Research On Corrosion Behavior Of Steel/Aluminum Electromagnetic Pulse Welded Joints Based On Electron Work Function

The use of electromagnetic pulse（EMP）welding technology for aluminum/steel dissimilar metal materials lap welding test,around the steel/aluminum EMP welding joint mechanical properties,macroscopic morphology,interfacial corrosion products and its corrosion mechanism to carry out research.The typical Fe-Al intermetallic compounds were used as the object of calculation,and the stability and corrosion resistance of jointed Fe-Al intermetallic compounds were explored by modeling them and calculating the results used the first-principles calculation method based on density generalized theory.The first was the use of scanning electron microscopy（SEM）,energy spectrum analysis（EDS）and X-ray diffraction analyzer（XRD）to analyze the welded joint bonding interface and its phase composition.Secondly,the atomic models of aluminum matrix,iron matrix and Fe-Al intermetallic compounds were constructed using the first nature principle calculation method,and the stability,surface energy,electron work function and intrinsic potential difference of their structures were calculated to analyze the corrosion resistance of intermetallic compounds.Finally,the welded joints for neutral salt spray corrosion test,the use of weight loss method to calculate its corrosion rate,analysis of welded joints corrosion morphology and corrosion products to reveal its corrosion mechanism.（1）The aluminum/steel EMP welded joint had a symmetrical ring shape and the joint morphology was observed by SEM.The welded joint was composed of three parts:the middle unwelded area,the welded area and the external unwelded area.The welded area of the bonding interface form mainly presented as a wavy interlocking structure,and along the welding direction,a certain range of bonding area,the size of the bonding interface waveform（wave height and wavelength）in the wavy bonding interface area was gradually increasing trend.The wave height was 3.081μm and the wavelength was 30.595μm when the wave-like structure starts to form near the middle of the unwelded area.When the waveform size reaches a wave height of 4.919μm and a wavelength of 51.622μm,the bonding interface exists in the form of near-direct bonding due to the impact angle and impact velocity until the final formation of the external unwelded area.（2）Aluminium/Steel EMP welding test during impact,the violent plastic deformation caused by the high speed collision leads to the formation of a high pressure and high temperature zone around the collision point,which promotes the metallurgical reaction in the region.The plastic deformation that occurs in a few microseconds can lead to local temperature increases or even melting,and the high temperature and pressure may lead to the formation of intermetallic compounds.Through SEM analysis of the weld fracture morphology,the particle flow generated during the welding process was partially trapped in the peripheral area of the straight weld when it impacts the surface of the steel plate,forming a band area with particle flow trapped.And the steel particles fly away after impacting the aluminum plate,while washing off the aluminum surface oxide film and leaving the pits,leaving the aluminum substrate exposed on the surface,and likewise forming a strip area containing impact holes at the periphery of the straight welding seam area.The influence of Lorentz force makes there was no corresponding band structure at the periphery of the annular weld area.EDS analysis of the fracture shows the presence of Fe₂Al₅ phase in the straight weld region on the steel side of the joint weld and Fe Al₃ phase in the annular weld region.The Fe Al phase was present in the straight weld area on the aluminum side of the joint weld and the Fe Al₃ phase was present in the annular weld area.（3）The structural stability and work functions of aluminum matrix,steel matrix,and iron-aluminum intermetallic compounds were calculated used the first-nature principle,and the theoretical results were consistent with the experimental generation of intermetallic compounds.It can be seen that the first-nature principle calculation can be used to determine the structural stability of Fe-Al intermetallic compounds and their prediction of corrosion tendency at the joints.After calculating the formation and binding energies,Fe₂Al₅has the smallest formation energy,which was very different from Fe Al₂,which has the largest formation energy.The binding energy was also the smallest,which was the most stable and the most easily generated phase compared to the other Fe_xAl_y phase.Fe Al₆ and Fe Al₂ have the largest binding energy and larger formation energy,indicating that Fe Al₆ and Fe Al₂ are the most unstable and more difficult to generate among the Fe_xAl_y phases.The stability of other intermetallic compounds is between Fe Al and Fe Al₂ and Fe Al₆.（4）By calculating the electron work function and intrinsic potential difference between the matrix and Fe-Al intermetallic compound,the difference of electron work function and intrinsic potential difference between the Fe-Al intermetallic compound and the aluminum matrix was positive.When it exists in the electrochemical environment,the aluminum substrate with a small electron work function acts as the anode phase preferentially corroded.The corrosion sequence between Fe-Al intermetallic compound and aluminum matrix was as follows:Al>Fe Al₆>Fe Al₂>Fe Al₃>Fe₄Al₁₃>Fe₂Al₅>Fe₂Al>Fe Al>Fe₃Al.（5）The calculated results were verified by neutral salt spray corrosion tests to prove the reliability of the simulation calculations.At the steel/Al EMP welded lap joint gap,the particle flow during impact washes away the surface oxide film,leaving the Al substrate bare and shiny,as well as the Al particles trapped on the steel side surface.Combined with the calculated results,the electron work function values of Al were lower than those of Fe-Al intermetallic compound and Fe matrix.In the neutral salt spray test,corrosion first occurred at the joint gap,and with the corrosion cycle,the erosion of the gap location from vertical to horizontal gradually spread,until the erosion of the entire weld.The main cause of corrosion was due to the potential difference between the aluminum substrate,the iron substrate and the Fe-Al intermetallic compound,and the corrosion sequence was consistent with the calculated results.