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Improving Aluminum Casting Alloy and Process Competitiveness
Report 07 #2 - A.1
Research Team: Animesh Mandal (508) 831 6503 [email protected] Makhlouf M. Makhlouf (508) 831 5647 [email protected]
Focus Group Members: Adam Kopper Mercury Marine (Chair) Chuck Leonard Citation Corporation Bob Logan Palmer Foundaries Fred Major Alcan International, Ltd. Nat Natesh Chem-Trend David Neff Metaullics Systems Company Cliff Roseen Rogers Corporation Krishnan Venkatesan Citation Corporation David Weiss Eck Industries
PROJECT STATEMENT
Objectives
The objective of this project is to obtain a new family of aluminum-silicon alloys that can be processed by high-pressure die-casting at significantly lower than usual temperatures and therefore is cost-effective; and that is heat-treatable, and therefore is capable of significantly higher ductility, with no sacrifice in other mechanical properties.
Strategy
The project is divided into three phases as follows:
Phase 1 concentrates on developing a new family of aluminum-silicon alloys that is castable at significantly lower than usual temperatures, and therefore are cost-effective; and that is heat-treatable, and therefore is capable of significantly higher than typical mechanical properties. Within the context of Phase 1 is the development of an optimum heat treatment schedule for the new alloy system.
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Phase 2 focuses on adapting the alloy system developed in Phase 1 to the die-casting process. The underlying premise is that die-casting at sufficiently low temperatures will cause negligible soldering of the alloy to the die. Accordingly, much lower iron content than that which is typical to die casting alloys will be required in the new alloy system, thus allowing significantly higher than typical mechanical properties to be attained.
Phase 3 explores the possibility of heat-treating die cast samples of the new alloys to further enhance their performance characteristics. Within the context of Phase 3 is measurement and documentation of the mechanical properties of heat-treated, die-cast samples.
PROJECT TASKS
The following tasks will be performed.
Phase 1
Task 1: Literature review and project planning
Task 2: Thermodynamic simulations and theoretical alloy design
Task 3: Sample production via permanent molds and microstructure analysis
Task 4: Optimization of heat treatment of permanent mold samples
Task 5: Mechanical property measurement and optimization
Sub Task 5.1: Production and heat treatment of tensile test bars (Permanent mold samples)
Sub Task 5.2: Measurement and optimization of room temperature mechanical properties (Permanent mold samples)
Phase 2
Task 6: Sample production via die-casting and microstructure analysis
Task 7: Mechanical property measurement and optimization
Sub-Task 7.1: Production and heat treatment of tensile test bars (Die cast samples)
Sub-Task 7.2: Measurement and optimization of room temperature mechanical properties
(Die cast samples)
Phase 3
Task 8: Optimization of heat treatment of die cast samples
ACHIEVEMENTS TO DATE
Literature Review is in progress (Task 1).
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Thermodynamic Simulations and Theoretical Alloy Design (Task 2).
Sample production via permanent molds and microstructure analysis (Task 3).
Mechanical property measurements and optimization (Task 5) – in progress.
See Appendix A for a status report.
CHANGES IN PROJECT STATEMENT
None
WORK PLANNED BEFORE THE NEXT ACRC MEETING
− Optimization of alloy composition for gravity pour permanent mold casting
− Optimization of heat treatment (gravity pour permanent mold castings)
− Mechanical property measurement and optimization (gravity pour permanent mold castings)
PROJECT DELIVERABLES
Deliverables from this project will include:
− A new family of aluminum-silicon alloys that is castable in permanent molds at significantly lower than usual temperatures, and therefore is cost-effective; and that is heat-treatable, and therefore is capable of significantly higher than typical mechanical properties.
− A new family of aluminum-silicon alloys that is die-castable at significantly lower than usual temperatures, and therefore is cost-effective. These alloys will contain less than usual iron and are heat-treatable; therefore they are capable of significantly higher than typical mechanical properties.
PROJECT SCHEDULE
The expected completion date of the project is December 2009.
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APPENDIX A Status Report
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INTRODUCTION Cast hypereutectic aluminum-silicon alloys have low-density, good high temperature mechanical properties, and are wear-resistant; therefore they are widely used in automotive engine components. However, in recent years, yet better performance has been required from these alloys in order to meet the ever-increasing demands for higher output power from engines. New alloys with lower coefficient of thermal expansion, higher elevated temperature strength, and better abrasion and wear resistance are now needed. The presence of silicon particles in hypereutectic Al-Si alloy castings lowers the castings’ coefficient of thermal expansion [1], and the morphology and distribution of these silicon particles determine the castings’ mechanical properties. The tensile strength of the castings increases with increasing the volume fraction and decreasing the size of the silicon particles, and spherical silicon particles are preferred over angular particles, as they do not act as stress concentrators. Generally, the silicon content of the alloy and the cooling rate during solidification are process parameters that control the volume fraction and size of the silicon particles, while chemical modifiers such as strontium and phosphorus are effective in changing the particle shape from angular to spherical.
Today, almost all aluminum engine blocks are produced by die-casting 390 (Al-17%Si-4.5%Cu-0.5%Mg) alloy [2], and a review of the open literature shows that only very little work has been done recently on developing new hypereutectic Al-Si alloys [3-6]. The literature review also reveals that most of the research effort in hypereutectic Al-Si alloys has been confined to alloys with less than 0.6 wt% Mg and less than 5 wt % Cu. However, it has been recently shown that magnesium contents in excess of the typical quantities found in commercial hypereutectic Al-Si alloys can produce alloys with enhanced microstructures, which may have attractive mechanical properties. Figure 1 shows isopleths and microstructures of hypereutectic Al-14Si-0.2Fe-xMg alloys. The isopleths were created using the commercial thermodynamic software Pandat and show that with the addition of Mg, the Al-Si phase diagram is no longer a simple eutectic invariant, but rather many phases form during solidification of these alloys. Most importantly is that formation of the primary silicon phase is suppressed upon adding Mg, and large Chinese script Mg2Si particles form and their amount increases with increased Mg. Mg2Si has high potential as a phase component in aluminum alloys because of its high melting temperature (1085oC), low density (1.99 g/cc), high hardness (4.5 GPa), low coefficient of thermal expansion (7.5 x 10-6 K-1), and reasonably high elastic modulus [7]. However, its presence in the form of large Chinese script particles detracts from the alloys’ mechanical properties.
In this report we present the progress made to date on optimizing the chemical composition of hypereutectic Al-Si-Mg alloys in which the Mg content is in excess of 2 wt%. Most of the research effort during this reporting period has been devoted to overcoming the negative effects that Mg2Si with Chinese script morphology has on the alloy by (i) manipulating the size and morphology of the Mg2Si particles, and (ii) by suppressing the formation of the Mg2Si particles.
After a description of the materials and procedures used in the various experiments, the report includes a section that details the research that has been performed to-date on controlling the morphology of Mg2Si particles by means of small quantities of chemical additives, including yttrium, cerium, strontium, phosphorus, and calcium. This is followed by a section that details the research that has been performed to-date on suppressing the formation of Mg2Si particles by the addition of copper. The presence of a sufficient amount of copper in the alloy favors the formation of the Q and θ phases over the formation of Mg2Si. Although sluggish, fragmentation,
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spherodization, and dissolution of the Q and θ phases are possible by an appropriate heat treatment regimen. Development of this heat treatment regimen will be one of the objectives of the work done during the next reporting period.
0 wt% Mg 1wt% Mg 2wt% Mg
3 wt% Mg 4 wt% Mg 5wt% Mg
Figure 1. Isopleths and microstructures of Al-14Si-0.2Fe-xMg alloys.
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MATERIALS AND PROCEDURES Melt and Specimen Preparation The chemical composition of the alloys used during this effort is presented in Table I. The alloys were produced from pure Al ingots, Al-50%Si, Al-52%Mg and Al-80%Fe master alloys.
For initial microstructure characterization, small amounts (approximately 0.5 lbs) of each alloy was prepared in a box type resistance furnace and poured into sand cups to obtain near equilibrium microstructure. The melt chemistry was checked using a Spectro-max spark test analyzer. Samples for microstructure characterization were obtained from the centre of the casting. All samples were sectioned from identical locations in the castings to ensure that th e microstructures represent similar cooling rates.
For making tensile test bars, approximately 40 lbs of melt were prepared for each alloy. The charge materials were melted in clean silicon carbide crucibles in a resistance furnace, the melts were heated to 750-800°C for at least 30 minutes in order to ensure dissolution of all the alloying elements. The melt chemistry was checked using a Spectro-max spark test analyzer and adjusted until the target composition was achieved. The melts were then degassed using a rotary degasser for 40 minutes. The melt chemistry was re-checked. Standard tensile test specimens were produced in a permanent mold. The mold was preheated and maintained at 425±5oC during pouring. Approximately twenty tensile test bars were produced from each alloy composition.
Table I. Composition of Al-14Si-0.2Fe-xMg alloys used for the study. Alloy Si Fe Mg Al
A0 14.05 0.23 - Bal. A1 13.95 0.22 1.08 Bal. A2 13.98 0.23 2.05 Bal. A3 14.01 0.21 3.06 Bal. A4 14.06 0.25 4.01 Bal. A5 14.04 0.25 5.02 Bal.
Heat Treatment and Tensile Testing The tensile test bars were solution heat treated immediately after casting. The heat treatment was performed in a ThemoLyne 30400 box furnace and the temperature was maintained to within ±5°C from the set temperature.
Five sets (each set containing 5 tensile test bars) were prepared as follows: Set 1: As cast condition Set 2: Solution-treated at 520°C for 12 hours and natural aged for 1 day (T4 temper) Set 3: Solution-treated at 520°C for 12 hours and natural aged for 10 days (T4 temper) Set 4: Solution-treated at 520°C for 12 hours and artificially aged at 200°C for 4 hours
(T6 temper) Set 5: Solution-treated at 520°C for 12 hours and artificially aged at 200°C for 8 hours
(T6 temper)
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Tensile tests were conducted in an Instron Universal Tensile Testing Machine, Model 5550. An MTS extensometer with a 2 inch gage length was used and was allowed to remain attached to the specimen until the specimen broke. The ramp rate of the machine was 0.1 in/min and all the tests were conducted at room temperature.
Metallographic Analysis The microstructure of the as-cast and heat-treated specimens was analyzed using optical and scanning electron microscopy, and particle size analysis was performed using an image analyzer. Images were taken from different regions in a sample and analyzed to arrive at an average value. For each case twenty frames from different areas of the sample were chosen in order to insure that the information is representative of the microstructure. The number of particles observed in each frame varied from frame to frame depending on the size of the Mg2Si particles. However, it was always ensured that the data reported is based on an average of at least two hundred particles.
All the samples were etched with Kellers reagent for optical microscopy and image analysis. For SEM studies, the samples were deep etched with 2% HF solution. The Cu containing alloys were etched with 1g NaOH dissolved in 100 ml distilled water at 50°C followed by swabbing with 5% HNO3 acid solution for 5-10 seconds. The size of the Mg2Si particle was evaluated as the square root of the product of the lengths of the particles in the X and Y directions.
RESULTS AND DISCUSION Establishing the Base Alloy Figure 2 shows typical microstructures of Al-14Si-0.2Fe-xMg alloys solidified in sand molds at 45 K/min. Figure 2a shows the typical microstructure of hypereutectic Al-14Si-0.2Fe-0Mg. The microstructure contains mainly eutectic silicon together with a few primary silicon particles dispersed in an α-Al matrix. Addition of 1 wt% Mg to this alloy (Figure 2b) leads to the formation of Mg2Si particles with a typical Chinese script morphology. Further addition of Mg leads to the formation of coarser Mg2Si particles without any change in their morphology (Figure 2c). In the Al-14Si-0.2Fe-3Mg alloy (Figure 2d), the eutectic silicon is refined and the size of Mg2Si particles does not increase appreciably. The alloy with 4 wt% Mg (Figure 2e) does not show any significant change in the size of Mg2Si particles with respect to the alloy containing 3 wt% Mg. Also the extent of refinement of eutectic silicon is somewhat diminished. It is interesting to note that in the alloy with 5 wt% Mg (Figure 2f) Mg2Si crystals (of about 40 µm) with sharp corners appear in the microstructure along with coarse Chinese script Mg2Si particles. This morphology of Mg2Si is not desirable as it may affect the ductility of the alloy.
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Figure 2. Optical micrographs of Al-14Si-0.2Fe-xMg alloy solidified at 45 K/min. (a) x = 0 wt% Mg (b) x = 1 wt% (c) x = 2 wt% Mg (d) x = 3 wt% Mg
(e) x = 4 wt% Mg (f) 5 wt% Mg.
a b
c d
e f
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Figure 3 shows that addition of Mg not only lowers the eutectic temperature of the alloy, but also shifts the eutectic composition towards higher Si contents. For example, in the alloy with 4 wt% Mg the eutectic Si content shifts to 13.5 wt% Si and the eutectic temperature drops to 560°C. This explains the presence of only very few primary silicon particles in the alloys with high Mg content.
0 1 2 3 412.6
12.8
13.0
13.2
13.4
13.6
560
564
568
572
576
Variation of Eutectic point in Al-14Si-0.2Fe-Mg alloys
Wt.% Mg
Eu
tec
tic
Si,
wt.
%
Eu
tec
tic
Te
mp
era
ture
, o
C
Figure 3. Variation of the eutectic point with the Mg content of Al-14Si-0.2Fe-xMg alloys.
Based on this analysis, the alloy with the chemical composition Al-14Si-0.2Fe-3Mg was selected as the base alloy for further optimization.
Effect of Cooling Rate – Figure 4 shows typical microstructures of Al-14Si-0.2Fe-xMg alloys solidified in steel molds at 220 K/min. Although there is a noticeable decrease in the size of the Mg2Si paticles at the higher cooling rate, there is no significant change in its morphology - the Chinese script still persists.
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Figure 4. Optical micrographs of Al-14Si-xMg-0.2Fe alloy solidified at 220 K/min. (a) x = 0 wt% Mg (b) x = 2 wt% (c) x = 3 wt% Mg.
Similarly, Figure 5 shows typical microstructures of Al-14Si-0.2Fe-xMg alloys solidified in copper molds at 300 K/min.
The microstructure is refined significantly as compared to that of samples solidified in the steel mold. In this case, both eutectic and Mg2Si particles show significant reduction in size. However, the Chinese script morphology of the Mg2Si particles still persists as seen in Figure 6.
a b
c
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Figure 5. Optical micrographs of Al-14Si-xMg-0.2Fe alloy solidified at 300 K/min. (a and b) x = 0 wt% Mg (c and d) x = 2 wt% (e and f) x = 3 wt% Mg.
a b
c d
e f
13
Figure 6. SEM micrograph showing the morphology of Mg2Si in Al-14Si-0.2Fe-3Mg alloy solidified at 300 K/min.
Figure 7 shows the average size of the Mg2Si particles in Al-14Si-0.2Fe-xMg alloys at different cooling rates. At slower cooling rates, the variation in size of the Mg2Si particles is high, but when the alloys are cooled at relatively higher cooling rates, there is hardly any variation in the size of the Mg2Si particles with increased Mg content.
2.0 2.5 3.00
20
40
60
80
100
120
140
160
180
200
Siz
e o
f M
g2
Si
wt.% Mg
Al-14Si-0.2Fe-xMg alloys
Sand Cast (45oC/min)
Metal Mold (220oC/min)
Copper Mold (300oC/min)
Figure 7. Change in size of Mg2Si particles in Al-14Si-0.2Fe-xMg alloys solidified at different cooling rates as a function of Mg content.
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The wear resistance of Mg2Si particles improves the overall wear characteristics of the alloy. In addition, a refined form of Mg2Si particles can enhance the tensile properties of the alloy, especially ductility. Since the optimized alloy will eventually be die-cast at cooling rates that will certainly exceed 300 K/min, the Mg2Si particle size will be small; however, the Chinese script morphology is detrimental irrespective of particle size. Hence an attempt was made to modify the morphology of the Mg2Si particles from Chinese script to a less detrimental form by means of chemical additives.
Modification of Mg2Si Morphology by Chemical Additives The base alloy, namely Al-14Si-0.2Fe-3Mg was treated with different chemical modifiers that were believed will alter the Chinese script morphology of Mg2Si into a more useful morphology. The chemical additives used are listed in Table II.
Table II. Master alloys used to modify the morphology of Mg2Si particles. Master Alloy Addition level (wt%) Al-87% Y 0.25, 0.50, 0.75, and 1.00 Al-99%Ce 0.25, 0.50, 0.75, and 1.00 Al-10wt%Sr 0.025, 0.050, 0.5, and 1.00 Cu-15wt%P 0.02, 0.05, 0.2, and 0.5 Al-10wt%Ca 0.2, 0.5, and 1.0
Yttrium – The optical micrographs in Figure 8 show that addition of yttrium does not alter the microstructure of the hypereutectic alloy significantly. The morphology of the eutectic silicon and Mg2Si particles does not change with the addition of Y; however, the Mg2Si particle size decreases somewhat with Y additions as seen in Figure 9.
a b
15
Figure 8. Optical micrographs of Al-14Si-0.2Fe-3Mg alloy. (a) Y = 0.5 wt%, (b) Y = 0.75 wt%, and (c) Y = 1 wt%.
0.50 0.75 1.00
165
170
175
180
185
Part
icle
siz
e o
f M
g2
Si
wt% Y
Figure 9. Size of Mg2Si particles in Al-14Si-0.2Fe-3Mg alloy with different Y contents.
Cerium – As seen in Figure 10, the addition of cerium to Al-14Si-0.2Fe-3Mg alloy has a pronounced effect on the size distribution of coarse Mg2Si particles. The typical Chinese script morphology of Mg2Si is modified and/or fragmented to smaller Mg2Si particles. This fragmentation is seen in the case of the alloys modified with 0.25 and 0.50 wt% Ce, however the beneficial effect of Ce seems to fade at higher addition levels. As seen in Figure 11, the Mg2Si
c
16
particle size shows a minimum at about 0.5wt% Ce. Thereafter, the size of Mg2Si particles increases. The mechanism of modification of Mg2Si by cerium will be investigated.
Figure 10. Optical micrographs of Al-14Si-0.2Fe-3Mg alloy. (a) Ce = 0.25 wt%, (b) Ce = 0.5 wt%, and (c) Ce = 0.75 wt%, and (d) Ce = 1.00 wt%.
a b
c d
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0.00 0.25 0.50 0.75 1.0020
40
60
80
100
120
140
160
180
Pa
rtic
le s
ize o
f M
g2
Si
wt% Ce
Figure 11. Size of Mg2Si particles in Al-14Si-0.2Fe-3Mg alloy with different Ce contents.
Strontium – As seen in Figure 12, addition of strontium to the base alloy has a similar effect on the morphology of the Mg2Si particles as cerium, though the effect occurs at much lower addition levels. At 0.025 and 0.050 wt%, the eutectic silicon and Mg2Si particles are significantly modified. The Mg2Si particles are modified from the typical Chinese script to various other morphologies as seen in Figure 13. However it should be mentioned that at few places in the microstructure, coarse Mg2Si particles still exist. At higher Sr levels the eutectic silicon particles coarsen and so does the Mg2Si particles suggesting over modification of the alloy at higher addition levels of Sr.
a b
18
Figure 12. Optical micrographs of Al-14Si-0.2Fe-3Mg alloy. (a) Sr = 0.025 wt%, (b) Sr = 0.05 wt%, (c) Sr = 0.5 wt%, and (d) Sr = 1.00wt%.
Figure 13. SEM micrograph showing the morphology of Mg2Si in Al-14Si-0.2Fe-3Mg-0.025Sr alloy.
Figure 14 shows that at 0.025 and 0.05 wt% Sr, the difference in size of Mg2Si particles is negligible. However, the size increases drastically when the alloy is over modified with 0.5 wt% and above.
c d
19
0.00 0.25 0.50 0.75 1.00
50
55
60
65
70
Part
icle
siz
e o
f M
g2
Si
wt% Sr
Figure 14. Size of Mg2Si particles in Al-14Si-0.2Fe-3Mg alloy with different Sr contents.
Cerium + Strontium – While keeping the level of Ce fixed at 0.25 wt%, strontium levels were varied between 0.025 and 0.050 wt%. In both cases, Mg2Si particles were refined significantly as seen in the optical micrographs of Figure 15. The microstructure was more uniform when the combined addition was done. This may be due to the different modifying action of each element. While Ce modifies the Mg2Si particles, Sr modifies both Mg2Si particles and the eutectic Si particles. There was no significant difference in the microstructure between the alloy with (0.25Ce + 0.025Sr) and the alloy with (0.25Ce + 0.05Sr). Figure 16 shows the modified morphology of the Mg2Si particles. Notice the presence of a few spheroids Mg2Si particles in this microstructure.
Figure 15. Optical micrographs of Al-14Si-0.2Fe-3Mg-0.25Ce alloy.
(a) Sr = 0.025 wt%, and (b) Sr = 0.05 wt%.
a b
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Figure 16. SEM micrograph showing the morphology of Mg2Si in Al-14Si-0.2Fe-3Mg-0.025Sr alloy.
Heat treated tensile test bars were used to measure the room temperature properties of Al-14Si-0.2Fe-3Mg-0.25Ce-0.025Sr alloy. After testing, the tensile test bars were sectioned and used for microstructure analysis. As seen in Figure 17a, the eutectic silicon in the as-cast condition is completely modified. A faster cooling rate in the cast iron tensile test bar mold changes the morphology of the Mg2Si particles to a mostly dot like submicron particles. After solution treatment there is complete spheroidization of the eutectic silicon and As Figure 17b and 17c show, there is no significant difference between the microstructure of the alloy in the T4 and T6 conditions.
c
a b
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Figure 17. Optical micrographs of Al-14Si-0.2Fe-3Mg-0.25Ce-0.025Sr. (a) as-cast, (b) T4 - naturally aged for 24 hours, (c) T6 - aged at 200°C for 4 hours.
Figure 18 clearly shows complete spheroidization of the eutectic Si in the T4 and the T6 condition.
Figure 18. SEM micrographs of Al-14Si-0.2Fe-3Mg-0.25Ce-0.025Sr. (a) T4 - naturally aged for 24 hours, and (b) T6 - aged at 200°C for 4 hours.
Table III shows the room temperature tensile properties of Al-14Si-0.2Fe-3Mg-0.25Ce-0.025Sr as a function of heat treatment.
Mg2Si
Eutectic Si
c
a b
22
Table III. Room temperature tensile properties of Al-14Si-0.2Fe-3Mg-0.25Ce-0.025Sr. Temper designation Heat treatment UTS (ksi) YS (ksi) % EL QI*
As-cast - 30.4 19.4 2.67 273.4 1 day 35.0 24.4 3.96 331.0
T4 10 days 39.2 26.0 4.42 367.3
4 hours – 200°C 45.5 42.8 0.89 305.9
T6 8 hours – 200°C 43.8 41.1 0.98 300.5
* QI ≡ Quality Index = UTS (MPa) + 150 log [% El.]
Figure 19 shows images of the fractured surface of the alloy. In the as-cast condition, the alloy has an elongation of 2.67%, the characteristics of brittle fracture is clearly evident from the fractograph in Figure 19a. In the T4 temper condition, the alloy shows an improvement in ductility to 4.42% and that is reflected by the presence of dimples on the fracture surface in Figure 19b. In the T6 temper condition, matrix cracking is evident (Figure 19c), which results in poor ductility of 0.89%.
a b
23
Figure 19. SEM fractographs of Al-14Si-0.2Fe-3Mg-0.25wt%Ce-0.025wt%Sr (a) as-cast (b) T4 – 1 day (c) T6 – 4 hours
Phosphorus – Phosphorus additions varied between 200 ppm to as high as 1 wt%. The lower additions (i.e., 200 and 500 ppm) did not change the size and morphology of the Mg2Si and eutectic silicon particles as seen in Figure 20. An increase in the phosphorus level to 0.2 wt% reduces the Mg2Si and eutectic silicon particle size, but not unfiformly throughout the structure. Further increase in the P content to 0.5 wt% leads to over modification. Figure 21 shows the change in size of the Mg2Si particles with P additions.
a b
c
24
Figure 20. Optical micrographs of Al-14Si-0.2Fe-3Mg alloy. (a) P = 0.02 wt%, (b) P = 0.05 wt%, (c) P = 0.2 wt%, and (d) P = 0.5 wt%.
0.1 0.2 0.3 0.4 0.5
80
100
120
140
160
Pa
rtic
le s
ize
of
Mg
2S
i
wt% P
Figure 21. Size of Mg2Si particles in Al-14Si-0.2Fe-3Mg alloy with different P contents.
Heat treated tensile test bars were used to measure the room temperature properties of Al-14Si-0.2Fe-3Mg-0.2P alloy. After testing, the tensile test bars were sectioned and used for microstructure analysis. As seen in Figure 22, phosphorus does not modify the eutectic Si through out the microstructure. Also a few coarse Mg2Si particles are present in the microstructure.
c d
25
Figure 22. SEM micrographs of Al-14Si-0.2Fe-3Mg-0.2P. (a) T4 - naturally aged for 24 hours, and (b) T6 - aged at 200°C for 4 hours.
Table IV shows the room temperature tensile properties of Al-14Si-0.2Fe-3Mg-0.2P as a function of heat treatment.
Table IV. Room temperature tensile properties of Al-14Si-0.2Fe-3Mg-0.2P. Temper designation Heat treatment UTS (ksi) YS (ksi) % EL QI*
As-cast - 26.1 19.2 1.01 180.5 1 day 34.8 25.3 1.84 279.7
T4 10 days 36.2 27.8 2.33 304.8
4 hours – 200°C 41.0 39.7 0.59 248.4
T6 8 hours – 200°C 42.0 37.6 0.54 249.2
* QI ≡ Quality Index = UTS (MPa) + 150 log [% El.]
Calcium – As seen in Figure 23, addition of 0.2 wt% Ca shows only a slight change in the microstructure of the base alloy. When the Ca level is increased to 0.5 wt%, the eutectic microstructure is significantly refined, but only in patches. A higher level of Ca (i.e., 1 wt%) decreases the Mg2Si particle size, but it also leads to the formation of complex elongated Ca-rich phases as shown in the micrograph of Figure 23c. Figure 24 shows the change in the size of the Mg2Si particles with Ca additions.
a b
26
Figure 23. Optical micrographs of Al-14Si-0.2Fe-3Mg alloy (a) Ca = 0.2 wt%, and (b) Ca = 0.5 wt%.
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
60
80
100
120
140
160
Pa
rtic
le s
ize
of
Mg
2S
i
wt% Ca
Figure 24. Size of Mg2Si particles in Al-14Si-0.2Fe-3Mg alloy with different Ca contents
Ca rich phase
a b
c
27
Table V shows the room temperature tensile properties of Al-14Si-0.2Fe-3Mg-0.5Ca as a function of heat treatment.
Table V. Room temperature tensile properties of Al-14Si-0.2Fe-3Mg-0.5Ca.
Temper designation Heat treatment UTS (ksi) YS (ksi) % EL QI* As-cast - 26.9 20.5 1.14 194.2
1 day 37.5 26.5 2.51 318.6
T4 10 days
4 hours – 200°C 42.2 40.5 0.8 276.3
T6 8 hours – 200°C 46.0 44.7 0.7 293.9
* QI ≡ Quality Index = UTS (MPa) + 150 log [% El.]
Suppressing the Formation of Mg2Si Hypereutectic Al-14Si-3Mg-0.2Fe with Cu addition was simulated using Pandat. The objective was to develop alloys with high Mg content while avoiding the formation of Mg2Si particles and forming only Al5Cu2Mg8Si6 (the Q phase) and possibly Al2Cu (the θ phase) in the alloy. The premise is that the morphology of the Q phase can be modified by suitable heat treatment which was not possible with Mg2Si particles.
Figure 25 shows the various phases that form in Al-14Si-3Mg-0.2Fe alloy as a function of Cu addition. At approximately 2 wt% Cu, the amount of Mg2Si retained in the alloy is negligible, and while the Al2Cu phase begins to form, the amount of the Al5Cu2Mg8Si6 phase remains more or less constant. Such an alloy is hoped to give better properties when the Q phase is spheroidized by an appropriate heat treatment.
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0 1 2 3 4 5 60
1
2
3
4
5
6
7
8
9
10
Al2Cu
Mg2Si
(retained)
Mg2Si
Al5Cu
2Mg
8Si
6
Co
nsti
tuen
t (w
t.%
)
Cu (wt.%)
Figure 25. Computer simulation showing the various phases in Al-14Si-3Mg-xCu alloy.
Figure 26a shows an optical micrograph of as the as-cast Al-14Si-3Mg-2.5Cu alloy slow cooled in a sand mold. In the as-cast condition, the eutectic silicon along with the Al2Cu and Q phase are observed in the microstructure. As predicted in Figure 25, the Mg2Si particles are absent in this alloy. When the alloy is solutionized at 480°C for 12 hours (Figure 20b), Al2Cu particles are dissolved completely and the size of the Q phase particles is significantly reduced. After 24 hours, the size of the Q phase particles decreases significantly but with no significant change in the morphology of the eutectic silicon (Figure 20c). However, after 36 hours, there is a noticeable change in the eutectic silicon morphology and the Q phase spherodizes (Figure 26d). Figure 27 shows the decrease in the average size of the Q phase particles as a function of increasing solutionizing time. After 36 hours at 480°C, the Q phase particles are less than 9 µm in diameter, moreover, their aspect ratio changed from 2.1 in the as-cast microstructure to 1.2 in the solutionized microstructure.
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Figure 26. Optical micrographs of Al-14Si-3Mg-2.5Cu-0.2Fe alloy. (a) as-cast, (b) solutionizing time, ST = 12 hours, (c) ST = 24 hours, and (d) ST = 36 hours.
Q phase
Q phase
Q phase
Q phase
Al2Cu
a b
c d
Si
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As cast ST = 12 h ST = 24 h ST = 36 h
8
10
12
14
16
18
Siz
e o
f Q
ph
as
e
Heat Treatment Condition
Figure 27. Effect of solutionizing time (ST) on the average size of the Q phase in Al-14Si-3Mg-2.5Cu alloy.
SUMMARY Hypereutectic Al-14Si-0.2Fe-xMg alloys were investigated. Based on microstructure analysis it was confirmed that Al-14Si-3Mg-0.2Fe alloy is a useful alloy for further investigation. However, Mg2Si particles with Chinese script morphology develop in these alloys and seem to unfavorably influence their room temperature tensile properties. Increasing the cooling rate from 45 to 300 K/min significantly reduces the size of the Mg2Si particles, but does not change their morphology. Hence, different chemical additions were employed in an effort to change the morphology of the Mg2Si particles from Chinese script to a morphology that is less detrimental to the tensile properties of the alloy.
Varying amounts of Y, Ce, Sr, and P were investigated. Y did not seem to have an effect on the morphology of Mg2Si. However, alloys modified with cerium, strontium and a combination of both elements showed promising results: The Mg2Si particles were fragmented and an average size of about 40 µm could be achieved during slow cooling. Moreover, finer dot-like Mg2Si particles developed in alloys that were solidified in permanent molds. As a result, an elongation of 4.42% in the T4 condition could be achieved in these alloys. Although the amount of Ce used in the alloy is somewhat high for commercial purpose, the amount required may be reduced considerably if higher cooling rates are employed, such as in die casting. The addition of P and Ca did not show much improvement in tensile properties as compared to combined effect of Ce and Sr. Addition of copper to the Al-14Si-3Mg alloy was assessed by thermodynamic simulations and a few exploratory experiments. Addition of Cu suppresses formation of Mg2Si in favor of the Q and θ phases. Experiments with solution treatment showed that the Q phase could be
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spheroidised if a proper solutionizing temperature and time are used. Further work will address optimizing the solutionizing treatment.
REFERENCES
1 ACRC Report #06-2 (2006). 2 J.E. Hatch, Aluminum, Properties and Physical Metallurgy, ASM, Metals Park, OH,
1984, pp. 346– 347. 3 P. Kapranos et al., J. Mater. Proc. Tech 135 (2003) 271. 4 S. Spigarelli, M. Cabibbo, E. Evangelista, S. Cucchieri, Mater Lett 56 (2002) 1059. 5 S. Spigarelli, E. Evangelista, S. Cucchieri, Mater. Sci. Eng. A387–389 (2004) 702. 6 L. Lasa, J.M. Rodriguez-Ibabe, Mater. Sci. Eng. A363 (2003) 193. 7 J. Zhang, Z. Fan, Y.Q.Wang, B.L. Zhou, Mater Sci Eng. A281(2003) 104-112.
