Wave strain hardening in combined and additive technologies

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BACKGROUND
One of the main tasks of mechanical engineering technology is to ensure the performance properties of the most loaded critical machine parts that determine the service life of the entire mechanism or product. Hardening treatment is usually used to solve this problem. Surface plastic deformation (SPD) is a well-known method of hardening machine parts, the advantages of which are the ease of implementation and wide technological capabilities for obtaining a surface layer with the required quality parameters.
Among the known SPD methods, wave strain hardening (WSH) by static-pulse treatment occupies a special place, since it allows creating a deep hardened surface layer of up to 6-8 mm or more, while it is possible to regulate the uniformity of hardening, which is extremely important for improving the performance properties of parts. The possibility of obtaining a large depth of hardening is achieved by concentrating shock waves of deformation in the contact patch of the tool and the hardened surface, which arise in the striker-waveguide impact system after the impact. Shock waves are controlled by changing the geometric parameters of the impact system. The deformation waves form impact pulses in the contact spot, the shape of which is adapted to transfer maximum energy to the metal material being strengthened. Strengthening by deformation waves is carried out under conditions of combined action of static and impact loads. The static load should ensure continuous contact of the impact system with the surface being strengthened and is usually at least 10% of the dynamic load. Continuous contact with the deformed body creates the effect of a prolonged impact pulse. This allows for a more complete transfer of impact energy from the impact system to the loaded material, maximizing the efficiency of the strengthening process. As a result of the action of deformation waves, plastic imprints of the required dimensions with a given overlap are formed on the surface, which allows for regulating the uniformity of strengthening, creating both a uniformly and heterogeneously modified structure, which is often more effective in improving the performance characteristics of parts operating under cyclic loads [1, 2]. The features of WSH allow it to be used both independently and as part of combined strengthening technologies, as well as in additive technologies, which have been developing quite intensively recently.

RESULTS AND DISCUSSION
Increasingly, traditional methods of hardening do not always meet modern requirements for the reliability and durability of machine parts. Therefore, the development of combined hardening technologies is currently becoming quite relevant. Approaches that combine effects on the hardened material with different physical natures have proven to be especially effective. An analysis of existing combined hardening methods has demonstrated the high efficiency of the combined use of SPD and chemical-thermal treatment. The use of SPD before the carburizing process increases the dislocation density, activates diffusion processes and allows achieving higher levels of carbon concentration in the diffusion zone. This makes it possible to reduce the time and energy costs of the hardening process, and also provides an additional resource for improving the performance characteristics of parts [3, 4]. The use of WSH in combined hardening allows the formation of a deeper diffusion zone for carbon penetration during subsequent carburizing. The conducted studies have shown that combined hardening by carburization with preliminary WSH results in the formation of a uniformly modified surface layer, which is the most saturated with carbon after carburization, and a heterogeneously modified sublayer located underneath. The alternation of hard and soft sections of the heterogeneously modified sublayer is changed by WSH modes [5-8]. The modes of combined hardening by carburization with preliminary WSH have been established, which contribute to an increase in durability under contact fatigue loads by up to 2.5 times. The use of preliminary WSH has reduced the time and energy costs associated with the carburization process and accelerated the carburization process by up to 6 times. This is especially important when obtaining deep carburized layers of up to 5 mm or more. It is traditionally believed that preliminary SPD of metallic materials before heat treatment (HT) is meaningless, since the hardening disappears during subsequent thermal exposure. However, a method of strengthening is known that uses a combined effect of preliminary volumetric plastic deformation (drawing, rolling, etc.) and subsequent heat treatment – ​​preliminary thermomechanical treatment (PTMT). The purpose of PTMT is to increase the strength characteristics of metallic materials due to the formation of a dislocation structure by plastic deformation, which retains its stability when heated by heat treatment [9, 10]. The disadvantage of this method is the impossibility of regulating the uniformity of strengthening and, accordingly, the creation of a heterogeneously modified structure.
The combination, when preliminary strengthening by SPD is carried out first, and then hardening by heat treatment, has not been used. The use of WSH in combined treatment will allow not only to create a heterogeneously modified structure, but also to obtain a deep hardened surface layer, which, as PTMT shows, is quite important for an additional effect when combined with heat treatment. As a result of the conducted studies of combined treatment with WSH+TO, it was established that the use of WSH allows achieving higher values ​​of average hardness in the surface layer relative to hardening with TO only. Thus, for 30KhGSA steel, the hardness can be increased to 25% with unchanged impact toughness, and for 10KhSND steel, the surface layer receives a simultaneous increase in hardness and impact toughness, respectively, up to 39% and 25% relative to non-hardened steel [11-15].

In WSH modes providing a uniformly modified structure, the combined WSH+TO treatment allowed to increase the abrasive wear resistance of 30KhGSA steel samples by 16%. When creating a WSH of a heterogeneously modified structure, the combined WSH+TO treatment allowed to increase the fatigue life by more than 60% compared to samples strengthened only by TO.
At present, additive technologies used to create metal machine parts are actively developing and being implemented in production [16-20]. However, despite the growing popularity, the main disadvantage of parts obtained in this way is the presence of internal defects in the structure of the grown metal material and, in this regard, low strength characteristics relative to materials obtained by traditional methods. To improve the structure and increase the strength characteristics of the grown metal materials, the use of strain hardening treatment with SPD is quite effective.
The problem of improving the structure and mechanical characteristics of the grown metal can be solved by using strain hardening treatment in the process of layer-by-layer synthesis. Currently, the use of various SPD methods in growing metal materials is actively developing, such as rolling, ultrasonic impact treatment, minting, laser impact treatment, when each subsequent deposited layer is subjected to plastic deformation in a hot or cold state, grinding and optimizing the grain size of the deposited metal and, thereby, improving the mechanical properties of the product material [21-26]. Such methods, as a rule, provide a hardened surface layer no more than 1-3 mm deep, while the synthesized layer during growth by surfacing can reach 1.5-2.5 mm or more. The surface layer of the part synthesized by the 3DMP method, heating to high temperatures by the heat flow from the melt pool, partially loses the effect of strain hardening. The problem of effective hardening can be solved by using WSH, which provides a greater depth of the hardened surface layer, allowing several deposited layers to be hardened simultaneously. The WTH process is advisable to carry out at a temperature of the deformed surface of 200...400°C, i.e. a short period of time after surfacing, without the need for complete cooling of the deposited material. Wave thermal deformation hardening (WTHH) has shown high efficiency in increasing the strength characteristics of the grown metallic material [27, 28]. As a result of the studies, it was found that, in contrast to non-hardened samples, samples synthesized using WTHH from steels and alloys of the Cr-Ni and Cr-Ni-Mo group have higher mechanical properties: hardness can be increased by 2.5-2.6 times, yield strength - by 2-2.2 times, tensile strength - by 1.5-1.7 times, which significantly (1.4-2.5 times) exceeds similar properties of rolled products from the same grade of material.

CONCLUSION
The technological capabilities of WSH allow creating a deep hardened surface layer with the required uniformity of hardening, forming, if necessary, both a uniformly and heterogeneously modified structure. Having a high potential, VDU can be used quite effectively not only independently, but also in combined cementation treatment with preliminary WSH, WSH + TO, as well as for hardening the synthesized layers in additive synthesis.

About the authors

Andrey Kirichek

Брянский государственный технический университет

Email: avkbgtu@gmail.com
ORCID iD: 0000-0002-3823-0501
SPIN-code: 6910-0233

д-р техн. наук, профессор, проректор по перспективному развитию

Russian Federation, Брянск

Dmitriy Solovyev

Владимирский государственный университет имени А. Г. и Н. Г. Столетовых

Email: murstin@yandex.ru
ORCID iD: 0000-0002-4475-319X

д-р техн. наук, профессор, профессор кафедры технология машиностроения

Russian Federation, Владимир

Alexandr Yashin

Владимирский государственный университет имени А. Г. и Н. Г. Столетовых

Email: yashin2102@yandex.ru
ORCID iD: 0000-0002-3186-1300
SPIN-code: 3473-4047

канд. техн. наук, доцент, доцент кафедры технология машиностроения

Russian Federation, Владимир

Sergey Silantyev

Владимирский государственный университет имени А. Г. и Н. Г. Столетовых

Author for correspondence.
Email: ppdsio@yandex.ru
ORCID iD: 0000-0002-3524-385X
SPIN-code: 2686-4678

канд. техн. наук, доцент, доцент кафедры технология машиностроения

Russian Federation, Владимир

References

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