Effect Of Surface Pretreatment And Post-plating Treatment On Microstructure And Properties Of Continuous Hot-dip Aluminizing

In this paper,the effect of pre-electroplated Fe/Ni process and over-aging treatment on the microstructure and mechanical properties of continuous hot-dip Al/Al-Si coating was studiedby hot dip galvanizing simulator in Ansteel Group Corporation.At the same time,the feasibility of the application of the process in the actual production is analyzed.The thermodynamic properties and kinetic properties of Fe-Al intermetallic compounds during the formation of the coatings were calculated and analyzed by models to further clarify the formation mechanism of the coatings.Finally,the finite element simulation of the three-point bending test of the coating is carried out by ANSYS Workbench software.The cracking form and the influence factors of the mechanical processing performance are analyzed.The results show that:In the formation of alloy layer,Fe2Al5 phase and Fe Al3 phase may exist simultaneously.Fe2Al5 phase first nucleates and grows in solid-liquid interface and forms a single-phase layer.Then Fe Al3 phase will form nucleus in Fe2Al5 phase and aluminum liquid interface.The faster the cooling rate,the thinner the thickness of the alloy layer.When the cooling rate exceeds a certain degree,the cooling capacity required to thin the same thickness will increase exponentially.Even if the cooling rate is 30?/s,the thickness of the alloy layer is still?5.6?m.Pre-plating Ni process can improve the wetting of aluminum liquid,reduce the thickness of alloy layer,improve the quality of aluminum layer on the surface of type II aluminum.Pre-plating Fe process makes the overall quality of hot-dip aluminized coating worse,the alloy layer thickness increases,cracks and holes and other defects increased,the surface layer of porous,coating surface is very rough.The over-aging process can improve the ductile toughness of the steel sheet,but it will increase the thickness of the alloy layer,and the defects such as the micropores in the surface aluminum layer increase.The overaging process should be selected according to the C and N atoms in the substrate.Aluminum plating in the bending process,regardless of the thickness of the alloy layer will inevitably occur micro-cracks fracture,the coating at the micro-cracks prone to stress concentration is destroyed.The lower surface coating is easy to be cracked,the upper surface coating is easy to fall off.With the increase of the thickness of the alloy layer and the substrate,the coating is easier to be cracked or extruded.With the increase of the thickness of the surface aluminum layer,the crack is less likely to expand through the entire surface aluminum layer,and the coating mechanical workability becomes better.