Surface Mechanical Attrition Treatment Research Articles

Surface nanocrystallization enhances the biomedical performance of additively manufactured stainless steel.

Additive manufacturing enables the fabrication of patient-specific implants of complex geometries. Although selective laser melting (SLM) of 316L stainless steel (SS) is well established, post-processing is essential to preparing high-performance biomedical implants. The goal of this study was to investigate surface mechanical attrition treatment (SMAT) as a means to enhance the electrochemical, biomechanical, and biological performances of 316L SS fabricated by SLM in devices for the repair of bone tissues. The SMAT conditions were optimized to induce surface nanocrystallization on the additively manufactured samples. SMAT resulted in a thicker oxide layer, which provided corrosion resistance by forming a passive layer. The fretting wear results showed that the rate of wear decreased after SMAT owing to the formation of a harder nanostructured layer. Surface modification of the alloy by SMAT enhanced its ability to support the attachment and proliferation of pre-osteoblasts in vitro. The study of the response in vivo to the additively manufactured alloy in a critical-sized cranial defect murine model revealed enhanced interactions with the cellular components after the alloy was subjected to SMAT without inducing any adverse immune response. Taken together, the results of this work establish SMAT of additively manufactured metallic implants as an effective strategy for engineering next-generation, high-performance medical devices for orthopedics and craniomaxillofacial applications.

Detail characterization on the microstructure evolution of nickel-base alloy 600MA via surface mechanical attrition treatment

Microstructure evolution of alloy 600MA was explored after surface mechanical attrition treatment (SMAT) with different durations via scanning electronic microscopy, electron backscattered diffraction, transmission electron microscopy and transmission Kikuchi diffraction. The grain refinement and coarsening occurred gradually as strain accumulated with increasing SMAT time. During grain refinement, the initiation and development of dislocation walls or cells came up at early stage. A strip of dislocation wall led to straight element concentration just prior to the formation of ultrafine-laminated (UFL) structures containing abundant twin boundaries (TB) and low angle grain boundaries (LAGB). The UFL structures with a texture orientation were subdivided by the development of dislocation arrays into ultrafine-grained (UFG) structures with random orientations. Meanwhile, the elements were dispersedly concentrated in a few grains. During grain coarsening, the dispersed element concentration still existed. The fraction of LAGB, TB and dislocation density was significantly reduced while the fraction of high angle grain boundary (HAGB) increased along with grain growth. • The grain refinement and coarsening occurred gradually as strain accumulated with increasing SMAT time. • During grain refinement, the initiation and development of dislocation walls or cells came up at early stage. • A strip of dislocation wall led to straight element concentration just prior to the formation of ultrafine-laminated (UFL) structures containing abundant twin boundaries (TB) and low angle grain boundaries (LAGB). • The UFL structures with a texture orientation were subdivided by the development of dislocation arrays into ultrafine-grained (UFG) structures with random orientations. • During grain coarsening, the fraction of LAGB, TB and dislocation density was significantly reduced while the fraction of high angle grain boundary increased along with grain growth.

Experimental and numerical vibration analysis of surface mechanical attrition treatment

The surface mechanical attrition treatment creates a nanocrystal layer with improved mechanical properties without changing the surface chemistry. Vibration produces residual stresses in a layer close to the surface by successive impacts on the material surface. Vibration treatments represent a “green” alternative to surface treatments without the use of chemicals and high temperatures. Depending on the operating frequency and amplitude of a particular vibration platform, the most decisive parameter to be controlled for the efficiency of the application is the velocity. Vibration platforms in the SMAT process are ultrasonic and mechanically based platforms. This work presents a vibration platform where velocity is below 1 m/s, which is the lowest operating speed defined in the SMAT process. This work includes both analytical and numerical solutions to the natural frequency values of the vibrating table. The operating conditions were determined by finding the natural frequency values. The amplitude, velocity, and acceleration relations of the vibrating table that will occur during the process were found using the frequency response analysis method. ANSYS and MATLAB were used for numerical and analytical solutions, respectively. Numerical and analytical results were in agreement with the experimental modal analysis.

Soybean-inspired nanomaterial-based broadband piezoelectric energy harvester with local bistability

Bistable nonlinear energy harvesters are effective structures for scavenging broadband energy from frequency-varying vibration sources. The bistable characteristic of such structures is typically induced by introducing magnetic fields, prestress, and nonuniform geometries or fixtures. However, the complicated structures or single deformation modes of these structures limit their practical application. Inspired by the structure of soybean pods, a bionic metallic nanomaterial-based bistable piezoelectric energy harvester is developed in this study. The nanoplate substrate with a local bistable region mimics the configuration of a soybean pod. To the best of the authors’ knowledge, the local bistable configuration represents a novel energy harvesting strategy. The local bistable nanoplate is fabricated using a mature surface mechanical attrition treatment technique that generates a gradient nanostructure to enhance the mechanical properties of the bistable structure. The energy harvesting performance and nonlinear dynamic characteristics of the energy harvester are evaluated through frequency-sweep and fixed-frequency vibration tests and numerical simulations with a new two-step finite element (FE) model. The harvester characteristics in three vibration modes (single-well vibration (SV), intermittent cross-well vibration (ICV), and continuous cross-well vibration (CCV)) are discussed. The experimental and numerical results demonstrate that the voltage output and working bandwidth of the proposed harvester increase by five times in the ICV and CCV modes when the excitation acceleration increases from 0.5 g to 3.0 g. The bionic nanomaterial-based bistable piezoelectric energy harvester can be potentially used in various applications such as vehicle suspension systems, tires, and vehicle-bridge systems.

Screening and analysis of key genes in the biological behavior of bone mesenchymal stem cells seeded on gradient nanostructured titanium compared with native pure Ti.

Titanium (Ti) and Ti-based alloy materials are ideal brackets that restore bone defect, and the mechanism of related genes inducing bone mesenchymal stem cells (BMSCs) to osteogenic differentiation is currently a hot research topic. In order to screen key genes of BMSCs during the osteogenic expression process, we acquired data sets (GSE37237 and GSE84500) which were in the database Gene Expression Omnibus (GEO). Investigations on differentially expressed genes (DEGs) and their enrichment of functions were conducted. We constructed relative protein-protein interaction (PPI) network by using Search Tool for the Retrieval of Interacting Genes (STRING) and visualized the expression of DEGs with Cytoscape. A total of 279 DEGs were discerned, which could be divided into 177 down regulated genes and 102 up regulated genes. In addition, the DEGs' enrichment and pathways included regulation of actin cytoskeleton, inflammatory mediator regulation of transient receptor potential (TRP) channels, peroxisome proliferator-activated receptors (PPAR) pathway, cell cycle, Rheumatoid arthritis, mitogen-activated protein kinases (MAPK) signaling pathway and Ras signaling pathway ect. It showed that 10 notable up regulated genes were mainly in AMP-activated protein kinase (AMPK) pathway. Then we used a technology named surface mechanical attrition treatment (SMAT) to prepare gradient nanostructured (GNS) surface Ti and seeded well-growing BMSCs on the surface of SMAT Ti and native pure Ti. Cell Counting Kits-8 (CCK-8), apoptosis experiment, immunofluorescence technology and staining experiments for alka-line phosphatase (ALP) and alizarin red staining (ARS) were used to research the proliferation, adhesion and differentiation ability of BMSCs seeded on SMAT Ti compared with native pure Ti. We used quantitative real-time PCR (qRT-PCR) technology so as to verify the expression of the most significant 5 genes. In summary, these results indicated novel point of views into candidate genes and potential mechanism for the further study of BMSCs' behaviors seeded on SMAT Ti.

Surface morphology and gradient microstructural evolutions in pure titanium via surface severe plastic deformation

A pure Ti plate with a smooth surface and a gradient nanostructured layer was achieved using surface mechanical attrition treatment method with 3 mm zirconia balls at room temperature. The twinning and dislocation slips layer, transition layer, ultrafine grains layer and nanograins layer appeared in sequence from the matrix to the topmost surface, representing different stages of microstructural evolutions. The dislocation slips and 10 1 ¯ 2 twins coordinated and competed with each other to adapt to plastic deformation. An additional misorientation and mirror symmetry line separation resulted from the interaction between <c + a> slips and 10 1 ¯ 2 twin boundaries, causing the twin boundaries to gradually transform into high angle grain boundaries. A stronger dislocation–twin boundary interaction in the transition layer triggered the continuous formation and accumulation of high angle grain boundaries, which subdivided the twins into blocks or submicronic polygonal grains. It is proved that a combination of continuous dynamic recrystallization and discontinuous dynamic recrystallization leads to the generation of nanograins. • A smooth surface with nanostructured layer was achieved. • The gradient nanostructured layer of Ti after SMAT was discussed. • The interaction of <c + a> slips and twin boundaries plays a dominant role in the mechanism of microstructure evolution. • cDRX and dDRX combine to generate the nanograins.

Influence of Surface Mechanical Attrition Treatment (SMAT) on Microstructure, Tensile and Low-Cycle Fatigue Behavior of Additively Manufactured Stainless Steel 316L

Direct Energy Deposition (DED), as one common type of additive manufacturing, is capable of fabricating metallic components close to net-shape with complex geometry. Surface mechanical attrition treatment (SMAT) is an advanced surface treatment technology which is able to yield a nanostructured surface layer characterized by compressive residual stresses and work hardening, thereby improving the fatigue performances of metallic specimens. In the present study, stainless steel 316L specimens were fabricated by DED and subsequently surface treated by SMAT. Both uniaxial tensile tests and uniaxial tension-compression low-cycle fatigue tests were conducted for as-built and SMAT processed specimens. The microstructure of both conditions was characterized by roughness and hardness measurements, scanning electron microscopy and transmission electron microscopy. After SMAT, nanocrystallites and microtwins were found in the top surface layer. These microstructural features contribute to superior properties of the treated surfaces. Finally, it can be concluded that the mechanical performance of additively manufactured steel under static and fatigue loading can be improved by the SMAT process.

Exfoliated nano-hBN additives for enhancing tribological performance of ATSP coatings deposited on AISI 316L steel: Role of SMAT pre-treatment

The current study investigates the role of hexagonal boron nitride (hBN) content (1, 5, and 10 wt%), the extent of exfoliation (magnetic stirring for 1, 5 and 12 h duration) and substrate conditions (with/without surface mechanical attrition treatment (SMAT)) on tribological performance (under dry sliding condition) of aromatic thermoset polymer (ATSP) coatings deposited on AISI 316L stainless steel using Mayers bar technique. The thickness of hBN particles is decreased from ~115 to ~14 nm after 12 h of stirring. The coating thickness is slightly higher for the SMATed (S) substrate than for the non-SMATed (NS) substrate. The coefficient of friction (COF) reduction of ~57 and ~86 % is observed for ATSP@hBN composite coating (containing 10 wt.% hBN) deposited on NS and S specimens (after 1 h stirring), respectively. The ATSP coating’s volume loss is reduced by ~70 % when deposited on SMATed substrate. Further, COF and volume loss are decreased with increased hBN content and stirring time. Composite coatings deposited on non-SMATed specimens show a considerable reduction in COF to ~0.036 after 12 h of stirring and 10 wt.% of hBN content. The minimum COF of ~0.018 and volume loss of ~0.0095 mm3 are achieved when this composite coating is deposited on the SMATed substrate. The ATSP-coated NS specimen suffers more severe damage during wear by adhesion, scratching, and oxide formation, which is considerably reduced after adding hBN. The wear track width and surface damage are negligible in the case of composite coated SMATed substrate. The factors like better load-bearing capacity, traces of coating on the wear track and thick transfer film on the counter ball are responsible for lowering the COF of ATSP and ATSP@hBN composite coatings deposited on SMATed substrate. The reduced COF generates less frictional heat, causing lower oxidation and enhanced coating durability.

Modelling residual stress and residual work hardening induced by surface mechanical attrition treatment

Shot peening induced residual stress is strongly related to residual work hardening which relies on both the process and the target material. In this work, residual stress and coupled residual work hardening are studied in a surface mechanical attrition treatment (SMAT) on a cylindrical structure by combining discrete element method (DEM) and finite element method (FEM). Shot, ultrasonic parameters, chamber structure, sonotrode surface, and cylindrical structure of the target are finely modeled in DEM simulation. On the basis of the DEM results, FEM simulation of the SMAT process is performed on a representative area considering the factual impact parameters and elastic-plastic behavior of the target material, and mechanism of residual stress generation is analyzed. It shows that isotropic hardening and kinematic hardening of the target material play a different role in residual stress generation. The obtained maximum compressive residual stress is mainly determined by residual yield stress, while the residual kinematic hardening is nearly unchanged when the coverage is increased. Further analyses indicate that the mechanisms of residual stress generation should be very different for the Johnson-Cook model and the model that includes kinematic hardening, even though these two kinds of models are reported to be able to well predict the shot peening induced residual stress. The gradient of the residual work hardening can be used for further evaluating mechanical properties of the SMATed cylindrical structure.

Bending fatigue life enhancement of NiTi alloy by pre-strain warm surface mechanical attrition treatment

Many structures like medical stents made of superelastic NiTi shape memory alloy (SMA) are subjected to cyclic bending loads and show a limited fatigue life due to crack nucleation and growth in the surface layers under local tensile stress. Here, we enhance the bending fatigue life of NiTi plates by pre-strain warm surface mechanical attrition treatment (pw-SMAT) where the austenite phase is directly subjected to severe plastic deformation and grain refinement without inducing phase transformation. Amorphous and grain size gradient (5–100 nm) microstructures, as well as a maximum compressive residual stress of 1093 MPa are produced in the surface layer of the NiTi plates via the pw-SMAT. The compressive residual stress notably reduces the surface tensile stress from bending. The grain size gradient layers with improved hardness and reduced hysteresis have high fatigue crack nucleation resistance, while the middle large-grained layers of the plates have high fatigue crack growth resistance. The combined effects of the gradient nanostructure and the compressive residual stress substantially increase the bending fatigue life of the NiTi plates from an original 103 cycles to over 1.3 × 104 cycles. The results open up a new route to improve the bending fatigue life of NiTi plates by heterogenous nanostructures.

The effect of mechanical incompatibility and strain delocalization on the mechanical properties of gradient structure Cu-4.5Al alloy

Gradient-structured (GS) metals have been reported to possess superior mechanical properties. In this study, the mechanical incompatibility and strain delocalization of Cu-4.5Al-SMAT specimen processed by surface mechanical attrition treatment (SMAT) during tensile was systematically investigated. The results showed that the hardness of specimen exhibited a gradient variation along the thickness after SMAT, which was related to the gradient defects (deformation twins and dislocations). EBSD observation demonstrated that the magnitude of mechanical incompatibility between gradient layers of specimen was associated to the degree of hardness gradient, which was further measured by load-unload-reload (LUR) test. The hetero-deformation induced (HDI) stress of Cu-4.5Al-SMAT specimen was more obvious compared to annealed specimen without SMAT, and the rate of HDI stress strengthening decreased with increasing tensile strain due to the reduction of degree of hardness gradient. Additional strain hardening was found for the Cu-4.5Al-SMAT specimen, which was confirmed by hardness and TEM. Furthermore, the combination of the gradient structure and the coarse-grained matrix can induce dispersed strain bands on the GS surface, which led to strain delocalization and thus obtained high yield strength and considerable ductility in the Cu-4.5Al-SMAT specimen.

Role of gradient nanograined surface layer on corrosion behavior of aluminum 7075 alloy

Gradient nano-grained structures have been a promising technique to evade the strength-ductility trade-off in metals and alloys. Therefore, in this work, the effect of surface mechanical attrition treatment (SMAT) on the microstructure and corrosion behavior of the high-strength aluminum alloy was investigated. SMAT was performed at room temperature and liquid-nitrogen (LN2) flow conditions to generate two distinctly different initial gradient microstructures. Potentiodynamic polarization, electrochemical impedance spectroscopy, and intergranular corrosion tests were performed. Surface film characterization of untreated and treated samples was performed using X-ray photoelectron spectroscopy and time of flight secondary ion mass spectroscopy techniques. Result reveals significant microstructural changes in SMAT processed samples such as the formation of precipitates and dissolution of inherent phases. In addition, a reduced anodic dissolution rate was observed with the SMAT processed samples. Furthermore, the surface film characterization revealed a thicker oxide film with Cu and SiO2 enrichment in SMAT samples.

Effect of Ultrasonic Surface Mechanical Attrition Treatment-Induced Nanograins on the Mechanical Properties and Biocompatibility of Pure Titanium.

Commercially pure titanium (Ti) is widely used in bio-implants due to its high corrosion resistance. However, Ti exhibits marginally low mechanical and tribological properties, which limit its applications in some orthopedic implants. In this work, the Ti samples were subjected to ultrasonic surface mechanical attrition treatment (SMAT) for various durations to improve their surface properties such as hardness, strength and surface energy. SMAT-induced grain refinement was analyzed using optical, scanning electron and atomic force microscopy techniques. A Vickers hardness test was performed to determine the through-thickness hardness. Mechanical testing was carried out to measure the yield strength, ultimate tensile strength and ductility of the specimens. Corrosion tests were performed on a Gamry Potentiostat. The surface energy of SMAT-modified samples was calculated using the Owens–Wendt method. It was observed that SMAT reduced the average grain size from 50 μm to as low as 100 nm. The grain refinement and the corresponding grain boundary density led to a significant improvement in mechanical properties and biocompatibility in terms of increased hardness, yield and tensile strengths, surface energy, corrosion rate and hydrophilicity.

A Review on the Effect of Mechanical and Thermal Treatment Techniques on Shape Memory Alloys

Despite of many interesting behaviors and attractive properties of Shape memory alloys (SMAs), there are some drawbacks and limitations that prevent them from being used in technology. But there are some treatment techniques that can be used to improve the behaviors of shape memory alloys. Also, they can remove or reduce the limitations of SMAs. In this study both and mechanical treatment techniques (Ball- and Roller-Burnishing Treatment, Surface mechanical attrition treatment, and Laser shock peening) and heat treatment techniques (Annealing, Normalizing, Hardening, and Tempering) have been clarified. And the effect of both treatment techniques of the properties of shape memory alloys have been reviewed.

Strain hardening behavior and microstructure evolution of gradient-structured Cu-Al alloys with low stack fault energy

Nanocrystallization can significantly improve the strength and hardness of metallic materials, but usually sacrifice ductility due to low work hardening capability. Heterostructured materials are an emerging class of materials with superior performances, because of their outstanding work hardening capability. In this work, a type of heterostructured material, a gradient structured Cu-Al alloy, was produced by surface mechanical attrition treatment (SMAT) at liquid nitrogen temperature. After SMAT processing, the yield strength was increased to more than 1.5 times, and the ductility remained almost unchanged. In conjunction with hetero-deformation induced (HDI) hardening, stacking fault energy is another important factor to increase the strain hardening in the system. Low stacking fault energy increased the density of stacking fault, and led to a finer spacing of nano twins (∼5.4 nm) and higher dislocation storage (8 × 1013 m−2) in the SMATed Cu-Al alloy at the intermediate strain stage. A significant up-turn of strain-hardening rate was also induced by low stacking fault energy.

Effect of Surface Mechanical Attrition Treatment on Torsional Fatigue Properties of a 7075 Aluminum Alloy

The effect of Surface Mechanical Attrition Treatment (SMAT) on torsional fatigue properties of a 7075 aluminum alloy was investigated. A number of fatigue samples were heat treated to increase the sensitivity of the material to SMAT. Compared with the as-machined (AM) samples, the fatigue lives of their SMATed counterparts (AM-SMAT) tested under torsional loading increased under high stress amplitudes, but decreased under low amplitudes. However, the fatigue lives of heated and SMATed samples (HT-SMAT) increased under all the investigated stress amplitudes, compared with those that were heat treated (HT). It was also revealed that the cracking mechanisms are different for the samples in different states, and they are dependent on the imposed stress levels. The results show that SMAT could have both beneficial and detrimental effects on the fatigue lives depending on the testing conditions. The roles played by various factors, including residual stresses, grain refinement, and surface roughness, were analyzed and discussed to interpret the results.

Literature Review on the Fatigue Properties of Materials Processed by Surface Mechanical Attrition Treatment (SMAT)

As a promising surface treatment technique, the surface mechanical attrition treatment (SMAT) has been applied to enhance mechanical properties of various materials. Through multidirectional severe plastic deformation, SMAT is able to nanocrystallize the near surface region of materials. The nanostructured layer associated with high compressive residual stresses coupled with a work hardening layer can provide the treated materials with an improved fatigue resistance. The present work gives a comprehensive review on the fatigue strength of SMATed materials. First of all, a brief introduction is given on the basic elements of SMAT and surface modifications induced by this treatment. The fatigue strength of a large variety of SMATed materials with different loading conditions is reviewed, including low-cycle fatigue (LCF), high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF). Then, the mechanism of enhancement or reduction is explained through a detailed review on the effects of several factors, such as residual stress, surface quality and nanocrystalline grains. In addition, the combined effect of SMAT coupled with other processes is also reviewed. Trends and prospects of the current research are summarized at the end.

Comparison of SP, SMAT, SMRT, LSP, and UNSM Based on Treatment Effects on the Fatigue Properties of Metals in the HCF and VHCF Regimes

This paper aims to provide a better understanding regarding the effects of shot peening (SP), surface mechanical attrition treatment (SMAT), laser shock peening (LSP), surface mechanical rolling treatment (SMRT), and ultrasonic nanocrystal surface modification (UNSM) on the fatigue properties of metals in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes. The work in this paper finds that SMRT and UNSM generally improve the high-cycle and very-high-cycle fatigue properties of metals, while SP, SMAT, and LSP can have mixed effects. The differences are discussed and analyzed with respect to the aspects of surface finish, microstructure and microhardness, and residual stress. SMRT and UNSM generally produce a smooth surface finish, while SP and SMAT tend to worsen the surface finish on metals, which is harmful to their fatigue properties. In addition to inducing a plastic deformation zone and increasing microhardness, surface treatments can also generate a nanograin layer and gradient microstructure to enhance the fatigue properties of metals. The distribution of treatment-induced residual stress and residual stress relaxation can cause mixed effects on the fatigue properties of metals. Furthermore, increasing residual stress through SP and SMAT can cause further deterioration of the surface finish, which is detrimental to the fatigue properties of metals.

Gradient-structured high-entropy alloy with improved combination of strength and hydrogen embrittlement resistance

High-strength materials usually exhibit poor hydrogen embrittlement resistance, and thus, there are demands for materials with high strength and good ductility under hydrogen. Here, gradient structures containing surface nanotwins are introduced in a CrMnFeCoNi high-entropy alloy by surface mechanical attrition treatment, and hydrogen embrittlement resistance is compared with coarse- and nano-structured alloys produced by high-temperature homogenization and high-pressure torsion, respectively. The coarse-grained alloy shows high ductility, but its yield stress is low. The nanostructured alloy shows ultrahigh yield stress, but with poor hydrogen embrittlement resistance. The gradient-structured alloys have both high yield stress (500–700 MPa) and good ductility (15–33%) under hydrogen.

Review of passivity and electrochemical properties of nanostructured stainless steels obtained by surface mechanical attrition treatment (SMAT): trend and progress

Abstract Surface nanocrystallization provides the opportunity to produce gradient-structured metallic materials with improved properties. Several attempts have been made to produce nanostructured stainless steel (SS), along with the study of the resultant corrosion resistance. However, the current knowledge is insufficient to address the corrosion mechanism and the possible ways of enhancing the corrosion resistance after surface treatment. The present work reviews the past significant works on the effect of surface treatment by surface mechanical attrition treatment (SMAT) method as well as its processing parameters on the corrosion properties of SS. The corrosion resistance of nanostructured SS is influenced by the extent of grain refinement, compactness, and homogeneity of the passive film, Cr content, grain boundary structure, composition, and alloying elements. In addition, the resulting corrosion properties can be controlled by choosing the right processing parameters during treatment. Progress on the corrosion behavior of nanostructured steels was summarized and new avenues for further research and developments are proposed.