Preparation Of Ni-Ni_xAl_y-Al₂O₃ Gradient Coatings By Electrodeposition Composite Slurry Method-induction Heat Treatment

Aluminum alloy die casting molds often fail prematurely in the form of thermal fatigue,melt loss,oxidation,erosion,and other forms.However,it is difficult to solve these problems with a single coating.Functional gradient coatings with a certain coating structure are required to effectively alleviate thermal stress at high temperatures while meeting performance requirements such as wear resistance and corrosion resistance.In this paper,a Ni-Ni_xAl_y-Al₂O₃functionally gradient coating was prepared on the surface of H13 mold steel substrate using a water-based electrodeposited Ni composite slurry coating method and induction heat treatment aluminizing technology.Then,the effects of induction heat treatment time and temperature on the microstructure and hardness of the coating cross section were analyzed,and the formation mechanism of the coating and the formation mechanism of the bonding interface were studied.Finally,the wear resistance and corrosion resistance of the coating were studied respectively.The main conclusions are as follows:（1）When the induction heat treatment time is the same,with the increase of the induction heating current（temperature）,the Ni layer and the Ni₂Al₃layer decrease,and the attached NiAl₃layer has little change,but the free bulk NiAl₃in the residual Al layer increases.When the induction heat treatment temperature is the same,with the increase of heating time,the Ni layer decreases,and the transition layer Ni₂Al₃-NiAl₃basically converts to Ni₂Al₃.（2）According to the design of the gradient structure of the coating,the optimal process for induction heat treatment is 500 A（770℃）and 7 minutes.The gradient coating obtained under the optimal process exhibits a structure from the substrate to the surface layer:Ni/Ni₂Al₃-NiAl₃/Al（NiAl₃）.The Al（NiAl₃）layer will be oxidized into the working layer Al₂O₃during idle and early service,and the hardness of the transition layer Ni₂Al₃-NiAl₃layer is as high as 750 HV_0.01.（3）With the increase of heating time,the thickness of the Fe-Ni diffusion layer and the Ni-Al diffusion layer increases.The diffusion activation energy at the Fe-Ni interface is 82.425 k J/mol,and the diffusion activation energy at the Ni-Al interface is99.269 k J/mol.The magnetic field generated by induction heat treatment can promote the diffusion between interfacial atoms,and to some extent,reduce the activation energy of interfacial diffusion.（4）The formation mechanism of the Ni/Ni₂Al₃/NiAl₃interface is divided into three stages:atomic dissolution and diffusion during heating,nucleation and growth of Ni₂Al₃,and desolvation and precipitation of NiAl₃.（5）The wear amounts of H13 steel,Ni coating,and Ni-Ni_xAl_y-Al₂O₃gradient coating are 1×10^-3g,4×10^-3g and 0.8×10^-3g.The average friction coefficients of H13steel,Ni coating,and Ni-Ni_xAl_y-Al₂O₃gradient coating are about 0.529,0.776,and0.449,respectively.The wear resistance of each sample:Ni-Ni_xAl_y-Al₂O₃gradient coating>H13 steel>Ni coating.H13 steel mainly adopts adhesive wear mechanism,supplemented by particle wear mechanism;The wear mechanism of Ni coating is adhesive wear;The wear mechanism of Ni-Ni_xAl_y-Al₂O₃gradient coating is particle wear.（6）The open circuit po potential and self-corrosion potential of H13 steel,Ni coating,and Ni-Ni_xAl_y-Al₂O₃gradient coating in acid,alkali,and salt solution is E_{Ni-Nix Aly-Al2O3}>E_Ni>E_H13,and the self-corrosion current is I_H13>I_Ni>I_{Ni-Nix Aly-Al2O3}.The corrosion resistance of the three samples is Ni-Ni_xAl_y-Al₂O₃gradient coating>Ni coating>H13 steel.When immersed in 0.5 mol/L HCl for 24,72,and 168 hours,the|Z|_0.01Hzof the three samples was:Ni-Ni_xAl_y-Al₂O₃gradient coating>Ni coating>H13base steel.After soaking in 3.5 wt.%Na Cl for 24 hours,the gradient coating of the three samples was|Z|_0.01Hz:Ni-Ni_xAl_y-Al₂O₃>Ni coating>H13 base steel.However,when soaked for 72 hours and 168 hours,the|Z|_0.01Hzof the Ni-Ni_xAl_y-Al₂O₃gradient coating is smaller than that of the Ni coating.In general,the corrosion resistance of Ni-Ni_xAl_y-Al₂O₃coatings is indeed improved compared to Ni coatings and substrates.