Designing and Fabricating Aluminum Weldments

Aluminum’s big chance as an engineering material came with the development of inert gas-shielded welding. Now you it can be found in boats, ships, truck bodies, railroad cars, storage tanks, and many other fabrications too large or too demanding to weld with stick or gas the old ways.

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, resistance welding, and bonding:

• 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.

Base metals

The designer has hundreds of aluminum alloys to choose from, but the needs of the job narrow the choices to one or two. He eliminates alloys on the basis of strength, workability, resistance to corrosion, and weldability, and chooses the ones whose characteristics are closest to the needs of the job.

Filler metals

The weld is an alloy of filler metal and base metal that determines largely the properties of the weld. The amount of dilution of the filler metal by the base metal controls weld ductility, strength, resistance to cracking and corrosion, and heat-treatability.

Joint preparation and spacing, and the welding technique used, have a lot to do with how much base metal is melted and dilutes the weld. The greater the included angle in a butt joint, the least dilution of the filler metal. In heat-treatable alloys, a V-groove - and the right filler metal - usually produces a weld that is the least vulnerable to cracking.

Weld cracking

A filler metal containing more alloy than the base metal usually minimizes cracking. Welds made in 6061 plate with 6061 filler metal are crack-sensitive. Made in the same alloy with 4043 filler metal, welds are not crack-sensitive, because the filler metal melts and solidifies at a lower temperature than the base metal, meaning it stays plastic after the base metal has cooled, relieving weld stresses plastically. Also, high magnesium filler metals, like 5356, 5183, and 5556, strengthen welds and decrease crack sensitivity.

But don’t weld high magnesium base metals 5086, 5083, and 5456 with filler 4043. It chances low ductility and cracking, Avoid dilution; it might result in a weld alloy that is sensitive to hot cracking.

Corrosion resistance

Some environments, or chemicals may require the use of unusually pure filler metals or ones having exceptionally tight limits on alloys. Examples: for hydrogen-peroxide service, weld 5254 plate with 5654 filler metal; for certain salt-water water jobs, weld 1100 plate with 1100 filler metal.

Elevated temperatures require careful selection of both base and filler metals. For jobs working continuously at 150°F (65°C) or more, avoid using base metals containing more than 3.5 percent magnesium. A good choice for that sort of work is 5454 plate welded with 5554 filler metal.

Weld properties

High magnesium filler metals 5356, 5183, and 5556 produce strong, ductile welds. Low-alloy aluminum filler metals, like 1100, produce the most ductile — but weakest, welds. Aluminum-silicon filler metals produce the least ductile welds.

Good weld properties depend on the use of filler metals that are free of gas and nonmetallic inclusions. Also, wires, especially for gas metal-arc welding, must have clean, smooth surfaces.

Effect of welding on strength

Like every other metal, aluminum has its own metallurgical peculiarities. Welding heat weakens the base metal next to the weld if it is other than annealed (O-temper). How much the weakening affects the performance of the weldment depends on its location, orientation, and extent. That is why transverse welds in columns and beams resist buckling best at points of lateral support.

Longitudinal welds in structural members are harmless if their weak zones make up less than 15 percent of the part. Girth welds in piping or tubing may reduce resistance to buckling, but longitudinal welds reduce it hardly at all, because their weak zones are small compared to the total area of the weldment.

When welding weakens the weldment, the weak zone is about an inch either side of the center of the butt or fillet welds. Metal beyond the zone is at base metal strength. Welding affects heat-treatable (HT) and non-heat-treatable (NHT) alloys in different ways. Heat treating strengthens HT alloys, but only cold working strengthens NHT alloys.

Heat-treatable alloys

The weakest part of a weldment is the base metal affected by the heat of welding. Reduced-section specimens, designed to load the weld more than the base metal, usually break on testing in the heat-affected zone next to the weld if the weld is made right and the right filler metal is used. Stress at fracture usually exceeds table values.

How much base metal is weakened by welding, and the width of the weakened zone, depends on how long the zone is at highest welding heat. Full annealing of HT alloys usually takes sustained heating above 200°C for at least 20 minutes, depending on metal thickness.

The ultimate tensile strength of 6061-T6 as-welded is 165 MPa. Providing it was welded with the right filler metal, the assembly will regain most of the strength lost as a result of welding heat, if it is solution heat-treated and aged after welding, but will lose some ductility. But usually the treatment is either too costly or impractical. A better choice may be 6061-T4, and aging after welding. The weldment will be stronger in the as-welded condition than T6, and warping will be avoided. Base metal thickness affects the tensile strength of HT alloys.

The above mentioned alloys are those commonly used in weldments of aluminum. Now, for special jobs there are newer, stronger alloys, like the copper-free HT alloys 7004, 7005, and 7039. Compared to the older alloys, the newer ones are much less sensitive to quenching speed; they regain most of their original strength with natural (room temperature) aging and gain additional strength with artificial aging.

Non-heat-treatable alloys

Annealing cold-worked NHT alloys does not depend on time at temperature; it occurs almost as soon as the alloy reaches annealing temperature. The only way to strengthen softened NHT alloys is with cold working (rolling or stretching) after welding, which is seldom practical.

The annealed base metal is the weakest part of the NHT alloy weldment. Reduced-section tensile specimens will fail either in the weld, or in the annealed base metal, if the right filler metal has been used and the right procedure followed. Stress at fracture usually exceeds table values. Usually, the minimum as welded strengths of NHT alloys are the annealed strengths of the base metals welded with the right alloy.