First Principle Calculations Of The Interfaces And The Intermetallic Compounds In A Laser Welded Steel/Aluminum Joint

The connection of dissimilar metals of steel and aluminum can take into account the advantages of the two metals and has wide application prospects in the field of automobile manufacturing.However,a large amount of brittle Fe-Al intermetallic compounds are prone to appear after welding steel and aluminum materials,which reduces the performance of the joint and restricts its development and application.To improve the strength and long-term reliability of steel and aluminum structures,we must start with the study of intermetallic compounds at the interface.At present,the research on the structure adjustment of the steel-aluminum welding interface and the performance of Fe-Al intermetallic compounds have become hot spots in the related fields.This article focuses on the problems and difficulties in the field of steel and aluminum welding,and has carried out systematic research work.Laser welding experiments of T-joints were carried out on 316L stainless steel and 6061 aluminum alloy.It was found that under the welding process conditions of laser power 800W and incident beam angle θ=20°,the joint morphology was well-formed.The microstructure,reaction phase,fracture morphology and mechanical properties of welded joints were analyzed by optical microscope,scanning electron microscope(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD)and nanoindentation.According to XRD and EDS analysis,Fe3Al,FeAl,Fe2Al5,FeAl2,FeA13 and Fe4Al13 were formed in the joint.To analyze the properties of Fe-Al intermetallic compounds and their impact on steel-aluminum welded joints,the first-principles calculation method based on density functional theory was used to calculate the equilibrium lattice constant,thermodynamic properties,mechanical properties and electronic properties.The results show that Fe3Al and FeAl2 show plastic characteristics;FeAl,Fe2Al5,FeAl3 and Fe4Al13 show brittle characteristics.Near the Fermi level,the 3d electron layer of Fe and the p electron layer of Al work together.Fe-Al intermetallic compounds have weak ionicity,high hardness,high melting point,and have typical metal properties.Using the edge-to-edge crystal matching model,the mismatch rate at each reaction phase interface was calculated.Steel-aluminum interface models were established,and Griffith micro-crack theory was used to calculate the fracture work of the interface system to evaluate the bonding strength of each interface.The experimental and calculated results were mutually verified,and the properties of Fe-Al intermetallic compounds and their influence on the steel-aluminum interface were studied.