Binary mixtures of ultrafine metal powders. II. Rolling of ultrafine powders into porous sheets

Abstract

The paper examines the possibility of producing porous sheets by rolling binary mixtures of ultrafine powders of nickel, copper, and cobalt. The effect of the rolling rate on the thickness and porosity of sheets and on the specific surface area of powders after rolling is studied. The best rollability is shown by nickel and copper powders and by powders produced from mechanical mixtures of formates.Ultrafine powders of metals and their mixtures produced by the thermal decomposition of formates are generally rolled into porous sheets with needed porosity θ, thickness τ, and surface roughness R a . This paper examines the production of thin porous sheets from ultrafine powders of nickel ( Fig. 1a), copper, and cobalt, and binary mixtures of these powders resulting from the thermal decomposition of formates of mixtures and mechanical mixtures of formates (Fig. 1b, c).Ultrafine powders and their mixtures were rolled using a YuD 2200 rolling mill (Fig. 2) with horizontal work rolls 25 mm in diameter. The diameter of the backup rolls is 250 mm and their length is equal to that of the working rolls. The linear rolling speed V r = 190, 470, 790, and 1570 mm/min was increased incrementally. In the experiment, the initial gap between the rolls was constant and equal to 40 µm; their surfaces had roughness of 0.04-0.02 µm after mechanical treatment and finishing. Note that the rougher the roll surfaces, the rougher the sheet and the higher its activity in sintering with the surfaces of compact materials. In our case, roughness of the sheet was 0.32-0.16 µm.The thickness and porosity of the sheet decrease with increasing rolling speed (Fig. 3). This is attributed to the gas phase squeezed out from the powder when it rolled [1,2] and to the flowability of the powder. The air squeezed out from the powder moves in the direction opposite to rolling and complicates powder supply to the compaction area, thus impairing roll biting. A larger amount of powder enters the deformation area at a lower rolling speed. The sheet thickness increases, and so is the time during which powder particles are subjected to the pressing force between the work rolls.