First-principles Calculation Research On The Welding Interface Of Magnesium Alloy And Steel

Magnesium alloy is the lightest engineering structural metal material currently available,which has the advantages of low density,high specific strength,good damping and shock absorption.Steel is currently one of the most widely used metal materials and has the advantages of high strength,good plasticity and toughness.The structural parts formed by magnesium alloy and steel can achieve the purpose of lightweight materials while ensuring performance,and have broad application prospects in aerospace,automotive and other fields.Magnesium alloy and steel are typical non-reactive and difficult-to-solid solution heterogeneous materials.It is a hot research topic around the world to obtain a high-quality metallurgical bonding interface and improve the performance of the welded joint.In this paper,the welding interface between Mg alloy and steel is taken as the research object.The mechanical properties of the compound between Mg alloy and steel,the adsorption energy of Mg on the surface of the compound and the separation energy of the Mg alloy and steel interface are studied using the first-principles calculations and test.The main research results are summarized as follows:A laser-induced arc welding technique was used to weld 100×50×1.5 mm magnesium alloy and 100×50×1.0 mm steel with a Ni interlayer,and a well-formed welded joint was obtained.The mechanical properties and fracture morphology of the Mg alloy and steel welded joint under different laser powers were studied.The results show that with the increase of laser power,the tensile properties of welded joints first increase and then decrease.When the laser power is161W,the tensile properties are excellent and the maximum tensile strength reaches 239 MPa(94.5%of the tensile strength of Mg alloy base material).The analysis results of SEM and EDS show that when the interface forms dispersed Al–Ni compounds,the mechanical properties of the welded joint are better.The fracture is a mixed fracture of plastic and toughness,and the fracture is located in the weld.Based on the above experimental results,the Al–Ni compound is selected as the main research object.The formation energy,mechanical properties,electronic properties and thermodynamic properties of 7 common Al–Ni compounds-AlNi₃,AlNi₂,Al₂Ni₃,Al₃Ni₅,AlNi,Al₃Ni₂,Al₃Ni are calculated using the first-principles calculations.The results show that the AlNi compound has the lowest formation energy and the lowst free energy at room temperature,which shows that AlNi is the easiest to form.The brittleness and toughness of Al–Ni compounds were discussed based on the G/B(shear modulus/bulk modulus)coefficient.The results show that Al₃Ni₂ is brittle while other compounds are tough.Among them,AlNi₂ and AlNi have the best plasticity.Based on the above calculation results,the AlNi compound is selected as the main research object.The adsorption energy and electronic properties of single and multiple Mg atoms at different positions on the AlNi surface are calculated.The calculation results show that the chemical adsorption is developed of Mg atoms on the AlNi surface and the chemical bonds are formed between Mg and Ni atoms.The adsorbed Mg atoms will promote the adsorption of other Mg atoms,which is related to the interaction between Mg and Mg atoms.The formation of chemical adsorption and chemical bonds are important factors that cause the interaction of Mg atoms with the AlNi surface.The Mg/AlNi interface model was established by adding strain,and the interface separation energy under three different combinations of Top,Bridge,and Hollow was calculated.The influence of the addition of alloying elements Mg,Fe,Mn on the interface separation energy was studied.The results show that the interface exists stably under the three bonding methods,and the Mg/AlNi interface tends to exist in the form of Bridge.The interface will become more stable when Fe and Mn elements are added while it will become more unstable when Mg elements are added.The changes in electronic properties caused by the addition of alloying elements are important reasons for the stable existence of interfaces and welded joints.