Modern Cold Spray

It is a fact that, during the operation of engineered products, the materials of which these products are made of will degrade over time. As a result, engineers and scientists constantly develop new materials and/or techniques to repair and maintain these products to ultimately prolong their service life at minimum cost and with minimum environmental impact. This chapter introduces the reader to an overview of the cold spray technology, an enabler of a broad range of applications, many of which have never been previously possible or commercially practical. To achieve the chapter’s objectives, the beginnings of thermal spray and cold spray technologies are reviewed. Then, the advantages and limitations of cold spray are presented along with a number of applications and a general comparison against conventional thermal spray. Finally, a discussion on the complexity of surface engineering leads to view cold spray technology not in isolation but as a component of a broad range of surface engineering techniques available to solve engineering problems.

In this chapter, the physics involved in cold spray is presented in order to get an understanding of the fundamentals that govern this complex spray process. After an overview of the process basics, the process parameters that influence the process outcome, namely the coating quality through the particle impact velocity, are introduced as well as an overview of materials that are known to be sprayable by cold spray. This is followed by a detailed description of compressible fluid dynamics theory, which is at the heart of the cold spray process, allowing to better understand why the nozzle geometry, gas pressure and gas temperature are the most crucial parameters in cold spray amongst all the spray parameters. This is followed by a complete description of the bonding physics involved in cold spray that includes a comprehensive review of the critical velocity concept and its prediction for various materials. It also includes a detailed description of the adiabatic shear instability phenomena that is present in cold spray when particles impact the substrate as well as a thorough review of the numerical models and results used to gain a better understanding of this complex process of particle deformation and bonding, including localized nanosize features.

Cold spray is a process which relies on the kinetic energy of accelerated particles to impart high strain-rate deformation upon impact, which consequently leads to bonding. Proper selection of process parameters and appropriate powder characteristics are critical for exploiting the process capabilities fully so as to obtain the maximum possible process efficiency and desirable coating properties. A good cold-sprayed coating requires a good quality feedstock powder. It is already known that there exists a critical particle velocity, a material property, for the deposition of particle to occur, as explained in the previous chapters. For a given material, the critical velocity depends on the particle characteristics and the temperature achieved. The characteristics of the powders, such as morphology, size distribution, porosity and chemical purity, contribute to the overall coating properties. This chapter describes the properties of the feedstock required for cold spraying and their various manufacturing routes. The effect of particle size, morphology, oxide content and composition of the feedstock on the cold sprayability is also discussed in detail in this chapter. Cold spraying is increasingly used to deposit composite coatings, and this chapter summarises the characteristic of composite powder manufacturing and their cold sprayability.

In this chapter, coating properties are discussed in the light of the coating microstructure which is the topic of a first specific subchapter due to specific features related to the cold spray itself. The chapter refers to particles, splats and coatings, which result in subheadings but not systematically. The content mainly elaborates on characteristics, phenomena and properties, but also on particular characterization methods provided that they are not conventional and especially suitable for the study of cold spray issues. The first subchapter (Sect. 4.1) is devoted to microstructure since this cannot be disconnected from that, and it is more commonly termed as coating properties. This emphasis is all the more justified as, all in all, microstructures are rather seldom discussed in publications dealing with cold spray. However, considering microstructures is highly relevant to discuss the mechanical and physical behaviour of cold spray coatings due to some specific aspects of cold-sprayed materials, at a submicronic scale in particular. The authors try to highlight what is typical of cold spray rather than adopting a systematic approach to properties per material or per application. For this reason, the scope remains mainly limited to metallic coating materials even though some parts may refer to polymers and ceramics, which can still be considered as exotic, not to say unsuitable, materials for cold spray.

Due to the high impact of particles with substrates in cold spray coating, local plasticity and hence local residual stresses are formed. In this chapter, the formation, magnitude, distribution, simulation, and life enhancement of the residual stress induced by cold spray are studied. First, the cause of residual stress is discussed. Then, the methods for quantifying these stresses are reviewed, and measurement results are presented. Second, using sound constitutive plasticity models and incorporating underlying factors in a cold spray process, the numerical procedure for modeling the cold spray process is laid out. Finally, the effect of cold spray on fatigue life enhancement of the substrate is discussed. A comprehensive review of the pertaining literature is also provided in this chapter.

There are two main methods of injecting the powder feedstock into the gas stream: downstream and upstream injection. These methods came about from two different development threads, as explained in Chap. 1 as well as, in more detail, in the last chapter of this book. These distinct threads equally produced two distinct families of commercial equipment, each one with its own unique attributes and challenges. Commercial cold spray equipment became available in the late 1990s with a surge of machine suppliers, many have already dissolved, and many others are well established today. This chapter describes the latest commercially deployed equipment as of the time of this writing, so keep in mind that the rapid evolution of cold spray technology may likely result in the availability of more advanced equipment by the time the book is published. The chapter discusses fundamental machine design features of each one of the families which determines their capacity, portability, applicability, and cost. As every cold spray application requires a unique set of conditions for it to be viable, each type of equipment poses advantages and disadvantages; therefore, proper equipment selection represents the best engineering practice in order to obtain the best results from cold spray technology. The last section of the chapter describes the options for equipment automation as it applies to typical applications of the cold spray technology.

In laser-assisted cold spray (LACS) technology, the cold spray gun is coupled, in a single device, with one or more laser heads resulting in an effective combination of laser pretreatment of the substrate and the build-up of coating. Compared to coatings which have been produced by conventional cold spray process, LACS coatings present improved properties: higher adhesion and cohesion strength, less porosity, higher hardness, etc. Moreover, LACS presents high deposition efficiencies, making this method an attractive solution for the formation of advanced coatings.

The evolution of the LACS process is presented in the current chapter. Various LACS set-ups have been developed over the past years. Cold spray gun has been coupled with heating laser and/or ablation laser of continuous or pulsed mode. These set-ups are stated in the chapter together with experimental results from several applications of the LACS process.

Over the past two decades, in the coating industry as in many others, quality assurance has become one of the sinews of war. There is a growing need for noncontact, nondestructive monitoring tools that can provide useful information about the process in order to better control it, make it repeatable, and prevent problems. In the cold spray process, particle velocity is widely recognized as the most critical parameter governing coating formation. Various methods to measure particle velocity are discussed in this chapter, as well as their advantages and drawbacks. Also discussed are the challenges of characterizing small, fast, and cold particles in general. Finally, we demonstrate how particle size and flow can be calculated.

Dust collection is necessary due to the fact that the deposition efficiency of any spraying process is rarely 100 % and therefore, depending on the application, any fugitive particulate or overspray must be collected to comply with the workplace environmental and safety requirements. The primary way for a powder to be reclaimed is through the use of a dust collector system designed to meet the environmental challenges of the particulate running through it. The most important part of the collector is the filter, which is designed with a filtration media to get the longest filter life possible while providing efficiency to meet environmental expectations. Dust collector’s themselves can also have technologies that enhance product performance and serviceability. To collect particulate properly, best practices guidelines are described for ventilating hoods and booths. Lastly, once powder is reclaimed, considerations for disposal and regulatory compliance must also be addressed.

In this chapter, we discuss some of the most common applications as well as application trends for cold spraying at the time of this writing. It is recognized that new technologies may often require a lengthy adoption incubation period, depending on the success and influence of early adopters. At the time of this manuscript, the strongest adoption pull appeared to come from aircraft manufacturers and maintenance depots willing to rescue hard-to-restore magnesium castings. Likewise, an increasing number of adopters had emerged from the heavy equipment sector, where restoration of large cast iron castings was extremely attractive as no other restoration methods existed. This chapter is divided into four subsections, including repair and restoration in aerospace, antimicrobial bactericide coatings, tribological coatings, and iron and aluminum casting repair.

As an emerging technology, cold spray has proven to be useful for many applications, many of which are described in this book. While cold spray equipment designs and operating techniques are approaching optimal forms, the benefits and costs of its use for specific purposes must be quantified before it can be considered to be commercially viable. This chapter presents an organized, cost estimating framework that can be employed prior to commitment to any well-defined application. The cost framework includes feedstock usage, such as gas and powder, as well as economic factors, such as utilization and depreciation. A method of organizing these factors within a convenient spreadsheet is described.

In the past 10 years, the cold spray process has gained in maturity leading to a significant increase in the patents released on system innovation and process implementation in number of industrial sectors. Since the initial intellectual property patent review published by Irissou et al. in 2008 and covering the early development of cold spray from unsuccessful attempts in 1915 to the modern development in Russia and USA, over 300 new patents were awarded. The motivation for this chapter was to provide an overview of the recent, key developments in the expanding field of the cold spray technology. The patent search was performed using the software ORBIT from Questel (Paris, France), and the definitions of the cold spray process included: cold gas dynamic spray (CGDS), kinetic spray (KS), supersonic particle deposition (SPD), dynamic metallization (DYMET) or kinetic metallization (KM). Only the US patents, which were found to be excusive to the cold spray process, were reviewed.

Three main topics covered by the patent review consist of: (1) apparatus and methods, (2) feedstock and (3) applications. The apparatus and method section includes a review of patents aimed towards the improvement of the cold spray systems and system control. The feedstock section includes feedstock material design for improved sprayability and targeted chemical, mechanical and microstructural material properties. The cold spray applications section covers the field of protective coatings, welding, brazing and component repair as well as additive and near-net-shape manufacturing, found to be of great interest to the automotive and aerospace industries. Other patents reviewed consist of cold spray process applications in the field of biomedical and electronic devices.