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

Ysir Mhmood ,Peng-wn Chen ,* ,I.A.Btev ,Xin Go

a State Key Lab of Explosion Science and Technology,Beijing Institute of Technology,100081,PR China

b Novosibirsk State Technical University,K.Marks 20,630073,Novosibirsk,Russia

Keywords: Explosive welding Ti6Al4V/CP-Ti/Cu mooth particle hydrodynamic (SPH) Microstructure Mechanical properties

ABSTRACT 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 Cu-Ti,i.e.CuTi,Cu2Ti,CuTi2,Cu4Ti,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.

1.Introduction

Explosive welding is a famously recognized welding technique to weld different and similar kinds of metal alloy plates that cannot be cladded by other means of welding[1-3].And the above welded composite materials feature an optimum balance between production and expenses,and a high potential applications in various industrial applications.Furthermore,these composite materials perform well under extreme conditions,such as high pressure and temperature.For example,bimetallic [4-6] and multilayered materials [7-9] are used in chemical,transport,mechanical and shipbuilding industries [10].

Copper is an excellent conducting material,while Ti6Al4V exhibits high corrosion resistance with a high strength/mass ratio.Ti/Cu composite plate features low resistivity and high strength which is conducive to the applications in the electromechanical process.For example,welded Ti/Cu material is an ideal cathode material of electrolyzers for electrohydropulse loading [11].Various approaches have been reported for the production of Ti/Cu composite materials [12-15].However,explosive welding is one of the best welding techniques that provide good quality with a large welding area.Earlier,Paul et al.[16]welded titanium with copper to observe the electromechanical effect with interface morphology.Paul et al.[17] examined the conversion of non-equilibrium phases of Cu/Ti into equilibrium phases through annealing process at 973 K.Likewise,Shmorgun et al.[18] studied the microstructural changes induced by heat treatment and hot rolling process on Cu-Ti plates produced by explosive welding process.Pushkin et al.[19]investigated the fractal changes from flat to a wavy interface of Cu/Ti plates by using different impact angles in a series of explosive welding experiments.Skuza et al.[20]investigated the influence of standoff values on the interface microstructure of the Ti/Cu composite plate obtained by explosive welding.Similarly,Kahraman et al.[21] joined Ti6Al4V with copper by using the explosive welding method.They examined the interface morphologies and joint strength of the Ti6Al4V/Cu composite plates under different loading conditions.Moreover,a lot of work reports demonstrates that the morphology of the interface is one of the essential parameters to check the welding quality[22-24].Two main types of interface morphology,wavy and flat,are formed during the explosive welding process.Bahrani et al.[22] observed that the wavy interface is formed due to the interpenetration of jets irrespective of material strength.Reid [23] investigated that the wavy interface pattern contains some discontinuities due to Helmholtz instabilities.Chemin et al.[24] compared the experimental and theoretical results of interface formation.They also demonstrated that material strength has a significant impact on interface shape at a lower plat velocity.

Note that explosive welding is a solid-state process leading to a series of extreme conditions and phenomena in micro-scale second,such as the rapid temperature increase in local zones,material mixture induced by jetting,plastic deformation and melting,and high cooling rate during welding process.Such extreme conditions lead to the formation of brittle intermetallics which may affect the joint quality of welded plates.With respect to Ti/Cu plates produced by explosive welding,residual stress is intensified due to both the transient welding stress during impact and the difference in mechanical properties of two materials[25].Multiple researches imply that the utilization of interlayer plate reduces the interface area thickness [26],and enhances the bonding strength with an improved interface micromorphology [27].Additionally,Manikandan et al.[28]further demonstrates that the energy loss during the collision of flyer plate and interlayer plate is one critical factor to improve the interface morphology and reduce the intermetallics.Consequently,insertion of an interlayer plate in the explosive welding setup is applied to improve the welding quality of explosive welding.However,Saravanan et al.[29] reported that,in explosive welding process,the application of multi interlayer plates can hardly further improve the welding quality compared with that of single interlayer plate.In addition,the controlled energy consumption during the collision induced by the utilization of interlayer plate also inhibits the formation of the adiabatic shear band.With respect to Ti alloys,especially Ti6Al4V,the low ductility may induce the formation of cracks or adiabatic shear bands at higher strain rates in explosive welding process [30-33].Thus,in this work,the interlayer plate is utilized in the explosive welding of Ti6Al4Vplate for a better welding quality.

Simulation is an excellent tool to understand the explosive welding process,such as jet formation,interface formation,transient pressure and temperature during impact[34].Mahmood et al.[35] simulated the welding process by using LS-DYNA to analyse the pressure,temperature and plastic deformation profiles during the welding process.Nassiri et al.[36].simulated the jetting phenomena through Arbitrary Lagrangian Eulerian (ALE) method and smoothed particle hydrodynamics (SPH) to obtain welding parameters.Moj˙zeszko et al.[37] also investigated the explosive welding process of Cu/Ti by SPH simulation.

In the present study,the commercially pure Ti(CP-Ti)interlayer effect on interface shape and welding quality was investigated through both experiments and coupled ALE-SPH simulation.The microstructure and mechanical properties of the welded composite plate (Ti6Al4V/CP-Ti/Cu) were analysed through various characterization techniques and mechanical tests.In addition,the simulation results were analysed to investigate interface morphology and parameters under different conditions of explosive welding.

Fig.1.Schematic experimental setup for explosive welding.

2.Experimental procedure

2.1.Materials and welding setup

Fig.1 shows the experimental setup.Three parallel plates,Ti6Al4V,CP-Ti and pure copper plates,were used for experimentation through explosive welding method.Ti6Al4V and copper plate with a dimension of 150 mm×200 mm×3 mm were used as flyer plate and base plate,respectively.The CP-Ti plate with a dimension of 150 mm×200 mm×0.2 mm was used as interlayer plate.ANFO explosive (packing density 0.7 g/cm3,detonation velocity 2700 m/s) was used to accelerate the flyer plate.The experiment was conducted in the open air at ambient pressure and temperature with sand anvil.

2.2.Mechanical and microstructure tests

Tensile tests,tensile shear tests and three-point bending tests of welded Ti6Al4V/CP-Ti/Cu composite plate specimen were performed to examine the mechanical properties.Stratified samples were cut from the welded composite plate.Fig.2 presents the schematic diagrams of the tensile test and tensile shear test based on the studies of Han et al.[27].Besides,microhardness tests were performed along the perpendicular direction of the interface.For microstructure investigation,samples were polished mechanically and etched with Kroll etchant (H2O 92%,HNO36%,HF 2%) for the 20 s.

Fig.2.Schematic diagrams of(a)tensile test sample and(b)tensile shear test sample.

2.3.Simulation methods

Fig.3 shows the schematic setup to investigate the micromorphology of welded interfaces with less computational efforts.For the current study,the SPH simulation scheme features the Euler-ALE coupling.The Euler formulation is used to simulate explosives,while the flyer and base plates are simulated in ALE formulations.The interlayer plate,the top portion of the flyer and base plates with thickness of 0.5 mm are simulated in the SPH method with a particle size of 20 μm.ANSYS-AUTODYN (V14.5 ANSYS Inc.USA)software was used to carry out the simulation.

2.4.Equation of states and constitutive models

2.4.1.Jones-wilkens-lee (JWL) equation of state

The explosive expansion can be described by using the JWL equation of state,which represents the pressure-volume relation at the isentropic level.Standard JWL equation of state form is written as

whereA1,B1,C1,R1,R2and ω all are constants and found experimentally.A1,B1andC1have units of pressure,and the remaining constants are dimensionless values.Moreover,the integration of the thermodynamic equation can enhance the application of JWL equation.

2.4.2.Mei-Gruneisen shock equation of state

Mei-Gruneisen shock equation of state was practiced to simulate the metal alloys (flyer,interlayer and base plates),providing a relation between pressure and volume under shock conditions at a given temperature.

2.4.3.Johnson-Cook material model

Johnson-Cook material model was applied to predict the high deformation and von-Misses yield stress of the material.Johnson-Cook equation is as follow

whereAis the yield strength of the material,B is strain hardening coefficient,Cis strain rate constant,m is softening exponent,ε is a plastic strain,is plastic strain rate,n is hardening exponent,andT* is homologous temperature and equal toT*=Material models parameters for Ti6Al4V,CP-Ti and Cu are shown in Table 2.

Table 1 JWL parameters for ANFO [38,39].

Table 2 Parameters of material models and equation of states.

3.Results and discussion

3.1.Microstructure analysis of welding interfaces

Fig.4a-d shows the simulation and optical microscopic (OM)images of welded composite plate interfaces.Since the densities of flyer and interlayer are similar,the corresponding interface of flyer and interlayer plate (Interface I) features a smooth shape without any notable melted region.However,microstructure of interlayerbase plate interface (Interface II) features a wavy micromorphology due to its formation from two different alloys,which improves the welding quality with a better joints[43].The OM image(Fig.4d) presents the wavy micromorphology with no obvious cracks and discontinuities.Furthermore,the simulation result indicates the formation of melted zones within the wave vortex and crest inclusion of the Cu plate near the interface.Moreover,Fig.4c also present the formation of wavy interface and melted zones within the crest of the wave microstructure,which is in accordance with the experimental results (Fig.4d) and previous related researches[16].The simulation results show that the kinetic energy is converted into plastic work to generate shear deformation during impact,leading to the temperature increase in local zones and the enhancement of welded plate joints.Such results are conducive to understand the explosive welding process which can hardly be observed through experiment due to its short period (within a micro-scale second process).

Fig.5a indicates the plastic deformation in the interfaces,especially that in Interface II with the plastic strain higher than 5.These results are in good agreement with the previous studies[44].Furthermore,the deformation value of Interface I is almost a half of that of Interface II,because majority of the kinetic energy is converted into the formation of Interface II.High pressure and plastic strain increase the local temperature up to the melting point of the constituent metal alloys.The temperature profile can be verified through Fig.5b,indicating that at Interface II area,the local temperature raises up to 2000 K,leading to a strong deformation within the interface area,which is supported by previous simulation research on Cu/Ti explosive welding[45].Under such extreme conditions,both titanium and copper behave like a fluid at the collision point area,causing the formation of jet,wavy interface,vortex and intermetallics during the explosive welding process.However,the local temperature rises only present in the area near the interface with a width of several micrometers.Subsequently,the high cooling rate (105-107K/s [46-49]) induces quenching in the vortices,increases the microhardness of the interface area,and may be conducive to the formation of amorphous or ultra-fine grains in the interface area.

Fig.3.Schematic simulation setup of Ti6Al4V/CP-Ti/Cu explosive welding.

Fig.4.(a) 2D simulation process of explosive welding,(b-c) enlarged simulation results of interfaces,and (d) OM image of interface.

Blazynski [47] demonstrated that the impact pressure which is 10 times higher than the yield strength of constituent material is conducive to a good welding quality.Fig.5c reveals that pressure during impact is approximately 10 GPa,which is higher than the yield strengths of both materials.Moreover,this increment of impact pressure is also observed at both interfaces.Furthermore,Fig.5c also shows the jet formation during the simulation of explosive welding process,which is essential for the explosive welding process by removing the oxidation layers and impurities.Fig.5b also reveals that the jets are trapped in the crest portion of interfaces,especially in that of Interface II,due to high cooling rate,adiabatic heating,variation in dynamic angle,and poor smoothness of metallic surfaces.

SEM observations of Interface I (Fig.6a) indicate that Ti6Al4V grains are elongated toward the detonation direction,especially near the interface due to high deformation and temperature.Fig.6b represents the SEM image of Interface II,indicating two interface types,flat interface(Fig.6c)and vortices(Fig.6d).The flat interface is with a thin permanent layer of intermetallics in the thickness of 0.2-0.3 μm.The vortices feature several intermetallics regions with an average area of approximate 6000 μm2surrounded by copper.Furthermore,these regions feature a large number of micro line structures,such as circular or spiral lines.It indicates that the mixture of the Cu-Ti is nonhomogeneous,leading to the formation of multiple Cu-Ti intermetallics.Moreover,some Ti-rich areas in island-like shape can be observed within circular spiral patterns,which were also reported in the literature [30,50].Additionally,some separated small areas are also noticed along the melted solid region.These complex mixed microstructures imply that the molten region consists of various intermetallics and multiple separated zones of pure metals.This phenomenon is conducive to the bonding quality of welded plates based on the study of Paul et al.[16] on thin layer of solid melted area.Furthermore,due to melting and fluid-like behavior during explosive welding process,both materials from interlayer plate and base plate are mixed in the circular spiral shapes featuring non-chemical equilibrium.It can be confirmed by experimental results of a high resolution SEM image(Fig.6d) and is also reported by many researchers [41,51,52].

Fig.5.Simulated contour plots of (a) plastic strain,(b) temperature,and (c) pressure.

Fig.6.(a) SEM image of Interface I,(b) SEM image of Interface II,(c) flat interface,(d) intermetallics enclosed in vortex.

EDS analysis of recovered samples (Fig.7) was carried out to investigate the elemental distribution in the Interface II area.Point scan analysis (Fig.7a) and line scan analysis (Fig.7b) of a selected melted region show the presence of different types of Cu-Ti equilibrium are randomly distributed,featuring a strong gradient of chemical compositions.It is in good agreement with the presence of spiral or swirl pattern structure in corresponding area.Previously,Gloc et al.[32]and Mousavi et al.[53]reported similar microstructures of swirl shapes within the melted region of explosively welded Ti-steel interface.However,Cu-Ti swirl patterns are more intense as compared to Ti-steel.It is attributed to the lower melting point and higher thermal conductivity of copper[54].For this reason,heat is rapidly distributed in copper as compared to steel during the explosive welding process.Moreover,Fig.7b shows that the intensity of copper mixing is gradually decreased at the middle of a melted region,which indicates that copper mixing is restricted in the middle portion of the melted region.Ti-rich regions are observed inside the melted area with island-like shape.During the explosive welding process,the local solid material was mixed within the melted area due to the reaction of the complex force to form these small islands.These Ti islandslike morphologies were also reported in the literature [30,50].

Different areas of chemical composition corresponding to equilibrium phases of Cu-Ti,i.e.CuTi,Cu2Ti,CuTi2and Cu4Ti,were observed during EDS analysis,based on the Cu-Ti phase diagram[55].Earlier,Paul et al.[17] also observed these four equilibrium phases.Besides that,they observed the mixture of complex multiple fragmented phases and amorphous phases that are not present in the Cu-Ti equilibrium phase diagram.Furthermore,they suggested that the thin nano level reaction layer between the two parent plates plays an important role in welding quality.Elrefaey and Tillmann[56]examined that CuTi and CuTi2phases are brittle and may affect the joint strength of the welded plate.Besides these four equilibrium phases,the EDS scan at Point 5 predicts another equilibrium phase Cu3Ti2at the boundary of the melted region.Saboktakin et al.[57]also identified Cu3Ti2at the transition line of Cu-Ti.Furthermore,line scan analysis indicates no distinct boundaries between copper-rich and titanium-rich regions,implying the presence of multiple intermetallics phase with different contents,which is similar with the report of Zhao et al.[58]on the study of multiple equilibrium phases of Cu/Ti composite plate.

Fig.7.EDS scans in the melted zone of Interface II.(a) point scan analysis and (b) line scan analysis of recovered sample.

3.2.Mechanical tests

3.2.1.Tensile test

The tensile test is used to examine the mechanical behaviour of the material under different loading conditions.Mechanical properties of the welded composite plate are different from those of raw materials.In this study,the tensile tests were performed to study the mechanical response of the welded material.Stress-strain curves and macroscopic images after the test are shown in Fig.8.Results (Table 3) exhibit that the welded composite plate features an ultimate tensile strength (UTS)almost 3 times higher than that of the weak parent plate(Cu).

Moreover,the weighted estimated results of mechanical properties of ideal composite plate were calculated based on the corresponding mechanical property of above materials with the plate thickness,for the judgment of the mechanical properties of welded samples[59,60].Table 3 exhibits that the welded sample features a better UTS(15%higher than the weighted estimated UTS).Its better performance is due to the high deformation and compressibility of the materials under shock loadings.Since the welded composite plate is formed from two different materials with an interface consisting of various complex microstructure and phase composition,the sample is broken at two points in the tensile test.Table 3 shows that the ductility of the joint is less than those of the parent material plates (Ti6Al4V,Cu),which may be due to the strain hardening and the formation of intermetallics during impact.

Table 3 Tensile test results of the welded composite plate with tensile properties of parent materials.

3.2.2.Tensile shear test

The tensile shear test was conducted to observe the joint quality,using a load applied along the longitudinal direction.Fig.9 shows that sample breaks from the copper side without interrupting the welding joint at a maximum load of 400 MPa,which is much higher than the copper yield strength (Cu～210 MPa).Furthermore,Fig.9 shows the graphical representation of force versus displacement.The change of loading indicates that the deformation starts in the copper side and the joint exhibits no disturbance,which is also reported by Livne et al.[6] and Durgutlu et al.[54].

Fig.8.Engineering stress-strain profile of welded composite plate specimen.

Fig.9.Shear test result of welded composite plate specimen.

3.2.3.Bending test

Three-point bending test was performed along the perpendicular direction of the detonation wave.Samples were bent up to 30%from flyer and base sides because one of the parent materials(Ti6Al4V) had elongation less than 20% (as shown in Fig.10).The American national standard document (AWS B 4.0:2007) shows that under the condition that one of the parent material elongations is less 20%,bending test would be qualified at 20%elongation of the bent sample.Thus,the result shows that the welded composite plate exhibits elongation property.

3.2.4.Microhardness test

Fig.10.Bending test results of welded composite plate specimen.

Fig.11.Vickers hardness profile perpendicular to interface.

Microhardness test was performed along the perpendicular direction of the interface.According to Fig.11,near the interface,material hardness is increased due to the intermetallics formed by the high-speed collision of the parent plates.The high-speed collision produces maximum deformation and heat,which causes annealing at the contact point [64].In addition,the annealing process increases the hardness of the formed intermetallics harden as compared to parent materials.Therefore,after the explosive welding test,the microhardness of interface increases.The microhardness of Ti6Al4V,CP-Ti and Cu plates are 350 Hv,150 Hv and 85 Hv,respectively.The microhardness at Interface I and at Interface II are 400 Hv and 250 Hv,respectively.Besides,microhardness inside the melted vortex at Interface II is 485 Hv,which is almost double the average values at Interface II.It is most likely due to the presence of multiple Cu-Ti intermetallics formed under the extreme conditions during explosive welding process,such as high deformation,high temperature and high cooling rate.According to EDS point scan analysis (Fig.7),the melted area in the Interface II consists of different kind of Cu-Ti intermetallics.Moreover,Paul et al.[17] examined that higher percentage of titanium in Cu-Ti equilibrium phase features low hardness value.Microhardness results show that Ti-Cu intermetallics possesses higher hardness compared to the normal hardness value of Ti and Cu,due to above extreme conditions.These results are in accordance with the Previous investigations of Bina et al.[43],Kwiecień et al.[65] and Acarer et al.[66].

4.Conclusions

In the present study,microstructural and mechanical properties of Ti6Al4V/CP-Ti/Cu welded composite plate were investigated.A simulation tool was used to understand the explosive welding process,including the formations of interface morphology,especially interlayer-base interface,through SPH method.The simulation results successfully depict the experimentally formed vortices and molten zones.Additionally,simulated results of pressure,temperature and plastic strain are in good agreement with the experimental welding conditions.Microstructural analysis,particularly EDS analysis,revealed that there are some identified regions of chemical composition that satisfy the different equilibrium phases of Cu-Ti,i.e.CuTi,Cu2Ti,CuTi2,Cu4Ti,etc.These regions are located within the vortices near the interface area.Furthermore,small island-like Ti-rich regions were also observed within the melted regions.Mechanical tests,i.e.,tensile test,bending test,shear test and Vickers hardness test,showed that the welded composite plate features optimized mechanical properties to meet the minimum welding standards.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Acknowledgment

This research is supported by the opening project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology,opening project number is KFJJ18-05 M) and National Natural Science Foundation of China (Grant No.11472054).

Appendix

a.Euler

The Eulerian method is mostly used for extremely high strainrate problems,such as explosive related simulations.In Euler formulation,the material model moves in fixed grid.This method is advantageous owing to no grid deformities during simulations with a special calculation to deal with solids.

b.Smoothed Particle Hydrodynamics (SPH)

SPH technique is a gridless method which is conducive to the study of high impact dynamics.SPH method is based on Kernel approximation,which simplifies the conservation equations.Field information can be calculated by using discrete particles and neighboring particles are used to solve the integrals.If nearby particles are represented by‘j’subscript,then field variable for non zero Kernel approximation can be expressed as

wherempand ρpexhibit particle mass and density,respectively,f(r)is a field variable,rshows the location of particle,his the smoothing length which is used to calculate the effect on a particular particle by other neighboring particles.Aizawa et al.[38].used the SPH-Lagrange coupling scheme to replicate the interface of bimetallic plate welding successfully.

c.Arbitrary Lagrangian Eulerian (ALE)

Arbitrary Lagrange Euler (ALE) method is a hybrid space-based discretization,which can overcome space-based errors created by Lagrangian technique.In the ALE method,the shape of elements within the mesh is improved,while the moving boundary is used to track mesh borderlines.Some extra steps are used in Lagrange,which supports to redefine and rearrange the mesh itself.This method is also conducive to the study of high deformation problems.