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Laser beam welding and additive manufacturing of duplex stainless steel 2205
Citation Link: https://doi.org/10.15480/882.13582
Publikationstyp
Doctoral Thesis
Date Issued
2024
Sprache
English
Author(s)
Advisor
Referee
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2024-10-22
Institute
TORE-DOI
Citation
Technische Universität Hamburg (2024)
Duplex stainless steels exhibit a unique combination of high mechanical strength, high ductility and good corrosion resistance making them promising candidates for utilization in many industries where parts are subjected to a combined mechanical and corrosive load. In comparison to the more commonly employed austenitic stainless steels, parts can be designed thinner, resulting in savings in material for the construction and energy during the service life. The ductility as well as the corrosion resistance depend upon their ferrite/ austenite duplex microstructure, which is highly susceptible to heat input during manufacture, making the fusion processing challenging. Manufacturing processes must be optimized to obtain sound material and part properties. Additionally, possible deviations from the optimal condition need to be determined. Thereby, the robustness of the fabrication method – when optimal results cannot be achieved due to process constraints, or the results cannot be controlled adequately – can be assessed.
The present work considers laser beam welding and laser additive manufacturing of duplex stainless steels and their influence on the microstructure, quasi-static and fatigue properties. For laser beam welding with subsequent laser surface remelting, it was demonstrated that fatigue properties can be brought up to the level of the base material. For the case of additive manufacturing with laser and wire, a novel process control method was developed. The influence of process and design parameters on the material properties were investigated. Demonstrator parts were manufactured and the residual stress distribution resulting from process design choices was evaluated.
The laser beam welding process is often performed utilizing the keyhole effect. This process mode is characterized by small fusion and heat affected zones. Evaporation of material from the process may lead to the formation of porosity and poor weld surface quality. The high cooling rates inhibit the formation of austenite from the as δ-ferrite solidifying duplex stainless-steel material. The ferritic fusion zone of laser beam weldments exhibits higher strength and reduced ductility compared to the duplex base material, protecting the joint against deformation in quasi-static loading conditions. Under fatigue loading conditions, the strength is determined firstly by geometric notches, which can be alleviated by a laser surface remelting treatment, bringing the fatigue limit of specimens containing joints up to the level of the base material. The modified geometric configuration in additive manufacturing leads to a reduced cooling rate and increased heat accumulation during the fabrication of parts. The heat accumulation can be described by limited exponential growth laws, reaching a steady state after multiple layers of the part have been deposited. The heat accumulation and process defined geometry of the part pose a major challenge during additive manufacturing. This was solved by developing a process control method based on the interaction force between wire-tip and melt-pool. The reduced cooling rates lead to a reversal of the situation from laser beam welding. High amounts of austenite are formed in the weld metal. Large variations of the microstructure could be obtained by the utilization of suitable sample geometries, which allow for good heat conduction, as well as control of the interpass temperature. The heat accumulation led to a nearly entirely austenitic microstructure, while the reheating effect leads to the precipitation of austenite in preceding layers of the samples. Thus, the microstructure is comprised of layers with high strength and low ductility as well as less strong and more ductile layers. This arrangement as well as the texture resulting from epitaxial growth of the δ-ferrite grains along the build direction of the parts leads to strong anisotropy in the quasi-static mechanical properties with the build direction exhibiting up 100 MPa higher yield strength and half the fracture strain. The fatigue properties are governed by defects to which the duplex stainless steel is vulnerable due to its low fatigue crack propagation threshold. Process induced defects include insufficiently remolten parts of the oxide layer and lack of fusion. The formation of defects is governed by the laser intensity distribution during the additive manufacturing process. Defocusing the laser leads to a higher irradiated area and, together with the developed process control method, a reduced feedstock melting rate. This facilitates increased remelting depth into the previous layer and better contact angles. The order of layers has a higher influence on the residual stress distribution than the order of tracks in a layer. The main findings of this thesis can be summarized as follows: Laser surface remelting can bring the fatigue properties of welded joints up to the level of the base material. This has not been shown to be possible with other post-processing methods. For laser and wire additive manufacturing, a closed-loop control method has been developed that can be implemented in almost any existing manufacturing equipment with minimal investment, ensuring process stability and accounting for differences between preset and actual layer heights. As an important step towards the future industrial application of this technology, in addition to the quasi-static properties, the fatigue properties of laser and wire additive manufactured stainless duplex steels have been investigated and evaluated using a fracture mechanics framework.
The present work considers laser beam welding and laser additive manufacturing of duplex stainless steels and their influence on the microstructure, quasi-static and fatigue properties. For laser beam welding with subsequent laser surface remelting, it was demonstrated that fatigue properties can be brought up to the level of the base material. For the case of additive manufacturing with laser and wire, a novel process control method was developed. The influence of process and design parameters on the material properties were investigated. Demonstrator parts were manufactured and the residual stress distribution resulting from process design choices was evaluated.
The laser beam welding process is often performed utilizing the keyhole effect. This process mode is characterized by small fusion and heat affected zones. Evaporation of material from the process may lead to the formation of porosity and poor weld surface quality. The high cooling rates inhibit the formation of austenite from the as δ-ferrite solidifying duplex stainless-steel material. The ferritic fusion zone of laser beam weldments exhibits higher strength and reduced ductility compared to the duplex base material, protecting the joint against deformation in quasi-static loading conditions. Under fatigue loading conditions, the strength is determined firstly by geometric notches, which can be alleviated by a laser surface remelting treatment, bringing the fatigue limit of specimens containing joints up to the level of the base material. The modified geometric configuration in additive manufacturing leads to a reduced cooling rate and increased heat accumulation during the fabrication of parts. The heat accumulation can be described by limited exponential growth laws, reaching a steady state after multiple layers of the part have been deposited. The heat accumulation and process defined geometry of the part pose a major challenge during additive manufacturing. This was solved by developing a process control method based on the interaction force between wire-tip and melt-pool. The reduced cooling rates lead to a reversal of the situation from laser beam welding. High amounts of austenite are formed in the weld metal. Large variations of the microstructure could be obtained by the utilization of suitable sample geometries, which allow for good heat conduction, as well as control of the interpass temperature. The heat accumulation led to a nearly entirely austenitic microstructure, while the reheating effect leads to the precipitation of austenite in preceding layers of the samples. Thus, the microstructure is comprised of layers with high strength and low ductility as well as less strong and more ductile layers. This arrangement as well as the texture resulting from epitaxial growth of the δ-ferrite grains along the build direction of the parts leads to strong anisotropy in the quasi-static mechanical properties with the build direction exhibiting up 100 MPa higher yield strength and half the fracture strain. The fatigue properties are governed by defects to which the duplex stainless steel is vulnerable due to its low fatigue crack propagation threshold. Process induced defects include insufficiently remolten parts of the oxide layer and lack of fusion. The formation of defects is governed by the laser intensity distribution during the additive manufacturing process. Defocusing the laser leads to a higher irradiated area and, together with the developed process control method, a reduced feedstock melting rate. This facilitates increased remelting depth into the previous layer and better contact angles. The order of layers has a higher influence on the residual stress distribution than the order of tracks in a layer. The main findings of this thesis can be summarized as follows: Laser surface remelting can bring the fatigue properties of welded joints up to the level of the base material. This has not been shown to be possible with other post-processing methods. For laser and wire additive manufacturing, a closed-loop control method has been developed that can be implemented in almost any existing manufacturing equipment with minimal investment, ensuring process stability and accounting for differences between preset and actual layer heights. As an important step towards the future industrial application of this technology, in addition to the quasi-static properties, the fatigue properties of laser and wire additive manufactured stainless duplex steels have been investigated and evaluated using a fracture mechanics framework.
Subjects
Additive Manufacturing
Laser directed energy deposition with wire
Laser beam welding
FatigueDuplex stainless steel
DDC Class
660: Chemistry; Chemical Engineering
670: Manufacturing
621: Applied Physics
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2024_TUHH_Odermatt_Laser-beam-welding-and-additive-manufacturing-of-duplex-stainless-steel-2205.pdf
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