Welding with a Bang
There are a number of ways to join two metals together. Most of these involve some form of welding or brazing with many variations in these techniques. One of the welding categories is solid-state welding, and one of the eight solid-state welding techniques – explosion welding (EXW) – meets a unique metal-joining need.
As a welding technique, explosion welding is a relative newcomer. Its origins go back to World War I when it was observed that shrapnel was sticking to armor plate. Since heat was not applied, the conclusion was that the explosive forces caused the welding, and this theory was tested and duplicated in the years following WWII.
In 1962, DuPont applied for a patent on the explosion-welding process, which was granted in 1964. Much of the early work identified the ability to bond different types of metal with very different crystal structures. This unique ability has led to most of the current applications of EXW.
An early application of this technology was the tri-layer bonded material – copper sandwiched between copper-nickel – used for coinage in 1965. For this application, 6-inch billets about the size of a desktop were explosion welded and then hot rolled. In the following three years, DuPont produced 70 million pounds of blanks for dimes, quarters and half dollars that had previously been made from silver. When roll bonding of strip came into use, EXW was no longer a competitive way to produce the coin stock.
During the explosion, an energetic jet forms between the two metals that cleans the surfaces. Although heat is not applied in making an explosion weld, investigation shows that the interface metal is molten during the EXW process. This interface melt line is only 0.05-0.2 microns thick and cannot be seen with an optical microscope. Consequently, the generated heat does not affect the material properties. The strength of the EXW joint has been found to be equal or greater than the strength of the weaker of the two joined metals.
The localized heat necessary to form the weld is generated from the shock wave associated with the collision. Heat is also released by the plastic deformation (think friction) at the interface. The EXW process is self-contained and portable, and it can occur over large areas quickly.
The ability to join large surfaces in short order has led to many of the applications of this technology. It is typically used for joining large flat surfaces. Large plates – some up to 100,000 pounds and more than 40 feet in length – are produced this way. These largest plates are produced for oil-refinery equipment, power generation and chemical-equipment manufacturers.
The largest applications and most exotic material combinations are the manufacturing areas best suited to EXW. In the 1980s, the dominant products were titanium and aluminum clads. In the mid-1980s, clad plates were made for the first zirconium-clad acetic acid reactors. This has developed into a major application for EXW products. A large titanium-steel clad-plate application to produce an autoclave about 14 feet in diameter and almost 100 feet long is one of “the most successful large titanium-clad jobs ever.” Some of the smaller applications include ultra-high-vacuum joints between aluminum, copper and stainless steel; corrosion-resistant claddings on mild steel; and alloy aluminum joined to low-expansion-rate metals for electronics.
By virtue of the explosive nature of the process, companies perfoming EXW have experienced challenges in finding the best location. Presently, the largest EXW manufacturer uses a limestone mine in Pennsylvania as the site of its explosions. The 70-foot-thick limestone with a 60-foot tall and wide entrance makes the perfect location.
While being somewhat limited by its manufacturing technique – who wants an EXW facility next door – the flexibility in material size, variety of metal combinations and quick delivery are all areas in which explosion welding leads the way. Now you know a little more about how thermal processing plays a role even when applied heat is not part of the process. IH