Influência da anodização sulfúrica na resistência à fadiga da liga de alumínio 7175-T74

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Influence of shot peening and hard chromium electroplating on the fatigue strength of 7050-T7451 aluminum alloy

Chromium electrodeposition is a technique for the production of functional coatings on engineering components. These coatings are extensively micro-cracked and present high level of hardness, resistance to corrosion and wear and low coefficient of friction. In this paper the shot peening influence on the fatigue strength of aluminum 7050-T7451 alloy chromium electroplated, was investigated.The shot peening process was carried out to create residual stresses using ceramic and glass shots. A hard chromium electroplated coating of 100 mu m thickness was performed on the base material and the shot peened base material surfaces. S-N curves were obtained in axial and bending fatigue tests and compared with the 7050-T7451 aluminum alloy. In order to study the influence of residual stresses on fatigue life, the behavior of compressive residual stress field was measured by an X-ray tensometry.An increase in the axial fatigue strength of 25% and 50% of ceramic and glass shots, respectively, was observed. The lower performance in fatigue life for ceramic-shot peening may be attributed to higher surface damage, as a consequence of the overpeening intensity performed. However, in bending fatigue the behavior was practically equivalent for both processes. Fracture surface analysis by scanning electron microscopy was used to observe crack origin sites from shot peened and chromium electroplated samples. (C) 2006 Elsevier Ltd. All rights reserved.

Influence of anodization on the fatigue strength of 7050-T7451 aluminium alloy

In recent years, with higher demand for improved quality and corrosion resistance, recovered substrates have been extensively used. Consequently residual stresses originated from these coatings reduce the fatigue strength of a component. Due to this negative influence occasioned by corrosion resistance protective coatings, an effective process like shot peening must be considered to improve the fatigue strength. The shot peening treatment pushes the crack sources beneath the surface in most of medium and high cycle cases due to the compressive residual stress field (CRSF) induced. The aim of this study was to evaluate the influence on the fatigue life of anodic films grown on 7050-T7451 aluminium alloy by sulphuric acid anodizing, chromic acid anodizing and hard anodizing. The influence on the rotating and reverse bending fatigue strength of anodic films grown on the aluminium alloy is to degrade the stress life fatigue performance of the base material.A consistent gain in fatigue life in relation to the base material was obtained through the shot peening process in coated specimens, associated to a residual stress field compressive near the surface, useful to avoid fatigue crack nucleation and delay or even stop crack propagation.

Effect of electroless nickel interlayer on the fatigue strength of chromium electroplated AISI 4340 steel

Deposition of wear-resistant hard chromium plating leads to a decrease in the fatigue strength of the base material. Despite the effective protection against wear and corrosion, fatigue life and environmental requirements result in pressure to identify alternatives or to improve conventional chromium electroplating mechanical characteristics. An interesting, environmentally safer and cleaner alternative for the replacement of hard chronic plating is tungsten carbide thermal spray coating, applied by high velocity oxyfuel (HVOF) process.To improve the fatigue strength of aeronautical steel chromium electroplated, shot peening is a successfully used method. Multiple lacer systems of coatings are considered to have larger resistance to crack propagation in comparison with simple layer.The aim of this study was to analyze the effect of nickel underplate on the fatigue strength of hard chromium plated AISI 4340 steel in two mechanical conditions: HRc 39 and HRc 52.Rotating bending fatigue tests results indicate that the clectroless nickel plating underlayer is responsible for the increase in fatigue strength of AISI 4340 steel chromium electroplated. This behavior may be attributed to the largest toughness/ductility and compressive residual stresses which, probably, arrested or delayed the inicrocrack propagation from the hard chromium external layer. The compressive residual stress field (CRSF) induced by the electroplating process was determined by X-ray diffraction method. The evolution of fatigue strength compressive residual stress field CRSF and crack sources are discussed and analyzed by SEM. (c) 2006 Elsevier Ltd. All rights reserved.

Effects of surface treatments on the fatigue strength of AISI 4340 aeronautical steel

Internal residual stresses significantly influence the fatigue strength of coated materials. It is well known that chromium plating is the most used electrodeposited coating for important industrial applications. However, pressure to identify alternatives or to improve the chromium electroplating process have increased in recent years, related to the reduction in fatigue strength of the base material and to environmental requirements. The high efficiency and fluoride free hard chromium electroplating there called accelerated) is an improvement to the conventional process. One environmentally safer and cleaner alternative to hard chromium plating is tungsten carbide thermal spray coating applied by the High Velocity Oxy-Fuel (HVOF) process. To increase the fatigue strength of chromium plated materials, coating thickness and microcracks density are important parameters to be controlled. Techniques as compressive residual stresses induced by shot peening and multilayers, are also used. The aim of this study was to analyse the effects on AISI 4340 steel, in the rotating bending fatigue behaviour, of the: tungsten carbide thermal spray coating applied by HP/HVOF process; chemical nickel underplate, and shot peening process applied before coating deposition, in comparison to hard chromium electroplatings. Rotating bending fatigue test results indicate better performance for the conventional hard chromium plating in relation to the accelerated hard chromium electroplating. Tungsten carbide thermal spray coating and accelerated hard chromium plate over nickel resulted in higher fatigue strength when compared to samples conventional or accelerated hard chromium plated. Shot peening showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel hard chromium electroplated. (C) 2001 Elsevier B.V. Ltd. All rights reserved.

Evaluation of WC-17Co and WC-10Co-4Cr thermal spray coatings by HVOF on the fatigue and corrosion strength of AISI 4340 steel

It is known that chromium electroplating is related to the reduction in the fatigue strength of base metal. However, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. Environmental requirements are an important point to be considered in the search for possible alternatives to hard chrome plating. Aircraft landing gear manufactures are considering WC thermal spray coating applied by the high-velocity oxygen-fuel (HVOF) process an alternative candidate, which shows performance at least comparable to results, obtained for hard chrome plating. The aim of this study is to compare the influence of WC-17Co and WC-10Co-4Cr coatings applied by HVOF process and hard chromium electroplating on the fatigue strength of AISI 4340 steel, with and without shot peening. S-N curves were obtained in axial fatigue test for base material, chromium plated and tungsten carbide coated specimens. Tungsten carbide thermal spray coating results in higher fatigue strength when compared to hard chromium electroplated. Shot peening prior to thermal spraying showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel. Experimental data showed higher axial fatigue and corrosion resistance in salt fog exposure for samples WC-10Co-4Cr HVOF coated when compared with WC-17Co. Fracture surface analysis by scanning electron microscopy (SEM) indicated the existence of a uniform coverage of nearly all substrates. (C) 2004 Elsevier B.V. All rights reserved.

Residual stress influence on fatigue lifetimes of electroplated AISI 4340 high strength steel

Residual stresses play an important role in the fatigue lives of structural engineering components. In the case of near surface tensile residual stresses, the initiation and propagation phases of fatigue process are accelerated; on the other hand, compressive residual stresses close to the surface may increase fatigue life. In both decorative and functional applications, chromium electroplating results in excellent wear and corrosion resistance. However, it is well known that it reduces the fatigue strength of a component. This is due to high tensile internal stresses and microcrack density. Efforts to improve hard chromium properties have increased in recent years. In this study, the effect of a nickel layer sulphamate process, as simple layer and interlayer, on fatigue strength of hard chromium electroplated AISI 4340 steel hardness - HRc 53, was analysed. The analysis was performed by rotating bending fatigue tests on AISI 4340 steel specimens with the following experimental groups: base material, hard chromium electroplated, sulphamate nickel electroplated, sulphamate nickel interlayer on hard chromium electroplated and electroless nickel interlayer on hard chromium electroplated. Results showed a decrease in fatigue strength in coated specimens and that both nickel plating interlayers were responsible for the increase in fatigue life of AISI 4340 chromium electroplated steel. The shot peening pre-treatment was efficient in reducing fatigue loss in the alternatives studied.

Fatigue strength of HVOF sprayed Cr(3)C(2)-25NiCr and WC-10Ni on AISI 4340 steel

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Improvement in the fatigue strength of chromium electroplated AISI 4340 steel by shot peening

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

The surface treatment influence on the fatigue crack propagation of Al 7050-T7451 alloy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Repair Welding Effects on the Bending Fatigue Strength of AISI 4130 Aeronautical Steel used in a Critical to the Flight-Safety Structure

The aim of this study was to analyze the effect of successive TIG (tungsten inert gas) welding repairs on the reverse bending fatigue strength of AISI 4130 steel, which is widely used in components critical to the flight-safety. In order to simulate the abrupt maneuvers, wind bursts, motor vibration and helixes efforts, which generate cyclic bending loadings at the welded joints of a specific aircraft component called motor cradle, experimental reverse bending fatigue tests were carried out on specimens made from hot-rolled steel plate, 1.10 mm (0.043 in) thick, by mean of a SCHENK PWS equipment, with load ratio R = -1, under constant amplitude, at 30 Hz frequency and room temperature. It was observed that the bending fatigue strength decreases after the TIG (Tungsten Inert Gas) welding process application on AISI 4130 steel, with subsequent decrease due to re-welding sequence as well. Microstructural analyses and microhardness measurements on the base material, heat-affected zone (HAZ) and weld metal, as well as the effects of the weld bead geometry on the obtained results, have complemented this study.

Effect of WC-10%Co-4%Cr coating on the Ti-6Al-4V alloy fatigue strength

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Evaluation of WC-10Ni thermal spray coating with shot peening on the fatigue strength of AISI 4340 steel

Fatigue failure is a result of a crack initiation and propagation, in consequence of a cyclical load. In aeronautical components as landing gear the fatigue strength is an important parameter to be considered in project, as well as the corrosion and wear resistance.The thermal sprayed HVOF technology it's normally used to protect components against wear and corrosion, and are being considerate an alternative to replace chromium by the aeronautical industry. With respect to fatigue life, the HVOF technique induces residual stress on the interface. In the case of tensile residual stresses, the initiation and propagation phases of fatigue process are accelerated; on the other hand, compressive residual stresses close to the surface may increase fatigue life. The technique to improve the coated materials fatigue strength is the shot peening process, which induces residual stress in the surface in order to delay the nucleation and propagation process.The aim of present study is to compare the influence of WC-10 Ni coating applied by HVOF on the fatigue strength of AISI 4340 steel, with and without shot peening. S-N curves were obtained in axial fatigue tests for material base, and tungsten carbide coated specimens. (C) 2010 Published by Elsevier Ltd.

Fatigue fracture behavior of Ti-6Al-4V PVD coated

Fatigue, corrosion and wear resistance are important parameters in aircraft components development as landing gear. High strength/weight ratio and effective corrosion resistance make of titanium alloys an alternative choice to replace steel and aluminum alloys. However, titanium alloys have poor tribological properties, which reduce devices performance under friction. PVD coatings tribological systems has been increased due to their attractive mechanical properties as low environmental impact, low friction coefficient, low wear rate and hardness up to 2000 HV.In this study the influence of TiN deposited by PVD on the fatigue strength of Ti-6Al-4V alloy was evaluated. Comparison of fatigue strength of coated specimens and base material shows also a decrease when parts are coated. It was observed that the influence is more significant in high cycle fatigue tests. Scanning electron microscopy technique (SEM) was used to observe crack origin sites and fracture features. (C) 2010 Published by Elsevier Ltd.

Evaluation of the fretting fatigue behaviour of commercially pure titanium

Fretting fatigue occurs when the contact surfaces of two components undergo small oscillatory movement while they are subjected to a clamping force. A cyclic external load gives rise to the early initiation of fatigue cracks, thus reducing their service life. In this paper, the fretting fatigue behaviour of commercially pure titanium flat samples (1.5 mm thick) is evaluated. A fretting device composed of a frame, load cell, and two screw-mounted cylindrical fretting pads with convex extremities was built and set to a servo-hydraulic testing machine. The fatigue tests were conducted under load control at a frequency of 10 Hz and stress ratio R = 0.1, with various contact load values applied to the fretting pads. Additional tests under inert environment allowed assessing the role of oxidation on the wear debris formation. The fracture surfaces and fretting scars were analysed via scanning electron microscopy in order to evaluate the surface damage evolution and its effect on the fatigue crack features. The effect of the fretting condition on the S-N curve of the material in the range of 10(4)-10(6) cycles is described. Fatigue crack growth calculations allowed estimating the crack initiation and propagation lives under fretting conditions. The effect of the fretting condition in fatigue life is stronger for the lower values of cyclic stress and does not seem to depend on the contact loading value.