DEFORMABILITY OF STRIPS OF NICKEL SUPERALLOYS INTENDED FOR SHELL ELEMENTS OF AIRCRAFTS

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1 , Plzeň, Czech Republic, EU DEFORMABILITY OF STRIPS OF NICKEL SUPERALLOYS INTENDED FOR SHELL ELEMENTS OF AIRCRAFTS Krzysztof ŻABA, Maciej NOWOSIELSKI, Michal KWIATKOWSKI, Marcin HOJNY AGH University of Science and Technology, Cracow, Poland, EU krzyzaba@agh.edu.pl, 2 mnowosie@agh.edu.pl, 3 mkw@agh.edu.pl, mhojny@metal.agh.edu.pl Abstract Investigations presented in the hereby paper concern technological assessments of plasticity and limiting deformability of strips made of various grades of nickel superalloys intended for complex shell elements of aircrafts. Exceptionally unfavorable relations between technological properties of the initial material and shapes of products justify the necessity of creating the new practical knowledge, indispensable in the process of shaping elements of the required exploitation properties. In addition, there are certain limitations due to a low plasticity and a high intensity of its loosing because of deformations of nickel superalloys. Strips surfaces are covered by high hardness oxides which causes a risk of material adhering onto classic tools under standard conditions or their damaging. Elastic effects occurring during the shaping procedure require an application of additional operations making use of relaxation effects. In order to determine the formability of strips the investigations of their mechanical properties and normal anisotropy were performed. Tests of bending at single angle to estimate the springing angle, drawability tests by the Erichsen s method were made and the forming limit diagrams (FLD) were determined. Investigations were realized in room temperature. Keywords: nickel, superalloys, shell elements, aircrafts 1. INTRODUCTION Nickel-based superalloys [1] are, among others, used in the aircraft industry for exhaust system elements. These materials should fulfill the exploitation requirements determined by the user (heat resistance, creep resistance) as well as requirements related to processing (plasticity, warranting deformability in processes of elements formation drawing, bending, expanding). These materials owe their special properties of a complicated structure which is subject to additional modifications by changing the chemical composition, performance development processes and heat treatment. Charge materials, accurately correlated and dependent on the size and wall thickness of the produced elements, are Ni superalloys. The paper presents the results of technological assessments of plasticity and limiting deformability of strips made of the heat-resistant nickel superalloys, and, designed for the manufacturing of innovative product exhaust mixer acting as silencer used in the production of a new generation jet engines. Exhaust mixer is spatially complicated structure (Fig. 1). Extremely negative relationship between technological characteristics of the starting material and the shape of the product justifies the need for a new practical knowledge that is necessary in the process of creating the desired shape of the product with the required supplies. Circuit of expanded part of the mixer in the expansion is about 3-4 times greater than the circumference of the tubular part which excludes the implementation of the conventional methods of the disc pressing from a strip of uniform thickness, such as Ni-based alloys are hardly deformable.

2 , Plzeň, Czech Republic, EU Fig. 1. Exhaust mixer This limitation can be minimized by interoperation annealing or use of controlled heating of the material during its deformation. The individual operations, tools and equipment will be designed using the mathematical and physical modeling. The aim of the tests presented in the publication of material were to collect data necessary to build a mathematical model of the shaping process of exhaust mixer and compare selected for this purpose Ni-based superalloys. In order to determine the formability of strips the investigations of their mechanical properties and normal anisotropy were performed. Tests of bending at single angle to estimate the springing angle, drawability tests by the Erichsen s method, forming limit diagrams were determined. Investigations were realized in room temperature. The properties results of various types Ni-based superalloys are among others presented in papers [2-6]. 2. EXPERIMENTAL SETUP Strips of superalloys and,,6 mm thick were chosen for testing. The chemical composition of the base of the is presented in Table 1 and for the is presented in Table 2. The chemical composition was examined by the optical emission spectroscopy method by means of the SPECTROLAB M7 device. Tab. 1. Chemical composition of C Mn P S Si Cr Al Mo Co Ti Nb Fe N Ni,4,2,1,2,26 21,9,2 8,31,5,2 3,4 4,1,14 bal. Tab. 2. Chemical composition of C Mn P S Si Cr Al Mo Co Ti Nb Fe Cu Ni,5,9,7,2,5 18,56 2,,16 1, 5,2 19,6,3 bal. Mechanical properties was measured during standard uniaxial tensile testing accordance to the standard [7]. Springback effect was measured by the method of bending at 9 angle, in the specially constructed singleangle bending device. Spring angle was measured on samples before deformation and after rolling on 35, 5 and 6% strip deformation. Coefficients of the strip plastic anisotropy were determined on the bases of investigations and calculations made for samples cut out from the strip at angles of, 45 and 9 versus the rolling direction, in accordance to the standard [8]. The mechanical properties and anisotropy measurements were realized in the universal testing machine of the Zwick Company. Technological investigations of a strip performed with regard to assessing its susceptibility for stamping and forming were based on the Erichsen s test according to [9], which allows to assess the strip crack resistance, prefaced with the stability loss (groove formation), during biaxial tension with the friction participation at the

3 Rm [Mpa] A [%] , Plzeň, Czech Republic, EU punch front. The investigations on samples cut out from a strip were performed by means of the Erichsen GmbH and CO KG 142/4. Test was conducted with down force of 2kN. The forming limit diagram (FLD) of the tested strips of blanks has been determined by a method of bulging the strips or discs which were cut and they were 2mm to 8mm wide (Fig. 2). Fig.2. Width of the samples in the form of strips made of a blank used for determining the forming limit diagram by the Vermen s method [1] On strips by the method of electrochemical etching was made measuring grid, in the form of points. The samples were then subjected to plastic shaping. After deformation current position of the measurement points were read with a series of pictures with special digital camera. The results of measurements were processed using computer software based on the method of triangulation. In that way, determined the coordinates and the distance to the measurement point after deformation. With information as the location of the base measurement points and after deformation obtained information about the movement of each point, and ultimately mapped the distribution of deformation. Research of material deformation and flow analysis based on the photogrammetric system TRITOP and ARGUS GOM mbh company (Fig. 3) [11]. Fig. 3. Measuring system TRITOP and ARGUS 3. RESULTS 3.1. Mechanical properties The results of the mechanical properties of the and 718 strips of are presented in Fig. 4 a) b) c) R,2 [Mpa] Fig. 4. results of the mechanical properties of the and 718 strips a) R m, b) R,2, c) A A mechanical strips characteristics was determined on the bases of the static tensile test. Ranges of values of the measured properties are equal: R.2 = MPa, R m = MPa, A=22,4%. for

4 Spring angle a [ ] Lankford anisotropy R , Plzeň, Czech Republic, EU and R.2= MPa, Rm= MPa for, A=6,36%. The results shows higher tensile properties and lower plastic for in comparison to The results of mechanical properties just a little depend on the sampling direction for, and strongly depend on the. The smallest value of R m and R, 2 were reached for samples of, taken at an angle of 45 to the rolling direction. These samples were obtained to have largest percentage elongation A. 3.2 Strip anisotropy Results of Lankford anisotropy measurement for and are presented in Fig. 5. 1,4 1,2 1,8,6,4, Fig. 5. Lankford anisotropy for and strips Coefficients of plastic anisotropy depend on the angle of cutting out samples. The highest value for is observed for the sample cut out in angle 45 to the strip rolling direction r =1.24, smaller for the direction 9 r 9 =1.14, while the smallest for the direction r 45 =.69. In case the highest value for is observed for the sample cut out in angle 45 to the strip rolling direction r =1.25, smaller for the direction 9 r 9 =1.12, while the smallest for the direction r 45 =.68. The results indicate the potential risk of bleeding irregularities on the edge of shaped elements and the need for varied downforce. 3.3 Spring angle Results of measuring spring angle depending on the strips deformation of and 718 are shown in Fig Deformation [%] Fig. 6. Spring angle for and after deformation Greater spring angle a,97 7; 16; 24 occurs in the case of in relation to - a,62 3; 1; 2 with deformation relatively, 3, 45 and 6%. With an increase of deformation increase of

5 , Plzeň, Czech Republic, EU spring angle is observed. Results indicate a greater risk of changes in the shape after deformation for. 3.4 Erichsen test Erichsen test results realized on the strips of and 718 are shown in Figure 7. a) b) IE=9,53mm IE=9,83mm Fig. 7. Erichsen test results-macroscopic observations and IE grade a), b) An average height of drawing equals IE=9,53mm for and IE=9,83 mm for. A maximum force during test equals: F=25,83kN for and F=25,42 kn for. An observation of samples deformed in the Erichsen s test (morphology of bulges and cracks) enables to assess the investigated material as satisfactorily fine-grained and homogeneous that is suitable for drawing. The surface of bulges is smooth, while cracks on the tested strip indicate minimal irregularities, but they are greater for strip form from. However, in each case the cracks occurred along the perimeter bend direction. The results show better susceptibility to deformation shape of. 3.5 Forming Limit Diagrams Example cups - on the measurements of which - the forming limit diagram was estimated is presented for in Fig. 8 and for in Fig. 9. a) b) c) Fig. 8. sample after bulging by a punch of 75 mm diameter a) sample, b) field of minor strain, c) field of major strain

6 , Plzeň, Czech Republic, EU a) b) c) Fig. 9. sample after bulging by a punch of 75 mm diameter a) sample, b) field of minor strain, c) field of major strain Forming limit curves have been determined by Argus System software. It strongly depends from material, and they are presented in the diagram (Fig. 1). Fig. 1. Diagram of forming limit curve of the examined materials 4. CONCLUSION The paper presents the results of research at technological plasticity and Forming Limit Diagrams of Ni strip targeted to obtain the data needed to build a mathematical model of the exhaust mixer manufacturing process which are used in the production of a new generation of jet engines. In the research was carried strips of and. Based on the test results it can be concluded that these materials are characterized by high strength properties and low plasticity. Higher stability of the mechanical properties results show but better plastic properties characterize. LITERATURE [1] ASM Metal Handbook vol. 14, 22 [2] Daniel F. Paulonis, John J. Schirra, Alloy 718 at Prat&Whitney Historical perspective and future challenges, Superalloys 718, 625, 76 and Various Derivatives, Edited by E.A. Loria, TMS, 21 [3] Sarwan Mannan, Ed Hibner, Brett Puckett, Physical metallurgy of alloys 718, 725, 725HS, 925 for service in aggressive corrosive environments, Special Metals Corporations, Huntington, WV2575 [4] Jung Han Song, Hoon Huh, The Effect of Strain Rate on the Material Characteristics of Nickel-based Superalloy Key Engineering Materials, Vols (27) pp [5] Y. Huang and P. L. Blackwell, Microstructure development and superplasticity in sheet, Materials Science and Technology April 23 Vol

7 , Plzeň, Czech Republic, EU [6] P. Roamer, C.J. Van Tyne, D.K. Matlock, A.M. Meier, H. Ruble and F. Suarez, Room Temperature Formability of Alloys 625LCF, 71X and 718SPF, Advanced Steel Processing and Products Research Center Colorado School of Mines Golden, CO 841 [7] ISO :29 Metallic materials Tensile testing Part 1.: Method of test at room temperature [8] ISO 1113:1994 Mechanical testing of metals Determination of plastic strain ratio for sheet and strips [9] ISO 2482:23 Metallic materials Sheet and strips Erichsen cupping test [1] F. Dohmann, Introduction to the Processes of Hydroforming, Papers of the International Conference on Hydroforming, Fell-bach/Stuttart, Germany, Edited by Klaus Siegert, (1999), [11] Materials owned by GOM mbh