High Compressive Residual Stress Research Articles

Incredible improvement in fatigue resistance of friction stir welded 7075-T651 aluminum alloy via surface mechanical rolling treatment

Friction stir welding (FSW) is able to provide high static strength for aluminum alloys, but it always leads to a remarkable loss in fatigue strength. By means of surface mechanical rolling treatment (SMRT), a near-polished, gradient nanostructured surface and high compressive residual stresses were generated on the FSW joint of the 7075-T651 aluminum alloy. Uniaxial fatigue tests showed that an enhanced fatigue property with two orders of magnitude extension in fatigue lifetime was achieved on the SMRT joints, exhibiting failure mode transitions from surface to interior. Transmission electron microscopy analyses showed that the Al18Ti2Mg3 constituent phase was responsible for the internal crack initiation. Furthermore, the enhanced fatigue mechanism and the role of SMRT were discussed. The findings offer a potential pathway to greatly compensate the fatigue resistance deterioration of materials resulting from welding.

Enhanced Fatigue Behavior in Quenched and Tempered High-Strength Steel by Means of Double Surface Treatments

The aim of this study is to maximize the fatigue life of a high-strength steel. For this purpose, two different kinds of double surface treatments were studied: shot peening plus vibratory finishing and shot peening plus grit blasting. First, a high-intensity peening treatment was applied, with the aim of inducing a deep region submitted to high compressive residual stresses, and a second surface treatment was followed, to reduce the roughness induced in the first treatment and mitigate the damage produced on the specimen surface. The use of such double treatments was demonstrated to be a good choice to achieve high surface hardening and deep compressive residual stress profiles and to minimize surface defects. The fatigue life of the steel was greatly enhanced following the application of both double surface treatments. A vibratory treatment applied for 24 h after the application of a previous shot peening treatment was able to increase the average fatigue life of the single shot-peened samples almost sevenfold, but a much greater fatigue enhancement was produced using grit blasting for only 60 s. Anyway, both second treatments are able to remove the damaged surface layer produced in the first high-intensity shot peening treatment. The obtained results also revealed that an appropriate combination of surface treatments (shot peening + secondary treatment) avoided the initiation of cracks at the surface of the specimens under cyclic bending loads. Instead, crack initiation took place beneath the compressive residual stress field, induced in alumina inclusions that acted as stress concentrators.

Well-defined (0001)-oriented aluminum nitride polycrystalline films on amorphous glass substrates deposited by ion plating with direct-current arc discharge

500-nm-thick aluminum nitride (AlN) polycrystalline films were deposited on amorphous glass substrates at a substrate temperature of 200 °C by ion plating (IP) with direct-current (DC) arc discharge, radio frequency (RF) and DC reactive magnetron sputtering (MS). The IP technique enables a very high deposition rate of 113 nm/min compared with that of MS techniques. The structural properties of AlN films were investigated by X-ray diffraction (XRD), Raman spectroscopy, and electron probe micro analyzer (EPMA) measurements. XRD results show that AlN films with a columnar grain structure deposited by IP have a texture with a well-defined (0001) orientation between grains normal to the substrate, whereas AlN films grown by MS have a mixture of (0001) planes and planes with other orientations. Raman spectroscopy and XRD measurement results revealed high residual compressive stress in AlN films deposited by IP compared with that in AlN films grown by MS. The analysis of the data obtained by EPMA measurement showed that IP produces AlN films with a small amount of remaining oxygen species, whereas MS produces AlN films with a large amount of residual oxygen species.

Effect of tangential misalignment in ultrasonic burnishing

Ultrasonic burnishing is a surface finishing process that produces fine surface roughness, improved geometrical accuracy, high compressive residual stresses and increased hardness. The method is suitable for finishing dies or other double curvature surfaces and it does not require changing of the fixturing setup after a machining process. The method has not however become commonly used in industry, because the technology has not been extensively tested. Technological parameters in ultrasonic burnishing includes the surface speed, distance between consecutive passes (or in machining terms, feed) and contact force. Ultrasonic burnishing is based on forging at an ultrasonic frequency of over 20,000 impacts per second and there is no material removal associated with the process. The technological parameters have been tested and recommended values have been recorded for some common work materials. One important parameter is the tangential alignment of the burnishing tool in regard to the workpiece surface normal. This parameter is easy to control in turning operations or with planar surfaces, but with a double curved surface the control of the angle is more difficult. All the previous research has been done on cylindrical or planar workpiece and therefore the tangential misalignment or the sensitivity of the surface integrity regarding the tangential misalignment has not been studied. This paper investigates the effect of the tangential misalignment of the burnishing tool and the effect on surface integrity. The results show that ultrasonic burnishing is suitable method for finishing workpiece surfaces even with inclined tool angle. The ultrasonic burnishing produced a beneficial surface integrity state on the workpiece surface with hardened material and improved surface quality. The surface micro-hardness increased by 1.4-6.7 %. At the same time, the average surface roughness (Ra) values decreased by 50-85 % depending on the misalignment angle.

Controlling chloride induced stress corrosion cracking of AISI 316L stainless steel by application of buffing

Abstract This paper presents the effect of buffing on the stress corrosion cracking (SCC) resistance of machined surfaces of AISI 316L stainless steel (SS). Surface finishing operations are known to lower the SCC resistance of austenitic stainless steels and yet are unavoidable. AISI 316L SS samples were given for different surface finishing operations like turning, milling and grinding. The surface roughness in each case was determined using stylus profiler (contact mode). Lattice strain and phase changes were detected by X-ray diffractometry (XRD) studies. SCC susceptibility has been studied before and after surface buffing operation by exposing to magnesium chloride hexahydrate solution (MgCl2.6H2o) as per ASTM standard procedure G36 for 3h. Microstructural characterization before and after SCC tests were characterize by using field emission SEM (FESEM). Results highlights that AISI 316L SS in turned, milled, and ground condition were highly prone to Cl– induced SCC. But on the application of buffing, the SCC resistance of the samples was enhanced and no cracking was observed. Crack enhancement was due to a) reduced surface roughness b) minimum plastic strain and c) high compressive residual stresses generated during buffing operation.

Effect of Post Treatment in Argon Environment of Plasma Nitrided Local Disc Brake

Plasma nitriding (also known as ion nitriding, plasma ion nitriding, or glow-discharge nitriding) is a method of surface hardening using glow-discharge technology to introduce nascent (elemental) nitrogen to the surface of a metal part subsequent diffusion into the material. Because of the formation of high compressive residual stresses in the case region, increasing surface hardness and depth parameters cause remarkable improvement in the mechanical properties of steels. In this study, the properties of a nitrided local disc brake were investigated. For that purpose, the material was nitrided at 400°C of temperature, 1.6 mbar of pressure, and 4 hours of time then followed by post-treatment in Argon (Ar) environment for various of holding time such as 10, 20, 30, 40, and 50 minutes. At these conditions the hardness increased in the range 123.1 - 223.3 VHN, the wear rate decreased in the range 21.7 × 10-9 - 3.1 × 10-9 mm3/mm kg, while before being nitrided the hardness and the wear rates were 113.5 VHN and 22.8 × 10-9 mm3/mm kg, respectively. The optimum condition for holding time during post treatments was 20 minutes. Besides that, the effect of various of N2: Ar mixture such as 90%: 10%, 80%: 20%, 70%: 30%, and 60%: 40% on the properties of local disc brake was studied. For various of gas mixing, the optimum hardness in the order of 252.7 VHN and wear rate in the order of 2.8 × 10-9 mm3/mm kg were achieved at 90%N2: 10%Ar of a gas mixture.

Effects of laser shock peening on corrosion fatigue behavior of 2Cr13 stainless steel

Effects of laser shock peening (LSP) with different coverage layers on corrosion fatigue (CF) resistance of 2Cr13 stainless steel were investigated by residual stress measuring, corrosion fatigue testing and fatigue fracture morphology observing. The results show that LSP can induce high compressive residual stress on the surface of 2Cr13 stainless steel, and the value of surface compressive residual stress increases with the increase of LSP coverage layer. The CF life of LSP specimens is much larger than that of as-machined specimens, and it increases with the increase of LSP coverage layers; the CF crack growth rate of LSP specimens decrease compared with that of as-machined specimens, and it decreases with the increase of LSP coverage layers. After the analysis, we can conclude that the fatigue fracture surface of as-machined specimens is flat and contains few secondary cracks. However, for LSP specimens, due to high surface compressive residual stress induced by LSP, the growth of corrosion fatigue crack is effectively restrained, resulting in a river pattern morphology.

Surface integrity of 2A70 aluminum alloy processed by laser-induced peening and cavitation bubbles

This paper presents a novel compound strengthening method based on laser-induced peening and cavitation peening (LIPCP). 2A70 aluminum alloy was strengthened by plasma shock wave and cavitation bubble collapse shock wave. The dynamic characteristic of cavitation bubble was captured by using a high-speed camera, and the strengthening mechanism of LIPCP was detailed elaborated. The peening depth was measured by an ultra-depth-of-field microscope and increases with increasing laser energies. But the surface roughness decreases with an increase in laser energies. The surface, after the compound strengthening, improved considerably with the laser energy and impact times, and has higher compressive residual stress. However, the compressive residual stress reaches a saturation value in the first 5 impacts due to the formation of a hardened layer on the material surface. In addition, ultrasonic cavitation erosion experiments were performed in 3.5% NaCl solution at room temperature. The results reveal that there are four typical cavitation erosion periods during the cavitation erosion process, and the mass loss rate of the treated specimen is relatively lower than that of the untreated specimen. The microscopic morphology analysis shows that there are large smooth areas on the LIPCP treated surface after ultrasonic cavitation erosion, which indicates that the fatigue life and cavitation resistance of 2A70 alloy aluminum has been greatly improved. The LIPCP provides a promising method to strengthen the hydraulic machinery beneath the water.

Investigating adhesion wear on belt and its effects on dry belt finishing

Adhesion wear on abrasive belt sometimes occurs as an abnormal abrasion way because of an improper selecting on working conditions during the belt finishing. In this paper, the forming process of the adhesion wear and its effects on the finished surface is investigated through a series of experiments. The results reveal that the applied pressure and the belt feed rate are supposed to be the two sensitive variables for forming the adhesion wear. A certain amount of accumulated chips is a trigger to cause adhesion wear initially, and then, the sufficient heat accumulation becomes the main factor for the following adhesion wear. Thus, shortening the finishing duration, decreasing applied pressure, rotating speed and introducing cooling/lubrication system are recommended to avoid adhesion wear. The belt finishing process with adhesion presents a potential to further decrease surface roughness by 30% maximum, improve bearing characteristics and obtain equivalently high compressive residual stresses; however, this merit has to go with the sacrifice of belt’s cutting ability.

Comparative analysis of the structure and internal stress in Ti-6Al-4V alloys manufactured by 3D printing and processing with screw extrusion

Study is based on the compare characterization of the structure and evaluation of the residual internal stresses in Ti-6Al-4V samples manufactured by the 3D printing (selective laser melting) and in the Ti-6Al-4V samples obtained with using of the severe plastic deformation by screw extrusion. The microstructure and residual stresses were examined by transmission electron microscope and X-ray diffractometer. High tensile internal residual stresses in the 3D printed sample were found. The high compressive residual internal stresses were found in the hot pressed and in the twisted samples. It was shown that arising of the high residual stresses in the studied samples under various technological processes occurred in various ways. The residual stresses in the severe plastic deformed samples arose due to non-uniform volumetric plastic deformation. In 3D printed sample, the residual tensile stresses arose from both phase (martensitic) transformation and thermal deformation.

Effect of Ni content on the microstructure and mechanical behaviour of CrAlNiN coatings deposited by closed field unbalanced magnetron sputtering

Ni incorporation is known to impose an intriguing influence on the composition, microstructure and mechanical properties of nitride-based coatings prepared by physical vapour deposition. This work examined the effect of varying Ni concentrations, controlled by NiCr alloy target current (INiCr), on the microstructures and properties of CrAlNiN coatings deposited on to M2 tool steel substrates. These coatings were deposited by closed field unbalanced magnetron sputtering (CFUMS). The composition and structure of the as-deposited coatings were investigated by XPS, XRD, FIB, and TEM. Residual stresses were determined using the XRD-sin2ψ method. Nanoindentation tests were performed to assess mechanical properties of the CrAlNiN outer layer. At relatively low INiCr values, below 3 A, the grains within CrAlNiN layer underwent refinement, yet maintained a columnar structure. At higher INiCr values, from 3 to 5 A, the columnar grains transitioned into more equiaxed grains. Hardness values of 25–28 GPa at low INiCr values were associated with the high residual compressive stress, solid solution hardening and grain refinement.

Effect of thermal relaxation on residual stress and microstructure in the near-surface layers of dual shot peened Inconel 625

Thermal relaxation behavior of residual stress and microstructure in the near-surface layers of dual shot peened Inconel alloy 625 was investigated by X-ray diffraction method. Residual stress on the top surface layer was significantly relaxed in the first 15 min at the elevated temperatures of 500°C, 600°C, and 700°C. However, there was still high maximum compressive residual stress in subsurface layers. The relaxation behavior of residual stress has contributed to the thermally activated process. The activation enthalpy Δ H and m were calculated according to the Zener–Wert–Avrami method, the values of which were 1.59 eV and 0.4934, respectively. Microstructural evaluation revealed that it was slightly changed in the near-surface layers after various isothermal treatments. Accordingly, high level of compressive residual stress and dislocation density resulted in the retained mechanical properties of dual shot peened Inconel 625, which was discussed based on the relaxation of microstructure and microhardness.

Cutting mechanism and surface integrity in milling of Ti-5553 processed by selective laser melting

Titanium alloys are of significant importance in several high performance applications such as aerospace components or medical implants. Advances in additive technologies lead to an increase of additively built workpieces, offering new possibilities regarding functional integration and lightweight structures. Several authors have shown that microstructural, mechanical and thermal material properties differ significantly from those of cast alloys. Although additively produced parts are near net-shape, most of them are machined after the building process, to achieve the requirements regarding surface finish and dimensional accuracy. Titanium is generally known as a hardto- cut material due to its thermo-mechanical properties. Although there is a profound knowledge about the machinability of conventionally cast and wrought titanium alloys, there is a lack of understanding regarding the machining of additively built titanium. In this paper, the machinability of an additively built Ti-5Al-5V-5Mo-3Cr alloy (Ti-5553) is analyzed regarding chip formation, cutting forces and tool wear. Three different generation methods, conventional wrought, selective laser melting and selective laser melting with in-process (insitu) heat treatment are investigated. It is shown that the machinability differs significantly compared to a conventional wrought alloy, which is linked to decreased contact length on the tool rake face and higher mechanical tool load (up to + 23 % force increase). Highest tool load is found for titanium alloy, which is built using selective laser melting and in-process heat treatment. Furthermore, the surface integrity after machining is analyzed regarding hardness, roughness and residual stresses. Hereby, high compressive residual stresses for additively built titanium with in-process heat treatment are obtained due to the higher mechanical tool load. Therefore, it could be shown that the generation method (e.g. selective laser melting) needs to be considered for the later process design in finish machining.

Enhanced fatigue performance and surface mechanical properties of AISI 304 stainless steel induced by electropulsing-assisted ultrasonic surface rolling process

Abstract

Surface Integrity Analysis of Ti6Al4V After Semi-finishing Turning Under Different Low-Temperature Cooling Strategies

During machining of difficult-to-cut alloys, the rapid tool wear as a consequence of the high values of cutting forces and temperatures significantly affects the surface integrity of the machined surfaces reducing the product in-service performances. The use of conventional cutting fluids limits the tool deterioration, but provides several environmental issues to deal with, such as the exhausted fluid disposal, cleaning of the parts after machining, health hazards for the machines operators. In recent years, several researches have proposed to use environmental-friendly cryogenic coolants, whose very low operating temperatures have demonstrated to be able to inhibit the thermally activated tool wear mechanisms leaving the machined surface almost free from pollutants. In this framework, the aim of this research work is to analyze the surface integrity of the machined surfaces produced in semi-finishing turning of the Ti6Al4V titanium alloy under different low-temperature cooling techniques, namely liquid nitrogen (LN2), carbon dioxide (CO2) and gaseous nitrogen (N2) cooled at different temperatures, using the dry and wet cutting conditions as baseline. The surface integrity was evaluated considering both the surface finish (surface defects, roughness and topography) and the surface microstructural and mechanical alterations (altered layer, nano-hardness and residual stresses). The results showed that the best performances were obtained using N2 cooled at − 150 °C, which determined both the absence of surface defects, as wrinkles or feed marks irregularities typical of the surface generated under LN2, and high compressive residual stresses that made the surface suitable for biomedical and aerospace applications.

Fatigue properties and material characteristics of additively manufactured AlSi10Mg – Effect of the contour parameter on the microstructure, density, residual stress, roughness and mechanical properties

Additive Manufacturing (AM) is in general seen as a game changing technology. In particular for the space industry, it seems a promising tool for the constant strive towards increasing performance and reducing the cost of space missions. The layer-wise approach allows an increased design freedom and optimisation. Current space applications are designed with a large safety margin since the AM process generates imperfections in the material, which can lower the mechanical properties. A profound understanding on the one hand of the link between process parameters and final properties, and on the other hand of the influence of possible imperfections on mechanical properties still requires more research and can result in a further design and performance optimisation. With this aim, this study characterised the fatigue behaviour of AlSi10Mg processed via AM and the influence of the process parameters, and possible resulting imperfections, on fatigue behaviour. First of all, the influence of building direction, platform temperature, powder layer thickness, surface finish and heat treatment on the fatigue properties was verified. Results showed that there is a direct correlation between the process parameters and the fatigue properties of AlSi10Mg parts, having received the same post processing. Observations indicate that changing the parameters influences the defect population characteristics, determining the fatigue life. In addition, the different surface finishing methods have, as expected, a strong impact on the fatigue behaviour of AM AlSi10Mg materials. Interestingly, vibratory polished specimens produced without contour parameter endured on average a larger amount of cycles to failure than specimens produced with contour parameter. Therefore, the dimensions, roughness, hardness, microstructure, defect population and residual stresses were verified for these two different scanning strategies to look for an explanation for this behaviour. The samples without contour showed lower surface roughness and higher compressive residual stresses in the load direction, potentially explaining the higher number of sustained cycles to failure. Overall, this study showed that an optimised combination of process parameters and surface finish can improve fatigue performance, without the need for machining of the AM parts.

Effect of Laser Shock Peening on Residual Stress, Microstructure and Hot Corrosion Behavior of Damage-Tolerant TC21 Titanium Alloy

The effect of laser shock peening (LSP) on the residual stress, microstructure and hot corrosion behavior of damage-tolerant TC21 alloy was investigated. The equilateral scanning pattern was adopted to obtain a large overlapping area for LSP. The hot corrosion experiments in a salt mixture composed of 75 wt.% Na2SO4 + 25 wt.% NaCl at 700 and 800 °C for a total exposure of 25 h were conducted. The results demonstrated that LSP caused a high compressive residual stress with a significant increase in micro-hardness and microstructural changes in the form of crystal defects, i.e., high density dislocations. The crystal defects promoted the outward diffusion of Al, Cr and Ti to form protective mixed oxides (Al, Cr, Ti)O2. These oxides improved the adhesion of outer oxide layer and hindered the inward diffusion of O, S and Cl, which was beneficial to improve hot corrosion resistance. The mass gains of LSP-treated specimens after hot corrosion at 700 and 800 °C were, respectively, 3.72 and 4.78 mg/cm2, which were much lower than those of untreated specimens (5.65 and 6.81 mg/cm2). Therefore, LSP leads to an improvement in resistance to hot corrosion of TC21 alloy.

Effect of laser shock peening on wear behaviors of TC11 alloy at elevated temperature

Titanium alloy is regarded as a high performance material which has wide application prospect in aerospace and medical field. However, their poor wear properties may lead to failures in early service stages. In this paper, the microstructure, micro-hardness and residual stress of TC11 alloy treated by laser shock peening (LSP) are investigated. The sliding wear experiments are performed to study the influence of various temperature and applied load on the wear behaviors of TC11 alloy with and without LSP. The results indicate that the LSPed specimen has a superior wear properties to that of as-received specimen under the same wear test conditions. The tribological properties are greatly dependent on wear testing temperature and applied load. For the specimens tested under 15 N, the variation trend of friction coefficient and wear rate reduces first (25–500 °C) and then increases (500–600 °C) with increasing the test temperature. When the sliding wear test conducted at 400 °C, the larger the applied load, the more the friction coefficient and wear rate. The prominent tribological performances of LSPed TC11 alloy is ascribed to the high compressive residual stress and grain refinement induced by LSP.

Increase in Strength and Fretting Resistance of Alloy 718 Using the Surface Modification Process

This work comparatively investigated the strength (hardness, yield strength, dynamic elastic modulus, and surface residual stress), fretting failure, and corrosion resistance of the as-received and treated Ni-based superalloy Alloy 718. The goal of the current research is to improve the hardness, fretting wear, and corrosion resistances of Alloy 718 through the ultrasonic nanocrystal surface modification (UNSM) process with the aim of extending the lifespan of aircraft and nuclear components made of Alloy 718. The experimental results revealed that the surface hardness increased by about 32%, the fretting wear resistance increased by about 14%, and the corrosion resistance increased by about 18% after UNSM process. In addition, the UNSM process induced a tremendous high compressive surface residual stress of about −1324 MPa that led to an increase in yield strength and dynamic Young’s modulus by about 14 and 9%, respectively. Grain size refinement up to ~50 nm after the UNSM process is found to be responsible for the increase in surface hardness as well. The depth of the effective layer generated by the UNSM process was about 20 µm. It was concluded that the UNSM process played a vital role in increasing the strength and enhancing the corrosion and fretting resistances of Alloy 718.

Residual stress and microstructure evolutions of SAF 2507 duplex stainless steel after shot peening

In this study, the SAF 2507 duplex stainless steel was subjected to surface shot peening (SP) treatment. The characteristics of processed samples such as surface morphology, residual stress distribution and microstructure were analyzed by scanning electron microscopy, X-ray diffractometry and transmission electron microscopy techniques. Results showed that SP had deleterious side effect on the surface roughness. After SP, high compressive residual stresses were induced on surface layer of the SAF 2507 alloy. The maximum stresses located at subsurface were approximately −930 and −715 MPa in γ and α phase. SP introduced satisfied surface nanocrystallization and microstructure refinement into the material. The minimum grain size located at the top surface was about 25 nm, the thickness of nanocrystalline layer with crystallites smaller than 100 nm reached to ∼125 μm. Moreover, a mass of dislocations and dislocation cells was formed in γ phase, but was rarely observed in α phase. The quantified dislocation density at surface was about 2.29 × 1015 m−2 and 1.28 × 1015 m−2 in γ and α phase, respectively. SP leaded to the martensitic transformation, and the transformation sequence was considered to be: γ → austenite twins → α′-martensite. SP treatments resulted in work hardening and increased the microhardness of the near surface material, which was mainly ascribed to the grain refinement effects and the formation of the strain-induced α′-martensite.