Impact Toughness Testing of Cold-Sprayed Copper Parts: Standards, Methods and Applications

Impact toughness testing evaluates the ability of cold-sprayed copper parts to absorb energy under high-strain-rate loading, delivering essential data for process qualification and structural reliability assessment. Charpy V-notch tests performed to ISO 148-1, ASTM E23, and GB/T 229 quantify absorbed energy, fracture appearance, and directional anisotropy in cold spray deposits. This article explains specimen preparation, test procedures, evaluation criteria, and typical applications for impact toughness testing of cold-sprayed copper components.

Technical Overview

Cold spray, also known as cold gas dynamic spraying (CGDS), is a solid-state deposition process in which fine copper powder particles (typically 5–45 µm) are accelerated to supersonic velocities of 300–1200 m/s by a heated high-pressure gas stream (nitrogen or helium) through a de Laval nozzle. Upon impact, the particles plastically deform and bond metallurgically without bulk melting, allowing oxygen-sensitive and highly conductive materials such as copper to be deposited with minimal oxidation and low residual thermal stress.

Impact toughness, defined as the energy absorbed by a material during fracture under dynamic loading, is a critical mechanical property for cold-sprayed copper parts used in load-bearing, vibration-exposed, or impact-critical environments. Because cold spray deposits solidify as stacked, plastically deformed particles, their toughness is governed by interparticle bonding quality, residual porosity (typically below 1% for optimized copper deposits), work hardening introduced during deposition, and the resulting microstructural anisotropy between the build direction and the in-plane direction.

Charpy V-notch impact testing is the most widely accepted method for characterizing the impact toughness of cold-sprayed copper. As-sprayed deposits generally absorb only a fraction of the energy of fully annealed wrought copper due to incomplete particle bonding and strain hardening; however, appropriate post-deposition heat treatment or hot isostatic pressing (HIP) can substantially restore ductility. Impact toughness testing therefore serves as a sensitive indicator of deposition quality and an effective tool for validating process parameters and post-processing schedules.

Testing Samples

Freestanding Deposit Specimens

Copper deposits are sprayed onto sacrificial or removable substrates and then detached to produce freestanding blocks from which standard impact specimens are machined. This approach characterizes the intrinsic toughness of the cold-sprayed material without the influence of a substrate, and is the preferred configuration for material data sheets and process qualification.

Full-Size and Sub-Size Charpy Bars

The standard Charpy specimen measures 10 mm × 10 mm × 55 mm with a 2 mm deep V-notch machined at a 45° included angle with a 0.25 mm root radius. When the deposited copper thickness is insufficient for a full-size bar, sub-size specimens of 7.5 mm, 5 mm, or 2.5 mm thickness are machined to the requirements of ASTM E23 and ISO 148-1, and the reduced dimension is clearly reported with the results.

Orientation-Specific Specimens

Because cold-sprayed copper is mechanically anisotropic, specimens are machined in defined orientations relative to the spray passes and build direction. Notch orientations are documented so that toughness perpendicular and parallel to the layer interfaces can be compared, revealing the degree of anisotropy and the quality of interlayer bonding.

Post-Processed Condition Samples

Specimens are frequently tested in multiple metallurgical conditions, including as-sprayed, stress-relieved, annealed, and HIP-processed states. Comparing absorbed energies across these conditions allows engineers to select heat treatment parameters that optimize the toughness–strength balance for the intended service environment.

Testing Items

Impact toughness test programs for cold-sprayed copper parts typically include the following evaluated parameters:

• Absorbed impact energy (KV2 or KV8), expressed in joules, determined from the pendulum rise after fracture
• Lateral expansion at the compression side of the fractured specimen, in millimeters, as a measure of ductility
• Percent shear (ductile) fracture appearance, assessed against a fracture appearance chart or by image analysis
• Crack initiation energy and crack propagation energy, obtained through instrumented impact testing
• Force–displacement curves characterizing yield, maximum force, and fracture behavior under dynamic loading
• Temperature-dependent toughness values at sub-ambient, ambient, and elevated test temperatures
• Anisotropy ratio comparing toughness in different specimen orientations relative to the build direction
• Fractographic examination of the fracture surface by scanning electron microscopy (SEM) to identify porosity, unbonded particle interfaces, or intergranular separation

Testing Methods

Specimen Preparation and Notching

Specimens are machined by milling or grinding with controlled coolant to avoid altering the microstructure of the cold-sprayed copper. The V-notch is machined carefully to achieve the specified depth, angle, and root radius, since notch geometry strongly influences the measured energy. Surface finish requirements per ASTM E23 and ISO 148-1 are verified by dimensional inspection before testing, and each specimen is permanently identified with its orientation and processing condition.

Charpy Pendulum Impact Test Procedure

Testing is performed using calibrated pendulum impact machines with capacities appropriate for copper, commonly 150 J, 300 J, or 450 J, in accordance with ISO 148-1, ASTM E23, and GB/T 229. The procedure includes verification of pendulum calibration, anvil and support geometry, and specimen centering within 0.5 mm of the anvil midspan. The specimen is fractured by a single pendulum swing, and the absorbed energy is recorded. For tests at controlled temperatures, specimens are conditioned in liquid media or environmental chambers and broken within the specified time window after removal, typically five seconds.

Instrumented Impact Testing

Instrumented systems fitted with calibrated strain gauges or force sensors record the dynamic force–displacement response throughout the test, in accordance with ISO 14556 and ASTM E2298. This provides a decomposition of total absorbed energy into crack initiation and crack propagation components, which is particularly valuable for cold-sprayed copper because it distinguishes well-bonded, ductile deposits from deposits that initiate cracks readily at weak particle interfaces.

Data Evaluation and Reporting

Test reports issued by accredited third-party laboratories include specimen dimensions, notch orientation, machine capacity, test temperature, individual and mean absorbed energy values, lateral expansion, shear fracture appearance, and instrumented curves where applicable. Validity criteria are checked, including complete or qualified partial fracture and energy values within the calibrated range of the machine. Statistical scatter is reported because the stochastic distribution of porosity and unbonded interfaces in cold-sprayed material can produce wider data dispersion than in wrought copper.

Applications

Impact toughness testing of cold-sprayed copper parts supports a broad range of industries and engineering decisions:

• Qualification of cold spray additive manufacturing processes for copper tooling, brackets, and structural demonstrators in aerospace programs
• Validation of repair procedures for copper alloy components such as bushings, bearings, electrical contacts, and conductors
• Acceptance testing of conductive copper deposits on busbars, connectors, induction coils, and grounding hardware in the electrical power industry
• Verification of copper heat sinks, heat exchanger fins, and thermal management deposits used in power electronics
• Assessment of electromagnetic interference (EMI) shielding and corrosion-protection copper coatings on marine and defense hardware
• Development and verification of process parameter windows, powder lot acceptance, and post-deposition heat treatment schedules
• Comparative studies of nitrogen versus helium propellant gas, powder morphology, and nozzle design on deposit quality

FAQ

What impact energy values are typical for cold-sprayed copper?

As-sprayed cold-sprayed copper commonly absorbs a few to several tens of joules in a full-size Charpy V-notch test, depending on porosity, particle bonding, and strain hardening. Fully annealed wrought copper, by contrast, is extremely ductile and often does not completely fracture, absorbing energy at or above machine capacity. Optimized cold spray deposits combined with suitable annealing can recover a large proportion of wrought-level toughness.

Which standards govern the impact toughness testing of cold-sprayed copper parts?

The primary standards are ISO 148-1, ASTM E23, and GB/T 229 for pendulum impact testing, supplemented by ISO 14556 and ASTM E2298 for instrumented impact testing. Accredited third-party laboratories apply these standards together with client specifications or industry process documents for cold spray deposition.

Why is the toughness of cold-sprayed copper lower than that of wrought copper?

Residual porosity, incompletely bonded particle interfaces, and extensive work hardening introduced during high-velocity particle impact all reduce the energy absorbed during fracture. Toughness can be improved by optimizing gas temperature and pressure, using higher-velocity gas mixtures, spraying with high-quality spherical powder, and applying post-deposition annealing or hot isostatic pressing to close porosity and strengthen particle bonding.

Can sub-size specimens be used when the deposit is thin?

Yes. When the deposit thickness is below 10 mm, sub-size Charpy specimens of 7.5 mm, 5 mm, or 2.5 mm thickness are tested, and the results are reported with the specimen dimensions clearly stated. Because absorbed energy scales with cross-section, sub-size values are not directly comparable to full-size values and are best used for relative comparisons within a test series.

Is anisotropy a concern in impact test results?

Yes. Cold-sprayed copper typically shows higher toughness when cracks must cross well-bonded particle layers than when cracks propagate along interlayer or interparticle boundaries. Testing specimens in complementary orientations quantifies this anisotropy and confirms whether the deposit is fit for the intended loading direction.

How should fracture surfaces be examined after testing?

Fracture surfaces should be examined visually and by scanning electron microscopy to classify the fracture mode and identify processing-related defects. Ductile dimpled fracture with uniform deformation indicates well-bonded material, while flat facets at particle boundaries, unbonded interfaces, and clustered porosity indicate deposition conditions that require optimization.