Carbon fibre reinforced polymer (CFRP) is a very competitive alternative to aluminum for aircraft structures for lightweighting purposes, but this leaves vulnerability against lightning strike. Cold spray is one coating approach to make the polymers lightning strike proof. The aim of this work is to use cold spray on aircraft quality CFRPs to produce a metal coating with sound mechanical properties and good electrical conductivity for lightning strike protection. Copper, aluminum and tin were used as the coating materials and cold sprayed with both a high-pressure and a low-pressure cold spray system. A number of different combinations of the gas pressure and gas preheating temperature were used for the cold spray process. Erosion was found to be the key obstacle to developing continuous coatings on the CFRP substrates with the high-pressure system. On the other hand, continuous tin coatings were successfully obtained on the CFRP with the low-pressure system, due to the very soft tin coating the substrate through a “crack filling” mechanism; a process map was generated to define appropriate cold spray conditions. The tin/CFRP adhesion, which was assessed by pull-off tests, revealed the bonding was mainly mechanical since the adhesion strength was at a low level. Copper, aluminum and zinc powders were mixed with tin with the aim of increasing conductivity, and the addition of copper and zinc led to much higher deposition efficiencies compared to the pure tin coating. To understand, more generally, the cold sprayability of metal powders on polymeric substrates, powders of various compositions and characteristics were sprayed on different polymer substrates. Thermoplastic polymers generally showed positive results; in particular, thick copper coatings were successfully deposited on two thermoplastics. The electrical conductivity of the cold sprayed coatings was measured by the four-point conductivity measurement method. The conductivity in the as-sprayed tin coating was approximately half of that of bulk tin, and increased to 73 per cent as conductive as bulk tin after annealed at 80°C for 72 hours. The addition of copper generally did not increase the overall conductivity due to the growth of the more resistive intermetallics. Last but not least, continuing current injection tests, which duplicated component C of a lightning waveform, were performed on the tin coatings. The results showed that the cold sprayed tin coatings can provide effective protection to the CFRP underneath when subjected to a 100A current.
Tian Liu, Jeremy D. Leazer, Sarath K. Menon, Luke N. Brewer.
This work describes the micro- and nanostructures of inert gas atomized Al-Cu alloy powder particles over a range of compositions. For cold spray deposition, the microstructure and elemental distribution within the feedstock powder play a critical role in determining the final microstructure and properties of the fabricated material. A series of Al-Cu binary alloy powders, ranging from 2 to 5 wt% Cu, were produced by inert gasatomization and were then thoroughly characterized using electron microscopy and X-ray diffraction. The gas atomized powder particles, nominally 20 μm in diameter, presented a cellular structure with θ (Al2Cu) phase forming along the cell boundaries. The continuity and phase fraction of the θ phase increased systematically with copper content. No Guinier-Preston zones were observed in the powders, but small, incoherent θ phase particles were observed in the matrix near the cell boundaries. The as-atomized particles were observed to be in a non-equilibrium state with a reduced amount of θ phase and altered lattice parameters for the α and θ phases. A mild, post-atomization annealing returned the θ phase fraction and lattice parameters to their equilibrium values.
This paper examines the fatigue properties of freestanding AA7075 and AA2024 Cold Spray (CS) deposits. In this study, high pressure cold spray was used to make deposits large enough to machine mechanical test specimens free from the substrate. To quantify the mechanical performance, monotonic tensile and stress-controlled fatigue experiments were carried out on the freestanding CS and wrought AA7075 and AA2024 specimens. The experimental fatigue results revealed a reduction in the stress-life behavior of the CS deposits compared to the corresponding wrought material. However, when normalized by their ultimate tensile strengths, the CS specimens compared very well with the wrought materials, especially for the CS AA7075. Post-mortem analysis of the CS specimens revealed that fatigue cracks typically initiated from layer interface defects and subsequent crack propagation was influenced by the CS prior-particle boundaries. The results from this study indicate that refining the layer interface bonding may increase the fatigue resistance of freestanding aluminum alloy CS deposits.
Cold spray technology has recently gained much attention for applications beyond its primary cause of protective coating deposition. It has found its way for depositing thin films for surface functionalization, thick coatings for geometrical restoration, and has exhibited a high potential for additive manufacturing. As cold spray paves its way towards more structural applications, evaluating its performance under cyclic loading is of significant importance. Herein, the current state of the art on the contribution of various forms of cold spray deposits to fatigue strength is reviewed. Future perspectives for enhancing their structural integrity and promising trends are discussed.
Cold-sprayed coatings made of A357 aluminum alloy, a casting alloy widely used in aerospace, underwent set of standard tests as well as newly developed fatigue test to gain an information about potential of cold spray for repair and additive manufacturing of loaded parts. With optimal spray parameters, coating deposition on substrate with smooth surface resulted in relatively good bonding, which can be further improved by application of grit blasting on substrate’s surface. However, no enhancement of adhesion was obtained for shot-peened surface. Process temperature, which was set either to 450 or 550 °C, was shown to have an effect on adhesion and cohesion strength, but it does not influence residual stress in the coating. To assess cold spray perspectives for additive manufacturing, flat tensile specimens were machined from coating and tested in as-sprayed and heat-treated (solution treatment and aging) condition. Tensile properties of the coating after the treatment correspond to properties of the cast A357-T61 aluminum alloy. Finally, fatigue specimen was proposed to test overall performance of the coating and coating’s fatigue limit is compared to the results obtained on cast A357-T61 aluminum alloy.
Anastasios G. Gavras, Diana A. Lados, Victor K. Champagne, Robert J. Warren, Dileep Singh
Cold-spray-processed aluminum alloys have static mechanical properties superior to those of aerospace cast alloys, and similar to those of their wrought counterparts, making them good candidates for structural applications. However, their broad and confident use relies upon systematic fatigue crack growth studies to investigate and demonstrate the materials’ performance in critical high-integrity components. In this work, the fatigue crack growth behavior in early stages (small crack growth regime) was investigated for cold-spray processed 6061 aluminum alloys and coatings, at stress ratio R = 0.1, in room temperature laboratory air. The effects of the characteristic microstructure and initial flaw size on the fatigue crack growth response were systematically examined, and the crack growth mechanisms at the microstructural scale were established and compared to those of long cracks. The mechanical interfacial stability of coatings was examined in cold-spray 6061–rolled 6061-T6 couples. An original method of quantifying the deposition–substrate interfacial strength, and correlating it to the response under cyclic loading via crack-interface stability maps, was developed. The proposed methodology is based on combined scratch testing and fracture mechanics formulations, and failure at the coating–substrate interface can be predicted for any crack growth scenario under cyclic loading. The method can be broadly used for the design and optimization of cold-spray and other coatings, as well as in structural repair.
Alexis T. Ernst, Peter Kerns, Aaron Nardi, Harold D. Brody, Avinash M. Dongare, Seok-Woo Lee, Victor K. Champagne, Steven L. Suib, Mark Aindow
Surface oxides formed on powder feedstocks used for cold spray deposition can play an important role in the bonding of the particles and in the development of defects in the deposit. A combination of scanning transmission electron microscopy and x-ray photoelectron spectroscopy was used to investigate the oxides formed on gas-atomized Al 6061 alloy feedstock powders. The powders were studied in the as-atomized condition and after two different thermal exposures that correspond to typical feedstock pre-treatment conditions. The surface features and internal microstructures are consistent with those reported previously for these powders. The as-atomized powders have 5.2 nm thick amorphous oxide layers, with an outer Mg-rich sub-layer and an inner Mg-lean sub-layer. Powders heat-treated at 230 °C in air exhibit slightly thicker oxide layers with a crystalline MgAl2O4 spinel outer sub-layer and an amorphous aluminum oxide inner sub-layer. Powders homogenized under Ar at 400 and 530 °C have significantly thicker (8.9 nm) oxide layers with evidence for a defect inverse spinel Al(Mg,Al)2O4 inner sub-layer between the MgAl2O4 spinel outer sub-layer and the alloy. Differences between these observations and those reported previously for oxidation of bulk alloys are explained on the basis of Mg surface segregation during the gas atomization process.
Bryer C. Sousa, Caitlin Walde, Victor K. Champagne, Aaron T. Nardi, Richard D. Sisson and Danielle L. Cote
In this work, three commercially available aluminum alloy systems (Al 2024, Al 6061, and Al 7075) were considered to explicitly capture the differences in material properties associated with a rapidly solidified, gas-atomized particulate feedstock as compared with their conventionally cast counterparts. Differences between the microstructural, thermodynamic, mechanical, and kinetic behaviors associated with gas-atomized and conventionally bulk counterparts have been tacitly assumed by the cold spray community. However, many researchers continue to utilize legacy properties from bulk materials when simulating particle impact phenomena in silico, for example. By way of recognizing the fact that bulk material properties may not serve as substitutes for gas-atomized powder property input parameters for cold spray process simulation and computation in silico, enhanced cold spray research and development will be more easily achieved. Therefore, understanding the feedstock powder characteristics for use in cold spray can lead to fine-tuning the properties of cold spray consolidations. Optical microscopy, scanning electron microscopy, nanoindentation, microhardness, differential scanning calorimetry, elemental analysis, and cooling rate calculations were utilized. This work confirms preliminary findings that powder alloys may not be treated the same way as their bulk counterparts in so far as the enactment of heat treatment processing parameters are concerned. Specifically, vast discrepancies were found in the grain size, secondary phases, and mechanical behavior between the powder and cast versions of each alloy.
Jiawei Kelvin Bi, Zhi Cheng Kelvin Loke, Chi Keong Reuben Lim, Kok Hoon Tony Teng and Pak Keng Koh
This study investigates the mechanical properties of aluminium 2024 (Al-2024) and aluminium 7075 (Al-7075) cold-sprayed materials and coatings for repairs. It aims to determine the acceptable data needed to meet regulatory requirement when substantiating cold spray repairs. The study focuses on repairs of non-principal structural element (PSE) structures such as skin and panels that are prone to corrosion and wear. For cold spray repair of such components, the microstructure, tensile, peel, bearing, and bending strength from the repair process and powder materials of Al-2024 and Al-7075, were identified and investigated in accordance with MIL-STD-3021. Results show an average coating porosity of <1.2% for both materials. Average tensile strength was 247.1 MPa (with elongation of 0.76%) for Al-2024 and 264.0 MPa (with elongation of 0.87%) for Al-7075. Al-2024 has an average peel strength of 71.9 MPa, while Al-7075 is at 48.9 MPa. The Al-2024 bearing test specimens gave a maximum load strength before failure of 633.6 MPa, while the Al-7075 gave 762.7 MPa. The bending tests show good flexibility for coating thickness ranges of typical skin and panel parts. The results show that cold spray can be used to restore thickness and oversized hole diameters for Al-2024 and Al-7075 skin and panels. The bearing test conducted in this study has also demonstrated a new test method to determine the bearing load and yield strength of a cold spray-repaired hole in a plate.
Cold gas dynamic spray is being explored as a repair technique for high-value metallic components, given its potential to produce pore and oxide-free deposits of between several micrometers and several millimeters thick with good levels of adhesion and mechanical strength. However, feedstock powders for cold spray experience rapid solidification if manufactured by gas atomization and hence can exhibit non-equilibrium microstructures and localized segregation of alloying elements. Here, we used sealed quartz tube solution heat treatment of a precipitation hardenable 7075 aluminum alloy feedstock to yield a consistent and homogeneous powder phase composition and microstructure prior to cold spraying, aiming for a more controllable heat treatment response of the cold spray deposits. It was shown that the dendritic microstructure and solute segregation in the gas-atomized powders were altered, such that the heat-treated powder exhibits a homogeneous distribution of solute atoms. Micro-indentation testing revealed that the heat-treated powder exhibited a mean hardness decrease of nearly 25% compared to the as-received powder. Deformation of the powder particles was enhanced by heat treatment, resulting in an improved coating with higher thickness (* 300 lm compared to * 40 lm for untreated feedstock). Improved particle– substrate bonding was evidenced by formation of jets at the particle boundaries.
A.C. Hall, R.L. Williamson, D.A. Hirschfeld, T.J. Roemer
An earlier study reported an investigation of the mechanical properties of cold sprayed aluminum and the effect of annealing on those properties. In that study, cold spray coatings approximately one centimeter thick were prepared using three different feedstock powders: Valimet H-10, Valimet H-20, and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings. Each material was tested in two conditions: as-sprayed and after a 300°C, 22 hour air anneal. The as-sprayed material showed a high ultimate strength and low ductility, < 1% elongation. The annealed samples showed a reduction in the ultimate strength but a dramatic increase in ductility, up to 10% elongation. Microstructural examinations and fractography clearly showed a change in the fracture mechanism between the as-sprayed and annealed material, but insufficient data was available to conclusively explain the ductility increase at that time. Since then, Kikuchi mapping of the Valimet H-10 material in the as-sprayed and annealed conditions has been conducted. Kikuchi mapping allows indexing of grains, identification of grain boundaries, and phase identification using backscattered diffraction patterns in an SEM. The data shows that significant recrystallization within the splats upon annealing has occurred. No significant crystal growth across splat boundaries is observed. The data demonstrate that the mechanism of ductility increase in annealed cold spray deposits is recrystallization of the base aluminum material.
A. C. Hall, D. J. Cook, R. A. Neiser, T. J. Roemer, D. A. Hirschfeld
Cold spray, a new member of the thermal spray process family, can be used to prepare dense, thick metal coatings. It has tremendous potential as a spray-forming process. However, it is well known that significant cold work occurs during the cold spray deposition process. This cold work results in hard coatings but relatively brittle bulk deposits. This work investigates the mechanical properties of cold-sprayed aluminum and the effect of annealing on those properties. Cold spray coatings approximately 1 cm thick were prepared using three different feedstock powders: Valimet H-10: Valimet H-20: and Brodmann Flomaster. ASTM E8 tensile specimens were machined from these coatings and tested using standard tensile testing procedures. Each material was tested in two conditions: as-sprayed; and after a 300°C, 22h air anneal. The as-sprayed material showed high ultimate strength and low ductility, with <1% elongation. The annealed samples showed a reduction in ultimate strength but a dramatic increase in ductility, with up to 10% elongation. The annealed samples exhibited mechanical properties that were similar to those of wrought 1100 H14 aluminum. Microstructural examination and fractography clearly showed a change in fracture mechanism between the as-sprayed and annealed materials. These results indicate good potential for cold spray as a bulk forming process.
Silvano Rech, A. Trentin, Simone Vezzù, S. Pozza, D. Magalini, L. Tecchio
Residual stress by curvature method. Square aluminium alloy (AA6061) substrates were coated in order to perform MLRM and XRD stress characterizations. The coatings were characterized in terms of microstructure, microhardness an porosity. Finally standard spray-salt test has been performed in order to study the corrosion protection of the aluminium Al104-3 coating on two different substrates: carbon structural steel Fe37 and magnesium alloy AZ91. The cold spray deposition process Cold Gas Dynamic Spray or simply Cold spray, is gaining more interest than conventional thermal spray techniques primarily because of the lower deposition temperature required to deposit metallic and composite coatings. Cold spray does not use a heat source, such as the conventional thermal spray processes, but instead uses a high-pressure gas jet to accelerate particles to supersonic speed through a convergent-divergent de Laval nozzle so that the particles achieve sufficient kinetic energy to undergo plastic deformation on impact. The main part of the gas flows through a heating system while a minor part of the gas goes through the powder feeder and drags the particles to the nozzle. The two flows come together at the entrance of the nozzle and they are accelerated to supersonic speed thanks to the geometry of the nozzle. The gas temperature ranges typically between 200 and 800 °C but the particle temperature is highly slower because of the very short time in which they get in contact with the gas so the cold spray process does not involve bulk melting and the material is mostly produced entirely in the solid state. Conclusions 1. High particles deformation and low porosity content indicate a good cohesion in coating 2. Hardening of deposited particles; there are no difference between the three powders 3. Stresses inside coating are compressive 4. Aluminium coating exhibit after 1000h of spray-salt test a good protection of Mg alloy (AZ91) and carbon steel (Fe37) substrates; passivate coating surfaces show no evident defect and no exhibit delaminations Experimental procedure For this study three different aluminium commercial (Praxial Al 104-3, Sultzer-Metco 54NS, Valimet H15) powders were used. The depositions were carried out by means of CGT-Kinetics 3000 Cold Spray system provided with the special polymeric nozzle designed for aluminium powder spraying. The distance between nozzle and substrate was 20 mm; the powder flow and carrier gas flow rates were kept constant for all depositions. The cold spray gas parameters were kept constant: the nitrogen stagnation temperature was 350 °C and the stagnation pressure was 2.5 MPa. The coatings were deposited on aluminium alloy (AA6061) Almen strips in order to evaluate the Residual stress analysis Almen Almen XRD XRD XRD MLRM MLRM 0 10 20 30 40 50 60 70 80 90 100 Al 104-3 54NS H15 compressive stress intensity [MPa] Stress values are independent of different powders Depth profile on Multipass coating Two of the three stress measurement techniques are comparable except MLRM method that shows higher values due to different layer removing methods Two tensile peeks are exactly on the interface between subsequent cold spray passes Almen strip Curvature XRD MLRM Measurement techniques Cold spray technique induces compressive stresses Magnesium alloy AZ 91 substrate Steel Fe37 substrate polished as dep polished as dep 0 h Spray-Salt Test Corrosion protection In all the samples tested the aluminium coatings passivate and the surface coatings appear similar to that of bulk aluminium. Corrosion resistance is sensible to surface roughness; two different coating finishing are tested: as deposited (R=14.65±3.63mm) and polished (Ra<0.31±0.02mm) Al 104-3 powders 54NS powders H15 powders q Gas atomized powders q Spherical particles with some satellites q Dendritic microstructure with sub-micrometric porosity Cold Spray deposition Average velocity of particles 700 m/s Temperature of powders < 200°C q Good interlocking coating-substrate and very low interface porosity q Particle microstructure was preserved due to low temperature deposition q Higher deformation for the larger mean diameter particles q Low oxygen content (< 0.1%vol) Al 104-3 coating, etched 54NS coating, etched H15 coating, etched Aluminium coatings 0 0,5 1 1,5 2 2,5 3 3,5 Al 104-3 54NS H15 areal porosity[%] Coating Porosity Lower porosity due to high deformation ratio and absence of smaller particles powder powder powder coating coating coating 0 10 20 30 40 50 60 70 80 Al 104-3 54NS H15 hardness [Vickers 5g] Coating Vickers Microhardness q Constant along coating thickness q Coating hardness is about twice higher than particles hardness q Hardening due to high particle deformation q Microhardness independent of powder type Hardness [HV0.005] Advantages: fast, Cheap Disadvantages: accuracy, need model to quantify Advantages: relatively fast, depth profile Disadvantages: need to know coating mechanical properties Advantages: non-destructive, depth profile, local investigation area Disadvantages: need coating mechanical properties, local investigation area and little penetration Cold spray coatings (Al 104-3 powder) Cold Spray gun With de Laval nozzle Porosity is equal to about 2-3% for Al104-3 and H15 coating Porosity is lower than 1% for 54NS coating.