Papers by Keyword: Microstructure Modeling

Abstract: Industrial hot forming of nickel-based superalloys is typically carried out under non-isothermal conditions, where rapid temperature changes between forming steps not only affect the thermo-mechanical response but also the microstructural evolution, including processes such as recrystallization. Most microstructure models are developed and calibrated for idealized isothermal conditions, and their applicability to realistic transient temperature paths is still unclear. Therefore, this study investigates the microstructure evolution of Inconel 718 under non-isothermal hot deformation by combining dilatometer tests with full-field simulations using DIGIMU® in order to provide detailed insight into the underlying microstructural mechanisms and to assess the capability of a Full-Field approach for modelling such non-isothermal forming conditions. For this aim, compression tests with temperature increases (1020 °C to 1070 °C) and decreases (1120 °C to 1070 °C) were performed, with the temperature change applied at different strains. The results reveal a path dependence, heating at low strain to 1070 °C leads to higher DRX-fractions and finer, more homogeneous grain structures, whereas later heating at higher strains produces coarser, partially recrystallized microstructures due to reduced strain at the higher temperature. For the temperature decrease, DRX occurs predominantly at 1120 °C, after a late temperature change, no additional DRX takes place at 1070 °C. While an earlier change still allows additional DRX. The Full-Field simulations reproduce these trends in dislocation density, recrystallized fraction and grain size distributions with good qualitative agreement and moderate quantitative deviations. Overall, the study demonstrates that the timing and direction of temperature changes affect the final microstructural state, and that DIGIMU®, a Full-Field approach, can capture path-dependent microstructure evolution in Inconel 718 and provides a useful digital tool for analyzing and designing transient temperature hot forming processes.

Abstract: The evolution of microstructural features such as local grain size and local grain size distribution are essential in view of the final physical and mechanical properties of the nickel base alloy 718 for aircraft parts forged in a multi-step production route. Due to increasing standards and the need of the prediction of fracture mechanical properties, a multi-class grain size model for a more detailed microstructure prediction is necessary. Therefore, a multi-class model considers the real initial non-uniform grain size distribution and structure of the pre-material at the beginning of the forging process, which affects the evolution of grain sizes during thermo-mechanical treatment and leads to different results than commonly used uniform grain structures. The initial distribution is defined by grain classes according the ASTM standard. It is shown that the presence of different classes and distributions of grains are as import as the applied strain, strain rate and temperature on dynamic, meta-dynamic and static recrystallization. Additionally, dissolution processes of delta phase and grain growth kinetics are included in the model to properly indicate the recrystallized fractions and represent the resulting multi-class microstructure. A series of simulations with different initial distributions is discussed and compared with examined forged samples in terms of the resulting microstructure for typical forging parameters. Based on these results the microstructure model can be used in combination with collected process data to predict the resulting properties and for the design of new aircraft parts.

Abstract: Ring rolling is a versatile incremental bulk forming process. Due to the incremental character of the process, it consists of a large number of deformation and dwell steps. Finite element (FE) simulations of bulk forming processes are capable of predicting loads, stresses and material flow. In recent years, the finite element analysis of ring rolling processes has become feasible both in terms of calculation time as well as regarding the closed loop control of the kinematic degrees of freedom [1]. Accordingly, the focus of interest now includes the prediction of the microstructure evolution. The accuracy of such numerical simulations strongly depends on the models characterizing the material behavior and boundary conditions. In this paper, a finite element based simulation study was conducted, in order to evaluate the impact of boundary conditions such as transfer time, radiation, heat transfer and friction on the target values of the ring rolling process. The results of the simulation study were compared to ring rolling experiments on an industrial size ring rolling device. A good accordance regarding the evolution of the outer diameter and radial force was observed. Strong contingencies of transfer time on the forces throughout the process were detected and considered in the simulation study. In a post processing step, the evolution of the microstructure considering the dynamic and static recrystallization as well as the grain growth was calculated using the FE results. The calculated grain sizes show good accordance with the experimentally observed microstructure of the ring before and after the rolling. Furthermore, the impact of process parameters on the evolution of the grain size was investigated.

Abstract: Under certain conditions of extrusion temperature and strain rate Al-Mg-Si alloys produce coarse recrystallized grains at and near the surface. Current FEM models are able to analyze grain size evolution for extruded profiles, but cannot predict the coarse recrystallized grains near the surface. A new model using DEFORM 2D and local state variables such as strain, strain rate and temperature is compared with Al-Mg-Si rods extruded at 440°C and 500°C for two extremes of strain rate. The model is found to be sensitive to the processing conditions and to accurately predict the recrystallized grain size and fraction.

Abstract: Heavy-duty components used in the automotive industry, in wind turbines and in many other industrial applications are often produced using hot forging processes. Nowadays the design of hot forging processes aims for the optimization of process efficiency on the one hand and final mechanical product properties on the other hand. Excellent mechanical properties needed for hot-forged components e.g. high load capacity and high fatigue resistance depend on a fine homogeneous microstructure distribution across the final product’s cross-section. Efficiency in hot forging can be optimized by increasing the temperature during processing, which allows for lower forging loads and lower die stresses, thus improving die life in terms of mechanical fatigue. To guarantee for a fine homogenous microstructure across the cross section of the forged good, dynamic recrystallization (DRX) has to be initiated during deformation and Grain Growth (GG) has to be avoided during dwell times and cooling. Due to the high computational costs of finite element simulations an optimization aiming for lowest possible forging loads and finest possible grain sizes is very time-consuming. In this paper a Response Surface Model (RSM) of the forging process is introduced, which allows for much faster evaluation of the outcome of forging simulations, albeit by interpolation of simulation results, and thus allows for optimization. The information required to create the RSM is obtained by Design Of Experiments (DOE) techniques using an FE-model of the forging process which was calibrated earlier. The process variables considered include the initial temperature of the billet and the die kinematics. Subsequently, an optimization algorithm is combined with the RSM to find the design variables giving minimum possible loads during deformation and finest possible grain sizes in the forged product. The RSMs results are validated by the use of the existing FE-model.

Abstract: A 3D grain-level formulation for the study of brittle failure in polycrystalline microstructures is presented. The microstructure is represented as a Voronoi tessellation and the boundary element method is used to model each crystal of the aggregate. The continuity of the aggregate is enforced through suitable conditions at the intergranular interfaces. The grain-boundary model takes into account the onset and evolution of damage by means of an irreversible linear cohesive law, able to address mixed-mode failure conditions. Upon interface failure, a non-linear frictional contact analysis is introduced for addressing the contact between micro-crack surfaces. An incremental-iterative algorithm is used for tracking the micro-degradation and cracking evolution. A numerical test shows the capability of the formulation.

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: In the present paper actual demands for modern simulation strategies for hot rolling are discussed. The main focus of the discussion is on material flow simulation for hot rolling and the computation of material inhomogeneities. An overview on simulation techniques for material flow and microstructure evolution is given. Approaches for new simulation strategies which give fast results with a high modeling depth are discussed. As a result of actual investigations a fast model for material flow is presented.

Abstract: Integrated model based control systems are an essential part of modern plant operations. The production requirements have evolved from achieving geometric tolerances to becoming a large, flexible, but accurate metallurgical instrument. Within the steel industry, on-line process and microstructure monitoring systems have been realised for many years on hot strip mills. At recent conferences (including ReX&GG IV), Siemens has described the successful implementation of these models in steel plate production; in applications where there is increased complexity in through thickness microstructure and the final products have safety critical applications. Even with advanced on-line systems and tailored production routes there is a relatively comfortable process window in modern steel manufacture. For the plant builder, light structural metals titanium and magnesium, and harder nickel based high temperature alloys represent another level of complexity and process control requirements. This paper examines the potential of microstructure modelling with respect to the process design for manufacturing wrought products in alternative alloy systems. Metallurgical aspects will be considered and their practical implications discussed. The continued expansion of these materials into markets where high strength/weight ratios, corrosion resistance or high temperature properties are desirable (e.g. automotive, offshore, chemical, aerospace) make this worthwhile from an industrial perspective.

Abstract: In the titanium alloy Ti-6Al-4V the dual-phase grain structure, which forms during thermo-mechanical processing, is of high importance due to its effect on the mechanical properties. In general the most significant microstructural parameters are the amount of alpha and beta phase as well as their grain size. For this reason a new cellular automata method (CA) was developed to predict the evolving grain structure during isothermal and non-isothermal heat treatment. The probabilistic CA model is based on the diffusion controlled movement of grain and phase boundaries. During temperature changes an algorithm is adjusting alpha and beta phase fraction to maintain equilibrium phase values. Hence, the CA is capable to calculate grain coarsening as well as grain growth and shrinking in the two-phase area while heating and isothermal holding at forging temperature. The initial microstructure can be imported form virtual created microstructures, real micrographs and EBSD-images. The results are mean grain diameters, grain size distributions and virtually simulated microstructures which can be easily compared with real micrographs. The predicted microstructures are showing a good correlation to data in literature and experimental results.