Cladding of flat plates by explosion welding process description
Introduction
The technology of laminating a cladding metal layer onto a base metal makes it possible to weld various metals combinations. The method is based on using for this purpose the energy of explosive detonation. Explosive composition is distributed on the cladding metal surface.
The process runs at the detonation velocity across the area limited by geometric dimensions of cladding layer, and therefore it is not controlled. However this process can be adjusted and controlled at the preparation step through selection of welding parameters.
The direct power source for welding of interacting cladding and base metals surfaces is the energy of their kinetic interaction, directed and distributed over the limited area. In this sense, the explosive detonation on the cladding layer surface is just the initial energy source, and if not being organized properly, the process will not reach the required interaction and proper welding of the surfaces. In this context, the energy capabilities of explosive are not decisive, as it is needed to provide the specified acceleration of cladding surface and its proper interaction with the base metal under the specified angle and within the limited area.
In order to provide the necessary parameters of impact of the surfaces, the base metal and cladding metal are positioned parallel to each other being fixed on the special supports, which during colliding of the surfaces are removed by a gas flow in the gap. The size of this gap is one of the decisive parameters of the process.
At the explosion, the pressure Р and temperature Т of the explosive layer on the cladding surface, being equal to environmental Р and Т, are transformed at the detonation velocity into Р and Т of detonation products. They start expanding through environment until they again reach the balance condition with environment.
In case the interaction of detonation products and the cladding layer were not limited in space and time, the interaction of these two objects would run according to the momentum conservation law as:
Thus, it may be said, that there is a relation, which determines the velocity of impact between cladding and base metals:
The explosion produced on the cladding metal surface is limited in space by the given surface, and the gas velocity on its surface is equal to zero. But this velocity reaches maximum at some distance from the cladding metal surface. Efficiency of transferring of explosion energy from detonation products to the cladding metal depends not only on the temperature and pressure, but also on the density of the gas mixture.
Momentary pressure impulse on the cladding metal surface, which can reach 60 GPa, is a result of affection of detonation products acceleration, expanding through the unlimited free space. In this sense, the pressure curve is “inversed” to the velocity curve (Fig.2)
.
According to the thermodynamics, this system (cladding layer – detonation product) is of an open type, and thus, the considerable part of energy of detonation products instantly dissipates in the form of heat and radiation, which is carried away by the detonation products.
The heating of the base and cladding metal during the cladding process takes place mostly due to their deformation and friction caused by impact and usually reaches 45-70 °С averaged over all mass.
The value of acceleration (impact velocity) is defined not only by the explosive properties, but also by the gap distance, as on its linear interval the velocity of the cladding metal layer changes from V=0up to VCM.
While developing of this welding method it was found out that using of chemically pure explosive and stoichiometric explosive mixtures do not lead to the desired result.
The specified parameters of impact of the base and cladding metals can be stably reached, so as to use the mixture of an explosive itself and a ballast substance. If so, the ballast substance provides not only the specified impulse, but also smoothes down effect of explosion parameters upon the cladding metal, because a part of detonation energy goes on “transformation”, including heating of the ballast.
Thus, the second integral parameter of the process is the chosen composition of the explosive mixture.
The joint kinetic properties of the cladding metal and the flow of detonation products directed outward the cladding metal define the necessary impact parameters. Due to this reason, the sizes of the gap and thickness of explosive layer differ by the different thicknesses of the plates to bond and different types of metals.
As it was mentioned above, the welding process runs on the limited area of impact of cladding and base metals, and not over all surface. In order to make the process successful, it is necessary to reach the optimal impact parameters.
Fig. 4 shows the «cross section» of the detonation wave.
Due to the specially selected way of exposure of detonation products, which follow directly the detonation wave front, occurs the momentary plastic kinetic displacement (precipitation) of the cladding plate upon the surface of the base metal. The strength of this exposure is defined by mass and velocity of cladding metal falling down on the base metal surface per unit time and pressure of detonation products just behind the detonation wave front:
At the same time, out of the impact point are distributed the stress waves, the velocity of which is equal to the sound velocity of this type of metal. The detonation wave and consequently the impact point are moving along the base metal surface with the subsonic (relatively to the metal) velocity. The optimal velocity for providing the reliable welding is within the range 2000-2400 m/sec. Alternation of elastic deformation zone and plastic deformation zone of the base metal forms the wave-shaped surface of the interface between cladding and base metals. Presence of the described surface is an indirect proof of the quality of the received bond.
As it follows from the Fig.4, expanding of cladding metal during its impact precipitation on the base metal surface causes its thinning in proportion to angle α and post-braking (fixing) on the base metal surface due to action of frictional force, because the impact velocity vector is not perpendicular to the surface of the base metal. The described mechanism of interaction between cladding and base metals determines the main heat source providing their reliable bonding, which is friction on the interaction area of extremely short length. Thus, this process by its meaning is close to another variety of pressure welding, which is friction welding. Being fixed relatively to the base metal due to bonding, the extension of the cladding metal causes the deflection of bimetal plate towards the cladding metal (the edges of the plate turn up).
To initiate the deformation process, it is necessary to form the specified angle α of colliding the cladding and base metals (see the Figure). When α ≤ 5, impact is “plane” and no welding occurs. The experiments with the liquid explosives based on nitric tetra oxide and kerosene (2-2,4 of trotyl equivalent) have shown that when the mass (specific weight) of the detonation product is small, the time of interaction with the cladding metal is so short, that even when the detonation velocity is within the specified range, the reflection of detonation products from the cladding metal runs so as the dynamic action and acceleration is not enough for its elastic-plastic precipitation. Some ballast added to this explosive mixture will help to attain the result near to the required. At large angles α the metal of cladding layer can be break.
To maintain all above parameters within the specified range, the surfaces of base and cladding metals should be specially pretreated. Friction and co-deformation of conjugating surfaces of base and cladding metals is the main source of heat, its intensity directly affects their reliable bonding. This process can cause both dissociation and ejection of oxide films and residual contamination inward the gap between the base and cladding metals, or form the ineligible compounds like carbides and intermetallics. The correct interaction can be reached by the balanced selection of bursting charge power, detonation velocity and gap distance (the dynamic component of the process), and by sufficient finishing and other pretreatment of the surfaces to provide the specified friction coefficient of the colliding surfaces, as the volume of the surface films created by the process and dissociating during bonding of the plates cannot be large.
Being thrown inward the gap between the base and cladding metals, the high-temperature products of interaction of their surfaces get in contact with the gas medium of the gap, which besides moves outward the conjugating line according to the “supersonic plunger” model, additionally heating it up. Thus, the total amount of gas in the gap and the products of interaction of the base and cladding metals surfaces are accumulating in a narrow zone, which moves in front of the bonding line, and are thrown outside beyond the dimensions of the base metal almost simultaneously with the completion of the bonding process.
Both excess of volume and pressure of the gas medium in the gap, as well as its additional heating over the designed value, due to improper pretreatment of the surfaces and wrong selection of an explosive, can form the faulty penetration zones and cause the outward gas medium emission.
Here are the minimum steps, which should be done about pretreatment of the bonding surfaces, in order to get success:
- the surfaces to be bond should not have any aside impurities and pollutions, except the alloys, of which they consist of. For that the surfaces are mechanically protected against the surface films and scale and are degreased.
- in order to provide the stable impact angle over all area of cladding and base metal surfaces, they should be of the specified flatness and roughness. These parameters are indicated in the present standards OST and TU and as a rule they do not exceed the following:
- flatness – 7 mm per 11 meters;
- roughness - Ra ≤ 6,3
- Mechanical parameters of the cladding and base metals should be within the specified combination depending on the properties of the metals to be bond, in order to reach the required parameters of the co-deformation in the zone of their interaction.
List of Operations
- Preparation of the components for cladding
- Preparation of the base metal
- Preparation of the cladding metal
- Preparation of the explosive
- Preparation of the site for cladding
- Transportation of the components to the site for cladding
- Transportation of the base and cladding metals
- Transportation of the explosive
- Assembling of the package Base Metal –Explosive-Cladding Metal for cladding and arrangements for saving the package parameters during the preparation stage.
- Cladding
- Parameters of impact
- Parameters of explosion
- Parameters of the process
- Machining of the clad plate
- 5.1. Transportation
- Straightening
- Heat treatment
- Surface finishing and cutting
- Protection of the surface
- Testing of clad plate and making the samples
- Enclosed documentation
- Approval by a Customer
- Packing
Description of Operations
1. Preparation of the components for cladding
1.1. Preparation of the base metal
Base metal is a massive hot-rolled or forged plate of high (adequate) physical properties and of lower than the cladding metal price.
Thus, the base plate should be heat treated in a proper way correspondingly to the alloy of the base, so as to reach the hardness, which could permit some plastic deformation of the surface necessary for the proper cladding, and also tempered for stress relief.
The slab of the base plate, which meets all above requirements, undergoes the operation of the surface treatment.
The purpose of this stage is as follows:
- to clean completely from scale and acid films
- to achieve the specified flatness, which should not exceed 2mm per 1 meter across all area of the base plate (the defects of the initial base plate are removed by hot rolling and forging)
- to achieve the specified uniformity of the surface up to the roughness, which usually does not exceed Ra ≤ 6,3 for different materials. Depending on geometrical parameters of the initial surface and a degree of its impurity with the rolling (forging) products, the surface is subjected either milling and polishing or only dressing and polishing. For the subsequent protection of a surface, up to installation of it into a package, it is possible to leave some residue of water-oil emulsion on its surface or coat it by a special film.
1.1.1. Plasma (vacuum-arc) pretreatment of the surface
Plasma pretreatment of the surface imparts the thin surface layer of the base plate (layer thickness is about 40-50 microns) with the unique physical properties unattainable at the other methods of preparation. The metal surface is cleaned in the cathode spots of electric arc. At the same time energy of density 1011 Wt/m2 releases. Under such exposure all chemical compounds on the metal surface dissociate, ionize and sublimate off the surface. Positive ions of metals, generated during dissociation of oxides and other impurities, are accelerated by electric field and settle back on the surface of base metal; atoms of non-metallic components generated gaseous substances and leave the exposure area. Thus, the layer of chemically pure metal is generated on the surface of the base metal. This layer is equal by its composition to the alloy of the base plate, but with no carbon, sulphur, phosphorus and other non-metallic components. If it is steel, the layer of chemically pure iron is generated. This layer has an ultra-thin structure of ferrite with grain size 100-150 nano-m. It is of high adhesive ability and not subject to corrosion both in humid environment and in water and even in salt solutions.
Besides, to modify the surface quality, the additional metals can be inserted into the arc. For the steel base plate such metals are Niobium and Vanadium. Such properties of the surface, either cleared or modified by insertions, make the weld more sufficiently stable and strong. The obtained properties can be kept during long period, it makes easier the transportation and preparation of cladding procedure on the fire ground.
Preparation of the base metal surface by plasma-treatment is not obligatory and to be additionally agreed.
1.2. Preparation of the cladding metal
As a rule, the cladding material is corrosion-resistant or has other special properties. It is rather valuable and to be economically used.
Thus, in most cases the cladding material is cold-rolled, of satisfactory geometry properties and low roughness.
That is why, the cladding metal is grinded up to the specified roughness Ra 6,3 micrometers (depends on the type of material), at the same time the surface is cleared of oxide films.
The same as for the base metal, if it is reasonable, the surface of cladding metal is protected until it is mounted in the package at the cladding site.
1.2.1. Plasma method of surface preparation
All mentioned in 1.1.1 also refers to the cladding plate surface. Such preparation is especially important for the metals having many dense oxide films. Formation of the thin dense layer of metal on the plate surface with the properties different to the properties of the cladding metal sufficiently widens the possibilities of the explosion welding.
Such treatment is not obligatory and to be additionally agreed.
1.2.2. Preparation of explosive
Preparation of the explosive includes the following operations:
- To check, if the initial parameters of explosive components meet the Standard requirements
- To reach the specified granularity
- To wet the explosive components up to the necessary condition.
- Preparing of the homogenous mixture of explosive and “ballast” filler.
- Packing for transportation to the site, where the package is assembled.
1.3. Preparation of the cladding site
Preparation of the cladding site consists in making of a special sand pedestal convenient for mounting of the package by personnel and lifting devices, and protecting the received article from the blast wave reflection.
2. Transportation of the cladding components to the cladding cite
2.1. Transportation of the base and cladding metals
Transportation should be arranged so as to keep the parameters and properties of the base and cladding metals, obtained during the preparation operations. Special handling and fastening facilities are used for this purpose.
2.2. Transportation of explosives
Transportation of the explosive components is performed according to the current rules, duty regulations and current legislation concerning the transportation of explosives.
3. Assembling of the package Base-Cladder-Explosive for cladding and arrangements for keeping parameters of the package obtained during preparation
A method to fulfill this item sufficiently depends on the season and weather conditions.
The base metal is installed horizontally onto the constructed pedestal. Then the surface of the base metal is finally cleared of protective coating and deoiled.
To protect the base metal from precipitation of condensed moisture, it is recommended to heat it to the temperature a few degrees higher than the air temperature.
Depending on the location of ignition point and geometry of the base metal, the support spacers are installed on the surface of the base metal. The support spacers should provide the guaranteed plate separation distance, prior calculated for the alloy types of the base and cladding plates, the type of explosive, mass and geometry. As a rule, the plate separation distance specifically corresponds with thickness of the cladding metal. It is a determinative parameter selected to achieve the desired impact velocity.
A frame (usually made of the wooden strips) filled with the explosive is installed around the perimeter of the cladding metal plate. The height of the frame corresponds to the prior determined depth of explosive layer.
An electrically driven detonator and a buster (it is some explosive of cylindrical form and of high detonation velocity, used when it is demanded by the explosive structure) are placed at the ignition point.
Mounting should be performed at the special weather conditions acceptable for this package during the specified period of time, in order to avoid precipitation of condensed moisture or other environmental pollutions.
When necessary, the preparation of the package can be performed under the tent and safety barriers against wind, atmospheric precipitates and other environmental factors occur during the installation.
Inert gases can be used for protection of the surfaces while cladding.
Safety barriers can be made of expendable foil.
4. Cladding
The type of explosive mixture and its amount is selected according to the principals described in the Introduction and should generate a specific energy release per square unit of cladding metal plate to provide a specific impact velocity. Besides, the detonation velocity should be subsonic relatively to the acoustic velocities of the base and cladding metals.
The explosive is uniformly and with equal density distributed on the cladding plate surface. Besides, a zone of forming (acceleration) of detonation wave is previously defective, thus it is necessary to predetermine correctly the location of ignition point, placing it either outside an article made impact afterwards or outside the zone of machining.
4.1. Impact Parameters
Impact parameters include the separation distance between the base and cladding plates and the detonation velocity. Both parameters determine an impact angle and impact velocity.
Since the detonation velocity for each specific explosive composition is an independent parameter, the desired impact angle and impact velocity are determined by the plate separation distance.
The optimal detonation velocity to achieve the required result is in the range 2000-2400 m/s. The impact angle is typically in the range 5-10°. The impact velocity is about 500 m/s, the pressure in the point of contact between base and cladding plates is developed up to 60GPa.
4.2. Explosion parameters
Parameters of explosion are the velocity of explosive detonation and its amount (depth of explosive layer).
The detonation velocity is determined by the explosive composition (type of explosive substance and type of ballast filler). Amount of explosive per 1 m2 of cladding plate surface is selected so, as to provide the specified impact parameters (please refer to item 4.1.). Amount of explosive grows in proportion to thickness and density of cladding metal.
4.3. Process parameters
All mentioned characteristics of the process could be united in a simple scheme showing the essential and non-essential parameters:
| Components of package | Parameters | Essential | Non-essential |
|---|---|---|---|
| Base metal | Material grade | Yes | |
| Thickness | Yes | ||
| Mechanical properties (heat treatment conditions) | Yes | ||
| Preparation of the plate surface | Yes | ||
| Cladding metal | Material grade | Yes | |
| Thickness | Yes | ||
| Mechanical properties (heat treatment conditions) | Yes | ||
| Preparation of the plate surface | Yes | ||
| Sizes and geometry parameters | Geometry | Yes | |
| Plate surface area | Yes | ||
| Ratio of plate thickness Base/Cladding | Yes | ||
| Location of zones of the ready product to be mechanically treated | Yes | ||
| Package parameters | Plate separation distance | Yes | |
| Explosive type and density, kg/m2 | Yes | ||
| Composition of explosive mixture | Yes | ||
| Ignition point | Yes | ||
| Detonation velocity | Yes | ||
| Material and shape of support spacer | Yes | ||
| Material of frame for explosive | Yes | ||
| Weather conditions | Yes* |
* - on condition of arranging protection of the package against dust, condensate precipitation on the surfaces to be bond and acceptable moisture content in explosive.
5. Operations performed on clad plate
5.1. Transportation
Loading and transportation of a ready cladding plate does not need the special technique and facilities.
5.2. Straightening
After cladding the plate is deformed by action of the blast wave. Thus, it should be submitted to a heavy mechanical treatment to restore its flatness.
Treatment can be as:
- Compacting, in case thickness and size are not large
- Rolling
5.3. Heat treatment
The obtained at the previous stage flat bimetal plate retains the stresses and higher hardness of the surface generated during cladding and afterward straightening.
To avoid these disadvantages, the plate is heat-treated according to the types of alloys and their combinations used at each specific case.
5.4. Surface treatment and edge cutting
The straightened and heat-treated plate is ready to be used for making the final product, i.e. bimetal plate of the specified geometry sizes, mechanical properties and surface parameters. For this purpose it should undergo all necessary machining operations prescribed in the Work Specifications.
5.4.1. Some part of the plate edge with possible defects is cut off (it is typically from 40 to 100 mm). Some edge defects are acceptable at this method of cladding. At this stage the test specimens are made and 100% ultrasonic test is performed according to the standards specified by the customer.
5.4.2. Some more finishing of the surface, like milling, polishing, etc. can be performed additionally.
5.5. Surface protection
The obtained bimetal plate is ready for sale and shipment.
On customer’s request the surface of bimetal can be coated by protective polymeric layer or primer, or be packed without protective coating.
6. Testing of clad plate and making specimens
The program and rules of testing are prescribed in TU5.961-11772-2001 «Steel/Titanium Bimetal Plates. Specifications».
7. Documentation to be enclosed
If it is not specially mentioned in a supply contract, it is obligatory to provide a customer with the certificates on a bimetal plate together with the test reports (described in item 6), as well as the certificates on initial materials (base and cladding metals).
8. Customer’s approval
The terms of approval by the Customer are stipulated by a supply contract at each case.
9. Packing
Packing of bimetal plate should save its geometry, and protect the surfaces and edges from mechanical damage.