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Production of rapidly quenched alloys in plasma-arc melting.

To increase the reliability and longevity of devices, machines and mechanisms, it is necessary to use new materials with high functional properties. The alloys with the crystal structure, used widely as constructional materials, cannot always ensure the required high level of the service properties. Therefore, there has been special interest in recent years in the problem of production and application of amorphous and microcrystalline metallic materials, whose properties greatly differ from those of the alloys produced by standard technology [1].

Superfast quenching of the melt is the most widely used method of producing amorphous and microcrystalline structures. Ceramic crucibles, produced from refractory materials, for example, quartz, are used for melting and collection of the melt in the currently available technologies. Consequently, a wide range of the rapidly quenched alloys can be produced.

However, the direct contact of the ceramic material with the melt in the melting of high-only activity alloys results in the chemical interaction and failure of the crucible. In addition, superfast quenching of the alloys with higher melting point is complicated by softening of the refractory material, the loss of rigidity of the structure and, consequently, destabilisation of the process. Therefore, the production of rapidly quenched alloys from the high-reactivity and refractory alloys using the current technologies is not feasible.

This problem can be solved by developing methods of superfast quenching which make it possible to avoid the contact of the ceramic material with the melt and maintain high temperature throughout a long period of time. This may be based on melting in a watercooled crucible with a powerful heat source.

In this case, the most suitable method of melting which can be used efficiently, is plasma-arc melting (PAM) in a skull crucible. The method is used in special electrometallurgy in remelting high-reactivity and refractory metals and alloys, titanium, tungsten, alloys with higher nitrogen content, and other materials.

The E.O. Paton Electric Welding Institute has developed laboratory equipment for superfast quenching of the melt in PAM. The external appearance and the diagram of equipment are shown in Fig. 1 (2). The melting chamber 10 is fitted with the inspection sys-tem and the technological hatch for loading the charts. The lower part of the chamber contains the copper water-cooled crucible 9 with a replaceable discharge nozzle 1 in the centre produced from a refractory material. The melting chamber is installed on the column 4 which is moved by the mechanism on the column 2, and the upper part of the melting chamber contains a direct action DC plasma torch 12, fitted with the displacement mechanism 11. The plate 6 carries the drum-cooler 5 with a drive and a mechanism for longitudinal displacement. To make sure that the outlet of the discharge and the surface of the drum-cooler are parallel, the chamber is fitted with a mechanism for fine adjustment 3. Equipment is connected to a gas supply system and there are also additional receiver 7 with the volume of 1.30 5[m.sup.3] and a vacuum system with a mechanical pump 8.

[FIGURE 1 OMITTED]
Technical characteristics of OP-133 equipment

* Volume of the crucible, l 0.5

* Speed of rotation of the drum, rpm 50--3000

* Thickness of the strip, [micro]m 10 --100

* Number of plasma torches 1

* Consumption of the plasma forming gas, 0.5
 [m.sup.3] / h

* Maximum working current, A 1000


The copper water-cooled crucible is produced in the form of an inverted truncated cone so that the construction is rigid and a sufficient volume of the melt can be collected (Fig. 2a). To maintain the optimum volume of liquid metal and ensure the required superheating of the entire melt, the coefficient of the shape of the crucible is equal to 0.4 (the ratio of depth to mean diameter).

[FIGURE 2 OMITTED]

As mentioned previously, the metal is discharged through a ceramic nozzle. The application of the replaceable ceramic nozzle did not solve the problem of the contact of molten metal with ceramics but greatly reduced the area of contact with the melt. The small dimensions and the large thickness of the nozzle wall also enable standard effective materials to be used at higher temperatures.

The material of the nozzle may include various types of ceramics and composites, depending on the composition of the alloy for producing rapidly quenched ribbons. In the experiments described in this article, the authors used a nozzle produced from Kersil pressed quartz ceramics (Fig. 2b), and a graphite crucible was used for the Cu--P alloy. The Kersil material showed high resistance and rigidity in the entire temperature range up to 1400[degress]C.
Technical characteristics of Kersil quartz ceramics

* Bending strength, MPa 15--45

* Apparent density, g/ 1.9--2.0
 [cm.sup.3]

* Open porosity, % 5--15

* Coefficient of linear thermal
 expansion,

* 1/[degress]C 5.8 *
 [10.sup.-7]

* Heat resistance in relation to 20-1300
temperature gradient, [degrees]C

* Heat conductivity coefficient, 0.7-1.0.
W/(m [degrees]C)


Equipment is fitted with replaceable drums-coolers of different diameters, produced from different materials (Table 1). The drum has the form of a hollow cylinder into which water is poured and maintained there as a result of the centrifugal force during rotation. In this study, the experiments were carried out using copper and steel drums with a diameter of 300 mm (Fig. 3).

[FIGURE 3 OMITTED]
Table 1. Dimensions and properties of the material of the drum-cooler.

Material Diameter, mm Width, mm Heat conductivity Hardness
 coefficient,
 W/(m*[degrees]C)

Copper 350 71 400 370... 420

--"-- 315 67 400 370... 420

--"-- 300 80 400 370... 420

--"-- 260 65 400 370... 420

Steel 45 300 47 47 800... 900


Melting of the charge and further melting were carried out in a hermetic chamber in argon. To simplify the process of investigating this technology, the melt was discharged on the spinning drum situated outside the chamber, i.e., in air (Fig. 4). If it is required to quench high-reactivity alloys, the drum-cooler can also be placed in the chamber with a controlled atmosphere. Investigations were 1.0 carried out on modelling alloys, characterised by the relatively low melting point, inert in relation to the surrounding atmosphere, wt.%: Ni--base, 7 Si, 3.5 B, 1 Cr, 0.5 Fe, Fe - base; 19 Cr; 11 Ni; 9B; Cu - base; 9--P.

[FIGURE 4 OMITTED]

The investigation of the thermal state of the melting chamber show that an increase of the power of the plasma torch in the range 20--40 kW is accompanied by an increase of the effective efficiency of the remelting process. A subsequent increase of the power of the plasma torch (W > 40 kW) has al-most no effect on the effective efficiency and the heat losses in the plasma torch and the melting chamber of equipment. However, a plasma torch power of more than 50 kW was characterised by a tendency for reducing the effective efficiency of the melting process with a small increase of the losses in the plasma torch and in the chamber. The calculation is carried out on the basis of the experimental data, shows that the specific consumption of electric energy was (4--6 kW*h/kg). The thermal efficiency of the plasma torch was 86--88%, the effective efficiency of the process 44--45% (3).

The plasma arc is a sufficiently powerful and concentrated heat source and, consequently, the melt contains a temperature gradient along the height of the pool. The control of the temperature gradient makes it possible to control and predict the melting and spinning processes. Measurements of the temperature of the melt in plasma-arc melting show that superheating the upper layers of the pool made each 500[degrees]C in relation to the temperature in the vicinity of the discharge orifice (Fig. 5). The coefficient of discharge of the melt from the skull crucible in relation to the powder plasma arc is shown in Fig. 6. The maximum value of the coefficient (approximately 0.72) was recorded at a power of approximately 40 kW.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

The subsequent increase of power did not lead to any changes in the volume of the pool and the discharge metal. This is explained by the redistribution of heating other elements of the melting chamber.

It is interesting to investigate the thermal load on the ceramic nozzle during recording of the melt. The temperature of the nozzle was measured in the immediate vicinity of the discharge orifice during the spinning and superfast quenching processes (Fig. 7) (4).

[FIGURE 7 OMITTED]

As indicated by the graph, the temperature is stabilised 5--6 min after activating the plasma torch and reaching the optimum technological conditions.

Further holding of the melting regimes is not rational because there are no visible temperature changes any longer and the system approaches the stationary state.

When reaching the given stable regime, the spinning drum-cooler is placed under the discharge nozzle with the required gap. At this moment, the main line of discharge of the gas from the melting chamber is closed, and the gas (argon) is supplied from the receiver at excess pressure.

When the spinning drum is placed in the position, the projecting part of the nozzle is cooled of a short period of time as a result of blowing an airflow during rotation. Fig. 7, 3 shows a rapid decrease of temperature, equalling 300--350[degress]C. However, as a result of the high rate of the process and the low heat conductivity of the ceramics, this does not have any strong effect on the melt temperature. A further discharge of the melt into the orifice is characterised by the supply of new superheated volume of the metal, and the model is rapidly heated. This corresponds to a rapid increase of temperature with a total gradient of approximately 900[degress]C. Discharge last several seconds.

The main section of equipment for producing the alloys with the amorphous structure is the drum-cooler and, therefore, further investigations were carried out to determine the thermal state of the cooler during super-fast quenching.

This was carried out using the procedure and equipment for content measurement of the temperature of the spinning drum on the internal side of the contact surface in quenching the Ni--Si--B alloy (Fig. 8) (5).

[FIGURE 8 OMITTED]

When spinning the copper--phosphorus alloy on the steel drum, it was possible to determine the general relationships of the variation of pressure in the chamber, the speed of rotation of the drum in the temperature of the internal surface of the disc (Fig. 9).

[FIGURE 9 OMITTED]

Since the copper--phosphorus alloy in the molten state is characterised by the fluid-flow state, discharge is carried out at a low excess pressure in the chamber. Minimisation of the distance between the discharge orifice and the drum-cooler in the range 0.6--0.2 mm results in the situation in which the rapid variation of the viscosity of the melt as a result of rapid solidification results in the formation of some loads preventing the free movement of the drum and causing its short-term arrest.

The full-size experiments have been used to construct the mathematical model for describing the process of propagation of heat in the body of the drum and determining methods of further improvement of the shape of the drum [6].

The experimental results were used to determine the optimum technological parameters of the process of melting point of the melt, produced the specimens are rapidly quenched ribbons in both the complexly amorphous structure (Ni--Si--B) (Fig. 10, 11) (7), and the Micro crystalline structure (Fe--Cr--Ni--B, Cu--P).

[FIGURE 10 OMITTED]

[FIGURE 11 OMITTED]

In metallographic examination, the structure of the Ni--Si--B amorphous alloys was investigated and the effect of annealing on the formation of nanocrystalline was determined. The results of the nonuniform distribution of the crystals in the subsurface layer and in the direction of the number of nanocrystal-line the dimensions from the surface into the thickness of the strip [8, 9].

The results of scientific investigations can be used for recommending the proposed technology for producing high-reactivity and refractory alloys with the amorphous structure and microcrystalline alloys.

References

(1.) Molotilov B.V., Stal', 2001, No. 1, 79-83.

(2.) Shapovalov V.A., et al., Vest. Vostochno-ukr. Nats. Un-ta im. V. Dalya, 2003, No. 11, 80--88.

(3.) Zhadkevich M.L., et al., Donbas. Gos. Mashinostr. Akad., 2006, No. 1, 160--163.

(4.) Zhadkevich M.L., et al., Sovremen. Elektrometallurgiya, 2005, No. 1, 26--29.

(5.) Shapovalov V.A., et al., Sovremen. Elektrometallurgiya, 2007, No. 4, 27--29.

(6.) Shapovalov V.A., et al., Sovremen. Elektrometallurgiya,, 2008, No. 3, 42--46.

(7.) Nikitenko Yu.O., Nanosistemi, Nanomateriali, Nan-otekhnologii, 2006, 4, No. 4, 797--801.

(8.) Kozhemyakin G.N., et al., Sovremen. Elektrometal-lurgiya, 2008, No. 4, 48--49.

(9.) Kozhemyakin G.N., et al., Crystallography Reports, 2009, 54, No. 7, 142--144,

Yu.A. Nikitenko

E.O. Paton Electric Welding Institute, Kiev
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Author:Nikitenko, Yu. A.
Publication:Advances in Electrometallurgy
Date:Jul 1, 2010
Words:2132
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