A Mechanism of Porosity Distribution in A356 Aluminum Alloy Castings

Transcription

1 Materials Transactions, Vol. 43, No. 7 (2002) pp to 1715 c 2002 The Japan Institute of Metals A Mechanism of Porosity Distribution in A356 Aluminum Alloy Castings Kun-Dar Li and Edward Chang Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, R.O. China The problem of porosity and shrinkage defects in metal casting is complex. Over the years there have been debates on the mechanisms responsible for their formation. In this study A356 aluminum alloy with different hydrogen contents in the melts were cast in a permanent mold and the porosity content and thermal parameters were measured. A simple mechanism was proposed to explain the porosity distribution in the casting. In the mechanism both the roles of interdendritic feeding resistance by Darcy s law and kinetic hydrogen diffusion into the pore are involved. By differentiating one factor in the model, and comparing the prediction with the experimental data, the study suggests that both factors should be taken into consideration to satisfactorily explain the porosity distribution in the castings. (Received March 22, 2002; Accepted May 16, 2002) Keywords: aluminum, porosity, Darcy s law, hydrogen, thermal parameters 1. Introduction The problem of porosity and shrinkage defects in metal casting is complex. Despite many efforts to elucidate the problem, the mechanisms of the phenomenon remain unsolved. 1 8) Some researchers believe the problem is caused by the resistance to the interdendritic liquid flowing from riser to feed the solidifying liquid at the advancing liquid/solid interface. This resistance causes a local pressure drop of the liquid by Darcy s law. 9) One advocator of the theory was Lecomte- Beckers 10) who, like others, 11 13) described the magnitude of mushy zone pressure drop index ( P) in the interdendritic liquid in directional solidification or equiaxed dendritic casting by the following expression: P = 24πµβ ˆnτ 3 ( ) T 2 ( ) dfs (1) ρ l g G dt where µ is the viscosity of liquid, β = β/(1 β) (β is the solidification contraction), ˆn is the number of interdendritic channels, τ is the tortuosity, ρ l is the density of liquid, g is the acceleration of gravity, T is the solidification range, G is the thermal gradient, and df s /dt is the average solidification rate ( f s is the solid fraction and t is the time). The pressure drop index based on Darcy s law within the interdendritic liquid was employed to estimate the formation of porosity in aluminum alloys 11, 13) and nickel-base superalloy castings. 10) Equation (1) suggests that internal soundness in a casting is favored by a short solidification range; a low dendrite number density, tortuosity and solidification rate; and a high fluidity and thermal gradient. For some directional solidified castings, however, Darcy s law does not appear to be a controlling factor in porosity formation ) The pressure drop associated with liquid flow is much smaller than the prevailing atmospheric pressure and could not be responsible for the shrinkage porosity. Poirier et al. 17) ignored the effect of interdendritic fluid flow on porosity. Formation of porosity in Al Cu alloys was treated thermodynamically by analyzing the equilibrium partition of hydrogen content at the liquid/solid interface during solidifica- Corresponding author: address: edchanghs@yahoo.com tion using the equilibrium lever rule: 18) [H l ]=[H 0 ]/[ f l + (1 f l )k H ] (2) where [H l ] is the hydrogen content in the liquid, [H 0 ] is the initial hydrogen content in liquid (ml/100 g Al), f l is the weight fraction of liquid, and k H is the equilibrium partition ratio of hydrogen between solid and liquid. In eq. (2) a complete diffusion of hydrogen from liquid into the gas pores has been assumed. The predicted porosity content by thermodynamics was considered too high since the time involved in diffusion of hydrogen to the pores was neglected. 14) The purpose of the paper is to present the important factor of time in the redistribution of hydrogen during the solidification of aluminum alloys for the mechanism of porosity formation. The mechanism intends to explain the porosity distribution in A356 aluminum alloy castings as affected by hydrogen contents. 2. Experimental Procedure Figure 1 shows the JIS SKD61 permanent mold for casting a φ25 mm L150 mm sample with φ50 mm L50 mm riser. The materials of A356 ingot with mass% Sr were melted in a graphite crucible placed inside an electric resistance furnace. For varying the hydrogen content, the melt was either untreated, degassed with moisture-free Ar, or using wet A356 alloy for the melt. The hydrogen contents in the melts were controlled at 0.10, 0.15, and 0.20 ± ml/100 g Al, detected by an Alscan hydrogen quantitative analyzer. The melts were poured at 720 C. Thermal measurements were used to obtain the thermal variables, e.g. thermal gradient (G), solidus velocity (V s ) and local solidification time (t f ), in the casting. The porosity content was calculated in accordance with the formula (D s D)/D s 100% where D s is the theoretical density of the standard specimen and D is the density of the test pieces measured by the Archimedes method. 3. Experimental Results The porosity distributions in A356 castings vary with hydrogen contents and locations in the castings (Fig. 2). The

2 1712 K.-D. Li and E. Chang Fig. 3 Variation of local solidification time, thermal gradient and solidus velocity with distance from the free end of the casting. Fig. 1 Schematic illustration of the experimental setup. time will be available for the liquid metal to feed a solidifying site. 22, 23) Both are adverse effects. It appears that the correlation between porosity content (Fig. 2) and thermal parameters (Fig. 3) along the casting length is not satisfactory. 4. Discussion Fig. 2 Variation of porosity content with distance from the free end of the castings with different hydrogen contents. porosity content is lowest near the free end, and it gradually increases to a maximum near the mid-length of the casting. Further in the direction toward riser, the porosity content lowers slightly and then increases again gradually. The general 19, 20) trend of the results has been agreed by other studies. The variation of thermal parameters with location in the castings is exhibited in Fig. 3. The figure shows the local solidification time increases toward the riser, which indicates prevailing of directional solidification. The thermal gradient is highest at the free end and it reduces gradually toward the riser. The solidus velocity, as also indicated in the figure, increases slightly and then decreases toward the riser. A lower thermal gradient will result in a longer interdendritic feeding length, 21) while a high solidus velocity means that less In this study, we propose that the volume content of porosity (V p (%)) at a location in the casting can be calculated in accordance with the ideal gas law: V p (%) = K [H p ]/P g (3) where K is a proportional constant in vol% atm/ml/100 g Al and [H p ] is the hydrogen content (ml/100 g Al) diffused into the pore at a certain location in the casting. When pore forms in the interdendrites, the gas pressure in the pore (P g, atm) can be represented as: 24) P g = P 0 + ρ l gh + 4σ C tf πτ 3 µβ ( T V ) ( ) s α ln (4) C 2 G f l (x) where P 0 is the atmospheric pressure, ρ l gh is the hydrostatic pressure of riser, σ is the surface tension of interdendritic liquid, C = 35.1 µm/min 0.286, 24) T = e 0.244t f C, 24) α is the liquid volume fraction at the end of mass feeding, f l (x) is the liquid volume fraction, and x is the distance from the liquid/solid interface. 13) The third term on the right side of eq. (4) is the surface tension effect of the liquid to overcome when a pore forms, and the fourth term on the right is the pressure drop from the interdendritic fluid flow based on Darcy s law. It has been observed and recognized that pores form in a solidifying alloy by the process of nucleation and growth. 14, 15) We further assume that this kinetic process is one of the more general phase transformations in materials. 25) At initiation of solidification, i.e. time is small, both the numbers of pore and the pore size are small, and hence the kinetics of reaction should be slow. 25) It is well known that as time increases the number of pore and pore size increases, which warrants an

3 A Mechanism of Porosity Distribution in A356 Aluminum Alloy Castings 1713 increasing kinetic reaction. If very sufficient time is allowed, all the hydrogen content in liquid except the solubility of hydrogen in solid (Cs 0 ) should be diffused into the pore thermodynamically. That is the supersaturated dissolved hydrogen in the melt becomes exhausted as mentioned by Poirier et al. 17) Under the limiting condition, hence [H p ] will be equal to ([H 0 ] Cs 0)ast f approaches infinity. For otherwise the time of diffusion is limited, the fraction of hydrogen ( f ), defined as [H p ]/([H 0 ] Cs 0 ), that can be diffused into the pore should be determined mathematically as: [ ( ) [H p ] n ] f = ([H 0 ] Cs 0) = 1 exp tf (5) t τ The concept and mathematics of the above formulation, though simple, is consistent with the rigorous Avrami s equation. 26) The argument is on the basis that the kinetics of nucleation and growth of the pore in the solidifying aluminum alloy is merely another kinetic phase precipitation in alloys and materials. 27) Where in eq. (5) n is an exponential constant, t f and t τ are the local solidification time and the relaxation time, respectively. n and t τ are associated with the characteristics of nucleation and growth of pore. The relaxation time (t τ ) is a measure of how fast the hydrogen diffuses into the pore; when t f = t τ, the hydrogen should transport a quantity equal to (1 1/e) of ([H 0 ] Cs 0), or 0.63 ([H 0] Cs 0 ). Larger values of t τ would mean a slow kinetic of hydrogen diffusion into the pore. If circumstances allow a long period of diffusion, the kinetics of the hydrogen precipitation would level off due to the exhaustion of hydrogen being diffused into the gas pore, and the hydrogen content inside the pore ([H p ]) should approachs ([H 0 ] Cs 0). Association of eq. (5) with eq. (3), the volume percent of porosity can be described as: [ ( ) V p (%)P n ] g K ([H 0 ] Cs 0) = 1 exp tf (6) t τ The physical properties of A356 alloy are required in evaluation of eq. (6). Emadi et al. studied the effect of Sr modification on surface tension of A356 alloy, and found the addition of Sr to A356 alloy decreased the surface tension of the liquid by about 19 pct to 0.64 N/m. 28) The viscosity of A356 melt at eutectic temperature (565 C) is regressed as kgm 1 s 1 (Fig. 4) from the data of Ref. 29). The solubility of hydrogen in the solid (Cs 0)was proposed as ml/100 g Al, 24) which is in general agreement with and ml/100 g Al by other studies. 17, 30) In addition, K in eq. (3) is a regression constant about 12 vol% atm/ml/100 g Al. 24) For simplicity, the term on the left of eq. (6) is defined as an index of porosity fraction (F). The relationship between the index of porosity fraction (F) and the local solidification time is shown in Fig. 5. From the figure, it is obvious that different initial hydrogen contents show the different kinetic rates of nucleation and growth of pore, but all of them appear to follow the Avrami s equation, which might serve to indirectly support the derivation. In eq. (6), the quantities of n and t τ can be determined semiempirically from the fitting curve of the plot of ln( ln(1 F)) vs. ln t f as shown in Fig. 6. According to the result, the values of n are regressed to 0.56 for three different hy- Fig. 4 The viscosity of A356 alloy as a function of temperature. Data from Ref. 29). Fig. 5 Relation between the index of porosity fraction (F) and local solidification time. drogen contents, while the quantities of t τ are 11.42, and s for the hydrogen contents at 0.20, 0.15 and 0.10 ml/100 g Al, respectively. The results imply that the kinetics of nucleation and growth of pore formation occurs more rapidly in the case of the melt containing more dissolved gas, which corresponds to a small relaxation time. This finding has been supported by Fang and Granger 31) and Sigworth and Wang. 14) In eq. (4), it is shown that P g varies with the location in a casting, the range of which being atm in the study. If taking an average of P g = 1.43 into eq. (6), then V p (%) can be depicted as a function of t f. The plot of the predicted variation of the volume percent of porosity content with the local solidification time at different hydrogen contents per eq. (6) with the averaged P g, together with the experimental data, are illustrated in Fig. 7. From the figure, the kinetic model appears to fit the experimental results except the deviations of porosity contents between prediction and measurements corresponding to the local solidification time of about 60 s. The higher experimental data of poros-

4 1714 K.-D. Li and E. Chang Fig. 7 The measured (symbols) and predicted (curves) porosity content as a function of local solidification time in A356 alloy castings with different hydrogen contents. In drawing the predicted curves of porosity content, an average of P g = 1.43 in everywhere of the castings was assumed. Fig. 6 Relation between ln( ln(1 F)) and ln t f in A356 alloy castings with different hydrogen contents: (a) 0.2 ml/100 g Al, (b) 0.15 ml/100 g Al, and (c) 0.1 ml/100 g Al. ity content than the prediction arises because P g at the above location of the castings should have been about 1.35 atm instead of 1.43 atm. An examination of Fig. 3 will find that the location that solidifies at about 60 s in the castings corresponds to a maximum solidus velocity. Thus the difficulties of interdendritic fluid feeding caused by this thermal parameter appears to result in the interdendritic fluid pressure drop based on Darcy s law as well as the maxima as shown in Fig. 2. In the predicted curves of Fig. 7 the variation of P g has been purposely neglected to differentiate the sole effect of dissolved gas redistribution without the interference of the factor of interdendritic feeding resistance. The aim was to elucidate a complex long debates among the two schools of thinking in this field of researches in the last 50 years: 14) which factors of the interdendritic feeding resistance by Darcy s law 9) or the solute redistribution that are responsible for the shrinkage porosity. 17) For the advocators who favor the role of hydrogen redistribution, the mechanism of which is unsloved. 17) Thermodynamic model predicted a too high porosity content, since the kinetics of diffusion of dissolved gas has been neglected. 14) Our present mechanism involving the roles of interdendritic feeding and kinetics of hydrogen redistribution appears to support that both mechanisms are exercising. If allowing variation of P g per (4) to vary in eq. (6), the predicted volume percent of porosity content along the length of the castings with different hydrogen contents, together with the experimental data, are displayed in Fig. 8. In this figure, both of the interdendritic feeding resistance and the kinetics of hydrogen redistribution are taken into consideration to describe the mechanism of porosity formation. It appears that a satisfactory quantitative porosity content at any location of the castings with different hydrogen contents can be predicted by the proposed model.

5 A Mechanism of Porosity Distribution in A356 Aluminum Alloy Castings 1715 differentiating one factor in the proposed model, we are able to prove that both roles mentioned above should exert an effect in order to satisfactorily explain the porosity distribution in A356 aluminum alloy castings. REFERENCES Fig. 8 Variation of measured (symbols) and predicted (curves) porosity content with distance from the free end of the castings with different hydrogen contents. 5. Summary The problem of porosity and shrinkage in metal casting is complex since many thermal parameters as well as physical properties are involved. Further complication of the problem is due to the behavior of fluid flow in the interdendritic space and the rejection of dissolved gas into the pore kinetically during solidification. Over the years, two main schools of thinking in this field have been in debate: which factors of the interdendritic feeding resistance by Darcy s law or the solute redistribution that are responsible for the shrinkage porosity. In this study the A356 aluminum alloy with three hydrogen contents were cast in a permanent mold and the porosity contents and thermal parameters in different locations of the casting were measured. A simple mechanism for kinetic diffusion of hydrogen into the pore disregarding the thermodynamic model was proposed. The concept and mathematics involved in the mechanism is assumed to obey the Avrami s equation in the more general phase transformations. The results of regression of n and t τ imply that the kinetics of nucleation and growth of pore formation occurs more rapidly in the case of the melt containing more dissolved gas, which corresponds to a small relaxation time. This finding has been supported by Fang and Granger 31) and Sigworth and Wang. 14) By 1) E. Niyama, T. Unchida, M. Morikawa and S. Saito: A Method of Shrinkage Prediction and Its Application to Steel Casting Practice, 49th International Foundry Congress, April (1982) p. 1. 2) S. Minakawa, I. V. Samarasekera and F. Weninberg: Metall. Trans. B 16B (1985) ) J. A. Spittle, M. Almeshhedani and S. G. Brown: Cast Metal 7 [1] (1994) 51. 4) R. A. Entwistle, J. E. Gruzleski and P. M. Thomas: Solidification and Casting of Metals, Proc. International Conference on Solidification (The Metal Society, 1979) p ) H. Huang and J. T. Berry: AFS Trans. 101 (1994) ) G. V. Kutumba Rao and V. Panchanathan: AFS Trans. 81 (1973) ) Y. W. Lee, E. Chang and C. F. Chieu: Metall. Trans. B 21 B (1990) ) J. A. Taylor: Cast Metals 8 [4] (1995) ) T. S. Piwanka and M. C. Flemings: Trans. Met. Soc. AIME 236 (1966) ) J. Lecomte-Beckers: Metall. Trans. A 19A (1988) ) H. Shahani: Scandinavian Journal of Metallurgy 14 (1985) ) P. S. Mohanty, F. H. Samuel and J. E. Gruzleski: Metall. Trans. A 24A (1993) ) S. T. Kao, E. Chang and Y. W. Lee: Mater. Trans., JIM 35 (1994) ) G. K. Sigworth and C. Wang: Metall. Trans. B 24B (1993) ) P. D. Lee and J. D. Hunt: Acta Mater. 45 [10] (1997) ) R. C. Atwood, S. Sridhar, W. Zhang and P. D. Lee: Acta Mater. 48 (2000) ) D. R. Poirier, K. Yeum and A. L. Maples: Metall. Trans. A 18A (1987) ) M. C. Flemings: Solidification Processing, International Edition (McGraw-Hill Book Co., 1974). 19) S. N. Tiwari, A. K. Gupta and S. L. Malhotra: The British Foundryman 79 [3] (1986) ) S. Saikawa, K. Nakai, Y. Sugiura and A. Kamio: Mater. Trans., JIM 40 (1999) ) W. H. Johnson and K. J. Kaur: AFS Trans. 67 (1959) ) V. de L. Davies: AFS Cast Metals Res. J. 11 (1975) ) V. de L. Davies: Metal Society (1979) ) K. D. Li, M. C. Cheng and E. Chang: AFS Trans. 109 (2001) ) J. W. Christian: The Theory of Transformation in Metals and Alloys Part I Equilibrium and General Kinetic Theory, 2nd Ed., (Pergamon Press, Oxford, 1975). 26) M. Avrami: J. Chem. Phys. 7 (1930) ) F. S. Ham: J. Phys. Chem. Solids 6 (1958) ) D. Emadi, J. E. Gruzleski and J. E. Toguri: Metall. Trans. B 24B (1993) ) Procast Build-in Database, Procast The Professional Casting Simulation System (UES, Inc., 1988). 30) J. E. Gruzleski and B. M. Closset: The Treatment of Liquid Aluminum- Silicon Alloys, (The American Foundrymen s Society, Inc., 1990). 31) Q. T. Fang and D. A. Granger: AFS Trans. 97 (1989) 989.