____________________ * Corresponding author: University Park, Nottingham, NG7 2RD, Tel (+44) 115 8467675; aditya.deshpande@nottingham.ac.uk THERMO-MECHANICAL ANALYSIS AND LIFING OF Ni-Cr SPF TOOL Aditya A Deshpande 1 *, Sean B Leen 2 , Wei Sun 1 and Thomas H Hyde 1 1 Department of Mechanical, Materials & Manufacturing Engineering, Faculty of Engineering, University of Nottingham, NG7 2RD, UK 2 Department of Mechanical and Biomedical Engineering, College of Engineering and Informatics, National University of Ireland, Galway, Ireland 2 sean.leen@nuigalway.ie ABSTRACT: The paper describes the aniso-thermo-mechanical analysis of a representative large industrial SPF (XN40F, high nickel-chromium) tool. Sequentially coupled thermo-mechanical analyses under realistic loading conditions are developed within a general purpose non-linear FE code, ABAQUS to predict and analyse the complex temperature-stress-strain cycles of the SPF tool. A temperature-dependent, two-layer visco-plasticity model which combines both creep and combined isotropic-kinematic plasticity is chosen to represent the material behaviour. The material constants are identified from multiple strain-range isothermal cyclic tests and stress relaxation tests, over a range of temperatures between 20 o C and 900 o C. The FE predicted stress-strain data is used in stress-strain-life equations obtained from isothermal fatigue lifing tests at 900 o C and the identified constants are validated against the TMF tests and simulative SPF tool test, designed to represent the temperature and stress-strain cycling associated with the most damaging phase of the tool cycle. The predicted SPF tool life is consistent with the simulative SPF tool test life and industrial observations. KEYWORDS: thermo-mechanical fatigue; superplastic forming; XN40F; tools; Ostergren model; Zamrik model; two- layer visco-plasticity model 1 INTRODUCTION This paper is concerned with the development of life prediction methods for the harsh thermo- mechanical conditions experienced by superplastic forming tools. This includes high temperature cyclic loading and steady mechanical loading for extended durations. The tools are manufactured by casting from specialized alloys. The cost of tool failure is significant and tool life is an important limiting aspect in the SPF process. Large SPF tools are susceptible to distortion due to thermal cycling and high temperature creep, as well as cracking due to fatigue, creep-fatigue and thermo-mechanical fatigue. Cyclic thermal gradients arise from the heating and cooling of tools (low frequency) and from the opening and closing of the SPF press during blank insertion and (formed) part removal (higher frequency). In addition, cyclic mechanical loads result from self-weight, clamping and pressurization (due to the forming cycle). A significant amount of work has been carried out by Bernhart and co-workers at Ecoles des Mines in Albi on hot working tools [1-4]. Atwo-layer visco-plasticity model, available within the commercial FE code ABAQUS, which combines both creep and combined isotropic-kinematic plasticity is chosen to represent the SPF tool material behaviour which is XN40F (40% Ni. 20% Cr, balance Fe). A series of isothermal strain-controlled fatigue, thermo- mechanical fatigue-creep and stress relaxation tests were conducted to identify the material and failure constants [5]. The process of material and failure constant identification and validation against experimental temperature-stress-strain loops is described in [6]. Sequentially coupled thermo- mechanical analyseswere performed to simulate the complex temperature-stress-strain cycles of the SPF tool. The temperature-dependent material model is furthermore applied to a simulative TMF test, designed to represent the temperature and stress-strain cycling associated with the most damaging phase of the tool cycle at a predicted critical location. Thermo- mechanical fatigue life predictions of the SPF tool were made using the Ostergren and the Zamrik model. 2 MATERIAL BEHAVIOUR MODEL The SPF tool is cast from nickel- chromium alloy. The composition of XN40F tool material is given in Table 1. Temperature-dependent thermal properties, such as thermal conductivity and specific heat, employed in the heat transfer analysis of the SPF tool are shown in Table 2. Table 1. Composition of the SPF tool material XN40F Elements C Ni Cr Fe Weight % 0.35 40.0 20.0 Balance The material constitutive model employed here is a temperature-dependent two-layer viscoplasticity model, where cyclic plasticity behaviour is captured using combined non-linear kinematic/isotropic hardening and viscoplasticity behaviour is captured via the Norton power law for secondary creep. The multiaxial equations for the two-layer viscoplastic model are defined as follows: © Springer-Verlag France 2010 Int J Mater Form (2010) Vol. 3 Suppl 1:1151 1154 – DOI 10.1007/s12289-010-0976-9