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Microstructural characterization and high cycle fatigue behavior of investment cast A357 aluminum alloy
International Journal of Fatigue 77 (2015) 154–159
Contents lists available at ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Microstructural characterization and high cycle fatigue behavior of investment cast A357 aluminum alloy S. Dezecot a,b,c, M. Brochu a,⇑ a
Ecole Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Qc H3C 3A7, Canada INSA-Lyon, MATEIS CNRS UMR 5510, F-69621 Villeurbanne, France c Centre des Matériaux, Mines ParisTech, CNRS UMR 7633, BP 87, 91 003 Evry Cedex, France b
a r t i c l e
i n f o
Article history: Received 28 September 2014 Received in revised form 27 February 2015 Accepted 5 March 2015 Available online 14 March 2015 Keywords: Aluminum alloy A357 Investment casting Microstructure High cycle fatigue Crack initiation
a b s t r a c t In order to observe the influence of strontium (Sr) modification and hot isostatic pressing (HIP) on an aluminum–silicon cast alloy A357 (AlSi7Mg0.6), the microstructure and the high cycle fatigue behavior of three batches of materials produced by investment casting (IC) were studied. The parts were produced by an advanced IC proprietary process. The main process innovation is to increase the solidification and cooling rate by immersing the mold in cool liquid. Its advantage is to produce finer microstructures. Microstructural characterization showed a dendrite arm spacing (DAS) refinement of 40% when compared with the same part produced by conventional investment casting. Fatigue tests were conducted on hourglass specimens heat treated to T6, under a stress ratio of R = 0.1 and a frequency of 25 Hz. One batch of material was unmodified but two batches were modified with 0.007% and 0.013% Sr addition, from which one batch was submitted to HIP after casting. Results reported in S–N diagrams show that the addition of Sr and the HIP process improve the 106 cycles fatigue strength by 9% and 34% respectively. Scanning electron microscopy (SEM) observation of the fracture surfaces showed a variety of crack initiation mechanisms. In the unmodified alloy, decohesion between the coarse Si particles and the aluminum matrix was mostly observed. On the other hand, in the modified but non HIP-ed alloy, cracks initiated from pores. When the same alloy was subjected to HIP, a competition between crystallographic crack initiations (at persistent slip bands) and decohesion/failure of intermetallic phases was observed. When compared to fatigue strength reported for components produced by permanent mold casting, the studied material are more resistant to fatigue even in the unmodified and non HIP-ed states. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Investment casting is one of the oldest foundry processes allowing the production of parts with an excellent surface finish, tight dimensional tolerances and complex shapes [1]. Such a manufacturing process has found interest in aeronautical applications because it increases the buy to fly ratio and can potentially save weight if mechanical properties are properly optimized. The metallurgical and technological progress of IC, have opened the doors for manufacturing fatigue critical components by IC [1,2]. However, the substantiation of new materials and processes, calls for a deep understanding and control of the relationships between mechanical properties, microstructure and manufacturing process parameters, justifying the goal of several research projects [3–8]. Within the reviewed literature on the topic, several authors showed that the fatigue strength of cast aluminum alloys is closely related to ⇑ Corresponding author. Tel.: +1 514 340 4711x4614; fax: +1 514 340 5867. E-mail address: [email protected] (M. Brochu). http://dx.doi.org/10.1016/j.ijfatigue.2015.03.004 0142-1123/Ó 2015 Elsevier Ltd. All rights reserved.
the fineness of the microstructure which is typically characterized in terms of dendrite arm spacing (DAS) which is in turn controlled by the cooling rate [8–10]. Fast cooling rates produce fine microstructures and smaller DAS. Since conventional investment casting produces coarse microstructures, the fatigue strength of IC components is reputed to be low when compared to the same alloy produced by permanent molding or sand casting [7,11]. Despite the difference in microstructure caused by processing conditions, fatigue mechanisms in cast Al–Si–Mg alloys are similar. Cracks typically initiate at defects [7,12–14], silicon particles [6,13,15–17], intermetallics [13,14,18] or in the primary alpha phase [18–20]. The size and morphology of the phase particles (eutectic, intermetallics) have an impact on the triggered mechanisms. Results have shown that higher fatigue strengths are associated to round and small phases [3,21]. Heat treatment and addition of eutectic modifier, such as strontium (Sr), are strategies used to refine and/or spheroidized eutectic silicon particles [3,10,22–24]. When these processing parameters are well gauged, optimized yield strength, elongation and fatigue strength can be obtained
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[20,25,26]. Nevertheless, Sr addition increases the number of pores [22,27] that can be responsible for crack initiation. To counteract the detrimental effect of pores, a HIP treatment that closes the foundry defects is often used [8,16,28,29]. This paper presents results of material characterization and fatigue tests performed on Al–Mg–Si alloy A357 produced by a proprietary investment casting process. The objective of this study is to understand the roles of the microstructure on the alloy fatigue strength.
volume fraction) of the primary alpha phase (f a), the grain equivalent spherical diameter (D), the secondary dendrite arm spacing (SDAS), the length and width of the eutectic silicon particles (LSi and Wsi), the porosity and the maximum length of pores (Lp). A Poulton macroetch was used to reveal the grains but other features were measured on as polished specimens. Results will be presented in Section 3. 2.3. Mechanical properties Hardness measurements were carried out according to ASTM E18. The average Rockwell B hardnesses (HRB) calculated from 10 measurements are reported in the first row of Table 3. The tensile properties were measured on subsize specimens according to ASTM B-557 and results are reported in Table 3. A comparison of the tensile properties of UM and Sr specimens reveals that the addition of strontium enhances the hardness, yield strength and ultimate strength of aluminum A357. Eutectic refinement increases the surface to volume ratio of silicon particles which favors a more homogeneous solution heat treatment. Fine and round silicon particles also contribute to the improvement of elongation as they postpone the formation of microvoids [18,21]. Comparing the tensile test results of Sr and Sr-HIP may indicate that hot isostatic pressing mainly contributes to the improvement of strengths. On the other hand, a small reduction of the elongation is observed after HIPing which contradicts the results of Lee et al. [16]. Based on this unexpected behavior, the authors propose that the modified quenching and tempering conditions applied to SrHIP specimens are mainly responsible for the differences in strengths and elongation. Elongation is very sensitive to temperature and aging time as reported in the work of Alexopoulos et al. [25].
2. Experimental procedure 2.1. Materials Fatigue specimens and samples for metallographic observations were prepared from cast plates similar to Fig. 1. The innovative investment casting process consists of placing the mold into a tank wherein a coolant is injected to accelerate and control the solidification. Moreover, the coolant is pressurized to 6 bars (600 kPa) to ‘‘increase the material resistance to forming gas pores and enhance the feeding capability’’ [5]. In this project, two aluminum plates were cast simultaneously in one ceramic mold. Liquid metal was gravity fed from zone III (see Fig. 1) down to zone I. Three hourglass fatigue specimens (Fig. 2) were machined from zone II as shown by the dotted lines in Fig. 1. The material studied is ASTM B179 cast aluminum alloy A357 having the nominal chemistry reported in the first line of Table 1. Three varieties of this alloy were studied: unmodified A357-T6, Sr-modified A357-T6 (0.007 wt.%Sr) and Sr-modified and HIP-ed A357-T6 alloy (0.013 wt.%Sr). They will be respectively identified UM, Sr and Sr-HIP and their specific chemistry, measured by optical spectroscopy, is reported in Table 1. All plates were heat treated to T6 according to the procedures described in Table 2. Tempering conditions were optimized for each material in order to maximize the yield strengths.
2.4. Fatigue tests Axial fatigue tests were performed on hourglass specimens with a rectangular cross section of 9.5 2.54 mm. Such a geometry was
2.2. Microstructural characterization Microstructural characteristics were measured from observations under an optical microscope equipped with a digital camera and image analysis software. A total area of 30 mm2 was observed for each material, to obtain: the surface fraction (equivalent to
Fig. 2. Fatigue specimen geometry.
Table 2 T6 Heat treatment procedures.
UM Sr Sr-HIP
Fig. 1. IC plate geometry.
Solution heat treatment Time/temperature
Quench Coolant/ temperature
Aging Time/ temperature
18 h/543 °C 18 h/543 °C 18 h/543 °C
Glycol/20 °C Glycol/20 °C Water/2 °C
20 h/160 °C 20 h/160 °C 18 h/160 °C
Table 1 Chemical composition of aluminum alloy A357 (wt.%).
A357 (ASTM) UM Sr Sr-HIP
Si
Fe
Cu
Mn
Mg
Zn
Ti
Sr
6.5–7.5 7.1 7.1 7.1
<0.12 0.06 0.06 0.06
<0.10 0.01 0.01 0.01
<0.05 <0.01 <0.01 <0.01
0.45–0.7 0.60 0.64 0.62
<0.05 <0.01 <0.01 <0.01
0.04–0.20 0.13 0.13 0.10
<0.03 0.0007 0.0070 0.0130
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Table 3 Tensile properties and hardness.
3. Results
Rockwell hardness – HRB Yield strength 0.2% – Re (MPa) Ultimate strength – UTS (MPa) Elongation (%)
UM
Sr
Sr-HIP
65 272 331 5.9
68 299 353 7.6
70 327 361 6.5
In this section, the results of microstructural characterization and material testing are presented.
3.1. Microstructure
chosen in order to produce results easily comparable to those previously published by Brochu et al. [4,20,30]. Specimens were polished with a 1 lm diamond paste. Constant stress amplitudes tests (ASTM E 466) were used to generate endurance curves (S– N) at a stress ratio R = 0.1 and a frequency of 25 Hz. The stress amplitudes were chosen to obtain fatigue life between 104 and 107 cycles. This corresponds to stress amplitudes ranging from 77 MPa to 122 MPa for UM and Sr materials but ranging from 99 MPa to 140 MPa for the Sr-HIP, which is more resistant to fatigue. After failure, each fracture surface was examined under scanning electron microscopy (JEOL JSM-840A).When second phase particles were observed at crack initiation sites, energy dispersive X-ray spectroscopy (EDX) was used to identify their chemical nature.
Pictures of the material microstructures are presented in Figs. 3 and 4 and measured microstructural characteristics are reported in Table 4. Fig. 3 and results in Table 4 reveal that the grain size, the fraction of primary alpha phase and the SDAS are in the same range for all investigated materials. This observation was supported by a statistical analysis of our measurements that is not reported in this paper. A comparison of Fig. 4A and C shows that the addition of strontium refined the eutectic silicon particles. A refinement from 11.1 to 3.5 lm in size is reported in Table 4 which is consistent with studies conducted by Ceschini, Zhongwei and Yang [23,28,31]. Fig. 4C and E shows that HIPping had no effect on the morphology and size of eutectic silicon, as expected. Further examination of the
Fig. 3. Microphotographs, etched with Poulton, showing the grain structure of the materials (A) UM, (B) Sr and (C) Sr-HIP.
Fig. 4. As polished micrographs of (A), (B) unmodified material, (C), (D) Sr modified material and (E), (F) Sr modified and HIP material.
Table 4 Mean microstructural features.
UM Sr Sr-HIP
D (lm)
f a (%)
SDAS (lm)
LSi (lm)
Wsi (lm)
Pores (mm2)
Defect length (lm)
721 691 771
63 67 65
47 51 49
11.1 3.5 3.4
8.7 1.7 1.9
5 9.5 /
8.9 9.6 /
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S. Dezecot, M. Brochu / International Journal of Fatigue 77 (2015) 154–159 Table 5 Basquin law parameters and ra at 106 cycles.
PM [30] UM Sr Sr-HIP
Fig. 5. Distribution of defect sizes for UM and Sr.
eutectic constituent showed that it is not only composed of silicon particles and a phase, it also contains other intermetallics. Fe-rich phases, in light gray, were identified by EDX. The largest Fe-rich particles were observed in the UM material (Fig. 4A) smaller ones were also visible in the Sr and Sr-HIP materials. Since the fatigue behavior of metals is very sensitive to casting defects, statistical distributions of pores from metallographic samples are reported in Fig. 5. The HIP-ed plates contained no visible (or quantifiable) defects, Fig. 5 gives the defect population for the UM and Sr material. The mean defect length and the defect density are reported in Table 4 to ease comparison. Based on the density value, the addition of strontium doubled the number of defects contained in this aluminum alloy but had no significant effect on their size.
3.2. Fatigue strength The results of the fatigue tests are represented in Fig. 6 in the form of SN curves, with the diamonds for UM, the stars for Sr and the triangles for the Sr-HIP material. For comparison, the results obtained by Brochu et al. [20], for permanent mold (PM) unmodified A357-T6, are presented by squares. The lines in Fig. 6 are Basquin type power law equations (ra = r0 f Nc), with the r0 f fatigue strength coefficient and c the fatigue strength exponent, that were fitted to our results. For each material, r0 f , c and the calculated fatigue strength at 106 cycles are reported in Table 5. For Sr-HIP the regression proposed does not contain the two lowest life
r0 f (MPa)
c
ra a 106 cycles (MPa)
284 473 449 289
0.1111 0.1352 0.1246 0.0720
62 73 80 107
results obtained at ra = 99 MPa because untypical casting defects were observed on their fracture surfaces. Both specimens come from the same plate that should have been discarded at quality control. Oxide films are sometime difficult to identify in radiographs because they are thin and have an attenuation coefficient very close to aluminum. Comparing the calculated fatigue strength obtained for PM and UM materials shows that the 106 fatigue strength of material produced by investment casting can be superior, by approximately 18%, to that of permanent mold castings. This is surprising considering that the microstructural features are slightly smaller in the PM component; secondary dendrite arm spacings are respectively 47 lm and 43 lm [30] for the IC and the PM components. Such a fine microstructure and high fatigue strength has not been reported for investments cast components before [2–16] and is attributed to the faster cooling rate achieved with the improved investment casting process. For the materials modified with strontium, the 106 cycles fatigue strengths are 80 MPa and 107 MPa respectively for Sr and Sr-HIP. The addition of strontium allowed a gain of 7 MPa (9%) at 106 cycles when compared to UM results. Strontium modification has two beneficial effects on fatigue. It increases the yield strength (see Table 3) which makes the material more resistant to plastic deformation and thus to the formation of persistent slip bands, and it refines and spheroidizes silicon particles (see Table 4) which postpone plastic deformation incompatibility between the phases to higher strain values. On the other hand, it was found to increase the density of pores which may explain that fatigue strength increased by only 9%. Applying a HIP treatment to the modified alloy showed the full potential of the material and the process as it further increased by 27 MPa (34%) the material fatigue strength. This is in accordance with the microstructural analysis in which no pores could be observed for the Sr-HIP material. HIP plays an important role as it closes the defects from which fatigue cracks initiate prematurely.
3.3. Crack initiation mechanisms
Fig. 6. Fatigue behavior of the different A357-T6 alloys at R = 0.1 and f = 25 Hz.
Fractographic analyses were performed to locate and identify the features responsible for crack initiation. All specimens were observed and crack initiation mechanisms could be classified in four categories: silicon particles (Fig. 7), casting defects (Fig. 8), intermetallics, crystallographic slip in the primary alpha phase (Fig. 9). The majority of the fatigue test specimens made from the UM material showed the presence of silicon particles at the initiation sites. Fig. 7a shows the footprint of a silicon particle that was debonded from the material. Traces of silicon (in black), confirmed by an EDX analysis, are visible. This type of initiation mechanism has already been reported by Buffière et al. [12], and is due to a strain incompatibility between the aluminum matrix and the silicon particle. On the other hand, in the PM material, large pores are the main cause of failure [20]. It should be noted that just one crack propagated from a pore within the UM material. On the other hand, the most frequent initiation sites (80%) observed in the strontium modified alloy (Sr) are pores. The size of these defects observed at crack initiation sites ranges between 100 lm and 290 lm (see Fig. 8). Pores became the limiting
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Fig. 7. Fracture surfaces at 500 for material UM (A) at ra = 122 MPa and Nr = 26,178 cycles, (B) at ra = 99 MPa and Nr = 117,276 cycles.
Fig. 8. Fracture surfaces at 100 for material Sr (A) at ra = 122 MPa and Nr = 53,762 cycles, (B) at ra = 99 MPa and Nr = 131,665 cycles.
Fig. 9. Fracture surfaces with the respective magnifications of 100 and 500 for material Sr-HIP (A) at ra = 122 MPa and Nr = 354,394 cycles, (B) at ra = 140 MPa and Nr = 60,814 cycles.
microstructural discontinuity for two reasons. Firstly because the addition of strontium refined the size of the silicon eutectic particles down to 11 lm. Secondly because the density of pores doubled with the addition of strontium. The most frequent crack initiation mechanism for the modified and HIP material is the formation of persistent slip bands in the primary alpha phase. This mechanism is characterized by the observation of flat surfaces usually oriented at approximately 45 ° from the loading direction (Fig. 9a). Sometimes the crystallographic propagation mode can extend over more than one grain as shown in Fig. 9a. This mechanism was also observed by Brochu et al. [4] in the study on short crack propagation in rheocast A357 alloy. A second interesting crack initiation mechanism observed in the Sr-HIP specimens is the cleavage of iron rich intermetallic particles as shown in Fig. 9b. In the presence of pores, this mechanism is not dominant as reported by Yi et al. [32]. On the other hand, in the Sr-HIP material, silicon particles are refined and pores are absent which triggers new mechanisms. Intermetallics are nevertheless not de main feature observed at crack initiation site because their volume fraction is low, reducing
the probability of finding it near the specimen surface. This is especially true in the case of hourglass specimens in which a limited amount of material is tested. 4. Discussion and conclusion This experimental study shows the influence of microstructural features on the fatigue strength and failure mechanisms of investment cast A357 alloy in T6 condition. Three different microstructures were tested to show the influence of strontium modification and HIP treatment on the fatigue behavior. The main conclusions of this research work are the followings: For 106 cycles at R = 0.1, the proprietary investment casting technology improves the fatigue strength by 11 MPa (18%) when compared with the same alloy produced by permanent mold casting. For unmodified and unHIP material, the main crack initiation mechanism observed is decohesion of large eutectic silicon particles in the size range of 100 lm.
S. Dezecot, M. Brochu / International Journal of Fatigue 77 (2015) 154–159
Strontium addition reduced the size of the silicon particles but doubled the number of pores which triggered crack initiation from pores rather than eutectic silicon, improving the fatigue strength by 9%. Application of HIP on the strontium modified material closes the defects which significantly improve (34%) the material fatigue strength. For the Sr-HIP specimens, cracks mainly initiated at persistent slip bands or at iron rich intermetallics. According to our results and analysis, refinement of eutectic silicon particles has a great potential of increasing fatigue strength of aluminum alloy A357 but only if pores and intermetallics are kept small and to a minimum density. The smallest microstructural discontinuity observed at crack initiation site was in the range of 45 lm. This size range should be the target for silicon particles, pores and intermetallics if one wishes to optimize fatigue life. Below this value, it appears that crack initiation from persistent slip bands will limit the fatigue resistance. Acknowledgments The authors would like to thank CRSNG for the financial support during this project and Alphacasting Inc. for providing the material. References [1] Pattnaik Sarojrani, Karunakar D Benny, Jha PK. Developments in investment casting process—A review. J Mater Process Technol 2012;212(11):2332–48. [2] Kennerknecht S, Dumant X, Tombari R, Biljon P. The effect of processing parameters on the fatigue properties of D357 investment castings. JOM 49 1997;11(1):22–8. [3] Barlas Bruno. Etude du comportement et de l’endommagement en fatigue d’alliages d’aluminium de fonderie. MAT – Centre des Matériaux: École Nationale Supérieure des Mines de Paris; 2004. [4] Brochu Myriam, Verreman Yves, Ajersch Frank, Bouchard Dominique. High cycle fatigue strength of permanent mold and rheocast aluminum 357 alloy. Int J Fatigue 2010;32(8):1233–42. [5] Wang QG, Jones P, Osborne M. The effects of applied pressure during solidification on the microstructure mechanical properties of lost foam A356 castings. In: Conference proceedings from materials solutions 2002, October 7, 2002 – October 9, 2002. Advances in Aluminum Casting Technology II. ASM International; 2002. p. 75–84. [6] Lee FT, Major JF, Samuel FH. Fracture behaviour of Al12 wt.%Si0.35 wt.%Mg(0– 0.02)wt.%Sr casting alloys under fatigue testing. Fatigue Fract Eng Mater Struct 1995;18(3):385–96. [7] Lados DA, Apelian D, De Figueredo AM. Fatigue performance of high integrity cast aluminum components. In: Conference proceedings from materials solutions 2002, October 7, 2002 – October 9, 2002. Advances in Aluminum Casting Technology II. ASM International; 2002. p. 185–96. [8] Wang QG, Apelian D, Lados DA. Fatigue behavior of A356/357 aluminum cast alloys. Part II - Effect of microstructural constituents. J Light Met 2001;1(1):85–97. [9] Spear RE, Gardner GR. Dendrite Cell Size. Mod Cast 1963;V43(5):209–15. [10] Ceschini L, Morri Alessandro, Andrea Morri A, Messieri Gamberini S. Correlation between ultimate tensile strength and solidification microstructure for the sand cast A357 aluminium alloy. Mater Des 2009;30(10):4525–31. [11] Zhu X, Shyam A, Jones JW, Mayer H, Lasecki JV, Allison JE. Effects of microstructure and temperature on fatigue behavior of E319–T7 cast aluminum alloy in very long life cycles. Int J Fatigue 2006;28(11):1566–71.
159
[12] Buffiere J-Y, Savelli S, Jouneau PH, Maire E, Fougeres R. Experimental study of porosity and its relation to fatigue mechanisms of model Al-Si7-MgO.3 cast Al alloys. Mater Sci Eng, A 2001;316(1–2):115–26. [13] Ammar HR, Samuel AM, Samuel FH. Effect of casting imperfections on the fatigue life of 319-F and A356–T6 Al–Si casting alloys. Mater Sci Eng, A 2008;473(1–2):65–75. [14] Linder Jan, Axelsson Markus, Nilsson Henrik. The influence of porosity on the fatigue life for sand and permanent mould cast aluminium. Int J Fatigue 2006;28(12):1752–8. [15] Lee FT, Major JF, Samuel FH. Effect of silicon particles on the fatigue crack growth characteristics of Al-12 wt pct Si-0.35 wt pct Mg-(0–0.02) wt pct Sr casting alloys. Metall Mater Trans A 1995;26(A (6)):1553–70. [16] Lee Min Ha, Kim Jae Joong, Kim Kyung Hoon, Kim Nack J, Lee Sunghak, Lee WEui. Effects of HIPping on high-cycle fatigue properties of investment cast A356 aluminum alloys. Mater Sci Eng, A 2003;340(1-2):123–9 (janvier 15). [17] Zeng Lei, Sakamoto Junji, Fujii Atsushi, Noguchi Hiroshi. Role of eutectic silicon particles in fatigue crack initiation and propagation and fatigue strength characteristics of cast aluminum alloy A356. Eng Fract Mech 2014; 115:1–12. [18] Ye Haizhi. An overview of the development of Al-Si-alloy based material for engine applications. J Mater Eng Perform 2003;12(3):288–97. [19] Yi JZ, Gao YX, Lee PD, Lindley TC. Microstructure-based fatigue life prediction for cast A356–T6 aluminum–silicon alloys. Metall Mater Trans B 2006;37(2): 301–11. [20] Brochu Myriam, Verreman Yves, Ajersch Frank, Bouchard Dominique. Propagation of short fatigue cracks in permanent and semi-solid mold 357 aluminum alloy. Int J Fatigue 2012;36(1):120–9. [21] Zhang B, Poirier DR, Chen W. Microstructural effects on high-cycle fatiguecrack initiation in A356.2 casting alloy. Metall Mater Trans A 1999;30(10): 2659–66. [22] Closset B, Fay D. Strontium modification of aluminum investment casting alloys. Trans Am Foundrymen’s Soc 1990:505–9. [23] Yang Ching-Yi, Lee Sheng-Long, Lee Cheng-Kuo, Lin Jing-Chie. Effects of Sr and Sb modifiers on the sliding wear behavior of A357 alloy under varying pressure and speed conditions. Wear 2006;261(11–12):1348–58. [24] Moustafa MA, Samuel FH, Doty HW. Effect of solution heat treatment and additives on the microstructure of Al-Si (A413.1) automotive alloys. J Mater Sci 2003;38(22):4507–22. [25] Alexopoulos ND, Tiryakioglu M. Relationship between fracture toughness and tensile properties of A357 cast aluminum alloy. Metall Mater Trans A 2009; 40(3):702–16. [26] Haro-Rodriguez Sergio, Goytia-Reyes Rafael E, Dwivedi Dheerendra Kumar, Baltazar-Hernandez Victor H, Flores-Zuniga Horacio, Perez-Lopez Maria J, et al. On influence of Ti and Sr on microstructure, mechanical properties and quality index of cast eutectic Al–Si–Mg alloy. Mater Des 2011;32(4): 1865–71. [27] Yang Li, Zheng bing Xu, Jian min Zeng. Effect of hydrogen content on impact property of A357 alloy. In: 6th International conference on physical and numerical simulation of materials processing ICPNS2010, November 16, 2010 – November 19, 2010. Materials Science Forum. Trans Tech Publications Ltd.; 2012. p. 704–5. [28] Ceschini L, Morri A, Sambogna G. The effect of hot isostatic pressing on the fatigue behaviour of sand-cast A356–T6 and A204–T6 aluminum alloys. J Mater Process Technol 2008;204(1–3):231–8. [29] Weihmuller Lance W, Mashl Stephen J. The effect of hot isostatic pressing on the mechanical properties of aerospace aluminum castings. In: 61st American helicopter society international annual forum 2005, june 1, 2005 – june 3, 2005, 2. Annual forum proceedings - ahs international. American Helicopter Society; 2005. p. 1447–56. [30] Brochu Myriam. Comportement en fatigue de l’aluminium 357 coulé par gravité et rhéocoulé. École Polytechnique de Montréal 2010. [31] Zhongwei Chen, Ruijie Zhang. Effect of strontium on primary dendrite and eutectic temperature of A357 aluminum alloy. Res Dev 2010. [32] Yi JZ, Gao YX, Lee PD, Lindley TC. Effect of Fe-content on fatigue crack initiation and propagation in a cast aluminum–silicon alloy (A356–T6). Mater Sci Eng, A 2004;386(1):396–407.
Contents lists available at ScienceDirect
International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue
Microstructural characterization and high cycle fatigue behavior of investment cast A357 aluminum alloy S. Dezecot a,b,c, M. Brochu a,⇑ a
Ecole Polytechnique de Montréal, C.P. 6079, Succ. Centre-Ville, Montréal, Qc H3C 3A7, Canada INSA-Lyon, MATEIS CNRS UMR 5510, F-69621 Villeurbanne, France c Centre des Matériaux, Mines ParisTech, CNRS UMR 7633, BP 87, 91 003 Evry Cedex, France b
a r t i c l e
i n f o
Article history: Received 28 September 2014 Received in revised form 27 February 2015 Accepted 5 March 2015 Available online 14 March 2015 Keywords: Aluminum alloy A357 Investment casting Microstructure High cycle fatigue Crack initiation
a b s t r a c t In order to observe the influence of strontium (Sr) modification and hot isostatic pressing (HIP) on an aluminum–silicon cast alloy A357 (AlSi7Mg0.6), the microstructure and the high cycle fatigue behavior of three batches of materials produced by investment casting (IC) were studied. The parts were produced by an advanced IC proprietary process. The main process innovation is to increase the solidification and cooling rate by immersing the mold in cool liquid. Its advantage is to produce finer microstructures. Microstructural characterization showed a dendrite arm spacing (DAS) refinement of 40% when compared with the same part produced by conventional investment casting. Fatigue tests were conducted on hourglass specimens heat treated to T6, under a stress ratio of R = 0.1 and a frequency of 25 Hz. One batch of material was unmodified but two batches were modified with 0.007% and 0.013% Sr addition, from which one batch was submitted to HIP after casting. Results reported in S–N diagrams show that the addition of Sr and the HIP process improve the 106 cycles fatigue strength by 9% and 34% respectively. Scanning electron microscopy (SEM) observation of the fracture surfaces showed a variety of crack initiation mechanisms. In the unmodified alloy, decohesion between the coarse Si particles and the aluminum matrix was mostly observed. On the other hand, in the modified but non HIP-ed alloy, cracks initiated from pores. When the same alloy was subjected to HIP, a competition between crystallographic crack initiations (at persistent slip bands) and decohesion/failure of intermetallic phases was observed. When compared to fatigue strength reported for components produced by permanent mold casting, the studied material are more resistant to fatigue even in the unmodified and non HIP-ed states. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Investment casting is one of the oldest foundry processes allowing the production of parts with an excellent surface finish, tight dimensional tolerances and complex shapes [1]. Such a manufacturing process has found interest in aeronautical applications because it increases the buy to fly ratio and can potentially save weight if mechanical properties are properly optimized. The metallurgical and technological progress of IC, have opened the doors for manufacturing fatigue critical components by IC [1,2]. However, the substantiation of new materials and processes, calls for a deep understanding and control of the relationships between mechanical properties, microstructure and manufacturing process parameters, justifying the goal of several research projects [3–8]. Within the reviewed literature on the topic, several authors showed that the fatigue strength of cast aluminum alloys is closely related to ⇑ Corresponding author. Tel.: +1 514 340 4711x4614; fax: +1 514 340 5867. E-mail address: [email protected] (M. Brochu). http://dx.doi.org/10.1016/j.ijfatigue.2015.03.004 0142-1123/Ó 2015 Elsevier Ltd. All rights reserved.
the fineness of the microstructure which is typically characterized in terms of dendrite arm spacing (DAS) which is in turn controlled by the cooling rate [8–10]. Fast cooling rates produce fine microstructures and smaller DAS. Since conventional investment casting produces coarse microstructures, the fatigue strength of IC components is reputed to be low when compared to the same alloy produced by permanent molding or sand casting [7,11]. Despite the difference in microstructure caused by processing conditions, fatigue mechanisms in cast Al–Si–Mg alloys are similar. Cracks typically initiate at defects [7,12–14], silicon particles [6,13,15–17], intermetallics [13,14,18] or in the primary alpha phase [18–20]. The size and morphology of the phase particles (eutectic, intermetallics) have an impact on the triggered mechanisms. Results have shown that higher fatigue strengths are associated to round and small phases [3,21]. Heat treatment and addition of eutectic modifier, such as strontium (Sr), are strategies used to refine and/or spheroidized eutectic silicon particles [3,10,22–24]. When these processing parameters are well gauged, optimized yield strength, elongation and fatigue strength can be obtained
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[20,25,26]. Nevertheless, Sr addition increases the number of pores [22,27] that can be responsible for crack initiation. To counteract the detrimental effect of pores, a HIP treatment that closes the foundry defects is often used [8,16,28,29]. This paper presents results of material characterization and fatigue tests performed on Al–Mg–Si alloy A357 produced by a proprietary investment casting process. The objective of this study is to understand the roles of the microstructure on the alloy fatigue strength.
volume fraction) of the primary alpha phase (f a), the grain equivalent spherical diameter (D), the secondary dendrite arm spacing (SDAS), the length and width of the eutectic silicon particles (LSi and Wsi), the porosity and the maximum length of pores (Lp). A Poulton macroetch was used to reveal the grains but other features were measured on as polished specimens. Results will be presented in Section 3. 2.3. Mechanical properties Hardness measurements were carried out according to ASTM E18. The average Rockwell B hardnesses (HRB) calculated from 10 measurements are reported in the first row of Table 3. The tensile properties were measured on subsize specimens according to ASTM B-557 and results are reported in Table 3. A comparison of the tensile properties of UM and Sr specimens reveals that the addition of strontium enhances the hardness, yield strength and ultimate strength of aluminum A357. Eutectic refinement increases the surface to volume ratio of silicon particles which favors a more homogeneous solution heat treatment. Fine and round silicon particles also contribute to the improvement of elongation as they postpone the formation of microvoids [18,21]. Comparing the tensile test results of Sr and Sr-HIP may indicate that hot isostatic pressing mainly contributes to the improvement of strengths. On the other hand, a small reduction of the elongation is observed after HIPing which contradicts the results of Lee et al. [16]. Based on this unexpected behavior, the authors propose that the modified quenching and tempering conditions applied to SrHIP specimens are mainly responsible for the differences in strengths and elongation. Elongation is very sensitive to temperature and aging time as reported in the work of Alexopoulos et al. [25].
2. Experimental procedure 2.1. Materials Fatigue specimens and samples for metallographic observations were prepared from cast plates similar to Fig. 1. The innovative investment casting process consists of placing the mold into a tank wherein a coolant is injected to accelerate and control the solidification. Moreover, the coolant is pressurized to 6 bars (600 kPa) to ‘‘increase the material resistance to forming gas pores and enhance the feeding capability’’ [5]. In this project, two aluminum plates were cast simultaneously in one ceramic mold. Liquid metal was gravity fed from zone III (see Fig. 1) down to zone I. Three hourglass fatigue specimens (Fig. 2) were machined from zone II as shown by the dotted lines in Fig. 1. The material studied is ASTM B179 cast aluminum alloy A357 having the nominal chemistry reported in the first line of Table 1. Three varieties of this alloy were studied: unmodified A357-T6, Sr-modified A357-T6 (0.007 wt.%Sr) and Sr-modified and HIP-ed A357-T6 alloy (0.013 wt.%Sr). They will be respectively identified UM, Sr and Sr-HIP and their specific chemistry, measured by optical spectroscopy, is reported in Table 1. All plates were heat treated to T6 according to the procedures described in Table 2. Tempering conditions were optimized for each material in order to maximize the yield strengths.
2.4. Fatigue tests Axial fatigue tests were performed on hourglass specimens with a rectangular cross section of 9.5 2.54 mm. Such a geometry was
2.2. Microstructural characterization Microstructural characteristics were measured from observations under an optical microscope equipped with a digital camera and image analysis software. A total area of 30 mm2 was observed for each material, to obtain: the surface fraction (equivalent to
Fig. 2. Fatigue specimen geometry.
Table 2 T6 Heat treatment procedures.
UM Sr Sr-HIP
Fig. 1. IC plate geometry.
Solution heat treatment Time/temperature
Quench Coolant/ temperature
Aging Time/ temperature
18 h/543 °C 18 h/543 °C 18 h/543 °C
Glycol/20 °C Glycol/20 °C Water/2 °C
20 h/160 °C 20 h/160 °C 18 h/160 °C
Table 1 Chemical composition of aluminum alloy A357 (wt.%).
A357 (ASTM) UM Sr Sr-HIP
Si
Fe
Cu
Mn
Mg
Zn
Ti
Sr
6.5–7.5 7.1 7.1 7.1
<0.12 0.06 0.06 0.06
<0.10 0.01 0.01 0.01
<0.05 <0.01 <0.01 <0.01
0.45–0.7 0.60 0.64 0.62
<0.05 <0.01 <0.01 <0.01
0.04–0.20 0.13 0.13 0.10
<0.03 0.0007 0.0070 0.0130
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Table 3 Tensile properties and hardness.
3. Results
Rockwell hardness – HRB Yield strength 0.2% – Re (MPa) Ultimate strength – UTS (MPa) Elongation (%)
UM
Sr
Sr-HIP
65 272 331 5.9
68 299 353 7.6
70 327 361 6.5
In this section, the results of microstructural characterization and material testing are presented.
3.1. Microstructure
chosen in order to produce results easily comparable to those previously published by Brochu et al. [4,20,30]. Specimens were polished with a 1 lm diamond paste. Constant stress amplitudes tests (ASTM E 466) were used to generate endurance curves (S– N) at a stress ratio R = 0.1 and a frequency of 25 Hz. The stress amplitudes were chosen to obtain fatigue life between 104 and 107 cycles. This corresponds to stress amplitudes ranging from 77 MPa to 122 MPa for UM and Sr materials but ranging from 99 MPa to 140 MPa for the Sr-HIP, which is more resistant to fatigue. After failure, each fracture surface was examined under scanning electron microscopy (JEOL JSM-840A).When second phase particles were observed at crack initiation sites, energy dispersive X-ray spectroscopy (EDX) was used to identify their chemical nature.
Pictures of the material microstructures are presented in Figs. 3 and 4 and measured microstructural characteristics are reported in Table 4. Fig. 3 and results in Table 4 reveal that the grain size, the fraction of primary alpha phase and the SDAS are in the same range for all investigated materials. This observation was supported by a statistical analysis of our measurements that is not reported in this paper. A comparison of Fig. 4A and C shows that the addition of strontium refined the eutectic silicon particles. A refinement from 11.1 to 3.5 lm in size is reported in Table 4 which is consistent with studies conducted by Ceschini, Zhongwei and Yang [23,28,31]. Fig. 4C and E shows that HIPping had no effect on the morphology and size of eutectic silicon, as expected. Further examination of the
Fig. 3. Microphotographs, etched with Poulton, showing the grain structure of the materials (A) UM, (B) Sr and (C) Sr-HIP.
Fig. 4. As polished micrographs of (A), (B) unmodified material, (C), (D) Sr modified material and (E), (F) Sr modified and HIP material.
Table 4 Mean microstructural features.
UM Sr Sr-HIP
D (lm)
f a (%)
SDAS (lm)
LSi (lm)
Wsi (lm)
Pores (mm2)
Defect length (lm)
721 691 771
63 67 65
47 51 49
11.1 3.5 3.4
8.7 1.7 1.9
5 9.5 /
8.9 9.6 /
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PM [30] UM Sr Sr-HIP
Fig. 5. Distribution of defect sizes for UM and Sr.
eutectic constituent showed that it is not only composed of silicon particles and a phase, it also contains other intermetallics. Fe-rich phases, in light gray, were identified by EDX. The largest Fe-rich particles were observed in the UM material (Fig. 4A) smaller ones were also visible in the Sr and Sr-HIP materials. Since the fatigue behavior of metals is very sensitive to casting defects, statistical distributions of pores from metallographic samples are reported in Fig. 5. The HIP-ed plates contained no visible (or quantifiable) defects, Fig. 5 gives the defect population for the UM and Sr material. The mean defect length and the defect density are reported in Table 4 to ease comparison. Based on the density value, the addition of strontium doubled the number of defects contained in this aluminum alloy but had no significant effect on their size.
3.2. Fatigue strength The results of the fatigue tests are represented in Fig. 6 in the form of SN curves, with the diamonds for UM, the stars for Sr and the triangles for the Sr-HIP material. For comparison, the results obtained by Brochu et al. [20], for permanent mold (PM) unmodified A357-T6, are presented by squares. The lines in Fig. 6 are Basquin type power law equations (ra = r0 f Nc), with the r0 f fatigue strength coefficient and c the fatigue strength exponent, that were fitted to our results. For each material, r0 f , c and the calculated fatigue strength at 106 cycles are reported in Table 5. For Sr-HIP the regression proposed does not contain the two lowest life
r0 f (MPa)
c
ra a 106 cycles (MPa)
284 473 449 289
0.1111 0.1352 0.1246 0.0720
62 73 80 107
results obtained at ra = 99 MPa because untypical casting defects were observed on their fracture surfaces. Both specimens come from the same plate that should have been discarded at quality control. Oxide films are sometime difficult to identify in radiographs because they are thin and have an attenuation coefficient very close to aluminum. Comparing the calculated fatigue strength obtained for PM and UM materials shows that the 106 fatigue strength of material produced by investment casting can be superior, by approximately 18%, to that of permanent mold castings. This is surprising considering that the microstructural features are slightly smaller in the PM component; secondary dendrite arm spacings are respectively 47 lm and 43 lm [30] for the IC and the PM components. Such a fine microstructure and high fatigue strength has not been reported for investments cast components before [2–16] and is attributed to the faster cooling rate achieved with the improved investment casting process. For the materials modified with strontium, the 106 cycles fatigue strengths are 80 MPa and 107 MPa respectively for Sr and Sr-HIP. The addition of strontium allowed a gain of 7 MPa (9%) at 106 cycles when compared to UM results. Strontium modification has two beneficial effects on fatigue. It increases the yield strength (see Table 3) which makes the material more resistant to plastic deformation and thus to the formation of persistent slip bands, and it refines and spheroidizes silicon particles (see Table 4) which postpone plastic deformation incompatibility between the phases to higher strain values. On the other hand, it was found to increase the density of pores which may explain that fatigue strength increased by only 9%. Applying a HIP treatment to the modified alloy showed the full potential of the material and the process as it further increased by 27 MPa (34%) the material fatigue strength. This is in accordance with the microstructural analysis in which no pores could be observed for the Sr-HIP material. HIP plays an important role as it closes the defects from which fatigue cracks initiate prematurely.
3.3. Crack initiation mechanisms
Fig. 6. Fatigue behavior of the different A357-T6 alloys at R = 0.1 and f = 25 Hz.
Fractographic analyses were performed to locate and identify the features responsible for crack initiation. All specimens were observed and crack initiation mechanisms could be classified in four categories: silicon particles (Fig. 7), casting defects (Fig. 8), intermetallics, crystallographic slip in the primary alpha phase (Fig. 9). The majority of the fatigue test specimens made from the UM material showed the presence of silicon particles at the initiation sites. Fig. 7a shows the footprint of a silicon particle that was debonded from the material. Traces of silicon (in black), confirmed by an EDX analysis, are visible. This type of initiation mechanism has already been reported by Buffière et al. [12], and is due to a strain incompatibility between the aluminum matrix and the silicon particle. On the other hand, in the PM material, large pores are the main cause of failure [20]. It should be noted that just one crack propagated from a pore within the UM material. On the other hand, the most frequent initiation sites (80%) observed in the strontium modified alloy (Sr) are pores. The size of these defects observed at crack initiation sites ranges between 100 lm and 290 lm (see Fig. 8). Pores became the limiting
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Fig. 7. Fracture surfaces at 500 for material UM (A) at ra = 122 MPa and Nr = 26,178 cycles, (B) at ra = 99 MPa and Nr = 117,276 cycles.
Fig. 8. Fracture surfaces at 100 for material Sr (A) at ra = 122 MPa and Nr = 53,762 cycles, (B) at ra = 99 MPa and Nr = 131,665 cycles.
Fig. 9. Fracture surfaces with the respective magnifications of 100 and 500 for material Sr-HIP (A) at ra = 122 MPa and Nr = 354,394 cycles, (B) at ra = 140 MPa and Nr = 60,814 cycles.
microstructural discontinuity for two reasons. Firstly because the addition of strontium refined the size of the silicon eutectic particles down to 11 lm. Secondly because the density of pores doubled with the addition of strontium. The most frequent crack initiation mechanism for the modified and HIP material is the formation of persistent slip bands in the primary alpha phase. This mechanism is characterized by the observation of flat surfaces usually oriented at approximately 45 ° from the loading direction (Fig. 9a). Sometimes the crystallographic propagation mode can extend over more than one grain as shown in Fig. 9a. This mechanism was also observed by Brochu et al. [4] in the study on short crack propagation in rheocast A357 alloy. A second interesting crack initiation mechanism observed in the Sr-HIP specimens is the cleavage of iron rich intermetallic particles as shown in Fig. 9b. In the presence of pores, this mechanism is not dominant as reported by Yi et al. [32]. On the other hand, in the Sr-HIP material, silicon particles are refined and pores are absent which triggers new mechanisms. Intermetallics are nevertheless not de main feature observed at crack initiation site because their volume fraction is low, reducing
the probability of finding it near the specimen surface. This is especially true in the case of hourglass specimens in which a limited amount of material is tested. 4. Discussion and conclusion This experimental study shows the influence of microstructural features on the fatigue strength and failure mechanisms of investment cast A357 alloy in T6 condition. Three different microstructures were tested to show the influence of strontium modification and HIP treatment on the fatigue behavior. The main conclusions of this research work are the followings: For 106 cycles at R = 0.1, the proprietary investment casting technology improves the fatigue strength by 11 MPa (18%) when compared with the same alloy produced by permanent mold casting. For unmodified and unHIP material, the main crack initiation mechanism observed is decohesion of large eutectic silicon particles in the size range of 100 lm.
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Strontium addition reduced the size of the silicon particles but doubled the number of pores which triggered crack initiation from pores rather than eutectic silicon, improving the fatigue strength by 9%. Application of HIP on the strontium modified material closes the defects which significantly improve (34%) the material fatigue strength. For the Sr-HIP specimens, cracks mainly initiated at persistent slip bands or at iron rich intermetallics. According to our results and analysis, refinement of eutectic silicon particles has a great potential of increasing fatigue strength of aluminum alloy A357 but only if pores and intermetallics are kept small and to a minimum density. The smallest microstructural discontinuity observed at crack initiation site was in the range of 45 lm. This size range should be the target for silicon particles, pores and intermetallics if one wishes to optimize fatigue life. Below this value, it appears that crack initiation from persistent slip bands will limit the fatigue resistance. Acknowledgments The authors would like to thank CRSNG for the financial support during this project and Alphacasting Inc. for providing the material. References [1] Pattnaik Sarojrani, Karunakar D Benny, Jha PK. Developments in investment casting process—A review. J Mater Process Technol 2012;212(11):2332–48. [2] Kennerknecht S, Dumant X, Tombari R, Biljon P. The effect of processing parameters on the fatigue properties of D357 investment castings. JOM 49 1997;11(1):22–8. [3] Barlas Bruno. Etude du comportement et de l’endommagement en fatigue d’alliages d’aluminium de fonderie. MAT – Centre des Matériaux: École Nationale Supérieure des Mines de Paris; 2004. [4] Brochu Myriam, Verreman Yves, Ajersch Frank, Bouchard Dominique. High cycle fatigue strength of permanent mold and rheocast aluminum 357 alloy. Int J Fatigue 2010;32(8):1233–42. [5] Wang QG, Jones P, Osborne M. The effects of applied pressure during solidification on the microstructure mechanical properties of lost foam A356 castings. In: Conference proceedings from materials solutions 2002, October 7, 2002 – October 9, 2002. Advances in Aluminum Casting Technology II. ASM International; 2002. p. 75–84. [6] Lee FT, Major JF, Samuel FH. Fracture behaviour of Al12 wt.%Si0.35 wt.%Mg(0– 0.02)wt.%Sr casting alloys under fatigue testing. Fatigue Fract Eng Mater Struct 1995;18(3):385–96. [7] Lados DA, Apelian D, De Figueredo AM. Fatigue performance of high integrity cast aluminum components. In: Conference proceedings from materials solutions 2002, October 7, 2002 – October 9, 2002. Advances in Aluminum Casting Technology II. ASM International; 2002. p. 185–96. [8] Wang QG, Apelian D, Lados DA. Fatigue behavior of A356/357 aluminum cast alloys. Part II - Effect of microstructural constituents. J Light Met 2001;1(1):85–97. [9] Spear RE, Gardner GR. Dendrite Cell Size. Mod Cast 1963;V43(5):209–15. [10] Ceschini L, Morri Alessandro, Andrea Morri A, Messieri Gamberini S. Correlation between ultimate tensile strength and solidification microstructure for the sand cast A357 aluminium alloy. Mater Des 2009;30(10):4525–31. [11] Zhu X, Shyam A, Jones JW, Mayer H, Lasecki JV, Allison JE. Effects of microstructure and temperature on fatigue behavior of E319–T7 cast aluminum alloy in very long life cycles. Int J Fatigue 2006;28(11):1566–71.
159
[12] Buffiere J-Y, Savelli S, Jouneau PH, Maire E, Fougeres R. Experimental study of porosity and its relation to fatigue mechanisms of model Al-Si7-MgO.3 cast Al alloys. Mater Sci Eng, A 2001;316(1–2):115–26. [13] Ammar HR, Samuel AM, Samuel FH. Effect of casting imperfections on the fatigue life of 319-F and A356–T6 Al–Si casting alloys. Mater Sci Eng, A 2008;473(1–2):65–75. [14] Linder Jan, Axelsson Markus, Nilsson Henrik. The influence of porosity on the fatigue life for sand and permanent mould cast aluminium. Int J Fatigue 2006;28(12):1752–8. [15] Lee FT, Major JF, Samuel FH. Effect of silicon particles on the fatigue crack growth characteristics of Al-12 wt pct Si-0.35 wt pct Mg-(0–0.02) wt pct Sr casting alloys. Metall Mater Trans A 1995;26(A (6)):1553–70. [16] Lee Min Ha, Kim Jae Joong, Kim Kyung Hoon, Kim Nack J, Lee Sunghak, Lee WEui. Effects of HIPping on high-cycle fatigue properties of investment cast A356 aluminum alloys. Mater Sci Eng, A 2003;340(1-2):123–9 (janvier 15). [17] Zeng Lei, Sakamoto Junji, Fujii Atsushi, Noguchi Hiroshi. Role of eutectic silicon particles in fatigue crack initiation and propagation and fatigue strength characteristics of cast aluminum alloy A356. Eng Fract Mech 2014; 115:1–12. [18] Ye Haizhi. An overview of the development of Al-Si-alloy based material for engine applications. J Mater Eng Perform 2003;12(3):288–97. [19] Yi JZ, Gao YX, Lee PD, Lindley TC. Microstructure-based fatigue life prediction for cast A356–T6 aluminum–silicon alloys. Metall Mater Trans B 2006;37(2): 301–11. [20] Brochu Myriam, Verreman Yves, Ajersch Frank, Bouchard Dominique. Propagation of short fatigue cracks in permanent and semi-solid mold 357 aluminum alloy. Int J Fatigue 2012;36(1):120–9. [21] Zhang B, Poirier DR, Chen W. Microstructural effects on high-cycle fatiguecrack initiation in A356.2 casting alloy. Metall Mater Trans A 1999;30(10): 2659–66. [22] Closset B, Fay D. Strontium modification of aluminum investment casting alloys. Trans Am Foundrymen’s Soc 1990:505–9. [23] Yang Ching-Yi, Lee Sheng-Long, Lee Cheng-Kuo, Lin Jing-Chie. Effects of Sr and Sb modifiers on the sliding wear behavior of A357 alloy under varying pressure and speed conditions. Wear 2006;261(11–12):1348–58. [24] Moustafa MA, Samuel FH, Doty HW. Effect of solution heat treatment and additives on the microstructure of Al-Si (A413.1) automotive alloys. J Mater Sci 2003;38(22):4507–22. [25] Alexopoulos ND, Tiryakioglu M. Relationship between fracture toughness and tensile properties of A357 cast aluminum alloy. Metall Mater Trans A 2009; 40(3):702–16. [26] Haro-Rodriguez Sergio, Goytia-Reyes Rafael E, Dwivedi Dheerendra Kumar, Baltazar-Hernandez Victor H, Flores-Zuniga Horacio, Perez-Lopez Maria J, et al. On influence of Ti and Sr on microstructure, mechanical properties and quality index of cast eutectic Al–Si–Mg alloy. Mater Des 2011;32(4): 1865–71. [27] Yang Li, Zheng bing Xu, Jian min Zeng. Effect of hydrogen content on impact property of A357 alloy. In: 6th International conference on physical and numerical simulation of materials processing ICPNS2010, November 16, 2010 – November 19, 2010. Materials Science Forum. Trans Tech Publications Ltd.; 2012. p. 704–5. [28] Ceschini L, Morri A, Sambogna G. The effect of hot isostatic pressing on the fatigue behaviour of sand-cast A356–T6 and A204–T6 aluminum alloys. J Mater Process Technol 2008;204(1–3):231–8. [29] Weihmuller Lance W, Mashl Stephen J. The effect of hot isostatic pressing on the mechanical properties of aerospace aluminum castings. In: 61st American helicopter society international annual forum 2005, june 1, 2005 – june 3, 2005, 2. Annual forum proceedings - ahs international. American Helicopter Society; 2005. p. 1447–56. [30] Brochu Myriam. Comportement en fatigue de l’aluminium 357 coulé par gravité et rhéocoulé. École Polytechnique de Montréal 2010. [31] Zhongwei Chen, Ruijie Zhang. Effect of strontium on primary dendrite and eutectic temperature of A357 aluminum alloy. Res Dev 2010. [32] Yi JZ, Gao YX, Lee PD, Lindley TC. Effect of Fe-content on fatigue crack initiation and propagation in a cast aluminum–silicon alloy (A356–T6). Mater Sci Eng, A 2004;386(1):396–407.