**2.1 Plastic deformation in explosive welding process**

*Material Flow Analysis*

shipbuilding application [18].

**2. Working principle of explosive welding**

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 [15]. There are various applications of explosive welding products such as in cryogenic pressure vessels [16], scram jet engine components [17],

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

*Schematic diagram of explosive welding process in parallel set-up, a) initial set-up, b) after the explosion has* 

**26**

**Figure 1.**

*initiated.*

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 [19–21]. 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 [22–24]. 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 [25]. Occurrence of high plastic deformation of the mating surfaces lead to grain refinement [26]. Difference in grain size adjacent to weld interface is observed due to severe plastic deformation [27]. 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 [28–30].
