Tashimova S.B., Bogomolov A.V.

Pavlodar State University named after S. Toraigyrov

MODERN METHODS OF PROCESSING BY PRESSURE IN POWDER METALLURGY

Specificity of reception of powder materials and products from them as initial raw materials for processing by pressure has caused possibility and necessity of working out of new methods of such processing conditionally named the nonconventional.

Hot pressing of metal powders allows combining pressing and sintering operations. It can be carried out two ways: two-phasic with preliminary cold formation of metal powders and one-phasic pressing at the raised temperature. Hot pressing of preliminary warm porous preparations affects insufficiently effective in case of manufacture of very small products because of excessively fast cooling of preliminary warm preparations or in general unsuitable at use difficultly pressed powders. Therefore in certain cases use direct hot pressing powder mix material with charge it in a compression mold warm or heated up together with it.

Porosity the hot pressed ring samples received at pressure 500 MPa with endurance of 1-2 mines and temperature 500°С, remained high (10,8-13,4 %) because of low pressure and pressing temperatures. As a result of annealing (1300°С, 12) it has decreased to 1,8-3,7 %. Powder-alloy use allows to avoid necessity for material homogenization, however such powder intended for reception permalloy and received by joint decomposition of carbonyls iron and nickel, has the lowest technological properties. It doesn't possess fluidity and is badly pressed in a cold condition, the bulk weight makes only 0,6 kg/m3 [1]. At pressure 300 MPa and above samples turn out with a considerable quantity longitudinal cracks. After hot pressing on the mode specified above they had a good surface, but high porosity (8,3-15 %) and low magnetic properties. The subsequent high-temperature annealing has led to occurrence on a face surface of swellings and bubbles. For prevention of their formation it is recommended to do isothermal endurances at 500-700°С, promoting an exit of gaseous products of reactions carbon depletion. Disappearance of swellings and bubbles provided also with cold calibration at pressure 1200 MPa and annealing then magnetic properties appeared at level of requirements.

Other way of reception of a material is described by hot pressing of metal powders in work [2] and has received the name "Osprej-process". Its essence consists that spray liquid metal a high-speed stream of inert gas and directs in established in the chamber of dispersion a collector where the formed powders having enough high temperature, cook among themselves owing to inertial efforts arising at impacts. Preparation in density to 99,9 % from theoretical with high properties and fine-grained structure of a material is as a result formed. Trial installation is equipped by individual furnaces capacity of 150 and 30 kg. Intensive researches on reception by this method of production from fast-cutting, special corrosion proof steels, stellite, heat resisting nickel alloys and layered materials are conducted.

Hot pressing in metal covers in which freely filled up metal powders consist, and also molding, the past and done not pass sintering, prevents interaction of a processed material with environment and promotes reduction of cooling of its blankets at contact to walls concerning the cold tool. Besides, at increase in a thickness of covers resistance of radial deformation of this material that leads to reduction of stretching pressure and danger of formation of cracks increases. Especially the specified factors get great value at processing of difficult formed powders from highly active materials with low plasticity. However application of covers increases labor input of process in connection with necessity of their manufacturing and the subsequent removal. The material expense increases also.

Use of metal covers at hot extrusion sintered preparations can be illustrated on an example of reception of samples from the sprayed powder iron-nickel of an alloy [3]. A powder pressed in a hydrostat under the pressure of 600 MPa and the received preparations by porosity of 12-13 % then sinter at 1173°С, 18 h. Sintered samples porosity of 6-8 % placed in a cover from a corrosion proof (corrosion-proof) steel with thickness of a wall of 5 mm. On pipe end faces welded a bottom and a cap. Heating of samples before extrusion spent in the environment of hydrogen at 12000С. At extrusion samples had temperature 1180 °С. A matrix greased with glass, and the container - graphite. Extrusion samples had porosity of 0,9-2,4 %, diameter of 20 mm, factor of an extract 14, degree of deformation of 73 %. Extruded the powder material on durability at room temperature doesn't concede, and in plasticity even surpasses received by molding. However its thermal stability is lowered that is connected with the raised gas saturation and presence of ferriferous particles in an initial powder.

Hot pressing sintered preparations from a powder of the same alloy spent in the container from a thin-walled pipe of stainless steel. In a pipe with the help roughing down pump VN-2 created (after three washings) depression about 1-10 Pases of the residual environment of argon. On residual oxygen this environment is equivalent to vacuum 10-3 Pases.

Trailer part of a pipe in which placed sintered preparations, heated up in the electric furnace of resistance to 1200-1280°С with endurance 3-5 h. Then a pipe took out from the furnace, and its warmed up part pressed out on a press at pressure 300 MPa to degree squeezing 30 %. For maintenance at hot pressing of conditions of all-round non-uniform compression of preparation heated up and pressed out in thick-walled (10 mm) holders from a steel of St3. Squeezing without holders led to occurrence of small cracks on lateral surfaces of preparations. Their extraction from holders after squeezing carried out machining.

Mechanical properties the hot pressed powder iron-nickel an alloy, both in an initial condition, and after annealing have appeared higher, than a compact alloy. Satisfactory was also it punching ability, however for thermal stability increase it is necessary to reduce gas content an alloy and quantity of harmful impurity (iron), and also to create conditions for reception of a coarse-grained structure.

Similar researches have been spent and at use of preparations, sintered from sprayed recovered a powder of heat resisting alloy JS6K. Sintered preparations placed in the container from stainless steel in steel holders with thickness of a wall of 5 mm. Despite high density of samples after hot pressing, they concede on properties cast that is connected basically with their fine-grained structure.

Covers use also at hot a proskating rink of preparations, sintered from highly active alloys. Thus preparations heat up before rolling (100°С, 30 mines, vacuum 6-10-2 Pases) and roll in a wrapper from an iron tape in the thickness of 1 mm. The surface of the samples not protected by a wrapper quickly is cooled at contact with cold sticks, the material loses plasticity and fissures. Almost full elimination of a time at hot a proskating rink is reached only at the general on height more than 65-70 %. Such big squeezing it is possible to reach at vacuum a proskating rink with application of a protective wrapper of all for one pass. At a proskating rink in the air environment use protective packages from a nickel foil in the thickness of 0,3-0,4 mm. Under optimum conditions of sintering and the subsequent thermal processing hot-rolled powder nickelid the titan possesses high enough operational properties [3].

One more version of such processes is consolidation at the atmospheric pressure, used for manufacture of fast-cutting steels and heat resisting alloys. Initial powders receive dispersion in the environment of nitrogen or argon, subject to mechanical activation, and then chemical processing for the purpose of clearing of a surface of particles and their improvement diffusive weldability. A powder fall asleep in the form, pumpdown to residual pressure 9,8 Pases and solder. The form with a powder place in the furnace and maintain 4-10 h 1100°С. Under the influence of temperature and atmospheric pressure there is a consolidation to 98-99 %. Under the published data, the alloys received in such a way have ultradisperse structure and are characterized by the improved low cyclic fatigue durability [1].

"Zonal" hot pressing at which the part of preparation which has undergone to heating to a plastic condition is mainly deformed, can be carried out both at free its deposit, and in a stamp [32]. In the first case for hot pressing sintered preparations from nickelid the titan (diameter of 18 mm, porosity of 30-40 %) used installation for diffusive welding of type DS-2. Preparations pressed in vacuum 10-3 Pases after heating to 1000°С from double-coil an inductor at pressure 3,8-20,9 MPa steel cylindrical punches. End faces of punches isolated from heat from freely pressed out by them sintered the sample thin mica.

Consolidation porous sintered the sample occurred in the zone of the maximum heating located in an average part of the sample opposite to an inductor. This zone on the size makes approximately about third of height of the sample. Under the influence of pressure porous rather cold and small plasticity the material of the sample was pressed from two parties into the softened zone of the maximum heating which was condensed. Heating up, the pressed material got high plasticity and was condensed with formation of a lateral thickening. This process passed continuously until all volume of the sample didn't appear in a zone of the maximum heating. After loading removal the sample maintained 30 mines at 1000°С and then cooled.

In view of that in the course of free hot reduction plastic deformation proceeds unequally in volume of the condensed sample, the density nickelid the titan has appeared lowered even after removal on the blanket lathe on depth more than 2 mm. The lowered density have squeezed out of puanson spaces a material of preparation in the absence of conditions of all-round volume compression, and also adjoining to end faces of punches layers of the sample because of cooling by them and plasticity falls. However positive sides of this method of hot pressing sintered porous nickelid the titan are obvious also, allowing to condense it to small residual porosity at use of insignificant efforts and simple equipment.

In mass production by more perspective the method at which in a special inductor all preparation transferred further in a stamp for hot pressing is exposed to non-uniform heating is. Heating also can be carried out only in separate zones of preparation, and its other parts remain cold or heat up partially at the expense of a heat transfer from the next zones.

Such distribution of temperatures in a preparation material causes also distinction in resistance to its deformation (consolidation), and consequently, distinction of density on preparation volume even at uniform it weighting in a stamp. In other cases degree of non-uniformity of distribution of density, on the contrary, can decrease.

In drawing 1 the scheme of punching of preparation with non-uniform heating on height is presented, distribution of its temperatures (t) and density (ρ) after the termination of punching of a product is shown. These dependences have a similar kind and at other character of distribution of temperatures, for example at heating only a peripheral layer of preparation. In this case this layer is condensed only, and "the cold" core remains don't luted. "Zonal" heating of preparation is easily carried out in the induction heating device with variable number of coils of an inductor or with its zonal placing.

The main advantage of a described method consists in maintenance of the set distribution of properties in volume of a product, weighting of its part which are exposed, for example, to intensive deterioration or influence of shock loadings, are as much as possible condensed and have the highest mechanical properties. Other, not loaded, parts remain don't luted, their porosity can strictly be supervised and in turn provide demanded properties of a material of these parts of a product - antifrictional, etc. Simultaneously is thus provided economy of a material (at the expense of porosity of not loaded parts), power expenses for preparation heating (all preparation heats up not), and firmness of tool equipment raises at manufacturing of products.

Conclusion. New nonconventional methods of processing of metals expand with pressure of powder materials technological possibilities of powder metallurgy and demand additional researches.

Literature:

1. Libenson G.A. Osnovy poroshkovoi metallurgii.M: Metallurgiya, 1987, 208s.

2. Promyshlennaya tehnologiya goryachego pressovaniya porowkovyh izdelii. Dorofeev Y.G., Gasanov B.G. I dr. – M: Metallurgiya, 1990, 206s.

3 . Devyatov B.A Maloothodnaya tehnologiya processov obrabotki metallov davleniem M: Mashinostroenie 1999, 348 s.