Balasubramanian Nagarajan

Introduction of Enhanced Compressive Residual Stress Profiles in Aerospace Components Using Combined Mechanical Surface Treatments

Mechanical surface treatments such as Shot Peening (SP) and Deep Cold Rolling (DCR) are being used to introduce Compressive Residual Stress (CRS) at the surface and subsurface layers of aerospace components, respectively. This paper investigates the feasibility of a combined introduction of both the surface and sub-surface compressive residual stress on Ti6Al4V material through a successive application of the two aforementioned processes, one after the other. CRS profiles between individual processes were compared to that of combination of processes to validate the feasibility. It was found out that shot peening introduces surface compressive residual stress into the already deep cold rolled sample, resulting in both surface and sub-surface compressive residual stresses in the material. However the drawback of such a combination would be the increased surface roughness after shot peening a deep cold rolled sample which can be critical especially in compressor components. Hence, a new technology, Vibro-Peening (VP) may be used as an alternative to SP to introduce surface stress at reduced roughness.

Modeling Effects of Compliance in Coated Abrasive Tools

Coated abrasive tools are popularly used in the industry for surface finishing. An important characteristic of these tools is their compliance (a grinding wheel is rigid in comparison) which permits smaller amounts of material removal, conformance to curved surface shapes, and assistance in blending the finished surfaces with the unfinished surfaces. This important characteristic is modeled using the finite element method by capturing suitable properties of the coated abrasive tool and its backing pad. Surface finishing experiments are conducted on a flat work surface in a machining center under position control (as opposed to force control) and the area of polished contact is measured along with the normal forces (using a dynamometer). The finite element model results of contact area and forces are compared with that of the experiment. The usefulness of this modeling approach is demonstrated by extracting force per grain and application to curved surfaces.

Microstructure Study of Nickel-Based Superalloys after Deep Cold Rolling

Mechanical surface treatments are conducted on aerospace components in order to improve their fatigue life through inducing compressive residual stresses, cold work and smoother surface finish. The microstructures of the component surface and subsurface after treatment influence the crack nucleation and crack propagation significantly. This paper studies the effect of Deep Cold Rolling (DCR), a subsurface process using hydrostatically controlled balls, on the resulting microstructure of RR1000, a nickel-based superalloy used in high temperature aerospace applications. In this study, DCR of RR1000 was conducted by varying the diameter of the roller ball with a constant fluid pressure and overlap. Vickers microhardness was measured to characterize the work hardening behavior during DCR. The microstructure of RR1000 subsurface before and after DCR along both the rolling and transverse directions is analyzed further. The results show that deep cold rolling results in a significant variation on the microstructure of RR1000 including elongation of matrix grains and the precipitates, within a depth of 10 μm from the rolling surface. The change in microstructure along the rolling surface is found to be more prominent than along the transverse directions irrespective of the ball diameter.

Finite Element Analysis of the Effect of Flexible Pad on the Deformation of Metal Foils in Flexible Pad Laser Shock Microforming

Flexible Pad Laser Shock Forming (FPLSF) is a new microforming process using laser-induced shock pressure and a flexible pad. This process involves high strain-rate (~105 s-1) plastic deformation of metallic foils along with the hyperelastic deformation of the flexible elastomer pad over which the foil is positioned. This paper studies the influence of flexible pad on the shockwave propagation behavior and the plastic deformation of metal foil in FPLSF using finite element analysis. The effect of flexible pad materials such as silicone rubber, polyurethane rubber and natural rubber on the deformation of copper foils has been analysed in detail. An increase in crater depth is observed with the reduction in flexible pad hardness. However, it is found that there exists an optimum hardness of the flexible pad to achieve perfect hemispherical craters on metal foils, as bending of foils at non-deformed region is observed with softer pads whereas flattening of crater surface occurs with harder pads. The effect of flexible pad thickness on the foil deformation was analyzed at six different thickness levels: 300 μm, 600 μm, 900 μm, 1200 μm, 1500 μm, and 2000 μm. Similarly, there exists an optimum flexible pad thickness to maximize the crater depth and achieve the hemispherical shapes. Analysis of flexible pad thickness indicates that the pad thickness influences the elastic recovery of the flexible-pad and hence the plastic deformation of the metallic foils.

Investigation of Copper Foil Thinning Behavior by Flexible-Pad Laser Shock Forming

This paper reports on a novel microforming technique, Flexible-Pad Laser Shock Forming (FPLSF) which uses laser-induced shock waves and a flexible pad to induce plastic deformation on metallic foils. Thickness distribution at the cross-section of the craters formed by FPLSF is analyzed experimentally with respect to laser fluence, which is a significant process variable that controls the deformation pressure. Furthermore, hardness of the deformed samples at the cross-section is measured by nanoindentation testing. It is found that the thinning of copper foil by FPLSF ranges from 7% to 25% for laser fluence ranging between 7.3 J/cm2 and 20.9 J/cm2. Thinning is maximum at the crater center, which can be attributed to the maximum compressive stresses in the thickness direction, and minimum at the edge portions. With increase in laser fluence, thinning of the foil increases whereas minimum change in hardness is observed. The variation in thinning across different crater locations ranges between 6% and 8% only, which indicates that FDLSF can be developed as a competitive technique to produce components with uniform thickness distribution.

The Effects of Ablative Coating Thickness at Various Laser Intensities and Multiple Laser Pulses on Thin Copper Sheet Formability in High Strain Rate Laser Shock Forming

High strain rate metal forming using laser induced shockwave pressure is used for micro‐scale deformation. Ablative coating is one of its significant process components as it generates shockwave pressure through vaporization into plasma by laser irradiation as well as impedes the shockwave propagation. The effect of ablative coating thickness on the forming behavior of thin copper foils is investigated in this paper. Various laser intensities have been used in parallel with changes in coating thickness to further study this effect. The amount of plastic deformation is analyzed with respect to coating thickness, laser intensity and the number of pulses. It is found that there is an optimum coating thickness for a given laser intensity to achieve maximum deformation depth at the center of the formed cavity. The deformation depth shows a monotonic relationship with the laser intensity. Multiple pulses improve the deformation characteristics by limiting the ablation of workpiece material.