Quoting a batch of 50 drone arms. 7075-T6 comes out about 35% more expensive than 6061-T6 on the quote. The arm is 180 mm long, 6 mm thick, with pockets.
Is the stiffness gain real for this geometry, or am I paying for strength I don't need? Fatigue is the main concern — motors produce constant vibration.
For a 6 mm-thick arm the stiffness is identical — both alloys have E ≈ 69–72 GPa. 7075 buys you yield strength (~500 vs ~275 MPa) and better fatigue, not stiffness.
If vibration fatigue is the concern, the honest fix is geometry: add a 1.5 mm rib along the pocket floor and fillet every internal corner to ≥ 2 mm. That gives you more fatigue margin than the alloy swap and costs almost nothing in machining time.
Go 7075 only if the arm is weight-limited and you can't add material.
Agree with Lukas. One practical note from the shop floor: 7075 machines beautifully but is much less forgiving if you later want to bend or weld anything. 6061 keeps your options open.
The 7075 (Al-Zn) alloy series boasts the highest tensile and yield strengths among aluminum alloys; specifically, the 7075-T6 alloy—having undergone heat treatment—exhibits strength and rigidity comparable to ordinary steel. However, it is difficult to bend and challenging to weld. Drone motors typically operate at speeds around 10,000 rpm, generating significant vibration. Materials lacking sufficient flexibility are prone to cracking under high-frequency mechanical vibration; indeed, structural failure and wear caused by cracking are common issues in the housings of high-speed rotating machinery. The 6061 alloy is the most common aluminum alloy; it is cost-effective and offers good flexibility, along with respectable rigidity and fatigue strength. While knowing the motor's power output would allow for a more precise recommendation regarding structure and thickness, I believe that using the 6061-T6 alloy is the most rational choice from both structural and economic standpoints.
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