1975 Volume 44 Issue 2 Pages 121-127
Mechanisms of beam (D-M) deflection observed in the cross sections of electron-beam-welded dissimilar metal joints were investigated.
Explanations based on mechanical missettings or several physical characteristics were rejected through experiments. Finally, the stray magnetic field theory was evaluated.
It was found that use of a magnetic shielding plate(having a small through-hole to pass the electron beam, a patented process of United Airecraft Corp., U.S.A.), which was placed parallel to and near the surface of the test plate to absorb the stray magnetic field, was effective to prevent the beam deflection.
Application of inverse magnetic field parallel to the welding direction to cancel out the opposite-directional component of the stray magnetic field around the impinging point of the electron beam was also effective for the purpose.
Thermo electromotive forces of several combinations of dissimilar metals were measured. It was found that.
1) When the thermo e.m.f. at 800°C was lower than 5.5mV, the beam deflection was hard to distinguish.
2) When the thermo e.m.f. went up higher than 5.8mV at 800°C, the beam deflection became recognizable, and the deflection became larger with the e.m.f. value.
3) The beam (and penetration) was always deflected towards plus (+) metals.
It was concluded from these findings that,
A) Hot and cold junctions are formed in welding dissimilar metals, and large electric current flows due to the thermo e.m.f.
B) Stary magnetic field is formed in the upper space of the weld joint by this current. Interaction of the electron beam and a component of the field parallel to the welding direction deflects the beam to the transverse direction.
C) The deflected beam produces an inclined penetration.
D) The deflection angle a is to be proportional to ΔV800// √ E
where ΔV800 is the thermo e.m.f. between two metals at 800°C and E is the accelerating voltage of the beam.
