CommunicationInvestigation on Formation Mechanismof Irregular Shape Porosityin Hypoeutectic Aluminum Alloyby X-Ray Real Time Observation

HENGCHENG LIAO, LEI ZHAO, YUNA WU,RAN FAN, QIGUI WANG, and YE PAN

The formation mechanism of irregular shape porosity inhypoeutectic aluminum silicon alloy (A356) was inves-tigated by X-ray real time observation on porosityevolution during solidification and re-melting. Porosityin the hypoeutectic aluminum A356 alloy with highhydrogen content (>0.3 mL/100 g Al) first forms in theliquid as small spherical gas bubbles, then expands alongwith the pressure drop in the mushy zone due toshrinkage and lack of feeding, and finally deforms intoirregular morphology by the impingement of aluminumdendrite network. Degassing is a key to eliminateporosity in aluminum alloy castings.

DOI: 10.1007/s11661-012-1193-6� The Minerals, Metals & Materials Society and ASMInternational 2012

Porosity in cast aluminum alloys has long beenrecognized as one of the most detrimental factorsaffecting mechanical properties,[1,2] especially fatigueresistance of the material.[3–8] In general, porosity incasting can be classified into gas porosity and shrinkageporosity. The large difference in hydrogen solubilitybetween the solid and liquid phases is the main cause forhydrogen gas porosity in aluminum castings.[9] Assolidification proceeds, the excess atomic hydrogen isrejected from the newly formed solid into the surround-ing liquid. When the partitioned hydrogen in thesolidification front reaches a critical solubility level,molecular hydrogen pores (gas porosity) form that maygrow or shrink depending on the local hydrogenconcentration levels and the rate of hydrogen diffusion.Theoretically, the Scheil model may be used to simplydetermine the hydrogen content in the remaining liquidand, subsequently, the fraction of solid at whichhydrogen saturation and associated pore nucleationwould occur. However, the Scheil equation should be

modified due to the high diffusion coefficient of hydro-gen in solid aluminum and the competition of porenucleation and growth for hydrogen solute. The gasporosity usually has a relatively regular spherical con-figuration. The shrinkage porosity refers to the densitydifference between solid and liquid. When the volumet-ric shrinkage cannot be fed by liquid metal, voids(shrinkage-porosity) form in the final solidifying liquidmetal pools, spreading along the configuration of thesolidified solid. The shrinkage porosity thus has a fairlyirregular morphology. In hypoeutectic aluminum alloys,shrinkage porosity is relatively easy to form due to thelarge mushy zone. The detrimental influence from theirregular shape porosity on mechanical properties ismuch more severe than the spherical shape gas porosity,due to the high stress concentration and thus crackingaround the sharp corners of the irregular pores.[4,10,11]

Porosity formation can be divided into two stages:nucleation and growth. In cast aluminum alloys, thenucleation of porosity is in no way homogeneous. Theheterogeneous nucleation occurs at sites such as oxideinclusions with gas gaps, grooves of mold wall, undis-solved fine gas bubbles in liquid, etc.[12–14] Duringmelting or pouring of aluminum alloys, oxide inclusionscan readily form, especially with turbulent flow. In oxideinclusions, there are a great number of nanoscale gasgaps that have strong ability to absorb hydrogen. Theoxide inclusions are almost not wetted by aluminumliquid, and thus, they are ideal nucleation sites forporosity formation. Recently, Campbell[9] put forwardthat nucleation of all pores is originated from bifilms ofaluminum oxides, which have a dual-layer structure andno bonding strength between the two dry layers. Theformation of porosity is thought to be that the bifilm ispulled open by all kinds of forces produced duringsolidification. The solidification shrinkage force enforcesthe bifilm to deform along the configuration of solidifiedsolid to form the irregular morphology of shrinkageporosity.Recently, X-ray imaging technology has been used to

observe in real time the porosity evolution duringsolidification of aluminum alloys.[15–19] However, theinvestigation on formation of irregular shape porosity incast aluminum alloys is still limited. In this article,microfocus X-ray imaging and directional solidification(XIDS) technology, which is described in detail inReference 17, was utilized to in-situ observe the mor-phology evolution of irregularly shaped porosity duringsolidification and remelting of a hypoeutectic Al-SiA356 alloy.A356 alloy with a nominal composition of Al-7 wt pct

Si-0.35 wt pct Mg was prepared in an electric resistantfurnace. After holding at 1033 K (760 �C) for 30 min-utes, the melt was cooled to 1003 K (730 �C) for Srmodification by controlling Sr content of 0.02 wt pct.After processing and further holding at a temperature of1003 K (730 �C) for 30 minutes, some molten aluminumwas first poured into a metal thin-wall mold preheatedat 573 K (300 �C) with a cavity of 3109 609 3 mm3,and after solidified, the thin plate casting was cut intotypical dimensions of 3109 109 3 mm3 for remeltingexperiments in XIDS. Some melt was poured into a

HENGCHENG LIAO and YE PAN, Professors, LEI ZHAO andYUNA WU, Doctoral Students, and RAN FAN, Master Student, arewith the Jiangsu Key Laboratory for Advanced Metallic Materials,School of Materials Science and Engineering, Southeast University,District of Jiangning, Campus of Southeast University, Nanjing211189, P.R. China. Contact e-mail: hengchengliao@seu.edu.cnQIGUI WANG, Materials Specialist, is with the Materials TechnologyDepartment, GM Global Powertrain Engineering, Pontiac, MI 48340.

Manuscript submitted October 12, 2011.Article published online May 16, 2012

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heavy Cu Ransley mold to obtain a chilled sample forhydrogen content measurement. The hydrogen contentmeasured by vacuum remelting technique (LECO*

Technical Services Laboratory) is about 0.329 mL/100 g Al in the 0.02 wt pct Sr modified melt. Theremaining liquid metal was quickly poured into a sampleboat (with a cavity of 3009 109 3 mm3) in the XIDSsetup. After liquid metal was poured into the XIDS, thecooling system in the XIDS was simultaneously acti-vated to start unidirectional solidification from thebottom to the top in the sample boat, and X-rayradiographs and video were also recorded automaticallyat a frequency of 25 Hz. The temperature gradient, G,was about 2.0 K/mm during solidification, and thesolidification velocity, R, was about 0.2 mm/s. Forremelting experiments in XIDS, the prepared thin platespecimen was placed into the sample boat in XIDSsetup, and then the XIDS was activated to startremelting from the top to the bottom and X-rayradiographs and video were also recorded.

In Figure 1, X-ray radiographs show morphologyevolution of porosity in Al-7 pct Si-0.3 pct Mg alloywith equiaxed dendrite solidification. At about 100 sec-onds after solidification starts (Figure 1(a)), the porosity(black shadow) is still approximately spherical. Thisoccurs at the early stage of solidification. At this time,the liquid feeding is very good during directionalsolidification, and the shrinkage force is thought to bevery small. In this case, the bifilms are impossibly pulledopen. As the initial hydrogen content is relatively high inthe studied alloy, it is rational to think that the pores arehydrogen-induced, namely, gas bubbles. This is similarto the results observed at the early stage of porosityformation in the same alloy with columnar dendritesolidification.[17] After solidification proceeds for afurther 30 seconds, configuration boundaries of some

pores become indistinct and the morphology of porescannot retain spherical shape (Figure 1(b)). This indi-cates that shrinkage force starts to influence the growthof porosity. At about 220 seconds, the morphology ofalmost all pores becomes irregular, and some pores seemto be interconnected or overlapped (Figure 1(c)).Figure 2 shows the porosity morphology in the finallysolidified sample at the same view field as X-ray. It isseen that many clustered pores are irregular in theinterdendritic region. In the case of solidification vari-ables (G = 2.0 K/mm, and R = 200 lm/s in this arti-cle), the primary Al phase in the hypoeutectic A356alloy grows as equiaxed dendrites. At the early stage ofsolidification, the solid fraction of the primary dendritesis small, and porosity can nucleate and grow as gasbubbles in the liquid (as shown in Figure 1(a)). Whenthe growing pores touch the dendrites, however, theycannot retain spherical shape due to the constraintsfrom the dendrite network, making the porosity bound-aries indistinct (Figure 1(b)). With the succeeding

Fig. 1—X-ray macrographs showing morphology evolution of porosity during equiaxed dendrite solidification in Al-7 wt pct Si-0.3 wt pct Mgalloy. G = 2.0 deg/mm, and R = 200 lm/s: (a) 100 s, (b) 130 s, and (c) 220 s.

Fig. 2—Morphology of porosity in the finally solidified Al-7 wtpct Si-0.3 wt pct Mg alloy with equiaxed dendrite growth in XIDScasting.

*LECO is a trademark of LECO Technical Services Laboratory,St. Joseph, MI.

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eutectic solidification, the liquid feeding becomes muchmore difficult, and porosity has to expand along thedendrite network channels to compensate for thepressure drop by shrinkage and lack of feeding. As aresult, the morphology of porosity also becomes muchmore irregular, and its boundary also becomes muchindistinct in X-ray radiographs (Figure 1(c)). From thepreceding observation, it can be concluded that porosityin hypoeutectic aluminum alloy first forms as a gasbubble, then expands along the surrounding dendritenetwork channels by solidification shrinkage, and finallyevolves into irregular morphology.

Figure 3 shows porosity morphology in the A356alloy casting made in the thin-wall metal mold. Because

of the higher cooling rate in the thin-wall metal moldcasting, aluminum dendrites are finer and porositybecomes much more irregular in comparison with thoseshown in the XIDS casting (Figure 2).Figure 4 shows porosity evolution during remelting of

the A356 casting sample (made by the thin-wall metalmold) with a heating rate of 0.15 K/s. Figure 4(a) is aradiographof the sample before remelting. It is hard to seeporosity boundaries clearly, because the pores are fairlyirregular and interconnected in three dimensions,as shown in Figure 3. After heating up for about110 seconds, the porosity boundaries gradually becomedistinct (Figure 4(b)). However, themorphology of poros-ity still presents as irregular shape due to the geometricconstraints from the dendrite network. Spheroidization ofporosity did not occur until about 165 seconds(Figure 4(c)).Afewmore seconds later (atabout170 seconds,Figure 4(d)), many pores except for the large pores (asmarked by 1 and 2) became spherical shape, indicatingthat they were fully surrounded by liquid. With contin-uous remelting of the aluminum dendrite network, allpores become spherical, as shown in Figure 4(e). It is alsointeresting to note, by comparing Figure 4(e) with 4(d),that some pores disappeared during remelting. This isprobably attributed to bubble conglomeration and espe-cially escape from the liquid. The succeeding evolution ofporosity clearly reveals bubble movement (as marked by1, 3, 4, etc.) and disappearance (asmarked by 1, 3, 4, 5), asshown in Figures 4(e) and (f). The disappearance ofporosity is too quick to be captured by the video used inthis study with a frequency of 25 Hz. At 290 seconds(Figure 4(g)), there is no pore surviving in the view ofX-ray.By comparing images betweenFigures 4(e) and (f),it is also noted that the sizes of pores 3 and 4 are decreased

Fig. 3—Porosity morphology in Al-7 wt pct Si-0.3 wt pct Mg alloycast in the thin-wall metal mold.

Fig. 4—X-ray radiographs showing porosity evolution during remelting of the hypoeutectic A356 alloy: (a) 0 s, (b)110 s, (c) 165 s, (d)170 s,(e) 240 s, (f) 270 s, (g) 280 s, and (h) 290 s. Black spots are pores.

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gradually, and their diameters change from about 600 lm(pore 3) and 700 lm (pore 4) to 500 and 550 lm,respectively. This may be due to dissolution of hydrogenfrom the porosity into liquid.

According to the Al-Si-Mg phase diagram, thevolume fraction of eutectic phase in A356 alloy is about50 pct, and this value becomes slightly lower under fastsolidification conditions. During remelting, the eutecticis first melted followed by the primary a-Al dendritenetwork. For the studied alloy, there is a large mushyzone. At the beginning of the remelting process, thealuminum dendrite network prevents the porosity fromfree movement even when pores become spherical. As aresult, the spherical pores cannot abruptly float off anddisappear like that observed in eutectic Al-Si alloyduring remelting.[19] Only when the aluminum dendritenetwork becomes fully disconnected, or is completelymelted, do the pores abruptly float away. Compared tothe remelting of eutectic Al-Si alloy, spherical pores inthe hypoeutectic A356 alloy during remelting cansurvive in mushy zone for a long time because of theimpingement of the aluminum dendrite network.

Whether the spherical pores that finally abruptly floataway are gas driven or bifilm induced needs furtherdiscussion. If they were bifilm induced, their sizes shouldnot have been reduced (as observed in Figures 4(e)through (g)), particularly when the impingement forcefrom the aluminum dendrite network is fully released.Furthermore, if those escaped pores were bilfim induced,by taking into account buoyancy (floating) force, gravity,and viscosity resistance, their moving velocity, v, could beestimated by Stocks’ equation: v = 2(qAl – qP)gr

2/9l,where qAl and qP are the densities of Almelt and porosity,respectively; l is the viscosity of Al melt; g is gravityaccelerated velocity; and r is the porosity radius. Consid-ering Al melt at 953 K (680 �C), l is 0.0012 Ns/m2, g is9.8 m/s2, and qAl is 2.389 103 kg/m3.[20] According to thedensity of a nanoporous anodic aluminum oxide film of2.8 to 3.1 g/cm3,[21] the qP in the preceding equation isgiven as 2.24 g/cm3, (80 pct of 2.8 g/cm3), and then a porewith 250-lm radius will move upward with a velocity ofonly about 16 mm/s. If that were the case, the escapingprocess of the pores should have been captured by X-rayreal time observation. In fact, however, the upwardfloating speed of the pores shown in Figure 4 was so fastthat no escaping trace of them was captured by the video.If we assume those spherical pores are gas driven, thefloating speed of a pore with only 200-lm radius can be ashigh as 170 mm/s, which exceeds the video resolutionused in the study. This value is also consistent with that(140 mm/s) in the transparent alloy by Han.[22] It is thusdemonstrated that the irregular shape pores in thehypoeutectic aluminum A356 alloy casting are mainlygas induced.

In summary, porosity in the hypoeutectic aluminumA356 alloy with high hydrogen content (>0.3 mL/100 g Al) first forms in the liquid as small sphericalgas bubbles, then expands along with the pressure dropin the surrounding liquid due to shrinkage and lack offeeding, and finally deforms into irregular morphologyexerted by the impingement of the aluminum dendritenetwork. Degassing is a key to eliminating porosity inaluminum alloy castings.

This work was funded by the GM ResearchFoundation under Contract No. GM-RP-07-211.

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