Research On Ablation Of Aluminum Alloy Piston Used In Turbocharge Diesel Engine

Clarifying the ablation mechanism of diesel engine piston is significant for improving its reliability,working stability and optimizing materials,but there are still much controversy so far.In the low temperature and idle speed environment,it is easy to cause the ablation problem,which makes the material design and optimization fall into the bottleneck.In order to solve the ablation problem of high-power turbocharged diesel engine pistons under this operating condition,experiments were carried out including bench test,thermal exposure and thermal fatigue,and oxyacetylene gas impact test.Surface profiler,3D confocal laser microscope,optical microscope（OM）,scanning electron microscope（SEM）,energy dispersive spectroscope（EDS）and X-ray diffractometer（XRD）were used to analyze the surface roughness,micromorphology,chemical composition and phase,and the hardness of the alloy was tested.The ablation mechanism of the piston after bench test,the relationship between the microstructure and properties of the piston aluminum alloy and the temperature,and the ablation mechanism of the piston aluminum alloy after high-temperature gas with short-term and high-frequency loading were obtained.Through the above studies,the ablation mechanism is mutually supplemented and confirmed.The main results and conclusions obtained are as follows:（1）The ablation mechanism of the piston alloy after bench test includes the following processes:unburned fuel cracked and carbonized to form carbon deposits which adhere to the phosphating film and react with the surface alloy.The carbon deposit layer and the reaction layer are peeled off and the alloy is exposed due to the softening of the alloy and the impact of high-speed gas.Each phase cracks at the interface of different phases due to thermal stress,and then flakes off in the form of large clusters.Ablation is surface damage,the depth of damage is about several hundred microns.Both the microhardness of the aluminum matrix and the Brinell hardness in the piston ablation zone decrease significantly with the increase of the ablation degree,and the hardness decrease more significantly nearer to the surface,from 132MPa of the original alloy to about 80MPa.（2）After thermal exposure at different temperatures for 2h,the surface roughness of the alloy increases with increasing temperature,and it increase significantly between 500℃and550℃.The growth velocity of surface roughness with phosphating coating is faster than that of the uncoated alloy.At 550℃,the interface between the silicon phase,the Al₃Cu Ni phase,and the matrix is severely cracked,and the primary silicon phase formed islands in the matrix.The matrix between the Al₃Cu Ni phase fall off,and a large number of holes are formed on the surface,which cause an increase in surface roughness.The increase in surface roughness is caused by phase transformation and oxidation.The surface roughness of the alloy after 500℃thermal fatigue is significantly higher than 450℃.After thermal fatigue at 450℃,there are no obvious cracks and holes,while a zigzag roughening crack with a depth of 150μm is formed after 200 times thermal cycling at 500℃and each phase at the notch has collapsed after 500times.When the thermal fatigue is performed at 450℃by different heating rates,the coupling failure point of heating temperature and time is the maximum while the heating rate is 2.25℃/s.During the thermal exposure and thermal fatigue experiments,the maximum surface roughness of the alloy is close to the degree of slight ablation after the bench test.（3）Piston alloy after oxyacetylene carbonization flame ablation,the surface roughness increases with the number of impacts,reaching a maximum after 2000 times.At this time,the roughness is between the slight ablation and moderate ablation of the bench test piston,which is about 0.5 times the moderate ablation.And the surface roughness of the alloy containing the phosphating film coating is significantly greater than that of the uncoated alloy,which is consistent with the evolution of the alloy after thermal exposure.After ablation,the alloy surface is covered with a large amount of carbon,and the carbon deposits react with the aluminum matrix to form Al₄C₃.Because the thermal expansion coefficients of the ceramic phase（Al₄C₃）and the aluminum alloy are significantly different,the material cracks and peels off due to thermal mismatch.This conclusion is consistent with the bench test.