Materials Science Forum Vol. 762

Abstract: Accurate modeling of dynamic recrystallization (DRX) is highly important for forming processes like hot rolling and forging. To correctly predict the overall level of dynamic recrystallization reached, it is vital to determine and model the critical conditions that mark the start of DRX. For the determination of the critical conditions, a criterion has been proposed by Poliak and Jonas. It states that the onset of DRX can be detected from an inflection point in the work hardening rate as a function of flow stress. The work hardening rate is the derivative of the flow stress with respect to strain. Flow curves are in general measured at a certain sampling rate, yielding tabular stress-strain data, which are per se not continuously differentiable. In addition, inevitable jitter occurs in measured flow curves. Hence, flow curves need to be interpolated and smoothed before the work hardening rate and further derivatives necessary for evaluating the criterion by Poliak and Jonas can be computed. In this paper, the polynomial interpolation originally proposed by Poliak and Jonas is compared to a new approach based on radial basis functions using a thin plate spline kernel, which combines surface interpolation of various flow curves and smoothing in a single step. It is shown for different steel grades that the interpolation method used has a crucial influence on the resulting critical conditions for DRX, and that a simultaneous evaluation by surface interpolation might yield consistent critical conditions over a range of testing temperatures.

Abstract: ncrease of requirements to quality of the flat-rolled steel causes the appearance of new methods and improvement of the well-known approaches to improving the geometry of thin-gage rolled steels. On modern rolling mills, control and management of the strips cross-section is carried out by a profile and flatness control system (PFC). The main channel of the influence of such systems on the geometric parameters of the rolled steel is the axial shifting of S-shaped working rolls in CVC (Continuous Variable Crown) mills. The performance of the PFC is determined by the correct choice of the initial (machining) profile of the working rolls. The paper discusses the way to improve the methodology of grinding S-shaped working rolls by taking into account the stresses, arising in the zone of contact strip with rolls, the elastic deformation of roll system, thermal deformations, arising from the contact with the hot strip and wear of the working layer of the rolls during the campaign. The developed mathematical model of stress-deformed state allows you to determine the places with the highest stresses in the zone of contact strip with working rolls and adjust the initial profile of the rolls to ensure uniformity of extracts of a strip in width with regard to shifting rolls in the axial direction. The mathematical model of the thermal condition of S-shaped rolls, which allows to predict the change of the rolls thermal profile, gives the possibility to create the algorithm for the grinding of the working rolls in hot condition and increase the accuracy of performance of their machining profile.

Abstract: In this paper a numerical investigation on the void nucleation behaviors under combined mechanical and thermal cycling conditions have been performed. A finite element unit cell model is conduct to calculate the local stress-strain field and describe the process of void nucleation from inclusion. Numerical results show that the thermal mismatch stress between the particles and matrix can assist the external load to cause interface debonding. Under certain mechanical and thermal cycling conditions, complicated stress and strain hystereses developed in the matrix. Both the plastic strain and plastic energy density of the interface will be accumulated during every thermal cycle. The plastic energy accumulation of the interface will first reach the debonding value and failure occurs. Based on the numerical calculation, a new energy based failure criteria is proposed to characterize the behaviors of void nucleation under combine mechanical and thermal cycling conditions.

Abstract: Simulation of some, or all, steps in a manufacturing chain may be important for certain applications in order to determine the final achieved properties of the component. The paper discusses the additional challenges in this context.

Abstract: Ring rolling is an incremental bulk forming process. Hence, the process consists of a large number of alternating deformations and dwell steps. For accurate calculations of material flow and thus ring geometry and rolling forces in hot ring rolling processes, it seems necessary to consider material softening due to static and post dynamic recrystallization which could occur between two deformation steps. In addition, due to the large number of cycles, the modeling results, especially the prediction of grain size, can easily be affected by uncertainties in the input data. However, for small rings and ring material with slow recrystallization kinetics, the interpass times can be short compared to the softening kinetics and the effect of softening can be so small, that microstructure evolution and the description of the materials flow behavior can be de-coupled. In this paper, a semi-empirical JMAK-based model for a stainless steel (1.4301/ X5CrNi18-9/ AISI304) is presented and evaluated by the use of experiments and other investigations published in [1],[2]. Finite Element (FE) simulations of a ring rolling process with a high number of ring revolutions and thus multiple, incremental forming steps were conducted based on ring rolling experiments. The FE simulation results were validated with the experimentally derived rolling force and evolution of ring diameter. The microstructure evolution was calculated in a post processing step considering the investigated evolution of strain and temperature. In this calculation the interrelations between the fraction of dynamically recrystallized microstructure, the evolution of post-dynamically recrystallized microstructure and the final grain size have been considered. Both, the calculated final microstructure and the evolution of rolling force and ring geometry calculated stand in good agreement with the experimental investigations.

Abstract: Local laser heat treatment is an efficient method to manufacture tailored heat-treated steel strips. It can be applied to soften narrow zones of the strip in order to improve its formability on desired areas. However, the properties achieved are dependent on several process parameters. An objective is to develop a predictive model to optimize the heat treatment parameters instead of using experimental trials. In the present study, a finite element model was applied to predict the maximum temperature and heating and cooling rates, as well as the heat distribution along the heat treated area. To develop the model and to test its feasibility, experiments were performed, in which process parameters were varied to study their effects on temperature distribution in a 6 mm thick abrasion resistant steel grade. Scanning of a laser beam was used to optimize the width and depth of the heat-affected zone.In practice, local laser heat treatment process parameters have to be optimized with care for successful results. The most important task is to minimize the temperature gradient between the surfaces and to keep the peak temperatures close to the austenitizing temperature. The results indicate that a simple model can be used to predict the outcome of the heat treatment, so that finite element modeling can be adopted as a suitable tool for design of local heat treatments, allowing more advanced treatments and applications with complex geometries.

Abstract: The study has been performed concerning the influence of temperature field created by heat conduction between sample and the anvils, and the influence of lubrication condition (friction coefficient) on thermal mechanical compression test with numerical simulation, with regard to deformation resistance and distributions of strain and strain rate. The results show that temperature field has an effect on deformation resistance and distributions of strain and strain rate. That will influence on the results from thermal mechanical simulation tests. Further, the results also show that friction coefficient has no influence on deformation resistance, but the friction coefficient will result in uneven distribution of strain and strain rate.

Abstract: Three-dimensional finite element simulation (3D FES) and experiments were carried out for analyzing the deformation behavior, homogeneity, microstructure and properties of 5052 Al alloy during groove pressing (GP) with two different processing conditions, that is, constrained groove pressing (CGP) and unconstrained groove pressing (UGP). The simulation results show that the values of the equivalent strain and its distribution depend strongly on the constrained conditions. Especially, the equivalent strain and its distribution are inhomogeneous along X direction, with lower strain regions located at both ends and larger strain regions with periodic variation located at intermediate section. CGP results in a higher accumulative rate of equivalent strain than that of UGP. The average strain is equal to the theoretical strain for CGP, but it is much lower than its theoretical strain for UGP. The experimental results show that grain sizes of 5052 Al alloy can be refined significantly by CGP or UGP, while CGP has a higher rate of grain refinement and finer grains than that of UGP. And the results of microhardness confirmed the prediction of 3D FES.

Abstract: The hot compression behavior of in situ TiB whiskers reinforced Ti6Al4V (TiBw/Ti6Al4V) composites with a novel network microstructure is investigated in the temperature range of 900-1100°C and strain rate range of 0.001-10 s^-1. The results show that all the stress-strain curves of the composites display peak flow, softening and steady-state. Moreover, the peak flow stress decreases with increasing temperatures and decreasing strain rates. Processing map of the composite is constructed using the dynamic material model (DMM). Dynamic recrystallization (DRX) of α phase is observed in the deformation region corresponding with peak efficiency of the processing map. However, the flow instability region ranged from 900 to 1100°C at strain rates higher than 1.0 s^-1 should be avoided.

Abstract: Bubble migrations in liquid titanium melt under hypergravity field is modeled using commercial computational fluid dynamics software FLUENT 6.3 (Fluent inc., USA). The two-phase fluid model, incorporated with the Multiple Reference Frames (MRF) method is used to predict the movement of the bubble in the melt. Simulated results are compared with experimental data from the water model measurement and reasonable agreements are obtained. Furthermore, the computed results show that the bubble migration under hypergravity field includes the movement forward to the casting rotating shaft and the movement opposite to the direction of the rotating mould. In addition, the initial bubble size and the surface tension between the melt and the gas bubble have an important effect on the distortion of the bubble.