Valutazione della formabilità di lamiere di titanio a freddo e a tiepido

The knowledge of stress and strain relationship and material fracture conditions is fundamental to optimize sheet stamping processes in terms of part feasibility. In fact, the numerical models commonly used in simulation can give reliable results only if the input parameters, such as material characterization at different temperatures and friction condition are correctly estimated. This is particularly important in the case of hard-to-deform materials, such as Titanium and its alloys. The simplest test to be carried out for the definition of material properties is the tensile test that can be executed at different temperatures (Figures 1 and 5). The main limitation of this test is related to the mono-axial stress state; on the contrary, the stress and strain conditions in actual stamping operations are more complex, being 3-dimensional. For this reason several tests have been developed, in particular Nakazima test (Figures 2-4). In this test a 2-dimensional stress state is generated and the maximum deformation at break can be easily identified allowing the definition of safe and unsafe deformation areas. In order to evaluate the influence of strain distribution in the material, several tests are carried out on sheet strips of different widths (Figure 8). In this way, several couples of limit membranal strain states can be obtained and plotted on a diagram reporting the so called FLC (Formability Limit Curves) identifying the safe and unsafe deformation areas (Figure 9). These curves can be obtained for different materials, thicknesses and temperatures. The present research focuses on warm forming of non-axisymmetric components made of titanium CP; this material is characterized by low formability and high springback. Uniaxial tensile and Nakazima tests were performed to investigate the material behavior at room temperature and 300 °C. The flow stress curves derived from uniaxial tensile tests (Figure 9 and Figure 10) have been used to simulate the stamping process of an automotive component (Figure 12) by using the commercial finite element code PamStamp 2G (Figure 13) at room temperature and at 300°C. The simulations confirm that the component cannot be produced at room temperature. However, the warm stamping process (Figure 15 and Figure 16) permit to obtain the component due to the higher material formability at 300°C. In order to validate the simulation results, the Nakazima test has been preliminary simulated and the results in terms of deformation have been compared with the experimental one obtaining a good agreement (Figure 11). Further investigations will be oriented to the evaluation of the optimal temperature for titanium-CP sheet stamping, to the study of the effect on friction of the oxide formed on the sheet surface and to the evaluation of springback effect during the process.

Il lavoro descrive i risultati di prove di trazione e di prove di formabilità limite Nakazima effettuate a temperatura ambiente e a tiepido (300° C) utili per lo sviluppo di un modello simulativo per la valutazione della fattibilità di un elemento di geometria complessa per l'industria automobilistica, da realizzare in lamiera di titanio commercialmente puro. Si dimostra come le caratteristiche del materiale considerato, espresse in termini di legge di flusso e di formabilità, cambino al variare della temperatura di lavorazione rendendo fattibile o meno il prodotto. Il modello FEM e l'applicazione delle curve di flow stress e di formabilità limite (FLC) sono state validate tramite simulazione preliminare delle prove Nakazima stesse. L'approccio proposto è particolarmente adatto per definire, in ambiente virtuale, geometria e parametri di processo per operazioni di stampaggio su materiali innovativi difficilmente deformabili a temperatura ambiente e consente notevoli risparmi di tempo e di costo, evitando operazioni di "trial-and-error" normalmente utilizzate nella fase di setup del processo produttivo.