How to Improve the Thermal Conductivity of Aluminum Alloys

Aluminum’s intrinsic high thermal conductivity is one of its most valuable attributes for heat-transfer and thermal-management applications.

Alloying, processing and defects reduce that conductivity—often substantially.

Improving the thermal conductivity of an aluminum alloy therefore requires a systems approach: choose the right alloy chemistry, control melting and solidification, apply appropriate heat treatments, and use forming and thermomechanical processing to reduce defects and optimize microstructure.

This article explains the physical mechanisms that control heat conduction in aluminum alloys and provides concrete, actionable strategies to raise conductivity while acknowledging the trade-offs with mechanical properties and manufacturability.

1. Why alloy thermal conductivity changes — the physics in brief

Heat conduction in metals is dominated by electrons. Two microscopic factors reduce thermal conductivity relative to pure aluminum:

• Solute scattering: Substitutional alloying atoms (Si, Mg, Cu, Mn, Cr, Li, etc.) and second-phase particles scatter conduction electrons, reducing the mean free path and hence thermal transport.
  Different solutes have different scattering strengths; some (Cr, Li, Mn) cause large reductions per at.% while others (Sb, Cd, Sn, Bi) have little measurable effect.
• Defect scattering: Point defects, dislocations, grain boundaries, porosity and oxide/inclusion particles further scatter electrons and phonons and lower conductivity.

A useful practical corollary: anything that reduces electronic scattering (lower solute content, fewer defects, fewer second phases, higher purity and larger mean free paths) tends to increase thermal conductivity.
The Wiedemann–Franz relation also links electrical and thermal conductivity in metals, so measures that increase electrical conductivity will often raise thermal conductivity as well.

2. Alloy selection and chemistry: the primary lever

Strategy: choose alloys or compositions with minimal strong-scattering elements for the application.

• Prefer high-purity/aluminum-rich alloys when conductivity is paramount (commercially pure Al grades have thermal conductivity near that of pure Al ≈ 237 W·m⁻¹·K⁻¹).
  Commonly used structural alloys have lower conductivity—typical cast Al alloys fall in the 100–180 W·m⁻¹·K⁻¹ range—because of alloying and porosity.
• Avoid or minimize elements that strongly reduce conductivity. The elements Cr, Li and Mn produce large conductivity penalties per unit concentration; where feasible their content should be minimized.
• Use low-alloy or binary systems (e.g., Al–Si with low Si, or Al with small Mg) rather than multi-component alloys when thermal conductivity is a priority.
• If mechanical strength is required, consider dual-approach: use a higher-conductivity core alloy where heat flow matters, and attach higher-strength local reinforcements mechanically or by local alloying/heat treatment.

Trade-off: reducing alloy content often reduces strength. The selection must balance conductivity with required mechanical performance.

3. Heat treatment: recover conductivity by removing solute/defects

Heat-treatment strongly influences conductivity because it changes solute distribution and defect density.

• Quench/age (T-tempers): Rapid quenching can retain a supersaturated solid solution (increasing electron scattering) and thus lower conductivity.
  Subsequent aging (precipitation) can improve conductivity by removing solute from the matrix into precipitates, but the net effect depends on precipitate type, size and distribution.
• Annealing / stress relief: Annealing reduces dislocation density and relaxes lattice distortions from cold work, yielding improved electron transport and higher thermal conductivity.
• Practical guidance: If parts are quenched for strength, consider controlled aging schedules that balance strength and conductivity; where conductivity is critical, prefer annealed tempers or lighter quench severity and then age to form coarse, electrically less-active precipitates.

4. Melting, refining and melting practice: reduce gases and inclusions

Defects introduced in the melt and casting stage (gas porosity, oxide films, non-metallic inclusions, bifilms) are major conductivity killers.

Key process controls:

• Degassing: use rotary degassing (argon or nitrogen) to remove dissolved hydrogen and minimize gas porosity.
• Fluxing & cover salts: appropriate covering agents (e.g., NaCl–KCl mixtures) and fluxes reduce oxidation and inclusions.
  The use of chlorinated refining agents (historic practice: hexachloroethane) can remove dissolved gases—apply modern, controlled practices and appropriate environmental controls.
• Melt cleanliness and filtration: ceramic filtration and careful skimming reduce oxide entrainment and bifilm formation, which improve both conductivity and mechanical properties.
• Temperature control: maintain an optimal superheat (user-recommended window is often ~700–750 °C for many Al melts) to avoid excessive dissolution of refractories or oxide generation; avoid overheated melts that increase gas pickup.
• Alloying additions control: add alloying elements in controlled fashion and avoid tramp impurities (Fe, Ni, Co) that can form coarse intermetallics or reduce conductivity.

5. Forming and thermomechanical processing: reduce porosity and refine microstructure

Mechanical working after solidification is an effective way to eliminate casting defects and improve conductivity:

• Hot extrusion / forging / rolling: these processes close porosity, break up and redistribute inclusions, and refine dendritic segregation. Increased deformation and subsequent recrystallization produce fine, equiaxed grains with fewer large defects—favourable for conductivity.
• Severe plastic deformation (SPD) and controlled hot working produce fine, homogeneous microstructures that reduce electron scattering from large inhomogeneities.
• Directional solidification / use of chills: during casting, control solidification to reduce segregation and form smaller dendrite arm spacings that respond better to subsequent homogenization treatments.
• Hot isostatic pressing (HIP): for critical components, HIP effectively eliminates internal pores and improves thermal and mechanical uniformity.

6. Practical roadmap & prioritized interventions

Use a staged approach that targets the largest gains first:

1. Alloy choice: pick the least alloyed, highest-conductivity alloy that meets strength/corrosion needs.
2. Melt practice: implement degassing, flux cover, filtration and strict temperature control to minimize pores and inclusions.
3. Casting route selection: prefer processes that yield low porosity (permanent-mold, squeeze casting, investment casting with vacuum) for heat-critical components.
4. Post-casting densification: use HIP for critical applications.
5. Thermal processing: anneal or design aging treatments to precipitate solute out of solution when possible.
6. Forming: apply extrusion/forging/rolling to close residual porosity and homogenize the microstructure.
7. Surface and joining practices: avoid weld zones and heat tints on primary heat paths; if welding required, plan localized treatments to restore conductivity where feasible.

7. Conclusions

Improving aluminum alloy thermal conductivity is a multidisciplinary task combining alloy design, melt metallurgy, heat treatment and forming.

Start with material selection—only then optimize process controls (degassing, filtration, casting method), followed by heat-treatment and mechanical processing to close defects and tune microstructure.

Where conductivity is mission-critical, quantify targets, require electrical/thermal testing, and accept the necessary trade-offs between mechanical strength, cost and manufacturability.

References: https://langhe-industry.com/how-to-improve-the-thermal-conductivity-of-aluminum-alloys/