Effect of Degassing Treatment on the Quality of Al–7Si and A356 Melts ∗1

Materials Transactions, Vol. 43, No. 11 (2002) pp. 2913 to 2920 c 2002 The Japan Institute of Metals EXPRESS REGULAR ARTICLE

Li-Wu Huang1, ∗2, Pi-Wein Wang1,∗2, Teng-Shih Shih1, ∗3 and Jenq-Hwu Liou2

1Department of Mechanical Engineering National Central University, Chung-Li, Taiwan 32054, R.O. China 2Department of Mechanical Engineering Chien Kuo Institute of Technology, Changhua, Taiwan 500, R.O. China

In melting aluminum alloys, inclusion particles suspended in melt can be effectively reduced by ﬂoatation and/or sedimentation. Never- theless, few researchers have focused on the effect of degassing bubbles on inclusion particles entrapped in the Al melt and the poured casting as well. The degassed bubbles evolved from a diffuser, ﬂoated up and collapsed on free surface of the melt. After explosion, the splashed droplets dropped on surface and then sank into melt following the movement of convection loop induced near the free surface of melt. Consequently, after degassing treatment, AlÐ7Si melts increased the inclusion particle counts and foggy marks revealing on the section of chilled samples. The A356.2 melts decreased the inclusion particle counts but increased the foggy marks on the poured chilled samples.

(Received August 1, 2002; Accepted September 26, 2002) Keywords: oxide ﬁlm, inclusion particle, foggy mark, convention loop

1. Introduction

Alloying element, such as Si and Mg, adding in aluminum melt significantly increases the inclusion particles and devel- ops a high pore density in solidified castings.1Ð4) Many meth- ods have developed to assess the inclusion content for a given melt. Weighting the filtered liquid metal in a closed cham- ber has developed as PoDFA.5) The filter was sectioned and then examined by a microscope to measure the quantity of inclusions. The Liquid Metal Cleanliness Analyzer (LiMCA) instrument uses a DC current to monitor the quality of melt.6) When inclusions entered the orifice of the analyzer, voltage measured from the capillary unit would correspondingly drop. Figure 1 shows the flow chart displaying the analytical chemistry procedure termed as phenol filtration method.7,8) A piece of sample cut from casting was dissolved into a se- lected chemical solution. After a period of time, the metal- lic aluminum was entirely dissolved in the solution but the inclusion left as residual. The whole solution was filtered in order to capture the inclusion particles. These particles were then removed to examine their morphologies and size, and to identify their constituents by SEM equipped with an EDAX. Qualiflash filtration technique,9) LAIS,10) a dual stage ceramic foam filtration system11) or Chlorine fluxing12) meth- ods can be another selection to reduce the inclusion content in the melt. Most researchers believe that inclusions serve as heterogeneous nucleation sites to develop pores in the cast- Fig. 1 Flow chart for filtration of particles existed in A356.2 sample via ing. Consequently, the pores deteriorate the quality of the phenol filtration method. casting.1Ð5,7,9,10,12)

2. Experimental Procedure desired levels. A high quality A356.2 ingot was melted to ob- tain the chilled samples. After melting, all melts were held at An induction furnace was used to melt different batches of 953 K and measured hydrogen content, 0.20Ð0.22 mL/100 g- 20Ð25 kg of aluminum alloys (3000 Hz, 150 kW) by using a Al, by BOMEM ALSCAN model F-HMK 100D. This melt SiC-graphite-clay crucible. Master alloy of AlÐ50%Si was held for 600 s and then poured to produce samples at 1033 K. added into the melt in order to bring silicon content to the If degassing treatment was desired, melt held at 953 K, and had degassed thoroughly with 2 L/min of nitrogen for 1800 s. A diffuser, Fig. 2, generated clouds of ﬁne gas bubbles ﬂoat- ∗1 Part of this Paper was Presented at the 7th Asian Foundry Congress, on ing up from the bottom of the melt. After degassing treatment, Oct. 14, 2001, held in Taipei, Taiwan, R.O. China. ∗2Graduate Students, National Central University. the hydrogen content was measured in the range of 0.09Ð ∗3Corresponding author: [email protected] 2914 L.-W. Huang, P.-W. Wang, T.-S. Shih and J.-H. Liou

Table 1 Chemical analyses of materials used in this study.

Main element Si (%) Fe (%) Mg (%) Sr (ppm) Ti (%) Al (%) AlÐ7%Si without degassing 7.06 0.09 0 — 0.01 Balance AlÐ7%Si with degassing 6.85 0.11 0 — 0.01 Balance A356.2 without degassing 6.75 0.07 0.34 214 0.13 Balance A356.2 with degassing 6.81 0.12 0.33 165 0.11 Balance A356.2 (low Sr) with degassing 7.04 0.13 0.35 <10 0.12 Balance

sured two sections and the average was counted as the area ratio of foggy mark. A high-speed camera, 1000 pictures per second, was also adopted to take photographs for showing bubble explosion on free surface during water simulation.

3. Experimental Results

3.1 Oxide film and foggy mark After the polished sample subjected to ultrasonic vibration for a period of time, foggy marks show on its surface with different shapes, reported by Shih et al.4) These foggy marks formed due to eroded oxide film and/or cracking oxide film on the surface of sample. These marks include foggy clouds, strips or spots. Adding Si in the AlÐSi alloy decreases the ex- tent of foggy clouds and the length of foggy strip but increases the number of foggy spots. Mg in the AlÐSiÐMg alloy tends to reduce the size of foggy clouds but greatly increases the number of foggy spots. Reaction may occur in the AlÐSiÐMg melt to form spinel (MgO·Al2O3) on the interface of oxide film and in the melt, according to the description of Shih.4) Fig. 2 Diffuser shown in (a) three dimensions and (b) detailed scales. Figures 3(a) and (b) show the foggy marks on the surfaces of chilled samples, after ultrasonic-vibration treatment. The 0.12 mL/100 gAl. The melt further held at 953 K for 600 s area ratios of foggy marks are 2.2% and 1.2% for AlÐ7Si and and then poured at 1033 K. A transferred ladle made of ce- A356.2 alloy respectively. This ratio is affected by the con- ramic fiber was used to pour the samples. The whole batch of stituent of melt and depends on sample taking from different melt could pour six spoons; each spoon in series was coded locations of melt as well. In fact, degassing treatment would as corresponding to locations counting from top to bottom of also affect the measured area ratio of foggy marks and will be the melt in crucible. In other words, the samples from the discussed latter. first spoon coming from top level of the melt and those from The elastic modulus and hardness of oxide film and the the last spoon taking from bottom level of the melt. In addi- matrix of alloys (AlÐ7Si and A356.2) are quite different. If tion to chill samples, each spoon also poured the keel block oxide film existed on the polished surface, it may protrude ir- castings, JIS H5202, at temperature about 1033 K. Chilled regularly and microscopically out of sample surface, because samples were prepared in dimensions, 50 mm in diameter and oxide film has a high elastic modulus than matrix. The pro- 10 mm in thickness. After polishing, samples were analyzed truding points reduce the driving force for nucleating micro- by spectrometer testing, Table 1. bubbles and consequently develop micro-jet impact on oxide After the spectrometer test, samples were polished again film. Micro-jet persistently impacts on the brittle oxide film 13) and removed to measure inclusion particle counts via opti- and explosion of micro-bubbles introduces the shock wave. cal microscope equipped with image analyzer. The particle The micro-jet impact tends to crack the oxide film and to counts were recorded. Each sample had been measured ten erode the film gradually, as shown in Fig. 4 along with EDAX times and the average of the total inclusion particle counts analyses. This sample has been treated in the ultrasonic fre- was obtained. quency of 46 kHz for 2400 s. The polished surface demon- After measuring the particle counts, chilled samples were strates foggy marks under optical, or visual, observation as polished again and placed in an ultrasonic cleaner filled with shown in Fig. 3. 500 mL of tap water. These samples were then ultrasonic- treated for 1800 s. Foggy marks would gradually show on 3.2 Particles and pores the polished surfaces of chilled samples during ultrasonic vi- Inclusion particles readily exist in many active metal melts. bration treatment. The morphology and area ratio of foggy The oxide inclusion particles entrapped in Al alloy castings marks were measured and recorded. Each sample was mea- significantly influence their surface appearance, mechanical Effect of Degassing Treatment on the Quality of AlÐ7Si and A356 Melts 2915

Measured at a white mark Foggy marks

(a)

Foggy marks

Element Mass At O K 22.63 51.47 MgK 0.81 1.21 Al K 25.02 33.74 Si K 3.03 3.92 AuK 47.56 8.79 CaK 0.96 0.87 Total 100.00 100.00 (b) Fig. 4 SEM and EDAX analyses showing the morphology of foggy area; Fig. 3 Chilled blocks of (a) AlÐ7Si and (b) A356.2 showing foggy area; sample from A356.2 melt and after ultrasonic vibration treatment with after polished and ultrasonic vibration treatment with 46 kHz for 40 min. 46 kHz for 40 min.

properties and machining performance. Therefore, monitor- ing and reducing the amounts of entrapped inclusions is a vital issue in producing aluminum alloy castings. In order to identify the entrapped oxide particles, a thin sample was sliced from chilled block and cooled by liquid nitrogen. This thin sample was then fractured into two pieces and removed to ob- serve their fractured surfaces via SEM. Figure 5 demonstrates a feather-like oxide film, or particles, accompanied with a pore. Apparently, this feather-like film serves as a heterogeneous nucleation site to form a pore. The surface of this pore is bumpy and shows the morphology similar to Al dendrite. This sample comes from AlÐ7Si melt and its constituents are confirmed by the EDAX analyses. Figure 6 shows the fractured surface of AlÐ7Si sample indicating the existence of inclusion particles. Phenol filtration method7,8) was adopted in this study to collect inclusion particles. Particles from the filtrated residue Fig. 5 SEM and EDAX analyses on a stringer-type oxide film accompanied coated with gold and were examined by SEM. Figure 7 con- with a pore; AlÐ7Si sample after fractured. firms that these particles are rich in alumina and some silica and/or mullite. The analyzed carbon may come from the con- tamination during extraction process. ing from top to bottom level of melt in the crucible. Figure 8(a) shows the measured area ratio of foggy mark 3.3 Quality of Al–7Si alloy on chilled samples from different spoons, or corresponding As mentioned in the experimental procedure, melt was to different levels in the melt. For the AlÐ7Si alloy without poured into a spoon and then transferred to produce chilled degassing treatment, sample poured from near bottom level samples and keel block (JIS H5202) castings. The whole shows a great deviation in the area ratio of foggy mark. After batch of melt can poured 5Ð6 spoons. Samples produced degassing treatment, the AlÐ7Si sample from upper level of from different spoons are coded as the locations correspond- melt contrarily illustrates a great deviation in the area ratio of 2916 L.-W. Huang, P.-W. Wang, T.-S. Shih and J.-H. Liou

Inclusion particles Al-7Si melts

with degassing (average:4.1 )

without degassing (average:2.2 ) 7 7

6 6

5 5

4 4

3 3

2 2

1 1 Area Ratio of Foggy Mark ( ) Area Ratio of Foggy 0 0 Top Bottom Corresponding Position in the Melt (a)

Fig. 6 Oxide particles appeared on a fractured surface of a thin plate of AlÐ7Si chilled specimen. Al-7Si melts

Measured at a black mark with degassing (average:245) without degassing (average:111) 350 350

300 300 -2

mm 250 250

200 200 /count N 150 150

100 100

Particle, Particle, 50 50

0 0 Top Bottom Corresponding Position in the Melt (b)

Fig. 8 Measured (a) ratio of foggy area and (b) particle count in AlÐ7Si chilled blocks versus different samples corresponding to varying positions Energy, E /keV in the crucible; melt with and without degassing treatment; melt degassing treated by nitrogen (2 L/min, 1800 s) and holding at 993 K for 600 s. Element Mass At Al K 36.17 47.10 OK 57.58 44.43 Si K 6.25 8.47 gassing treatment, samples poured from near bottom level of −2 Total 100.00 100.00 melt show higher particle counts (N/count·mm , where N is number), Fig. 8(b). This may result from fact that inclusion Fig. 7 SEM and EDAX analyses of an inclusion particle; pure aluminum particles sank in the molten melt during holding. Assuming sample after phenol filtration process. that the average moving velocity of the molten melt is small during holding period and the particle is spherical, formula of 2 = 2R (ρs−ρ)g 14) foggy mark. This difference can definitely be attributed to the Vt 9µ can be adopted in a creeping flow; where Vt influence of degassing treatment. If melt is without degassing is the sinking speed of a mullite particle, m/s; ρs is the density treatment and held for a period of time (more than 900 s), ox- of mullite, 2.8 × 103 kg/m3; R is the radius of the mullite par- − ide film may sink and cluster on the bottom of melt, since ticle, 5×10 6 m; g is gravitational acceleration, 9.8 m/s2. The . × −5 the density of mullite (3Al2O3·2SiO2) and aluminum liquid sinking speed of mullite particles is about 1 1 10 (m/s), − is 2.8 × 103 and 2.3 × 103 kg/m3 respectively. Degassing which is close to experimental value of 8.3 × 10 5 (m/s) from treatment produces stirring action and forms convection loop that a particle sank and moved 50 mm in distance at 600 s in in the melt. Oxide film floats up and moves following the creeping flow. convection patterns developed in the melt. If holding time is not long enough or the intensity of convection remains active, 3.4 Quality of A356.2 alloy oxide films are more likely to locate at the upper portion of Figures 9(a) and (b) show the relation of area ratio of foggy the melt since the difference in density of mullite and alu- mark and particle count of sample versus corresponding loca- minum melt is small, Fig. 8(a). The samples poured from tion in the A356.2 melt. If the melt did not conduct degassing melt with degassing treatment show a greater ratio of foggy treatment, samples poured from different levels remain a sta- mark, 4.1% on the average of six samples, than those from ble ratio of foggy mark, 1.2% on the average, and particle melt without degassing treatment, 2.2%. Regardless of de- count, 191 counts/mm2 on the average. Effect of Degassing Treatment on the Quality of AlÐ7Si and A356 Melts 2917

Table 2 The size distribution and inclusion particle counts existed in chill samples of A356.2 alloy with degassing treatment.

Composition A356.2 with high Sr Particle size <2 µm3Ð5 µm6Ð10 µm11Ð15 µm >15 µm Total Count/mm2 (%) count (%) count (%) count % count % count % count %

Without degassing 78 40.84 73 38.22 31 16.23 7 3.66 2 1.05 191 100 treatment particle size below 10 µm contains 182 counts (95.29%) 42 35.59 41 34.75 22 18.64 8 6.78 5 4.24 118 100 Degassing treatment particle size below 10 µm is 105 counts (88.98%) Composition A356.2 with low Sr content 24 29.27 28 34.15 18 21.95 4 4.88 8 9.75 82 100 Degassing treatment particle size below 10 µm contains 70 counts (85.37%)

A356.2 melts samples with a high Sr content, 165 ppm, develop a greater A356.2 (low Sr) with degassing (average:8.1 ) particle count in a greater deviation but a slightly lower ra- A356.2 with degassing (average:6.7 ) tio of foggy mark than those with a low Sr content, less than A356.2 without degassing (average:1.2 ) 14 14 10 ppm. Table 2 demonstrates the size distribution of inclusion par- 12 12 ticle existed in the A356.2 samples. These samples poured 10 10 from the melt without degassing treatment possessing a 8 8 greater number of inclusion particles than those poured after degassing treatment, 191 versus 118 count/mm2. Particles 6 6 mostly have the size less than 10 µm, 90Ð95%. Decreasing 4 4 the Sr content in A356.2 melt can apparently reduce the par- 2 2 ticle count from 118 to 82 count/mm2, especially for particle

Area Ratio of Foggy Mark ( ) Area Ratio of Foggy µ 0 0 size ﬁner than 10 m. Top Bottom Summarily speaking, after degassing treatment, the inclu- Corresponding Position in the Melt (a) sion particle counts in A356.2 samples cut down to almost half but those in AlÐ7Si samples increase more than two-fold, as shown in Table 3. Most particle sizes are under 10 µm.

A356.2 melts Strontium is commonly added into A356.2 to modify the morphology of eutectic silicon. It increases the oxidation behav- A356.2 without degassing (average:191) ior of Al melt and inclusion particles in Al casting, reported A356.2 with degassing (average:118) by Emadi.1) This study conﬁrms that adding Sr in A356 alloy A356.2 (low Sr) with degassing (average:82) 250 250 indeed increases the tendency for oxidation of A356.2 melt and lifts up particle counts.

-2 200 200 4. Discussion 150 150

/count mm 4.1 Surface tension of Al alloy N 100 100 The surface tension of pure aluminum (99.99%) is about

50 50 0.86 N/m. Adding 5%Si into aluminum melt slightly de-

Particle, Particle, creases its surface tension, 0.82 N/m, but alloying 5%Mg into 0 0 aluminum melt sharply cut the surface tension to 0.6 N/m.15) Top Bottom Corresponding Position in the Melt Adding Sr in A356.2 alloy not only decreases the surface ten- (b) sion of melt but also produces the Sr-base inclusion as AlÐSiÐ 1,16Ð18) Fig. 9 Measured (a) ratio of foggy area and (b) particle count in A356.2 Sr and/or SrÐAlÐO type compound. chilled blocks versus different samples corresponding to varying positions The surface of Al molten melt is normally covered with a in the crucible; melt with and without degassing treatment; melt degassing thin oxide film during melting and degassing treatment. If treated by nitrogen (2 L/min, 1800 s) and holding at 993 K for 600 s. the oxide film is perfectly composed of alumina, mullite or spinel, these oxides provide a great strength, which can sup- press degassed bubble floating and protruding out of free sur- After degassing treatment, samples increase the ratio of face. In fact, degassed bubbles float up, protrude out of free foggy mark but decrease the particle count with the depth in surface and explode as can be seen in the experiment. This the melt regardless of Sr content. In addition, samples poured indicates that the covered oxide films are possibly contami- from different levels display a high degree of deviation either nated with impurities, such as Ca, Na or Cl and so on. This in the ratio of foggy mark or the particle counts. The A356.2 2918 L.-W. Huang, P.-W. Wang, T.-S. Shih and J.-H. Liou

Table 3 The inclusion particle counts and their size distribution for samples of AlÐ7Si alloy with and without degassing treatment.

Composition AlÐ7%Si Particle size <2 µm3Ð5 µm6Ð10 µm11Ð15 µm >15 µm Total Count/mm2 (%) count (%) count (%) count % count % count % count %

Without degassing 44 39.64 37 33.33 20 18.01 6 5.4 4 3.62 111 100 treatment particle size below 10 µm contains 101 counts (90.98%) 83 33.20 97 38.67 53 21.1 12 4.99 5 2.04 245 100 Degassing treatment particle size below 10 µm contains 232 counts (92.97%)

Fig. 10 Phototgraphs taking from high-speed camera indicating the progressive sequence for forming a bubble on free surface of water and its explosion, time step = (10−3 s). greatly reduces the constraining force of covered oxide film, truding degassed bubbles. The thermal fluctuation of the melt especially at high temperature. The electro-magnetic force can even be other possible factor to fracture the covered sur- induced by the high frequency induction furnace may also be face oxide film, once the degassed bubble is approaching to another driving force to break the covered oxide film for pro- the free surface. When a degassed bubble floats up near the Effect of Degassing Treatment on the Quality of AlÐ7Si and A356 Melts 2919 free surface of melt, surface oxide fractures and bubble pro- trudes out from the site of fracture. All of these sequences occur in a short time. The new open surface is fresh without oxide film developed and accumulated. The surface tension of molten metal provides a constraining force for protruding a degassed bubble and for remaining its shape on free surface, once it protruded out of surface. Alloying elements influence the surface tension of aluminum melt and the size of bubble protruding out of free surface. If a molten melt possesses a high surface tension, such as pure Al or AlÐSi alloy, a degassed bubble would protrude out of free surface significantly causing big chunk of oxide films and/or great amounts of inclusion particles after degassed bubble exploded. For A356.2 Fig. 11 Schematic illustration showing the sequence of degassed bubble melt, a degassed bubble can protrude less degree out of sur- floats up to the free surface of melt while the air stream flows downward face of melt due to the addition of Mg, and therefore produce and convection loops in the melt are induced. a low amount of splashed droplets after explosion.

4.2 Bubble explosion in water simulation melt, different oxide particles of SiO2, MgO and spinel ac- A cylindrical tank, which made of transparent acrylic, was companied with alumina may be possibly existed after de- filled with water in this study. The bottom plate of the tank gassing treatment. These oxide particles in the A356.2 melt was drilled a small hole and sealed with silica gel. A bubble may react with alumina to form pyrope, Mg3Al2Si3O12 with was shot into water from the hole via a needlepoint of an in- 3.56 × 103 kg/m3 in density, via the stirring action during de- jector. A high-speed camera was used to record the dynamic gassing treatment. Clustering of particles with oxide film in- change of free surface of water during the period of bubble crease the number of foggy spots in the previous study,4) and − approaching and explosion with time step of 10 3 s.19Ð23) the area ratio of foggy mark in the A356.2 sample after de- Figure 10 displays a series of photographs taking from ex- gassing treatment, Fig. 9(a). The alumina, spinel and pyrope periment. A small amount of air injected into the bottom of are with a greater density than liquid aluminum. After de- tank will instantly form bubble floating up to free surface of gassing treatment, the A356.2 samples display high area ra- water. When the bubble reaches at the free surface, it remains tios of foggy marks distributed at location near bottom level stable in its shape for a short period of time, as shown in of melt, as illustrated in Fig. 9(a). Figure 10(a). The bubble explodes from top area and then collapses, as shown from Figs. 10(b) to (d). There are several 4.4 Effect of inclusion particles on fatigue of Al–7Si cast- shinning spots shown on Figs. 10(d) and (e). These shinning ings spot are more likely the splashing water droplets produced In the experiment, molten metal poured into permanent from the collapsed bubble. mold, JIS H5202. The degassing-treated AlÐ7Si samples are Simply, ignore the effect of convection loop induced by prepared for fatigue test. The samples subjected to a rotat- floating bubbles and its dynamic interaction on the size and ing and bending test with stress amplitude of 80 MPa. At shape of bubble. Decreasing surface tension of solution re- this stress amplitude, the cycles to failure of specimens were duces the constraining force for remaining bubble on free sur- recorded. These samples were then sectioned to assess the in- face causing an early explosion. Therefore the size of bubble clusion particle counts after polishing. Figure 12 displays the protruding out of free surface is decreased; in other words, relation of inclusion particles and cycles to failure of each the bubble contour protruding out of free surface is decreased. sample. The inclusion particle counts are 187 count/mm2 Adding magnesium in AlÐSi melt greatly lowers the surface for specimen 1 developing 2.15 × 105 life cycles. Speci- tension of the melt. Therefore, the stable size of bubble pro- men 5 possesses particle count 49 count/mm2 and performs truding out of free surface of A356.2 melt is correspondingly 3.11 × 105 life cycles. All five specimens have above 95% decreased and most degassed bubbles develop early explo- of particles with size less than 10 µm. The inclusion particles sion during degassing treatment. Fewer or finer droplets can existed in the matrix of tested bars may serve as the initiator be produced from each bubble explosion comparing with the for forming and growing micro-cracks. This shortens the in- case of AlÐ7Si melt. Droplets fall down on free surface and cubation period of crack initiation and accelerates the crack sinks into melt. The droplets or piece of oxide films move into propagation during constructing a crack. Consequently, fa- the melt following the movement of convection loop within tigue life is decreased, if the matrix of alloy casting possesses melt, as schematically illustrated in Fig. 11. Theoretical anal- significantly high particle counts. yses for showing convection loops in the melt and downward air stream above the free surface refer to Appendix 1. In this 5. Summary analysis, the K -ε turbulence model was used and the comput- ing domain was set to be 40000 grids. After degassing, the AlÐ7Si melts increased inclusion particle counts. The inclusion particles usually contained Al, Si 4.3 Clustering of oxide film and inclusion particles and O elements and formed the compound of mullite in AlÐ The inclusion particles or oxide film in AlÐ7Si melt is 7Si melt and casting as well. Degassing treatment produces mainly mullite, which is stable in the melt. In the A356.2 2920 L.-W. Huang, P.-W. Wang, T.-S. Shih and J.-H. Liou

250 250 (1999) 49Ð65. Inclusion Particle 10) D. Sampath, P. G. J. Flick, J. Pool, W.Boender and W.van Rijswijk: The 2 187 Minerals, Metals & Materials Society, Light Metals (1996) 817Ð821. 200 below 10 um 200 96.26 11) L. S. Aubrey, M. A. Cummings and C. L. Oliver: The Minerals, Metals total count & Materials Society, Light Metals (1996) 845Ð855. 150 150 121 12) C. Nyahumwa, N. R. Green and J. Campbell: AFS Trans. 106 (1998) 95.04 215Ð224. 100 75 74 100 13) J.-J. Chen, L.-W. Huang and T.-S. Shih: Mater. Trans. (JIM), 2002, to 98.67 90.54 49 be submitted. 100 14) G. H. Geiger and D. R. Poirier: Transport Phenomena in Metallurgy, 50 50 (Addison-Wesley Publishing Company, 1980) pp. 68Ð71. inclusion particle, count/mm particle, inclusion 15) ASM Metals Handbook, Vol. 2, aluminum foundry products, 1999, 0 0 pp. 35Ð50. failure to cycles: 272800 310900 215100 264800 275800 16) R. A. Robie, B. S. Hemingway and J. R. Fisher: Thermodynamic Prop- Specimen: 2 345 1 erties of Minerals. (1979) pp. 152Ð174. 17) N. B. Pilling and R. E. Bedworth: J. Inst. Of Metals 29 (1923) 529Ð591. Fig. 12 Relationship of inclusion particle counts and cycles to failure for 18) J. E. Gruzleski and B. M. Closset: The Treatment of Liquid AluminumÐ each sample from AlÐ7%Si melt tested at the stress amplitude of 80 MPa. Silicon Alloys, (The American Foundrymen’s Society, 1990) pp. 57Ð 105. 19) W. Lauterborn and H. Bolle: J. Fluid Mech. 72 (1975) 391Ð399. stirring action forming convection loops in the melt. Oxide 20) J. R. Blake and D. C. Gibson: J. Fluid Mech. 111 (1981) 123Ð140. film floated up and moved following the convection patterns 21) M. C. Chou, G. P. J. Too, C. J. Huang, C. M. Jian and J. J. Hwang- developed in the melt. If the holding time is not long enough Fuu: J. of the Society of Naval Architects and Marine Eng., (R.O.C.) 20, No. 2 (2001) 31Ð40. or the intensity of convection remains active, oxide films are 22) C.-R. Lin, G.-P.Too and C.-J. Huang: J. of the Chinese Institute of Civil more likely to distribute on the upper portion of the melt. Be- and Hydraulic Eng. 10 (1998) 9Ð16. fore collapse, a degassed bubble could remain a greater stable 23) D. Dey, J. M. Boulton-Stone, A. N. Emery and J. R. Blake: Chem. Eng. size out of free surface in AlÐ7Si melt than in A356.2 melt. Sci. 52 (1997) 2769Ð2783. This produced a great amount of splashed droplets in AlÐ7Si Appendix melt after bubbles explosion. After degassing treatment, the A356.2 samples increased (1) Using the Fluent 5.5 edition to solve the equations in- the ratio of foggy marks but decreased the particle counts. cluding Continuity equation and Momentum equation The splashed droplets of A356.2 melt formed oxide parti- under K - Turbulence Model. cles of alumina, mullite, and/or spinel. Clustering of particles (2) Using VOF (Volume of fraction) model to determine the with oxide film led to form pyrope during degassing. This convection term of gas-liquid interface under processing increased the number of foggy spots and area ratio but de- of Second order upwind. creased the particle counts. Decreasing Sr content in A356.2 (3) Using explicit term of time (∆t = 5 × 10−5 s), Solv- melt significantly decreased the particle counts. ing the transient convection field by taking the numeri- cal method of PISO (Pressure-implicit with splitting of Acknowledgements operators). The authors greatly appreciate the National Science Coun- cil, ROC, for financial support of this research, NSC 91-2216- E-008-006.

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