Why Relative Density Alone Cannot Predict Copper AM Conductivity

Relative density and conductivity are related material-quality indicators, but one does not determine the other. Accept density with a defined method for continuity or porosity risk, and accept electrical or thermal conductivity with its own method, temperature, direction, specimen, final state, and functional context.

A copper AM supplier may report a high relative density and a buyer may infer high conductivity. That inference can fail even when both numbers were measured correctly.

This guide closes the test-selection decision. Use the quality and acceptance decision path for the wider evidence chain. For the broader property discussion, use the electrical conductivity guide.

Density and Conductivity Ask Different Questions

Relative density asks how much measured material density approaches a selected theoretical or reference density, or how much void content is detected by a specified method.

Conductivity asks how readily the final material carries electrical current or heat under stated conditions.

The correct acceptance package may include several rows because the part has several failure modes.

Why Two Dense Parts Can Conduct Differently

Conductivity can change with:

• Alloying additions and solute state.
• Precipitation and heat-treatment condition.
• Oxygen and other impurities.
• Oxide films and contamination at powder or melt-track boundaries.
• Pore shape, connectivity, size, and orientation.
• Microstructure, residual strain, and thermal history.
• Test temperature and direction.
• Surface condition and method suitability.
• Sampling location and build-platform position.

A density method may not detect or weight all of these contributors in the same way. A part can show similar measured relative density before and after a thermal treatment while its CuCrZr conductivity changes substantially because precipitation state changes.

The current EOS CuCrZr material and process data provides a clear route-specific example. For a named EOS process, the reported average defect percentage remains a process characteristic while typical electrical conductivity differs greatly between as-manufactured, conductivity-optimized, and tensile-optimized heat-treated conditions. The lesson is not to transfer those exact values to another supplier. The lesson is that density alone cannot identify the final precipitation and conductivity state.

Why Conductive Parts Can Still Fail with Small Defect Populations

An average bulk result can hide a critical local condition.

Examples:

• A connected pore path intersects a pressure wall.
• Lack of fusion sits at a high-current neck.
• A defect reaches a machined sealing face or threaded port.
• A thin RF wall contains local roughness or geometry error.
• A thermal interface distorts after heat treatment.
• One internal channel retains powder while the surrounding bulk is dense.

The acceptance method should therefore consider defect location and morphology, not only an average percentage. ASTM F3637-23 is a guide to relative-density measurement methods for metal AM finished parts. Its value is methodological consistency; it does not turn one density result into proof of every finished-part property.

Select and Report the Density Method

Possible routes include geometric density, Archimedes-type methods, metallographic image analysis, X-ray CT, and other qualified techniques. Each has different sample, resolution, surface, access, uncertainty, and defect-sensitivity limitations.

The report should state:

• Method and governing procedure.
• Theoretical/reference density used.
• Specimen geometry and final state.
• Build orientation and specimen location.
• Surface preparation.
• Number of specimens or fields.
• Measurement uncertainty or method limitations.
• Whether results are bulk averages, local sections, or CT-derived values.
• Acceptance rule and handling of outliers.

Do not compare an Archimedes result from one route with image-analysis or CT output from another as if they were identical metrics.

Specify Conductivity Independently

Electrical acceptance should define:

• Conductivity or resistivity target and unit.
• Test method, such as an agreed eddy-current or direct-resistance method.
• Test temperature and correction.
• Specimen or part location.
• Build and measurement direction.
• Final heat-treated and finished state.
• Surface requirements for the instrument.
• Sample count and lot rule.

Thermal acceptance should define whether thermal conductivity is measured directly or derived from diffusivity, heat capacity, and density, and at which temperature. For a finished thermal part, add the relevant assembly or fluid-system test.

The pure copper finished-conductivity specification guide provides a complete requirement structure.

Use a Property-and-Function Matrix

Density may support the material or local column. It does not replace the functional column.

Distinguish Process Qualification from Part Acceptance

Process qualification

Use density, conductivity, tensile or hardness, chemistry, and microstructure on representative specimens to establish a controlled material-process-final-state route. Include orientation and build-location strategy where relevant.

First article

Connect route evidence to the actual geometry with internal inspection or sectioning where justified, critical dimensions, finished interfaces, and part-level functional tests.

Repeat production

Use frozen process controls, witness strategy, sampling, part-level acceptance, and change notification. The exact mix should reflect process maturity and failure consequence.

NIST’s powder-bed-fusion measurement research highlights powder spreading, in-process measurement, modeling, and control as distinct measurement challenges. A high final density number does not eliminate the need for the process and finished-part evidence chain.

Avoid These Procurement Errors

• Writing “99.9% dense” with no method or reference density.
• Treating density as a guaranteed conductivity value.
• Comparing supplier values measured by different methods and states.
• Accepting CuCrZr conductivity before the property-setting heat treatment.
• Using one flat coupon as proof of a current neck or thermal interface.
• Accepting CT as proof of leak tightness.
• Using a data-sheet typical value as a drawing minimum.
• Omitting test temperature, direction, and final finishing state.

RFQ Checklist for Density and Conductivity Evidence

Provide:

1. Exact material or permitted alternatives.
2. Part function and controlling electrical, thermal, pressure, or RF requirement.
3. Required final heat treatment, machining, polishing, plating, and cleaning state.
4. Relative-density or porosity question and selected method.
5. Conductivity/resistivity target, method, temperature, direction, and specimen.
6. Critical defect locations, walls, channels, ports, interfaces, and contact pads.
7. Same-build witness relationship and sampling plan.
8. Part-level resistance, temperature rise, thermal, RF, proof, leak, or flow test.
9. Reports, calibration, uncertainty, laboratory, and traceability expectations.
10. Repeat-build and change-control requirements.

Submit these through the secure RFQ form. COPPER 3DP coordinates supplier evidence and quotation scope; project-specific acceptance values remain the responsibility of the customer and selected manufacturing route.