Walid Jomaa

Surface Finish and Residual Stresses Induced by Orthogonal Dry Machining of AA7075-T651

The surface finish was extensively studied in usual machining processes (turning, milling, and drilling). For these processes, the surface finish is strongly influenced by the cutting feed and the tool nose radius. However, a basic understanding of tool/surface finish interaction and residual stress generation has been lacking. This paper aims to investigate the surface finish and residual stresses under the orthogonal cutting since it can provide this information by avoiding the effect of the tool nose radius. The orthogonal machining of AA7075-T651 alloy through a series of cutting experiments was performed under dry conditions. Surface finish was studied using height and amplitude distribution roughness parameters. SEM and EDS were used to analyze surface damage and built-up edge (BUE) formation. An analysis of the surface topography showed that the surface roughness was sensitive to changes in cutting parameters. It was found that the formation of BUE and the interaction between the tool edge and the iron-rich intermetallic particles play a determinant role in controlling the surface finish during dry orthogonal machining of the AA7075-T651 alloy. Hoop stress was predominantly compressive on the surface and tended to be tensile with increased cutting speed. The reverse occurred for the surface axial stress. The smaller the cutting feed, the greater is the effect of cutting speed on both axial and hoop stresses. By controlling the cutting speed and feed, it is possible to generate a benchmark residual stress state and good surface finish using dry machining.

An Investigation of Machining-Induced Residual Stresses and Microstructure of Induction-Hardened AISI 4340 Steel

Excessive induction hardening treatment may result in deep-hardened layers, combined with tensile or low compressive residual stresses. This can be detrimental to the performance of mechanical parts. However, a judicious selection of the finishing process that possibly follows the surface treatment may overcome this inconvenience. In this paper, hard machining tests were performed to investigate the residual stresses and microstructure alteration induced by the machining of induction heat-treated AISI 4340 steel (58–60 HRC). The authors demonstrate the capacity of the machining process to enhance the surface integrity of induction heat-treated parts. It is shown how cutting conditions can affect the residual stress distribution and surface microstructure. On the one hand, when the cutting speed increases, the residual stresses tend to become tensile at the surface; and on the other hand, more compressive stresses are induced when the feed rate is increased. A microstructural analysis shows the formation of a thin white layer less than 2 µm and severe plastic deformations beneath the machined surface.

Identification and Validation of Marusich’s Constitutive Law for Finite Element Modeling of High Speed Machining

This study aims to identify the coefficients of Marusich’s constitutive equation (MCE) for the aluminum AA7075-T651. Material constants were identified inversely form orthogonal machining tests and from dynamic tests. The proposed material model was successfully implemented in a finite element model (FEM) to simulate the high speed machining of the aluminum AA7075-T6. Deform 2D® software was used. A reasonable agreement between predictions and experiments was obtained. The comparison was based on cutting forces, chip morphology, and tool/chip contact length.© 2014 ASME

FEA-Based Comparative Investigation on High Speed Machining of Aluminum Alloys AA6061-T6 and AA7075-T651

Independent research studies have shown notable dissimilarity in the machining behaviour of aluminum alloys AA6061−T6 and AA7075−T651 commonly used in automotive and aeronautical applications. The present work attempts to investigate this dissimilarity based on experimental and numerical data with a focus on chip formation and generated residual stresses under similar high−speed machining (HSM) conditions. The numerical data were calculated by a finite element modeling (FEM) developed using DeformTM 2D software. The results showed that both studied alloys exhibit different chip formation mechanisms and residual stress states at the machined surfaces. On one hand, the AA6061−T6 alloy generates continuous chips and tensile residual stresses whereas the AA7075−T651 alloy produces segmented chips and compressive residual stresses. FEM results showed that the AA6061−T6 alloy generates lower cutting temperature at the tool−chip interface along with higher equivalent total strains at the machined surface as compared to the AA7075−T651 alloy. Based on the experimental and numerical results, it was pointed out that the differences in terms of thermal conductivity and initial yield stress are the main reasons explaining the dissimilarity observed.

Surface Roughness Effects on the Fatigue Behavior of As-Machined Inconel718

Surface finish of machined components plays a key role in their life performance. The aim of this research is to investigate the effect of different roughness parameters on high cycle fatigue behavior of Inconel718. Rotating bending fatigue tests have been performed on Inconel718 specimens with various surface roughnesses produced by turning. Height and amplitude distribution roughness parameters were investigated. Statistical analyses show that a valley material component (Mr2), as one of the amplitude distribution parameters, is the most relevant parameter for the high cycle fatigue life of machined specimens. Observations conducted at the surface of broken specimens gage length, have shown the impact of surface roughness and residual stresses on the crack propagation mode. When the roughness increases, valleys were shown to be deeper and larger, leading to a higher Mr2 value and an increase of stress concentration.

Identification of material constitutive law constants using machining tests: A response surface methodology based approach

The finite element modeling (FEM) of chip formation is one of the most reliable tool for the prediction and optimization of machining processes; thanks to the high performance of advanced computers and robust finite element codes which made the modeling of complex machining processes (turning, milling, and drilling) becomes possible. The success of any FEM depends strongly on the constitutive law which characterizes the thermo-mechanical behavior of the machined materials. Johnson and Cook’s (JC) constitutive model is widely used in the modeling of machining processes. However, one can find in the literature, different coefficients of JC’s constitutive law for the same material which can affect significantly the predicted results (cutting forces, temperatures, etc.). These differences were attributed to the different methods used for the determination of the material parameters. In the present work, an inverse method, based on orthogonal machining tests, was developed to determine the parameters of the JC constitutive law. The originality of this study lies in the use of the response surface methodology (RSM) as a technique to improve the existing inverse method. The studied material is a 6061T6 high strength aluminum alloy. It is concluded that the calculated flow stresses obtained from the proposed approach were in a good agreement with the experimental ones. Moreover, the material parameters obtained from the present study predict more accurate values of flow stresses as compared to those reported in the literature.