Oberflächenbehandlung nach spanender Formgebung durch ein einfaches Zusatzverfahren

A supplementary mechanical treatment to strengthen surfaces after machining In this work, a surface treatment is presented in which a hard metal tool insert with rounded edges is pressed on a machined surface at low speed and optimized pressure. This treatment can be executed on the same lathe used previously for machining. The effects on surface topography and surface layer properties (microhardness and X-ray peak halfwidth profiles, as well as residual stresses) have been quantitatively determined. In copper specimens, the surface treatment leads to compressive residual stresses and to strong surface layer deformation and, thus to an increase of near-surface hardness. This effect decreases the amplitude of plastic deformation in the surface layer during fatigue and results in a 25% increase of the fatigue strength at 10⁷ cycles. In Al-2024 T6, the surface treatment causes high compressive residual stresses, an increase in superficial hardness and smoothing of the surface topography. This results in an improvement of the fatigue strength at 10⁷ cycles of 48%.     

Influence of turning parameters on the high‐temperature fatigue performance of Inconel 718 superalloy

The safety‐critical rotating parts of aircraft engines are mainly designed using experimental material data, based on standard specimens and procedures, while few data are available on the effect of manufacturing anomalies on fatigue life. In this context, the paper investigates the effects of different machining parameters on the high‐temperature fatigue resistance of Inconel 718 superalloy specimens, cut from engine disk forgings, machined by turning on a vertical lathe. An unconventional specimen was designed in order to have the machining marks aligned with the fatigue loading axis, so to reproduce the hoop stresses in engine disks. For the test campaign, three machining parameters were chosen (depth of cut, cutting speed and insert wear) that typically may generate non‐geometrical anomalies. A correlation has been found between the machining parameters, the residual stresses, the surface roughness, and the distorted and amorphous layer thicknesses. Correlations of such data with fatigue life are also presented and discussed.     

Ensuring Fatigue Life of Parts During Finish Turning with Cubic Boron Nitride Tools

The paper addresses a relationship between the surface state of a steel 40Kh part upon finish turning with cBN tools and its fatigue life. The authors are the first to put forward a mathematical model that shows the dependence of fatigue life of a machined part on the machining conditions in turning using cBN tools. This study has demonstrated the possibility of finish turning of essential parts that have to work under loading of variable magnitudes and directions.     

THE INFLUENCE OF MACHINING DEFECTS AND INCLUSIONS ON THE FATIGUE PROPERTIES OF A HARDENED SPRING STEEL

Abstract In hardened steel with high tensile strengths the fatigue strength is often limited by defects, either surface defects or interior defects. In this work the influence of different surface conditions on the fatigue strength of a hardened spring steel has been studied. The fatigue limit of hour-glass shaped specimens with turned, ground or polished surface was found using load-controlled testing combined with the stair-case method. A model based on a statistical treatment of the machining defects (surface) and the inclusions (interior) combined with fracture mechanics has been used to predict the fatigue limit. It was intended to consider the total effect of surface defects and interior defects. The model also makes it possible to predict the failure mode, i.e. surface-defect initiated failures or inclusion-initiated failures.     

EVALUATION OF LOW CYCLE FATIGUE UNDER NON-PROPORTIONAL LOADING

Abstract— A series of low cycle fatigue experiments have been conducted on a 42CrMo steel under tension-torsion loading. Thin-walled tube specimens were used. Low cycle fatigue under various loading paths, including circular and square paths, have been investigated.
The plastic work criterion for low cycle fatigue failure has previously been generally accepted, but it is difficult to calculate stress and strain for complex loading paths, especially for non-proportional loading. This present study suggests a simple method for the calculation of the stable cyclic stress and strain values based on a Modified Endochronic Constitutive Theory (MECT) that redefines an intrinsic time scale. The loading path effect under non-proportional loading is also considered when evaluating fatigue life.
The results show that the plastic work approach using the MECT method in multiaxial fatigue calculations correlates reasonably well the data and is a reflection of loading path dependence.     

Investigating the effect of machining processes on the mechanical behavior of composite plates with circular holes

The influence of the machining quality on the mechanical behavior of CFRP composites is yet not fully understood. There are only few works in the literature that have investigated the effect of the machining quality on CFRP. In fact, most of these works focus only on conventional machining such as axial or orbital drilling. The aim of this paper is to examine the influence of two machining processes namely conventional machining (CM) and abrasive water jet machining (AWJM) on the mechanical behavior of composite plates under cyclic loading. For this purpose, an experimental study using several composite plates drilled with a cutting tool and an abrasive water jet machining was carried out. In order to study the impact of the process of machining on the mechanical behavior, thermographic infrared testing and fatigue cyclic tests were performed to assess temperature evolutions, stiffness degradation, and the damage evolution in these plates. Fatigue testing results have shown that the damage accumulation in specimens drilled with CM process was higher than the AWJM specimens. Furthermore, the endurance limit for a composite plate drilled with CM was approximately 10% inferior compared to specimens drilled with AWJM. This difference can be related to the initial surface integrity after machining induced by the difference in the mechanism of material’s removal between the two processes used.     

Influence of shot peening on high-temperature corrosion and corrosion-fatigue of nickel based superalloy 720Li

High-temperature corrosion fatigue, a combination of corrosion with a fatigue cycle, is an emerging generic issue affecting power generation and aero gas turbine engines and has the potential to limit component life. Historically, surface treatments, such as shot peening have been used to improve component life and have been optimised for fatigue response. Research into optimisation of shot peening techniques for hot corrosion and high-temperature corrosion fatigue has shown 6–8A 230H 200% coverage to provide overall optimum performance for nickel-based superalloy 720Li based on the limited data within this study. Utilisation of electron backscatter diffraction techniques, in combination with detailed assessment of corrosion products have been undertaken as part of this work. The resultant cold-work visualisation technique provides a novel method of determining the variation in material properties due to the shot peening process and the interaction with hot corrosion. Through this work it has been shown that all three shot peening outputs must be considered to minimise the effect of corrosion fatigue, the cold work, residual stress and surface roughness. Further opportunity for optimisation has also been identified based on this work.     

The effect of cutting speed and heat treatment on the fatigue life of Grade 5 and Grade 23 Ti–6Al–4V alloys

High speed machining is a necessary manufacturing method for ensuring productivity and profitability. However, research has demonstrated that the high speed machining process impairs the surface characteristics of materials such as Ti–6Al–4V including surface roughness and subsurface microstructural damage. Therefore, there is concern that high speed machining detrimentally influences the fatigue properties of Ti–6Al–4V components. This paper investigates the effect of cutting speed on the surface integrity and fatigue properties of Ti–6Al–4V (ASTM Grade 5) and Ti–6Al–4V ELI (ASTM Grade 23) alloys in the beta annealed and mill annealed heat treated conditions. It was found that the surface roughness and fatigue properties are not significantly influenced by cutting speed, however, the microstructure substantially influences the properties.     

Setup and fixture planning in automated process planning systems

This research deals with the problems of setup and fixture planning for the machining of prismatic parts. The overall aim is to design the minimum number of setups that can be fixtured and machined using the available fixture elements and tools on the shop-floor. Setup planning takes into account machining sequence constraints, machine tools, as well as the feasibility of fixturing. A general scheme for search strategies in such planning is developed and implemented. The greater part of this work is aimed towards the development of an automatic fixture planner. A generalized representation scheme for a variety of fixture elements using geometric as well as functional properties is developed. A methodology is described to build up assemblies of fixture elements complete with the workpiece. The proposed approach has been implemented as part of an existing automated process planning system called the Quick Turnaround Cell.     

Automated tool-wear monitoring and tool changing using intelligent supervisory control†

Process performance is very dependent upon the unique characteristics of the process equipment and the work/tooling interactions. To address this problem, research has been conducted to explore the feasibility of using an intelligent supervisory controller to build a local process database and provide control for the process. The research concerns the milling process as used in a machining centre. This paper will report on the data analysis, proposed control approaches, and how these results relate to the intelligent supervisory controller     

Heat treatment,surface roughness and corrosion effects on the damage mechanism of mechanical components in the very high cycle fatigue regime

Many engineering components are subjected to combined torsion and axial loading in their working conditions, and the cyclic combined loading can result in fatigue fracture after a very long life fatigue regime. The present investigation extends over a wider range of test conditions involving surface treatment and manufacturing effects such as machining, so as to understand the fatigue properties and damage mechanisms of the material beyond 10⁹ cycles.This work reviews the effect of surface conditions on the fatigue behaviour of mechanical components in the gigacycle regime. Evidently, surface conditions can be variable and are due to very different reasons such as manufacturing effects like machining or final surface processes on the parts, heat treatment before and after manufacturing or environmental conditions like corrosion. In fact, this is a detailed comparative study based on the results of experiments carried out by our research team working in this domain. For this reason, it reveals a continuous decrease of the fatigue strength in the VHCF domain for the investigated materials under different surface conditions as important information for design engineers.Experimental investigation on the test specimens was performed at a frequency of 20 kHz with different stress ratios varying between R = ?1 and R = 0.7 at room temperature. All of the fatigue tests were carried out up to 10¹⁰ cycles. The damage mechanism was evaluated by Scanning Electron Microscopy (SEM).     

Polycrystal modelling of fatigue: Pre-hardening and surface roughness effects on damage initiation for 304L stainless steel

The 304L stainless steel is a major component of residual heat removal circuits of pressurized water reactors (PWRs). The main purpose of this study is to understand the risk of thermal fatigue damage resulting from the machining of the 304L steel pipes inner surface (pre-hardening gradient, residual stresses and scratches), at the scale of the microstructure. This work is based on previous results obtained for pipe specimens thanks to a macroscopic elasto-visco-plastic model. Applied to the pipe specimens, this modelling showed that a thermal loading with temperature gradient, induced a cyclic non-linear biaxial loading at the inner surface of the pipe. In this paper, a polycrystal plasticity model, implemented in a Finite Element (FE) code, is adapted to cyclic loading. An elementary volume (3D aggregate), representing the inner surface and sub-surface of the 304L steel tube, is built from successive polishings and orientation mappings thanks to an Electron Back Scattering Diffraction method. At the grain scale, the polycrystal model is used as a “numerical microscope” to compute the local mechanical fields. Different fatigue criteria are tested to determine their sensitivity to surface properties (roughness, residual stress and pre-hardening) and to the microstructure of the material (crystallographic orientation and grain size). Pre-hardening leads to a lower and more homogeneous distribution of local strain amplitudes in the aggregate, but slightly higher stresses when compared to initial material without hardening. By contrast, surface roughness leads to large localized strain and stress fields in grains located at the bottom of scratches. To determine the surface micro-structural “hot spots” features and to test the sensitivity of different surface conditions, three different fatigue criteria (Manson-Coffin, Fatemi–Socie and Dissipated Energy criteria) have been computed. We point out that the pre-hardening may have a complex effect on fatigue resistance, since it reduces local plastic strain amplitudes, but increases local stresses. Moreover, the pre-hardening has a positive effect on fatigue since it delays damage initiation. By contrast, the surface roughness leads to a negative effect. However, we have shown that the three different fatigue criteria do not deliver similar quantitative predictions. Relevant criteria for high cycle fatigue, such as stress based criteria, are not considered in this paper, since the thermal loading used for computation is large enough to reduce cyclic plastic strain straining within all grains of 304L pipe inner surface for midlife of experiments.     

Influence of microstructural defects and the surface topography on the fatigue behavior of “additively-subtractively” manufactured specimens made of AISI 316L

As additive manufacturing offers only low surface quality, a subsequent machining of functional and highly loaded areas is required. Thus, a sound knowledge of the interrelation between the additive and subtractive manufacturing process as well as the resulting mechanical properties is indispensable. In this work, specimens were manufactured by using laser-based powder bed fusion (L-PBF) with substantially different sets of process parameters as well as subsequent grinding (G) or milling (M). Despite the substantially different surface topographies, the fatigue tests revealed only a slight influence of the subtractive manufacturing on the fatigue behavior, whereas the different laser-based powder bed fusion process parameters led to pronounced changes in fatigue strength. In contrast, a significant influence of subtractive finishing on the fatigue properties of the defect-free continuously cast (CC) reference specimens was observed. This can be explained by a dominating influence of process-induced defects in laser-based powder bed fusion material, which overruled the influence of surface machining. However, although both laser-based powder bed fusion parameter sets resulted in substantial defects, one set yielded similar fatigue strength compared to continuously cast specimens.     

Influence of the build orientation on the fatigue strength of EOS maraging steel produced by additive metal machine

This paper presents a research dealing with the dependence of the fatigue strength of maraging steel parts, manufactured by direct selective laser sintering, on the production build orientation. Three sets of specimens have been manufactured according to the ISO 1143 Standard (2010) by EOSINT M280 additive manufacturing machine, with the following heat and mechanical treatments, in agreement with the recommendations by the material manufacturer and current literature. The expected outcomes are the Fatigue Limit values of the material and the maximum number of cycles observed at different stress levels for three different build orientations (three different angles, 0°, 45° and 90°, between the build direction and the longitudinal axis of the samples). The results have been processed and compared by statistical methods in order to determine the fatigue curves in the finite life domain and the fatigue limits, along with their confidence bands and intervals, and to investigate the significance of the build orientation factor.     

Conducting the failure examination

Conclusion Performing failure analyses is an exceptionally challenging and exciting job. The investigator must have a sound background in the areas that are central to testing and analytical work and must also be knowledgeable enough to know when to call on other fields of specialization that interface with the work. New problems are always encountered that tax the ingenuity of the investigator, and they will sharpen and expand ones skills if we are willing to go on learning. By the same token, analysis of failures by many investigators in the ferrous and nonferrous metal industries here and abroad has, over the years, expanded the practical knowledge for processing and product developments and also contributed to basic scientific insights. Those who frequently conduct failure examinations agree on several main points. Most failures could easily be prevented if certain well-documented precautions are taken. Many failures occur as a result of fatigue, and one or more of the following may play a decisive role: original design deficiencies, material inadequacies, machining or fabrication, imperfections, abuse or neglect, improper maintenance, improper repairs, and metallurgical factors such as decarburization. Heat-treatment irregularities are another major contributing factor in failures.     

基于有限元分析的高精密柔性夹具优化方案设计

针对高精密加工过程中柔性夹具的变形问题进行理论计算和实验分析.首先应用CAE软件对现有柔性夹具进行有限元分析,分析夹具变形问题.然后通过CAD软件改变该夹具模型的结构参数和尺寸参数,优化夹具构造,以减少定位过程的夹具变形,并再次通过CAE软件进行分析,验证优化效果.通过增加夹具体主平面刚度、增加夹具体肋板刚度和改变夹具定位点位置等方法,实现夹具体的最大变形量满足加工精度的要求.实验分析以定位点变形为研究对象,测量得到4个定位点在夹具装夹过程中的变形量.实验结果基本符合理论计算结果,得到最终优化方案的4个定位点变形量均小于001 mm,满足了该高精密加工中对夹具体的精度要求.优化方案在实际生产中得到实施,效果良好.     

Use of Eddy Current Technology to Assist in the Evaluation of the Fatigue Damage of Electrical Conductors

The article deals with the evaluation of fatigue damage on overhead conductors which have been in service for a period exceeding 50 years. Under wind excitation, electrical transmission lines are subjected to aeolian vibrations that may induce wire fatigue of conductor at the exit of supporting equipment. This fatigue phenomenon is a fretting fatigue problem which is located at contact points between wires belonging to adjacent wire layers or between external layer wire and the supporting equipment. This article reports the methodology followed: 1) in selecting wires with the more severe damaged contact point using eddy current technology and 2) in comparing, through a metallographic examination, the contact points of those selected wires coming from in situ worn conductors and from laboratory fatigue tested virgin conductors used as reference.     

Experimental force modeling for deformation machining stretching mode for aluminum alloys

Deformation machining is a hybrid process that combines two manufacturing processes—thin structure machining and single-point incremental forming. This process enables the creation of complex structures and geometries, which would be rather difficult or sometimes impossible to manufacture. A comprehensive experimental study of forces induced in deformation machining stretching mode has been performed in the present work. A table-type force dynamometer has been used to record the deforming forces in three Cartesian directions. The influence of five process parameters—floor thickness, tool diameter, wall angle, incremental step size, and floor size on the deforming forces—is investigated. Individual as well as combined empirical models of the parameters with regard to the forces have been formed. The results of this study indicate that the average resultant force primarily depends on the floor thickness to be deformed and the incremental depth in the tool path. This could be due to the variation in local stiffness of the sheet with change in floor thickness. The effect of tool diameter, deforming wall angle, and floor size is not significant.     

Effect of residual surface stress on the fatigue behavior of a low-alloy powder metallurgy steel

In this study we have characterized the fatigue behavior of a low-alloy powder metallurgy (P/M) sintered steel, composed mainly of martensite and bainite with 10.4% porosity. After each processing step, the residual axial surface stress was measured by X-ray diffraction. Significant compressive surface stresses were generated during the machining of the fatigue specimens. A heat-treatment at 175 °C after machining had no effect on these residual stresses, but polishing the surface resulted in a 20% reduction in compressive stresses. The residual surface stresses had no apparent effect on the fatigue behavior of the sintered steel. Rather, the fatigue behavior was controlled by sub-surface defects consisting of weakly bonded particles located in regions of high porosity.     

Fatigue behaviour of fastener holes in high‐strength aluminium plates repaired by cold spray deposition

This work examines the fatigue behaviour of additive cold spray (CS) repairs of AA7075 and AA2024 fastener holes. Structural ring repairs around fastener holes were made by machining blend‐outs ranging from 1/8 to 1/2 the thickness of the plate, then refilling the section of removed material with CS deposition. The repairs were then tested in a lap shear geometry with the repair on both the free (outside) and the mating (inside) surfaces, as well as in remote uniaxial tension. CS repairs for the inside lap shear AA7075 repair configuration and the outside lap shear AA2024 repair configuration were found to have significantly increased fatigue lives even exceeding the number of cycles to failure of the undamaged, unrepaired control plates. Further, none of the CS repairs caused any detrimental impact on fatigue life, and microhardness results indicate that no thermal damage to the substrate occurred. Some interface cracking was seen in the CS repairs; however, no separation of the repair from the substrate was observed.