Page 59 - MSAM-1-2

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

the other hand, methanol (with lower molecular weight) in the form of laser, oxy-fuel combustion, flame spray, or
allows multilayer formations on the powder particles. detonation spray. Compared with these technologies, cold
The multilayer formation can reduce the cold welding spray neither uses high temperature (e.g., selective laser
[14]
[15]
of the powder particles while milling . Kollo et al. melting or direct metal deposition) nor engages in complex
investigated that there was no sticking of aluminum silicon chemical processes (e.g., electroplating). This makes cold
carbide powders on the milling tool when heptane was spray the best fit for working with geometries of complex
utilized instead of stearic acid as a surfactant. Some other shapes and simultaneously accomplishing deposition
factors governing the quantity of surfactant required for without thickness limitation. According to ASTM F2792-
mechanical milling/alloying processes are speed of milling, 12A standard, cold spray is a promising AM technology in
milling temperature, and ball-to-powder ratio. These the industry-scale manufacturing landscape. This standard
factors can greatly affect the adsorption of surfactants on is later replaced by ISO/ASTM 52900:2021.
the surface of the powder particles. In the cold spray process, the parameters that
Lu and Zhang demonstrated the effect of the amount determine a successful deposition also include particle
[16]
of surfactants used during milling on the milling time. morphology shape, type of the material, and the standoff
They reported that the aluminum-magnesium powder distance between substrate and nozzle. The gases used for
yield after milling with 4 wt.% stearic acid was much cold spray are helium, nitrogen, and air . Helium is costly
[7]
higher than the milling done with 1 wt.% stearic acid for and is helpful in cases where powder particles should reach
the same milling time. Another critical observation by high critical velocity required for highly dense coatings.
Shaw et al. was the decrease in crystallite size with the Helium provides a good working temperature; however,
[17]
increase in milling time. However, with the increase in the nitrogen and air can cut the cost of the manufacture of
amount of surfactants, the actual crystallite size increases. coatings . Nitrogen can also prevent oxidation of the
[18]
[7]
They reported that powders milled for 4 h without any coatings . Furthermore, annealing treatments could be a
surfactant had the same crystallite size as that of powders good alternative to increase the denseness of the coatings
milled for 8 h with 1 wt.% stearic acid and almost the instead of using a costly process gas .
[19]
same for powders milled for 16 h with 2 wt.% stearic acid. The typical size range of the powder particles required
Furthermore, the hardness of powders is affected by the for a successful cold spray should be less than 100 µm.
reaction of surfactants with powder particles. Kollo et al. The particles having a size larger than 100 µm may not
[15]
observed that when heptane was used as a surfactant, the get cold sprayed as they are difficult to get accelerated
hardness of powder particles was less compared to the to supersonic velocity by the carrier gas. In general, for
case when stearic acid was used as a surfactant during depositing composite powders, the spray conditions and
milling. Therefore, the choice of surfactant and its amount parameters are typical, and they may not be the same as
to be used for milling have great significance in deciding the parameters used for single-powder deposition. Before
the milling parameters before cold spraying the milled spraying, the different powders are mechanically milled
powders. together to make composite agglomerate. In a cermet

1.4. Cold spray process powder feedstock, the metal powder particles act as a
binder and help to ameliorate bonding. A combination of
Among additive manufacturing-based technologies, there soft metal powders and hard ceramic powders can prevent
are two distinct groups of techniques primarily divided damage to the hard ceramic particles and, hence, help in
based on their functional task, material types being retaining the desired properties intended. This mechanical
handled, or the degree of complexity in their deposition milling also enhances the deposition efficiency and forms
process. These are powder based and non-powder based. thick coatings [7,20] . Figure 2 shows the parameters required
In the non-powder feed method, such as wire arc AM or to obtain a successful cold-sprayed coating.
laser melt deposition wire, a wire feed is melted in a nozzle
through plasma arc or laser. Whereas in the powder feed 1.5. The metal-ceramic interface bonding
deposition method, the powder as feedstock is sprayed characteristics in the cold spray process
onto a substrate in a supersonic/transonic atmosphere Combining ceramics and metals is difficult due to their
to develop 3D coatings or freeform objects through self- different bonding characteristics. In ceramics, atoms have
consolidation. The technique thus works as a direct AM ionic and covalent bonds; on the other hand, metals are
process in high productivity requirements. normally associated with metallic bonds. The flow of

The common denomination among the two groups is electrons is restricted in ceramics. Moreover, the stable ionic
the requirement of a high thermal energy source, either and covalent bonds in ceramics reduce surface interactions,

Volume 1 Issue 2 (2022) 3 https://doi.org/10.18063/msam.v1i2.12