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The Influence of Inoculation on the Metal Expansion Penetration With Respect to the Primary and Eutectic Solidification

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The Influence of Inoculation on the Metal

Expan-sion Penetration With Respect to the Primary and

Eutectic Solidification

Izudin Dugic, Attila Diószegi and Ingvar L Svensson

Research report 2005:2, ISSN 1404-0018

Jönköping University

Dept of Mechanical Engineering/Component Technology P.O. Box 1026

SE.-551 11 Jönköping Sweden

Jönköping, 2005

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Abstract

The mechanism of metal expansion penetration of grey cast iron components is depend-ent on both solidification anomalies at the metal – mould interface and the inclination of the sand mould to permit the metal liquid to penetrate between the sand grains. The pre-sent work utilizes the latest development of primary austenite inoculation in combination with classic eutectic inoculation to limit the metallurgical contribution to metal expansion penetration. A solid shell containing the primary austenite dendrite network constitutes the barrier between the liquid metal and mould interface. Inoculants of both the primary- and eutectic phase control the permeability of the dendrite network.

Keywords: Grey iron, primary austenite, hot spot, metal expansion penetration, dendrite, inoculation.

NOMENCLATURE

D Diameter of eutectic cell, µm

DAS Secondary Dendrite Arm Spacing, µm

NV Number of eutectic cells, m-3

NA Number of austenite grains, per 25 mm

NAV Number of austenite grains, m-3

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1. INTRODUCTION

The solidification of grey cast iron is a complex process. The most widely used commer-cial grey cast irons are hypoeutectic, in which solidification starts with the nucleation and growth of austenite grains followed by nucleation and growth of an austenite-graphite eutectic.

The nucleation and growth of primary austenite coupled to the nucleation and growth of the eutectic phase was elucidated recently in a work by Diószegi et al [1]. They investi-gated different type of inoculants with respect to both the primary and the eutectic nu-cleation and found that refinement of the primary austenite grains contributed to an in-creased number of eutectic cells.

Much research on the nucleation mechanism of flake graphite has been carried out worldwide [2-6]. Metal expansion penetration was briefly investigated by Levelink and Julien [7] who described and investigated the phenomena of metal exudation penetration. The aim of this work is to utilize the latest development of primary austenite inoculation in combination with classic eutectic inoculation to limit the metallurgical contribution to metal expansion penetration.

2. MATERIALS AND EXPERIMENTAL PROCEDURES 2.1. Casting cup

The inoculants were tested using a specially designed casting cup, see figures 1 to 4. The optimization of the casting cup was made with help of the casting simulation program MAGMASOFT®. The aims of the casting simulation were:

- to optimise the design of the casting cup - to locate the position of the thermal centre

The reason for using the core (figure 1) was to enable metal penetration formation in the casting cup. The diameter of the test cup was 80 mm with a height of 80 mm. The inter-nal core diameter was 30 mm, and was rounded at the end in contact with the bulk metal with a radius of 15 mm. The sand wall of the cover and bottom of the test cup was 30 mm thick, while the sand wall of the radial mould surface was 20 mm thick. The test cups were produced in quartz sand, bonded by an organic binder using SO2 –gas as catalyst.

The mould surface was not treated with any traditional coating. A thermocouple (type K) was placed in the cover to measure the temperature of the test cup during the experiment, see figures 5 and 6.

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Figure 1. Casting cup Figure 2. Cover of the casting cup.

Figure 3. Casting cup with cover Figure 4. Solidification time.

2.2. Chemical composition

The molten iron used in the experiments was directly tapped from an induction holding furnace on a production line. The temperature of the iron was in the range 1460-1480 ºC. The chemical composition was determined from cast coin specimens (see table 1), which were cast immediately before the melt was poured into the moulds. During the progres-sion of the experiments the chemical composition in the base melt changed slightly, which affected the chemical composition of the samples.

2.3. Nucleating agents

The choice of inoculant used in grey cast iron production today is probably one of the most important parameters in obtaining good quality castings. The inoculant selected de-termines the structure of the grey cast iron and it is believed that it may also affect the degree of metal expansion penetration. Various different nucleating agents were added to the iron, to promote both the nucleation of primary austenite and the nucleation of the austenite-graphite eutectic. The specification of the nucleating agents used in the experi-ment is listed in table 2, and their chemical compositions are shown in table 3.

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2.4. Experimental setup

The casting experiments were carried out at the Volvo Foundry in Skövde. Twenty cast-ing cups were cast uscast-ing the different nucleatcast-ing agents and compared to the untreated melt. Each pouring was done as soon as possible after transferring the melt from the hold-ing furnace into the ladle. After pourhold-ing, the cover containhold-ing the thermocouple was placed on the top of the test cup. Two casting cups were cast from each of the same treated melts, see figure 6, and from one cup the cooling curve was registered. One sam-ple was cooled under normal conditions (denoted: as cast) and the other was direct austempering after solidification (DAAS)-treated. A description of each sample is listed in table 4.

Figure 5. The casting cup Figure 6. Casting cups

2.2. DAAS Treatment

DAAS treatment – Direct Austempering After Solidification, was used by Rivera, Boeri and Sikora [8] to investigate the macrostructure of grey cast iron. In this special thermal process the austenite is retained and keeps the crystalline orientation defined during so-lidification. After using special colour metallographic techniques the grain structure can be revealed. The procedure is as follows:

- after pouring, the cast parts are shaken out from the mould when their tempera-ture is approximately 950 ºC,

- the parts are transferred into a furnace held at 900 ºC and held for 30 min to al-low temperature equilibration,

- the parts are then austempered in a molten salt bath held at 360 ºC and held for 90 min. before finally air-cooling to room temperature.

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3. EXPERIMENTAL RESULTS

3.1. Macro- and microstructure examination

The macro- and microstructure examination was carried out at the Department of Com-ponent Technology-Castings, Jönköping University. The samples obtained after cooling to room temperature were cut in two symmetrical parts through the thermal hot spot and the metal-mould interface was examined. The internal section of the sample which was exposed for a long solidification time was examined with respect to metal penetration. A scale from 1 to 5 to classify the degree of penetration was used (5 is a high degree of penetration, 1 is a low degree of penetration). Figures 7 to 9 show some typical examples demonstrating various degrees of metal penetration. Data from the penetrated area from the experiments are collected in appendix 1.

Figure 7. The metal penetra-tion area, degree 1.

Figure 8. The metal penetra-tion area, deg. 2 – 3.

Figure 9. The metal penetra-tion area, deg. 4–5.

Figure 10. The microstructure close to the metal penetration area, degree 1.

Figure 11. The microstructure close to the metal penetration area, degree 2-3.

Figure 12.The microstructure close to the metal penetration area, degree 4-5.

In addition, one half of the as cast and one half of the DAAS treated samples were ppared for microstructure investigation using a colour etching method. The etching re-agents were slightly different for the macro- and microstructure investigations and are described in table 5. Figures 10 to 12 show the microstructure close to the metal penetra-tion area.

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Both the macrostructure and microstructure examinations were conducted for three dif-ferent zones on the cross-section of the sample. Figure 13 shows the positions of the zones investigated.

Eight of the twenty DAAS treated samples were investigated with respect to the number of primary austenite grains. The ground and polished sample was colour etched, and the austenite grain distribution thereby revealed. A reproduction of the austenite grain by photography is very difficult due to the various crystallographic orientations of austenite grains. Instead, a sketch of the austenite grain borders is presented in figure 14. The grey area represents austenite forming the outer shell of columnar grains. The light area repre-sents the internal equiaxed austenite grains. The results of macrostructure investigation are presented in table 6a and b. In appendix 2 the sketches of the austenite grain distribu-tion from the cross secdistribu-tions of the casting cups are collected.

Figure 13. The zones (1 to 3) of the cast-ing cup which were investigated.

Figure 14. Primary austenite grain distri-bution

on the cross section of a penetration test cup.

The as cast samples were examined with respect to the size/number of eutectic cells and with respect to the secondary dendrite arm spacing. The results of microstructure investi-gation are presented in tables 6a and 6b.

4. DISCUSSION

In this report two different types of metal expansion penetration were observed:

- in the first group - metal expansion penetration occurred before the columnar to equiaxed transition, as shown in figure 15

- in the second group - metal expansion penetration occurred after the columnar to equiaxed transition, as shown in figure 17.

1 2

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Figure 15. Metal expansion penetra-tion before the columnar to equiaxed transition

Figure 16. Metal expansion penetration before the columnar to equiaxed transition.

In a parallel study we have described the mechanisms of metal expansion penetration [9]. From the metallographic analyses a certain amount of sand grains were found to be ad-hering to the metal surface, and a metallic strip connected to the test cup surface was found (figure 15).

The microstructure at the metal-sand interface for the first type of penetration is pre-sented in figure 16. The bulk of the sample has a normal structure consisting of a dendrite network with interpenetrating eutectic cells. Eutectic phases with graphite flakes and eutectic austenite were observed in the adherent metal strip. From the metallographic in-vestigation we can see that the graphite flakes are mainly oriented perpendicularly to the mould surface, and connect the bulk material and the mould surface.

Figure 17. Metal expansion penetration after the columnar to equiaxed transi-tion

Figure 18. Metal expansion penetration after the columnar to equiaxed transition.

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The microstructure at the metal-sand interface for the second type of penetration is shown in figure 17, where it can be seen that the curved surface (which has the highest penetra-tion tendency) has deformed the mould, and pushed back the sand. Some indicapenetra-tions of an anomalous phase which solidified mainly as white cast iron were observed.

The cross-sections of the castings show different degrees of penetration with respect to the deformation of the metal-sand interface.

When the metal expansion penetration occurs before the columnar to equiaxed transition, a fluid metal of near eutectic composition is in contact with the sand grains of the mould. When metal expansion penetration occurs after the columnar to equiaxed transition an already solidified metallic surface is in contact with the mould surface. In both cases the hot sand grains are barely pressed back, and an extremely high compaction of the sand is expected. The interactions which occur between the sand grains due to the high degree of compaction is not known.

The two types of penetration mechanisms described have been observed separately, and even a transition between the two mechanisms has been observed. The outcome of both mechanisms is that liquid metal comes into contact with the sand grains at the metal-mould interface. The criteria required to force the liquid metal to penetrate between the sand grains are not discussed here, however this has been briefly discussed by Stefanescu at. al [10].

5. CONCLUSIONS

From the present investigation it is understood that a solid phase barrier between the pressurized fluid metal and the sand mould interface is required to obstruct direct contact between the fluid metal and the mould. A surface coating intended to form the solid bar-rier is widely used. A drawback of this coating is that it can deform and break due to thermally induced stresses, and allow liquid metal to come into contact with the sand. A metallurgical way to create a solid phase barrier has been demonstrated by addition of inoculants for primary phase inoculation. However the same solid phase barrier is also exposed to expansion forces, which arise from eutectic solidification and the eutectic nu-cleation, and if the expansion forces are too great the solid metallic shell will also break and allow the liquid metal to come into contact with the sand. A further drawback of the broken metallic shell is that the shear stresses of the metallic shell can easily crack the core and thereby contributing to sand crack formation (see samples 10 and 11 in appendix 1)

The most efficient combination of parameters for reduction of the sand penetration is be-lieved to be the promotion of a tough metallic shell at the metal-mould interface in com-bination with a eutectic growth resulting in low liquid pressure of the last solidifying metal.

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6. ACKNOWLEDGEMENT

The authors would like to thank the partner of this research project namely the Skövde Foundry of Volvo Truck Component Corporation.

7. REFERENCES

1. A. Dioszégi, K.Z. Liu and I.L. Svensson, “Inoculation of Primary Austenite in Grey Cast Iron”, Conference Proceeding ISCP8, Beijing, October 2006.

2. H. Miyake and A. Okada, “Nucleation and Growth of Primary Austenite in Hypoeu-tectic Cast Iron”, AFS Transactions, 106, pp. 581-587, 1998.

3. K. Nagaoka, “Effect of Irreversible migration of graphite upon the growth of cast iron”, 35th Int. Foundry Congress, paper 39, 1968.

4. B. Lux and H. Tannenberger, “Inoculation effect on graphite formation in pure Fe-C-Si”, Modern Casting, 41, No. 3, 1962.

5. G. F. Ruff and J. F. Wallace, “Effects of solidification structure on the tensile proper-ties of grey iron”, AFS Transactions, 85, pp. 179-202, 1977.

6. L. Nastac and D.M. Stefanescu, “Micro/Macro Scale Phenomena in Solidification”, C. Beckerman et al. (editors), Am. Soc. Mech. Eng., NY, HTD-vol. 218, AMD-vol. 139 p.27, 1992.

7. W. Weis, in Proc. 2nd Int. Symp. on “The metallurgy of cast iron”, Geneva,

Switzer-land, 1974. 8. G.L Rivera, R.E. Boeri and J.A. Sikora, “Solidification of Gray Cast Iron”, Scripta

materialia, Paper 50, pp 331-335, 2004.

9. A. Dioszégi and I. Dugic, “The Mechanisms of Metal Expansion Penetration in Grey Cast Iron”, Conference Proceeding, ISCP8, Beijing, October 2006.

10. D.M. Stefanescu, T.S. Piwonka, S. Giese, and A. Lane, “Cast Iron Penetration in Sand Moulds, Part I: Physics of Penetration Defects and Penetration Model”, AFS Transactions, 104, pp 1233-1248, 1996.

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Table 1. The chemical composition in wt % of experimental castings No C Si Mn P S Cr Ni Mo Cu Sn Pb Al Ti 1 3.18 1.77 0.56 0.045 0.09 0.15 0.07 0.24 0.93 0.05 0.002 0.005 0.012 2 3.24 1.75 0.56 0.045 0.09 0.15 0.07 0.23 0.92 0.05 0.002 0.005 0.012 3 3.25 2.03 0.55 0.043 0.09 0.15 0.069 0.23 0.9 0.05 0.002 0.005 0.012 4 3.34 1.8 0.55 0.044 0.1 0.15 0.069 0.23 0.9 0.05 0.002 0.005 0.012 5 3.25 1.88 0.56 0.043 0.09 0.15 0.069 0.23 0.89 0.05 0.002 0.005 0.012 6 3.22 1.77 0.55 0.043 0.09 0.15 0.069 0.23 0.89 0.05 0.002 0.005 0.012 7 3.27 1.8 0.56 0.042 0.09 0.15 0.067 0.23 0.89 0.05 0.002 0.005 0.012 8 3.24 1.76 0.56 0.043 0.1 0.15 0.067 0.23 0.89 0.05 0.002 0.005 0.012 9 3.21 1.74 0.56 0.045 0.1 0.15 0.069 0.22 0.88 0.05 0.002 0.005 0.013 10 3.24 1.77 0.57 0.047 0.09 0.15 0.07 0.23 0.89 0.05 0.002 0.005 0.013 11 3.24 1.77 0.57 0.048 0.1 0.15 0.07 0.23 0.89 0.05 0.002 0.005 0.013 12 3.24 1.77 0.56 0.046 0.1 0.15 0.07 0.23 0.88 0.05 0.002 0.005 0.013 13 3.24 1.78 0.57 0.046 0.09 0.15 0.07 0.23 0.88 0.05 0.002 0.005 0.013 14 3.23 1.76 0.56 0.045 0.09 0.15 0.07 0.23 0.87 0.05 0.002 0.005 0.013 15 3.19 1.75 0.57 0.044 0.09 0.15 0.07 0.22 0.88 0.05 0.002 0.005 0.013 16 3.19 1.75 0.57 0.045 0.09 0.15 0.07 0.22 0.87 0.05 0.002 0.005 0.013 17 3.21 1.77 0.57 0.045 0.09 0.15 0.07 0.22 0.87 0.05 0.002 0.005 0.013 18 3.16 2.06 0.55 0.042 0.09 0.15 0.068 0.22 0.86 0.05 0.002 0.005 0.013 19 3.26 1.82 0.56 0.044 0.09 0.15 0.068 0.22 0.86 0.05 0.002 0.005 0.013 20 3.22 1.92 0.57 0.044 0.09 0.15 0.069 0.22 0.86 0.05 0.002 0.006 0.013

Table 2. Material of nucleating agents

Name Nucleating agents

AA1 Pure Fe powder AA2 SiC powder AA3 SiO2 quartz

AA4 SiO2 cristobalite

AB1 Graphite powder E1 Inoculant A E2 Inoculant B E3 Inoculant C

Table 3. Chemical composition in wt % of the inoculants

Inoculants Si Ca Al Sr C Ba Zr

A 73-77 max 0.1 max0.5 0.6-1.0 - - -

B 32 0.5 0.7 - 50 4.5 -

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Table 4. Description of samples Description Casting no. Sample no As cast/ DAAS Nucleating agent Addition amount Treatment method Coating* 1 DAAS - - - - 1 1A As cast - - - - 2 DAAS - - - + 2 2A As cast - - - + 3 DAAS E1 0.4 %, wt in ladle - 3 3A As cast E1 0.4 %, wt in ladle - 4 DAAS E2 0.2 %, wt in ladle - 4 4A As cast E2 0.2 %, wt in ladle - 5 DAAS E3 0.4 %, wt in ladle - 5 5A As cast E3 0.4 %, wt in ladle -

6 DAAS AA1 0.3 %, wt in ladle -

6 6A As cast AA1 0.3 %, wt in ladle -

7 DAAS AA2 0.2 %, wt in ladle -

7 7A As cast AA2 0.2 %, wt in ladle -

8 DAAS AA3 0.2 %, wt in ladle -

8 8A As cast AA3 0.2 %, wt in ladle -

9 DAAS AA4 0.2 %, wt in ladle -

9 9A As cast AA4 0.2 %, wt in ladle -

10 DAAS - - - AB1

10 10A As cast - - - AB1

11 DAAS - - - AA1

11 11A As cast - - - AA1

12 DAAS - - - AA3

12 12A As cast - - - AA3

13 DAAS - - - AA4

13 13A As cast - - - AA4

14 DAAS AA1 0.3 %, wt in ladle AB1

14 14A As cast AA1 0.3 %, wt in ladle AB1 15 DAAS AA1 0.3 %, wt in ladle AA1

15 15A As cast AA1 0.3 %, wt in ladle AA1 16 DAAS AA1 0.3 %, wt in ladle AA3

16 16A As cast AA1 0.3 %, wt in ladle AA3 17 DAAS AA1 0.3 %, wt in ladle AA4

17 17A As cast AA1 0.3 %, wt in ladle AA4

18 DAAS AA1 E1 0.3 %, wt 0.4 %, wt in ladle in ladle - 18

18A As cast AA1 E1 0.3 %, wt 0.4 %, wt in ladle in ladle - 19 DAAS AA1 E2 0.3 %, wt 0.2 %, wt in ladle in ladle - 19

19A As cast AA1 E2 0.3 %, wt 0.2 %, wt in ladle in ladle - 20 DAAS AA1 E3 0.3 %, wt 0.4 %, wt in ladle in ladle - 20

20A As cast AA1 E3 0.3 %, wt 0.4 %, wt in ladle in ladle -

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Table 5. Description of etching.

Etching reagent Temperature. [°C] Time. [min] Application

NaOH 10g. KOH 40g. Picric acid 10 g. destilled water 75.5 ml

110 2.5 Austenite grain in DAAS

NaOH 10g. KOH 40g. Picric acid 10 g. destilled water 50 ml

110 15 Eutectic cell in as cast

Table 6a. The results of microstructure investigation

D Nv Nv DAAS DAAS P Casting no Area [µm] [m-3] [m-3] average [µm] [µm] av-erage 1 1758 184053206 62 1 2 1486 304749900 2.55E+08 62 61 5 3 1537 275409171 60 1 1394 369157485 45 2 2 1344 411909866 3.73E+08 54 49 2 3 1435 338410864 49 1 1000 1000000000 43 3 2 1294 461527062 5.72E+08 50 47 2 3 1581 253048863 47 1 1105 741162036 52 4 2 1337 418413586 5.89E+08 58 55 1 3 1181 607086125 55 1 1321 433801997 52 5 2 1194 587471794 5.33E+08 54 53 2 3 1200 578703704 53 1 1836 161577904 51 6 2 2221 91275521.8 1.29E+08 58 54 5 3 1954 134037501 52 1 1156 647331138 50 7 2 1425 345585416 4.61E+08 58 54 5 3 1368 390608516 53 1 1493 300483464 45 8 2 1658 219404950 2.59E+08 52 49 5 3 1575 255951881 50 1 1735 191470364 50 9 2 1646 224238669 2.09E+08 56 54 5 3 1681 210521696 56 1 1607 240964114 49 10 2 1346 410076442 3.07E+08 56 51 5 3 1548 269579666 49 1 1547 270102783 57 11 2 1650 222611793 2.23E+08 56 58 5 3 1786 175531731 60 1 1820 165876872 48 12 2 1564 261390473 2.35E+08 55 50 5 3 1535 276487092 48

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1 1927 139750978 48

13 2 2240 88972531 1.20E+08 54 50 4

3 1967 131397445 49

Table 6b. The results of microstructure investigation

D Nv Nv DAAS DAAS P Casting no Area [µm] [m-3] [m-3] average [µm] [µm] av-erage 1 2064 113728922 48 14 2 1650 222611793 1.90E+08 56 51 3 3 1627 232186701 48 1 1773 179421215 60 15 2 1511 289872227 2.06E+08 58 59 5 3 1887 148827881 60 1 1947 135488410 57 16 2 2038 118137414 1.46E+08 62 57 5 3 1753 185632602 52 1 1817 166699853 53 17 2 2027 120071173 1.68E+08 60 57 5 3 1660 218612875 58 1 1141 673198387 51 18 2 986 1043204028 7.46E+08 63 55 1 3 1243 520698845 50 1 1165 632444214 52 19 2 1079 796041420 7.27E+08 49 50 1 3 1100 751314801 50 1 1341 414680551 42 20 2 1020 942322335 6.16E+08 56 50 2 3 1267 491665976 53

Table 7. The results of microstructure investigation.

Casting no Area NA NAV [m-3] NAV average [m-3] Nv/NAV Nv/NAV Average 1 4.8 7077888 26.00 1 2 4 4096000 4.89+06 74.40 59.61 3 3.8 3511808 78.42 1 3.8 3511808 105.12 2 2 4.2 4741632 3.74+06 86.87 101.77 3 3.6 2985984 113.33 1 5.8 12487168 80.08 3 2 4.8 7077888 9.52+06 65.21 57.80 3 5.2 8998912 28.12 1 4.2 4741632 156.31 4 2 4.8 7077888 5.52+06 59.12 114.49 3 4.2 4741632 128.03 1 5.6 11239424 38.60 5 2 5.4 10077696 9.77+06 58.29 56.41

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3 5 8000000 72.34 1 5.4 10077696 16.03 6 2 4.8 7077888 8.38+06 12.90 15.23 3 5 8000000 16.75 1 3.8 3511808 184.33 7 2 4.8 7077888 4.89+06 48.83 109.51 3 4 4096000 95.36 1 4.8 7077888 42.39 8 2 4 4096000 6.39+06 53.47 42.62 3 5 8000000 32.00

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Appendix 1

Casting 1 Casting 1A

Casting 2 Casting 2A

Casting 3 Casting 3A

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Casting 5 Casting 5A

Casting 6 Casting 6A

Casting 7 Casting 7A

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Casting 9 Casting 9A

Casting 10 Casting 10A

Casting 11 Casting 11A

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Casting 13 Casting 13A

Casting 14 Casting 14A

Casting 15 Casting 15A

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Casting 17 Casting 17A

Casting 18 Casting 18A

Casting 19 Casting 19A

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Casting 1 Casting 2

Casting 3 Casting 4

Casting 5 Casting 6

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Figure

Figure 5. The casting cup  Figure 6. Casting cups
Figure 9. The metal penetra- penetra-tion area, deg. 4–5.
Figure 13. The zones (1 to 3) of the cast- cast-ing cup which were investigated.
Figure 15. Metal expansion penetra- penetra-tion before the columnar to equiaxed  transition
+5

References

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