Residual stresses after orthogonal machining of AlSl 304: numerical calculation of the thermal component and comparison with experimental results

Residual stresses due to the thermal influence of orthogonal machining have been calculated with a finite element model using stationary workpiece temperatures during cutting calculated with the finite difference method. Calculated results are compared with experimental data obtained with the X-ray diffraction method. In this way, the thermal and mechanical/frictional influences of the machining operation on the workpiece residual stress state can be separated. The influence of cutting speed and cutting depth on machining residual stresses is discussed. It is shown that the thermal as well as the mechanical impact of the orthogonal cutting process causes tensile residual stresses. The mechanical impact of the machining operation causing tensile residual stresses is due to (a) compressive plastic deformation in the surface layer ahead of the advancing tool and (b) greater elastic relaxation upon unloading with respect to the underlying material of a thin, strongly work-hardened surface layer. CHRISTOPH WIESNER, formerly Research Assistant with the Laboratoire de Métallurgie Mécanique, Ecole Polytechnique Fédérale de Lausanne, MX-D Ecublens, 1015 Lausanne, Switzerland.     

Coupled temperature,stress, phase transformation calculation

The quenching of steels involves thermal, mechanical, and structural phenomena and their couplings. In this paper, a coupled thermal, phase transformation, internal stresses calculation model is presented. Especially, the stress-phase transformation interactions (transformation plasticity and kinetics modifications through internal stresses) are taken into account in this model, not only for martensitic transformation but also for diffusion dependent transformation. Using a specific case, the cooling of a cylinder made of eutectoid carbon steel, an analysis of how the stress phase transformation interactions affect the internal stresses, and plastic strain evolutions during cooling are performed. The calculated results show that internal stresses have an important effect on the kinetics of pearlitic transformation. These changes in transformation kinetics modify the levels of the internal stresses themselves and the residual stresses.     

Numerical Simulation of Continuous Tension Leveling Process of Thin Strip Steel and Its Application

 Cold rolled thin strip steel of high flatness quality undergoes multistage deformation during tension leveling. Thus, the parameters of set up and manipulating are more difficult. With the aid of FE code MSCMARC, the tension leveling process of thin strip steel was numerically simulated. Concentrating on the influence of the roll intermeshes in 2# anti cambering on the distribution and magnitude of residual stresses in leveled strip steel, several experiments were done with the tension leveler based on the results from the simulation. It was found from the simulation that the magnitude of longitudinal residual stresses in the cross section of the leveled strip steel regularly presents obvious interdependence with the roll intermeshes in 2# anti cambering. In addition, there is a steady zone as the longitudinal residual stresses of the surface layers in leveled strip steel vary with the roll intermeshes of 2# anti cambering, which is of importance in the manipulation of tension levelers. It was also found that the distribution of strains and stresses across the width of strip steel is uneven during leveling or after removing the tension loaded upon the strip, from which it was found that 3D simulation could not be replaced by 2D analysis because 2D analysis in this case cannot represent the physical behavior of strip steel deformation during tension leveling.     

Measurement and calculation of residual stresses after die forging

For die forgings, fabricated from steels Ck 45, 42 CrMo 4 and 38 MnSiV S 6 3, TTT and CCT diagrams were determined, and after different heat treatments, measurements of mechanical properties, fatigue strength and residual stresses were carried out. The diameter of the specimens after the forging process was 32 and 45mm. The residual stresses after quenching between ?450 and +160N/mm² could be reduced to about 60N/mm² by tempering up to 620°C. The fatigue strength in the range of 300N/mm² depends more on the strength than on the residual stresses. Calculations with the program Antras-Thepla correlate well with the measured microstructures and residual stresses. This shows that the materials and processing data used for the calculation are conform to real processes.     

A Benchmark Study on Casting Residual Stress

A benchmark study was undertaken for casting residual stress measurements through neutron diffraction, which was subsequently used to validate the accuracy of simulation prediction. The “stress lattice” specimen geometry was designed such that subsequent castings would generate adequate residual stresses during solidification and cooling of ductile cast iron, without any cracks. The residual stresses in the cast specimen were measured using neutron diffraction. Considering the difficulty in accessing the neutron diffraction facility, these measurements can be considered as a benchmark for casting simulation validations. Simulations were performed using the identical specimen geometry and casting conditions for predictions of residual stresses. The simulation predictions were found to agree well with the experimentally measured residual stresses. The experimentally validated model can be subsequently used to predict residual stresses in different cast components. This enables incorporation of the residual stresses at the design phase along with external loads for accurate predictions of fatigue and fracture performance of the cast components.     

Effects of Burnishing Parameters on the Surface Characteristics,Microstructure and Microhardness in EN Series Steels

Surface finish and surface hardness of the components play vital role in quality of products/components, in general and failure resistance, in particular. One of the finishing process involving surface plastic deformation that introduces compressive residual stresses and thereby improves fatigue resistance is “Burnishing”. Even though the burnishing process is widely employed, its process parameters were not systematically studied till date and not fully established for various important structural materials. The burnishing process parameters include force, speed, feed, and number of tool passes. In the present study, the data obtained from systematically conducted burnishing experiments are correlated with theoretical design using Taguchi method in case of EN series steels (EN 8, EN 24 and EN 31). The surface characterization employed includes optical microscopy, microhardness and magnitude of residual stress. The study revealed a one-to-one correlation between burnishing depth, increase in average microhardness and magnitude of compressive residual stresses and a peak in all these three at intermittent extent of burnishing (either after first or second pass) in all the three alloy steels.     

Material property database for heat treatment simulation of tool materials

A materials data base has been set up with mechanical, thermophysical and transformation properties of tool and creep resistant steels. In particular powder metallurgical materials have been covered. The intended use of the data is for numerical simulation of heat treatment with focus on computation of residual stresses and distortion in Compound materials (macro composites). The main body of the data was generated in new experiments. In this paper data for the powder metallurgical high speed steel ASP 2060 will be presented. The material properties determined include thermophysical data such as thermal conductivity and specific heat, mechanical data such as yield strength and work hardening rate, and thermal expansion data. Most data were determined at different temperatures and after heat treatments to different microstructures. Additional data added to the database include literature data for elastic properties and transformation induced plasticity. Quenching of a Compound ring consisting of an inner high speed steel and an outer tool steel was simulated numerically using two codes, DistSIMR and Trast7. With the help of a database interface input data adjusted to the formats of the two codes were generated. The results of the simulations showed good agreement between the two models both for the computed residual stresses and distortions, which confirms the reliability of the results.     

An Experimental Investigation into Additive Manufacturing-Induced Residual Stresses in 316L Stainless Steel

Additive manufacturing (AM) technology provides unique opportunities for producing net-shape geometries at the macroscale through microscale processing. This level of control presents inherent trade-offs necessitating the establishment of quality controls aimed at minimizing undesirable properties, such as porosity and residual stresses. Here, we perform a parametric study into the effects of laser scanning pattern, power, speed, and build direction in powder bed fusion AM on residual stress. In an effort to better understand the factors influencing macroscale residual stresses, a destructive surface residual stress measurement technique (digital image correlation in conjunction with build plate removal and sectioning) has been coupled with a nondestructive volumetric evaluation method (i.e., neutron diffraction). Good agreement between the two measurement techniques is observed. Furthermore, a reduction in residual stress is obtained by decreasing scan island size, increasing island to wall rotation to 45 deg, and increasing applied energy per unit length (laser power/speed). Neutron diffraction measurements reveal that, while in-plane residual stresses are affected by scan island rotation, axial residual stresses are unchanged. We attribute this in-plane behavior to misalignment between the greatest thermal stresses (scan direction) and largest part dimension.     

Elastoplastic finite element analysis of short-fiber-reinforced SiC/Al composites: effects of thermal treatment

Elastoplastic finite element analyses of realistic models of short-fiber-reinforced composites were extended to include the effects of prior thermal treatments on predictions of subsequent mechanical properties. Two three-dimensional models were used, one in which the fiber ends were transversely aligned and another in which they were staggered. Both models were found to be necessary for accurate predictions of the behavior of higher volume fraction composites. The temperature dependence of the yield stress of the matrix material was explicitly included in the analysis. The spatial and temporal history of calculated. The room temperature residual stresses were also predicted. Both the plastic deformation and the residual stresses in the matrix were spatially non-uniform and varied rapidly from the regions near the ends of the fiber to those near the midpoint. Predictions of subsequent tensile stress-strain properties were in good quantitative agreement with experiments. The presence of residual stresses and locally deformed regions caused the tensile behavior to differ from the compressive behavior. These differences were complex and depended on the volume fraction and aspect ratio of the reinforcement. The analyses provide detailed insight into the deformation mechanisms of these composites.     

Bauschinger effect and residual phase stresses in two ductile-phase steels: Part II. The effect of microstructure and mechanical properties of the constituent phases on Bauschinger effect and residual phase stresses

A finite-element method has been employed to calculate the effect of the prestrain, particle size, volume fraction, and yield strength ratio of the constituent phases on the Bauschinger effect (BE) and residual phase stresses for ferrite-martensite two-phase steels. The relationships be-tween the BE parameters and residual phase stresses are given, and the basics of inelastic un-loading and the effect of reverse deformation on the BE and residual phase stresses are also discussed. Based on the decaying of the residual phase stresses (commonly called “back stresses”) during reverse loading, the relationship between back stresses and permanent softening has been elucidated. Some of the calculated results are compared with experimental ones, and good agree-ment between them is found.     

Pretension-Dependent Residual Stress of Alumina Fiber-Reinforced Composite Wire

The relationship between pretension and residual stress of an aluminum wire reinforced with 45 vol pct continuous Nextel? 610 alumina fibers is investigated. It is shown that as pretension stress increases, the matrix residual stress decreases. A transition in matrix residual stress from tension to compression occurs at a pretension stress of about 80 MPa. The initial rapidly decreased residual stress caused by pretension at relatively low pretension stresses is a result of matrix elastic compressive deformation; while the later gradually decreased residual stress at higher pretension stresses comes from matrix plastic compressive deformation. As the matrix yield stress and hardening exponent increase, the decrease in matrix residual stress with pretension stress is more rapid and the absolute value of matrix residual stress increases. An analytical model suitable for fiber-reinforced metal matrix composites (MMCs) with strong interfacial bonding is developed to describe the relationship between pretension and matrix residual stress and is shown to be in good agreement with the experimental and finite-element calculated results. The pretension-dependent matrix residual stress phenomenon suggests that the mechanical properties of fiber-reinforced MMCs associated with matrix residual stress may be effectively improved by applying tensile loads.     

基于密集冷却工艺的700 MPa级高强带钢减小残余应力研究

目前密集冷却工艺已广泛用于生产高强度带钢,但是该技术冷却速率较快的特点易造成带钢冷却不均匀等问题,导致带钢残余应力过大,进而产生边浪等板形缺陷.本文利用有限元方法,使用ABAQUS有限元软件建立某700 MPa级高强度带钢在密集冷却工艺下的模型,实现温度-相变-应力耦合计算,并进行多个实验验证了模型的准确性.通过修改有限元模型边界条件和初始条件,研究边部遮挡和初始温差对带钢层流冷却阶段产生的残余应力分布的影响规律.对于减小带钢层流冷却过程中产生的残余应力,减小带钢进入层流冷却前的初始温差更加有效.本研究成果经过现场试验验证,可靠性较高,可用于指导该种类型高强带钢生产,以减少带钢的残余应力,提高带钢板形质量.      

Dependence of thermal residual stress on temperature in a SiC particle-reinforced 6061Al alloy

The thermal stresses (TS) in the matrix of a SiC _p /6061Al composite during thermal cycling were measured by X-ray diffraction. Also, the TS during thermal cycling and residual stress distribution (RSD) at room temperature in the two phases of composite were calculated by finite element modeling (FEM). The measured and calculated results indicated that the closed stress-temperature loop was formed during thermal cycling. The stress state in the matrix changed from tension to compression during heating and from compression to tension during cooling. Plastic deformation took place in the matrix of the composite during thermal cycling. The general change trend of TS with temperature during thermal cycling was in agreement between the experiment and calculation.     

电弧焊接数值模拟中热源模型的研究与发展

焊接过程的数值模拟作为一种有效的计算手段,在焊接温度场及残余应力分布的评价中获得了广泛应用,而焊接热源模型的选择及模型参数的确定直接影响到计算和评价结果的准确性.本文通过对近年来常用的电弧焊接热源模型进行梳理,介绍了其研究进展,分析了不同热源模型的特点及适用性.高斯面热源模型和双椭球体热源模型作为基础热源模型,广泛应用于较小尺寸工件和规则轨迹的焊接过程数值模拟,且具有较高的计算精度;简化热源模型和温度替代型热源模型多用于大厚工件的多层多道焊接及复杂轨迹焊接过程的数值模拟,能够实现效率和精度的统一;多丝电弧焊接热源较为复杂,采用修正后的双椭球体叠加热源模型,计算结果能保证一定的精度;结合型热源模型对熔池形状的描述更灵活,在深熔电弧焊的数值模拟中具有优势.本文可为电弧焊接过程数值模拟的热源模型选择和模型参数确定提供有益参考.      

Hydrologic and Water Quality Integration Tool: HydroWAMIT

A spatially distributed and continuous hydrologic model focusing on total maximum daily load (TMDL) projects was developed. Hydrologic models frequently used for TMDLs such as the hydrologic simulation program—FORTRAN (HSPF), soil and water assessment tool (SWAT), and generalized watershed loading function (GWLF) differ considerably in terms of spatial resolution, simulated processes, and linkage flexibility to external water quality models. The requirement of using an external water quality model for simulating specific processes is not uncommon. In addition, the scale of the watershed and water quality modeling, and the need for a robust and cost-effective modeling framework justify the development of alternative watershed modeling tools for TMDLs. The hydrologic and water quality integration tool (HydroWAMIT) is a spatially distributed and continuous time model that incorporates some of the features of GWLF and HSPF to provide a robust modeling structure for TMDL projects. HydroWAMIT operates within the WAMIT structure, developed by Omni Environmental LLC for the Passaic River TMDL in N. J. HydroWAMIT is divided into some basic components: the hydrologic component, responsible for the simulation of surface flow and baseflow from subwatersheds; the nonpoint-source (NPS) component, responsible for the calculation of the subwatershed NPS loads; and the linkage component, responsible for linking the flows and loads from HydroWAMIT to the water quality analysis simulation program (WASP). HydroWAMIT operates with the diffusion analogy flow model for flow routing. HydroWAMIT provides surface runoff, baseflow and associated loads as outputs for a daily timestep, and is relatively easy to calibrate compared to hydrologic models like HSPF. HydroWAMIT assumes that the soil profile is divided into saturated and unsaturated layers. The water available in the unsaturated layer directly affects the surface runoff from pervious areas. Surface runoff from impervious areas is calculated separately according to precipitation and the impervious fractions of the watershed. Baseflow is given by a linear function of the available water in the saturated zone. The utility of HydroWAMIT is illustrated for the North Branch and South Branch Raritan River Watershed (NSBRW) in New Jersey. The model was calibrated, validated, and linked to the WASP. The NPS component was tested for total dissolved solids. Available weather data and point-source discharges were used to prepare the meteorological and flow inputs for the model. Digital land use, soil type datasets, and digital elevation models were used for determining input data parameters and model segmentation. HydroWAMIT was successfully calibrated and validated for monthly and daily flows for the NSBRW outlet. The model statistics obtained using HydroWAMIT are comparable with statistics of HSPF and SWAT applications for medium and large drainage areas. The results show that HydroWAMIT is a feasible alternative to HSPF and SWAT, especially for large-scale TMDLs that require particular processes for water quality simulation and minor hydrologic model calibration effort.     

万能轧制H型钢复合镶套式轧辊应力分析和过盈量的优化设计

通过对万能轧制Ｈ型钢复合镶套式轧辊应力场的分析和计算，采用可靠性优化设计方法，仿真研究了材质性能、残余应力和摩擦系数对轧辊系统可靠度的影响，并给出了最佳过盈量。其结论可用于指导复合轧辊材质选择、热装方法的制定和轧辊结构设计。     

Influence of heat transfer on the development of residual stresses in quenched steel cylinders

In this paper results of systematic FE-calculations about the influence of characteristic points of the temperature dependent heat transfer coefficient, especially the Leidenfrost point and the point of maximum heat transfer coefficient on the development of residual stresses are discussed. The numerical investigations were carried out for SAE 1045 and 4140 steel cylinders with 10 and 20 mm 0 quenched in water and oil, respectively. In this work experimentally determined h, T-curves are linearly approximated in the successive stages of heat transfer. Changes of the Leidenfrost-temperature do not influence the middle plane residual stresses of the cylinders investigated. Increasing maximum heat transfer coefficients and low temperatures of maximum heat transfer coefficient, respectively, cause higher magnitudes of residual stress. The development of residual stresses is determined by the temperature dependent gradient of the heat flux density δq/δT in the temperature range of martensitic transformation. Increasing Leidenfrost-temperatures cause more homogeneous stress and residual stress states at the surface of quenched cylinders due to the symmetrical cooling of the sample in axial as well as in radial direction. In particular, it was shown that during immersion cooling of cylindrical parts the heat transfer is locally dependent. Simulating immersion cooling this dependence has to be considered using effective local heat transfer coefficients.     

As-Cast Residual Stresses in an Aluminum Alloy AA6063 Billet: Neutron Diffraction Measurements and Finite Element Modeling

The presence of thermally induced residual stresses, created during the industrial direct chill (DC) casting process of aluminum alloys, can cause both significant safety concerns and the formation of defects during downstream processing. Although numerical models have been previously developed to compute these residual stresses, most of the computations have been validated only against measured surface distortions. Recently, the variation in residual elastic strains in the steady-state regime of casting has been measured as a function of radial position using neutron diffraction (ND) in an AA6063 grain-refined cylindrical billet. In the present study, these measurements are used to show that a well-designed thermomechanical finite element (FE) process model can reproduce relatively well the experimental results. A sensitivity analysis is then carried out to determine the relative effect of the various mechanical parameters when computing the as-cast residual stresses in a cylindrical billet. Two model parameters have been investigated: the temperature when the alloy starts to thermally contract and the plasticity behavior. It is shown that the mechanical properties at low temperatures have a much larger influence on the residual stresses than those at high temperatures.     

Hydride embrittlement in ZIRCALOY-4 plate: Part II. interaction between the tensile stress and the hydride morphology

The effect of an applied tensile stress on the hydrides morphology in ZIRCALOY-4 was studied. To this end, the residual stresses around the hydride caused by the hydride precipitation was first evaluated. Considering the disability to predict hydride transformation stresses by ordinary macroscopical mechanical calculation in previous studies, X-ray diffraction (XRD) profile analysis and transmission electron microscopy (TEM) observations were carried out to quantify the microstructural evolution in hydrided ZIRCALOY-4. The residual microstrains and microstresses in the matrix and around the hydride were thus estimated. The big discrepancy between our results and the existing studies were explained by the major self-accomodation of phase transformation deformation remaining inside the hydrides and the local plastic accommodation of ZIRCALOY-4. In order to study the stress effect on hydride orientation and to estimate the hydride orientation threshold stresses, hydrogen was introduced into the specimens under tensile stress. A quantitative technique was used to evaluate the susceptibility to perpendicular hydride formation under the influence of texture, residual stresses, and externally applied tensile stresses, following an improved approach that had been first developed by Sauthoff and then applied to Zr-H system by Puls. Both analytical and experimental results indicate that the threshold stress for producing perpendicular hydrides varies with the microstructural features, the yield strength, and the residual stresses.     

Effect of the cooling rate on the properties of a Si-10% Al alloy for magnetron sputterint targets

The effect of the cooling rate on the micromechanical properties and residual thermal stresses in Si-10% Al alloy targets has been studied. To reach the required quality, these targets should be cooled at rates of 10–40 K/s. In this case, the alloy has high strength properties, the level of residual stresses in a target is minimal, and the stress distribution is most uniform.