| Firstly,the existing formula of molten salt is improved unharmfully.Na2CO3 was used to replace NaNO2 components in original formula to improve the thermal stability of formulation and greatly reduced the environmental impact.The theoretical melting point and the measured melting point of the improved formula were obtained by combining the calculation of multiple(quasi)eutectic phase change material melting point and latent heat of fusion prediction model and the test of DSC.And combining with both corresponding response surface figure and contour map,the variation of theoretical melting point and measured melting point with the proportion of components in the formula was analyzed.Results showed that the change trend of theoretical melting point with formula composition was the same as that of measured melting point,and the maximum absolute value error was only 3.5%.Through the response variable optimizer obtaining the lowest melting point and the corresponding component ratio,it was found that both optimal component ratios are similar and the data fit well.So the optimal improved formula is determined as m(NaOH)=0.3,m(NaNO3):m(KNO3):m(Na2CO3)=0.4435:0.4302:0.1263.Then,the thermal stability of the optimal improved formula and its removal effect on contaminants in the cylinder cover were studied.Results showed that the formula loss weight was only 1%in the range of 25～500℃,indicating that the improved formula had good thermal stability,and its operating temperature range increased from 250℃-300℃ to 250℃-500℃,which meets the requirements of future remanufacturing cleaning.Besides,the removal effect of optimal improved formula on black attachments and corrosion-scale mixture contaminants was obvious.There are no contaminants on both oil channel and water channel.Meanwhile,some light blue areas appeared on the channel matrix,indicating that the improvement formula improved the corrosion resistance of the matrix surface to a certain extent.Secondly,the ability of the optimal improvement formula to penetrate the complex channel structure and the mechanical properties of the matrix surface were studied.Rotational rheometer was used to measure the optimal improved formula viscosity value under different temperature.Results showed that the viscosity decreased with temperature rise,which was from 33Pa·s in 280℃ to 0.022Pa·s in 360℃and was close to the viscosity of 10#diesel engine under normal temperature.And the formula’s density was only 1.682 kg/cm3 when 360℃,which met the requirements of cleaning cylinder cover complex structure,and 360℃ was determined as the cleaning temperature of optimal improved formula when cleaning engine cylinder cover.With introducing the viscosity and density of improvement formula under 360℃ to Fluent to simulate formula’s penetration,the pressure contours and the velocity vector diagram of different diameter,shape and length to diameter ratio were obtained.Result showed that the optimal improvement formula in complex channel structure had a certain velocity and the pressure decreased from inlet to outlet,indicating that the optimal improvement formula could completely pass through the complex channel and had a good ability of penetration.By comparing the surface hardness and residual stress of HT250 before and after formula cleaning,surface hardness of HT250 appeared a certain degree of rise and residual stress appeared a certain degree of decline.However,the substrate surface still showed a certain amount of residual compressive stress after cleaning,not adversely affecting the performance of substrate surface,and reducing the risk of surface transverse crack occurrence caused by the high residual stress.Meanwhile,the substrate still maintained the positive influence on fatigue resistance because of the remaining residual stress.Finally,the cleaning mechanism of optimal improved formula on contaminants removal in engine cylinder cover was studied.SEM,FTIR and XRD were used to analyze the black attachment contaminants in oil channel and corrosion-scale mixture contaminants in water channel.Results showed that the black attachment contaminants were mainly organic matter containing hydroxide radical,methyl group and aromatic group,with an average thickness of 38.91μm.The corrosion-scale mixture contaminants were mainly composed of inorganic materials such as iron oxide,CaCO3 and a small amount of organic matter and contained a certain amount of crystal water inside.CaCO3 was tightly adhered to the surface layer and the iron oxide was located in the middle layer.There were many gaps inside the contaminants.Besides,HS-GC was used to analyze the gas production in cleaning process.Results showed that the gas production contained a variety of components,including alkanes,alkenes,aromatic hydrocarbons,aldehydes,esters,alcohols,ketones and hydroxides.As a result,the cleaning mechanism of optimal improvement formula on contaminants removal in engine cylinder cover could be described in to three aspects:thermal effect,physical flow effect and reactions of oxidation and pyrolysis.By comparing with contaminants reacted in air,the reaction activation energy of contaminants in air and in optimal improved formula was obtained.Then,the reason for good cleaning effect of contaminants in optimal improved formula is explained from the perspective of cleaning dynamics. |
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