Laser-assisted cold-spray has been recognized for over a decade as a technique capable of depositing high quality coatings. By laser heating (and hence softening) the surface being coated, deposition can occur at particle velocities lower than those normally associated with the cold spray process. This can be used to increase deposition rate. However, it can also be used to facilitate the deposition of higher hardness material combinations, normally more out of the reach of the conventional cold spray process. Laser heating can also reduce the requirements of the process on gas usage and gas heating for a given combination, making it more cost-effective. In the work reported below, the capability of a novel co-axially laser-assisted system (COLA) to deposit higher hardness materials, relevant to a range of different industrial applications, has been evaluated. This system can be retrofitted to conventional cold spray equipment.

Cold gas dynamic spraying (CGDS) can be used to deposit oxygen sensitive materials, such as titanium, without significant chemical degradation of the powder and with minimal heating of the substrate. The process is thus believed to have potential for the deposition of corrosion resistant barrier coatings. However, to be effective a barrier coating must not allow ingress of a corrosive liquid and hence must have minimal interconnected porosity. Thus the aim of the present study was to investigate the effects of processing, including a post-spray annealing treatment, on the deposit meso- and microstructures and corrosion behavior. Commercially pure titanium powder was deposited using pre-heated nitrogen as main and powder carrier gas using a CGT Kinetiks 4000 system to produce coatings on stainless steel. Selected coatings were debonded from the substrate, and the resultant free standing deposits heat treated at 1050° C in vacuum for 60 minutes. Changes in microhardness were measured and correlated with microstructural changes. Optical microscopy, scanning electron microscopy, X-ray diffraction (XRD), helium pycnometry and mercury porosimetry were all employed to examine the microstructural characteristics of coatings and free standing deposits, before and after heat treatment. Their corrosion performance was also investigated using potentiodynamic polarization tests in 3.5 wt% NaCl. The influences of heat treatment and corrosion behavior will be analyzed and discussed in terms of pores structure evolution and microstructural changes.

Despite much research effort and many application development studies, corrosion resistant alloy (CRA) coatings, prepared by thermal spraying still provide greatly inferior corrosion resistance when compared to parent material of the same composition, due to a network of oxides and pores in the coatings. However, the recent development of Cold Gas Dynamic Spray (CGDS) technology has made possible the deposition of low porosity and oxide-free CRA coatings. This paper describes the corrosion performance of Ni-based Alloy 625 following cold spraying onto steel substrates both as-sprayed and following heat treatment. Microstructures and oxygen content of powders and coatings were investigated, and coating microhardness and strength were determined in both as-sprayed and post treated conditions. Mercury intrusion was used to measure the interconnected porosity in the as-deposited and post-treated coatings. Finally, the corrosion behaviour of the sprayed and post-treated coatings was measured using a standard test (ASTM B117-07a).

High-temperature nickel-based superalloys such as 1N718 are widely used in gas turbine components as they remain stable at operating temperatures up to ~750°C. There is now a growing interest in the repair and refurbishment of such components using spray deposition techniques. Although many investigations have been carried out to study the effect of conventional processing on the microstructure and mechanical properties of 1N718, much less attention has been given to the alloy when sprayed to form a coating. The purpose of the present study was to investigate and compare 1N718 deposits produced by HVOF spraying and cold gas spray deposition. Optical microscopy, scanning electron microscopy and X-ray diffraction were employed to examine the microstructural evolution of the coatings and to compare the deposition behaviour of the two different processes. Particular attention was paid to porosity, oxide content and the formation of secondary intermetallic phases. Coating microhardness and bond strength were also measured. Results will be presented and discussed in the context of the different thermal histories of the powder particles in the two processes.

Al-Sn plain bearings for automotive applications traditionally comprise a multilayer structure. Conventionally, bearing manufacturing involves casting the Al-Sn alloy and roll¬bonding to a steel backing strip. Recently, high velocity oxy- fuel thermal spraying has been employed as a novel alternative manufacturing route. The present project extends previous work on ternary Al-Sn- Cu alloys to quaternary systems, which contain specific additions for potentially enhanced properties. Two alloys were studied in detail, namely Al-20wt.%Sn-lwt.%Cu-2wt.%Ni and Al-20wt.%Sn-lwt.%Cu-7wt.%Si. This paper will describe the microstructural evolution of these alloys following HVOF spraying onto steel substrates and subsequent heat treatment. Microstructures of powders and coatings were investigated by scanning electron microscopy and phases identified by X-ray diffraction. Coating microhardnesses were determined in both as-sprayed and heat treated conditions and differences related to the microstructures which developed. Finally, the wear behaviour of the sprayed and heat treated coatings in hot engine oil was measured using an industry standard test and compared with that of conventionally manufactured Al-Sn bearings.

Over the past five years, interest in cold gas dynamic spraying (CGDS) has increased substantially. Considerable effort has been devoted to process development and optimization for low melting point metals such as copper and aluminium. This paper will describe work undertaken to expand the understanding of deposition of titanium by cold spray methods. CGDS deposits have been produced from commercially pure Ti. Using room temperature helium gas, a range of processing conditions, powder size ranges, substrates and substrate preparation methods have been employed to study their impact on deposition of powders. Scanning electron microscopy has been employed to examine deposit microstructures, and microhardness testing of deposits has been conducted. Samples for pull-off bond strength test have been prepared from a number of the more promising sets of parameters and adhesive strengths have been determined. Computational estimates of gas velocity and in-flight particle velocity have been made focusing specifically on the influence that these factors have on the process deposition efficiency. Differences will be discussed in terms of powder feedstock characteristics and the underlying physical and mechanical properties of the powders and substrates.