Surface Residual Stress State Research Articles

Data-driven prediction of the surface layer state in hard-turning for optimization of component quality

Data-driven approaches are an effective solution for modeling problems in machining. To increase the service life of hard-turned components, it is important to quantify the correlation between the cutting parameters such as feed rate, cutting speed and depth of cut and the near-surface properties. For obtaining high-quality models with small data sets, different data-driven approaches are investigated in this contribution. Additionally, models that enable uncertainty quantification are crucial for effective decision-making and the adjustment of cutting parameters. Therefore, parametric multiple polynomial regression and Takagi–Sugeno models, as well as non-parametric Gaussian process regression as a Bayesian approach are considered and compared regarding their capability to predict residual stress and surface roughness values of 51CrV4 specimens after hard-turning. Moreover, a novel method based on optimization of data driven non-linear models is proposed that allows for identification of cutting parameter combinations, which at the same time lead to satisfactory surface roughness and residual stress states.

A unified model for bending fatigue life prediction of surface-hardened gears

The surface hardness and residual stress (RS) states significantly affect gear bending fatigue, and further mainly determines the reliability of the high-end equipment such as helicopters and electric automobiles. To date, there has been limited research focusing on the study of the quantitative connection between fatigue life and such surface integrity characteristics. In this paper, a damage-coupled gear bending fatigue model based on the continuum damage theory is established to calculate the gear fatigue life considering RS and hardness states. Then, unified gear bending stress-life (S-N) equations originating from the Basquin equation are derived to evaluate the quantitative effects of RS and hardness on gear bending fatigue life. The RS and hardness impact coefficients in unified equations are determined using the fatigue data obtained from a gear damage-coupled model. The unified gear bending S-N curves are verified using the experimental fatigue life data of shot peening and un-peening gears. These lead to the establishment of the unified equation integrating both the residual stress and surface hardness states for 18CrNiMo7-6 gear.

Analysis of the Influence of Surface Modifications on the Fatigue Behavior of Hot Work Tool Steel Components.

Hot work tool steels (HWS) are widely used for high performance components as dies and molds in hot forging processes, where extreme process-related mechanical and thermal loads limit tool life. With the functionalizing and modification of tool surfaces with tailored surfaces, a promising approach is given to provide material flow control resulting in the efficient die filling of cavities while reducing the process forces. In terms of fatigue properties, the influence of surface modifications on surface integrity is insufficiently studied. Therefore, the potential of the machining processes of high-feed milling, micromilling and grinding with regard to the implications on the fatigue strength of components made of HWS (AISI H11) hardened to 50 ± 1 HRC was investigated. For this purpose, the machined surfaces were characterized in terms of surface topography and residual stress state to determine the surface integrity. In order to analyze the resulting fatigue behavior as a result of the machining processes, a rotating bending test was performed. The fracture surfaces were investigated using fractographic analysis to define the initiation area and to identify the source of failure. The investigations showed a significant influence of the machining-induced surface integrity and, in particular, the induced residual stress state on the fatigue properties of components made of HWS.

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.

Effect of SiC Deposition Behavior on Microstructure and Mechanical Properties of Cold-Sprayed Al5056/SiC Composite Coatings

To study the effect of deposition behavior of the reinforcement on microstructure and property of cold-sprayed metal matrix composite coating, four Al5056 composite powders containing 15 wt.%, 30 wt.%, 45 wt.% and 60 wt.% SiC were mixed to prepare the composite coatings by cold spraying. Effect of the SiC deposition behavior on microstructure, SiC distribution, micro-/nanohardness, surface residual stress and shear strength of the coatings were studied. Experimental results show that the increase in SiC content in the powder increased the SiC collision with the deposited SiC particles, which caused the difference in SiC content and distribution in the composite coatings. The SiC deposition behavior caused the SiC morphology to evolve from integrity to small cracks and complete fragmentation. The SiC deposition behavior effectively influenced its peening effect on the deformed Al5056 particles, that is, as the SiC fragmentation threshold arrived, the nanohardness, surface residual stress and shear strength of the coating showed the corresponding transition point. The surface residual stress state of the deformed Al5056 matrix could in turn affect the SiC deposition by increasing the SiC deposition efficiency at the minimum compressive stress.

Scanning Manufacturing Parameters Determining the Residual Stress State in LPBF IN718 Small Parts

The influence of scan strategy on the residual stress (RS) state of an as‐built IN718 alloy produced by means of laser powder bed fusion (LPBF) is investigated. Two scan vector rotations (90°‐alternation and 67°‐rotation), each produced following two different scan vector lengths (long and short), are used to manufacture four rectangular prisms. Neutron diffraction (ND) and laboratory X‐ray diffraction (XRD) techniques are used to map the bulk and surface RS state, respectively. The distortion induced upon removal from the baseplate is measured via profilometry. XRD measurements show that the two long scan vector strategies lead to higher RS when compared with the equivalent short scan vector strategies. Also, the 67°‐rotation strategies generate lower RS than their 90°‐alternation counterparts. Due to the lack of reliable stress‐free references, the ND results are analyzed using von Mises stress. In general, ND results show significant RS spatial non‐uniformity. A comparison between ND and distortion results indicates that the RS component parallel to the building direction (Z‐axis) has a predominant role in the Z‐displacement. The use of a stress balance scheme allows to discuss the variability along the length of the specimens, as well as examine the absolute RS state.

Prediction of near surface residual stress states for hard turned specimens using data driven nonlinear models

Advantageous near surface residual stress states, being of crucial importance for performance of components, can already be established by adequate machining strategies avoiding cost intensive post treatments. In the present work, residual stress and integral width distributions in hard-turned specimens (AISI 6150) are characterized in dependence of processing parameters and workpiece hardness. In order to provide a basis for robust control schemes, experimental results are analyzed using various data driven methods. It is revealed that sophisticated approaches such as Takagi-Sugeno multi-models accurately predict process-property relationships and eventually outperform standard approaches such as multiple linear regression.

Cooling control for castings by adopting skeletal sand mold design

The cooling control of the melt during the casting process is of great significance. A comprehensive closed-loop cooling control of castings by adopting a skeletal sand mold design was proposed. The skeletal sand mold consisting of an adaptive shell, functional cavities and a support was designed and created based on the finite difference meshes of a casting. It was applied to a round wall test casting. Two kinds of skeletal sand molds, one with lattice support and the other with enforcing ribs for this casting were designed and printed out by the 3D printing (3DP) method. Aluminum alloy A356 was cast by using these two sand molds. The first mold was cooled by natural convection, the other one by water spray cooling. Two sound castings were obtained. The sand mold temperature, cooling curves, microstructures, mechanical properties, residual stress and deformation were measured, compared and discussed. Water spray cooling hastened the cooling rate by 62%, increased the content of Mg and Cu in the α-Al matrix, improved the mechanical properties, and altered the surface residual stress state.

Experimental study on additive/subtractive hybrid manufacturing of 6511 steel: process optimization and machining characteristics

The additive/subtractive hybrid manufacturing (ASHM) method, which takes the advantages of both additive manufacturing (AM) and precision subtractive machining (SM), is a promising technique for the surface quality improvement of an AMed part. However, machining characteristics and the residual stress evolution during the ASHM process have not been investigated in detail for the materials with phase transformation. In this study, an experimental study is conducted on manufacturing of a 6511 martensitic stainless steel with the ASHM method that combines selective laser melting (SLM) and end milling. The process window of SLM is determined in terms of the relative density. Milling forces, surface roughness, and residual stress of the ASHM samples with different cutting parameters are studied. The phase transformation of the martensitic steel is considered in the analysis of the residual stress. It is found that the surface residual stress state and distribution can be controlled by the milling parameters. This study provides guidance for the optimization of the process parameters for ASHM to manufacture martensitic stainless steel.

Development of surface residual stress and surface state of 42CrMo4 in multistage grinding

Abstract In this study, cylindrical grinding of a 42CrMo4 (AISI 4140) steel with varied grinding sequence stages is analyzed regarding the effect of the different loads during multistage grinding up to the resulting final residual stress state and material structure. The grinding process was adjusted in a way to enhance the mechanical load with varying intensity and generate a pronounced compressive residual stress state in the final grinding step. Furthermore, consecutive multistage grinding processes were carried out and investigated. Using a mobile XRD system with a cos α method mounted on a robotic positioning arm, repetitive measurements of the ground surfaces were performed in the machine tool without sample removal, giving information about the surface residual stress development for each step of the process sequence. The information about the surface integrity development and possible effects of the grinding process during each step is relevant for the contact conditions and modification mechanisms which lead to the final surface integrity. It is shown that the final material state, normally accessible in post-process investigations, can also be achieved reliably by sequencing the process into several steps of a regular grinding process and depends strongly on the contact parameters in a non-linear way. It could be shown, that even the spark out step can lead to significant changes in the surface residual stress state.

Machining for an increased fatigue life for a Ti-6Al-4V ELI component

Abstract Fatigue life is an important design criterion for many structural parts. The surface integrity induced by a typical machining operation may have a marked effect on fatigue life. Literature evidence suggests that the compressive surface residual stress state may change appreciably from the state obtained at the recommended conventional cutting speeds when intermediate cutting speeds are utilized. Titanium alloys typically show an increased compressive residual stress state at intermediate speeds before eventually tending towards tensile stresses at elevated cutting speeds. This paper investigates the effect of an optimised cutting strategy to control the residual stress state of a machined Ti-6Al-4V ELI (extra low interstitial) component to thereby control the fatigue life. An experimental program is conducted whereby stepped shafts are machined at various cutting speeds before being subjected to a full cyclic in-plane bending moment fatigue test. Cutting speed is varied between the typically recommended speed of 40 m/min to an intermediate speed of 110 m/min. Residual stresses are measured by XRD and compared to the fatigue life achieved. Other surface integrity descriptors including surface roughness, sub surface microstructure and selected surface damage effects are presented and discussed in relation to the fatigue performance. In essence the paper reports that fatigue life can be improved by machining at the optimum cutting speeds. These optimum intermediate speeds do however introduce other less desirable effects such as an increase in surface damage including the presence of weldments and ploughing grooves that may have a negative effect on the reliability of the improvements obtained.

Experimental Study on a "Snake-Type" Vibration Cutting Method for Cutting Force and Cutting Heat Reductions.

Cutting is the foundation of manufacturing in industry. The main cutting objects include metals, ceramics, glasses, compositions, and even biological materials such as tissues and bones. The special properties of each material such as hardness, ductility, brittleness, and heat conductivity lead to either a large cutting force or a high cutting temperature. Both of these factors result in poor machinability due to rapid tool wear or break or unsatisfactory surface integrity of the material finishing surface using the conventional cutting (CC, conventional cutting) types. In nature, snakes have their own way of reducing heat accumulation on their body when moving on the hot desert surface. They move forward along an “S”-type path, so that the bottom of their body separates from the desert intermittently. In this way, the separation interval both reduces the cutting heat accumulations and effectively achieves cooling by allowing the air to go through. In addition, the acceleration of Odontomachus monticola’s two mandibles when striking a target can reach 71,730 g m/s2 within 180 ms, which can easily break the target surface by the transient huge impact. Therefore, based on a snake’s motion on the desert surface and Odontomachus monticola’s striking on the target surface, respectively, an ultrasonic-frequency intermittent cutting method, also called “snake-type” vibration cutting (SVC, snake-type vibration cutting), was proposed in this study. First, its bionic kinematics were analyzed, then the SVC system’s design was introduced. Finally, cutting experiments were conducted on a common and typical difficult-to-cut material, namely titanium alloys. Cutting force, cutting temperature, and the surface integrity of the material finishing surface were measured, respectively. The results demonstrated that, compared to conventional cutting methods, SVC achieved a maximum of 50% and 30% reductions of cutting force and cutting temperature, respectively. Moreover, the surface integrity was improved both in surface roughness and residual stress state.

Quality Control and Structural Assessment of Anisotropic Scintillating Crystals

Nowadays, radiation detectors based on scintillating crystals are used in many different fields of science like medicine, aerospace, high-energy physics, and security. The scintillating crystals are the core elements of these devices; by converting high-energy radiation into visible photons, they produce optical signals that can be detected and analyzed. Structural and surface conditions, defects, and residual stress states play a crucial role in their operating performance in terms of light production, transport, and extraction. Industrial production of such crystalline materials is a complex process that requires sensing, in-line and off-line, for material characterization and process control to properly tune the production parameters. Indeed, the scintillators’ quality must be accurately assessed during their manufacture in order to prevent malfunction and failures at each level of the chain, optimizing the production and utilization costs. This paper presents an overview of the techniques used, at various stages, across the crystal production process, to assess the quality and structural condition of anisotropic scintillating crystals. Different inspection techniques (XRD, SEM, EDX, and TEM) and the non-invasive photoelasticity-based methods for residual stress detection, such as laser conoscopy and sphenoscopy, are presented. The use of XRD, SEM, EDX, and TEM analytical methods offers detailed structural and morphological information. Conoscopy and sphenoscopy offer the advantages of fast and non-invasive measurement suitable for the inspection of the whole crystal quality. These techniques, based on different measurement methods and models, provide different information that can be cross-correlated to obtain a complete characterization of the scintillating crystals. Inspection methods will be analyzed and compared to the present state of the art.

On the impact of deep rolling at different temperatures on the near surface microstructure and residual stress state of steel AISI 304

AbstractThe consequences of deep rolling at cryogenic temperature, room temperature and high temperature were investigated for steel X5CrNi18‐10 (AISI 304). Near surface microstructures, martensite content and residual stresses were analyzed. It is shown that, applying different deep rolling temperatures, the near surface states of the processed specimen are altered in a characteristic way. Particularly important is that different depth distributions of strain hardening, martensite volume fraction and residual stresses can be created.

Residual Stress Evaluation Induced by Fatigue Crack in Shot Peened, Notched Specimens

The advantages of applied compressive residual stress on fatigue properties of materials is a well-described topic, but the residual stress state of machine elements can change during application, therefore it is necessary to describe how these changes are related to the operational parameters. So the monitoring of residual stress relaxation gives more details to improve the lifetime. The surface residual stress state evolution of hardened (quenched and tempered), shot peened, 2 side notched quadratic fatigue specimens made of 42CrMO4 steel was investigated nondestructively by X-ray diffraction method. Residual stress state was monitored on the surface with 1 mm spatial resolution in 5 mm environment of the notch. The compressive residual stress relaxation associated the notching and the fatigue test and the reversal of sign of the residual stress (from compression to tension) associated the crack propagation were experimentally showed.

Materials and process engineering aspects of warm deep rolling

For industrial applications efficient and short processes are essential, justifying the increased complexity of deep rolling treatments at elevated temperature. For this purpose, the direct integration of deep rolling manufacturing steps in heat treatment processes is a promising strategy. Combining inductive heat treatment with deep rolling leads to encouraging results concerning near surface material states as well as resulting fatigue strengths. Different process concepts are introduced and discussed. Characteristic results of near surface microstructures and residual stress states produced in this way for steels SAE 1045 and SAE 4140 are presented. In addition, the consequences of the material states and the microstructures on cyclic deformation behaviour and fatigue strength are outlined. The integrated system introduced allows the flexible application of appropriate process parameters, the analysis of their effects on the fatigue behaviour and the determination of optimised process conditions.

Residual stress in nano-structured stainless steel (AISI 316L) prompted by Xe+ ion bombardment at different impinging angles

The effect of low energy (<1 keV) xenon (Xe+) ion bombardment on the residual stress of polycrystalline iron alloy (AISI 316L steel) is reported. The results take into account the influence of the ion incident angle maintaining constant all other bombarding parameters (i.e., ion energy and current density, temperature, and doses). The bombarded surface topography shows that ions prompt the formation of nanometric regular patterns on the surface crystalline grains and stressing the structure. The paper focalizes on the study of the surface residual stress state stemming from the ion bombardment studied by means of the “sin2 ψ” and “Universal Plot” methods. The analysis shows the absence of shear stress in the affected material region and the presence of compressive in-plane residual biaxial stress (∼200 MPa) expanding up to ∼1 μm depth for all the studied samples. Samples under oblique bombardment present higher compressive stress values in the direction of the projected ion beam on the bombarded surface. The absolute value of the biaxial surface stress difference (σ11-σ22) increases on ion impinging angles, a phenomenon associated with the momentum transfer by the ions. The highest stress level was measured for ion impinging angles of 45° (σ11 = −380 ± 10 MPa and σ22 = −320 ± 10 MPa). The different stresses obtained in the studied samples do not affect significantly the formation of characteristic surface patterns.

Fatigue life equality of polished and electrical discharge machined WC-Co hard metal achieved solely by wet blasting

WC-Co hard metals are materials widely used in wear applications, metal machining and highly fatigue loaded tools. Shaping tools and components made of hard metals often requires electrical discharge machining (EDM) that is well-known to negatively influence the material's fatigue properties.The current work demonstrates that it is possible to achieve a fatigue limit of electrical discharge machined specimens equal to the one of finely ground and polished reference samples. This is achieved by the application of advanced wire EDM technology combined with a sole surface post-treatment step of abrasive blasting. Stress amplitude-life curves were determined in a four point bending test setup. Depth profiles of residual stress were determined by means of X-ray diffraction for electrical discharge machined samples prior to and after blasting. Surface defect size was documented in metallographic cross sections by scanning electron microscopy. The surface residual stress state was found to be crucially important regarding the observed fatigue behavior. If the applied EMD technology produces cracks shallow enough to allow for their removal by abrasive blasting and compressive residual stress is introduced as deep into the surface as damage reaches, the negative effect of EDM on the fatigue limit is neutralized.

Assessing the heterogeneity of residual stress for complementing the fatigue performance comprehension

The residual stress analysis is a well-established method for predicting fatigue failures of mechanical components. Within industrial constraints, the X-ray diffraction is a technique usually applied to measuring a small spot of the workpiece surface. This punctual and averaged outcome does not allow the proper representation of the residual stress. The objective of this study is to define a feasible method for assessing the heterogeneity of the surface residual stress state. The proposal is based on the theoretical relationship between the deviation of the residual macrostress and the intensity of the microstress. Steel shot peened gears were produced and their microstresses were assessed by means of the diffraction profiles broadening. The reference database was composed of topography measurements, metallographic analyses and residual macrostress maps. The stress heterogeneity was reasonably correlated to the intensity of the Gauss integral breadth. Applied to ground parts, the correlation’s parameter filled a comprehension gap between the measured residual stress intensity and observed contact fatigue failures. Using the same data from the macro residual stress measurement, the method proved to be feasibly applied. Moreover, by providing a deviation perspective to the residual stress state, the heterogeneity assessment enhances the analysis of a fatigue failure.

Residual Stress Evaluation of a High Performance Machined Pre-formed Ti6Al4V Part

Near net-shape technologies are increasingly used to enhance resource efficiency in high performance machining for various applications, including aerospace. In this paper the effect of a pre-formed based manufacturing route on the residual stress state of an aerospace part in Ti6Al4V is presented. The manufacturing strategies, including the pre-form and machining stages, are outlined in detail. Two similar preformed billets were manufactured. High performance machining was then conducted at two different laboratories to manufacture a similar part utilizing different machining strategies. The effect of the manufacturing route and strategies utilized on the residual stress state of selected regions of the part were then evaluated by XRD (surface) and neutron diffraction (body) techniques. The results show that productivity improvements can be realized by utilizing a pre-form without incurring noteworthy residual stresses as a result of the forming process. The surface residual stress state, however, is substantial and found to be largely a function of the cutting strategy applied.