Surface Fatigue Strength Research Articles

Surface integrity and fatigue behavior of AISI 4340 steel after hybrid laser-ultrasonic assisted ball burnishing process

The final surface modification processes enhance the fatigue strength, wear resistance, and corrosion resistance of the workpiece by improving the surface quality, enhancing the microhardness, generating compressive residual stresses on the surface, and creating favorable changes in the microstructure. In this regard, using processes with different mechanisms is regarded as an optimal method to increase the efficiency of the final surface modification processes. This research seeks to experimentally investigate the hybrid laser-ultrasonic assisted ball burnishing (LUAB) process and its effect on the surface integrity and fatigue strength of 4340 alloy steel. The effect of the process on the sample’s microstructure was evaluated by the images obtained from the optical microscope and SEM. Further, the parameters of mean surface roughness and microhardness, as well as the results of the uniaxial tensile test and fatigue test of the workpiece were evaluated. In the case of using appropriate parameters in the process, the results indicated a 93.38% reduction in mean surface roughness and an increase of surface microhardness by 3.4 times compared to the initial sample. Additionally, the investigation of microstructure revealed the accessibility of homogeneous and refinement martensitic structures without surface melting. The results of uniaxial tensile and fatigue tests indicated a 6.29% increase in tensile strength and a 20.71% increase in the endurance limit of the treated sample (LUAB) compared to the initial sample. Finally, the results also showed a change in the fracture mode in the uniaxial tensile test and a reduction in the final fracture area in the fatigue test.

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.

Analysis of the Possibility of Import Substitution in the Production of Sucker Rod Pump Cylinders at JSC “Neftemash”

Chemical-thermal treatment is aimed at saturating the surface layer with elements that change the microstructure and properties of the surface layer of the part. The most common chemical-thermal treatment processes are carburization, nitriding and nitrocarburization. However, the use of these processes, despite increasing surface hardness, fatigue strength and wear resistance, also has disadvantages leading to loss of productivity. These include the duration of the chemical-thermal treatment process and, most importantly, the need for subsequent mechanical processing to eliminate possible deformations and warping when obtaining high-precision surfaces. For cylinders of sucker rod pumps operating under conditions of contact friction, dimensional accuracy after chemical-thermal treatment without subsequent mechanical treatment is of great importance. The article presents the results of experimental studies to test the effectiveness of the carbonitration process of rod pump cylinders, in order to ensure the longevity of the tubular parts of rod pumps at the Izhevsk enterprise Neftemash JSC and to search for possible ways of import substitution of German technology. As a result of experimental studies, it was found that in order to achieve the hardness required by the design documentation, it is necessary to either significantly increase the exposure time during gas carbonitration, or, to increase productivity and efficiency, conduct experimental studies using the process of liquid carbonitration in a molten salts of cyanates and carbonates; and also, the carbonitration process is certainly more effective than ion nitriding, which is currently carried out for 26 hours for cylinders of sucker rod pumps, since even with a holding time of 1.5 hours, the hardness results obtained are somewhat better than those of Chinese samples.

Inductive Heating and Quenching of Planetary Shafts for Diesel Engine Starters

High mechanical and temperature cyclic loading of the final products for automotive, transport, construction and agriculture mechanization industry, demands sufficient mechanical properties of all of their components during its exploitation. Majority of the components is made of steel, by different cold forming processes. Their main demanded characteristics are surface wear resistance and fatigue strength under pulsating stress in combination with cyclic temperature loading, which could be achieved only by appropriate heat treatment. The efficiency of the combined inductive heating and water quenching heat treatment and quality of the planetary shafts were analyzed, with the use of thermographic analysis, hardness measurements, and metallographic examination. Combination of inductive heating and water quenching is the most effective heat treatment process of carbon steel planetary shafts for the diesel engine starters. Long life span of carbon steel planetary shafts it’s essential for their economical production. The replacement of starter is expensive from both: money and working time point of view. Surface temperature measurements during the inductive heating process were performed in the industrial environment. The intensity and homogeneity of the planetary shaft surface temperature field was measured by thermographic camera. On the base of theoretical knowledge and measurements, a mathematical model for temperature conditions determination in the shaft during the entire process of heating and quenching was carried out, and used for analyses and optimization of planetary shafts induction hardening process.

Effects of discrete laser surface melting on the fatigue performance of 20CrMnTi steel gear

Bionics research has revealed the remarkable fatigue resistance exhibited by intertidal zone shellfish, attributed to their radial ribs and multi-layered microstructure. Leveraging the analogous stress and working conditions shared by shellfish and gears, we adopted the discrete laser surface melting (DLSM) technology to create discrete laser surface melted (DLSMed) units on gear tooth surfaces. This paper aimed to scrutinize the influence of discrete laser surface melting on the fatigue performance of 20CrMnTi steel gears. In terms of laser selection, we opted for the Nd: YAG pulsed laser. Across various laser parameters, the proportion of DLSMed units on gear tooth surfaces varied. Scanning Electron Microscopy (SEM) facilitated the observation of microstructures and fatigue morphology of the DLSMed gear. X-ray Diffraction (XRD) measurements gauged residual stress along both the surface and cross section of the DLSMed unit, unveiling the residual stress distribution. Microhardness and surface roughness were assessed through a microhardness tester and a surface profilometer, respectively. Fatigue tests were conducted using both the Forschungsstelle für Zahnräder und Getriebebau (FZG) testing machine and a high-frequency fatigue testing machine. Experimental findings disclosed that the DLSMed unit comprised a melting zone and a heat-affected zone. The depth of the residual compressive stress layer increased alongside current intensity, albeit with the unexpected emergence of residual tensile stress. Enhanced current intensity corresponded to a noticeable elevation in the DLSMed gear's hardness value. Differing fatigue behaviors were evident between the as-received gear and the DLSMed gear. The DLSMed gear exhibited reduced fatigue damage compared to the as-received gear. The DLSMed units effectively curbed crack initiation and propagation, markedly enhancing gear fatigue performance. A direct relationship between the DLSMed unit's area proportion on the gear surface and gear fatigue strength was established. Finally, we delved into the anti-fatigue mechanism underlying the DLSMed gear's performance.

Effect of Finishing Protocols on the Surface Roughness and Fatigue Strength of a High-Translucent Zirconia.

In case of need for esthetical improvement of zirconia restorations, an individualization using extrinsic staining can be applied. This study aimed to evaluate the surface roughness and fatigue strength (survival) of high-translucency zirconia (3Y-TZP, YZ®HT, Vita Zanhfabrik) with extrinsic characterization and/or glaze. Sixty (60) zirconia discs (12 × 1.2 mm) were obtained, sintered, and randomly distributed among three groups (n = 20) according to the surface finishing protocol: C (control), C + G (extrinsic characterization followed by a glaze layer), and G (glaze layer). The surface roughness (Ra) was analyzed with a contact profilometer. Subsequently, the specimens were subjected to a fatigue load profile starting at 120 N during 20,000 cycles at 4 Hz frequency, with a 5% increase at each step until failure. The failed specimens were evaluated under a stereomicroscope. Surface roughness analysis was evaluated by using one-way ANOVA and post hoc Tukey tests (95%); while fatigue survival probability was analyzed with Kaplan-Meier and Mantel-Cox (log- rank, 95%). One-way ANOVA revealed that surface roughness was affected by the finishing protocol, where C + G showed the highest mean value (0.46 ± 0.18 µm)A followed by G (0.30 ± 0.10 µm)B, and C (0.19 ± 0.02 µm)C. While for fatigue strength, the G protocol presented a higher mean value (243.00, and 222.36-263.63)A, followed by C + G (192.75 and 186.61-198.88)B and C (172.50 and 159.43-185.56)C. Surface finishing protocols modify the surface roughness and fatigue strength of high-translucent zirconia. Regardless of the surface roughness, both glazing protocols improved the ceramic fatigue strength, favoring the restoration's long-term survival.

Study on the Surface Enhancement of Thin-Walled Metallic Materials Using a Novel Double-Side Burnishing Tool

Burnishing is a surface finishing process to produce a smooth surface. Since it can improve the mechanical properties of the material, such as surface hardness, wear resistance, and fatigue strength, in one process, it has been widely used in industry to enhance the surface quality of mechanical parts. However, due to the high burnishing force, it is difficult to use the process of thin materials, because such materials can easily be deformed in the process. In this research, a novel double-sided burnishing (DSB) tool that can process the thin plate material from both sides is designed and fabricated. The developed DSB tool symmetrically moves the burnishing tips on both sides with a single output to facilitate position control. The burnishing forces from the two sides cancel each other, and the moment that causes bending deformation is suppressed during the processing. Different thin plate metallic materials are processed using the developed tool, it is confirmed that deformation during burnishing is suppressed. By investigating the surface properties of the processed specimen, it is found that surface roughness, surface hardness, tensile strength, and fatigue strength can be improved by using the developed DSB tool. This makes it possible to process complicated, thin-walled parts, such as engine and turbine blades, which are supposed to have great potential applications in the automobile and aerospace industries.

Influence of Contour Scan Variation on Surface, Bulk and Mechanical Properties of LPBF-Processed AlSi7Mg0.6.

Metal additive manufacturing technologies have great potential for future use in load-bearing aerospace applications, requiring a deeper understanding of mechanical performance and influencing factors. The objective of this study was to investigate the influence of contour scan variation on surface quality, tensile and fatigue strength for laser powder bed fusion samples made of AlSi7Mg0.6 material and to create high-quality as-built surfaces. The samples were produced with identical bulk and different contour scan parameters to accommodate the investigation of the impact of as-built surface texture on mechanical properties. The bulk quality was evaluated by density measurements according to Archimedes' principle and tensile testing. The surfaces were investigated using the optical fringe projection method, and surface quality was assessed by the areal surface texture parameters Sa (arithmetic mean height) and Sk (core height, derived from material ratio curve). Fatigue life was tested at different load levels, and the endurance limit was estimated based on a logarithmic-linear relation between number of cycles and stress. All samples were found to have a relative density of more than 99%. Surface conditions distinctive in Sa and Sk were successfully created. The resulting mean values of the ultimate tensile strength σult are between 375 and 405 MPa for 7 different surface conditions. It was confirmed that the influence of contour scan variation on bulk quality is insignificant for the assessed samples. Regarding fatigue, one as-built condition was found to perform as well as surface post-processed parts and better than the as-cast material (compared to literature values). The fatigue strength at the endurance limit for 106 cycles is between 45 and 84 MPa for the three considered surface conditions.

Effects of Heat Treatment and Diamond Burnishing on Fatigue Behaviour and Corrosion Resistance of AISI 304 Austenitic Stainless Steel

The surface cold working (SCW) of austenitic stainless steel (SS) causes martensitic transformation in the surface layers, and the percentage fraction of the strain-induced martensite depends on the degree of SCW. Higher content of α′−martensite increases the surface micro-hardness and fatigue strength, but deterioration of the corrosion resistance is possible. Therefore, the desired operational behaviour of austenitic SS can be ensured by the corresponding degree of SCW and heat treatment. This article evaluates the effects of SCW performed by diamond burnishing (DB) and heat treatment on the surface integrity (SI), rotating fatigue strength, and corrosion resistance of AISI 304 austenitic SS for two initial states: as-received hot-rolled bar and initially heat-treated at 1100 °C for one hour followed by quenching in water. Then, DB was implemented as a smoothing and hardening process, both alone and in combination with heat treatment at 350 °C for three hours after DB. The electrochemical performance was examined by open circuit potential measurements, followed by potentiodynamic tests. For both initial states, smoothing DB provided the lowest roughness, whereas an improvement in the maximum surface micro-hardness was obtained after hardening DB and subsequent heat treatment. The maximum fatigue strength was obtained by hardening multi-pass DB without subsequent heat treatment for the as-received initial state. Smoothing DB and subsequent heat treatment maximised the surface corrosion resistance for the two initial states, whereas a minimum corrosion rate was obtained for the initially heat-treated state. For the as-received state, smoothing DB and subsequent heat treatment simultaneously lead to a high fatigue limit (equal to that obtained by hardening single-pass DB) and a low corrosion rate.

Effect of Diamond Burnishing on Fatigue Behaviour of AISI 304 Chromium-Nickel Austenitic Stainless Steel.

The disadvantages of widely used austenitic stainless steels are their low hardness and relatively low fatigue strength. Conventional chemical-thermal surface treatments are unsuitable for these steels since they create conditions for inter-granular corrosion. An effective alternative is a low-temperature surface treatment, creating an S-phase within the surface layer, but it has a high cost/quality ratio. Austenitic steels can increase their surface micro-hardness and fatigue strength via surface cold working. When the goal is to increase the rotating bending fatigue strength of austenitic chromium-nickel steels, and the requirements for significant wear resistance are not paramount, diamond burnishing (DB) has significant potential to increase the fatigue strength and, based on the cost/quality ratio, can successfully compete with low-temperature chemical-thermal treatments. The main objective of this study is to establish the effect of DB on the rotating fatigue strength of AISI 304 L chromium-nickel austenitic steel. The influence of DB parameters on the surface integrity (SI) characteristics was studied. Optimal DB parameters under minimum roughness and maximum micro-hardness criteria were obtained. Rotating bending fatigue tests of the diamond burnished (in a different manner) and untreated specimens were performed. DB implemented via parameters providing maximum micro-hardness increased fatigue limit by 38% compared to untreated specimens.

Sintering mode of a translucent Y-TZP: Effects on its biaxial flexure fatigue strength, surface morphology and translucency.

This investigation evaluated the effect of two sintering modes of a translucent zirconia (Y-TZP) on its surface roughness, topography, phase-transformation (t → m), translucency and biaxial flexure fatigue strength. To do so, 50 Y-TZP discs (Ø=15 mm; thickness=1.2 mm; IPS e.max ZirCAD LT) were prepared and divided into two groups: Standard mode (SM) and Fast mode (FM). Staircase fatigue testing was performed (piston-on-three balls set-up, ISO 6872:2015), as well as surface roughness, profilometry, scanning electron microscopy (SEM-FEG), energy dispersive X-ray spectroscopy (EDX), phase transformation (t → m) using X-ray diffraction analysis (XRD), translucency parameter analysis (TP and TP00 ) and fractography. The results showed no statistical significant differences for roughness parameters (p > 0.05, SM: Ra=0.13 ± 0.02, Rz=1.21 ± 0.26 and RSm=24.91 ± 2.19; FM: Ra=0.14 ± 0.03, Rz=1.32 ± 0.25 and RSm=24.68 ± 2.16) or flexural fatigue strength (SM: 512 (464-560) MPa; FM: 542 (472-611) MPa) between the groups. In addition, similarity in surface morphological features (SEM and profilometry), composition and phases (EDX and XRD) was observed between the firing protocols. Fractography showed that the failure origin occurred on the tensile side. Sintering mode did not affect the TP (F=0.001, p=0.97) and TP00 (F=0.12, p=0.72). Therefore, the fast-sintering mode is suggested as a viable alternative to the standard mode since it does not influence the evaluated surface morphology, microstructure, fatigue strength and translucency of a translucent monolithic zirconia. The fast sintering mode is a viable alternative for zirconia without compromising its topography, microstructure, mechanical performance or translucency.

A Probabilistic Fatigue Strength Assessment in AlSi-Cast Material by a Layer-Based Approach

An advanced lightweight design in cast aluminium alloys features complexly shaped geometries with strongly varying local casting process conditions. This affects the local microstructure in terms of porosity grade and secondary dendrite arm spacing distribution. Moreover, complex service loads imply changing local load stress vectors within these components, evoking a wide range of highly stressed volumes within different microstructural properties per load sequence. To superimpose the effects of bulk and surface fatigue strength in relation to the operating load sequence for the aluminium alloy EN AC 46200, a layer-based fatigue assessment concept is applied in this paper considering a non-homogeneous distribution of defects within the investigated samples. The bulk fatigue property is now obtained by a probabilistic evaluation of computed tomography results per investigated layer. Moreover, the effect of clustering defects of computed tomography is studied according to recommendations from the literature, leading to a significant impact in sponge-like porosity layers. The highly stressed volume fatigue model is applied to computed tomography results. The validation procedure leads to a scattering of mean fatigue life from −2.6% to 12.9% for the investigated layers, inheriting strongly varying local casting process conditions.

Enhancing surface integrity of A7050-T7451 aluminium alloy by pneumatic machine hammer peening

Aerostructures withstand cyclic mechanical loads and therefore, their design must fulfil surface integrity and fatigue strength requirements. This paper studies the effect of pneumatic machine hammer peening on surface integrity of 7050-T7451 aluminium alloy. Specimens were hammer peened using two different stepover distances (0.07 and 0.35 mm) and initial offset (0.3 and 0.5 mm). A Kistler dynamometer was used to measure the forces generated by hammer peening. Then, the surface topography, residual stresses and microstructural damage of the specimens were characterised. The results demonstrate that the magnitude of residual stresses and the thickness of the affected layer increases when reducing the stepover distance, while surface roughness reduces. Additionally, none of the tested conditions produced microstructural defects. These findings suggest that pneumatic hammer peening is an additional process to manufacture thin-walled structural aluminium alloy components.

Ti6Al4V titanium alloy fatigue strength after AM- and machining-based process chains

The paper proposes the prediction of the fatigue strength of additive manufactured Ti6Al4V titanium alloy machined under flood and cryogenic cooling conditions, making use of the approach based on linear elastic fracture mechanics that was used for the first time in the context of such process chains. The role of the machining-generated surface texture, hardening, microstructural characteristics and residual stresses states is discussed in detail, giving new insights about the correlation between surface integrity and fatigue strength when addressing defective metal alloys, as those fabricated through AM.

Variability of Surface Roughness and Fatigue Strength Across the Build Platform in Laser Powder Bed Fusion

Variability of Surface Roughness and Fatigue Strength Across the Build Platform in Laser Powder Bed Fusion

Impact of the drilling process on the surface integrity and residual fatigue strength of 2024-T351 aluminum parts

This paper investigates the influence of the drilling process on an aluminum 2024-T351 part in terms of surface integrity and fatigue strength. Two drilling processes (axial; orbital) are considered. The surface integrity is studied through surface roughness measurements, microstructure and hardening analysis and residual stress assessment. An innovative Hole Opening Comparative Technique is set up to assess the residual stress state. Fatigue tests are conducted with open-hole specimens, and on filled-hole samples (double-lap bearing strength tests). The strain hardening of the hole subsurface seems to be the main factor influencing the fatigue behavior, associated with a residual stress state.

Effect of Carburizing and Nitriding on Fatigue Properties of 18Cr2Ni4WA Steel in Very High Cycle Fatigue Regime

The work aims to study the influence of carburizing and nitriding on fatigue properties of 18Cr2Ni4WA high strength steel in very high cycle fatigue regime. Very high cycle fatigue tests were carried out on 18Cr2Ni4WA Steel after carburizing and nitriding respectively. The micro morphology of fatigue fracture was observed by scanning electron microscope, the failure mode and failure mechanism were discussed. The relationship between fatigue life and defect size, FGA size, fish eye size of fracture was analyzed. The characteristic size of defects is evaluated by Gumbel, Weibull and GEV distribution functions, and a modified Akiniwa fatigue life prediction model considering the relationship between FGA size and inclusion size was established. The results showed that, nitriding and carburizing treatment improve the surface fatigue limit of the steel. The fatigue life decreases with the increase of internal defect size and FGA size. After carburizing and nitriding treatment, the internal fatigue strength of the specimen decreases slightly. When the failure probability is 99%, the internal defect sizes of nitrided specimens calculated by Weibull, Gumbel and GEV distributions are 141.5 μm, 148.4 μm and 211.7 μm respectively. The calculated internal defect sizes of carburized specimens are 47 μm, 67.8 μm and 40 μm respectively. Compared with the experimental data, the fatigue strength predicted by GEV is the most appropriate. carburizing and nitriding treatment can improve the surface fatigue strength of 18Cr2Ni4WA steel, but slightly reduce the internal fatigue strength. The prediction result of the new model is conservative when the failure probability is 99%, which is suitable for engineering application.

Minimizing printing time and volumetric error by GPU-accelerated adaptive slicing

Staircase defects in additive manufacturing significantly reduce the surface roughness, dimensional accuracy, and fatigue strength of printed parts. Adaptive layer thicknesses are promising to reduce the printing time and staircase defects simultaneously. However, conventional adaptive slicing algorithms aim to minimize the number of layers, instead of the actual printing time, and are computationally expensive to evaluate all possible slicing combinations and find the optimal ones. This paper presents a GPU-accelerated slicing algorithm to minimize both the printing time and the volumetric error explicitly and find all the optimal solutions among all possible combinations of layer thicknesses. The experimental results validated that the algorithm could obtain not only the optimal slicing but also the dimension accuracy along the printing direction.

Precision manufacturing of turbine wheel slots by trim-offset approach of WEDM

The present study aims to precisely manufacture the turbine wheel slot using a novel trim-offset approach of wire-electrical-discharge-machining (WEDM) in order to overcome the challenges of a conventional broaching process. Two dimensional model of turbine wheel slot is prepared in ‘ELCAM’ software to generate the computer-numerical-control (CNC) code of fir-tree-profile geometry. The experimental plan based on a trim-offset approach is executed to minimize the thermal damage and to improve the surface fatigue strength, which is most desirable for a long operating life of the jet engine. An average cutting speed of ~1.5 mm/min is observed for rough-cut and ~5 mm/min for trim-cut operation. Six categories of wires are used to manufacture the turbine wheel slots without any wire breakage and gap-short issue. The fabricated profile slots have demonstrated an average roughness (Ra) of 0.65 μm, profile accuracy within ±5 μm, minimum hardness alteration (34.87 Hv), and negligible recast layer (<5 μm) using B-150 wire and fulfilled the essential requirements of the gas turbine industries. Compared to wire-electrochemical-machining (WECM), WEDM trim-offset approach is quite advantageous and should be beneficial for the researchers working in ‘Precision machining of aerospace materials’.

Optimal weight design problem of spur gears

Gear is one of the most common and important components in machinery. Evaluation on durability of gears plays crucial role in the assessment of the whole system reliability and service life. For other parts like shafts, the gears also act as loads. Therefore, dimensions and weight of the gears should be reduced as much as possible, contributing the size and weight reduction of the whole systems, which is essential to be cost-effectiveness. The current research focuses on optimal weight design problem of spur gears, such that the weight is minimized under the constraints taken from working conditions. The weight is a function of six variables, i.e. face width, shaft diameter of pinion, shaft diameter of gear, number of teeth on pinion, module and hardness. Constraints are derived based on AGMA standard and engineering handbooks, including the bending strength, the surface fatigue strength, the interference condition, the condition for uniform load distribution, the torsional strength of shaft on pinion and gear, and the center distance. The set of optimum design variables is determined by the heuristic algorithm Grey Wolf Optimizer (GWO). The accuracy and efficiency of the GWO in the optimal weight design problem of spur gears are assessed based on comparison with other popular methods, such as Genetic Algorithm (GA), Particle Swarm Optimization (PSO) and Simulated Annealing (SA). It is noted that in previous works, some of the constraints are still violated. Therefore, a penalty term is taken into the objective function, such that any set of design variables that violates constraints will be considered as ``unfit'' by the algorithm. It is demonstrated that using the proposed approach by current work, the optimal weight and the corresponding set of design variable are very close to reference data. Yet the advantage of the proposed approach is exhibited in the fact that all of the constraints are satisfied.