96
Remanufacturing and Advanced Machining
parameters having a complex influence over thermal behavior of the melt pool. One
of the most common factors for these defects is wrong heat input rates during hightemperature solidification. Surface defects usually occur because of high cooling
rates and repeated thermal loading. Speidel et al. (2021) named other defects induced
by PBF processes that are widely used, such as poor surface quality generated by
layer-by-layer supply, unfused powder, balling, and subsurface porosity. The authors
emphasize that a combination of discretely sized powder feed-stocks influencing
roughness and the very layer-based manufacturing approaches leading to staircase
effects mean that most AM parts require some form of post-processing and surface
finishing prior to application.
Among the post-processing techniques, metallurgical methods with hot isostatic
pressing should be mentioned, as well as principal heat treatments and stress relief
methods (Singh et al., 2020). In the heat treatment process, it is very important to
obtain a proper microstructure that affects mechanical properties of metal. The thermal treatment is required also in order to reduce porosity and other defects. In the
case of EBM parts, due to the higher processing temperature, there is little need for
stress relief, but some material may still benefit from HIP in the heat treatment process to get a more optimal microstructure (Singh et al., 2020).
Solution, Quench, Age, and Temper Heat Treatments. For precipitate hardened
materials, a solution heat treatment is required. It should be long enough to dissolve
the precipitate but short enough to limit grain growth. These processes of solution,
quench, age, and temper heat treatment tend to be alloy specific, but they give different starting conditions of AM materials compared to cast and wrought materials of
the same composition (Singh et al., 2020).
Hot Isostatic Press (HIP). Most metals are heat-treated by this method to close
down pores and cracks developed in the material. This process involves pressure
and high temperatures working simultaneously to reduce porosity that may lead to a
poorer tensile and fatigue performance. Most post-processed AM parts can undergo
HIP treatment. Although it is not necessary to treat all additively manufactured parts
this way, when high performance or safety is required, HIP is included in the standard process for a wide variety of materials (Singh et al., 2020).
Stress Relief. It is the process of material recovery. At an elevated temperature,
the atomic diffusion rate increases and atoms from the region of higher stress start
to move toward the region of lower stress, which results in relief of internal strain
energy. PBF produces a high thermal slop layer by layer, which generates a high
degree of localized residual stresses throughout the part, thus a stress relief is necessary to equalize the stress gradients (Singh et al., 2020).
2.1.3 am meThods relaTed To The ProducT life cycle
Sanchez et al. (2020), trying to identify sustainability benefits of AM, distinguish
four main stages of a product lifecycle:
1. Design
2. Production
Remanufacturing and Advanced Machining
parameters having a complex influence over thermal behavior of the melt pool. One
of the most common factors for these defects is wrong heat input rates during hightemperature solidification. Surface defects usually occur because of high cooling
rates and repeated thermal loading. Speidel et al. (2021) named other defects induced
by PBF processes that are widely used, such as poor surface quality generated by
layer-by-layer supply, unfused powder, balling, and subsurface porosity. The authors
emphasize that a combination of discretely sized powder feed-stocks influencing
roughness and the very layer-based manufacturing approaches leading to staircase
effects mean that most AM parts require some form of post-processing and surface
finishing prior to application.
Among the post-processing techniques, metallurgical methods with hot isostatic
pressing should be mentioned, as well as principal heat treatments and stress relief
methods (Singh et al., 2020). In the heat treatment process, it is very important to
obtain a proper microstructure that affects mechanical properties of metal. The thermal treatment is required also in order to reduce porosity and other defects. In the
case of EBM parts, due to the higher processing temperature, there is little need for
stress relief, but some material may still benefit from HIP in the heat treatment process to get a more optimal microstructure (Singh et al., 2020).
Solution, Quench, Age, and Temper Heat Treatments. For precipitate hardened
materials, a solution heat treatment is required. It should be long enough to dissolve
the precipitate but short enough to limit grain growth. These processes of solution,
quench, age, and temper heat treatment tend to be alloy specific, but they give different starting conditions of AM materials compared to cast and wrought materials of
the same composition (Singh et al., 2020).
Hot Isostatic Press (HIP). Most metals are heat-treated by this method to close
down pores and cracks developed in the material. This process involves pressure
and high temperatures working simultaneously to reduce porosity that may lead to a
poorer tensile and fatigue performance. Most post-processed AM parts can undergo
HIP treatment. Although it is not necessary to treat all additively manufactured parts
this way, when high performance or safety is required, HIP is included in the standard process for a wide variety of materials (Singh et al., 2020).
Stress Relief. It is the process of material recovery. At an elevated temperature,
the atomic diffusion rate increases and atoms from the region of higher stress start
to move toward the region of lower stress, which results in relief of internal strain
energy. PBF produces a high thermal slop layer by layer, which generates a high
degree of localized residual stresses throughout the part, thus a stress relief is necessary to equalize the stress gradients (Singh et al., 2020).
2.1.3 am meThods relaTed To The ProducT life cycle
Sanchez et al. (2020), trying to identify sustainability benefits of AM, distinguish
four main stages of a product lifecycle:
1. Design
2. Production