Effects of laser shock processing on stress corrosion cracking susceptibility of AZ31B magnesium alloy

Laser shock processing (LSP) is a new technique for strengthening metals. The effects of LSP on the stress corrosion cracking (SCC) susceptibility of AZ31B magnesium (Mg) alloy were investigated. Water-immersed specimens of AZ31B magnesium alloy were shocked by Q-switched Nd: glass laser with a wavelength of 1064 nm. A fine-grained structure with an average sub-grain size of 5.8 μm was obtained after four laser impacts. Residual stress distribution as a function of depth was assessed by using X-ray diffraction technology. It was observed that with increasing the number of laser impacts, the compressive residual stress near the surface increased. The depth of the compressive residual stress induced by LSP exceeded 0.8 mm from the surface. SCC test in 1 wt.% NaOH solution showed that LSP retarded the SCC initiation and growth in AZ31B Mg alloy.     

K417材料激光冲击强化残余应力松弛预测

针对某型发动机涡轮叶片材料K417激光冲击强化后残余应力在高温下松弛的现象,分析了残余应力松弛的机理,并运用灰色理论建立了温度作用下残余应力松弛的灰色预测模型.该模型的计算结果和实验结果吻合良好,精度较高,为激光冲击强化后残余应力松弛的预测提供一种可行的方法.     

Simulation of residual stress induced by a laser peening process through inverse optimization of material models

Laser peening (LP) is a surface enhancement technique that induces compressive residual stresses in the surface regions of metallic components to increase fatigue life. Simulation of the LP process is a complex task due to the intensity of the pressure loading (order of GPa) in a very short time period (in nanoseconds). A finite element technique is used to predict the residual stresses induced by the LP process. During the LP process, strain rates could reach as high as 10⁶ s⁻¹, which is very high compared to conventional strain rates. A reliable material model is needed to determine the dynamic response of a material. In this work, an optimization-based approach is developed to obtain the material model constants when there is very little or no experimental data of material behavior available. The approach is presented by comparing the residual stress prediction from simulation with available experimental results for Ti–6Al–4V material. To demonstrate the consistency of the approach, LP experiments have been performed at LSP Technologies on Inconel^®718 with different laser power densities, and the residual stress results are compared with the simulation. The Johnson–Cook, the Zerilli–Armstrong, and the Khan–Huang–Liang material models are used during the simulation procedure. The performance of each model is assessed by comparing the residual stress results between simulation and experiments.     

Thermal relaxation of residual stress in laser shock peened Ti–6Al–4V alloy

Laser shock peening (LSP) induced residual stresses in Ti–6Al–4V, and their thermal relaxation due to short-term exposure at elevated temperatures are investigated by an integrated modeling/simulation and experimental approach. A rate and temperature-dependent plasticity model in the form of Johnson–Cook (JC) has been employed to represent the nonlinear constitutive behavior under both LSP and thermal loads. By comparing the simulation results with experimental data, model parameters for Ti–6Al–4V are first calibrated and subsequently applied in analyzing the thermal stability of the residual stress in LSP-treated Ti–6Al–4V. The analysis shows that the magnitude of stress relaxation increases with the increase of applied temperature due to material softening. Most of stress relaxation occurs before 10 min to 20 min exposure in this study, and stress distribution becomes more uniform after thermal exposure. An analytical model based on the Zener–Wert–Avrami formula is then developed based on the simulation results. The activation enthalpy of the relaxation process for laser shock peened Ti–6Al–4V is determined to be in the range of 0.71 eV to 1.37 eV.     

Thermal stress analysis of spiral laser-welded tube

Laser welding of mild steel tube was carried out under the nitrogen assisting gas ambient. Finite element method (FEM) was used to predict the temperature and stress fields in a laser spirally welded tube with 1.6 mm wall thickness and 75 mm diameter. Temperature-dependent thermal and mechanical properties of steel were incorporated in the model. Laser was modeled as a 3D volumetric moving heat source. The residual stresses produced after the completion of the laser welding process were investigated. Moreover, the effect of welding speed on the level of residual stress was studied. It was found that von Mises stress attained high values in the cooling cycle after the solidification of the molten zones. The residual stress developed in the welding region was measured using the XRD technique and results were compared with the predictions. Optical microscopy and SEM were used for metallurgical examination of the welding sites. The numerically predicted residual stress was in good agreement with the XRD results. Increasing the welding speed with constant power resulted in reduction of the width of the high stress zone.     

Transgranular SCC mechanism of thermally treated alloy 600 in alkaline water containing lead

This work aims to understand a SCC failure mode of thermally treated steam generator tubing materials in high temperature water containing lead. The effect of lead contents on the anodic polarization curves of alloy 600 (UNS NO6600) and alloy 690 (UNS NO6900) has been studied in a solution of pH 10 at 200 °C and 315 °C. Lead increased the active peaks of alloy 600 and alloy 690 in mild alkaline water at high temperatures. A reduction of PbO to a metallic lead in alloy 690 is easier than that of alloy 600. When lead was added into the solution, a relative ratio of Cr from among the main metallic elements (Cr, Fe, and Ni) of alloy 600 and alloy 690 decreased in the outer corrosion film. Alloy 690 TT showed a transgranular stress corrosion cracking (TGSCC) in a 10 M NaOH solution with 5000 ppm of lead. Intergranular stress corrosion racking (IGSCC) was observed in the 100 ppm lead condition, and some TGSCC was detected on the fracture surface of the alloy 600 MA cracked in the 10,000 ppm lead solution. IGSCC seemed to be retarded by a crack blunting around the grain boundaries, and the TG cracking mode of the thermally treated alloy 600 and 690 seemed to be related to a crack blunting at the grain boundary carbide and a film dissolution by lead in an alkaline solution.     

Tribological performance of hybrid filtered arc-magnetron coatings. Part II: Tribological properties characterization

Nano-structured coating architectures were developed to provide a best blend of corrosion and wear resistance for high chromium content steels used in aerospace bearing and gear applications. A hybrid filtered arc-magnetron deposition process was employed to deposit functionally graded, multilayered and nanocomposite TiCrN/TiCrCN + TiBC cermet coatings on carburized steel substrates. Coatings exhibited excellent adhesion to the carburized surfaces and had hardness in the range of 23-25 GPa. Tribological properties of the coatings were characterized by: pin-on-disk COF, lubricated sliding, reciprocating sliding, and 3 ball half thrust bearing tests in dry and lubricated environments at high contact stresses. Both polyester and perfluoropolyalkylethers (PFPAE) based lubricants were used to evaluate coating performance with neutral and chemically aggressive lubrication. Sliding friction and reciprocating sliding wear tests were performed using modified disk-on-ring and point-on-disk arrangements, respectively. Contact stresses were estimated using the Hertzian contact formula (sliding friction), and through direct measurements of contact areas by SEM (reciprocating sliding). Low-speed thrust bearing high load rolling contact was evaluated at 350 °C, using Si₃N₄ balls and PFPAE-based lubricant, at contact stresses of ∼ 3.2 GPa. Aggressive corrosion testing was performed on coated samples using MIL-STD-810F “salt-fog” testing. Wear and corrosion behavior was investigated using SEM/EDS, EDX, AFM, profilometry, and optical microscopy. The influence of coating architecture on wear properties was investigated. Multifold improvements in the surface dry and lubricated wear life, reduction of the dry friction coefficient, prevention of corrosion attack from the products of PFPAE lubricant degradation, and improvement of salt-fog corrosion resistance are demonstrated.     

Die Praktische Bedeutung niedrig-frequenter Lastwechsel bei dehnungsinduzierter Spannungsrißkorrosion

Practical importance of low-cycle loads in connection with strain-induced stress corrosion cracking New investigations on stress corrosion cracking have shown that besides stresses the strain rates are essential factors. In general, stress corrosion cracking occurs within critical ranges of strain rate only. From practical point of view, stress corrosion cracking can be divided into two modes with respect to the lower limiting value of strain rate: classical and non-classical mode. In the first case this value is ≤ 0. Thus, stress corrosion cracking occurs at constant load. In the second case the value is > 0. Thus, stress corrosion cracking can only occur within periods of increasing stresses. Theoretical considerations are in keeping with testing results and show that these conditions exist when slow cycle and low amplitudes are present. Aspects to differentiate between stress corrosion cracking and corrosion fatigue are discussed.     

Effects of laser shock processing on electrochemical corrosion resistance of ANSI 304 stainless steel weldments after cavitation erosion

Effects of laser shock processing (LSP) on electrochemical corrosion resistance of weldments after cavitation erosion were investigated by X-ray diffraction (XRD) technology, scanning electron microscope (SEM), roughness tester and optical microscope (OM). Some main factors to influence erosion and corrosion of weldments, residual stresses, surface roughness, grain refinements and slip, were discussed in detail. Results show that LSP impacts can induce compressive residual stresses, decrease surface roughness, refine grains and generate the slip. Thus, the erosion and corrosion resistance with LSP impacts is improved.     

Damage tolerant functionally graded WC-Co/Stainless Steel HVOF coatings

In this paper, effective damage tolerance of a functionally graded coating (FGC) deposited by high velocity oxygen fuel (HVOF) spraying is observed. The thick FGC (≈ 1.2 mm) consists of 6 layers with a stepwise change in composition from 100 vol.% ductile AISI316 stainless steel (bottom layer) to 100 vol.% hard WC-12Co (top layer) deposited onto an AISI316 stainless steel substrate. Damage tolerance is observed via 1) an increase in compliance with depth, and 2) an increase in fracture resistance by containment, arrest and deflection of cracks. A smooth gradation in the composition and hardness through the coating thickness is found by scanning electron microscopy and depth-sensing microindentation, respectively. The in-situ curvature measurement technique reveals that during the deposition of the FGC, compressive stresses exist in the lower, metallic layers owing to peening effect of successive impact, and these gradually evolve to high tensile, in the top layers. Tensile stresses appear to be due to quenching alone; thermal stresses are low because of the gradation. All of this is beneficial for the deposition of a thick coating.The FGC structure shows the ability to reduce cracking with increased compliance in the top layer during static and dynamic normal contact loading, while retaining excellent sliding wear resistance (ball-on-disk tests). Results are discussed in comparison to the behavior and properties of coatings of similar individual compositions and thicknesses, as well as a thick monolithic WC-12Co sprayed coating. Further improvements in the processing are proposed to enhance the adhesion strength and avoid coating delamination under high load contact-fatigue conditions.     

Enhancement of abrasion and corrosion resistance of duplex stainless steel by laser shock peening

The results for laser shock peening of duplex stainless steel (22% Chromium-5% Nickel) using a pulsed Nd:YAG laser (wavelength = 532 nm, pulse width = 8 ns) for the application to high-capacity pumps for reverse-osmosis type seawater desalination plants are reported. By properly selecting the process parameters such as laser intensity of 10 GW/cm², laser pulse density of 75 pulse/mm², and 100 μm thick aluminum foil as a protective coating layer, wear volume and corrosion rate of duplex stainless steel could be reduced by 39% and 74.2%, respectively. The number and size of corrosion pits produced on wear track during copper accelerated acetic acid salt spray test decreased approximately by half as a result of laser shock peening. It is shown that laser shock peening is a practical option to improve abrasion and corrosion properties of a seawater desalination pump parts.     

Micro-abrasive wear of DC and pulsed DC titanium nitride thin films with different levels of film residual stresses

In this work, six specimens with titanium nitride (TiN) thin films and cemented carbide (WC-Co) substrates were analyzed in terms of their micro-abrasive wear behavior. These specimens were obtained from a previous work, in which film depositions were conducted varying parameters such as bias (0, − 50 or − 100 V), type of target power (DC or pulsed DC) and, in the cases where substrate bias was zero, substrate condition (ground or floating). As a result, the level of film residual stresses varied from specimen to specimen, in the range from 4 to 11 GPa (compressive). In this work, micro-abrasive tests were run on these six specimens, using balls of AISI 1010 steel and an abrasive slurry with distilled water and silicon carbide particles with average particle size of 5 μm. Results were analyzed in terms of the wear mechanisms observed at the worn surface and also in terms of the wear resistance, characterized by the wear coefficient (k). Trends indicate a decrease in film wear rate with an increase in the value of film residual compressive stresses, as long as the adhesion was not impaired. Different values of film wear coefficient (k_c) were calculated for specimens obtained with ground and floating voltage substrates, although similar values of film residual stresses were measured in both cases.     

Efficient numerical prediction of residual stress and deformation for large-scale laser shock processing using the eigenstrain methodology

Laser shock processing (LSP) is an effective but costly process for inducing compressive residual stresses and deformation that are primarily applied in the aerospace industry. Accurate modeling of the LSP process with optimization is helpful to reduce development time and cost, but the simulation time is computationally expensive due to the long duration to capture the transient response of the material for each shock. In the present research, the eigenstrain modeling method is developed to predict the effect of large-scale LSP more efficiently compared with previous methods. In the developed eigenstrain-based method, residual stress and deformation fields are analyzed elastically using the simulated eigenstrain as initial strain, which is incorporated into the model by thermal expansion with a predefined unit temperature field and different anisotropic thermal expansion coefficients. For the large-scale LSP application, the eigenstrain in one representative cell identified through an explicit analysis is proposed as an approximation of the actual full eigenstrain field for efficient prediction. The predictions are verified by the predicted results from the explicit/implicit method for laser peening (LP) and the pure explicit method for laser peen forming (LPF) and are also validated by the experimental results of a single LP surface treatment of Ti6Al4V and a LPF bending of 1060 pure aluminum plates. Compared with the previous methods, the eigenstrain modeling method is proved to be effective and much more computationally efficient.     

Stress corrosion cracking of gas pipelines - Effect of surface roughness, orientations and flattening

The primary corrosion mitigation of the external surface of high pressure steel gas pipelines is protective coatings with secondary protection usually by cathodic protection. Adhesion and resistance to cathodic disbondment of the coating is critical for its integrity and grit blasting is an important process in achieving this adhesion. The effect of surface roughness, from grit blasting, on the intergranular stress corrosion cracking resistance of X70 gas pipelines was investigated using slow strain rate testing in carbonate/bicarbonate solution at 75 °C. The effect of orientation of test pieces with respect to the axial direction of pipes was also investigated.Time to failure ratios decreased with increasing surface roughness indicating reduced stress corrosion cracking resistance. The reduced resistance to cracking with increasing roughness would be predominantly associated with stress concentration effects related to the surface roughness resulting from the grit blasting. Crack concentration decreased with increasing roughness, which is likely to be associated with the concentration of surface damage from the grit blasting using varying sized grit. As formed pipe surfaces, with no grit blasting, resulted in some of the lowest time to failure ratios and hence some of the lowest resistances to stress corrosion cracking. These also showed some of the deepest cracks. The influence of roughness and residual stresses on threshold stress is currently being investigated.Time to failure ratios indicated a greater resistance to stress corrosion cracks for circumferentially orientated test pieces compared to those longitudinally orientated. Whilst further testing would be required for confirmation, the current results suggest that flattening the test pieces had only a minor, if any, effect on stress corrosion cracking susceptibility as measured by slow strain rate testing to fracture.     

含应力特性试件的设计制作及湿H2S应力腐蚀实验

为了预测石油化工设备在材料特性、应力特性和环境特性下的应力腐蚀开裂，设计制作了能实现材料特性和应力特性的新型试件，用电测法监控加载，实测相应的应力值，再将含应力特性试件在NACET－1F－9标准实验溶液中进行试验，达到了预期的目的。新型试件结构简单、加载方便，它既能在实验室实验，也能模拟设备的应力水平，随设备一起长期运行。从而可以为设备在实际的应力腐蚀环境中安全运行提供重要的数据，为研究设备的应力腐蚀开裂提供了一种行之有效的方法。     

Ion beam assisted deposition of TiN-Ni nanocomposite coatings

Hard and tough nanocomposite coatings consisting of hard TiN nanograins embedded in a soft metallic intergranular phase of Ni have been produced using ion beam assisted deposition. The chemical composition has been obtained by Rutherford Backscattering and the microstructural properties: phases, grain size, and texture of the coatings have been investigated by X-Ray Diffraction. In the composition range 0-22.5 at.% Ni, δ-TiN is the only crystalline phase and Ni appears as an X Ray amorphous phase. The hardness increases up to a maximum of 41 GPa at ~ 7 at.% Ni which corresponds to a TiN crystallite size of ~ 8 nm and a Ni intergranular phase thickness of roughly 1 monolayer. It is shown that the hardness enhancement in TiN-Ni nanocomposite coatings is not correlated with residual stresses, but rather with the intrinsic properties of the nanostructure. An important improvement in wear resistance is obtained for the coatings exhibiting the highest toughness and not the highest hardness. These results show that ion assisted processing is an effective tool for producing dense TiN-Ni nanocomposite coatings and tailoring their structure and mechanical properties.     

激光冲击处理诱导AZ31B镁合金表面纳米化

根据优化的激光工艺参数,利用激光冲击处理技术在AZ31B镁合金上制备出纳米结构表层,采用X射线衍射仪(XRD)和透射电镜(TEM)表征了AZ31B镁合金激光冲击处理后表面纳米层的微观结构,分析了纳米晶粒内微挛晶的成因,探讨了激光冲击处理诱导AZ31B镁合金晶粒细化的机理。晶粒细化机理归纳如下:在原始晶粒内,位错滑移导致位错缠结,应力集中诱发机械孪生;在亚晶粒和已经细化的晶粒内,继续形成位错缠结和位错胞;位错缠结转变成小角度取向差的亚晶界,细分粗大晶粒成亚晶粒;亚晶界演变成大角度晶界,最终形成等轴状、取向随机分布的纳米晶组织。     

Mechanical property of Fe-base metallic glass coating formed by gas tunnel type plasma spraying

Metallic glass has excellent functions such as high toughness and corrosion resistance. Therefore it is one of the most attractive materials, and many researchers have conducted various developmental research works. However, the metallic glass material is expensive and a composite material is preferred for the industrial application. Thermal spraying method is one of potential candidates to produce metallic glass composites. The gas tunnel type plasma system, which has high energy density and efficiency, is useful for smart plasma processing to obtain high quality ceramic coatings such as alumina (Al₂O₃) and zirconia (ZrO₂) coatings. Also, the gas tunnel type plasma spraying can produce metallic glass coatings. In this study, the Fe-base metallic glass coatings were formed on the stainless-steel substrate by the gas tunnel type plasma spraying, and the microstructure and mechanical property were investigated. The Fe-base metallic glass coatings of about 200 μm in thickness were dense with a Vickers hardness of about Hv = 1100 at plasma current of 300 A. The abrasive wear resistance of Fe-base metallic glass coating was higher than the SUS substrate.     

Introduction of compressive residual stress into stainless steel by employing a cavitating jet in air

In order to eliminate stress corrosion cracking, a method of introduction of compressive residual stress using cavitation impacts was proposed, without use of shots. The cavitation impacts were successfully produced by a cavitating jet in air, without the requirement of a water-filled chamber. The injection condition of the jet was optimized and the introduction of compressive residual stress into stainless steel was demonstrated using the jet. The maximum compressive residual stress introduced by the optimized jet was 500 MPa on the surface, while the thickness of the modified layer was up to 400 μm. A method for estimation of the introduced compressive residual stress by the jet as a function of processing time was proposed, considering the stochastic phenomena of the cavitation impacts. Both the intense impact at 0.2 Hz and relatively weak impact at 4.5 Hz affect the introduction of compressive residual stress. The value of the residual stress and the thickness of the modified layer can be estimated by the proposed experimental equation.     

Understanding the effect of nitrogen in austenitic stainless steel on the intergranular stress corrosion crack growth rate in high temperature pure water

Understanding the effect of nitrogen content on the crack growth rate (CGR) due to intergranular stress corrosion cracking (IGSCC) in high temperature (288 °C) pure water, in non-sensitised and strain-hardened stainless steel (SS) type 304 LN was the focus of this study. Non-sensitised SS containing two different levels of nitrogen (0.08 and 0.16 wt.%) in the solution annealed condition was strain-hardened by cross-rolling at 200 °C (warm rolling). It has earlier been reported that SS with a higher nitrogen level in the warm rolled condition has a higher CGR in high temperature pure water. Tensile testing was carried out using both the SS in the warm rolled as well as in the solution annealed condition at 288 °C. Samples were prepared for transmission electron microscopy (TEM) from the warm rolled SS and from the tensile tested (at 288 °C) specimens. TEM studies indicated that twinning and shear band formation were the major modes of deformation due to rolling at 200 °C and these feature were observed to terminate at grain boundaries, leading to regions of higher strain and stresses at grain boundaries. Higher nitrogen SS has higher grain boundary strain and stresses making the grain boundary regions more susceptible to IGSCC, resulting in higher CGR values. At 288 °C dislocation entanglement and cross-slip were the predominant modes of deformation.