This work specifically focuses on time-averaged centerline bow shock standoff distance (Δ) which serves as a simple descriptor of the complex impinging jet flow field. Synthesis of physical and computational experiments conclusively demonstrates that a commonly cited correlation in cold spray literature involving Δ is quantitatively valid for the case of supersonic flow over stationary spheres, which is the situation for which this correlation was specifically developed. However, the present work shows that this correlation is quantitatively and qualitatively invalid for supersonic jet impingement flows relevant to CS.

This research proposes an experimental methodology towards visually observing high strain rate polymer deformation characteristics at scales relevant to cold spray particle impacts. Macro-scale (~ 3 mm) polymer impact testing via a light gas gun has shown evidence of cold spray indicative features at certain (material, particle/substrate temperature, velocity, etc.) conditions.

The polymer cold spray (CS) process has been demonstrated as a promising coating and repair technique for fiber-reinforced polymer composites (FRPs). However, a noticeable variation in coating thickness (herein referred to as checkerboard pattern) often occurs in the initial (bond) layer of low-pressure CS deposition. The checkerboard pattern occurs due to essentially periodic variations in matrix thickness above the subsurface fiber weave pattern. When the bond layer exhibits the so-called checkerboard pattern, the CS deposition for subsequent layers may be negatively affected in terms of deposition efficiency, porosity, adhesion, surface roughness, and surface thickness consistency. The present work compares results of both numerical simulations and experimental studies performed to reveal the governing mechanisms for and elimination of checker-boarding. Numerical single particle impact simulations are conducted to observe various thermomechanical domains for CS impact on the FRP surface in different regions of the composite material. Complementary experimental CS studies of exemplar powders onto FRPs with various surface interlayer thicknesses are also presented. Experimental analyzes of deposits include microstructural observations to compare against the simulations while also providing practical strategies for the elimination of checkerboarding effects.

High-performance polymers such as poly(ether ether ketone) (PEEK) are appealing for a wide variety of industrial and medical applications due to their excellent mechanical properties. However, these applications are often limited by relatively low thermal stability and conductivity compared to metals. Many methods developed to metallize polymers, including vapor deposition and thermal spray processes, can lead to poor quality control, low deposition rate, and high cost. Thus, cold spray is a promising potential alternative to rapidly and inexpensively produce polymer-metal composites. In this study, we investigated the deposition characteristics of metalpolymer composite feedstock, composed of PEEK powder with varying volume fractions of copper (Cu) flake added, onto a PEEK substrate. We prepared the Cu-PEEK composite powder in varying compositions by two methods: hand-mixing the powders and cryogenically milling the powders. Scanning electron microscopy (SEM) of the feed mixtures shows that cryogenically milling the polymer and metal powders together created uniformly distributed micron-scale domains of Cu on PEEK particle surfaces, and vice versa, as well as consolidating much of the porous Cu flake. In lowpressure cold spray, the relatively large volume fractions of PEEK in the composite mixtures allowed for lower operating temperatures than those commonly used in PEEK metallization (300-500 °C). While the deposition efficiencies of each mixture were relatively similar in single-layer experiments, deposits formed after multiple passes showed significant changes in deposition efficiency and composition in PEEK-rich feedstock mixtures. SEM of deposit surfaces and cross-sections revealed multiple co-dominant mechanisms of deposition, which affect both the porosity and final composition of the deposit. Though present in all samples analyzed, the effects of cryogenic milling were more prevalent at lower Cu concentrations.

Metallization of polymers and fiber-reinforced polymer composites is gaining attention due to the widespread application of these components in various industries, such as wind energy, aerospace, and automotive industries. Cold spray is a promising new technique to achieve the metallization of polymer and fiber-reinforced polymer composites. This work investigates the deposition mechanisms of polymer-coated metallic particles on polymer-based substrates by finite element analyses. Impact mechanics of PEEK-coated nickel particles impacting PEEK and carbon fiber-reinforced PEEK substrates are modeled. Results show the prominence of mechanical interlocking of metallic particles in the substrate, which occurs due to their entrapment inside the substrate, caused by the high energy impact-induced welding of scraped PEEK coating. The PEEK coating acts as a cushioning component, effectively mitigating the impact energy of the metallic component. The scraped PEEK coating also accumulates on the upper half of the particle, forming a cap welded to the substrate and sealing the metallic particle inside. It is observed that the depth of the carbon fiber mat in the substrate affects the mechanism and the success of deposition.

Polymers have proven to be challenging to cold spray, particularly with high efficiency and quality when using inexpensive nitrogen (N 2 ) and air propellants. Helium (He) when used as a process propellant can improve spray deposit properties but is often undesirable due to its limited availability and high cost. In this study, additives of multiple particle sizes and materials were mixed with polymer powder in an effort to improve the performance of polymer sprays using mainly N 2 as a process propellant. The effects of additives on deposit microstructure were investigated by precise ion-beam polishing of deposit cross sections and subsequent electron microscope imaging. Additional metrics including the density and post - spray composition of deposits were investigated to quantify the peening effect and the amount of embedded additive. Additives, regardless of size, were observed to embed in the spray deposits. Additionally, hard-phase additives demonstrated nozzle-cleaning properties that continually remove polymer fouling on the nozzle walls. Inversely, sprays with polymer powder and no additives tended to clog the nozzle throat and diverging section as a result of continual fouling.

The promising structural properties of fiber-reinforced polymer composites make them widely popular in the energy, automotive, defense, and aerospace industries. One of the most challenging limitations associated with the use of composites in the above applications is the maintenance and repair protocols. In this study, a novel cold spray approach is introduced as an efficient alternative for the structural repair of fiber composites. Damages in the form of circular tapered holes are created in glass fiber-reinforced polymer (GFRP) composite substrates using a conventional drilling process. The in-lab created damages are repaired by cold spray with thermoplastic (nylon 6) and thermoset (polyester epoxy resin, PER) materials. The fundamental adhesion mechanisms are investigated through microstructural observations, which point to adiabatic shear instability due to the occurrence of severe plastic deformation as a governing factor. Microstructural examinations also suggest that no significant fiber damage or surface degradation occurs after the repair by cold spray. Mechanical tests performed on neat, damaged, and repaired composites reveal the partial recovery of structural performance and load-bearing capacity after cold spray repair. Results obtained in this work highlight cold spray as a promising alternative technique for onsite structural repair of composite structures with minimal pre/post-processing requirements.

The feasibility of processing various polymers by cold spray has been exemplified by depositions with low porosity and properties comparable to the bulk material. However, cold sprayed polymers are generally deposited with low deposition efficiency compared to more extensively studied metal sprays. Low efficiencies in polymer sprays are attributed to characteristic differences in material properties between metals and polymers. Notably, the thermophysical properties of polymers limit heat transfer and promote intra-particle thermal gradients that develop during cold spray processing. These properties (e.g., thermal conductivity, heat capacity, density) and low deposition efficiencies demand alterations to the cold spray process equipment outside typical metal powder spray conditions. Herein, a modified powder feed tube is used to pre-heat powder to temperatures (~84 °C) below the powder melting point, or cool it (~-55 °C) below room temperature before contacting the high velocity carrier gas in the nozzle of a CSM 108 cold spray system. Numerical simulation demonstrated that pre-heating/cooling the powder feedstock is a viable means of adjusting particle temperature upon impact with the substrate; however, this technique has generally not been deliberately utilized in the cold spray of polymers. In the present work, no significant increase in deposition efficiency (~65% for all sprays) was found by increasing the pre-heat temperature. However, pre-heated particles had a mechanical strength 28% higher than particles injected at room temperature and -55 °C. Despite this, scanning electron microscope images indicated no notable differences between the deposit microstructures. Future works are planned to study the effect of pre-heat at higher particle impact velocities and degrees of pre-heat to improve powder consolidation.

Polymer cold spray has yielded lower deposition efficiency (DE) and quality deposits compared to metal cold spray. The disparity stems from metals being studied far longer than polymers in cold spray; in addition, polymers exhibit richer thermo-mechanical behavior. An experimental study was conducted to examine the effects of polymer feedstock degree of crystallinity (D) on cold sprayed deposits of polyetherketoneketone (PEKK), a thermoplastic used in aerospace and other high-performance applications. As deposition relies on the plastic deformation of the impacting particle, polymers with high D may inhibit deposition, reducing deposit quality and efficiency. This study evaluates three PEKK grades produced using different ratios of terephthalic (T) to isophthalic (I) monomer moieties (T/I = 60/40, 70/30, 80/20). The ratios control D, with higher proportions of T monomers corresponding to higher crystallization rates and degrees of crystallinity. A parametric study was completed to evaluate functional process set points of system carrier gas temperature and powder mass flow rate. Using operational parameters common among the PEKK grades, spray cycles were completed for each material and quantitative responses to variation in crystallinity were evaluated through a suite of analyses. DE of the materials was assessed gravimetrically, deposit porosity was evaluated by scanning electron microscopy, and thermophysical changes to the feedstock during the spray cycle were determined by differential scanning calorimetry. Overall, we found that cold spray processing of powders of lower D formed less porous deposits with a higher DE than more crystalline powders sprayed at the same process conditions. PEKK grades with lower T/I ratios achieved DEs in the range of 60-75%, whereas the most T enriched grade only reached ~10% DE.

The cold spray process is sensitive to variations in feedstock and requires consistent powder properties, particularly flowability, to produce uniform structures. Poor powder flow causes a cascade of effects arising from erratic feeding including deposits with void spaces and inconsistent geometries. These issues result in deposits which are not suitable for testing and prevent sample replication, hindering experimental evaluation of deposits. Powder flowability is largely affected by the material preconditioning and storage conditions; with flowability directly affecting the deposit properties of deposition efficiency (DE), porosity, and surface finish. In this study, the flowability and deposit quality of a fluoropolymer-based powder was evaluated with changing pretreatment conditions. Powder flowability was analyzed by mass flowrate (g/s), the Carr angle of repose, and the Hausner ratio. Flowability was evaluated for powders as received, after sieving (45-100 μm), with drying at elevated temperature (80 °C), with inert gas vacuum purging, and after 72 hrs. of exposure to high relative humidity (95% RH). Powders exposed to humid conditions were also dried under inert gas vacuum purging to determine the effectiveness of the process as a reconditioning method. Preconditioned powders with the highest flowability according to these tests were sealed in metal containers, stored under 95% RH for one week, and reevaluated to determine the ability of this preconditioning and storage method to protect materials from exposure to undesirable conditions. Next, the effect of preconditioning on cold spray deposit quality was evaluated for the fluoropolymer-based powder with the best and worst flowability. The choice of spray conditions was informed by simulation of particle velocity and temperature distribution at impact using one-dimensional compressible flow modeling, couple with thermal analysis of the powder. The DE was determined gravimetrically, surface roughness was evaluated using a profilometer, and microstructure was evaluated using a scanning electron microscope (SEM). The ability to manipulate powder flowability through simple preconditioning methods and quickly evaluate the properties of the feedstock before use in the manufacturing process, coupled with straightforward and rapid evaluation the resultant deposit; will save time and money, and accelerate research efforts, compared to evaluating powder suitability by trial and error.

In the thermal spray business quality assurance reports from a high end system usually contain only the values of the thermal spray process. But what about handling system values like spray distance and surface velocity. The paper shows the possibilities that are available today on a MultiCoat Vision thermal spray installation and focuses on a quality assurance report that includes the values of the entire system, as well as, the details from the sprayed workpiece.