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New explosive coating technology and structural performance
Zagirniak M.V.
KremenchukMykhailoOstrohradskyNationalUniversity
Dragobetskii V.V.
KremenchukMykhailoOstrohradskyNationalUniversity
Summary
Purpose analysis of the variety of factors of the physical phenomena accompanying the process of the powerexplosive effect for development of new processes of metal working: explosive film coating, superfast crystallization of hardening and updating of a superficial layer a piece. Industrial approbation of cladding technologies by explosion of piece surfaces of complex configuration and determination of parameters of the process of the explosive welding of high-strength pig-iron (graphite of the spherical form) with Godfield steel.
Approach the analysis of the physical phenomena accompanying the process of explosive loading of materials is carried out. The opportunity of use of the explosive metal working for production of pieces and materials with unique properties which cannot be produced by other methods of working is found. As a result of experimental researches new alloys and technologies of the explosive metal working for the sphere of manufacturing are received.
Findings–the parameters of asteady explosive cladding of piece surfaces with coating and film thickness of 200300 nm and graphite coatings and films withthe thickness of 10100 nm are determined. Thetechnology of modifying effect of the explosive loading and production of monolithic edges of three layer spherical bottoms is tested. The parameters and borders of the explosive welding of Godfield steel with high-strength pig-iron are determined.
Research and practical implications the technology of applying thin metal coatingswith the thickness of less than 0.5 mm allowing to receive a clad metal connection with the strength of bond, equal or exceeding the strengthof a substrate is developed; the use of modifying effect of shock waves prior to chemical thermal treatment has reduced the time of the process of cementation 1.5…2 times; coatingsof superfirm powder materials are produced; the technologies of the explosive cladding of basic surfaces of aluminium and pig-iron axle-box cases are developed, this has increased their wear resistance 1012 times.
Value the essentially new processes of application of thin metal coatings, superfast crystallization, dynamic hardening and diffusive saturation of a superficial layer are developed.
Key words: coating technology, shock waves, cladding, fast crystallization, metastable, impact.
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Coating Technology and Structural Performance.
The introduction. As a result of the power impact formed at the explosion in the material of a piecethere arisesdisturbance of various nature, which results in phase transformations,irreversible plastic deformations, convertible increase of volume of the material under the action of the heat released at the impact compression, formation of connections and destructions. At the various schemes of the impulse impact there can be achieved the required forming, hardening, connection, splitting and continuity disturbance.
The character and intensity of the impulse impact on a piece even in one process can essentially differ and open opportunities for changing geometrical, mechanical, physical, etc. properties. The disturbance extending with certain final speeds as waves of stress(loading, unloading and also reflected ones), forms in the piece the disturbance areas which in time expand. Each area of disturbancehas its stressed state characterized by the tensor of stress and tensor of deformation and is determined by the disturbance nature.
In the vicinity of the direct action of the pulse power factor the area of loading disturbancearises which in time spreads with the final speed. At time, when the growth of deformation stops, the process of unloading begins. Disturbance appropriate to the process of unloading spreads in the material with the final speed as a wave of unloading. The secondary area of disturbance of the wave of unloading located inside the area of loading disturbance is formed. At theexit of the wave of stress on the surface orat the interaction of the waves of stress in the body there appears a phenomenon of reflection. The reflected wave of loading, spreading in the opposite direction, forms the secondary area of disturbance of the reflected wave. At transition of the front of the wave of stress from one area of disturbanceto another the movings of particles of the environment are continuous under the condition of preservation of the environment continuity.
In addition, the process is further accompanied by the influence of expanding products of detonation directly or in the transmitting environment on theworkable material, causing additional deformation.
High power parameters and the impulse character of the explosive loading in their elementary application are extremely diverse, and not all their aspects are investigated. During the hydroexplosive forming besides thenecessary forminga significant hardening of a piece under working is achieved, and in a number of cases there takes place the welding of the piece to a matrix. The process of the explosive weldingis also accompanied by hardening, plastic deformation and forming. During the explosive pressing besides the welding by friction arising at the adiabatic compression of particles and theirliquid phase of sintering, also microprocesses of the explosive welding by explosion of particles occur.
At a choice and substantiation of the processes of working for specific conditions of manufacturing, probably, energetically optimum is that process which under other equal conditions provides the least consumption of energy necessary for reception of the required physical and mechanical properties of products with the required operational properties.
For transition from elementary processes of the explosive metal working to the real ones and revealing essentiallynew ones, it is necessary to proceed to consideration of the processes using high-energy sources of energy and the phenomena, accompanying the process of the explosion impacton the material being worked.
The purpose of the work. The analysis of the variety of factors of the physical phenomena accompanying the processes of the explosivepower impact for the development of the new processes of metal working: explosive film coating, superfast crystallization, hardening and updating of a superficial layer of a piece, explosive cladding of the contact surfaces of a piece of complex configuration,production of compositions with Godfieldsteel.
Material and results of researches.Ashock-wave character of the explosive loading causes a number of the physical phenomena not inherent to the static loading,such asoccurrence of high temperatures at the front of a powerful shock wave with formation of high-temperature phases, polymorphic transformations, crushing of grains and formation oftwin defects, formation of cumulative jets, "multiple" chipping under the influence of shock waves of high amplitude, sintering, destruction, deformation and welding. These physical phenomena can find application in the following technologies: 1) fast crystallization; 2) production of amorphous materials; 3) modifying effect of shock waves; 4) production of thin and film coatings; 5) cladding, hardening and welding. The processes, parameters of which correspond to the boundary characteristics for each of the processes, have large opportunities for creation of new technologies of the explosive working.
Recently scientists haveshownan interest to a method of fast crystallization, the essence of which is in cooling the melted metal with the speed of about one million degrees per second. The quickly cooled alloys are rather homogeneous, as there is no time for formation and growth of large grains. The materials with a homogeneous structure are strong and have a high melting temperature. Fast crystallization can cause formation ofmetastable phases: crystal amorphous, less stable phases formed at a slow cooling. The metastable phases have a number of untrivial properties. For example, quick-crystallized aluminium alloys have specific strength, equal or exceeding the strength of titanium alloys under moderate and high temperatures. They are also extraordinary corrosion resistant. Quick-cooled aluminium alloys are capable to replace titanium in the parts of compressors of gasturbine engines, pig-iron in brake disks of automobile wheels, pig-iron axle-box cases of railway cars, etc.
There are some methods of production of quick-cooled alloys. The elementary of them is superfirm hardening, at which dropsof a melted metal are thrown out on a cooling surface. The other method is dispersion: fine dispersed drops are cooled by the inert gas. An installation is developed, in which a thin jet of melt falls on a quickly rotatingdisk, which splits it into drops and throws them out into a cold atmosphere. This method helps to produce fine-grain powders of a quick-cooled alloy, which are then compacted by hot compaction.
It is more effectively to carry out the process of dynamic hardening with the use of explosive substances. For this purpose powder or a pieceunder work is thrown by a charge of an explosive substance on the cooled surface of a liquid or metal. At a collision of the item with the surface with the speed of about 2.53.0 thousand m/sec there is a fast cooling and shock-wave compression resultingin the formation of metastable phases.
The process can be carried out as follows. In a cavity formed in a high-strength alloy, an explosive substance and powder are put. During the explosion of the charge the powder brings up to speed, and at a collision with a cooled surface the processes, characteristic for fast crystallization, take place.
It is possible to estimate the maximal pressure arising at a collisionof the powder with the firm surface under the formula [2]
(1)
where о, Соdensity of the powder and speed of the sound in it; v speed of powder particles;k constant, describing increase of speed of a wave at a shock compression.
The pressure necessary for formation of a metastable phase corresponds to hundreds of kilobars and is achievable at the speeds of collision, exceeding several times the speed of the sound in the air. It is possibleto achieve it applying impulse sources of energy. The task of throwing powder by products of the explosion in a simplified variant is solved in reference [3]. Speed v1of the thrown powder will make
.(2)
Here parameter of products of explosion; speed of detonation;,
where densities of the explosive substance and powder; extent of the explosive substance and powder. Depending on the kind of the explosive substance and ratio of weights of the explosive substance and powder speed V1 reaches 2,500 m/sec.
A number of processes in the explosive metal working (cutting, welding) is connected with formation of cumulative jets in the workable materials. This phenomenon can be used for production of amorphous materials. It is known, that the speed of the temperature fall makes up to 3.5106degrees per/sec [3]at a collision of powder with the surface of the other material at speeds of collision of about 1,000 m/sec-1 inthe interval of temperatures of 700-350o. At such gradients of temperatures amorphous materials from the melt are produced. Such parameters of collision are characteristic of the processes of the explosive welding, and a number of researchers studying the structures of the connection zone of the pieces welded by explosion have found inclusionsof an amorphous metal in a weld [3]. Probably, the formation of amorphous structures occurs from the melted metal. In other words,collision of powder with the substrate should result inmelting of the contact zone of the material under work. At some modes of the explosive welding in the zones, adjacent to a weld, cast inclusions are formed, which reduce the quality and strength of the welded connection and are extremely undesirable. For formation of a layer from the amorphous material or material having a large number of amorphous components the modes of collision should ensuremelting of the contact layers and a high speed of cooling. The quantity of cast inclusions increasesas the speed of detonation D increases, and consequently, the speed of the point of contact. The limiting value of the speed of the point of contact is taken within the limits of the zone of wave formation3. The size of the welding gaph and the parameter of welding r correspond to the maximal speed of collision. As a rule, at such modes welding does not take place, and a pseudo-amorphous layer is formed having a number of unique properties. Thus, on a joint of the processes of the explosive welding and explosive hardening for a number of materials the process of fast crystallization is possible. And this process, probably, enables production of monometals with the superficial structure and properties, characteristic for clad metal. It is possible to intensify and stabilize the process of applying coatings with a pseudo amorphous structure by giving to the surface of the substrate the relief which would ensurethe formation of counter cumulative jets from the melted metal and oxides. For example, the relief can be of triangular (fig. 1), trapezoidal, etc. shapes. At a collision of jets a fine-grain sheet and dusting of jets onto a massive body of the substrateis formed. In such conditions the speed of cooling the components of cumulative jets reaches 106 degrees per second, creating conditions for fast crystallization.
In the processes of self-propagating high-temperature synthesis (SHS) the task of fixing the intermediate products when multistep reactions in the SHS wave take place is solved at the speeds of cooling of 104-105K/sec, close to the speeds of high-speed hardening. In these conditions metastable structures are formedand substancesacquire special properties. The required speed of cooling can be received using a high-speed jet of water directed perpendicularly to the front of the wave of synthesis [4].
The use of the multiple chipping effect in the methods of the explosive workingopens new opportunities in the field of applying thin and film coatingswiththe thickness of about 100 nm and less.
The coatings received during a joint plastic deformation at the explosive welding surpass all existing methods of claddingin the strength of bond with a substrate. It enables to use such pieces with coatings in the conditions of the dynamic loading. Besides, the coatingsare corrosion resistant, antifriction, hydrostable, heat-conducting and durable.
Certain difficulties arise at applyingcoatings with the thickness of less than 1 mm. In this case there is deformation, distortion, breaks, etc. of the clad layer, and what is especially essential, the required charge of the explosive substance in its mass and geometry is less than the critical diameter of detonation. To solve the problem of welding additional run-in plates are used, to which a foil ofclad material is pasted. As a rule, after welding because of presence of the waves of stretching separation of the run-in plate occurs. The quality of the clad layer depends on the technology of pasting. Besides pasting, a coatingcan be appliedon the run-in plate by any other method,for example, spraying, chemical, etc. At a collision with the substrate the transfer of the material of thecoating onto the substrate takes peace. The strength of bond thus surpasses the initial strengthwith the run-in plate. To producecoatings from graphite and also graphite films on the run-in plate the coating can be applied by rubbing with a graphite core. To receive films, it is necessary to dissolve the substrate.
Fig. 1 Scheme of hardening with formation of cumulativejets:
1 explosive substance;2 striker;
3 item of the substrate subjected to hardeming; 4 jet
The next direction of formation of thin metal coatings is the explosive welding of powders with monolithic metals.
To apply powders on metals by shock waves the schemes similar to the unilateral compaction [2] are used. The process of throwing a porous (or loose) layer, as a rule, is split into two stages at the theoretical analysis. It is considered that at the first stage the speed up occurs in a shock wave strongly condensing the substance, making it close to a compact condition. At the second stage the products of detonation further accelerate the compacted porous layer. Between the explosive substance and powder a blanket is used. The application of powder coatings on a metal basis by a shock-wave working is made both in the firm and liquid condition. The change of the mechanism of formation of layers from the firm to liquidcondition is determined by the values of the speed of collision of powder with the substrate and speed of sliding of the shock wave along the substrate. For the process in the firm phase the layer of a coating consists of compacted, deformed and welded among themselves particles of the initial powder, and the strength of bond is close to the strength of the monolith. Collision of the powder with the speed exceeding critical results in the formation of layers from a liquid phase with the formation of cast structures and the strength of bond at the level of the monolith. The thickness of layers can reach 10−100 nm.
The monolithic metal substrate at impulse working remains cold. Liquidformation of superficial layers can be considered as dynamic hardening from a liquid state. A number of features is inherent in the given method: powder melting occurs witha high speed of heating, that allows to avoid burning out and oxidation of elements when working high alloyed and powder mixes; a high speed of physical-chemical reactions in the shock front allows to receive layers ofmetastable phases or oversaturated solutions and compounds of powder mixes directly during the time of working; transfer of powder metal in a melted state and its cooling with the speed of about 1 mln. degrees per second create conditions for fast crystallization.
During the explosive welding at modes r < rmin and r > rmax (r - parameter of welding) the connection of the welded materials does not occur, but nevertheless, on the colliding surfaces a thin metal layer is formed. At r < rmin the basic mechanism of the formation of a coating is connected with a frictional interaction of colliding surfaces, and at r > rmax, when the pressure in the zone of contact exceeds2т (т – a limit of fluidity), multiple chipping from the colliding surface of a thrown plate is observed. As a result of it a collision and welding of metal chips to the surface of the substrate take place, then additional caulking of chips in the surface by the impact of a cladding plate with the following recoil of the latter occurs.
The connection also does not occur in the case, when the received connection is collapsed by the waves of unloading because during the deformation of the metal in the zone of connection a large amount of energy changes in heat, and the most deformed surfaces of the zone melt and then harden when heat transfers in thesurrounding layer of metal. At the movement of the point of contact the zone of high pressure is replaced by the zone of the stretching stress. When the melted section of the metal has no time to harden, there is destruction of the zone of connection. The zone of mixingof melted at a collision metals remains on one of the surfaces.