COMPARISON OF THE CREEP THRESHOLD OF PISTON ALUMINUM ALLOYS WITH TIME

Abstract

The direction of work associated with the increase and improvement of efficiency and environmental performance while maintaining the trend of increasing specific power, leads to an increase in the thermal stress of the combustion chamber parts. This leads to cases of loss of parametric reliability of the side surface of the pistons both during experimental studies and during initial operation. Such cases are characterized by the appearance of rubbing in the area of the finger hole. These facts indicate the high relevance of additional studies of the properties of aluminum piston alloys, especially at the initial stage of thermal loads of structures. The aim of the work is to obtain comparative information about the creep threshold of materials, the excess of which causes deformation of the piston side surface in time and, accordingly, the occurrence of an unpredictable decrease in the gap in the friction pair piston-cylinder. This work presents a fundamental approach, in accordance with which the creep threshold of materials in the process of their hardening is determined. The creep process of the material at the first two stages is taken into account. The creep threshold of AL25 and AK4 alloys in their initially non-hardened and hardened with time state has been studied. Dependences between the level of temperatures and thermal stresses, which determine the creep thresholds of the studied piston aluminum alloys, are established. It is shown that the hardening of the studied alloys occurs during the first 10 hours of thermal loading. It has been established that the creep limit of non-hardened AL25 and AK4 alloys is 1.5-2 times less than the creep limit of hardened materials. An approach is proposed to ensure the parametric reliability of the piston side surface, which consists in the impossibility of thermal loading of the material by a level exceeding the creep threshold of unhardened materials. The presented approach is recommended to be used taking into account the concept of guaranteed reliability of structures at the initial stages of their design. A detailed presentation of the main material of the study is presented on the example of the AK4 alloy. The presented approach to the determination of the time-varying creep threshold can be used for arbitrary materials.