In this work, cold spray shot peening was used to treat interstitial-free (IF) steel. Different numbers of impact passes were evaluated and the microstructure and hardness evolution in IF steel were analyzed. Based on the experimental results, the potential of CSSP to achieve surface strengthening and properties enhancement in metallic materials is demonstrated.

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.

Metal surface characteristics play a significant role in interacting with their biological environment. Copper surfaces have been identified for their antimicrobial properties. Improvement of antibacterial and antiviral performances can be tailored by surface microstructure modification. Severe plastic deformation is an effective surface modification procedure to improve the mechanical performance of metal surfaces. This technique can be adapted to obtain surface grain refinement and induce surface roughness. In this work, cold spray shot peening is used to modify copper substrate surfaces and study the effects on their antibacterial properties. To modify the grain structure of copper, different shot-peening parameters were examined. The surface roughness and microstructure were investigated by employing optical and scanning electron microscopy. The bactericidal activity of copper substrates after shot peening treatment is discussed and a comparison between the bacterial load on treated (shot-peened surface and cold sprayed copper coating) and untreated surfaces (as-received) is provided. Testing of the surfaces after their exposure to the biological environment demonstrated improved microbial inactivation performances for surfaces that had undergone grain refinement without exceeding a certain roughness value.

Metal structures in offshore facilities are usually protected from corrosion using Zn-Al coatings even though they are subjected to collective stress conditions. This paper evaluates a post-treatment called machine hammer peening and its effect on surface finish, induced residual stresses, and near-surface microstructure of thermally sprayed ZnAl4 coatings. As expected, coating roughness was reduced from about Rz = 53.5 μm in the as-sprayed condition to 10.4 μm after treatment and coating densification was revealed in the near-surface zone. Residual stresses, which were surprisingly compressive in the as-sprayed condition, were likewise affected by the peening process, reaching a maximum of 200 MPa. The influence of peening direction and other such parameters were also investigated as part of the study.

In this work, a new cold-spray shot-peening process was used to achieve surface nanocrystallization on a magnesium alloy deposit. The results of various examinations and tests show that nanocrystalline layers up to 40 µm thick with an average grain size in range of 50-80 nm can be prepared on AZ91D deposits using the new process. The nanocrystalline layers also exhibit good microhardness and tribological properties.

In this study, pure Al coating was deposited via in-situ shot-peening-assisted cold spray method in order to study the effect of the in-situ tamping effect which was caused by the impact of large sized shot-peening particles on grains size evolution of coatings. The microstructures of the as-sprayed Al coating were observed by using Scanning Electron Microscope and Electron Backscatter Diffraction. A commercial gas atomized Al powder with a grain size range of 10-20 μm was used as the spraying powder. The cross section of the as-sprayed Al particles presented elongated rectangular morphologies, which indicated that the nearly spherical particles experienced severe plastic deformation by the impact of large sized shot-peening particles. It was found that dynamic recrystallization of dislocations-ridden regions was responsible for the grain refinement of cold sprayed coating. Aluminum grains with size of several tens to several hundred of nanometers can be apparently recognized at the whole cross section of the particle. Therefore, in-situ shot-peening-assisted cold spray method can deposit completely nanocrystalline coating using micrometer-grain powder, and thus can be employed to develop high quality coatings of commercial importance.

NiCoCrAlY coatings are widely used as bond coats for ceramic thermal barrier coatings (TBCs) and oxidation and corrosion protective overlay coatings in industrial gas turbines. High temperature oxidation behaviour of NiCoCrAlYs has a great influence on the coating performance and lifetime of TBCs. A promising route to decrease the oxidation rate of such coatings is post-coating surface modification which can facilitate formation of a uniform alumina scale with a considerably slower growth rate compared to the as-sprayed coatings. In this work, the effect of surface treatment by means of shot peening and laser surface melting (LSM) on the oxidation resistance of high velocity air-fuel (HVAF) sprayed NiCoCrAlY coatings was studied. Isothermal oxidation was carried out at 1000 °C for 1000h. Results showed that the rough surface of as-sprayed HVAF sprayed coatings was significantly changed after shot peening and LSM treatment, with a compact and smooth appearance. After the exposure, the oxide scales formed on surface-treated NiCoCrAlY coatings showed different morphology and growth rate compared to those formed on as-sprayed coating surface. The oxidation behaviour of surface treated HVAF-sprayed NiCoCrAlY coatings were revealed and discussed.

Three Fe-based powder alloys, Höganäs Fe SP529, Fe SP586 and 6AB, have been deposited by HVOF and HVAF spraying onto mild steel plates. The sprayed samples were first ground and then shot peened using glass shot in order to seal the surface interconnected pores and other surface imperfections. The samples as ground and ground/glass shot peened were tested by salt spray (fog) exposition for 238 h according to ASTM B117/ISO 9227. FeSP586, HVOF and HVAF sprayed and glass shot peening samples achieved surface sealing enough to pass the test with appearance rating RA = 9 according to ISO 10289. All other samples achieved moderate to excessive pitting and/or moderate to excessive staining types of corrosion defects.

In this study, pure Al coatings were deposited on ZK60-T5 Mg alloy substrates via in-situ shot-peening assisted cold spray in order to study the effect of the Al coating on fatigue behavior of coated samples. Fatigue behavior of the coated and un-coated samples has been investigated through experimental tests. The size and shape distribution of powders, microstructural characteristics of coatings and fractography of fatigue test samples have been studied using scanning electron microscopy. The average microhardness of pure Al coating is higher than 70 HV50. In order to obtain the fatigue S-N diagram for each set, coated and un-coated samples have been tested in a load-controlled condition. The tension-compression fatigue experiments reveal that the fatigue property of ZK60-T5 alloy coated with pure Al coatings has significantly deteriorated compared with un-coated samples.

This study evaluates the effect of hammer peening on the wear behavior of cermet coatings. WC-FeCMnSi coatings were produced by twin wire arc spraying and post-treated on a five-axis machining center equipped with pneumatic peen. The surface topography of the peened coatings was examined and compared to as-sprayed and polished samples. Dry sliding friction and abrasive wear tests showed that the treated coatings had lower friction coefficients, but were less wear resistant than non-treated samples. Likewise, strain hardening effects revealed by nanoindentation testing were offset by process-induced cracking of embedded carbides, which contributes to break-outs and third-body wear.

This study assesses the effect of machine hammer peening (MHP) and carbide grain size fraction on the friction and wear behavior of arc-sprayed WC-W 2 C FeCMnSi coatings. SEM examination shows that post-treatment by MHP compresses the coating, reducing both thickness and porosity, particularly in coatings with ultrafine carbides. The treatments also cause cracking, however, especially in carbide phases. Ball-on-disk tests were carried out on as-sprayed and treated samples to determine sliding wear and friction properties, and dry sand rubber wheel tests were used to evaluate abrasion resistance. SEM and EDX analyses before and after wear testing show how coating microstructure and grain size correlate with the friction and wear test results obtained and the given surface treatments.

This study demonstrates a novel method for improving the corrosion resistance of cold sprayed Al6061 coatings. Large stainless steel particles were added to a commercial Al6061 powder and the mixture was deposited on Mg alloy AZ31B substrates using nitrogen gas at low working pressure and temperature. It is shown that the stainless steel particles had a shot-peening effect, thus increasing the density as well as the corrosion resistance of Al6061 coatings. SEM examination showed that no stainless steel particles were incorporated in the coating.

In this study, in-situ shot-peening approach was introduced by mixing large stainless steel (SS) shots with diameters over 150 μm with IN718 spray powders to aim at developing a novel approach to achieve dense coating. Effect of mixed stainless steel shot content on the microstructure and mechanical properties of the IN718 coating was examined. Results show that IN718 coating can be deposited without any shot peening particle inclusions for their relatively low velocity. It was revealed that the deposition efficiency of the IN 718 powders is improved by the in-situ shot peening effect. With increasing shot fraction in the powder mixture from 0% to 50 vol.%, the coating porosity decreased from 5.6% to 0.2% only by using N 2 accelerating gas. Remarkable work hardening induced by impact of the shot peening particles was detected.

The HVOF process is characterized by the deposition of a low porosity and oxide content coating and the particles projected by this process have high kinetic energy, resulting in a high density coating. However, the presence of non-melted particles can affect the porosity, tensile adhesion, hardness and surface finishing. In this article, the influence of the shot peening treatment on morphology, porosity, tensile adhesion and roughness of some FeMnCrSi(Ni/B) HVOF coatings have been studied. High carbon steel particles with spherical geometry were used for shot peening. The results show that the shot peening reduces the porosity, mainly in the coatings with higher porosity level, and generates an increase of hardness with no phase transformations. The increase in adherence is also a benefit generated by the treatment. The surface quality analyses show a significant roughness reduction. With shot peening a compressive residual stress state was achieved in the coating, improving mechanical properties.

This study evaluates the influence of shot peening on the fatigue life of cold spray aluminum alloy 6082 coatings. A pneumatic blast machine with standard steel shot was used to peen both uncoated and coated substrates. Six test groups representing different treatment protocols were characterized in terms of residual stress, roughness, and rotating bending fatigue. The results show that the best fatigue performance is obtained by intense shot peening prior to cold spraying. Post-treatment shot peening, in contrast, had a detrimental effect as a large portion of the kinetic energy is absorbed in the coating, resulting in surface damage rather than further work hardening.

Fundamental properties of Titanium coating prepared by low temperature HVOF process aided by injection of water had been published by our research group. The results showed that low temperature HVOF process provided as a good means for the deposition of comparatively dense Ti coating, however the interconnected pores observed in Ti coating degraded corrosion resistance of Ti coating, therefore it was necessary to manufacture denser Ti coating. Two processes were respectively applied for the densification of Ti coating. One process was to perform post heat treatment for as-sprayed Ti coating with conventional Ti powder as feedstock. The other process was to modify thermal spray powder. Ti powder mixing with spherical glass powder was used to deposit denser Ti coating as a result of shot peening effect of hard glass powder. Finally the corrosion resistance for densified Ti coatings was evaluated by electrochemical characterization.

Residual stress build up in thick thermal spray coatings is a property of concern. The adhesion of these coatings to the substrate is strongly influenced by the residual stress generation during the coating deposition process. In the HVOF spray process, due to lower processing temperature and higher particle velocity as compared to plasma spraying, significant peening stresses are generated during the impact of semi molten particles on the substrate. The combination of these peening stresses together with quenching and cooling stresses that arise after deposition can be of significant importance. In this paper both a numerical finite element analysis (FEA) method, to calculate peening, quenching and cooling residual stresses, and experimental methods, as Modified Layer Removal Method (MLRM) and Neutron Diffraction analysis, are applied. The investigation is performed for thick Inconel 718 coatings on Inconel 718 substrates. Combined, these numerical and experimental techniques yield a deeper understanding of residual stress formation and a tool for process optimisation. The relationship between the stress state and deposit/substrate thickness ratio is given particular interest.

Laser post treatment is a well known procedure in order to homogenize the geometry, the microstructure and aspect of the surface of thermal sprayed coatings. Furthermore the properties of coatings, as for example corrosion resistance can be improved, as a sealing layer is induced by remelting. High power laser sources as CO 2 -, Nd:YAG as well as diode laser are used for this aim. They allow a very deep remelting zone in a depth-range between 0,2-2 mm. Contemporarily the hard phases as well carbides in the coating degenerate due to the very high heat input and the wear resistance of the coating decreases. Short-pulse laser are very advantageous for processing surfaces in a very narrow band (under 50 µm). Applications are nowadays developed for the surface treatment of aluminium cylinder running surfaces and of titanium implants. The short-pulse laser post processing can be a very profitable methods to improve the behaviour of coatings without damaging hard phases. Investigations on remelting of PTA- and thermal sprayed coatings with a Nd:YAG-short-puls laser and with an excimer laser are reported in this paper. Surface quality as for example roughness of PTA-coatings with tungsten carbides (NiCrBSi+60%WSC) could be improved and a slight surface sealing layer was induced. Depending on the adopted laser system, different surface roughness profiles can be reached. In further investigations the suitability of the method was proven for thermally sprayed coatings of NiCrBSi

Coating-substrate adhesion in cold spray is a paramount property, the mechanisms of which are not yet well elucidated. These mechanisms are governed by metallurgical and morphological phenomena occuring when cold-sprayed particles impinge on the substrate. To go into these mechanisms, due to the intrinsic characteristics of the cold spray process, i.e. the low-temperature and high velocity of the particles, direct observation and control of inflight particles and related phenomena (especially when impinging) cannot be done easily. For this reason, an experimental simulation of the particle-substrate reactions at the particle impingement was developed. This simulation is based on original filter impact experiments from laser shock acceleration of plates/foils (fliers). These were applied to the Cu-Al metallurgically-reactive system to simulate Cu cold-sprayed onto Al. The velocity of the plate was selected in the range of actual cold spray velocities. Relevant Cu-Al interaction phenomena were featured and studied as a function of filter impact conditions, i.e., primarily, shearing, plastic deformation, phase transformation (including rapid melting/solidification and formation of intermetallics). These phenomena were shown to be similar to those involved in cold spray. This was ascertained by a parallel study of cold-sprayed Cu coating of Al using SEM, TEM, EPMA, and an energy balance and diffusion calculations. In addition, this simulation can be used to feed FE modeling of cold spray particle impingement on the substrate. Preliminary results are discussed from modeling using the “RADIOSS®” code. More generally, laser shock flier impact experiments were demonstrated to result in a powerful tool capable of simulating cold spray coating-substrate interface mechanisms. Major assets rest on their high significance, reproducibility, flexibility and potential for substituting for direct laborious cold spray optimization testing.