Thermal Conductivity in Metals and Alloys

## What Is Thermal Conductivity? Thermal conductivity (k) is the rate of heat flow per unit area per unit temperature gradient, in W/(m-K). Fourier's law: Q = -k x A x (dT/dx). ## Values for Common Metals | Metal/Alloy | k at 25 degrees C (W/m-K) | |------------|---------------------------| | Silver | 429 | | Copper (C11000) | 391 | | Aluminum (1100) | 222 | | Aluminum (6061-T6) | 167 | | Carbon steel (1018) | 51 | | 304 Stainless | 16 | | Titanium (Grade 2) | 16 | | Inconel 625 | 9.8 | ## Why Alloying Reduces Conductivity Heat is carried by free electrons in a regular lattice. Alloying disrupts periodicity, scattering electrons. Pure copper: 391 W/m-K. Cupronickel (70Cu-30Ni): 29 W/m-K (93% reduction). ## The Wiedemann-Franz Law Thermal and electrical conductivity are proportional in metals (k/sigma = L x T) because both are carried by free electrons. ## Engineering Applications **Heat exchangers**: Fluid-side resistance usually dominates. A 316L tube (k=16) has only 5-10% worse overall U than copper (k=391) in turbulent flow. **Heat sinks**: Aluminum 6063 (k=201) dominates due to conductivity + low density + extrudability. Copper for high-power-density applications. **Welding**: High conductivity (copper, aluminum) dissipates heat rapidly, requiring higher heat input. Low conductivity (stainless, titanium) concentrates heat, enabling deep penetration but increasing distortion. **Thermal stress**: Low thermal conductivity materials develop steeper temperature gradients, increasing susceptibility to thermal shock and fatigue.