New developments in advanced welding

Weld characterization

The explosion welded interface typically exhibits a wavy morphology. When explosion welding paramenters are correctly selected, there is minimal evidence of melting along the back and crest of the wave. There is typically a swirl of material in the break of the wave which frequently contains solidified melt. Explosive energy and detonation rate have significant effect upon bond morphology. At low explosion detonation rates, the bond is typically flat. As the detonation rate is increased, the bond transitions from flat to wavy, and then at higher rates exhibits large waves and excessive swirling.3 Explosion welding parameters and bond morphology have been shown to have a relationship which can be described in a manner similar to a Reynolds number. Bond zones will typically show no indication of melt layers when examined at the upper limits of present day optical microscopy. Further, there is no evidence of metal mixing or diffusion when examined at the current limits of commercial scanning electron microscopy. The bond appears to truly be a cold weld.

Transmission electron microscopy has been used to study the bond at a significantly higher magnification. At least three research groups have presented results from limited TEM studies of bond morphology.17-19 All three studies

found evidence of what appears to be prior molten metal in a region of 0.05 to 0.2 mm thick at the interface. This region exhibits mixing of the two metal types, but does not exhibit stable crystallographic or solidification structures. In the as-welded condition it appears to have a metastable amorphous atomic structure. The three studies have hypothesized that a thin layer of metal at the collision point has been heated well above the melting points and then resolidified at an extremely high cooling rate, in the range of 1 x 10-5K/s. Under these conditions, there is insufficient time at a given temperature for steady state structures to form. If the interface is reheated to temperatures at which stable microstructures and intermetallics can form, the bondline gradually develops all of the thermally stable features that are observed in a steady state phase diagram between the component metals. Figure 8.6 presents a schematic of the as-welded bond zone region. These studies suggest the EXW weld is more realistically described as a hypercooled, micro-fusion weld. The cold weld characteristics of the interface result from the rapidity of the process, combined with highly localized melting and fusion.

8.6 Conclusions

EXW is a robust, well-developed welding technology. Its primary application is in the manufacture of clad plates and speciality products that are derived from clad plates, such as welding transition joints. The cold welding features of EXW provide unique capabilities for joining a large range of metals where traditional fusion welding technologies cannot be applied. Very high magnification analyses of explosion welds suggest that the unique interface conditions result from a hypercooled, microfusion weld. The cold welding characteristics of EXW are attributed to the rapidity of the heating and cooling rates, combined with highly localized melting and fusion.

8.7 References

1. Blazynski T. Z., Explosive Welding, Forming, and Compaction, Applied Scientific Publishers Ltd., Essex, UK, 1983

2. Holtzman A. H. and Cowan G. R., ‘Bonding of metals with explosives’, Welding Research Council Bulletin, 1965, 104, April

3. Pocalyko A., ‘Explosively clad metals’ Encyclopedia of Chemical Technology, Vol 15, 3rd ed., John Wiley & Sons, 1981, 275-96

4. Banker J. G. and Reineke E. G., ‘Explosion welding’, ASM Handbook, Vol 6, Welding, Brazing, and Soldering, 1993, 303-5

5. Patterson A., ‘Fundamentals of explosion welding’, ASM Handbook, Vol. 6, Welding, Brazing, and Soldering, 1993, 160-4

6. Linse V., ‘Procedure development and process considerations for explosion welding’, ASM Handbook, Vol. 6, Welding, Brazing and Soldering, 1993, 896-900

7. Cowan G. R., Douglass J. J. and Holtzman A. H., US Patent 3137937, ‘Explosive bonding’, 1964

8. Smith L. M. and Celant M., Practical Handbook of Cladding Technology, Edmonton, Alberta, CASTI Publishing, 1998

9. Banker J. G., ‘Try explosion clad steel for corrosion protection’, Chemical Engineering Progress, AICHE, July 1996, 40-4

10. Banker J. G. and Winsky J. P., ‘Titanium/steel explosion bonded clad for autoclaves and vessels,’ Proceedings of ALTA 1999Autoclave Design and Operation Symposium, Alta Metallurgical Services, Melbourne, Australia, May 1999

11. Banker J. G., ‘Commercial applications of zirconium explosion clad’, Journal of Testing and Evaluation, ASTM, W. Conshohocken, PA, March, 1996, 24(2) 91-5

12. Banker J. G. and Cayard M. S., ‘Evaluation of stainless steel explosion clad for high temperature, high pressure hydrogen service’, Proceedings of Hydrogen in Metals Conference, Vienna, Austria. Oct. 1994, NACE International, Houston, TX

13. Banker J. G., ‘Recent developments in reactive and refractory metal explosion clad technology’, NACE Paper 03459, NACE International, Houston, TX 2003

14. McKinney C. R. and Banker J. G., ‘Explosion bonded metals for marine structural applications’, Marine Technology, Society of Naval Architects and Marine Engineers, July 1971, 8(3), 285-92

15. Banker J. G. and Nobili A., ‘Aluminum-steel electric transition joints, effects of temperature and time upon mechanical properties’, in Schneider W. (ed.), Light Metals 2002, Warrendale, PA, The Minerals, Metals, and Materials Society, 2002, 439-45

16. Banker J. G., US Patent #6,772,934 Kinetic Energy Welding Process, 2003.

17. Chiba A., et al., ‘Microstructure of bonding interface in explosively-welded clads and bonding mechanism’, Materials Science Forum, 465, 465-74. Trans Tech Publications, Switzerland, 2004

18. Yamashita T., Onzawa T. and Ishii Y., ‘Microstructure of explosively bonded metals as observed by transmission electron microscopy’, Transaction of Japan Welding Society, Sept 1975, 4(2), 51-6, Tokyo, Japan

19. Nobili A., Masri T. and Lafont M. C., ‘Recent developmets in characterization of a titanium-steel explosion bond interface’, Reactive Metals in Corrosive Applications Conference Proceedings, Haygosth J. and Tosdale J. (eds), Wah Chang, Albany OR, 1999, 89-98