Effect Of The Anode Oxidation On The Diesel Engine Piston On The Structural Reliability

In order to meet the requirements of modern industrial development,the diesel engine is continuously developing towards high power and high load.As one of the core components of the diesel engine,the piston will be subjected to greater heat load and mechanical load.At present,the maximum explosion pressure on diesel piston is more than 20 MPa.Such a harsh working environment gives a serious challenge to the reliability of the piston.After the hard anodizing on the top surface of piston,the temperature of the piston can be effectively reduced because of the low thermal conductivity of the oxide layer.At the same time,because the thermal expansion coefficient is different between the oxide layer and the piston,and the oxide layer is closely bound to the piston,the top surface of the piston covered by the oxide layer is subjected to tensile stress to change the original stress state of the piston top surface.If the tensile stress is used reasonably,it can improve the stress state of piston top to effectively reduce the cracks.In addition,the oxide layer has good abrasion resistance and corrosion resistance,the reliability of the piston is further improved.Therefore the study that how to make a reasonable hard anodic oxidation treatment on piston top of diesel engine is of great significance to reduce the fatigue failure of piston and improve the reliability of piston.In this paper,a diesel engine piston was used as the research object.We designed three plans according to the difference of oxidation regions,each of which includes seven different thicknesses.The temperature field,the stress field and the fatigue life of the piston were simulated by finite element software.Combined with the results of hardness plug temperature test,thermal fatigue test and cavitation cycling test,the effect of hard anodic oxidation treatment on the reliability of piston was analyzed comprehensively.The major content of this dissertation are as follows:(1)The thermal conductivity of the oxide layer was only one-eighth of the thermal conductivity of the piston.The good thermal insulation performance caused the oxide layer to be a thermal barrier at the top of the piston,thus reducing the temperature of the piston head as a whole,and the maximum thermal stress and thermal deformation of the piston were correspondingly reduced.The simulation results showed that: In No.1 plan,the thickness of oxide layer increased by 25 microns,and the highest temperature of the piston would drop by 1.63 c.In No.2 plan,the thickness of oxide layer increased by 25 microns,and the highest temperature of the piston would drop by 1.35 c.In No.3 plan,the thickness of oxide layer increased by 25 microns,and the highest temperature of the piston would drop by 1.08 c.The general trend was that the thicker the oxide layer and the wider the coverage area of the oxide layer,the more obvious the temperature drop in the high temperature area of the piston,but the temperature change in the low temperature area of piston was not obvious.Only from the perspective of lowering the temperature,the No.1 plan was the best solution.(2)The original piston top was subjected to tensile stress in the direction of the piston pin,and was subjected to compressive stress in the direction perpendicular to the piston pin.The stress at the top surface and throat of the piston was the largest and most prone to cracking.At high temperature,reasonable arrangement of oxide layer at the top of the piston can effectively improve the original stress state of the piston top surface to reduce the crack in the piston top and improve the reliability of the piston.The simulation results showed that: Using No.1 plan would increase the tensile stress on the side and bottom of the combustion chamber,which tended to cause cracking.In No.2 plan,the throat of piston was oxidized in the direction of the piston pin,which further increased the tensile stress in this area and leaded to cracking.Using No.3 plan can best improve the stress state of piston top.(3)The results of thermal fatigue test for original piston and hard oxygen piston showed that the reasonable hard anodic oxidation treatment for piston top surface can greatly improve fatigue life of piston.The results of cavitation cycling test showed that the reliability of the hard oxygen piston was higher than that of the ordinary piston,but the improper arrangement of oxide layer can lead to new cracking.The good wear resistance and corrosion resistance of the oxide layer also played an important role in improving the reliability of piston.