Prediction of Machining Induced Surface Integrity using Elastic- Viscoplastic Simulations and Temperature-Dependent Flow Softening Material Models in Titanium and Nickel-based alloys

In this study, the feasibility of predicting surface integrity and residual stresses by using elastoviscoplastic finite element simulations and temperature-dependent flow softening constitutive material modeling is investigated. A friction determination method is proposed to identify friction coefficients in presence of tool flank wear. Serrated and cyclical chip formation has been simulated for using tools with and without flank wear. The predicted residual stresses and surface integrity is compared against experimental results from literature. Effect of friction on the residual stress profiles is also investigated. These results are highly essential in predicting machining induced microstructure alterations that are detrimental to fatigue life of nickel and titanium alloy components. Introduction Titanium and nickel alloys are difficult-to-machine materials with considerable manufacturing problems such as machining induce surface integrity and residual stresses [1, 2]. Titanium and its alloys are today used in many industries including aerospace, automotive and medical device. Specifically, Ti-6Al-4V alloy is the most suitable because it offers favorable mechanical characteristics such as high strength-to-weight ratio, toughness, superb corrosion resistance and bio-compatibility. Nickel-base super alloys are often used in mission critical components such as in aircraft/industrial gas turbine engines. Particularly, IN718 nickel alloy is widely utilized and rated as extremely difficult due to the high toughness and work hardening behavior in which a work hardened layer forms in response to the machining induced deformations on the subsurface [3]. When these critical structural components in 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 [4]. There are many studies on the issue of surface integrity of machined parts, and an extensive review of such studies has already been done [4-6]. Recently, with the use of modified material behavior models and elasto-viscoplastic deformations based Finite Element (FE) simulations have begun to offer solutions for a rich set of field variables, providing much detailed insight for the chip formation processes in titanium alloys [7,8]. Elastoviscoplastic FE simulations can also enable realistic prediction of machining induced surface integrity, residual stresses and optimization of tool micro-geometry and machining parameters without running costly experimentation. However, most FE modeling approaches suffer from numerical convergence in elasto-viscoplastic analysis (especially in 3D analysis), lack of reliable material models to represent micromechanical and microstructural changes such as dynamic recrystallization and phase transformation during chip formation and friction characteristics to simulate a realistic chip formation process and accurate calculation of output variables such as strain, stress and temperature distributions. In this paper, elasto-viscoplastic finite element simulations with newly developed material models are utilized to predict machining induced residual stresses on Ti-6Al-4V titanium and IN718 nickel-based alloys. Advanced Materials Research Vol. 223 (2011) pp 401-410 Online available since 2011/Apr/19 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.223.401 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 192.12.88.154-22/06/12,00:25:08) Temperature Dependent Flow Softening Material Models Titanium Alloy Ti-6Al-4V. In machining titanium alloys, it is commonly known that plastic instability and adiabatic shearing related chip serration occurs. The underlying cause of chip serration is often associated with adiabatic shearing. Recent studies have considered the effects of flow softening and adiabatic shearing effect on the behavior of titanium Ti-6Al-4V alloy at high strains. Flow softening phenomenon can be described as offering less resistance to local plastic deformations due to rearrangement of dislocations caused by subsequent cycling or dynamic recrystallization in the material. This phenomenon is usually observed during an increase in strain beyond a critical strain value together with a rapid rise in material’s temperature. Flow softening is believed to cause adiabatic shearing within the primary shear zone and chip segmentation with shear bands are formed as the deformed material leaves this zone [8]. For this reason, modified material constitutive models with flow softening resulting from strain softening and temperature softening are sought in literature. By developing such a modified Johnson-Cook (JC) constitutive model and implementing it into Finite Element software, Forge-2D by Calamaz et al. [7] and Deform-2D by Sima and Özel [8] were able to simulate serrated chip formation in machining of titanium alloy Ti-6Al4V. In the modified JC material model given in Eq. (1), the influence of strain, strain rate, temperature and temperature dependent strain softening on the flow stress is defined by four multiplicative terms. σ = A + Bε ε 1 + Cln ε ε 1 − D + 1 − D tanh  ! " # (1) where D = 1 − $ , p = ' , σ is flow stress, ε is true strain, ( is true strain rate, ε0 is reference true strain, and T, Tm, T0 are work, material melting and ambient temperatures respectively. The experimental flow stress data by Lee and Lin [9] has been taken as the base for this modified material model. The most optimum set of model parameters that was identified with inverse analysis are; A=724.7 MPa, B=683.1 MPa, n=0.47, C=0.035, m=1.0, Tm=1604°C, a=2, s=0.05, r=2, d=1, b=5. The details of this methodology are outlined by Sima and Özel [8] and flow stress curves are given in Fig.1. Fig. 1: Modified flow stress curves for Ti-6Al-4V versus the JC model by Lee and Lin [9]. Nickel Based Alloy IN718. On the other hand, dynamic material behavior data for Inconel 718 do not appear in the published literature with a very few exceptions. Zhang et al. [10] investigated strain-rate sensitivity of IN718 at high temperatures and presented compression test results at temperatures from 960 to 1040°C, with strain rates from 0.001 to 1.0 s. One of the revealing results of that study was the flow softening behavior of the flow stress curve at strain rate of 1.0 s and temperature of 980°C above the strain value of 0.3. DeMange et al. [11] have studied high strain rate compression behavior of IN718 under annealed and aged conditions at strain rate ranges of 1796-3506 s for annealed and 1681-4581 s for aged IN718 material. Flow stress curves of the annealed material exhibited strong strain hardening at all strains, but the curves of the aged material showed sharp softening effect around a strain value of 0.1 and remained constant after strain value of 0.25. Pereiera and Lerch [12] referred to that data and suggested the JC constitutive model without considering thermal softening effects (A= 1350 MPa, B= 1139 MPa, n= 0.6522, C= 0.0134) for 100 300 500 700 90