Infrared measurements of the melt puddle in planar flow casting

Materials Science and Engineering, 98 (1988) 29 32

29

Infrared Measurements of the Melt Puddle in Planar Flow Casting* G. STEPHANI, H. M~HLBACH, H. FIEDLER and G. RICHTER

Zentralinstitut fiir Festk6rperphysik und Werkstofforschung der Akademie der Wissenschq[tender D.D.R., 8027 Dresden, Pos([?tch (G.D.R.)

Abstract

Infrared (IR) measurements of the melt puddle behaviour during preparation of 45-200 #m high-speed steel ribbons reveal an inhomogeneous temperature distribution, with hot and cold regions. The experiments with ribbons 200 #m thick show that the liquid phase is extended outside the melt puddle and that it reaches the final ribbon thickness before the onset of solidification. The results of these measurements lead to a better understanding of ribbon formation during melt spinning in connection with the behaviour of the melt puddle and its hydrodynamics. 1. Introduction

Rapid solidification of ribbons of metallic alloys directly from the melt by planar flow casting [ 1, 2] has been intensively investigated in recent years. Quenching rates as Characteristic features of the solidification process and further ribbon cooling have been obtained by temperature measurements [3~5], and agree well with those derived from microstructural measurements of crystalline alloys [7 11]. Heat-transfer coefficients were estimated from cooling rates on the basis of heat-transfer analysis [3, 4, 6, 11-13]. No information has yet been published on the temperature distribution within the melt puddle in the single-roller technique. This paper deals with infrared (IR) temperature measurements from a lateral view to the melt puddle. The thickness dependence of the temperature profiles in the melt puddle was studied. The solidification process and ribbon formation are discussed in terms of the temperature distribution within the melt puddle. 2. Experimental procedure

Rapidly quenched high-speed steel ribbons of the alloy X82WMo6.5 (M2-type) with the chemical com-

*Paper presented at the Sixth International Conference on Rapidly Quenched Metals, Montreal, August 3-7, 1987. 0025-5416/88/$3.50

position 0.83 wt.% C, 6.09 wt.% W, 5.08 wt.% Mo, 1.92 wt.% V, 4.14 wt.% Cr were prepared by planar flow casting onto the surface of a steel wheel (heatresistant alloy). To improve the wetting conditions, the wheel was preheated to 120 °C. Ribbons of different thicknesses were prepared by varying the wheel surface speed (5-15 m s J) and the gap distance between nozzle and wheel. Ribbons more than 100 #m thick were obtained using a special nozzle with a channel for increasing the mass flow. The temperature of the melt puddle was measured with an IR device especially designed by the Technical University of Magdeburg. The IR arrangement consists of a modified X-ray television camera with a silicon-diodenendicon as an IR detector, and produces 50 picture frames per second. Measuring data of the IR camera were fed into a computer-controlled analyzer system. The temperature dependence of the emissivities of the ribbons and the melt puddle were determined experimentally under an inert gas atmosphere. The emissivity was e = 0.35 at 1000 °C and increased linearly to e = 0.45 at 1600 °C. The position of the IR measuring device in all cases was parallel with the slot length laterally. The total error in the temperature was + 4%, from the temperature failure of the emissivity coefficient and the measuring error of the IR camera.

2.1. Ribbons 80 #m thick Figures l(a) and (b) show the results of IR measurements of melt puddles during ribbon preparation at intervals of 0.1 s and 1.0 s respectively. Both reveal significant features dependent on the time concerning size, shape and temperature distribution of the puddle. The length extension of the melt puddle amounts to 2.5mm after 0.1 s process time, whereas an increase to 3.6 mm is given after 1.0 s. For the melt puddle extended convexly under the second orifice lip after 1 s of ribbon formation, the extension is no further than the wall of the slot after 0.1 s. Differences of the temperature distribution dependent on process time between 0.1 s and 1.0 s are revealed by Figs. 2(a) and (b) showing the isotherms within the range of solidification. Measurements after 0.1 s gave a rather f'? Elsevier Sequoia/Printed in The Netherlands

30 Process time 0.1s

IJ50~'365 (a) Process time ls

12~ 12_~ ' ' ~ - ~ , ~U U

>~It,60

1570

/

wheel

jO,Smm

(b) Fig. 1. IR isotherms of the melt puddle, for ribbons 8 0 # m thick, after process times of (a) 0.I s and (b) 1.0 s.

long melt puddle in the temperature range between the liquidus and solidus, which was reduced substantially after 1.0 s. In this case, the temperature gradient was shifted away from the orifice lip down the ribbon running direction (Fig. l(b)). Taking into account other IR melt-puddle data, an average gradient of 1.2 × 103 K mm -1 along the puddle can be given for the temperature range between liquidus and solidus. Similarly for the melt puddle formed for the solidification of a ribbon 45 #m thick, areas of inhomogeneous temperature distribution observed 0.1 s 1600 1500

,~,~__

"TL

~/L__ __ _

lZtOO

/~-¢"

1300

,-~-/ "~

i

110( /,5

4.0

3' .5

Process time 0.1s ~--~ 50pro o---olO0 pm o----o150pm

' ' 3.0 Z5 2'0 1.5 Puddle length Cmm]

,

1.0

&

(a)

1600 Process time 1.0s f

./'-

1500

TL

oa

"6

--

_

,,

_ ~2.._J

lt+O0

1300 TS - - - - ~ ' / ~ 1201 110

(b)

t"---

/

~,.5

,,--4 50/Jm ,P--.--.,100pro

I

I

t,.0

3.5

I

I

I

I

3.0 25 2.0 1.5 puddle length [ram]

1.0

0,5

Fig. 2. Temperatures at 50/tm, 100/~m and 150/~m from the wheel surface vs. the puddle length, for high-speed steel ribbons 80 # m thick, after process times o f (a) 0,1 s and (b) 1.0 s.

after the beginning of the process becomes equalized at 1.0 s. This behaviour seems to indicate that the process is quasi-steady-state at this time, as observed in Fig. 2(b). For this state, the time dependence is influenced by tracer heating, improved wetting conditions, increased adhesion time of the ribbon to the wheel surface as evidenced by indications of some wear with increasing time. The discontinuity of the curves in Figs. 2(a) and (b) between 0.1 s and 1.0 s running time is also influenced by the process starting conditions. During this initial time of instability with oscillations in puddle length and temperature, the quenching rate varies from 3 x 105 K s -1 to 7 × 105 K s - ' on average for the ribbons 80 #m thick, with the higher rate at the beginning of the process. 2,2. Ribbons 200 /~m thick In preparing ribbons 200/~m thick, tunnelled orifices were adopted to increase the mass flow. The maximum height of the melt in the gap was 600 #m, approximately half the distance to the vortex of the tunnel, leaving the constraint of the gap between the lips of the orifice and the roll surface. At 10 mm from the point of impingement, measured from the edge of the lip, the height of the melt was the thickness of the solidified ribbon, i.e. 200 #m. Isothermic areas resulting from IR measurements of melt puddles at 0.2 s and 3.0 s process time are shown in Figs. 3(a) and (b). The time dependence of temperature distribution is again evident. Isothermal areas of the same order of magnitude were extended down the ribbon running direction simultaneously as the process time increased. Other conclusions can be drawn for the variation of the puddle lengths with time, the length increasing to 11.2 mm at 4.0 s. Comparing the isotherms at 200/~m thickness, it is noticeable that the metal is still partially liquid. When modelling ribbon formation, it is understood that a liquid layer has been dragged out of the melt puddle, and solidification finally takes place further on in adhesion to the roll surface. Melt puddle measurements laterally, using convential high-speed motion pictures [14], pose another problem. More accuracy would be attained by IR temperature measurements, fixing isotherms. The temperatures at the top side of the melt puddle and the wheel side as a function of the puddle length are given in Fig. 4. Above the liquidus temperature, an average cooling rate of T1 = 3.2 × 105 K s -1 was calculated from the IR measurements at the top side. At liquidus temperature, the cooling rate had decreased to ~r2 = 1. I x 105 K s - t, probably because of the release of latent heat. At the chill side of the ribbon there is an abrupt temperature gradient under the

31 Process hme 0.2s

Process lime 35 13~0

~

~

?

1420

~

:

1,490

'

/ / 1520

:

1620

1560 1,590

-

. .......

1300

.~

~I~nozzle

.....

'I

s o.f

Fig. 3. IR isotherms of the melt puddle for a high-speed steel ribbon 200 l~m thick, after process times of (a) 0.2 s and (b) 3.0 s.

tunnel of the orifice near the liquidus temperature, with a cooling rate ~ = 1.05 x 106 K s 1. The release of latent heat results in an increase of temperature at this part of the melt puddle, shown by the distortion of the dashed line at the liquidus temperature in Fig. 4, given by the average value of all the measured data.

!'\ ":"1"

1600 L1

~

T,

M~If puddle fop 'side

\, \

f3

Process time 3s

----

Melf puddte

-c

\

T2

I bOttom stde

=

L

\

5-

\ '\

1300

;

\

i

mS

- ~

.

.

.

.

.

.

.

3. Conclusions (i) The melt puddle is the most significant area of the melt-spinning process and the determining spot for ribbon formation. IR temperature measurements using a specially developed camera offered additional information about its behaviour and hydrodynamics. (ii) At the beginning of the process, the temperature distribution within the puddle was inhomogeneous, but it equalized with the ribbon thickness, i.e. with increasing mass flow, and for ribbons less than 80/~m thick with increasing process time. This time was at least 1 s; it is defined by the quasi-steady-state attained, given by a stabilized puddle length with a symmetrical temperature sequence beginning at the point of impingement and the highest temperature being the temperature of solidification. (iii) In preparing high-speed steel ribbons 200/~m thick, the liquid flow attained its final layer thickness before the onset of solidification. Supercooling could not be seen from IR temperature measurements but from the release of latent heat, shown by a change of the temperature gradient at the liquidus temperature. This could not be measured in ribbons 40-80/~m thick, presumably because of the higher cooling rates within a low melt puddle. (iv) The cooling rates derived from IR temperature measurements within the puddle agree with recent publications and seem to be significant for rapid quenching of steels.

I

120C

I 0

~ 5 t0 Melt puddle length I (mm)

Fig. 4. Temperature of the top and bottom of the melt puddle vs. the puddle length after 3.0 s process time, for high-speed steel ribbon 200 j~m thick.

Acknowledgment The authors are very grateful to Dr. Roessler from TU Magdeburg for his kind assistance in performing the experiments precisely.

32

References 1 M. C. Narasimhan, U.S. Patent 4142571, March 6, 1979. 2 H. Fiedler, H. M/ihlbach and G. Stephani, J. Mater. Sci., 19 (1984) 3229-3235. 3 A. G. Gillen and B. Cantor, Acta Metall., 33 (1985) 1813 1825. 4 M. J. Tenwick and H. A. Davies, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals V, North-Holland, Amsterdam, 1985, pp. 67-70. 5 E. Vogt and G. Frommeyer, in P. W. Lee and R. S. Carbonara (eds.), Rapidly Solidified Materials, American Society for Metals, Metals Park, OH, 1986, pp. 291-296. 6 K. Takeshita and P. H. Shingu, Trans. Jpn. Inst. Met., 27 (1986) 454462.

7 M. Matyja, B. C. Giessen and N. J. Grant, J. Inst. Met., 96 (1986) 30-32. 8 H, Jones, Mater. Sci. Eng., 65(1984) 145-156. 9 F. Duflos and J. F. Stohr, J. Mater. Sci., 17(1982) 36413652. 10 H. Gahm, W. Ruhs and A. Gruber, Prakt. Metallogr., 21 (1984) 173-189. 11 E. Vogt and G. Frommeyer, Z. Metallkd., 78(1987) 262267. 12 H. A. Davies, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals V, North-Holland, Amsterdam, 1985, p. 101. 13 H. Miihlbach, G. Stephani, R. Sellger and H. Fiedler, to be published in Int. J. Rapid Solidification. 14 J. L. Walter, General Electric Report 78 CRD 172, November 1978.