Materials Transactions, Vol. 46, No. 2 (2) pp. 263 to 271 #2 The Japan Institute of Metals Effects of and Fluxing on the Quality of Al-7%Si and A36.2 Alloys Teng-Shih Shih* 1 and Kon-Yia Wen* 2 Department of mechanical Engineering, National Central University, Chung-Li, Taiwan 324, 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 A36 alloy castings. (Received May 1, 24; Accepted December 6, 24) Keywords: degassing, fluxing, pore counts, relative porosity 1. Introduction A degassing treatment is commonly used in producing aluminum alloy castings. Sigworth 1) 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 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 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 techniques based only upon the relationship between the hydrogen content and the degassing time. Reasonably, reducing the hydrogen content will certainly improve the mechanical properties of the resultant aluminum alloys. The 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 Nakamura 3) 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 gas flow rate and/or reducing the nozzle diameter effectively decreased the bubble volume. Decreasing the bubble volume of course can improve the effectiveness of degassing. Increasing the gas flow rate introduces a strong convection in the melt, a matter which has been rarely addressed and should be of concern. During the melting of aluminum alloys, the inclusion of particles suspended in the melt can be effectively reduced by floatation and/or sedimentation. Oxide inclusions are mostly heavier than the base melt and so tend to fall to the bottom of the crucible. In addition, filtration can be used to remove inclusions, significantly improving the density of the poured castings. 4) Shih and coworkers developed an ultrasonic-vibration treatment which could reveal foggy marks on the polished * 1 Corresponding author, E-mail: T331@cc.ncu.edu.tw * 2 Graduate student, National Central University (c) (d) (e) (f) 1 mm Fig. 1 Foggy marks on the polished specimen surface; using ml of tap water and vibration treated for 3 s: aluminum ingot; A36 boat mold; (c) A36 tensile bar from a ASTM B18 mold; (e) A36 squeezed mold; (f) A36 aluminum wheel casting; (g) wrought 661 bar. 6) surface of chilled samples. 7) After this ultrasonic vibration treatment, the oxide film entrapped in the Al-Si-Mg alloys would be revealed as a foggy area caused by cavitation micro-jet impacts, as shown in Fig. 1. 6) Pores may contain oxide film, oxide particle(s), as shown in Fig. 2, or be free of particles. In this work, the effects of the degassing treatment on the foggy marked areas and the pore count of the Al-7%Si and A36 melts are evaluated. 8) The fatigue life cycles of aluminum alloy castings are indeed closely related to the pore count and the area of foggy marks on the chilled samples. The number of pores will increase mainly due to the degassed bubbles protruding and exploding at the free surface of the melt. Debris that falls in the craters is eventually entrapped in (g)
264 T.-S. Shih and K.-Y. Wen Fig. 2 SEM micrograph showing an oxide particle entrapped in the pore. the melt and the poured casting as well. Magnesium added to Al-Si alloys changes the surface tension of the melt, therefore the size of bubbles protruding from the free surface of the melt is different, producing variation in the amounts of debris and of pores entrapped in the castings. Fluxing is also used in the processing of the aluminum alloy casting. A proper fluxing procedure can improve the cleanliness of the melt by accelerating the separation of inclusions. 9) Ye and Sahai have discussed the conventional mechanisms for the removal of oxide film from an aluminum surface by molten salts. 1) They suggested that the oxide film is stripped as a result of the force induced by the interfacial tension gradient that exists between the liquid aluminum and the oxide film. Roy and Sahai have studied the coalescence behavior of aluminum alloy drops in molten salt. They explained that the removal of the oxide layer from the Al surface did not take place in equimolar NaCl-KCl. 11) Shih and Weng studied the effect of a degassing treatment on the quality of Al-7%Si and A36 melts using different degassing diffusers. 12) The degassing bubbles rose in the melt floating with convection loops during the degassing treatment. The effects of magnesium added to the A36 alloy on the pore count and the shape of the oxide film is very significant, compared with the Al-7%Si and A36 alloys. 12) Adding Mg reduces the surface tension of the A36 alloy melt, producing few splashed droplets after the explosion of bubbles at the free surface during degassing, which results in a lower pore count than in the Al-7%Si alloy. In the Al-7Si alloy, the oxide, if formed, tends to form mullite and in the A36 alloy, the oxide is more likely to form pyrope. The formation of different oxides had an influence on the shape of the oxide film and the foggy marked area of the Al-7%Si and A36 alloys. This study extends the previous work and assesses the effect of a degassing treatment with/without the covering flux, on the foggy marked areas (oxide film or lumps) and the pore counts of the Al-7%Si and the A36.2 melts. The effects of the foggy marked area and/or pores on the fatigue properties of the resultant A36 alloy castings are also assessed. 2. Experimental Procedure In this study, an induction furnace equipped with an SiCgraphite-clay crucible was used to melt 2 kg batches of an aluminum alloy (3 Hz, 1 kw). A master alloy Al- %Si alloy was added to the pure Al melt in order to raise the silicon content to the desired levels. High quality A36.2 ingots, with and without Sr, were melted to obtain chilled samples, reduced pressure samples and permanent mold castings, JIS H22. After melting, all the melts, with or without a covering flux, were held at 93 K and degassed by nitrogen (1 L/min) via a porous bar diffuser. A covering flux, comprised of NaCl (6%) and KCl (44%), was used with a melting point of about 983 K. 8) After being completely dried at 673 K for 36 s, the flux mixture was carefully spread over the surface of the melt. A 2 kg batch of the melt required 1 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 6 s. After the degassing treatment, the melt was held for 6 s, and then poured at 133 K. A transfer ladle made of ceramic fiber was used to pour the samples. A single batch of melt could produce five to six groups of samples. Each group was coded in a series that indicated its corresponding location in the melt, from top to bottom. In other words, the samples from the first spoon came from the top level of the melt and those from the last spoon from the bottom level of the melt. Each group of samples included two chilled samples ( mm in diameter and 1 mm thick), one reduced pressure sample, and about four permanent mold castings. After polishing, the chilled samples were analyzed via spectrometer testing; see Table 1. Each sample was tested three times and the average was recorded. After the spectrometer tests, the chilled samples were polished again. The pore count was measured via an optical microscope equipped with an image analyzer (magnification 1X). Each pore might or might not contain particle(s) and in some cases several particles could be trapped in one pore. Each sample was measured ten times and the average of the total was the pore count. After measuring the pore count, the chilled samples were placed in an ultrasonic cleaner filled with ml of tap water. The samples were then ultrasonically treated for 18 s. Differently shaped foggy marks were revealed on the surfaces of the chilled samples. The morphology and the areas of the foggy marks were recorded. The foggy marked areas on two sections of each sample were measured and the average counted as the area ratio. Water simulation was adopted to observe the floating and movement of the bubbles. In the experiment, a high-speed camera, capable of taking 1 pictures per second, was used to take photographs of floating bubbles and their movements. The reduced pressure sample was sectioned into two pieces. One piece was polished to reveal the morphology and distribution of the pores and the gas holes. The density of the other piece was measured by the Archimedes method d r. The density d c of the chilled sample was also measured. The relative porosity of each set of samples was computed based on the density differences between the chilled and the reduced pressure samples, RP ¼ðd c d r Þ=d c.
Effects of and Fluxing on the Quality of Al-7%Si and A36.2 Alloys 26 Table 1 The chemical compositions of the alloys studied; including Al-7%Si, A36.2 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 A36.2 (degassing) 6.7.34 11.62.124 Bal. B A36.2 high Sr (degassing) 6.9.32 184.68.146 Bal. C Al-7%Si (degassing) 6.6.1.8 Bal. D A36.2 (degassing 7.43.29.74.141 Bal. and fluxing) E A36.2 high Sr (degassing 7.67.32.61.126 Bal. and fluxing) F Al-7%Si (degassing and fluxing) 7.13.99.7 Bal. > degassed using a porous bar fluxing by NaCl + KCl ( g/1 kg of melt) content is less than 1 ppm per at least three tests. 3. Experimental Results 3.1 Foggy marks and pore counts For an Al-7%Si melt, degassed with or without a covering flux, the chilled samples showed quite different results in terms of the foggy areas and the pore count. Figures 3 and show that, for the melt degassed with a covering flux, the ratio of the foggy marked area was reduced, from 3.7 to 1.3% (the average of samples from one melt), but the pore count had obviously increased, from 187 to 313 (poresmm 2 ), compared to samples prepared from the degassed melt. Figure 3 displays the measured ratios of the foggy marked areas. The ratios are high and significantly scattered. A pore, if it exists, is usually accompanied by an inclusion particle (or particles). Thus the measured pore count will surely be affected by any particles trapped in the melt and consequently in the chilled samples. Figure 3 illustrates that in the samples from the melt degassed with the covering flux the pores were relatively fine. Fine particles will float during the holding period after degassing, and likely be distributed in the top of the melt. If the melt was degassed without the covering flux, the particles were coarse and sank to the bottom of the melt; see Fig. 3. Figures 4 and show the measured ratios of foggy marked areas and the pore counts for the A36.2 melt, degassed both with and without the covering flux. The melt degassed with the covering flux showed decreased ratios of foggy marked areas, from.8 to.7%, but an increased pore count, from 84 to 174 (poresmm 2 ), compared to the melt that was degassed only. Again, the use of a covering flux significantly increased the pore count but decreased the foggy marks in the A36 alloy, as in the Al-7%Si alloy. During the degassing treatment, the oxide film and the chunky inclusion particles will move counter-clockwise, following the motion of convection loops in the melt. 12) Any oxide films or chunky inclusion particles entrapped in the melt will be pushed upward to an area near the top of the melt during degassing, to be caught by the surface flux and become dross. After skimming, most of the oxide films and chunky inclusion particles can therefore be removed. The pictures in Fig. show the evidence displaying the cleanliness of the chilled samples prepared from the A36.2 melt degassed with (right) and without the covering flux (left). Using a covering flux can significantly decrease the foggy marks (oxide film/chunky inclusion particles). In the A36 melt, without the covering flux, the foggy marks included mostly lumps, some strips and spots; see left-hand column in Fig.. In the A36 melt, using the covering flux, fewer strips or fine spots remained on the chilled sample; see right-hand column in Fig.. Figures 6 and show the ratios of the foggy marked area, and the pore count on the A36.2 sample with Sr, degassed with and without the covering flux. Using the covering flux decreased the foggy marked area from 2.9 to 1.2%, but increased the pore count from 96 to 194 (poresmm 2 ). The two A36.2 melts, with and without Sr, had similar results that are an increase of pore count and a decrease in the foggy marked area. The experimental observations show that Sr added to the A36.2 alloy slightly increases the number of foggy spots but reduces their size. The pore counts increased slightly probably due to the entrapment of Sr-based compounds or inclusions. The oxide films in the Al-7%Si melt contained a high fraction of mullite while those in the A36.2 melt were rich in pyrope, Mg 3 Al 2 Si 3 O 12. Mullite is lighter than pyrope, 2:8 1 3 vs. 3:6 1 3 kg/m 3, so the heavy oxide film or particles tended to sink to the bottom of the A36.2 melt; compare Figs. 3 and 4 to 6. Convection loops persisted throughout the whole degassing period. The primary Al 2 O 3 oxide film in the melt gradually reacted with the oxide particles, SiO 2, MgO etc., or the alloying elements, Si, Mg and oxygen, to form mullite in the Al-7%Si melt and pyrope in the A36.2 melt. The resultant oxides could therefore be lumpy, feather-like, strips or chunky spots,
266 T.-S. Shih and K.-Y. Wen Pore, N/pores. mm -2 Area Ratio of Foggy Mark(%) 7 6 4 3 2 1 4 37 3 32 3 27 2 22 2 17 1 12 1 Al-7Si(aveg:3.7%) Porous bar Al-7Si(aveg:1.3%) Porous bar & Fluxing 1 2 3 4 6 Position from to in crucible Al-7Si(aveg:187pores. mm -2 ) Porous bar Al-7Si(aveg:313pores. mm -2 ) Porous bar & Fluxing 1 2 3 4 6 Position from to in crucible Fig. 3 a) the relation between the area ratio of foggy mark; b) the pore count versus the samples prepared from different locations; from the top to the bottom of the Al-7%Si alloy melt. Pore, N/pores. mm -2 Area Ratio of Foggy Mark(%) A36.2 Low Sr (aveg:.8%)porous bar A36.2 Low Sr (aveg:.7%)porous bar & Fluxing 14 13 12 11 1 9 8 7 6 4 3 2 1 2 22 2 17 1 12 1 7 2 1 2 3 4 6 Position from to in crucible A36.2 Low Sr (aveg:84pores. mm -2 )Porous bar A36.2 Low Sr (aveg:174pores. mm -2 ) Porous bar & Fluxing 1 2 3 4 6 Position from to in crucible 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 A36.2 alloy melt. depending on the reaction or coalescence of the oxide and the fragmentation of the oxide film in the melt. In the present study we used a composite salt, 44% KCl and 6% NaCl. This covering salt decomposed and melted on the free surface of the melt during degassing. Figures 7 (d) show the illustrations of floating bubbles, bubble explosions and convection loops in the different melts, based on experimental observations. Figures 7 and schematically illustrate the convection loop, oxide film movement and floating of bubble in the Al-7%Si alloy melt with and without the covering flux. In the Al-7%Si alloy degassed with the covering flux, the bubbles exploded in the areas near the diffuser and the crucible wall. The surface oxide that accumulated in the area near the diffuser increased in thickness following an increase in the degassing time. Subsequently, the finer bubbles coalesced and grew beneath the surface oxide layer. They moved toward the wall, finally protruding and exploding at cracks that existed in the covering flux, as shown in Fig. 8, or at the junction of the free surface and the crucible wall. In addition, the Al-7%Si melt possessed a greater surface tension than the A36.2 melt did. Thus the bubbles would protrude more in the Al-7%Si melt than in the A36 melt, generating more explosive droplets that would fall in the craters to be entrapped in the melt. The degassing treatment trapped greater amounts of inclusion particles in the Al-7%Si melt than in the A36 melt. The Al-7%Si alloy clearly had a higher pore count than the A36 alloy; see Figs. 3, 4 and 6.
Effects of and Fluxing on the Quality of Al-7%Si and A36.2 Alloys 267 only A36 alloy and fluxing A36 alloy Sample No. (3) (1) (4) (2) () Fig. Photographs showing the foggy marks revealed on the sections of the chilled A36.2 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 mm. Oxide films in the melt can originate either from the ingot or from the reaction of oxide particles in the melt, or be generated from the bursting of bubbles during degassing. Such films move counter-clockwise in the melt, following the motion of convection loops, to be pushed up close to the surface layer, where large particles and the oxide films will be captured by the surface flux. After being degassed with a covering flux and skimming, the chilled samples will reveal a significant decrease in the foggy marked area. The A36.2 melt with and without the addition of Sr showed the same type of bubble explosions at the free surface during degassing. In the experiments, we see that the A36.2 melt with the addition of Sr developed a heavy (or thick) covering flux near the diffuser. Consequently, the bubbles exploded in an area slightly away from the diffuser. Bubbles that exploded at the free surface near the diffuser generated droplets that fell on the covering flux. Some of the droplets that fell in the newly opened craters became oxide inclusions and sank in the melt. These entrapped oxide inclusions were driven by convection loops toward the covering flux. They were captured by the covering flux and increased the flux layer s thickness. This meant that the particles entrapped in the melt and the pores shown in the chilled samples increased slightly. According to Ref. 12, coarser degassing bubbles will float and move to the outer ring of the degassing bubble cloud in the melt. They coalesce with floating and moving in the melt. They finally explode at the junction of the free surface and the wall, creating craters which entrap oxide inclusions in the melt. The bubbles burst and droplets fall on the craters to form oxide inclusions. These oxide particles will then sink, following the motion of the melt (convection loops) and consequently became trapped as inclusion revealing pores in the chilled samples; see Figs. 7 (d). In addition, Mg added to the A36.2 melt further decreases its surface tension, which decreases the size of bubbles protruding from the free surface. Therefore, fewer droplets and/or inclusion particles formed in the A36.2 melt than in the Al-7%Si melt. After the degassing treatment and skimming process, the chilled A36.2 samples revealed lesser amounts of pores than the chilled Al-7%Si samples did; compared Figs. 3, 4 and 6. An A36 alloy can benefit from the addition of Sr but this decreases the surface tension of the melt and produces Sr-based inclusions such as Al-Si-Sr and Al-Si-Sr-O type compounds. 13,14) The A36 alloy with Sr resulted in a thicker covering flux during degassing and produced a higher pore count than for the A36 alloy, due to the Sr-based inclusions or compounds; see Figs. 4 and 6.
268 T.-S. Shih and K.-Y. Wen A36.2Sr (aveg:2.9%) Porous bar A36.2Sr (aveg:1.2%) Porous bar & Fluxing 8 7 6 Area Ratio of Foggy Mark(%) 4 3 2 1 1 2 3 4 6 Position from to in crucible A36.2Sr (aveg:96pores. mm -2 ) Porous bar A36.2Sr (aveg:194pores. mm -2 ) Porous bar & Fluxing (c) (d) Pore, N/pores. mm -2 22 2 17 1 12 1 7 2 1 2 3 4 6 Position from to in crucible Fig. 7 Schematic illustrations showing convection loop, trapped oxide 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) A36.2 melt degassed with the covering flux; d) A36.2 melt added with Sr and degassed with the covering flux. Cracking 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 A36.2 added with Sr melt. Cracking 3.2 Variation of alloying elements In the experiments, chilled samples were poured from 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 of three tests was obtained. Figures 9 (c) respectively show the results for the three melts (degassed only). Data before the degassing treatment are also included for comparison. These results indicate that the degassing treatment did not have any significant effect on changing the alloying elements of samples taken from different levels of Fig. 8 Photograph shows the covering flux, a porous bar diffuser and the cracking surface layer for Al-7%Si alloy melt during degassing. the melt. The high-density elements, such as Fe and Ti, were significantly scattered at different levels of the melt. The A36.2 samples, with and without Sr behaved differently. The former scattered more significantly than did the latter, especially for Mg; see Figs. 9 and. The test results were affected by the distribution and the size of the entrapped oxide film, the inclusion particles trapped in the matrix, and
Effects of and Fluxing on the Quality of Al-7%Si and A36.2 Alloys 269 Deviation of Alloy Concentration(%) Deviation of Alloy Concentration(%) Deviation of Alloy Concentration(%) 3 2 2 1 1 - -1 3 2 2 1 1 - -1 3 2 2 1 1-1 2 3 4 6 7 1 2 3 4 6 7 the precipitates. The chilled samples solidified rapid at 4. K/ s, therefore the effect of precipitates could be reasonably excluded from the comparison. Figures 1 (c) illustrate the alloying elements measured in samples prepared from different levels of the melt, after being degassed with the covering flux. Surprisingly, the alloying elements in the melts degassed with the covering flux were very close regardless of the level of the melt from which the samples were prepared. Note that Mg and Sr (from 184 to less than 1 ppm), showed a great loss after the degassing treatment. Reasonably, Sr would be consumed due to reaction with Cl to form a compound, and then became dross. During degassing this dross would be caught when it moved near the surface layer. Magnesium was also affected by Cl, and was partly consumed due to the bursting of the bubbles at the melt surface. If we compare Figs. 9 and 1 with Figs. 3, 4 and 6, we see that the foggy marks apparently decreased, and the number of fine particles trapped in the A36.2Sr Al-7Si -1 (c) 1 2 3 4 6 7 Position from to in crucible A36.2 mean Si(%):6.7% Fe(%):.62% Mg(%):.34% Ti(%):.124% mean Si(%):6.9% Fe(%):.68% Mg(%):.32% Ti(%):.146% Sr:184ppm mean Si(%):6.6% Fe(%):.1% Ti(%):.8% Fig. 9 The relation between deviation of alloy concentration and the chilled sample poured from different levels of the melt a) A36.2; b) A36.2 with Sr addition; c) Al-7%Si alloy, degassed only. Deviation of Alloy Concentration(%) Deviation of Alloy Concentration(%) Deviation of Alloy Concentration(%) 3 2 2 1 1 - -1 3 2 2 1 1 - -1 3 2 2 1 1 - -1 1 2 3 4 6 7 1 2 3 4 6 7 1 2 3 4 6 7 Position from to in crucible A36.2 Fluxing mean Si(%):7.43% Fe(%):.74% Mg(%):.29% Ti(%):.141% matrix increased. The uniformity of the alloy elements had clearly improved, according to the measurement of samples from a given melt. 3.3 Relative porosity and fatigue life Figure 11 shows the rotational bending test data for samples prepared from permanent A36.2 alloy castings. Each value is an average of two tests. Solid marks represent samples subjected to degassing and fluxing treatments, and the blank marks show degassing only. The enclosed numbers correspond to the permanent mold castings prepared from different levels of the melt. Number 1 represents castings from the top of the melt. The solid line was taken from the work of Sugiyama. 1) Samples from two melts, degassed with A36.2Sr Fluxing mean Si(%):7.13% Fe(%):.61% Mg(%):.32% Ti(%):.126% Al-7Si Fluxing mean Si(%):7.67% (c) Fe(%):.99% Ti(%):.7% Fig. 1 The relation between deviation of alloy concentration and the chilled sample poured from different levels of the melt a) A36.2; b) A36.2 with Sr addition; c) Al-7%Si alloy, degassed with the covering flux.
27 T.-S. Shih and K.-Y. Wen Stress Amplitude, σ /MPa 18 16 14 12 1 8 6 4 Sample No. (heat A36.2 with low Sr) 1 2 3 4 flux 1 flux 2 flux 3 flux 4 flux Sugiyama 1) run out 1 mm 1 mm 1.36 1 cycles 2.8 1 cycles 1E+3 1E+4 1E+ 1E+6 1E+7 Number of Cycles to Failure, Nf Fig. 11 The relation between stress amplitude and number of cycles to failure for a rotating bending test; A36.2 alloy samples degassed with or without covering flux. 1 mm (c) 1.33 1 cycles (d) 2.7 1 cycles 1 mm Table 2 The calculated relative porosities for samples prepared from different melts. Heat No.: A: A36.2 (degassing) B: A36.2 with high Sr (degassing) C: Al-7Si (degassing) D: A36.2 (degassing + fluxing) E: A36.2 with high Sr (degassing + fluxing) F: Al-7Si (degassing + fluxing) Sample NO.: From top to bottom 16: after degassing treatment coded as poured form top to bottom of the melt (unit in percent) Heat NO. Sample NO. A B C D E F 1 1.7 1.74 2.1 1.79 1.94.82 2 1.9 1.94 1.46 1.7 1.49.67 3 1. 2. 1.17 1.71 1.23.97 4 2. 2.13 3.2 1.9 1.3 1.19 2.8 2.24 3.13 1.98 1.4 1.27 6 2.3 2.74 3.92 2.9.6.93 Mean 1.72 2.13 2.1 1.84 1.37.98 The listed number is the percentage of the relative porosity. and without the covering flux, showed inferior fatigue life cycles to those discussed in the reference work. Possible factors for this include differences in the heat treatment, the Sr content and the pore count. In the current study, the as-cast A36.2 melt had a very low Sr content (less than 1 ppm) but in the reference data their sample contained 7 8 ppm of Sr was subjected to a T6 treatment. Strontium would modify the silicon structure and toughen the matrix of the A36 alloy, leading to the longer fatigue life cycles. Table 2 lists the calculated relative porosities of A36 alloy samples degassed with and without the covering flux. Referring to Figs. 4, 11 and Table 2, we see that increasing the pore count from 84 to 174 (poresmm 2 ) Fig. 12 Fracture surfaces of samples A36 (low Sr) degassed and without; (c) and (d) with using the covering flux; life cycles included for comparison; 1:36 1 and 2:8 1 ; (c) 1:33 1 and (d) 2:7 1 cycles; subjecting to 1 MPa stress amplitude. increases the relative porosity from 1.72% to 1.84%. Figure 11 indicates that at 1 MPa stress amplitude the A36 alloy samples (degassed only) showed 9:4 1 4 1:9 1 life cycles, but those degassed with the covering flux showed 9:7 1 4 1:4 1 life cycles; at 16 MPa stress amplitude the A36 alloy samples (degassed only) show 4: 1 3 1:8 1 4 life cycles, but those degassed with the covering flux showed 1:3 1 3 1:3 1 4 life cycles. Pores stimulate crack initiation and accelerate crack propagation. Therefore an oxide film is more likely to decrease the matrix s resistance to crack propagation. Both factors would lead to a deterioration in the life cycle of the A36 alloy castings. Figures 12 (d) show the surface fractures of A36 samples degassed, without and with the covering flux for samples subjected to a 1 MPa stress amplitude. Note that more wrinkles are shown on the periphery of the A36 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 more potent sites for crack initiation than those that were degassed only; Figs. 12 and. Increasing the pore count clearly stimulated crack initiation during the rotating bending tests and significantly decreases the fatigue life cycle. 4. Conclusions For the Al-7%Si and A36.2 alloys studied, the melts degassed with a covering flux showed a significant decrease in the area ratios of the foggy marks but a two-fold increase in the pore count compared to the melts that were degassed only. The Al-7%Si alloy possessed a pore count far greater than did the A36 alloys, regardless of the Sr levels (as high
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