E-Thesis 450 views 222 downloads
The corrosion and mechanical performance of industry relevant additively manufactured steels and alloys / IWAN GRECH
Swansea University Author: IWAN GRECH
DOI (Published version): 10.23889/SUthesis.63487
Abstract
Laser Powder Bed Fusion (LPBF) is a manufacturing technique that allows for the production of metallic components directly from CAD files. The technique allows for partsof increased strength to weight ratio to be produced in comparison to wrought parts. Complex geometries with internal lattices can...
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Swansea, Wales, UK
2023
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| Institution: | Swansea University |
| Degree level: | Doctoral |
| Degree name: | Ph.D |
| Supervisor: | Sullivan, James. and Lavery, Nicholas. |
| URI: | https://cronfa.swan.ac.uk/Record/cronfa63487 |
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| Abstract: |
Laser Powder Bed Fusion (LPBF) is a manufacturing technique that allows for the production of metallic components directly from CAD files. The technique allows for partsof increased strength to weight ratio to be produced in comparison to wrought parts. Complex geometries with internal lattices can be printed without the need for post manufacture machining. Parts manufactured by LPBF suffer from inherent build defects including porosity that can negatively impact both the mechanical and corrosion properties of these parts. Whilst the mechanical properties of the materials manufactured using this method have been investigated since the infancy of the technique the corrosion performance of these materials is still largely not understood. This PhD thesis aims to improve and expand upon the current knowledge of the corrosion properties of LPBF manufactured parts, specifically in chloride environments. In order to accomplish this LPBF was used to produce components of various build parameters in Invar, 17-4PH and 316L. The corrosion properties of LPBF manufactured Invar was compared to its wrought counterpart through a range of techniques including time-lapse microscopy (TLM), scanning vibrating electrode technique (SVET), white light interferometry and electron back scatter diffraction (EBSD). It was determinedthat the inherent porosity of the LPBF manufactured Invar resulted in the material being susceptible to localised corrosion attack. This attack occurred predominantly at porosity defects with the majority of localised corrosion initiating at porosity defects located at the melt pool boundary region of the microstructures.316L was also built using LPBF with the distribution of density across the build plate being investigated as well as the corrosion performance of the material. CT scanning was also implemented to evaluate the morphology and location of porosity within the material. To reduce the porosity within LPBF manufactured 316L post production processing via Hot Isostatic Pressing (HIP) was investigated. A total of 12 HIP treatment cycles were devised varying temperature, pressure and time with an additional treatment being further treated by a furnace treatment and water quenching. The mechanical and corrosion performance of the components post HIP was investigated using a range of techniques including potentiodynamic scanning, hardness, fatigue and tensile testing as well as microscopy and x-ray diffraction (XRD). It was found that Post-processing HIP cycles at mid (1125 °C) and high (1200 °C) temperatures remove the melt pools, melt pool boundaries and sub-grain cellular features that are present in the microstructure from the additive process. The post-process HIP cycle at the lowest of the temperatures (700 °C) was found to result in an increased yield strength and a reduced elongation to fracture compared to other treatments. Variations in pressure did not induce a significant difference in properties. HIP treatment 4 - HIP (T700 P137) was seen to have the highest Nf at higher stress over 10 – As built, whilst being intermediate on ultimate tensile strength and recording the lowest average pitting potential. Samples subject to treatment4 - HIP (T700 P137) retained their cellular substructures and grain boundaries have not become enlarged, whilst density has been increased by the HIP treatment. HIP treatments succeeded in reducing the spread of the pitting potentials by 52.5 % compared to as-built parts with the largest reduction in the spread of pitting potentials being achieved in low temperature cycles. However, there is still too large a variation in the corrosion performance of AM parts compared to wrought to instil confidence in their service in corrosive environments over wrought counterparts. LPBF 17-4PH samples were manufactured and the density distribution across the build plate was investigated as well as the corrosion properties of the material. CT scanning was also used to evaluate the porosity of these manufactured samples which contained irregularly shaped lack of fusion defects. |
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| College: |
Faculty of Science and Engineering |