Stop the Steel
This nugget discusses the many applications where aluminum is becoming the metal of choice over steel.
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The industrial revolution has been driven by use of steel as the material of choice for structural applications – bridges, ships, cars, trucks, pipelines, buildings, and many others. One of the salient benefits of using steel is the ease of welding – a process commonly used for assembly and fabrication. However, one of the main drawbacks of steel has been losses due to poor corrosion resistance. By some estimates, it costs the United States $275 billion dollars a year in losses due to corrosion. An alternative structural alloy is aluminum which has great natural resistance to corrosion due to a tenacious oxide film on the surface. Additionally, aluminum has better malleability that allows it to be easily extruded, and also can be cast to form complex shapes. While aluminum alloys are not as strong as steel, their density is much lower than steel resulting in strength-to-weight ratio that is actually better than steel. The main drawback for wider use aluminum has been greater challenges in welding. However, advances in welding and joining technologies (in addition to well established welding processes such as resistance welding and arc welding) has now made aluminum alloys a worthy alternative, and in some cases preferred option for mobile applications such as electric vehicles. In this newsletter, we will review the advances in welding and joining technologies for aluminum and benefits of using aluminum alloys over steel.
A major breakthrough in welding of aluminum occurred in early nineties with the invention of the friction stir welding process. The process uses heat generated from friction to soften aluminum and then stir it with the rotor to mix aluminum alloy from both components to form a bond. While commonly used for butt welding, it can also be used for lap welded designs. Applications include rocket fuel tanks, heat exchangers, and even thin sheets used to make computer monitor housings. Since it is not a fusion welding process, aluminum alloys that are otherwise considered difficult to weld using existing arc welding techniques, can now be welded with this solid-state process that prevents loss of volatile constituents such as lithium during arc welding processes. Aluminum alloys also undergo considerable volume contraction when solidifying from liquid to solid state resulting in significant distortion which can avoided by friction-stir welding.
Solid-state welding processes can also be employed in situations where aluminum has to be welded to other metals but fusion welding is not a viable option due to formation of brittle intermetallic compounds. Commonly used processes for solid-state welding are ultrasonic welding, friction welding, and roll bonding. Roll bonding allows manufacture of bimetal strips where one side is aluminum and the other side can be another alloy such as copper. A bimetal connector now allows for welding the aluminum side to an aluminum component, and copper alloy to the other component with traditional processes. When large area bonding is required, explosion welding is an option – yes, there is a process called explosion welding which uses explosives to force two parts together with extreme force that produces a strong solid-state bond. Here again, a bimetal component can be machined out of the welded structure and used as a suitable transition structure.
Another development since 2000 is the rapid improvement in laser beam quality with fiber and disk lasers. The higher beam quality and higher peak powers have allowed for use of smaller spot size at higher power densities to the point where aluminum can be easily fusion welded with laser beams that produce deep and narrow welds, mimicking profiles produced with electron beam welding. Higher peak power also allows for rapid linear speed during welding resulting in very narrow heat affected zones which limits the loss of strength due to welding that is encountered with arc welding processes. Rapid heating and cooling during laser welding also avoids excessive heat dissipation from the weld and the corresponding collateral damage to sensitive components in the vicinity.
In situations where welding is not the preferred mode of joining, Aluminum is also well suited for adhesive bonding. Adhesive bonding has many benefits including uniform distribution of loads over larger areas, good fatigue life, capability to provide vibration damping, formation of sealed joints along with bonding, and avoids degradation of aluminum strength due to welding. With proper preparation of component surfaces with pre-treatments, adhesive bonds are quite strong and have a long life suitable for structural applications.
Similar to steels, Aluminum also offers a wide variety of alloy options based on families as listed in the table below along with common applications.
Use of aluminum, already dominant in aircraft construction and with a substantial share in rail and marine transportation, is exhibiting rapid growth in the automotive industry as EVs and hybrids are becoming widely popular all over the world. By some industry estimates, use of Aluminum will be ten times present usage levels in the next two decades; new options for welding and bonding aluminum will play a significant role in this growth. A design engineer would do well to give aluminum serious consideration as a worthy alternative where steel would have been the material of choice till now.