The essence of powder metallurgy is in the production of powders and the manufacture of products, coatings or materials of multifunctional purpose from non-waste technology. Powders are obtained from metal and non-metallic raw materials, as well as secondary raw materials of machine-building and metallurgical production. The technological process of production and processing of products and materials by the methods of powder metallurgy involves the production of powders, their shaping into blanks, sintering (thermal treatment) and, if necessary, final processing (finishing, calibration, compressive compression, heat treatment).
Methods for the production of powders are subdivided into mechanical (without changing the chemical composition of the raw materials), physico-chemical and combined.
The mechanical method involves mechanical grinding of compact materials, carried out by crushing, grinding or abrasion in special aggregates-mills (vortex, planetary, centrifugal, ball, vibrating, rotating, etc.).
Physico-chemical methods for obtaining metallic powders. Metal halide compounds that are reduced either by hydrogen or by active metals (sodium and magnesium). The mechanism for the reduction of most solid compounds by gaseous reducing agents is based on an adsorption-autocatalytic theory.
Recovery agents used in the recovery of powders.
The reducing agents are gases (hydrogen, carbon monoxide, dissociated ammonia, natural convertible, water, coke or blast furnace gases), solid carbon (coke, charcoal, soot) and metals. The choice of a reducing agent depends not only on the thermodynamic estimates, but also on the volatility, which should be minimal, since otherwise the process must be conducted at an elevated pressure due to argon or other inert gases.
Iron powder is the basis of a large-tonnage PM. There are methods for obtaining powders from FeCl2. The hydrogen powder recovered by hydrogen has a high purity and cost.
The reduction by carbon monoxide is carried out at temperatures above 1000 ° C based on the adsorption-catalytic mechanism. Recovery with solid carbon occurs at 900-1000 ° C.
The soda method is used to produce a powder of increased purity. Add 10 – 20% of the soda to the charge, with which impurities react when the reduction forms, forming water-soluble sodium aluminates.
The combined process includes magnesium reduction, and after washing with calcium, the consumption of which is reduced by half. The reduction with calcium hydride gives a powder of titanium and its hydride. Reduction of titanium chloride by sodium. Titanium chloride is obtained by chlorinating the ore concentrate, purifying and fractional distillation. Recovery of titanium chloride by magnesium is the most economical way. The reaction occurs at 800 – 900 ° C. The steel sealed device is filled with magnesium ingots, pumped out with air, filled with argon, magnesium is melted, a limited amount of titanium chloride is supplied from above, so that there is no overheating.
Recovery from solutions, gaseous compounds and in plasma. From the solutions of Ni, Cu, Co compounds, metals are displaced by hydrogen in autoclaves. Shift the hydrogen potential to the negative side by raising the pH or increasing the pressure of hydrogen. It is more effective to change the pH, the increase, which by one is equivalent to a change in the hydrogen pressure by a factor of 100. Thermal calculations show that these metals can be precipitated already at 25 ° C and 0.1 MPa. The reduction of gaseous compounds by hydrogen is carried out in a fluidized bed of halides of tungsten, rhenium, molybdenum, niobium and titanium. The preparation of highly disperse powders in plasma is promising for metals, carbides, nitrides, etc. The reducing agents are hydrogen or products of plasma conversion with a high temperature and without oxidizers. Nickel oxide is reduced in an Ag-H2 or Ag-CO jet, the hydrogen content being close to the stoichiometric, and the heat exchange and plasma formation occur due to argon. The reaction is limited by the dissociation of NiO, its complete reduction is achieved at 7000 ° C.
Physico-chemical basis for the production of powders by electrolysis. The process is a kind of restoration: the transfer of electrons to the metal with a simultaneous restructuring of the structure occurs not by means of reducing agents, but by electric energy. The method is universal, ensures high purity of powders. Electrolysis is one of the most complex physico-chemical processes for the production of powders. The process consists in the decomposition of aqueous solutions of the compounds of the released material. The presence of chlorine or fluorine on the anode makes it necessary to take measures to prevent its interaction with the electrolyte and powder. The electrolyte from the powders is separated by distillation by heating or centrifugation and washing.
Electrolysis of aqueous solutions. A method for producing powders of copper, silver, iron, nickel, cobalt, tin, etc. Nickel, zinc, cobalt form uniform dense fine-grained sediments, regardless of the nature of the electrolyte. Silver or cadmium grows in the form of separately highly branched crystals during the electrolysis of simple salts, from the solution of cyanide salts they are released as a smooth smooth layer.
Preparation of copper, nickel, iron powder. Copper powder is obtained from a solution of copper sulphate, it has a high purity and controlled dispersity. Nickel powder is obtained by electrolysis of ammoniacal solutions of chloric acid nickel. Features of obtaining iron powder are due to the fact that in the stress range, iron is located to the left of hydrogen, so the latter is released together with hydrogen, worsening the current yield and the quality of the powder.