Damage and fracture during sheet-metal forming of alloy 718

Abstract

Forming nickel-based superalloy aero-engine components is a challenging process, largely because of the risk of high degree of springback and issues with formability. In the forming tests conducted on alloy 718 at room temperature, open fractures are observed in the drawbead regions, which are not predicted while evaluating the formability using the traditional forming-limit diagram (FLD). This highlights the importance of an accurate prediction of failure during forming as, in some cases, may severely influence the springback and thereby the accuracy of the predicted shape distortions, leading the final shape of the formed component out of tolerance. In this study, the generalised incremental stress-state dependent damage model (GISSMO) is coupled with the isotropic von Mises and the anisotropic Barlat Yld2000-2D yield criteria to predict the material failure in the forming simulations conducted on alloy 718 using LS-DYNA. Their effect on the predicted effective plastic strains and shape deviations is discussed. The failure and instability strains needed to calibrate the GISSMO are directly obtained from digital image correlation (DIC) measurements in four different specimen geometries i.e. tensile, plane strain, shear, and biaxial. The damage distribution over the drawbeads is measured using a non-linear acoustic technique for validation purposes. The numerical simulations accurately predict failure at the same regions as those observed in the experimental forming tests. The expected distribution of the damage over the drawbeads is in accordance with the experimental measurements. The results highlight the potential of considering DIC to calibrate the GISSMO in combination with an anisotropic material model for forming simulations in alloy 718.

Prediction of shape distortions during forming and welding of a double-curved strip geometry in alloy 718

“…As stated previously in this section, the results regarding the strip geometry are based on the forming simulation coupled with the GISSMO model. Previous work by Pérez et al [2] shows the use of the GISSMO model in a forming simulation of a double-curved component when the presence of microcracks in some of the drawbeads could not be predicted using a forming limit diagram (FLD). The mapping of the results from the forming mesh in Fig.…”

“…Figure 14 a and b illustrates the simulated draw-in at the final stage of the forming procedure. The effect of adding the generalised incremental stress-state dependent damage model (GISSMO) proposed by Pérez Caro et al [2] for the simulated draw-in was also studied, as shown in Fig. 14 c and d.…”

“…The GISSMO model assures that the effects of material failure, such as local thinning and strain localization, are captured during the forming procedure. The GISSMO model was calibrated including regularization for the mesh refinement during the FE analysis [2]. Based on the difference between the predicted and measured draw-ins for the two considered cases in Table 4, the friction coefficient was assumed 0.275, similar to previous results by Wiklund and Larsson [23] using pin-ondisc tests.…”

“…Odenberger et al [1] studied the compensation of deep-drawing prototype tools for springback in alloy 718 sheets, leading to a significant reduction of lead time, while maintaining high accuracy of the predicted shape deviations. By considering the anisotropic plastic properties of alloy 718 and including models for damage and failure, as Pérez Caro et al [2] investigated, new suitable forming procedures have been developed. Additionally, thermomechanical finite element analyses (FEA) in studies on welding and heat treatments have greatly contributed to manufacturing processes in various industries.…”

Springback prediction and validation in hot forming of a double-curved component in alloy 718

“…The mechanical properties of the material are determined between 20 and 1000°C and used as input to the novel thermo-mechanical version of the Barlat Yld2000-2D material model based on previous work by Barlat et al [16]. In this work, the influence of using an isotropic yield criterion in the predicted amount of springback in a hot forming simulation has been assumed a similar behaviour as previously reported by Odenberger et al [3,5] and Pérez Caro et al [17]. A qualitative analysis of the shape of the stress-strain curves at high temperatures is performed.…”