An analysis is presented of mechanisms involved in forming rapidly solidified surface layers on a substrate material by an electron beam. This can be achieved either by (i) point‐source or by (ii) line‐source melting. In mode (i) it is critical that minimal cooling occur between successive beam oscillations so as to avoid solid‐state induced transformations where adjacent rapidly solidified zones overlap. In mode (ii) a linear heat source is formed by oscillating the beam sufficiently rapidly and moving this line source transversely over the surface to form a homogeneous rapidly solidified layer. The conditions for this to occur are mathematically predicted. A theoretical model yields the minimum oscillation frequency for this to occur. Under steady‐state heat flow conditions the following features were studied: stability of the cavity, geometry of the melt zone, and rate of cooling during solidification. It is shown that a linear vapor cavity exists in mode (ii), provided the oscillation frequency of the beam exceeds a critical value, typically ≊250 Hz. The predicted cavity depth is shown to correspond to the depth of the turbulent flow region, as revealed by microstructural observations. The molten pool length is governed by the steady‐state heat conduction requirements, whereas the depth is found to be empirically related to process parameters. Predicted values of the cooling rate are compared with those deduced from the microstructural scale and the solidification morphology for the case of a molybdenum high‐speed steel (M7). Theoretical and experimentally obtained values are shown to be in good agreement.
展开▼
