Research On Evolution Mechanisms Of Interfacial Meso-structures In Explosive Welded Bimetals

Explosive welded metallic composites are widely used in industrial fields,mechanical properties are important indicators for evaluating the quality of the weldment,closely related to the meso-structures(1 nm～1 mm)such as the interfacial morphology,defects,and crystals of the composites.Understanding the evolutionary characteristics of these interfacial meso-structures during collision is the basis for designing the explosive welding parameters and predicting interface quality.To address the concern,this work comprehensively utilized various material characterization and numerical simulation methods to systematically study the meso-characteristics and evolution mechanisms of interfacial morphology,defects such as void and crack,and crystal structures under different conditions.We improved the classical model for the wave evolution and proposed a new way for defects suppression,which provide theoretical support for promoting the comprehensive performance of explosive welded metallic composites and precisely regulating joint interfaces.Firstly,this work conducted multiple groups of exploratory explosive welding experiments by controlling the trend of changes in collision parameters and the physical properties between the base and flyer plates.Material characterizations were carried out to analyze the influence characteristics of different factors on the interfacial morphology.The results indicated that collision parameters variation mainly changes the size and intensity of the vortex,while the difference in the wave impedance between the base and flyer plates is the key factor for the shape of the wave.Euler simulation results showed that increasing the collision parameters of explosive welding contributes to a wider plastic deformation zone near the collision point,leading to an increase of the jet thickness and amplitude in direction.The shear instability of the contact surface in front of the collision point facilitates the interfacial morphology to transit from straight to waveform.With the increase of the wave impedance difference between the base and flyer plates,the shear strain distribution at the front and back of the wave loses balance.The material in front of the wave is squeezed more severely,leading to the continuous backward tilting of the wave until it becomes a curled wave structure.On this basis,the SPH simulation is used to track the evolutionary process of three typical wave structures,and a more detailed wave evolution model was established after summarizing their common processes,which makes up for the shortcomings of the classical indentation mechanism in describing phenomena such as jet sources,interfacial starting disturbances,substrate depressions,protrusions formation and vortex development.Secondly,a number of explosive welding experiments with different types of material combinations were designed in this work.Interfacial defects such as voids and cracks were captured through material characterization and classified in detail based on their morphology,size and position.Combined with Euler simulation,the formation mechanisms of voids and cracks with different types and characteristics were studied.The excessive jets and their direction fluctuations induce void between the plates,which are subsequently incorporated into the circular motion of the interface.Under the combined influence of centrifugal force,physical stirring and rapid cooling,microvoids are formed at the center of the vortex.The rapid rise of local temperature and pressure will induce material melting,followed by isothermal solidification and rupture in the subsequent rapid cooling and pressure drop process,which favors the nanoscale voids formation.The irregular shape and internal pressure distribution of the vortex region result in random size,number and position of nano voids.The continuous molten layer at the interface commonly appeared in alloys with low-melting point and combinations that are prone to react,the residual kinetic energy of the flyer plate after impact would vertically compress the continuous molten layer to generate stress parallel to the interface,leading to vertical cracks caused by the fracture of brittle intermetallic compounds.The cracks in the localized molten region can be divided into voidsurrounding ones and random ones,the shape of the vortex and void will affect the direction of cracks,the miscibility of molten materials and whether two metals react or not does not affect the occurrence of cracks,intermetallic compounds are unnecessary conditions for cracks formation.In addition,the brittle jet fragments within the molten zone can break the propagation of cracks and reduce their length.This fact indicates that the ductile materials around the vortex can limit cracks to a local area,and therefore,avoiding the continuous thick molten layer is a key guarantee for welding quality.Based on this finding and defects analysis,this work proposed a method to improve the interface bonding quality by low-temperature pretreating the materials to be welded.Comparative experimental results showed that this method can effectively reduce the strain at the interface during collision,thereby reducing the width of the interfacial molten zone and decreasing the density of defects including voids and cracks.The method enhances the bonding strength while slowing down the impact-induced grain structure changes,and better retains the initial performance of the parent materials.Finally,the work studied the deformation characteristics of crystal structures within the impact affected area with different bonding morphologies.The results showed that with the increase of plastic deformation,crystals near the interface will appear in a sequence of refinement,elongation along wave-shaped boundaries,and growth along the temperature gradient direction.The evolution of the crystal structure within the vortex is influenced by thermal-mechanical history,vortex morphology,and difference in the thermal conductivity between the flyer and base plates.The rapid condensation of the molten material within the vortex and the temperature gradient between the vortex and surrounding matrix promote the formation of columnar crystals.The morphology of the vortex and the difference in thermal conductivity between the flyer and base plates affects the distribution characteristics of columnar crystals by changing the direction of the temperature gradient.There is a competition between the hardening of materials caused by grain refinement near the interface and the softening of materials triggered by the recrystallization process.Materials combinations that are difficult to blend together and easy to react respectively enhance the nano-hardness of the vortex region by promoting the diffusion and formation of intermetallic compounds after solidification.