A paper recently published in the journal Additive Manufacturing has demonstrated the feasibility of using cold spray additive manufacturing for carbon-fiber-reinforced plastic (CFRP) metallization with tin (Sn) coatings.
Study: Dynamic recrystallization of Sn coatings on carbon-fiber-reinforced plastics during cold spray additive manufacturing. Image Credit: Peter Sobolev/Shutterstock.com
Background
CFRP is used extensively in the automotive and aerospace industries as an effective lightweight composite material. However, CFRP possesses a low electrical conductivity, which is a major drawback as lightning strikes can severely damage CFRP components in aircraft, posing a risk to the crew and passengers.
Several lightning strike protection (LSP) solutions were developed and investigated to address this issue. Among these solutions, polymer surface metallization by depositing a coating over CFRP can effectively increase the electrical conductivity of the composites.
Cold spray, an additive manufacturing process, primarily involves powder spraying at low temperatures that lead to the deformation of particles after their impact on the substrate. Solid-state densification occurs in this process due to the kinematic energy of the particles accelerated to a high velocity through a converging-diverging needle.
Moreover, the high material deposition rate of 100 g min−1 and above based on the processing condition can help in decreasing the manufacturing costs. Thus, the cold spray technique is considered a suitable surface treatment technology for polymer-based material metallization with the least thermal damage during cold spray processing.
Metal coatings were deposited successfully on the thermoplastic CFRP substrates using the cold spray technique. However, thermosetting CFRP surface metallization using the cold spray technique was challenging due to the surface damage by erosion.
Sn has attracted considerable attention as a coating material for LSP through cold spraying due to its ease of deformation against polymer-based substrates and low melting point. Thus, cold-sprayed Sn coatings can act as an effective LSP for CFRP.
A continuous Sn coating was applied successfully to epoxy-based thermosetting CFRP substrates using a low-pressure cold spray technique at gas temperatures between 473 and 523 K. However, the cold-sprayed coating was hindered at temperatures above 573 K due to the multi-laminate erosion of the CFRP substrate.
Dynamic recrystallization (DRX) is a form of recrystallization that takes place during deformation. During cold spraying, DRX occurs exclusively at substrate-particle and/or particle-particle interfaces. For instance, the formation of ultrafine grains was observed in a study when a nickel coating was deposited on a carbon steel substrate. Similarly, DRX was observed at the interface between a copper substrate and a cold-sprayed copper particle.
Although previous studies have investigated DRX on the coatings during the cold spraying process, the microstructural evolution of the deposited Sn coatings and the impact of the evolution on the mechanical properties of coatings have not been studied sufficiently until now.
The Study
In this study, researchers deposited pure Sn coatings on an epoxy-based thermosetting CFRP composite using a cold spray additive manufacturing technique with three different pass numbers of 96, 48, and 24 at 473 K gas temperature. Later, they investigated the effects of variable pass numbers on the mechanical properties and microstructural evolution of the Sn coatings and analyzed the DRX occurrence in the deposited Sn coatings after cold spraying.
In cold spray processing, the pass number is a variable parameter that can significantly impact the coating quality. Thus, the pass number is a crucial factor in additive manufacturing. Liquid nitrogen was used to maintain the cryogenic temperature at the sample holder during sample preparation to avoid thermal damage.
Water-atomized Sn powder, with an average particle diameter of 24 μm, was used as the starting material. Non-spherical Sn particles were obtained by attaching satellite particles to coarser particles. The CFRP composite was composed of carbon fiber reinforcements and a thermosetting epoxy matrix. Acetone was used to degrease the 30 × 30 × 2 mm3 CFRP samples before the cold spraying process.
The Sn coatings were deposited using a commercial cold spray system. A de Laval nozzle with a tailored converging-diverging structure was employed to deliver the heated compressed air. The material deposition rate and spray distance were maintained at 7 g min-1 and 20 mm, respectively. An as-rolled Sn plate was used as a reference for the study.
An InfRec R300 high-resolution infrared thermography was utilized to measure the surface temperature of the samples during cold spray experiments. The cross-sectional morphology and surface profile were characterized using a wide-area three-dimensional (3D) measurement system.
X-ray computed tomography measurements and electron backscatter diffraction (EBSD) analysis were performed to evaluate the porosity of Sn coatings and monitor the microstructural evolution of the coatings, respectively. Transmission electron microscopy was used to characterize the Sn coatings. Vickers hardness tests were performed to determine the Vickers hardness values.
Observations
Pure Sn coatings were deposited successfully on the thermosetting CFRP using the cold spray technique. The temperature at the Sn coating surface measured in real-time was lower compared to the gas temperature.
However, the temperature reduced with the increasing pass number due to the increased Sn deposition volume as Sn possessed a higher thermal conductivity compared to the CFRP substrate.
The grain size of the cold-sprayed Sn coatings decreased gradually with the increasing pass numbers. Relatively finer-grained homogenous microstructures with a 4.5 µm average grain size were obtained with 96-pass deposition.
The geometrically necessary dislocation (GND) density in grain interiors/ in-grain local misorientation of Sn coatings increased with the increasing pass number, indicating continuously accumulated dislocations during multipass cold spraying.
The simultaneous observation of dislocation substructure formation and grain refinement indicated DRX in the Sn coatings during the multipass cold spraying process. The occurrence of DRX significantly improved the strength of the coating microstructure.
The relatively high normalized deformation temperature (T/Tm) during the Sn cold spraying process accelerated the DRX kinetics. Additionally, the tamping effect due to repeated particle collisions increased the DRX grain volume fraction. The DRX fraction increased with the increasing pass number and eventually exceeded 60% after the 96-pass deposition.
The Vickers hardness of the deposited coatings increased considerably with the increasing pass number, leading to a more homogeneous hardness distribution in the building direction, which indicated a synergistic strengthening effect of accumulated dislocations and grain refinement.
To summarize, the findings of this study demonstrated the effectiveness of cold spray additive manufacturing of Sn coatings as an LSP method for metallization of CFRP.
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Source
Sun, J., Zhou, S., Chiba, A. et al. Dynamic recrystallization of Sn coatings on carbon-fiber-reinforced plastics during cold spray additive manufacturing. Additive Manufacturing 2022. https://www.sciencedirect.com/science/article/pii/S221486042200344X?via%3Dihub
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