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Resource and energy saving technologies in the manufacture of large-sized castings from magnesium alloys

https://doi.org/10.38013/2542-0542-2020-1-77-86

Abstract

This article considers an approach to the development of low-waste resource- and energy-saving technologies for the manufacture of large-sized geometrically-complex hull castings from magnesium alloys in combined forms (chill moulds with an internal sand сore). This approach implies solving a set of tasks, such as the provision of conditions for directional solidification and effective risering; the creation of low-cost non-deficient gas protective environments; automatizaton of simultaneous flux-free smelting, modification and refining processes; reduction of gas consumption required for heating moulds; as well as the improvement of labour conditions. In order to solve these tasks, the authors: 1) developed and implemented software application packages; determined the composition of gas protective environments and gas modifying mixtures; calculated the modes of melt processing; 2) developed and implemented automated installations for processing the melt and ensuring the controlled and adjustable heating of large-sized moulds under a predetermined temperature gradient at the height of the mould; 3) developed a technology for recycling magnesium alloy shavings.

For citation:


Bobryshev B.L., Popkov D.V., Moiseev V.S., Koshelev O.V., Bobryshev D.B., Moiseev K.V. Resource and energy saving technologies in the manufacture of large-sized castings from magnesium alloys. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(1):77-86. https://doi.org/10.38013/2542-0542-2020-1-77-86

Introduction

A major structural feature of modern aviation and rocket equipment is the use of large-sized geometrically-complex hull parts made from high-strength magnesium alloys ensuring effective relation between weight and power of an article and thus its operational characteristics in general.

In order to solve this task, Moscow Machine-Building Plant “AVANGARD”, JSC in cooperation with AVANGARD-LIT, Ltd. organized a section for the manufacture of largesized geometrically-complex high-duty castings from magnesium alloys, where several basic resource and energy saving technology fields are developed.

The first field consists in the development of computer-aided-design (CAD) of magnesium alloys casting processes and development of complete sets of design and technical documentation.

Based on the analysis of physical and chemical processes employed in the course of smelting and casting of magnesium alloys, simulation of the processes of filling and crystallisation of castings in ProCast environment, ensuring forming of solid casting with minimum process overlapping and deadhead elements, a modern high-performance, high-precision, low-waste, resource-saving, environmentally friendly process of manufacture of large-sized castings from magnesium alloys was organized, complete sets of design and technical documentation were developed.

The development included application of a genuine scientifically grounded approach to the design of processes for large-sized castings manufacture from magnesium alloys in combined forms, which comprised the solution of a set of tasks aimed at ensuring the conditions for directional solidification and continuous risering. For solving these tasks, for each stage of mould-filling and casting forming based on the methods of casting hydraulics and thermal theory of casting, the expressions were obtained for calculating the required process parameters from the filling temperature to the time of casting hold in the chill mould, which were combined in software application packages [1, 2].

The second field consists in the development of an advanced method of melt processing that combines the main operations in the technology of products manufacturing from magnesium alloys - modification and refining (gas removal) [3].

The modification process ensures fine crushing of castings crystalline grains, as well as the required stable mechanical properties of alloys: an increase of strength, yield, and elongation (σB, σ0,2, δ).

The standard process of modification with magnesite proceeds by the reactions:

MgCO3 = MgO + CO2, 2Mg + CO= 2MgO + C and 4Al + 3C = Al4C3.

As a result, the produced aluminium carbide creates multiple crystallizing nuclei, crushes the structure of alloy in the cast and increases the level of properties.

At the same time, the alloy gets contaminated with magnesium oxide and requires performance of an additional operation - flux refining. At that, the alloy gets saturated with chloride ions, which leads to a reduction in corrosion resistance. Besides, approximately 15-20 % of the smelting crucible volume shall be drained into an intermediate ingot for further cleaning and processing. A significant disadvantage of this melt processing method is short duration of the modification effect (40-60 min maximum), which is unacceptable for the manufacture of small- and mid-size castings. The microstructure of ML5 alloy modified with magnesite is shown in Figure 1. It is established that just after 70 minutes of melt holding at 740 °c the alloy structure becomes almost unmodified.

All widespread modification methods have certain disadvantages: high process temperature (up to 780 °С), saturation of liquid melt with hydrogen, contamination with magnesium and calcium oxides, bumping of melt leading to an increased oxidability and additional melt contamination with oxides and slag, significant reduction of corrosion resistance due to blending of chlorine-containing fluxes into the melt.

Fig. 1. Microstructure of ML5 alloy. Modifi cation with magnesite. Melt holding at 740 °С. Т4. ×200 (marker 100 μm): а - after processing; b - 70 min hold period; c - 240 min hold period

Fig. 2. Microstructure of ML5 alloy. Modifi cation with a mixture of freon 12 and argon at a ratio of 1:3. Melt holding at 740 °С. Т4. ×200 (marker 100 μm): а – after processing; b – 70 min hold period; c – 240 min hold period

The developed method of modifying magnesium alloys of the Mg-Al-Zn-Mn system includes smelting of magnesium, introduction of alloy components in protective gas without using flux and purging of melt with a modifying additive (mixture of oxygen-free carbon gas (freons) with inert gas (argon, helium) at a ratio of (1:1)-(1:3)) at a temperature of 730-750 °С.

In case of using freons 12 and 14, the process proceeds by the reactions:

CCl2F2 + 2Mg = MgCl2 + MgF2 + C and 4Al + 3C = Al4C3, CF4 + 2Mg = 2MgF2 + C and 4Al + 3C = Al4C3.

Pure carbon is released from the gaseous phase without intermediate transformations, i. e., the possibility of its more complete release and recovery increases.

In purging modes employed at the enterprise, the introduced carbon amounts to 0.02-0.07 % of the melt weight (Table 1), at that, mechanical properties are higher by almost 20 % (Fig. 2).

Total consumption of the modifying mixture amounts to 30-60 l/min. Considering a significant (by 3-5 times) reduction of slag produced during modification, it is possible to reduce either the consumption of the modifying additive or the purging time. Reduction of the purging time makes the process difficult. Reduction of the modifying additive concentration by diluting the active gas with an inert one appears to be more effective.

The microstructure of ML5 alloy modified with a mixture of freon 12 and argon at a ratio of 1:3 is shown in Figure 3. It is established that even after 240 minutes of melt holding at 740 °C the modified alloy structure remains unchanged.

Apart from reducing active gas consumption, decreasing melt contamination with interaction products and increasing metal quality, the presence of argon in a mixture with freons additionally leads to a reduction of hydrogen content in the melt through diffusional release of the latter from the solution into the cavity of a bubble rising through the melt.

The developed technology ensures stable mechanical properties in state Т4 (more than 235÷240 MPa) and corrosion resistance (Table 2).

Table 1

Modification modes

Modifying additive

Processing temperature, °С

Estimated consumption of a modifying additive, carbon % wt of melt weight

Processing time, min

50 % freon 12 + 50 % argon

740

0.07-0.1

4-5

25 % freon 12 + 75 % argon

0.02-0.05

5-6

Magnesite

0.1-0.12

8-10

Corrosion resistance of samples was determined by the content of hydrogen released during their testing in 3 % solution of sodium chloride for 48 hours, and it conforms to the requirements of the regulatory documents (8 cm3/cm2).

The technology neither involves nor leads to toxic substances formation, its application doesn’t require any additional equipment, and the products have higher reliability, longer service life and can be operated under all climatic conditions.

Application of the developed melt processing method allowed to increase the duration of the modification effect for ML5 alloy at least up to 4 hours [4], compared with 40 minutes at using the conventional method of modification with magnesite with subsequent flux refining, which is supported by the data shown in Figure 3.

The third field consists in the development of automated systems of flux-free smelting.

When using the conventional method of flux smelting, a significant amount of alloy components of magnesium-based alloys (in particular, those containing rare-earth metals) turns to slag as a result of exchange reactions with flux components, i. e., is lost.

Flux-free smelting with the use of gas protective environments has become an effective substitute for this smelting method. In the USSR and Russia it was pioneered by the scientific schools of Foundry Engineering Technology department of MATI named after K. E. Tsiolkovsky (M. V. Sharov, V. V. Serebryakov, B. L. Bobryshev, Yu. P. Aleksandrova and others) [5-7], VIAM (A. A. Lebedev, I. Yu. Mukhina and others) [8, 9] and VILS (B. I. Bondarev, A. M. Ponomarenko, O. V. Detkova and others) [10, 11].

Today the challenges of minimizing gas protective environments consumption during smelting and development of methods for combining melt protection with modification and refining are still pressing.

Table 2

Impact of the modifying additive composition on ML5 alloy properties

Modifying additive

σВ, MPa

σ0,2, MPa

δ, %

Ск, см3/см2

50 % freon 12 + 50 % argon

281

104

14.2

4.3

25 % freon 12 + 75 % argon

278

104

13.8

4.3

Magnesite

244

91

7.2

3.9

An equipment complex, which allowed to automatize and develop controlled processes of magnesium alloy protection at flux-free smelting, modification and refining of melt, was first developed and introduced at the enterprise, which gave an opportunity to drastically reduce process expenses and castings price, as well as to improve operation conditions in a shop [12].

The control system of gas protective environments supply under the crucible cover and modifying gases supply into the melt consists of two units: a control unit and a monitoring unit.

The melt monitoring unit is intended for displaying on the monitor screen a change in the monitored parameters in the course of smelting, their recording in the computer memory for subsequent analysis and generation of smelting reports (Fig. 4).

Fig. 4. Appearance of the gas supply system: а - control cabinet; b - melt process monitoring unit

The gas supply control unit sets gas consumption in the course of smelting and modification. Gas consumption is set by the operator prior to the smelting process start.

At that, the values of gases consumption are displayed on the monitor screen in the melt monitoring unit.

Gas protective environments and modifying gases are supplied to the input of the control unit under pressure of 1...1.5 atm. In the beginning of smelting process, the operator switches on the supply of gas protective environments under the crucible cover. In the course of smelting, when the crucible cover is opened for process operations, the consumption of gas protective environments is automatically increased. The signal for increasing gas consumption is generated using infrared optical pyrometer directed to the crucible cover.

During modification, the operator additionally switches on the supply of modifying gases, which are supplied to the device for melt purging via the corresponding valves and mixer.

The monitoring unit includes a computer with touch-screen and modules of analogue input and discrete input-output. The signals of gas consumption, the temperature inside the crucible and on the crucible cover are sent to the input of the analogue input module. During smelting, all input signals are displayed on the monitor screen in the form of diagrams, and in the digital form (Fig. 5).

Fig. 5. Record of the results of process operations at ML5 alloy smelting

Upon smelting completion, the changes of input signals occurred during smelting are saved in the computer memory and can be displayed on the monitor screen and recorded onto a USB carrier for the analysis. At that moment, a smelting report is generated, which is also saved in the computer memory and can be displayed on the monitor screen and recorded onto a USB carrier.

The smelting report includes the preset (alloy brand, casting number, melt weight, emergency temperature, crucible cover temperature) and monitored (time of smelting and modification start, duration of smelting, modification and holding, actual temperature values, average and total consumption of gases during smelting and modification) parameters of the smelting process.

Apart from reducing the consumption of gas protective environments, increasing melt purity, improvement of labour conditions in the shop, application of the developed technology and the gas protective environments control system allowed to reduce the smelting time by 5-7 % (15-20 min), which provides additional economic benefit due to energy saving.

The fourth field consists in the development of automated systems of ensuring controlled heating process of large-sized moulds for creating the preset heating gradient of mould matrices.

High quality of a large-sized geometrically-complex casting of significant height (up to 1.5 m) depends not only on a perfect melt, but also on correct preparation of moulds (in particular, metal moulds), ensuring the process of directed controlled crystallization of the melt after mould-filling.

A widespread method of large-sized moulds heating constitutes in heating mould matrices with flare burners (Fig. 6) or special electric heaters.

Fig. 6. Mould heating by flare burners

Both these methods are inefficient, since they generally heat the surrounding environment, and above all, they do not ensure sufficiently accurate adjusted gradient oftemperature distribution throughout the height of the mould.

The appearance of a large-sized metal mould equipped with infrared burners and a controlled heating system is shown in Figure 7.

Fig. 7. Automated equipment complex for creating controlled directed gradient of large-sized metal moulds heating: а - heating of large-sized mould; b - monitoring system control cabinet; c - process of large-sized casting manufacture from ML5 alloy

An automated equipment complex for creating controlled directed gradient of large-sized metal moulds heating was first developed and introduced into production at the enterprise, which gave an opportunity to almost completely exclude porosity defects in the castings from wide-range magnesium alloys [13].

The algorithm of mould heating device (MHD) operation control ensures automatic fitlfilment of the requirements for the mould preset temperature gradient in the modes of mould preparation to filling and immediately in the course of casting filling process, as well as in the mode of applying paint coating to the mould working surface without active intervention of section maintenance staff.

MHD operation is based on the mechanism of gas-air mixture burning on porous penetrable burner

plates, ensuring the oxidation process at low СО and NOx emissions and retention of oxygen in the shop facility atmosphere, an increase of efficiency of the devices against flare burners due to low-temperature oxidation process, a reduction of the temperature of external mould surface and surrounding atmosphere.

The fifth field consists in the development of compositions of inhibiting admixtures to the cold hardening mixtures (CHM) and anti-stick coatings and parting paints.

In order to avoid ignition in case of magnesium alloys contact with moulds and rods material, the enterprise developed the compositions of available, low-cost domestic inhibitors (silicon carbide and oleophilic bentonite) added to CHM [14] and parting paints (oleophilic bentonite and talc) [15]. The physical and chemical properties of the inhibitors ensure an increase of heat-conductivity and thermal resistance of CHM and paint, chemically inhibit magnesium preventing its oxidation and ensuring absence of oxidic defects on the castings surface, suppression of burning in case of magnesium ignition, significantly reduce the possibility of gas release leading to gas defects on the casting surface and surface cracking, and are also capable of ensuring high casting surface purity without carbonization.

The sixth field consists in the recycling of shavings produced during machine processing of castings from magnesium alloys. The technology of shavings compaction and remelting was developed, it ensures recycling of up to 65-70 % of specification-grade ML5 alloy completely complying with the GOST requirements (Fig. 8).

Fig. 8. Fractures of ingot alloys: а - ML5 alloy after shavings recycling; b - purchased MA8Ts alloy

Currently, the organized magnesium casting section ensures manufacture of large-sized high-duty hull castings of a wide range and of up to 2 t of non-defective castings per month. The set of performed works allowed to reduce castings cost by 10-15%.

References

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About the Authors

B. L. Bobryshev
AVANGARD-LIT, Ltd.

Bobryshev Boris Leonidovich – Cand. Sci. (Engineering), Assoc. Prof., General Director

Research interests: foundry engineering.



D. V. Popkov
Moscow Engineering Plant “AVANGARD”, JSC

Popkov Denis Vladimirovich – Cand. Sci. (Economics), General Director

Research interests: foundry engineering.



V. S. Moiseev
AVANGARD-LIT, Ltd.

Moiseev Victor Sergeevich – Dr. Sci. (Engineering), Prof., Deputy General Director

Research interests: foundry engineering.



O. V. Koshelev
Moscow Engineering Plant “AVANGARD”, JSC

Koshelev Oleg Viktorovich – Chief Metallurgist

Research interests: foundry engineering.



D. B. Bobryshev
AVANGARD-LIT, Ltd.

Bobryshev Danila Borisovich – Head of the Casting Department

Research interests: foundry engineering.



K. V. Moiseev
Moscow Engineering Plant “AVANGARD”, JSC

Moiseev Kirill Viktorovich – Cand. Sci. (Engineering), Deputy Chief Metallurgist

Research interests: foundry engineering.



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For citation:


Bobryshev B.L., Popkov D.V., Moiseev V.S., Koshelev O.V., Bobryshev D.B., Moiseev K.V. Resource and energy saving technologies in the manufacture of large-sized castings from magnesium alloys. Journal of «Almaz – Antey» Air and Space Defence Corporation. 2020;(1):77-86. https://doi.org/10.38013/2542-0542-2020-1-77-86

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