INFLUENCE OF SHRINKAGE POROSITY ON THE FATIGUE BEHAVIOR OF CAST AlSi7Mg

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1 INFLUENCE OF SHRINKAGE POROSITY ON THE FATIGUE BEHAVIOR OF CAST AlSi7Mg Stanislava FINTOVÁ a, Radomila KONEČNÁ a, Gianni NICOLETTO b a Department of Materials Engineering, University of Žilina, Univerzitná 1, Žilina Slovakia, stanislava.fintova@fstroj.uniza.sk b Department of Industrial Engineering, University of Parma, Viale G.P. Usberti, 181/A, Parma, Italy, gianni.nicoletto@unipr.it, Abstract Cast defects are always present in aluminum castings due to the production process. Because of the negative influence of porosity on the fatigue properties, the porosity knowledge is the key-step in the fatigue life prediction of Al-Si castings. This contribution presents rotating bending fatigue tests that were planned and performed on specimens of modified AlSi7Mg cast alloy based on the largest defect size characterization by metallography. For the production of the fatigue test specimens, separately cast bars were produced with different treatments of the liquid metal before casting. The objective was to determine the role of modifiers on fatigue response using a limited number of test specimens. The largest defect size on individual specimens was predicted using a statistical pore size distribution obtained with metallography and used in fatigue test planning. Predicted and actual sizes of critical pores observed on the fatigue fracture surface of broken specimens were compared. 1. INTRODUCTION Aluminum alloys are increasingly used in structural applications controlled by low weight requirements, (i.e. aircraft and automotive industry) for their excellent specific strength (i.e. strength-to-density ratio). The Al-Si alloys are the most used Al alloys when the casting process is considered [1]. The presence of pores in cast Al-Si alloys reduces fatigue life of an order-of magnitude or more compared to a defect-free material because practically eliminates the crack initiation phase. It was also found that there is a threshold defect size (i.e. in the range μm, [1]) for fatigue crack initiation from a pore. Below this threshold size, fatigue crack initiation occurs at eutectic particles or by slip band formation. Typically, fatigue endurance is reduced when the size of porosity increases. Therefore, the quality of the castings is strictly related to the porosity control [2]. In this contribution rotating bending fatigue tests were planned based on the largest defect size prediction according to the Largest Extreme Value Distribution theory [3]. For the defect size characterization, the pore area1/2 definition was used according to [4]. The defects observed on the fatigue fracture surfaces after the fatigue test were compared with the predicted largest defect sizes. 2. EXPERIMENTAL MATERIAL AND PROCEDURE The experimental material was the cast aluminum alloy AlSi7Mg heat treated to T6. The material of the set 4 was modified by LVE tablets based on sodium salts and degassed by nitrogen for 10 minutes. The bars of the set 5 were treated with using the same way as for the set 4 and cast after the subsiding of the modification effect of LVE tablets. The bars were separately cast to the steel moulds. The metallographic specimens were extracted from the cast bars before the production of the fatigue tests specimens. Rotating bending fatigue tests were performed at the University of Parma in Italy. The structural analysis was carried

2 out applying metallographic techniques and digital image analysis software on metallographic polished cross sections and was performed at the University of Žilina. Typical examples of the microstructure with defects of the AlSi7Mg aluminum alloy specimens are shown in Fig. 1. The microstructure consists from primary dendrites of α-phase (solid solution of Si in Al) and eutectics (α-phase + Si particles) located in interdendritic spaces. The Si particles had a rounded shape on the metallographic section due to the optimal alloy modification. Porosity was present in all materials due to the casting process. The typical examples of the defects observed on metallographic specimens are also shown in Fig. 1. a) set 4 b) set 5 Fig. 1 Typical examples of microshrinkages observed on metallographic cross-sections The metallographic evaluation, the measurements of defect size and the pore morphology evaluation were performed with a light microscope and the image analysis program NIS Elements 3.0. The aim was the statistical characterization of pore sizes to be used for the prediction of the largest defect size according to the Largest Extreme Value Distribution (LEVD) theory [3]. The porosity was evaluated at 50x magnification and the controlled area S 0 was 1.62 mm 2. The rotating bending fatigue tests for the two material sets were planned based on the results of the previous pore size characterization by metallographic methods and porosity prediction using LEVD theory. Smooth 6 mm diameter fatigue specimens were tested at different stress amplitude levels (from 80 to 30 MPa) at 50 Hz frequency. 3. EXPERIMENTAL RESULTS 3.1 Porosity evaluation, characterization and prediction The metallographic specimens of set 4 were characterized by quite small casting defects with morphology of microshrinkage pores as it is shown in Fig. 1a. Except in one case (i.e. specimen 4B with large defects in the central part) the defects in the specimens of set 4 were uniformly distributed on the metallographic sections. The porosity of the specimens of set 5 was in the shape of the microshrinkage pores, Fig. 1b. Except the specimen 5E the amount of porosity was small and the sizes of observed defects were smaller compared to the defects of set 4. Specimen 5E had the largest defects in the set. The value of the (area) 1/2 was used as characterizing pore size and as severity indicator in fatigue. The results of the porosity evaluation according to the LEVD theory, [3], are summarized in the Fig. 2. The pore size observed on the metallographic specimens of set 4 had a large scatter. The smallest pore sizes were measured on the specimens 4D and 4C, then 4A, 4E and the largest defects were present in the specimen

3 4B. On the specimen 4B the largest defect sizes were measured and also the scatter of the measured values was the largest one in the case of this specimen. Similar differences in the measured largest defects sizes for the specimens from the set 5 cast after the subsiding of the LVE tablets modification effect were observed. In this case the smallest porosity was measured on the specimens 5D, 5C, 5A and 5B and the largest one on the specimen 5E. a) set 4 b) set 5 Fig. 2 LEVD plots The plots of Fig. 2 can be used for the comparison of individual specimens and planning of fatigue experiments. For a given stress amplitude, the longest fatigue life was expected for the specimens with smallest pores because of the influence of pore size on fatigue properties. For a given stress amplitude, the number of cycles to the fatigue fracture is assumed to increase with decreasing pore size. The predicted largest defect size for the area S = 10 mm 2, the representative area of the most loaded area of the rotating bending fatigue specimens, were calculated using the appropriate return period T in the plot of Fig. 2 and are given in the Tab. 1. Tab. 1 Predicted defect sizes and rotating bending fatigue tests results SDAS [m] predicted largest defect size S a [MPa] N f [cycles] SDAS [m] predicted largest defect size S a [MPa] N f [cycles] 4C D D C A A E B B E For all metallographic specimens, the predicted largest defect sizes were larger then the threshold defect size for the fatigue crack initiation (i.e. in the range μm, [1]). And so in the comparison of both sets, the smaller porosity was observed on the specimens from the set 5 cast after the subsiding of the modification effect compared to the set 4 modified by LVE tablets and degassed. Therefore superior fatigue properties for the set 5 were assumed compared to set 4.

4 For completeness, the values of Secondary Dendrite Arm Spacing (SDAS), obtained according to the straight line method (i.e. line length=12 cm) [5], are summarized in the Tab. 1 because SDAS characterize the material structure. It is observed that the values of SDAS are similar for all specimens of both sets. 3.2 Fatigue tests results The fatigue test results are shown in the Fig. 3, where the number of cycles to the fracture (N f ) is plotted versus the nominal stress amplitude S a (i.e. S N curve). The fatigue specimens were tested considering the porosity evaluation and the largest defect size prediction for the area S = 10 mm 2 given in Tab C 5C 5D Sa [MPa] E 4E 4D 5A 5B 4B 4A run out set 4 set ,E+00 1,E+01 1,E+02 1,E+03 1,E+04 1,E+05 1,E+06 1,E+07 1,E+08 log Nf [cycles] Fig.3 S-N curve The specimens from each set with the smallest predicted largest defect size were tested at the largest stress level amplitude. With increasing of the predicted largest defect size, the applied stress level amplitude decreased. The prediction according to the porosity characterization that the set 4 is worse in terms of fatigue behavior than set 5 was verified. The plot shows a very large scatter between the two material sets and among specimens of the same material. It demonstrates that the largest pore sizes in the tested material volume affect dramatically the fatigue response. Specimens failed within 100 load cycles. While 4C and 5C were subjected to relatively high stress amplitude, specimens 4E and 5E apparently contained large initial defects with essentially propagated by brittle fracture mechanisms. 3.3 Fractographic analysis Some chosen specimens broken during the fatigue tests were examined in SEM. The fracture surfaces and the crack initiation places (IP) are shown in Fig. 4. The fracture surfaces of the specimens, except the specimen 4D, were characterized by the fatigue region and the final static fracture. The results show, that for all the examined specimens the fatigue fracture occurred because of the presence of a casting defect on or near the free surface of the specimen. This is due to the rotating bending load condition that develops the highest stress on the surface of the tested specimen.

5 a) specimen 4B, S a = 30 MPa, N f = cycles b) specimen 4B, IP c) specimen 4D, S a = 50 MPa, N f = cycles d) specimen 5B, S a = 45 MPa, N f = cycles e) specimen 5B, IP f) specimen 5B, defects below the specimens free surface g) specimen 5D, S a = 80 MPa, N f = cycles h) specimen 5D, IP 1 i) specimen 5D, IP 2 Fig. 4 Fatigue fracture surfaces

6 Only in the case of the specimen 5D the multiple fatigue crack initiation occur, Fig. 4g. In the other cases only one IP on the fatigue fracture surfaces were found, Fig. 4a and Fig. 4d. In the case of the specimen 4D to identify the fatigue crack initiation place was not possible. Also it was not possible to distinguish fatigue region on the fracture surface. In the cases of the specimens 5B and 5D it was possible to estimate the size of the defects observed on IP on the fatigue fracture surfaces. The critical defect at the IP of the specimen 5B, Fig. 4e, had the size area 1/2 = 75 m, which is almost the same of the predicted largest defect size for this specimen for the area S = 10 mm 2, see Tab.1. In the case of the specimen 5D the predicted largest defect was smaller than the measured critical size of 107 m. 4. CONCLUSIONS The rotating bending fatigue tests on two sets (i.e. 4 and 5) of limited number of specimens from cast AlSi7Mg alloy were planned and performed after the metallographic evaluation of the respective pore size population. The following conclusions can be proposed: - the fatigue behavior of AlSi7Mg cast after subsiding of the modification effect of LVE tablets is slightly better that AlSi7Mg modified with LVE tablets of sodium salts and degassed by nitrogen for 10 minutes, - similar largest defects sizes for the area S = 10 mm 2 were predicted for both sets, - the number of cycles to the fracture reasonably correlate with the predicted largest defect sizes, - the different scatter in defect sizes between two material sets obtained by LEVD analysis and metallography correlate with fatigue data scatter, - the porosity evaluation can be used for the fatigue test planning and comparison of different materials using a limited number of specimens. ACKNOWLEDGEMENTS This work was done as a part of KEGA grant No.3/6110/08. LITERATURE [1] WANG, Q. G. APELIAN, D. LADOS, D. A. Fatigue behavior of A356-T6 aluminum cast alloys. Part I. Efect of casting defects. Journal of Light Metals 1, 2001, p [2] KONEČNÁ, R. NICOLETTO, G. MAJEROVÁ, V.: Largest extreme value determination of defect size with application to cast Al-Si alloys porosity, METAL 2007, Ostrava: TANGER 2007, CD ROM [3] MURAKAMI, Y. Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, Elsevier, Oxford, [4] KONEČNÁ, R. FINTOVÁ, S. NICOLETTO, G.: Influence of the casting pores origin on the fatigue properties of Al-Si cast alloys, Metallography 2010, In press. [5] Atlas métalographique des alliages d aluminium. CTIF, Paris, 1980.