Thermostability of Ionic Nitriding of Aluminium Alloys at Isothermal and Thermal-cycle Influence
Authors:   National Academy of sciences of Ukraine G.S. Pisarenko institute for problems of strength, Rutkovskіy Anatoly , Markovych Sergiy ,   Central Ukrainian National Technical University, Myhajlyta Sergiy ,   Central Ukrainian National Technical University
Published: January 2021

Тhe study of the main parameter that limits the use of pistons made of aluminum alloys in boosting engines - heat resistance and its dependence on diffusion ion-nitrided layers. In the analysis of recent studies and publications, the most common methods of heating samples during tests, their advantages and disadvantages are identified. The task of the study is to identify the relationship of factors that determine the heat resistance of ion-nitrided aluminum alloys under isothermal and thermocyclic effects for effective analysis of the mechanism of phenomena and control of the technological process of strengthening the pistons. The availability of this information will increase the strength and reliability of both the coatings themselves and the parts with coatings at the stage of their design. A set of laboratory equipment based on the Nutcracker installation was used for the study, which provides cyclic change of temperature and load of the sample, control and recording of temperature values, loading and deformation and operation in automatic mode. Flat samples of aluminum alloy AL2I with heat-protective diffusion ion-nitrided layers applied in the VIPA-1 installation and samples without treatment were used. Characteristics of the thermal cycle: 500 ° C ↔ 720 ˚C. heating time - 10 sec., cooling 15 sec., which corresponds to the real operating conditions of the pistons. The fixed value of the load was 0.8σВ according to DSTU 2637-94 and the criteria for determining the allowable stress in the Standards of ASME (80% of the average value of stresses that lead to a total deformation of 1%). The curves of isothermal and thermocyclic creep having three clearly expressed sites of creep are defined, depending on temperature their duration is different. Isothermal and thermocyclic creep curves of aluminum alloy AL21 with heat-protective diffusion ion-nitrided layers have three distinct creep areas, depending on the temperature their duration is different. This is due to the presence in the surface layer of the reinforcing phase AlNi; uniformity of distribution of components of a covering on a surface and degree of their refractoryness. This is due to the complexity of the movement of dislocations, which on their way face the need to overcome large obstacles in their movement, caused by the presence of a reinforced layer. Conclusions. Based on the obtained curves of isothermal and thermo cyclic creep, it is established that the type of creep curves depends on the temperature and conditions of its influence on the composition "base-coating"; the time to failure in the thermo cyclic test mode is less than in the isothermal; in the process of creep of aluminum alloy AL21 with heat-protective diffusion ion-nitrided layers, two competing processes are observed: hardening due to plastic deformation and removal of hardening under the influence of elevated temperature; despite the different composition of heat-protective diffusion ion-nitrided layers, the resistance to isothermal and thermo cyclic creep of the reinforced aluminum alloy AL21 increased by 1.8 ... 2.2 times.