Mechanical response of type 17-4 precipitation hardened stainless steel produced by laser engineering net shaping: influence of heat treatment procedure.
Mochinya, Takalani
Mochinya, Takalani
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Abstract
Precipitation hardened stainless steel, of 17-4 PH type, has a weight percentage of 15–17.5 wt% chromium, 3-5 wt% copper and 3-5 wt% nickel. In addition, 17-4 PH is the most widely used grade of precipitation hardened stainless steels due to its affordability, high tensile strength, resistance to corrosion and comparatively good fracture toughness. Consequently, 17-4 PH SS finds usage in many industries for applications like biomedical tools, chemical industries, aircraft components and nuclear power plants because of its combination of superior mechanical properties and good corrosion resistance. Laser Additive Manufacturing (LAM) is a processing technique that builds parts layer upon layer by melting metal powders using a laser beam. Furthermore, Laser Engineered Net Shaping (LENS) is a type of LAM technique that uses a direct energy source (laser) to fabricate components from metallic powders. LENS is the most common Direct Energy Deposition (DED) process, but LENS is associated with some imperfections during fabrication, which are of major concern, ultimately limiting the mechanical properties of the fabricated parts. Moreover, it is common knowledge that by optimizing and standardizing of the LENS process, would result in comparable desired properties of the wrought 17-4 PH SS. This could lead to the successful implementation of LENS for the processing 17-4 PH SS in various industries such as the aerospace. In this study, fabrication of the 17-4 PH SS was carried out using the LENS technology. The optimized LENS process parameters were selected based on the build samples’ microhardness, build density and porosity of the as-built 17-4 PH SS coupons. Analysis of the microstructure was performed using an Optical Microscope (OM) and porosity analysis was conducted on the OM micrographs. Heat treatment protocol was conducted according to H900 and H925 standard conditions. Microstructural evolution, compositions and phases present were examined using Scanning Electron Microscope Equipped with Energy Dispersion Spectroscopy (SEM/EDS), Electron Backscatter Diffraction (EBSD) and X-ray Diffraction (XRD) techniques to establish the relationship between microstructure and processing parameters. The mechanical properties were evaluated by Vickers microhardness and tensile testing techniques. From the results obtained, it was deduced that optimization of the LENS process parameters (laser power, scanning speed and hatch space) produced 17-4 PH SS samples with an efficient energy density to eliminate defects such as lack of fusion and porosity while producing dense sample of up to 99.96 wt.% build density. It was observed that the application of heat treatment led to grain refinement, which transformed the large coarse ferrite dominant microstructure to a finer ferrite phase with retained austenite in small quantities of 0.01 wt% to 0.02 wt%. Additionally, heat treatment of the 17-4 PH SS samples resulted in the decrease of grain sizes and increased the microhardness. The microhardness results reveal that the values increased from 336 HV0.3 to 487 HV0.3 and 512 HV0.3. Furthermore, the tensile tests results showed a yield strength (YS) of 1097.11 MPa (ageing at H900, for 1 hour) and 1112.04 MPa (ageing at H925, for 4 hours); while the ultimate tensile strength (UTS) was 1040.28 MPa (ageing at H900, for 1 hour) and 1132.76 MPa (ageing at H925, for 4 hours). Consequently, the increase in strength of the heat-treated 17-4 PH SS was
credited to Cu-precipitates produced by aging treatment, which hinders dislocation movement. However, it was observed that the increase in tensile strength compromised the plasticity properties of the heat-treated 17-4 PH SS produced. The research's goal was accomplished when heat treatment profiles for 17-4 PH SS produced by LENS-DED were determined. This shows that AM technique can produce parts with the right mechanical quality for aerospace application, which may result in lower manufacturing costs when used commercially.
Description
Submitted in partial fulfilment of the requirements for the degree, Master in Engineering: Metallurgical Engineering in the Department of Chemical, Metallurgical and Materials Engineering within the Faculty of Engineering and the Built Environment at the Tshwane University Of Technology.
Date
2023-11-01
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Tshwane University of Technology
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Keywords
Precipitation, Laser Engineered Net Shaping(LENS), Stainless steel