the study of structure development regularities …the connection of various elements of the...

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International Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 11, November 2018, pp. 1471–1478, Article ID: IJCIET_09_11_142

Available online at http://www.iaeme.com/ijciet/issues.asp?JType=IJCIET&VType=9&IType=11

ISSN Print: 0976-6308 and ISSN Online: 0976-6316

© IAEME Publication Scopus Indexed

THE STUDY OF STRUCTURE DEVELOPMENT REGULARITIES IN

VT35 ALLOY AFTER STRENGTHENING THERMAL PROCESSING

Skvortsova S.V

Doctor of engineering sciences, professor, Moscow Aviation Institute (National Research

University), Material Science Department, Moscow, Orshanskaya st., 3

Gvozdeva O.N

Candidate of engineering sciences, associate professor, Moscow Aviation Institute (National

Research University), Material Science Department, Moscow, Orshanskaya st., 3

Orlov A.A

Post-graduate student, Moscow Aviation Institute (National Research University), Material

Science Department, Moscow, Orshanskaya st., 3

Stepushin A.S

Post-graduate student, Moscow Aviation Institute (National Research University), Material

Science Department, Moscow, Orshanskaya st., 3

Volodin A.V

PJSC “Normal”, Nizhny Novgorod, Litvinova st., 74

ABSTRACT

The article studied the effect of the temperature and aging time on the kinetics of

high-temperature β-phase decay in the titanium alloy VT35. It was shown that VT35

alloy has a high technological plasticity in the hardened single-phase β state: the

maximum compression ratio during a sediment at room temperature makes 75 - 80%

with the maximum strength of about 800 MPa. They determined the temperature and

the time intervals of the hardening heat treatment, which makes it possible to reach the

values of the tensile strength up to 1400 MPa and shear stress values up to 815 MPa

Keywords: pseudo-β-titanium alloys, decay, aging, technological plasticity,

mechanical properties, shear stress, tensile strength, sediment, fastening details

Cite this Article: Skvortsova S.V, Gvozdeva O.N, Orlov A.A, Stepushin A.S and

Volodin A.V, the Study of Structure Development Regularities in Vt35 Alloy after

Strengthening Thermal Processing, International Journal of Civil Engineering and

Technology, 9(11), 2018, pp. 1471–1478

http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=9&IType=11

The Study of Structure Development Regularities in Vt35 Alloy after Strengthening Thermal

Processing


1. INTRODUCTION

At present, the main trend of aviation equipment development is the replacement of metallic

materials with composite ones in order to increase the efficiency of aircraft due to the structure

weight reduction [1]. The connection of various elements of the airframe design is done by

various fastening elements (rivets, bolts). In previous studies [2] they showed that the main

drawback of steel and aluminum fasteners in the structures made of polymer composite

materials is the electrochemical corrosion of rivets, especially during the use of aircraft from

aircraft carriers. And of all the materials used to make metal fastening parts, titanium-based

alloys have the best corrosion resistance [3, 4].

The fastening elements are related to mass production products. The technology of their

manufacture includes hot deformation of the billet into a rod of the required size, a cold or

warm landing, intermediate technological operations and final hardening treatment of a

finished product [2, 5]. Therefore, alloys must be sufficiently technological at room

temperature and capable of subsequent hardening at low temperatures at the same time [6-9].

This paper is the continuation of the research carried out by the authors in this direction [9-

11]. In previous studies [9, 10] it was shown that the most promising alloy for the production

of high-strength fastening parts is the pseudo-β-titanium alloy VT35. In work [11], they

substantiated the optimization of its chemical composition, which makes it possible to obtain

semi-finished products with high technological plasticity. However, the products manufactured

from them have insufficient strength in comparison with the currently widely used alloy VT16

[2, 12, 13]. Therefore, in this work, the assessment was performed concerning the influence of

temperature-time parameters of aging on the decay of the metastable β phase and the

development of a thermal treatment regime on this basis that provides a balanced set of

technological and operational properties that allow to obtaining quality products from VT35

alloy by cold plastic deformation methods.

2. MATERIALS AND METHODS OF RESEARCH

The studies were carried out on hot-rolled bars of VT35 alloy, obtained by the advanced

technology. The chemical composition of the alloy is shown in Table 1. The heat treatment was

carried out in an electric resistance furnace SNOL-2.2.5.1.8/10-I3 in the air atmosphere.

Microstructure studies were performed on an optical microscope AXIO Observer.A1m at

the magnifications up to 1000 times. They used light field method was used in the air medium.

The analysis of the obtained images was carried out using ImageExpert Pro3 software package.

Rockwell hardness was determined by Macromet 5100T device in accordance with GOST

9013-59. The determination of mechanical properties at room temperature was carried out on

a universal tensile machine TIRAtest 2300 using specialized grippers. Mechanical tests for

tension, draft and cut were conducted in accordance with GOST 1497-84, GOST 8817-82 and

OST 1.90148-74, respectively.

3. EXPERIMENT RESULTS AND DISCUSSION

A pilot batch of bars made of VT35 alloy was manufactured for the studies. The technological

scheme of bar manufacturing from VT35 alloy, from 18 mm to 9 mm in diameter, included

forging and subsequent rolling in the β-region. As the diameter of the workpiece was reduced,

the deformation temperature was gradually reduced from 1060 °C to 850 °C.

The structure of the bars from the VT35 alloy after hot rolling is identical and is represented

by the grains of the β phase, the size of which depends on the rolling temperature. The final

stages of ∅ 18 mm rod deformation were carried out at a temperature of 1000 °C and 9 mm

Skvortsova S.V, Gvozdeva O.N, Orlov A.A, Stepushin A.S and Volodin A.V


rod - 850 °C, therefore the average grain size is 220 μm in the rod of ∅18 mm, and 70 μm in

the rod of ∅9 mm. To remove the stresses, all the rods after rolling were heated to the

temperature of 800 °C (Tpp + 70 °C) and were cooled in air after isothermal hardening. The

alloy VT35 refers to pseudo-β-alloys, so the cooling in air is the hardening for it (Figure 1).

Table 1. The chemical composition of the ingot from VT35 alloy

Semi-product Alloying elements, mass. % Admixtures, mass. %

Al V Cr Sn Mo Zr Nb Fe C N О

Test ingot 2,9 14,9 2,36 2,82 0,55 0,53 0,02 0,0

5

0,0

1

0,0

2 0,12

Requirements

by passport 2,0-4,0

14,0-

16,0

2,0-

4,0

2,0-

4,0

0,5-

2,0

0,5-

2,0

0,01-

0,4 0,3 0,1

0,0

5 0,15

а) б) в)

Figure 1. The structure of ∅ 18 (a), 12 (б) and 9 mm (в) rods from alloy VT35 after hardening at the

temperature of 800 °C

At present, there are no mechanical property requirements for VT35 alloy bars, which are

preferred for the manufacture of fastening parts, but such data are available for VT16 alloy

[12]. Therefore, in this paper, the mechanical properties of the bars made of VT35 alloy were

compared with the requirements for the bars made of VT16 alloy.

The analysis of the mechanical test results for tensile, draft and shearing of samples from

the VT35 alloy shows that they have high processability in a hardened state, which was

evaluated by the maximum compression ratio during the draft at room temperature. Its values

were not less than 78%. However, in the hardened state, the alloy has a sufficiently low level

of tensile strength (σв ≈ 790 MPa) and shear stress

(τср≈575 MPa), which is significantly lower than the required values for the alloy VT16

(Table 2).


Processing


Table 2. Mechanical properties of bars made of VT35 alloy

∅, mm Hardness,

HRC

Draft tests Tensile tests Cut tests

εпр, % σв, MPa δ, % ψ, % τср, МПа

18 20,5 78 775 26 66 580

16 25,0 78 775 26 66 575

14 25,0 78 780 25 62 570

12 23,5 78 800 23 61 570

10 25,0 78 810 25 71 575

9 23,5 80 810 25 71 590

According to

TR 1-809-

987-2002 for

VT16

− ≥ 75 813 − 931 ≥ 14 ≥ 60 ≥ 630

Thus, it is possible to conduct cold plastic deformation on the obtained experimental rods

with such a complex of mechanical properties, but they do not have the necessary margin of

strength and shear stress. Strength characteristics can be increased on the products due to

additional hardening heat treatment.

Therefore, at the next stage of the work, they studied the influence of the heating

temperature and the holding time on the kinetics of the β phase decay. Aging was carried out

in the temperature range 475° - 600 °С with the increments of 25 °С. The maximum isothermal

holding time at each selected temperature was 60 hours. The cooling after aging was carried

out in the air. The degree of β phase decay was controlled by metallographic and X-ray

diffraction analysis, and the degree of hardening - by the change of hardness.

The carried out studies have shown that in the course of aging at all studied temperatures

the decay starts in the interval from 1 - 5 hours, but it proceeds with different intensities (Fig.

2a, c, d). Thus, at the aging temperatures of 475°, 500°, and 525° after the aging for 5 hours,

approximately the same degree of β phase decay is observed, but the size of the emitted

particles of the α phase differs - it gradually increases with aging temperature increase (Fig. 2a,

c, d). Besides, the separation of the α phase by the volume of β-grains is quite uneven, which

is expressed by the presence of grains in the structure, both fully decayed and free of

precipitates, which is characteristic of pseudo- titanium alloys [14-16]. The increase in the

aging temperature to 550 °C practically does not affect the intensity of the β phase decay, but

leads to a substantial increase of β phase particles. It should be noted that there are no β-grains

sharply differing by the degree of decay in the structure (Fig. 2c). The increase of the aging

temperature to 600 °C leads to a sharp inhibition of the decay processes despite the activation

of diffusion processes. So, after the holding for 5 hours there is an insignificant amount of α-

particles in the structure, which are allocated mainly along the boundaries of β-grains (Fig. 2d).

This is due to the decrease in the driving force of β→α- transformation due to the approach to

the transition temperature (α+β)/β, which makes 730 °C for the given alloy.



475°°°°С 500°°°°С 600°°°°С

5 hours

а)

в)

д)

50 hours

б)

г) е)

Figure 2. The structure of the experimental rod of ∅18mm from the alloy VT35, depending on the

heating temperature and the holding time

The described changes in the structure of aged samples are also reflected in the change in

their hardness (Fig. 3). The most intensive increase in hardness after the aging for 5 hours is

observed in the samples aged in the temperature range 475° - 525°С. And the lower the aging

temperature, the more dispersed the α-particles and the higher the alloy hardness level (the

degree of hardening) in comparison with the quenched state (25 HRC units). So the increase of

hardness with the aging at 475 °C during the first 5 hours of aging is 1,7 HRC/hour; 500 °С -

1,5 HRC/hour; 525°С - 1,0 HRC/hour. With the aging temperature increase, the intensity of

hardening decreases and at 550 °C it makes 0.2 HRC/hour, and at 600 °C - 0 HRC/hour.


Processing


Figure 3. The change of sample hardness from the experimental rod made of VT35 alloy, depending

on the heating temperature and the holding time

The increase of aging time to 10 hours at the temperatures of 475°, 500°, and 525 °C retains

a general pattern in the course of β phase decay and the degree of hardening (Fig. 3). However,

the intensity of hardening is reduced and makes 0.7 HRC/hour at 475 °C; 0.6 HRC/hour at 500

°C and

0,4 HRC/hour at 525 °C, and at 550 °C it increases to 0.6 HRC/hour, which is associated

with the intensification of the decay process. The separation of the α phase at 600 °C proceeds

weakly and the increase of hardness obtained after 10 hours of exposure makes only 1 HRC

(Figure 3). A further increase of aging duration at all temperatures leads to the increase of

hardness, but the intensity of its decay decreases (Fig. 3).

Thus, the maximum hardness of 43.5 HRC is achieved with the aging of the alloy VT35 at

475 °C. The increase of the aging temperature leads to the gradual decrease of hardening rate

and the maximum hardness. This is due to the degree of dispersion of the developed particles

of α phase, which decreases with the aging temperature increase [17]. As the diffusion mobility

of the atoms increases, the time for the complete decay of the β-solid solution decreases. If the

decomposition at 475 ° C is completed within 40 hours, then at 525 °C the decomposition

makes 20 hours. A further increase in the aging temperature, despite of an even greater

activation of the diffusion processes, leads to the increase of time for equilibrium state reaching,

which is conditioned by the decrease of the driving force for β→α- transformation, due to the

approach of aging temperatures to the temperature of the phase transition.



Based on the analysis of the obtained results, it can be concluded that it is not advisable to

carry out the process of product hardening at the temperatures of 600° and 550°С. Since the

duration of aging to the maximum hardening is 35 hours at least on the average, the achieved

level of hardness does not exceed 34.0 HRC. The most optimal aging temperatures are 500°

and 475°С, providing the hardness at the level of 42.0 - 43.0 HRC for the same aging time.

At the final stage of work mechanical properties were determined on the experimental bars

made of alloy VT35 in the thermally strengthened state. The modes of hardening heat treatment

included the quenching from the β-region at the temperature of 800 °C and the subsequent

aging at the temperature of 475 °C for 40 hours and at the temperature of 500 °C for 25 hours.

The tests showed that the aging at a temperature of 475 °C provides the strength of about

1400 MPa for VT35 alloy and a shear resistance of more than 800 MPa with satisfactory

plasticity. The increase of aging temperature to 500 °C reduces the strength level up to 1200

MPa and increases the ultimate elongation almost 2 times (Table 3).

Table 3. Mechanical properties of experimental bars made of VT35 alloy after hardening heat

treatment (HHT)

HHT mode Hardness in

HRC

Extension test Shear tests

σв, MPa δ, % ψ, % τср, MPa

800°С, 1 hour, air

475°С, 40 hours 43,5 1410 5 13 815


500°С, 25 hours 42,0 1210 9 18 745


525°С, 20 hours 36,5 1090 15 43 680

4. SUMMARY

Thus, the conducted studies have shown that the VT35 alloy in the quenched single-phase β

state has a good plasticity, which provides the maximum compression ratio during draft at a

room temperature of 78 - 80% with a relatively low strength level of 775 - 810 MPa.

Subsequent aging at the temperatures of 475° - 525°С allows to increase significantly both the

strength values up to 1100 - 1400 MPa and the shear stress up to 680 - 815 MPa, which allows

to consider this alloy as one of the promising materials for fastening part manufacture.

ACKNOWLEDGEMENTS The work was financially supported by RF Ministry of Education and Science within the framework of state

support for the cooperation of Russian higher educational institutions, state scientific institutions and

organizations implementing complex projects for the creation of high-tech production approved by RF

Government Resolution No. 218 (April 9, 2010) CC No. 02.G25.31.0154 by the equipment of TSKP "AKMiT"

MAI.


Processing


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