BACKGROUND OF THE INVENTION

1 . Field of the Invention

The present invention relates to a method of bending metallic objects of uniform or variable thickness and also such objects made of brittle and/or very hard materials, such bending being performed along straight lines.

2. Brief Description of the Background of the Invention Including Prior Art

It has been of long-standing interest to find a method which allows to bend metal objects of constant or variable thickness, even made of brittle or very hard materials.

The presently known methods of bending this type of metallic objects are based on imposing a plastic deformation on the material by applying external forces of appropriate strength and direction. The bending is performed with the aid of specially designed machines and/or tools, often of a high output power, such as angle bending machines, press tools, and pressers.

External forces applied for bending the material produce plastic deformations accompanied by elastic strain. A permanent change in shape of the material is produced by the plastic deformation, whereas a part of the elastic strain originating from internal stresses which are not self-compensated disappears as soon as the application of external forces is terminated. This results in an undesirable additional shape alteration which is difficult to control during the bending process. The other part of the elastic strain which is due to the presence of self-compensated stresses remains in the material and may shorten the lifetime of the material when put to its intended use. This latter part of the stresses may vanish in the effect of a rheological process, in particular, when the material is exposed to high temperatures when in use. This produces further undesirable and hard to predict shape alterations of the object subjected to such a bending process. Therefore, presently known methods are not applicable for bending brittle metals, nor for bending materials of a very high strength and/or hardness.

Before explaining the method of the present invention, it appears proper to point out some of the basic thermomechanical properties of metals and their alloys.

A first basic property of metals and their alloys is their good thermal conductivity. This good thermal conductivity causes a fast even distribution of heat within a metal object, even between distant portions of the object at different temperatures. Therefore, if a large difference in temperature is to be produced on a relatively small strip, a very strong and concentrated source of energy must be provided and this is only possible for a very short time period. After the heating process has been terminated,

maintaining the obtained temperature difference in this strip portion requires, consequently, an intensive cooling.

Metals and their alloys used in the production of rods and plates are further characterized in that their mechanical properties, such as the limits of elasticity, the yield point and the maximum deformation beyond which the metal cracks and breaks, depend on the prevailing state of stresses and/or temperature. These dependencies are very complicated and are different for different materials. Therefore, it is practical to use certain simplified models and/or schematic diagrams which describe these dependencies as long as they are only used to allow a proper description of the processes. For the representation of this method, the literature of the subject knows two such models, which are described below and which correspond to two states of stresses: in a test of simple tension and in a test of simple uniaxial uniform compression.

Fig. 1 a is a schematic diagram of stress-temperature relationship in a simple tension test. The ordinate indicates the values of the stresses Re corresponding to the yield point and the maximum relative elongations A, which correspond to the cohesion strength/rupture stress. The temperature T is marked on the axis of abscissae. Fig. 1 a shows two principal temperature ranges, which correspond to two principal behaviors of a metal. In the temperature range 0 < T < T _{1 f} the yield point and the maximum relative elongations remain practically unchanged. This means that upon reaching the yield point Re, the material starts to flow and, at the same time, its relative elongations increase but only up to a maximum value A shown in the drawing, beyond which maximum value A cracking and splitting of the metal occur. In the temperature range T ₁ < T < T ₂ , the stresses Re corresponding to

the yield point diminish but, in contrast, the maximum relative elongations A increase. The limiting temperature f ₂ is a temperature at which maximum relative elongations A reach a value A = A. The value A is defined in the description to follow.

Figure 1 b is a schematic diagram of stress-temperature relationship in the uniaxial compression test. On the ordinate of the diagram there is shown the course of the stresses Re corresponding to the yield point Re and the maximum relative elongation (here contraction), which correspond to the cohesion strength. On the abscissa there is shown the temperature T. Fig. 1 b also shows two principal temperature ranges and the corresponding two principal behaviors of a metal . In the temperature range 0 < T < T the yield point and the maximum elongation (contraction) remain practically unchanged. This means that upon reaching the yield point Re, the material starts to flow and, at the same time, its relative elongations (contractions) increase but only up to a maximum value A, beyond which maximum value A cracking and splitting of the metal occur. In contrast, in the temperature range T, < T < T ₂ , the stresses Re corresponding to the yield point diminish, and the maximum relative elongations A (contractions) increase. The limiting temperature T ₂ is a temperature at which maximum relative elongations (contractions) A = A. The value A is defined in the description to follow.

The schematic diagrams of Figs. 1 a and 1 b represent linear approximations of the real curves of the course of the stresses Re and maximum relative elongations A in relation to temperature T, which real curves were obtained from a number of experiments performed on a given material. This linear behavior emphasize the approximate character of the diagrams in Figs. 1 a

and 1 b since approximation to an experimental curve can be made in a variety of ways. Although the diagrams such as in Figs. 1 a and 1 b may look quite the same, the characteristic parameters, i.e. Re, A, A, T^ T ₂ are different for the uniaxial tension and the uniaxial compression. The temperatures T ₂ and the corresponding maximum relative elongation (resp. contraction) A are determined experimentally to be such maximum values where metal creep effects are still practically non-existent, which metal creep effects are known to be intensified with the increasing temperature. The function Re (T) can be linearly extended to the intersection with the T axis at the point T = T ₂ , where Re = 0. The melting point T _m is also shown in both Figs. 1 a and 1 b. In the temperature range T ₂ < T < T _m the plasticity limit of the material is zero = (Re = 0), and beyond the temperature Tm the material becomes a liquid. In the process of bending with a laser beam a small overshoot of temperature above the melting point T _m to a temperature T _m = 1 .1T _m is admissible. Since the time spans of action of such temperatures are very short, practically only irregularities of the surface of the metal are subject to melting.

The above mentioned uniaxial tension and compression tests and their linear approximations allow to determine the characteristic temperatures T^ T ₂ , T ₂ , T _m , T _m for a given material subjected to bending by laser beam and, thereby, allow to designate the appropriate parameters for such a process.

There obviously exist metals (or alloys) whose thermomechanical properties cannot be described with a sufficient accuracy by linear models of Figs. 1 a and 1 b. These metals require other, more complex models based on respective experimental results. This problem, however, is outside the scope of the present invention.

The method of the invention comprises a two-phase process of heating and cooling of the material along a designated bending line.

In the first phase the material is subjected to heating by a concentrated energy stream which generates a thermal effect. By moving the energy stream having a power/energy/ SE with a velocity V along the bending line, the heating is performed such that the maximum temperature T _g of the irradiated surface disposed symmetrically on both sides of the bending line, measured immediately after the passage of the energy stream, satisfy the inequality: T, < T _g < T _m , while the maximum temperature T _d on the side opposite to the irradiated surface has to satisfy the following two inequalities: for plastic materials 0 < T _d < T ₂ for brittle materials 0 < T _d < T ₂ wherein the temperatures T _{1 t} T ₂ , T _m can be adapted from Fig. 1 a, which corresponds to a simple tension test, since both outside layers of the bent material are subject to extension during the bending process.

In contrast, Fig. 1 b which shows a schematic diagram of thermomechanical properties in an uniaxial compression test, allows to determine the degree of deformation of the material in the region of the neutral axis of the bent object, since this part of the material is subject to compression during the bending process with an energy stream.

When the maximum temperature T _g on the side irradiated by the energy stream is within the range T ₁ < T _g < T _m , plastic state is assured and a partial outflow of the liquid metal towards

the energy stream is generated and, consequently, a bending in this direction is obtained .

When the maximum temperature T _d on the side opposite to the irradiated side is within the range: 0 < T _d < T ₁ for plastic materials, an increased temperature gradient is induced on the overall thickness of the material and, consequently, the bending angle is thereby increased. In this case exceeding of the maximum relative elongations A on the surface opposite to the irradiated side is not associated with any crack hazard. However, the occurrence of plastic flow is possible.

The maximum temperature T _d on the surface opposite to the irradiated side is within the range: 0 < T _d < T ₂ for brittle materials and this is due to the necessity of increasing the maximum elongations A on this surface in order to prevent cracking. This measure is necessary even though the temperature gradient over the material thickness is thereby diminished and, in consequence, the bending angle is also decreased.

The temperature ranges determined above for a given material where the thermomechanical properties of this material are shown in Fig. 1 a, are obtained by a proper selection of the parameters of the energy stream, such as the power SE, the diameter D measured at the contact with the metal surface and the velocity V, in relation to the thickness L of the material. The heating process is considered to be correct, i.e. a concavity is produced on the irradiated side of the sheet, when the diameter D satisfies the inequality: D = 3L and the heating is accompanied by immediate cooling either by means of a natural heat distribution outwardly from the heated region into the surrounding material or, when necessary, by an artificial cooling, with the aid of a cooling

stream directed at the surface of the metal at an appropriate distance b behind the energy stream.

In the second cooling phase of the bending process, it is required that:

1 ) an amount of heat has to be removed from the material, so that the average temperature T _r during the time between the two subsequent heating and cooling phases of the bending process remain uniform and lower than T _{1 t} 2) the above described temperature conditions of the heating process, can be obtained not only in the sections located immediately behind the moving energy stream, but also within a certain distance b behind the stream. This distance must be reduced in some situations, where such a reduction is imposed by the requirements of the bending process. This can be achieved by artificial cooling with a stream of cold liquid or cold water spray, where the cooling stream is directed on the metal surface at a designated distance b < b ₀ behind the axis of the energy stream at its contact with the metal surface. The distance b is determined by the conditions of the bending process.

A permanent bending is effected during the cooling stage, since the material is previously heated on the irradiated side to a temperature T _g (such that T ₁ < T _g < T _m ) and then a shortening occurs perpendicular to the bending line, thus forming a concavity on the irradiated side. This results in a bending by a certain angle δ which bending angle δ corresponds to a single passage of the energy stream along the bending line. In case this angle is insufficient, the whole operation of heating and cooling must be repeated until the desired result is obtained. An analysis of the state of stresses generated in the cooling phase shows that the material being bent using the present method is being elongated

in the superficial layers in a direction perpendicular to the bending line, and shortened in the central layer.

These elongations should not exceed the maximum relative elongations defined by Figs. 1 a and 1 b because of the risk of cracking which may occur especially in the outermost layers.

The method according to the present invention allows bending of metal objects without applying external forces or metal forms. By using the method to bend flat sheet metal along a designated family of straight lines, various developable surfaces such as cylinders and cones can be obtained.

SUMMARY OF THE INVENTION

1 - Purposes of the Invention

It is an object of the present invention to obtain an improvement of bending metal plates.

It is another object to provide a bending method for brittle materials.

It is yet a further object of the present invention to provide for a controlled bending of metallic objects.

These and other objects and advantages of the present invention will become evident from the description which follows.

2. Brief Description of the Invention

The method according to the present invention comprises that the object is submitted to a two-phase process of heating and cooling along predetermined straight lines.

In the first phase the material is submitted to heating with a concentrated energy stream, which concentrated energy stream produces a thermal effect. The heating is applied either to the whole length of the straight bending line or point by point, by shifting the concentrated energy stream SE, with a speed V, along the bending line. The heating is performed in such a way that the depth of the heated strip of material which is subjected to deformation is smaller than the thickness of the bent object. This is obtained by moving a laser beam symmetrically along a predetermined straight line, on one side of the plate, which laser beam heats at every moment the surface of a circle in such way that the maximum temperature in the points of this surface, laying in the neighborhood of the straight bending line, reach immediately after the passage of the laser beam a value Tg, where the value T _g satisfies the inequality T-, < T _g < T _m . On the opposite side of the plate the maximum temperature reaches a value T _d , where the value T _d satisfies the inequality T ₁ or T ₂ , where: T→ - for plastic materials, where the yield point is practically constant at temperature T→ and should be T ₂ > T→ .

T ₂ - for brittle materials wherein the value of maximum relative elongations in the simple tension test increases with the temperature growth, the lowest possible temperature should be adapted, at which the elongations will already be larger than maximum relative elongations on the opposite surface, which prevents cracking and breaking of the material.

- for materials wherein the value of maximum relative elongations in a simple tension test decreases with the temperature growth, the highest possible temperature should be assumed, at which the elongations are still larger than the maximum relative elongations which occur on the opposite surface, in order to prevent cracking and breaking of the material.

- for plastic materials for which the maximum relative elongations in a simple tension test remain practically unchanged up to the temperature T _{1 t} and upon exceeding the temperature T→ the elongations increase, the lowest possible temperature T ₂ > T→ should be assumed, at which the elongations are larger than the maximum relative elongations on the opposite surface, in order to prevent breaking and cracking.

T _m - is an experimentally determined limiting temperature which is higher than the melting point of a given material, beyond which temperature T _m , eddy currents are generated and corrugations of surface occur, caused by the laser beam moving with a determined speed.

The heating is effected by means of a concentrated laser beam such that the predetermined temperatures are reached by a proper adjustment of the parameters of the laser beam such as power and moving speed in coordination with the above specified temperature Tm, as well as the ratio of the diameter D of the laser beam to the thickness L of the object being bent. The condition D < 5L should be maintained, where:

D - is the diameter of the laser beam

L - is the overall thickness of the bent object at its heated portion.

The material is plasticized and partially melted to the depth smaller than the thickness of the object.

The alternative phases of heating and cooling are repeated at least twice, while the predetermined temperatures have been maintained, until the desired bending angle is achieved.

In the second phase, the object is being cooled at ambient temperature or, additionally, with a stream of blown gas to a state, wherein the material looses its plasticity. This is achieved by cooling the locally heated portion with a cooling means, immediately upon shifting of the laser beam, until the local disappearance of temperature gradients on the whole thickness of the object, i.e. until the temperature difference between the temperature of the previously heated surface and a temperature T _r surface disappears, while maintaining the condition T _r < T→ , where: T→ - is the temperature in the cooling phase at the moment of fading out of the temperature gradient and difference within the thickness of the object, i.e. between the previously heated surface and the opposite surface.

During the cooling phase, the material within the previously deformed strip portion is contracted perpendicularly to the heated surface as an effect of thermal shortening of the material. This contraction results in bending the object by a certain angle δ in relation to the irradiation line, such that elongations ε _r on the unheated surface are smaller than the maximum elongation, relative to A and occurring during the effected process, within a temperature range, selected according to the above-described diagrams, where:

A - is the maximum relative elongation for the bent material in the effected process within the selected temperature range, ε _r -is the maximum elongation on the unheated side.

Frequently repeating the above-described process allows to bend the object to the desired bending angle. The method according to the invention allows the bending of metallic objects without requiring an application of external forces. It is possible with this method to change the curvature of the object from a distance under conditions when the access to the object is not possible. Besides, the method of the present invention allows to bend objects made of brittle and very hard materials which was impossible with the conventional methods.

Another very important feature of the present method is the speed of the processes of local alternative heating and cooling of the metal. This high speed results in that the time during which the individual crystallic grains are exposed to the increased temperature is very short, and thus a growth of the individual crystallic grains is prevented. Thanks to this feature, the present method of bending causes relatively small changes in the crystalline structure and mechanical properties of the material. In certain cases it is even possible to provide a certain control of these changes in a desired direction.

According to the present invention it is possible to generate deep strips of plasticized material which are surrounded by the original material , where the temperature of the surrounding material only slightly exceeds the ambient temperature and, subsequently, to cool the previously plasticized strips. The depth of this strip can be as large as 85% of the thickness of the sheet of metal. This allows to obtain a very high efficiency of bending measured by the obtained angle δ of permanent bending, which angle δ can reach up to 6 ^° after a single passage of the laser. A further advantage of the present method is that the concavity is always generated on the heated side.

In case of materials characterized by a considerable degree of reflection of the laser beam, an appropriate coating of an energy-absorbing substance such as carbon should be applied to the surface. This coating must exhibit a necessary durability in order to maintain its absorbing properties during frequently repeated process of heating and cooling .

The novel features which are considered as characteristic for the invention are set forth in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, in which are shown several of the various possible embodiments of the present invention:

Fig. 1 a - is a view of a schematic diagram of a simple tension test illustrating the relationship of stresses corresponding to a yield point and of maximum relative elongations corresponding to a cohesion strength as a function of the temperature of material; Fig. 1 b - is a view of a schematic diagram of a uniaxial compression test illustrating the yield point in relationship to the temperature of a material;

Fig. 2a - is a schematic side view of a bent object, in this case a plan-parallel plate; Fig. 2b - is a front view of the plan-parallel plate illustrated in Fig. 2a;

Fig. 3 - is a perspective view of the plan-parallel plate shown in Fig. 2a illustrating traces of an energy stream and of a cooling stream, and showing regions in different stages of heating and cooling; Fig. 4 - is a cross-sectional view of a heated fragment of a plate, showing regions of possible melting, regions of plasticity, and a bending angle δ characteristic for the final stage of the heating phase; Fig. 5 - is a schematic diagram of temperature distribution as a function of the thickness of a plate during the final stage of the heating phase, showing maximum, admissible temperatures on an irradiated side and on an unirradiated side;

Fig. 6 - is a schematic diagram of the distribution of the isotherms and the regions of flowing and plasticized material as illustrated in the cross-sectional view shown in Fig. 4;

Fig. 7 - is a cross-sectional view of a heated fragment of a plate during a final stage of a cooling process showing a region of maximum deformations and an angle of permanent bending; Fig. 8 - is a schematic diagram illustrating an internal stress distribution as a function of the thickness of a plate after a termination of a cooling phase showing maximum values of tensile stresses 5, and d, in the superficial layers and of the contracting stresses d. in the region of the neutral axis of the object; Fig. 9 - is a perspective view of a plan-parallel plate illustrating a bending during the final stage of a heating phase, where a slight convexity is produced on an irradiated side in relation to the degree and distribution of heat within the thickness of an object as well as to the stiffness of a material surrounding a heated region; Figs. 10a and 10b - are schematic diagrams illustrating the isotherm distribution at the initial and at the final stages of the cooling phase in the material of the plate shown in Fig. 5;

Fig. 1 1 - is a perspective view of the plan-parallel plate illustrating the bending during the cooling phase, where a concavity is produced on the irradiated side in relation to Fig. 3;

Fig. 12 - is an exemplified diagram of the distribution of stresses and elongations in a bent object in a cross-section of a bent plate;

Fig. 13 - is a perspective view substantially from the top illustrating the movement of the bending "wave" in case of a long bending line; Fig. 14a - illustrates a planar sheet metal before its forming into a cylindrical surface;

Fig. 14b - shows an already formed cylindrical surface obtained by performing a sequence of bendings;

Fig. 15a - is a schematic diagram of thermomechanical properties of a sheet metal in a simple tension test;

Fig. 15b - is an approximated diagram of the thermomechanical properties shown in Fig. 15a;

Fig. 16a - illustrates a section of a planar sheet metal before its formation into a conical surface; Fig. 16b - shows an already formed conical surface obtained as a result of a sequence of bendings.

DESCRIPTION OF INVENTION AND PREFERRED EMBODIMENT

The laser bending method described in the present invention allows, for example, bending of sheet metal of uniform surfaces.

In the first phase, the material of the bent object is submitted to the heating with a concentrated stream of energy SE of laser radiation. During the application of the energy stream of laser radiation SE moving with a speed V along the bending line

AA, a local change of the state of the material is generated, which material exhibits changed properties to a depth G. Within this region two strips may be distinguished. In the first strip S ₁ the material is in a liquid state, and in the second strip S ₂ the material is completely or partially plasticized. The line U designates the limit of the region incorporating the strip of the liquid and of the plasticized material.

Comparing now the position of the section surfaces i-i (i = 1 , 2, 3, 4) and the position of the track left on the surface of the sheet metal by the passing energy stream (as shown in Fig. 7), it may be assumed that the smallest quantity of energy penetrates to the layer surrounding section 1 - 1 , and the largest quantity penetrates to the layer 4 - 4. The distributions of the isotherms in these sections illustrate successional steps of the heating phase. The isotherms corresponding to the highest temperatures are present in the section 4 - 4. They are shown in Fig. 6, which illustrates the end of the heating phase. On the heated surface the temperature has reached the value T _g < T _m , and the isotherms T _m , T ₂ and T ₁ have extended deeper into the material. Between the isotherms T _m and T _m the metal is in a flowing state. Between the isotherms T _m and T ₂ the metal is in a plastic state. Between the isotherms T ₂ and T ₁ the metal is characterized by a lowered yield point. The material is in its original solid state only between the isotherms T ₁ and T _d .

Thus, during the heating process a defined sequence of strata characterized by different properties may be distinguished along the thickness of the material. The specific strips are indicated in Fig. 6 with the symbols G ₄ , G ₃ , G ₂ , G., , wherein G ₄ + G ₃ + G ₂ + G→ = G. Practically the sum of these depths Gj is slightly larger than G, because a slight bulging occurs to the

convex meniscus which is formed on the heated surface. Fig. 6 shows a sequence which is characteristic for such a process, where the maximum bending angle δ is to be obtained. If a bending angle δ to be obtained is smaller than the maximum obtainable bending angle, then the amount of energy furnished to the material in the heating process must be reduced. Consequently, in case of bending very brittle materials, a slightly different sequence of strips has to be produced within the depth of the material. In this case, the strip of material in primary state should be totally eliminated by bringing the unheated surface to a temperature T→ < T _d < T ₂ .

Finally, it must be emphasized that the effectiveness of the heating process depends on the ratio (G ₃ + G ₂ )/L. The closer is the ratio to 1 , the more effective is the bending . In case the power of an employed laser is not limited, and the plates or bars to be bent are relatively thick, then an attempt to increase the ratio results in the generation of a strip of liquid metal of a considerable width G ₄ relative to the thickness L. When a favorable ratio (G ₃ + G ₂ )/L can be obtained without generating the strip of liquid metal , such situation is advantageous with respect to the crystalline structure of the metal. Therefore, the appearance of the liquid metal strip (G ₄ ) is acceptable only if such a necessity exists. In contrast, the presence of a small strip of original material G→ > 0 is favorable for the bending process because it improves the thermal gradient (Fig. 5) within the thickness of the sheet metal and prevents the excessive increase of the temperature T _d of the unheated surface beyond the value T ₁ within the time lapse between the termination of the heating phase and the beginning of the cooling phase. Fig . 5 shows the distribution of temperatures as a function of the thickness L of the material ; Fig . 5 shows the melting point T _m and the temperatures

T _d and T _g on both sides of the object. This temperature distribution in relation to the melting point T _m determines the widths of the regions S ₁ and S ₂ relative to the thickness L of the material.

As it was already mentioned, during the heating phase, internal stresses of rather insignificant strength are generated, which stresses may cause a slight bending of the sheet metal such that a convexity is formed on the irradiated side as shown in Figs. 4 and 9.

The schematic representation of the temperature distribution within the thickness L of the heated material as shown in Fig. 5 indicates additionally the melting point of the material T _m . During the heating phase, the material of the first strip S ₁ and of the second strip S ₂ , under the influence of stresses generated in the effect of thermal expansion, grows in volume and flows out of the previously assured volume space.

The temperature distribution in relation to the melting point value T _m determines the depths of the first strip S T and of the second strip S ₂ relative to the thickness L of the material.

In the second phase of the process the material cools down at ambient temperature or, additionally, with a stream of blown gas. The state of the material in the region of the bending line, i.e. the liquid state in the first strip S ₁ and the plasticized state in the second strip S ₂ , changes back to the solid state, which is the original state of the material. The borderline of the region including during the heated phase both, the flowing material strip and the plasticized material strip is represented on Fig. 7 as the line U .

In addition to the above-described process of heating sheet metal with a laser, it is appropriate to mention the natural cooling of the heated strip which occurs continuously due to quick dispersion of the heat within the whole volume of the sheet, which is due to good thermal conductivity of the metals. Therefore, additional external cooling prevents to a certain extent the natural tendency to even distribution of excessive temperature within the thickness of the material , i .e. if the corresponding uniform temperature T _r within the whole thickness of the material has reached, for example, T _r = T ₂ , then the premature termination of the bending process would occur. Consequently, the achieved bending angle δ would be too small. In order to prevent this natural tendency and, consequently, to increase the permanent bending angle δ , the temperature balancing must be achieved at the level T _r < T ₁ or, for brittle materials, T _r < T ₂ . This can be achieved by such artificial cooling which allows to maintain a large temperature gradient within the overall thickness of the sheet metal. The method of such cooling is illustrated in Fig. 10a. The principal cooling stream S' _ch is directed onto the heated strip within some distance behind the trace of the beam. An auxiliary correcting stream of liquid S" _ch is directed onto the unheated surface disposed underneath the heated strip, practically next to the trace of the laser beam. The purpose of the principal cooling stream S' _ch is to obtain a fast lowering of the temperature T _g . In contrast, the auxiliary correcting stream S" _ch prevents the temperature T _d from exceeding T ₁ or T ₂ before the temperature within the overall thickness L balances to the temperature T _r . The distribution of the isotherms in these sections illustrates successional stages of the cooling phase. Fig. 10a illustrates the distribution of maximum-temperature isotherms at the beginning of the folding process, while Fig. 10b shows the termination of this

process, which is also the termination of the bending process. The temperatures within the overall thickness L of the sheet metal have balanced at the level T _r < T, or, in case of very brittle materials, at the level T _r < T ₂ . The internal stresses reach then their maximum value. The crystalline structure of the material , however, is not exactly the same as the crystalline structure of the original material, even though, for most materials these changes are practically insignificant and limited to the regions where the material is flowing or plasticized.

As a result of internal stresses δ caused by the shrinking of the material in a low temperature, a shortening of the material occurs along the fibers indicated with arrows, which represents the distribution of the stresses along the thickness L of the object, as shown in Figs. 8 and 1 1 .

During the cooling phase of the above-described process, an omnidirectional shrinking of the material occurs. Stresses are generated in the material in those directions, where the shrinking does not occur unrestrained and freely and where the temperature is lower than T ₂ . This results in a permanent bending of the sheet metal. The distribution of the stresses and deformations at the end of the bending process is shown in Fig. 12. In Fig. 12 there is shown that the strongest tensioning stresses are present in the superficial layers while the strongest compressing stresses are located in the central layer. In case of bending of plastic metals which are not very hard, it can happen that the maximum tensioning stresses exceed the yield point. Then the flowing of the material occurs which decreases the effectiveness of the bending process, but does not involve any further negative consequences. However, in case of brittle materials, when certain limiting values of tensioning stresses are exceeded, it may result in cracking and

bursting in the superficial layer. This may be prevented if a balancing of the temperature in the bent section and, consequently, the termination of the bending process are effected at the temperature level T _r > T ₂ . In this case, the stresses δ _r is associated with elongations ε _r which are smaller than maximum elongations A (ε _r < A) corresponding to the temperature T ₂ . In such case no cracking or bursting of the unheated surface occurs. Maintaining this condition also minimizes the risk of cracking on the previously heated surface.

Fig. 12 illustrates sufficiently well the distribution of stresses and deformations present in the material of bent sheet metal or bars, where the bending line is relatively short. When the bending line is L > 15L, then the configuration of the states of stresses and deformations is different. Configurations for a symmetrical bending of sheet metal are illustrated in Fig. 13. Fig. 13 shows the state of deformations in a form of two waves moving in a short distance behind the laser-beam track. After the passage of these waves, the angle between the two panels of sheet metal decreases by a value δ. These stresses appear and operate in the final stage of the cooling phase and reach their maximum value at the end of the cooling phase. These stresses actually cause deformation in the form of the two waves moving together with the laser beam as shown in Fig. 13. The degree of the angle δ of the permanent deformation depends not only on the strength of these stresses, but also on the stiffness of the sheet metal being deformed. The solution of this problem is a very complicated task for the thermomechanics of coatings. In the present description, it should be only emphasized that the wavelength L ₀ extenuates this stiffness and favors the increase of the degree of the permanent bending angle.

In the example performance of the bending process according to the invention, a section of a sheet metal plate was used. The beam is directed perpendicular to the surface of the plate.

A plate selected for conical bending, having a thickness of 3 mm, made of steel ST35 of the following chemical composition: 0.28C, 0.1 SI, 0.05P, 0.05S. Fig . 10a illustrates the plate in the original state.

Fig. 12 illustrates a schematic diagram Re (T) for this steel. The diagram covers the temperature range 0 < T < 500°C. With respect to the substantially linear course, an approximation is adapted, where the temperature T ₂ corresponds to Re = 0. The melting point T _m = 1490 ^° C was determined based on experimental results. Fig. 15a is a schematic diagram showing the course of Re(T) and A(T) for the steel ST35. The diagrams shows the temperature T ₂ = 420 ^β C, T ₂ = ~700'C and T _m = 1530'C.

Consequently, the parameters of the bending process must be adjusted, i .e. the laser beam power SE, width D of the strip, and the relative velocity of the beam V as a function of the position of the point on the straight line AA.

The method of bending metallic objects according to the present invention can be used for forming of the objects made of brittle or very hard materials. Furthermore, this method can be used in forming of the objects in the conditions, where the access to these objects is difficult as for example in case of objects which are placed in vacuum or which are under hazard (such as under high voltage, or dangerous radiation). Furthermore, the

invention method can be applied for the formation of objects of a conical or cylindrical surface out of flat sections of sheet metal.

Cylindrical bending of sheet metal - consists in performing a sequence of bendings of the sheet metal along a pencil of straight parallel lines. In effect of the permanent bending of the sheet metal along each of the straight lines of a bending angle δ a cylindrical surface is obtained, wherein the form of the obtained cylindrical surface depends on: the distance between the straight lines AA as well as on the angles δ realized on each of the straight lines. The bending angles on each of the straight lines should be identical in case of production of a cylindrical bending.

Conical bending of sheet metal - consists in performing a sequence of bendings of sheet metal along a pencil of straight intersecting lines. In this case, for bending of sheet metal along a predetermined line, it must be considered that for each point of this line the distance between the neighboring lines is different. Consequently, the permanent bending angle δ must be a proper function of the position of the laser beam on the straight line.

It will be understood that each of the elements described above, or two or more together, may also find a useful application in other types of bending methods for metallic objects differing from the types described above.

While the invention has been illustrated and described as embodied in the context of a bending method for metallic objects, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.