Titanium and nickel alloys represent a significant metal portion of the aircraft structural and engine components. When these critical structural components in aerospace industry are manufactured with the objective to reach high reliability levels, surface integrity is one of the most relevant parameters used for evaluating the quality of finish machined surfaces. The residual stresses and surface alteration (white etch layer and depth of work hardening) induced by machining of titanium alloys and nickel-based alloys are very critical due to safety and sustainability concerns. This review paper provides an overview of machining induced surface integrity in titanium and nickel alloys. There are many different types of surface integrity problems reported in literature, and among these, residual stresses, white layer and work hardening layers, as well as microstructural alterations can be studied in order to improve surface qualities of end products. Many parameters affect the surface quality of workpieces, and cutting speed, feed rate, depth of cut, tool geometry and preparation, tool wear, and workpiece properties are among the most important ones worth to investigate. Experimental and empirical studies as well as analytical and Finite Element modeling based approaches are offered in order to better understand machining induced surface integrity. In the current state-of-the-art however, a comprehensive and systematic modeling approach based on the process physics and applicable to the industrial processes is still missing. It is concluded that further modeling studies are needed to create predictive physics-based models that is in good agreement with reliable experiments, while explaining the effects of many parameters, for machining of titanium alloys and nickel-based alloys.

Machining induced surface integrity in titanium and nickel alloys: A review

Surface integrity in material removal processes: Recent advances

A review of optimization techniques in metal cutting processes

This paper presents an overview of the past research on Surface Integrity (SI) studies in the context of machined components from a range of work materials including stainless steels, Ni and Ti alloys, hardened steels for dies and moulds, bearings and automotive applications. Typical surface alterations such as phase transformations, microhardness and residual stress are discussed and correlated with the functional performance of the machined products. A summary of past and current modelling efforts is then presented along with projections for developing predictive models for SI and means for enhancing product sustainability in terms of its functional performance. Reference to this paper should be made as follows: M'Saoubi, R., Outeiro, J.C., Chandrasekaran, H., Dillon Jr., O.W. and Jawahir, I.S. (2008) 'A review of surface integrity in machining and its impact on functional performance and life of machined products', Int. J. Sustainable Manufacturing, Vol. 1, Nos. 1/2, pp.203-236.

A review of surface integrity in machining and its impact on functional performance and life of machined products

The affected layer is generated within the machined surface layer through the cutting process. Cutting conditions such as the nose radius of the tool, feed rate and shape of cutting edge at the finishing operation affect the residual stress, surface hardness, and surface roughness. In this paper, it is shown that such machined surface property could be controlled by the setting of the cutting conditions to some extent. Then the effect of the machining conditions on the fatigue life was investigated through a fatigue test using the specimen finished under various cutting conditions. It was shown that it is possible to get longer fatigue life for machined parts than the virgin material or the carefully finished material without affected layer, only by setting the proper cutting conditions. Such a situation was realized when the generated residual stress was small and the induced surface hardness was high. A longer fatigue life for the machined components can be obtained by applying such cutting conditions as a low feed rate, a small corner radius and a chamfered cutting edge tool.

The effect on fatigue life of residual stress and surface hardness resulting from different cutting conditions of 0.45%C steel

This paper introduces a more comprehensive experimental model which has the capability of predicting residual stress profile. The main advantage of this model over the existing models that it provides the effect of machining parameters on maximum residual stress and determines both the location and depth of this maximum residual stress. Five different materials namely; stainless steel-304, steel-37, 7001 and 2024-aluminum alloys and brass were machined by turning utilizing one of experimental design techniques based on response surface methodology. Tensile strength of these materials and both cutting speed and feed rates are considered as three input parameters affecting residual stress distribution. The residual stress distribution in the machined surface region was determined using a deflection-etching technique. It is proposed here that the residual stress profile is a deterministic function of the three input parameters used. Also, it is postulated that the residual stress profile along the depth beneath surface is a polynomial function of the depth beneath surface and the coefficients of this polynomial are, in turn, functions of the input parameters. The model has been developed and has been checked for accuracy.

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A method of modeling residual stress distribution in turning for different materials

Roller-burnishing is used in place of other traditional methods to finish 6061-T6 aluminum alloy. How to select the burnishing parameters to improve surface integrity (reduce surface roughness, increase surface microhardness and produce compressive residual stress) is especially crucial. This paper presents an investigation of the effect of roller-burnishing upon surface roughness, surface microhardness and residual stress of 6061-T6 aluminum alloy. The residual stress distribution in the surface region that was burnished is determined using a deflection-etching technique. Mathematical models correlating three process parameters: burnishing speed, burnishing depth of penetration and number of passes, are established. A Group Method of Data Handling Technique, GMDH, is used. It is shown that low burnishing speeds and high depths of penetration produce much smoother surfaces, whereas a combination of high speed with high depth leads to rougher surfaces because of chatter. The optimum number of passes that produces a good surface finish was found to be 3 or 4. The maximum value of compressive residual stress decreases with an increase in burnishing speed. The maximum compressive residual stress increases with an increase in burnishing depth of penetration and/or number of passes.

M.H. El-Axir

Papers

A method of modeling residual stress distribution in turning for different materials

Experimental techniques for studying the effects of milling roller-burnishing parameters on surface integrity