Bonding Mechanism And Microstructure Characteristics In An Explosively Welded Al/Mg Alloy

Magnesium alloy,due to its lower density,higher specific strength,rich resource and recyclability,is used in automotive,aerospace and other fields.However,poor corrosion resistance of magnesium and magnesium alloys hinders their further applications.Al/Mg alloy cladding plate has been fabricated by explosive welding of an AZ31B magnesium alloy?base?plate with a 6061aluminum alloy?flyer?plate.The bonding mechanism and microstructure characteristics of Al/Mg alloy cladding plate was studied,aiming to provide a theoretical foundation for the engineering fabrication of Al/Mg alloy cladding plate by explosive welding.ANSYS simulations in our thesis have been conducted to investigate the force-heat interaction during the explosive welding process of Al/Mg alloy cladding plate.In addition,the formation mechanisms of wavy interface,vortex structure and local melting zone were discussed through both numerical simulations and theoretical studies.The interfacial bonding mechanism was analyzed by conducting both experimental studies and molecular dynamics?MD?simulations.Microstructure evolution was characterized at/near the bonding interface by using optical microscopy?OM?,scanning electron microscopy?SEM?,electron backscatter diffraction?EBSD?and transmission electron microscopy?TEM?techniques.The formation mechanism,microstructure evolution,micro-nano mechanical properties of the adiabatic shear bands?ASBs?,and key process parameters were investigated.Furthermore,the interfacial bonding strength and integrated mehcanical properties of the cladding plate were characterized.The interfacial bonding morphologies of various explosively welded Al/Mg alloy cladding plates were studied.Results show that there are four typical interfacial morphologies:1)flat interface;2)micro-wave interface;3)wavelet interface with small vortex structure;and 4)large wave interface with large vortex structure.The formation of the vortex structure was found to be sensitive to some welding parameters.Along the detonation wave direction,the interface morphology of the cladding plate transited from a flat interface to a wavy interface and from a wavelet interface to a large wavy interface.Formation of the wavy interface is due to interaction between the collision point and the periodic jetting caused by the plastic deformation of the material in the impact zone occurred under high shear force;The vortex structure formation at the bonding interface was mainly due to capture of the jet particles by the base plate under large plastic deformation.During the collision,the larger the plastic deformation of the base plate,the more captured jet particles and the larger the vortex structure formed.The motion of the atoms at the bonding interface was studied by MD simulations.Results showed that atomic inter-diffusion were found during the unloading period of the explosive impact load.Pressure distribution and plastic deformation at the bonding interface were revealed by ANSYS numerical simulations.Plastic deformations were clearly found at the bonding interface and the degree of deformation decreased with the increasing distance away from the interface.The microstructure in the bonding interface was characterized by using focused ion beam?FIB?and transmission electron microscopy?TEM?.A continuous and nonuniform thickness transition layer with an average thickness of about 2?m was found and the phase composition of this transition layer was?-Mg₁₇Al₁₂ phase.The microstructure evolution at/near the bonding interface of the Al/Mg alloy cladding plate was characterized by means of OM,SEM,EBSD and TEM.The results showed that microstructure in the aluminum alloy plate near the interface changed from its original polygonal grain to the elongated deformed grain,and a large number of high-density dislocations and subgrain structures were formed in the grains and grain boundaries.ASBs structure was found in the magnesium alloy plate near the interface.The ASBs started at the bonding interface of the aluminum/magnesium alloy cladding plate and diminished gradually with increasing distance away from the interface.With the distance increasing away from the interface,in the magnesium alloy,the percentage of the recrystallized grains decreased,while that of the shear bands,compressive twins and the small angle grain boundaries increased.In the vortex structure,both Mg₁₇Al₁₂ and Mg₂Al₃ phases were found,and the percentage of Mg₁₇Al₁₂phase was higher than that of Mg₂Al₃ phase.In the melting zone,only Mg₁₇Al₁₂phase was found.The microstructure evolution in the ASBs in the magnesium alloy plate near the interface was characterized under high-speed explosive impact.The micro-nano mechanical properties of the ASBs structure were studied using the nano-indenter.HCP crystal structure in the ASBs was found only in the magnesium alloy plate.The formation of the ASBs was mainly due to the periodic impact load on the magnesium alloy plate and the local plastic deformation occurred under high shear force.The microstructure in the ASBs was found to be fine recrystallized grains.The driving force for the recrystallization was the temperature rise in the material due to the local plastic deformation of the material.Results from the nano-indentation experiments showed that the hardness of material at the center of the ASBs zone?1.22 GPa?was higher than that of material outside the ASBs zone?0.87 GPa?.The ASBs structure in the magnesium alloy plate was found at both flat interface and wavy interface,and the width,density and length of the ASBs structure increased with the increasing size of the wavy interface.Studies of the bonding strength;the post-heat treatment microstructure;and the micro-zone mechanical properties at the interface and the mechanical properties of the cladding plate showed that the bonding strength at the micro-wave interface,wavelet interface and large wave interface were 188.4,201.2 and 159.6 MPa,respectively.Mg₁₇Al₁₂ intermetallic phase was found form near the magnesium alloy plate and the Mg₂Al₃ intermetallic phase was formed near the aluminum alloy plate at the interface after heat treatment.Corresponding EBSD analysis revealed that the Mg₁₇Al₁₂ intermetallic phase was consisted of columnar grains and the Mg₂Al₃ phase was consisted of equiaxed grains.Nanoindentation testing results showed that the average hardness of the Mg₁₇Al₁₂2 and Mg₂Al₃ intermetallic phases were 3.77 and 4.27GPa,respectively.Both the tensile strength and elongation rate of the Al/Mg cladding plate were improved after heat treatment.Furthermore,the tensile strength of the cladding plate reached the highest value of 189 MPa after annealing at 200?,and the elongation of the cladding plate reached 22.6%after annealing at 400?.