New Technology Of Knurling-Friction Coating Diffusion Aluminizing And Study On Diffusion Mechanism

Aluminizing process is the most widely used metal corrosion protection technology,including hot dip aluminizing,hot spray aluminizing,powder embedding aluminizing,slurry aluminizing and vapor deposition aluminizing.However,the traditional aluminizing process generally has problems such as high energy consumption,large aluminum consumption,uneven thickness of the dip coating and difficult control.In addition,high temperature diffusion treatment leads to coarse microstructure of the material,which affects the mechanical properties of the material at room temperature.In order to meet the needs of modern industrial production,it is necessary to further study the new aluminizing process with good quality,low cost and little influence on the mechanical properties of the workpiece.This paper presents a new process of’rolling+friction coating+induction diffusion annealing’aluminizing.The 20 steel and gray cast iron HT150 were treated by friction coating aluminizing and diffusion annealing.The microstructure changes near the rolling area,the thickness of aluminized layer,the thickness and composition of alloy layer after diffusion annealing were analyzed by metallographic microscope,scanning electron microscope and energy spectrum analyzer.The experimental results show that the thickness of pure aluminum layer can be well controlled by rolling treatment,but the effect of rolling treatment on the thickness of iron-aluminum metallurgical bonding zone between substrate and coating layer is small.Before diffusion annealing,the thickness of the alloy layer is only 2～4μm,and the coating layer from outside to inside is a small amount of Al₂O₃,pure aluminum layer（main part）and Fe Al₃phase;After diffusion annealing,the thickness of the alloy layer increases obviously,from the surface to the internal of the matrix are Al₂O₃,a small amount of pure aluminum layer,Fe Al₃phase,Fe₂Al₅phase（the main part）.The diffusion coefficient of Fe and Al elements varies greatly during diffusion annealing,and the discontinuous distribution of elements in some regions causes vacancy phenomenon.The effects of rolling amount,friction coating speed,diffusion annealing temperature and holding time on the thickness of Fe-Al alloy layer were analyzed in detail by orthogonal test.The effects of two diffusion annealing methods on the thickness of the alloy layer and the diffusion mechanism of aluminum atoms were analyzed.The test shows that the rolling amount and diffusion annealing heating temperature have great influence on the thickness of Fe-Al alloy layer.Due to deformation,the surface microstructure of carbon steel is refined,the grain boundaries are increased,and the aluminum element diffuses rapidly along the grain boundaries.At the same time,induction heating produces eddy current effect,which increases the heating rate and effectively reduces the diffusion holding time:20 steel,the alloy layer thickness reaches 30μm～250μm in 2～8 minutes;Gray cast iron HT150,in 2～6 minutes within the alloy layer thickness of 20μm～160μm,ordinary annealing requires 2～5 hours.According to the friction coating aluminizing test of 20 steel,the expression of diffusion coefficient was deduced by Fick’s second law,and the diffusion coefficient of aluminum element in the process of friction coating aluminizing was obtained.The diffusion depth of aluminum element in the process of 20 steel friction coating was analyzed by commercial finite element software,and the calculated data were consistent with the experimental data.Using commercial finite element analysis software,the temperature field of 20 steel in the process of friction coating aluminizing under different rotational speed and pressure was analyzed,and the optimum value of main process parameters of carbon steel friction coating aluminizing was preliminarily determined.