Microstructural evolution in solution heat treatment of gas- atomised Al alloy (7075) powder for cold spray

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 um for untreated feedstock). Improved particle–substrate bonding was evidenced by formation of jets at the particle boundaries

Structure property relationship of suspension thermally sprayed WC-Co nanocomposite coatings.

Tribomechanical properties of nanostructured coatings deposited by suspension high velocity oxy-fuel (S-HVOF) and conventional HVOF (Jet Kote) spraying were evaluated. Nanostructured S-HVOF coatings were obtained via ball milling of the agglomerated and sintered WC-12Co feedstock powder, which were deposited via an aqueous-based suspension using modified HVOF (TopGun) process. Microstructural evaluations of these hardmetal coatings included transmission electron microscopy, x-ray diffraction, and scanning electron microscopy equipped with energy dispersive x-ray spectroscopy. The nanohardness and modulus of the coated specimens were investigated using a diamond Berkovich nanoindenter. Sliding wear tests were conducted using a ball-on-flat test rig. Results indicated that low porosity coatings with nanostructured features were obtained. High carbon loss was observed, but coatings showed a high hardness up to 1000 HV2.9N. S-HVOF coatings also showed improved sliding wear and friction behavior, which were attributed to nanosized particles reducing ball wear in three-body abrasion and support of metal matrix due to uniform distribution of nanoparticles in the coating microstructure

Structure-property relationships in a CoCrMo alloy at micro and nano-scales

This investigation considered the multiscale tribo-mechanical evaluations of CoCrMo (Stellite®21) alloys manufactured via two different processing routes of casting and HIP-consolidation from powder (Hot Isostatic Pressing). These involved hardness, nanoscratch, impact toughness, abrasive wear and sliding wear evaluations using pin-on-disc and ball-on-flat tests. HIPing improved the nanoscratch and ball-on-flat sliding wear performance due to higher hardness and work-hardening rate of the metal matrix. The cast alloy however exhibited superior abrasive wear and self-mated pin-on-disc wear performance. The tribological properties were more strongly influenced by the CoCr matrix, which is demonstrated in nanoscratch analysis

Post deposition heat treatment of cold sprayed C355 deposits for repair: Microstructure and mechanical properties

Cold gas dynamic spray is increasingly used for dimensional repair in the aerospace sector as it is capable of producing dense, oxide-free deposits of significant thickness and with good levels of adhesion and inherent mechanical strength. There is significant interest in extending the application of cold spray deposits to include structural, load-bearing repairs. However, particularly for high strength aluminium alloys, cold spray deposits can exhibit high levels of porosity and micro-cracks, leading to mechanical properties that are inadequate for most load bearing applications. In this work, heat treatment was investigated as a potential means of improving the properties of a cold sprayed Al alloy C355 deposit. C355 alloy deposits were produced using two process gas temperatures (350°C and 500°C) and three gas pressures (40, 50 and 60 bar) using a commercially available HPCS system. Microstructural analysis of the coatings revealed that the optimal microstructure (ca. 1% porosity) was obtained at 500°C and 60 bar. Therefore, coatings produced with process conditions of 500°C and 60 bar were heat treated at 175, 200, 225, 250°C for 4h in air and the evolution of the microstructure and microhardness was analysed. The results show that heat treatment at 225°C can decrease porosity (<0.2%) and retain high hardness (105 HV0.05vs 130 HV0.05as-sprayed). Further investigation was performed on as-sprayed and 225°C heat treated deposits. The results show that this heat treatment can halve residual stress (-50 MPa vs -100 MPa as-sprayed), and improve tensile properties (UTS). Therefore, this work has demonstrated that the heat treatment of C355 cold sprayed deposits at 225°C can significantly improve their properties

Suspension plasma sprayed coatings using dilute hydrothermally produced titania feedstocks for photocatalytic applications

SPS titania coatings, with applications in water purification, were formed using continuous hydrothermally produced feedstocks for the first time. Coating photoactivity was compared with CVD and P25 analogues.</p

A Comparison of the Potential Capability of SFS, SPS and HVSFS for the Production of Photocatalytic Titania Coatings

The photocatalytic capabilities of titanium dioxide are widely published. Reported applications of titania coatings include air purification, water purification and self-cleaning. Suspension spray has been highlighted as a possible route for the deposition of highly active nanostructured TiO2 coatings. Published work has demonstrated the capabilities of suspension plasma spray and high-velocity suspension flame spray; however, little work exists for suspension flame spray (SFS). Herein, these three suspension spray processes are compared as regards their capability to produce photocatalytic TiO2 coatings and their potential for industrial scale-up. A range of coatings were produced using each process, manipulating coating parameters in order to vary phase composition and other coating characteristics to modify the activity. The coatings produced varied significantly between the processes with SFS being the most effective technique as regards future scale-up and coating photoactivity. SFS coatings were found to be up to nine times more active than analogous coating produced by CVD