In the present work, fracture forming limit diagram (FFLD) of two different solution treated i.e. 970 °C (HT970) and 1070 °C (HT1070) Inconel-718 sheets were evaluated by carrying out stretch forming experiments. The obtained failure limiting strains were then transformed into effective plastic strain versus triaxiality locus (ƞEPS-FFLD) and major versus minor principal stress space (σ-FFLD). It was observed that the HT1070 material possessed higher ƞEPS-FFLD and lower σ-FFLD indicating improved formability with reduced deformation load. In order to predict the fracture, six different ductile fracture models incorporating anisotropic properties were calibrated with experimental data. Among all the models, Oh model showed comparatively better prediction with an average absolute error of 10.2% and 10.1% for HT970 and HT1070 materials respectively. However, the model was not able to capture the fracture limits along complete triaxiality path ( 0.33 ≤ η ≤ 0.66 ). Therefore, finite element (FE) analysis of deep drawing and stretch forming processes were performed incorporating experimentally evaluated ƞEPS-FFLD and σ-FFLD. The formability in terms of fracture location, limiting draw ratio, wrinkling evolution, thickness distribution, and limiting dome height were predicted successfully using Hills-48 plasticity theory. Also, the microhardness at critical regions of deformed cups was determined and the maximum hardness was found at cup junction of HT970 material especially in case of hemispherical dome cups. Thus, it was suggested that the deep drawn components of HT970 material have better part performance as compared to that of HT1070 material.

Prediction of fracture and deep drawing behavior of solution treated Inconel-718 sheets: Numerical modeling and experimental validation

Forming and fracture limits of IN718 alloy at elevated temperatures: Experimental and theoretical investigation

A promising approach to decrease deformation load and improve ductility during forming of Inconel-718 (IN718) is through the use of solution treated blanks. In the present work, IN718 sheets were separately solution treated to different temperatures in the domain of 970 – 1070 ° C at an interval of 50 ° C to dissolve the strengthening precipitates ( γ ′ and γ ′ ′ ), and subsequently, were oil quenched to retain the microstructure. Due to significant difference in average grain size, the solution treated samples of 970 ° C (HT970) and 1070 ° C (HT1070) were selected for tensile characterization, deep drawing analysis, and micro-texture evolution. The tensile test response showed approximately 30% improvement in ductility and 27% reduction in load in case of HT1070 material with respect to that of HT970 material. The deep drawing process window was evaluated using flat bottom and hemispherical dome punch geometries, and it was found that the limiting draw ratio (LDR) improved marginally by 4.5% in case of HT1070 material. This improvement was due to the dominating presence of cube component in initial texture of HT1070 material. The high presence of cube and goss components in initial texture and the large difference in Taylor factor of individual grains at critical regions of deep drawn cups led to higher unsatisfactory surface roughness in the case of HT1070 material. Hence, it is suggested that the deep drawn components of HT970 material has better part performance in terms of surface roughness. Further, an attempt was made to correlate the fractographs with the fracture and formability behavior of sheet metal.

Effect of solution treatment on deep drawability of IN718 sheets: Experimental analysis and metallurgical characterization

Prediction of earing defect and deep drawing behavior of commercially pure titanium sheets using CPB06 anisotropy yield theory

Improvement in limiting drawing ratio of aluminum tailored friction stir welded blanks using modified conical tractrix die

Recently, aerospace industries have shown increasing interest in forming limits of Inconel 718 sheet metals, which can be utilised in designing tools and selection of process parameters for successful fabrication of components. In the present work, stress-strain response with failure strains was evaluated by uniaxial tensile tests in different orientations, and two-stage work-hardening behavior was observed. In spite of highly preferred texture, tensile properties showed minor variations in different orientations due to the random distribution of nanoprecipitates. The forming limit strains were evaluated by deforming specimens in seven different strain paths using limiting dome height (LDH) test facility. Mostly, the specimens failed without prior indication of localized necking. Thus, fracture forming limit diagram (FFLD) was evaluated, and bending correction was imposed due to the use of sub-size hemispherical punch. The failure strains of FFLD were converted into major-minor stress space (σ-FFLD) and effective plastic strain-stress triaxiality space (ηEPS-FFLD) as failure criteria to avoid the strain path dependence. Moreover, FE model was developed, and the LDH, strain distribution and failure location were predicted successfully using above-mentioned failure criteria with two stages of work hardening. Fractographs were correlated with the fracture behavior and formability of sheet metal.

S. V. S. Naryana Murty

Papers

Microstructures, Forming Limit and Failure Analyses of Inconel 718 Sheets for Fabrication of Aerospace Components