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In this work, the segregation of solute elements in magnesium alloys AZ91D and AAI60B in a crucible during hot-chamber die casting process has been investigated The sampling of molten metal for chemical composition analysis was carried out at three positions within a crucible of 600 mm in depth. The picked molten metal was poured into a steel mold to cast into a rod 40mm diameter and 120mm length.
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KEUN YONGSOHN, Dept, of Ach'ancedMaterials Research, Korea Institute of Machinery and Materials (KIMM),
KANG MIN CHEOL, Korea Magnesium Technology Research Association (KhITRA), KYUNG HYUN KIM, Dept, of Advanced Materials Research, Korea Institute of Machinery and Materials (KIMM)
SEGREGATION OF SOLUTE ELEMENTS IN CRUCIBLE DURING DIE CASTING
YflK 621 74
Introduction
The uses of magnesium alloys to structural components of automobiles, aircraft, electronics, and computers have been increased and are expected to further grow during next decades because they olfer good castability and high specific strengths [1, 2]. Currently, most of the magnesium-alloy components are produced by die-casting process, during which, in many cases, molten metals are maintained in the crucible for up to several hours before subsequent charging. Commercial magnesium alloys contain aluminum, zinc, or manganese as major alloying elements, whUe they limit the presence of copper, nickel, and iron because of their harmful effect on corrosion resistance [3]. The segregation of the alloying or impurity elements in the crucible may results in severe variations in mechanical properties or corrosion resistance of the magnesium products. Thus, it is important to maintain the compositional consistency of molten metals in the crucible during the casting process.
The objective of this study is to investigate the variations in chemical compositions of magnesium alloys AZ91D and AM60B within a melting crucible during hot chamber die-casting process. The variations on the position from the bottom of crucible and on the holding times have been investigated.
Experimental Procedure
The sampling of molten metal for chemical composition analysis was carried out at three positions (A; bottom, B and C; 100 and 300 mm upside from the bottom, respectively) within a crucible of 600 mm deep (Fig. 1, a) after stirring, using a tool shown in Fig. 1, b. The entrance of gooseneck was located at 300 mm upside (position C) from the bottom of crucible. The samplings were carried out three times at each condition. The picked molten metal was poured into a steel mold to cast into a rod 40mm dia. and 120mm long. The specimen for
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Fig. 1.
Schematic diagram showing (a) sampling position in crucible and (6) sampling tool for chemical composition analysis
chemical composition analysis was cut from the casting into a disc 40mm dia. and 20 nun long, which was polished using a #800 emery paper prior to the analysis.
The chemical analysis was carried out using Hilger Poyvac E2000 emission mass spectrometer. The data are the average of five time measurements. In order to investigate the effect of holding time, the
specimens were sampled from the position 'C' at variable holding times ranging from 0 (right after stirring) to 120 minutes. The stirring was carried out manually for 10 minutes using a mechanical stirrer. Magnesium alloys used in this study were AZ91D and AM60B. The ASTM standard chemical compositions of the alloys were shown in Table 1.
Table 1. ASTM standard chemical compositions of AZ91D and AM60B
Alloys/ Density Al Zn Mn Cu Si Fe Ni Other Mg
AZ91D 8.3-9.7 0.35-1.0 0.15-0.5 <0.03 <0.1 <0.005 <0.002 <0.3 bal.
AM60B 5.5-6.5 <0.22 0.25-0.6 <0.01 <0.1 <0.005 <0.002 <0.3 bal.
Density (g/cm3) 2.70 7.14 7.43 8.96 2.33 7.86 8.90 - 1.74
Results Discussion
and
50 100 150 200 250 300 Distance from Bottom (mm)
A1
1. Effect of position in crocible
Fig. 2 shows the variation in the concentration of
alloying elements and impurity in magnesium-alloy melts on the distance from the bottom of crucible. The concentration of Al, Zn, and Mn was lower at the top of molten metals. Similar trend was also observed for impurity element Fe, as shown in Fig. 2(d). The concentration of the alloying elements increased as the sampling position approaches to the bottom of the cru-cible. At the bottom of the crucible, Al, Fe, Mn showed higher concentrations than the maximum limits of each alloy.
The macro-segregation of alloying elements and impurities within the crucible is related to the relative density and reactivity between the elements. In this study, the sampling was carried out immediately after mechanical stirring. However, the bottom of crucible did not completely mixed with the top or middle regions of the melt because of increased viscosity at the bottom. Although the viscosity of molten metal was not measured, it was found qualitatively that the resistance of stirrer was some-
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50 100 150 200 250 300 Distance from Bottom (mm) Zn
50 100 150 200 250 300 Distance from Bottom (mm)
Mn
c
50 100 150 200 250 300 Distance from Bottom (mm)
Fe
d
Fig. 2. Variations of the concentration of alloying flements and impurity on the position of crucible during
hot-chamber die casting process
what greater at the bottom compared to the other regions. The higher viscosity of the melts indicates that the heavier elements than magnesium such as Al, Zn, Mn, and Fe were concentrated at the bottom, where they form complex intennetallic phases as shown in Fig. 3 by reacting one another during long term-exposure of hot chamber die-casting process.
Fig. 3 shows the SEM microstructures of AZ91D obtained from the bottom, 10 and 30 cm higher region of crucible. The microstructure of AZ91D obtained from the bottom of crucible (Fig. 3, a) showed considerable volume of second phases including complex Al-Fe-Mn
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Element A B c D
C 9.25 13.95 22.55 33.69
O 18.79 3.35 6.35
Mg 0.76 57.95 52.06 53.91
AI 35.51 7.80 18.20 5-31
Mn 49.12
Fe 5.10
ZD 1.51 3.84 0.74
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Element wt(%) atomic(%)
C 8.53 23.33
Mg 0.77 1.05
AI 34.66 42.18
Mn 50.01 29.89
Fc 5.62 3.30
Total 100.00 100.00
Predicted Al-Fe-Mii compound
b 10cm liigh from the bottom
C 30cm high from the bottom
Fig. 3. SEM microstructures of AZ91D samples obtained from different positions of crucibles
compound (A), oxides (B), and inclusions (C). However, the microstructure of AZ91D sample obtained at around gooseneck (Fig, 3, b) showed only trivial amount of Al-Fe-Mn phase. Thus, the increased viscosity at the bottom of crucible is mainly due to Al-Fe-Mn intennetallic phase, which caused macro-segregation during long-time operations.
2. Effect of post-stirring holding time
Post-stirring holding up to 250 minutes did not significantly change the composition of the molten
metals. Fig. 4 shows the variations in the concentration of aluminum and zinc in AZ91D and AM60B on holding time. In both cases, no remarkable variations in the concentration of alloying elements were observed although the concentration of AI in AZ9ID slightly decreased after 240 minutes. In contrast, the concentration of Fe in AZ91D was relatively high (-0.007% in AZ91D) immediately after stirring, which was reduced under 0.005%, maximum limit in AZ91D, after 80 minutes holding. The initial high Fe content is considered as the result of flotation
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50 100 150 200 250 Holding Time (min)
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50 100 150 200 250 Holding Time (min)
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Fig. 4. Variations of the concentration of (a) AJ and (b) Zn on holding time
of Fe-rich melts at the bottom of the crucible due to the stirring.
The above results indicate that the concentration of alloying elements such as A1 and Zn does not significantly vary during short-periods of holding time at around the entrance of gooseneck in hot-chamber die casting system. However, repeated casting operation without bailing out may induce macro segregation in heavier elements such as Al, Zn, Mn and Fe, etc. The increased Fe-concentration at the bottom of crucible may cause higher corrosion rate of die cast products, which need to be completely cleaned up periodically.
Conclusions
During a hot-chamber die casting process, the content of alloying elements and impurities varied with the position of crucible. Heavier elements such
as AI, Zn, Mn Fe, etc. were concentrated at the bottom of crucible, which is resulted from long-time, repeated casting operations without bailing out. The macro-segregation of alloying elements and impurities within the crucible is related to the relative density and reactivity between the elements, by which considerable volumes of Al-Fe-Mn phase were formed at the bottom of crucible. Short-periods of holding time, however, did not significantly vary the concentration of Al and Zn at around the entrance of gooseneck.
References
1. Brown R„ Magnesium Technology 2000 ed. by Kaplan H.I.. Hryn J„ Clow B.. 2000 TMS. March 12-16. NashviUe. Tennessee. USA. 2000. P.3.
2. Sohn K.Y.. Kang M.C.. Kim K.H. Mat. Sci. Forum. 2003. Vol. 419-422. P.135.
3. Han wait J.D. etal. Trans AIME. 1942. Vol. 147. P.273.
