Materials Science Forum Vol. 949

Abstract: Isothermal forging processes are typically used for near-net shaping of high-performance components such as turbine discs and blades. Recent developments have introduced isothermally forged titanium aluminides into commercial jet engines. Titanium aluminides are lightweight intermetallic compounds with excellent creep properties but very limited ductility. Their low workability requires isothermal forging at slow strain rates, which is typically kept constant in the process. This work explores the possibility of controlling the strain rate during the process using model predictive control, so that the process time is reduced while the microstructure transformation and the amount of damage introduced into the workpiece are controlled. The results of isothermal compression with friction show that both an acceleration of the process and a reduction of damage are possible using the suggested control strategy.

Abstract: Rebielskij-type design of a rolled con rod preform and appropriate upper and lower dies are presented. The proposed process was tested by Simufact Forming program and modifications made to satisfy the criteria of shape, stress and strain state and temperature distribution during processing. Based on the CAD documentation, a STL file was prepared for 3D polymer printing the die, the con rod preform and the forging. The results will enable 3D printing of prototypes in the appropriate metallic materials, leading to tool manufacture and processing in industrial conditions.

Abstract: Process maps according to Parasad et al. are already widely used to make statements about the formability of materials and their forming energy. However, these process maps only apply to conventional incompressible materials. At the TU Bergakademie Freiberg, these process maps have already been extended for particle-reinforced incompressible solid materials with a homogeneous particle distribution. The next step is to adapt the model for compressible particle-reinforced matertials so that they can also be used in powder metallurgy. The problem here is that the volume decreases as a result of compaction during powder forming. In powder metallurgy, however, compaction plays an important role. On the one hand, the compaction of the components leads to an increase in the material properties. On the other hand, pores pose a high risk of fractures and cracking. For this reason, it is the aim of this paper to make the existing process maps for incompressible materials usable for compressible materials by corresponding adaptations of the models prevailing in powder metallurgy. Furthermore, the effects of a homogeneous particle distribution and a graded particle distribution within the TRIP matrix composites on the process maps will be investigated. For this reason, process maps are produced in the temperature ranges between 700 – 1050 °C, with forming speeds of 0.001 – 100 s^-1 and residual porosity of 10 – 30 %. For this purpose, specimens with corresponding residual porosity and homogeneously distributed ZrO₂ 5 vol.%, 10 vol.%, 15 vol.% and 20 vol.% as well as a graded layer structure of corresponding ZrO₂ proportions are prepared. With the aid of these specimens, flow curves are determined and adjusted at appropriate temperatures and forming speeds during compression tests. The energy dissipation and an instability map are then modelled from these flow curves and a process map is derived. It was found that with increasing ZrO₂ content in the homogeneous and the graded structure, the areas that allow damage-free forming become smaller. The same applies with decreasing residual porosity. Nevertheless, the areas, which allow failure-free forming, are larger than the possible forming areas of solid components. However, the power dissipation efficiency of incompressible specimens is significantly lower than that of compressible specimen [1]. In addition, it was observed that with increasing ZrO₂ content and decreasing residual porosity, the efficiency of the power dissipation in the formable areas decreases. It was also found that the distribution of the reinforcing particles has a significant influence on the flow curves and the associated process maps, then the graded specimen do not represent a superposition of the individual process maps of the homogeneous specimens.

Abstract: The main objective of presented research is an attempt of application of techniques taken from a dynamically developing field of image analysis based on Artificial Intelligence, particularly on Deep Learning, in classification of steel microstructures. Our research focused on developing and implementation of Deep Convolutional Neural Networks (DCNN) for classification of different types of steel microstructure photographs received from the light microscopy at the TU Bergakademie, Freiberg. First, brief presentation of the idea of the system based on DCNN is given. Next, the results of tests of developed classification system on 8 different types (classes) of microstructure of the following different steel grades: C15, C45, C60, C80, V33, X70 and carbide free steel. The DCNN based classification systems require numerous training data and the system accuracy strongly depend on the size of these data. Therefore, created data set of numerous micrograph images of different types of microstructure (33283 photographs) gave the opportunity to develop high precision classification systems and segmentation routines, reaching the accuracy of 99.8%. Presented results confirm, that DCNN can be a useful tool in microstructure classification.

Abstract: In this work we apply a numerical inverse analysis procedure based on the ICME framework to ensure a required microstructure and combination of properties in the quenched plate. The microstructure is decided first, and the cooling profile needed to obtain such microstructure is then calculated using the CALPHAD approach. Subsequently, an inverse analysis of the heat transfer problem provides the description of the convection mechanism in quenching that results in the demanded cooling profile. An additional constraint is set on the through-thickness thermal gradient to achieve a homogeneous microstructure. Finally, the resultant microstructure is computed. By means of the proposed model we are able to retrieve the necessary quenching process parameters and to quantify the influence of these parameters on the temperature and microstructure distribution within the plate after the quenching.

Abstract: The present paper deals with the influence of the duration of isothermal spheroidization annealing on the evolution of pearlite bands in various initial states. In this study, two initial conditions of the steel 16MnCrS5 are considered: a) industrially hot-rolled pearlite structures in their ferritic matrix and b) a specifically adjusted microstructure in the lab condition. Based on the experimental investigations and quantitative microstructural analyses, an empirical model for the prediction of pearlite banding within a broad range of annealing durations could be derived. Both, experiment and model, agree that pronounced pearlite bands in the initial state almost disappear after 25 h of spheroidization annealing. On the other hand, a marginal degree of pearlite banding in the initial state increases slightly during annealing. This fact could be explained by inhomogeneous cementite formation inside and outside the primary segregation regions of manganese.

Abstract: One effect of high influence on the dimensional accuracy during bending is springback. It inevitably occurs due to the elastic proportion in the material behavior. The impact is notably high when producing springs made of ultra-high strength spring strips of the steel grade 1.4310 (X10CrNi18-8). The high yield ratio needed to fulfil the functionalities required during application leads to dimensional inaccuracies that have to be compensated during the production process. This paper reports a simulation-based approach to predict the springback behaviour of ultra-high strength spring strips with tensile strengths TS = 1500-1800 MPa. Based on the results of advanced material testing and modelling, the numerical prediction of the springback behavior of an exemplary bending process (free bending) has been investigated in detail. This helps to obtain deeper knowledge and understanding of the springback phenomenon and to achieve suitable strategies for a more efficient industrial tool and process design while processing ultra-high strength spring strips.

Abstract: Deformation behavior of an as-cast Mg-6.8Y-2.5Zn-0.4Zr alloy during plane strain compression was characterized in present work by high-temperature testing. Based on the experimental data, the values of strain rate sensitivity, efficiency of power dissipation and the instability parameter under the condition of various hot working parameters were investigated. Processing maps were established by superimposing the instability map over the power dissipation map, this being connected with microstructural evolution analysis in the hot deformation processes. Accompanied microstructure characterization of the binary α-Mg/ Long Period Stacking Ordered (LPSO) microstructure reveals that the flow behavior is related to the deformation mechanisms. At lower temperatures (350 – 400 °C) formation of kink bands is observed, which normally occur when deformation twinning is inhibited and other slip systems are strongly hindered by the complex LPSO structures. Dynamic recrystallization (DRX) was initiated at higher temperatures above 400 °C, influencing the softening behavior of the material significantly. DRX was the main softening mechanism when deformation takes place at 500 °C and the kink band deformation decreased.

Abstract: The world steel market is in a phase of maturity. Worldwide growth in steel demand is expected to be in the range of 1% per year and the world’s biggest market, China, is facing a phase of stagnating or even diminishing steel demand. The situation has changed from a supplier driven to a customer driven market meaning that customer demands are increasing and margins are dropping. Besides increasing levels of digitization and service orientation as well as the development of new sales channels the delivery of higher quality, introduction of new steel grades and further customization of high-end products to cover niche markets are of prime importance to keep or increase market shares. Besides efficient organization and execution of the daily business ranging from production to quality management and customer services paired with data-based insight, highly efficient research and development (R&D) with increasingly limited amount of resources is a must. This paper addresses the latter topic regarding trends for future steel grade demands and strategies for efficient R&D.