Effect of heat treatment on the microstructure and mechanical properties of structural steel–mild steel composite plates fabricated by explosion welding

Two dissimilar steel plates,structural steel and mild steel,were joined by explosion welding to form a composite.The composite was then heat-treated by quenching at 840℃ for 30 min followed by tempering at 200℃ for 3 h.The microstructure was investigated under an optical microscope and a scanning electron microscope.The mechanical properties were measured using Vickers microhardness and Charpy impact tests.The results show a deformed interface with typical wave features at the welding zone,but no defects were observed.Moreover,the ferrite in the parent plate in the weld zone was elongated due to the strong plastic deformation from the explosion.After heat treatment,the hardness of the flyer plate(structural steel)was over HV0.2800,while that of the parent plate(mild steel)was HV0.2200.The increase in hardness was due to the presence of martensite.Moreover,the average impact energy was increased from 18.5 to 44.0 J following heat treatment;this is because of the formation of recrystallized grains at the weld interface,which is due to the dynamic recovery and local recrystallization,and the strong elemental diffusion that occurred between the two plates.

Theoretical analysis of the elastic Kelvin-Helmholtz instability in explosive weldings

By considering the joint effects of the Kelvin-Helmholtz(KH) and Rayleigh-Taylor(RT) instabilities, this paper presents an interpretation of the wavy patterns that occur in explosive welding. It is assumed that the elasticity of the material at the interface effectively determines the wavelength, because explosive welding is basically a solid-state welding process. To this end, an analytical model of elastic hydrodynamic instabilities is proposed, and the most unstable mode is selected in the solid phase. Similar approaches have been widely used to study the interfacial behavior of solid metals in high-energy-density physics. By comparing the experimental and theoretical results, it is concluded that thermal softening,which significantly reduces the shear modulus, is necessary and sufficient for successful welding. The thermal softening is verified by theoretical analysis of the increase in temperature due to the impacting and sliding of the flyer and base plates, and some experimental observations are qualitatively validated.In summary, the combined effect of the KH and RT instabilities in solids determines the wavy morphology, and our theoretical results are in good qualitative agreement with experimental and numerical observations.

The role of physical properties in explosive welding of copper to stainless steel

This paper investigates the effects of the physical properties on the microstructure and weldability of explosive welding by joining two metals with a significant contrast in thermophysical properties:stainless steel and copper.Sound welds between stainless steel and copper were obtained,and the interfacial morphology was wavy,regardless of the position of the materials.The weldability of dissimilar pairs was found to be more dependent on the relationship between the physical properties of the base materials than on the absolute value of the material property.When there is a significant difference in thermal conductivity between the flyer and the base plate,together with a material with a low melting temperature,the weldability of the pair is often poor.The relative position of the plates affects the interfacial microstructure even when similar morphologies are found.For the metallic pairs studied,the wave size was bigger for the configuration in which the ratio between the density of the flyer and the density of the base plate is smaller.The same phenomenon was observed for the impedance:bigger waves were found for a smaller ratio between the impedance of the flyer and the impedance of the base plate.

Design and test of a protective structure for the double vertical explosive welding of large titanium/steel plate

A comprehensive protective structure with rigidity and flexibility was put forward and designed in view of the quality and safety problems for the double vertical explosive welding of large titanium/steel cladding plate.The movement speed and displacement of the protective structure was calculated by establishing its physics model.The dynamics and stabilization properties were analyzed,and the protective structure parameters were optimized and devised.The comprehensive protective structure,which is composed of rigidity unit and flexibility wall,can bear the impact of detonation wave and the high-speed movement of the cladding plate.There are no damage and deformation in the protective structure and the cladding plate.The protective structure can be used many times.The bonding rate of the Ti/steel plate obtained was nearly 100%,and there is no deformation,surface cracks,and big wave and micro-defects.Therefore,the protective problems of the double vertical explosive welding can be solved effectively by the protective structure.

Thermal fatigue behavior of copper/stainless steel explosive welding joint

Thermal fatigue performance of copper/stainless steel explosive welding joint was investigated by using a highly effective thermal fatigue test device. The testing device adopted induction coil to heat and carry out two groups of thermal fatigue test at the same time. Metallurgical microscope and scanning electron microscope were used to respectively measure the surface crack and cross-section crack propagation morphology of the explosive welding joint specimen that were conducted thermal cycling for different upper limit temperatures and different cycle time.Experimental results indicated that the cyclic thermal stress and oxidation corrosion was the major factors for fatigue damage behavior of explosive welding joints, where the oxidation corrosion of the interface has become more serious with the increasing the upper limit temperature or the number of cycles rising. Thermal fatigue cracks initiation was mainly beginning from the wavy interface between copper and stainless steel, the vortex-like cast microstructure formed by explosive welding can prevent the crack from propagating along the interface edge and change the direction of crack propagation.The initiation and expansionof thermal fatigue cracks were observed in the copper matrix.

NEW ACHIEVEMENTS ON THE THEORY AND TECHNOLOGY OF EXPLOSIVE WELDING

There are four new achievements of this work on the theory and technology of explosive welding.(1) It has been found and defined three kinds of bonding interfaces: big wavy, small wavy and micro wavy, and the micro wavy interface is the best. In a cladding plate, it is for the first time to find that the form of interface presents regular distribution.(2) Although the interface has the features of melt, diffusion and pressure welding in the mean time, the seam and 'hole' brought by the melt weaken the bonding strength of interface greatly, and the effect of melt on interface must be eliminated in explosive welding, so explosive welding is not a melt weld. The diffusion welding is a kind of form of pressure welding, and the diffusion is not the reason of the bonding of interface but the result of interface high pressure. So the diffusion welding cannot also explain the bonding mechanism of it. The experiment and theory make clear that explosive welding is a special pressure one.(3) To get good interface of no melt, explosive charge must be selected on the low limit of welding windows. In explosive welding, the drive plate should be treated as the viscous and plastoelastic body, not incompressible fluid. The bending moment under the explosive welding loading must be greater than that under dynamic limit of drive plate. According to the condition, the lower limit of explosive welding is obtained. It is about 20% less than that obtained by tradition calculation, and suitable for engineering application.(4) It is for the first time to test and study on soil anvil characteristics and change regularity under explosive welding impact loading. Through soil anvil parameter optimization analysis, it is the best for explosive welding with sandy soil of water content 17.00% and density 1.74q/cm3.

Experimental and numerical investigations of interface properties of Ti6Al4V/CP-Ti/Copper composite plate prepared by explosive welding

Explosive welding technique is widely used in many industries.This technique is useful to weld different kinds of metal alloys that are not easily welded by any other welding methods.Interlayer plays an important role to improve the welding quality and control energy loss during the collision process.In this paper,the Ti6Al4V plate was welded with a copper plate in the presence of a commercially pure titanium interlayer.Microstructure details of welded composite plate were observed through optical and scanning electron microscope.Interlayer-base plate interface morphology showed a wavy structure with solid melted regions inside the vortices.Moreover,the energy dispersive spectroscopy analysis in the interlayer-base interface reveals that there are some identified regions of different kinds of chemical equilibrium phases of CueTi,i.e.CuTi,Cu_(2)Ti,CuTi_(2),Cu_(4)Ti,etc.To study the mechanical properties of composite plates,mechanical tests were conducted,including the tensile test,bending test,shear test and Vickers hardness test.Numerical simulation of explosive welding process was performed with coupled Smooth Particle Hydrodynamic method,Euler and Arbitrary Lagrangian-Eulerian method.The multi-physics process of explosive welding,including detonation,jetting and interface morphology,was observed with simulation.Moreover,simulated plastic strain,temperature and pressure profiles were analysed to understand the welding conditions.Simulated results show that the interlayer base plate interface was created due to the high plastic deformation and localized melting of the parent plates.At the collision point,both alloys behave like fluids,resulting in the formation of a wavy morphology with vortices,which is in good agreement with the experimental results.

Study on numerical simulation of TA1-304 stainless steel explosive welding

In order to guide the explosive welding experiment of titanium-stainless steel,Three-dimensional numerical simulation of explosive welding,which select TA1 as flyer plate and 304 stainless steel as base plate,is carried out by using the LS-DYNA software and SPH-FEM coupling algorithm in the present study.The explosive welding window is calculated and established.It is found that the numerical simulation results are in good agreement with the experimental results.The displacement,velocity and pressure-time curves of characteristic elements show that the quality of explosive welding composites is superior.It is proved that SPH-FEM coupling algorithm is effective for explosive welding of TA1/304 stainless steel and can effectively guide the selection of explosive welding parameters.

Calculation of temperature field near stagnation point in explosive welding

Energy deposition at the interface of explosive welding is analyzed by symmetrical impaction model of uneompressible liquid. Equation of energy in the flow field of explosive welding is deduced and the distribution of temperature in the flow field is solved by finite difference method on the basis that the adiabatic compression is considered. The results show that the temperature rise increases with the increasing of the velocity of approaching flow and impact angle, under appropriate velocity of approaching flow and impact angle the temperature rise near the welding interface will be higher than the melting point of the material and the thin melted layer is localized on the region near welding interface.

Interfacial Characterization and Mechanical Property of Ti/Cu Clad Sheet Produced by Explosive Welding and Annealing

It was aim to investigate the interfacial microstructure and shear performance of Ti/Cu clad sheet produced by explosive welding and annealing. The experimental results demonstrate that the alternate distribution of interfacial collision and vortex of flyer layer forms in the interface a few of solidification structure. TEM confirms that the interfacial interlayer contains obvious lattice distortion structure and intermetallic compounds. It interprets the explosive welding as the interfacial deformation and thermal diffusion process between dissimilar metals. The interfacial shear strength is very close to the Cu matrix strength, which is determined by the mixture of the mechanical bonding and metallurgical bonding. Several cracks exist on the shear fracture owing to the intermetallic compound in the interfacial solidifi cation structure and also the probable welding inclusion.

Investigation on the explosive welding mechanism of corrosion-resisting alu-minum and stainless steel tubes through finite element simulation and experi-ments

To solve the difficulty in the explosive welding of corrosion-resistant aluminum and stainless steel tubes, three technologies were proposed after investigating the forming mechanism through experiments. Then, a 3D finite element model was established for systematic simulations in the parameter determination. The results show that the transition-layer approach, the coaxial initial assembly of tubes with the top-center-point the detonation, and the systematic study by numerical modeling are the key technologies to make the explosive welding of LF6 aluminum alloy and 1Cr18Ni9Ti stainless steel tubes feasible. Numerical simulation shows that radial contraction and slope collision through continuous local plastic deformation are necessary for the good bonding of tubes. Stand-off distances between tubes （D1 and D2） and explosives amount （R） have effect on the plastic deformation, moving velocity, and bonding of tubes. D1 of 1 mm, D2 of 2 mm, and R of 2/3 are suitable for the explosive welding of LF6-L2-1Cr18Ni9Ti three-layer tubes. The plastic strain and moving velocity of the flyer tubes in-crease with the increase of stand-off distance. More explosives （R2/3） result in the asymmetrical distribution of plastic strain and non-bonding at the end of detonation on the tubes.

MECHANISM OF WAVE FORMATION AT THE INTERFACE IN EXPLOSIVE WELDING

Some of the main progress on the investigation of the mechanism of the wave formation in explosive welding at the Institute of Mechanics is summarized and otters'previous works are re- viewed.Our systematic experiments and analysis do not substantiate the theory of wave formation based on Karman vortex-street analogy or Helmholtz instability.On the contrary,they show that materi- al strength insensitive to strain rate plays an important role.A simple hydro-plastic model is presented to explain the main features regarding the interracial wave formation and to estimate the magnitude of wave length.The result is in broad agreement with experiment.

Atomic diffusion across Ni_(50)Ti_(50)Cu explosive welding interface:Diffusion layer thickness and atomic concentration distribution

Molecular dynamics simulations are carried out to study atomic diffusion in the explosive welding process of NisoTis0-Cu （at.%）. By using a hybrid method which combines molecular dynamics simulation and classical diffusion the- ory, the thickness of the diffusion layer and the atomic concentration distribution across the welding interface are obtained. The results indicate that the concentration distribution curves at different times have a geometric similarity. According to the geometric similarity, the atomic concentration distribution at any time in explosive welding can be calculated. NisoTis0- Cu explosive welding and scanning electron microscope experiments are done to verify the results. The simulation results and the experimental results are in good agreement.

High strain rate deformation of explosion-welded Ti6Al4V/pure titanium

Multilayer materials are widely used in military,automobile and aerospace industries.In this paper,the response of an explosion-welded Ti6Al4V/pure titanium with a flat interface to dynamic loading is investigated.An SHPB apparatus is used.Then,the dynamic behaviour of a bimetal sample is explored with a DIC system coupled to the SHPB.Result indicates that in the bimetal sample pure titanium is deformed and failed before Ti6Al4V.The stress curve of the sample shows two different peaks in a striker velocity higher than the 18.3 m/s.When the incident wave encounters the interface of the Ti6Al4V/pure titanium sample,only a small fraction of the wave is reflected owing to similar impedance.Using the direct interpretation stress-strain curve is unreasonable in this case because of unhomogenised plastic deformation.The micro structure of the sample is investigated after loading.An adiabatic shear band is formed in the pure titanium side before failure,and the interface of the sample remains intact under different loading conditions.The FEM simulation result for the sample is in good agreement with experimental observations.

Behavior and mechanism of the bonding interface of 0Cr18Nig/16MnR by explosive welding

In order to investigate the bonding behavior and mechanism of the interface prepared by explosive welding, the bonding interfaces of 0 Crl 8Ni9/16MnR were observed and analyzed by means of optical microscope （OM） , scanning electron microscope （SEM） and electron probe microanalysis （ EPMA ）. It is found that the welding interfaces are wavy due to the wavy explosive loading. There are three kinds of bonding interfaces i. e. big wave, small wave and micro wave. There are a few seam defects and all elements contents are less than both of the base and .flyer plate in the transition zone of big wavy interface. Moreover, some ＂holes＂ result in the lowest bonding strength of big wavy interface nearby the interface in the base plate. All elements contents of the small wavy interface are between two metals, and there are few seam and hole defects, so it is the higher for the bonding strength of small wavy interface. There is no transition zone and defects in the micro wavy interface, so the interface is the best. To gain the high quality small and micro wavy bonding interface the explosive charge should be controlled.

Formation, Evolution and Final Structure of Interface in 2024Al Joints Fabricated by Explosive Welding

The wavy interface for similar or the same metal explosive welding（EXW） and the universal mechanism of wavy interface formation in EXW were studied in this work. Based on a new established model, it was deduced that the evolution frequencies of the instability were constrained in a limited range. Then experiments of identical metal EXW were performed and welding interfaces were characterized for examining the final morphology. By calculating the fractal dimensions and multifractal spectra of welding interface, the fractal characteristics of interface were revealed and a quantitative description was achieved for EXW interface structure. Thus, the formation, evolution and final morphology of wavy interface were systemically researched.

Calculation of the Distribution Rule of Equivalent Strain Rate near Explosive Welding Interface

The objectives of this study were to analyze the distribution of equivalent strain rate near the stagnation point and probe into the effects of colliding angle on strain rate. An ideal fluid model of symmetrically colliding was used to research them. Calculations showed the equivalent strain rate and the colliding half angle are closely related to each other with the material geometrical size and explosive velocity selected, the equivalent strain has large gradient within several jet thicknesses near the stagnation point, the maximal strain points are lined up along a beeline, but a curve near the stagnation point. With different colliding angles, they can be fitted by using exponential curve. That is, the exponential curve can be regarded as the token curve in explosive welding..

Preliminary study of impaction mechanism of interfacial wave in explosive welding

The characteristics of both the detonation and explosive loading are studied comparatively. The width of the detonation reaction zone is from 0. 1 to 1 mm approximately, and it about equals the wavelength of interfacial wave in explosive welding. Both of them increase with the increasing of the thickness and detonation velocity of the explosive. The pressure of the detonation reaction zone is also wavy. The amplitude of the interfacial wave is decided by the detonation loading and the rate of the strength of the base and drive plate materials, and the wavelength is equal to the width of detonation reaction zone. So, a new impaction mechanism of pressure and inlaying due to wavy loading is put forward.

Construction and experimental investigation of the R-δf-type welding window

In this study, a new R-δf-type welding window on charge parameters is constructed by determining the boundary conditions of the explosive ratio. The R-δf-type welding window synthetically considers the explosive properties and material properties, and can quickly determine the explosive charge at different welding materials with different thickness. In order to test the practicability and accuracy of the R-δf-type welding window, 410S stainless steel and Q345R low alloy steel plates were explosively welded using 15 mm charge determined by the new window. Microstructures of the bonded sections were examined and then shear tests were carried out on the bonded specimens. Microstructural examination showed the joint interface was transformed from micro wavy to small wavy appearance without microstructure defects. Shear tests results showed inter inacial shear strength was much higher than that of the national first grade standard. So the R-δf-type welding window has strong practicability and accuracy and is helpful to the practical production and application of explosive welding composite materials.

Study on an Explosion Treated Steel Weld Metal with Prepared Crack

A softening zone with recrystallized grain around the prepared crack tip in the shock waves treated C-Mn steel weld metal was observed. It is suggested that a dynamic recovery occurred around the prepared crack tip even at a low explosion pressure (5 GPa) because of the stress and strain concentration at the crack tip when shock waves pass through. This result supports the previous work on the improved mechanical properties of a shock treated welded joint with residual crack from the viewpoint of microstructure.