Surface Tensile Residual Stresses Research Articles

The role of retained austenite on the formation of the nanostructured hard-turned induced white layer in AISI 52100 bearing steel

Interest in hard-turning is steadily increasing due to its obvious benefits in terms of desirable surface integrity and improved operational efficiency. Surface microstructural variations can occur during machining due to cutting speed, tool geometry, and process conditions. These variations create nanostructured white layers (WL), categorized as mechanically induced white layers (M-WL) or thermally induced white layers (T-WL). This study explored the role of retained austenite (RA) content (<2%, 12%, and 25%) on WL generation in AISI 52100 bearing steel, offering insights for optimizing hard-turning. The findings showed that, regardless of RA content, samples exhibited M-WL with no dark layer beneath the white layer when utilizing a cutting speed (VC) of 60m/min using a fresh insert. Increasing tool flank wear to 0.2mm led to the formation of T-WL and surface tensile residual stresses in specimens with higher RA content (12% and 25%). This effect was also observed at 260m/min with a fresh cutting insert. Machining at 260m/min with a worn tool (VB) of 0.2mm resulted in T-WL and surface tensile residual stresses, independent of RA content. Additionally, a 0.2mm tool wear caused a significant shift in the maximum subsurface compressive residual stresses to greater depths, irrespective of RA content.

Influence of a cutting fluid on the surface integrity and fatigue strength of 42CrMo4 drilled parts.

This study investigates the influence of a cutting fluid during the drilling operation on the surface integrity and fatigue strength of 42CrMo4 hardened steel. The investigation includes characterizing surface topography, microstructure at different hole locations, and residual stresses. Fatigue tests were conducted, and the results were correlated with observations on surface integrity. The findings reveal that crack initiation occurs in a transition zone within the hole, where the cutting process evolves from pure cutting to a region with a strongly adhesive layer. The absence of cutting fluid leads to increased surface roughness and tensile residual stresses, resulting in a 28% reduction in fatigue strength.

3D FEM heat transfer simulation of surface grinding of cryogenic pre-cooled parts

Cooling lubricants are used in grinding to minimize heat-related problems such as surface cracks, burn and tensile residual stresses. An alternative is the cryogenic cooling, where the cryogenics like LN2 are supplied at very low temperatures. The feasibility of different cryogenic cooling strategies depends on the workpiece volume-to-surface ratio. In this study 3D FEM heat transfer simulations are presented, which map the temporal and spatial temperature development, to assess the potential of LN2 pre-cooling when dry surface grinding. Different clamping strategies in terms of insulation are investigated simulatively and experimentally.It could be shown that clamping covers are required to maintain the cryogenic temperature in the workpiece during surface grinding due to their sufficient insulation. The temperatures during grinding of pre-cooled workpieces were successfully predicted by the developed FEM heat transfer simulation model.

Correlation between residual stress and geometric tolerances in ultrasonic assisted turning

High accuracy in production parts and manufacturing quality are the requirements of production. Residual stress affects the surface quality and dimensional accuracy of manufactured parts. Production of parts with geometric tolerance, optimal residual stress, and suitable surface quality is always required by manufacturing industries. When machining parts, tensile residual stresses are created in them, which is one of the main reasons for the loss of geometric tolerances and distortion in the workpiece. This study investigated the effect of the ultrasonic-assisted vibration compared with conventional machining in various conditions of a 2024 aluminum workpiece on the surface quality, runout, straightness, and tensile residual stresses. In this article, the effects of geometric tolerance parameters, tensile residual stress, and surface roughness have been investigated simultaneously. A total of 18 experiments in different cutting speeds and feed rates in two machining states of conventional and ultrasonic vibration states were performed. The results showed that ultrasonic vibrations could be used to achieve 31% lower run out and 36% lower straightness in the parts. Also, the values of tensile residual stress in the ultrasonic vibration machining were 11% lower compared to conventional machining. Ultrasonic vibrations can also be used in most cases to achieve better surface quality, tolerance, and tensile residual stress according to machining parameters compared to conventional machining. The experimental results show that ultrasonic-assisted turning is a reliable method for machining, which can obtain lower tensile residual stress and improve the machined surface integrity and better tolerances.

Influence of laser shock peening on the residual stresses in additively manufactured 316L by Laser Powder Bed Fusion: A combined experimental–numerical study

Detrimental subsurface tensile residual stresses occur in laser powder bed fusion (LPBF) due to significant temperature gradients during the process. Besides heat treatments, laser shock peening (LSP) is a promising technology for tailoring residual stress profiles of additively manufactured components. A multi step process simulation is applied aiming at predicting the residual stress state after applying LSP to a cuboid shaped specimen manufactured by LPBF in two different building directions as well as comparing it with a post-build heat treatment. The validity of the numerical simulation is evaluated based on comparisons of residual stresses determined by incremental hole drilling technique within different stages of the multi step process: in the as-build condition, after subsequent heat treatment as well as after applying LSP to the as-build and heat treated specimens, showing overall a good experimental-numerical agreement throughout each of the process stages. Applying a heat treatment to the as-build LPBF sample at 700 °C for 6 h showed not to be effective in eliminating the surface tensile stress entirely, reducing the tensile residual stresses by 40%. However, the application of LSP on LPBF components showed promising results: LSP was able even to convert the detrimental near surface tensile residual stresses in the LPBF component into compressive residual stresses next to the surface, which is known to be beneficial for the fatigue performance.

Effects of ultrasonic shot peening on fatigue behavior of TA15 titanium alloy fabricated by laser melting deposition

Due to the influence of tensile residual stress and pores, the fatigue resistance of titanium alloy fabricated by laser melting deposition (LMD) still needs to be improved. In this study, ultrasonic shot peening (USP) was utilized to strengthen the TA15 titanium alloy manufactured by LMD. The surface topography, microstructure, microhardness, residual stress and fatigue behavior were analyzed. After USP treatment, the root-mean-square deviation of the three-dimensional profile (Sq) and the local stress concentration factor (Kst) increased. High-density dislocations (dislocation tangles, dislocation walls, dislocation cells) and deformation twins were generated on the specimen subsurface. The existence of high-density dislocations improved the surface microhardness. Meanwhile, changes in microstructure affected the state and distribution of residual stress. The surface tensile residual stress was completely transformed into compressive residual stress. The fatigue behavior was significantly influenced. Under 720 MPa and 760 MPa stress levels, the safe fatigue life (reliability R = 0.95, confidence level C = 0.95) of TA15 titanium alloy manufactured by LMD increased by 200 % and 43 %, respectively. In the pore-profile valley competition, the pore showed greater fatigue sensitivity. The high-density dislocations and compressive residual stress accounted for the improvement in fatigue life. The high-density dislocations hindered the crack nucleation and short crack propagation at pores near the surface. The compressive residual stress reduced the crack propagation rate in the stable crack propagation stage. The results indicated USP as an effective method to improve the fatigue performance of the TA15 titanium alloy manufactured by LMD.

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.

Influence of Shot Peening on the Isothermal Fatigue Behavior of the Gamma Titanium Aluminide Ti-48Al-2Cr-2Nb at 750 °C

One possibility to improve the fatigue life and strength of metallic materials is shot peening. However, at elevated temperatures, the induced residual stresses may relax. To investigate the influence of shot peening on high-temperature fatigue behavior, isothermal fatigue tests were conducted on shot-peened and untreated samples of gamma TiAl 48-2-2 at 750 °C in air. The shot-peened material was characterized using EBSD, microhardness, and residual stress analyses. Shot peening leads to a significant increase in surface hardness and high compressive residual stresses near the surface. Both effects may have a positive influence on lifetime. However, it also leads to surface notches and tensile residual stresses in the bulk material with a negative impact on cyclic lifetime. During fully reversed uniaxial tension-compression fatigue tests (R = −1) at a stress amplitude of 260 MPa, the positive effects dominate, and the fatigue lifetime increases. At a lower stress amplitude of 230 MPa, the negative effect of internal tensile residual stresses dominates, and the lifetime decreases. Shot peening leads to a transition from surface to volume crack initiation if the surface is not damaged by the shots.

Optimization of turn-milling process in roughing and finishing regimes: Trade-off between productivity, cutting force and surface integrity

Turn-milling is a hybrid machining process which used benefits of interrupted cutting for proceeding of round bars. However, number of controllable parameters in the hybrid process is numerous that makes optimizing the process complicated. In the present study, an optimization work has been proposed to investigate the trade-off between production rate and cutting force in roughing regime as well surface roughness and tensile residual stress in finishing regime. Number of 43 experiments based on response surface methodology was designed and carried out to gather required data for development of quadratic empirical models. Then, the adequacy and importance of process factors were analyzed using analysis of variances. Finally, desirability function was used to optimize the process in rough and finish machining regimes. The obtained results showed that selection of eccentricity and cutter speed at their maximum working range can effectively enhance the quality characteristics in both the roughing and finishing regimes.

Comprehensive study on effect of orthogonal turn-milling parameters on surface integrity of Inconel 718 considering production rate as constrain

Round bars made of Inconel 718 are widely used in aerospace industries due to their excellent properties of working at harsh environment. However, machinability of this alloy by conventional turning process results in excessive tool wear and poor surface integrity. The former increases the production cost and latter reduces the fatigue life. In the present paper, interrupted cutting of Inconel 718 round bars was introduced as an effective way for enhancing the life time. Turn-milling as newly emerging cutting technology was utilized here to machine cylindrical samples of Inconel 718. Effect of process factors such as tool rotational speed, workpiece rotational speed, feed rate and eccentricity was analyzed on surface roughness and residual stress. The obtained results showed that the feed rate is most influential parameter that determines value of surface roughness and residual stress and. Furthermore, by considering production rate as a constraint, it was logically discussed that a setting of 1000 rpm cutter speed, 300 rpm work rotational speed, 0.12 mm/rev feed rate and 0.2 mm eccentricity can guarantee maximum production rate as well minimum surface roughness and tensile residual stress.

Effect of surface mechanical attrition treatment on high cycle and very high cycle fatigue of a 7075-T6 aluminium alloy

Effects of surface mechanical attrition treatment (SMAT) on fatigue performance of a 7075-T6 aluminium alloy were studied in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes. Fatigue test results showed that SMAT can improve fatigue strength in HCF regime, but decrease it in VHCF regime. Fracture surface observations indicated that the low SMAT intensity is quite optimal, which can protect the surface from crack initiation. However, increasing SMAT intensity could lead to over-peening by inducing deep impact dimples, micro-cracks at the surface and high tensile residual stresses in the subsurface, which deteriorated fatigue performance in VHCF regime.

Optimization of process parameters for the minimization of surface residual stress in turning pure iron material using central composite design

Surface residual stress, induced by process parameters, has a significant effect on the performance of workpiece. Focusing on the minimization of surface residual stress in turning DT4E pure iron, the influence of cutting speed (vc, 80–240 m/min), feed rate (f, 0.04–0.20 mm/rev) and depth of cut (ap, 0.05–0.25 mm) on surface tensile residual stress (στ in cutting direction, σr in feeding direction) is analyzed, and the optimal combination of process parameters is found. Central composite design (CCD) method is first used to design the turning experiments of pure iron material. By means of range analysis and the main effect plot, the relations between the process parameters and responses are discussed in detail. Subsequently, the non-linear mapping models of στ and σr with regard to turning parameters are established respectively using support vector regression (SVR) method. The experimental database is utilized to validate the effectiveness of SVR models. Taking the total stress σ as the optimization objective of process parameters, the optimal combination of turning parameters within given parameter ranges is finally obtained by combining SVR models with improved particle swarm optimization (PSO) algorithm. Further, the comparisons between genetic algorithm (GA), traditional and improved PSO algorithms are conducted, which indicate that the improved PSO algorithm has an advantage in convergence efficiency. Finally, verification experiments are performed to confirm the minimization of surface residual stress. The results show that the proposed optimization strategy of process parameters is effective and reliable, while at the same time providing a good prediction accuracy.

On selective laser melting of Inconel 718: Densification, surface roughness, and residual stresses

The current study investigates the effects of a wide range of process parameters on three part properties; density, surface roughness, and surface residual stresses simultaneously for selective laser melting of Inconel 718. In addition to the lack of investigations on surface roughness and residual stresses in selective laser melting of Inconel 718, process maps were developed for the selection of the best process parameters to achieve the desired values for the three parameters combined. Five laser powers, six scan speeds and three hatch spacings were chosen from the stable single tracks tests. Based on each property, a 99.5% density or a 2 μm surface roughness or the least surface tensile residual stress of 248 MPa were possible. However, no single process parameter combination was able to achieve good values for all three parameters. Prioritizing density and surface roughness, being crack initiators, over residual stresses for their effect on fatigue failure, it was found that 99.2% density and relatively low roughness of 3.5 μm are feasible at 320 W, 600 mm/s and 0.12 mm hatch spacing. Finally, opposite to the commonly observed columnar grain in Inconel 718, mixed grain structure was obtained at 600 mm/s and 1000 mm/s, indicating reduced anisotropy.

Fatigue Strength of 316 L Stainless Steel Manufactured by Selective Laser Melting

In this study, the fatigue strength of 316 L stainless steel manufactured by selective laser melting (SLM) is evaluated. The effect of powder layer thickness and postmachining is investigated. Specimens were produced with 30 and 50 µm layer thickness and tested under high cycle fatigue in as-printed and postmachined conditions. Examination of the specimens reveals that in the as-printed condition, fatigue strength suffers from high roughness and surface tensile residual stresses as well as defects such as pores and lack of fusion voids. After machining, the fatigue strength was improved due to lower surface roughness, presence of compressive residual stresses, and removal of surface porosity. The results show that increasing the layer thickness (within the range tested) has a minor negative impact on fatigue strength; however, it has a major positive impact on the productivity of the SLM process. In addition, it is clear that the impact of postmachining on fatigue is far greater than that of the layer thickness.

The Effect of Milling Cooling Conditions on the Surface Integrity and Fatigue Behavior of the GH4169 Superalloy

The GH4169 superalloy has high strength at high temperatures. Cooling conditions have a major impact on the machined surface integrity, which further affects the fatigue properties of specimens of the GH4169 superalloy. The influence of cooling conditions on the surface integrity of the GH4169 superalloy is first studied during the side milling. Then, the effect of surface integrity under different cooling conditions on the fatigue behavior of specimens of the GH4169 superalloy is investigated by a standard tensile and tensile–mode fatigue testing. The results obtained show that surface roughness and the depth of the plastic deformation layer in wet milling and dry milling makes little difference, the surface microhardness rate in dry milling is slightly lower than that in wet milling, the surface tensile residual stress in dry milling is significantly higher than that in wet milling, and the fatigue behavior in dry milling is only about 50% of that in wet milling. In addition, the crack initiation of specimens of the GH4169 superalloy utilizing wet milling is on the subsurface, while that from dry milling is on the surface. Thus, cooling conditions have an important impact on the fatigue behavior of specimens of the GH4169 superalloy, and micro defects in dry milling are the main factors of decreasing of fatigue behavior of specimens of the GH4169 superalloy.

An experimental investigation of surface integrity in selective laser melting of Inconel 625

Inconel 625 is a Ni-based superalloy widely used in nuclear and aerospace applications because of its high strength and corrosion resistance. The current paper investigates selective laser melting of Inconel 625 using a wide range of process parameters: four laser powers, five scan speeds, and three hatch spacings. Cube coupons are produced and their end properties are measured, specifically, relative density, surface roughness, microhardness, and surface residual stresses. Process maps are constructed to correlate processing parameters to the part surface integrity, and the possible underlying causes are highlighted. Experimental results show that highly dense parts can be produced using selective laser melting and low average surface roughness can be obtained in both the scan and hatch directions. Hatch spacing is the most prominent factor to attain relative densities above 99% while high laser powers are key to lower the average surface roughness. Microhardness and microstructure examination are done for a subset of the process parameters and found to not significantly change with laser power. Surface residual stresses are measured on the top surface in the scan and hatch directions and process maps are developed. There is no particular parameter that solely affects surface residual stresses, despite several cases where the increase in laser power reduced the surface residual stresses. 90% of measurements showed surface tensile residual stresses except for a few cases that resulted in surface compressive residual stress.

Laser Additive Manufacturing using directed energy deposition of Inconel-718 wall structures with tailored characteristics

A process window is developed for fabricating thin walled Inconel 718 (IN718) structures for Laser Additive Manufacturing using Directed Energy Deposition (LAM-DED). In-house developed LAM-DED setup deployed to investigate the effect of process parameters on geometry and quality. The influence of post-heat treatment on microstructure, surface and mechanical properties is also studied. Full factorial experiments are carried out to derive the process window yielding requisite geometry, deposition rate and quality. The optimized process parameters are deployed for thin wall fabrication. The maximum and minimum limits for laser energy per unit length (E) and powder feed rate per unit length (F) are experimentally found to be 210 kJ/m, 105 kJ/m, 12.5 g/m and 4 g/m, respectively for fabricating thin walls of requisite geometry. The fabricated thin walls are observed to be defect free with dendritic microstructure. The microstructural examination of heat-treated samples reveals recrystallized equiaxed grains with reduction in surface tensile residual stress up to 50%, and modified surface topography. The reduction of average hardness by 12%, significant improvement in ductility by 62.5% and improvement in energy storage capacity by 2.4 times are observed during the study.

Effects of mechanical polishing treatments on high cycle fatigue behavior of Ti-6Al-2Sn-4Zr-2Mo alloy

In this study, high cycle fatigue (HCF) behavior of Ti-6Al-2Sn-4Zr-2Mo (wt.%) (Ti-6242) alloy sheets subjected to mechanical polishing treatments (MPTs) were investigated at room temperature. The effects of surface roughness and residual stress induced by the MPTs (cold rolling polishing (CRP), sandpaper polishing (SP) and nylon cloth polishing (NCP)) were studied. Results indicate that the fatigue limits of Ti-6242 alloy sheets polished by CRP, SP and NCP were 632.6, 593.0 and 639.0 MPa, respectively. The average surface roughness (Ra) values of the Ti-6242 alloy sheets for CRP, SP and NCP were 0.36, 0.31 and 0.18 μm, respectively. Top surface tensile residual stress (TRS), surface and deep subsurface TRS and top surface compressive residual stress (CRS) were induced by CRP, SP and NCP, respectively. HCF behavior of the Ti-6242 alloy sheets was dominantly influenced by deep subsurface residual stress induced by the MPTs, whereas the effects of surface roughness and top surface residual stress were negligible.

Effects of Cutting Edge Microgeometry on Residual Stress in Orthogonal Cutting of Inconel 718 by FEM.

Service performance of components such as fatigue life are dramatically influenced by the machined surface and subsurface residual stresses. This paper aims at achieving a better understanding of the influence of cutting edge microgeometry on machined surface residual stresses during orthogonal dry cutting of Inconel 718. Numerical and experimental investigations have been conducted in this research. The cutting edge microgeometry factors of average cutting edge radius , form-factor K, and chamfer were investigated. An increasing trend for the magnitudes of both tensile and compressive residual stresses was observed by using larger or introducing a chamfer on the cutting edges. The ploughing depth has been predicted based on the stagnation zone. The increase of ploughing depth means that more material was ironed on the workpiece subsurface, which resulted in an increase in the compressive residual stress. The thermal loads were leading factors that affected the surface tensile residual stress. For the unsymmetrical honed cutting edge with K = 2, the friction between tool and workpiece and tensile residual stress tended to be high, while for the unsymmetrical honed cutting edge with K = 0.5, the high ploughing depth led to a higher compressive residual stress. This paper provides guidance for regulating machine-induced residual stress by edge preparation.