METHOD FOR BUTT WELDING TWO PIECES OF MEDIUM OR HIGH CARBON CONTENT STEEL, WITH A LASER BEAM

Abstract: The method enables the butt welding of two metal pieces (1,2; 10,12) which are difficult to weld, particularly pieces of medium or high carbon content steel, with the use of a laser beam (L). Very briefly, it comprises the steps of: placing the surfaces of the pieces (1,2; 10,12) to be welded in mutual contact, directing two laser beams (L) at homologous points of two opposite sides (4,5) of the zone (S) of contact of the surfaces of the two pieces (1,2; 10,12), the beams (L) having a power such to ensure a thin, deep-penetration weld (6,7; M) having a substantially constant section and forming solidifi­ cation menisci (6a, 7a) with convex profiles.

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for welding metal pieces. More particularly, the invention concerns a method for the butt welding of two metal pieces which are difficult to weld, particularly pieces of different metals and more specifically of medium or high carbon content steel, with the use of a laser beam. STATE OF THE ART It is well known that the welding of metal pieces of medium or high carbon content steel is particularly difficult. Some difficulties arise from the evolution of carbon monoxide resulting from the reduction by the carbon of the iron oxide which may form during welding, and from a tempered structure resulting from the quick cooling of the weld and the surrounding zones. Moreover, the martensitic structure which results from the partial tempering has irregular grains at the interfaces of which impurities from low-solvents may separate, for example, small percentages of sulphur and phosphorous present in the commercial steels; these impurities involve the creation of microcracks. The latter differ from tempered structures resulting from welding in that they cannot be removed by subsequent reheating and like processes. Such phenomena result in the welds in such steels being very fragile and reduces their usefulness, particularly for pieces which have to withstand considerable dynamic forces. On the other hand, steels having medium-high carbon contents are those most used for the manufacture of mechanical parts since they are easy to work, easy to temper, and relatively cheap. It has been attempted to solve the problem of welding of medium/high carbon content steels both by conventional techniques, such as the MIG (metal inert gas), TIG (tungsten inert gas) and electron beam techniques, and by the recent laser welding method but satisfactory results have not been obtained. Hence, until now, the joining of parts of such steels has been achieved for the most part by screws or like mechanical fixing means which result in higher manufacturing costs and the consequent size limitations, difficulty of assembly, etc. OBJECT OF THE INVENTION The object of the present invention is to provide a method which enables the problem of welding of metal pieces which are difficult to weld, particularly pieces of medium or high content steel, to be solved with the use of a laser beam. STATEMENT OF THE INVENTION This object is achieved according to the present invention by means of a method characterised in that it comprises the steps of: placing the surfaces of the pieces to be welded in mutual contact, directing at least one laser beam at homologous points of two opposite sides of the zone of contact of the surfaces of the two pieces, the beams having a power such as to enable deep-penetration welding with the formation of thin welds which have substantially constant sections and form solidification menisci with convex profiles. For reasons which will become more apparent below, the use of two laser beams acting simultaneously on homologous points of the two sides of the zone of contact is advantageous from the point of view of the quality of the weld, the productivity and redundancy, and reliability of the system. By virtue of the aforesaid characteristics, both the stresses within each weld, called "auto-stresses" below, and the stresses induced thereby in the pieces and/or in other welds, called "tele-stresses" below, are minimised. The reduction in the auto-stresses and the tele-stresses reduces to a minimum the probability of there being dangerous cracks in the welding zones which cannot be eliminated by subsequent tempering/annealing of the pieces. These cracks would make the welded joint not very reliable, particularly for applications in which high dynamic stresses are foreseen. The welds carried out by a high-power laser beam improve the efficiency of the process or the efficiency of welding, this being understood as the ratio between the energy needed to create a unit volume of weld and the energy absorbed by the pieces and transmitted through conduction to the pieces. This efficiency may reach particularly high values of the order of 80-90%. With a suitable choice of the laser welding parameters, such as the power of the beam used and the speed of advance of the beam, it is possible to refine the base metal, that is, obtain a uniformly-distributed fine grain in the weld zone. The high temperature and recirculating movements of the molten metal induced by the high temperature gradients resulting from the high concentration of energy which can be obtained with a laser beam cause impurities to volatilize during the welding and the welds are free from such impurities, with a fine grained structure and consequent characteristics of uniformly hardness. Notwithstanding the great hardness, the welds have good characteristics of ductility and enable the use of welded pieces for applications in which considerable dynamic and fatigue stresses are foreseen. In other words, the method achieves a stress-relieving action, the elimination of low-melting substances, refinement, and a uniformity of the grain structure, such that the resilience and strength of the welds obtained by the method of the invention, on the basis of tests carried out, are so satisfactory as to render almost any further subsequent stress-relieving heat treatment practically superfluous. BRIEF DESCRIPTION OF THE DRAWINGS Further characteristics and advantages of the invention will become apparent from the detailed description which follows with reference to the appended drawings, provided purely by way of non-limiting example, in which: Figure l is a perspective view showing two metal pieces before their welding by the method of the invention, Figure 2 illustrates the pieces shown in Figure l during welding, Figure 3 is a schematic view showing another two pieces prepared for welding in another embodiment of the method of the invention, Figure 4 is a schematic perspective view showing the pieces of Figure 3 during welding, Figure 5 is a detail of Figure 4 on an enlarged scale, Figure 6 is a schematic perspective view illustrating another embodiment of the method of the invention, and Figures 7 to 9 show a further embodiment of the method. With reference to Figure l, a first steel piece with a high carbon content, for example C43 steel, is indicated l and is intended to be welded to a second piece, also of high carbon content steel, for example of the C33 type. The pieces l and 2 are placed with their surfaces to be welded together in mutual contact. The zonal area of contact between the surfaces of the pieces l and 2 is indicated 3. In a first embodiment of the method of the invention, two laser beams L with the same characteristics are directed at homologous points on two opposite sides 4 and 5 of the zone of welding 3, as shown in Figure 2. The rates of advance of the laser beams L along the sides 4 and 5 of the zone of welding are preferably the same. The power of these beams is also such as to achieve deep-penetration welding with the formation of thin welds 6 and 7 having substantially constant sections and forming solidification menisci 6a and 7a with convex profiles. As stated, the welds must penetrate deeply so as to form welds having the maximum possible slimness, that is the maximum depth/thickness ratio, and hence the minimum thickness (and volume) relative to the depth. For this purpose, however, it is necessary to stress that it is the depth of the weld which determines the strength of the weld. If the weld were metallurgically perfect, the weld could even be only a few hundred angstroms to make the structure formed by the two welded pieces effectively one piece. However, technological limitations, among which are the precision required to effect such a thin joint, make this limit unachievable. The thickness of the weld is also of particular importance. This thickness must be as uniform as possible. This considerably reduces the stresses induced in the during its solidification, and thus achieves a minimum auto-stress. Tele-stresses induced by one weld on another are also considerably reduced. The welds must be made, as stated above, with convex solidification menisci. This characteristic is extremely important. In fact, menisci fulfil essentially two extremely important functions: in the first place, they act as reserves of liquid material to be brought to the zones which solidify and in which it is necessary to compensate for shrinkage; furthermore, the menisci help create compressional stresses which improve the resistance to the formation of microcracks and cracks and fatigue stresses in general. Experiments carried out by the inventors have shown that if the weld parameters are such as to involve the formation of concave menisci, dangerous states of local tensional stresses are created in the weld. Cracks, microcracks and fragility of the welds result from these. In particular, the bead welds formed with concave menisci are incapable of withstanding strong external loads and particularly the tele-stresses induced by the other weld. In Figure 3, another two pieces of high carbon content steel are indicated l0 and l2 and are to be welded together along respective facing surfaces l0a and l2a. The surface l2a of the piece l2 is flat. A longitudinal groove l4 is formed in the surface l0a of the piece l0 and is defined laterally by two ribs l6 with substantially identical flat frontal surfaces l6a. Before welding, the pieces l0 and l2 are brought together and clamped against each other, as shown by the arrows F in Figure 4. Two power laser sources S are then disposed on opposite sides of the zone of joining of the pieces l0 and l2, indicated G. These sources are moved parallel to the joint zone G, as indicated by the arrows V in Figure 4. The laser beams L emitted by the sources achieve the simultaneous welding of each of the ribs l6 to the facing portions of the surface l2a of the piece l2. As shown in particular in Figure 5, the weld, indicated M, has a substantially rectangular section and forms external and internal soldification menisci l8, l9 with substantially convex profiles. These menisci result essentially from the forces acting on the molten metal (temperature gradients, vapour pressures, surface tensions, viscosity, and gravity) which make the molten part or weld M leave the zone of contact between the pieces. The convex menisci in practice eliminate dangerous tensional forces which would otherwise occur at the apices of the weld and contribute, as stated above, to the forcing of the free surface of the meniscis into compression during the solidification phase, a situation which prevents the formation of cracks and contributes to the improvements of the fatigue strength of the joint. As may be seen in Figure 5, the aspect figure of the weld, a parameter defined as the ratio between the depth h of the weld and its width d, is particularly high and in any case is greater than 3. By virtue of the almost rectangular section of the weld M, the auto-stresses are particularly low, in that the flow of heat from the weld to the pieces l0 and l2 is substantially in a single dimension, and in that the piece l2 is free to move towards the piece l0 and vice versa. Moreover, since the solidification of the molten metal M starts substantially from a point P at the centre of the molten metal itself, the tele-stresses transmitted from one weld to an adjacent weld are also minimised. In fact, if the solidification were to start, for example, at the internal meniscus l9, the further molten parts M undergoing solidification would subject the weld forming around the already solidified meniscus l9 to tension. This would create tension in the weld adjacent the other rib l6. Another way of carrying out the method of the invention provides for the use of a welding material in the form of a sheet to be inserted between the frontal surfaces l6a of the ribs l6 and the portions of the surface l2a of the piece l2 in contact with these ribs. In Figure 6, two U-shaped sheets are indicated 22 and are constituted by a nickel - based alloy, for example Inconel. The sheets 22 are positioned on the ribs l6 of the piece l0 with their respective concavities facing the frontal surfaces l6a. These sheets may be fixed to the piece, for example, by spot welding. To advantage, the sheets 22 are several tenths of a millimetre thick, particularly two tenths of a millimetre. After the pieces l0 and l2 have been placed together, the welding is effected in a manner similar to that described previously. The presence of the welding material in the form of the sheets 22 adds material to the molten metal M, reducing the percentage of carbon and extending the martensitic structure. Furthermore, it provides material for forming the convex menisci l8 and l9. In addition, with the nickel alloys, the sheets 22 may be formed of low carbon content steel or stainless austenitic steel. A further embodiment of the method of the invention will now be described with reference to Figures 7 to 9. Figure 7 shows a sheet of welding material 23 having a rectangular aperture 24 and a plurality of bendable tabs or appendages 25 and 26. The sheet 23 is coupled to a piece l0 to be welded in the manner illustrated in Figure 8: the portion of the sheet having the aperture 24 is applied to the face of the piece l0 intended to be welded to the other piece l2, and the tabs 25 and 26 are bent over the piece l0. By virtue of the clamping achieved, the sheet 23 is mechanically fixed to the piece l0, which is advantageous from the point of view of handling of the piece l0 and from a logistical point of view. The piece l0 is then placed against the piece l2, as shown in Figures 8 and 9: the aperture 24 in the sheet 23 determines a spacing between the facing surfaces of the pieces l0 and l2 and, in practice, has the same function as the groove l4 formed in one of the pieces in the embodiment of the method explained with reference to Figures 3 to 5. As shown in Figure 9, two laser beams L are then directed at the junction zone of the pieces l0 and l2 and achieves the weld by fusing the longitudinal sides 23a of the sheet 23. If the thickness of the piece to be welded, which has previously had the sheet of welded material coupled to it, is such that it is not convenient to fold the sheet, it is possible to form a groove in this piece in which the ends of the tabs of the sheet are engaged. From experiments carried out by the inventors, it is found that the weld achieved by the method of the present invention, and in particular by the method explained with reference to Figures 3 to 5, is able to withstand extremely high static loads under tension and flexion. Two pieces welded in the manner illustrated with reference to Figures 3 to 5 with the use of a Spectra Physics 973 laser operating at a power of l.5KW, a translational speed of 2.5 m/min, and a penetration of l.5mm withstood static loads of 3000-4000 Kg. Tests carried out on the same pieces with the Charpy pendulum showed that the structure welded in this manner had a very high resilience, even in the crude state without heat stress-relieving after welding. It is thought that the optimum characteristics for the weld achieved depend on the fact that, if correctly carried out, the laser welding subjects the molten metal to a self-stress-relieving process. This process occurs during welding in substantially the following manner. The molten metal is subjected to a refining action. It is due to rapid antisymmetrical circulating movements (at frequencies of the order of 300-800Hz) with respect to the axis of the weld, which are induced in the deep-penetration weld by the inversion of the temperature gradient (see A.V. La Rocca, "Laser welding in some mass-production industries", Proceedings of the International Conference on Laser '79, Orlanda, Florida, U.S.A. - Dec. l979, pages 6lss). It is yet to be established whether the point of highest temperature is produced within the piece by the plasma. The circular movements are directed towards the surface and bring with them the impurities which are exposed to the laser beam and, in effect, are made to sublime instantaneously. Furthermore, cooling occurs rapidly (but is uniform, however, if the section of the weld is substantially constant) and results in a fine crystalline grain, also uniformly distributed. Hence, even if the material is hard or has a poor capacity for elongation, the grain is in any case distributed uniformly within the volume of the weld without having tension peaks. The material may then withstand greater elongation without breaking. In other words the weld, being self-stress-relieved during the process, exhibits properties which are entirely similar, and even better, than those of a weld which has been stress-relieved by heating after the process.

Claims: 1. Method for the butt welding two metal pieces (l, 2; l0, l2) which are difficult to weld, particularly pieces of medium or high carbon content steel, with the use of laser beams (L), characterised in that it comprises the steps of: placing the surfaces of the pieces (l, 2; l0, l2) to be welded in mutual contact, directing at least one laser beam (L) at homologous points of two opposite sides (4, 5) of the zone of contact (3) of the surfaces of the two pieces (l, 2; l0, l2), the beams (L) having a power such as to enable deep-penetration welding with the formation of thin weld (6, 7; M) which have substantially constant sections and form solidification menisci (6a, 7a; l8, l9) with convex profiles. 2. Method according to Claim l, characterised in that at least one longitudinal groove (l4) is formed in that surface (l0a) of one of the pieces (l0) to be welded which is intended to be welded to the other piece (l2), so as to define a pair of longitudinal ribs (l6) having flat frontal surfaces (l6a), the frontal surfaces (l6a) of the ribs (l6) then being placed in contact with the surface (l2a) of the other piece (l2) in the welding position, the ribs (l6) of one piece (l0) then being welded to the other piece (l2) by means of the laser beams (L), each of which is made to be incident on the zone of contact between a rib (l6) of one piece (l0) and the surface (l2a) of the other (l2), so as to achieve a penetrating weld. 3. Method according to Claim 2, characterised in that the ribs (l6) of one piece (l0) are welded to the other piece (l2) simultaneously by means of two laser beams (L) having the same characteristics and speed of advance relative to the pieces (l0, l2) along the junction zone (G) of the pieces (l0, l2). 4. Method according to Claim 2 or Claim 3, characterised in that the frontal surfaces (l6a) are equal to each other. 5. Method according to any one of the preceding claims, characterised in that the two pieces (l0, l2) are clamped against each other during the welding with a force (F) such that, after welding, each junction zone (G) between a rib (l6) of one piece (l0) and the edge (l2a) of the other piece (l2) has a weld (M) with a meniscus (l8; l9) with a convex profile. 6. Method according to Claim l, characterised in that the power of the laser beams (L) is between 2.l0⁵ Watts/cm² and 9.l0⁶ Watts/cm². 7. Method according to Claim l, characterised in that the relative speed of advance of the laser beams (L) relative to the pieces (l, 2; l0, l2) is greater than lm/minute and is typically 3-5m/minutes. 8. Method according to Claim l, characterised in that the slimness (h/d) of the weld (M) is greater than or equal to 3. 9. Method according to Claim l, characterised in that a shaped sheet (22; 23) of welding material is interposed between the two pieces (l0, l2). l0. Method according to Claim 9, characterised in that the sheet (22; 23) is of stainless austenitic steel. 11. Method according to Claim 9, characterised in that the sheet (22; 23) is of a nickel-based metal alloy. 12. Method according to Claim 9, characterised in that the shaped sheet (22) is previously fixed to one of the pieces (l0) by folding or spot welding. 13. Method according to Claim l2, characterised in that the sheet (23) has an aperture (24) in its portion intended to be between the pieces (l0, l2) to be welded. 14. Method according to Claim l3, characterised in that the sheet (23) has a plurality of tabs (25, 26) which can be bent to anchor the sheet (23) to one of the pieces (l0) to be welded.

Inventors: CANTELLO, MAICHI, CAPRA, GUGLIELMO, LA ROCCA, ALDO VITTORIO
Applicants: FIAT AUTO S.P.A.
Application year: 1987
Main classification: B23K26
IPC codes: B23K26, B23K1