Parameters of rocket engine chambers, obtained by selective laser fusion

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Introduction

High quality requirements are imposed on the products of the rocket and space industry. At all stages of aircraft engines (AE) development, parameters are evaluated based on the efficiency of design solutions, the quality of materials used, design development of prototypes, and the results of product tests during production and operation [1].

The development of additive technologies (AT) in comparison with traditional production in the future will significantly reduce the time and cost of manufacturing products as a result of improving manufacturability, reducing the number of parts of assemblies, obtaining structural materials with improved properties [2]. The differences in the material properties of parts produced using additive technologies and those produced by traditional molding methods have already been revealed. Currently, the use of 3D printing in the production of rocket engines is constrained by insufficient development of technology parameters.

The purpose of the research is to study the properties of the material obtained by SLM printing method, experimental development of printing modes and identification of controllable parameters affecting the physical and mechanical characteristics of products operating at high temperatures.

Description of selective laser metal melting technology

The Reshetnev Siberian State University of Science and Technology together with industrial partner LLC "Polychrome" are conducting research on the development of technology for the production of low thrust rocket engine chamber by selective laser melting (SLM) of Inconel 718 material.

Inconel 718 is a heat-resistant, high-quality alloy that was developed for the reliable operation of products at temperatures up to 980 °C. Almost immediately after its creation, it became highly demanded and already in the 70s of the last century it accounted for more than 50% of the total gross production of industrial heat-resistant alloys in the USA. Inconel 718 is now considered one of the most sought-after materials in the Inconel group.

This alloy has high impact toughness and strength up to a temperature of 900 °C, and at low temperature (–78 °C) Inconel has no sensitivity to notching. Inconel alloys are resistant to oxidation and corrosion. When heated, Inconel forms a thin stable passivating oxide film protecting the surface from further destruction. Inconel maintains its strength over a wide temperature range and is therefore a universal material used in many industries.

Initially, experiments were conducted to identify controllable laser parameters and to work out printing modes of the printer from Inconel 718 powder of PR08-HN53BMTU grade. Then experimental samples were printed to determine the porosity, chemical composition, mechanical properties of the obtained material (Fig. 1).

 

Рис. 1. Образцы для механических испытаний

Fig. 1. Samples for mechanical testing

 

A part of samples was manufactured without heat treatment. A part of the samples was heat-treated according to the following regime: hardening at 980 ± 10 °C, holding for 1 hour, cooling in air (argon), aging at 720 ± 10 °C, holding for 8 hours, cooling in the furnace to 620 °C for 2 hours, then holding for 8 hours at 620 ± 10 °C, cooling in air (argon). The obtained samples were subjected to studies to determine the chemical composition, structure, and mechanical properties of the material. The density of Inconel 718 alloy is 8.19 g/cm³.

According to the content of alloying elements, the material of samples corresponds to Inconel 718 grade. According to the structure, the material of the samples has increased porosity [3; 4]. The surface of the samples has increased roughness.

To investigate the mechanical characteristics, the material of standard samples of the chamber-demonstrator was tested for tensile properties and microhardness was determined by the Vickers method. As a result of the studies, the values of yield strength, strength, tensile strain and hardness of the material were obtained [5].

After processing the test results, the values of residual material deformations and elastic moduli for heat-treated samples and samples without heat treatment were obtained (Table 1).

 

 

Table 1. Inconel 718 alloy test results at room temperature

Sample number	Modulus of elasticity (E), MPa	Permanent strain at break (δост), %
1. With heat treatment	0,214·105	5,27
2. With heat treatment	0,209·105	5,38
3. With heat treatment	0,176·105	5,95
4. With heat treatment	0,156·105	6,95
5. Without heat treatment	0,139·105	6,51
6. Without heat treatment	0,139·105	6,43
7. Without heat treatment	0,116·105	8,7
8. Without heat treatment	0,113·105	5,23

 

Heat treatment of the samples led to an increase in their strength characteristics, hardness, and a decrease in elasticity and plasticity. The obtained stress characteristics are close to the results of [6], while the strain characteristics are significantly lower (Table 2).

 

Table 2. Mechanical properties of Inconel 718 alloy at room temperature

Characteristics	Inconel 718 (after heat treatment)
	With heat treatment	Without heat treatment
Tensile strength σв, MPa	1400	1380
Yield strength σ0,2, MPa	1150	1240
Relative elongation δ, %	15	18

 

Затем были напечатаны две камеры и смесительные головки ракетного двигателя (рис. 2).

The two chambers and mixing heads of the rocket engine were then printed (Fig. 2).

 

Рис. 2. Напечатанные камера (а) и смесительная головка ракетного двигателя (б)

Fig. 2. Printed rocket engine chamber (а) and mixing head (b)

 

To confirm their performance, a whole range of tests was carried out. First, vibration tests of the chambers were successfully carried out. Then strength and tightness tests using a special device were carried out.

According to the technological requirements for test devices, the test device must ensure tightness, simulate and withstand the loads that the product experiences in real operation. For testing the strength and tightness of the engine chamber, a special device was designed (Fig. 3). It was manufactured by the company Variant-999 Ltd. 

 

Рис. 3. Приспособления для испытаний

Fig. 3. Test devices

 

According to OST 92-4291-75 the following types of tests were chosen: for strength by pressure test (using distilled water), for tightness by «aquarium» method (using air). Tightness test is performed immediately after the strength test using the same special device. Test modes were developed
(Table 3).

 

Table 3. Parameters of tightness test modes

Parameter	Strength	Tightness
Pressurization pressure, kg/sm2	11,2	12,65
Pressurization duration, min	5	5
Test pressure, kg/sm2	12,75	11,5
Test duration, min	5	3

 

The main requirement for test devices is that they must ensure tightness and withstand the loads that the product is subjected to in actual operation. Therefore, all elements of the device operating under loads were designed on the basis of calculations carried out under the condition of strength. The scheme of the test fixture is shown in Fig. 4.

 

Рис. 4. Схема приспособления для испытаний: 1 – камера; 2 – кольцо упорное; 3 – прижим; 4, 5, 8 – уплотнительные кольца; 6 – шток; 7 – фланец; 9 – шпилька; 10 – штуцер

Fig. 5. The scheme of the device for testing: 1 – chamber; 2 – thrust ring; 3 – clamp; 4, 5, 8 – sealing rings; 6 – rod; 7 – flange; 9 – stud; 10 – fitting

 

The working body for testing is fed into the cavity «A» through the inlet fitting 10 and removed through the outlet fitting. The tightness of the cavity is ensured by sealing rings 4, 5, 8. Therefore, special requirements on roughness and dimensional accuracy are imposed on the mating surfaces of the chamber-demonstrator 1, as well as flange 7 and clamp 3 with rings. For uniform pressing of the sealing ring 8 to the surfaces of the flange 7 and the demonstrator chamber 1, six studs 9 arranged evenly around the circumference are used. Pressing of sealing rings 4 and 5 from the side of critical section is provided by means of rod 6. The strength calculation of the loaded elements of the device showed the necessary safety margin for the test.

During engine chamber tightness testing, increased porosity and leaking of the chamber material was found.

Analysis of identified parameters of selective laser melting technology

As a result of working out the technology of printing samples and rocket engine chambers on a particular printer, the following features were revealed [7]:

– printed products have isotropic mechanical properties;

– low printing speed;

– internal porosity of the obtained product;

– shrinkage of the product after heat treatment [8];

– possibility of warping of the product after heat treatment;

– necessity of post-processing of the product surface to reduce roughness [9-14];

– limitation of the product size by the dimensions of the space of the printing device;

– high cost of powder.

Further, by changing the printing modes of the engine chambers, satisfactory results were achieved to improve the quality and reliability of the products in strength and tightness tests.

Based on the results of this work, four groups of controllable SLM printing parameters affecting the properties of the resulting product material can be identified:

1. laser characteristics F_л:

– laser power N_л;

– print spot temperature T;

– print spot size d_л;

$F_{\text{л}}(N_{\text{л}},T,d_{\text{л}})$.

2. Properties of the melted layer F_с:

– powder granularity d_з;

– height of the melted powder layer h [15];

– the amount of overlap of the beam edges when printing with the next laser pass in the layer Δ_d;

$F_{\text{с}}(d_{\text{з}},h,{\Delta }_d)$.

3. Printing time parameters F_t :

– speed of laser movement during printing v;

– laser return time when printing with the next pass in the layer t;

$F_t(\nu ,t)$.

4. Geometric parameters of the product F_r:

– transverse dimension of the product b;

– product wall thickness δ;

– angle in the horizontal plane between the product and the laser beam direction a;

$F_r(b,\delta ,\alpha )$.

Conclusion

According to the results of the conducted researches the following conclusions can be formulated:

– the technology of selection of printer parameters for SLM printing, which allows to obtain products with the necessary mechanical properties of the material, has been experimentally worked out;

– recommendations on control tests of products manufactured by 3D printing method have been developed;

– the properties of products manufactured by 3D printing method were investigated and the differences from the products manufactured by traditional methods were revealed;

– controlled parameters of SLM printing affecting the properties of the product material were systematized.

To implement new technology in production, it is necessary to study the influence of various factors on individual physical and mechanical properties of materials in the form of functional or statistical dependencies F_i (F_л, F_с, F_t, F_r). First of all, this applies to such characteristics of printers as the ability to control the laser parameters, shape and size of the printed product. This will make it possible to develop theoretical bases for new production technologies, significantly reducing the experimental development of the manufacturing technology for each new product [16-18].

About the authors

Viktor Yu. Zhuravlev

Reshetnev Siberian State University of Science and Technology

Author for correspondence.
Email: vz@sibsau.ru

Cand. Sc., Аssistant Professor of a department of Aircraft Engines

Russian Federation, Krasnoyarsk

Elvira S. Manokhina

Reshetnev Siberian State University of Science and Technology

Email: xim96@inbox.ru

Master of the Department of Aircraft Engines

Russian Federation, Krasnoyarsk

Matvey A. Shinkarev

Reshetnev Siberian State University of Science and Technology

Email: 89134467140@bk.ru

Master of the Department of Aircraft Engines

Russian Federation, Krasnoyarsk

References

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