Surface Compressive Residual Stress Research Articles

Reprint of: Residual stress inclusion in the incrementally formed geometry using Fractal Geometry Based Incremental Toolpath (FGBIT)

Single Point Incremental Sheet Forming (SPISF) is a well-known flexible alternative to conventional generative manufacturing processes. In SPISF, the geometry to be formed is fragmented into series of 2D slices and the plastic deformation is achieved through layer by layer movement of a Numerically Controlled (NC), hemispherical or ball end forming tool. The whole plastic deformation is the sum of all localized strains developed during each increment. Spiral, constant z incremental toolpaths, and their variants are common conventional toolpaths for SPISF. Several researchers have investigated these toolpaths extensively. Fractal Geometry Based Incremental Toolpath (FGBIT) is a recently developed toolpath for SPISF that improves the process formability and stress distribution. Unlike conventional toolpaths, FGBIT deforms the base region of the formed geometry which induces work hardening and residual stresses into the work piece. This may lead to the forming of high strength components. The residual stress distribution over the base region of the formed component (square cup) has been investigated in this study. Further, a comparison based on residual stress distribution between FGBIT and conventional incremental toolpaths is presented. Residual stresses have been measured by using nanoindentation technique. Pile up generation near the periphery of the indent is investigated for conventional and FGBIT based toolpaths. It has been observed from the experimental results that, the strength of the formed component increases due to induced compressive surface residual stresses while using FGBIT hence, metal components with high fatigue life and better strength-to-weight ratio can be formed.

Stability of low-temperature-gaseous-carburization layer in AISI316L stainless steel at high temperature

In this paper, the stability of low-temperature-gaseous-carburization (LTGC) layer in AISI316L stainless steel was investigated by exposing at temperature between 100 ℃ to 400 ℃ from 150 h to 3000 h in air. Test results show that, after carburization, a carburized layer with high carbon content, hardness and huge compressive residual stress form on the surface of 316L. The carburized layer is phase stable at the temperature not exceeding 400 ℃ that there are no undesired Cr carbides (such as M23C6) precipitate after heat exposure for 3000 h. The carburized layer thickness and hardness is basically stable at the temperature not exceeding 300 ℃. While for the stability of beneficial huge surface compressive residual stress, the highest service temperature should not exceed 200 ℃. Test results also showed that thickness reduction of carburized layer due to oxidation is also an important factor that affecting the stability of carburized layer. Because of high carbon atmosphere in the carburizing progress, a soot layer also forms on the surface of carburized layer and carbon atoms in the soot layer will diffuse in the carburized layer in the early stage of heat exposure progress.

The critical assessment for the fatigue limit of nanocrystallized surface with micro-notches obtained by ultrasonic surface rolling processing

The fatigue limits of micro-notched specimens with nanocrystallized surface fabricated by ultrasonic surface rolling processing (USRP) are determined at room temperature. Two kinds of metal materials of specimens, IF steel with single ferritic phase (body-centered cubic, BCC) and 316L with single austenitic phase (face-centered cubic, FCC), are selected. The experimentally determined fatigue limits of micro-notched USRP specimens, with and without stress relieving treatment, are both much higher than that of coarse grain specimens due to surface hardening and compressive residual stress. Furthermore, it is verified that the predicted value from Murakami’s model agrees well with the experimental value of not only the coarse grain specimen but also the specimen with nanocrystallized surface if the surface Vickers hardness is selected.

Effects of U‐Bending on Pitting Corrosion of 304L Stainless Steel

The influence of cold U‐bending on pitting corrosion of 304L stainless steel in an acidic NaCl solution is investigated using surface analyses and electrochemical measurements. The pitting behavior of the exterior and interior surfaces of U‐bended specimen exhibiting tensile and compressive residual stress respectively are both considered. The results show that microstructural modification by U‐bending promotes the pit nucleation. Considering the passive film properties after U‐bending, no significant change in both the thickness and Cr/Fe ratios of the passive films formed on the two deformed surfaces can be found. The accompanying residual stresses after U‐bending, especially the tensile residual stress, exhibits a remarkable acceleration of the metastable pit growth rate. Therefore, the pitting potential declines significantly for the exterior surface of U‐bended specimen. However, the compressive residual stress in the interior surface facilitates the repassivation of the metastable pit, can nullify the negative effect of microdefects for the pitting corrosion resistance, so that the pitting potential of the interior surface of U‐bended specimen does not decrease.

Thermo-mechanical coupling effect on surface residual stress during ultrasonic vibration-assisted forming grinding gear

Abstract Gears are important motion and power transmission components and enhancing their machining accuracy and performance. In this work, a novel processing of ultrasonic vibration-assisted forming grinding gear (UVAFGG) is proposed, which involves the superimposition of ultrasonic vibration on the gear, and the thermo-mechanical coupling effect on surface residual stress is explored as well. Firstly, based on the local contact geometry relationship for forming grinding, the three-dimensional (3D) distribution model of the heat flux density along the tooth profile is established. In addition, the mathematical relationship models between the grinding force, grinding heat, residual stress, and grinding parameters are also given. Then, the effect of grinding parameters such as spindle speed and ultrasonic amplitude for the characteristics of residual compressive stress accumulation is obtained by comparing the single factor experimental and finite element simulation results. It is found that under the action of ultrasonic vibration, the grinding heat and grinding force reduce by 38.7% and 40.5 % and 40.5 %, respectively. The surface residual compressive stress gradually decreases with the increase in spindle speed, while with the increase in ultrasonic amplitude, it increases first and then decrease. Comparing with the conventional grinding gear (CGG), the average rate of increase of the surface residual compressive stress under the both parameters is 17.8 %–31.3 % and 20 % respectively. Meanwhile, the residual compressive stress along involute shows a negative correlation with the increase in the rolling angle. At the same time, the experimental and simulation results agree well with the theoretical model. In conclusion, the results of this study provide a novel process technology for gear machining, and a reference for the investigation on residual stress during UVAFGG.

Machining of bearing trunnions of rolls by heavy lathes in conditions of repair production

Technology has been developed, tested on plants and theoretically justified for repair of precision bearing trunnions of oversized ‘raw’ rolls in which emphasis is done on use of unusual technique in machining of such details on heavy lathes essence of which is to regulate upper slides of lathe support taking into account distances between repairing trunnions. It is not always possible to obtain required surface parameters with roughness Ra = 0.8...0.4 μm as well as surface compressive residual stress since machining on lathe is carried out using cutting tool and especially in conditions of repair production. It is proposed to use additional device for grinding in order to solve this problem.

Mechanical properties and cracking behavior of low-temperature gaseous carburized austenitic stainless steel

In the present paper, in order to study the effect of low-temperature gaseous carburization (LTGC) on the mechanical properties of AISI 316 L stainless steel, uniaxial tension tests were performed on carburized specimens with different thickness. Cracking behavior of carburized layer at different depths was investigated by using scanning electron microscope (SEM). After carburizing, a ~30 μm thick carburized layer with ~ −2.2 GPa residual stress is formed on the surface region. Correspondingly, tensile stress in the non-carburized core compensating the surface residual compressive stress occurs. The yield strength (YS) of carburized specimen determined from tensile test reduces due to the existence of the pre-tensile stress in the core. Because of high hardness of carburized layer, ultimate tensile strength (UTS) of carburized specimen increases. After carburization, the ductility of the outmost carburized layer is significantly reduced and the cracking behavior of the carburized layer is dependent on layer depth. During the uniaxial tension process, the brittle surface layer (<10 μm) firstly cracks perpendicular to the loading direction at a low strain (≤ 4.3%), then cracks along different directions. Besides, intergranular cracking also occurs because of the carburization-induced lattice rotation. The cracking strain at the carburized layer depth of ~10 μm and ~20 μm increases to ~25% and ~56%, respectively.

Influence of Nitrocarburizing and Post-Oxidation on Surface Characteristics, Fatigue, and Corrosion Fatigue Behaviour of AISI 4330V Steel

In the present work, gaseous nitrocarburizing followed by water spray oxidation in vacuum was carried out on 4330V steel. The X-ray diffraction analysis showed that the thermochemically modified surface layer mainly consisted of e-Fe2-3(N,C), γ′-Fe4(N,C), and magnetite (Fe3O4). To quantify the residual stress generated due to surface treatment, the Sin2ψ method was used. Residual stress of compressive nature up to a depth of 1 mm was observed, and its maximum value was 383 MPa. Nitrocarburizing followed by oxidation not only increased the surface hardness but also decreased the wear loss. The synergistic effect of corrosion-resistant phases formed after surface treatment improved surface hardness and compressive residual stress which resulted in the enhancement of corrosion fatigue life.

Surface Integrity of TA19 Notched Simulated Blades with Laser Shock Peening and Its Effect on Fatigue Strength

To reveal the fatigue strengthening mechanism of TA19 notched simulated blades with laser shock peening (LSP), surface integrity and fatigue strength were investigated. The surface morphology, residual stress, near-surface microstructure, fatigue strength and fatigue fracture morphology were analyzed by surface profiler, x-ray diffraction (XRD), transmission electron microscopy (TEM), QRG-100 servo-hydraulic fatigue test machine and scanning electron microscope (SEM). Results indicated that LSP induced surface micro-dents plastic deformations with a few microns in depth, surface compressive residual stresses and surface nano-grains of TA19 notched simulated blades. Compared with the as-received material, fatigue strengths of TA19 notched simulated blades were improved by 162% for LSP-1 and 218% for LSP-3. In addition, fatigue crack initiation (FCI) locations reduced after LSP, fatigue crack growth (FCG) rate decreased after LSP, and secondary cracks were observed in the FCI and FCG regions of fatigue fracture morphologies after LSP. Fatigue strengthening mechanism of TA19 notched simulated blades with LSP was compressive residual stresses and refined grains. Results could be beneficial to the application of LSP process in civil engines blades.

Analysis on the Fatigue Properties of Shot-Peened Al-Si-Mg Alloy and Its Fatigue Life Prediction

Ceramic micro-shot peening (CMSP) and steel micro-shot peening (SMSP) were utilized to investigate the effect of micro-shot peening (MSP) on the high-cycle fatigue properties of Al-7Si-0.3Mg casting aluminum alloy in a previous study. However, the improvement effects of CMSP and SMSP on the fatigue strength (at 5 × 107 cycles) were only 33% because the depth of harden layers was only 20 and 55 μm while the depth of compressive residual stress affected layers was only 37 and 68 μm. In this study, conventional shot peening (CSP) was utilized, and the results were compared with those of MSP, with the expectation that CSP would provide a greater improvement in the fatigue strength. The affected surface layers of the shot-peened specimens were characterized using surface morphology, microhardness, and residual stress analyses. In addition, the effect of CSP on the fatigue strength at 5 × 107 cycles was investigated using a rotating bending fatigue test (R = − 1). An investigation of the extensive surface compressive residual stress relaxation process for the three different shot-peened specimens during cyclic loading was conducted using x-ray diffraction. In addition, the initiation sites for fatigue cracks on the fracture surface were observed using scanning electron microscopy. Furthermore, the fatigue life of the samples with the internal casting defect failure mode was predicted using linear elastic fracture mechanics, while that for samples with the surface crack initiation failure mode was predicted using the modified Morrow model considering the residual stress.

Study on surface heat-affected zone and surface quality of Ti-6Al-4V alloy by laser-assisted micro-cutting

This paper takes Ti-6Al-4V alloy as an example to carry out laser-assisted machining experiment and simulation. It mainly studies the influence of laser power and cutting speed on the heat-affected zone of the material surface. The effects of laser power, cutting speed and cutting depth on tool wear, surface quality, and residual stress on this basis were also investigated. The experimental results show that laser power (P) and cutting speed (v) are important factors affecting the width (Wd) of the heat-affected zone of the workpiece surface. The relationship can be described as follows Wd = 116.278 + 2.587 × P − 5.191 × v. The results of simulation and experiment show that Ti-6Al-4V alloy discolors when exposed to temperatures from 105 to 130 °C under laser irradiation. During the laser-assisted machining experiment, tool flank wear occurs and the main wear patterns are adhesive wear, flaking, and micro-chipping. When the laser power is 40 W, the cutting speed is 18 m/min, and the cutting depth is 10 μm, an improved roughness value of 0.271 μm can be obtained. The residual compressive stress and microhardness of the machined surface obtained by laser-assisted machining are higher as compared to the conventional machining method. Moreover, as the laser power increases, the surface residual compressive stress exhibits an increasing trend while the microhardness first increases and then decreases.

Effect of grinding parameters on surface quality, microstructure and rolling contact fatigue behaviors of gear steel for vacuum pump

In this study, the different grinding parameters were introduced to gear steel for vacuum pump. The effect of grinding parameters including grinding velocity (VS), grinding depth (Ap) and feed rate (Vf) on the grinding force, surface quality, residual stress distribution and rolling contact fatigue (RCF) behaviors were evaluated by piezoelectric force sensor, scanning electron microscopy (SEM), X-ray diffraction and ball-on-disc fatigue tester. Results showed that the suitable grinding parameters could effectively improve the surface quality, dislocation structures and residual compressive stress. The surface quality and residual compressive stress interacted to affect RCF behaviors. The RCF life of grinding processed samples (VS = 30 m/s, Ap = 0.02 mm, Vf = 500 mm/min) exhibited the maximum mean life of 6.97 × 105 cycles. And the main damage modes of grinding processed samples were spalling and peeling.

A comprehensive study for mechanical behavior of functionally graded porous nanobeams resting on elastic foundation

In this paper, a comprehensive study is presented for mechanical analysis of functionally graded porous (FGP) nanobeams resting on an elastic foundation. The nanobeam is modeled according to different beam theories along with nonlocal strain gradient theory and Gurtin–Murdoch surface elasticity theory. Using Hamilton’s principle, the set of governing equations are derived, and exact solutions are presented using Navier’s method for the static bending, mechanical buckling and free vibration analyses of simply supported FGP nanobeams. The effects of various parameters on static deflection, critical buckling load and fundamental natural frequency of FGP nanobeams are studied. It is shown that for all types of porosity distribution patterns, an increase in porosity parameter leads to an increase in static deflection, and a decrease in critical buckling load and effect of porosity parameter on fundamental natural frequency is dependent on the porosity distribution pattern. Numerical results confirm that tensional residual surface stress decreases static deflection and increases both critical buckling load and fundamental natural frequency, but compressive residual surface stress has opposite effects.

Enhancement of Fatigue Strength on SAE 1541 Steel Link Plate with Slip Ball Burnishing Technique

This research paper describes a technique for the enhancement of the fatigue strength of the chain link plate in the drive system of a military armoured vehicle. SAE 1541 steel link plates of chains were subjected to cyclical tensile stress due to repeated loading and un-loading conditions. The crack was getting originated from the pitch hole and growth perpendicular to the chain pulling load, due to fatigue mechanism. In general plate holes are manufactured using the conventional process. An additional novel technique called the slip ball burnishing (SBB) method is applied for improving the hole properties. The improvement is made by producing local plastic deformation, improving surface finish and compressive residual stress throughout in the pierced hole. Both the conventional process (CP) and the SBB technique have been evaluated by optical, profile, surface roughness and micro harness tests. Experimental fatigue test validations were done in both chain samples using the Johnson-Goodman method. SBB chains passed 3x106 cycles at the load of 17.61 kN and CP chains passed 3x106 cycles at the load of 13.92 kN. The conclusion was that SBB made a significant improvement of 26.51 per cent of fatigue strength compared to CP.

Improving the Mechanical Properties of Aluminum Alloys 6061-T6 Friction Stir Welding Joints Using Ultrasonic Peening

The aim of this paper is to study the possibility of joining plates from aluminum alloy 6061-T6 by friction stir welding method. Plates were prepared to dimensions (200 × 60 × 6) mm by using (CNC) milling machines. The FSW was then applied to join the operated plates by a vertical milling machine using different rotation welding speed of 1000,1200,1400, rpm and linear welding speed of 30 mm/min for the production of many weld joints. Microstructure and micro hardness, tensile, bending, temperature distribution testing were implemented. The results observed that the effect of the increasing of rotating speed for friction stir welding leads to increase the mechanical properties due to the increase of heat input. The best result was obtained at rotational speeds (1400) rpm, the specimens that are welded at 1000 rpm and had decreases in mechanical properties are subjected to ultrasonic peened surface treatment at 1,2,3 passes to improve the mechanical properties of welded joint, Compared to the non-peening weld joint, the mechanical properties increased when the number of ultrasonic peened passes increased due to the surface hardening and compressive residual stresses produced by the ultrasonic peening process.

Influence of high-energy shot peening on the microstructure and mechanical properties of Ti–6Al–4V welded joints

In this study, Ti–6Al–4V welded joints were subjected to a high-energy shot peening (HESP) treatment. The structural characteristics of the Ti–6Al–4V welded joints were investigated by optical microscopy (OM) and x-ray diffraction (XRD). The Rockwell hardness and residual stress in different regions of the Ti–6Al–4V welded joints were assessed before and after treatment. Tension tests were then performed to investigate the effect of HESP on the welded samples. Experimental results showed that an Surface nanocrystallization (SNC) layer can be prepared on a Ti–6Al–4V welded joint via HESP. The average grain size of the SNC layer was approximately 30 nm after HESP. HESP also resulted in a higher surface hardness compared with that of the original sample. After treatment, surface compressive residual stresses formed in welded joints, and the residual peak stress of the weld zone was decreased from 650 MPa to −638 MPa. The surface hardness and surface residual stress were distributed more uniformly on the surface of the welded joint. After treatment, the tensile properties were improved.

Enhancement of Fatigue Endurance Limit through Ultrasonic Surface Rolling Processing in EA4T Axle Steel

Fatigue property is a key evaluation index for the service reliability of railway axle. In this work, the effect of ultrasonic surface rolling processing (USRP) on the surface characteristic and fatigue property was investigated in an EA4T axle steel used on high speed trains by several characterization techniques and the staircase method fatigue testing. The surface characteristics were initially studied in EA4T axle steel under different static loads of 1.0 kN, 1.5 kN and 2.0 kN, and served as the important USRP parameter. It was found that the larger static load greatly improved the surface microstructure, microhardness and compressive residual stress, but also increased the surface roughness. Furthermore, the rotating bending fatigue endurance limit of the USRP specimen with a static load of 1.5 kN was obviously enhanced by ~14% (from ~352 MPa to ~401 MPa) relative to the untreated specimen. The enhanced fatigue limit induced by USRP was attributed to the synergistic effect of the grain refinement, as evidenced by transmission electron microscope (TEM) observation, work-hardening, the increased compressive residual stress and the reduced surface roughness. Moreover, the fatigue limit of the USRP specimen was ~4% higher than that of the rolling specimen with turning off the ultrasonic system, ~386 MPa, which showed that the role of the ultrasonic impact could enhance the fatigue property. These findings demonstrate the validity of this technique in modifying the surface characteristics and thus improving the fatigue resistance of axle material, further ensuring its service safety and reliability.

Effects of different mechanical surface treatments on surface integrity of TC17 alloys

The effects of shot peening (SP), ultrasonic surface rolling (USR), and laser shock peening (LSP) processes on the surface integrity of TC17 alloys, including the surface topography, in-depth residual stress and microhardness distributions, and microstructure, are investigated and compared. The results show that the lowest surface roughness values of 0.04–0.12 μm and a smooth surface topography are achieved from the USR process, while those after the SP and LSP processes are 0.50–1.07 μm and 0.36–0.73 μm, respectively. The depth of the compressive residual stress layer induced from the USR and LSP processes is mostly in the range of 0.7–2.0 mm, which is larger than that induced from the SP process. The surface compressive residual stresses generated from the SP, USR, and LSP processes are in the range of −570 to −870 MPa, depending on the applied parameters. The induced work hardening is observed and the increased surface microhardness is approximately 6.4% after all treatments compared with the initial states. Gradient changes in the plastic deformation and grain refinement are observed in the subsurface microstructure. Moreover, the USR and LSP processes demonstrate a deeper plastic deformation layer compared with the SP process. The impact intensity of the mechanical surface treatments has a notable effect on the surface integrity characteristics compared with the impact coverage. The USR process is more effective at improving the surface integrity characteristics of TC17 alloys compared with the SP and LSP processes due to its relatively smooth surface, high-amplitude and deep layer of compressive residual stress, deep work-hardened layer, and large gradient microstructural changes.

Exploring grinding and burnishing as surface post-treatment options for electron beam additive manufactured Alloy 718

Numerous additive manufacturing (AM) techniques have been developed over the past decade. Features like immense freedom of intricate part design and shorter lead time make AM routes promising for a wide range of applications spanning aerospace, marine and automobile sectors. Among the various metal AM processes, Electron Beam Additive Manufacturing (EBAM) is being widely explored to realise the potential of Ni-based superalloys and Ti alloys for varied high-performance applications. A novel attempt has been made in this paper to assess the surface integrity of as-built EBAM nickel-based superalloy 718 (AB) subjected to grinding (G), Low Plasticity Burnishing (LPB) and their sequential combination. Apart from their influence on sub-surface microstructures, the effect of process variables during the above post-treatments on the residual stress profiles was also investigated. Results revealed that G + LPB results in about 0.6 μm lower surface roughness, 17% improved microhardness compared to AB + LPB, and higher compressive surface residual stress as compared to LPB processed EBAM samples. The sequential grinding and LPB - improved microhardness, was also found to extend about 500 μm more when compared to the LPB process. The G + LPB, which is greatly influenced by the prior grinding, smoothens the surface and thus results in a better surface finish. Highest hardness, superior surface finish, reduced porosity and improved compressive residual stress were observed in samples that adopted the AB + G + LPB sequence over other samples, with the LPB step at 40 MPa yielding the best results.

Effects of different shot peening parameters on residual stress, surface roughness and cell size

Shot peening is widely used to strengthen the surface of metal parts, and its effect is affected by various parameters, such as shot peening coverage, peening intensity, peening velocity, shot size and shot material. In this study, a random multi-shots model which incorporated dislocation density-based constitutive equations was established. With the developed model, the effects of peening velocity, peening coverage and double peening on the surface integrity in terms of the residual stress, surface roughness and dislocation cell size were analyzed. The simulated results show that increasing the peening velocity can significantly thicken the refining layer and the compressive residual stress layer (CRSL), and deepen the location of the maximum compressive residual stress (MCRS). Meanwhile, increasing the peening coverage can heighten the magnitude of MCRS in subsurface, but hardly affects the surface compressive residual stress (SCRS) and the thickness of CRSL. Compared to simply pursuing the improved peening velocity and enhanced coverage, the use of an appropriate double peening process can induce a larger SCRS while reduce the surface roughness slightly.