ZIRCON SAND - A VIABLE ALTERNATIVE MOULDING SYSTEM FOR ...

3 downloads 0 Views 1MB Size Report
The experimental observations of casting titanium in sodium silicate bonded zircon sand mould are presented in this paper. Metal-mould reactions. in general, ...
ZIRCON SAND - A VIABLE ALTERNATIVE MOULDING SYSTEM FOR TITANIUM CASTINGS R.L Saha, T.K. Nandy, D. Mukherji, R.D.K. Misra Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad 500 258

K.T. Jacob Indian Institute of Science, Bangalore 560 012 (Received 3 October 1989)

ABSTRACT The experimental observations of casting titanium in sodium silicate bonded zircon sand mould are presented in this paper. Metal-mould reactions. in general, involved dissolution of oxides in liquid titanium resulting in contamination of the casting. Minimal metal-mould reactions occurred when titanium was cast in zircon sand mould containing about 7.5 wt% of Zr0 2 . It has been further shown that the metal~mould reaction is considerably reduced if moulds were fired at high temperatures (> 1273K). This ensured elimination of moisture from the mould and also resulted in some beneficial changes in the mould chemistry. The reduction in metal-mould reaction is reflected in the decrease in oxygen and hydrogen contamination and decrease in hardness. Thus microhardness profile and oxygen analysis seems to provide a good index for evaluation of severity of metal-mould reaction. The method has been demonstrated to be satisfactory for casting titanium components.

1. INTRODUCTION Metal-mould r~action is a perennial problem in al­ most all the cast alloys. It has been difficult to quantify these reactions because of number of variables involved. In order to obtain castings with good surface finish and close tulerance it is necessary to minimise metal-mould reaction. This problem is in fact more severe for titanium in view of its highly reactive nature in molten condition. Adaptation of rammed graphite mould is one such step for mini­ misation of metal-mould reaction. This process is, however, cumbersome and expensive l . The selection of mould materials for commercial grade castings is generally based on the availability, cost and more importantly its stability against liquid titanium. The binder should also have refractory properties similar to that of the mould material and preferably provide fast hardening to obtain good green strength in the mould. Zircon sand has been found to be a potential mould sand for titanium because of its high refractoriness. The process described makes use of sodium silicate as a binder and CO 2 gas for subsequent hardening. Initial attempts were carried out with 10% sodium silicate binder and the resultant casting exihibited

poor details, and extensive pin holes on the cast surface 2 . However, improved results were reported using 4% sodium silicate in the mould l ,3. In a recent systematic study on the effect of sodium silicate on the strength of mould it was shown that it is possible to obtain reasonable mould strength even with the addition of only 2.5-3.0% sodium silicate 4•5 • The mould hardening in sodium silicate CO 2 process provides high green strength which is attained almost instantaneously without heating. The CO 2 process is more versatile and is appropriately suited for mass production and in addition to these advantages it is also less expensive than organic binders6,7. The sodium silicate CO 2 hardening reaction yields a three dimensional continuous network of silica gel which provides the necessary binding between the sand particles. The chemistry of the process is highly complex in nature. It is, however, possible to represent the overall hardening reaction in a simpli­ fied form as: Na 2SiO j

+ CO 2 + H 20

-) Na 2C0 3

+ Si0 2 + H 20 (1)

Out of the above hardening products, water and Si0 2 molecules are expected to react with liquid titanium

53

TRANS. INDIAN INST. MET VOL.44, NO.1, FEB 1991

and result in extensive contamination of titanium casting. Some reaction of titanium is also possible with zircon sand (Zr5iO). It was therefore realised that improvement in the performance of zircon sand moulds is possible if (1) thermodynamically stabler compounds like Zr0 2 are introduced in the moulds and (2) react:ve constituents of mould like water and Si0 2 are eliminated. Since all the mould constituents are oxide based it is likely that the oxygen will be one of the main contaminants in the casting. It is also known that oxygen has high solubility (4 at %) in titanium and exhibits high hardening effect. It wa5, therefore, thought that determination of microhardness and oxygen contamination levels in titanium cast in oxide base moulds may provide some semi-quantitative idea on the extent of metal-mould reaction. In fact, the oxygen microhardness relationship in titanium has been correlated 8 ,9. This relationship is, however, expected to marginally vary from system to system since it is influenced by the presence of impurities and variations in microstructure. In the present study, an attempt has been made to minimise metal-mould reaction in zircon sand moulds and quantify its effects in cast components.

3. RESULTS AND DISCUSSION 3.1 Effect of zr0 2 addition Microhardness data as a function of oxygen content in titanium is presented in Fig. 1. Figure 1 shows that microhardness is directly related to oxygen content.

OXYGEN (ppm)

i 35 : B

8"' '""'.

1:00

r

r-

I

CL

25

-T-­

SOlO

7950

12600 20.000 31500

I

I



EXPERIHENTAl IVPNi - THIS STUDY

T'0

630

J,.

SHIHASAKI ,I .1 !1~801 :HBI

1'00

J,. I
( /Jm)

Fig. 2; Effect of Zr0 2 addition in zircon sand mould-mix on micro hardness of cast titanium rod.

turbulant liquid state. During this period the metal­ mould reaction products are homogeneously distri­ buted in the casting. The second regime exhibits decrease in microhardness as a function of distance from the surface. This regime represents the process which is considered to be dependent on the diffusion of reaction products and occurs after quiescent conditions are established in the liquid and solidifi­

cation commences at the metal-mould interface. It may, however, be noted that for a constant depth, the microhardness values in both the regimes decrease with increase in Zr0 2 content in the mould mix. The decrease in microhardness in the first regime can be speculated as due to decrease in metal-mould reaction with increase in Zr0 2 content. It is, however, expected that the increase in hardness in the cast rods compared to the starting material can be because of increase in contamination levels in liquid titanium during reaction of the metal with the mould. From the present data it is observed that the oxygen content of the cast rod decreases with increase in Zr0 2 content of the mould. However, no appreciable influence on hydrogen contamination was observed in the castings in moulds with different ZrO. contents. " The validity of correliltion between oxygen and hardness values presented in Fig. 1 can be verified by converting the analysed oxygen values (1910 to 2510 ppm) to hardness values from Fig. 1. We obtain corresponding hardness values of 175-200 VPN. These estimated values are lower than the measured hardness values by about 30-40 VPN (Fig. 2). This discrepancy may be due to either hardening effect caused by other elements or probably due to microstructural variations due to different cooling rates obtained for titanium buttons and the cast titanium rods. The extent of contamination with respect to oxygen and hydrogen can also be obtained by subtracting the contents of these elements in the starting material

Table 1.

EFFECT OF

zr0 2

ADDITION IN ZIRCON SAND MOULD MIX ON BULK AND HYDROGEN CONTAMINATION IN CAST

TITANIUM RODS

% Zr0 2 Addition

Concentration of and H in in the cast rods

°

(ppm)

0.0 2.5 5.0 7.5 Ti buttons used for casting

°

H

2320 2140 2000

110 110 105 105

800

40

2510

Contamination over base values (Ti buttons) due to mould reaction (ppm)

° 1710 1.S20 1340 1200

Oxygen Oxygen contamination contamination due to due to mould oxides*'* moisture* (ppm) (ppm)

H

70 70

65 65

560 560 520 520

1150 960 820 680

• Calculated assuming that H 20 dissolved completely as Hand 0 into liquid titanium. Increase in hydrogen contamination will thus represent the amount of moisture absorbed by titanium. The oxygen pick up from moisture is calculated from stoich iometry . .. Difference between total increase in oxygen and the oxygen picked up from moisture .

.ss

TRANS. INDIAN INST. MET. VOL.44, NO.1, FEB 1991

removal of moisture from mould mix and is described below.

from the oxygen and hydrogen content of the castings (Table 1). It is seen that oxygen contamination decreases from 1710 ppm to 1200 ppm over base value of 800 ppm of oxygen (charge for melting) when 7.5% Zr0 2 was added in the mould mix. It is believed that the increase in the gas content (0 and H) is possible because of reaction of liquid titanium with moisture and oxide constituents of moulds. The increase in hydrogen content can be attributed largely to the presence of moisture and from this correspon­ ding increase in oxygen due to moisture can be determined by assuming complete dissolution of moisture in titanium.

3.2 Removal of moisture in zircon sand moulds It is essential to ensure complete elimination of moisture from the mould in view of high solubility of oxygen and hydrogen in titanium. The removal of moisture from the mould, which is considered to be a typical capillary porous body, depends on the mode of heat transfer, i.e., conduction, convection or radiation. The moisture in the mould which is considered to be a complex process 10 • However, drying of moulds is considerably accelerated at high tem peratures.

(2)

In the CO 2 hardening process where sodium silicate is used as a binder, a three dimensional continuous network of colloidal silica is produced which can be characterised as a two phase colloidal mixture of liquid in solid. The surface of the silica gel, may contain silano!, Si(OH) groupsJ which are frequently viewed as polymers of silicic acid [(Si(OH)/l. The colloidal particles link together to form a solid network and the liquid is contained in very fine capillaries between particles or very small holes in the framework. In view of the aforementioned complexities and observations, moisture seems not to be completely removed from mould when it is fired at 873 K. Therefore, to determine the temperatures required for complete removal of moisture from the mould firing temperatures upto 1473 K were employed in this study.

The difference between total increase in oxygen and the increase in oxygen due to moisture provides the data on increase in oxygen resulting from reaction of liquid titanium with oxides. The results of these caicul:J.tions are listed in (Table I) and presented graphically in Fig. 3. The increase in oxygen content of 520 ppm (Table I) due to moisture is not much affected by the increase in Zr0 2 content and can be considered to be appreciable in the casting (Fig. 3). However, the increase in oxygen contamination due to reaction with mould oxides decreases linearly from 1150 ppm to 680 ppm with Zr0 2 content from 0 to 7.5% (Fig. 3). The appreciable oxygen contamina­ tion due to moisture, suggests that firing at 873K for 2 h is insufficient for complete elimination of moisture. This observation led to deta:led study on ZIRCON SAND MOULJ-SODIUM SILICATE BONDED

0. D..

Z 0

z -

V1 V1

TO REACTION WITH

2000

• •

~

o

MOISTURE REFRACTORY OXIDES

-J

l:J lLJ

1600

lLJ

>

1200



l..J

-J

800

w

l:J

>-

X

3.1

~

0

:z

3.2

3

L:

:z

3.3

:r:

«

f-

3.4r-------------­

~

" TOT AL



«

ZIRCON SAND MOULD-SODIUM SILICATE BONDED

OXYGEN CONT AMINA TION DUE

E

400

l:









::::>

LJ

2.9

~

0

0

0

2.8

o0

25

50

7.5

Zr0 2 . ADDITION (% ) F;g. 3: Oxygen contamination in titanium as a function o Zr0 2 content in zircon sand mould.

473

673

873

1073 1273

1473

TEMPERA TURE (K) Fig. 4: % Cumulative weight 1088 in zircon sand mould mix fU'ed at increasing temperatures.

SAHA et. al. : ZIRCON SAND - ALTERNATIVE MOULDING SYSTEM FOR TITANIUM CASTINGS

The compacted mould mix was fired for two hours at each temperature and cumulative loss in weight was recorded. The temperature was increased from 474 to 1473 K at intervals of 200 K. The increase in % cumulative weight loss in sodium silicate bonded zircon sand with temperature is presented in Fig.4. It may be seen from Fig. 4 that % cumulative weight loss continues to increase upto 1273 K and then becomes constant, indicative of complete removal of moisture. The cumulative loss in moisture is rapid upto 873 K and becomes gradual at higher tempera­ tures. It is now confirmed that higher firing tempera­ tures () 1273 K) are essential to ensure complete removal of moisture from zircon sand moulds using sodium silicate as binder.

Sintering and fusion of zircon particles is expected to be initiated in moulds at elevated temperatures, particularly in the presence of free silica 12 • Fig. 5a shows that sintering of some zircon particles had commenced when mould was fired at 873 K. Small Zr0 2 particles are also seen to be present on and around zircon particles. However, moulds fired at 1473 K displayed enhanced sbtering of zircon particles (Fig. 5b) and considerable reduction in the number of Zr0 2 particles implying sintering of these particles on to the zircon grains. The surface of zircon particles also displayed a kind of roughening which might have been caused by the operating sintering phenomenon involving zircon grains, silica gel and Zr0 2 particles.

3.3 Chemical and physical changes in mould mix on firing at higher temperatures

The effect of mould firing temperatures on the change in chemistry of mould mix was studied by identifying the compounds at each firing temperature by X-ray diffraction. Typical X-ray diffraction patterns obtained are presented in Fig.6 and results are summarised in Table 2. Table 2 shows that sodium zirconium silicate and sodium carbonate compounds are present besides zircon, Zr0 2 and Si0 2 when moulds are fired at 873 K and above. However, the formation of sodium zirconate was observed at temperature of 1273 K and above. The formation of Na-Zr- silicate and Na­ zirconate is possible since it is known that zircon may decompose by fused alkalies or alkali carbonates at their melting points to form ortho silicates and ortho zirconates 13 • The observations also suggest that the volume fraction of sodium zirconium silicate and sodium zirconate may increase at higher temperatures. The for.nation of silicates and zirconates, which are relatively more stable compounds than Si0 2, are expected to assist in minimising the metal-mould reaction.

Application of higher mould firing temperatures is likely to affect chemistry of mould constituents and may also promote sintering of zircon sand particles. Scanning electron microscopy was used to study the effect of firing temperature on sintering. Attempts were also made to identify the induced changes in chemistry of mould constituents by X-ray diffraction studies.

Table 2 ~OMPOUNDS PRESENT IN SODIUM SILICATE BONDED ZIRCON SAND MOULD MIX WHEN FIRED AT HIGH TEMPERATURES (X-RAY DIFFRACTION)

Compounds Present

Na 2CO) Na 1ZrSi 4 0

11

Na 1Zr0 3 P Present Fig. 5: Effect of flring temperature on sintering of zircon particles in zircon sand moulds using sodium silicate as binder. a) 873 K and b) 1473 K.

Mould firing temperature (K)

873

1073

1273

1473

P

P

P

P

P

P

P

P

A

A

P

P

A Absent

The major constituents zircon, Zr0 2 and Si0 2 have also been observed at all the temperatures.

57

TRANS. INDIAN INST. MET. VOL.44, NO.1, FEB 1991

--!i .N

.:

MOULD FIRED AT 873 K

!i

r

:::

o~

.!;;

N

I

0

N

I

\e:

n

8

N

.:; 6"0

.

,.

N

'"

i;;;

VI

I

o!:i

r=

15

.~

Q

r= 0

VI

[;

z

[;

Ii

.1: N

MOULD FIRED AT 1473 K !i .N

.:

!i

.:

~

N

of

0

~ 0

.:;

Ii

Iiu

...

N

.

N C C

N

iii


o c f0o o 0

L

o

LJ

Z UJ

°

00

00

c::=:? VV (7000

000

&>0

E

400 f-

l.:J



X

0

;

200

.... -

_""0

-

-0- -

-

0-



-

-

-

-

--

.. -

--­

0

-----p c::::::>

­

L-L__ 873

1073

12"1

IL 70.

MOULD FIRING TEMPERATURE, K

Fig. 9 : Hoist tube cast in zircon sand mould fired at 873 K.

Fig. 8: Effect of mould firing temperature on oxygen contamination in cast titanium rod in sodium silicate bonded zircon sand mould containing 7..5 'Ye Zr0 2.

a) Casting with riser showing surface condition. b) Schematic radiograph showing porosity distribution in the tubular portion

59

TRANS. INDIAN INST. MET . VOL.44, NO.1, FEB 1991

F

.4t .

Fig. 10

... _­ - - -

...

----­

-'

.,...-.!?/

--------------­

----­ _.­ -----­

- --­

"'---- ......._-- -­

Hoist tube cast in zircon sand mould fired at 1273 K. a) Casting surface showing improved finish and considerable decrease in surface pin holes b) . Schematic radiograph showing reduced poro8ity

the casting. Results on one such cast component are presented. A hydraulic hois~ tube (weighing about 5.4 kg) was selected and cast using 25 kg skull casting furnace. The mould was prepared from the sand mix containing zircon + 7.5% Zr0 2 + 3% sodium silicate + 2.5 temper water. The mulled sand mix was rammed over the wooden pattern and holes were poked in the mould for the escape of gases during ca sting. The mould waS then fired along with mould box at either 873 K or 1273 K for 4 h before charging it in the furnace for casting. The core was also prepared with the same mould mix composition using appropriate core box and it was fired along with the mould. About 15 kg of titanium was melted in a consumable vacuum arc melting furnace at a vacuum of 0.133 Pa (10-3 torr) in a water cooled copper crucible and poured in the mould. After completion of solidification, casting was removed from the mould and sand blasted . The quality of casting was visually

inspected for surface defects and subsequently radio­ graphed for internal defects. The casting made in mould fired at 873 K indicated surface roughness and extensive pin holes on the surface (Fig. · 9a) and radiography exhibited large number of internal poro­ sities schematically shown in Fig. 9b. The same com­ ponent was also cast in zircon sand mould fired at 1273 K for 4 h. It showed definite improvement in the quality of casting; the casting surface was smooth and the number of pin holes were considerably less (Fig. lOa) as compared to the casting made in the mould fired at 873 K. The internal porosity in the tubular portion of the casting also exhibited remarkable reduction as is apparen.t from the corresponding radiograph schematically showr. in Fig. lOb. The tensile properties for both the castings were deter­ mined and results are listed in Table 5. It may be seen that reduction in the extent of oxygen contamination due to firing of moulds at higher temperatures

Table 4 TENSILE PROPERTIES OF TITANIUM CASTINGS AS SPECIFIED

* Grade ** Moulds

Oxygen wt. %

UTS MPa

(~STM)

AND OBTAINED IN THIS STUDY

0.2 % YS MPa

% EI

Hardness HV

fi red at (K)

* Specified Grades as per ASTM B367-1978 (max)

(min)

(min)

(min)

(m2x)

C1

0.18

240

170

24

190

C2

0.25

345

275

20

210

C3

0.35

450

380

15

235

** PROPERTIES OBTAINED IN THIS STUDY Hydraulic hoist tube 873K

0.24

350

298

20.5

200HV

1273K

0.18

320

280

26

180HV

60

I

SAHA et. al. : ZIRCON SAND - ALTERNATIVE MOULDING SYSTEM FOR TITANIUM CASTINGS

ACKNOWLEDGEMENTS

(1273 K) led to improvement in ductility (Table.4). The observations are in good accord with the earlier findings on the beneficial effect of decrease in metal­ mould reaction on firing of sodium silicate bonded zircon sand moulds at temperatures greater than 1073K. It can therefore be concluded that mould firing at higher temperatures is an effectiv2 method for minimising mttal-mould reaction in zircon sand moulds.

The authors (RLS, TKN, DM and RDKM) wish to thank Dr. P. Rama Rao, Director, DMRL for encouragement and permission to publish this work.

REFERENCES 1. Burrus J.M. and Koch R.K., Foundry Mangement and Technology 109 (May 1982) 43. 2. Lang R.M., Development of processes for making of Ti casting other than ingot, Report No.DRDTB-l­ 12046, Battelle rYternorial InstiMe, Columbus, (1954)

4. CONCLUSIONS 4.1 Metal-mould reaction occurring during casting of titanium in sodium silicate bonded zircon ·sand moulds can be effectively minimised by:

3. Koch R.K., Hoffman J.L., Transue M.L. and Beall

R.A., Casting titanium in zircon sand moulds, U.S. Bureau of mines, Report RI-8208, (1977)

(a) addition of stabler oxide (Zr02) in the mould mix and

4. Saha R.L., Mukherji D., Chakravorty C.R., proc. Ann. Convention of Inst. of Indian Fouf"dryman, New Delhi (March 19-21, 1984) 1 (1984) 54

(b) ensuring complete removal of moisture from moulds by firing the moulds at tem­ peratures of 1273 K and above.

5. Mukherji D., Saha R.L., Chakravorty C.R., Trans. lnd. Inst. Met., 38 (1985) 465.

4.2 The microhardness profile and oxygen con­ tamination analysis provides a good index for determination of relative degree of metal­ mould reaction.

6. Atterton D. V., Proc. Sr. Foundryman, 48 (1955) B.45.

7. Sarkar A.D., Foundry Core and Mould making by

CO 2 process, Pergamon Press, Oxford, 196c.

4.3 The microhardness profile exhibits two distinct regimes. The first of which is characterised by the invariant part of the microhardness profile that corresponds to the situation when the metai is in turbu!ant liquid state and the metal­ mould reaction products are homogeneously . distributed in the casting. The second regime which exhibits decrease,in microhardness as a function of distance from the surface represents a process which is considered to be dependent on the diffusion of reaction products. This occurs after quiescent conditions are estab­ lished in the liquid and solidification com­ mences at the metal-mould interface.

8. Shimaskii k., Ono K., Tsuruno T., Private Commu­ nication, (1980).

9. Weidman K.E., Shenoy R.N., Met. Trans. l8A (1987) 1503. 10. Veinik A. I., Thermodynamics for Foundryman, McClean & Sons Pvt. Ltd., London, 1968. 11. Willey J. D., Kirk-Othmer Encyclopaedia of Chemi­ ca/Tech., Eds. Mark H.F., eta!., John Wiley & Sons, USA, 20 (1982) 766-781 12. Taylor D.A., Trans. Am Foundrymen Soc., 69 (1961) 153. 13. Faster W.R., J. Am. Ceram Soc., 34 (1951) 302.

4.4 The component castings produced correspond to C1 class castings of ASTM specifications, when moulds are fired at 1273 K.

61