Different explosives used in the explosive welding process.
Explosive welding is a solid-state process, which is an advanced form of joining two metal plates with dissimilar metallurgical properties, irrespective of the differences in physical and chemical properties. In this process, high pressure of explosive is used to accelerate one metal plate over another to form the bimetallic product. The pressure needs to be sufficiently high and for enough length of time to achieve inter-atomic bonds. During the explosive welding process, a jetting phenomenon occurs at the collision point which cleans the top oxide layer over metals and leaves the virgin surfaces that help in the joining process. The metals are joined without losing their pre-bonded properties with higher bond strengths than the strength of the weaker parent material. There are various critical factors such as explosive type, mass of explosive, stand-off distance, type of plate material, velocity of detonation etc. which affect the bond quality. Researchers mainly play with all these parameters to bring out the best characteristics of the bimetallic product that can be used for the desired applications such as heat exchanger, pressure vessels etc.
- bond strength
- dissimilar materials
- detonation velocity
- explosive welding
- metallurgical properties
Welding is a process of joining two materials together through pressure, heat and sometimes with the addition of filler materials. The important condition for any welding technique is that the two surfaces that need to be joined should be cleaned and uncontaminated. Moreover, if the two surfaces are brought together in such a way that the surfaces exchange the outer orbit of the valence electrons and form interatomic bonds, the weld formed will be very strong in terms of mechanical properties. But this kind of bond formation is not possible through conventional means. In most of the welding techniques melting is involved in joining the two components. There are also some welding processes such as solid-state welding processes where heat required is below the melting point of the base material being welded and therefore, no melting is observed during joining for example ultrasonic welding [1, 2], friction welding [3, 4, 5], cold welding , explosive welding [7, 8, 9, 10] and diffusion welding [11, 12]. All of the welding methods have some advantages and disadvantages in their particular field and are applied as per the need of the applications. In the current world, there is an increasing trend of using dissimilar material combinations for various applications such as automobile, shipbuilding, military, aerospace and oil industries etc. The bi-metallic product takes the mechanical advantage of both the materials such as wear resistance, corrosion resistance, high tensile strength and lightweight. To meet such requirements many researchers are extensively working in this field to produce such combinations. In which explosive welding is considered as one of the potential welding technique and is gaining more attention due to its vast features as mentioned [13, 14]. Explosive welding is one of the solid-state welding processes in which explosive energy is used to create a high-velocity impact collision between the two plates to be joined. The process can join a wide area of non-compatible material combination irrespective of the difference in mechanical and chemical properties and which cannot be joined by any other conventional means. It is a surface bond welding, which provides a strong metallurgical bond at the molecular level and provides strength higher than the base materials . There are various applications of explosive welding products such as in cryogenic pressure vessels , scram jet engine components , shipbuilding application .
2. Working principle of explosive welding
In the explosive welding process, the explosive is used as a source of energy to accelerate one of the metal plates into another. Figure 1a shows the initial set-up of the explosive welding process showing the two plates i.e. base plate which is kept stationary and the movable flyer plate is kept at a particular calculated distance called stand-off distance. The explosive box is placed with a buffer sheet over the plates. This buffer sheet protects the flyer plate from damage due to explosion. To initiate the main explosive detonator is used, which is placed above explosive. Figure 1b shows the schematic diagram after the detonation of explosive has initiated in the explosive welding process. Here we can observe the collision point, where the two plates collide and the bond formation occurs. Along with this jetting phenomenon is witnessed which is one of the most important criteria and also an essential condition for bond formation. Jetting occurs during an oblique collision at the collision point, in which it cleans the mating surfaces and Leaves behind a virgin surface free from oxide layers and contaminants. This helps to interact two mating materials at the atomic level when subjected to high impact pressure waves arising from the explosion effects. This process is capable of joining large surface area due to its ability to distribute high energy density. Explosive welding can be basically defined in two steps; first jet phenomenon occurs and cleans up the oxide layers and second, the high impact pressure forces the mating surfaces into such intimate contact that they meet at the interatomic level and results in strong metallurgical bond.
2.1 Plastic deformation in explosive welding process
In explosive welding process due to detonation effect of explosive many critical phenomena occur such as release of large gas product i.e. explosion, high impact collision between mating surfaces, high temperature, generation of heat, plastic deformation in the metal plates, pressure generation, jetting and bonding occurs for a very short period of time i.e. microseconds [19, 20, 21]. Out of these, plastic deformation that occurs at the weld interface due to high impact pressure is considered as one of the important factor responsible for good bond formation. Pastic deformation in explosive welding process occurs when pressure at the collision front overcomes the yield strength of the materials. Through plastic deformation an intimate contact is formed where the two mating surfaces are brought too close together that atomic reaction occurs between the mating surfaces [22, 23, 24]. Plastic deformation can be examined using visioplastic methods without disturbing the original properties of materials. The most distinctive form of plastic deformation is the wave formation in explosive welding . Occurrence of high plastic deformation of the mating surfaces lead to grain refinement . Difference in grain size adjacent to weld interface is observed due to severe plastic deformation . Various researchers have witnessed high hardness value at the weld interface of explosively welded specimens in microhardness examination study. It was mainly attributed to intense plastic deformation developed across the weld interface. The level of plastic deformation in explosively welded specimens decrease gradually with increase in distance from the weld interface [28, 29, 30].
2.2 Types of experimental set-up
There are two types of explosive welding set-up i.e. parallel and the inclined set-up . Figure 1 shows the parallel set-up where the two plates are placed parallel to each other. This kind of configuration is used for joining large and thick plates. While the inclined set-up is shown in Figure 2 in which flyer plates are inclined at a particular angle (α). This kind of configuration is generally applied for joining small and thin plates.
2.3 Terminology used in explosive welding
3. Different parameters affecting the explosive welded products
There are various parameters which influence the final product of the explosive welding process. Therefore, careful control of welding parameters is very critical. The criteria for selection of the welding parameters depends upon the mechanical properties of the matting surfaces [32, 33]. Many researchers change the magnitude of these parameters by playing with the different parameters such as detonation velocity, stand-off distance, explosive type etc. The various process parameters are discussed below.
|Al alloy -Al alloy||Tube||PETN|||
|Steel-steel plates||Parallel set-up||Elbar-5||[56, 57, 58]|
|Copper-copper alloy||Parallel set-up||Powder emulsion explosive|||
|Dissimilar materials combinations|
|Titanium and magnesium alloy AZ31||Inclined set-up (Under water)|||
|Aluminum to stainless steel||Parallel set-up||Cu, Ti & Ta|||
|C103 niobium alloy and C263 nimonic alloy||Parallel set-up||Not used|||
|Titanium and aluminum||Parallel set-up||Not used||[61, 62]|
|Aluminum and copper||Parallel set-up||Al5052, Cu & SS304|||
|Sn and Cu||Inclined set-up (Under water)||Not used|||
|Al and Mg alloy||Parallel set-up||Not used||[64, 65]|
|Aluminum and carbon steel and Aluminum-stainless steel||Parallel set-up||Aluminum AA1050|||
|Aluminum and copper||Parallel set-up||Not used||[67, 68, 69]|
|Aluminum and steel||Parallel set-up||Not used||[70, 71, 72, 73]|
3.1 Weldability window
The condition that should satisfy for proper bonding to take place is defined by weldability window. Detailed view with the description of weldability window is shown in Figure 3. It is plotted between collision angle (β) and collision velocity (Vp), where it is well defined by four different lines [50, 51]. The first limit is placed at the rightmost side in which formation of the jet at the collision point is considered. As jetting is one of the important criteria in explosive welding. Abrahamson  linked welding velocity with the collision angle β as shown in Eq. 1 for the first limit. The second limit is placed at the left side of the weldability window which is related to the formation of wavy morphology at the weld interface. Cowan et al. introduced Reynolds number for describing the laminar and turbulent flow  as shown in Eq. 2. The third limit is related to the minimum flyer plate velocity (Vpmin) which ensure that the impact pressure developed at the collision point exceeds the yield strength of the materials. Lower boundary equation was developed for third limit as shown in Eq. 3. While the fourth limit corresponds to the maximum flyer plate velocity (Vpmax) which maintains the required impact pressure below the value so that the melting does not occur at the weld interface. To avoid melting Eq. 4 was developed by Wittman . Therefore, in order to obtain good bond, selection of welding parameters should be with in the described limits of weldability window [20, 34, 50, 54].
Where : ρ Density of flyer plate and base plate
: Hardness value of flyer plate and base plate
Where : Vickers hardness no.
Ρ: Density of the material
K: Constant value
Take value 0.6: Plate surface is very clean
1.2: Imperfectly cleaned plate surface
Where Tm: Melting temperature,
Cp: Specific heat capacity,
K: Thermal conductivity,
h: Thickness of flyer plate,
Cb: Bulk sound speed
3.2 Different materials combination joined by explosive welding
Explosive welding process is capable of joining similar and dissimilar material combinations irrespective of the difference in physical and chemical properties. The various material combinations joined using explosive welding process i.e. similar and dissimiliar combinations are given in Table 2. In this process, different authors have also used the concept of the interlayer to minimize the kinetic energy loss as well as the formation of meting zone at the weld interface.
3.3 Important points in explosive welding process
Following points should be considered for explosive welding process to produce a strong metallurgical bond.
The pressure generated at the collision point should be enough in magnitude so as to exceed the dynamic elastic limits of the mating materials in order to ensure that deformation has occurred at the weld interface .
Stand-off distance should be calculated properly to ensure that the flyer plate can accelerate to the required impact velocity needed for good bonding. Use of high stand-off distance will result in edge instability and can also affect the bonding quality [74, 75].
The explosive used should provide sufficient energy in order to accelerate the flyer plate to the preferred velocity. The high detonation velocity of explosive should also be avoided as it can lead to spalling and damage of the joining materials. Therefore, the velocity of detonation must be less than 120% of the sonic velocity of the materials being welded [76, 77].
Flyer plate velocity (Vp) and collision velocity (Vc) should be less than the velocity of sound in either of the participant material. In order that the reflected stress waves do not interfere with the incident wave at the collision point [19, 78, 79].
Explosive welding is a solid-state welding process capable of joining any material combination which cannot be joined by any other conventional means. It can join materials irrespective of the difference in chemical and physical properties.
Jetting is one of the important criteria in explosive welding process which removes the oxide layers present at the mating surfaces. This jetting freely exit at the corners of the joint if the welding parameters are selected properly else if it gets trapped will result in the defects.
Plastic deformation is caused due to high impact pressure and is considered as one of the important condition for joint formation in explosive welding process. Plastic deformation leads to the intimate contact of the two mating surfaces and results in strong metallurgical bond formation. It is responsible for grain refinement as well as increase in hardness value across the weld interface of explosively welded samples.
To obtain a good bond, various welding parameters such as type of explosive, stand-off distance, flyer plate velocity, and collision velocity need to be selected very carefully. As these parameters will directly or indirectly affect the product of the weld.
During explosive welding, there are various defects which are uncounted especially intermetallic formation at the weld interface. To minimize these defects researchers are using different approaches such as interlayer concept and low velocity of detonation explosives which will reduce the kinetic energy loss at the collision point.
In the explosive welding process, we can join two materials and take the mechanical advantage of both the materials in the final product. Due to its enormous advantages, it has great application in the field of aerospace, automobiles, oil industries, defense and ship industries.
I would like to deeply acknowledge Dr. Pal Dinesh Kumar, Scientist-F, Joint director (MEMWD), Terminal Ballistics Research Laboratory (TBRL-DRDO) and Dr. Sachin Tyagi, Sr. Scientist, Central Scientific Instruments Organization (CSIO-CSIR), Chandigarh, India for their motivation and guidance during the whole journey.
Conflict of interest
The authors declare no conflict of interest.