Ultrasonic Nanocrystalline Surface Modification Research Articles

Prediction of residual stress distribution induced by ultrasonic nanocrystalline surface modification using machine learning

Ultrasonic Nanocrystalline Surface Modification (UNSM) offers an efficient and cost-effective approach for enhancing material mechanical properties by inducing Severe Plastic Deformation (SPD). It leads to grain refinement and substantial residual stress generation beneath the workpiece surface. This study investigates the influence of key modification parameters, specifically static load, vibration amplitude, and strike tip size on compressive residual stress (CRS) distribution. A Finite Element Method (FEM)-based model for the UNSM process is developed, and validated against experimental outcomes, yielding a dataset of 45 unique cases across various modification scenarios. The Balancing Composite Motion Optimization (BCMO), as a meta-heuristic algorithm is used to optimize the hyperparameters of the Support Vector Regression (SVR) model. Additionally, the performance of Artificial Neural Network (ANN), Polynomial Chaotic Extension (PCE), and Kriging algorithms is evaluated in parallel. Among these Machine Learning (ML) models, the SVR-BCMO emerges as a pioneer for its accuracy in estimating residual stress. A sensitivity analysis employing Sobol’ indices further clarifies the distinct impact of each input parameter on residual stress distribution resulting from UNSM. In essence, this research offers a tool for rapidly estimating residual stress, even in cases of limited datasets. Furthermore, the findings help in making prompt decisions regarding of UNSM conditions. This is achieved by elucidating the effect of each input parameter and facilitating the determination of residual stresses in specific scenarios.

Controlling defects of laser-powder bed fusion processed 316L stainless steel via ultrasonic nanocrystalline surface modification

Metal additive manufacturing (MAM) offers an excellent capability for designing complex geometries with topology-optimized and near-net-shaped structures. The optimization-designed MAM parts with constrained volume require superior mechanical properties to broaden their utilization in the industry. However, the intrinsic defects generated during the building process deteriorate the mechanical functionality and limit utilization in various industrial applications. In this study, we propose a new strategy to reduce generated defects using ultrasonic nanocrystal surface modification (UNSM) on laser powder bed fusion (LPBF) processed 316L stainless steel. Subsurface pores and high surface roughness in MAM parts were significantly improved through the impact of UNSM treatment, and a gradient structure with mechanical incompatibility was developed. Consequently, the LPBF-processed samples after UNSM treatment show the excellent combination of strength and ductility, which is attributed to the high synergistic hardening from the gradient microstructure and the suppression of damage evolution by controlling built defects.

Processing aluminum alloy with hybrid wire arc additive manufacturing and ultrasonic nanocrystalline surface modification to improve porosity, surface finish, and hardness

In this paper, a novel hybrid wire arc additive manufacturing (WAAM) and ultrasonic nanocrystal surface modification (UNSM) on porosity manipulation and surface properties of aluminum 5356 alloys was studied. The goal is to improve the quality of the WAAM-built part by eliminating bigger pores and reducing its size, reducing surface roughness, and increasing surface hardness. The as-built WAAM and WAAM-UNSM-treated samples were quantitatively studied for porosity using an X-ray micro-computed tomography (μ-CT). The surface roughness was measured on the surface profile of the same samples before and after UNSM treatment. Followed by the Vickers micro-hardness tests to evaluate the hardness modified by the influence of the UNSM treatment. It was found that the bigger pores in the as-built WAAM samples were eliminated and the medium-sized pores were shrunk to almost half the size after the UNSM treatment. Further, the UNSM treatment showed a significant improvement in both surface roughness and hardness on the WAAM Al5356 samples. This experimental work demonstrates the critical advantages of hybrid WAAM-UNSM in improving the qualities of the WAAM processed parts.

Superior effects of hybrid laser shock peening and ultrasonic nanocrystalline surface modification on fatigue behavior of additive manufactured AlSi10Mg

The surface texture of metallic parts produced by laser powder bed fusion (L-PBF) in the as-built condition detrimentally affects their mechanical properties, especially fatigue behavior. Accordingly, applying surface post-treatments has become an attractive approach to improve the mechanical performance of these materials. In the present study, both the individual and combined effects of post-processing methods, i.e., laser shock peening (LSP) and ultrasonic nanocrystalline surface modification (UNSM) with the same intensity of 10–12 A [0.001 in.], were systematically investigated on mechanical properties and fatigue behavior of L-PBF AlSi10Mg specimens. A wide range of experiments involving microstructural characterization, hardness and residual stresses measurements, porosity and surface texture analyses, tensile tests, and rotating bending fatigue tests were conducted. The results revealed that the hybrid LSP + UNSM process resulted in significant improvement in mechanical properties and fatigue behavior due to (i) sub-surface pores closure up to the depth of 517 μm, (ii) 60 % surface hardness improvement, (iii) inducing −420 MPa surface compressive residual stresses, and (iv) surface roughness reduction up to 70 %. The fatigue life was improved up to 75, 56, and 35-fold compared to the as-built state after applying LSP + UNSM, UNSM, and LSP treatments, respectively.

Microstructural evolution and mechanical properties of laser-powder bed fusion processed 316L stainless steel with an ultrasonic-nanocrystalline surface modification

Because the surface roughness of additively manufactured (AM) metallic alloys affect their mechanical properties, surface post-treatment has become an attractive strategy for improving the performance of AM parts. Although several studies have revealed that surface treatment improves the mechanical properties of AM parts, changes in their surface morphologies and deformation mechanisms have not been well investigated. To solve this problem, in this study, the role of ultrasonic nanocrystalline surface modification (UNSM) treatment on the plastic deformation behavior of AM 316L stainless steel was investigated. The shear strain from the UNSM treatment induced severe grain refinement and crumbled traces at the surface of the as-built samples, causing a strength-ductility combination improvement compared to the non-treated sample. Moreover, the UNSM treatment removed the unmelted particles that relieved strain localization at the surface of the as-built sample during the plastic deformation process. Meanwhile, UNSM treatment in the machined sample reduced the surface quality, such as microcracks, which causes strength-ductility combination degradation. This result indicates that the post-surface mechanical treatment of the as-built metallic parts effectively improves the mechanical properties. This result was accomplished by inducing a gradient microstructure and a crumbled trace, which contribute to the strength improvement and the crack growth inhibition, respectively.

Post-building successive treatments using surface laser remelting and ultrasonic nanocrystalline surface modification for rough surface built via directed energy deposition

In this study, surface laser melting (SLR) and ultrasonic nanocrystalline surface modification (UNSM) were applied as post-building treatments to modify the surface of additively manufactured parts via directed energy deposition (DED). The DED building of STS316L powder was conducted on a substrate, and the deposited surface was irradiated with a laser of the DED equipment to perform SLR on the surface. Subsequently, the remelted surface was re-treated with UNSM, and the changes in each step were examined. Via the experiment, the as-built surface roughness and waviness of 12.5-μm Ra and 8.22-μm Wa, were improved to 2.3 μm and 2.5 μm, respectively, by the successive treatments of SLR and UNSM (denoted by SLRed-UNSMed). From the perspective of grain refinement by UNSM treatment, performing SLR with a small overlap followed by UNSM was determined to be effective in maximizing the effect of UNSM. The SEM and EBSD analysis results indicated that grain refinement occurred in a deeper region in SLRed-UNSMed than in the sample (denoted by UNSMed) that was treated solely with UNSM and no SLR. In addition, according to the pole figure analysis, the primary surface modification and grain reorientation in the (100) direction by SLR allowed the UNSM energy to be transferred more evenly and deeper. Meanwhile, high hardness and compressive residual stress were observed in SLRed-UNSMed, and a nano-indentation test confirmed an increase in the effective depth of UNSM relative to that of UNSMed. Based on this study, an SLR treatment followed by UNSM can be expected to achieve surface modification and improve the mechanical properties of additively manufactured products.

Multi-layered heterostructured CoCrFeMnNi high-entropy alloy processed using direct energy deposition and ultrasonic nanocrystalline surface modification

The advantages of heterostructured materials as structural materials include superior mechanical properties and the ability to tailor the strength-ductility combination via microstructure customization. To maximize heterostructure effects, this study combined direct energy deposition and ultrasonic nanocrystalline surface modification processes to create a CoCrFeMnNi equiatomic high-entropy alloy with multi-layered microstructures. The fabricated CoCrFeMnNi high-entropy alloy has a novel microstructure composed of multiple layers of repetitive microstructures with heterogeneity and demonstrates a remarkable synergetic strengthening effect in comparison to conventional heterogeneous materials. The outstanding mechanical properties derived from various hard and soft layer interfaces, as well as the effects of each layer and interface, were quantitatively analyzed using grain-scale digital image correlation technology. By combining the direct energy deposition and ultrasonic nanocrystalline surface modification processes, this study presents a method for fabricating a new class of heterostructured materials with multi-layered microstructure that exhibit deformation heterogeneity and grain size heterogeneity. The multi-layered microstructure with multiple heterogeneous boundaries breaks the conventional wisdom regarding heterostructured materials having only one or two heterogeneous interfaces.

Individual and synergistic effects of thermal and mechanical surface post-treatments on wear and corrosion behavior of laser powder bed fusion AlSi10Mg

Materials fabricated by laser powder bed fusion (LPBF) are generally characterized with internal defects, inhomogeneous microstructure and rough surface with multiple forms of irregularities in their as-built state. Post-treatments are needed to resolve the issues associated with these imperfections. In this study, the individual and synergistic effects of different post-treatments including heat treatment, shot peening and ultrasonic nanocrystalline surface modification on microstructure, surface morphology, roughness, mechanical, corrosion and tribological properties of LPBF AlSi10Mg specimens have been investigated. Two different sets of parameters were considered for the surface treatments, both with mechanical energy source. Apart from surface characteristics, various properties including microhardness and residual stresses as well as wear and corrosion behavior of post-processed specimens were analyzed and compared with those of the as-built condition. The results indicated that hybrid post-treatments have the highest positive effects on wear and corrosion resistance as their kinetic energy affected multiple physical aspects; these include the microstructure leading to surface grain refinement paired with deep compressive residual stress field and improved surface morphology offering a more regular surface.

Effects of different mechanical and chemical surface post-treatments on mechanical and surface properties of as-built laser powder bed fusion AlSi10Mg

Laser powder bed fusion (LPBF) materials have various surface defects, which can detrimentally affect the mechanical properties and fatigue behavior of these materials. Surface post-processing can be crucial to reduce or eliminate these surface imperfections. In this study, the effects of different post-processing methods including shot peening, ultrasonic nanocrystalline surface modification and severe vibratory peening as well as electro-chemical polishing and other hybrid treatments obtained from their combination were surveyed on microstructure, surface and mechanical properties of LPBF AlSi10Mg specimens. Two different processes with varying levels of kinetic energy were considered for each mechanical surface treatment. The results indicate that mechanical treatments can effectively modify the surface morphology, improve surface hardness and induce compressive residual stresses. In addition, electro-chemical polishing was highly efficient in surface roughness reduction and increasing the wettability of the hybrid mechanically and chemically treated specimens.

Effect of ultrasonic nanocrystalline surface modification process on fretting wear behavior of laser surface textured 20CrMoH steel

Laser surface texturing (LST) followed by ultrasonic surface rolling (USR) was applied to 20CrMoH steel to improve the friction and wear behavior. In this paper, the fretting friction and wear mechanisms of the modified (USR, LST + USR) 20CrMoH steel under different loads and lubrication conditions were systematically investigated. Results indicated that the regular coupling distribution of micro-texture and gradient nanostructure (GNS) was formed on the surface layer of the combined treated material. Simultaneously, the depth of the GNS was up to 22 μm and the surface microhardness was increased from 800 HV0.2 to 860 HV0.2. Besides, the surface roughness Ra value was decreased from 0.24 μm to 0.01 μm and the maximum residual compressive stress (RCS) value was −610 MPa. Under dry friction and high load conditions, compared with the untreated specimen, the average friction coefficient and worn losses of the combined treated specimen were reduced by about 10% and 30%, respectively. In comparison with the untreated specimen, the average friction coefficient of the combined treated specimen was decreased by 26% under the conditions of low load and oil lubrication. Especially, the fretting wear resistance was obviously improved and its worn loss was significantly reduced by 92.05% compared with the untreated material. The predominant wear mechanisms of the combined treated material under dry friction and oil lubrication were oxidation wear and abrasive wear, respectively.

Intergranular SCC mechanism of ultrasonic nanocrystalline surface modified AISI 304 SS in H2SO4 solution containing chloride

In this work, the stress corrosion cracking (SCC) susceptibility of AISI 304 stainless steel processed using ultrasonic nanocrystalline surface modification (UNSM) treatment, was studied in 1 M H2SO4 containing 5 wt.% NaCl by means of combined slow strain rate test with electrochemical methods. The mechanical properties are improved by the UNSM, however the time to failure decreases due to the higher anodic dissolution kinetics of the UNSM sample, which remain in the order of mA/cm2 compared to µA/cm2 of the as-received (AR) sample. Adsorption of chloride ions has been identified by the presence of an inductive loop in the Nyquist plot, indicating a more reactive surface of the UNSM sample, which promotes dealloying of the metal. The development of several transgranular SCC (TG‒SCC) areas in the UNSM sample, compared to the AR, plus the nucleation of intergranular SCC (IG‒SCC) in the UNSM region is studied by scanning electron microscopy (SEM), X-ray micro-computed tomography (micro-CT), and electron back scattered diffraction (EBSD).

Gradient-structured ferrous medium-entropy alloys with enhanced strength-ductility synergy by ultrasonic nanocrystalline surface modification

In this study, gradient-structured Fex(CoNi)90−xCr10 ferrous medium-entropy alloys were fabricated by using an ultrasonic nanocrystalline surface modification process. The processed alloys with gradient structures exhibited the improved room-temperature yield strength by up to 158% and tensile strength by up to 31% at reasonable ductility compared to homogeneous microstructured counterparts.

Gradient plasticity in gradient nanocrystalline metals: Extra toughness from dislocation migration

Gradient nanocrystalline (GNC) metals exhibit an unprecedented combination of high strength and high ductility. In this study, GNC copper was obtained using ultrasonic nanocrystalline surface modification (UNSM), and its plasticity mechanism was investigated using tensile tests. It was found that UNSM treatment can grant copper superior yield strength and ductility which is beyond the capacity of conventional cold-working. Moreover, the UNSM-treated copper has a reduced strain-hardening capacity which does not lead to early necking and deteriorated ductility. The strain energy analysis shows that the yield strength of GNC material can be estimated using the root mean square of yield strengths of the coarse-grained (CG) layer and the gradient structure (GS) layer. The strain energy analysis also predicted a strong interaction between GS layer and CG layer during tensile deformation. Furthermore, a dislocation-based constitutive model was applied to understand the interaction between CG layer and GS layer. The migration of dislocations from the CG layer to the GS layer was proposed to explain the interaction. The migration of dislocations induces strain softening and, thus, releases internal strain energy. The proposed mechanism is supported by our experimental observations: the reduction in strain-hardening capacity is positive-related to the depression in strain localization in UNSM-treated GNC copper. The improved ductility is attributed to the depressed strain localization and the refreshed capacity for dislocation accumulation, both resulted from dislocation migration from the CG layer to the GS layer.

Mechanical property enhancement in gradient structured aluminum alloy by ultrasonic nanocrystalline surface modification

The large strength difference between hard and soft components in heterogeneous structured materials leads to the evolution of high back-stress hardening, which increases the strength and ductility of materials simultaneously. Moreover, the combination of high shear strain and elevated temperature allows an increase in the strength of low-melting temperature metallic alloys by grain refinement, solute migration, and clustering. In this study, to design a new heterogeneous microstructure in the aluminum alloy, both room temperature (RT) and high-temperature (HT) ultrasonic nanocrystalline surface modification (UNSM) were conducted, and their mechanical properties and microstructural evolutions were investigated. The large shear strain from the UNSM treatment reduces the grain size at the sample surface and creates a gradient structure. The combination of shear strain and elevated temperature during UNSM treatment induces solute migration at a certain depth of the specimens, resulting in the nano-sized Mg-rich particles at the surface region. Both grain refinement and precipitation at the surface region of the HT sample provide strong back-stress hardening in the early stages of deformation that enhances the strength and ductility of materials. Therefore, a high shear strain and control of processing temperature allow the design of a unique heterogeneous microstructure in low-melting temperature metallic alloys, which is a good strategy for enhancing the mechanical properties of sheet or thin metallic products.

Correction to: “Ultrasonic nanocrystalline surface modification of low strength aluminum alloy: trade-off between surface integrity and production rate aiming at desired fatigue life”

In processing of metallic material by ultrasonic nanocrystalline surface modification (UNSM), obtaining high quality sample with desired surface property needs a careful control on processing factors. The condition would be more critical when the process is designed for match production. In such condition, both of surface integrity and production rate should be taken into account. To meet both of goals, in the present study, an experimental work was made to analyze how the UNSM process factors influences the surface integrity under desired values of production rate. In order to do so, series of experiments based on small fraction central composite design were carried out including static pressure, vibration amplitude, spindle speed, feed rate, and ball size as input parameters. Also, the surface roughness, compressive residual stress, and machining time were considered performance measures. A hybrid optimization approach including gray relational analysis and desirability function was proposed to obtain desired surface integrity under minimum value of machining time. Different criteria based on importance of production rate and surface integrity were separately defined and controlled by variation of weight factors. It is found that considering the production rate as dominant criterion leads to production of samples with maximum high cycle fatigue life. This is due to selection high spindle speed that enhances both production rate and surface quality. On the other hand, considering overall value of surface integrity as dominant criterion of optimization leads to producing sample with desired values of low cycle fatigue life. This effect can be attributed to selection of low spindle speed that deteriorates surface finish but enhances the compressive residual stress.

Optimization of residual stresses generated by ultrasonic nanocrystalline surface modification through analytical modeling and data-driven prediction

In this work, we have developed a fast and robust analytical model based on the Hertzian contact theory and the Neuber's plasticity law to predict the residual stress generated by ultrasonic nanocrystal surface modification (UNSM). The measured residual stresses data for a number of different metals was used to validate the analytical model. Sensitivity studies as well as the linear regression analysis were conducted to study the effects of UNSM parameters (striking amplitude, static load, spacing, scanning speed and temperature) on the distribution of residual stresses and the surface finish. The analysis shows that a balance between residual stress and surface finish needs to be considered when choosing the UNSM parameters: the increase of ultrasonic striking amplitude and static load improve the magnitude and depth of the residual stress at the risk of deteriorating the surface finish; a higher temperature leads to a lower magnitude but deeper residual stress as well as a better surface finish; smaller tip size can significantly increase the magnitude of residual stresses but it also deteriorates the surface finish. Finally, an experimental design based on a large database (with a total of 18876 sets of UNSM parameters) was introduced to find the optimal UNSM parameters for different engineering needs.

Ultrasonic nanocrystalline surface modification of low strength aluminum alloy: trade-off between surface integrity and production rate aiming at desired fatigue life

In processing of metallic material by ultrasonic nanocrystalline surface modification (UNSM), obtaining high quality sample with desired surface property needs a careful control on processing factors. The condition would be more critical when the process is designed for match production. In such condition, both of surface integrity and production rate should be taken into account. To meet both of goals, in the present study, an experimental work was made to analyze how the UNSM process factors influences the surface integrity under desired values of production rate. In order to do so, series of experiments based on small fraction central composite design were carried out including static pressure, vibration amplitude, spindle speed, feed rate, and ball size as input parameters. Also, the surface roughness, compressive residual stress, and machining time were considered performance measures. A hybrid optimization approach including gray relational analysis and desirability function was proposed to obtain desired surface integrity under minimum value of machining time. Different criteria based on importance of production rate and surface integrity were separately defined and controlled by variation of weight factors. It is found that considering the production rate as dominant criterion leads to production of samples with maximum high cycle fatigue life. This is due to selection high spindle speed that enhances both production rate and surface quality. On the other hand, considering overall value of surface integrity as dominant criterion of optimization leads to producing sample with desired values of low cycle fatigue life. This effect can be attributed to selection of low spindle speed that deteriorates surface finish but enhances the compressive residual stress.

Correlation of orientation relationships and strain-induced martensitic transformation sequences in a gradient austenitic stainless steel

Orientation relationships (ORs) play a crucial role in the phase transformation of face-centered cubic austenite (γ-fcc) to body-centered cubic martensite (α′-bcc) upon plastic deformation. So far, ORs between γ-fcc and α′-bcc are reported frequently; however, the correlation to the transformation sequences is not clear. Here, the gradient structured austenitic stainless 304 steel prepared by ultrasonic nanocrystalline surface modification was examined to clarify the ORs that were involved in the strain-induced martensitic transformation (SIMT) behaviors. The results showed changes in the ORs from Nishiyama–Wassermann (N-W) to Kurdjumov–Sachs (K-S) and Pitsch with an increase in depth from the top-treated surface. The OR of α′-bcc formed directly on γ-fcc was identified as N-W, while those via the hexagonal closed packed e-plate (e-hcp) were the K-S and Pitsch as they were nucleated at the single and two e-plates intersections, respectively. Thus, the SIMT sequence was the direct γ → α′ with the presence of the N-W OR in the high strain depth, while it was the γ → e → α′ as the existence of K-S and Pitsch ORs in the low strain depth. The findings provide essential data for the in-depth and comprehensive study of the polymorphic martensitic transformation mechanisms in various steels and alloys.

Optimization of ultrasonic nanocrystalline surface modification of copper and its influence on fatigue life

In processing of metallic material by ultrasonic nanocrystalline surface modification (UNSM), obtaining high quality sample with desired surface property needs a careful control on processing factors. In the present investigation, an attempt was made to find optimal setting of UNSM factors viz static pressure, vibration amplitude, spindle speed, feed rate and ball size on surface integrity factors such as surface roughness and hardness. Number of 32 experiments were conducted using small fraction central composite design to correlate input-output relationship. Grey relational analysis and desirability function was also applied to experimental data to identify a setting corresponds to maximum hardness and minimum surface roughness. The optimum samples were also subjected to fatigue tests as well the microscopic observations to find how much improvement occurs as a result of optimization with respect to as turned sample. It is found from both approaches that the setting of 0.03 MPa static pressure, 10 μm vibration amplitude, 100 rpm spindle speed, 0.08mm/rev feed rate and 8 mm strike size is an optimal parameter setting that results in surface roughness of 0.356 μm and hardness of 155 HV. The sample machined by optimum condition result in fatigue life cycles improvement more than 2.5 times in different levels of applied stress. The improvement can be attributed to combined effect of smoothed surface topography, fine microstructure, hardened surface layers and exerting compressive residual stress.

Control of magnesium in vitro degradation based on ultrafine-grained surface gradient structure using ultrasonic nanocrystalline surface modification

Here, a gradient structure based on an ultrafine-grained Mg surface was prepared using ultrasonic nanocrystalline surface modification (UNSM). UNSM is expected to be commercially attractive because it enhances mechanical properties and does not lead to surface roughening, which promotes corrosion. The modified surface increases corrosion resistance by accelerating the development of compact corrosion products that provide protection. The ultrafine-grained (UFG) surface gradient structure improved corrosion resistance and maintaining strength, which increased as the grain size became smaller. In particular, the UFG structure generated by the UNSM process exhibits a dramatic decrease in corrosion rate compared to the fine-grained structure, resulting in the slope change in the plot of corrosion rate versus grain size in the UFG regime. The surface gradient structure is an optimized approach for early-stage fast bone healing and later-stage fast degradation considering the relationship between grain size, pH, and strength change.