rely on the hot-curing operation to harden the aluminium (up to T6).

Use of extruded sections

Availability

The relatively low cost of extrusion dies often makes it economic to design one’s own section or ‘suite’ of sections to suit the job in hand. The use of such sections can reduce fabrication costs and produce an improved final product provided, of course, the quantities are sufficient.
Extrusion is mainly confined to the 6000 and 7000 alloy groups, the order of merit for extrudability being: (1) 6063; (2) 6082 or 6061; and (3) 7019 or 7020. Complex sections, including hollows, are produced in all of these. Extrusions are also possible in 2014 (high-strength) and 5083 (high-ductility), but with severe limitations on profile and at much higher cost. Hollow sections are normally produced using a ‘bridge die’ in which a mandrel, defining the internal shape, is supported on feet locating on the body of the die (which defines the outer shape).
Since the hot plastic metal has to flow around these feet and reunite, the final section contains longitudinal welds. These cannot be seen and, in the vast majority of applications, are quite acceptable. But there are some situations where they
would be regarded as a potential danger. Hollow sections extrude more slowly than nonhollows, and thus cost more per kilogram; the die charge is also higher.
Apart from custom-made profiles, the designer has a wide range of conventional sections from existing dies to choose from, such as channels, angles, T- and !sections and boxes.
Stockists hold these, usually in 6082-T6 or equivalent. Sections are extruded in long lengths and can be supplied up
to 20 m long to meet special needs. The normal limit on length is much less than this and is dictated by handling and transport.

Limiting dimensions

Sections generally are available up to about 300 mm wide from small and medium extrusion presses. With large presses, using special die assemblies, it is possible to extrude sections up to 600 mm wide, depending on the shape. But relatively few mills contain such equipment.
The designer often wants a section to be as thin as possible, for the economy. In 6063 alloy the lower limit on thickness can very roughly be taken as the lesser of 1.0 mm and width/120. In 6082 (or equivalent) the corresponding values are l. 5fmm and width/80, while in 7019 they are somewhat more. Sections of 6063 at the limit of slenderness can be supplied on the T5 condition (air-quenched) to reduce the amount of post-extrusion straightening needed to correct distortion.

Section design


Figure 14.4 shows a few of the devices that can be incorporated in the design of extruded shapes. Figure 14.4(a) shows a lipped channel space-frame chord, which is a more efficient shape than a plain (unlipped) channel, having greatly increased local buckling resistance. The planking section (Figure 14.4(b)) incorporates various features, including integral stiffeners, interlock, and anti-slip surface. Planking sections, first developed as flooring for trucks, have also been employed in bridge decks and (after piercing) for open-work flooring. Figure 14.4(c) shows a doublesided planking section, again interlocking.

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