Anisotropy and tension–compression asymmetry modeling of the room temperature plastic response of Ti–6Al–4V

Constitutive modeling and the effects of strain-rate and temperature on the formability of Ti–6Al–4V alloy sheet

A novel hot stamping process for Ti6Al4V alloy using cold forming tools and a hot blank was presented in this paper. The formability of the material was studied through uniaxial tensile tests at temperatures ranging from 600 to 900°C and strain rates ranging from 0.1 to 5 s−1. An elongation ranging from 30% to 60% could be achieved at temperatures ranging from 750 to 900°C respectively. The main microstructure evolution mechanisms varied with the deformation temperature, including recovery, phase transformation and recrystallization. The hardness of the material after deformation first decreased with the temperature due to recovery, and subsequently increased mainly due to the phase transformation. During the hot stamping tests, qualified parts could be formed successfully at heating temperatures ranging from 750 to 850°C. The forming failed at lower temperatures due to the limited ductility of the material. At temperatures higher than 900 °C, extensive phase transformation of α to β occurred during the heating. During the transfer and forming, the temperature dropped significantly which led to the formation of transformed β, reduction of the formability and subsequent failure. The post-form hardness distribution demonstrated the same tendency as that after uniaxial tensile tests.

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Formability and microstructure evolution mechanisms of Ti6Al4V alloy during a novel hot stamping process

In this work constitutive models suitable for thermo-mechanical forming of the titanium alloy Ti-6Al-4V are evaluated. A tool concept for thermo-mechanical forming of a double-curved sheet metal component in Ti-6Al-4V is proposed. The virtual tool design is based on finite element (FE) analyses of thermo-mechanical sheet metal forming in which two different anisotropic yield criteria are evaluated and compared with an isotropic assumption to predict global forming force, draw-in, springback and strain localisation. The shape of the yield surface has been found important and the accuracy of the predicted shape deviation could be slightly improved by including the cooling procedure. The predicted responses show promising agreement with the corresponding experimental observations when the anisotropic properties of the material are considered.

Thermo-mechanical sheet metal forming of aero engine components in Ti-6Al-4V—PART 2: Constitutive modelling and validation

This paper compares the predictions of an isotropic thermo-elasto-viscoplastic approach and of an anisotropic thermo-elastoplastic one with experimental results representative of the mechanical behavior of Ti–6Al–4V at moderate temperatures and low strain rates. The first model is the well known Norton–Hoff viscoplastic constitutive law with isotropic von Mises yield locus identified by using monotonic tension tests performed at strain rates from 10 −3  s −1 to 10 −1  s −1 and at temperatures up to 400 °C. The second model is a thermo-elasto-plastic one defined by the orthotropic yield criterion CPB06. It takes into account the anisotropy and the strength differential (SD) effect in tension–compression of Ti–6Al–4V at RT, 150 °C and 400 °C. The identification of the SD effect is done by using tension and compression tests and the anisotropy behavior is identified by using shear, plane strain, tension and compression tests performed in three orthogonal material directions. The accuracy of the load and displacement predictions of the two macroscopic constitutive models are compared to experimental results obtained from tests performed on specimens with multiaxial loadings and large strain at several temperatures.

Impact of anisotropy and viscosity to model the mechanical behavior of Ti–6Al–4V alloy

Ti-6Al-4V is one of the most frequently used titanium alloy in aerospace applications such as for load carrying engine structures, due to their high strength to weight ratio in combination with favourable creep resistance at moderate operating temperatures. In the virtual development process of designing suitable thermo-mechanical forming processes for titanium sheet metal components in aero engine applications numerical finite element (FE) simulations are desirable to perform. The benefit is related to the ability of securing forming concepts with respect to shape deviation, thinning and strain localisation. The reliability of the numerical simulations depends on both models and methods used as well as on the accuracy and applicability of the material input data. The material model and related property data need to be consistent with the conditions of the material in the studied thermo-mechanical forming process. In the present work a set of material tests are performed on Ti-6Al-4V at temperatures ranging from room temperature up to 560°C. The purpose is to study the mechanical properties of the specific batch of alloy but foremost to identify necessary material model requirements and generate experimental reference data for model calibration in order to perform FE-analyses of sheet metal forming at elevated temperatures in Ti-6Al-4V.

J. Hertzman

Papers

Thermo-mechanical sheet metal forming of aero engine components in Ti-6Al-4V – PART 1: Material characterisation