Fatigue Properties Of Alloy Research Articles

Investigation on high cycle fatigue performance of additively manufactured Alloys: Synergistic effects of surface finishing and Post-Heat treatment

The high-cycle fatigue (HCF) behavior of additively manufactured (AM) materials in structural applications remains insufficiently explored when contrasted with conventionally fabricated alloys. Addressing this research gap, we systematically investigate the potential to enhance HCF performance in AM alloys through a strategic interplay of surface finishing and post-heat treatment design. Concentrating on laser powder bed fusion, we evaluate surface roughness and post-heat treatment effects on HCF life across a300to1000 MPa stress range. Findings show that, at700to1000 MPa, surface finishing significantly extends HCF life by mitigating crack initiation. Paradoxically, at300to700 MPa, it shortens HCF life due to induced local residual stress on the sample surface. Additionally, post-aging finishing moderately enhances HCF life compared to pre-aging. This underscores the crucial role of sequence in surface finishing and heat treatment, emphasizing their collective impact on fatigue properties. Our study advances the understanding of post-processing factors influencing AM alloy fatigue properties, providing valuable insights for enhancing mechanical performance.

Impact of laser powder bed fusion processing on the cyclic and fatigue properties of Ni20Cr alloy

Additive manufacturing is a new-age technology specialising in intricate fabrication in the manufacturing industry. However, extremely high cooling rates and far-from-equilibrium kinetics produce heterogeneous microstructure and induce high initial dislocation density, porosity, and residual stress. These unique microstructural features are known to severely impact cyclic and fatigue properties of alloys. In such a context, we investigated the cyclic and fatigue properties of a Ni-20 wt%. Cr alloy manufactured via laser powder bed fusion process (LPBF) and compared with its cast counterpart. The fatigue testing was carried out with three different strain amplitudes depicting Low, Medium and High Cycle Fatigue (LCF, MCF, HCF). LPBF samples exhibited higher fatigue resistance than that of cast samples, however experienced early failure in all the conditions. Despite strong planar glide, LPBF samples exhibited softening behaviour; the degree of softening is similar for LCF and MCF and less pronounced for HCF. The cast samples did not undergo any softening whatsoever indicating huge differences in cyclic strain mechanisms. Masing analysis was proposed to graphically analyse this softening behaviour. Further, the flow stress was categorised into backstress and effective stress for LPBF and cast samples. Majority of softening in LPBF samples for LCF and MCF occurred via backstress, whereas effective stress is associated to HCF. To investigate the origin of this softening for LPBF samples, microstructure characterisations were performed during the softening and at fracture. Post-fatigue microstructure indicates a clear modification of the dislocation structures from those inherited from LPBF to those linked to fatigue. This gradual change in microstructure is expected to induce the cyclic softening.

Assessment of Ti6Al4V alloy fatigue properties based on specimens produced by DMLS technology

The Ti6Al4V alloy produced by additive methods is the most studied material, which can be attributed to its wide application and the resulting need for a thorough understanding of this material and its behavior under static and fatigue loads. For this reason, the paper presents the results of tests of the Ti6Al4V alloy produced by the additive method and in the standard smelting process. The publication contains the results of the alloy tests under static and variable loads based on strain control.

Anisotropic fatigue properties of Alloy 718 manufactured by Electron Beam Powder Bed Fusion

In this study, Alloy 718 specimens manufactured by Electron Beam Powder Bed Fusion process are subjected to two different post-treatments to have different microstructural features. Low cycle fatigue testing has been performed both parallel and transverse to the build direction. EB-PBF Alloy 718 exhibits anisotropic fatigue behaviour; the fatigue life is better along the parallel direction compared to the transverse direction. The anisotropy in fatigue life is related to the anisotropy in the Young's modulus. The pseudo-elastic stress vs. fatigue life approach is presented as a potential solution to handle anisotropy in fatigue life assessment of additively manufactured engineering components.

High cycle fatigue performance of AlSi10mg alloy produced by selective laser melting

Selective laser melting (SLM), which is an additive manufacturing (AM) technology, has been widely used in industrial fields. The inherent defects resulting from the SLM process are inevitable. The effect of building directions on the static mechanical properties has been observed. However, certain critical functional components are under cyclic loads and extreme service conditions. The effect of inherent defects and building directions on the high cycle fatigue (HCF) performance of SLM alloys is not very clear. More testing data should be provided. Therefore, in the present work, the rotation bending fatigue (RBF) tests of SLM AlSi10Mg bar samples that included three building directions are carried out. These results indicate that most fatigue cracks initiated from the unmelted defects of the sample surfaces (or subsurface). The SLM building directions have an obvious impact on the AlSi10Mg alloy fatigue properties. The 0° samples exhibit a higher fatigue limit value than the 45° and 90° samples. Moreover, the crack propagation mechanism is revealed by fracture analysis. The negative effect of multiple-crack initiation on the RBF specimen fatigue lives under low cyclic loads is explained in detail. Finally, the evaluation method for the fracture toughness (KIC) by the fracture surfaces of the RBF samples is provided.

Data Assessment Method to Support the Development of Creep-Resistant Alloys

This work introduces a methodology to assess data quality for the tensile, creep/stress relaxation, and fatigue properties of alloys (as well as metadata associated with manufacture) as a part of a project to develop new materials for extreme environments. The extreme environments in question deal with those found in the power generation sector. Data quality assessment is needed to ensure the reliability of data used in analytics to develop new materials for the power generation sector and to predict the performance of established materials in current use. As data quality metrics have not been standardized for material properties data, quality rating guidelines are developed here for the aspects of data completeness, accuracy, usability, and standardization. The specific design requirements for heat-resistant alloy development were considered in creating each metric. Establishing the quality of a dataset in these areas will enable robust analysis. High-quality data can be set aside to develop predictive models. Lower-quality data need not be discarded but can be used for experimental design. Determining the quality of a materials dataset will also provide additional metadata with the data resource and will promote data reusability. A sample high-quality dataset is presented to indicate the typical data attributes collected from relevant mechanical property testing results, which were considered when generating the data quality metrics. A data template of these attributes was created as a tool for data generators and collectors to promote uniformity and reusability of alloy data. The sparsity of the sample dataset was calculated in order to highlight the areas where data gaps pose a challenge for reliable prediction of creep rupture lifetime.

Influence of phase coarsening on fatigue crack growth in precipitate strengthened alloys

The precipitate size and distribution within precipitate strengthened alloys will change significantly after phase coarsening, and it will further affect their mechanical properties. In this paper, the fatigue properties of alloys during phase coarsening processes are investigated. The fatigue crack growth in various microstructures under the low cycle fatigue condition is analyzed systemically, and several aspects such as the cyclic deformation and failure morphologies, cyclic stress, and crack growth rate, are discussed in detail. Especially, a mechanism analysis is conducted, which is based on evolutions of internal variables within the material, e.g., stress state and plastic strain. The influence of phase coarsening on the crack propagation is also discussed. Related analysis demonstrates that the microstructure of alloys plays an important role in their fatigue properties. During the phase coarsening process, the fatigue strength of alloys descends gradually, the fatigue crack deflection becomes more serious, the propagation becomes more unstable, and the crack growth rate increases. Besides, related degradations derived from phase coarsening are more distinct under the higher external load. The corresponding fatigue properties are closely related to the plastic deformation as well as the dislocation movement within the alloys. The variation of the precipitate size and distribution alters the interaction manners between the precipitate particles and the plastic deformation bands or cracks within the microstructure, which further leads to different fatigue characteristics of alloys.

Microstructure and Fatigue Behavior of a Ni-Cu-Sn Alloy

In this paper, the static and fatigue properties of a Cu-Ni-Sn alloy are investigated. Tensile tests, hardness tests and microstructural analyses using optical and scanning electron microscopy (SEM) were performed and two sets of fatigue tests, with load ratio (R) R = − 1 and R = 0 , respectively, were carried out. The results showed the capability of the alloy to bear high static stress, thanks to its good strength properties. However, the fatigue tests showed a strong sensitivity of the alloy fatigue properties depending on the raw material batch. The comparison between microstructural analyses and fatigue test results showed a strong correlation; in particular, the specimens having a more inhomogeneous microstructure showed lower durability. In addition, the different microstructure also affected the fracture surface morphology as highlighted by SEM analyses.

Influence of defects and as-built surface roughness on fatigue properties of additively manufactured Alloy 718

Electron beam melting (EBM) and Selective Laser Melting (SLM) are powder bed based additive manufacturing (AM) processes. These, relatively new, processes offer advantages such as near net shaping, manufacturing complex geometries with a design space that was previously not accessible with conventional manufacturing processes, part consolidation to reduce number of assemblies, shorter time to market etc. The aerospace and gas turbine industries have shown interest in the EBM and the SLM processes to enable topology-optimized designs, parts with lattice structures and part consolidation. However, to realize such advantages, factors affecting the mechanical properties must be well understood – especially the fatigue properties. In the context of fatigue performance, apart from the effect of different phases in the material, the effect of defects in terms of both the amount and distribution and the effect of “rough” as-built surface must be studied in detail. Fatigue properties of Alloy 718, a Ni-Fe based superalloy widely used in the aerospace engines is investigated in this study. Four point bending fatigue tests have been performed at 20 Hz in room temperature at different stress ranges to compare the performance of the EBM and the SLM material to the wrought material. The experiment aims to assess the differences in fatigue properties between the two powder bed AM processes as well as assess the effect of two post-treatment methods namely – machining and hot isostatic pressing (HIP). Fractography and metallography have been performed to explain the observed properties. Both HIPing and machining improve the fatigue performance; however, a large scatter is observed for machined specimens. Fatigue properties of SLM material approach that of wrought material while in EBM material defects severely affect the fatigue life.

Study on Microstructure of directional solidified T91 steel

Using the method of directional solidification to prepare T91 steel is an effective way to reduce the elastic modulus and improve the thermal fatigue properties of alloy. In this paper, T91 steels were fabricated through liquid metal cooling method by the solidification rate of 30μm/s, 90μm/s and 180μm/s, and the crystal morphology was analysed. The results demonstrate that deflection angle of columnar crystals is increased with the raising of solidification rate, the maximum angle is 20 degrees, and the solidification structure is refined in general. The central solidification zone is the concentration area of directional microstructures, in which lath ferrites are arranged in parallel and the martensites are mainly distributed in the beginning and the end of the sample. The range of elastic modulus values of three specimens is 141.05~186.73GPa, and E90μm/s<E180μm/s<E30μm/s<Ecommon cast. The thermal fatigue property of T91 obtained by directional solidification method is better than that by common casting. When the solidification rate is 90μm/s, the thermal fatigue resistance of the specimen is best, which increases by about 25%.

Effect of Strain Range on the Low Cycle Fatigue in Alloy 617 at High Temperature

The aim of this study is to investigate the fully‐reversed low cycle fatigue properties of Alloy 617 in the air at 950 °C; these tests were conducted at total strain ranges from 0.9% to 1.5% with a constant strain rate of 10−3/s. The result of the fatigue tests showed a decrease in fatigue resistance with an increasing total strain range. The reduction of fatigue resistance was due to the effect of the total strain range and microstructure evolution during high temperature, such as brittle oxides cracking. At all testing conditions, the cyclic softening mechanism was observed as a function of the total strain range in the current high temperature condition. An analysis of low cycle fatigue resistance was performed using the Coffin–Manson relationship and the total strain energy density; it was found that Alloy 617 followed these relationships well. In addition, this study compared well with previous work reported in the literature for a similar testing condition. Post‐fracture analysis on the fracture surfaces of failed specimens revealed a more severe damage cracking at the periphery of specimens due to the increase in the total strain range. The surface connected grain boundary cracks induced by oxidation were obvious at low strain range. Thus, the primary crack propagation occurred in transgranular mode from persistent slip bands.

Effect of Frequency on the Fatigue Behavior of IN718 Superalloy

IN718 alloy possesses excellent mechanical properties at high temperatures, good process ability, therefore, it has been widely used in aero engine turbine disks, compressor disks, and power turbine shafts (i.e., rotating components). The fatigue properties of the alloy are a key factor that determines the safety and reliability of the engine. In this paper, the fatigue properties of IN718 alloy are investigated under low-and middle-frequency conditions at 600 °C and 455 °C, the initiation of fatigue cracks, and the relation between fatigue life and grain size are discussed. The results show that the carbides response as a crack initiation site at low-frequency fatigue condition (1 Hz), and string-type or heap-type carbides distribution promotes crack propagation and shortens fatigue life, the twin boundaries in large grains are act as a crack initiation site at middle-frequency fatigue condition (10 Hz). The grain size is smaller, and the low cycle fatigue properties of the alloy are better.

Effects of stress ratio on the temperature-dependent high-cycle fatigue properties of alloy steels

This paper addresses the effects of stress ratio on the temperature-dependent high-cycle fatigue (HCF) properties of alloy steels 2CrMo and 9CrCo, which suffer from substantial vibrational loading at small stress amplitude, high stress ratio, and high frequency in the high-temperature environments in which they function as blade and rotor spindle materials in advanced gas or steam turbine engines. Fatigue tests were performed on alloy steels 2CrMo and 9CrCo subjected to constant-amplitude loading at four stress ratios and at four and three temperatures, respectively, to determine their temperature-dependent HCF properties. The interaction mechanisms between high temperature and stress ratio were deduced and compared with each other on the basis of the results of fractographic analysis. A phenomenological model was developed to evaluate the effects of stress ratio on the temperature-dependent HCF properties of alloy steels 2CrMo and 9CrCo. Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.

Enhanced fatigue crack propagation resistance of Al-Cu-Mg alloy by intensifying Goss texture and refining Goss grains

The effect of intensifying Goss texture and refining Goss grains on the enhancement of fatigue crack propagation (FCP) resistance in an Al-Cu-Mg alloy was systematically investigated. The way to improve fatigue performance mainly includes two steps: intensifying Goss texture component and then refining these Goss grains. Hot rolling at elevated temperature and pre-solution treatment are capable of Goss texture formation. Then large cold rolling reduction can refine grains and obtain more these Goss grains, which increases the twist angle boundary components with their adjacent grains to enhance fatigue crack deflection and FCP resistance. By contrast, refining Brass etc. rolling-oriented grains can increase the tilt angle boundary components and promote fatigue crack growth. And the reason why grain refinement cannot always improve fatigue properties in alloys with micrometer grains can depend mainly on grain orientations and the relative boundary components in this Al-Cu-Mg alloy. Besides, coarse Fe-, Si- and Mn-rich inclusions are detrimental to fatigue properties, but the fine, globular and homogeneous distribution of these particles can improve fatigue properties to some extent.

Effect of Rolling on High-Cycle Fatigue and Fracture of an Al – Mg – Sc Alloy

The tensile strength and fatigue properties of alloy 1575 of the Al – Mg – Sc system are studied after hot deformation (at 360°C) and subsequent cold rolling with different reduction ratios. The effect of the deformed structure on the properties and mechanisms of fracture of the alloy under cyclic tests is determined.

Thermal oxidation of medical Ti6Al4V blasted with ceramic particles: Effects on the microstructure, residual stresses and mechanical properties

Roughening of Ti6Al4V by blasting with alumina or zirconia particles improves the mechanical fixation of implants by increasing the surface area available for bone/implant apposition. Additional thermal oxidation treatments of the blasted alloy have already shown to be a complementary low-cost solution to enhancing the in vitro biocompatibility and corrosion resistance of the alloy. In this work, the effects of oxidation treatment on a grit blasted Ti6Al4V biomedical alloy have been analysed in order to understand the net effect of the combined treatments on the alloy fatigue properties. Synchrotron radiation diffraction experiments have been performed to measure residual stresses before and after the treatments and microstructural and hardness changes have been determined. Although blasting of Ti6Al4V with small spherical zirconia particles increases the alloy fatigue resistance with respect to unblasted specimens, fatigue strength after oxidation decreases below the unblasted value, irrespective of the type of particle used for blasting. Moreover, at 700°C the as-blasted compressive residual stresses (700MPa) are not only fully relaxed but even moderate tensile residual stresses, of about 120MPa, are found beneath the blasted surfaces. Contrary to expectations, a moderate increase in hardness occurs towards the blasted surface after oxidation treatments. This can be attributed to the fact that grit blasting modifies the crystallographic texture of the Ti6Al4V shifting it to a random texture, which affects the hardness values as shown by additional experiments on cold rolled samples. The results indicate that the oxidation treatment performed to improve biocompatibility and corrosion resistance of grit blasted Ti6Al4V should be carried out with caution since the alloy fatigue strength can be critically diminished below the value required for high load-bearing components.

A Study on Very High Cycle Fatigue Properties of Low Flammability Magnesium Alloy in Rotating Bending and Axial Loading

In order to use a low flammability magnesium alloy as structural components, very high cycle fatigue properties of this alloy (AMCa602) were investigated. S-N properties obtained in both rotating bending and axial loading were compared with each other. It was found that S-N curve in the axial loading appeared a little lower than that in the rotating bending due to the differences of stress distributions and critical volumes for both loading types. Moreover, the statistical aspect on the fatigue property was analyzed as P-S-N characteristics in the rotating bending. After fatigue tests, fracture surfaces of failed specimens were observed by means of a scanning electron microscope (SEM) and the microstructures at the crack initiation site and the propagation path were also observed by combining FIB technique and EBSD analysis. Thus, it was found that some specimens failed from surface inclusions and their fatigue lives were lower in comparison to those of the specimens without surface inclusions. In addition, the fracture surfaces of this alloy revealed very rough in the usual life region, whereas a characteristic smooth area was observed on the fracture surfaces of specimens failed in the surface inclusion-initiated fracture and in very high cycle regime. A stress intensity factor range at the front of the smooth area (ΔKsmooth) tended to a definite value so that the fracture mechanism of this alloy was governed by a concept of ΔK.

High-cycle Fatigue Properties of Alloy718 Base Metal and Electron Beam Welded Joint

High-cycle fatigue properties of Alloy 718 plate and its electron beam (EB) welded joint were investigated at 293K and 77K under uniaxial loading. At 293K, the high-cycle fatigue strength of the EB welded joint with the post heat treatment exhibited somewhat lower values than that of the base metal. The fatigue strengths of both samples basically increased at 77K. However, in longer life region, the EB welded joint fractured from a blow hole formed in the welded zone, resulting in almost the same fatigue strength at 107 cycles as that at 293K.

Low Cycle Fatigue Properties of Alloy 617 base Metal and Weld Joint at Room Temperature

Alloy 617 is the one of the leading candidate materials for intermediate heat exchangers (IHX) of a Very High Temperature Reactor (VHTR). System start-ups and shut-downs as well as power transients will produce low cycle fatigue (LCF) loadings of components. As a series of the work to better understand the LCF properties of Alloy 617 weld joints at high temperature, firstly, in this work, strain-controlled LCF testing of Alloy 617 base metal (BM) and weld joint (WJ) by a gas tungsten arc weld process (GTAW) were carried out. Fully reserved total-strain controlled LCF tests have been conducted at room temperature with four total strain ranges of 1.5, 1.2, 0.9, and 0.6%. For the LCF tests triangular test waveforms with a frequency of 0.25Hz were applied. The present paper is to characterize the LCF properties for Alloy 617 base metal (BM) and weld joint (WJ) from the cyclic stress response behavior and fatigue fracture behavior, with the comparative method. The cyclic stress response behavior was influenced by the level of total strain ranges and the material properties. Though base metal (BM) had shown higher plastic strain accumulation, the observed fatigue life of the weld joint (WJ) is lower than the base metal (BM). Coffin-Manson relationship and energy-life models can be used to determine the fatigue life.

Fatigue properties of alloy 718 overlay-coated with a Co-based X40 alloy by the Micro Spark Coating

Micro Spark Coating (MSC) has been developed as a new functional coating process for Ni-based superalloys used in advanced gas turbines. In this study, some metallurgical and mechanical properties of a MSC layer made of a Co-based wear resisting alloy (X40), and its influence on the high temperature fatigue properties of Ni-based superalloy, Alloy718, were investigated. Prior evaluation of the metallurgical and mechanical properties of the MSC layer that the cavity fraction of MSC layer significantly decreased during the thermal exposure period at 650°C associating with the generation of an oxide phase, progressive sintering and the subsequent increase in hardness and elastic modulus of MSC layer. However, at 480°C these changes were not significant even after 1000hrs exposure. It was found from the high temperature fatigue tests at 480°C and 650°C that the fatigue life of the specimen with MSC layer was almost comparable to that of bare Alloy718 specimen at 480°C, while at 650°C the life of the former was slightly longer than that of the latter. These results suggested that the MSC would have a potential to add a new function to Ni-based superalloy without a reduction in fatigue properties at elevated temperature.