Deep Rolling Research Articles

Fatigue life improvement of similar and dissimilar aluminum friction stir welds by deep rolling

At the moment, high production costs prevent the friction stir welding (FSW) process from further industrial applications even if comparable high fatigue strength of the joints can be reached. Higher welding speed may reduce the production costs but decrease the fatigue strength of the manufactured FSW joints. A potential solution is a mechanical post weld treatment directly after welding to increase the fatigue strength of the FSW joint again. In this study, hydrostatic deep rolling was applied for fatigue strength improvement of similar and dissimilar FSW joints made of EN AW 5083 and EN AW 6082 alloys manufactured with different welding speed. Additionally, the fatigue strength was directly compared to conventional joints manufactured by metal inert gas (MIG) welding and to base material specimen made of EN AW 5083. The surface state in as-welded condition was characterized by surface roughness and residual stress measurements. Fatigue tests were performed to quantify the fatigue strength of the FSW joints. Similar compressive residual stresses were determined after deep rolling for similar and dissimilar joints. No fatigue life improvement was determined for deep-rolled similar joints made of EN AW 5083. However, in this condition, the fatigue life of the specimen was within the range of the base material. Thus, significant lower fatigue life and a high fatigue life improvement by deep rolling were reached for dissimilar joints. An increase of the welding speed from 300 to 800 mm/min strongly decreased the fatigue strength of dissimilar welded joints in the investigated case.

Influence of initial stress on the residual stress generated by deep rolling

Deep rolling is a finishing process and as such, usually used at the end of a process chain. The preceding processes often already have a substantial effect on the surface integrity. However, regarding residual stress, it is normally assumed that the information about pre-existing residual stress is lost during deep rolling, i.e. the pre-existing stress is irrelevant for the residual stress after the process and can therefore be neglected. In the present paper, the validity of this assumption is tested for a range of different combinations of initial stress profiles and deep rolling setups by using finite element simulations.

Designing an Optimal LSST Deep Drilling Program for Cosmology with Type Ia Supernovae

The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) is forecast to collect a large sample of Type Ia supernovae (SNe Ia) expected to be instrumental in unveiling the nature of dark energy. The feat, however, requires accurately measuring the two components of the Hubble diagram, distance modulus and redshift. Distance is estimated from SN Ia parameters extracted from light-curve fits, where the average quality of light curves is primarily driven by survey parameters. An optimal observing strategy is thus critical for measuring cosmological parameters with high accuracy. We present in this paper a three-stage analysis to assess the impact of the deep drilling (DD) strategy parameters on three critical aspects of the survey: redshift completeness, the number of well-measured SNe Ia, and cosmological measurements. We demonstrate that the current DD survey plans (internal LSST simulations) are characterized by a low completeness (z ∼ 0.55–0.65), and irregular and low cadences (several days), which dramatically decrease the size of the well-measured SN Ia sample. We propose a method providing the number of visits required to reach higher redshifts. We use the results to design a set of optimized DD surveys for SN Ia cosmology taking full advantage of spectroscopic resources for host galaxy redshift measurements. The most accurate cosmological measurements are achieved with deep rolling surveys characterized by a high cadence (1 day), a rolling strategy (at least two seasons of observation per field), and ultradeep (z ≳ 0.8) and deep (z ≳ 0.6) fields. A deterministic scheduler including a gap recovery mechanism is critical to achieving a high-quality DD survey.

A method to reduce computation time in finite element simulations of deep rolling

A method to reduce computation time in finite element simulations of deep rolling

Combination of Thermal and Mechanical Strategies to Compensate for Distortion Effects during Profile Grinding

This paper describes the investigations and the results of an analysis of distortion compensation processes for profile grinding. Steel workpieces often change their residual stress state due to machining in a seemingly uncontrolled matter. Furthermore, in research as well as in the industry, the accurate representation of shape deviations during the cutting of slim profiled workpieces and their deformation handling is a major challenge. In this paper, a valid predictive model, developed for the compensation of distortions resulting from the effect of a laser-based treatment and a deep rolling, was calibrated by experimental data. The numerical design of these strategies provided a model for predicting compensation parameters to minimize profile grinding distortions.

Surface integrity modification of CoCrMo alloy by deep rolling in combination with sub-zero cooling as potential implant application.

Alloys made of CoCrMo are well established as implants materials since decades in orthopedic surgery. The good mechanical properties, biocompatibility and especially the corrosion resistance are important rationales for the use of these alloys. Nevertheless, retrieved implants from revision surgery showed the occurrence of abrasion and corrosion. The wear mechanisms and the occurring corrosion processes might be reduced with a functionalization of the surface. The hexagonal phase of the cobalt chromium matrix plays an important role in the surface functionalization. It can be specifically transformed and set during the manufacturing process. One possibility for the induction of the transformation is the use of a deep rolling process in combination with a novel "sub-zero" cooling strategy during machining. The influence of force and temperature during the deep rolling process on the formation of the hexagonal Co-phase is examined in this study. The results from the targeted setting of the hexagonal Co-phase in the subsurface are shown. For this purpose, EBSD studies have been carried out to detect and quantify the proportion of Co-hex phase in the subsurface of the modified alloys. To analyze the mechanical properties, we measured the residual stress and hardness in the near surface layer under conditions close to the application. Furthermore, we performed biological tests to show a potential influence of the modification on the biocompatibility when using the sub-zero cooling approach. We observed no negative effect on the osteoblastic cell line which attached similarly to all tested surfaces. The investigations provide first insights into the potential use of "sub-zero" cooling in modifying orthopedic implant materials, but also the respective limits with regard to the surface functionalization. Deep rolling in combination with an innovative cooling strategy has a great potential to improve the mechanical properties of CoCr28Mo6 wrought alloy, by subsurface hardening and phase transformation.

Effect of surface integrity on fatigue life of 7075-T6 aluminum alloy by combination of fine turning with hydrostatic deep rolling

Effect of surface integrity on fatigue life of 7075-T6 aluminum alloy by combination of fine turning with hydrostatic deep rolling

The comprehensive performance and strengthening mechanism of the columnar crystal Cu-Ni-Si alloy after two large deformation rates of cryogenic rolling-aging

In order to improve the strength and electrical conductivity of Cu-Ni-Si alloy, the columnar crystalline C70250 copper alloy strip suffer from two deep cryogenic rolling-aging treatments with large deformation, and the relationship between the cryogenic rolling-aging process and microscopic structure, mechanical, conductive properties is studied, and the precipitation behavior is analyzed. The research findings show that the columnar crystalline C70250 copper alloy produced by continuous directional solidification can be directly processed by two cryogenic rolling-aging treatment with large deformation, eliminating the traditional solution treatment process and obtaining a fibrous tissue structures containing high-density dislocations and multi-scale precipitates, the yield strength, tensile strength and electrical conductivity of the alloy reached 879 MPa, 923 MPa and 38.7 % IACS respectively, combination of strength and electrical conductivity is excellent. Two deep cryogenic rolling with large deformation can force the matrix to change to amorphous, favorable to atomic diffusion. Solute atoms are continuously transformed into Ni2Si phase and Cu atoms are continuously expelled, forming large-sized pure Ni2Si precipitated phase and small-sized NiSi cluster phase containing Cu atoms, which together impede dislocation movement and reinforce alloy. With the uniform precipitation of multi-scale nano-precipitation phases and reduction of grain boundary density, the electrical conductivity along the rolling direction is significantly reduced.

Influence of induction hardening on the damage tolerance of EA4T railway axles

Induction hardening, a more efficient and effective surface hardening technique of railway axles, has received considerable attention for its potential of replacing deep rolling to be the surface strengthening technique of EA4T axles. In order to establish guidance for industrial utilization of induction hardened railway axles, damage tolerance assessment on induction hardened EA4T axles is a necessity. The purpose of this paper is to evaluate the influence of induction hardening on the damage tolerance of EA4T axles subjected to flying ballast impact damage. Impact damages were introduced artificially by means of shooting balls of 2 mm in diameter to induction hardened specimens and untreated ones. The damage modes of two kinds of specimens were analyzed and the resultant loading capacity debit was evaluated. Based on the characterization of damages and fracture surfaces, failure mechanisms were discussed. Due to the high strength of hardened layer, it is more significant to resist plastic deformation under a moderate impact. Under ballistic impact, microcracks, material loses and adiabatic shear bands (ASBs) present in damage of hardened specimen. Fatigue strength of smooth hardened specimen was improved by 61 % in comparison with that of untreated one. As a result of stable and considerable compressive residual stress of induction hardened (IH) specimen, fatigue strength of damaged hardened specimen is markedly larger than the as received (AR) ones with similar defect size.

Mechanical Properties Enhancement of Dissimilar AA6061-T6 and AA7075-T651 Friction Stir Welds Coupled with Deep Rolling Process.

The main purpose of this research was to enhance the mechanical properties of friction stir welds (FSW) in the dissimilar aluminum alloys 6061-T6 and 7075-T651. The welded workpiece has tensile residual stress due to the influence of the thermal conductivity of dissimilar materials, resulting in crack initiation and less fatigue strength. The experiment started from the FSW process using the 2k full factorial with the response surface methodology (RSM) and central composite design (CCD) to investigate three factors. The experiment found that the optimal rotation speed and feed rate values were 979 and 65 mm/min, respectively. Then, the post-weld heat treatment process (PWHT) was applied. Following this, the 2k full factorial was used to investigate four factors involved in the deep rolling process (DR). The experiment found that the optimal deep rolling pressure and deep rolling offset values were 300 bar and 0.2 mm, respectively. Moreover, mechanical property testing was performed with a sequence of four design types of workpieces: FSW, FSW-PWHT, FSW-DR, and FSW-PWHT-DR. It was found that the FSW-PWHT-DR workpiece had an increase in tensile strength of up to 26.29% and increase in fatigue life of up to 129.47% when compared with the FSW workpieces, as well as a maximum compressive residual stress of −414 MPa.

Innovative X-ray diffraction and micromagnetic approaches for reliable residual stress assessment in deep rolled and microfinished AISI 4140 components

The residual stress state in the subsurface is a key element of surface integrity. It is well known to have a significant impact on a component's properties in terms of fatigue behavior and resistance to wear and corrosion. For this reason, adjusting residual stresses during manufacturing is a major challenge in modern production engineering, to improve and ensure a component's fatigue strength. In this context, hydrostatic deep rolling of the workpiece surface using adapted parameters enables the targeted induction of compressive residual stresses into subsurface layers. Due to specific properties regarding subsurface and topography for functional components in tribological applications, a further machining operation by microfinishing following deep rolling seems to be purposeful. In particular with regard to the production of components exposed to periodic load changes when used, the process combination can enable a substitution of the typically required conventional subsurface zone hardening. With the aim of economical process design, the corresponding parts can be manufactured with significantly reduced time and costs. Efficient and well-founded methods for monitoring the resulting influence on the subsurface zone properties are essential for a reproducible and target-oriented process design. The prevailing method for the non-destructive assessment of residual stresses in both academia and industry is X-ray diffractometry using the sin2 ψ-method. However, this method is time-intensive and requires complex instrumentation. Thus, efforts have been undertaken in past decades to develop alternative methods for the efficient and reliable characterization of residual stresses. In this research, the applicability of the cos α-method in X-ray diffractometry and a micromagnetic approach for residual stress assessment was investigated, analyzing deep rolled and microfinished AISI 4140 specimen conditions. In addition to the diffractometric and micromagnetic measurements, metallographic and topographic analyses of machined surfaces were carried out. Deep rolling was found to induce significant compressive residual stresses of up to −1100 MPa. After microfinishing of the deep rolled surfaces, favorable compressive residual stresses remain in the subsurface, reaching approximately up to −750 MPa. Based on this, the production of tailored surfaces with respect to a suitable combination of topography and subsurface is possible. For all surface states investigated, a good agreement between the two approaches in X-ray diffraction was found. Magnetic Barkhausen noise (MBN) measurements prove to be well applicable for an efficient and holistic assessment of surface integrity in the subsurface of deep rolled and microfinished AISI 4140 specimens.

Failure causes and hardening techniques of railway axles – A review from the perspective of structural integrity

Fatigue is a classic and longstanding issue, which challenges railway axles’ structural integrity. In spite of its almost 200 years of research history, it has always been a hot topic considering the increasing service speed and safety concern. Except for inclusions, defects from axle steel ingot, corrosion pit, fretting and flying ballast impact damage also bring potential failure risk. A broad overview on the four kinds of defects responsible for axle failures is given in present paper. Surface hardening techniques are utilized during axle manufacturing aiming to improve surface integrity of railway axles by means of both retarding crack initiation and decelerating crack growth rate due to introduction of residual compressive stress and formation of hardened layer. Mechanisms of surface hardening techniques, including shot peening, deep rolling and induction hardening, are briefly introduced and their applications to axles are reviewed. Based on state-of-the-art of axle manufacturing, induction hardening and anti-corrosion/impact coating is recommended as an efficient and prospective combination with regard to structural integrity of railway axles.

Automated geometry measurement and deep rolling of butt welds

During the joining of two metal sheets by welding, a process-specific geometry of the weld is created. The local geometry of the created weld has a decisive influence on its fatigue strength. This is due to stress concentration at the geometric notches. In this paper, a process known from mechanical engineering called deep rolling is applied on butt welds. The influence on the local weld geometry and the local stress concentration after deep rolling is investigated. Additionally, a novel automated measurement system using optical laser line scanning is presented. The system is qualified for the evaluation of the local weld geometry regarding its flank angles and toe radii. The presented investigations show that the deep rolling process influences the stress concentrations determined by 2D-FE-simulations using real scan data. A correlation between the difference in toe radii or local notch stresses before and after deep rolling and the initial flank angle was found. This indicates that there are process and geometry specific conditions for the successful application of the deep rolling process.

Modeling of Deep Rolling as a Distortion Compensation Strategy during Profile Grinding

This paper analyzes the possibility to induce residual compressive stresses utilizing deep rolling in a long, slim steel workpiece that is profile ground and that resembles a linear guide rail. These intentionally caused residual compressive stresses at the side surfaces of the workpiece are intended to counteract the distortion due to residual stresses in the V-groove of the guide rail induced by grinding. A finite element model was prepared based on the initial state of the workpiece. The amount of respective stresses depended on the load intensity and duration of the process. The experimental data regarding the residual stress profiles due to the deep rolling process and the resulting workpiece distortion were used to validate the simulation, so that an adjustment or a calibration of the thermometallurgical and thermomechanical material data was possible. The key findings are the numerical design of appropriate strategies for the distortion potential induction, the experimental distortion results, knowledge on the mechanical treatment by deep rolling, and the successful modeling of the mechanical treatment and the process strategy.

Reduced FFT-Based Simulation of a Mechanically Loaded Clustered Microstructure Using an Adaptive Set of Fourier Modes

Processes, such as deep rolling or induction hardening, have a remarkable influence on the material properties within the surface layer of a work piece. Our overall goal is to develop efficient two-scale methods, which are able to show the microstructural evolution of the machined material. The calculation of a spatially resolved microstructure comes along with a high computational effort. To reduce the computational costs, we combine a clustered description of the structure [1] with a model order reduction technique for the performed fast Fourier transformations (FFT) [2]. We choose a reduced set of Fourier modes, which is adapted to the underlying microstructure and thus based on the occurring strain field [3]. By that, we analyze the influence of a mechanical impact on an elasto-plastically deforming material.

Effects of Deep Rolling on the Microstructure Modification and Fatigue Life of 35Cr2Ni4MoA Bolt Threads

Stress concentration on a bolt thread, resulting from its own special shape, poses a threat to the fatigue strength of the bolt, which directly affects the safety and reliability of aircraft. In this paper, deep rolling was applied to a bolt thread to improve its fatigue resistance. The properties of the plastic deformation layer, including the surface morphology, microstructure, hardness, and residual stress, as well as the fatigue life of the bolt, were characterized by means of SEM, white light interferometer, EBSD, and fatigue tests. The results showed that the surface roughness of the bottom of the thread was reduced to 0.255 μm, and a plastic deformation layer of about 300 μm in depth was formed after rolling. A more compact streamlined fibrous microstructure, composed of refined grains, with increased dislocation density and hardness and decreased tensile residual stress, was formed in the plastic deformation layer. The fatigue life of the bolts after rolling increased by about 113%, evidencing the comprehensive result of these microstructure modifications.

Mechanical surface treatment of EBM Ti6Al4V components: Effects of the resulting surface layer state on fatigue mechanisms and service life

Although additive manufacturing of the titanium alloy Ti6Al4V has been an established process for years, problems still exist with regard to the design of components subjected to cyclic loads. In this context, process-related defects play an important role, such as surface roughness and pores. Typically, AM Ti6Al4V components are machined and subjected to a hot isostatic pressure (HIP) treatment, which, however, strongly limits the cost advantage. Mechanical surface treatment processes can be used locally where high loads are expected and thus make a cost-effective contribution to the service life of additively manufactured components. In the present work, the influence of the mechanical surface treatment process of deep rolling (DR) on the surface state of electron beam melted (EBM) and EBM+HIP components is investigated. In addition, rotating bending tests are presented in SN curves and these are evaluated in the context of the surface layer condition and the failure mechanisms. The fatigue strength could be improved from RAB=157MPa in the as built state (AB) to RAB+DR=251MPa in the deep rolled state (AB+DR). Through the deep rolling treatment, the surface could be strengthened to the extent that the location of the failure shifts into the component volume. The comparison of the fracture surfaces analysis of deep rolled samples shows that pores below the surface are the cause of failure, in a depth that is not influenced by the mechanical surface treatment. In an additional series of tests, the fatigue strength of the HIP and deep-rolled (HIP+DR) state was determined. Since HIP EBM specimens are nearly pore-free, the failure mechanism switches back to the strengthened surface, resulting in an increased fatigue limit of RHIP+DR=413MPa.

Achieving High Strength-plasticity of Nanoscale Lamellar Grain Extracted from Gradient Lamellar Nickel

Traditional metallic materials usually face a dilemma between high strength and poor strain hardening capacity. However, heterogeneous structured metallic materials have been found to obviously overcome the trade-off. Herein, gradient lamellar structure was fabricated through ultrasound-aided deep rolling technique in pure Ni with high stacking fault energy after heat treatment. The gradient lamellar Ni was successively divided into the four regions. In-situ micropillar compression tests were conducted in different regions to reveal the corresponding microscopic mechanical properties. Microscopic characterization techniques were performed to explore underlying deformation mechanisms and the effects of microstructural parameters on deformation behaviors. This work demonstrates that the micropillar with near nanoscale lamellar thickness possesses excellent strength and plasticity. On one hand, the reason for high strength of near nanoscale micropillar is that the strength of micropillar increases with the decrease of lamellar thickness according to the Hall-Petch effect. On the other hand, numerous lamellar grain boundaries perpendicular to the loading direction is found to hinder the motion of slip bands, resulting in great strain hardening capacity in the near nanoscale lamellar micropillar.

Influence of deep rolling on surface layer condition and fatigue life of steel welded joints

Deep rolling is an established mechanical surface treatment method to modify surface and subsurface properties and increase the wear- and fatigue strength of metallic parts and components. However, until now, deep rolling has not been considered for use as a post treatment method of welded joints to extend fatigue life of steel constructions. In this study, butt joints made of construction steels S355J2+N and S355G10+M were treated by means of hydrostatic deep rolling with tool diameter. The surface and subsurface properties after treatment were quantified by roughness-, hardness and residual stress measurements. Reduced roughness of 50–80%, increase of surface near hardness of up to 45% and compressive residual stresses of up to -1,000MPa were determined after deep rolling. Furthermore, the stress concentration factor of the weld toe was evaluated for each specimen based on surface scans. Due to the comparable low contact force of the deep rolling tool-process parameter combinations investigated, the stress concentration of the weld toe was mostly unaffected by deep rolling. However, a significant fatigue life improvement by deep rolling was determined for all test series, showing a maximum increase of FAT-class of approximately 60%. A tendency is shown that a higher tool diameter leads to higher fatigue life improvement.

High temperature behaviour of a mechanically surface hardened single crystal nickel-based superalloy

Surface hardening procedures such as mechanical shot peening and deep cold rolling introduce extensive surface cold work and compressive residual stress. While both methods are designed to increase material fatigue strength, single crystal nickel-based superalloys used for turbine blade applications are routinely exposed to temperatures above 900 °C. The thermal stability of the cold work and the residual stress induced in the sub-surface of the material is therefore a key design requirement. In this study, mechanically shot peened and deep cold rolled samples of the superalloy CMSX-4 were annealed in vacuum at either 900 °C or 1100 °C for between 100 and 1000 h. Using scanning electron microscopy, electron backscattered diffraction and centre hole drilling, the cold work, the residual stress and the material response to annealing heat treatments were all studied. Significant recrystallisation of the cold worked material was observed, typically after 500 h at 900 °C and after 5 h at 1100 °C. Undesirable topologically close-packed precipitates began forming on slip bands generated by both surface hardening techniques after only 10 h at 900 °C, whereas at 1100 °C their formation was hindered by the recovery of slip bands. Rafting of the γ/γ' microstructure took place at 1100 °C, with the extent of the rafting found to be limited by the depth of compressive residual stress. Samples treated by deep cold rolling generally displayed smaller recrystallisation depths, lower rates of topologically close-packed precipitate formation and deeper rafting than those treated by mechanical shot peening.