Static and fatigue strength of a die-cast aluminium alloy under ...

Static and fatigue strength of a die-cast aluminiumalloy under different feeding conditions

M Avalle, G Belingardi, M P Cavatorta* and R DoglioneDipartimento di Meccanica, Politecnico di Torino, Torino, Italy

Abstract: This paper reports investigations of the in�uence of porosity and casting defects on the staticand constant-amplitude fatigue strength of a die-cast aluminium alloy. Three batches of specimens, differingin the sprue-runner design and consequently in content and types of defects, were tested in ‘as-cast’conditions. Defects consisted of gas and shrinkage pores as well as cold �lls, dross and alumina skins.Casting defects were observed to reduce signi�cantly the static and fatigue properties of the material.Whereas for the static characteristics the decrease was progressive with the porosity range, for the fatiguestrength the decrease was most signi�cant from the lowest to the middle porosity range. The batches wereclassi�ed with regard to the porosity level, as the metallurgical defects were not detectable through X-rayexamination. The content and size of metallurgical defects were observed to increase together with theporosity level. Scanning electron microscopy (SEM) observation of the fracture surfaces demonstrated theimportant role played by dross, alumina skins and, above all, cold �lls on the fatigue fracture.

Keywords: die casting, porosity, casting defects, fatigue aluminium

NOTATION

A% elongation at rupture (%)E Young’s modulus (GPa)Rm ultimate strength (MPa)Rp0:2 yield strength (MPa)

r material density (kg=m3)

Abbreviations

SEM scanning electron microscopyS–N curves fatigue curves of stress–number of cycleswt % weight (mass) percentage

1 INTRODUCTION

Casting is the most economical way to transfer raw materialsinto readily usable components. However, one of the majordrawbacks of conventional and even more advanced castingtechniques is the formation of shrinkage and gas cavities,often coupled with other defects: cold �lls, alumina skins,dross [1].

The literature on the in�uence of casting defects on thematerial properties of cast aluminium alloys is quite broad[2–9]. It is generally acknowledged that the static andfatigue strengths of materials containing defects are lowerthan those of a defect-free material. In the case of fatiguestrength, defect distance from a free surface needs to betaken into account. Recent data have also shown theimportance of the distribution and morphology of phasesin the microstructure. In reference [9], fatigue strength isobserved to vary with solidi�cation parameters and hencelocation in the casting.

In general, literature data are almost exclusivelyconcerned with the effect of porosity, as investigations arecarried out on samples obtained by sand or permanentmould casting. Moreover, often the difference between thesamples consists in the adjustment of the melt gas content,which is likely to in�uence only the porosity level. On theother hand, the aforementioned metallurgical defects, morecommonly encountered in die casting of components, areknown to affect the mechanical properties adversely [1], butquantitative data are still lacking. Understanding the in�u-ence of these defects can be quite important, consideringthat, in contrast to shrinkage and gas porosity, they areusually not detected by the foundry on-line radioscopicquality control.

The study reported here investigated the in�uence ofcasting defects on the static and constant-amplitude fatigue

The MS was received on 31 July 2001 and was accepted after revision forpublication on 16 November 2001.*Corresponding author: Dipartimento di Meccanica, Politecnico diTorino, Corso Duca degli Abruzzi 24, Torino 10129, Italy.

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strength of a die-cast aluminium alloy. Three batches ofstandard specimens were tested, which were characterizedby a different sprue-runner design and consequently differ-ent feeding conditions. Defects consisted of gas and shrink-age pores as well as cold �lls, dross and alumina skins. Thematerial was EN AC-46000–UNI EN 1706 (ex GD–Al Si8.5Cu3.5Fe–UNI 5075). Hypoeutectic aluminium–siliconalloys are widely used in industrial applications owingto their favourable strength–weight ratios and goodmanufacturability.

2 EXPERIMENTAL

The chemical composition (wt %) and mechanical proper-ties of the alloy used in this study are given in Tables 1 and 2respectively (UNI EN 1706). The geometry and dimensionsof static and fatigue specimens are depicted in Fig. 1.

Specimens were obtained directly from die casting andwere tested in ‘as-cast’ conditions (i.e. with an as-castsurface). The only modi�cation to the cast surface was thetrimming of �ash. Three different batches of specimens weretested. In an attempt to study the in�uence of casting defectsrather than of porosity alone, the batches were characterizedby a different sprue-runner design, i.e. different feedingconditions. Variations in the sprue-runner geometricalarrangement are intended to simulate non-optimized feedingconditions, frequently encountered in industrial practice.

Figure 2 depicts the sprue-runner design for the high-porosity range: the feeding occurred at the level of the two�llets with a lateral in�ow and no feeder. For the middleporosity range, one feeding channel was closed and twosmall feeders were added at the specimen extremities. Forthe low-porosity range, the sprue-runner was changedcompletely. A single large feeding channel was used tofeed one extremity of the specimen. The in�ow was there-fore coaxial with the specimen. A large feeder was added atthe other extremity of the specimen.

Prior to testing, each specimen was examined with X-raysin order to assess the specimen porosity level. The threebatches were classi�ed as ranges 0, 2, 4 (category A)according to the ASTM E505 standard [10]. Densitymeasurements of the three batches were also taken in

support of the X-ray classi�cation (Table 3). Regardingporosity, it should be noted that even specimens classi�edas having a porosity range A0 were not free from pores.They had scattered=isolated micropores less than 0.1 mm insize. These micropores was also found adjacent to largerpores in specimens with a higher range of porosity. Withporosity range A2, pore diameters up to 0.2 mm were found,and with porosity range A4, pore diameters up to 0.5 mm.

Scanning electron microscopy (SEM) images taken of thefracture surfaces demonstrated that the cavities wererounded and more concentrated in the core of the castings(Fig. 3). The pore shape and limited size of the samplesindicate that porosity arises mainly from entrapped airbubbles during the feeding of the die and eventually fromhydrogen development during solidi�cation. However,shrinkage is also present at the centre of the samples inthe high-porosity ranges. As mentioned above, the changes

Table 2 Mechanical properties of the aluminium alloy used in thestudy

E …GPa† r …kg=m3† Rp0:2 …MPa† Rm …MPa† A%

70 2730 140 240 1

Table 1 Chemical composition of the aluminium alloy used inthe study (wt %)

Si Cu Fe Zn Mn Mg Ni Ti Pb

9.83 2.99 0.98 1.11 0.19 0.04 0.064 0.052 0.19

Fig. 2 Sprue-runner design for the high-porosity range (A4)

Fig. 1 Static and rotating bending fatigue specimen

Table 3 Density measurements (kg=m3) on the specimen batches

A0 A2 A4

2730 2570 2230

Proc Instn Mech Engrs Vol 216 Part L: J Materials: Design and Applications L02601 # IMechE 2002

26 M AVALLE, G BELINGARDI, M P CAVATORTA AND R DOGLIONE

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in the sprue-runner design introduced by the foundry arelikely to increase the porosity level as well as the presence ofother casting defects due to a non-optimization of thefeeding channel. However, casting defects such as dross,cold �lls and alumina skins are not detectable through X-rayexamination. An initial classi�cation for the three batcheswas therefore made with regard only to the porosity level.

3 RESULTS AND DISCUSSION

Figure 4 shows static stress–strain characteristics for thethree batches. Owing to important variations in density(Table 3), a notable decrease in the yield and ultimatestrength with the degree of porosity was observed. Asexpected, the elongation at rupture also decreased withincreasing levels of porosity.

Fatigue testing was carried out by means of a cantileverbending machine at 100 Hz. Cantilever rotating bendingtests were run rather than tensile fatigue tests, considering

that specimens were tested in as-cast conditions and diecasting is known to enhance material properties at thesurface. A �rst series of tests was run on the A0 batchwith the purpose of investigating the material under study.

S–N curves were taken up to 107 cycles and the fatiguecrack initiation sites were characterized by SEM. Thefatigue strength at 107 cycles was determined by means ofa staircase procedure. A median value for the fatiguestrength of 143 MPa was found, with a standard deviationof 3.9 MPa. Thus, the fatigue strength falls very close to theyield strength of the material. It is generally acknowledged[4] that, for materials for which there is little differencebetween the endurance limit and the yield strength, the S–Ncurves are quite �at. To verify the hypothesis, a secondstaircase was run to estimate the fatigue strength at 26106

cycles. A median value for the fatigue strength of 152MPawas found, with a standard deviation of 4.9 MPa. Thedifference between the fatigue strength at 26106 and 107

cycles is limited, as expected.A series of fatigue tests was run to failure at different peak

stress levels, from 150 to 190 MPa. The stress range was

Fig. 3 Representative fracture surfaces for the A2 (left) and A4 (right) batches; £30

Fig. 4 Static stress–strain curves for the three batches

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STATIC AND FATIGUE STRENGTH OF A DIE-CAST ALUMINIUM ALLOY 27



Fig. 5 S–N fatigue curve for the A0 batch. Percentages refer to probability of survival

Fig. 6 Comparison for the fatigue strength at 26106 cycles for the three batches

Fig. 7 Decrease of static and fatigue properties as a function of material density





limited, considering that the stress levels are above the yieldstrength of the material. Figure 5 shows S–N curves for theA0 specimens.

For comparison of the three batches, the fatigue strengthat 26106 cycles was evaluated by means of a staircaseprocedure. Figure 6 shows results obtained for the differentbatches for the fatigue strength median value and standarddeviation. The decrease in fatigue strength appears to bemost signi�cant when the degree of porosity is increasedfrom A0 to A2, while it is distinctly lower from A2 to A4.This trend of results is in agreement with literature datapublished by Sonsino and Ziese on silicon–magnesium–aluminium cast alloys [4].

Figure 7 summarizes the results obtained for the staticand fatigue properties as a function of material density. Forthe yield and ultimate strength, the decrease appears to bealmost linear with the density. This is not unexpected, sincethe material is macroscopically quite brittle (2–3 per centelongation at rupture only). For the fatigue strength, thedecrease is most signi�cant initially. Only for the A0batch does the fatigue strength fall very close to the yieldstrength.

SEM images of the fracture surfaces showed that for theA0 batch the fatigue fractures originated from an area withno visible defects. The fatigue initiation region contains onlyregular microstructural features, i.e. the Al matrix, the Siparticles and the Cu–Fe based intermetallic particles (Fig.8). It is easily concluded that the fatigue crack originated

Fig. 8 Microstructure of the alloy as seen in the metallo-graphic microscope: coarse Si and intermetallic parti-cles (grey features in the light aluminium matrix)provide favourable sites for microcrack nucleation

Fig. 9 Fatigue cracks originating from casting defects: (a) gas cavities (£150); (b) dross (£250); (c) cold �lls(£500); (d) alumina skins (£140)

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STATIC AND FATIGUE STRENGTH OF A DIE-CAST ALUMINIUM ALLOY 29



from the decohesion or breaking of the second-phaseparticles. The mechanism of fatigue crack initiation in adefect-free material should be the following: pile-up ofdislocations at the matrix=particle interface due to the loadcycling [11]; build-up of strong local stress concentration;breaking of the particles having a high aspect ratio orcontaining notches (usually the biggest, as also shown inFig. 8); initiation of a microcrack. This is consistent with thefact that the fatigue strength lies close to the yield strength,since it is known [12] that in cast aluminium alloys someplastic strain is necessary to initiate particle breaking.Finally, the result is consistent with fatigue data reportedfor similar alloys [5].

On the other hand, for the A2 and A4 batches, the fatiguecracks initiated from near surface gas cavities (Fig. 9a) aswell as dross (b), cold �lls (c) and alumina skins (d). Indeed,the changes in the sprue-runner design made by the foundrycaused an increase in the porosity level as well as in thecontent of other casting defects due to non-optimization ofthe feeding channel. The combination of pores and othermetallurgical defects caused the decrease in material proper-ties observed among the three batches. This combination ofcasting defects might well be the reason for the observedlarge decrease in material characteristics compared to litera-ture sources [2–5].

In the authors’ opinion, a change in the sprue-runnerdesign may be more representative of real work conditions,i.e. different design of the die, than the simple addition ofgas content to the melt as is often done in the literature. Inthis respect, it is worth underlining that cavities tend toconcentrate in the middle of thick sections, while cold �llsrepresent a discontinuity in the material at the edge of thespecimen, where fatigue cracks tend to nucleate.

4 CONCLUSIONS

Changes in the sprue-runner design have been observed toaffect the porosity level of the casting as well as the contentof other casting defects such as dross, alumina skins andcold �lls. Casting defects are observed to reduce the staticand fatigue strength of the material. For the static character-istics the decrease is progressive with the defect content; forthe fatigue strength the decrease is most signi�cant initially.The observed decrease in material characteristics is greaterthan found in literature data because the combined effect ofporosity and metallurgical defects is considered, whereas inmost experimental studies batches differ only in the melt gascontent, i.e. the porosity level.

ACKNOWLEDGEMENTS

This research was supported by the European Union underContract BRST-CT98-5328. The authors acknowledge IngM. Albertinazzi for helping with the fatigue tests.

REFERENCES

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9 Seniw, M. E, Conley, J. G. and Fine, M. E. The effect ofmicroscopic inclusion locations and silicon segregation onfatigue lifetimes of aluminum alloy A356 castings. Mater.Sci. Engng, Part A, 2000, 285, 43–48.

10 Standard Reference Radiographs for Inspection of Aluminumand Magnesium Die Casting, ASTM E 505-96. In Annual Bookof ASTM Standards (American Society for Testing and Materi-als, Philadelphia).

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12 Doglione, R., Douziech, J. L., Franccois, D. and Berdin, C.Tensile damage stages in a cast A356T6 alloy. Mater. Sci.Technol. (in press).





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