7075 or 6061 aluminium: is the stronger alloy always the better pick?
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TL;DR: 7075-T6 has about 80 % more yield strength than 6061-T6, but the same stiffness, less ductility, lower thermal conductivity, and it is usually not welded. Start with 6061 and only move up when strength is the real limit. The situation: you are designing a bracket or a frame and the numbers make 7075 look like a free win. Our archive lists 7075-T6 at 503 MPa yield and 572 MPa tensile (Kaiser, tube and pipe) against 276 MPa and 310 MPa for 6061 in T6 and T651 (ASM), with a density of 2.8 versus 2.7 g/cm3. Why would anyone pick 6061? Stiffness does not change Elastic modulus is about 71 GPa for 7075 and 68.9 GPa for 6061. If a part fails because it flexes too much, the stronger alloy will not make it stiffer. Strength stops yielding and breaking; it does nothing for deflection. Plenty of designs are stiffness-limited, and for those the extra money for 7075 buys nothing. Ductility goes the other way Elongation is 11 % for 7075-T6 and 17 % for 6061-T6 in these tables. The stronger alloy forgives less before it breaks, which matters in fatigue-sensitive or impact-prone parts and anything you bend. Heat flow Thermal conductivity is 130 W/m.K for 7075-T6 and 167 W/(m.K) for 6061. If the part carries heat (a heatsink, a bracket under an electronics box, an enclosure wall), 6061 moves it noticeably better. Welding and forming 6061 is the one people weld, and it machines and anodizes easily. 7075 is generally treated as a no-weld alloy because it tends to crack in the heat-affected zone, and it is more sensitive to stress corrosion in some tempers and grain directions. That is general industry experience, not a number from our tables, so check the temper and orientation for your part. The T6 trap These numbers are for T6. Strength depends on temper and thickness, and the Kaiser figures come from tube and pipe. Plate and thick sections can differ, so check the temper and form on the certificate. What I would do Start with 6061 and calculate the margin. Move to 7075 only when the shortfall is genuinely strength, not stiffness, heat or weld access. 7075 earns its price in highly loaded, weight-critical machined parts such as aerospace fittings, structural brackets and tooling plates. Anyone swapped 6061 for 7075 and found it did not fix the problem? I would like to hear what the real limit turned out to be.