Being at the metal supplier’s counter and trying to choose between 6082 and 7075 is like trying to choose between two different engines in motorsport. Both will do what you need them to do, but one will be completely incompatible with the end goal you are trying to achieve. The two grades are not priced very differently either, with the difference hovering at around 20-30%. The difference in pricing is not the reason you need to choose the grade. It is about understanding what each grade is capable of and aligning that with the need at hand.
The figure that is quoted the most is tensile strength. In this case, 7075-T6 has a tensile strength of about 570 MPa while 6082-T6 has a tensile strength of approximately 310 MPa. The tensile strength of 7075 is almost double that of 6082, making 7075 seem like an obvious choice. However, this does not consider the different load conditions that the two grades might be subjected to (cyclic load, impact, etc) or how easily the two grades can be welded to each other.
Let’s begin discussing weldability because it rules out a great deal of potential applications. The 6082 grade has weldability as one of its key design features. Applying TIG welding heat to a work piece does weaken the heat affected zone, however, the zone typically still retains about 60-70% of the strength of the base material. More importantly, the gradient between the heat affected zone, and the unaffected zone is gradual. The 7075 grade, however, loses strength to welds in a catastrophic way. You can drop to 30% of the strength of the base material. If your load bearing component requires welding, 6082 is the only option that makes any sense.
This is also the case for chassis tubes, fabricated brackets, and rollcages, which unanimously use 6082, or the US equivalent 6061. These structures need joints that can be welded, and trying to use 7075 to make these structures is counterproductive. You’d just end up making a structure that fails at the welds after only a small amount of use. Sure, you would have saved a kilogram, but you built a structure that failed.
For engineered pieces that don’t need welding, 7075 gets really useful. Welding is irrelevant for suspension wishbones, uprights, wheel hubs, and pedal boxes, which are usually machined from solid billets. The greater strength means that more material can be removed while still keeping needed structural integrity. A 7075 wishbone can weigh 30-40% less than a similar part made from 6082, which is a significant advantage in reducing unsprung mass or satisfying a minimum weight requirement.
The 7075 alloy machines more easily, and this is a difference that can be seen in both the cost and quality of the end result. The 7075 alloy produces less stick, does not clog cutting tools as easily, and takes a lot less effort to create a nice finish than 6082, which is a little stickier. 6082 does not end quality machining, however, more care to the tools and cutting speeds are needed. This is of little concern in one-off fabrication, but for mass production runs, it can be a significant factor.
Corrosion resistance tips in favour of 6082. No grade is immune to corrosion. However, 7075 is more susceptible to it, especially when there is stress corrosion cracking in marine or high humidity environments. If you are racing in salt air or storing the car outside, 6082 is better for holding up over time. Anodizing does help both grades, but 7075 requires more careful prep to get a good quality finish.
Cost is different depending on the type of building you are doing. 7075 raw material costs about 30% more than 6082, but when you are machining from solid billet, material costs are only 10 to 20% of the total part cost. This is because labor and machine time are the major part of the cost. If you are buying pre cut tube for fabrication, material costs more for the project. In this case, 6082 becomes more attractive for the project.
The performance of fatigue is more intricate than simply looking at the strength numbers. 7075 grade handles high cycle fatigue pretty well, but 6082 is not far behind, especially in welded constructions where 7075 becomes the weak link due to the heat affected zone. For components which are subject to millions of cycles of stress – for example, wheel hubs – it’s best to go beyond the assumption that a material’s higher strength makes it better and look at actual fatigue data for your design.
The practical decision tree is simple enough. If welding’s involved, go for 6082. If you are solid machining and weight is of high importance, go for 7075. If the application is in a corrosive environment, or if one needs to anodize to a high standard, go for 6082. If you are building something that needs to be tough and damage tolerant rather than lightweight, 6082’s better ductility may be a plus.
Neither grade is better in absolute terms – they’re just optimized for different applications. When you match the material to the job, you are able to create components that will stand the test of time.
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