As most of the failures in engineering components initiate from the surface layer, applying surface treatments can play a crucial role in controlling material performance and lifetime. In this study, ...different surface severe plastic deformation techniques including severe shot peening, laser shock peening and ultrasonic nanocrystal surface modification have been considered. The effects of process parameters and the kinetic energy of each treatment on the microstructure, mechanical properties and fatigue behavior of nickel-based super-alloy Inconel 718 have been investigated. The results revealed that using the proper parameters to increase the kinetic energy of the applied surface treatments, it is possible to effectively promote surface grain refinement and induce a deep compressive residual stress field in Inconel 718 samples. Among the applied treatments, ultrasonic nanocrystal surface modification was found to be the most efficient one in improving the mechanical properties as it led to the most significant fatigue performance, followed by severe shot peening and laser shock peening.
Selective laser melting fabricated materials regularly exhibit excellent static strength, thanks to the very high thermal gradient that characterizes the process; however, their performance under ...dynamic loading is somehow restricted due to the limited ductility, tensile residual stresses, the defect density and the inadequate surface morphology in the as built configuration. Currently mechanical or electrochemical surface polishing steps combined with various heat treatments are integrated into the production lines to respectively address these issues. However, these methods are reported to occasionally lead to inconsistent results apart from the burden of additional costs. Herein, we applied various post treatments including shot peening, sand blasting and heat treatment to evaluate their individual and synergetic effect to tackle the aforementioned challenges. Physical, microstructural and mechanical properties of SLM fabricated AlSi10Mg specimens were investigated with special focus on fatigue strength. The results highlight that appropriate post treatments can significantly enhance the fatigue performance of SLM specimens resulting in characteristics that are comparable and even better than conventionally manufactured material. Surface treatments, in particular, were found to be efficient in significantly enhancing the fatigue strength, while eliminating the necessity of polishing steps that are currently applied on as built material.
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•Surface treatments and T6 heat treatment were applied to additive manufactured parts.•Peak hardening considerably promoted ductility and elongation.•Fatigue properties were significantly increased by proper kinetic surface treatment.•Post processed parts had better fatigue strength compared to conventional material.
Abstract
Shot peening is widely used for improving mechanical properties especially fatigue behavior of metallic components by inducing surface hardening, compressive residual stresses and surface ...grain refinement. In air blast shot peening, projection pressure and surface coverage (an index of peening duration) have been considered as major controlling process parameters; the combination of these parameters plays a critical role in the beneficial effects of shot peening. Generally in severe shot peening aimed at obtaining surface grain refinement, constant values of pressure are considered with different peening durations. Considering very high peening duration, however, the phenomenon of over shot peening, which can be identified with the formation of surface defects could occur. The present study introduces a novel shot peening treatment, here called gradient severe shot peening (GSSP) that instead of using constant projection pressure, implements gradually increasing or decreasing pressures. The gradual increase of the projection pressure acts as a pre-hardening stage for the following higher projection pressure boosting the potential of the material to tolerate the sequential impacts and thus become less prone to the formation of surface defects. The results of the experiments indicate significant fatigue life improvement obtained for GSSP treated specimens compared to the standard treatment with constant pressure. GSSP avoids the detrimental effects of over-peening, while maintaining the beneficial effects of surface nano-crystallization, surface hardening and compressive residual stresses. The notable difference in fatigue strength enhancement for GSSP treated material can be also attributed to the modulated surface morphology with lower surface roughness compared to a standard shot peening treatment with the same exposure time.
Recent advances in the field of cold spray have put forward the potential of this deposition technique to be used as a non-thermal additive manufacturing process with significantly high deposition ...rates. In this study, we use the additive manufacturing potential of cold spray for fabrication of freestanding three-dimensional Inconel 718 samples, which is a challenging material for cold spray due to its high hardness and limited deformability. Additionally, we fabricated samples with similar geometry using one of the most common additive manufacturing methods, i.e. selective laser melting. Microstructural characteristics, distribution of residual stresses, porosity and structural integrity of the cold spray deposited samples were compared with those obtained by selective laser melting before and after different heat treatments. The results of the first time study of axial fatigue strength of cold spray deposited freeform samples indicate the notable efficiency of cold spray for fabrication of freestanding objects for structural components, with similar characteristics to those obtained from laser based additive manufacturing technique and even comparable to bulk material properties. The low working temperature of the cold spray method, suggests it as a promising additive manufacturing technique with a high potential to address many challenges regarding laser based approaches.
► Requirements and evaluation approaches of surface coverage have been reviewed. ► The numerical models available in the literature have been statistically studied. ► A complementary technique for ...coverage assessment has been developed. ► The developed approach permits realistic modelling with reasonable computation time.
Surface coverage is a major control parameter for shot peening and has substantial influence on the reliability and uniformity of the process. It restrains quality variations that may cause premature failure or malfunctioning of the treated part. Despite the role of surface coverage in process quality, a general description and a unique standard method for its practical control and prediction are not yet established. This paper provides a general review of the definitions and requirements of surface coverage, its practical, theoretical and computational evaluation approaches. In the end a statistical study is performed on a coverage estimation method already developed by the authors. The results of the statistical study result in development of a complementary technique for coverage assessment which ensures full coverage with an optimum number of impacts. Different models are explained and critically discussed.
Cell–substrate interface is known to control the cell response and subsequent cell functions. Among the various biophysical signals, grain structure, which indicates the repeating arrangement of ...atoms in the material, has also proved to play a role of significant importance in mediating the cell activities. Moreover, refining the grain size through severe plastic deformation is known to provide the processed material with novel mechanical properties. The potential application of such advanced materials as biomedical implants has recently been evaluated by investigating the effect of different substrate grain sizes on a wide variety of cell activities. In this review, recent advances in biomedical applications of severe plastic deformation techniques are highlighted with special attention to the effect of the obtained nano/ultra-fine-grain size on cell–substrate interactions. Various severe plastic deformation techniques used for this purpose are discussed presenting a brief description of the mechanism for each process. The results obtained for each treatment on cell morphology, adhesion, proliferation, and differentiation, as well as the in vivo studies, are discussed. Finally, the advantages and challenges regarding the application of these techniques to produce multifunctional bio-implant materials are addressed.
A two-step severe shot peening (SSP) method was designed to investigate the softening mechanism of nanostructured Mg-8Gd-3Y (GW83) alloys and its effects on the surface characteristics of the ...deformation layer. The experimental results indicated that a gradient deformation layer with nanocrystalline (50–100 nm) near the surface layer was fabricated by single SSP (0.54 mmN). The corresponding formation mechanism of nanocrystalline was mainly owing to continuous dynamic recrystallization (cDRX). By applying the two-step SSP (0.54 + 0.23 mmN) treatment, softening occurred near the surface layer (<50 μm) due to the occurrence of partial discontinuous dynamic recrystallization (dDRX). The reason for dDRX behavior during SSP treatment resulted from the combined effects of high-level stored strain energy and heat production. This softening behavior was characterized by the increase of domain size, the slight enhancement of basal texture, and the decrease of dislocation density and microhardness. Therefore, appropriate SP parameters should be selected to avoid material softening when obtaining nanocrystalline Mg alloys.
•SSP developed nanostructured deformation layer and induced high-level CRS.•Two-step SSP method was designed to investigate the work softening mechanism in nanostructured GW83 alloy.•Partial dDRX behavior should be responsible for the softening phenomenon.•The softening phenomenon is characterized by the increase of domain size, the slight enhancement of basal texture, and the decrease of dislocation density.
Abstract Substrate grain structure and topography play major roles in mediating cell and bacteria activities. Severe plastic deformation techniques, known as efficient metal-forming and grain ...refining processes, provide the treated material with novel mechanical properties and can be adopted to modify nanoscale surface characteristics, possibly affecting interactions with the biological environment. This in vitro study evaluates the capability of severe shot peening, based on severe plastic deformation, to modulate the interactions of nanocrystallized metallic biomaterials with cells and bacteria. The treated 316L stainless steel surfaces were first investigated in terms of surface topography, grain size, hardness, wettability and residual stresses. The effects of the induced surface modifications were then separately studied in terms of cell morphology, adhesion and proliferation of primary human osteoblasts (bone forming cells) as well as the adhesion of multiple bacteria strains, specifically Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa , and ampicillin-resistant Escherichia coli . The results indicated a significant enhancement in surface work hardening and compressive residual stresses, maintenance of osteoblast adhesion and proliferation as well as a remarkable decrease in the adhesion and growth of gram-positive bacteria ( S. aureus and S. epidermidis ) compared to non-treated and conventionally shot peened samples. Impressively, the decrease in bacteria adhesion and growth was achieved without the use of antibiotics, for which bacteria can develop a resistance towards anyway. By slightly grinding the surface of severe shot peened samples to remove differences in nanoscale surface roughness, the effects of varying substrate grain size were separated from those of varying surface roughness. The expression of vinculin focal adhesions from osteoblasts was found to be singularly and inversely related to grain size, whereas the attachment of gram-positive bacteria ( S. aureus and S. epidermidis ) decreased with increasing nanoscale surface roughness, and was not affected by grain refinement. Ultimately, this study demonstrated the advantages of the proposed shot peening treatment to produce multifunctional 316L stainless steel materials for improved implant functions without necessitating the use of drugs.
Laser powder-bed fusion (LPBF) process, as one of the most widely used technologies of additive manufacturing, enables fabrication of parts with intricate geometries. The choice of process parameters ...in this technology plays a major role in defining the microstructural, mechanical and surface properties of the fabricated parts. In this study, the effects of LPBF process parameters on static tensile properties (including yield strength and ultimate tensile strength and elongation) of Ti-6Al-4V samples were investigated using artificial intelligence methods. Deep learning approach was employed by using neural networks for prediction, optimization and parametric and sensitivity analyses. Relevant experimental data available in the literature were collected to feed the network. Stacked auto-encoder was assigned to the networks for high accuracy pre-training. LPBF process parameters including laser power, scanning speed, hatch spacing, layer thickness and sample direction were regarded as inputs while yield strength, ultimate strength and elongation were considered as outputs of the neural networks. The obtained results indicate the high potential of neural networks to be used as a powerful tool for process parameter optimization for enhanced mechanical performance of additive manufactured parts.
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