Exploring the Fundamentals of Explosion Welding

【Case Introduction】

In 1944, Carl in the United States accidentally discovered during an explosive test that two brass thin circular plates, each with a diameter of about 1 inch and a thickness of 0.035 inches, were solidly welded together due to the sudden impact of the explosion. In 1957, an American named Philip Jack successfully achieved the explosive welding of aluminum and steel.

Since then, through continuous efforts of welders, explosive welding technology has been widely used in industries such as petrochemicals and shipbuilding.

Principles and Characteristics of Explosive Welding

Explosive Welding (EW) uses explosives as an energy source and harnesses the impact force generated during the explosion to cause violent collisions, plastic deformation, melting, and interatomic diffusion on the workpiece, thereby achieving a connection through a pressure welding method.

The weld seam is formed between two or more layers of the same or different metal materials in a brief explosion process that lasts only a few milliseconds.

Welding Process

Explosive welding is a dynamic welding process. Figures 8-1 and 8-2 show schematic diagrams of a typical explosive welding process. During explosive welding, the explosive, detonator, and workpiece are first installed, and then the explosive is detonated by the detonator. The explosion of the explosive moves at a constant speed (generally 1500~3500m/s) from left to right.

The chemical energy released by the explosive during the instant of the explosion generates a pressure of up to 700MPa, brief local temperatures of up to 3000°C, and a shock wave with a speed of 500~1000m/s. This shock wave impacts the workpiece and causes a violent collision.

Under the impact, a metal jet is generated in front of the contact point of the base plate and cladding, forming a jet stream. Its scouring action removes impurities and dirt from the surface of the workpiece, allowing clean metal surfaces to contact each other.

During the process of plastic deformation of pure metal on both sides of the interface, the impact kinetic energy is converted into heat energy, causing the thin layer of metal near the interface to heat up and melt. Under high temperature and high pressure, the metal atoms within this thin layer diffuse into each other, forming metal bonds. After cooling, a solid joint is formed.

Figure 8-1 Schematic Diagram of the Explosive Welding Process (Parallel Method)

a) Base Plate and Cladding
b) Pre-weld Installation
c) An Instant During the Explosion Process
d) Completed Weld

  • 1-Base Plate
  • 2-Cladding
  • 3-Base
  • 4-Buffer Protection Layer
  • 5-Explosive
  • 6-Detonator
  • β-Collision Angle
  • S-Collision Point
  • VD-Explosive Detonation Speed
  • VP-Cladding Speed
  • VCP-Collision Point Speed
  • h-Gap
Figure 8-2 Schematic Diagram of Angle Explosive Welding Process

a) Before the Explosion b) After the Explosion

  • α-Installation Angle
  • β-Collision Angle
  • γ-Bend Angle
  • S-Collision Point
  • VD-Explosive Detonation Speed
  • VP-Cladding Speed
  • VCP-Collision Point Speed

Characteristics of Interface Bonding

Based on different metallurgical bonding forms between two metal materials under various explosive welding conditions, explosive welding can be divided into the following types:

(1) Straight-line Bonding

The characteristic of this type of bonding is that a straight, clear bonding line can be seen on the interface, with the base metal directly in contact and bonded, without any noticeable plastic deformation or melting microstructures.

The main reason for this bonding characteristic is a low impact speed. This type of explosive welding is rarely used in production because it is very sensitive to changes in welding parameters, leading to unstable joint quality and defects like non-fusion.

(2) Wavy Bonding

When the impact speed is above a certain critical value, the joint’s bonding area presents a regular continuous wavy shape as shown in Figure 8-3, with large or small discontinuous vortex areas formed on the interface. The vortex is made up of melted material, also known as a melting groove, exhibiting a cast structure.

Figure 8-3 Features of the Explosive Welding Joint Area

a) Wave-shaped Joint Area
b) Continuous Melting Layer Joint Area
c) Mixed Type Joint Area

The composition of the front vortex is mainly base plate, and the back vortex is mainly cladding. If the material in the vortex forms a solid solution, it is ductile; if it forms intermetallic compounds, its brittleness increases. A good welding joint surface should consist of uniformly small waves, with isolated melting grooves.

(3) Continuous Melting Layer Bonding

When the interface impact speed and angle are too large, large vortices are produced, even forming a continuous melting layer. If this large vortex or melting layer is a solid solution, it generally does not damage the joint strength.

However, if brittle intermetallic compounds are formed, the joint will become brittle, and it often contains a large number of shrinkage holes and other defects, so welding operations that can form a continuous melting layer bonding must be avoided.

Characteristics of Explosive Welding

(1) Advantages of Explosive Welding

1) Explosive welding can form a high-strength metallurgical bonding weld between the same or different metals. For instance, connections between Ta, Zr, Al, Ti, Pb, and non-alloy steel, alloy steel, stainless steel, which are difficult to achieve with other welding methods, can be easily realized with explosive welding.

This is mainly because explosive welding reduces the brittle compound layer to a minimum or even eliminates the brittle metal layer.

2) It can weld a wide range of parts with a weldable area range from 13 to 28m2. In explosive welding, if the base plate is stationary, its thickness is unrestricted; the cladding thickness ranges from 0.03 to 32mm, achieving a high cladding ratio.

3) It can perform welding of double-layer and multi-layer composite plates, and it can also be used for butt, overlap welding seams, and spot welding of various metals.

4) The explosive welding process is relatively simple, does not require complex equipment, has abundant energy, requires less investment, and is easy to apply.

5) There is no need for filler metal, and the structure design uses composite plates, which can save precious scarce metals.

6) The welding surface does not require complex cleaning, just remove thicker oxides, scale, and oil stains.

Explosive composite materials have varying degrees of hardening and strengthening, known as “explosive hardening” and “explosive strengthening”. The hardening of the cladding material contributes to its corrosion resistance and wear resistance, and strengthening is beneficial to the strength design of these materials to a certain extent.

(2) Disadvantages of Explosive Welding

1) The metal materials to be welded must have sufficient toughness and impact resistance to withstand the impact of the explosion and intense collision. High-strength alloys with a yield strength greater than 690MPa are difficult to explosively weld.

2) During explosive welding, the high-speed jet between the metals being welded shoots out in a straight line, so it is generally only used for the welding of planar or cylindrical structures, such as the welding of plates, tubular components, and tubes to plates.

The welding of complex-shaped components is limited. At the same time, the thickness of the cladding cannot be too thick, and the thickness of the base layer cannot be too thin. If welding is performed on a thin plate, additional support is needed.

3) Most explosive welding is performed outdoors, with a low degree of mechanization and poor working conditions, and it’s easily affected by weather conditions.

4) The noise and blast wave produced during explosive welding have a certain impact on the surrounding environment. Although it can be performed underwater, in a vacuum, or buried in sand, these methods would increase the cost.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top