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

Shi Changgen,Sun Zerui,Fang Zhonghang,Zhao Linsheng,Shi Hesheng

1.PLA Army Engineering University,Nanjing 210007,China;

2.Jiangsu Runbang New Materials Group,Nanjing 201803,China

Abstract 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.

Key words large Ti-steel cladding plate,double vertical explosive welding,comprehensive protective structure,protective mechanism

0 Introduction

Explosive welding is an irreplaceable way to make composite plates;however,common parallel explosive welding not only has a low energy utilization rate,but also has a huge impact on surrounding environment[1-4].The double vertical explosive welding is a new method which places the explosive between the two flyer plates and adopts a form of closed charge.This method makes full use of the energy generated in the detonation process and overcomes the shortcomings of the current parallel welding.It saves about 2/3 of the explosive charge,improves the production efficiency,and reduces the impact damage effect of explosive welding[5].

The double vertical explosive welding has achieved the welding test of composite materials of big plates face metal,and most of the motion of the plates has converted into the interface binding energy and the deformation energy of the composite plate,but there is a small part of the energy[6-7].The remaining energy will be converted to the kinetic energy,which moves the composite plate welded at a high speed (above 100 m/s in theory)and causes its macroscopic deformation.The high speed movement brings great risk and uncertainty for welding work[8].As shown in Fig.1,there were two composite plates of stainless steel/steel with a length of 4 m and a width of 1.5 m welded together by double vertical explosive welding.Since the problem of movement protection is not solved,serious deformation and damage occur in the composite plate.

In order to solve the problems of quality and safety in the combination of composite plates after the explosion welding,here we put forward and designed a comprehensive protective structure of impact resistance for the double vertical explosive welding,and studied its protective mechanism.Through the kinematics research of composite plates,this protective structure can be reusable in evenly on both sides of the double vertical explosive welding device.The design and research of the protective structure is the key to the application of the double vertical explosive welding.

Fig.1 Two large composite plates (T2/ Q235B)welded by double vertical explosive welding in one detonation (a)Composite board 1 (b)Composite board 2

1 Comprehensive protective structure

According to the mechanisms of rigid structure collision protection and flexible absorbing energy dissipation[9-10],we designed a comprehensive protective structure used in the double vertical explosive welding by combining the energy absorption buffer characteristics of flexible protective wall and the excellent shock resistance of rigid protective structure,as shown in Fig.2.It is mainly composed of a flexible protective wall and a rigid protective structure,and the two sides of the protective wall are independent of each other and without rigid connection.After welding,there is only a small range of movement,which will not cause deformation and damage of the rigid protective structure and is conductive to the recycling of the protective structure.

Fig.2 Unilateral schematic diagram of flexible protective structure(a)Integrated protective structure(b)Rigid protective structure

2 Analysis of protective mechanism of comprehensive protective structure

The collision and movement process of the composite plate and the protective structure were analyzed by establishing a mathematical model,and the movement displacement change rule of the protective structure was calculated and the stress situation was analyzed.

2.1 Protective structure and protection process

The metal composite plate and the rigid protective structure were simplified into two rigid mass blocks respectively;the flexible protective wall was simplified as elastic medium with two physical elements-spring and damper.The elastic recovery coefficient and the energy dissipation of the flexible protective structure were considered.The physical model of the protection process is shown in Fig.3.

Fig.3 Physical model of the protection process

According to the theory of collision dynamics,the collision between two objects must be between a completely elastic collision and a completely inelastic collision.Under the condition of completely elastic collision,the two objects satisfy the law of conservation of kinetic energy and the law of conservation of momentum,namely:

The displacement of the rigid fender was assumed to zero in the high-speed collision process of the metal composite plate and the rigid fender.Ignoring the external forces in the horizontal direction,given the initial conditions ofvf=vF,andvF=0,the velocities of the two objects after collision are as follow:

Under the condition of completely inelastic collision,the two objects after collision have the same speed.Eq.(3)reduces to

According to the physical model shown in Fig.3 and the theory of structural dynamics,the motion equation of the metal composite plate with the comprehensive protective structure after the collision in the horizontal direction is

whereMis the mass of the object after collision (M=m1+m2in the case of completely inelastic collision andM=m2in the case of completely elastic collision).kis the elasticity coefficient of the flexible protective wall;cis the damping coefficient of the flexible protective wall;xrepresents the horizontal displacement of the rigid protective structure.Ffis the friction force on the ground during the movement of the protective structure.

According to the motion equation above,the initial condition ofx(0)=0 andx'(0)=v'Fis substituted,and the motion process of the rigid protective structure can be obtained by solving the quadratic differential equation:

2.2 Analysis of Force and Stability of Protective structure

The load acting on the rigid protective structure mainly includes the impact force of composite plate collision,the gravity of the protective structure itself (including the vertical plate,floor plate and flexible protective structure)and the earth pressure from the side of the flexible protective wall.The specific stress is shown in Fig.4.Here,W1,W2,W3are the weight of rigid protective structure vertical plate,floor plate and flexible protective structure,respectively;γHKPis the pressure strength at bottom;hpare the height of earth pressure acting point fromOpoint.

(1)Impact force of composite plate

According to the collision mathematical model,the contact forceFafter high-speed collision can be obtained by the contact mechanics theory:

whereEis the relative elastic modulus,R*is the equivalent radius of the curvature of the impactor;δ*is the maximum invasion depth;andTeare elastic contact time,elastoplastic contact time and total contact time,respectively;the dimensionless parametere=1.185(Vy/V0)0.25is the recovery coefficient.Vyis the yield velocity of the material;V0is the initial impact velocity,and 0 ＜e＜ 1;whene=0,2e2/(1+e2=0)is the case of complete plastic contact;whene=1,2e2/(1+e2)=1 in the case of complete elastic contact.

(2)Soil pressure

Since the vertical plate of the rigid protective structure is smooth and the filling surface of the flexible protective wall is approximately horizontal,the earth pressure of the rigid protective structure can be calculated by using the passive earth pressure calculation method in Rankine earth pressure theory[11].

It is assumed that the flexible protective wall is constructed of clay-free soil,the pressure strength at the top of the rigid protective structureσ0=0,σ1=γHKP,that's the net force of the earth pressureEp:

whereγis the weight of the soil,which is related to the compactness and water content of the soil,usuallyγ=16 -20 kN/m3,His the height of the flexible protective wall,Kpis the soil pressure coefficient,andKD=tan2(45+φ/2),whereφis the internal friction angle of the soil,the action line of earth pressure,Ep,passes through the centroid of the triangle.

(3)Stability analysis

When the composite plate impacts the comprehensive protection structure,various forces acting on the protection structure will superimpose and generate corresponding torque,it may tip around O point while the protection structure shifts along the direction of the applied force.In order to avoid unstable phenomena such as overturning,it is necessary to check the anti-overturning stability of the protection structure.It must be required that the anti-overturning safety factorKt(the ratio of anti-overturning moment to overturning moment at pointO)is not less than 1.6,that is

whereS1,S2,S3are the horizontal distance of gravity center of vertical plate,floor plate and floor plate fromOpoint,respectively;htis the height of impact force acting point fromOpoint of composite plate.

On the soft soil foundation,the floor of the rigid protection structure may be trapped in the soil in the process of movement,so that the center of the moment moves inward,resulting in the reduction of the anti-overturning safety coefficient.The check calculation should pay attention to the compressibility of the soil.At the same time,when the floor area at the side of the floor is large,the soil weight above the floor can also be included in the dead weight of the protective structure.If the checking results cannot meet the requirement of Eq.(9),it can be solved by increasing the thickness of the protective structure and the gravity,or by extending the bottom plate and increasingS1.

2.3 Law of displacement variation of protective structure

In the process of double vertical explosive welding,composite panels and comprehensive protective structure collide,and then the comprehensive protective structure moves on both sides at a certain speed until the speed drops to zero.At this time,the displacement of the protective structure reaches maximum,and its displacement magnitude is directly related to the safety of the welding protection,which is an important content of the design and research of the comprehensive protective structure of the double stand explosive welding.

As it is complex,the displacement of the comprehensive protective structure is related to the material properties,size and other parameters of the composite plate and the protective structure.In order to simplify the research on the displacement change of the protective structure,the size of the titanium/steel composite plate was 0.3 m × 0.2 m,the thickness of the cladding plate was 5.5 mm,and the thickness of the base plate was 18,26,34 and 42 mm respectively.The vertical plate size of the rigid protective plate was 0.6 m × 0.5 m,and the thickness was 50,60,70,80,90 and 100 mm respectively.The thickness of the flexible protective wall was 1 m.

The stiffnesskand the damping coefficientcof the flexible protective wall were calculated according to the Lysmer solution[12]:

whereρ,vsandλare,respectively,the density of sand,shear wave velocity and poisson's ratio of the flexible protective wall,ρ=1.5 g/cm3,vs=110 m/s,λ=0.2,Sis the contact area between the rigid protective board and the flexible protective wall.

It is assumed that the collision between the composite plate and the protective structure is completely inelastic.Due toM=mf+mFandv'F=(mfvf)/(mf+mF),the numerical solution of the displacement of the comprehensive protective structure can be obtained by substituting it,as shown in Table 1.

It is assumed that the collision between the composite plate and the protective structure is a completely elastic collision.Due toM=mFandV'F=(2mfvf)/(mf+mF),the displacement of the comprehensive protective structure can be obtained,as shown in Table 2.

According to the collision theory,the two colliding objects under the condition of fully elastic collision satisfy the conservation law of kinetic energy,that is,the energy loss of the objects is the minimum,while the kinetic energy loss is the maximum.When all else are invariant,the displacement of the protection structure reaches its maximum under the condition of completely elastic collision (represented bySmax),and its minimum value under the condition of completely inelastic collision (represented bySmin).The actual displacement of the protection structure is betweenSmaxandSmin.The displacement of the protective structure increases linearly with the thickness of the cladding plate after the explosion.

3 Titanium/steel double vertical explosion welding protection test and interface test

3.1 Titanium/steel double stand explosive welding and protection test

According to the detonation mechanism of the doublevertical explosive welding and the characteristics of the lateral dispersion of the double vertical explosive welding of composite plate,the practical explosion test of the double vertical explosive welding of titanium/steel was carried out under the protection of the optimized comprehensive protective structure.The size and mechanical properties of the test materials are shown in Table 3.The explosive was No.50,the stable detonation speed was about 1 800 m/s,the charging thickness was 25 mm,and the spacing was designed to be 8 mm.

Table 1 Displacement of comprehensive protective structure under completely inelastic impact (mm)

Table 2 Displacement of comprehensive protective structure under complete elastic impact (mm)

Table 3 Test material size and mechanical properties

The installation and protection of the double vertical explosive welding test for titanium/steel composite plate are shown in Fig.5a and b respectively.Fig.5c and d are the protective effect diagram after the explosion and the titanium/steel composite plate obtained,respectively.

As can be seen from Fig.5,the movement range of the protective device and the composite board after welding was within the safe distance;the rigid protective device had a certain deflection but had no damage and deformation.Most of the sandbags were basically well preserved,which can be used repeatedly;after the appearance inspection,the extension of the cladding plate was completely removed,and the composite plate was not damaged or cracked,and the ultrasonic inspection showed that the composite rate of each composite plate reached more than 98%,and the performance indicators met the requirements of national standards.

3.2 Interface morphology of titanium/steel double vertical explosive welding

The titanium/steel composite plate was corroded and observed under the metallographic microscope at 200× -400×.Fig.6 shows the metallographic structure of the interface of No.1 sample composite plate.Fig.7 shows the metallographic structure of one side of steel plate,indicating ferrite and a small amount of pearlite.Fig.8 shows the metallographic structure of one side of titanium plate.

Fig.5 Double vertical explosion welding test of titanium/steel composite plate (a)The installation (b)Protection (c)Protective effect after explosion (d)T2/Q235B composite plate after explosion

Fig.6 No.1 interface microstructure of Ti/steel composite plate(a)OM position 1 (b)OM position 2

Fig.7 Metallographic structure of ferrite and a small amount of pearlite on one side of steel plate(a)OM 200× (b)OM 400×

Fig.8 Metallographic structure of one side of titanium plate(a)OM 200× (b)OM 400×

4 Conclusions

(1)An integrated protective structure for large plate double vertical explosive welding was proposed and optimized.The composite protective structure composed of rigid protective device and flexible protective wall can withstand the impact of detonation wave and high-speed composite plate.The rigid protective device and the composite plate had no damage and deformation and can be used repeatedly,which effectively solved the protection problem of doublestand explosive welding.

(2)The metal composite plate and the rigid protective structure are simplified into two rigid mass blocks;the flexible protective wall is simplified as an elastic medium composed of two physical elements-spring and damper.The elastic recovery coefficient and the energy dissipation of the flexible protective structure are considered.

(3)According to the physical model,the analytical solution of the motion velocity of the rigid protective structure is obtained by using the complete elastic collision theory and the complete inelastic collision theory,and then the dynamic performance,structural stability and displacement change rule of the protective structure are analyzed.

(4)By using the protective structure,we carried out the explosion welding and protection test of Ti/steel plate,and the protection effect was good.The welding rate of Ti/steel double stand explosive composite plate was nearly 100%.The composite plate had no deformation,surface crack,over-melted crack and other large-wave microstructure defects.