Experiments on Dynamic Behavior of Molten Metal and Solid Particles Under Direct Current Passing

At electrodischarge sintering of powder mixtures by electric direct current, some essential displacements of molten metal and solid particles were revealed. Such displacements at conventional sintering were not known to us. The objects were powders (mixtures): Sn–Cu (dispersivity 100–200 µm); Sn–high-temperature alloy on nickel base or electrocorundum; copper alloy-electrocorundum (dispersivity 63–500 µm); Ni (dispersivity 10 µm). The electric direct current was passed in vertical direction parallel with the axis of cylindrical mould or through the series circuit steel-powder nickel layer-hard alloy. If the cylindrical container (mould) did not have the upper punch (a load on top was absent), the sample after solidification had the form of a bullet. This sample became convex on top and concave on bottom. At the application of prepressing (start pressing) on top, the movement of suspension (molten matrix and solid particles) occurred in a similar way. Under the action of electromagnetic forces, a redistribution of solid particles in volume of suspension occurs. Particles with more high conductivity than liquid phase begin to cluster at the center. For the case of lower conductivity, the motion of the particles to the outer side surface of mould prevails. As the current passed through the series of circuit steel-nickel powder layer-hard alloy, three named parts of the circuit were sintered into a single block. After destruction at testing, the interlayer had the form of a ring. This shows the nonuniformity of the passing of electric current through the interlayer. The cause of displacements of solid (nonmolten) particles may be the simultaneous influence of thermic convection and squeezing out by the electromagnetic Lorentz-forces, which are directed radially. The phenomena described may be applied in the production of objects with a gradient (variable in volume) concentration of particles added specially. This refers to products of their interaction with molten matrix as well.