Patent Application
20100032413
The invention relates to the welding of metallic components made of materials susceptible to hot cracking. Objects in which its application is expedient and advantageous are all components which comprise multiphase solidification alloys having a broad solidification interval or are constructed from alloys which contain alloy elements or contamination elements which form a low-melting-point eutectic material with one or more main alloy elements and which are to be joined using fusion welding methods of high power density. Such materials, which have only been able to be welded crack-free inadequately up to this point, are, for example, ferritic, ferritic-perlitic, or austenitic machining steels, hardenable aluminum alloys, austenitic steels endangered by hot cracking, nickel alloys, etc. The invention is especially advantageously usable for all welding tasks in which, for reasons of method technology, properties, or cost-effectiveness, no welding additive material may be applied to ensure the ability to weld without hot cracking or the use of welding additive materials is inadequate to reliably avoid hot cracking in processing.
Further potential fields of use are the avoidance of so-called middle rib defects in laser beam welding of figure plates made of construction steels, of middle rib cracks in the welding zone of thin plates made of austenitic stainless steels, and of very rigid or very hard clamped components.
In addition, the method may also be used to avoid hot cracks in the welded material during repair or buildup welding.
Hot cracks are a severe welding problem, which prevents the use of economically important alloys, which are advantageous in use, in an array of welding structures. They predominantly occur in multiphase solidification alloys, in alloys having secondary alloy or contaminant elements, which form a low-melting-point eutectic material with one or more alloy elements, and in cases of very rapid solidification running in the direction of the plate plane or in very rigid weld seam surroundings.
Correspondingly, extensive and manifold efforts and attempted solutions known up to this point have been made to solve the problem of hot cracking.
Thus, for example, the attempt has been made to remove the metallurgical causes of the hot crack formation—the formation of low-melting-point phases or of grain boundary films—by the use of suitable welding additive materials. However, in spite of its broad technical application, this method is not suitable in every case. Thus, for example, suitably adapted welding additive materials do not exist for every alloy susceptible to hot cracking. In addition, the welding process typically becomes more expensive. Furthermore, it may be disadvantageous that in ultra-high-strength materials, the mechanical carrying capacity of the weld seam may decrease due to the use of welding additive materials, which shift the composition of the welded material in the direction of eutectic solidification. Furthermore, it is generally disadvantageous that the primary cause of the hot crack formation—exceeding critical tensile elongations and/or critical elongation rates during the solidification in the two-phase region—is not thus combated.
Various methods have become known for avoiding critical elongation and/or elongation rates during the solidification in the two-phase region. Thus, for example, a method is described in WO 03/031108 (W. Kurz, J.-D. Wagniere, M. Rappaz, F. de Lima: “Process for Avoiding Cracking in Welding”), with the aid of which the hot cracking occurring during the laser beam welding of aluminum alloys is combated, in that a second heat source—preferably a laser—follows a first heat source—preferably also a laser—at a constant distance, the second heat source is oriented directly on the solidification zone, and the power of the second heat source is set so that the local cooling rate of surface-proximal areas of the solidification zone is reduced or these are even briefly heated locally once again. The additional trailing heat source may comprise an electron beam, laser beam, electric arc, or plasma source or also a combination of two sources and operates using a lower power density than the first heat source.
Using a 1.7 kW CO2 laser as the welding laser and a 1.2 kW pulsed Nd:YAG laser as the second heat source, 1.0 mm thick plates made of the aluminum alloy 6016 may be welded in an I-butt without hot cracking. The best results were achieved when, at a feed rate of vs=3.6 m/minutes, the Nd:YAG laser beam, which was focused on a focus diameter of df=0.6 mm, was situated at a distance of 3 mm behind the center point of the CO2 laser beam. Due to the local second energy introduction using a pulse energy of 8 J, a pulse intensity of 30 W/cm2, and a frequency of 150 Hz, an enlarged melt bath and a reduction of the local quenching rate from 2600 K/s to 1500 K/s were achieved. It proved to be decisive for the action mechanism that the second laser beam acts directly on the solidification zone. The following effects, which counteract the hot crack formation, are thus achieved according to the findings of the inventor:
The disadvantage of this solution in application technology is that only thin plates may be welded therewith out hot cracking. The reason for this is that the laser energy of the second laser is only absorbed on the surface and the thermal penetration depth during the very brief interaction time of the laser beam with the surface of the solidification zone, of at most Δt=(df/vs)=(0.6 mm/3600 mm)min=0.01 seconds, is only very small. Therefore, the depth of the zone having reduced cooling speed is very small. Similar behavior occurs upon the application of the other claimed energy sources for the second heat source. The depth of the zone having reduced cooling speed becomes even less if steels, having their much lower thermal conductivity, are to be welded according to this method.
Similarly to this above-mentioned solution of the prior art, for the welding of thin aluminum plates without hot cracking (see, for example: V. Ploshikin, A. Prikhodovsky, M. Makhutin, A. Ilin, H.-W. Zoch “Integrated Mechanical-Metallurgical Approach to Modeling of Solidification Cracking in Welds” in: Th. Böllinghaus, H. Herold (editors): Hot Cracking Phenomenon in Welds, Springer Verlag 2005, ISBN 3-540-2232-0, pages 223-244), situating a second defocused laser beam adjacent to the strongly focused laser beam for welding and moving the second laser beam parallel to the first laser beam and at the same speed, has also become known. A 2.0 mm thick aluminum plate of the alloy AA6056, which was solidly clamped on one side, was welded using a laser power of 1.8 kW and a feed rate of 2.8 m/minute. At a distance of the weld seam of approximately 25 mm from the lateral sample edge, a complete sample separation occurred due to longitudinally running hot cracks after the welding. If the plate was continuously heated locally at a laser power of 750 W using the second laser, which was located at a distance of approximately 20 mm adjacent to the weld seam on the free, unclamped plate side, hot cracks were able to be avoided.
It is also disadvantageous in this solution that the method is only suitable for thin plates. The cause of this shortcoming is, as in the above-mentioned first example, the laser also only acts as a surface energy source in this method. In addition, this solution is also too costly for crack avoidance for many practical applications. The reason is that an expensive laser must also be used for the second heat source.
Preventing cracks in the thermal influence zone in that the welding and heating of the weld seam surroundings are caused quasi-simultaneously by the same electron beam, in that, in very rapid succession, the focused electron beam welds in pulses using a high power density and is then defocused and deflected for the heat treatment, has become known from the field of the electron beam welding (see GB 2,283,448 A, Th. K. Johnson, Al. L. Pratt: “Improvements in or relating to electron beam welding”). The surface temperatures may thus be set in a targeted way in front of, adjacent to, and behind the welding zone.
It is also disadvantageous here that the energy source for generating the secondary temperature fields represents a surface energy source, whose effectiveness does not extend far enough into the material to also be sufficiently effective for the case of avoidance of hot cracks in the welding zone with deeper weld seams and materials of worse thermal conductivity. The cause of this is again that the energy of the electron beam is completely absorbed in the uppermost boundary layers and propagates too slowly into the depths in relation to the high welding speed of the beam welding method. In contrast, if the positions of the regions to which the electron beam is applied for the heat treatment are placed so far in front of the welding position that the additional temperature field also reaches the weld base in the position of the welding zone, the temperature field no longer acts locally, but rather more like a general homogeneous preheating. From our own experiments it is known, however, that a homogeneous preheating is not sufficiently effective for hot crack avoidance. In addition, it is disadvantageous that these methods may only be used for electron beam welding under vacuum.
The object of the invention is therefore to specify a novel and effective method and a novel device for crack-free welding, repair welding, or buildup welding of materials susceptible to hot cracking, which is also suitable for greater weld seam depths and greater plate thicknesses, for multiple welding methods—also particularly usable in atmosphere, for a broader palette of metallic materials, and particularly also those materials having worse thermal conductivity, and which may additionally be implemented significantly more cost-effectively than the known prior art.
The invention is based on the object of specifying a welding method and a device usable therefor, which allows the tensile elongations, which occur during the cooling in the temperature interval of brittleness, to be avoided or at least suppressed to a harmless level in the solidification zone.
The object is achieved according to the invention by a novel method for crack-free welding, repair welding, or buildup welding of materials susceptible to hot cracking, as described in Claim 1, and a corresponding device, as specified in Claim 10.
According to Claim 1, the solution according to the invention for welding methods using high power density is that instead of the surface energy sources used according to the prior art, electromagnetic volume sources are used as the auxiliary energy source in such a way that, in the interior of the component, they generate two specially implemented inhomogeneous temperature fields, which travel with the welding zone, run parallel or nearly parallel to the welding direction on both sides, and extend longitudinally to the welding direction. The two temperature fields begin in front of the welding zone viewed in the welding direction. Their temperature maxima are located outside the thermal influence zone and behind the solidification zone of the weld seam in the welding direction, their depths at least reaching the weld seam depths at the location of the temperature maxima.
Welding methods for which the method according to the invention may be employed are specified in Claims 2 through 4.
The idea that the invention is not restricted, as stated in Claim 2, solely to the welding method of high power density being a laser beam welding method. As specified in Claims 3 and 4, plasma, TIG, WIG, or non-vacuum electron beam welding facilities may just as well be used.
Claim 5 contains an especially advantageous variant for generating the additional temperature fields using inductive heating. As stated in Claim 6, the temperature field according to the invention may also be generated using conductive heating.
Method-influencing variables for setting the depth and extension of the additional temperature fields by selecting the induction frequency, the length, shape, and extension of the two inductor branches, the attachment of field amplification elements, and the induction frequency are specified in Claim 7.
Claims 8 and 9 give ideas for the design of the temperature fields as a function of the component geometry and if different materials are to be welded with one another.
Device Claim 10 states that the auxiliary energy source is a volume source which is connected to the welding head in such a way that it follows the movement of the welding head at the same speed. It is stated in Claim 16 that the device may advantageously be used to perform the method according to at least one of Claims 1 through 9.
Claims 11 through 13 refine the idea of the invention for the case that the auxiliary energy source is an inductor. A solution alternative thereto by the use of conductive heating is stated in greater detail in Claims 14 and 15.
The advantages of the solution according to the invention in relation to the prior art are that it
The invention is explained in greater detail on the basis of the following exemplary embodiments. Identical features are provided with identical reference numerals in the figures.
In the figures:
a: shows a temperature field implementation according to the invention to avoid hot crack formation
b: shows a longitudinal section AA through one of the two temperature fields generated by the auxiliary energy source, and an associated longitudinal section BB along the line of symmetry of the weld seam
c: shows a cross-section CC through the weld seam and the superimposed temperature fields of welding energy source and auxiliary energy source in a plane through the solidification zone
The solution according to the invention will be explained on the basis of the fundamental construction of the device and the general method steps.
Two plates (1, 2) made of a material sensitive to hot cracking are to be bonded to one another by welding through an I-butt (see
After the auxiliary energy source (22) and the welding energy source (32) are turned on, the welding process is started. In general, the auxiliary energy source (22) moves at the same feed rate vS as the welding energy source (32). During the movement, the auxiliary energy source (22) generates two additional temperature fields 1 and 2 (9, 10), see
In the general case of curved weld seams (7), different plate thicknesses, or different materials of the two components 1 and 2 (1, 2) to be welded, the temperature fields (9 and 10) and the levels of the temperature field maxima Tmax1 and Tmax2 (13′, 13″) do not necessarily have to be equal and lie completely symmetrical to the weld seam (7). The induction frequency is selected as a function of the plate thickness and the electromagnetic properties of materials so that the depth of the temperature fields (9, 10) at least reaches the weld seam depth tS (12) at the location of the temperature field maxima Tmax1 and Tmax2 (13′, 13″) (see also
Machining steels have an increased sulfur content to improve the cutting ability and the formation of short breaking chips. This sulfur forms low-melting-point eutectic materials with the iron upon fusion, which result in hot cracking upon welding. Machining steels are therefore considered non-weldable. This increasingly applies to the heat-treating steels, which additionally have a carbon content greater than approximately 0.3% to ensure their temperability. Although the procedure according to the invention may also be applied advantageously to other materials endangered by hot cracking, such as austenitic steels, aluminum alloys, and nickel alloys, the suitability of the method is to be shown on the basis of the example of heat-treating machining steels because of the special difficulty and the lack of suitable alternative solutions, such as welding additive materials which avoid hot cracking.
Two plates, which are 250 mm long, 100 mm wide, and 6 mm thick, and are made of heat-treating machining steel 45S20 (chemical composition: approximately 98% iron; 0.43% carbon, 0.201% sulfur; 0.25% silicon; 0.94% manganese; 0.018% phosphorus) are to be joined on their longitudinal side using laser beam welding. A cross-flow 6 kW CO2 laser is to be used as the welding energy source (32) for the laser beam welding. The laser beam power is set to 5.5 kW. The welding speed vS is vS=1.5 m/minute. Helium is supplied in a quantity of 15 l/minute using a trailing nozzle configuration as a protective gas.
Although the weld seam is well implemented, it has a plurality of transverse and longitudinal hot cracks, as transverse and longitudinal grinds show (see
To avoid hot cracking, an inductive energy source is used as the auxiliary energy source (22). The induction generator has a frequency of 9 kHz. The double-armed inductor (schematic illustration in
The inductor is positioned centrally to the weld seam (7). A value ax≈20 mm is selected as the distance ax between the beginning of the temperature fields 1 and 2 (18, 19) and the center point of the welding zone (4), approximately measured as the smallest distance between the center line of the energy beam of the welding method (3) and the connection line between the two front edges of the inductor branches 1 and 2 (18, 19). The inductive power is set to an effective power display on the induction generator of 20 kW.
The length lSEZ of the welding zone (4) and the solidification zone (6) totals lSEZ≈22 mm. To perform the welds, the same welding parameters are set as for the welds without auxiliary energy source. The inductor (15) is moved simultaneously with the welding head (23). Upon reaching the starting position, the inductor (15) and the laser beam are turned on, the laser beam with a time delay.
Using the setting parameters, a temperature maximum Tmax=Tmax1=Tmax2=850° C. is reached. The temperature maxima Tmax1 and Tmax2 are approximately bx+cx≈32 mm behind the position of the center point of the energy beam of the welding method (3). The distance bx between the center point of the welding zone (4) and the end of the solidification zone (6) is approximately bx≈20 mm. Therefore, for the selected length of the inductor branches, li1=li2≈3*lSEZ, and for the distance bi between the inductor branches, bi≈5*bSZ.
The cause of the avoidance of the hot cracking is that it is possible during the solidification and cooling of the weld seam, at least in the temperature interval ΔTIS, which is critical for hot cracking, to compensate for the thermal shrinking of the weld seam (7) sufficiently by the thermal volume increase of the two temperature fields (9, 10) generated by the volume energy source (22).
Tubular parts made of an austenitic rustless steel, which is susceptible to hot cracks, are to be bonded by laser beam welding. Conventional laser beam welding does not permit reliable avoidance of hot cracks.
The tube wall thickness is 6 mm. Because inductive energy coupling into the austenitic material is not as effectively possible as in a ferritic material, but, on the other hand, the electrical resistance and the resistive heating which may be generated are relatively great, a conductively acting auxiliary energy source suggests itself as the volume energy source for this case. For this purpose, as shown in
Before the start of the welding process, the conductive current flow through the power collectors (24-26 or 25-27) and the components (1) and (2) is started. Two temperature fields, which penetrate the plate thickness d and are inclined to the surface, are generated by the resistive heating along the approximately tubular current path, which result in a thermal expansion of the heated volumes of the components (1) and (2). When the desired target temperature is reached, the laser used as the welding energy source (32) is switched in and the feed is started at the speed vS. The two temperature fields (9, 10) thus generated result in a reduction of the tensile elongations in the solidification zone (7) during the passage of the temperature interval of brittleness ΔTIS and thus ensure welding free of hot cracks.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102006048580.7-45 | Oct 2006 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2007/008786 | 10/10/2007 | WO | 00 | 4/13/2009 |
