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.
Composition
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.
Cold Spray-Ready Feedstock Powders
If all of this sounds daunting, consider selecting a feedstock that has already been used in cold spray. The scientific community has been researching cold spray for years and there are many peer-reviewed publications that you can use as a starting point for selecting a feedstock powder and cold spray parameters – no need to reinvent the wheel!
For folks in the Department of Defense, the Qualified Products List (QPL) can also be a great place to start! The US Army Research Laboratory has spearheaded an effort to redefine cold spray powder specifications, and qualified powders are listed on the QPL [5]. Listed products have associated cold spray data sheets containing spray parameters that can be used as a starting point with the materials when developing your application. Additionally, products listed on the QPL have strict quality metrics to adhere to, ensuring a robust supply chain for your application.
Additionally, Powders On Demand sells powders from multiple vendors that have been developed and optimized specifically for cold spray. Reported product specifications for a given feedstock powder can include hardness when cold sprayed (for powders for wear and corrosion applications) or tensile properties in the as-sprayed condition (for powders for structural applications), in addition to standard morphology and size distribution metrics.
Cold Spray Parameters
Once you’ve selected a powder or two, you can narrow down your cold spray parameters. If you selected a powder from the QPL or from Powders on Demand, those sources can provide you with recommended spray parameters, significantly reducing your development time. If you need to develop parameters yourself, you have come to the right place!
If you’re feeling fundamental, you can calculate the critical velocity for your material and selected size distribution and then calculate the system parameters to get you there. If this sounds overwhelming, a reduced Edisonian approach can be used. First, review literature for any cold spray work on your material or similar materials; this can get you to the right ballpark of conditions. Then, we recommend the use of line studies at multiple parameter sets to rapidly evaluate multiple conditions in a short time frame. At this stage in your research, there is no need to spray a large block to home in on conditions. Spray 5-6 lines at different conditions modulated around the ballpark you found in literature, prepare a cross section of each line, and evaluate each region for porosity and deformation at the substrate/coating interface, as well as percent inclusion if you are working with a multi-phase feedstock.
Deposit Porosity
Porosity will give you an indication of the strength and quality of your deposit; when optimized, cold spray is capable of achieving fully dense (less than 1% porosity) parts. A caveat here is that just because you have a fully dense part does not mean you have good particle to particle adhesion; this will need to be evaluated and optimized in subsequent testing. But to get you started, limiting porosity will be enough.
Substrate Deformation
Deformation at the substrate/coating interface will give you an indication of the degree of bonding at the interface; generally, higher deformation (a “wavy” appearance) at the interface indicates a high degree of mechanical interlocking and strong bonding between the substrate and cold spray coating, while a smoother interface means the opposite. One is not inherently better than the other and is determined by your application; a protective coating where adhesion to the substrate is critical would require different adhesion than a bulk additive part where the substrate will be removed. If you require a strong bond between coating and substrate but have a smooth interface, you may be able to introduce a bond coat. On the contrary, if you have a deformed interface but require low adhesion, you may be able to introduce a release layer. Additionally, depending on the thickness of your substrate in your final application, interface roughness and high deformation may indicate high amounts of residual stress that can be detrimental to thin substrates. These are nuanced and are beyond the scope of this quick guide, but suffice it to say that you have options for further optimization.
Percent Inclusion
If you have a multi-phase feedstock, percent inclusion will quantify the amounts of different phases present in your deposit. This is particularly important if you require a specific amount of a given phase in the coating. For example, cold-sprayed wear coatings leverage ductile metals to deposit brittle ceramics, but if the ceramic content in the deposit is too low, the required wear properties will not be met. The requirements for percent inclusion will greatly vary by application, and it is important to quantify this early on in your development process.
Cold Spray Parameters at Your Fingertips
As previously mentioned, products listed on the QPL for the MIL-DTL-32495 Powders for Cold Spray Deposition have associated cold spray parameters [5]. Additionally, reach out to others in the cold spray community. Members of the Cold Spray Action Team can help connect you with an expert in your particular material or direct you to someone who has qualified a process similar to yours. The Information Age can feel overwhelming at times, but the Cold Spray Action Team can help guide you through it!
Works Cited
5. MIL‐DTL‐32495, Detail Specification: Aluminum‐based powders for cold spray deposition, Department of Defense, 2014.