Materials Transactions, Vol. 46, No. 2 (2005) pp. 263 to 271 #2005 The Japan Institute of Metals
Effects of Degassing and Fluxing on the Quality of Al-7%Si and A356.2 Alloys
Teng-Shih Shih*1 and Kon-Yia Wen*2
Department of mechanical Engineering, National Central University, Chung-Li, Taiwan 32054, R.O.China
A covering flux is commonly used to prevent an aluminum melt from reacting with the surrounding atmosphere or from re-oxidizing. In this study, melts were degassed with and without a covering flux using a porous bar diffuser. After degassing and holding, the melts were then poured to obtain chilled samples, reduced pressure samples and permanent mold castings. The chilled samples were polished and treated by ultrasonic vibration to reveal any foggy marks and the area of the foggy marks and the pore count were measured. The densities of the chilled samples and the reduced pressure sample were also measured to compute the relative porosities. The factors influencing the relative porosities of the aluminum alloy castings were then discussed. Rotational bending fatigue tests were also conducted to assess the effect of the pore count and the relative porosity on the fatigue life cycles of the A356 alloy castings.
(Received May 10, 2004; Accepted December 6, 2004) Keywords: degassing, fluxing, pore counts, relative porosity
1. Introduction (a) (b)
A degassing treatment is commonly used in producing aluminum alloy castings. Sigworth1) found that small purging bubbles are effective in removing gas, due to the large surface area for a given volume of purging gas, and the slowed (c) movement in the melt. Building on the work of Engh and Pedersen,2) he compared the effectiveness of hydrogen reduction, evaluated using a lance and a porous plug. Using (d) a porous plug can improve the efficiency of the degassing compared with using a lance degasser. However, most research work has assessed the effect of different degassing (e) techniques based only upon the relationship between the hydrogen content and the degassing time. Reasonably, reducing the hydrogen content will certainly improve the (f) (g) mechanical properties of the resultant aluminum alloys. The 10 mm effect of the degassing technique on the quality of the aluminum alloys includes not only controlling the hydrogen content but also the resultant quality of the melt and its cleanliness. Gnyloskurenko and Nakamura3) took advantage of in-situ observations at the melt. They found that the gas bubble volume increased as the wettability of the melt worsened, such as after using alumina nozzle. Increasing the Fig. 1 Foggy marks on the polished specimen surface; using 500 mL of tap gas flow rate and/or reducing the nozzle diameter effectively water and vibration treated for 300 s: (a) aluminum ingot; (b) A356 boat decreased the bubble volume. Decreasing the bubble volume mold; (c) A356 tensile bar from a ASTM B108 mold; (e) A356 squeezed of course can improve the effectiveness of degassing. mold; (f) A356 aluminum wheel casting; (g) wrought 6061 bar.6) Increasing the gas flow rate introduces a strong convection in the melt, a matter which has been rarely addressed and should be of concern. surface of chilled samples.5–7) After this ultrasonic vibration During the melting of aluminum alloys, the inclusion of treatment, the oxide film entrapped in the Al-Si-Mg alloys particles suspended in the melt can be effectively reduced by would be revealed as a foggy area caused by cavitation floatation and/or sedimentation. Oxide inclusions are mostly micro-jet impacts, as shown in Fig. 1.6) Pores may contain heavier than the base melt and so tend to fall to the bottom of oxide film, oxide particle(s), as shown in Fig. 2, or be free of the crucible. In addition, filtration can be used to remove particles. In this work, the effects of the degassing treatment inclusions, significantly improving the density of the poured on the foggy marked areas and the pore count of the Al-7%Si castings.4) and A356 melts are evaluated.8) The fatigue life cycles of Shih and coworkers developed an ultrasonic-vibration aluminum alloy castings are indeed closely related to the pore treatment which could reveal foggy marks on the polished count and the area of foggy marks on the chilled samples. The number of pores will increase mainly due to the degassed *1Corresponding author, E-mail: [email protected] bubbles protruding and exploding at the free surface of the *2Graduate student, National Central University melt. Debris that falls in the craters is eventually entrapped in 264 T.-S. Shih and K.-Y. Wen
graphite-clay crucible was used to melt 20 kg batches of an aluminum alloy (3000 Hz, 150 kW). A master alloy Al- 50%Si alloy was added to the pure Al melt in order to raise the silicon content to the desired levels. High quality A356.2 ingots, with and without Sr, were melted to obtain chilled samples, reduced pressure samples and permanent mold castings, JIS H5202. After melting, all the melts, with or without a covering flux, were held at 953 K and degassed by nitrogen (1 L/min) via a porous bar diffuser. A covering flux, comprised of NaCl (56%) and KCl (44%), was used with a melting point of about 983 K.8) After being completely dried at 673 K for 3600 s, the flux mixture was carefully spread over the surface of the melt. A 20 kg batch of the melt Fig. 2 SEM micrograph showing an oxide particle entrapped in the pore. required 100 grams of the covering flux. After the flux completely covered the surface of the melt, a porous bar nitrogen pumping diffuser was carefully set into the melt. The flow rate was 1 L/min and the degassing time was 600 s. the melt and the poured casting as well. Magnesium added to After the degassing treatment, the melt was held for 600 s, Al-Si alloys changes the surface tension of the melt, therefore and then poured at 1033 K. A transfer ladle made of ceramic the size of bubbles protruding from the free surface of the fiber was used to pour the samples. A single batch of melt melt is different, producing variation in the amounts of debris could produce five to six groups of samples. Each group was and of pores entrapped in the castings. coded in a series that indicated its corresponding location in Fluxing is also used in the processing of the aluminum the melt, from top to bottom. In other words, the samples alloy casting. A proper fluxing procedure can improve the from the first spoon came from the top level of the melt and cleanliness of the melt by accelerating the separation of those from the last spoon from the bottom level of the melt. inclusions.9) Ye and Sahai have discussed the conventional Each group of samples included two chilled samples (50 mm mechanisms for the removal of oxide film from an aluminum in diameter and 10 mm thick), one reduced pressure sample, surface by molten salts.10) They suggested that the oxide film and about four permanent mold castings. After polishing, the is stripped as a result of the force induced by the interfacial chilled samples were analyzed via spectrometer testing; see tension gradient that exists between the liquid aluminum and Table 1. Each sample was tested three times and the average the oxide film. Roy and Sahai have studied the coalescence was recorded. behavior of aluminum alloy drops in molten salt. They After the spectrometer tests, the chilled samples were explained that the removal of the oxide layer from the Al polished again. The pore count was measured via an optical surface did not take place in equimolar NaCl-KCl.11) microscope equipped with an image analyzer (magnification Shih and Weng studied the effect of a degassing treatment 100X). Each pore might or might not contain particle(s) and on the quality of Al-7%Si and A356 melts using different in some cases several particles could be trapped in one pore. degassing diffusers.12) The degassing bubbles rose in the melt Each sample was measured ten times and the average of the floating with convection loops during the degassing treat- total was the pore count. ment. The effects of magnesium added to the A356 alloy on After measuring the pore count, the chilled samples were the pore count and the shape of the oxide film is very placed in an ultrasonic cleaner filled with 500 mL of tap significant, compared with the Al-7%Si and A356 alloys.12) water. The samples were then ultrasonically treated for Adding Mg reduces the surface tension of the A356 alloy 1800 s. Differently shaped foggy marks were revealed on the melt, producing few splashed droplets after the explosion of surfaces of the chilled samples. The morphology and the bubbles at the free surface during degassing, which results in areas of the foggy marks were recorded. The foggy marked a lower pore count than in the Al-7%Si alloy. In the Al-7Si areas on two sections of each sample were measured and the alloy, the oxide, if formed, tends to form mullite and in the average counted as the area ratio. Water simulation was A356 alloy, the oxide is more likely to form pyrope. The adopted to observe the floating and movement of the bubbles. formation of different oxides had an influence on the shape of In the experiment, a high-speed camera, capable of taking the oxide film and the foggy marked area of the Al-7%Si and 1000 pictures per second, was used to take photographs of A356 alloys. This study extends the previous work and floating bubbles and their movements. assesses the effect of a degassing treatment with/without the The reduced pressure sample was sectioned into two covering flux, on the foggy marked areas (oxide film or pieces. One piece was polished to reveal the morphology and lumps) and the pore counts of the Al-7%Si and the A356.2 distribution of the pores and the gas holes. The density of the melts. The effects of the foggy marked area and/or pores on other piece was measured by the Archimedes method dr. The the fatigue properties of the resultant A356 alloy castings are density dc of the chilled sample was also measured. The also assessed. relative porosity of each set of samples was computed based on the density differences between the chilled and the 2. Experimental Procedure reduced pressure samples, RP ¼ðdc drÞ=dc.
In this study, an induction furnace equipped with an SiC- Effects of Degassing and Fluxing on the Quality of Al-7%Si and A356.2 Alloys 265
Table 1 The chemical compositions of the alloys studied; including Al-7%Si, A356.2 with or without Sr addition; melts degassed with or without the covering flux; mass%. (mass%) Alloy elements Si Mg Sr Fe Ti Al Heat No. (%) (%) (ppm) (%) (%) (%) A A356.2 6.57 0.34 11 0.062 0.124 Bal. (degassing) B A356.2 high Sr 6.59 0.32 184 0.068 0.146 Bal. (degassing) C Al-7%Si 6.56 0 0.105 0.008 Bal. (degassing) D A356.2 (degassing 7.43 0.29 0.074 0.141 Bal. and fluxing) E A356.2 high Sr (degassing 7.67 0.32 0.061 0.126 Bal. and fluxing) F Al-7%Si (degassing 7.13 0 0.099 0.007 Bal. and fluxing) > degassed using a porous bar fluxing by NaCl + KCl (5 g/1 kg of melt) content is less than 10 ppm per at least three tests.
3. Experimental Results to be caught by the surface flux and become dross. After skimming, most of the oxide films and chunky inclusion 3.1 Foggy marks and pore counts particles can therefore be removed. For an Al-7%Si melt, degassed with or without a covering The pictures in Fig. 5 show the evidence displaying the flux, the chilled samples showed quite different results in cleanliness of the chilled samples prepared from the A356.2 terms of the foggy areas and the pore count. Figures 3(a) and melt degassed with (right) and without the covering flux (b) show that, for the melt degassed with a covering flux, the (left). Using a covering flux can significantly decrease the ratio of the foggy marked area was reduced, from 3.7 to 1.3% foggy marks (oxide film/chunky inclusion particles). In the (the average of samples from one melt), but the pore count A356 melt, without the covering flux, the foggy marks had obviously increased, from 187 to 313 (pores mm 2), included mostly lumps, some strips and spots; see left-hand compared to samples prepared from the degassed melt. column in Fig. 5. In the A356 melt, using the covering flux, Figure 3(a) displays the measured ratios of the foggy marked fewer strips or fine spots remained on the chilled sample; see areas. The ratios are high and significantly scattered. right-hand column in Fig. 5. A pore, if it exists, is usually accompanied by an inclusion Figures 6(a) and (b) show the ratios of the foggy marked particle (or particles). Thus the measured pore count will area, and the pore count on the A356.2 sample with Sr, surely be affected by any particles trapped in the melt and degassed with and without the covering flux. Using the consequently in the chilled samples. Figure 3(b) illustrates covering flux decreased the foggy marked area from 2.9 to that in the samples from the melt degassed with the covering 1.2%, but increased the pore count from 96 to 194 flux the pores were relatively fine. Fine particles will float (pores mm 2). The two A356.2 melts, with and without Sr, during the holding period after degassing, and likely be had similar results that are an increase of pore count and a distributed in the top of the melt. If the melt was degassed decrease in the foggy marked area. The experimental without the covering flux, the particles were coarse and sank observations show that Sr added to the A356.2 alloy slightly to the bottom of the melt; see Fig. 3(b). increases the number of foggy spots but reduces their size. Figures 4(a) and (b) show the measured ratios of foggy The pore counts increased slightly probably due to the marked areas and the pore counts for the A356.2 melt, entrapment of Sr-based compounds or inclusions. degassed both with and without the covering flux. The melt The oxide films in the Al-7%Si melt contained a high degassed with the covering flux showed decreased ratios of fraction of mullite while those in the A356.2 melt were rich in foggy marked areas, from 5.8 to 0.7%, but an increased pore pyrope, Mg3Al2Si3O12. Mullite is lighter than pyrope, 2:8 count, from 84 to 174 (pores mm 2), compared to the melt 10 3 vs. 3:56 10 3 kg/m3, so the heavy oxide film or that was degassed only. Again, the use of a covering flux particles tended to sink to the bottom of the A356.2 melt; significantly increased the pore count but decreased the foggy compare Figs. 3(a) and 4(a) to 6(a). Convection loops marks in the A356 alloy, as in the Al-7%Si alloy. During the persisted throughout the whole degassing period. The degassing treatment, the oxide film and the chunky inclusion primary Al2O3 oxide film in the melt gradually reacted with particles will move counter-clockwise, following the motion the oxide particles, SiO2, MgO etc., or the alloying elements, of convection loops in the melt.12) Any oxide films or chunky Si, Mg and oxygen, to form mullite in the Al-7%Si melt and inclusion particles entrapped in the melt will be pushed pyrope in the A356.2 melt. The resultant oxides could upward to an area near the top of the melt during degassing, therefore be lumpy, feather-like, strips or chunky spots, 266 T.-S. Shih and K.-Y. Wen
With degassing treatment With degassing treatment Al-7Si(aveg:3.7%) Porous bar A356.2 Low Sr (aveg:5.8%)Porous bar Al-7Si(aveg:1.3%) Porous bar & Fluxing A356.2 Low Sr (aveg:0.7%)Porous bar & Fluxing 14 7 13 12 6 11 10 5 9 4 8 7 3 6 5 2 4 3 1 2
Area Ratio of Foggy Mark(%) Area Ratio of Foggy 1
Area Ratio of Foggy Mark(%) Area Ratio of Foggy 0 0 145623 123456 Top Bottom Top Bottom Position from Top to Bottom in crucible Position from Top to Bottom in crucible (a) (a) With degassing treatment With degassing treatment Al-7Si(aveg:187pores . mm-2) Porous bar -2 Al-7Si(aveg:313pores . mm-2) A356.2 Low Sr (aveg:84pores . mm )Porous bar Porous bar & Fluxing A356.2 Low Sr (aveg:174pores . mm -2) Porous bar & Fluxing 400 375 250 350 225 -2 325 -2 200
mm 300 mm . 275 . 175 250 150 225 125 N/pores , 200 N/pores 175 , 100
Pore 150 75 Pore Pore 125 50 100 25 123456 Top Bottom 1234 5 6 Top Bottom Position from Top to Bottom in crucible (b) Position from Top to Bottom in crucible
Fig. 3 a) the relation between the area ratio of foggy mark; b) the pore (b) count versus the samples prepared from different locations; from the top to the bottom of the Al-7%Si alloy melt. Fig. 4 a) the relation between the area ratio of foggy mark; b) the pore count versus the samples poured from different locations; from the top to the bottom of the A356.2 alloy melt. depending on the reaction or coalescence of the oxide and the fragmentation of the oxide film in the melt. in the degassing time. Subsequently, the finer bubbles In the present study we used a composite salt, 44% KCl coalesced and grew beneath the surface oxide layer. They and 56% NaCl. This covering salt decomposed and melted on moved toward the wall, finally protruding and exploding at the free surface of the melt during degassing. Figures 7(a)– cracks that existed in the covering flux, as shown in Fig. 8, or (d) show the illustrations of floating bubbles, bubble at the junction of the free surface and the crucible wall. In explosions and convection loops in the different melts, based addition, the Al-7%Si melt possessed a greater surface on experimental observations. Figures 7(a) and (b) schemati- tension than the A356.2 melt did. Thus the bubbles would cally illustrate the convection loop, oxide film movement and protrude more in the Al-7%Si melt than in the A356 melt, floating of bubble in the Al-7%Si alloy melt with and without generating more explosive droplets that would fall in the the covering flux. craters to be entrapped in the melt. The degassing treatment In the Al-7%Si alloy degassed with the covering flux, the trapped greater amounts of inclusion particles in the Al-7%Si bubbles exploded in the areas near the diffuser and the melt than in the A356 melt. The Al-7%Si alloy clearly had a crucible wall. The surface oxide that accumulated in the area higher pore count than the A356 alloy; see Figs. 3(b), 4(b) near the diffuser increased in thickness following an increase and 6(b). Effects of Degassing and Fluxing on the Quality of Al-7%Si and A356.2 Alloys 267
Degassing only Degassing and fluxing A356 alloy A356 alloy Sample No. (3)
(1) (4)
(2) (5)
(a) (b) (a) (b)
Fig. 5 Photographs showing the foggy marks revealed on the sections of the chilled A356.2 samples a) degassed only; b) degassed with the covering flux. Sample number corresponds to the locations for the sample poured from different levels of the melt in the crucible. The diameter of a chilled sample is 50 mm.
Oxide films in the melt can originate either from the ingot will float and move to the outer ring of the degassing bubble or from the reaction of oxide particles in the melt, or be cloud in the melt. They coalesce with floating and moving in generated from the bursting of bubbles during degassing. the melt. They finally explode at the junction of the free Such films move counter-clockwise in the melt, following the surface and the wall, creating craters which entrap oxide motion of convection loops, to be pushed up close to the inclusions in the melt. The bubbles burst and droplets fall on surface layer, where large particles and the oxide films will the craters to form oxide inclusions. These oxide particles be captured by the surface flux. After being degassed with a will then sink, following the motion of the melt (convection covering flux and skimming, the chilled samples will reveal a loops) and consequently became trapped as inclusion significant decrease in the foggy marked area. revealing pores in the chilled samples; see Figs. 7(a)–(d). The A356.2 melt with and without the addition of Sr In addition, Mg added to the A356.2 melt further decreases showed the same type of bubble explosions at the free surface its surface tension, which decreases the size of bubbles during degassing. In the experiments, we see that the A356.2 protruding from the free surface. Therefore, fewer droplets melt with the addition of Sr developed a heavy (or thick) and/or inclusion particles formed in the A356.2 melt than in covering flux near the diffuser. Consequently, the bubbles the Al-7%Si melt. After the degassing treatment and exploded in an area slightly away from the diffuser. Bubbles skimming process, the chilled A356.2 samples revealed that exploded at the free surface near the diffuser generated lesser amounts of pores than the chilled Al-7%Si samples droplets that fell on the covering flux. Some of the droplets did; compared Figs. 3(b), 4(b) and 6(b). An A356 alloy can that fell in the newly opened craters became oxide inclusions benefit from the addition of Sr but this decreases the surface and sank in the melt. These entrapped oxide inclusions were tension of the melt and produces Sr-based inclusions such as driven by convection loops toward the covering flux. They Al-Si-Sr and Al-Si-Sr-O type compounds.13,14) The A356 were captured by the covering flux and increased the flux alloy with Sr resulted in a thicker covering flux during layer’s thickness. This meant that the particles entrapped in degassing and produced a higher pore count than for the the melt and the pores shown in the chilled samples increased A356 alloy, due to the Sr-based inclusions or compounds; see slightly. According to Ref. 12, coarser degassing bubbles Figs. 4 and 6. 268 T.-S. Shih and K.-Y. Wen
With degassing treatment A356.2Sr (aveg:2.9%) Porous bar A356.2Sr (aveg:1.2%) Porous bar & Fluxing
8 7 6 5 4 (a) (b) 3 2 1 0 Area Ratio of Foggy Mark(%) Area Ratio of Foggy 134256 Top Bottom Position from Top to Bottom in crucible (a) With degassing treatment A356.2Sr (aveg:96pores . mm -2 ) Porous bar A356.2Sr (aveg:194pores . mm -2 ) Porous bar & Fluxing (c) (d)
Fig. 7 Schematic illustrations showing convection loop, trapped oxide 225 film, floating bubbles and surface layer in a) Al-7%Si alloy melt degassed only; b) Al-7%Si alloy melt degassed with the covering flux; c) A356.2 -2 200 melt degassed with the covering flux; d) A356.2 melt added with Sr and 175 mm
. degassed with the covering flux. 150 125 N/pores , 100 75
Pore Cracking 50 25 123456 Top Bottom Position from Top to Bottom in crucible (b)
Fig. 6 a) the relation between the area ratio of foggy mark; b) the pore count versus the samples poured from different locations; from the top to the bottom of the A356.2 added with Sr melt. Cracking
3.2 Variation of alloying elements Fig. 8 Photograph shows the covering flux, a porous bar diffuser and the In the experiments, chilled samples were poured from cracking surface layer for Al-7%Si alloy melt during degassing. different levels (or locations) in the melts. After grinding and polishing, the samples were removed for spectrometer testing. Each sample was tested three times and the average the melt. The high-density elements, such as Fe and Ti, were of three tests was obtained. Figures 9(a)–(c) respectively significantly scattered at different levels of the melt. The show the results for the three melts (degassed only). Data A356.2 samples, with and without Sr behaved differently. before the degassing treatment are also included for The former scattered more significantly than did the latter, comparison. These results indicate that the degassing treat- especially for Mg; see Figs. 9(a) and (b). The test results ment did not have any significant effect on changing the were affected by the distribution and the size of the entrapped alloying elements of samples taken from different levels of oxide film, the inclusion particles trapped in the matrix, and Effects of Degassing and Fluxing on the Quality of Al-7%Si and A356.2 Alloys 269
Degassing Degassing A356.2 Fluxing mean A356.2 mean 30 Si(%):7.43% Si(%):6.57% 30 25 Fe(%):0.062% Fe(%):0.074% 25 20 Mg(%):0.34% Mg(%):0.29% 15 20 Ti(%):0.124% Ti(%):0.141% 10 15 5 10 0 5 -5 -10 0 (a) Deviation of Alloy Concentration(%) of Alloy Deviation 01234 5 67 -5
Degassing A356.2Sr mean -10 (a) 30 Si(%):6.59% 02314567
25 Fe(%):0.068% Degassing A356.2Sr Fluxing mean Mg(%):0.32% 20 Si(%):7.13% Ti(%):0.146% 30 15 Fe(%):0.061% Sr:184ppm 25 10 Mg(%):0.32% 5 20 Ti(%):0.126% 0 15 -5 10 -10 5 0125367 4 (b) 0 Deviation of Alloy Concentration(%) of Alloy Deviation -5 -10 Degassing Al-7Si mean (b) 30 Si(%):6.56% Concentration(%) of Alloy Deviation 01234567 Concentration(%) of Alloy Deviation 25 Fe(%):0.105% Degassing Al-7Si Fluxing mean 20 Ti(%):0.008% 30 Si(%):7.67% 15 25 Fe(%):0.099% 10 20 Ti(%):0.007% 5 15 0 10 -5 5 -10 (c) 0 Deviation of Alloy Concentration(%) of Alloy Deviation 0123 4 5 67 -5 Top Bottom Position from Top to Bottom in crucible -10 (c)
Deviation of Alloy Concentration(%) of Alloy Deviation 02356147 Fig. 9 The relation between deviation of alloy concentration and the Top Bottom chilled sample poured from different levels of the melt a) A356.2; Position from Top to Bottom in crucible b) A356.2 with Sr addition; c) Al-7%Si alloy, degassed only. Fig. 10 The relation between deviation of alloy concentration and the chilled sample poured from different levels of the melt a) A356.2; the precipitates. The chilled samples solidified rapid at 4.5 K/ b) A356.2 with Sr addition; c) Al-7%Si alloy, degassed with the covering s, therefore the effect of precipitates could be reasonably flux. excluded from the comparison. Figures 10(a)–(c) illustrate the alloying elements meas- ured in samples prepared from different levels of the melt, matrix increased. The uniformity of the alloy elements had after being degassed with the covering flux. Surprisingly, the clearly improved, according to the measurement of samples alloying elements in the melts degassed with the covering from a given melt. flux were very close regardless of the level of the melt from which the samples were prepared. Note that Mg and Sr (from 3.3 Relative porosity and fatigue life 184 to less than 10 ppm), showed a great loss after the Figure 11 shows the rotational bending test data for degassing treatment. Reasonably, Sr would be consumed due samples prepared from permanent A356.2 alloy castings. to reaction with Cl to form a compound, and then became Each value is an average of two tests. Solid marks represent dross. During degassing this dross would be caught when it samples subjected to degassing and fluxing treatments, and moved near the surface layer. Magnesium was also affected the blank marks show degassing only. The enclosed numbers by Cl, and was partly consumed due to the bursting of the correspond to the permanent mold castings prepared from bubbles at the melt surface. If we compare Figs. 9 and 10 different levels of the melt. Number ‘‘1’’ represents castings with Figs. 3, 4 and 6, we see that the foggy marks apparently from the top of the melt. The solid line was taken from the decreased, and the number of fine particles trapped in the work of Sugiyama.15) Samples from two melts, degassed with 270 T.-S. Shih and K.-Y. Wen
Sample No. (heat A356.2 with low Sr) 1 2 3 4 5 flux 1 180 flux 2 flux 3 flux 4 160 flux 5
140 σ 120
1 mm 1 mm
100 run out 5 5 (a) 1.36 10 cycles (b) 2.08 10 cycles 80 Stress Amplitude, /MPa /MPa Stress Amplitude, 60 Sugiyama15) 40
1E+003 1E+004 1E+005 1E+006 1E+007
Number of Cycles to Failure, Nf
Fig. 11 The relation between stress amplitude and number of cycles to failure for a rotating bending test; A356.2 alloy samples degassed with or
without covering flux. 1 mm 1 mm
(c) 1.33 105 cycles (d) 2.75 105 cycles
Table 2 The calculated relative porosities for samples prepared from Fig. 12 Fracture surfaces of samples A356 (low Sr) degassed (a) and (b) different melts. without; (c) and (d) with using the covering flux; life cycles included for 5 5 5 Heat No.: A: A356.2 (degassing) comparison; (a) 1:36 10 and (b) 2:08 10 ; (c) 1:33 10 and (d) 2:75 105 cycles; subjecting to 100 MPa stress amplitude. B: A356.2 with high Sr (degassing) C: Al-7Si (degassing) D: A356.2 (degassing + fluxing) E: A356.2 with high Sr (degassing + fluxing) increases the relative porosity from 1.72% to 1.84%. F: Al-7Si (degassing + fluxing) Figure 11 indicates that at 100 MPa stress amplitude the 4 Sample NO.: From top to bottom A356 alloy samples (degassed only) showed 9:4 10 5 1 6: after degassing treatment coded as 1:95 10 life cycles, but those degassed with the covering 4 5 poured form top to bottom of the melt flux showed 9:7 10 1:4 10 life cycles; at 160 MPa (unit in percent) stress amplitude the A356 alloy samples (degassed only) show 4:5 103 1:8 104 life cycles, but those degassed Heat NO. 3 4 ABCDEFwith the covering flux showed 1:3 10 1:3 10 life Sample NO. cycles. Pores stimulate crack initiation and accelerate crack 1 1.75 1.74 2.15 1.79 1.94 0.82 propagation. Therefore an oxide film is more likely to 2 1.09 1.94 1.46 1.57 1.49 0.67 decrease the matrix’s resistance to crack propagation. Both 3 1.05 2.00 1.17 1.71 1.23 0.97 factors would lead to a deterioration in the life cycle of the 4 2.05 2.13 3.20 1.90 1.53 1.19 A356 alloy castings. Figures 12(a)–(d) show the surface 5 2.08 2.24 3.13 1.98 1.45 1.27 fractures of A356 samples degassed, without and with the 6 2.30 2.74 3.92 2.09 0.56 0.93 covering flux for samples subjected to a 100 MPa stress Mean 1.72 2.13 2.51 1.84 1.37 0.98 amplitude. Note that more wrinkles are shown on the The listed number is the percentage of the relative porosity. periphery of the A356 specimens degassed with the covering flux; Figs. 12(c) and (d). Samples degassed with the covering flux possessed a higher pore count and correspondingly had and without the covering flux, showed inferior fatigue life more potent sites for crack initiation than those that were cycles to those discussed in the reference work. Possible degassed only; Figs. 12(a) and (b). Increasing the pore count factors for this include differences in the heat treatment, the clearly stimulated crack initiation during the rotating bending Sr content and the pore count. In the current study, the as-cast tests and significantly decreases the fatigue life cycle. A356.2 melt had a very low Sr content (less than 10 ppm) but in the reference data their sample contained 70–80 ppm of Sr 4. Conclusions was subjected to a T6 treatment. Strontium would modify the silicon structure and toughen the matrix of the A356 alloy, For the Al-7%Si and A356.2 alloys studied, the melts leading to the longer fatigue life cycles. degassed with a covering flux showed a significant decrease Table 2 lists the calculated relative porosities of A356 in the area ratios of the foggy marks but a two-fold increase in alloy samples degassed with and without the covering the pore count compared to the melts that were degassed flux. Referring to Figs. 4(b), 11 and Table 2, we see that only. The Al-7%Si alloy possessed a pore count far greater increasing the pore count from 84 to 174 (pores mm 2) than did the A356 alloys, regardless of the Sr levels (as high Effects of Degassing and Fluxing on the Quality of Al-7%Si and A356.2 Alloys 271 as 184 ppm). For A356 alloys degassed with the covering 2) A. Engh and T. Pedersen: Light Metal, (TMS-AIME, 1984) 1329–1343. flux, the foggy marked area significantly decreased, from 3) S. V. Gnyloskurenko and T. Nakamura: Mater. Trans. 44 (2003) 2298– 2302. 5.8% to 0.7%, but the pore count greatly increased, from 84 4) D. V. Neff and P. V. Cooper: AFS Trans. 98 (1990) 579–584. 2 to 174 (pores mm ), and the relative porosity increased, 5) Y. J. Chen and T. S. Shih: Journal of the Chinese Society of Mechanical from 1.72% to 1.84% compared with the A356 alloy Engineers (ROC) 23 No. 1 (2002) 55–67. degassed only. The fatigue life cycles of A356 alloy castings 6) Y. J. Chen, L. W. Huang and T. S. Shih: Mater. 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