THE ANALYSIS OF FORGING INCONEL 718 ALLOY. Aneta ŁUKASZEK-SOŁEK, Janusz KRAWCZYK, Piotr BAŁA, Marek WOJTASZEK

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1 THE ANALYSIS OF FORGING INCONEL 718 ALLOY Aneta ŁUKASZEK-SOŁEK, Janusz KRAWCZYK, Piotr BAŁA, Marek WOJTASZEK AGH University of Science and Technology, Krakow, 30 Mickiewicza Av., address: Abstract The analysis of changes in the microstructure of hot forged Inconel 718 alloy was presented in his paper. After heating in electric chamber furnace, the Inconel 718 alloy billet was forged at 1150 º C. The temperature of heating, forging and cooling the billet was constantly monitored. Processing of the investigated alloy was preceded by dilatometric tests, the analysis of microstructure of as-received and hot deformed in compression under various thermomechanical conditions on Gleeble simulator Inconel 718 alloy as well as by numerical simulations of Inconel 718 alloy forging. The numerical simulations were conducted using QForm 3D software. The obtained results of both simulations and microstructural analysis were used to determine the conditions of Inconel 718 alloy forging in industrial conditions. The correlation of the results of microstructural observations, hot compression tests and forging the investigated alloy in industrial conditions were discussed. Keywords: nickel alloy, microstructure, forging, numerical modelling 1. INTRODUCTION Alloy Inconel 718 is a wrought nickel alloy precipitation hardening [1-4]. Due to the creep resistance up to 650 C and a lower cost (due to high iron content) in comparison to the other nickel alloys [5,6], there is the practical application of the alloy [1,7]. Although Inconel 718 alloy is considered ductile, only the selection of appropriate parameters of forming allows for obtaining forgings with no defects and favorable microstructure [8]. The purpose of this study was to determine the parameters for the implementation of forgings of an elongated, compact shape, characterized by a differentiated way to fill the die cavity. 2. INVESTIGATED MATERIAL The object of the study was Inconel 718 alloy with the chemical composition shown in Table 1. As supplied the rod diameter was = 50 mm. On delivery within the alloy investigated there are: phase separation (needle-shaped precipitates within the grains bounduaries), NbC and TiC carbides in the matrix of the fcc crystal structure (Fig. 1). The material in the initial state was therefore not oversaturated. Table 1 Chemical composition (weight %) of the investigated Inconel 718 alloy C Fe Co Nb+Ta Ti Cr Mo Al Ni Bal.

Fig. 1 Microstructure of the investigated alloy in as-delivered condition 3.

Figure 2 shows the heating curves of the alloy in as-delivered condition recorded at the rate of 0.08 C/s to 1200 C.

This effect is associated with the precipitation of Ti and/or Cr carbides, or the precipitation and growth of intermetallic phases.

phases. At the temperature of 880 C there is a reduction in expansion associated with the beginning of the dissolution of phase.

2 Fig. 1 Microstructure of the investigated alloy in as-delivered condition 3. DILATOMETRIC RESEARCH In order to determine the temperature range of the investigated alloy forging dilatometric tests were performed. Figure 2 shows the heating curves of the alloy in as-delivered condition recorded at the rate of 0.08 C/s to 1200 C. During the heating at about 400 C, thermal expansion up to a temperature of about 650 C follows. This effect is associated with the precipitation of Ti and/or Cr carbides, or the precipitation and growth of intermetallic phases. At temperature of 650 C there is a reduction in expansion (contraction on the differential curve) probably associated with the initiation of precipitation of intermetallic phases. At the temperature of 880 C there is a reduction in expansion associated with the beginning of the dissolution of phase. At a temperature of about 1000 C, there is temporary stabilization of expansion, and at about 1050 C its reduction appears, which may indicate the dissolution of intermetallic phases in the investigated alloy. Fig. 2 Dilatometric curve of heating with a rate of 0.08 C/s with coresponding differential curve. 4. NUMERICAL MODELLING AND INDUSTRIAL TESTING For the purposes of the analysis the elongated forging with a compact shape was chosen, characterized by a differentiated way to fill the die (Fig. 3). Fig. 3 Forging model: a) with b) without the flash.

The process of forging included: pre-flattening the billet size Ø30x114 mm to 25 mm square, then the forming in the process of roughing pass and finishing pass was carried out.

3 The process of forging included: pre-flattening the billet size Ø30x114 mm to 25 mm square, then the forming in the process of roughing pass and finishing pass was carried out. Numerical modeling was performed using commercial software based on the finite element method QForm 3D. The analyzed material flow characteristics, necessary for the forging process simulations, were determined by compression tests under various temperature-stress-strain conditions using Gleeble 3800 thermo-mechanical simulator. Effective mean stress and strain distributions were obtained from the performed simulations. The results of numerical modeling of forging billet Inconel 718 are presented in Figures 4 and 5. The mean stress distribution (Fig. 4) and effective strain (Fig. 5) were analyzed in the forging cross-sections. The highest value of compressive stress occurs at the bottom of the forging and ranges from 1800 to 2200 MPa. Fig. 4 Mean stress distribution. Fig. 5 Effective strain distribution. The material flow in the cavity during forging proceeds by upsetting and piercing. Proper material flow was observed in the whole volume of the forging. A high degree of strain inhomogeneity within the volume of the workpiece was noticed. Several regions of different strain levels were also distinguished in the forged part. When assessing the distribution of effective strain, large non-uniformity was identified. A few specific areas of strain that affect the properties of the material (forging) can be noticed (highlighted). In the analyzed forging of the cell, the maximum values of effective strain are concentrated in a web of the forged part and in a flash, what is connected with the intense material flow in these parts of the forging. In case of forging the forging of the cell, because of the manner (method) of filling the die, the maximum value of the strain intensity is concentrated in the bottom region of the forging and in the area of flash, which is related to the movement of large amounts of material in these areas. Large deformation intensity values indicate a better throughput of material (reduction ratio), what should result in better mechanical properties of the finished product. One can observe the presence of areas with little processing (strain) caused by a slight movement

of the material. These areas are located at the radius of the outer part at both the lower and the upper surface of the die cavity.

Simulation of the strain rate at 1150 C in a Gleeble thermomechanical simulator (heating rate 2.

The microstructure was considered appropriate for the final forging. Therefore, the above forging conditions were considered appropriate for industrial testing. Fig.

4 of the material. These areas are located at the radius of the outer part at both the lower and the upper surface of the die cavity. Moreover, the analysis of the forging process indicates that throughout the volume of the forging, the intensity of strain rate is about 0.1 s -1 (Fig. 6). Simulation of the strain rate at 1150 C in a Gleeble thermomechanical simulator (heating rate 2.5 C/s, holding time 10 s, cooling in the compressed air, true strain 1) results in obtaining a uniform grain microstructure as an outcome of dynamic recrystallization (Fig. 7). The microstructure was considered appropriate for the final forging. Therefore, the above forging conditions were considered appropriate for industrial testing. Fig. 6 Effective strain rate distribution Fig. 7 Microstructure of the test sample as a result of deformation of the alloy in the thermomechanical simulator Gleeble at temperature of 1150 C and a strain rate of 0.1 s -1. Forging trials of a model forging (Fig. 8) from the Inconel 718 alloy were carried out in the Forge Group Grelowski in Goleszów. Heated in a chamber gas furnace (with the controlled heating of the billet) to a temperature of 1250 C Inconel 718 alloy was forged in the screw-hydraulic press at a maximum pressure of 1000 t. The temperature of the billet was constantly monitored by thermocouples attached to it. Die temperature was 300 C and the initial temperature of the forging was 1150 C. The parameters of industrial trials matched the ones set out in numerical calculations.

Fig. 8 Forging of the cell in an industrial environment with the selected area for microstructure tests.

As can be seen that it was possible to get a finer grain in industrial environment than the one predicted by

$8) Unfortunately, in the studied area a fracture in the industrial forging was observed (Fig.$ 10), which was not indicated by calculations performed.

and strains. Fig. 10 Crack observed in the industrial forging 5.

5 Fig. 8 Forging of the cell in an industrial environment with the selected area for microstructure tests. The microstructure of the studied forging cross-section is shown in Figure 9. As can be seen that it was possible to get a finer grain in industrial environment than the one predicted by computer simulations and laboratory tests carried out in thermomechanical simulator Gleeble'a. Fig. 9 Microstructure of the industrial forging in the analyzed area (Fig. 8) Unfortunately, in the studied area a fracture in the industrial forging was observed (Fig. 10), which was not indicated by calculations performed. However, the crack was observed near the area of the flash, where the numerical calculations show large stresses and strains. Fig. 10 Crack observed in the industrial forging 5. CONCLUSION Methodology used to develop technology to produce alloy Inconel 718 forging made it possible to obtain proper microstructure in the industrial forging. Applied numerical modelling indicated the area of particularly high material effort, in which a fracture in an industrial forging was found. However, the results of the

6 computer simulation did not allow the unambiguous statement that there is a danger of crack formation in this area. The results obtained in this study will be used to find the boundary conditions for numerical simulation for more effective predictions of the formation of cracks in Inconel 718. ACKNOWLEDGEMENTS Financial support of Structural Funds in the Opera-tional Programme - Innovative Economy (IE OP) financed from the European Regional Development Fund - Project WND-POIG /09 is gratefully acknowledged. REFERENCES [1] DONACHIE, M.J., DONACHIE, S.J. Superalloys A Technical Guide. Second Edition, ASM International, [2] ASTM B637 06, Standard specification for precipitation hardening nickel alloy bars, forgings, and forging stock for high temperature service. [3] OLOVJÖ, S., WRETLAND, A., SJÖBERG, G. The effect of grain size and hardness of wrought Alloy 718 on the wear of cemented carbide tools. Wear, 2010, vol. 268, page [4] DAVIES, J.R. (ed.) Heat-Resistant materials (ASM Specialty Handbook), ASM International, [5] BAŁA, P. Microstructure characterization of high carbon alloy from the Ni-Ta-Al-Co-Cr system. Archives of Metallurgy and Materials, 2012, vol. 57, nr. 4, page 937. [6] BAŁA, P. Microstructural characterization of the new tool Ni-based alloy with high carbon and chromium content. Archives of Metallurgy and Materials, 2010, vol. 55, nr. 4, page [7] BHADESHIA, H.K.D.H. Nickel Based Superalloy. superalioys.html. [8] KRAWCZYK, J., ŁUKASZEK-SOŁEK, A., ŚLEBODA, T., BAŁA, P., BEDNAREK, S., WOJTASZEK, M. Strain induced recrystallization in hot forged inconel 718 alloy. Archives of Metallurgy and Materials, 2012, vol. 57, nr. 2, page 593.

ISOTHERMAL FORGING OF P/M FeAl ALLOYS. T. ŚLEBOD, S. BEDNAREK, A. Łukaszek-SOLEK

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