Cold Spray Additive Manufacturing

Cold spray is a solid-state coating deposition technology which has recently been applied as an additive manufacturing process to fabricate individual components and to repair damaged components. In comparison with fusion-based high-temperature additive manufacturing processes, cold spray additive manufacturing (CSAM) has shown to retain the original properties of the feedstock, to produce oxide-free deposits, and to not adversely influence underlying substrate materials during manufacture. Therefore, CSAM is attracting considerable attention from both scientific and industrial communities. Although CSAM is an emerging additive manufacturing technology, a body of work has been carried out by the authors’ and other research groups worldwide, and the technology has been applied across a range of manufacturing areas. This chapter aims to provide a brief introduction of CSAM and acts as a start of this book.

In CSAM, successful particle deposition requires the particles to achieve a higher velocity than critical velocity. Therefore, a thorough understanding of particle acceleration and heating behavior inside and outside a cold spray nozzle is critical for developing high-performance cold sprayed deposits. Over the years, much effort has been devoted to investigate the supersonic gas flow and the consequent particle acceleration and heating behavior inside and outside the nozzle. Experimental investigation is the most direct way to clarify these physical phenomena involved in cold spray process. However, the relatively high money- and timing- cost, especially the infeasibility to capture all the flow features (e.g. flow velocity, temperature, density and turbulence properties) inside and outside the nozzle significantly limits the wide application of experimental approach. To deal with this problem, analytical and numerical modeling were developed and employed in many studies. In the early stage, various analytical models were developed to predict the gas flow properties and particle velocity. These analytical models, mainly one-dimensional (1D) model, normally introduced assumptions and simplifications to the real physical problems, thus the prediction accuracy may be not satisfactory. Thanks to the rapid growth of the computer power, computational fluid dynamics (CFD) technique has become a popular approach to predict the gas flow properties and particle velocity in cold spray. It is highly flexible to simulate the gas flow at different working conditions and costs less than experiment. This chapter provides a comprehensive introduction on the gas flow, particle acceleration, and heat transfer behavior in cold spray.

Manufacturing parameters are critical for the fabrication of cold sprayed deposits with desirable microstructure and excellent performance. In CSAM, key manufacturing parameters that determine the deposition process include gas pressure, gas temperature, gas type, powder feed rate, nozzle traverse speed, scanning step, standoff distance, spray angle and trajectory. These parameters must work in unison and under careful control to produce high-quality deposits. In this chapter, the aforementioned key manufacturing parameters and their roles in cold spray deposition are introduced and the relationship among these parameters are discussed.

The microstructure of cold sprayed deposits directly determines the properties of CSAM products and thus it is of importance to well understand the microstructural evolution mechanism of cold sprayed deposits. The microstructure is largely dependent on the plastic deformation behaviour of cold spray feedstock particles during their deposition. As the plastic deformation of cold spray particles is not homogenous with severe plastic deformation occurring at the contact interfaces, cold sprayed deposits typically have heterogenous microstructure and grain structures. A large number of dislocations and newly formed grains are only localized in the shear zones at the interparticle interfaces, while the microstructure in the interior region is mainly characterized by coarse grains. This chapter thoroughly discusses the formation mechanism of dislocations and ultra fine grains (UFGs) in various cold sprayed deposits during cold spray deposition. The formation mechanisms of intermetallic compounds and amorphous phases are also interpreted and discussed.

Cold sprayed deposits normally have unfavorable mechanical properties in their as-fabricated state, such as lower ductility, lower electrical conductivity, and lower thermal conductivity when compared to conventionally fabricated or fusion-based additive manufactured counterparts due to the inherent defects (i.e., porosity and unbonded interparticle boundary) found in the cold sprayed deposits. These defects generally exist in as-fabricated deposits, particularly when particle impact velocity is low. As structural weaknesses, defects have a negative impact on the mechanical properties of cold sprayed deposit. It is well known that increasing gas pressure and temperature can improve the mechanical properties of as-fabricated deposits. However, gas pressure and temperature cannot be infinitely increased due to the technological limitation of current cold spray systems. Fortunately, in recent years, some newly developed strengthening processes have been found effective to promote the mechanical properties of cold sprayed deposits. In this chapter, various strengthening strategies that have been frequently used for cold sprayed deposits, including conventional annealing, hot isostatic pressing, hot rolling, in-situ shot peening, in-situ densification, in-situ laser assistance, friction stirring and powder heat treatment will be introduced and discussed.

Metal matrix composites (MMCs) are composite materials for which reinforcing particles and/or flakes (e.g., ceramic, diamond, 2D materials) are incorporated into metal or alloy matrix so that the new material can take the advantages of both and to gain new properties. CSAM has demonstrated superiority over conventional fusion-based AM technologies for fabricating MMCs due to its short production time, unlimited product size, high flexibility and suitability for damaged component repair [1, 2]. Over the years, extensive studies have been conducted to investigate the deposition mechanisms, microstructures and properties of cold sprayed MMC deposits. To date, various starting powders have been applied as the feedstocks for the cold spray deposition of MMCs. Typically, the MMC deposits fabricated using different feedstocks are governed by different deposition mechanisms and exhibit different microstructures. This chapter focuses on the MMC deposits made via CSAM, from feedstock preparation, deposition mechanism, microstructure to mechanical properties.

It is known that nanostructured materials generally have improved properties as compared with conventional materials. Therefore, it is of importance for CSAM to gain the capability to produce deposits with nanostructure. However, nanoparticles, due to their low weights, are difficult to deposit on the substrate via CSAM. They are easy to be picked up by the driving gas and thereby suffer from dramatic deceleration when passing through the compressed bow-shock in front of the substrate. Consequently, the impact velocity of nanoparticles is very low so that deposit is hard to form on the substrate. Fortunately, although nanoparticles cannot be deposited directly by CSAM, cold sprayed metal deposits can still exhibit nanostructure. Firstly, the starting feedstock for CSAM can be nanocrystalline powders; in this case, the deposit retains the nanostructure of the starting powders. Secondly, using nanomaterials to reinforce MMC deposits can also enable the deposit to present nanostructure. This chapter aims to provide an overview of the metal deposits with nanostructure produced by CSAM, particularly focusing on the deposit microstructure and properties.

Metallic glasses (MGs) and high entropy alloys (HEAs) are both emerging multi-component alloys with unique microstructures and extraordinary properties. However, there are many challenges during the manufacturing process of MGs and HEAs using conventional fabrication techniques which hinders their development as engineering materials. Cold spray as a burgeoning metal deposition and solid-state additive manufacturing technology provides an alternative to allow the fabrication of large-scale MGs and HEAs. This chapter provides an overview of MGs and HEAs deposits fabricated by cold spray additive manufacturing technique. Firstly, the background of MGs and HEAs are briefly introduced. Then, the feedstock powder for spraying and deposit microstructure evolution are provided. Finally, the particle deformation behavior and bonding mechanism are discussed.

CSAM as an emerging additive manufacturing process has great potentials in fabricating free-stranding components and repairing damage components. It can be used in a wide range of industrial sectors such as aerospace, aviation, marine, automotive and nuclear. Although CSAM has been regarded as an additive manufacturing process in recent years, it typically leaves a rough, undulating and porous surface after fabrication. For practical industrial applications, such as in the repair of a worn surface, the as-fabricated surface is not suitable for immediate use, either due to surface finish or dimensional accuracy requirements. This necessitates the use of a machining or finishing process. For clarifying the machinability of CSAM deposits and explore their potential applications, this chapter provides a thorough review and discussion on the post machining and the industrial applications of CSAM deposits.

CSAM as an emerging additive manufacturing process has been given special attentions by researchers and engineers worldwide. This new additive manufacturing process has shown great potentials in fabricating free-stranding components and repairing damage components. To date, a large number of studies have been carried out to explore the bonding mechanism, microstructures and properties of cold sprayed deposits. Various methods have been developed to strengthen the cold sprayed deposits so that they can be used by industry. This chapter summarizes the main findings of this book, and the future perspectives of CSAM are also proposed.