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Explosion-welding:Know how to weld Clad Plates.SOLUTIONS with Effective, Practical Advice
Explosion-welding is a solid state process. It provides metallurgical bond between two bodies, by using controlled explosion to generate high speed impact. A short note on Explosion welding was published (2) in Issue 9 of Practical Welding Letter for May 2004. Click on PWL#09 to see it. In Explosion-welding considerable heat is generated in the explosive detonation. Explosion creates the propelling force, but there is no time for heat transfer. Therefore no appreciable temperature increase of the metal bodies occurs and The result is an ideal metal to metal bond without melting or diffusion. Therefore a truly cold joint is formed without heat affected zone. There is almost no reduction of the mechanical properties exhibited by the materials before Explosion-welding. In fact hardness increases and ductility decreases somewhat because of the severe localized plastic flow at the interface. Also the thickness of the original materials is reduced and the other dimensions increase a little. However this does not matter, as most of the plates are re-rolled to final dimensions.
As a consequence, joining of dissimilar metals becomes feasible even for those difficult material combinations.
Those are not suitable for fusion welding, because of widely differing melting points and formation of brittle intermetallic compounds. Typical Advantages of Explosion-welding are:
Limitations of Explosion-welding are:
One of the most common uses of Explosion-welding is the application of a relatively thin cladding on a thick base. The covering is some material providing special characteristics (like outstanding resistance to corrosion, or wear, or heat). The base metal, generally less expensive, is designed to provide a suitable support for engineering applications. The combination of a relatively inexpensive support and an expensive, specialized cladding provides a cost effective solution. Especially for those cases where making the parts with monolithic (one piece) material would result in a prohibitively costly solution. Another use is for manufacturing of transition pieces of dissimilar materials. Those permit the subsequent regular welding of additional components of the materials by common welding processes. Two surfaces to be joined by Explosion-welding are initially spaced at a small stand-off distance that typically varies from 0.5 to 4 times the thickness of the cladding sheet. The material to be clad must exhibit sufficient ductility and fracture toughness to stand the mechanical deformation needed to perform Explosion-welding without being shattered. Usual limits are minimum elongation of 15 % and notch toughness value of 30 J at process temperature. Explosion-welding deforms the metals at very high rates (104 up to 105 per sec). The usual setup includes a massive plate or anvil supporting the base plate. The cladding material, called clad or flyer plate, is covered with the chemical explosive, in uniformly distributed granular form or as a plastic sheet. It is held parallel or at an angle to the base metal at the correct standoff distance, that is necessary to develop the speed needed at the impact. Explosion-welding is produced as the cladding material is accelerated at high speed against the static base metal. The explosive material is ignited at a predetermined point on the plate surface using a high velocity explosive booster. The explosive force impacts the two surfaces together, progressively, at the collision front. The detonation travels away from the initiation point and across the plate surface at the specified detonation speed. The gas expansion of the explosive detonation accelerates the cladding plate across the standoff gap, resulting in an angular collision at the specified collision velocity. Upon progressive detonation of the explosive, from the starting edge to the opposite one, the deformation force runs at high speed across the whole surface of the cladding material. This force causes the clad metal to form a moving bend that collides with the base metal at a rapidly sweeping line, made of all the collision points in every section of the plates. The resultant impact creates very high localized pressures at the collision points. The interfacial pressure at the collision front must exceed the yield strength of the materials, so that plastic deformation will occur. These pressure waves travel away from the collision point at the acoustic velocity of the metals. Since the collision line is moving forward at a lower, subsonic rate, pressures are created at the immediately approaching adjacent surfaces, forming a jet of metal just ahead of the collision front. The jet, included within the two component surfaces, is sufficient to spall a thin layer of metal from each surface and eject it away from the interface. Surface contaminants, oxides and impurities are stripped away in the jet. At the collision point, the newly created clean metal surfaces impact at very high pressures. The so cleaned surfaces meet and become welded under the powerful pressures of the explosion, sufficient to squeeze the metals to behave like viscous fluids. This behavior creates the characteristic wave pattern bond line as seen in cross section examination under a microscope. The geometry and the characteristics of the wavy line depend upon the materials and the parameters of the process. The processing parameters include:
The explosive detonation velocity is an independent variable selected to achieve the required impact conditions. The flyer plate approach velocity at the impact point, is typically in the speed range of 250 to 500 m/s (800 to 1600 ft/s). The collision front's velocity of 1500 to 3500 m/s (4900 to 11000 ft/s) must be lower than the speed of sound in the materials, so that the shock wave precedes the bond being formed. A collision angle between 5 to 25 degrees is usually selected. Explosion-welding is done on the following Base Metals: Carbon and Low Alloy Steels, Austenitic Stainless Steels and Aluminum. The following Cladding Materials are applied: Stainless steels of all kinds, Nickel and its alloys, Copper and its alloys, Aluminum and its alloys, Titanium, Zirconium, Tantalum. For fabricating and welding assemblies made with explosion welded bimetal plates, careful precautions must be adopted. For any material combinations, the different characteristics of both materials must be taken into account. A short note on Titanium Clad Steel was published (11) in Issue 23 of Practical Welding Letter for July 2005. Click on PWL#023 to see it. An Article on Welding Clad Steel was published (7) in Issue 63 of Practical Welding Letter for November 2008. An Article on VFAW - A new welding technique to watch was published (7) in Issue 149 of Practical Welding Letter for January 2016.
Find further details on the process in: AWS WHB-3.9
It is possible to purchase only the required Chapter: 9: Explosion Welding - WHC3.09
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