Patent Publication Number: US-8124912-B2

Title: Method for heating components

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for heating of structural components prior to and/or during and/or after a further machining thereof. 
     BACKGROUND INFORMATION 
     Structural components, such as for example turbine blades of gas turbines, must be heated during production or maintenance work or for repair thereof for the performance of most varied working or processing operations. Such heating is also referred to as pre-heating. It is also customary to heat gas turbine structural components subsequent to a working operation in the sense of a heat treatment. 
     In connection with the maintenance of turbine blades, so-called deposit welding is used, for example. In connection with the deposit welding, pre-heating to a desired process temperature of a machining (or working) area or welding area of the turbine blades to be welded is required. A reliable deposit welding can be performed only when the turbine blade to be welded has been heated at least in the machining area to the process temperature and is kept at the desired process temperature during the deposit welding. 
     According to the prior art, so-called inductive systems are used for heating or pre-heating of structural components. Such inductive systems may involve coils, for example, which heat the structural component based on an inductive energy introduction. The heating or pre-heating of structural components by means of inductive systems has the disadvantage that during the heating or pre-heating high-temperature tolerances of up to 50° C. may develop at the structural component to be heated. Such an inexact temperature distribution on the structural component to be heated is disadvantageous. Moreover, such inductive systems consume very much energy. Another disadvantage of inductive systems resides in the fact that during the heating or pre-heating, higher temperatures may develop inside the structural component than on the surface of the structural component. This may lead to damages of the structural component. 
     SUMMARY OF THE INVENTION 
     Starting from the foregoing, the invention is based on the problem to provide a new method for heating structural components. 
     The above object has been achieved according to the invention in a method of heating a processing area of a structural component. According to the invention, the processing area or machining area (area to be processed or worked) is irradiated by several laser sources for heating, whereby each laser source directs an energy beam onto the machining area in such a way that each laser source produces one respective energy spot on the machining area, which energy spots together heat the machining area, and whereby each of the laser sources produces a static or quasi-static (stationary or quasi-stationary) energy spot on the machining area in such a way that the position of the respective energy spot on the machining area is stationary or quasi-stationary. Thereby, it is possible to avoid problems which occur in connection with an inductive heating. Furthermore, difficulties which can occur when the energy spots move due to the motion of the laser source, are avoided. 
     According to an advantageous embodiment of the invention, a temperature measuring device is allocated to each laser source, which device measures the heating of the machining area produced by the respective laser source or rather by the energy spot of the respective laser source and compares the measured heating with a respective temperature rated value, whereby, depending on the comparing, the radiation energy of the respective energy beam is individually fixed for each of the laser sources. Hereby optimal preconditions are given for adapting the heating of the structural component or the machining area to the varying structural component cross sections. 
     Preferably, each of the laser sources produces a quasi-stationary energy spot on the machining area in such a way that the position of the respective energy spot on the machining area varies maximally between respective neighboring energy spots in order to thereby heat the transition area between two neighboring energy spots. Thereby, a still more homogeneous heating of the machining area is achievable while simultaneously avoiding the problems of movable systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred further embodiments of the invention are derived from the dependent claims and the following description. Example embodiments of the invention will be explained in more detail with reference to the drawing without being limited thereto. Thereby, the Figures show: 
         FIG. 1  a substantially schematized arrangement with a structural component to be heated shown in cross-section for illustrating a first embodiment of the method according to the invention; 
         FIG. 2  a substantially schematized arrangement with the structural component to be heated shown in a side view for the further illustration of the first embodiment of the method according to the invention; and 
         FIG. 3  a substantially schematized arrangement with a structural component to be heated shown in cross-section for illustrating a second example embodiment of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION 
     In the following, the method according to the invention for heating or pre-heating of structural components is described with reference to  FIGS. 1 to 3  illustrating the pre-heating of a turbine blade of a gas turbine. 
       FIG. 1  shows, in a substantially schematized manner, a turbine bucket  10  of a high-pressure turbine of an aircraft engine, in a cross-section, namely through a blade  11  of the turbine bucket  10 .  FIG. 2  shows the turbine bucket  10  in a side view whereby a blade foot or root of the blade  11  is designated with reference number  12 . It is within the teaching of the present invention to heat the turbine bucket  10  of the high-pressure turbine prior to and/or during and/or after a further machining of the same, namely in a machining (working) area  13  of the blade  11  shown in  FIG. 2 . 
     According to the present invention, the turbine bucket  10  is irradiated on one side by several laser sources  19  for heating the machining area  13 , as shown in  FIGS. 1 and 2 , whereby each of the laser sources  19  respectively directs an energy beam  14  onto the machining area  13  of the turbine bucket  10 .  FIG. 1  shows a total of seven of such energy beams  14 . The energy beams  14  produce on the turbine bucket  10 , namely in the machining area  13  thereof, respective energy spots  15 . The energy spots  15  together heat the machining area  13  of the turbine bucket  10 . The energy spots  15  are dot-shaped or circular. 
     According to the present invention, the laser sources  19  produce stationary or quasi-stationary energy spots  15  in the machining area  13  of the turbine bucket  10 . The term stationary energy spot is intended to mean that the position of the respective energy spot in the machining area  13  is “static”, thus it does not change. On the other hand in connection with a quasi-stationary energy spot a small motion of the same is possible. 
     In a first alternative embodiment of the present invention, the laser sources produce stationary energy spots. More specifically, the position of the respective energy spots  15  in the machining area  13  does not change. If the spacing between such stationary energy spots is selected to be small enough, it is possible to obtain a homogeneous heating of the entire machining area  13 . 
     According to an alternative of the present invention, the laser sources  19  produce quasi-stationary energy spots  15  in the machining area  13 . In connection with a quasi-stationary energy spot  15  a small motion of the same within the machining area  13  is permissible, whereby a position of an energy spot  15  changes maximally between the respective immediately neighboring energy spots  15 . Thereby, an even more homogeneous heating of the machining area  13  can be achieved, namely preferably in the transition area  18  between two neighboring energy spots  15 . 
     A temperature measuring device  20  is allocated to each laser device  19 . Each of the temperature measuring devices  20  measures or ascertains the heating caused by the respective laser source  19  or by the respective energy spot  15  in the machining area  13  of the turbine bucket  10 . The actual temperature values ascertained by each of the temperature measuring devices  20  are compared in a control unit  21  with a respective rated temperature value. Thus, a separate temperature rated value is allocated to each laser device  19  or each energy spot  15  produced by the respective laser device. 
     The radiation power of the respective energy beam  14  and thus the power of the respective energy spot  15  of each laser device is individually adapted on the basis of this temperature rated value. Thus, a pre-defined temperature profile can be exactly adjusted in the machining area  13 . Furthermore, in this manner it is possible to take into account the varying cross-section of the turbine bucket  10  along the machining area. Thus,  FIG. 1  namely shows that the cross-sectional profile of the turbine bucket  10  noticeably varies between the edges  16  and  17 . In so far, with the help of the present invention the radiation energy can be easily adapted with certainty to the cross-section of the turbine bucket  10  that varies over the machining area  13 . 
     In the example embodiment of  FIGS. 1 and 2 , the machining area  13  of the turbine bucket  10  is heated from one side by laser sources  19 . In distinction hereto, in the example embodiment shown in  FIG. 3 , it is possible to heat the machining area  13  from two sides. Thus, in the example embodiment of  FIG. 3 , energy beams  14  are directed onto the machining area  13  from both sides of the turbine bucket  10 . Thereby, the quality of the heating can be still further improved. 
     In accordance with the present invention, diode lasers are preferably used as the laser sources  19 . The use of diode lasers which have a linear power output in response to a linear control is particularly preferred. Diode lasers make it possible to direct the radiation energy with a narrowly limited specific wavelength onto the turbine bucket  10  or onto the machining area  13  to be heated. The defined wavelength of the diode lasers makes possible a good and defined limitation of the energy spreading and a precise heating of the turbine bucket  10  or rather of the machining area  13 . However, alternatively other laser sources can be used for the heating, for example a CO 2 -laser, an Nd-laser or a YAG-laser should be mentioned here. 
     The heating as well as the measuring of the heating at the turbine bucket  10  takes place in a contactless manner. Pyrometers are particularly used for a contactless temperature measurement. As already mentioned, a pyrometer  20  is allocated to each laser source  19  in order to ascertain the heating caused by the respective laser source. 
     The invention is preferably used in the heating of turbine buckets  10  in connection with a repair or a maintenance work of the same. A machining that requires heating of the turbine bucket is for example the so-called deposit welding. The use of the method according to the invention is, however, not limited to repair works on turbine buckets. Rather, the present method can also be used on other structural components of a gas turbine, for example, when repairing a housing.