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Protection and Restoration
form a desired geometry. Some systems use a solid wire feed, most commercial
methods employ metal powder. The two most common commercialized methods for
MAM include powder bed fusion (PBF) and direct energy deposition (DED) with
laser powder bed fusion (LPBF) printers being the most widely available systems on
the market. LPBFs attain a superior resolution with a typical layer thickness between
10 and 50 μm, while powder-fed DED printers have around a 250 μm layer thickness. However, an important factor in both precision and quality of the build begins
with preparation of the starting alloy powder. Consistent powder geometry, composition, and flowability are required to ensure repeatability of metal printed structures.
A specific size of particles must ensure a good packing behavior and limit porosity.
The average particle size distribution is 10–45 μm in LPBF and 20–200 μm in DED.
Powder flowability improves with increased sphericity, which is reflected in quality
of a finished product; but if the powder is too flowable, then spreading defects can
be produced during deposition. If metal powder properties such as size, sphericity, surface texture, and chemical composition vary within a single batch, a printed
part may be produced with more prominent defects, such as high porosity and poor
surface finish.Cooke et al. (2020) present several techniques for atomizing metal for
PBF and DED applications, including gas atomization, high-pressure water atomization, plasma rotating electrode, plasma atomization, electrolytic processing, and
mechanical crushing. They point out gas atomization and sometimes high-pressure
water atomization are the most common and preferred methods in the industry. Both
the processes pour molten metal through a nozzle. When it comes in contact with a
high-pressure fluid stream, the stream separates and quenches the metal into small
particles which can then be used as a starting powder for MAM (Cooke et al., 2020).
Before describing application of structural materials to additive manufacturing,
Liu et al. (2021) introduce the concept of four-dimensional (4D) printing, which has
emerged with involvement of versatile shape-morphing systems. Typical examples
of shape-morphing assemblies are origami and kirigami, the former being the art
of folding thin sheets into 3D objects with rich geometric algorithms, and the latter a variation of origami in which a material is cut when the structure is folded.
In 4D printing, certain environmental stimuli, such as exposure to heat, magnetic
fields, liquids, electricity, light, gas, prestress, or their combinations are posed on a
3D-printed material. As a result, in response to these stimuli, the material autonomously and programmably changes its configuration or function. With 4D printing,
a multi-material strand can be folded into a desired shape and in the last decade
various smart materials have been developed for 3D printing and self-shaping assembly. Multiple prospects for structural materials AM have been raised, including
multi-material AM (MMa-AM), multi-modulus AM (MMo-AM), multi-scale AM
(MSc-AM), multi-system AM (MSy-AM), multi-dimensional AM (MD-AM), and
multi-function AM (MF-AM) (Liu et al., 2021).
Additively manufactured materials, apart from a microstructure typical for each
respective AM method, may possess some process-induced defects. In a part produced by PBF, defects are often similar to those after welding (Singh et al., 2020).
Defects like cracking, high surface roughness, unintended anisotropic mechanical
and physical properties, or anisotropic shrinkage depend on controllable process
Protection and Restoration
form a desired geometry. Some systems use a solid wire feed, most commercial
methods employ metal powder. The two most common commercialized methods for
MAM include powder bed fusion (PBF) and direct energy deposition (DED) with
laser powder bed fusion (LPBF) printers being the most widely available systems on
the market. LPBFs attain a superior resolution with a typical layer thickness between
10 and 50 μm, while powder-fed DED printers have around a 250 μm layer thickness. However, an important factor in both precision and quality of the build begins
with preparation of the starting alloy powder. Consistent powder geometry, composition, and flowability are required to ensure repeatability of metal printed structures.
A specific size of particles must ensure a good packing behavior and limit porosity.
The average particle size distribution is 10–45 μm in LPBF and 20–200 μm in DED.
Powder flowability improves with increased sphericity, which is reflected in quality
of a finished product; but if the powder is too flowable, then spreading defects can
be produced during deposition. If metal powder properties such as size, sphericity, surface texture, and chemical composition vary within a single batch, a printed
part may be produced with more prominent defects, such as high porosity and poor
surface finish.Cooke et al. (2020) present several techniques for atomizing metal for
PBF and DED applications, including gas atomization, high-pressure water atomization, plasma rotating electrode, plasma atomization, electrolytic processing, and
mechanical crushing. They point out gas atomization and sometimes high-pressure
water atomization are the most common and preferred methods in the industry. Both
the processes pour molten metal through a nozzle. When it comes in contact with a
high-pressure fluid stream, the stream separates and quenches the metal into small
particles which can then be used as a starting powder for MAM (Cooke et al., 2020).
Before describing application of structural materials to additive manufacturing,
Liu et al. (2021) introduce the concept of four-dimensional (4D) printing, which has
emerged with involvement of versatile shape-morphing systems. Typical examples
of shape-morphing assemblies are origami and kirigami, the former being the art
of folding thin sheets into 3D objects with rich geometric algorithms, and the latter a variation of origami in which a material is cut when the structure is folded.
In 4D printing, certain environmental stimuli, such as exposure to heat, magnetic
fields, liquids, electricity, light, gas, prestress, or their combinations are posed on a
3D-printed material. As a result, in response to these stimuli, the material autonomously and programmably changes its configuration or function. With 4D printing,
a multi-material strand can be folded into a desired shape and in the last decade
various smart materials have been developed for 3D printing and self-shaping assembly. Multiple prospects for structural materials AM have been raised, including
multi-material AM (MMa-AM), multi-modulus AM (MMo-AM), multi-scale AM
(MSc-AM), multi-system AM (MSy-AM), multi-dimensional AM (MD-AM), and
multi-function AM (MF-AM) (Liu et al., 2021).
Additively manufactured materials, apart from a microstructure typical for each
respective AM method, may possess some process-induced defects. In a part produced by PBF, defects are often similar to those after welding (Singh et al., 2020).
Defects like cracking, high surface roughness, unintended anisotropic mechanical
and physical properties, or anisotropic shrinkage depend on controllable process