Dark spots in the radiography film of the weld may not be always indication of porosity or segregation, it may be because of scattering of the rays, have a close look on it next time you observe.
A special form of scattering caused by x-ray diffraction is encountered occasionally. It is most often observed in the radiography of fairly thin metallic specimens whose grain size is large enough to be an appreciable fraction of the part thickness. The radiographic appearance of this type of scattering is mottled and may be confused with the mottled appearance sometimes produced by porosity or segregation. It can be distinguished from these conditions by making two successive radiographs, with the specimen rotated slightly (1 to 5 degrees) between exposures, about an axis perpendicular to the central beam. A pattern caused by porosity or segregation will change only slightly; however, one caused by diffraction will show a marked change. The radiographs of some specimens will show a mottling from both effects, and careful observation is needed to differentiate between them.
A relatively large crystal or grain in a relatively thin specimen may in some cases “reflect” an appreciable portion of the x-ray energy falling on the specimen, much as if it were a small mirror. This will result in a light spot on the developed radiograph corresponding to the position of the particular crystal and may also produce a dark spot in another location if the diffracted, or “reflected,” beam strikes the film. Should this beam strike the film beneath a thick part of the specimen, the dark spot may be mistaken for a void in the thick section
The mottling caused by diffraction can be reduced, and in some cases eliminated, by raising the kilovoltage and by using lead foil screens. The former is often of positive value even though the radiographic contrast is reduced. Since definite rules are difficult to formulate, both approaches should be tried in a new situation, and perhaps both used together.
Reference: Radiography in Modern Industry
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VFA welding process, a new process variant in the field of collision welding technique basically to weld dissimilar materials. Researchers from The Ohio State University have discovered this technique.
The flyer, instead of being driven by chemical explosives (explosive welding) or magnetic forces (magnetic pulse welding), is launched toward the target by the pressure created from the electrically driven rapid vaporization of a thin metallic conductor.
In this technique a high-voltage capacitor bank generates a short electrical pulse, which is passed through a piece of thin aluminum foil. The foil vaporizes within microseconds, and a burst of hot gas pushes both the pieces of metal together at speed of thousands of miles per hour. The metals don’t melt, so there’s no weakening of metal; instead the impact directly makes a bond of atoms of one metal to atoms of the other metal.
Mechanical impulse is developed from 0.0762 mm thick aluminum foils, which are vaporized using capacitor bank discharge with nominal charging voltage of 5.5 kV and peak current on the order of 100 kA delivered with rise times of about 12 μs. Welding couples of copper–titanium, copper–steel, aluminum–copper, aluminum–magnesium and titanium–steel have been successfully created with the same input parameters such as foil geometry, input energy and standoff distance. Instrumented peel tests, lap shear tests and optical and scanning electron microscopy reveal a wide spectrum of both strengths and interface structures. Copper–titanium and copper–steel welds are strong and have characteristic wavy interfaces with little intermetallics or void formation. The other combinations are seen to have brittle interfaces with intermetallics and defects, with the collision welding parameters used presently. For the titanium–steel system, a thin nickel interlayer is introduced and all the layers are welded in a single experiment. Peel strength of the weld was observed to be quadrupled. Peak velocities of up to 560 m/s were obtained for titanium flyer sheets
The technique uses a lesser amount of energy because the electrical pulse is so short, and because the energy essential to vaporize the foil is less than what would be used to melt the metal parts. Thus far, the team have used this method to join different combinations of copper, magnesium, aluminum, nickel, iron, and titanium.
- Collision Welding of Dissimilar Materials by Vaporizing Foil Actuator: A Breakthrough Technology for Dissimilar Materials Joining, Glenn Daehn, Anupam Vivek
- Vaporizing foil actuator: A tool for collision welding, A. Vivek, S.R. Hansen, B.C. Liu, Glenn S. Daehn
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