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QUALITY IMPROVEMENT BY STRONTIUM MODIFICATION OF LOW PRESSURE ALUMINIUM CASTINGS
QUALITY IMPROVEMENT BY STRONTIUM MODIFICATION OF LOW PRESSURE ALUMINIUM CASTINGS B. CLOSSET TIMMINCO METALS 130 Adelaide Street West Toronto, Ontario, Canada M5W 1G5
ABSTRACT The effect of strontium modification on microstructures, tensile properties and impact strength of both A356.0 and 413.0 alloys has been studied. It is shown that the impact strength depends very strongly on the microstructure and the degree of modification. The microstructure and impact strength of unmodified and modified A356.0 and 413.0 alloys have been related to the electrical conductivity. Finally, the influence of heat treatment on microstructures, mechanical properties and electrical conductivity has also been studied.
KEYWORDS Strontium modification; microstructures; aluminium-silicon casting; mechanical properties; impact strength; electrical conductivity; thermal analysis.
INTRODUCTION The usage of Al-Si castings for automotive applications has increased steadily in recent years especially in North America. Typical automotive parts include cylinder heads, intake manifolds, brake calipers and wheels. The penetration of aluminium castings in the automotive market has been accompanied by significant progress in the molding technology and the molten metal treatment. As a result higher quality products are achieved in compliance with the specifications of the automotive industry. It has been shown that the use of strontium as an eutectic modifier has a significant effect on the ductility of hyper-eutectic Al-Si alloys (Closset and Gruzleski, 1982). Strontium has a long-term modifying effect that persists over extended holding times. Its usage has almost doubled in the last three years. Strontium is now widely used in the low pressure casting of aluminium automotive wheels (K. Alker and U. Hielscher, 1972). Other casting parts can be produced economically by the low pressure casting technique. In this study two types of castings were obtained in the absence of risers and with a minimum of gating. One of the main objective of this paper was to demonstrate that strontium modification improves the quality of different parts cast by a low pressure method. For each type of parts produced a different alloy was cast. An electrical conductivity technique and thermal analysis was applied in order to determine the degree of modification corresponding to different levels of strontium addition.
243
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REDUCTION AND CASTING OF ALUMINUM
EXPERIMENTAL Alloy Composition and Strontium Modification The A356.0 and 413.0 type alloys were molten at 760C in a Si-C crucible heated by an electric resistance furnace. The composition of both alloys is given in Table 1. TABLE 1 Chemical Composition of A356.Q and 413.0 Allovs Alloy Si A356.0 7.4 413.0 12.5
Fe 0.09 0.35
Cu 0.006 0.016
Mn 0005 0.017
Mg 0.42 0.017
Elements Zn 0.016 0.008
(wt.%) Ti 0.07 0.02
Sr -
Al Bal. Bal.
The metal is transferred from the melting furnace to the low pressure machine holding furnace by using a 108 kg transfer laddie where the molten metal treatment is carried out. Before strontium addition, a minimum of flux is added at the surface of the melt to remove oxides. Then strontium is added in the form of a 90%Sr-10%Al alloy and dissolution is completed after 5 minutes. Finally the metal is poured into the holding furnace. During the entire melting and casting process, the metal is not degassed for hydrogen removal. Casting and Sampling Two different molds, containing two identical cavities, mounted on the top of the low présure machine, were filled with the corresponding A356.0 or 413 alloys. Four series of casting comprising a set of non-modified parts and three series of modified parts, are cast in a preheated mold. A set is constituted by 15 to 30 castings and each modified set contains a different strontium level. Castings were randomly selected from each set and submitted to an analysis and testing program including spectro-chemical and microstructure analysis, tensile and impact strength testing, thermal analysis, and electrical conductivity measurements. The sample location for analysis and testing is shown in Figure 1. Heat Treatment Before heat treatment each set was divided in two lots of equal size. The first lot was analysed and testing in the as-cast state while the second lot was heat-treated according to the schedules shown in Table 2. TABLE 2 Heat Treatment Cycles of A356.0 and 413.0 Allovs
Solution Treatment Water Quenching Precipitation
Alloy A356.0 Temperature Time (C) (Hours) 538 13 25 154 8
Alloy 413.0 Temperature Time (C) (Hours) 538 13 25
The heat treatment cycles of both A356 and 413.0 alloys differ only by the precipitation hardening sequence. The 413.0 alloy containing only 0.017% magnesium and is not designed for precipitation treatments.
REDUCTION AND CASTING OF ALUMINUM
la. 413.0 Alloy
lb. A356.0 Alloy
Fig. 1. Sample Locations: 1. Metallography 4. Impact Strength 2. Spectrochemestry 5. Electrical Conductivity 3. Tensile Strength Electrical Conductivity Conductivity measurements were carried out at room temperature on cast surfaces before and after each heat treatment cycle. An electrical conductivity meter developed by K.J. Law (Model M4900B) was used. This device is temperature compensated and measures the conductivity by an eddy current technique as a percentage of the International Annealed Copper Standard (%IACS). Thermal Analysis The system used for thermal analysis has been described in our earlier work (S. Argyropoulos, B. Closset, J.E. Gruzleski, and H. Orger, 1983). Samples are cut from each selected casting and approximatively 100g are remelted in an alumina crucible before thermal analysis. The eutectic transformation characteristics of each cooling curves are analysed.
METALLOGRAPHY AND MICROSTRUCTURES A356 Alloy The effect of various strontium levels on the as-cast microstructure is shown in Figure 2. The non-modified structure contains a relatively coarse acicular silicon. Figure 2a. Modification is complete at a 0.016% strontium level. Figure 2b. Higher strontium levels
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REDUCTION AND CASTING OF ALUMINUM
(0.020% and 0.024%) do not improve the fineness of the eutectic. Figures 2c and 2d. Heat treatment acts to change the cast structure of both non-modified and modified melts. Figure 3. The fine silicon particles of the as-cast modified structure (Figure 2b) undergo considerable coalescence on heat treatment, and hence the heat treated structure (Figure 3b) appears coarser than the as-cast, but remains finer than the non-modified structure. The coarse eutectic silicon of the non-modified structure is made less angular. Figure 3a.
t
2a: 0%Sr
2c: 0.020%
15 0μ
Il
2b: 0.016%Sr
Γ
-· > * w x m 1 5 Ou
>
«
2d: 0.024%Sr
Fig. 2. Effect of Strontium on Microstructure of As-Cast A356.0 Alloy
Î-J
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REDUCTION AND CASTING OF ALUMINUM
3a: 0%Sr 3b: 0.016%Sr Fig. 3. Effect of Strontium on Silicon Morphology of Heat Treated A356.0 Alloy Because of higher solidification rate obtained in a permanent mold, the non-modified (0%Sr) A356.0 alloys present large lamelles of eutectic silicon. Figure 2a. An identical alloy cast in a sand mold exhibits a non-modified structure with a coarse acicular silicon (B. Closset and J.E. Gruzleski, 1981). In both cases, an addition of strontium in the order of 0.015% is sufficient to modify the structure and to obtain a finely dispersed eutectic. Figure 2b. In general, the microstructural change occuring with modification in permanent mold parts is less marked than in sand mold parts. 413.0 Alloy The addition of strontium to a 413.0 melt changes dramatically the morphology of the eutectic silicon. Figure 4.
4a:0%Sr
4b:0.010%Sr
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REDUCTION AND CASTING OF ALUMINUM
150/ί
4c: 0.0135%Sr
4d: 0.0190%Sr
Fig. 4. Effect of Strontium Microstructure of As-Cast 413.0 Alloy At 0%Sr, the non-modified alloys appears hyper-eutectic and exhibits a large amount of coarse eutectic silicon. Figure 4a. An addition of 0.010% strontium results in a mixed structure of modified and non-modified eutectic silicon, and the absence of the primary silicon phase. Figure 4b. At higher silicon levels (0.0135% and 0.019%), the structure becomes finer (Figure 4c) and finally a complete fibrous eutectic can be observed (Figure 4d). The solution treatment changes the morphology of both non-modified (0%Sr) and modified (0.019%Sr) eutectic silicon. Figure 5.
150 u LJ
5a: 0%Sr
5b: 0.019%Sr
Fig. 5. Effect of Strontium on Silicon Morphology of Heat Treated 413.0
Alloy
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REDUCTION AND CASTING OF ALUMINUM
The non-modified (0%Sr) acicular silicon undergoes little coalescence and spheroidization. The most noticeable effect is an angularity loss of the silicon particles. Figure 5a. At 0.019% strontium, the solution treated microstructure is characterized by an important coalescence resulting in spherical silicon particles. Figure 5b. Tensile Properties A356.0 Allov A summary of the as-cast and heat treated tensile properties is given in Table 3. The results reported in Table 3 are the average of four test pieces machined from samples cut from two locations shown on the casting represented in Figure 1. In the as-cast stage, the elongation increases from 3.6% to 6.45% with an addition of strontium from 0% to 0.02%, and the tensile and yield strength are unaffected. After heat treatment, the A356.0 remains ductile while the elongation increases only from 5.1% to 6.9% with a corresponding strontium increase from 0% to 0.020%. Table 3 shows also a dramatic increase in tensile and yield strength upon heat treatment. TABLE 3. Tensile Properties of As-Cast and Heat Treated A356.0 Allov
(%) 0 0.016 0.020 0.024
As-Cast 173.7 169.8 171.8 169.7
Heat Treated 268.9 272.1 280.0 276.0
E(%)
YS (MPA)
UTS (MPA)
Sr
As-Cast 106.1 106.7 104.9 101.3
Heat Treated 213.4 213.6 223.9 220.3
As-Cast Heated Treated 3.60 4.80 5.25 6.45
5.10 5.50 6.90 5.80
Elongation of both as-cast and heat treated A356.0 alloys is only marginally improved by strontium modification. This can be explained by a relative small difference between the morphology of silicon platelets (0%Sr) and fibers (0.016%Sr). Microstructure differences are even less obvious after heat treatment. Figure 3. 413.0 Allov The tensile properties of the as-cast and heat treated samples are summarized in Table 4. TABLE 4. Tensile Properties As-Cast and Heat Treated 413.0 Allov Sr (%) 0 0.010 0.0135 0.019
UTS (MPA) As-Cast Heat Treated 143.2 167.8 172.4 168.9
170.5 187.5 182.0 182.0
E(%)
YS (MPA) As-Cast 83.8 81.4 83.8 81.7
Heat Treated 87.5 85.0 86.0 90.0
As-Cast 3.3 9.7 13.5 14.2
Heat Treated 6.3 14.1 18.1 18.1
Both elongation and tensile strength increase gradually with the strontium content while the yield strength is relatively unaffected. In the as-cast stage, elongation increases from 3.3% for a non-modified structure (0%Sr) to 9.7% for an undermodified silicon (0.010%Sr) and then
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REDUCTION AND CASTING OF ALUMINUM
to 13.5% for a well modified eutectic (0.0135%Sr). In a less dramatic manner, the tensile strength is significantly improved from 143.2 Mpa to 172.4 MPa when the structure changes from lamellar-acicular (0%Sr) to fibrous (0.0135%). After solution treatment, the effect of strontium modification is still very noticeable. For example, elongation of a non-modified alloy (0%Sr) is increased from 6.3% to 14.1% for an undermodified structure (0.010%Sr). Subsequent additions of strontium resulting in a complete modification bring elongation values to 18.1%. Modification also improves tensile strength from 170.5 MPa for a non-modified alloy (0%Sr) to 182.0 MPa after full modification (0.019%Sr) is obtained. Yield strength of both ascast and heat treated 413.0 are not significantly affected by modification. Improvements in tensile strength and elongation as a result of solution treatment, are less substantial than those occuring with strontium modification. For example, solution treatment increases the elongation of a non-modified alloy by approximatively 100%, while the microstructure modification alone is responsible for a 400% improvement in elongation. Modification has a stronger impact on the tensile strength and elongation of 413.0 alloys than A356.0 alloys. This difference can be attributed to the presence of an eutectic phase close to 100% in 413.0 alloys compared to an approximatively 40% eutectic phase in A356.0 alloys. It should also be noted that the as-cast tensile strength of modified 413.0 alloys are in the order of 170 MPa, and are comparable to non-modified and modified A356.0 alloys. Impact Strength
2
Impact testing (Charpy) was carried out on unotched samples having lxcm section. Impact strength improvement due to microstructure modification is shown in Figure 6 for both 2 treated A356.0 alloy. In the as-cast stage, the impact 2 as-cast and heat strength increases from 9.5 Jxcm" for an non-modified structure (0%Sr) to 18.7 Jxcm" for a modified structure 2 2 (0.016%Sr). The heat treated samples exhibit also an impact strength increase upon modification from 23.7 Jxcm" at 0%Sr to 33.7 Jxcm" at 0.020%Sr. Impact strength of both as-cast and solution treated 413.0 alloy is greatly improved by the 2 modification. Figure 7. The impact strength of as-cast 2 samples increases from microstructure 2 6.6 Jxcm" for a coarse lamellar structure (0%Sr) to 34.2 Jxcm" for a 2complete modified eutectic (0.019%Sr). An improvement in impact strength from 15.7 Jxcm" to 65.5 Jxcm" for solution treated samples is even more remarkable when the structure changes from nonmodified (0%Sr) to modified (0.019%Sr). The impact strength of both heat treated samples is significantly greater than the impact 2 2 alloys increase the impact strength of strength of an as-cast alloy. Heat treatment of A356.0 2 case of 413.02alloys the impact non-modified samples from 9.5 Jxcm" to 18.7 Jxcm' . In the strength increases upon 2heat treatment, from 6.6 Jxcm" to 15.7 Jxcm" , is less than an increase from 6.6 Jxcm" to 34.2 Jxcm"2 resulting from modification treatment. The impact strength of both as-cast2and heat treated modified 413.0 (0.019%Sr) alloys2is respectively 34.2 Jxcm^ and 65.5 Jxcm" which in both cases is higher than the 33.7 Jxcm" impact strength of a heat treated and modified A356.0 alloy. The impact strength increase confirms the ductility improvement resulting from the eutectic modification of Al-Si alloys. At a higher silicon level (12.5%), the increase in as-cast impact strength upon modification is in the order of 500%, compared to approximatively 100% for an alloy containing between 6.5% and 7.5% silicon. Again this difference in impact strength in favor of the high silicon containing alloy can be attributed to an amount of eutectic phase close to 100%.
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REDUCTION AND CASTING OF ALUMINUM
0.01 Sr (wt
0.03
0.02
0.01
%)
Sr
0.03
0.02 (wt%)
Fig. 7 Impact Strength of As-Cast & Heat Treated 413.0 Alloy · : As-Cast • : Solution Treated
Fig.6 Impact Strength of As-Cast & Heat Treated A356.0 Alloy · : As-Cast • : Heat Treated Electrical Conductivity
The electrical conductivity was measured on the cast surface at location 5 shown in Figure 1. The electrical conductivity values for both as-cast and heat treated A356.0 and 413.0 alloys, are given in Table 5. TABLE 5. Electrical Conductivity of A356.0 and 413.0 Alloys Alloy Sr(%)
413.0
A356.0 0
0.016 0.020
0.024
As-Cast 35.0
39.0
38.7
39.0
Heat Treated 37.2
38.1
38.1
38.4
0
0.010
0.0135
0.0190
32.4
39.0
40.9
41.5
35.0
37.2
37.2
37.2
Conductivity (%IACS)
Modification of the as-cast structure due to an increase of the strontium level from 0% to 0.016%, is accompanied by an increase in electrical conductivity from 35.0% IACS to 39.0% LACS. Higher strontium levels (0.020% and 0.024%) do not result in higher electrical conductivity. After heat treatment, the electrical conductivity increases only from 37.2% IACS to 38.1% IACS. Modification improves the electrical conductivity of as-cast A356.0 alloy by approximatively 11.5%. The conductivity of non-modified heat treated A356.0 alloy (37.2%) IACS) is
REDUCTION AND CASTING OF ALUMINUM
252
significantly higher than the conductivity of a corresponding as-cast alloy (35.0% IACS). For modified alloys, the conductivity of heat treated structures (38.1% IACS) is slightly inferior to the conductivity of as-cast structures (39.0% IACS). Similar results have been obtained by using a direct current (DC) electrical resistivity technique (B. Closset, K. Pirie, J.E. Gruzleski, 1985). The net increase in conductivity (2.2% IACS) upon heat treatment of non-modified A356.0 alloy is primarily due to the dissolution of alloying elements (Mg) and impurities in the aluminium matrix. The slight decrease in conductivity (0.6% IACS) upon heat treatment of a modidfied alloy (0.020%Sr) is the result of a conductivity increase by the dissolution of alloying elements and impurities, and a conductivity decrease caused by the coalescence of the fibrous eutectic forming larger spherical silicon particles.
Fig. 8 Comparison Between Electrical Conductivity & Impact Strength of As-Cast A356.0 Alloy A rElectrical Conductivity • .Impact Strength
Impact Strength ( J x cm"2)
Ο οι Ο
Electrical Conductivity ( % IACS >
Impact Strength ( J χ cm"2)
Electrical Conductivity ( % IACS >
Figure 8 shows a good correlation between impact strength and electrical conductivity of ascast A356.0 alloy. It can be seen that electrical conductivity and impact strength evolve similarly when the structure changes from non-modified (0%Sr) to modified (0.016%Sr). At 0.020%Sr, the A356.0 alloy is well modified (Figure 3d), and both electrical conductivity and impact strength reach a plateau.
Fig. 9 Comparison Between Electrical Conductivity & Impact Strength of As-Cast 413.0 Alloy • .Electrical Conductivity # .Impact Strength
The electrical conductivity of both as-cast and heat treated 413.0 samples are shown in Table 5. The electrical conductivity of the as-cast alloy increases gradually from 32.4% IACS for a nonmodified eutectic (0%Sr) to 41.5% IACS for a complete modified structure (0.019%Sr). The increase in conductivity is less marked when the 413.0 alloy has been submitted to a solution treatment. Conductivity increases only from 35.0% IACS for a non-modified (0%Sr) sample to 37.2% IACS for a modified alloy (0.019%Sr). The conductivity (35.0% IACS) of a non-modified
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REDUCTION AND CASTING OF ALUMINUM
solution treated 413.0 alloy is significantly higher than the corresponding conductivity (32.4% IACS) of as-cast samples. A modified 413.0 alloy exhibits higher conductivity in the as-cast stage (41.5% IACS) than after solution treatment (37.2% IACS). Modification increases the electrical conductivity of as-cast 413.0 samples by 28% which is largely superior to the 8% increase obtained by solution treatment. This demonstrate the dominant effect of modification over heat treatment to change the morphology of eutectic silicon. Conductivity increase upon solution treatment of a non-modified eutectic can be attributed to the dissolution of impurities in the alloy. A 10% conductivity decrease upon heat treatment of a modified 413.0 alloy is directly related to the coarsening of the eutectic silicon by coalescence. Figure 9 shows the change of electrical conductivity and impact strength of a 413.0 alloy with modification. At 0.020%Sr, the structure is well modified and both conductivity and impact strength reach a maximum. Thermal Analysis The thermal analysis characteristics obtained from A356.0 alloy cooling curves are shown in Table 6. TABLE 6. Thermal Analysis Characteristics of A356.0 Alloy Sr (%) 0 0.016 0.020 0.024
Eutectic Nucleation Temperature TC(C) 572.2 568.2 568.8 567.9
Eutectic Growth Temperature TE(C)574.0 569.7 570.2 569.4
ΔΤ2= TE-TC (C)
ΔΤ= TE-575.0 (C)
1.8 1.5 1.4 1.5
-1.0 -5.3 -4.8 -5.6
The increase of the strontium level in the melt from 0% to 0.016% and the eutectic modification result in a decrease of the eutectic temperature from 574.0C to 569.7C, which corresponds to an increase of ΔΤ from -1.0C to -5.3C while ΔΤ2 remains practically unchanged. Further strontium additions do not change the thermal analysis characteristics. A microstructure change from lamellar (0%Sr) to fibrous (0.016%Sr) are characterized by a decrease in Δ Τ from -1.0C to -5.3C. Therefore, the choice of Δ Τ as a parameter to distinguish a non-modified structure from a modified alloy appears to be adequate. Thermal analysis curves were also obtained by remelting 413.0 alloys, and their characteristics are given in Table 7. Silicon modification decreases the eutectic temperature in the order of 3.0C which is substantially less than in the case of a A356.0. The decrease in the eutectic temperature, AT, is not sufficient to distinguish a non-modified structure (0.%Sr) from a undermodified (0.010%Sr) structure as shown by the microstructure in Figure 2. A second parameter, ΔΤ2, seems to reflect more adequately the gradual changes in microstructures; ΔΤ2 decreases from 3.5C to 2.3C when the structure changes from non-modified (0%Sr) to undermodified (0.010%Sr). Further strontium additions (0.0135% and 0.019%) improve the fineness of the eutectic silicon. At a 0.0190%Sr level, the structure is fibrous which corresponds to Δ T2=0.6C.
REDUCTION AND
254
CASTING OF ALUMINUM
TABLE 7. Thermal Analysis Characteristics of 413.0 Sr
(«)
0 0.010 0.0135 0.0190
Eutectic Nucleation Temperature TC(C) 576.8 575.1 575.4 576.8
Eutectic Growth Temperature TE(C)
ΔΤ2= ΔΤ= TE-TC TE-580.0 (C) 3.5 2.3 1.4 0.6
580.3 577.4 576.8 577.4
Allov
(C) + 0.3 -2.6 -3.2 -2.6
CONCLUSIONS The following main conclusions can be drawn from this work: 1.
Strontium modification substantially improves the elongation, and to a lesser extent the tensile strength of an Al-Si eutectic type alloy (413.0). At lower silicon levels (A356.0 alloy), the improvement in elongation is less marked.
2
Impact strength of both 413.0 and A356.0 alloys is very sensitive to the microstructure and is greatly improved by the strontium modification of the eutectic.
3.
Electrical conductivity increases with the fineness of the eutectic silicon and is an excellent method to determine, non-destructively, microstrutural changes in as-cast 413.0 and A356.0 alloys. The conductivity technique is more sensitive to the silicon morphology in the as-cast than in the heat treated stage.
4.
Thermal analysis can be used to evaluate quantitatively and qualitatively the degree of modification of both A356.0 and 413.0 alloys. Both characteristics ΔΤ and Δ Τ 2 are necessary to define the modification of the alloy, especially for the 413.0 alloys. ACKNOWLEDGEMENTS
This work was made possible with the technical collaboration of Grenville Castings. author expresses his thanks to M. Bray and K.D.B. Budd for their technical assistance.
The
REFERENCES B. Closset and J.E. Gruzleski (1982). Met. Transaction A n a 945-951. G 5rtt%M7 U R Zg rl e e S( l1 d9 8 3 ) K. Alker and U. Hielscher (1972).JK Aluminium 19 ' ° · American Foundrymen. R r\?™îvHS 1 ^ A f r i c a n Foundrvmen's Tmn.^tiW so sol-808
^SS^SSA
B. Closset, K. Pine, J.E. Gruzleski (1985). American
Fnu nr^^PfÇ^^^ n
07-316.
3
ABSTRACT The effect of strontium modification on microstructures, tensile properties and impact strength of both A356.0 and 413.0 alloys has been studied. It is shown that the impact strength depends very strongly on the microstructure and the degree of modification. The microstructure and impact strength of unmodified and modified A356.0 and 413.0 alloys have been related to the electrical conductivity. Finally, the influence of heat treatment on microstructures, mechanical properties and electrical conductivity has also been studied.
KEYWORDS Strontium modification; microstructures; aluminium-silicon casting; mechanical properties; impact strength; electrical conductivity; thermal analysis.
INTRODUCTION The usage of Al-Si castings for automotive applications has increased steadily in recent years especially in North America. Typical automotive parts include cylinder heads, intake manifolds, brake calipers and wheels. The penetration of aluminium castings in the automotive market has been accompanied by significant progress in the molding technology and the molten metal treatment. As a result higher quality products are achieved in compliance with the specifications of the automotive industry. It has been shown that the use of strontium as an eutectic modifier has a significant effect on the ductility of hyper-eutectic Al-Si alloys (Closset and Gruzleski, 1982). Strontium has a long-term modifying effect that persists over extended holding times. Its usage has almost doubled in the last three years. Strontium is now widely used in the low pressure casting of aluminium automotive wheels (K. Alker and U. Hielscher, 1972). Other casting parts can be produced economically by the low pressure casting technique. In this study two types of castings were obtained in the absence of risers and with a minimum of gating. One of the main objective of this paper was to demonstrate that strontium modification improves the quality of different parts cast by a low pressure method. For each type of parts produced a different alloy was cast. An electrical conductivity technique and thermal analysis was applied in order to determine the degree of modification corresponding to different levels of strontium addition.
243
244
REDUCTION AND CASTING OF ALUMINUM
EXPERIMENTAL Alloy Composition and Strontium Modification The A356.0 and 413.0 type alloys were molten at 760C in a Si-C crucible heated by an electric resistance furnace. The composition of both alloys is given in Table 1. TABLE 1 Chemical Composition of A356.Q and 413.0 Allovs Alloy Si A356.0 7.4 413.0 12.5
Fe 0.09 0.35
Cu 0.006 0.016
Mn 0005 0.017
Mg 0.42 0.017
Elements Zn 0.016 0.008
(wt.%) Ti 0.07 0.02
Sr -
Al Bal. Bal.
The metal is transferred from the melting furnace to the low pressure machine holding furnace by using a 108 kg transfer laddie where the molten metal treatment is carried out. Before strontium addition, a minimum of flux is added at the surface of the melt to remove oxides. Then strontium is added in the form of a 90%Sr-10%Al alloy and dissolution is completed after 5 minutes. Finally the metal is poured into the holding furnace. During the entire melting and casting process, the metal is not degassed for hydrogen removal. Casting and Sampling Two different molds, containing two identical cavities, mounted on the top of the low présure machine, were filled with the corresponding A356.0 or 413 alloys. Four series of casting comprising a set of non-modified parts and three series of modified parts, are cast in a preheated mold. A set is constituted by 15 to 30 castings and each modified set contains a different strontium level. Castings were randomly selected from each set and submitted to an analysis and testing program including spectro-chemical and microstructure analysis, tensile and impact strength testing, thermal analysis, and electrical conductivity measurements. The sample location for analysis and testing is shown in Figure 1. Heat Treatment Before heat treatment each set was divided in two lots of equal size. The first lot was analysed and testing in the as-cast state while the second lot was heat-treated according to the schedules shown in Table 2. TABLE 2 Heat Treatment Cycles of A356.0 and 413.0 Allovs
Solution Treatment Water Quenching Precipitation
Alloy A356.0 Temperature Time (C) (Hours) 538 13 25 154 8
Alloy 413.0 Temperature Time (C) (Hours) 538 13 25
The heat treatment cycles of both A356 and 413.0 alloys differ only by the precipitation hardening sequence. The 413.0 alloy containing only 0.017% magnesium and is not designed for precipitation treatments.
REDUCTION AND CASTING OF ALUMINUM
la. 413.0 Alloy
lb. A356.0 Alloy
Fig. 1. Sample Locations: 1. Metallography 4. Impact Strength 2. Spectrochemestry 5. Electrical Conductivity 3. Tensile Strength Electrical Conductivity Conductivity measurements were carried out at room temperature on cast surfaces before and after each heat treatment cycle. An electrical conductivity meter developed by K.J. Law (Model M4900B) was used. This device is temperature compensated and measures the conductivity by an eddy current technique as a percentage of the International Annealed Copper Standard (%IACS). Thermal Analysis The system used for thermal analysis has been described in our earlier work (S. Argyropoulos, B. Closset, J.E. Gruzleski, and H. Orger, 1983). Samples are cut from each selected casting and approximatively 100g are remelted in an alumina crucible before thermal analysis. The eutectic transformation characteristics of each cooling curves are analysed.
METALLOGRAPHY AND MICROSTRUCTURES A356 Alloy The effect of various strontium levels on the as-cast microstructure is shown in Figure 2. The non-modified structure contains a relatively coarse acicular silicon. Figure 2a. Modification is complete at a 0.016% strontium level. Figure 2b. Higher strontium levels
245
246
REDUCTION AND CASTING OF ALUMINUM
(0.020% and 0.024%) do not improve the fineness of the eutectic. Figures 2c and 2d. Heat treatment acts to change the cast structure of both non-modified and modified melts. Figure 3. The fine silicon particles of the as-cast modified structure (Figure 2b) undergo considerable coalescence on heat treatment, and hence the heat treated structure (Figure 3b) appears coarser than the as-cast, but remains finer than the non-modified structure. The coarse eutectic silicon of the non-modified structure is made less angular. Figure 3a.
t
2a: 0%Sr
2c: 0.020%
15 0μ
Il
2b: 0.016%Sr
Γ
-· > * w x m 1 5 Ou
>
«
2d: 0.024%Sr
Fig. 2. Effect of Strontium on Microstructure of As-Cast A356.0 Alloy
Î-J
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REDUCTION AND CASTING OF ALUMINUM
3a: 0%Sr 3b: 0.016%Sr Fig. 3. Effect of Strontium on Silicon Morphology of Heat Treated A356.0 Alloy Because of higher solidification rate obtained in a permanent mold, the non-modified (0%Sr) A356.0 alloys present large lamelles of eutectic silicon. Figure 2a. An identical alloy cast in a sand mold exhibits a non-modified structure with a coarse acicular silicon (B. Closset and J.E. Gruzleski, 1981). In both cases, an addition of strontium in the order of 0.015% is sufficient to modify the structure and to obtain a finely dispersed eutectic. Figure 2b. In general, the microstructural change occuring with modification in permanent mold parts is less marked than in sand mold parts. 413.0 Alloy The addition of strontium to a 413.0 melt changes dramatically the morphology of the eutectic silicon. Figure 4.
4a:0%Sr
4b:0.010%Sr
248
REDUCTION AND CASTING OF ALUMINUM
150/ί
4c: 0.0135%Sr
4d: 0.0190%Sr
Fig. 4. Effect of Strontium Microstructure of As-Cast 413.0 Alloy At 0%Sr, the non-modified alloys appears hyper-eutectic and exhibits a large amount of coarse eutectic silicon. Figure 4a. An addition of 0.010% strontium results in a mixed structure of modified and non-modified eutectic silicon, and the absence of the primary silicon phase. Figure 4b. At higher silicon levels (0.0135% and 0.019%), the structure becomes finer (Figure 4c) and finally a complete fibrous eutectic can be observed (Figure 4d). The solution treatment changes the morphology of both non-modified (0%Sr) and modified (0.019%Sr) eutectic silicon. Figure 5.
150 u LJ
5a: 0%Sr
5b: 0.019%Sr
Fig. 5. Effect of Strontium on Silicon Morphology of Heat Treated 413.0
Alloy
249
REDUCTION AND CASTING OF ALUMINUM
The non-modified (0%Sr) acicular silicon undergoes little coalescence and spheroidization. The most noticeable effect is an angularity loss of the silicon particles. Figure 5a. At 0.019% strontium, the solution treated microstructure is characterized by an important coalescence resulting in spherical silicon particles. Figure 5b. Tensile Properties A356.0 Allov A summary of the as-cast and heat treated tensile properties is given in Table 3. The results reported in Table 3 are the average of four test pieces machined from samples cut from two locations shown on the casting represented in Figure 1. In the as-cast stage, the elongation increases from 3.6% to 6.45% with an addition of strontium from 0% to 0.02%, and the tensile and yield strength are unaffected. After heat treatment, the A356.0 remains ductile while the elongation increases only from 5.1% to 6.9% with a corresponding strontium increase from 0% to 0.020%. Table 3 shows also a dramatic increase in tensile and yield strength upon heat treatment. TABLE 3. Tensile Properties of As-Cast and Heat Treated A356.0 Allov
(%) 0 0.016 0.020 0.024
As-Cast 173.7 169.8 171.8 169.7
Heat Treated 268.9 272.1 280.0 276.0
E(%)
YS (MPA)
UTS (MPA)
Sr
As-Cast 106.1 106.7 104.9 101.3
Heat Treated 213.4 213.6 223.9 220.3
As-Cast Heated Treated 3.60 4.80 5.25 6.45
5.10 5.50 6.90 5.80
Elongation of both as-cast and heat treated A356.0 alloys is only marginally improved by strontium modification. This can be explained by a relative small difference between the morphology of silicon platelets (0%Sr) and fibers (0.016%Sr). Microstructure differences are even less obvious after heat treatment. Figure 3. 413.0 Allov The tensile properties of the as-cast and heat treated samples are summarized in Table 4. TABLE 4. Tensile Properties As-Cast and Heat Treated 413.0 Allov Sr (%) 0 0.010 0.0135 0.019
UTS (MPA) As-Cast Heat Treated 143.2 167.8 172.4 168.9
170.5 187.5 182.0 182.0
E(%)
YS (MPA) As-Cast 83.8 81.4 83.8 81.7
Heat Treated 87.5 85.0 86.0 90.0
As-Cast 3.3 9.7 13.5 14.2
Heat Treated 6.3 14.1 18.1 18.1
Both elongation and tensile strength increase gradually with the strontium content while the yield strength is relatively unaffected. In the as-cast stage, elongation increases from 3.3% for a non-modified structure (0%Sr) to 9.7% for an undermodified silicon (0.010%Sr) and then
250
REDUCTION AND CASTING OF ALUMINUM
to 13.5% for a well modified eutectic (0.0135%Sr). In a less dramatic manner, the tensile strength is significantly improved from 143.2 Mpa to 172.4 MPa when the structure changes from lamellar-acicular (0%Sr) to fibrous (0.0135%). After solution treatment, the effect of strontium modification is still very noticeable. For example, elongation of a non-modified alloy (0%Sr) is increased from 6.3% to 14.1% for an undermodified structure (0.010%Sr). Subsequent additions of strontium resulting in a complete modification bring elongation values to 18.1%. Modification also improves tensile strength from 170.5 MPa for a non-modified alloy (0%Sr) to 182.0 MPa after full modification (0.019%Sr) is obtained. Yield strength of both ascast and heat treated 413.0 are not significantly affected by modification. Improvements in tensile strength and elongation as a result of solution treatment, are less substantial than those occuring with strontium modification. For example, solution treatment increases the elongation of a non-modified alloy by approximatively 100%, while the microstructure modification alone is responsible for a 400% improvement in elongation. Modification has a stronger impact on the tensile strength and elongation of 413.0 alloys than A356.0 alloys. This difference can be attributed to the presence of an eutectic phase close to 100% in 413.0 alloys compared to an approximatively 40% eutectic phase in A356.0 alloys. It should also be noted that the as-cast tensile strength of modified 413.0 alloys are in the order of 170 MPa, and are comparable to non-modified and modified A356.0 alloys. Impact Strength
2
Impact testing (Charpy) was carried out on unotched samples having lxcm section. Impact strength improvement due to microstructure modification is shown in Figure 6 for both 2 treated A356.0 alloy. In the as-cast stage, the impact 2 as-cast and heat strength increases from 9.5 Jxcm" for an non-modified structure (0%Sr) to 18.7 Jxcm" for a modified structure 2 2 (0.016%Sr). The heat treated samples exhibit also an impact strength increase upon modification from 23.7 Jxcm" at 0%Sr to 33.7 Jxcm" at 0.020%Sr. Impact strength of both as-cast and solution treated 413.0 alloy is greatly improved by the 2 modification. Figure 7. The impact strength of as-cast 2 samples increases from microstructure 2 6.6 Jxcm" for a coarse lamellar structure (0%Sr) to 34.2 Jxcm" for a 2complete modified eutectic (0.019%Sr). An improvement in impact strength from 15.7 Jxcm" to 65.5 Jxcm" for solution treated samples is even more remarkable when the structure changes from nonmodified (0%Sr) to modified (0.019%Sr). The impact strength of both heat treated samples is significantly greater than the impact 2 2 alloys increase the impact strength of strength of an as-cast alloy. Heat treatment of A356.0 2 case of 413.02alloys the impact non-modified samples from 9.5 Jxcm" to 18.7 Jxcm' . In the strength increases upon 2heat treatment, from 6.6 Jxcm" to 15.7 Jxcm" , is less than an increase from 6.6 Jxcm" to 34.2 Jxcm"2 resulting from modification treatment. The impact strength of both as-cast2and heat treated modified 413.0 (0.019%Sr) alloys2is respectively 34.2 Jxcm^ and 65.5 Jxcm" which in both cases is higher than the 33.7 Jxcm" impact strength of a heat treated and modified A356.0 alloy. The impact strength increase confirms the ductility improvement resulting from the eutectic modification of Al-Si alloys. At a higher silicon level (12.5%), the increase in as-cast impact strength upon modification is in the order of 500%, compared to approximatively 100% for an alloy containing between 6.5% and 7.5% silicon. Again this difference in impact strength in favor of the high silicon containing alloy can be attributed to an amount of eutectic phase close to 100%.
251
REDUCTION AND CASTING OF ALUMINUM
0.01 Sr (wt
0.03
0.02
0.01
%)
Sr
0.03
0.02 (wt%)
Fig. 7 Impact Strength of As-Cast & Heat Treated 413.0 Alloy · : As-Cast • : Solution Treated
Fig.6 Impact Strength of As-Cast & Heat Treated A356.0 Alloy · : As-Cast • : Heat Treated Electrical Conductivity
The electrical conductivity was measured on the cast surface at location 5 shown in Figure 1. The electrical conductivity values for both as-cast and heat treated A356.0 and 413.0 alloys, are given in Table 5. TABLE 5. Electrical Conductivity of A356.0 and 413.0 Alloys Alloy Sr(%)
413.0
A356.0 0
0.016 0.020
0.024
As-Cast 35.0
39.0
38.7
39.0
Heat Treated 37.2
38.1
38.1
38.4
0
0.010
0.0135
0.0190
32.4
39.0
40.9
41.5
35.0
37.2
37.2
37.2
Conductivity (%IACS)
Modification of the as-cast structure due to an increase of the strontium level from 0% to 0.016%, is accompanied by an increase in electrical conductivity from 35.0% IACS to 39.0% LACS. Higher strontium levels (0.020% and 0.024%) do not result in higher electrical conductivity. After heat treatment, the electrical conductivity increases only from 37.2% IACS to 38.1% IACS. Modification improves the electrical conductivity of as-cast A356.0 alloy by approximatively 11.5%. The conductivity of non-modified heat treated A356.0 alloy (37.2%) IACS) is
REDUCTION AND CASTING OF ALUMINUM
252
significantly higher than the conductivity of a corresponding as-cast alloy (35.0% IACS). For modified alloys, the conductivity of heat treated structures (38.1% IACS) is slightly inferior to the conductivity of as-cast structures (39.0% IACS). Similar results have been obtained by using a direct current (DC) electrical resistivity technique (B. Closset, K. Pirie, J.E. Gruzleski, 1985). The net increase in conductivity (2.2% IACS) upon heat treatment of non-modified A356.0 alloy is primarily due to the dissolution of alloying elements (Mg) and impurities in the aluminium matrix. The slight decrease in conductivity (0.6% IACS) upon heat treatment of a modidfied alloy (0.020%Sr) is the result of a conductivity increase by the dissolution of alloying elements and impurities, and a conductivity decrease caused by the coalescence of the fibrous eutectic forming larger spherical silicon particles.
Fig. 8 Comparison Between Electrical Conductivity & Impact Strength of As-Cast A356.0 Alloy A rElectrical Conductivity • .Impact Strength
Impact Strength ( J x cm"2)
Ο οι Ο
Electrical Conductivity ( % IACS >
Impact Strength ( J χ cm"2)
Electrical Conductivity ( % IACS >
Figure 8 shows a good correlation between impact strength and electrical conductivity of ascast A356.0 alloy. It can be seen that electrical conductivity and impact strength evolve similarly when the structure changes from non-modified (0%Sr) to modified (0.016%Sr). At 0.020%Sr, the A356.0 alloy is well modified (Figure 3d), and both electrical conductivity and impact strength reach a plateau.
Fig. 9 Comparison Between Electrical Conductivity & Impact Strength of As-Cast 413.0 Alloy • .Electrical Conductivity # .Impact Strength
The electrical conductivity of both as-cast and heat treated 413.0 samples are shown in Table 5. The electrical conductivity of the as-cast alloy increases gradually from 32.4% IACS for a nonmodified eutectic (0%Sr) to 41.5% IACS for a complete modified structure (0.019%Sr). The increase in conductivity is less marked when the 413.0 alloy has been submitted to a solution treatment. Conductivity increases only from 35.0% IACS for a non-modified (0%Sr) sample to 37.2% IACS for a modified alloy (0.019%Sr). The conductivity (35.0% IACS) of a non-modified
253
REDUCTION AND CASTING OF ALUMINUM
solution treated 413.0 alloy is significantly higher than the corresponding conductivity (32.4% IACS) of as-cast samples. A modified 413.0 alloy exhibits higher conductivity in the as-cast stage (41.5% IACS) than after solution treatment (37.2% IACS). Modification increases the electrical conductivity of as-cast 413.0 samples by 28% which is largely superior to the 8% increase obtained by solution treatment. This demonstrate the dominant effect of modification over heat treatment to change the morphology of eutectic silicon. Conductivity increase upon solution treatment of a non-modified eutectic can be attributed to the dissolution of impurities in the alloy. A 10% conductivity decrease upon heat treatment of a modified 413.0 alloy is directly related to the coarsening of the eutectic silicon by coalescence. Figure 9 shows the change of electrical conductivity and impact strength of a 413.0 alloy with modification. At 0.020%Sr, the structure is well modified and both conductivity and impact strength reach a maximum. Thermal Analysis The thermal analysis characteristics obtained from A356.0 alloy cooling curves are shown in Table 6. TABLE 6. Thermal Analysis Characteristics of A356.0 Alloy Sr (%) 0 0.016 0.020 0.024
Eutectic Nucleation Temperature TC(C) 572.2 568.2 568.8 567.9
Eutectic Growth Temperature TE(C)574.0 569.7 570.2 569.4
ΔΤ2= TE-TC (C)
ΔΤ= TE-575.0 (C)
1.8 1.5 1.4 1.5
-1.0 -5.3 -4.8 -5.6
The increase of the strontium level in the melt from 0% to 0.016% and the eutectic modification result in a decrease of the eutectic temperature from 574.0C to 569.7C, which corresponds to an increase of ΔΤ from -1.0C to -5.3C while ΔΤ2 remains practically unchanged. Further strontium additions do not change the thermal analysis characteristics. A microstructure change from lamellar (0%Sr) to fibrous (0.016%Sr) are characterized by a decrease in Δ Τ from -1.0C to -5.3C. Therefore, the choice of Δ Τ as a parameter to distinguish a non-modified structure from a modified alloy appears to be adequate. Thermal analysis curves were also obtained by remelting 413.0 alloys, and their characteristics are given in Table 7. Silicon modification decreases the eutectic temperature in the order of 3.0C which is substantially less than in the case of a A356.0. The decrease in the eutectic temperature, AT, is not sufficient to distinguish a non-modified structure (0.%Sr) from a undermodified (0.010%Sr) structure as shown by the microstructure in Figure 2. A second parameter, ΔΤ2, seems to reflect more adequately the gradual changes in microstructures; ΔΤ2 decreases from 3.5C to 2.3C when the structure changes from non-modified (0%Sr) to undermodified (0.010%Sr). Further strontium additions (0.0135% and 0.019%) improve the fineness of the eutectic silicon. At a 0.0190%Sr level, the structure is fibrous which corresponds to Δ T2=0.6C.
REDUCTION AND
254
CASTING OF ALUMINUM
TABLE 7. Thermal Analysis Characteristics of 413.0 Sr
(«)
0 0.010 0.0135 0.0190
Eutectic Nucleation Temperature TC(C) 576.8 575.1 575.4 576.8
Eutectic Growth Temperature TE(C)
ΔΤ2= ΔΤ= TE-TC TE-580.0 (C) 3.5 2.3 1.4 0.6
580.3 577.4 576.8 577.4
Allov
(C) + 0.3 -2.6 -3.2 -2.6
CONCLUSIONS The following main conclusions can be drawn from this work: 1.
Strontium modification substantially improves the elongation, and to a lesser extent the tensile strength of an Al-Si eutectic type alloy (413.0). At lower silicon levels (A356.0 alloy), the improvement in elongation is less marked.
2
Impact strength of both 413.0 and A356.0 alloys is very sensitive to the microstructure and is greatly improved by the strontium modification of the eutectic.
3.
Electrical conductivity increases with the fineness of the eutectic silicon and is an excellent method to determine, non-destructively, microstrutural changes in as-cast 413.0 and A356.0 alloys. The conductivity technique is more sensitive to the silicon morphology in the as-cast than in the heat treated stage.
4.
Thermal analysis can be used to evaluate quantitatively and qualitatively the degree of modification of both A356.0 and 413.0 alloys. Both characteristics ΔΤ and Δ Τ 2 are necessary to define the modification of the alloy, especially for the 413.0 alloys. ACKNOWLEDGEMENTS
This work was made possible with the technical collaboration of Grenville Castings. author expresses his thanks to M. Bray and K.D.B. Budd for their technical assistance.
The
REFERENCES B. Closset and J.E. Gruzleski (1982). Met. Transaction A n a 945-951. G 5rtt%M7 U R Zg rl e e S( l1 d9 8 3 ) K. Alker and U. Hielscher (1972).JK Aluminium 19 ' ° · American Foundrymen. R r\?™îvHS 1 ^ A f r i c a n Foundrvmen's Tmn.^tiW so sol-808
^SS^SSA
B. Closset, K. Pine, J.E. Gruzleski (1985). American
Fnu nr^^PfÇ^^^ n
07-316.
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