New explosive coating technology and structural performance

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Kremenchuk Mykhailo Ostrohradsky National University

New explosive coating technology and structural performance

Zagirniak M.V. mzagirn@kdu.edu.ua

Dragobetskii V.V. vldrag@kdu.edu.ua

Summary

explosive processing metal industrial

Purpose analysis of the variety of factors of the physical phenomena accompanying the process of the power explosive 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 a steady explosive cladding of piece surfaces with coating and film thickness of 200300 nm and graphite coatings and films with the thickness of 10100 nm are determined. The technology 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 coatings with the thickness of less than 0.5 mm allowing to receive a clad metal connection with the strength of bond, equal or exceeding the strength of 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; coatings of 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.

Categorize your paper under one of these classifications: technical paper.

Coating Technology and Structural Performance.

The introduction. As a result of the power impact formed at the explosion in the material of a piece there arises disturbance 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 disturbance has 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 disturbance arises 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 the exit of the wave of stress on the surface or at 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 disturbance to 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 the workable 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 the necessary forming a 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 welding is 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 their liquid 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 essentially new 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 impact on 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 explosive power 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 Godfield steel.

Material and results of researches. A shock-wave character of the explosive loading causes a number of the physical phenomena not inherent to the static loading, such as occurrence of high temperatures at the front of a powerful shock wave with formation of high-temperature phases, polymorphic transformations, crushing of grains and formation of twin 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 have shown an 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 of metastable 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 drops of 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 rotating disk, 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 piece under 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 resulting in 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 collision of the powder with the firm surface under the formula [2]

(1)

where _о, С_о density 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 possible to 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 v₁ of 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 3.510⁶ degrees 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 in the interval of temperatures of 700-350^o. 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 inclusions of 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 in melting 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 ensure melting of the contact layers and a high speed of cooling. The quantity of cast inclusions increases as 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 formation 3. The size of the welding gap h 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 ensure the 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 substrate is 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 formed and substances acquire 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 working opens new opportunities in the field of applying thin and film coatings with the thickness of about 100 nm and less.

The coatings received during a joint plastic deformation at the explosive welding surpass all existing methods of cladding in the strength of bond with a substrate. It enables to use such pieces with coatings in the conditions of the dynamic loading. Besides, the coatings are corrosion resistant, antifriction, hydrostable, heat-conducting and durable.

Certain difficulties arise at applying coatings 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 of clad 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 coating can be applied on 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 the coating onto the substrate takes peace. The strength of bond thus surpasses the initial strength with the run-in plate. To produce coatings 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 cumulative jets:

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 liquid condition 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. Liquid formation 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 with a 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 of metastable 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 < r_min and r > r_max (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 < r_min the basic mechanism of the formation of a coating is connected with a frictional interaction of colliding surfaces, and at r > r_max, when the pressure in the zone of contact exceeds 2_т (_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 the surrounding 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 mixing of melted at a collision metals remains on one of the surfaces.

The stable production of coatings with the thickness of 10?100 nm is mostly expedient for carrying out at r<r_min and activation of the frictional interaction. It is achieved by sliding of a clad layer on the substrate. For this purpose a charge is placed on a face surface of the cladding piece, or the cladding piece is made wedge shaped.

By its nature the process is close to the processes of frictional brass plating, bronzing and copper plating of steel surfaces. The purpose of the process is protection of pieces from setting at run-in, reduction of deterioration in the subsequent work. In the layer containing copper it is possible to excite a selective transfer.

It is technologically convenient to apply a coating, using adhesive compounds. Glue is put on the technological plate, for example, silicate or liquid glass. A cladding layer of powder of the necessary composition is put on the glue. At a collision with the cladding surface the layer of powder is transferred on it. The hardened glue recoils.

The interaction of the falling and reflected shock waves in the loaded material results in occurrence of increasing stretching stresses, which can result in destruction named chipping. This phenomenon is most of all influenced by the form of the wave of stress and a limiting value of the destroying stress _в of the loaded material. The location of a chipping crack in a solid material or the place of separation in a layered material depends on the form of the wave of stress. When the wave propagates in the material, the steepness of curve (t) behind the front of the wave decreases with the increase of the thickness of the loaded poly- or monomaterial , that results in the increase of chipping thickness . At propagation of the wave of stress of large intensity 2_в multiple chipping occurs, i.e. several following one after another parallel chipping 3.

Modifying influence of shock waves is used for intensification of diffusion processes in the technologies of chemical-thermal treatment.

As diffusion is made by consecutive moving of separate atoms, for movement of any given atom in a solid the atom must acquire certain energy of activation. It is established, that any factor promoting the increase of the initial energy of the atom, thus promotes reduction of that additional energy, which is necessary for activation of the atom at diffusion.

The factors promoting a directed flow of the substance are the following: a gradient of temperature, gradient of stress, gradient of an electrical field, etc.

In our research a gradient of stress and temperature represents the greatest interest. When shock waves pass through metals the gradient of stress makes up about hundreds of thousands atmospheres per some micrometres, and it is obvious, that in this case D is considerably important. The estimation of the factor of diffusion under the influence of shock waves is connected with large difficulties as the time of the process is not enough, and it is difficult to allocate this effect in the pure state. The passage of the shock wave is inevitably connected with the increase of temperature, change of defects of the crystal structure, effect of the temperature field after unloading, concentration heterogeneity and other factors.

The essential influence on the factor of self-defusion the speed of deformation at plastic deformation exerts. The increase of the speed of deformation from 10^-4 up to 210^-6 sec^-1 at the temperature of 750^o Centigrade results in the increase of the factor 2,500 times. Abnormal acceleration of diffusion in various combinations of metals and iron carbon couple was observed at the impulse deformation with the speed of 20 sec^-1, and also at the impulse welding with the speed of 10² sec^-1. The received values D for self diffusion surpass the factor of diffusion of iron in a liquid state. In the range of speeds from 10^-2 up to 10² sec^-1 there is also an increase of the factor of diffusion, resulting from the increase of the average concentration of vacancies, the number of which is estimated with the help of the dependence

,(3)

Where =0.25 factor which takes into account a share of formed dislocation thresholds, which can move convectively; distance, at which every dislocation moves; the Burgers vector; distance between dislocation dipoles at the front of the shock wave 4. The considered process is applied at hardening matrixes from low alloyed steel for manufacturing lime-and-sand bricks by cementation (fig. 2). Their preliminary explosive loading has reduced the time of endurance in a furnace 3 times. Thus their durability has increased 1.52 times.

Fig. 2 Matrix for lime-and-sand brick

Hence, one of the areas of application of the explosive working can become a modifying effect of shock waves and high-speed deformation on a material, which further will be subjected to chemical-thermal and other kinds of treatment. After the explosive loading the majority of metals change to in the activated state, and a number of the processes of chemical-thermal treatment and application of coatings (cementation, borating, aluminizing, nitriding, etc.) proceeds much faster at the same parameters of quality.

Further we shall consider the technological applicability of the chipping phenomenon. The interaction of the falling and reflected shock waves in a loaded material results in the occurrence of increasing stretching stresses, which can lead to destruction named chipping. This phenomenon is most of all influenced by the form of the wave of stress and the limiting value of the destroying stress _в in the loaded material. The location of a chipping crack in a solid material or a place of separation in a laminated material depends on the form of the wave of stress. At propagation of the wave in a material the steepness of the curve (t) behind the front of the wave decreases with the increase of the thickness of the loaded poly or monomaterial , that results in an increase of the chipping thickness . At propagation of the wave of stress of large intensity 2_в multiple chipping occurs, i.e. some following one after another parallel chippings occur.

During the explosive working which is not connected with splitting and destruction, chipping occurrence is an undesirable phenomenon. Usually for prevention of chipping a reflecting surface of the material is directly supported against the other body absorbing energy 2. The prevention of chipping is also made by the use of fascinating plates and mastics 2. Thus chips absorbing a significant quantity of energy, can be used for performance of a number of auxiliary operations.

The useful application of chipping is, first of all, separation of layered products for their recycling after performance of their designation. For this purpose on the outside surface a layer of the explosive substance is applied, the parameters of which are chosen so that the destruction takes place on the interface.



Fig. 3 Scheme of production of items with the monolithic edges:

1 explosive substance; 2 layered piece; 3 spacer; 4 matrix

At connection of multilayered items among themselves by the methods of fusion welding large technological difficulties arise at separation of the stack as a result of thermal heating by an electrical arch. It is desirable to ensure solidity of edges and dense, without bonding, bearing of item layers. To solve this problem is possible as follows (fig. 3). Preliminary made items 1 are collected in a stack and deformed unless a required configuration of bottom 2 is produced. Then separation of the edges of bottom 2 is made. For this purpose the bottom is placed in matrix 3 so that the edges of bottom 2 bulge out above the plane of the top end face of matrix 3, for what a spacer 4 is put in a cavity of the matrix. After that along the outside perimeter of the edge of bottom 2 a ring charge of an explosive substance 5 is put and its initiation is made.

As a result of the effect of the explosion there is separation of layers of multi-layer bottom 2 at its edge. After that the multi-layer bottom is placed in matrix 3 and the explosive welding of the bottom edges is made with calibration and compaction of layers.

Thus, the chipping effect finds its use as a functional and intensifying factor and is connected with working and production of layered items and use of lamination both of constructive and technological methods. This method has found application at manufacturing layered vessels of high pressure (fig. 4).



Fig. 4 Vessels of high pressure

The process of the explosive welding can find application not only for cladding plates and cylindrical shells, but also for cladding and hardening surfaces of pieces of complex configuration.

In connection with the development of manufacturing special profiles by extrusion, the opportunity of manufacturing axle-box cases of railway cars from aluminium alloys has appeared, a high corrosion resistance of which allows to exclude expenses for anticorrosive treatment, to reduce repairs because of corrosion, to lower the weight of running parts of railway cars by 2030 %. However aluminium alloys have no sufficient antifrictional properties, therefore it is expedient that the surfaces of an aluminium axle-box case, contacting with steel surfaces of neighboring parts, are also made of steel. In comparison with the other known ways of applying steel coatings on the working surfaces of aluminium pieces a lot of advantages are accumulated in the way of applying coatings on the basis of the explosive cladding. They are the following: strong welded connection of metals along the surface, preservation of structural characteristics of the clad metal because of complete absence of thermal effect on it, production of coatings of high durability, caused by the processes of hardening of welded materials in the zone of contact, the durability grows in to the depth of a coating.

The process of cladding axle-box cases by durable steel plates is mastered. The axle-box cases of railway cars are made of aluminium-magnesium alloy. While in service, deterioration of lugs on axle-box cases made 3040 mm (the friction couple is steel - aluminium alloy). For wear prevention it is recommended to clad axle-box case lugs by plates from carbon steel with the thickness of 46 mm. The shear strength of the clad layer should be in the limits of 1215 MPa. The processes of spraying, overlaying, knurling by rollers did not provide the necessary parameters of the clad layer. The decision therefore was made to test the process of explosive welding.

Aluminium and iron form a chemical compound at interaction, therefore metals of this group are difficult to weld by all kinds of welding, including by explosion, which is carried out in a rather narrow range of parameters of collision. Besides in this case the process of the explosive welding is complicated for some reasons. First of all, the area of the welded surfaces is small. And if the length of lugs fits in the range of a steady mode of welding, the width of 40 mm is not enough. The axlebox case contains two lugs, the distance between which is 150 mm. The air shock wave covers this distance approximately in 4.510^-4 sec. Therefore at a simultaneous cladding of two lugs synchronization of blasting of two charges should be in the limits of (1.52)10^-4 sec. Besides, there is deformation of the axlebox case and propagation of wave disturbance along the metal at a shockwave loading.

The cousideration of all these factors has allowed to solve the task of cladding lugs of axlebox cases by steel plates. Steel plates preliminary were cladded by plates of technically pure aluminium. The thickness of a layer of aluminium was chosen under the conditions of the minimal extrusion of metal on edges and subsequent minimizing the parameter of welding r (a ratio of the weight of the explosive substance to the weight of the welded plate). It practically excluded edge incomplete fusion. For elimination of the incomplete fusion in the initial zone of the process connected with the absence of an oblique collision in this zone and clearing of the welded surfaces, we applied the scheme of the explosive welding folding a corner of the plate. Steel aluminium clad metal (2,000x40x4x2 mm) was welded to lugs of axlebox cases. The modes of welding were selected to provide the required strength of the welded joint and minimal deformation of the lugs and axlebox case as a whole, i.e. the parameter of welding r=0.6 and the speed of collision 500600 msec^-1 corresponded to the minimals.

At the dynamic loading of case blanks produced by extrusion, deformation was not observed. Finished cases had an elliptic internal aperture. Therefore at the explosive cladding of finished products it was necessary to use expanding mandrels, preventing deformations of the case and dampening the refracted and reflected shock waves resulting in decrease of the strength of the welded joint or even in separation of a clad layer.

The following parameters of the explosive welding were determined, r=0.6, h=3, D=2,200msec^-1, Н=35 mm. Here h welding gap; thickness of a plate, D speed of detonation, Н height of a charge.

The received welded joints were subjected to standard tests for breaking off and shearing. The break of the samples which were cut out along the weld, occurred on a less strong material, that testifies to a high quality of the welded connection. An experimental batch of axle-box cases with lugs was made. The lugs were cladded by steel plates and sent for operational tests. One of the samples with a plate welded to one of the lugs (for presentation) is given in fig. 5.

Fig. 5 Axle-box case with welded steel plates

Not less effective is the direct explosive hardening of lugs of aluminium cases either by a shock wave or by an impact of a solid body, thrown by explosion. The process of hardening by a superimposed charge of the explosive substance (shock wave) is technically simpler and easy to perform. In this case the labour input of the process several times reduces as there is no necessity for manufacturing clad metal steel-aluminium plates and folding their corner. The operations of installation of remote elements and clad plates are excluded. After hardening the hardness of lugs through all the thickness of the lugs grows by 4045 %, the limit of fluidity by 50 %, and the limit of strength by 1015 of % and corresponds to the properties of carbon steel approximately by 20 %.

Higher parameters of strength and hardness are reached at hardening lugs by the impact of a solid body, thrown by explosion, at the parameters of collision, which provide melting-down of the superficial layer of lugs of the axle-box case, without formation of a welded joint, i.e. the parameters of collision are outside the limits of the parameters of the explosive welding (speed of detonation of the explosive substance more than 1.3 times surpasses the speed of a sound of aluminium-magnesium alloy), but also do not reach the parameters of hardening by explosion. If the surface of the axle-box case lug is given a relief ensuring formation of counter cumulative jets of a melted aluminium-magnesium alloy at a collision with the thrown plate, the microlayer with microhardness appropriate to the hardness of titanium alloys is formed. Under such conditions the speed of cooling of the counter cumulative jets reaches about 1 mln. degrees per second 3, conditions for fast crystallization are created.

The process of cladding basic surfaces of axle-box cases, made from high-strength pig-iron plates, by Godfield steel plates was tested under the offer of German railway car constructors.

One of the basic research problems of the process of the explosive cladding is the opportunity of forecasting formation of a welded joint at certain modes of collision for materials with known physical-mechanical properties. Despite of abundance of the scientific and technical saurces on determination of the area of welding of various material combinations, there are no data on welding pig-iron and Godfield steel. However the arising demand for such combinations has resulted in the necessity of creation of the explosive welding of pig-iron and Godfield steel with copper.

Some uncertainty at the experimental determination of the modes of collision, ensuring welding by explosion, is connected with the absence of the precise criterion ascertaining the result of welding of pieces. Basically pieces are considered to be welded only in that case, when the strength of a seam is as strong as the initial materials. Such a criterion of weldability seems to be acceptable at the explosive cladding. Usually a welded seam at such working is strengthened and exceeds the strenght of less strong metal from the combination. If this condition at the explosive welding is not fulfilled then, as a rule, it results in stratification during the subsequent working and operational loadings. But in a number of cases a low strength of bonding layers is required. In such cases it is considered, that the welding did not work when the strength of the seam tends to zero. Probably, from our point of view, it is necessary to limit the strength of a welded seam to a minimum in each specific case, proceeding from the conditions of operation of the clad product. For example, the association of boiler installations workers in Japan as a criterion uses shear-strength of binary combinations welded by explosion. For the majority of clad metals the criterion is accepted equal to 140 MPa, for steel cladded by copper or copper alloys it is 100 MPa, for steel cladded by aluminium it is 50 MPa. For steel with a corrosion-resistant coating used for vessels of high pressure the allowable shear strength makes 200 MPa.

The strength of joints received by the explosive welding of various combinations of metals insignificantly changes over a wide range of change of the modes of collision. Transition from the modes with a satisfactory strength to the modes not ensuring formation of a welded seam (the strength of a seam is equal to zero), occurs in the narrow limits of changing of the modes of collision.

When the model of an ideal liquid 1 is used, two parameters characterizing an oblique collision out of a class of parameters are chosen as the most essential: either the angle of collision and the speed of the contact point v_к, or the angle (dynamic angle of rotation) and the relative speed of the contact point (a ratio of the contact point speed to the sound speed).

The area of the explosive welding is limited on the plane , v_к by four curves. Physical sense of these curves is the following: the right border corresponds to transition to supersonic modes of collision, the top border corresponds to excessive heating of the connection zone. The bottom and left borders depend on the termination of wave formation. The welding of high-plastic metals (aluminium, copper and some their alloys) is possible without formation of waves. The true left border of welding is set for them.

The area of the explosive welding can be presented in a three-dimensional space (angle of collision, relative speed of the point of contact, average weight or thickness of colliding plates). The three-dimensional area of the explosive welding practically is used rather seldom. The introduction of the third parameter is connected with critical conditions of the existence of a viscous current.

The physical mechanisms limiting the values of viscosity at superhigh speeds of deformation, depend on a specific nature of the deformable body. In the given concrete case, it is mostly probable, that for fragile pig-iron restriction are due to a minimum of the shear deformation . A criterion of the bottom border is used in this case .

The minimum of the thickness of a plastic soft layer is determined such that not involving a fragile material for copper and pig-iron into the process of deformation

, (4)

Where , dynamic limits of fluidity of copper and steel; intensity of speeds of deformation; B, n - hardening constants.

Approximately the minimal size of the thickness of the plastic layer can be determined from the ratio for compression of the plastic layer on a rigid surface. The maximal pressure is taken either in the acoustic approximation p=v², or in hydrodynamical p=0,5v². The thickness of the layer ignoring the inertial components will make:

(5)

Where limit of fluidity of a sublayer; length of the piece.

We take this value for minimal, which corresponds to ~1.8 mm.

As in practice it is inconvenient to use the spatial area of welding, it makes sense to determine the minimal and optimum thickness of the plastic layer. The optimum thickness for small values of the dynamic angle of rotation is determined from the energy criterion and makes

(6)

Where optimum value of the kinetic energy.

This kinetic energy corresponds to the optimum size of a welding gap ensuring speed-up of the piece to the maximal speed of collision. At the same time it depends on the weight of a mobile piece, and hence, on its thickness.

With the purpose of unification of the experiments in determination of the quality of welding depending on the thickness of a layer, it makes sense instead of the thickness of a layer to consider a ratio of weights of the clad plate and a layer within the area of welding on parameters and v_к and their invariance. The samples welded by explosion were tested for destruction at shear (operational requirements regulate shearing forces). As a result, the dependence f (h/h_пр,_ср) for the steel+copper+pigiron couple (fig. 4) is received.

Having determined the optimum allowable size of a layer, we build the area of welding in the coordinates (v_к;) at the fixed value of h.

The connection of copper with steel by traditional methods of welding is a rather complex task. Iron and copper have strongly differing temperatures of crystallization, bad wetting with respect to each other and a limited reciprocal solubility in a solid state. Nevertheless, the explosive welding provides a strong connection for this couple in a rather wide range of the parameters of collision. The dependence between the parameters of collision and copper strength is not determined, since practically on all the modes of welding destruction occurs in copper. Tests for formability do not result in stratification.

The process was carried out as follows. The explosive welding was carried out under a parallel scheme. In this case v_к=D. The variation of the speed of detonation was carried out by addition of phlegmatizers in the explosive substance. The dynamic angle of rotation was varied by the change of the parameter of welding, that resulted in a change of height of a charge of the explosive substance, and consequently, the speed of detonation. In the experiments plates from technically pure copper with the thickness of 46 mm and cast blanks from Godfield steel and high-strength pig-iron were used. After the construction of the copper Godfield steel area of welding the technology of the explosive cladding of lugs of the axle-box cases by plates from Godfield steel welded to copper plates was tested. The mode of the welding excluded formation of eddy zones and minimal presence of melt.

The speed of detonation of the explosive substance was preliminary measured according to the Dotrish method and the method of registration of time of unfolding strips of foil put in the layer of the explosive substance. The received data on the area of welding of the researched combinations were compared with the data received from the experiments on throwing a copper plate by a wedge charge of the explosive substance. The height of the charge of the explosive substance was chosen so that on the initial sector there was a sound welding of plates, and on the final sector the welding did not work. The thickness of the copper plate was changed in the limits from 2 to 18 mm. Such experiments have allowed to determine the area of welding and the dependence =(r). As the plates thickness grows the divergence of the experimental data takes place in the interval from 5 % to 20 %. At thickness of the plate higher than 12 mm the welding by explosion does not occur, that is probably connected with dependence of the parameters of the external loading necessary for bending of a thrown plate on its thickness.

The results of the experiments are given in tables 13 and fig. 7. The necessity for performance of the experiments was connected with the absence of data on welding copper with pig-iron and Godfield steel and a significant disorder of experimental and theoretical data on the assessment of the area borders of welding homogeneous and heterogeneous materials.

Table 1. Limiting modes of welding

№	Godfield steel	High-strength pig-iron
	vк=D	r	, radian	vк=D, msec-1	r	, radian
1	1,790	0.42	0.12	1,820	0.39	0.12

Table 2. Modes of welding copper with Godfield steel and pig-iron

vк, msec-1	, degrees	Result (Godfield steel)	Result (pig-iron)
4,450 4,330 4,240 3,860 3,920 3,990 2,830 2,200 1,780 3,100	14 12 16 17 10 14 10 10 20 12	- - - - + - + + - +	- + + + + - + + - +

Table 3. Characteristics of ammoniac saltpeter explosive substances

Description of Explosive Substance	Composition %	Speed of detonation D Hвв=dкр, msec-1	Critical diameter dкр, mm
	Ammoniac saltpeter	Trinitrotoluene
Ammonite 6ZhV	71	29
Welding Ammonite АS1	85	15		1820
Welding Ammonite АS2	90	10	(3,0003,100)*
Welding Ammonite АS3	94	6		2428
Welding Ammonite АS4	99	2	1,7001,900*	3840

* data received experimentally. Others are taken from references 1,2.

Fig. 6 Dependence of the shear strength of connection on the thickness of a layer



Fig. 7 the Left and bottom borders of the area of welding of a clad metal of Godfield Steel+copper with high-strength pig-iron

For charges with the thickness of 30-100 mm the use of dependence (7) gives a divergence with the experimental data within the limits of 18 %.

D=1300+37,0H-10,62C- 8,04H²+0,6008C²+6,145HC-0,004628HC², (7)

Where С percentage of ammoniac saltpeter; Н - thickness of a charge, mm.

Thus, the optimum sizes of thickness of a layer and the border of the area of welding of pig-iron with plates from Godfield steel, cladded by copper are received.



Fig. 8 The axle-box case made from high-strength pig-iron

Conclusions

1. The study and estimation of the variety of factors of the physical phenomena accompanying the process of power effect of the explosion allowed to develop essentially new processes of application of thin metal coatings, superfast crystallization, dynamic hardening and diffusive saturation of the superficial layer.

2. The technology of application of thin metal coatings with the thickness of less than 0.5 mm (restrictions on the process of the explosive welding) allows to produce clad connections with the strength of bond, equal or exceeding the strength of the substrate.

3. Modifying effect of shock waves prior to chemical-thermal treatment allows to reduce the time of the process of cementation 1.5…2 times.

4. The processes of superfast crystallization allow to produce superfirm powder materials, application of which allows to increase hardness of the superficial layer of a product 3…4 times.

5. The technology of the explosive cladding of basic surfaces of aluminium and pig-iron axle-box cases is developed. The parameters are determined and the area of the explosive welding of pig-iron with Godfield steel is constructed.

Reference

1. Materials Science and Engineering - It Past and Its Future. M.C.Flemings in Transactions of the Iron and Steel Institute of Japan, Vol. 26, No. 2, pages 93100; 1986.

2. Palmour H., Lains W.D., Springs R.M. Recent trends in indestanding dynamic Compaction of powders // Internat.: Round Table Conference ondientering 5 / Procledings of / Amstodam. Partoroz. - 1982. C.611618.

3. Crossland A.B., Braithwaite B.M., Chapman H.E. Explosive Plugging Development of Nuclear Heat Exchangers // Welding and Fabrication in the Nuclear Industry. London. 1979. P. 297303.

4. Schwars, A. Theory for the Shock Wave Consolidation of Powders // Acta Met. 1984. V. 32. № 8. P. 1243 1252.

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