Updated: May 4
Getting started with a new material for cold spray can be daunting. You have an application, you know your performance requirements, and you have a material in mind, but you’re not really sure where to start with all of the different available feedstock and possible cold spray parameters. This post aims to address those challenges and serve as a guide for getting started with your material in cold spray.
There are two primary components in developing a successful cold spray deposition: selecting a feedstock powder and selecting cold spray parameters. This guide is broken down into sections to address each of those components to get you started in creating a well-bonded, fully dense cold spray consolidation.
The Feedstock Powder
Once you have a material in mind, a quick internet search will return many different powder vendors – so, how to choose? Powder characteristics most often reported and specified are composition, particle size, and morphology, and density and flowability. These all play an important role in cold spray.
Cold spray is a solid-state process, meaning there is no melting. Therefore, the composition and any features present in the feedstock prior to cold spray will be retained during the cold spray process. This can be used to your advantage, but also means that you really need to understand the effect of composition on your final property evaluations. These effects are nuanced and are beyond the scope of this quick guide, but suffice it to say that if compositional tolerances are important to you in your final part, you must pay attention to them in the feedstock.
Particle Size and Morphology
The solid-state nature of cold spray relies on mechanical deformation and metallurgical bonding of particles in order to build up layers. Whether a particle adheres to a substrate or rebounds is dependent upon its velocity [1-4]. This is known as the critical velocity, and is dependent upon the density of the material and the size of the particle. Understanding the required velocity for your material can be a good place to start for determining the size distribution to use, but also goes hand-in-hand with cold spray parameter selection – different cold spray system configurations affect the achievable velocities of a given sized particle. Particle morphology can also affect velocity and flight dynamics. Spherical, angular, and even agglomerated powders have been successfully cold sprayed. Approximately equiaxed aspect ratio is preferred over flake-like powders.
Powder Density and Flowability
Powder density and flowability metrics are very important for other AM processes particularly relevant for their spreadability and packing efficiency in a powder bed process. In relation to cold spray, density and flowability come in to play when considering powder feeding. While not an important factor when selecting a feedstock, these characteristics are important to know as you move into the cold spray parameter selection process. An important to note to consider is that just because a powder is not flowable does not mean that it is not able to be fed and cold sprayed, so you can still consider fine, non-flowable powder for your application, assuming the powder meets your other criteria.