Implementation and development of the incremental hole drilling method for the measurement of residual stress in thermal spray coatings

The experimental measurement of residual stresses originating within thick coatings deposited by thermal spray on solid substrates plays a role of fundamental relevance in the preliminary stages of coating design and process parameters optimization. The hole-drilling method is a versatile and widely used technique for the experimental determination of residual stress in the most superficial layers of a solid body. The consolidated procedure, however, can only be implemented for metallic bulk materials or for homogeneous, linear elastic, and isotropic materials. The main objective of the present investigation was to adapt the experimental method to the measurement of stress fields built up in ceramic coatings/metallic bonding layers structures manufactured by plasma spray deposition. A finite element calculation procedure was implemented to identify the calibration coefficients necessary to take into account the elastic modulus discontinuities that characterize the layered structure through its thickness. Experimental adjustments were then proposed to overcome problems related to the low thermal conductivity of the coatings. The number of calculation steps and experimental drilling steps were finally optimized.     

Characterization and residual stresses of WC-Co thermally sprayed coatings

The present investigation has been conducted in order to determine the residual stresses of an as-ground WC-12Co coating of two different thicknesses, by means of two different methods. Firstly, X-ray diffraction techniques, which allowed the determination of the surface residual stresses of the coating by means of the method called “sin²ψ” method. Secondly, an incremental hole drilling technique together with the integral method, which allowed the analysis of the non-uniform through-thickness residual stresses present in the coatings. It has been determined that the surface residual stresses are of a compressive nature, which could be due to the grinding that was applied to the coatings in order to achieve the desired thicknesses. On the contrary, the results of the incremental hole drilling tests indicated that the through-thickness residual stress distributions are not uniform and are characterized by the presence of tensile peak stresses, at depths in the range of ~ 50-125 μm. Such stresses were observed to decrease towards the coating-substrate interface where the compressive component of the stress state becomes greater than the tensile component. It has been found that the mean residual von Mises stress is higher in the thinner coating than in the thicker one, of approximately 180 and 107 MPa, respectively.     

搅拌摩擦焊接头残余应力的试验

搅拌摩擦焊是 90年代出现的一种新型焊接技术 ,特别适用于熔化焊接性差的铝合金等材料。搅拌摩擦焊接头的纵向残余应力分布具有高应力梯度的特点 ,传统的应变片钻孔法不能满足测量要求。提出了云纹干涉钻孔法测量非均匀分布残余应力的计算公式和试验方法。该方法由云纹干涉法测量钻孔释放的位移条纹 ,通过确定孔边待测区域内三个测量点的条纹值 ,可直接得到该区域内的残余应力。利用该方法测量了铝合金薄板搅拌摩擦焊接头纵向残余应力沿深度和横向的分布 ,其分布规律表现为在搅拌带内为拉应力 ,搅拌带外残余应力的值迅速下降 ,并变为压应力以保持平衡。     

Modeling Residual Stress Development in Thermal Spray Coatings: Current Status and Way Forward

An overview of analytical and numerical methods for prediction of residual stresses in thermal spray coatings is presented. The various sources and mechanisms underlying residual stress development in thermal spray coatings are discussed, then the various difficulties associated with experimental residual stress measurement in thermal spray coatings are highlighted. The various analytical and numerical models used for prediction of residual stresses in thermal spray coatings are thoroughly discussed. While analytical models for prediction of postdeposition thermal mismatch stresses are fully developed, analytical quenching and peening stress models still require extensive development. Various schemes for prediction of residual stresses using the finite element method are identified. The results of the various numerical and analytical models are critically analyzed, and their accuracy and validity, when compared with experiments, are discussed. Issues regarding the accuracy and applicability of the models for predicting residual stresses in thermal spray coatings are highlighted, and several suggestions for future development of the models are given.     

X-ray residual stress measurement in metallic and ceramic plasma sprayed coatings

Processing-induced residual stresses play an important role in the production and performance of thermally sprayed coatings. Their precise determination is a key to influence the coating properties by modification of process variables and to understand the processing-structure-property relationship. Among various methods for residual stress measurement, x-ray diffraction holds a specific position as being non-destructive, phase distinctive, localized, and applicable for real parts. The sin² ω methods is commonly applied for bulk materials as well as coatings. However, the results are often reported without sufficient experimental details and the method is used in its simplified form without justification of certain assumptions. In this investigation, the sin² ω x-ray diffraction method was used to measure residual macrostress in plasma sprayed metallic (nickel, NiCrAlY, and molybdenum) and ceramic (ZrO₂ + 8% Y₂O₃) coatings. Reproducibility of the method was tested and the assumptions allowing its use are discussed and experimentally verified. For nickel coatings, a comparison with hole drilling and neutron diffraction measurements is presented. The influence of processing factors such as deposition temperature and coating thickness is studied and the results are discussed.     

Methods and application of residual stress analysis on thermally sprayed coatings and layer composites

Coating and layer composite manufacturing most commonly involves high temperature gradients and intensive heat transfer between the different composite materials. This can be noticed not only for thermal spraying, but also for other coating techniques. The combination of temperature gradients and materials with different thermophysical properties leads to the formation of thermal stresses in the composite, which are superimposed by stress generating effects during coating solidification, phase transformation or recrystallization. The final state of residual stresses affects the structural and functional properties of the coating as well as the component reliability during operation. Therefore, residual stress analysis is an important tool for the optimization of coatings and layer composite manufacturing processes in order to ensure stability of the processes, adhesion and compatibility of the coating, and finally, the reliability of the components in various technical systems.The most common residual stress measurement techniques are described and compared, with the focus on the incremental hole drilling and milling method. The advantages and disadvantages of the methods are discussed with respect to their application on industrial machine parts. The typical application fields for the different methods are given with respect to the specific measurement principles. The incremental hole drilling method is presented in more detail with application examples that illustrate the suitability of this method for the optimization of thermal spraying processes in industrial layer composite manufacturing by managing the heat and mass transfer in a most appropriate way.     

钻孔法中的残余应力场Ⅰ.理论分析

钻孔法测量残余应力时,由于切削力的作用,会在孔口附近产生加工硬化层。硬化层内材料的特性会发生明显变化,从而影响残余应力的释放。通过增大孔口附近区域材料的弹性模量的方式,将硬化层简化为一个异质圆环,利用弹性力学方法求得了无限大板在双向均布载荷作用下钻孔后的释放应力的解析解。应用有限元软件MSC／Patran ＆ Nastran,对304不锈钢板在二向应力状态下的释放应力进行了数值计算,并与文中的解析解进行了比较。结果表明,将硬化层简化为异质圆环的分析模型是有效的,文中推导的解析解是正确的,钻孔法测量残余应力时考虑硬化层的影响会有助于提高测量精度。     

Residual stress analysis in thermally sprayed layer composites,using the hole milling and drilling method

Residual stresses are related to the thermophysical properties of substrate and coating materials and occur after the coated component has undergone thermal spraying and machining processes. All residual stresses in layer composites result from different individual stress mechanisms occurring during the manufacturing process, mainly based on heat and mass transfer during the coating deposition. Using the hole-milling-and-drilling method, residual stress fields can be measured in a quasi-nondestructive way over the drilling depth with appropriate resolution. In several drilling and milling operations, a cylindrically shaped hole is brought step by step into the component surface. The residual stresses are locally relieved due to material removal, deform the surface around the drilled microhole, and are measured by high-resolution measurement tools (e.g., strain gages (DMS)), for every drilling step in the form of relaxed surface strains. Using calibration curves and material data (E, μ), the measured surface strains are converted into nominal strains at the bottom of the drilled hole for every drilling step. Out of the differentiated strains, in-plane stress fields can be incrementally determined by Hooke’s law. This study describes residual stress measurement features, the finite-element method (FEM) calculation, and the idealization of calibration curves, as well as the results of exemplary stress measurements. The original version of this article was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), May 5–8, 2003, Basil R. Marple and Christian Moreau, Ed., ASM International, 2003.     

可计及温度与层状结构影响的超高温陶瓷基复合材料涂层残余热应力理论表征模型

目的创建可计及温度与层状结构共同影响的超高温陶瓷基复合材料涂层与基体层因热不匹配导致的残余热应力的理论表征模型。方法基于经典的层合板理论与超高温陶瓷基复合材料热物理性能参数对温度的敏感性研究,引入温度和层状结构对涂层与基体层所受残余热应力的影响,形成各层残余热应力温度相关性的理论表征方法,并以ZrB_2-SiC复合材料涂层为例,利用该理论方法系统地研究了各种控制机制对残余热应力的影响及其随温度的演化规律。结果超高温陶瓷基复合材料涂层与基体层所受的残余热应力随着温度的变化而变化,涂层热膨胀系数与基体层热膨胀系数差别越大,变化幅度越大。当涂层材料热膨胀系数大于基体层材料热膨胀系数时,涂层材料遭受残余拉应力,基体层材料遭受残余压应力;随着涂层厚度的增加,涂层所受拉应力减小,而基体层所受压应力增大;当涂层材料热膨胀系数小于基体层材料热膨胀系数时,涂层材料遭受残余压应力,基体层材料遭受残余拉应力;随着涂层厚度的增加,涂层所受压应力减小,而基体层所受拉应力增大。低温下,各层所受残余热应力对层厚与每层材料组成的变化比较敏感,随着温度的升高,敏感性降低。结论对于涂层材料,应设计涂层材料的热膨胀系数小于基体层材料的热膨胀系数,使涂层遭受残余压应力,这不仅能够降低材料表面产生裂纹的危险,同时可以抑制表面已有缺陷的扩展。同时应当设计相对较小的涂层厚度,以增大涂层所受的残余压应力,降低基体层所受的残余拉应力,有效提高整体材料在不同温度下的强度性能。     

超音速等离子喷涂FeCrBSi涂层组织和残余应力分析

采用超音速等离子喷涂技术制备了FeCrBSi涂层,利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)和纳米压痕仪等研究了涂层的微观组织和力学性能。采用X射线应力仪对不同厚度及不同温度退火后涂层的表面残余应力进行测试。结果表明涂层表面的残余应力为拉应力,且随着涂层厚度的增加而增加;对试样进行退火处理可以有效地缓和涂层表面的残余应力,随着温度的升高涂层表面的残余应力不断降低,到260℃左右变为压应力;压应力值随着退火温度的升高而变大,但当温度升高到大约400℃以上时,保持在80 MPa左右。     

Residual Stresses in Thermal Spray Coatings and Their Effect on Interfacial Adhesion: A Review of Recent Work

An overview is presented of the development of residual stresses in thermal spray coatings and their ef-fects on interfacial debonding. The main experimental techniques for measurement of residual stresse are briefly described, with particular attention given to the method of continuous curvature monitoring. Boundary conditions satisfied by all residual stress distributions are identified and expressions derived for the curvatures and stress distributions arising from a uniform misfit strain between coating and sub-strate.It is noted that stress distributions in thick coatings rarely correspond to the imposition of such a uniform misfit strain, so that recourse to numerical methods becomes essential for quantitative predic-tion of stress distributions. Relationships are presented between residual stresses and corresponding strain energy release rates during interfacial debonding. The effect on this of superimposing stresses from an externally applied load is outlined. The initiation of debonding is then considered, covering edge effects and other geometrical considerations. Finally, some specific case histories are briefly outlined to illustrate how the various theoretical concepts involved relate to industrial practice.     

Cross-Sectional Residual Stresses in Thermal Spray Coatings Measured by Moiré Interferometry and Nanoindentation Technique

A plasma-sprayed thermal barrier coating (TBC) was deposited on a stainless steel substrate. The residual stresses were firstly measured by moiré interferometry combined with a cutting relaxation method. The fringe patterns in the cross-section of the specimen clearly demonstrate the deformation caused by the residual stress in thermal spray coatings. However, restricted by the sensitivity of moiré interferometry, there are few fringes in the top coat, and large errors may exist in evaluating the residual stress in the top coat. Then, the nanoindentation technique was used to estimate the residual stresses across the coating thickness. The stress/depth profile shows that the process-induced stresses after thermal spray are compressive in the top coat and a tendency to a more compressive state toward the interface. In addition, the stress gradient in the substrate is nonlinear, and tensile and compressive stresses appear simultaneously for self-equilibrium in the cross-section.     

Correlation between residual stress and abrasive wear of WC–17Co coatings

This investigation had been conducted to determine the influence of residual stresses on the abrasive wear resistance of HVOF thermal spray WC–17 wt.% Co coatings, as well as to derive stress relaxation after cutting by wire electric discharge machining (EDM). The abrasive wear properties of the coatings were characterised using an ASTM-G65 three body abrasive wear machine with silica sand as the abrasive. The residual stress was measured by means of X-ray diffraction techniques, on the coated samples before and after the abrasive wear tests. Compressive residual stresses were observed in the surface layer of the large coated samples. However, stress relaxation results after cutting into small sizes were distinctly different. There was strong correlation between residual stresses in the surface layer and abrasive wear resistance, as well as yield strength of a material.     

Effect of shot peening on residual stresses and crack closure in CVD coated hard metal cutting inserts

Chemical vapour deposited TiCN-Al₂O₃ hard coatings applied on hard metals may remain in a residual tensile stress state after production and sometimes they show a surface crack network. Shot peening treatments are used to introduce compressive residual stresses in the coatings and hard metal. The current work investigates dry shot peening treatments of hard metal coated by TiCN as base layer and α-Al₂O₃ as upper layer by means of finite element modelling. The simulations use experimentally parameterized material models for substrate and coatings. The influence of the coating roughness on plastification and stress development after shot peening is regarded and a number of 2D simulations with various combinations of speed, impact angles, and particle sizes are carried out. Additionally, the effect of compressive residual stresses and associated crack closure is investigated. The study reveals a number of recommendations for effective shot peening in terms of introduced compressive stresses and crack closure. Residual stress depth profiles from the calculations and experiments show good agreement for a chosen set of reasonable process parameters.     

Microstructural Characteristics and Residual Stresses in Arc-Sprayed Cermet Coatings Using Different Carbide Grain Size Fractions

Different studies have emphasized the technological relevance of residual stresses in engineered surfaces, such as thermally sprayed coatings, and their effect on the fracture and fatigue behavior. In arc-sprayed coatings, the microstructural characteristics and resulting residual stresses are determined primarily by the inherent process characteristics and feedstock material. With the scope of this work, a study on the residual stress field in coatings formed by an arc spraying process for different electric parameter settings has been carried out using different wire configurations. Thus, iron-based cored wires with different grain-sized tungsten carbides as filler material were used as feedstock. The coatings are mainly composed of eutectic carbides, and eta carbides such as M₆C, M₁₂C, or M₂₃C and some iron-rich oxide phases, as well as characterized by an inhomogeneous, lamellar microstructure. The results demonstrated that the magnitude of the residual stresses in the coating depends on the carbide grain size fraction used as filling for cored wires. A smaller carbide grain size leads to a more pronounced dissolution of eutectic carbides, resulting in the formation of eta carbides, which in turn is accompanied by decreased tensile residual stresses across the coating. With respect to the spray parameter settings, reduced tensile residual stresses are observed when an increased voltage is applied, which can be attributed to phase evolution phenomena during spraying and thermal effects on the substrate-coating system.     

Through-thickness residual stress evaluations for several industrial thermal spray coatings using a modified layer-removal method

Residual stresses are inherent in thermal spray coatings because the application process involves large temperature gradients in materials with different mechanical properties. In many cases, failure analysis of thermal spray coatings has indicated that residual stresses contribute to reduced service life. An estab-lished method for experimentally evaluating residual stresses involves monitoring deformations in a part as layers of material are removed. Although the method offers several advantages, applications are lim-ited to a single isotropic material and do not include coated materials. This paper describes a modified layer-removal method for evaluating through-thickness residual stress distributions in coated materials. The modification is validated by comparisons with three-dimensional finite-element analysis results. The modified layer-removal method was applied to determine through-thickness residual stress distributions for six industrial thermal spray coatings: stainless steel, aluminum, Ni-5A1, two tungsten carbides, and a ceramic thermal barrier coating. The modified method requires only ordinary resistance strain-gage measuring equipment and can be relatively insensitive to uncertainties in the mechanical properties of the coating material.     

盲孔法测量非均匀残余应力时的释放系数

盲孔法测量沿深度非均匀的残余应力时,积分法是最常用的计算方法,其释放系数矩阵[a],[b]对计算精度有很大影响.由于在标定释放系数矩阵时假定各层上的残余应力均匀分布,当应力梯度变化较大时会引起较大的计算误差.文中首先建立了标定释放系数矩阵的三维有限元模型,并通过模拟均匀残余应力场验证了其有效性.在此基础上分析了积分法标定的释放系数矩阵用于计算沿深度非均匀残余应力时的误差,并提出了二次标定的修正方法.结果表明,采用二次标定的释放系数矩阵计算得到的残余应力值更接近于真实应力.     

Recent advances in the hole drilling method for residual stress measurement

The main activities in the hole drilling residual stress measurement technique recently developed at the University of Pisa are reviewed and presented. Particular attention was paid to developing tools for increasing the limits indicated by the presently applied standard procedures for residual stress evaluation. For residual stresses that were assumed to be uniform through-thickness, the effect of plasticity was numerically analyzed and results formed the basis for a procedure that allows an increase in the maximum measurable residual stress up to 0.9 of the material yield strength. For nonuniform through-thickness residual stress, accurate analytical influence functions are proposed by which arbitrary interpolation of the influence coefficients is avoided and all the experimentally obtained strains, with no regard to their number, can be used as input for residual stress evaluation.     

Residual welding stresses in laser beam and tungsten inert gas weldments of titanium alloy

Abstract

The residual welding stresses in laser beam (LB) and tungsten inert gas (TIG) weldments of a titanium alloy in thin plate form were investigated experimentally in the present work. A hole drilling technique was used to measure the residual stresses in the weldments. The effects of the welding method and post-weld heat treatment (PWHT) on the residual stresses were analysed. The results show that (i) the residual stress distribution in the LB welded joints is similar to that obtained for traditional fusion welding processes, although the distribution zone is much narrower in LB welding, (ii) the residual stress in the heat affected zone for LB welding is about 100 MPa lower than that for TIG welding, and (iii) PWHT in vacuum greatly relieves the welding residual stress.