Semen Naumovich Lekakh

Three-Dimensional Nonlinear Finite Element Analysis of Hot Radial Forging Process for Large Diameter Tubes

A nonlinear coupled finite element model is developed to predict the behavior of large diameter tubes subjected to mechanical and thermal loadings during hot radial forging process. The model is formulated in a three-dimensional (3D) framework to account for both axial and circumferential effects. This model considers both material and geometric nonlinearities. A rate-dependent material model is used to describe the viscoplastic behavior of the workpiece subjected to high temperature and large strain. A tubular workpiece with the mandrel inside and four forging dies outside is modeled in commercial finite element code. A subroutine is developed and implemented to simplify the modeling process for radial forging simulation. Numerical results presented include residual stress, plastic strain, and temperature distribution along the axial and hoop directions in the deformed workpiece. Results are also presented for contact force to evaluate the performance of the die in the forging process. Finite element model predictions are compared with experimental and two-dimensional (2D) axisymmetric simulation results available in literature. A variety of case studies are conducted for hot radial forging process using the developed 3D model.

Corner Strength of Investment Casting Shells

During the investment casting process, the shell is subjected to high internal pressure and thermal stress, particularly during pattern removal and when pouring steel into the free standing ceramic shell. Most testing methods investigate the properties of the ceramic shell in flat regions while cracks typically form in the sharp corners and edge regions. The corners and edge regions have different structure and thickness when compared to flat regions and experience large mechanical stress during processing. In this study, experimental methods were combined with finite element modeling to predict failure stress in the internal corner regions of the shell. The model takes into consideration the mechanical properties of the ceramic shell to determine the stress developed during loading. The effect of shell porosity on stress concentration in sharp corners was evaluated. A general equation was developed to predict the force necessary for crack formation in the shell based on various geometric variables. The results from the model were experimentally verified and the failure stress in flat and corner regions of the shell were compared in order to develop an improved equation.

Thermo-Chemistry of Non-Metallic Inclusions in Ductile Iron

Non-metallic inclusions formed during multi-stage ductile iron melt treatment play an important role in solidification, structure, and casting quality. Professor Carl Loper made fundamental studies on the role of inclusions in heterogeneous nucleation of graphite. In this article, a comparison of inclusions in steel and cast iron was done using thermodynamic calculations (FACTSAGE software) and automated SEM/EDS inclusion analysis (ASPEX system). Statistics of non-metallic inclusions (size, shape, composition) were studied in the iron and steel samples collected by quenching after different stages of melt treatment as well as from castings. The suggested methodology of inclusion analysis has the potential to be applied together with adaptive thermal analysis (ATAS) for solving practical problems such as decreasing shrinkage defects in ductile iron.

Age Strengthening of Gray Iron-Kinetics Study

Kinetics of gray cast iron strengthening at room and elevated temperature (100°C, 182°C and 285°C) were studied using 100 specimens cast from one industrial heat of gray iron. Tensile strength and Brinell hardness was measured during age strengthening. Peak aging was observed at shorter times for higher temperatures and over-aging was observed at 182°C and 285°C. Statistically significant data was used for evaluating aging kinetics and the determination of activation energies for precipitation.Tensile strength-temperature-time curves were described using Arrhenius and Avrami-Johnson-Mehl kinetics and an attempt was made to create a predictive model for age strengthening in gray iron. Earlier literature and a current study with another foundry indicate improvement of machinability with aging. The proposed model will help in optimizing the process for maximum tool life.

Crack Formation During Foam Pattern Firing in the Investment Casting Process

The application of rigid polymeric foam for large investment casting patterns with complex geometries can improve the dimensional tolerances and the surface quality of the casting. However, these pattern materials have a tendency to promote crack formation in investment casting shells during pattern removal by firing. Experimental methods were combined with finite element modeling to predict stress in the shell. The model takes into consideration the thermal and mechanical properties of the pattern and the shell materials to determine the heat transfer and thermal expansion stresses developed in the shell during firing. The thermal and mechanical properties of the pattern and shell were obtained from experimental tests. A 3D nonlinear finite element model was developed to predict possible crack formation in the shells during pattern removal. The effects of the thermo-mechanical properties of the foam and the shell, as well as the firing process parameters were modeled, and extreme cases were experimentally validated. Recommendations for firing process parameters and pattern design to decrease stress and eliminate crack formation in the shell were formulated.