VTechWorks staff will be away for the winter holidays starting Tuesday, December 24, 2024, through Wednesday, January 1, 2025, and will not be replying to requests during this time. Thank you for your patience, and happy holidays!

Browsing by Author "Mills, Robert Jeffrey"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Abrasive Blasting with Post-Process and In-Situ Characterization
    Mills, Robert Jeffrey (Virginia Tech, 2014-07-25)
    Abrasive blasting is a common process for cleaning or roughening the surface of a material prior to the application of a coating. Although the process has been in practice for over 100 years, the lack of a comprehensive understanding of the complex interactions that exist with the process can still yield an inferior surface quality. Subsequently, parts can be rejected at one of many stages of the manufacturing process and/or fail unexpectedly upon deployment. The objective of this work is to evaluate the effect of selected input parameters on the characteristics of the blasted surface characteristics so that a more useful control strategy can be implemented. To characterize surface roughness, mechanical profilometry was used to collect average roughness parameter, Ra. Decreasing blast distance from 6” to 4” gave ΔRa = +0.22 µm and from 8” to 6” gave ΔRa = +0.22 µm. Increasing blast pressure from 42 psi to 60 psi decreased the Ra by 0.33 µm. Media pulsation reduced Ra by 0.56 µm and the use of new media reduced Ra by 0.47 µm. Although blasting under the same conditions and operator on different days led to ΔRa due to shorter blast times, there was no statistically significant variance in Ra attributed to blasting on different days. Conversely, a ΔRa = +0.46 µm was observed upon blasting samples with different cabinets. No significant ΔRa was found when switching between straight and Venturi nozzles or when using different operators. Furthermore, the feasibility of fiber optic sensing technologies was investigated as potential tools to provide real time feedback to the blast machine operator in terms of substrate temperature. Decreasing the blast distance from 6” to 4” led to ΔT = +9.2 °C, while decreasing the blast angle to 45° gave ΔT= -11.6 °C for 304 stainless steel substrates. Furthermore, increasing the blast pressure from 40 psi to 50 psi gave ΔT= +15.3 °C and changing from 50 psi to 60 psi gave ΔT= +9.9 °C. The blast distance change from 8” to 6” resulted in ΔT = +9.8 °C in thin stainless steel substrate temperature. The effects of substrate thickness or shape were evaluated, giving ΔT= +7.4 °C at 8” distance, ΔT= +20.2 °C at 60 psi pressure, and ΔT= -15.2 °C at 45° blasting when comparing thin stainless steel against 304 stainless steel (thick) temperatures. No significant ΔT in means was found when going from 6” to 8” distance on 304 stainless steel, 40 psi and 60 psi blasting of thin SS, as well as angled and perpendicular blasting of thin SS. Comparing thick 304 and thin stainless steel substrates at a 6” blast distance gave no significant ΔT.
  • Thumbnail Image
    Surface Orientation Dependent Corrosion Damage and Temperature Dependent Mechanical Property Degradation of Sensitized AA5083-H116 Alloys
    Mills, Robert Jeffrey (Virginia Tech, 2018-11-06)
    This study relates the sensitization process microstructural changes of 5083-H116 to its resulting corrosion resistance and mechanical performance. Alcoa 5083-H116 was sensitized in an environmental chamber at 100°C for up to ~1500 hours and 150°C up to ~2000 hours, revealing different degrees of sensitization based on exposure times. Microstructural characterization was conducted on etched sensitized samples. Additionally, samples were subjected to accelerated corrosion scenarios for subsequent microstructural examination and subsequent mechanical (tension and tensile creep) testing. To connect the laboratory studies to the field exposure, Novelis 5083-H116 was sensitized at 100°C; dog bone samples were created and exposed for two years in a beach environment to investigate possible sensitization and corrosion effects. It was found that the sensitization at 100°C and 150°C of Alcoa 5083-H116 led to recrystallization from the asreceived (AR) state of the material (3 mg/cm²). The degree of sensitization of 61 mg/cm² recrystallized the grain size the most from the AR state. The higher sensitization temperature of 150°C caused higher thickness loss and mass-loss rates (MR) for the intergranular corrosion (IGC) susceptible sensitization levels. Accelerated corrosion on different surface orientations led to different corrosion mechanisms (parallel IGC vs. perpendicular IGC). While 5083-H116 material corroded on the rolled surface led to a uniform exfoliation damage on 150°C sensitization exposure, the 100°C rolled surface only exhibited pitting corrosion damage. The through plate thickness corrosion damage, however, exhibited a corrosion susceptible-resistant-susceptible (CSRS) pattern. Mechanical properties were assessed for the various conditions in terms of room temperature tension testing and elevated temperature creep tests. Sensitization affected yield strength but did not play a role in ultimate tensile strength. The presence of corrosion damage lowered yield strength and ultimate tensile strength of the IGC susceptible sensitized 5083-H116, with the through thickness corrosion damage reducing the properties more than corrosion of the rolled surface. Material sensitized at 150°C and then corroded had a greater reduction in room temperature mechanical properties. Creep testing was performed at elevated temperatures, and it was found the solely sensitized 5083-H116 at 100°C or 150°C behaved the same as as-received 5083-H116. When corrosion damage was introduced, creep rupture times and secondary creep rates were changed. Once the corroded section area was accounted for, no significant difference in Larson-Miller parameters was observed.