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Materials Science in Additive Manufacturing Cold spray additive manufacturing of Cu-based materials

powder particles are accelerated with the help of carrier have atomistic clean surfaces that might allow them to
gas. Hence, the particles impact the substrate with high get combined, resulting in welding. However, with time,
kinetic energy and form the coatings [3-5] . plastic deformation of powder particles also increases,
leading to the fracture of particles. In this process, the
1.1. Production of metal powders powder particles may probably adhere to the vial and
There are several methods to produce metal powders ball surfaces. Ductile particles are more prone to the cold
required for spraying purposes, additive manufacturing, welding process. The cold welding of ductile particles can
and other fields. These methods include mechanical milling, be controlled either by altering the surface of these particles
[14]
atomization, chemical precipitation, and reduction. In the or by milling at cryogenic temperatures .
atomization process, the liquid metal alloy is continuously The altering of the surface of powder particles can be
dispersed in the form of fine droplets due to the impact of achieved using process control agent (PCA) or commonly
the medium used for atomization. The different types of referred to as surfactants. The surfactants get absorbed
atomization processes are gas, water, plasma, and centrifugal into the powder particle surfaces, as shown in Figure 1,
atomization; plasma and gas atomization techniques are which results in a decrease in cold welding and an increase
the most popularly used . However, the spherical powders in fracturing. This may be due to the decrease in surface
[6]
produced by such techniques have a low cold spray energy of the newly formed surfaces of powder particles,
deposition efficiency compared to irregular-shaped powders. which prevents them from cold welding. The use of
For this reason, mechanical milling remains a popular choice surfactants in the milling of ductile powder particles
for creating powders for the cold spraying process . increases powder yield.
[7]
1.2. Powder feedstock preparation for cold spraying The amount of surfactant used should be appropriate
enough to cover the surfaces of the powder particles
A prominent powder processing technique for processing completely so as to serve as a deterrent to cold welding of
the powder feedstock before the cold spraying is mechanical the particles. Moreover, the quantity of surfactant required
ball milling. In this process, the powder mixtures are for the purpose of milling is dependent on: (i) The ductility
subjected to high-energy collision by the impact of metallic of the powders to be milled, (ii) thermal as well as chemical
balls, leading to two opposing mechanisms, and these cost ability of the PCA or surfactants, (iii) quantity of
mechanisms are cold welding and fracturing of the powder powder to be milled and its initial size, and (iv) duration
particles. A higher amount of ceramic reinforcements of milling. For example, the amount of surfactant for
leads to more fracturing of powder particles; hence, ductile materials is higher than that of the brittle materials.
smaller powder sizes can be achieved. Furthermore, these Surfactants with a large molecular weight (like stearic acid)
ball-milled powders are then cold sprayed with the help of form monolayers by adsorption on powder particles. On
a convergent-divergent nozzle and a carrier gas to increase
the speed of powder particles to supersonic velocity . The
[8]
convergent-divergent nozzle has an inlet and outlet with a
pressure difference existing between them. The gas flows
in the convergent part of the nozzle at a subsonic speed
and then accelerates to supersonic velocity in the divergent
part of the nozzle. The parameters such as dimensions
of the nozzle, type of the gas used, gas temperature, and
pressure help determine the in-flight characteristics of the
powder particles .
[7]
1.3. Role of surfactants in mechanical alloying
Mechanical treatment by ball milling is advantageous for
the preparation of composite powders [9-13] . Mechanical
alloying involves mixing different metals and forming
an intimate powder mixture as a product. On the other
hand, mechanical milling is a process of reducing the Figure 1. Surfactant added to the powders for ball milling. Surfactants
powder sizes with uniform chemical composition. These get adsorbed to the surface of the powder particles preventing them
[14]
two processes take place due to the high-energy impact of from agglomeration . (Reprinted from Critical Reviews in Solid State
and Materials Sciences, 39(2), Nouri, A., and Wen, C, Surfactants in
the balls on the powder particles, resulting in cold welding Mechanical Alloying/Milling: A Catch-22 Situation, 81 – 108, 2014, with
and fracturing. As a result, the powder particles always permission from Taylor and Francis).

Volume 1 Issue 2 (2022) 2 https://doi.org/10.18063/msam.v1i2.12