Residual Stress Analysis in Deep Drawn Twinning Induced Plasticity (TWIP) Steels Using Neutron Diffraction Method

In Twinning Induced Plasticity (TWIP) steels, delayed fracture occurs due to residual stresses induced during deep drawing. In order to investigate the relation between residual stresses and delayed fracture, in the present study, residual stresses of deep drawn TWIP steels (22Mn-0.6C and 18Mn-2Al-0.6C steels) were investigated using the finite element method (FEM) and neutron diffraction measurements. In addition, the delayed fracture properties were examined by dipping tests of cup specimens in the boiled water. In the FEM analysis, the hoop direction residual stress was highly tensile at cup edge, and the delayed fracture was initiated by the separation of hoop direction and propagated in an axial direction. According to the neutron diffraction analysis, residual stresses in 18Mn-2Al-0.6C steel were about half the residual stresses in 22Mn-0.6C steel. From the residual strain measurement using electron back-scatter diffraction, formation of deformation twins caused a lot of grain rotation and local strain at the grain boundaries and twin boundaries. These local residual strains induce residual stress at boundaries. Al addition in TWIP steels restrained the formation of deformation twins and dynamic strain aging, resulting in more homogeneous stress and strain distributions in cup specimens. Thus, in Al-added TWIP steels, residual stress of cup specimen considerably decreased, and delayed fracture resistance was remarkably improved by the addition of Al in TWIP steels.     

Residual Stress Measurement of Laser-Engineered Net Shaping AISI 410 Thin Plates Using Neutron Diffraction

In an attempt to relate the laser-engineered net shaping (LENS) process parameters, laser power and laser travel speed, to the quality of LENS-produced parts, strain measurements were taken at several predetermined points within seven LENS AISI 410 thin plates using the neutron diffraction method. The residual stresses at these points were then calculated using the measured strain values to ascertain how the internal stress varies as a function of the input parameters and location. It is found that the component of the stress in the vertical direction (i.e., perpendicular to the raster direction of the laser/powder nozzle) is dominant, in agreement with previous reports, and relatively insensitive to variations in process parameters. This was confirmed with numerical simulations performed with a thermomechanical model developed using the commercial program SYSWELD. The simulations also showed a good qualitative agreement with the measured simulated stresses. This article is based on a presentation given in the symposium entitled “Neutron and X-Ray Studies for Probing Materials Behavior,” which occurred during the TMS Spring Meeting in New Orleans, LA, March 9–13, 2008, under the auspices of the National Science Foundation, TMS, the TMS Structural Materials Division, and the TMS Advanced Characterization, Testing, and Simulation Committee.     

Effect of Treatment Area on Residual Stress and Fatigue in Laser Peened Aluminum Sheets

Two 2.0-mm-thick aluminum sheets were laser peened and the resulting residual stresses were measured using incremental hole drilling, surface X-ray diffraction, and synchrotron X-ray diffraction techniques. Laser peening was applied to two samples using the same laser peening parameters, but one of the samples has a larger peened area. The aim of this research was to discover the effect of peen area on residual stress, for application in aerospace structures for fatigue life enhancement. It was found that a larger peened area has higher and deeper compressive stresses in the crack-opening direction, leading to greater enhancement of fatigue life.     

The relaxation of residual stresses with postweld heat treatment in a high-performance weld measured with neutron diffraction

The residual stresses in a cylindrical weldment of HP-9-4-30 steel were measured with neutron diffraction in the as-welded (AW) state and after postweld heat treatment (PWHT). Large residual stresses are present in the interior of the material in the as-welded condition. The maximum principal stresses measured were found around the edges of the cap-pass heat-affected zone and reached up to 1045 MPa (76 pct of the base metal yield strength) in the as-welded condition. The principal stress directions for the residual stress tensors do not in general follow the hoop, axial, and radial axes of the weld and change from position to position within the weld, although the highest values are generally in the hoop direction. The postweld heat treatment relaxed the largest residual stresses, with the maximum value being 30 pct of the base metal yield stress. The need for position-dependent stress-free standards and the implications of stress gradients over the measurement volumes are discussed.     

Friction Stir Welding of HSLA-65 Steel: Part II. The Influence of Weld Speed and Tool Material on the Residual Stress Distribution and Tool Wear

A set of single pass full penetration friction stir bead-on-plate and butt welds in HSLA-65 steel were produced using a range of traverse speeds (50 to 500?mm/min) and two tool materials (W-Re and PCBN). Part I described the influence of process and tool parameters on the microstructure in the weld region. This article focuses on the influence of these parameters on residual stress, but the presence of retained austenite evident in the diffraction pattern and X-ray tomographic investigations of tool material depositions are also discussed. The residual stress measurements were made using white beam synchrotron X-ray diffraction (SXRD). The residual stresses are affected by the traverse speed as well as the weld tool material. While the peak residual stress at the tool shoulders remained largely unchanged (approximately equal to the nominal yield stress (450?MPa)) irrespective of weld speed or tool type, for the W-Re welds, the width of the tensile section of the residual stress profile decreased with increasing traverse speed (thus decreasing line energy). The effect of increasing traverse speed on the width of the tensile zone was much less pronounced for the PCBN tool material.     

Use of neutron and synchrotron X-ray diffraction for evaluation of residual stresses in a 2024-T351 aluminum alloy variable-polarity plasma-arc weld

The residual stress fields associated with variable-polarity plasma-arc (VPPA) welds in 2024-T351 aluminum alloy plates have been measured nondestructively using neutron and synchrotron X-ray diffraction. Neutron diffraction allows in-depth measurements of the full strain tensor to be made in thick components; synchrotron X-rays allow for rapid measurements of strains inside components, although their penetration is less than that of the neutrons and constraints arising from the diffraction geometry generally lead to only two strain components being easily measurable. Hence, a combination of the two techniques, applied as described herein, is ideal for a detailed nondestructive evaluation of residual stresses in plates. The residual stresses in a 12-mm-thick VPPA-welded aluminum 2024-T351 alloy plate have been measured using neutron diffraction. The stresses were then remeasured by a combination of neutron and synchrotron X-ray diffraction after the plate had been reduced in thickness (or, skimmed) to 7 mm by machining both sides of the weld, mimicking the likely manufacturing operation, should such welds be used in aerospace structures. A strong tensile residual stress field was measured in the longitudinal direction, parallel to the weld, in both the as-welded and skimmed specimens. There was only a slight modification of the residual stress state on skimming.     

Energy-input-based finite-element process modeling of inertia welding

A coupled thermal and mechanical finite-element (FE) model has been developed to describe the inertia welding of RR1000 nickel-base superalloy tubes using the DEFORM 7.2 FE package. The energy input rate is derived from measurements of torque, angular rotation speed, and upset taken from actual inertia welding trials. The model predicts the thermal history of the joint as well as the deformation pattern and final residual stresses. The thermal variation has been validated by a microstructural study of the weld region of the trial joints. Thermal profile predictions have been made for three welds having the same initial kinetic rotational energy but different levels of flywheel inertia and rotational velocity. The concomitant residual stress predictions have been compared with nondestructive neutron diffraction residual stress measurements. The implications of the results for inertia welding are discussed.     

Thermal Relaxation of Residual Stresses in Nickel-Based Superalloy Inertia Friction Welds

This article describes an experimental study aimed at characterizing the extent of residual stress relaxation during thermal treatment of inertia friction-welded alloy 720Li nickel-based superalloy welded tubular rings. In the as-welded condition, yield level tensile hoop stresses were found by neutron diffraction in the weld region along with axial bending stresses (tensile toward the inner diameter (ID)/compressive toward the outer). The evolution of these residual stress levels during postweld heat treatment (PWHT) was mapped experimentally over the weld cross section. After 8 hours of PWHT, the axial stresses relaxed by 70 pct, whereas the hoop stresses reduced by only 50 pct. Some scatter of residual stress evolution was found between samples, particularly for the axial stress direction. This was attributed to substandard tooling to grip the rings. The results on subscale samples were transferred to a full-scale aeroengine (650-mm diameter) compressor drum assembly that was postweld heat treated for 8 hours. It was found that the residual stresses, particularly in the axial direction, were noticeably lower in this full-scale weld component compared to the subscale weld heat treated for the same time. The differences seem to be best rationalized by the different standards of jigging used during joining these two types of welds.     

Verification of the Simulated Residual Stress in the Cross Section of Gray Cast Iron Stress Lattice Shape Casting via Thermal Stress Analysis

The residual stresses in the thick part of the stress lattice shape casting consist of the residual stress due to the temperature differential between the thick part and the thin part and the residual stress due to the temperature differential in the radial direction of the thick part. In this study, the gray cast iron stress lattice shape castings were cast and both types of the residual stresses were separately measured. Thermal stress analyses based on the casting experiment were conducted. Next, the measurements in this study were compared with both types of the simulated residual stresses. The thermal stress analyses estimated the residual stress due to the temperature difference in the radial direction of the thick part to be significantly higher than the measurement, although the residual stress due to the temperature difference between the thick part and the thin part was successfully predicted within a 10 pct error. Thus, this study suggested the introduction of the mechanical melting temperature, above which the very low yield stress is applied conveniently to describe the losses of the deformation resistance of the casting, to more accurately predict the residual stress due to the temperature difference in the radial direction of the thick part. From the verification of the suggested model, this study demonstrated that the conventional elasto-plastic model must introduce the mechanical melting temperature to predict the residual stress due to the temperature difference in the radial direction of the thick part and thus the overall residual stress in the stress lattice.     

Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of Laser Beam Welded TiAl-based Alloy

Post-weld heat treatment is carried out on the laser beam welded γ-TiAl-based alloy Ti-48Al-1Cr-1.5Nb-1Mn-0.2Si-0.5B (at. pct). The macro/microstructure and mechanical properties of both as-welded and heat-treated specimens are investigated by radiography, SEM, and tensile tests. Moreover, high energy synchrotron X-ray diffraction is performed to measure the residual stresses and evaluate the microstructure evolution. It is found that the residual stresses are distributed in a three-peak shape in the region of the weld zone and heat-affected zone of the as-welded specimen due to the microstructural transformation and heat softening. The residual stresses are largely relieved after the heat treatment. The heat-treated specimens have a near fully lamellar microstructure and show balanced mechanical properties of strength and ductility. The diffraction shows that the phase transformation from α ₂ to γ takes place under tensile load at 1023 K (750 °C), and the grain size and lamellar spacing are refined in the weld zone. Finally, the fracture mechanisms are found to be controlled by the local stress concentration-induced strain misfit between α ₂ and γ phases in the near γ grains and delamination and debonding in the lamellae. Boride ribbons of 5 μm in the near fully lamellar microstructure are found not to be detrimental to the tensile properties.     

The Residual Stress Relaxation Behavior of Weldments During Cyclic Loading

Accurate measurement of residual stress is necessary to obtain reliable predictions of fatigue lifetime and enable estimation of time-to-facture for any given stress level. In this article, relaxation of welding residual stresses as a function of cyclic loading was documented on three common steels: AISI 1008, ASTM A572, and AISI 4142. Welded specimens were subjected to cyclic bending (R = 0.1) at different applied stresses, and the residual stress relaxation existing near the welds was measured as a function of cycles. The steels exhibited very different stress relaxation behaviors during cyclic loadings, which can be related to the differences in the microstructures of the specimens. A phenomenological model, which treats dislocation motion during cyclic loading as being analogous to creep of dislocations, is proposed for estimation of the residual stress relaxation.     

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.     

Residual stress evaluation with X-rays in steels having preferred orientation

Two phenomena limit the use of X-ray stress evaluation in practice: preferred orientation and coarse grained materials. Measurements with Cr-radiation and the (211)-peak showed nonlinearD vs sin ² ψ distributions in the case of a 75 pct cold-rolled steel, due to preferred orientation, and for a deep-drawing steel spotty diffraction rings on a film exposed in front of the detector, due to large grain size, were observed. However, when Mo-radiation and the (732 + 651)-peak were used, theD vs sin² ψ distributions proved to be linear, so that residual stresses could be determined. The X-ray elastic constants (XEC) needed to compute the stresses from the measured lattice strains were determined under uniaxial load using samples cut at various angles with respect to the rolling direction. For the deep drawing steel in particular, the XEC differ from the values calculated for Fe-materials having randomly oriented crystallites. Furthermore, the XEC for the two investigated steels proved to depend on the angle of sample cut with respect to the rolling direction.     

Residual stresses in high-velocity oxy-fuel metallic coatings

X-ray based residual stress measurements were made on type 316 stainless steel and Fe₃Al coatings that were high-velocity oxy-fuel (HVOF) sprayed onto low-carbon and stainless steel substrates. Nominal coating thicknesses varied from 250 to 1500 μm. The effect of HVOF spray particle velocity on residual stress and deposition efficiency was assessed by preparing coatings at three different torch chamber pressures. The effect of substrate thickness on residual stress was determined by spraying coatings onto thick (6.4 mm) and thin (1.4 mm) substrates. Residual stresses were compressive for both coating materials and increased in magnitude with spray velocity. For coatings applied to thick substrates, near-surface residual stresses were essentially constant with increasing coating thickness. Differences in thermal expansion coefficient between low-carbon and stainless steels led to a 180 MPa difference in residual stress for Fe₃Al coatings. Deposition efficiency for both materials is maximized at an intermediate (∼600 m/s) velocity. Considerations for X-ray measurement of residual stresses in HVOF coatings are also presented.     

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.     

A Comparison of Residual Stress Development in Inertia Friction Welded Fine Grain and Coarse Grain Nickel-Base Superalloy

The effect of the base material microstructure on the development of residual stresses across the weld line in inertia friction welds (IFWs) of high-strength nickel-base superalloy RR1000 was studied using neutron diffraction. A comparison was carried out between tubular IFW specimens generated from RR1000 heat treated below (fine grain (FG) structure) and above (coarse grain (CG) structure) the γ′-solvus. Residual stresses were mapped in the as-welded (AW) condition and, after a postweld heat treatment (PWHT), optimized for maximum alloy strength. The highest tensile stresses were generally found in the hoop direction at the weld line near the inner diameter of the tubular-shaped specimens. A comparison between the residual stresses generated in FG and CG RR1000 suggests that the starting microstructure has little influence on the maximum residual stresses generated in the weld even though different levels of energy must be input to achieve a successful weld in each case. The residual stresses in the postweld heat treated samples were about 35 pct less than for the AW condition. Despite the fact that the high-temperature properties of the two parent microstructures are different, no significant differences in terms of stress relief were found between the FG and CG RR1000 IFWs. Since the actual weld microstructures of FG and CG RR1000 inertia welds are very similar, the results suggest that it is the weld microstructure and its associated high-temperature properties rather than the parent material that affects the overall weld stress distribution and its subsequent stress relief.     

预热对铍环激光束钎焊过程的影响研究

研究预热对铍环激光束钎焊过程温度场和应力场分布的影响。采用轴对称模型和热力解耦的有限元方法，并假定沉积到钎缝表面的激光束能量为Gauss分布，预热通过在焊接加热前添加一个能量密度低、有效加热半径大的单独工况实现。结果表明，预热使镀环钎缝外表面焊接最高温度增加，温度梯度减小，但焊深明显增加；采用预热工况焊接后，钎缝附近塑性变形区焊接残余应力明显减小，而热影响区残余应力增大。从整体分布来看，预热使铍环外表面焊接残余应力分布均匀化。对铍环外表面钎缝附近焊接残余应力进行X射线应力测试，并与有限元分析结果对比，二者应力变化趋势基本一致。     

Experimental Analysis of Slip Systems Activation in Neutron-Irradiated Zirconium Alloys and Comparison with Polycrystalline Model Simulations

Textures, stresses, and strains, as well as the overall so-called real structure, are crucial for properties of thin films deposited by different methods and can have both positive and negative effects depending on the film and its application. They were studied by a combination of different X-ray diffraction (XRD) techniques for several ZnO films. The films prepared by pulsed laser deposition (PLD) on MgO and sapphire single-crystalline substrates and amorphous-fused silica showed different kinds of strong preferred orientation and also different stresses that could be estimated only from the analysis of quite narrow, nonzero intensity regions of diffraction spots. XRD line broadening was analyzed by a combination of different asymmetric scans. Fiber (0001) texture and tensile residual stresses were found on fused silica, while domains with local epitaxy and huge compressive stress were detected on MgO substrate, and surprisingly, very strong local epitaxy but not parallel to the (0001) sapphire substrate was observed. No residual stress was detected there. Some methodological aspects of the XRD studies of thin nanocrystalline films with strong preferred orientation are discussed.