Despite its long-term cost-saving and performance-related implications, the selection of aluminium alloys is frequently overlooked. This choice often comes under the spotlight when component failures occur on the shop floor. For fabricators, machine shops and OEMs purchasing plate, sheet, bar, and extrusions, the distinction between 5000- and 6000-series alloys is critical. This distinction affects weldability and machinability, a component’s performance in service due to corrosion, and other service-related performance factors.
The 3000 and 5000 series, which are predominantly non heat-treatable, acquire their strength from work hardening and, in the case of the 5000 series, from magnesium as the primary alloying element. These alloys are typically employed where welding is a primary process. Magnesium-alloyed aluminium retains strength adjacent to a weld and does not experience the softening that can be observed in certain heat-treatable grades. For this reason, 5083 and 5052 are extensively utilised in the construction of marine structures, pressure vessels, storage tanks, and transport equipment. They also exhibit excellent resistance to corrosion in marine and industrial environments, making them an excellent choice for facilities in the vicinity of marine and/or industrial environments.
Heat treatable alloys of the 2000, 6000 and 7000 series gain strength through a process of controlled age and not through work hardening. The 6000 series alloys, such as 6082 and 6061, provide good strength, adequate weldability (with the correct filler), and moderate strength when combined with a good machinability rating and moderate cost. Because of these pros and cons, 6000 series aluminium alloys are used for structural frames, machining bases, jigs, and fixtures as well as general machine fabrication, given the optimum balance of strength, formability, and cost. 7000 series alloys, which have a higher strength, are built predominantly with zinc as the primary alloying addition, and provide higher strength, but at the expense of weldability and stress corrosion resistance. These alloys are used primarily in the aerospace industry, or other applications where high performance and demanding structural loads are required.
Machinability of the material that is to be constructed is of great importance. The trade-off that is made between weldability and corrosion resistance and improved machinability in free-machining grades that contain lead, bismuth and higher silicon must be addressed. It is important that the priorities of the end use of a component are addressed by the design and procurement teams. If improved machinability is of utmost concern, then a free-machining grade may be used despite its lack of weldability and increased potential for corrosion. Likewise, the balance between all three must be considered to avoid a sub-optimal situation if a single ‘do everything’ grade is specified.
Stock management offers significant savings opportunities to prospective buyers and facilities managers engaging in long-term production. As aluminium alloys undergo surface oxidation and stress relief after periods of time, defining a grade and gauge range with the support of a forecast, rather than ordering reactively, enables a stockholder to offer lower unit pricing due to their ability to stock more consistently. When a business can allocate a range of products for a long-term supply, working with a specialist supplier to secure availability (rather than ordering reactively grade by grade) helps reduce both lead time and cost risk.
Prior to committing to a grade, certification is another area worth checking. For safety-critical and pressure-bearing applications that will have a high level of exposure or a high degree of structural integrity, it is imperative to request mill test certificates, due to potential variation in the chemical composition and mechanical properties, and verify that the temper designation of the certificate agrees with the temper in the design calculation.
In general manufacturing, striking the appropriate balance of joining methods, machining, environmental exposure, and procurement strategies is essential. The specification stage, rather than the stage after fabrication has begun, is always the fastest and least costly option for ensuring the final product performs as required.