An approach to microstructure modelling in nickel based superalloys

Mechanical properties of components made from nickel based superalloys rely on the microstructure that forms during their thermomechanical processing. The ability for predicting and controlling microstructure during the processing is of the utmost importance for this class of alloys. In this work, the applicability of JMAK-type (Johnson-Mehl-Avrami-Kolmogorov) models is studied in the context of industrial manufacturing processes. The results of FEA (finite element analysis) based predictions of microstructure evolution in ATI 718Plus® alloy during the hot deformation process are presented. The limitations of the JMAK-type approach are discussed in the paper and concepts for an alternative modelling approach for microstructure prediction in nickel based superalloys are presented

On the applicability of JMAK-type models in predicting IN718 microstructural evolution

Nickel-based superalloys are widely used in the aerospace sector for their mechanical properties, which are directly related to the microstructural and physical properties of these materials. JMAK-type models have been applied to this class of superalloys for the prediction of microstructural evolution phenomena such as recrystallisation. However, these models often lack a clear range of applicability. The majority of the successful applications normally address rather idealised processes (relatively slow forging, simple geometry). However, the industrial production environment generally involves complex strain paths and thermal histories. Thus, there arises the question of whether the JMAK-type models can be applied to such cases. This paper’s research focus is to investigate the applicability of JMAK-type models for such processes. To do this, screw press forging of disks was used to validate the in-built JMAK-type model of Inconel 718® available in DEFORMTM. In particular, the applicability of the model was examined using a comparison between the results from simulation and from metallographic analysis. At first, the appropriateness of the JMAK outputs in describing the observed microstructures was investigated and then quantitative results were evaluated. The model’s outputs were found to be insufficient in describing the observed microstructural states and additional parameters were deemed necessary. The model’s predictions ranged from a broadly good match, for which the model could be calibrated with a proposed new methodology, to a qualitative mismatch that highlights the limits of the model’s applicability

Al-Li Alloys – The Analysis of Material Behaviour during Industrial Hot Forging

Al-Li alloys are a promising class of aerospace materials that combine light weight with high strength, comparable to those of steels. In the case of critical components, it is well known that providing the required reliability is impossible without tailoring the output microstructure of the material. This, in turn, requires a clear understanding of the logic behind microstructure formation depending on the total processing history (especially temperature and strain-rate history). However, uniaxial isothermal laboratory tests provide very limited information about the material behaviour. Real forging processes, especially involving complex geometries, sometimes develop quite complicated temperature-strain-rate paths that vary across the deformed part. A proper analysis of the microstructural transformations taking place in the material under these conditions is therefore very important. In this paper, the correlation between the loading history and microstructural transformations was analysed for AA2099 alloy using the hot forging of a disk-shaped component at selected forging temperatures and strain rates. The obtained results were compared to industrial processing maps based on uniaxial tests

Benchmarking mean-field models available in commercial FE software in application to two-blow forging of IN718 alloy

This paper analyses the potential of standard mean-field models available in commercial FE software Deform®, Forge® and QForm® for a microstructural prediction during multistage forging process of Inconel 718 at conditions close to industrial ones. The special set of experimental trials including heating, forging, reheating and final forging were conducted on 5 MN hydraulic press with detailed measurements of temperature distributions, timings and forging parameters. The microstructure distribution was investigated after each stage of the process (optical and EBSD) and compared with the predictions obtained in three softwares. Standard and optional capabilities as well as limitations and challenges of the models were investigated, and some improvement ideas were proposed

On the specifics of modelling of rotary forging processes

Rotary forging process, in spite of its various advantages, has still not reached industrial production scale owing to its complex nature. With the advent of sophisticated finite-element modelling capabilities, it is now possible to make rotary forging more predictable and optimise it for industrial production standards. However, modelling by nature involves a series of assumptions and simplifications that can help us make reasonable predictions. It is important to know the important factors that affect the results, and what compromises can be made, with a genuine understanding of what the compromises will result in. This paper reports some initial findings from our attempt towards robust modelling for the design of the rotary forging process. Herein, we have taken the simple case of rotary upsetting of cylinders using a custom-designed rotary forging machine and modelled it using commercial metal-forming software QForm

Simulation of the material softening during hot metal forming

Deformation softening is quite often observed during hot working of different alloys. Steels, aluminium, titanium or nickel alloys can demonstrate a decrease in flow stress under active deformation at constant temperatures and strain rates. Though the background microstructural mechanisms as well as the softening rates can be quite different, the treatment of such processes requires special attention. Deformation softening can cause significant non-uniformity of the metal flow resulting in flow localization, formation of shear bands and variation of the microstructure across the workpiece. This paper is devoted to the investigation of the specific issues which arise in this respect in FEM simulation of processes involving softening. The possible role of softening in shear band formation is studied using numerical simulation and physical modelling. The effect of the softening rate on the probability of flow localization is discussed. The interplay of deformation softening with the stain rate and temperature sensitivity is demonstrated using as an example the simulation of Equal Channel Angular Pressing (ECAP). An approach to account for the deformation softening in FEM simulations via process modelling of the microstructure refinement is proposed

Aspects of high strain rate industrial forging of Inconel 718

The major part of all material and microstructural data used for the modelling of nickel superalloy forgings is obtained from uniaxial laboratory tests with limited plastic strain and very simple thermo-mechanical history. At the same time, new challenges in near net shape industrial forging require a high level of reliability of modelling prediction of metal flow, for predicting the risk of defects and microstructural transformation. A few recently conducted benchmarking studies have shown that despite the availability of various material models (including microstructural ones) embedded in commercial FE software, in many cases, the level of prediction remains unsatisfactory. This is especially true for fast industrial forging processes (like screw press or hammer forgings). This paper suggests a methodology for processing the results from industrial forgings for obtaining robust data for calibration, validation, and improvement of material and microstructural models. This also can provide additional information on the material science behind the microstructural phenomena, which are problematic to capture and study using simple uniaxial tests

The effect of elasto-plastic properties of materials on their formability by flow forming

FEA process modelling, which has seen plenty of development in recent decades, has significantly simplified and broadened our capabilities for designing and optimising metal forming processes. It has become relatively easy to find the stress-strain state at any point and instant in the process, analyse the kinematics of metal flow or test different fracture criteria. However, it is frequently the case that all this information cannot compensate for the lack of a fundamental understanding of the process. Flow forming is a case in point. Although much research has been carried out since the 1960’s and has resulted in considerable industrial experience, still many aspects remain as “know how” and many basic questions do not have exact answers. This work reported herein is focused on the role of the elasto-plastic properties of a material with respect to its use in flow forming. Can the flow formability of a material be assessed using data from a uniaxial tensile test? If there exists the possibility of tailoring mechanical properties by heat treatment, what should be prioritised

Modelling challenges for incremental bulk processes despite advances in simulation technology : example issues and approaches

Incremental bulk deformation processes have traditionally been difficult to simulate. This paper will argue that, despite advances in computation and software, they remain difficult to model. The main reason for this is the shortage of ideas on what is the real objective of FE modelling for such processes. Even a very detailed model and data obtained in simulation does not give answers to the main question - how to optimise the process parameters? High computational time and volume of information only aggravate the situation. All modern mathematical techniques of dimensionality reduction (such as POD/PGD) lose their power when the priorities and acceptable compromises of modelling are not clear. This paper tries to use a large volume of available experimental and modelling experience to illustrate this problem and look for possible break-through directions