COMPARISON OF THE EXPERIMENTAL AND THE SIMULATION METHOD OF ESTABLISHING RESIDUAL STRESS IN Al-ALLOYS

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1 11/14 Archives of Foundry, Year 2004, Volume 4, 14 Archiwum Odlewnictwa, Rok 2004, Rocznik 4, Nr 14 PAN Katowice PL ISSN COMPARISON OF THE EXPERIMENTAL AND THE SIMULATION METHOD OF ESTABLISHING RESIDUAL STRESS IN Al-ALLOYS J. ČECH 1, K. PALÁN 2, R. ZALABA 3, K. ŠVAŘÍČEK 4, D. BAŘINOVÁ 5 1, 2, 3, 4 Brno University of Technology, Technická 2, Brno, CZ 5 Škoda Auto a.s. Mladá Boleslav, CZ SUMMARY The radiographic and the strain-gauge (drill hole) methods were used in evaluating the residual stress fields in Al-alloys. Two simulation software programs (, ) were used to compare the residual stress values established. Discussed in the paper are the potentials of individual methods, possible errors and the determination of residual stress at the critical points in the wall thickness of the casting. Key word: simulation software, residual stress, die-casting, squeeze casting 1. INTRODUCTION In the present paper we have tried to compare residual stress values obtained experimentally (using the drill-hole strain-gauge method, the microradiographic method) and by simulation (using the and the software). The measurement was performed on a casting of three-cylinder block for the EA 111 R3 engine, with embedded cast-iron liners. The casting is made of AlSi9Cu3 alloy. At the same time, comparisons were made of places produced by die-casting (DC) and places produced by the technology of local squeeze casting (LSC). We also examined the effect of heat treatment (annealing in our case) on residual stress. 1 Prof.Ing., CSc., cech@ust.fme.vutbr.cz 2 Ing., kamil.p@ .cz 3 Ing., romancast@ .cz 4 Ing., svaricek@umtn.fme.vutbr.cz 5 Ing., dagmar.barinova@skoda-auto.cz

2 Division of simulation software At present, there are a number of competing simulation software programs on the market. We used the two programs that were available to us, namely and. To calculate residual stress in the simulation, different methods are used: the finite elements method (FEM), the finite difference method (FDM), the finite volume method (FVM/CVM), etc. The accuracy of simulation calculations is not so much a matter of the method applied but rather a matter of generating the grid required for the application of the respective method. There are two types of grids structured and nonstructured grids. The software programs using the FEM (sometimes the FVM) are based on non-structured grids. The software programs using the FDM (and most of those using the FVM) use non-structured grids (Fig. 1) [1]. Fig. 1. Illustration of the FEM and FDM grids used Rys. 1. Rysunek zastosowanej siatki MES i MRS The FEM (): Advantages: the possibility of local grid refinement, a precise geometry if the number of elements is small, smooth surfaces, easy calculation of heat flow, ordinary calculation of stresses and deformations (the grid can be changed), easy inclusion of thermal radiation effects, there is no false viscosity when flowing is calculated, suitable for very complex surfaces, excellent visualization of the results, small computer memory space the method is clearly better suited for the calculation of stress/deformation. Disadvantages: demanding grid generation, much less robust solution of flowing melt, lengthy calculation (with respect to the number of elements). The FDM (): Advantages: easy grid generation, more robust and more accurate solution of the melt flow, suitable for simple surfaces, fast calculation (with respect to the number of elements. Disadvantages: the same size of elements over the whole domain, for a precise geometry a great number of elements, staggered (not smooth) surfaces, because of the staggered surface corrections are necessary for the thermal flow, very difficult

3 95 inclusion of stress and deformations (the grid cannot be changed), difficult calculation of radiation heat effects, the necessity of correcting false viscosity of the melt (see staggered surface), not very good visualization of results, greater computer memory space for the calculation less suitable for the calculation of stress/deformation [1]. 2. EXPERIMENTAL The experimental part of our work is based on the drill-hole strain-gauge method, which is describe in detail in the literature [2]. The three-cylinder block casting for the EA 111 R3 engine with embedded cast-iron liners (Fig. 2) was used in the experiment. Three groups of castings were compared: die-castings (DC), castings where local squeeze casting (LSC) was used, and castings with LSC exposed to different types of annealing. The individual castings measured were divided into groups according to the production technology as given in Table 1. The numbering of specimens is as follows: The first number is the number of a block produced by a certain technology, the second number refers to a specific place on the casting. For example.: block number (1 11) site where the specimen was taken from the block (1-6) 8-4 Table 1. The technologies used Tabela 1. Wykorzystane technologie Block No 1N 2N 3P 4O 5N 6N 7N 8N 9O H 10O S 11O H Technology LSC TL LSC LSC LSC LSC LSC LSC LSC+A LSC+A LSC+A N non-machined casting, P pre-machined casting, O H machined upper part of the block, O S machined lower part of the block, LSC local squeeze casting, TL diecasting, LSC + A local squeeze casting + annealing to remove residual stress. Note: The vacuum was not no when casting No 5N was produced. Fig. 2. Places where specimens were taken Rys. 2. Miejsca pobrania próbek

4 Determining residual stress by the radiographic method When analysing the state of stress by the sin 2 method the motion of one selected high-angle diffraction is examined when the specimen is being tilted from the focussing position. The deviations measured for different directions are interpreted as distortions (Fig. 3). Fig. 3. Description of orientation using angles, Rys. 3. Opis orientacji przy użyciu kąta Fig. 4. Representation of stress Rys. 4. Przedstawienie naprężeń Measuring by the sin 2 method yielded the stress value, which is stress in a direction deviated by angle from direction 1 (Fig. 4). Homogeneous stress is assumed that acts in parallel with the specimen surface, and also a minimum gradient in the direction toward the surface such that we can write 3 = 0. The measurement was carried out on a D 500-/501 diffractometer when the specimen was being tilted from the focussing position. The deviations measured for different directions of were entered into a program for stress calculation. Tilting angles ranged from -60 to +60 (for specimens 1-1, 1-5, 2-1 and 2-5 in 10 steps and for specimens 8-1 and 9-1 in 5 steps). The stress values obtained by the radiographic method are in Table 3. Table 3 Residual stress values established radiographically Tabela 3 Wartości naprężeń ocenionych rentgenograficznie Specimen No tilting [ ] The measurement was performed on specimens whose surface had been etched into a depth of ca 0.5 mm. Etching was performed in order to eliminate stresses

5 97 introduced here while the specimen was machined. In all the cases residual compression stress is concerned. The measurement took place in hundredths of mm Stress results obtained by simulation The simulation was performed using the software of Magmasoft Company and the software of RWP Company. was used to simulate both the upper and the lower part of the engine block whereas was used for the upper part only. Simulation using the program was performed in SKODA AUTO a.s. Mlada Boleslav while the -based simulation was conducted at the Faculty of Mechanical Engineering of Brno University of Technology in cooperation with the firm Optimal Technology in Brno [3]. In both the and simulations the places with LSC were taken treated as compression stress as the respective module for squeeze casting was not available. Table 4 gives the values obtained in the simulation for the upper part of the block, Table 5 those for the lower part of the block. Table 6 gives the simulation values for the upper part of the block. Table 4. Residual stress values for the upper part of engine block Tabela 4. Wartości naprężeń w górnej części bloku silnika Read-off point Max. axial Min. axial Residual stress Mises x-axis y-axis z-axis Table 5. Residual stress values for the lower part of engine block Tabela 5. Wartości naprężeń w dolnej części bloku silnika Read-off point Max. axial Min. axial Residual stress Mises x-axis y-axis z-axis

6 98 Table 6. Residual Ssress values for the upper part of engine block Tabela 6. Wartości naprężeń w górnej części bloku silnika Read-off point Principal Residual stress Mises Comparison of residual stress values obtained by experiment and simulation The following places were compared: 1) local squeeze casting 2) die-cast 3) as-annealed Places with local squeeze casting Table 7 gives a comparison of individual residual stress values obtained for different depths. Fig. 5 shows the maximum residual stress vs. depth plot for specimen 1 1. To a depth of 0.35 mm specimen 1 1 exhibits compression stress that at a depth of 1.5 mm changes to tensile stress. Table 7. Residual stress values for specimen 1 1, obtained by experiment and simulation Tabela 7. Wartości naprężeń dla próbki 1 1, eksperymentalne i sumulacyjne Depth Specimen 1 1 [mm] red red 1 red

7 Maximum residual stress 99 Maximum residual stress at selected depths Depth [mm] Specimen 1-1 Magma Simtec Fig. 5. Maximum residual stress vs. depth plot for specimen 1 1 Rys. 5. Naprężenia maksymalne w funkcji głębokości dla próbki Die-cast places Specimens 2-1, 2-5 and 1-5 are compared here. Specimens 2-5 and 1-5 were taken from places not affected by LSC while specimen 2-1 was taken from a casting in the production of which LSC had not been applied. Measuring results for specimen 2-1 are given in Table 8and the plot of maximum residual stress vs. depth is shown in Fig. 6. Measuring results for specimens 1-5 and 2-5 are in Table 9 and the plot of residual stress vs. depth is given in Fig. 7. Table 8 Residual stress values for specimen 2-1 obtained by experiment and simulation Tabela 8. Wartości naprężeń dla próbki 2-1, eksperymentalne i sumulacyjne Depth Specimen 2 1 [mm] red red 1 red Table 9 Residual stress values for specimens 1-5 and 2-5 obtained by experiment and simulation Tabela 9. Wartości naprężeń dla próbek 1-5 i 2-5, eksperymentalne i sumulacyjne Specimen 1 5 Specimen 2 5 Depth [mm] red red red 1 red ,14 27,

8 Maximal residual stress Maximum residual stress 100 Maximum residual stress at selected depths depth [mm] Specimen 2-1 Fig.6. Maximum residual stress vs. depth plot for specimen 2-1 Rys. 6. Naprężenia maksymalne w funkcji głębokości dla próbki 2-1 Maximum residual stress at selected depths Depth [mm] Specimen 1-5 Specimen 2-5 Fig. 7. Maximum residual stress vs. depth plot for specimens 1-5 and 2-5 Rys. 7. Naprężenia maksymalne w funkcji głębokości dla próbki 1-5 i Annealed places Specimens for measurement were taken from the upper part of the engine block (casting No 8 annealed at BUT) and from the lower part of the block (casting No 9 - annealed at SKODA AUTO a.s. Mlada Boleslav). Measuring results for the casting 8-1 are given in Table 10 and the maximum residual stress vs. depth plot is in Fig. 8. Measuring results for the casting 9-1are given in Table 11 and the maximum residual stress vs. depth plot is in Fig. 9.

9 Maximum residual stress 101 Table 10 Residual stress values for specimen 8-1 obtained by experiment and simulation Tabela 10. Wartości naprężeń dla próbki 8-1, eksperymentalne i sumulacyjne Depth Specimen 8 1 [mm] 1 2 red red red Table 11 Residual stress values for specimen 9-1 obtained by experiment and simulation Tabela 11. Wartości naprężeń dla próbki 9-1, eksperymentalne i sumulacyjne Hloubka Vzorek 9 1 [mm] 1 2 red red Maximum residual stress at selected depths Depth [mm] Specimen 8-1 Fig. 8. Maximum residual stress vs. depth plot for specimen 8-1 Rys. 8. Naprężenia maksymalne w funkcji głębokości dla próbki 8-1

10 Maximum residual stress 102 Maximum residual stress at selected depths Depth [mm] Specimen 9-1 Fig. 9. Maximum residual stress vs. depth plot for specimen 9-1 Rys. 9. Naprężenia maksymalne w funkcji głębokości dla próbki CONCLUS ION At smaller depths in die-cast places and in places of local squeeze casting there occurs residual compression stress that gradually changes to tensile stress at greater depths see Table 12. Table 12. Distribution of residual stress for individual measured places and depths Tabela 12. Rozkład naprężeń dla poszczególnych głębokości i miejsc pomiarowych Depth small (ca to 0.15 mm) medium (ca from 0,15 to 1 mm) greater (ca from 1 to 6 mm) residual stress measured residual stress from simulation LSC die-cast annealed Specimen 1-1 compression (of the order of tens of transient (of the order of units to tens of tensile (of the order of units to tens of Specimen 2-1 Specimen 1-5 Specimen 2-5 compression (of the order of tens of transient (of the order of units to tens of tensile (of the order of units to tens of Specimen 8-1 Specimen 9-1 transient (of the order of units to tens of transient (of the order of units to tens of transient to tensile (of the order of un its of specimens X -1 specimens X -5 specimens X -1 specimens X transient (of the order of tenths to tens of transient (of the order of units of probably compressi on or transient (of the order of units or tens of probably compressi on or transient (of the order of units or tens of

11 103 In the group of annealed specimens there were no major differences. The specimen annealed at BUT yielded more uniform results, the simulation programs did not reckon with subsequent heat treatment. During the calculation, neither of the simulation software programs (, ) was provided with modules for the calculation of squeeze casting and thus all the simulation calculations were performed for die-casting. On the basis of the application of the integral method of calculating residual stress we can determine the critical places where experimentally established stress values reach their maxima. REFERENCES [1] Gremaud, M., Rappaz,M.: Modeling of Foundry Processes Diferences Between Various Solutions, AFS Transactions , str. 7-18, [2] Čech, J., Zalaba, R., Švaříček, K.: Možnosti aplikace numerických metod ke zpřesnění výpočtu zbytkové napjatosti odvrtávací metodou u odlitků z Al slitin, Technológia 2003, Bratislava, 2003 [3] Krautschneider, R.: Srovnání experimentu a simulace u odlitků z litiny a Al slitin, Diplomový úkol VUT FSI, Brno, The work was supported by the Grant Agency of the Czech Republic, project No 106/02/0352, project No 106/04/0691 and research plan CZ PORÓWNANIE EKSPERYMENTALNEJ I SYMULACYJNEJ METODY OCENY NAPRĘŻEŃ SZCZĄTKOWYCH W STOPACH ALUMINIUM STRESZCENIE Do oceny pola naprężeń w stopach aluminium użyto metody rentgenograficznej i metody naprężenia kontrolnego. Dwa programy s ymulacyjne ( i ) zostały użyte do porównanie wyznaczonych wartości naprężeń. W artykule omówiono potencjał użytych metod, możliwe do popełnienia błędy i wyznaczenie wartości naprężeń szczątkowych w krytycznych miejscach na grubości ścianki odlewu. Recenzowała Prof. Ewa Majchrzak