Laser Welding Steel And Aluminium Intermetallic Mechanical Properties And Microstructure Research

Automotive lightweight becomes the frontier and hot spots in the21st centuryautomotive technology. The lightweight materials using of the vehicle body plays animportant role in the lightweight vehicle. For the multi-material car body structure,two different types of connecting materials（such as steel-aluminum, castiron-aluminum, aluminum-magnesium, etc）are required. Because of the lowsteel-aluminum solid solution, their different physical and chemical properties, FeAlbrittle intermetallic compounds are produced inevitably. Due to their naturedifferences, it is difficult to connect steel and aluminum together by usingconventional welding methods. So the laser welding will be an important way toconnect steel and aluminum.The mechanical properties of laser welded joints are seriously affected by FeAlbrittle intermetallic compounds. In this paper, first-principles calculation method isapplied to the laser welding field innovatively. started from the corresponding causalrelationship of microscopic structure, macroscopic properties and the physicalproperties in FeAl intermetallic compounds, it assesses the corresponding physicalquantities of intrinsic mechanical properties, and analysis of the impact of the brittlebehavior by adding elements to the FeAl intermetallic compounds. Finally, somespecific elements to improve FeAl brittle intermetallic compounds are elected toreveal the differences between intrinsic mechanical properties and electronic structure.On this basis, the galvanized steel and6016aluminum alloy are chosen as theresearch object, laser lap welding experiments of steel-aluminum with or withoutadding metal sandwich are carried out. Analyzed and optimized the processparameters, by means of using horizontal microscope, electron micro-hardness,scanning electron microscopy, X-ray diffraction, computer-controlled electronicuniversal testing machine to study the welded joint’s regional microstructure,hardness, fracture surface of the main phase and the joint mechanical properties. Theresults can provide an important theoretical guidance and technical supporting for thebody structure of the multi-material of laser welding.Fe₈Al₈and （Fe₇X）Al₈（X=Pb、Sn、Ti、Cu、Mn、Si、Zn） supercell model’s elasticitymoduli and the electronic structure are calculated by the use of first-principle methodwhich is based on the density functional theory. The results show that the brittle phase of FeAl compound attributes to the electron orbit hybridization between the states ofFe-sd and Al-sp which is obvious covalent bond characteristic. The brittleness of FeAlcompound was reduced with the alloy addition elements, and the order is as follows:（Fe₇Pb）Al₈,（Fe₇Sn）Al₈,（Fe₇Ti）Al₈,（Fe₇Cu）Al₈,（Fe₇Mn）Al₈,（Fe₇Si）Al₈,（Fe₇Zn）Al₈,Fe₈Al₈. The root of alloying differences in FeAl compounds brittleness electronicstructure is the complex results of systematic covalent bond and ionic bonding.Cu and Pb are considered as the better elements on reducing the brittleness ofFeAl intermetallic compound. The laser lap welding experiment with the Cu or Pbmetal sandwich addition are carried out by using the thickness of1.2mm DC56D+ZFgalvanized steel and the thickness of1.15mm6016aluminum alloy. The best formingof welding are obtained by adjusting the welding parameters. The element distribution,phase composition and microstructure of the steel-aluminum interface are changed bythe addition of Metal sandwich. Pb as the best brittleness reducing element, thewelded joints not only generate Fe-Al, Mg-Zn intermetallic compounds, but alsoproduce a new intermetallic compound called Mg2Pb.This compound has goodductility and plastic. In the conditions of high temperature, Mg2Pb structure is morestable than the FeAl intermetallic compounds, and it is benefit to control theformation of FeAl intermetallic compounds. Therefore, the mechanical properties ofweld metals are improved by Pb alloying.