Explosive welding of unequal surface using groove method

EXPLOSIVE WELDING OF UNEQUAL SURFACE USING GROOVE METHOD

Mohammad Tabatabaee Ghomi1,2, Jafar Mahmoudi2, Abolfazl Khalkhali3, Gholamhossein Liaghat41Technology Development Institute (TDI) and ACECR Researcher, Tehran, Iran

2Mälardalen University (MDH), Sweden3School of Automotive Engineering, Iran University of Science and Technology, Tehran, Iran

4Tarbiat Modares University, Tehran, IranE-mail: [email protected]

Received August 2011, Accepted May 2012No. 11-CSME-60, E.I.C. Accession 3300

ABSTRACTBond strength of welded joints is an important factor in the explosive welding process. In such weld-

ing process, stress waves produced by explosive energy propagate at the free surface and produce tensionstresses. These waves result in spalling and scabbing at the edges of metals and reduce the tensile bondstrength of explosive welding. The most common method for solving this problem is cutting and sizing theedges. However, this is not possible when the two metal parts to be joined are of unequal surfaces (a smallplate to a large plate). This paper focuses on applying a new technique (Groove Method) for solving thestrength problem at the edges for obtaining uniform welding. In this way, experimental and numerical ana-lyses are performed to evaluate the Groove Method. The obtained results show the success and effectivenessof the groove method suggested in this paper.

Keywords: explosive welding; groove method; bond strength; numerical simulation.

SOUDAGE PAR EXPLOSION DE SURFACE INÉGALE UTILISANT LA MÉTHODE DESOUDAGE PAR RAINURE

RÉSUMÉL’adhérence des joints soudés est un facteur important du processus de soudage par explosion. Dans ce

procédé les ondes de stress produites par l’énergie explosive se propagent à la surface libre et produisent descontraintes de traction. Ces ondes entraînent l’écaillage et le croûtage sur les bords des métaux et réduisentla force d’adhérence de la soudure. La méthode la plus courante pour résoudre ce problème est la coupe etle dimensionnement des bords. Toutefois, cela n’est pas possible lorsque les deux pièces de métal à assem-bler sont de surfaces inégales (une petite plaque sur une plaque de plus grande taille). Cet article traite del’application d’une nouvelle technique, la méthode de soudage par rainure (Groove Method), pour résoudrele problème d’adhérence sur les bords afin d’obtenir une soudure uniforme. Des analyses expérimentale etnumérique sont effectuées pour évaluer la méthode de soudage par rainure. Les résultats obtenus montrentle succès et l’efficacité de cette méthode de soudage.

Mots-clés : soudage par explosion; méthode de soudage par rainure; force d’adhérence; simulationnumérique.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 36, No. 2, 2012 113

1. INTRODUCTION

Explosive welding process which can be employed to join most metal combinations, including mis-matched metals and those that cannot be welded by conventional methods, is now used extensively in manyindustries. In addition, the process can clad one or more different metal layers onto one or both faces of abase plate. In the common explosive welding process, an explosive detonation is used to push a flyer platetowards a base plate (see Fig. 1). Different metals can be bonded together by the high energy produced inoblique collisions at high velocity produced by an explosive charge. The impact velocity Vp, and collisionangle b , shown in Fig. 1, determine the pressure and shear stress at the collision point [1]. The combinedpressure and shearing create a jet which contains the surfaces of the two materials and brings them togetherto form a metallurgical bond. The pressure has to be high enough and last for a sufficient length of time tocomplete inter-atomic bonds. The velocity at the collision point Vc determines the time available for bond-ing. The quality of the bond depends on precise control of the process parameters, including material surfacepreparation, plate separation, explosive load, detonation power and the detonation velocity Vd . Selection ofparameters is based on the mechanical properties, density, and shear wave velocity of each component.

Fig. 1. A typical explosive welding process and its important parameters.

Some researchers [2] have considered the explosive welding method to be fundamentally a fusion weldingprocess which relies on the dissipation of the kinetic energy at the interface. Godunov et al. [1] describedexperiments designed to study the mechanism of initiation of waves. They explained that waves appear at acertain distance from the point of collision between the plates, and then develop stepwise until they attain asteady state after a small number of oscillations.

McKee and Crossland [3] mentioned observed abnormal behavior in their experiments when a step wascreated in the base plate surface (see Fig. 2). At low velocity the waves initiated before the step weremaintained by the step, and in other cases the waves before the step were completely damped out by the stepand there was no indication of reinitiating. At high velocity the step was shown to have a negligible effecton the waves. A number of authors have worked on the geometry and mechanism of the waves [3–9].

Fig. 2. Step in base plate to initiate waves [1].

114 Transactions of the Canadian Society for Mechanical Engineering, Vol. 36, No. 2, 2012

A stress wave mechanism of wave formation was proposed by El-Sobky and Blazynski [8]. The authorsexplained that waves were observed on the surfaces of metals that had been subjected to oblique collision,where neither welding nor jetting had occurred. They considered the problem of surface disturbances bothahead of and behind the collision point to be caused by successive interference from sound waves in bothplates as well as multi-layered welding. In this mechanism, the waves were generated in front of the collisionpoint, followed by several vortices. The wave formation mechanism was attributed by Plaksin et al. [9], tothe regular instabilities that were induced by oscillating detonation waves and that were transmitted throughthe interface of the impacting materials. Al-Hassani and Akbari-Mousavi [10–13] have performed variousnumerical analyses, simulations and experiments in the field of the mechanism of wavy interface formationin explosive welding.

The tensile bond strength (TBS) of explosive welded joints is very important parameter in the EXWprocess. The ultimate limit of strength is equal to the yield point of the weaker metal and even less becauseof residual stresses. The results must be uniformed in the entire welding area. It can be checked by tensionexperiments. This test also measures the shear strength. The chisel test or peel off test, which involves tryingto peel off the flyer plate, is a very simple yet important test. Other tests are NDT tests such as ultrasonic andradiography tests. Metallographic imaging is a useful method for identifying the nature of the bond usingSEM microscopy.

In the explosive welding process the pressure produced in the area of the detonation front of the explosivecharge is used to accelerate the flyer plate to a high velocity before it impacts the base plate. Reflections canbe reduced by using a suitable velocity of explosive charge and suitable regulation of welding. The pressureproduced in the detonation front is transmitted into the flyer plate as a stress wave. A shock wave is reflectedfrom a free surface as a tensile wave. The reflected tensile wave broadens out into a normal stress wave. Asthe wave passes the front surface of the plate, the surface is displaced by a distance equal to the product ofthe particle velocity and the duration of the wave. When these compression waves reach the back surface ofthe metal slab, they are reflected back as a tension wave and the velocity of particles is doubled.

This phenomenon reduces the tensile bond strength (TBS) in the edges, and it is considered to be a majorproblem. Shock waves reflected at the end of the plates produce spalls and scabs. These can be reducedby using a suitable velocity of explosive charge and suitable regulation of welding, but experiments show areduction in the strength of bonding in the edges, and formation of bonding in the middle of welded metals(see Fig. 3).

Fig. 3. Bonded areas measured ultrasonically in an Al-Cu joint [5].

Scab and spall problems can be solved by cutting the edges or making the plates larger. Both thesemethods are not economical and are used when both plates are of the same size and shape. Another methodis to make the flyer plate bigger than the base plate and then cut the joint after welding. However, thismethod cannot solve the problem when the flyer plate is smaller than base plate.


In this paper, a new method to remove the edge effect is suggested. Thin grooves near the edges of the flyerplate are considered to solve the problem. This is an improvement on the Crossland method presented in theFig. 2. In the new method there is no need to cut and size the plates to improve the tensile bond strength,because the loading of impact waves cuts off the edges beyond the grooves and thereby makes a uniformweld. In order to show the efficiency of the new method over the existing simple method, experimental testsand numerical analyses are performed. The results show the superiority of the proposed method.

2. EXPERIMENTAL PROCEDURE

In order to show the effectiveness of the groove method, two types of experimental tests are carried out.A simple and a grooved plate are considered as flyer plates in the first and second experiments, respectively.The simple method setup and the improved groove method setup proposed in this paper are shown in Fig. 4.Position of grooves on the flyer plate in the groove method is also clear in this figure. The arrangement forwelding of two different materials with different surface areas which is used in this paper is shown in Fig. 5.The sizes of flyer and base plates in the first and second experiments are presented in the Table 1.

Fig. 4. Simple setup (flyer plate without grooves) and Improved Groove setup (flyer plate with grooves).

According to this table, the area specified with the grooves on the grooved flyer plate is equal to thearea of the simple plate. Therefore, both tests resulted in the same sized joint as shown in Fig. 6. In theseexperiments, aluminum and copper are used for flyer and base plates, respectively. The explosive used isPETN with detonation velocity of 6600 m/s. A constant stand-off distance (8 mm) is considered in bothexperiments.

Test Method Flyer Plate Base Plate

Width Length Thickness Width Length Thickness [mm] [mm] [mm] [mm] [mm] [mm]

Simple Method 50 150 5 150 250 15 Groove Method 70 180 5 150 250 15

Table 1. Experiment parameters.

In order to evaluate the tensile bond strength, mechanical tests are performed. Eight pieces were cut fromeach plate along length and width direction and were subjected to tensile test. The results are shown inTables 2 and 3. After welding, it is not possible to improve the edge problem in the simple setup. In thegrooved setup however, the impact waves in the edges reflect from the grooves and the parts of the flyer


Fig. 5. Arrangements for welding of two different materials with unequal surface areas.

Fig. 6. The welded plates after explosion.


plates that lie outside the grooves fall off. This means that the region of weak metal to metal bonding thatlies outside the area enclosed by the groove falls off, and what remains is a uniformly bonded with highstrength.

L [mm]

σ (simple) [N/mm2]

σ (grooved) [N/mm2]

0 0 147 5 69 177

10 177 177 20 177 186 30 177 177 40 177 177 45 69 177 50 0 137

Table 2. Results of TBS in width direction.

L [mm]

σ (simple) [N/mm2]

σ (grooved) [N/mm2]

0 0 157 15 78 177 35 177 177 60 167 186 80 177 177

115 177 177 135 69 167 150 0 147

Table 3. Results of TBS in length direction.

The TBS of 16 pieces of the two welds which are presented in the Tables 2 and 3 are shown in Fig. 7.Tensile bond strengths were constant throughout the contact surface in the improved test with the groovedflyer plate. In the absence of the groove, TBS was very low in the edges and improved with distance fromthe edge, reaching a maximum in between 1t and 1.5t from the edge, where t is the plate thickness. Thegrooved plate had the same maximum strength throughout the joint, and had uniform edges. This experimentconfirms previous works where the step method has been used in the base plate, but instead used a groove inthe flyer plate. This is a novel and useful method joining two different metals with different surface areas.

3. MODELING AND SIMULATION

3.1. Material modelingThe material model is an important factor affecting the accuracy of results of a finite element simula-

tion. The Johnson-Cook constitutive model [14] duplicates several important material responses observedin impact of metals. The three key material responses are strain-rate effects, strain hardening, and thermal


(a) (b)

Fig. 7. TBS in width and length direction of flyer plates: (a) TBS in width direction, (b) TBS in length direction.

softening. These properties are combined in the Johnson-Cook constitutive model [14]:

s =hA+B(e pl)n

i1+C ln

✓ ˙e pl

e0

◆�✓1�✓

T �T0

Tmelt �T0

◆m◆, (1)

where ˙e pl is the equivalent plastic strain rate, e pl is the equivalent plastic strain, s is the equivalent stress,e0 is a reference strain rate, A, B, C, n, and m are material parameters.

The fracture model suggested by Johnson and Cook [15] takes into account the result of stress, tempera-ture and strain rate. Failure is assumed to occur when the damage parameter D exceeds unity. The damageparameter, D, is defined as follows:

D = Â

De pl

e plf

!, (2)

where e plf is the equivalent strain at failure, De pl is the increment of the equivalent plastic strain, and the

summation is performed over all increments of deformation. ˙e pl

e0is a dimension-less pressure-deviatory

stress ratio. The strain at failure e plf is assumed to be dependent on a non-dimensional plastic strain rate, sm

s(where sm is the mean stress). The strain is defined as follows:

e plf =

hD1 +D2 exp

⇣D3

sm

s

⌘i1+D4 ln

✓ ˙e pl

e0

◆�+

✓1+D5

✓T �T0

Tmelt �T0

◆◆, (3)

where D1 �D5 are material constants.For the explosive, the Jones-Wilkins-Lee [16] equation of state was selected to indicate the expansion of

the explosive materials. The JWL equation of state defines pressure as function of internal energy per initialvolume, E, and specific volume (inverse of density), V , as [16]:

P = A✓

1� wR1V

◆e�R1V +B

✓1� w

R1V

◆e�R2V +

wEV

, (4)

where P is the pressure, E is the internal energy, V is the specific volume, w is the Gruneisen parameter,and A, B, R1 and R2 are constants which satisfy the mass, momentum, and energy saving equations. Thematerial parameters for the model in this study are presented in Table 4.


Material Behaviour Material Parts No. Aluminium Copper Explosive Sand

1 Johnson-Cook Damage

D1 .112 −0.191 D2 .123 5.91 D3 1.5 4.73 D4 .007 −0.052 D5 0 0

T melt [k] 925 0 T transition [k] 297 0

2 Damage Evolution Type Energy Energy Softening Exponential Exponential

Degradation Maximum Maximum Fracture energy 500 MPa 500 MPa

3 Density [kg/cm3] 2700 8940 880 8000

4 Elastic Isotropic scale : Long-term for Viscoelasticity

Young’s Modulus [GPa]

69 117 - 318

Poisson Ratio .3 .3 .3

5 Plastic Hardening Johnson-Cook

[MPa] A 369 95 [MPa] B 684 280

n .73 0.35 m 1.7 0

0ε 1 1

C .011 .02

6 Explosive Model Detonation

velocity [m/s]

5170 A [GPa] 348

Type: JWL B [GPa] 11.28 ω 0.24 R1 7 R2 2 Pre-detonation

Bulk modulus 0

Table 4. Input material parameters for the models in Abaqus.


3.2. Numerical modelingThe finite element model for improved groove setup is shown in Fig. 8. The model is constructed of three

parts: part 1: base plate of copper material; part 2: flyer plate of aluminum material and part 3: explosivematerial. Eight-node linear brick reduced integration C3D8R [17] elements are used in Abaqus/Explicit fordiscretization of all parts. Number of nodes and elements for each part are presented in the Table 5. In theareas close to the collision zones, a mesh size of about 1 mm was used, whereas in other areas the meshwas made larger (about 5 times). Therefore in part 1 at the contact point the mesh was refined ( 1

5 ). Theconstitutive materials of the flyer and base plates were aluminum and copper, respectively. The Johnson-Cook plastic strain hardening and failure criteria parameters used in this study were presented in materialmodeling section. The JWL EOS [16] was used for the explosive. In the ABAQUS code, energy andmomentum transfer occurred through contact surfaces between the base of the explosive and the uppersurface of the flyer plate and between the lower surface of the flyer plate and the upper surface of thebase plate. The lower surface of the base plate was fixed in a direction perpendicular to its surface. Theinteractions were specified by appropriate contact algorithms. The loading is due to the detonation of thePETN explosive.

Fig. 8. Mesh design of the model in Abaqus.

Parts No. Elements No. Nodes Simple Grooved Simple Grooved

Base plate 61776 61776 70310 70310 Flyer plate 15300 40756 21204 52326 Explosive 204 340 490 770

Table 5. Input material parameters for the models in Abaqus.

Bulk viscosity introduces damping associated with volumetric straining. Its purpose is to improve theModeling of high-speed dynamic events [17]. Abaqus/Explicit contains two forms of bulk viscosity: linearand quadratic. Linear bulk viscosity is included by default in an Abaqus/Explicit analysis. Linear bulkviscosity parameter and quadratic bulk viscosity parameters were chosen .06 and 1.2 respectively. Frictionwas also included in the modeling with a value of 0.3. Comprehensive descriptions of the modeling can befound in [18].

3.3. Simulation resultsThe simulations provided graphical output showing contour maps and profiles of a number of physical

parameters, such as contact pressure, shear stress, normal stress, plastic strain, effective strain, strain rate,internal energy, kinetic energy and velocity of the flyer plate at the point of contact and the angle of contact.For the grooved flyer plate, when the collision takes place, the damage parameter D (Eq. 2) exceeds unity inthe elements located on the grooves and the edges cut off beyond the groove. Figure 9 shows the Johnson-


Cook damage initiation criterion at integration points after explosion. It is clear from this figure that thedamage criterion has high values on the grooves. Figure 10 shows the result of shear contour in flyer plate

Fig. 9. Johnson-Cook damage initiation criterion at integration points after explosion.

at the time of collision. It is now desired to compare the bonding quality between the simple method andimproved groove method. Akbari Mousavi et al. [13] proposed two criteria for successful bonding in theexplosive welding process. These criteria which are proposed for numerical simulation are as below:

1-Plastic strain criteria:A threshold value of effective strain for bonding to take place should exist for different combinations ofmaterials. It appeared that an effective strain higher than 0.35 was required for bonding of stainless steelto steel plates. For titanium to mild steel it was slightly lower at 0.25. The plastic strain was lower for thenon-bonded case than that for the bonded case.

2-Shear stress criteria:The shear stresses in the two plates would have different sign (i.e. in opposite direction) magnitudes forbonding to occur whereas the shear stresses would have the same sign if bonding did not occur.

Fig. 10. Shear stress contour in the flyer plate.


Figure 11 shows the effective strain profiles in flyer at 0.06 m from the edge of the plate for simple andgroove method. In the case of simple method, the strain levels were highest in the areas in which weldingtook place. The figure suggests that the lowest value of effective strain required for bonding to take place isof the order of 0.1. The simulations predicted a lower value of effective strain for the non-bonded edges. Inthe case of improved groove method, the effective strain profile in flyer at 0.06 m from the edge shows thatthe strain levels were higher than 0.1 in the all of area.

Shear stress profiles are plotted in Figs. 12 and 13 for the simple method and improved groove method,respectively. According to the Fig. 12, in the edges where bonding did not take place, the stresses are of thesame sign. Whereas Fig. 13 shows that the shear stress in the flyer plate and base plate were of opposite signin the all regions in the case of improved groove method. According to the Figs. 11 and 13 the successfulbonding takes place between flyer and base plate using improved groove method.

Fig. 11. Effective plastic strain profiles in the simple and groove method.

Fig. 12. Shear stress profiles for flyer and base plates in the simple method.


Fig. 13. Shear stress profiles for flyer and base plates in the groove method.

4. CONCLUSIONS

A new method in explosive welding of unequal surfaces was presented. In this way making groove onthe flyer plate was suggested. The influence of this method was shown using experimental and numericalanalysis. Experimental and numerical results showed the effectiveness of the groove method. Comparisonof the numerical and experimental results revealed the accuracy of criteria formerly suggested by AkbariMousavi et al.

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Explosive welding of unequal surface using groove method