Showing papers on "Residual stress published in 2011"

Microstructure and residual stress of laser rapid formed Inconel 718 nickel-base superalloy

Abstract: The microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718 was investigated. The as-deposited microstructure of an LRFed Inconel 718 alloy is composed of columnar dendrites growing epitaxially along the deposition direction, and the columnar dendrites transformed to unevenly distributed equiaxed grains after annealing treatment at high temperature. Residual stress evaluation in microstructure scale by Vickers micro-indentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample, and it has a significant effect on the recrystallization during solution annealing treatment. The residual stress is introduced by rapid heating and cooling during laser rapid forming. There is an alternative distribution between high residual stress regions and low residual stress regions, within a single deposited layer, resulting in a similar distribution of recrystallized grain size.

Analysis of transient temperature and residual thermal stresses in friction stir welding of aluminum alloy 6061-T6 via numerical simulation

Abstract: Residual stress is lower in friction stir welding (FSW) compared with other melting weldment processes. This is due to being solid-state process in its nature. There are several advantages in utilizing stir welding process. Lower fluctuation and shrinkage in weldment metal-enhanced mechanical characteristics, less defects, and ability to weld certain metals otherwise impractical by other welding processes are to name just a few of these advantages. These have caused an ever increasing attention by the concerned to the process of FSW. In this investigation, three-dimensional numerical simulation of friction stir welding was concerned to study the impact of tool moving speed in relation with heat distribution as well as residual stress. Simulation was composed of two stages. Firstly, thermal behavior of the piece while undergoing the welding process was studied. Heat is generated due to the friction between tool and the piece being welded. In the second stage, attained thermal behavior of the piece from previous stage is considered as inlet heat of an elasto-plastic, thermo-mechanical model for the prediction of residual stress. Also, in the second stage, tool is eliminated and residual stress distribution is found after complete cooling of the piece and disassembly of the clamp. Material characteristic are introduced into the proposed model as temperature-dependent parameters. Obtained residual indicate that heat distribution along thickness varies and is asymmetrical enormously. Moreover, longitudinal residual stress in the weld which increases as speed of process and tool movement ascends. In the prediction of results of residual stress, only heat impact was studied. This was recognized as the main element causing minor difference in results obtained for simulation in comparison with that of actual experiment.

Residual stresses in laser direct metal deposited Waspaloy

Abstract: This paper reports a study into the effect of laser pulse length and duty cycle on the residual stress distributions in multi-track laser direct metal deposits of Waspaloy onto an Inconel 718 substrate. The residual stresses have been evaluated using neutron diffraction and the contour method, while electron microscopy and micro hardness indentation have been used to map the concomitant microstructural variation. In all cases, near the tops of the deposited walls, the longitudinal stresses are tensile towards the mid-length of the wall, while the stresses perpendicular to the substrate are negligible. By contrast near the base of the walls, the stresses along the direction of deposition are small, while the stresses perpendicular to the substrate are compressive at the centre and tensile towards the ends. Consistent with previous observations, the stresses parallel to free surfaces are tensile, balanced by compressive stresses in the interior (an inverse quench stress profile). These profiles have been found to be weakly dependent on the laser pulse parameters, most notably an increase in tensile stress gradient with increasing duty cycle, but the maximum residual stresses are largely unaffected. Furthermore, microstructural analysis has shown that the effect of laser pulse parameters on grain morphology in multi-track thick walls is less marked than previously reported for single-track wall structures.

Improvement of fatigue property in 7050–T7451 aluminum alloy by laser peening and shot peening

Abstract: The fatigue strength for 1 × 107 cycles of 7050–T7451 aluminum alloy was determined for machined, laser-peened, and shot-peened specimens. Moreover, fatigue lives were compared under the same load conditions. Results show that the laser peening induces a deeper compressive residual stress layer and better surface finish, therefore, it improves fatigue properties more effectively. Fractographic examination and analysis shows that the fatigue cracks initiate in the subsurface layer beneath the compressive residual stress field for laser- and shot-peened specimens, whereas the fatigue cracks form at surface for as-machined ones.

Numerical modelling of shot peening process and corresponding products: Residual stress, surface roughness and cold work prediction

Abstract: An investigation of the effects of controlled shot peening (CSP) process parameters on the treated material is presented. For this purpose, a three dimensional numerical model is developed, comprising the target plate and a number of shot impacts; their number is defined as the minimum required for a realistic simulation and minimum computational cost. The numerical model is verified by comparing the predicted residual stress (RS) fields to experimental. A parametric study of the shot velocity and impinging angle on the CSP products is performed for 4 shot types, i.e. S110, S230, S330 and S550. The main advantages of the present numerical model are: a) the relatively high number of shots introduced in the simulation compared to other publications that use only one shot, b) the number of shots dependency on the desired coverage, c) the thorough selection of numerical parameters, d) the high-strain rate material behaviour used for the target plate, e) the capability to calculate CSP effects on the target plate as function of coverage, f) the computed data which include RS field, surface roughness, cold work and geometrical stress concentration factor (Kt) and finally g) the computed results which are validated by experimental measurements.

Depth-resolved residual stress analysis of thin coatings by a new FIB–DIC method

Abstract: A new methodology for the measurement of depth sensitive residual stress profiles of thin coatings with sub-micrometer resolution is presented. The two step method consists of incremental focused ion beam (FIB) ring-core milling, combined with high-resolution in situ SEM-FEG imaging of the relaxing surface and a full field strain analysis by digital image correlation (DIC). The through-thickness profile of the residual stress can be obtained by comparison of the experimentally measured surface strain with finite element modeling using Schajer's integral method. In this work, a chromium nitride (CrN) CAE-PVD 3.0 μm coating on steel substrate, and a gold MS-PVD 1.5 μm on silicon were selected for the experimental implementation. Incremental FIB milling was conducted using an optimized milling strategy that produces minimum re-deposition over the sample surface. Results showed an average residual stress of σ = −5.15 GPa in the CrN coating and σ = +194 MPa in the Au coating. These values are in reasonable agreement with estimates obtained by other conventional techniques. The depth profiles revealed an increasing residual stress from surface to the coating/surface interface for both coatings. This observation is likely related to stress relaxation during grain growth, which was observed in microstructural cross sections, as predicted by existing models for structure–stress evolution in PVD coatings. A correlation between the observed stress gradients and the in-service mechanical behavior of the coatings is proposed. Finally, critical aspects of the technique and the influence of microstructure and elastic anisotropy on stress analysis are analyzed and discussed.

Stability of shot peening induced residual stresses and their influence on fatigue lifetime

Abstract: Mechanical surface treatment methods such as shot peening may improve the fatigue strength of materials. In this study, the effect of shot peening on strain controlled constant amplitude fatigue lo ...

Effects of laser shock processing on mechanical properties and micro-structure of ANSI 304 austenitic stainless steel

Abstract: The aim of this article is to address the effects of a single laser shock processing (LSP) impact on the nano-hardness, elastic modulus, residual stress and phase transformation of ANSI 304 austenitic stainless steel. Residual stress distribution of the LSP-shocked region is determined by X-ray diffraction (XRD) with sin 2 ψ method, and the micro-structural features in the near-surface layer are characterized by using cross-sectional optical microscopy (OM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). By comparing with the untreated samples, LSP can clearly improve nano-hardness, elastic modulus, and residual stress in the LSP-shocked region. The underlying enhancement mechanisms of LSP on nano-hardness, elastic modulus and residual stress of stainless steel ANSI 304 are also revealed. These studies may provide some important insights into surface modification for metal materials.

Effect of laser shock processing on fatigue crack growth of duplex stainless steel

Abstract: Duplex stainless steels have wide application in different fields like the ship, petrochemical and chemical industries that is due to their high strength and excellent toughness properties as well as their high corrosion resistance. In this work an investigation is performed to evaluate the effect of laser shock processing on some mechanical properties of 2205 duplex stainless steel. Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field which increases fatigue crack initiation life and reduces fatigue crack growth rate. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switched Nd:YAG laser, operating at 10 Hz with infrared (1064 nm) radiation. The pulses are focused to a diameter of 1.5 mm. Effect of pulse density in the residual stress field is evaluated. Residual stress distribution as a function of depth is determined by the contour method. It is observed that the higher the pulse density the greater the compressive residual stress. Pulse densities of 900, 1600 and 2500 pul/cm2 are used. Pre-cracked compact tension specimens were subjected to LSP process and then tested under cyclic loading with R = 0.1. Fatigue crack growth rate is determined and the effect of LSP process parameters is evaluated. In addition fracture toughness is determined in specimens with and without LSP treatment. It is observed that LSP reduces fatigue crack growth and increases fracture toughness if this steel.

Experimental study of nonlinear Rayleigh wave propagation in shot-peened aluminum plates—Feasibility of measuring residual stress

Abstract: Shot-peening is widely used in the aerospace industry to enhance the resistance of structural components to fatigue damage and stress corrosion by putting the outside layer of a component under an initial, residual compressive stress. The ability to measure these near-surface residual stresses is useful from a quality control and certification perspective, and can help predict the fatigue life of shot-peened components. This paper presents experimental results to examine the feasibility of measuring near-surface residual stresses using nonlinear Rayleigh surface waves. Experiments are conducted on aluminum alloy (AA 7075) samples shot-peened at different peening intensities and thus with different levels of residual stresses. The surface roughness of these samples is also measured. The nonlinear ultrasonic results show a large increase in the acoustic nonlinearity parameter, indicating the potential of nonlinear ultrasonics for the in situ measurement of near-surface residual stresses. The effects of surface roughness and the driving frequency on the measured acoustic nonlinearity parameter are briefly discussed. Finally, a preliminary model is used to interpret some experimental results. Future work to evaluate the separate contributions of cold work, residual stress and surface roughness to the total measured nonlinearity is also discussed.

Measuring Inaccessible Residual Stresses Using Multiple Methods and Superposition

Abstract: The traditional contour method maps a single component of residual stress by cutting a body carefully in two and measuring the contour of the cut surface. The cut also exposes previously inaccessible regions of the body to residual stress measurement using a variety of other techniques, but the stresses have been changed by the relaxation after cutting. In this paper, it is shown that superposition of stresses measured post-cutting with results from the contour method analysis can determine the original (pre-cut) residual stresses. The general superposition theory using Bueckner’s principle is developed and limitations are discussed. The procedure is experimentally demonstrated by determining the triaxial residual stress state on a cross section plane. The 2024-T351 aluminum alloy test specimen was a disk plastically indented to produce multiaxial residual stresses. After cutting the disk in half, the stresses on the cut surface of one half were determined with X-ray diffraction and with hole drilling on the other half. To determine the original residual stresses, the measured surface stresses were superimposed with the change stress calculated by the contour method. Within uncertainty, the results agreed with neutron diffraction measurements taken on an uncut disk.