Understanding the Structure-property Relationships of Solid-state Additively Manufactured Stainless-steel Coatings
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Authors
Ralls, Alessandro Mauricio
Issue Date
2024
Type
Dissertation
Language
Keywords
Cold Spray , Corrosion , Friction Stir Processing , Laser Shock Peening , Tribo-Corrosion , Tribology
Alternative Title
Abstract
Global expenses due to premature machine failure have been progressively increasing due to the inevitable degradation of machine components in moving mechanical assemblies. Components such as gears, bearings, and metal forming tools made from 316L stainless steel (SS) suffer from early part failure due to the influence of abrasion, corrosion, sliding wear-corrosion (i.e., tribo-corrosion), and fretting corrosion degradation. As a method to mitigate these handicaps, cold spray additive manufacturing (CSAM, or more frequently referred to as cold spray (CS)) has proved to be a reliable tool to form protective coatings and repair these components, ultimately prolonging their lifespans. However, these components still degrade over time due to the presence of porosity and brittleness induced by the CS process. To date, there is no comprehensive understanding of how the physical, chemical, and mechanical properties of CS 316L SS coatings influence their resistance to material degradation. To address this scientific gap, the present work studies the structure-property relationships for CS 316L. Specifically, the influence of friction stir processing (FSP) and laser shock peening (LSP) post-surface treatments on the degradation resistance of CS 316L SS was investigated. By exploring these mechanisms, a useful roadmap for robust CS 316L SS development can be realized, which can control their inevitable degradation. The initial emphasis of this work was geared toward understanding the structural formation of CS 316L SS and comparing its mechanical, tribological, and electrochemical characteristics to traditionally casted 316L SS. Through this analysis, the advantages and disadvantages of CS SS coatings were determined, which served as a fundamental framework for the remaining portion of this work. It was observed that the effect of CS resulted in a refinement of crystallinity due to the localized adiabatic heating and stresses of the severely deformed particles. These effects translated to the reduced friction and wear observed from the CS deposit. Electrochemical tests also revealed that the CS sample had a relatively similar corrosion rate as the wrought sample with an enhanced re-passivation ability. From these findings, it was found that the presence of porosity, crystal defects, non-heterogeneous microstructure, and lack of ductility limited its physical/chemical performance. To address these defects, F SP was employed as a post-surface processing technique to modify the tribological, corrosion, tribo-corrosion, and fretting-corrosion behavior of CS 316L SS deposits. Results indicate that the effect of FSP induced an austenitic phase transformation due to the combination of frictional heat and plastic deformation. In tribological conditions, friction and wear loss were reduced due to the closure of pores and localized grain refinement. Electrochemical measurements revealed that the corrosion rate decreased due to the densified surface and full austenitic phase transformation from FSP. In tribo-corrosion environments, less material loss was observed due to the surface exhibiting less cracking/delamination/wear-corrosion synergy during tribo-corrosion testing. In fretting corrosion conditions, the improved stiffness of the FSP surface transitioned the fretting regime from the reciprocating sliding regime (RSR) to the full stick regime (FSR). The formation of Ni within the wear scar due to fretting corrosion also reduced the wear corrosion synergy and total material loss. Based on these findings, it was inferred that FSP was found to be a viable method to improve the degradation resistance of CS 316L SS deposits. The effect of laser shock peening (LSP) at various intensities was also employed as a secondary post-processing technique to improve the tribological, corrosion, tribo-corrosion, and fretting-corrosion resistance of CS 316L SS. Results indicate that LSP reduced surface porosity and refined the microstructural features of the CS surface due to the combination of severe plastic deformation and rapid cooling rates of the peening process. Tribological tests indicated that the LSP surfaces had a greater frictional resistance due to the smoothened and strengthened surface. The corrosion rate was also decreased due to the densified and refined surface. In tribo-corrosion conditions, the repassivation kinetics of the passive film between sliding cycles became increasingly active, which prevented accelerated corrosion from taking place. In fretting corrosion conditions, the surface response transitioned from the reciprocating sliding regime (RSR) to the partial stick regime (PSR). However, due to the formation of wear debris near the wear track, galvanic corrosion occurred, which increased the wear corrosion synergy. Despite this, the surface strengthening effect of LSP treatment had a dominant role in fretting corrosion degradation resistance, which resulted in less material loss. Based on these findings, it was found that LSP was also a viable technique to improve the surface characteristics of CS 316L SS deposits. Collectively, these works allow for a novel and fundamental understanding of how post-processing techniques can modify the structure-property relationships of CS 316L SS coatings. By establishing this new multi-disciplinary knowledge, the lifespans of CS coatings can be further improved, which will set high-quality standards for CS research in the years to come.