Alloy Fasteners Research Articles

The Impact of Surface Treatment Processes on the Fatigue Life of Fasteners

Titanium alloy fasteners are extensively used in advanced fields such as aerospace due to their corrosion resistance, high-temperature endurance, low density, and high strength-to-weight ratio. In practical applications, fatigue failure is the primary failure mode for these fasteners. Besides the operational environment, the manufacturing process, especially surface treatment techniques, plays a crucial role in affecting the fatigue life of titanium alloy fasteners. This paper examines the impact of three surface treatment processes-rolled fillet, pulsed anodization, and molybdenum disulfide coating－on the fatigue life of titanium alloy fasteners through orthogonal experiments. The study finds that both rolled fillet and molybdenum disulfide coating significantly influence the fatigue life. This effect is associated with residual stresses, where compressive residual stress initially increases with rolling pressure but subsequently decreases, and reduces as the thickness of the molybdenum disulfide lubricating film rises.

Numerical simulation study on the cold thread rolling forming of Ti-6Al-4V alloy fasteners with external threads and their defects.

Abstract A three-dimensional numerical finite element model for cold rolling forming of external threads on Ti-6Al-4V alloy fasteners for aerospace applications was established using DEFORM-3D. This model was employed to investigate the characteristics of the cold rolling forming process and analyze the defects associated with the rolling operation. The stability of the ratio between torque and tangential load at the radius value of the rolling wheel (72.5) validates the accuracy of the model. The research results indicate that due to the unevenness of the extrusion force, the metal flow velocity is higher adjacent to the tooth-shaped surface compared to the central position. This causes the metal on both sides to accumulate and squeeze towards the middle position at the tooth top, ultimately resulting in tooth top folding defects.

Effects of solution aging treatments on microstructure features, tensile properties and low-cycle fatigue behavior of Ti-6.2Al-4.26Mo-1.33Mn alloy

High-performance titanium alloy fasteners play an important role in the aerospace industry as important general basic parts. This work developed a high-elastic-modulus Ti-6.2Al-4.26Mo-1.33Mn alloy with good balance of strength and plasticity through regulating its primary equiaxed α (αp) phase with solution aging treatment. The influence of αp on the tensile properties and low-cycle fatigue (LCF) behavior of the alloy was investigated systematically, and the crack propagation mechanism of fatigue fracture of titanium alloy was revealed. The results show that with increasing solution temperature, the content of αp phase decreases from 43.6 % to 15.1 % in the range of 825 °C–925 °C, and the LCF life is increased by almost 2 times. However, the size of αp phase is almost unchanged with a diameter of about 2.3 μm. With the increase in solution temperature, the tensile strength of the alloy is improved, while the plasticity decreases gradually. At the solution temperature of 875 °C, the ultimate tensile strength (UTS) of the alloy is 1350 MPa, which is 18.6 % higher than the as-forged alloy, with an elongation (El) of >7 %. Moreover, the fatigue performance is also significantly improved, which is associated with the reduced αp phase that was characterized to act as crack initiations and crack propagation path.

Environmental Scanning Electron Microscopy of copper alloy artefacts from unidentified shipwreck sites: Clues to the identity of shipwrecks on Kenn Reef, Coral Sea

This study investigates the use of Environmental Scanning Electron Microscopes (ESEM) as a tool for identification of historic shipwrecks. Copper alloy fasteners from unidentified shipwreck on Kenn Reef in the Coral Sea, were sampled and analysed to determine the chemical composition of each artefact. When combining this data with known evolution of copper alloying for wooden ship construction, historical and archaeological data, the results provided valuable insight into the possible date range for these wrecks. Specifically, comparison to known wrecks in the vicinity of Kenn Reef could be linked to these previously unidentified sites. Results are promising and ESEM analyses proved to provide valuable, reliable data, yet also highlighted limitations and areas for further investigation and research.

Effect of rolling-texture intensity on fretting damage and subsurface deformation behavior in a high-strength titanium alloy

Fretting damage is common in the high-strength titanium alloy fastener widely used in the aeronautic industry, leading to the failure of fastening fit or the initiation of crack. The titanium alloy fasteners often have typical preferred orientation characteristics (i.e., texture), and this is one of the important factors affecting its performance. However, the investigations on the mechanism of β rolling-texture intensity on fretting damage resistance and subsurface deformation are less addressed. Hence, fretting wear tests were carried out on samples with different rolling texture intensities. The results demonstrate that the samples with quite low (A-10 % sample) and quite high (D-70 % sample) rolling-texture intensity both exhibit excellent fretting wear resistance, but their mechanisms are completely different. Uniformly dispersed grain orientation renders the A-10 % sample with good recovery ability and a positive friction effect during wear. Low stress only concentrating at grain boundaries (GBs) weakens cracks’ initiation and propagation. The unique orientation-layered structure (OLS) leads to excellent recovery ability and a positive friction effect. Crack propagation is inhibited and only propagates along the OLS boundary without a connected trend. However, samples with moderate rolling texture intensity exhibit severe wear. Dislocations are restricted in local areas, so the poor recovery ability makes them have a negative friction effect. Crack propagation driving force continuously increases. Appropriate rolling texture intensity can reduce wear by three times. This study can provide information on the principle for designing fretting damage-resistant alloys.

Optimal selection of protective coatings for stainless steel-titanium alloy fasteners based on corrosion simulation

The Bearing Quick Release Latches (BQRL) are subjected to rainfall and other influences, which lead to galvanic corrosion due to the difference in free corrosion potential between the materials of each component. In order to improve the BQRL in the marine atmospheric environment, their surfaces are treated with a protective coating, therefore, the choice of coating is an important issue. In order to select the optimum coating, this paper develops a numerical model for the corrosion of the BQRL in the marine atmospheric environment based on the physical field of the secondary current distribution in COMSOL Multiphysics®. The electrochemical parameters of the different protective coatings are measured by electrochemical experiments and imported into COMSOL Multiphysics®. By comparing the anode current density and corrosion depth of various models, the best and worst corrosion protection solutions were identified. The current density difference between them is two orders of magnitude. The reasons for the large differences in corrosion rates are analyzed and compared with the results of experiments. This paper hopes to provide a new idea for optimal coating selection through COMSOL Multiphysics®, saving time and economic costs.

Wear- and Surface-Fatigue-Mediated Damage during Fretting in a High-Strength Titanium Alloy

Fretting damage is a key factor restricting the reliability and long life of high-strength titanium alloy fasteners. The fretting damage mechanisms correlated by the interplay relationship between wear damage and surface fatigue damage still need to be addressed. In the present work, fretting damage mechanisms of a high-strength titanium alloy (Ti-15Mo-2.7Nb-3Al-0.25Si, in wt %) were studied in depth. To represent the different matches of strength and toughness, three microstructures with different volume fractions of the primary α phase (A-20%, B-10%, C-1%) were applied to fretting wear tests. The evolution process of fretting wear effected by different microstructures is simulated and quantitatively combined with the sectional inclined angle of scars and the antiwear velocity of wear debris. The results demonstrate that the final worn volume of B-10% is the greatest and the best fretting wear resistance is achieved in A-20%. For wear damage, due to both the poor protective effect of the tribolayer and the fact that the tribolayer is hard to compact, B-10% has the highest wear velocity. For surface fatigue damage, the slip ratio was calculated and used as a bridge to correlate and predict wear damage and fatigue damage. The microcrack damage is most severe in C-1% but slight in A-20%. This study provides valuable experimental data for the prediction of fatigue life for fretting of titanium fasteners, and the quantitative analysis method could be generalized to other alloys.

Machine Learning Approaches to Model Galvanic Corrosion of Coated Al Alloy Systems

Previous studies have shown how galvanic coupling susceptibility between stainless steel 316 or titanium alloy fasteners and coated aluminum alloy 7075-T6 depends on the chosen coating system and environmental factors such as relative humidity (RH) and chloride concentration. In this study, several machine learning models were developed to predict, analyze, and quantify galvanic corrosion arising between relatively noble fasteners and coated aluminum alloy panels. Different independent factors including pretreatment, primer coating, topcoat, RH, chloride concentration, fastener material, fastener quantity, existence of a defect, type of environment, and time of wetness were evaluated for their effect on galvanic coupling lost volume. Artificial neural networks (ANN), random forest regression (RFR), and multiple linear regression (MLR) were used to develop damage functions for galvanic corrosion. ANN, RFR, and MLR models all showed a reasonable fit for lost volume as a function of different inputs.

Design and fabrication of isotropic high-strength Ti-6Al-4V fastener

• A method on industrial preparation of Ti-6Al-4V fasteners was proposed. • The yield strength of present fastener material reaches ∼1100 MPa. • In-situ DIC analysis was conducted to study the mechanical behavior of fastener material. We report a method on high-efficiency preparation of isotropic Ti-6Al-4V fasteners with super high strength. The fabrication process contains repeated rolling, drawing, heading, and heat treatment. The yield strength ( σ 0.2 ) of present fastener material reaches ∼1100 MPa in both axial and radial directions, which is among the highest value of all the Ti alloy fastener materials reported in the literature. The isotropic high strength of present material results from the fine bimodal microstructure and reduced texture. Present method to process Ti-6Al-4V fastener is both cost-effective and readily scalable, and it is therefore expected to be conducive to industrial production.

Improvement in fatigue performance of thin fasteners via electromagnetic strengthening process

The fatigue behaviors of 1.5 mm-thick 2A12-T4 aluminum alloy fasteners processed by the electromagnetic cold expansion (EMCE) method are studied. The fatigue life of the fasteners processed by the EMCE method is 1.77 times that of the conventional direct mandrel cold expansion method at the maximum applied stress of 120 MPa. Finite element simulation shows that the EMCE process can introduce tensile stress in both hoop and radial directions, which avoids warping in thin fasteners. Moreover, the EMCE process achieves uniformly distributed compressive residual stress along the thickness direction, which is also beneficial to the fatigue improvement of the fasteners.

Development of C/SiC Fasteners for High-Temperature Applications

Continuous carbon fiber reinforced silicon carbide (C/SiC) ceramic matrix composites are the candidate materials for high-temperature structural applications owing to their high specific strength, low coefficient of thermal expansion, and moderate thermal conductivity. C/SiC based fasteners provide reliability with oxidation resistance up to 1,650°C for joining such C/SiC structural components used in reusable launch vehicles for space applications. The present study describes the methodology to realize C/SiC based M8 fasteners by isothermal-isobaric chemical vapor infiltration process using 2D-stitched carbon fabric preforms. The realized fasteners exhibited a density of 2.10 g/cm3, a tensile strength of 191 ± 3 MPa, a shear strength of 230 ± 69 MPa in non-threaded and 150 ± 86 MPa in threaded regions of shank at room temperature, a high-temperature tensile strength of 170 ± 12 MPa, and a tensile modulus of 70 ± 8 GPa at 1,100°C in the atmospheric conditions. The tensile failure of the C/SiC bolts was observed to depend upon the relative dimensions of fillet radius and thread root radius, wherein a small fillet radius was observed to be detrimental, as it resulted in failure at the head-shank interface of the bolts. The scanning electron microscope images of tensile tested C/SiC bolts indicate extensive fiber pullout, characteristic of quasi-ductile failure of bolts. It is clearly evident from the studies that the C/SiC fasteners are a promising alternative over the currently used molybdenum alloy fasteners.

Microstructure Evolution Simulation of GH4169 Fasteners in Forging Process

This paper mainly calculated the microstructure evolution characteristics of GH4169 alloy fasteners during hot forging deformation. The constitutive equation and the microstructure evolution models of the material were embedded in the finite element software. By analysis, the grain size and dynamic recrystallization volume fraction were calculated, and the key points on the fastener in forging process were analyzed. As a result, the microstructure evolution of the GH4169 alloy fastener during the hot forging deformation process was obtained. The results show that, through the plastic deformation by the hot forging process, the grain size on the fastener’s head is gradually refined, the minimum average grain size is reduced from the initial 100μm to less than 10μm. During the deformation process, the splinted part of the fastener begins to undergo dynamic recrystallization first. After that, dynamic recrystallization volume fraction gradually increases from the central region to the edge, with the highest value of 60%. These results provide a basic data reference for GH4169 fastener forging process development and parameters optimization.

Degradation behavior of metastable β Ti-15-3 alloy for fastener applications

Environmentally induced degradation is a serious issue governing the use of titanium alloy fasteners for marine applications. The workhorse alloy Ti6Al4V which has been the most used titanium alloy till date as fastener material has its limitations and search has been on for improved titanium-based materials. This paper deals with researches aimed at evaluating the metastable beta titanium alloy Ti15V3Cr3Al3Sn (Ti15-3) in solution annealed condition as a fastener material for medium strength applications. A variety of tests was carried out to rate the resistance of the alloy in this condition to seawater environments slow strain rate testing (SSRT), crevice corrosion testing, potentiodynamic polarization studies, and immersion testing. Limited SSRT was also done on Ti6Al4V for the sake of comparison. Analysis of test results shows that solution annealed Ti15-3 offers itself as a good candidate material for medium strength fastener applications in marine environments.

Study on the effect of laser peening with different power densities on fatigue life of fastener hole

Laser shock processing can improve the fatigue life of fastener holes. The purpose of this study is to investigate the effect of laser power density on fatigue life and fracture characteristics of 7050-T7451 aluminum alloy fastener hole specimens with 6 mm thickness. ABAQUS software is used to analyze the three-dimensional stress distribution. The fatigue test was carried out and the treated specimens were evaluated by scanning electron microscope. The results show that with the increase of laser power density, the fatigue life gain of fastener hole specimens increases firstly, then decreases, and then increases again. The fatigue life gain of the fastener hole specimen is related to the initiation position of the fatigue crack and the direction of the crack propagation. It is speculated that this is closely related to the three-dimensional stress distribution of the fastener holes after laser shock processing in different laser power density.

Computational and Experimental Analyses of Galvanic Corrosion of AA7075-T6 Induced By Dissimilar Metal Fasteners and Mitigated By Sol-Gel Coatings

In modern aircraft, there is a need to protect aluminum structures from galvanic corrosion due to dissimilar metals used in complex aerospace structures. Galvanic corrosion between corrosion resistant fasteners (e.g., stainless steel or titanium alloys) and aluminum alloy structures represent a recurring problem. One means of mitigating such attack is via the application of surface treatments to the fasteners designed to limit their cathodic current density. Detailed measurements and modeling of these effects are important to optimization of the surface treatment and estimates of the level of improvement likely to be achieved. The present work combines experimental measurements of galvanic corrosion damage and computational modeling of galvanic current distributions. The goal is quantitatively connect observations of decreased galvanic corrosion damage on AA7075-T6 coupons due to the presence of stainless steel fasteners under thin electrolyte films as would be present in aerospace service. In particular, sol-gel surface treatments were applied to stainless steel and titanium alloy fasteners which were then placed in AA7075-T6 panels before exposure to salt spray testing. Experimental analyses of corrosion damage were performed using white light interferometry generated depth profiles. These data were used to create data for total volume mass loss (TVML). Computational modeling of galvanic current distributions was performed using a commercial FEM package for thin electrolyte film conditions with experimentally-derived electrochemical kinetics as boundary conditions. The polarization curves used and the current distributions for a 0.1 mm electrolyte layer are shown in Figure 1. Integration of the anodic current densities on the AA7075 over the surface produces a corrosion damage metric, the total anodic current (TAC). The effects of sample geometry, including spacing between fasteners and their location relative to the edges of the sample, as well as water layer thickness and solution concentration on the TAC will be reported. The TVML and TAC can be correlated to one another and to total measured mass loss via Faraday’s Law. Comparisons between the TVML and the TAC will be discussed in detail. Of particular importance will be analyses of the effective throwing power of the fasteners as a function of fastener material and the presence or absence of sol gel surface treatments. Figure 1: (a) Polarization curves in 0.6 M NaCl of the materials of interest. (b) False color plot of anodic (positive) and cathodic (negative) current distribution (A/m2) for a 0.1 mm electrolyte layer of 0.6 M NaCl Acknowledgements: The financial support of the Office of Naval Research through Contract # N68335-16-C-0121 (William Nickerson, PM) via Luna Innovations (Adam Goff) is gratefully acknowledged. Figure 1

Ductility of Titanium Alloy and Stainless Steel Aerospace Fasteners

This paper presents results from tests aimed to assess the relative ductility of titanium alloy Ti 6Al-4V and stainless steel A286 aerospace fasteners of comparable size and tensile strength. A test procedure is developed, and tensile tests are performed on test fasteners. All test fasteners fracture in the threaded region. Elastic and plastic deformation at rupture are extracted from the resulting load versus displacement curves and used to compute the ductility index for each test fastener. The ductility index quantifies the relative ductility between the different fastener materials. The average ductility index for the titanium alloy fasteners is about one-tenth the average value for the A286 fasteners. In addition, the fracture surfaces of the titanium alloy test fasteners fracture perpendicular to the axis of tensile loading, whereas the A286 test fasteners fracture across three or four threads, which corresponds to about a 45° angle. Both the relative ductility index values and fracture surface characteristics indicate much less ductility in the titanium alloy fasteners. These results are not intended to discourage the use of titanium alloy fasteners but rather to provide additional data for use in proper joint design when the benefits of lower weight or extreme temperature use are required.

Fatigue Crack Propagation Life Prediction of 2A12 Aluminum Alloy Based on VCCT

The fatigue crack propagation behavior of the prior corrosion 2A12 aluminum alloy fastener involving a central hole was investigated. The virtual crack closure technique (VCCT), is straightforward and not sensitive to the FEA mesh size, was carried out to calculate strain energy release rate and SIFs of AA 2A12 under different stress levels and corrosion years. Based on the VCCT, the simulation carried out to analyze the corrosion fatigue crack growth behavior. It was proved to be convenient to simulate the crack propagation life and the predicted crack growth curve was in good agreement with the experimental results.

Design and behavior of recentering beam-to-CFT column connections with super-elastic shape memory alloy fasteners

This study investigates the performance of new composite (steel-concrete) moment connections through numerical simulations. The innovative aspects of this research lay in the use of end-plate connections between steel beams and concrete-filled tube (CFT) columns that utilize a combination of low-carbon steel and shape memory alloy (SMA) components. In these new connections, the intent is to use the recentering effect provided by super-elastic SMA tension bars to reduce the level of building damage and residual drift after a major earthquake. The low-carbon steel components provide excellent energy dissipation. The analysis and design of these structures is complicated because the connections cannot be modeled as being simply pins or full fixity ones; they are partial restraint (PR). A refined finite element (FE) model with sophisticated three dimensional (3D) solid elements was developed to perform numerical experiments on the PR-CFT joints with a view to obtaining the global behavior of the connection. Based on the behavioral information obtained from these FE tests, simplified connection models were formulated using 2D joint elements with nonlinear spring components. The behavior of entire connections under cyclic loads was examined. The results were compared with the connection behavior obtained from the 3D FE simulations and corresponding connection tests. Good agreement was found between the simple and sophisticated models, confirming the robustness of the approach.

Numerical investigation on the cyclic behavior of smart recentering clip-angle connections with superelastic shape memory alloy fasteners

Superelastic shape memory alloy materials have become increasingly prevalent for recentering devices that have the ability to recover their plastic deformation automatically. For this reason, this study proposed new clip-angle connections incorporating superelastic shape memory alloy bolts. Including component spring models, mechanical joint models of steel bolted connections and shape memory alloy bolted connections are created for numerically simulating their cyclic behavior. The numerical analysis results are then compared to each other in terms of ultimate strength, energy dissipation, and permanent deformation. In particular, over 60% of the total displacement was recovered during unloads in case of shape memory alloy bolted connections, indicating that the proposed smart connections display obvious recentering features in their behaviors.

Development of Hardening and Tempering Cycle for High Strength Low Alloy Fastener Grade Steel

Medium carbon low alloy Ni-Cr-Mo steel is used in the fabrication of aerospace fasteners. It finds application in different heat treated conditions to meet the desired strength level. The alloy was realized through double melting route. Heat Treatment studies have been carried out by following different tempering temperatures to obtain varying strength levels ranging from 1200MPa to 1400MPa. Microstructural analysis has been carried out to find out reasons for variation in mechanical properties. Tempering cycle has been suggested to obtain fully tempered martensitic structure. This paper presents the different hardening and tempering cycles studied to obtain the desired strength level for the intended application.