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Plasma Transferred Arc Additive Manufacturing Using 17-4 Precipitation Hardened Stainless Steel

  • Author / Creator
    Ibrahim, Sandy
  • Plasma transferred arc additive manufacturing (PTA-AM) is a newly developed additive manufacturing (AM) process for the manufacturing of large industrial parts for the resource sector. A common material used in numerous industries is 17-4 precipitation hardened (PH) stainless steel, mainly due to its good corrosion resistance, ease of fabrication and an UTS of about 1310 MPa at peak age hardening. The objectives of the current research are to identify the operating parameters of 17-4PH stainless steel on the PTA-AM system. This is achieved by identifying process conditions that minimize printing defects in the final parts and identifying the printing parameters for basic shapes. The mechanical properties of printed shapes will be compared with parts manufactured using conventional processing techniques. In the initial stages of printing, after identifying operating parameters for printing 17-4PH stainless, two types of voids were discovered in the AM parts: oxide layers and porosity. The oxide layer was attributed to the lack of continuous shielding atmosphere over the AM part while it is being printed. Supplying a continuous source of argon gas over the entirety of the part during the printing process ensured the elimination of the oxide layers. The other source of voids, porosity, was attributed to porosity in the initial powders and insufficient heat input during the printing process. Changing the powder supplier and raising the heat input during printing resulted in greatly reducing porosity in the final AM part. Peak aging was performed on the final part, with minimum defect. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were conducted to verify the change of the lath martensite matrix into tempered martensite and the observed precipitated copper, respectively. No change in the martensitic matrix was noted. Hardness before and after aging was unaffected by changing processing parameters. It was demonstrated that an operating solution to avoid oxide formation is the use of 5% hydrogen in the shielding, center and powder gas feeds. In the second part of the project, processing parameters that yield minimum porosity and at least of comparable hardness and UTS relative to other manufacturing processes were desired. The PTA-AM system has numerous process parameters including shielding gas pressures and flowrates, current, voltage, angle of powder nozzle deposition relative to the welding direction and speed of an automated table connected to the PTA-AM system. The different combinations of the above parameters can result in a range of mechanical properties and microstructures of the AM samples. Taguchi design of experiments (DOEs) was used to identify the most influential parameters on the final mechanical properties and microstructure. Powder feed rate, current and table speed were determined to be key variables during the printing of the DOE, while all other parameters were kept constant. The printed parts had some variations, but all their mechanical properties fell within, or exceeded, the expected ranges of 17-4PH stainless steel parts manufactured with conventional techniques, before and after heat treatment. Electron backscatter diffraction (EBSD) analyses was used to verify that only a BCC microstructure is present in all stages of heat treated 17-4PH stainless printed using the PTA-AM system.

  • Subjects / Keywords
  • Graduation date
    Spring 2021
  • Type of Item
    Thesis
  • Degree
    Master of Science
  • DOI
    https://doi.org/10.7939/r3-f9yp-8w43
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.