Effect Of Microstructure On The Reliability Of Piston

Diesel locomotives are the core power of the development of the national transportation industry,heavy machinery and other important areas.In order to meeting the requirements of the development of modern industry and adapt to the higher requirements,the core components of the modern diesel locomotives are subjected to an unprecedented high thermal load and mechanical load.At present,the maximum explosive pressure of diesel engine pistons has reached 20 MPa,and the high pressure of the explosion has brought severe challenges to the performance of pistons.The bearing capacity of aluminum alloy pistons has been already nearing the limits.Heavy load brings a series of problems of piston fatigue failure.Therefore,it is of great significance to study how to improving the performance of the pistons and reducing the fatigue failure of the pistons.In this paper,a new type of diesel engine piston was considered as the research object.The microstructure of the piston material was improved by improving the casting process.The physical properties and mechanical properties of different metallographic grades of pistons have been tested.The temperature field,the thermal stress field and thermo-mechanical coupling stress field were simulated and analyzed by using the finite element analysis software,and the piston temperature field were tested by the templugs.The main research contents are as follows:(1)The material properties of 4 grades of pistons with different microstructure were tested in the laboratory,and the curves of the linear expansion coefficient,thermal conductivity and ultimate strength with variable temperatures were obtained.(2)The temperature field of pistons in cylinder No.1,No.3 and No.5 were tested by templugs on the engine pedestal.The average temperature at all measuring points on the piston surface was obtained.(3)From the temperature distribution of pistons,it can be seen that the maximum temperature of 4 grades metallographic of pistons showing an increasing trend with the decreasing of the metallographic grade.The difference of temperature of the 4 grades metallographic of pistons was significant in the piston head,and the high-temperature region of the piston combustion chamber decreased with the decreasing of the metallographic grade.The direction of heat transfer was studied from the heat flux field,the cooling oil chamber and the first ring groove ares played a leading role in the cooling of the piston head.In the thermal stress field,the thermal stress was concentrated at the throat,and the maximum thermal stress value was showing an increasing trend with the decreasing of the piston metallographic grade.The maximum of the thermal deformation of the 4 grades metallographic of pistons were less than the gap between piston and cylinder wall.(4)The thermo-mechanical coupling stress fields of 4 grades metallographic pistons were analyzed.The results showed that the coupling stress and deformation were showing an increasing trend with the declining of piston metallographic grade.From the overall distribution of the coupling stress field,it can be seen that the effect of thermal load on the coupling stress was mainly concentrated on the piston head,and the influence area of the mechanical load was concentrated on the pin seat.The distribution of coupling stress of the pin seat,the second groove and the throat area was concentrated.The difference between allowable stress values and the coupling stress values were showing a decreasing trend with the metallographic grade declining.The higher grade of piston metallographic,the stronger reliable performance of piston would be.(5)The top of piston was remelted by remelting technology.The simulation results of the remelted piston show that,compared with the unremelted piston,the maximum temperature of remelted piston decreased by 2℃,the thermal stress on the throat of remelted piston decreased by 6.6MPa.During the thermal fatigue test,the fatigue fracture behavior of the top and throat area of the remelted piston was not occured.Compared with the conventional piston,the life of remelted piston was improved by 3 times.