Custom parts
6061 is a strong general-purpose starting point. 7075 makes sense when higher strength justifies its tradeoffs.
Strength is not stiffness
7075 in common high-strength tempers can provide substantially greater strength than 6061-T6. Their elastic stiffness is much closer, so substituting 7075 does not dramatically reduce deflection in an otherwise identical shape. If deflection controls the design, section geometry may matter more than alloy strength.
Joining and corrosion matter
6061 is commonly selected where weldability and general corrosion resistance are useful. Welding still changes the local properties of heat-treated stock. 7075 is not a general-purpose welding alloy, and temper and exposure influence its corrosion performance.
Cost and availability
6061 is often an economical baseline for brackets, fixtures, and housings. 7075 can be justified for compact loaded components where the higher strength is used by the design. Stock dimensions, temper, and the amount of machining also affect total part cost.
Specify the complete material
State alloy and temper consistently in the quote and drawing. Use the actual grade data for structural calculations. A higher-strength alloy does not validate fatigue life, fastener placement, or a sharp stress concentration. Both grades remain subject to the same need for machinable geometry.