experimental study of porosity and fatigue...

METAL 2008 13. -15. 5. 2008, Hradec nad Moravicí

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EXPERIMENTAL STUDY OF POROSITY AND FATIGUE BEHAVIOR

OF CAST Al–Si ALLOYS

Stanislava Fintováa Viera Konstantováa

Radomila Konečnáa Gianni Nicolettob

a Department of Materials Engineering, University of Žilina, Univerzitná 1, 01026 Žilina Slovakia , [email protected]

b Department of Industrial Engineering, University of Parma, Viale G.P. Usberti, 181/A, 43100 Parma, Italy, [email protected],

Abstract

The sand casting process usually generates porosity that is highly detrimental to the fatigue behavior of cast Al-Si alloys. Since pores favor early fatigue crack initiation, the total fatigue life for a given stress level strongly depends on the initiating pore characteristics. Experiments show that shrinkage pores are especially detrimental because of considerable size and also irregular in morphology.

The study reports of fatigue experiments performed on specimens of Al-Si alloys produced in different ways tested under rotating bending conditions. Pores on the surface or just below the surface were found to be preferred locations of fatigue cracks initiation. The fatigue data are examined in the light of a thorough characterization of the pore population in these specimens using light optical microscopy and the image analysis program LUCIA Metallo 5.0. The Murakami’s approach to the statistical description of equivalent pore size was used. The link among fatigue life, nominal stress amplitude and defect size is discussed in the paper. The complex 3D shape of shrinkage pores is demonstrated by fracture surface inspection in the scanning electron microscopy. The correlation between initiating pore sizes and the pore size population determined by metallography is discussed with the aim of defining critical pore size prediction based on extrapolation criteria. 1. INTRODUCTION

Cast aluminum alloys are seeing increasing uses in the automotive industry due to their excellent castability, corrosion resistance, and especially their high strength to weight ratio. The increasing use of high integrity shaped cast aluminum components under repeated cyclic loading has focused considerable interest on the fatigue properties of cast Al-Si alloys. Fatigue properties of cast aluminum components strongly depend on casting defects and microstructural characteristics [1].

Porosity is the term used for the voids or cavities that form within a casting during solidification. It is the most common defect found in Al–Si casting alloys and is the major cause of rejection of such castings, as it often results in poor mechanical properties such as limited strength and ductility, variable fracture toughness, irregular crack initiation and crack propagation characteristics, as well as a lack of pressure tightness [2, 3].Two major fundamental effects contribute to the formation of porosity in solidifying Al–Si casting alloys: (i) shrinkage resulting from the volume contraction accompanying solidification, as well as inadequate liquid metal mobility (bad feeding), and (ii) gas evolution (mainly hydrogen) resulting from the decrease in gas solubility in solid metal compared to the liquid [2].


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There exists a critical defect size, [1], (in Sr modified cast A356 alloy, the critical defect size is in the range of 25-50 µm) for fatigue crack initiation, below which the fatigue crack initiates from other intrinsic initiators such as eutectic particles and slip bands.

Compared with porosity, the eutectic structure and intermetallic phases play a minor role in crack initiation. The effect of porosity on fatigue life has been summarized as follows: pores reduce the time for crack initiation by creating a high stress concentration in the material adjacent to the pores; because of this, most of the fatigue life is spent in crack growth [2].

Typically, fatigue endurance is reduced when the size of porosity increases [1, 4, 5]. Casting defects have a detrimental effect on fatigue life by shortening not only fatigue crack propagation, but also the initiation period. The decrease in fatigue life is directly correlated to the increase of defect size [1].

The present work is aimed to identify the influence of porosity (pore population and shape) to the fatigue properties response of castings. The Fatigue experiments on specimens of Al-Si alloys were performed. Light microscopy was used to analyze the microstructure and porosity of materials after fatigue testing. The statistical method proposed by Murakami, [6], to approach the statistical description of equivalent pore size is initially used. 2. MATERIAL AND EXPERIMENTAL PROCEDURES

The experimental material is cast Al-Si alloy delivered from the University of Parma, where the material was analyzed in term of mechanical properties. The specimens for fatigue tests were extracted from cast parts. The metallographic specimens were extracted from specimens broken during the fatigue tests. Two batches of material, according to the origin of the material were analyzed, B and C.

The structural analysis was carried out applying metallographic techniques and digital image analysis software on polished cross-sections of metallographic specimens. Typical microstructures are shown in Fig. 1. The microstructure is characterized by primary dendrites of α-phase (solid solution of Si in Al with maximal solubility limit 1.59 % at eutectic temperature 577 °C) and by a eutectic structural component (α + Si) located between the secondary dendrite arms. The eutectic silicon grew as thin, interconnected rods between dendrites of α-phase because the liquid metal was modified by an optimal amount of Sr.

Fig. 1 Microstructure of Al-Si alloy, etched with 0,5 % HF

The modified silicon rods appeared as round particles on the metallographic section.

Intermetallic phases are also located in interdenritic eutectic areas. Evaluation of the


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secondary dendrite arm spacing (SDAS) was performed according to the linear method (i.e. line with the length = 12 cm) [7].

The size of casting porosity was studied on metallographic specimens using a light optical microscope at the University of Žilina. However, random 2-D sections through pores cannot provide good estimates of the largest defect size without further data analysis. Pores originating fatigue fractures, observed on fracture surfaces, significantly larger than pores observed on the metallographic sections regardless of alloy and casting process. Therefore, the maximum pore size in a cast component can be estimated from the metallographic data using the largest extreme value statistics [8]. Here, the Murakami’s method was applied to the characterization of the pore size population according the largest extreme value distribution (LEVD) [6]. The image analysis program LUCIA Metallo 5.0 was applied for extensive and detailed measurement of porosity. The method is based on evaluation of the largest defect size on the field of view. The fracture surfaces were analyzed on SEM at the IPM in Brno to specify places of crack initiations. 3. RESULTS AND DISCUSION 3.1 Fatigue testing

The fatigue test results are presented in Tab.1. Several smooth specimens were tested at different stress amplitude levels (from 20 to 80 MPa) according to the rotating bending configuration at 50 Hz frequency. Since the objective was the correlation of stress amplitude, number of cycles and largest pore size on a specimen to specimen basis, a preliminary characterization of microstructure uniformity as described by SDAS was carried out. The SDAS results are also shown in Tab. 1 and the conclusion is that all specimens had quite similar SDAS except specimens 1B and 3C.

However, relation between stress amplitude and fatigue life shows quite high scatter that it is not generally unexpected for cast Al-Si alloys because of the inhomogeneity of the cast material and also because of the cast defects. One-order-of-magnitude difference in number of cycles to fracture at the same stress amplitude is observed in Tab. 1 and similar fatigue lives are associated to quite different stress amplitude.

Table 1 Fatigue test results, SDAS and characteristic value of defect size

Specimen S

[MPa]

N SDAS

[µm] Estimated largest pore size [µm]

for S = 10 mm2 for S = 100 mm

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1B 60 2 041 210 64 89 181 2B 70 10 388 728 58 51 99

3B * 80 2 074 770 53 60 122 4B 60 223 080 55 53 93

5B * 70 669 562 56 71 136 6B * 60 870 745 58 113 262 7B 30 2 269 663 55 61 129

1C * 50 3 611 605 59 101 187 2C 30 1 259 996 50 42 72

3C * 20 184 361 39 25 45

Such scatter can find an explanation in the presence of a pore population with a large scatter in size. Therefore, the broken specimens marked with an asterisk in Tab. 1 were


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examined in SEM. All the fracture surfaces were characterized by the fatigue region and the final static fracture. Except for specimen 3C, for which the fractography analysis detected a macroscopic mechanical damage, the fatigue fracture surfaces of the other four specimens revealed clearly the fatigue crack initiation places. The fracture surfaces and the crack initiation places are shown in Fig. 2. The results show, that for all the examined specimens the fatigue fracture occurred because of the presence of a pore on or near the surface. This is due to rotating bending load condition that develops the highest stress on the surface. Multiple fracture initiation places were also typical of all fracture surfaces of Fig. 2.

a) specimen 1C, Nf = 3 611 605 cycles b) specimen 3B, Nf = 2 074 770 cycles

c) specimen 5B, Nf = 669 562 cycles d) specimen 6B, Nf = 870 745 cycles

Fig. 2 Fracture surfaces after fatigue testing, SEM

The SEM image of the fatigue fracture surface of the specimen 1C is shown on the Fig. 2a where the fracture occurred at pores near the surface. The image shows the presence of two pores at the fracture sample surface, which acted as the main crack initiation places. The specimen 3B fractured because of the presence of surface porosity which acted as the main crack initiation places, Fig 2b. The Fig. 2c, where is shown the specimen 5B, shows that the fatigue fracture occurred as a result of the porosity presence at the sample free-surface which

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Table 2 Results of SEM investigation on fatigue fracture origin

Specimen S

[MPa]

N Number

of crack

initiations

Fatigue critical

pore size [µµµµm]

Estimated largest

pore size for

S = 10 mm2 [µm]

3B * 80 2 074 770 2 66 60 5B * 70 669 562 3 373 71 6B * 60 870 745 3 69 113 1C * 50 3 611 605 2 482 101

acted as the main crack initiation places. For this specimen were observed three initiating places. Three initiating places from pores on the surface were found on the fracture surface of the specimen 6B, Fig. 2d. These observations are summarized in Tab. 2 showing a correlation between number of initiation places and fatigue life: fatigue life increases if initiation places reduce in number.

a) initiation place 1 b) initiation place 2

c) fatigue/static fracture d) static fracture

Fig.3 Fracture surface of specimen 3B, SEM


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The Fig. 3 shows the examples of the SEM images obtained from the fatigue fracture surface of the specimen 3B which was tested on the stress amplitude 80 MPa. The arrows in Fig. 3a and 3b point to the pores that initiated the fatigue crack. The Fig. 3a shows the single pore below the surface acting as the crack initiation place. The single surface pore associated with the crack initiated place is shown in the Fig. 3b. The detailed image of the fracture surface (Fig. 3c) shows the changing between the fatigue region and the final static fracture. An attempt was made to measure a reference size of the initiating pores and their measures are given in Tab.2.

The pore contour was outlined as shown in Fig. 4 and the area computed to arrive to a pore size measurement. However, alternative measuring schemes are currently under investigation. The following correlations are found considering the results of Tab.2: - The fatigue crack initiations were influenced by size and localization of casting pores, - Fatigue life increases if initiation places reduce in number, - For a given number of initiations, an increase in pore size seems to reduce fatigue life.

3.2 Porosity characterization

The experimental results determined with the LEVD method are plotted in Fig. 5. The data are separately plotted according to the origin of the material. The 50 x magnification was used to evaluate pores size in this study.

The data of Fig. 5 show that the linearity requirement for the applicability of LEVD is only partially satisfied. In many cases of Fig. 5a data show an inflection point for a pore size that discriminate small fore from large, fatigue critical, pores. Such inflection points are associated to pore sizes 30-40 µm suggesting that pores above a threshold size distribute differently.

The control area of the measurements was S0 = 1.86 mm2. For prediction of maximum pore size in real castings the measured values were extrapolated to cross-sectional areas S = 10 mm2 (typical of a fatigue specimen) and S = 100 mm2 (representative of a small component) and the predicted pores sizes of the examined specimens are shown in Table 1 along with the fatigue data.

The predicted largest pores sizes for S = 10 mm2 are larger than a threshold defect size for fatigue crack initiation for all specimens from batch B as suggested from [1] (i.e. 50 µm). For the specimens 2C and 3C from the batch C, the largest pores sizes are predicted for the area S = 10 mm2 smaller than threshold of defect size. For the specimen 1C the predicted largest pore size is larger than the threshold of defect size. All the predicted largest pores sizes for

a) specimen 5B b) specimen 6B

Fig. 4 Scheme of the measurement of the initiating pore size


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area S = 100 mm2 with the exception of the specimen 3C are larger than the threshold of defect size for the fatigue crack initiation.

The assumption behind the pore size characterization with metallography is that fatigue life and fatigue strength decrease with an increasing expected largest pore size. The direct comparison of fatigue data and predicted pore sizes however is not completely satisfactory at this time. This is possibly due to the pore sizing procedure used that needs fine tuning for the special nature of casting pores. For example, in the LEVD plots of Fig. 5 the measurements do not fit well to the linearity requirement for applicability of the Gumbel distribution. One approach is the elimination of the pore measurements below the threshold size. This leads to a linear correlation that could be used to repeat the pore size prediction. Since in many cases, see for example batch C in Fig. 5b, elimination of measurements below the threshold leaves too few data points, the characterization phase should be improved acting on the image magnification and extension of areas investigated. These issues are subject of on-going work.

a) batch B

b) batch C

Fig. 5 LEVD plots of equivalent pore size

Another consideration follows when the predicted pore sizes for 10 mm2 are compared in Table 2 with the actual measurements for the fracture surface of fatigue specimens discussed in the previous section. In most cases the predicted critical pore size underestimate the observed pore size. However considering that these are the first direct comparisons and on a limited number of specimens they appear encouraging for additional study. 4. CONCLUSIONS

The pores occur typically in Al-Si casting. Because of their negative influence on fatigue behavior, the largest pore size should be estimated. In this paper the Murakami’s statistical method for determination of the largest pore size in a real casting on the basis of metallographic non etched sections was used. The fatigue fracture surfaces of four specimens were examined using scanning electron microscopy (SEM) to identify the fatigue crack initiation places in each case and to explain the results from the fatigue tests. The following conclusions are reached:

- Casting pore sizes appear to correlate with SDAS measurement. Casting technology and process influence not only SDAS but also largest pore sizes.

- Murakami’s statistical method for determination of the largest size of defect in metallic materials is applicable for determination of largest size of casting pores.

- Fatigue cracks were initiated from the pores on the free – surface or immediately below the surface.


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- The surface porosity is mainly responsible for decreasing the fatigue life of the specimens because the surface porosity creates regions of high stress concentration.

- The fatigue cracks of tested specimens were influenced by the amount of initiating pores and by the size of those pores. Fatigue life increases when the amount of initiating places decrease.

- The assumption that fatigue life and fatigue strength decrease with the increasing expected largest pore size is tested by direct comparison with actual critical pore sizes. Although the results are not completely satisfactory at this time, additional study is warranted.

Acknowledgments

This work was done as a part of the SK/IT project No10/NT and of VEGA grant No.1/3194/06. It is also consistent with the objectives of MATMEC, an Emilia-Romagna regional net-lab (http://www.matmec.it/).

REFERENCES [1] WANG, Q. G., APELIAN, D., and LADOS, D. A. Fatigue behavior of A356-T6

aluminum cast alloys. Part I. Efect of casting defects. Journal of Light Metals 1, 2001, s. 73-84.

[2] AMMAR, H. R., SAMUEL, A. M., SAMUEL, F. H Effect of casting imperfections on the fatigue life of 319-F and A356-T6 Al–Si casting alloys. Mater. Sci .Eng. A, 2007, doi:10.1016/j.msea.2007.03.112.

[3] AMMAR, H. R., SAMUEL, A. M., SAMUEL, F. H. Porosity and the fatigue behavior of hypoeutectic and hypereutectic aluminium-silicon casting alloys. International Journal of Fatigue, 2007, doi:10.1016/j.ijfatigue.2007.08.012.

[4] SKALLERUD B. Fatigue life assessment of aluminium alloys with casting defects. Eng Fract Mech; 44:857-74, 1993.

[5] SONSINO CM, ZIESE J. Fatigue strength and applications of cast aluminium alloys with different degree of porosity. International Journal of Fatigue; 15:75-84, 1993.

[6] MURAKAMI, Y. Metal Fatigue: Effects of Small Defects and Nonmetallic Inclusions, Elsevier, Oxford, 2002. p. 320.

[7] Atlas métalographique des alliages d´aluminium. CTIF, Paris, 1980. [8] WANG Q. G., JONES P. E. Prediction of Fatigue Performance in Aluminum Shape

Castings Containing Defects. The Minerals, Metals & Materials Society and ASM International 2007, doi: 10.1007/s11663-007-9051-4.

experimental study of porosity and fatigue...

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