Density and Weight Optimization in Alloy Selection

Density determines component weight and directly affects fuel consumption, payload capacity, and material cost. Specific properties (strength/density, stiffness/density) are the true metrics for weight-critical design.

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## Density of Engineering Metals | Metal/Alloy | Density (g/cm3) | vs Steel | |------------|-----------------|----------| | Magnesium AZ31B | 1.77 | 0.23x | | Aluminum 6061 | 2.70 | 0.34x | | Titanium Ti-6Al-4V | 4.43 | 0.56x | | Steel | 7.85 | 1.00x | | Inconel 718 | 8.19 | 1.04x | ## Specific Strength Specific strength = UTS / Density. Determines weight of a tension-loaded component. | Alloy | UTS (MPa) | Specific Strength (kN-m/kg) | |-------|-----------|-----------------------------| | A36 steel | 400 | 51 | | 7075-T6 aluminum | 572 | 204 | | Ti-6Al-4V | 930 | 210 | Ti-6Al-4V and 7075-T6 lead in specific strength. A titanium part carrying the same tensile load as steel weighs approximately 60% less. ## Specific Stiffness Specific stiffness = Elastic modulus / Density. | Alloy | E (GPa) | Specific Stiffness (MN-m/kg) | |-------|---------|------------------------------| | Steel | 200 | 25.5 | | Aluminum | 69 | 25.6 | | Titanium | 110 | 24.8 | | Beryllium | 287 | 155 | Steel, aluminum, titanium, and magnesium all have nearly identical specific stiffness (~25 MN-m/kg). For deflection-limited designs, changing from steel to aluminum does NOT save weight if cross-section geometry scales proportionally. Weight savings come from strength-limited designs. ## The Cost of Weight Savings | Industry | Value per kg Saved | |----------|-------------------| | Space launch | 10,000-50,000 USD | | Military aircraft | 200-500 USD | | Commercial aircraft | 100-300 USD | | Automotive (ICE) | 3-10 USD | These values guide material substitution economics. A titanium component saving 5 kg over steel is justified in aerospace (1,500 USD savings) but rarely in automotive (50 USD savings vs 100+ USD material premium).