Characterization of Explosion Weld Interface: From Swirls to Waves

Characterization of Explosion Weld Interface: From Swirls to Waves

Stephen Liu¹, John Banker² and Curtis Prothe²

¹Center for Welding, Joining and Coatings Research

Colorado School of Mines

Golden, Colorado 80401, U.S.A.

²Dynamic Materials Corporation

Boulder, Colorado 80301, U.S.A.

Abstract

In explosion welds, extremely large deformation within a distance measured in micrometers can be found next to the interface. Detailed characterization of the weld interface had been limited in the past by the availability of tools such as HVTEM, HAADF, EBSD, OIM, and FIB. In this work, explosion bonds from copper-steel, stainless steel-nickel-titanium, titanium-steel, stainless steel-mild steel, and titanium-aluminum were characterized. 

The results show that improper explosion welding parameters created large swirls that contained entrapped oxides, solidified melt pockets, intermetallic compounds, and even cracks. In the melt pockets, solidification occurred via heterogeneous nucleation, in contrast to epitaxial growth that takes place in arc welds. Heating at interfacial region led to dynamic recovery and recrystallization as evidenced by equiaxed submicron grains immediately next to the interface. Elongated grains in a short distance away from the interface indicated high degree of deformation. These grains, however, exhibited little internal defects which is an evidence of recovery.

Bonds produced using good industrial practice exhibited interfaces with uniform waves at regular periodicity and amplitude. Away from the wave crests of greater turbulence, amorphous layer of about 100 nm wide was observed in titanium-steel bond. Sharp composition gradient across only 20 nm was observed in copper-steel system.  

            Whether melting occurs in explosive bonds is questionable because of the very high cooling rate. Indirect measurements using tracer elements embedded in the joint and phase transformation characteristics showed 2600°C as the minimum temperature excursion in copper-steel bonds. However, melting of Cu or steel was not widespread but limited only to local pockets.