Prediction of fracture limits of IN625 alloy by coupling ductile damage models and anisotropic yielding functions with the classical Marciniak and Kuczyński model

The fracture forming limit diagram (FFLD) is gaining special attention in high strength materials where the necking tendency rarely occurs during sheet metal forming processes. In the present work, the classical Marciniak and Kuczyński (MK) model has been modified by coupling it with different ductile damage models (Cockcroft and Latham, Brozzo, Oyane, Ko, Oh, Rice and Tracey, McClintock and Clift) and anisotropic yielding functions (Hill 1948 and Barlat 1989) to predict the fracture limits of Inconel 625 (IN625) alloy at different temperatures. Firstly, uniaxial tensile testing has been conducted for the determination of important mechanical properties. Consequently, stretch forming experiments have been performed to analyze the forming limits of a material. It has been found that the safe and fracture forming limits of the material increased by approximately 17.26% and 22.22%, respectively, on increasing the temperature from 300 to 673 K. From the comparative analysis of different combinations of ductile damage models and yielding functions, the Cockcroft and Latham (C-L) damage model in combination with the Barlat 1989 yielding function helped in best predicting the theoretical FFLD as it displayed the least average root mean square error (RMSE) of 0.033. The other ductile damage models used for predicting the theoretical fracture limits displayed large error; hence, they should not be considered while designing a critical component in the manufacturing industry using IN625 alloy.

Formability of wire-arc deposited AISI 316L sheets for hybrid additive manufacturing applications

This paper is focused on the formability of wire-arc additively manufactured AISI 316L stainless steel sheets with the purpose of analysing the feasibility of including this material in hybrid additive manufacturing chains involving sheet metal forming operations. Conventional tensile tests performed in specimens obtained from different orientations to the building direction were carried out to characterise the mechanical properties, the strain loading paths and the limiting strains at fracture. Microstructure observations combined with fractography analysis add insight to the results by establishing a link between the forming limits by fracture and the crack opening mechanisms. Results obtained for wrought commercial AISI 316L stainless steel sheets are included for comparison purposes and reveal that the additively manufactured sheets have a much stronger anisotropic behaviour and poorer formability due to their dendritic-based microstructure. Still, the forming limits obtained from the experiments allow concluding that the additively manufactured sheets can withstand large plastic deformations and, therefore, can be used in hybrid additive manufacturing routes.

Development of Measurement Equipment and Experimental and Numerical Simulation Studies for Warm Forming Limits of High-Strength Steel

This paper describes the research and development of a set of measurement equipment for the warm forming limits of high-strength steel based on the Nakazima bulging test method and the digital image correlation method. The equipment could provide an argon shield and a water-cooling atmosphere, as well as two heating options: heating the specimen, dies, and environment to the test temperature simultaneously or heating the specimen to the test temperature at a higher speed than that for the dies and environment. The equipment was applied to measure the forming limit curves for high-strength DP600 steel at room temperature and at the temperature of 300 °C to verify its performance. The DYNAFORM software was then applied for the digital simulation of the bulging test method. A new limit-strain-fitting method was proposed to eliminate the impact of the distorted grid on the digital simulation process. The change trend of the forming limit curve acquired in the test had sound consistency with the test results.

Determination of Forming Limits Based on Finite Element Simulations

Two categories of experiments have been performed to obtain the experimental forming limits of a ferritic stainless steel from uniaxial to equibiaxial tension, including Nakajima tests and tensile tests of flat specimens with different geometries of the central hole as well as the notched dog bone. The plasticity behavior of the investigated material is described using an evolving non-associated anisotropic plasticity model, which is calibrated based on experimental results of uniaxial tensile tests along different loading directions. A damage mechanics model is calibrated and validated based on the global force and displacement response of tensile tests. Finite element simulations of the Nakajima tests and the tensile tests of various geometries have been performed using the anisotropic material model. A novel spatio-temporal method is developed to evaluate the forming limits under different stress states by quantitatively characterizing the plastic strain distribution on the specimen surface. The forming limits have been independently determined from finite element simulation results of tensile specimens and Nakajima specimens using the spatio-temporal evaluation method. The forming limits obtained from numerical simulations of these two types of experiments are in good agreement with experimental results.