TWI Industrial Member Report Summary 172/1982
By P J Blakeley and A Sanderson
The successful electron beam welding of steels and other ferromagnetic alloys may depend upon the magnetic properties of the workpiece. The effects of magnetic fields on electron beams both outside and inside the material are examined. Internal magnetic fields are usually sufficiently screened from the beam by ferromagnetic components but external fields can deflect the electrons out of the joint plane to give a miSSed joint defect. The magnitude and direction of possible magnetic flux densities are discussed and related to the extent of the potential defects. Beam deflection due to residual magnetism can be avoided in most situations by the use of a simple magnetic shield.
Magnetic fields may also arise when dissimilar metals are welded. These are caused by large thermoelectric currents which are established in the joint due to the existence of a hot junction at the welding zone and a cold junction along the joint face. A thermoelectric current model is proposed to explain the generation of magnetic fields which cause the observed curved fusion zones. Tests indicated the validity of this model. Welds were then made between alloys with a range of relevant physical properties and the extent of missed joint defects which arise is shown to be related to the thermoelectric potential difference existing between the alloys during welding, to the relative permeability, and to the resistivity of the components of the weld. Beam deviation in this case can be minimised by the use of an external magnetic shield and internal deflections accommodated to a certain extent by tilting the joint plane with respect to the beam-axis.