ISOTHERMAL FORGING OF P/M FeAl ALLOYS T. ŚLEBOD, S. BEDNAREK, A. Łukaszek-SOLEK AGH University of Science and Technology, Faculty of Metals Engineering and Industrial Computer Science, Al. Mickiewicza 30, 30-059 Kraków, Poland Abstract This study evaluates the possibility of P/M FeAl alloys forging, with a special emphasis on the processing parameters. Fe-40at.%Al alloy powder was used in this research. The powder was consolidated to full density by hot pressing. Cylindrical compression specimens were machined from the compacts. Compression tests were performed on Gleeble 3800 thermomechanical simulator in order to characterize the material behavior under various temperature-strain-strain rate conditions. The tests were carried out at various strain rates over the temperature range of 700 1100 C. Next, cylindrical specimens with a height of 10 mm and a diameter of 30 mm were machined from fully-dense compacts and used as a starting material for forging. The specimens were isothermally forged at 900 o C on a 100 t hydraulic press equipped with a specially designed forging dies. The research showed, that properly chosen forging conditions result in obtaining high quality FeAl alloy forgings without necessity of preform canning. Moreover, in addition to the experimental tests, numerical simulations of FeAl alloys forging were performed using QForm software. The boundary conditions for the simulations were prepared on the base of the experimental tests. The results of the simulations showed to be in very good agreement with the experimental observations. Keywords: FeAl alloy, powder metallurgy, microstructure, forging 1. INTRODUCTION Iron aluminides have been of great interest over the last several decades for possible use as moderate to high temperature structural materials. These alloys are especially attractive because they combine low density with good strength, excellent corrosion and oxidation resistance at elevated temperatures, a wide range of chemical stability, and relatively low costs [1]. Despite the desirable properties mentioned above, FeAl alloys, like aluminide compounds in general, are known to have relatively high ductile-to-brittle transition temperatures, on the order of half their melting point. This results in the limited ductility and toughness of these materials as one approaches ambient temperatures. Such limited ductility in the case of iron aluminides is thought to be the result of environmental embrittlement caused by the presence of moisture which, when it reacts with aluminum on the surface, causes the release of hydrogen. These limitations in ductility have greatly restricted the low-to-moderate temperature processing of these materials and subsequently, their general use to date. Powder metallurgy (P/M) processing of FeAl-based alloys provides many advantages over more traditional casting techniques. The room temperature mechanical properties of these alloys, particularly ductility and strength, are microstructure sensitive [2]. In this respect, powder metallurgy methods are very promising, since much finer microstructures can be produced [3]. Recent studies have pointed out the importance of starting powder particle nature (particle shape, size or powder surface oxide content) [4] as well as thermomechanical parameters in relation to the processing of P/M FeAl alloys [5, 6]. This research evaluates the possibility of FeAl alloys forging, with a special emphasis on the processing parameters.
2. EXPERIMENTAL PROCEDURE -100/+325 mesh water atomized FeAl alloy powder was used in this research. The chemical composition of FeAl powder is shown in Table 1. Tab.1 Chemical composition (at. %) of the FeAl alloy powder used in the investigation Fe Al Zr Mo Si B C O FeAl alloy Bal. 39.3 0.05 0.19 0.31 0.02 0.22 0.85 FeAl alloy powder was compacted to full density by hot pressing under an argon atmosphere at the temperature of 1100 o C. The density of the compacts was determined according to Archimedes method. Cylindrical specimens with a height of 12 mm and a diameter of 8 mm were machined from FeAl powder compacts and used for compression tests performed on Gleeble thermomechanical simulator. Plastometric tests performed on FeAl alloy powder compacts allowed determining true stress-true strain curves for the investigated alloy at various temperature-strain-strain rate conditions. These results were used in designing the parameters of P/M FeAl alloy processing as well as one of the boundary conditions for the performed numerical simulations of P/M FeAl alloy forging. Basing on the results of plastometric tests and the results of the numerical modeling of forging process, thermomechanical parameters of forging P/M FeAl alloy were determined. These parameters were used in laboratory tests of P/M FeAl alloy forging. 3. PLASTOMETRIC TESTS AND NUMERICAL MODELING OF FORGING Plastometric tests involved compression tests performed at strain rates of 0.01 s -1, 0.1 s -1, 1 s -1, and 10 s -1 at 100 o C intervals over the temperature range of 700 1100 C. Gleeble thermomechanical simulator was used for compression testing. The load vs. displacement data obtained from the experiments were converted into true stress true strain curves. Moreover, the inverse method was applied to interpret the results of the axisymmetrical compression tests performed on FeAl alloy samples. The examples of the calculated by the inverse method true stress true strain curves for P/M FeAl alloy are shown in Fig. 1. Fig. 1 True stress true strain curves for FeAl alloy powder compacts deformed in compression at a strain rate of: 0.1 s -1 (a), and 10 s -1 (b). Numerical modeling of the investigated forging process was performed using QForm 3D software. Calculated by the inverse method flow stress curves were applied to numerical simulations as one of the boundary conditions. 1 mm/s ram speed of the forging press was assumed in the performed simulations. Chosen results of numerical modeling of forging P/M FeAl alloy billet are shown in Fig. 2.
a b c d e f g h Fig. 2 Mean stress (a), effective strain (b), effective strain rate (c), and temperature (d) distribution in isothermally forged P/M FeAl alloy.
Mean stress (Fig. 2a, 2b), effective strain (Fig. 2c, 2d), effective strain rate (Fig. 2e, 2f), and temperature (Fig. 2g, 2h) distributions in the forged part were analyzed. Some degree of strain inhomogeneity within the volume of workpiece was noticed. Several regions of different strain levels were also distinguished in the forged parts. In the analyzed forging of P/M FeAl alloy billet, the maximum values of effective strain are concentrated in a web of the forging, what is connected with intense material flow in this area of the forged part. The highest values of compressive stresses occur in the vicinity of web of the forged part (Figure 2a). It can be predicted, that in this area the wear of forging dies will be increased. The simulations also revealed local increases of the effective strain rate in certain parts of the forged material (Figure 2e,f), what can potentially cause local gradients of stresses leading to surface cracking of the forgings. 4. FORGING TESTS Cylindrical specimens with a height of 10 mm and a diameter of 30 mm were machined from fully-dense compacts and used as a starting material for forging. The specimens were isothermally forged at 900 o C on a 100 t hydraulic press equipped with a specially designed forging tooling (Fig. 3). Ram speed of the forging press was 1 mm/s. Fig. 3 A schematic representation of isothermal forging device: 1 stamp, 2 die shield, 3 top die, 4 heating elements, 5 furnace shield, 6 thermocouple, 7 forging charge, 8 bottom die. Fig. 4 shows forging dies designed for isothermal forging as well as obtained P/M FeAl alloys forgings.
a b Fig. 4 P/M FeAl alloy forging: forging dies (a) and obtained forgings (b). The microstructure of P/M FeAl alloy powder compacts and forgings obtained from FeAl alloy powder compacts are shown in Fig. 5. a b Fig. 5 Microstructure of the compact (a) and forging (b) obtained from FeAl alloy powder. The nature of the microstructure of P/M FeAl alloy compacts as well as forgings, reflecting only the powder particle shape and size distributions, shows that both hot compacting and forging under assumed thermomechanical conditions resulted solely in densification and deformation of the grains in the case of forgings. 5. CONCLUSIONS The following conclusions can be drawn from the present study: - Experimentally designed parameters of thermomechanical processing of FeAl alloys were applied to both numerical simulations of forging process and to experiments performed in laboratory conditions. - Thermomechanical parameters used in the experimental investigations of isothermal forging of P/M FeAl alloy allow obtaining high quality FeAl alloy forgings. - The microstructure of forgings indicates, that the assumed parameters of isothermal forging did not cause any significant changes in the microstructure of the investigated alloy.
- The performed research on FeAl alloys forging showed good agreement of the experimental and numerical modelling results, what confirmed a proper design of the boundary conditions used in this study. - The research showed, that properly chosen forging conditions result in obtaining high quality FeAl alloy forgings without necessity of preform canning. ACKNOWLEDGEMENTS Financial support of Structural Funds in the Operational Programme - Innovative Economy (IE OP) financed from the European Regional Development Fund - Project WND-POIG.01.03.01-12-004/09 is gratefully acknowledged. REFERENCES [1] N.S. Stoloff: Iron aluminides: present status and future prospects. Materials Science and Engineering, 1998, A258, pp. 1-14. [2] T. Sleboda, J. Kane, R.N. Wright, N.S. Stoloff, D.J. Duquette: The effect of thermomechanical processing on the properties of Fe-40 at.%al alloy, Materials Science and Engineering, A368 (2004), pp. 332-336. [3] T. Sleboda, K. Muszka, J. Majta, P. Hale, R.N. Wright: The possibilities of mechanical property control in fine grained structures, Journal of Materials Processing Technology, 2006, Vol. 177, No. 1-3, pp. 461 464. [4] R.N. Wright, T. Sleboda, J. Kane, N.S. Stoloff, D.J. Duquette, S.C. Deevi: The effect of powder characteristics on thermomechanical processing of P/M FeAl, From Conference Proceedings: the 2004 International Conference on Powder Metallurgy & Particulate Materials, Metal Powder Industries Federation (MPIF), Vol. III, Part 9: Advanced Particulate Materials & Processes, Chicago, Illinois, 2004, pp. 44-53. [5] T. Sleboda, J. Kane, R.N. Wright, N.S. Stoloff, D.J. Duquette, S.C. Deevi: Microstructural refinement of Fe- 40a%Al alloy by thermomechanical processing. JOM A publication of The Minerals, Metals & Materials Society, 2003, Vol. 55, No. 11, pp. 249-250. [6] T. Śleboda: Influence of processing history on the mechanical behavior of P/M FeAl alloys. Steel Research International, 2008, Vol. 79, pp. 493 498.