High temperature oxidation behavior of Ti(Al,Si)_3 diffusion coating on γ-TiAl by cold spray

A Ti(Al,Si)3 diffusion coating was prepared on γ-TiAl alloy by cold sprayed Al?20Si alloy coating, followed by a heat-treatment. The isothermal and cyclic oxidation tests were conducted at 900 °C for 1000 h and 120 cycles to check the oxidation resistance of the coating. The microstructure and phase transformation of the coating before and after the oxidation were studied by SEM, XRD and EPMA. The results indicate that the diffusion coating shows good oxidation resistance. The mass gain of the diffusion coating is only a quarter of that of bare alloy. After oxidation, the diffusion coating is degraded into three layers: an inner TiAl2 layer, a two-phase intermediate layer composed of a Ti(Al,Si)3 matrix and Si-rich precipitates, and a porous layer because of the inter-diffusion between the coating and substrate.

Surface modification of magnesium alloys using thermal and solid-state cold spray processes:Challenges and latest progresses

Potential engineering applications of magnesium(Mg)and Mg-based alloys,as the lightest structural metal,have made them a popular subject of study.However,the inferior corrosion and wear characteristics significantly limit their application range.It is widely recognized that surface treatment is the most commonly utilized technique for remarkably improving a substrate’s surface characteristics.Numerous methods have been introduced for the surface treatment of Mg and Mg-based alloys to improve their corrosion behavior and tribological performance.Among these,thermal spray(TS)technology provides several methods for deposition of various functional metallic,ceramic,cermet,or other coatings tailored to particular conditions.Recent researches have shown the tremendous potential for thermal spray coated Mg alloys for biomedical and industrial applications.In this context,the cold spray(CS)method,as a comparatively new TS coating technique,can generate the coating layer using kinetic energy rather than combined thermal and kinetic energies,like the high-velocity oxy-fuel(HVOF)spray method.Moreover,the CS process,as a revolutionary method,is able to repair and refurbish with a faster turnaround time;it also provides solutions that do not require dealing with the thermal stresses that are part of the other repair processes,such as welding or other TS processes using a high-temperature flame.In this review paper,the recently designed coatings that are specifically applied to Mg alloys(primarily for industrial applications)employing various coating processes are reviewed.Because of the increased utilization of CS technology for both 3D printed(additively manufactured)coatings and repair of structurally critical components,the most recent CS methods for the surface treatment,repair,and refurbishment of Mg alloys as well as their benefits and restrictions are then discussed and reviewed in detail.Lastly,the prospects of this field of study are briefly discussed,along with a summary of the presented work.

Microstructure and Properties of Low-pressure Cold Sprayed Al-based Composite Coatings on Magnesium Alloy Surface

Pure Al and Al-30%Al_(2)O_(3)composite coatings are prepared on the surface of AZ31B magnesium alloy by low-pressure cold spraying.The morphology and structure of the coatings are analyzed by scanning electron microscope (SEM),energy dispersive spectroscopy (EDS),and X-ray diffraction (XRD),and the effects of the addition of Al_(2)O_(3)on the microstructure of the Al-based coatings are discussed.The mechanical properties and corrosion resistance of the coatings are fully evaluated by the micro-hardness tester,electronic tensile machine,and electrochemical workstation.The results show that the coating structure is more uniform and denser,and the porosity is significantly reduced after the addition of Al_(2)O_(3).The interfaces between the coatings and the magnesium alloy substrate are distinct,and the coatings and the substrate are mechanically combined.Compared with the pure Al coating,the microhardness of the Al-Al_(2)O_(3)composite coating is increased to 61.1 HV_(0.2),and the bonding strength reaches above 53.1 MPa.The self-corrosion potential of the two coatings is higher than that of the magnesium alloy,and the self-corrosion current density is significantly lower than that of the magnesium alloy substrate.The Al-based coatings prepared by low-pressure cold spraying have high hardness,good bonding strength,and good corrosion resistance,which can be used for the repair and protection of magnesium alloy structural parts.

Towards high-strength cold spray additive manufactured metals:Methods,mechanisms,and properties

Cold spray,as a solid-state additive manufacturing process,has been attracting increasing attention from both scientific and industrial communities.However,cold-sprayed deposits generally have unfavorable mechanical properties in their as-fabricated state compared to conventionally manufactured and fusion-based additive-manufactured counterparts due to the inherent microstructural defects in the deposits(e.g.,porosity and incomplete interparticle bonding).This downside reduces its competitiveness and limits its wide applications as an additive manufacturing process.In the past years,many strengthening technologies have been developed or introduced to adjust the microstructure and improve the mechanical properties of cold-sprayed deposits.The term“strengthening”in this work specifically refers to improving the mechanical strength,particularly the tensile strength of the cold-sprayed bulk deposits.According to the stage that the strengthening technologies are used in the cold spray process,they can be classified into three categories:pre-process(e.g.,powder heat treatment),in-process(e.g.,powder heating,in-situ micro-forging,laser-assisted cold spray),and post-process(e.g.,post heat treatment,hot isostatic pressing,hot rolling,friction stir processing).Therefore,a comprehensive review of these strengthening technolo-gies is conducted to illuminate the possible correlations between the strengthening mechanisms and the resultant deposit microstructures and mechanical properties.This review paper aims to help researchers and engineers well understand the different strengthening methods and provide guidance for the cold spray community to develop new strengthening strategies for future high-quality mass production.

Fast strength-ductility synergistically adjusting of cold spray additive manufactured Cu deposits via electric pulse processing

Electric Pulse Processing(EPP)treatment was innovatively introduced to optimize the strength and ductility of the CSAMed Cu deposits.The results show that EPP is an efficient and fast post-treatment to improve the strength and ductility(within tens of seconds).The larger the pulse current and number of pulses,the better the mechanical properties.Interestingly,this research found that when the heat input determined by pulse current and number of pulses exceeds a certain threshold(pulse current intensity is 2000 A,number of pulses is 10),increasing the number of repeat time could also effectively improve the mechanical properties.A tensile strength of 210 MPa and a ductility of 14.0%could be obtained with reasonable EPP parameters(pulse current intensity is 2000 A,number of pulses is 10,and repeat number is 2),which is similar to those of conventional annealing(e.g.,tensile strength is 272 MPa,elongation is 28.3%).The microstructure evolution analysis shows that EPP can effectively improve the bonding quality between the deposited particles by recrystallization,promote grain growth and the formation of twins,which is the main reason for the improvement of mechanical properties.

Ductile and high strength Cu fabricated by solid-state cold spray additive manufacturing

In this work,pure Cu with excellent strength and ductility(UTS of 271 MPa,elongation to fracture of 43.5%,uniform elongation of 30%)was prepared using cold spray additive manufacturing(CSAM),realizing a breakthrough in the field.An in-depth investigation was conducted to reveal the microstructure evolution,strengthening and ductilization mechanisms of the CSAM Cu,as well as the single splats.The results show that the CSAM Cu possesses a unique heterogeneous microstructure with a bimodal grain structure and extensive infinitely circulating ring-mounted distribution of twinning.Based on the single splat observation,the entire copper particle forms a gradient nano-grained(GNG)structure after high-speed impact deposition.The GNG-structured single splat serves as a unit to build the heterogeneous microstructure with bimodal grain distribution during the successive deposition in CSAM.The results also show that CSAM can achieve synergistic strengthening and ductilization by controlling the grain refinement and dislocation density.This work provides potential for CSAM technique in manufacturing various metallic parts with the desired combination of high strength and good ductility without additional post-treatments.

Influence of particulate morphology on microstructure and tribological properties of cold sprayed A380/Al2O3 composite coatings

In this study,three kinds of A380/Al2O3 composite coatings were prepared by cold spray(CS)using spherical,irregular and spherical+irregular shaped Al2O3 particulates separately mixed in the original A380 alloy powders.The influence of Al2O3 particulates’morphology on the microstructural characteristics(i.e.retention of Al2O3 content in coatings,coating/matrix interfacial bonding,pore size distribution and morphology etc.)and wear performance of the coatings was investigated by scanning electron microscopy(SEM),X-ray computed tomography(XCT)and 3-D optical profilometry.Results indicated that the spherical Al2O3 showsobvious tamping effect during deposition process.As a result,the interfaceshowedawavy shape while the matrix and particulates were mechanical interlocked with much improved adhesion.In addition,the porosity of the coating was minimized and the pores exhibited curved spherical structure with reduced dimensions.The irregular Al2O3 particles predominantly displayed the embedding effect together with fragmentation of Al2O3 particulates.Consequently,poor coating/matrix interfacial bonding,high porosity and the formation of angular-shaped pores were resulted in the coating.Dry sliding wear tests results revealed that the wear resistance of the coating is directly related with the retained content of Al2O3 in the coating.The coating containing irregular Al2O3 particulates displayed superior wear performance with its wear rate one seventh of that of the pure A380 alloy coating.The coating containing both kinds of Al2O3 particulates showed mixed characteristics of above two kinds of Al2O3 composite coatings.

Cold spray additive manufacturing of Invar 36 alloy:microstructure,thermal expansion and mechanical properties

In this work,the Invar 36 alloys were manufactured using cold spray(CS)additive manufacturing technique.The systematic investigations were made on the microstructural evolution,thermal expansion and mechanical properties under as-sprayed(AS)and heat-treated(HT)conditions.XRD(X-ray diffraction)and ICP-AES(inductively coupled plasma atomic emission spectroscopy)analyses show that no phase transformation,oxidation,nor element content change have occurred.The X-ray computed tomography(XCT)exhibited a near fully dense structure with a porosity of 0.025%in the helium-produced sample under as-sprayed condition,whereas the nitrogen-produced samples produced at 5 MPa and 800℃show more irregular pore defects.He-AS sample shows a more prominent grain refinement than that of nitrogen samples due to the more extensive plastic deformation.The post heat-treatment exhibited a promoted grain growth,inter-particle diffusion,as well as the formation of annealing twins.Between25℃and 200℃,the nitrogen samples possessed lower CTE(coefficient of thermal expansion)values(1.53×10^(-6)/℃)compared with those produced by casting and laser additive manufacturing.The He-AS samples exhibited a noticeable negative CTE value between 25℃and 200℃,which may due to the significant compressive residual stress(-272 MPa)compensating its displacement with temperature increase during CTE test.The N2-HT and He-HT Invar 36 samples present a notable balance between strength and ductility.In conclusion,the CS technique can be considered as a potential method to produce the Invar36 component with high thermal and mechanical performance.

Advanced cold spray technology:Deposition characteristics and potential applications

Cold spray technology,originated from the Institute of Theoretical and Applied Mechanics Siberian branch of the Russian Academy of Sciences,is a rapidly emerging industrial coating technology.Cold sprayed particles with high-velocity impact onto a substrate so as to induce severe plastic deformation and then create a deposit.For its low temperature and high velocity compared with thermal spraying,the cold spraying process is increasingly used in the industries for protective coating.The deposition characteristics of the particles,coating formation and bonding mechanism of the cold spraying process are different from thermal spraying.Many theory investigations of the cold spraying process contribute to the development of the high performance coatings,which makes the cold spraying process as a popular research field.Presently,the deposition characteristics,bonding mechanism,process optimization as well as classical applications of the cold spraying technology in the past are reviewed,and the interesting points for the further development,optimization and applications of this technology are also recommended.

Microstructure and bioactivity of a cold sprayed rough/porous Ta coating on Ti6Al4V substrate

To improve the bioactivity and biocompatibility of titanium implants,a rough/porous tantalum(Ta)coating was firstly prepared on Ti6Al4V substrate by cold spraying.The results indicated that the surface of Ta coating is extremely rough with lots of visible holes,because of poor deposition quality.Microstructure and microhardness results showed that different layers appeared in the inner and outer parts of coatings,corresponding to porosity,due to lack of subsequent compaction.The simulated body fluid(SBF)soaking test showed that spherical apatite sediments were mineralized on rough/porous surface in SBF after 2–4 weeks,which demonstrated that the cold sprayed Ta coating had good bioactivity.This was mainly attributed to the rough/porous surface of Ta coating obtained by cold spraying,which is conducive to the heterogeneous nucleation of apatite on it.

Influence of Gas Temperature on Microstructure and Properties of Cold Spray 304SS Coating

In the present study, 304 stainless steel coatings were deposited on interstitial-free steel substrates by cold gas dynamic spray technology. The effect of gas temperature on microstructure, micro-hardness, cohesive strength, and electrochemical property of the coatings were investigated and compared. The results showed that increasing gas temperature had a great contribution to enhancing the bonding strength between the deposited particles and making the microstructure more density. Therefore, the porosity of the coatings decreased from 0%4-0.5% to 2%4-0.3%, and the tensile strength of the coatings increased from 564-4 MPa up to 734-3 MPa. In addition, the corrosion resistance of the coatings was also deeply influenced by process gas temperature. The corrosion kinetics of the coatings were affected by both of the plastic deformation of deposited particles and the porosity in the coatings.

Electrochemical characterization and microstructure of cold sprayed AA5083/Al_(2)O_(3) composite coatings

This work focused on the role of Al_(2)O_(3) particles in the corrosion behavior of cold sprayed AA5083 aluminum alloy matrix composite(Al-MMC)coatings.The electrochemical characterization of the coatings was investigated in a 3.5 wt.%NaCl solution as a function of time.The results show that fragmentation of Al_(2)O_(3) particles is not clearly observed in the case of AA5083/20 vol.%Al_(2)O_(3) coating,while the broken Al_(2)O_(3) particles can be seen clearly in AA5083/40 vol.%Al_(2)O_(3) and AA5083/60 vol.%Al_(2)O_(3) coatings.The addition of 20 vol.%Al_(2)O_(3) particles yield the coating with the lowest porosity,and different volume fractions of Al_(2)O_(3)in the feedstock have important effects on the electrochemical behavior of composite coatings.The Al-MMC coating reinforced with 20 vol.%Al_(2)O_(3) particles exhibits the highest Ecorrand the lowest icorrcompared with the other conditions.The order of current density is as follows:AA5083/20vol.%Al_(2)O_(3)<AA5083<AA5083/40 vol.%Al_(2)O_(3)<AA5083/60 vol.%Al_(2)O_(3).

Dynamic evolution of oxide scale on the surfaces of feed stock particles from cracking and segmenting to peel-off while cold spraying copper powder having a high oxygen content

The oxide scale present on the feedstock particles is critical for inter-particle bond formation in the cold spray(CS)coating process,therefore,oxide scale break-up is a prerequisite for clean metallic contact which greatly improves the quality of inter-particle bonding within the deposited coating.In general,a spray powder which contains a thicker oxide scale on its surface(i.e.,powders having high oxygen content)requires a higher critical particle velocity for coating formation,which also lowers the deposition efficiency(DE)making the whole process a challenging task.In this work,it is reported for the first time that an artificially oxidized copper(Cu)powder containing a high oxygen content of 0.81 wt.%with a thick surface oxide scale of 0.71μm.,can help achieve an astonishing increment in DE.A transition of surficial oxide scale evolution starting with crack initiations followed by segmenting to peeling-off was observed during the high velocity particle impact of the particles,which helps in achieving an astounding increment in DE.Single-particle deposit observations revealed that the thick oxide scale peels off from most of the sprayed powder surfaces during the high-velocity impact,which leaves a clean metallic surface on the deposited particle.This makes the successive particles to bond easily and thus leads to a higher DE.Further,owning to the peeling-off of the oxide scale from the feedstock particles,very few discontinuous oxide scale segments are retained at inter-particle boundaries ensuring a high electrical conductivity within the resulting deposit.Dependency of the oxide scale threshold thickness for peeling-off during the high velocity particle impact was also investigated.

Effect of carrier gas species on the microstructure and compressive deformation behaviors of ultra-strong pure copper manufactured by cold spray additive manufacturing

Cold spray(CS)which has recently become a promising additive manufacturing(AM)technology,was used to fabricate ultra-strong pure copper.In addition,the effects of carrier gas species on the microstructural characteristics,mechanical properties and deformation mechanisms were systemically explored.The CSAM copper manufactured with N_(2) carrier gas reveals a heterogeneous bimodal microstructure consisting of ultra-fine grains at the particle interface and relatively coarse grains in inner particles.With He carrier gas,a homogeneous grain structure consisting of ultra-fine grains in most areas was obtained.Compressive tests showed that N_(2) and He carrier gasses enabled ultra-high yield strengths of 340 and415 MPa,respectively.These values are comparable to severely plastic deformed copper,which has extremely low ductility and shape fidelity.On the other hand,both samples showed a strain-softening phenomenon that does not commonly occur at room temperature.The deformation microstructures revealed that dynamic recovery(DRV)and dynamic recrystallization(DRX)phenomena were generated despite being deformed at room temperature.Based on the above findings,the overall deformation mechanisms according to the carrier gas species in the CSAM copper manufacturing process were discussed.Furthermore,the work hardening and softening behaviors of CSAM Cu are predicted by using a constitutive equation.

Experimental Study on Shock Wave Structures in Constant-area Passage of Cold Spray Nozzle

Cold spray is a technique to make a coating on a wide variety of mechanical or electric parts by spraying solid particles accelerated through a high-speed gas flow in a converging-diverging nozzle. In this study, pseudo-shock waves in a modeled cold spray nozzle as well as high-speed gas jets are visualized by schlieren technique. The schlieren photographs reveals the supersonic flow with shock train in the nozzle. Static pressure along the barrel wall is also measured. The location of the head of pseudo-shock wave and its pressure distribution along the nozzle wall are analytically explained by using a formula of pseudo-shock wave. The analytical results show that the supersonic flow accompanying shock wave in the nozzle should be treated as pseudo-shock wave instead of normal shock wave.

Numerical investigation of the rebounding and the deposition behavior of particles during cold spraying

Particles may rebound from a substrate surface during cold spraying, which affects the quality of the coating. In this paper, the rebound phenomenon and its consequences on deposition behavior have been analyzed using the finite element analysis software ANSYS/LS-DYNA version 970. In a range of particle velocities of 50-1000 m/s, increases of the impact velocity caused a rapid decline of the rebound coefficient R to a low point Rmin. After that, R began to rise slowly. Then the effect of the impact velocities and material properties on the rebound phenomenon were analyzed. Both the material strength and density influenced this rebound phenomenon. Four stages of the impact process and a model of strain distribution were proposed in detail to explain the rebound phenomenon.

Experimental and numerical study of deposition mechanisms for cold spray additive manufacturing process

Cold spray is an attractive and rapidly developing process for additive manufacturing with high efficiency and precision,repairing and coating,especially in aircraft and aerospace applications.Cold spray additive manufacturing deposits micro-particles with large plastic deformation below their melting point,eliminating heat effect zone which could deteriorate the quality of repairing zone.The particle deposition in cold spray is a complex process which involves high strain rate,high contact pressure and high temperature.Here we develop,utilize and validate a thermomechanical model to provide a definitive way to predict deposition mechanics and surface deformation evolution for particle deposition process in cold spray additive manufacturing.Both a single particle and dual particles models were developed to investigate the contact interaction between particle/substrate and particle/particle.Different combinations of particle/substrate materials(Cu/Cu,Al/Al,steel/steel,and nickel/nickel)and process parameters were considered in this study.The experimental study was conducted to validate simulation results,providing useful information for understanding the limitations and challenges associated with cold spray additive manufacturing.The framework provides insights into improving the quality and precision of stress/strain formation,particle interactions and particle deposition in cold spray additive manufacturing process.

Optimization of cold spray additive manufactured AA2024/Al_(2)O_(3) metal matrix composite with heat treatment

In this study,the effects of annealing and solution aging heat treatment on the microstructure and mechanical properties of cold spray additive manufactured(CSAMed)AA2024/Al_(2)O_(3) composite were investigated.The results showed that both annealing and solution aging enhanced the interfacial bonding between the deposited AA2024 particles in the composite.The tensile properties of CSAMed AA2024/Al_(2)O_(3) composite were significantly improved by 13.8%for ultimate tensile strength and 47.8%for elongation after solution aging.Microstructural examination and fractographic analysis showed that the fracture mechanism between the adjacent AA2024 particles changed from brittle to ductile-dominant mode after heat treatment.However,the interfacial bonding between the AA2024 and Al_(2)O_(3) particles cannot be improved.For the improvement of mechanical properties,the solution aging was slightly better than annealing.In addition,low-temperature annealing greatly reduced the anisotropy of the mechanical properties of CSAMed AA2024/Al_(2)O_(3) composite.

Microstructure and Mechanical Properties of Ti–Ta Composites Prepared Through Cold Spray Additive Manufacturing

In this study, an innovative approach was used to fabricate Ti–Ta composite biomaterials through cold spray additive manufacturing followed by a diff usion treatment. The microstructure and mechanical properties of the composites were investigated in detail using field emission scanning electron microscopy, electron backscatter diff raction, 3D X-ray computed tomography, tensile test, nanohardness test and resonance vibration test. The obtained results indicated that the prepared composites have inhomogeneity in their microstructure and composition. A unique microstructure, composed of Ti-rich, Ta-rich and diff usion regions, was evolved in the composites due to incomplete diff usion between Ti and Ta splats. Further, Kirkendall pores were formed in the composites due to uneven diff usion of the two phases(of Ti and Ta) during high-temperature heat treatment. The prepared composites simultaneously showed low elastic modulus and high tensile strength which is required for a good biomaterial. Low elastic modulus was associated with the residual pores and the alloying eff ect of Ta in Ti, while high tensile strength was related to the solid solution strengthening eff ects. The obtained results indicated that the prepared Ti–Ta composites have a great potential to become a new candidate for biomedical applications.

Influence of annealing treatment on microstructure and properties of cold sprayed stainless steel coatings

304 stainless steel coatings had been deposited on carbon-steel substrate by cold spray technique, vacuum annealing treatment was applied to the coatings with different temperatures, and the influence of annealing treatment on the microstructure and electrochemical behavior of the coatings in 3.5% NaCl were analyzed. The results indicated that, the cold sprayed coating was constituted by the flattened particles, and the interfaces were clearly observed between the deposited particles. It was also found that annealing treatment led to the recovery and recrystallization of the elongated grains in the as-sprayed coatings with the increase of annealing temperature. When the annealing temperature achieved to 950 ？ C, the interface of particles was disappeared and the coating s structure was made of new recrystallization austenite grains. Annealing treatment increased the potential volts, and reduced the corrosion rate with the increase of annealing temperature. The electrical corrosion morphology indicated that, the corrosion was firstly at the distortion area and the crevice corrosion mechanism played an important part. Based on these results, processes active during annealing treatment are kinetically dependent and strongly influenced by the annealing treatment temperature.