Laser powder bed fusion of steels : case studies on microstructures, mechanical properties, and notch-load interactions
Afkhami, Shahriar (2022-11-10)
Väitöskirja
Afkhami, Shahriar
10.11.2022
Lappeenranta-Lahti University of Technology LUT
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Konetekniikka
Kaikki oikeudet pidätetään.
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-856-0
https://urn.fi/URN:ISBN:978-952-335-856-0
Tiivistelmä
Additive manufacturing has evolved over the last few decades from utilising a prototyping approach to favouring a fabrication method. Consequently, this technology has become recognised as a potential replacement for traditional fabrication methods employed in the processing of metals, polymers, and biomaterials. In particular, the additive manufacturing of metals can play a significant role in the future of manufacturing due to its capabilities for sustainable production and reducing material waste when compared to conventional methods. However, the additive manufacturing of metals has yet to reach its full potential due to technological drawbacks. Although laser powder-bed fusion technique is one of the most frequently used methods for metal additive manufacturing, the metallic components fabricated still suffer from associated inhomogeneities and weak points. Further research is needed, therefore, to identify factors causing these disadvantages. Acquiring this knowledge is necessary for expanding the applicability of metal additive manufacturing throughout contemporary industry and construction.
On this basis, different aspects of the additive manufacturing of steel – one of the most economical alloy types – have been investigated in this research. This involved examining the microstructures and mechanical properties of two common types of steel – stainless steel 316L and tool steel 18Ni300 – in order to identify some of the effective parameters affecting their properties. Results show that the steels processed by laser powder-bed fusion suffer from the microstructural features associated with the intense thermal gradients present within manufacturing methods, e.g. segregation and cellular/dendritic subgrain structures. Nevertheless, mechanical properties were comparable to those of their conventional counterparts. Furthermore, parameters related to the manufacturing method, building orientation and surface quality were found to have a determining role in influencing the mechanical properties as material characteristics. This leads to the observation that an inappropriate building direction or surface quality can result in inferior mechanical performance.
The interactions between external loads, geometrical notches, inherent defects, and surface features were investigated in 18Ni300 and processed by laser powder-bed fusion. The results showed that notch strengthening under uniaxial quasi-static loads is common in this ductile steel. However, the existence of geometrical notches reduced the specimens’ fatigue performance. Finally, the applicabilities of numerical/analytical approaches such as the Hall–Petch model, Ludwigson equation, Solberg–Berto equations, and Murakami approach were also evaluated for the investigated materials. These models and equations showed relatively good agreement with the experimental results in some cases but, on other occasions, required recalibration or modification to become suitable for metals processed by additive manufacturing.
On this basis, different aspects of the additive manufacturing of steel – one of the most economical alloy types – have been investigated in this research. This involved examining the microstructures and mechanical properties of two common types of steel – stainless steel 316L and tool steel 18Ni300 – in order to identify some of the effective parameters affecting their properties. Results show that the steels processed by laser powder-bed fusion suffer from the microstructural features associated with the intense thermal gradients present within manufacturing methods, e.g. segregation and cellular/dendritic subgrain structures. Nevertheless, mechanical properties were comparable to those of their conventional counterparts. Furthermore, parameters related to the manufacturing method, building orientation and surface quality were found to have a determining role in influencing the mechanical properties as material characteristics. This leads to the observation that an inappropriate building direction or surface quality can result in inferior mechanical performance.
The interactions between external loads, geometrical notches, inherent defects, and surface features were investigated in 18Ni300 and processed by laser powder-bed fusion. The results showed that notch strengthening under uniaxial quasi-static loads is common in this ductile steel. However, the existence of geometrical notches reduced the specimens’ fatigue performance. Finally, the applicabilities of numerical/analytical approaches such as the Hall–Petch model, Ludwigson equation, Solberg–Berto equations, and Murakami approach were also evaluated for the investigated materials. These models and equations showed relatively good agreement with the experimental results in some cases but, on other occasions, required recalibration or modification to become suitable for metals processed by additive manufacturing.
Kokoelmat
- Väitöskirjat [1037]