Materials Science Forum Vols. 768-769

Abstract: The roller straightening process is a common method for straightening long products like beams after rolling and cooling. This process often causes an adverse residual stress state. All previous investigations operate only with the roller adjustment as correcting variable. However, this cannot properly describe the consequences on the cross section of the beam during bending. The present paper presents a concept to consider the development of curvature during the straightening process. In finite element analyses using Abaqus/Standard a beam with a rectangular cross section and simplified material properties is modeled for a fundamental and clear demonstration. The theo-retically determined residual stress state depends on the development of curvature during straight-ening. Vice versa it is possible to design a trend of curvature with the goal of tailoring the final re-sidual stress distribution to the desired optimum. The necessary roller adjustment is found in a simulation using the Abaqus user subroutine UAMP, where the curvature is permanently tuned by controlling the roller adjustment. The residual stress state resulting from these 1D considerations (“the theoretical stress state”) is verified in a subsequent 2D analysis giving the “actual stress state”. A comparison of the theoretical and the actual residual stress state illustrates the influence of the roller contact. The concept presented in this work can be applied to complex cross sections in com-bination with realistic material properties. However, for this purpose a large amount of calculation resources due to an extensive 3D modeling are needed.

Abstract: The samples studied in this paper were performed from carbon steel plates, cladded in one of the faces with stainless steel filler metals by submerged arc welding (SAW). After cladding work, the samples were submitted to post-weld heat treatments at different conditions and afterwards stainless steel coating surfaces were milled and mechanically polished, as in the industrial application. The residual stress analysis was performed by X-ray diffraction (XRD) and incremental hole-drilling methods (IHDM). The residual stresses profiles presented different in depth values in each sample, depending on the heat treatment conditions. The hole-drilling method was applied in several points of each stainless steel sample surface and the results presented similar evolution profiles. However compressive stresses increase with the increase of heat treatment temperature.

Abstract: Machining induced residual stresses were investigated in Inconel 718 prepared by high speed turning under dry cut condition. The influence of cutting tool wear and the use of cutting fluid were studied. By x-ray diffraction measurements, characteristic residual stress distributions with tensile stresses in the top layer and compressive stresses in the layer below were found in all the investigated samples. The magnitude of surface tensile stresses and size of the tension as well as compression zones varied depending on the cutting condition. The application of cutting fluid for turning using new tool has a minor effect, giving a somewhat larger subsurface compressive zone but reducing the surface tensile stresses. Tool flank wear has shown a much stronger effect. While a flank wear of VBmax=0.15 mm enhanced mostly the surface tensile residual stresses, a severer wear of VBmax=0.3 mm greatly increased the thickness of the subsurface compression zone and at the same time resulted in strong stress anisotropy. Microstructural study by electron channelling contrast imaging shows that the observed influence of tool flank wear or cutting fluid on residual stresses are related to different contributions from increased plastic deformation and cutting heat, which changed with the cutting conditions.

Abstract: Shot peening is a common procedure used to improve the static and cyclic strength of metal components and for forming of thin walled components. The underlying mechanisms are localized plastic deformation, work hardening and the introduction of compressive stresses into the near-surface region. During the last decade we have been establishing damage-free shot peening processes for brittle materials such as ceramics. Based on these results we are now developing processes for peen-forming of ceramic components. This paper describes the first successful experiments aimed at shaping ceramic specimens using shot peening. Strips of different thicknesses, made of silicon nitride ceramic, were shot-peened using different shot sizes, peening pressures and coverage. The residual stress-depth distributions were determined using X-ray diffraction. Based on the experimentally determined stress states, the curvatures of the strips were calculated analytically and using Finite Element calculations (FEM). The results of the curvature measurements and calculations agree well.

Abstract: Composite castings exhibit high residual stresses, mainly because of different thermal expansion of the used materials. Similar to the in-cast cylinder liners in a motor block, a composite specimen, consisting of a steel insert and an aluminum cast surrounding, was analyzed by neutron diffraction. The temperature- and time-dependent change of lattice spacing and thus the strain evolution was investigated by in-situ experiments directly after casting and during the cooling of the part. Different cooling conditions were investigated using two different molds, namely a sand and a permanent (steel) mold, optimized for in-situ neutron diffraction.

Abstract: 800x600 The short and long Japanese swords “WAKIZASHI” (back up sword of the main sword, “KATANA”) were made by one sword craftsman. The short and long swords “WAKIZASHI” were machined by rough grinding and final hand polishing, respectively. The microstructure, carbon content and hardness of short sword were measured experimentally on the cross section of sword blade. The 2θ-sin²ψ diagrams from Fe-211 plane using Cr-Kα radiation on the ground and polished blade were measured. An influence of final hand polishing process on the surface residual stress field of sword blade was examined. As a result, biaxial principal compressive residual stresses were generated and had constant stress gradients in depth on the ground and polished surfaces because the ψ-splitting was not observed and the measured 2θ vs. sin²ψ relations could be approximated as a parabolic curve. Large compressive residual stresses more than −1.0 GPa were distributed on the ground surface from “HASAKI” to “HAMON” of short sword. The surface compressive residual stress and its gradient were diminished gradually from “HAMON” to “MUNE” (the ridge of sword). On the other hand, compressive residual stresses more than −650MPa were distributed on the pol­ished surface from “HASAKI” to “HAMON” of long sword. The surface compressive residual stress and its gradient were also diminished gradually from “HAMON” to “MUNE”, and the residual stress gradient in the transverse direction were greatly degraded in comparison to the short sword. Additional compressive residual stress field induced by rough grinding was superimposed on the residual stress field after tempering process. The residual stress field near the blade surface after rough grinding was released partly by final hand polishing. Normal 0 21 false false false DE X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Century","serif";}

Abstract: This paper presents a 3D model that simulates an ultrasonic shot peening (USP) operation, using realistic process parameters and peening setups (part and chamber geometries). By simulating the shot dynamics (shot trajectories and impacts), statistical and spatial data are obtained for the peened component, i.e. surface coverage and coverage rate, impact speeds and angles, dissipated energy... This data can then be used for i) optimizing the design of peening chambers and process parameters and ii) predicting the residual stress and displacement fields induced by USP in the peened component. In fact, data from the 3D model can be used as initial data in existing residual stress prediction models. A chaining methodology was developed for this purpose and allows linking the choice of process parameters and USP setup to the induced residual stress displacement fields.

Abstract: The manufacturing of electron beam welded blade integrated disk (blisk) rotors sets new demands on mechanical surface treatment technologies. High durability and high efficiency of the blades are in general strict requirements for the component and have ideally to be increased by a mechanical surface treatment. The high complex 3D shape of the blades and the need to treat the blades on the rotor caused by a post weld heat treatment are additional challenges to solve. Limited clearance between the blades and low space between the individual blisk’s increase as well the requirements on the technology. Conventional technologies reach their limits and have to be improved. This paper gives an insight into the capabilities of different mechanical surface treatments regarding the treatment of blades on blisk-rotors. Compared with the benchmark shot peening, surface roughness, residual stress depth distribution and high cycle fatigue (HCF) are investigated on deep rolled and vibropeened specimen. Assets and drawbacks are shown and discussed.

Abstract: The paper deals with the effect of different shot peening (SP) treatment conditions on the ENAW 7075-T651 aluminium alloy. Suitable residual stress profile increases the applicability and life cycle of mechanical parts, treated by shot peening. The objective of the research was to establish the optimal parameters of the shot peening treatment of the aluminium alloy in different precipitation hardened states with regard to residual stress profiles in dynamic loading. Main deformations and main residual stresses were calculated on the basis of electrical resistance. The resulting residual stress profiles reveal that stresses throughout the thin surface layer of all shot peened specimens are of compressive nature. The differences can be observed in the depth of shot peening and the profile of compressive residual stresses. Under all treatment conditions, the obtained maximum value of compressive residual stress ranges between -200 MPa and -300 MPa at a depth between 250 μm and 300 μm. Comparison of different temperature-hardened aluminium alloys shows that changes in the Almen intensity values have greater effect than coverage in the depth and profile of compressive residual stresses. Positive stress ratio of R=0.1 was selected. Wöhler curves were determined in the areas of maximum bending loads between 30 - 65 % of material's tensile strength, measured at thinner cross-sections of individual specimens. The results of material fatigue testing differ from the level of shot peening on the surface layer.

Abstract: Piezo peening is a new alternative mechanical surface treatment process. Thereby a piezo actuator with an indenter is causing mechanical deformation of the surface area by multiple impacts in a defined way. First results for quenched and tempered AISI 4140 show a great potential: large surface compressive residual stresses of up to -1200 MPa could be generated. In order to obtain a process understanding different process parameters are systematically changed. After the mechanical surface treatments the residual stress and full width at half maximum (FWHM) depth distribution were measured using X-ray diffraction technique. By varying the process parameters path distance, feed rate and amplitude several combinations of residual stress and FWHM depth distributions were generated. The dependency of surface residual stresses, penetration depth, FWHM on the process parameters is analyzed.