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2 Archives of Civil and Mechanical Engineering

2 2021

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Prediction of forming limits and microstructural evolution during warm stretch forming of DP590 steel

100%

Pandre S. , Morchhale A. , Kotkunde N. , Singh S. K. , Ravindran S.

Archives of Civil and Mechanical Engineering

2021

tom Vol. 21, no. 3

397--419

Dual-phase (DP) steel has an excellent blend of various mechanical properties; hence it is used immensely in the automotive industries. It is challenging to form high strength DP steel into desirable complex shapes because of their limited formability at room temperature conditions. One of the proven alternatives is warm/hot forming. In-detail investigation of forming limits over DP590 steel has been carried out in present work. Firstly, various constitutive models and yield criteria have been formulated for DP steel at different temperatures and strain rates. The modified Arrhenius (m-A) constitutive model and Barlat 1989 yielding function displayed the best prediction of flow stress and anisotropic yielding behavior, respectively. The experimental forming limits (FLD) were evaluated at 300, 473 and 673 K temperatures using Nakazima tests. The forming limits of the material are improved by approximately 24% on increasing the temperature from 300 to 673 K. The textural analysis of the deformed surface has been done using electron back scattered diffraction (EBSD) studies, and γ fibers are found to be responsible for improvement in the formability of the material. Additionally, Marciniak and Kuczynski (MK) model was used to predict the theoretical FLD using all the possible combinations of constitutive models and yield criteria. Finally, the m-A constitutive model, along with Barlat 1989 yielding function has shown the best prediction for forming limits at all the temperatures. The finite element study has also been performed using mentioned material models for accurate prediction of dome height, surface strain and thickness distribution across the specimens.

Fracture limit analysis of DP590 steel using single point incremental forming: experimental approach, theoretical modeling and microstructural evolution

86%

Pandre S. , Morchhale A. , Mahalle G. , Kotkunde N. , Suresh K. , Singh S. K.

Archives of Civil and Mechanical Engineering

2021

tom Vol. 21, no. 3

138--157

The single point incremental forming (SPIF) process is gaining special attention in the aerospace, biomedical and manufacturing industries for making intricate asymmetric components. In the present study, SPIF process has been performed for forming varied wall angle conical and pyramidal frustums using DP590 steel. Initially, the conventional stretch forming process has been performed for finding the fracture forming limit diagram (FFLD). Further, it has been validated with the limiting strains found using SPIF process. The conical and pyramidal frustums deformed near to the plane strain and biaxial region, respectively. The theoretical FFLD has been predicted using seven different ductile damage models. The effect of sheet anisotropy while predicting the fracture strains has been included using Hill 1948 and Barlat 1989 yielding functions. Among the used damage models, the Bao-Wierzbicki (BW) model along with Barlat 1989 yield criterion displayed the least error of 2.92% while predicting the fracture locus. The stress triaxiality in the different forming region has been thoroughly investigated and it has been found that the higher triaxiality value reveals high rate of accumulated damage which lead to early failure of the material in the respective region. The stress triaxiality and effective fracture strains have also been found to be significantly affected by the anisotropy. The micro-textural studies have also been performed and it has been found that the increase in local misorientations and shift in the textural components from γ-fiber to ε-fiber in the corner region of the frustums worked towards limiting the formability of material and ultimately leading towards the fracture of frustums.