Subsurface Residual Stress Research Articles

Targeted Minimum Quantity Fluid Application in Machining

The surface integrity of a machined component is crucial for its service life part. One of the main final specifications that a machined part is inspected for is the surface integrity metrics, including surface residual stresses, surface microhardness, surface roughness, and microstructure. In this paper, the cutting fluid is strategically targeted to utilize heat energy effectively in the primary, secondary, and tertiary shear zones to positively affect the surface integrity metrics and machining mechanics. In this study, a lower quantity of the cutting fluids is targeted at the high-temperature zones to reduce the machining temperatures, thereby effectively simulating the effect of a ‘flood coolant’. The cutting fluid is applied simultaneously as a targeted Minimum Quantity Fluid (MQF) on the cutting tool’s flank and rake faces to improve the surface integrity metrics and chip formation. Also, this study analyzes the effect of the cutting fluid composition, the type of cutting fluid, and the amount of fluid quantities. The machining-induced surface integrity metrics are analyzed to understand the effects of targeted minimum quantity fluid application. The impact of the targeted application of cutting fluid on machining mechanics metrics, such as cutting forces and chip formation, is analyzed. Applying a targeted MQF application at the flank face of the cutting tool leads to higher compressive subsurface principal residual stresses. The results indicate that using MQF on both the flank and rake faces simultaneously enhances the surface integrity. The effect of a cutting fluid jet on the flank face is modeled to highlight the thermophysical properties that are crucial for selecting the appropriate cutting fluid to lower the machining-induced temperatures. With targeted MQF application, the fluid jet acts as a dynamic and external chip control mechanism. Overall, effectively managing temperatures in machining could enhance subsurface residual stresses and surface roughness using various cutting fluid combinations. Also, this paper presents a targeted cutting fluid application that improves the microstructural formation, enhancing chip control and producing machined surfaces and components with better surface integrity.

High-efficiency green machining of single crystal 4H–SiC based on tribo-oxidation

High efficiency, high quality and green machining of single crystal silicon carbide (SiC) is an important issue in the third-generation semiconductor industry. In this paper, a novel machining method for single crystal SiC based on tribo-oxidation is proposed. The formula and preparation process of polysaccharide bonded soft abrasive tool were designed. Dry machining of C-face of 4H–SiC substrate was carried out with the prepared environmentally friendly abrasive tool sample. The material removal mechanism of the proposed machining method was preliminarily analyzed, and the material removal rate, surface roughness and subsurface residual stress were studied. The experimental results show that the maximum material removal rate of 4H–SiC can reach 1.03 μm/min with a corresponding average surface roughness of Sa 0.816 nm. The machined surface has no brittle flaw, and subsurface residual compressive stress exists with a maximum value of 58.9 MPa. The material removal mainly attributed to the active oxidation of SiC surface induced by the friction between alumina abrasives and SiC substrate. The proposed machining method has potential application value in thinning of SiC wafers.

Surface and subsurface integrity of monocrystalline silicon in impulse-discharge driven abrasive machining

In abrasive machining of monocrystalline silicon, surface and subsurface behaviors are changed by the brittle-to-ductile removal transfer due to uncertain mechanical force, thus leading to surface micro-burr, surface roughness, subsurface residual stress, subsurface damage and so on. In mechanical diamond grinding, an impulse-discharge between wheel metal and semiconductor silicon is proposed to drive a loose-abrasive flow along with the flexible pressure for nano-scale surface removal, micro-scale mechanical removal and thermal modification. The formation behavior of abrasive-Si interface is researched to improve the surface and subsurface integrity by integrating modification, flexible removal and cut-copying. First, the surface formation procedure was modelled in relation to impulse-discharge energy, hydrodynamic pressure and loose-abrasive diameter, etc. Then, the dynamic characteristic of fixed-diamond, loose-abrasive machining as well as impulse-discharge machining was investigated in relation to the ductile-brittle transition. Finally, the surface integrity of monocrystalline silicon was investigated in hybrid machining. It is shown that the hybrid machined formation chain is formed in the abrasive-workpiece interface with cut-copying the surface profile. The impulse-discharge drives the loose-abrasive and modifies the interface by thermal ductile transition. The loose-abrasive obtains a removal force to eliminate the modified Si–O bond and expose the Si2p3/2, Si2p1/2 substrate with ductile formation. Decreasing the impulse-discharge energy and hydrodynamic pressure promote brittle to ductile transition for high surface and subsurface integrity. Accordingly, the surface integrity is subject to impulse-discharge energy, hydrodynamic pressure and loose-abrasive size. With the thermal tensile and abrasive driven effect of impulse-discharge, the residual compressive stress, surface micro-burr and subsurface damage thickness were decreased by 72 %, 65 % and 88 %, respectively. As a result, the mechanical cut-copying with impulse-discharge thermal modifying action and abrasive flow driving ensures the surface integrity and form accuracy in process.

Grinding EB-PBF based additive manufactured Ti6Al4V: A surface integrity study

Grinding is a finishing process typically done in most metallic manufacturing centers, primarily to achieve precision and surface improvement. Currently, the grinding process of titanium alloys generally requires flood coolant application. Electron Beam Powder Bed Fusion (EB-PBF) is an additive manufacturing process that uses an electron beam as the heat source to melt and fuse powder particles to build layer by layer to build a three-dimensional component. Grinding is a major secondary process applied to additively manufactured metals, but with the current methodologies, grinding may impart tensile residual stress on the surface, and thus the performance of the material under fatigue conditions is reduced. In this paper, a targeted cutting fluid application approach for grinding an additively manufactured titanium alloy is used to possibly impart a compressive residual stress upon the subsurface while also providing an improved surface roughness. This study uses samples ground with a traditional flood coolant and samples with targeted cutting fluid applications developed by the researchers. Metrics such as surface residual stress, surface roughness, microstructure, and microhardness were used to determine imparted qualities using the various grinding cooling methodologies. The results show that the subsurface maximum principal residual stresses decreased by 108%, the average surface roughness decreased by 33%, and the microhardness at 5 μm increased by 1% using targeted air as the cutting fluid compared to flood cooling while grinding additively manufactured Ti6Al4V. Overall, the targeted grinding cooling fluid application induced compressive subsurface residual stresses and reduced the average surface roughness.

Establishment and experimental verification of a three-dimensional finite element model for residual stress in surface processing of Inconel 718 alloy by laser cladding

The local load and residual stress changes of conventional forged Inconel 718 alloy can be simulated in a two-dimensional (2D) cutting simulation model, but the type and distribution of global residual stress field of the alloy cannot be predicted during laser cladding layer cutting. To address the shortcomings of conventional 2D cutting simulation models for forging Inconel 718 alloy, this paper proposed a 3D closed-loop finite element model, and taking the dry-side milling process of Inconel 718 alloy laser cladding layer as an example, the generation mechanism and distribution of residual stress are predicted during the machining process. Further testing of surface, near-surface, and subsurface residual stresses was conducted to analyze the specific distribution of residual stresses in the workpiece. The results indicated that the uneven distribution of cutting stress and temperature on the workpiece had a significant non-stationary thermal-mechanical effect on the surface of the machined workpiece; the distribution of surface residual stress was uneven, with the significant tensile residual stress at the workpiece boundary. As the depth increases, the influence of mechanical effects dominated, and the compressive residual stress was generated on the subsurface of the workpiece. Residual stress testing was conducted on the surface/near-surface, and the experimental results were consistent with the simulation results of the closed-loop simulation model. In the subsurface, residual stress showed a similar trend of change. This study compensated for the shortcomings of conventional finite element simulation and provided the reference value for the application of three-dimensional finite element simulation in laser cladding materials.

Multi-objective optimization of the subsurface residual stress field of TC4 alloy in machine hammer peening

Machine hammer peening (MHP) is a surface strengthening process by introducing residual compressive stress at the part's surface. Many indicators of residual stress, such as the surface residual compressive stress and depth of residual compressive stress, significantly impact the fatigue performance of parts and thus need to be optimized. The present study focuses on two main problems in multi-objective optimization of the subsurface residual stress field induced by MHP: the relationship between a specific residual stress field and a combination of impact velocity and indentation distribution and how to implement the MHP process that could produce such a combination. The latter problem was first addressed by a kinematic model of the MHP device, which can optimize the MHP device input parameters to achieve the target axial impact velocity. For the former problem, a finite element model was first established to simulate the residual stress field induced by MHP. Then, the response surface method was used to optimize the simulated residual stress field with multiple objectives. Based on the above two models, we can obtain the appropriate MHP parameters to induce a target subsurface residual stress field. A verification experiment was carried out, and the residual stress field obtained was relatively close to the theoretical one, with a deviation of 1.7 % for the depth of residual compressive stress and 13.8 % for the surface residual compressive stress.

Experimental study on the effect of the milling condition of an aluminum alloy on subsurface residual stress

Aluminum alloys used in monolithic parts for aerospace applications are subjected to distortion and residual stress (RS) generated by milling, affecting the product fatigue life. Particularly, the change in RS with depth (z) has a characteristic distribution with a maximum compressive RS at a z several tens of micrometers from the surface; however, the RS value depends on the measurement method used. In this study, the RS distribution with z from the surface after milling was measured for the AA7050-T7451 aluminum alloy by two-dimensional X-ray diffraction (2D method). The results were compared with those of four prior measurement methods, and the validity of 2D method was verified. The changes in subsurface RS with z showed similar distributions under all measurement conditions except when cos(α)-XRD was employed. The 2D method provides high repeatability. The in-plane RS distribution was also measured using 2D method to investigate the effect of milling conditions on this distribution. The RS values varied markedly depending on the measurement position, particularly at a small collimator diameter of 0.146 mm, allowing detection of localized extreme RS values. The maximum RS at z = 0 mm was − 85.6 MPa at a cutting speed of vc = 200 m/s and feed per tooth of fz = 0.05 mm, while it was − 16 MPa for vc = 450 m/s and 6.8 MPa for fz = 0.2 mm, revealing that the compressive RS changes to tensile RS as vc and fz increase.

Implementation of Gaussian Process Regression to strain data in residual stress measurements by hole drilling

Incremental hole drilling is a prominent tool in the surface and sub-surface residual stress measurements. Generally, strains are measured at drilling steps not corresponding to the stress calculation steps of the integral method. Consequently, strain measurements are fitted using polynomials or splines. However, this process is greatly influenced by user’s experience, affecting the calculated stresses as an additional source of uncertainty. The present work assesses the uncertainty associated with the strain fitting procedure by exploiting Gaussian Process Regression (GPR), a probabilistic machine learning framework which can yield uncertainties on the fit. These uncertainties were propagated to residual stresses by using Monte Carlo simulation. Laser shock peened AA 7050-T7451 specimens were considered as case study.The proposed methodology was corroborated by comparing residual stress results with those obtained exploiting traditional fitting procedures and X-ray diffraction measurements. Finally, the GPR-based approach demonstrated its ability to successfully discriminate different strain signal sources.

Surface smoothening and strengthening combined effect of mechanical hammer peening on inconel 718 superalloy

ABSTRACT In this study, the surface strengthening and smoothening combined effect of the mechanical hammer peening (MHP) treatment is demonstrated. On a IN718 nickel-based superalloy workpiece surface, the MHP treatment introduced a subsurface residual stress field featured with a maximum stress around −1.4 GPa and a penetration depth exceeding 1 mm, and an increase of the micro-hardness from 286.43 HV to 426.27 HV. Simultaneously, it produced a significantly smoothened surface (Ra 27 nm) compared with the untreated sample (Ra 555 nm). Moreover, applying an electric pulse of 100 A would further reduce the surface roughness, without deteriorating the surface strengthening effect. The surface strengthening effect of MHP treatment mainly originates from the increase of dislocation density, grain misorientation and refinement, which are caused by impacts of the punching head; while the surface smoothening effect is the result of precisely controlled and regularly aligned punching indents. Utilizing MHP in the manufacturing process chain of aircraft parts could combine the surface strengthening and smoothening processes into one single process, which substantially improves both the production efficiency and enhances the surface finish. Especially, the working principle of MHP makes it suitable for arbitrary or complicated surfaces, that is, aeroengine blades.

Study on Residual Stress in Disc-Milling Grooving of Blisks.

The disc-milling method is expected to increase the grooving efficiency of blisks. However, there are few studies about the residual stress on a blisk during disc-milling grooving. In this study, a single-factor experiment and an orthogonal experiment of blisk disc-milling and grooving were designed to obtain the residual stress. Surface subsurface residual stress were also studied. The results showed that the surface of the milling groove bore compressive stress. Residual stress decreased with increasing spindle speed and increased with increasing feed speed and spindle rotation angle. Moreover, residual stress was most sensitive to spindle rotation angle and least sensitive to feed speed. A higher residual stress produced on the machined surface led to a deeper layer of residual stress.

A Review of Non-Destructive Evaluation (NDE) Techniques for Residual Stress Profiling of Metallic Components in Aircraft Engines

Residual stresses are one of the main factors determining the failure of aircraft engine materials. It is not possible to reliably and accurately predict the remaining service life of aircraft engine components without properly accounting for the presence of residual stresses. The absolute level and spatial distribution of the residual stress is uncertain in aircraft engines because the residual stress profile is highly susceptible to variations in the manufacturing process. In addition, residual stresses keep evolving under complex thermal-mechanical loadings. Non-destructive techniques are desired by the aerospace industries for the regular monitoring of subsurface residual stress profile in aircraft engine components. The insufficient penetrating capability of the only currently available non-destructive residual stress assessment technique X-ray diffraction has prompted an active search for alternative non-destructive techniques. This paper provides an overview of the principle, practical applications, advantages, and limitations of four categories of nondestructive (diffraction, ultrasonic, and electromagnetic) techniques for residual stress profiling of metallic components in aircraft engines.

Subsurface residual stress and damaged layer in high-speed grinding considering thermo-mechanical coupling influence

Subsurface residual stress and damaged layer play a vital role in determining the accuracy maintenance and fatigue performance of parts. Due to the advantages of machining quality and efficiency, high-speed grinding technology is being applied to the machining of precision parts. At present, the influence of high-speed grinding on the damaged layers generation and residual stress distribution has not been completely recognized, and there has been little quantitative research on the mechanism of thermo-mechanical coupling influence on the distribution of residual stress generated in the high-speed grinding process. In this study, a finite-element model comprehensively considering grinding force and thermal field was proposed to investigate the subsurface residual stress and heat influenced layer of AISI 52,100 bearing steel. The subsurface damaged layer and residual stress fields were measured to validate the analytical results. Mathematical models were proposed to quantitatively analyze the thermo-mechanical coupling influence on the distribution characteristics of subsurface residual stress. The theoretical and experimental results demonstrate that when the grinding speed surpasses the critical value of 45 m/s, the depth of residual stress and damaged layer decrease simultaneously with the increase of grinding speed. Higher grinding speeds exceeding 60 m/s are supposed to restrain the maximum value of both the tensile and compressive residual stress, which helps to enhance the precision retention and fatigue performance of components. Base on this study, the performance of the subsurface can be controlled by selecting the proper grinding speed in the high-speed range.

Numerical and Experimental Study on Reasonable Coverage of Shot Peening on ZGMn13 High Manganese Steel

Shot peening technology is usually employed to improve the ability of mechanical parts to resist failure due to fatigue and wear. It is often used to strengthen the surface of a target, but the induced residual stress and its distribution with respect to the coverage can affect the performance of the shot peening process. In this study, a comprehensive numerical and experimental study was conducted to overcome these issues. Using numerical simulation we found that both the surface and subsurface residual stress increases with the increase of the coverage before stabilizing. Quantitative analysis using the Entropy Method indicates that under the shot peening parameters considered in the simulation coverage of 200% is best for the shot peening of ZGMn13 High Manganese Steel. The following experimental study agreed with the corresponding numerical data for the residual stresses at varied depths from surface to subsurface with errors of less than 25%. Thus, the related research outcomes can guide the shot peening process to obtain the optimized surface strengthening of the target.

Subsurface and Bulk Residual Stress Analysis of S235JRC + C Steel TIG Weld by Diffraction and Magnetic Stray Field Measurements

BackgroundDue to physical coupling between mechanical stress and magnetization in ferromagnetic materials, it is assumed in the literature that the distribution of the magnetic stray field corresponds to the internal (residual) stress of the specimen. The correlation is, however, not trivial, since the magnetic stray field is also influenced by the microstructure and the geometry of component. The understanding of the correlation between residual stress and magnetic stray field could help to evaluate the integrity of welded components.ObjectiveThis study aims at understanding the possible correlation of subsurface and bulk residual stress with magnetic stray field in a low carbon steel weld.MethodsThe residual stress was determined by synchrotron X-ray diffraction (SXRD, subsurface region) and by neutron diffraction (ND, bulk region). SXRD possesses a higher spatial resolution than ND. Magnetic stray fields were mapped by utilizing high-spatial-resolution giant magneto resistance (GMR) sensors.ResultsThe subsurface residual stress overall correlates better with the magnetic stray field distribution than the bulk stress. This correlation is especially visible in the regions outside the heat affected zone, where the influence of the microstructural features is less pronounced but steep residual stress gradients are present.ConclusionsIt was demonstrated that the localized stray field sources without any obvious microstructural variations are associated with steep stress gradients. The good correlation between subsurface residual stress and magnetic signal indicates that the source of the magnetic stray fields is to be found in the range of the penetration depth of the SXRD measurements.

Investigation on effects of single- and multiple-pass strategies on residual stress in machining Ti-6Al-4V alloy

Residual stresses in the surface and subsurface of the machined part are important factors for surface integrity, which can significantly influence its performance and service life. Generally, the final machined surface could be finished by single- or multiple-pass cutting operation and the formation of residual stresses in each process have been widely studied, however few studies pay attention to the differences of residual stress generated by single- and multiple-pass strategy on the final machined surface. In single-pass cutting, the original unprocessed surface is often employed as initial state. While in multiple-pass, the previous cut often cause accumulated strain/stress and temperature on the machined surface/subsurface, which will affect the cutting forces, thermal distributions, plastic deformation in the subsequent cuts and eventually influence the final residual stresses. Thus this paper investigates the cutting forces, temperatures and residual stresses distributions in machining Ti-6Al-4V with the special emphasis on the differences between the single- and multiple-pass strategies. Coupled Eulerian-Lagrangian (CEL) method is used in FE modelling and experiments are also implemented. Good agreements were found between the experimental and numerical results, then further discussions based on the FE results can draw the conclusions that, the first cut in multiple-pass cutting strategy can lead to a decrease in cutting force and surface tensile residual stress in the second cut. The multiple-pass cutting strategy can result in a lower magnitude and depth of compressive residual stress in subsurface, and this is more obvious when a smaller ap is implemented in the sequential cut. This work can provide practical guidance for optimizing the cutting parameters to get the desired residual stress.

Minimum Quantity Cutting Fluid Application for Grinding Weld Flash: Surface Integrity Evaluation

Abstract The effect of the grinding process for weld flash removal on the surface integrity of the welded joint has not been researched. The surface integrity of the welded joint is essential for the bandsaw blade life and to prevent any premature failure at the weld joint due to fatigue loading (a band saw blade undergoes mainly cyclic bending fatigue during its service). In this study, the effects of using different cutting fluid combinations on the grinding of weld flash in medium carbon alloy steel were carried out. The use of compressed air (CA) as a sustainable solution for grinding weld flash was explored. An experimental investigation of four different cutting fluid applications (dry/no cutting fluid, compressed air, minimum quantity lubricant using vegetable oil, and minimum quantity coolant using water-soluble oil) was carried out. The surface roughness, sub-surface residual stresses, and microhardness of the ground region were measured. This is a first-of-the-kind study on the effect of the flash removal process on the surface integrity of the welded joint. The results show that the surface integrity of the welded joint is significantly influenced by the cutting fluid application used during the grinding process of the flash. Dry grinding, the current industry standard for grinding weld flash in band saw blades, produced surface tensile residual stresses (24.82 MPa), lowest sub-surface microhardness (43.28 HRc), and the highest surface roughness (3.40 µm). In comparison, the air application had the highest surface compressive residual stresses (−289.57 MPa), highest sub-surface microhardness (48.67 HRc), and relatively low surface roughness (1.61 µm). This study provides the road map for selecting the cutting fluid application for grinding weld flash produced by the resistance welding process in the band sawing industry.

A Finite Element Analysis Based Approach to Understand the Effects of Targeted Minimum Quantity Cutting Fluid Application on Surface Integrity

Minimum quantity cutting fluid application is targeted upon the rake face and the flank face of the tool to improve the tool life and surface integrity. By strategically targeting/applying the cutting fluid on the tool rake and flank surfaces, the temperatures along the rake face and flank face are altered, thereby affecting surface machining-induced residual stresses. The magnitude of temperature alteration and thereby the machining-induced residual stresses can be affected by the amount of targeted cutting fluid and the composition of the targeted cutting fluid. In this paper, the authors use a numerical finite element approach to understand the effects of the strategically targeted cutting fluid on the flank face improves the sub-surface residual stresses by reducing the tool-tip machining temperatures when machining AISI 1045 annealed steel with an uncoated cemented carbide tool. The machining-induced residual stresses generated from the finite element model have shown that the temperatures near the tool tip reduce by targeting cutting fluid on the flank face. The targeted cutting fluid reduced the machined workpiece temperatures and also assisted in cooling the cutting tool from the flank and rake side.

Residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion as a function of process atmosphere and component design

The influence of the process gas, laser scan speed, and sample thickness on the build-up of residual stresses and porosity in Ti-6Al-4V produced by laser powder bed fusion was studied. Pure argon and helium, as well as a mixture of those (30% helium), were employed to establish process atmospheres with a low residual oxygen content of 100 ppm O2. The results highlight that the subsurface residual stresses measured by X-ray diffraction were significantly lower in the thin samples (220 MPa) than in the cuboid samples (645 MPa). This difference was attributed to the shorter laser vector length, resulting in heat accumulation and thus in-situ stress relief. The addition of helium to the process gas did not introduce additional subsurface residual stresses in the simple geometries, even for the increased scanning speed. Finally, larger deflection was found in the cantilever built under helium (after removal from the baseplate), than in those produced under argon and an argon-helium mixture. This result demonstrates that complex designs involving large scanned areas could be subjected to higher residual stress when manufactured under helium due to the gas’s high thermal conductivity, heat capacity, and thermal diffusivity.

The Importance of Subsurface Residual Stress in Laser Powder Bed Fusion IN718

The residual stress (RS) in laser powder bed fusion (LPBF) IN718 alloy samples produced using a 67°‐rotation scan strategy is investigated via laboratory X‐ray diffraction (XRD) and neutron diffraction (ND). The location dependence of the strain‐free (d0) lattice spacing in ND is evaluated using a grid array of coupons extracted from the far‐edge of the investigated specimen. No compositional spatial variation is observed in the grid array. The calculated RS fields show considerable non‐uniformity, significant stress gradients in the region from 0.6 to 2 mm below the surface, as well as subsurface maxima that cannot be accounted for via XRD. It is concluded that failure to determine such maxima would hamper a quantitative determination of RS fields by means of the stress balance method.

Influence of High-Productivity Process Parameters on the Surface Quality and Residual Stress State of AISI 316L Components Produced by Directed Energy Deposition

The production of large components is one of the most powerful applications of laser powder-directed energy deposition (LP-DED) processes. High productivity could be achieved, when focusing on industrial applications, by selecting the proper process parameters. However, it is of crucial importance to understand the strategies that are necessary to increase productivity while maintaining the overall part quality and minimizing the need for post-processing. In this paper, an analysis of the dimensional deviations, surface roughness and subsurface residual stresses of samples produced by LP-DED is described as a function of the applied energy input. The aim of this work is to analyze the effects of high-productivity process parameters on the surface quality and the mechanical characteristics of the samples. The obtained results show that the analyzed process parameters affect the dimensional deviations and the residual stresses, but have a very little influence on surface roughness, which is instead dominated by the presence of unmelted particles.