Surface enhancement technologies such as shot peening, laser shock peening (LSP), and low plasticity burnishing (LPB) can provide substantial fatigue life improvement. However, to be effective, the compressive residual stresses that increase fatigue strength must be retained in service. For successful integration into turbine design, the process must be affordable and compatible with the manufacturing environment. LPB provides thermally stable compression of comparable magnitude and even greater depth than other methods, and can be performed in conventional machine shop environments on CNC machine tools. LPB provides a means to extend the fatigue lives of both new and legacy aircraft engines and ground-based turbines. Improving fatigue performance by introducing deep stable layers of compressive residual stress avoids the generally cost prohibitive alternative of modifying either material or design. The X-ray diffraction based background studies of thermal and mechanical stability of surface enhancement techniques are briefly reviewed, demonstrating the importance of minimizing cold work. The LPB process, tooling, and control systems are described. An overview of current research programs conducted for engine OEMs and the military to apply LPB to a variety of engine and aging aircraft components are presented. Fatigue performance and residual stress data developed to date for several case studies are presented including: * The effect of LPB on the fatigue performance of the nickel based super alloy IN718, showing fatigue benefit of thermal stability at engine temperatures. * An order of magnitude improvement in damage tolerance of LPB processed Ti-6-4 fan blade leading edges. * Elimination of the fretting fatigue debit for Ti-6-4 with prior LPB. * Corrosion fatigue mitigation with LPB in Carpenter 450 steel. *Damage tolerance improvement in 17-4PH steel. Where appropriate, the performance of LPB is compared to conventional shot peening after exposure to engine operating temperatures.

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Case Studies of Fatigue Life Improvement Using Low Plasticity Burnishing in Gas Turbine Engine Applications

The present study is aimed at filling the gaps in scientific understanding of the burnishing process, and also to aid and arrive at technological solutions for the surface modifications based on Burnishing of some of the commonly employed engineering materials. The effects of various burnishing parameters on the surface characteristics, surface microstructure, micro hardness are evaluated, reported and discussed in the case of EN Series steels (EN 8, EN 24 and EN 31), Aluminum alloy (AA6061) and Alpha-beta brass. The burnishing parameters considered for studies principally are burnishing speed, burnishing force, burnishing feed and number of passes. Taguchi technique is employed in the present investigation to identify the most influencing parameters on surface roughness. Effort is also made to identify the optimal burnishing parameters and the factors for scientific basis of such optimization. Finally, a brief attempt is made to construct the Burnishing maps with respect to strength level (in this case, average micro hardness of unburnished material).

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Optimization of burnishing parameters and determination of select surface characteristics in engineering materials

Burnishing, a plastic deformation process, can be used to finish surfaces by plastic deformation of surface irregularities. A simple inexpensive and newly designed balburnishing tool, with interchangeable adapter for using different ball diameters, was used throughout the experimental work. In present paper reports on the optimization of the surface finish produced by ball burnishing process, using the surface response (RMS) methodology. A second order mathematical model correlating four predominant process parameters, viz, burnishing force, speed, ball diameter and number of tool pass, with the surface roughness parameter has been obtained. The model can be used in selecting the optimum process parameters for obtaining a desired control surface finish.

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Effect of ball burnishing parameters on surface roughness using surface roughness methodology

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.

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

Burnishing is a method of finishing and hardening machined parts by plastic deformation of the surface. Experimental work based on central composite second order rotatable design has been carried out on a lathe machine to establish the effects of ball burnishing parameters on the surface roughness of brass material. Analysis of the results by the analysis of variance technique and the F-test show that the parameters considered, have significant effects on the surface roughness. Keywords— Ball burnishing, Response surface Methodology.

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Determination of Surface Roughness by Ball Burnishing Process Using Factorial Techniques

Burnishing is a plastic deformation process in which force is applied on to a metal surface through a hard roller or ball. In this paper, an attempt has been made to investigate the optimum burnishing force, change in dimension and the variations of tangential and normal forces in the process. Expressions were derived to calculate analytically the following: (i) optimum burnishing force, (ii) size change in burnishing. The accuracy of these expressions was verified by suitably conducted experiments. Five non-dimensional numbers of importance to burnishing were established. The variation of normal and tangential forces in roller burnishing (and so the coefficient of friction) was investigated experimentally.

B. Kotiveerachari

Papers

Study of some aspects of burnishing