Surface finishing of Additively Manufactured metallic components- effect of prior surface condition and post-processing routes
Restricted (Penn State Only)
- Author:
- Rifat, Mustafa
- Graduate Program:
- Industrial Engineering (PHD)
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 07, 2023
- Committee Members:
- Saurabh Basu, Chair & Dissertation Advisor
Ed De Meter, Major Field Member
Todd Palmer, Outside Unit & Field Member
Robert Voigt, Major Field Member
Steven Landry, Program Head/Chair - Keywords:
- Indentation
Surface Roughness
Additive Manufacturing
Microstructure
Electron Beam Melting
Direct Metal Laser Sintering
Bead-blasting
Residual Stress
Nano-scratch test
Pile-up Behavior
Hardening
Centrifugal Disc Finishing
Surface Finishing - Abstract:
- The ability to fabricate complex parts using additive manufacturing (AM) is leveraged to create performance-critical components. Herein, a challenge is high surface roughness that naturally results from AM and may lead to premature failure unless mitigated. This is specifically true in components that are designed to undergo cyclic (e.g., fatigue) loading. Shear-based processes involving repetitive impact and abrasion are often utilized to address these challenges. These processes smooth the rough surfaces by a combination of smearing and material removal. Classical understanding of the mechanics of shear-based surface finishing processes cannot be extrapolated directly to parts produced by AM for process optimization. This is because surface roughness and microstructures in components that are specific to AM can substantially complicate their mechanics, which can produce unexpected results. In this dissertation, the mechanics of shear-based surface processing of parts resulting from AM is delineated using a mix of empirical characterization, and numerical modeling. For this, processes including bead blasting, and centrifugal disc finishing were used as research platforms. The complicating effects that originate from surface roughness, crystallographic microstructure, and porosity defects were delineated. This was done by studying the mechanics of material response during the unitary deformation events associated to these processes, viz., indentation, and nano-scratching, respectively. During bead blasting, the rough surface of a component is bombarded with numerous glass beads at high impact velocity to produce surface smoothing. Each of the impacts can be likened to a high-speed indentation event on the rough surface of the component. In centrifugal disc finishing, the workpiece is put inside a stationary bowl filled with abrasive media. A rotary disc beneath the bowl creates a relative motion between the workpiece and media that then smooths the rough surface by combined actions including cutting, ploughing, and indentation. These cutting and ploughing actions can be likened to a scratching event. Heat-treatment procedures that are often utilized to heal porosity defects, and tailor crystallographic microstructures for enhancing mechanical properties of components also complicate the deformation behaviors during indentation and scratching. Hence, to gain an understanding of the process consequences of bead blasting, and centrifugal disc finishing, controlled indentation, and nano-scratching studies were carried out on additively manufactured titanium alloy Ti6Al4V, and nickel alloy Inconel 718. Isolated and combined effects of microstructural, and surface texture aspects of parts created by AM were tested using directed numerical studies. Results obtained from these numerical studies were cross-validated using empirical characterizations. The following key results were produced: (i) The mechanics of bead blasting is heavily influenced by surface roughness, and near-surface crystallographic microstructures, and porosity defects. (ii) These effects can be captured using the numerically quantifiable parameter associated to a rough surface, viz., its local mean curvature. This association is mediated by the mechanics of energy absorption by a component during shear-based surface processing. (iii) The attributes of crystallographic microstructures, viz., grain boundaries, and second-phase precipitates affect the mechanics of abrasion by directly altering hardening rates of the material constituting the component. (iv) Mechanics of surface finishing can be tuned by changing surface-processing media.