WO2013154451A1

WO2013154451A1 - Method for a welding process control of nickel based superalloy products

Info

Publication number: WO2013154451A1
Application number: PCT/RU2012/000281
Authority: WO
Inventors: Alexander Vladimirovich KONDRATIEV; Denis Yurievich SARAEV
Original assignee: Siemens Aktiengesellschaft
Priority date: 2012-04-12
Filing date: 2012-04-12
Publication date: 2013-10-17
Also published as: EP2836331A4; RU2014145348A; EP2836331A1

Abstract

A method for a welding process control of Nickel-based superalloy products, especially for gas turbine components, which method includes generating a welding pattern basing on data relating to the geometry of welded products, the material of welded products and the parameters of the selected welding technology and welding equipment, performing a virtual welding using said welding pattern and assessing the quality of welding joint. According to the invention one can make a conclusion whether the selected welding parameters are appropriate for the given alloy composition, welding technology and geometry of the products to be welded and geometry of welding joint, and subsequent realize the welding process in optimal conditions basing on the welding pattern. The invention will bring substantial economic benefit since the number of expensive trials and rejection of welded products will be significantly reduced or even eliminated.

Description

METHOD FOR A WELDING PROCESS CONTROL OF NICKEL BASED

SUPERALLOY PRODUCTS

FIELD OF THE INVENTION

The present invention relates to a method for a welding process control of Nickel based superalloy products.

BACKGROUND OF THE INVENTION

Nickel-based superalloys are widely used as materials for service at high temperatures, in particular, in the hot zones of gas turbines. Most important properties of superalloys are high-temperature creep resistance, fatigue life, oxidation and corrosion resistance, as well as their weldability and processability.

It is well known that, in general, Ni-based superalloys have poor weldability. This issue becomes even much more complicated in case of dissimilar welding, i.e. when two different superalloys are joined, or a superalloy has to be joined with steel. Poor weldability is attributed to several defects which can form during welding in the weld bead and heat-affected zone (HAZ). As outlined in Henderson, M. B., Arrell, D. D., Larsson, R.R., Heobel, M. M., and Marchant, G. (3. (2004), Nickel based superalloy welding practices for industrial gas turbine applications. Science & Technology of Welding & Joining, 9(1), 13-21 , possible defects are: strain age cracking, solidification cracking, grain boundary formation at the weld centerline, and grain boundary liquation cracking.

Strain age cracking is observed in γ'-Νϊ₃(ΑΙ,Τϊ) precipitate strengthened alloys during post weld heat treatment, or subsequent service at high temperatures. It is characterized by intergranular micro-cracking due to residual stresses developed during manufacture, or applied stresses arising from service exposure. The strain age cracking is promoted by high additions of Ti and Al, as well as C, S, B.

Solidification cracking occurs in the newly formed weld bead when the mushy zone experiences tensile stresses. This defect is promoted by a wide solidification range for the alloy and low welding traverse speeds.

Centerline grain boundary forms along the centerline of the weld bead at intermediate to high input heat levels and high welding speeds. The defect is promoted by higher alloying additions and impurity levels. It is characterized by a sharp teardrop-shaped weld pool, a coarse columnar grain structure across the weld bead and a high volume fraction of eutectic and brittle phases along the centerline.

Grain boundary liquation cracking occurs within the HAZ adjacent to the weld bead as a consequence of local dissolution of grain boundary phases (mainly carbides and Laves phases). Susceptibility to grain boundary liquation cracking is dependent upon alloy composition, grain size (microsegregation is promoted by coarser microstructures), and welding speed.

Weldability of superalloys is also very sensitive to the process parameters, namely the effective power of a heat source and welding speed. Therefore one of the defect provoking factors consists in incorrect choice of operating parameters of welding process, the especially critical of which are power of the heat source (welding current, voltage) and the welding speed (electrode motion). Other parameters causing the defects are the composition of protective atmosphere or of a weld pool and the like as well as environmental conditions in the weld bead and heat-affected zone during welding.

Due to the complexity of metallurgical processes in the weld pool, as well as sensitivity to the variation of welding parameters, it is often necessary to make a number of trials before the welding is successful. This makes manufacturing process very expensive, especially in case when large parts, which cost hundreds or even millions of Euro, are welded together.

Ni-based superalloys are often used, e.g., in various hot components of gas turbines, comprising for example vanes, blades, discs, shafts and so on, thereby imposing additional requirements to the quality of a weld bead.

In addition to high quality requirements high cost of products made of superalloys should be taken into account and rejection of parts due to welding defects should be minimized.

The modern welding process control systems of Ni-based superalloys enable control of the welding process operating parameters and correction for variation of the parameters. To perform the correction, valid experimental data relevant to the previous production are needed.

There are trials and patents that are devoted to optimization of chemical composition of Ni-based superalloys, with the final goal to improve certain properties, like high temperature strength, creep and corrosion resistance and to reduce rejection of welded Ni-based superalloys products (e.g. EP2196551A1, US4938805A1, US5207846A1 , US5232662A1, US20070284018A1 , US20090291016A1 , WO 1997/038144A1 , W02009/109521 Al , EP2248923A1), weldability in general (e.g. EP225270A1 , GB2205109A, US6284392B1 , US20030170489A1), or weldability with a different material (e.g. US20100059146A1, EP2172299A2). Other developments are devoted to modification of processing of superalloys, i.e. the casting operation (e.g. EP2151292A1), specific complex thermo-mechanical treatment (e.g. in WO 1999007902 A 1), the welding process itself (e.g. US20110089150A1 , W0201 1054864A1 , W0201 1058045 A 1 ), pre-, post-weld heat treatment (e.g. US5509980A 1 , US6120624A1 , US20060042729A1 , US20060219758A1 ), repairing process of turbine components (e.g. US20070283560A1), or manufacturing method (e.g, US5413647A1).

It has to be noted that most of these developments are based on experimental trial-and-error approach.

At that, due to design complexity and high cost of products made of superalloys to be welded, there is often the lack of sufficient experimental data, previous production data or similar statistics that allow for reliable adaptation and verification of the welding process control system to obtain high quality welded products. Therefore, the amount of rejected products of Ni-based superalloys due to defects in the weld bead or in a heat-affected zone remains unacceptably high.

As pointed out in Cam, G. and Kocak, M., Progress in joining of advanced materials, International Materials Reviews, 1998, 43(1), 1-44, the weldability, in particular, the susceptibility to strain age cracking can also be roughly assessed in terms of aluminum and titanium contents, since these two alloying elements are responsible for formation of y' -precipitation strengthening. Taking into account information on assessment of weldability for a range of nickel based superalloys provided in US 20060042729 (see Figure 1), alloys with smaller amounts of Al and Ti (below the dashed line) are believed to be generally weldable, and those with high amounts of Al and Ti (above the dashed line) are difficult to weld due to enhanced strain age cracking phenomenon.

However, there exist no means available on the market for complex evaluation of alloys' weldability based on alloy compositions, geometry of products to be welded, including 3D configuration and geometry of a joint, welding technology and welding parameters.

SUMMARY OF THE INVENTION

The aim of this invention is to provide a method for a welding process control for Nickel based superalloy that eliminates one or more of the above mentioned disadvantages singly or in any combination.

In particular, it may be seen as an object of the present invention to provide an improved way for Nickel based superalloy welding control, which could be used to assess virtually (by means of computer implemented process) the welding conditions (alloy composition(s), welding power, welding speed, geometry of the joint etc.) and which should give a basis for the decision that a welding can be successful or not.

In the first aspect of invention provided is a method for a welding process control of Nickel-based superalloy products, the method comprising

- providing of data, relating to geometry of welded products, including geometrical parameters of products, welded bead and its position in the welded products,

- providing of data, relating to the material of welded products, including chemical composition of welded products,

- selecting of a welding technology and welding equipment,

- providing of data, relating to parameters of the selected welding technology and welding equipment, including type and power of a welding source, power distribution, welding electrode feed rate,

- generating, via a computing means, of a welding pattern basing on data relating to the geometry of welded products, the material of welded products and the parameters of the selected welding technology and welding equipment,

- performing, via a computing means, a process of virtual welding using said welding pattern to determine the parameters of the stress-strain state, temperature field and phase distribution for each section of a welding bead or a heat-affected zone of the superalloys to be welded at any time moment of the welding process,

- determining weldability properties of superalloys under given conditions at a given time moment, basing on the obtained parameters of the stress-strain state, temperature and phase distribution for each section of a welding bead or a heat- affected zone, and using the result of the determination of weldability properties to assess the quality of welding joint,

wherein:

with satisfactory weldability property a notification is provided about possibility of obtaining high-quality welded products and welding of products is performed using the welding parameters determined during the process of virtual welding according to said welding pattern, and

with unsatisfactory weldability property, a warning and information is provided about the possibility of a welding defect and its location, and recommendations are provided for changing at least one welding parameter to correct said defect.

The method may further comprise displaying of a determined weldability property on a display device.

In the proposed method, determining of weldability properties of superalloys may include at least one of determining of localized weldability for each of the welding sections and determining of overall weldability for all welding sections.

Such localized weldability could be used for determining local weldability, may be referred as spot weldability, of complicated shaped products, for example for transition portions between two portions of products having different shapes.

The overall weldability could be determined as integrally weldability or as averaged weldability. It depends only on a method, which shall be implemented by producing the overall weldability from the local weldability basing on the properties and parameters of finish welded products of Ni based superalloys. Since such methods are well known in the art, further specification of such approach is omitted.

Weldability properties of superalloys are determined basing on at least one criterion selected from the group consisting of a weld cracking criterion caused by tension stress and/or extending motion above the critical value aft the welding source, a faulty fusion criterion, a liquation cracking criterion and a grain boundary at centerline criterion.

In the second aspect the invention provides a computer-readable medium carrying a computer program comprising instructions for implementing said method for a welding process control of Nickel based superalloy products.

In general, the various aspects of the invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the invention will be apparent from and elucidated with reference to the non-limiting embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates an assessment of weldability for a range of nickel based superalloys according to US 20060042729;

Fig. 2 schematically illustrates a mechanical-metallurgical approach used in the method;

Fig.3 illustrates flow diagram of the claimed method;

Fig.4 to 13 relate to an example of method of Fig. 3, wherein

Fig. 4 illustrates a CAD model of welded parts including filler material in between,

Fig. 5 illustrates a finite element model based on CAD model;

Fig. 6 shows a temperature distribution at a certain time moment after onset of welding;

Fig. 7 shows Von Mises stress distribution at a certain time moment after onset of welding;

Fig. 8 illustrates a distribution of tensile stresses (o_yy) at a certain time moment after onset of welding;

Fig. 9 illustrates a distribution of plastic strains (von Mises) at a certain time moment after onset of welding;

Fig. 10, 1 1 illustrate an example showing places in which incomplete weld penetration occurred after welding with the heat input of 150 J/mm and welding speed of 5 mm/s: Fig.10 - location of defects; Fig.11 - temperature distribution showing melted area (in black);

Fig. 12, 13 illustrate the example in which full weld penetration occurred after welding with the heat input of 240 J/mm and welding speed of 5 mm s: Fig.12 - location of defects (no defects); Fig.13 - temperature distribution showing melted area (in black).

DETAILED DESCRIPTION OF EMBODIMENTS

With the above mentioned problems of the prior art taken in account, the present invention relating to a method for a welding process control of Nickel based superalloy products is proposed, the method providing increased quality of a welding joint and reduced amount of rejected welded products made of Ni-based superalloys.

The authors of the present invention propose a new complex approach to the welding process control.

Inability of performing multiple welding trials and the fact that the majority of products of superalloys are single items, not meant for mass reproduction, inferred the idea for the authors of the present invention of controlling the welding process basing on numerical simulation of the welding process, this numerical simulation may serve as basis for reliable conclusion about the quality of an obtained welded products of Nickel-based superalloys.

Such a numerical simulation, also referred to as virtual welding, is a reliable tool for controlling the welding process and allows for assessing the feasibility of welding under the current welding conditions, including the parameters of welding equipment, composition of products to be welded and their geometry taken into account.

The present invention differs from the conventional methods for a welding process control, based on adjustment of operating parameters within the predetermined limits, in that the method for a welding control provide, through the virtual welding, at any given time moment during the welding process a reliable response about feasibility of welding of Ni-based superalloys with a high quality welded products obtained and with the available operating parameters of welding equipment, material and geometry of welded products.

The apparent advantage of the present invention is the ability to determine the quality of the resulted welding joint while making changes to the welding process parameters, such as changes introduced by the operator. The control provides an appropriate response to the changes introduced by the operator and indicates the feasibility of welding considering the entire combination of welding conditions, characterized by the operating parameters of the welding equipment, material and geometry of the welded products.

The present invention also provides for a virtual welding, which is based on assessment of the actual weldability of Nickel-based superalloys.

Weldability of superalloys can be regarded as a comprehensive and the most reliable criterion for determining a quality of the welding. Referring to Fig.1 one could see one approach for an assessment of weldability for a range of nickel based superalloys according to aluminum and titanium contents according to US 20060042729. Such approach does not allow assessing of weldability of superalloys to determine the quality of the resulted welding joint while making changes to the welding process parameters.

The evaluation procedure for weldability of superalloys shall be based on using predetermined criteria, such as those proposed in the article of Dye D., Hunziker O. and Reed R.C, Numerical analysis of the weldability of superalloys, Acta materialia, 2001 , 49, 683-697, comprising mathematical descriptions of criteria for formation of certain defects, namely, for centerline grain boundary, constitutional liquation at second phases, and solidification cracking. In the claimed method can be used additional criteria.

However, the known approach is not designed for control and evaluation of alloys' weldability based on alloy compositions, 3D geometry of a product to be welded and geometry of a joint, welding technology and welding parameters.

In order to determine whether the welding of given superalloys, predefined geometry of the welded joint and other conditions (e.g. additional heating/cooling during welding), can be successful, it is proposed to use an integrated mechanical- metallurgical approach, which is schematically shown in Figure 2.

It is assumed that the design of welding joint, welding technology and material composition and properties are specified. Namely, the complete geometry of welding joint, power of heat source, its power distribution and speed, preheating conditions and/or additional heating/cooling during welding, alloy composition, temperature dependant thermal properties (thermal conductivity A) and temperature dependant thermo-mechanical properties (thermal expansion coefficient a, yield stress a_y, elastic moduli, etc) of alloys are defined.

The approach of Fig. 2 consists of two blocks.

A "metallurgy" block is based on thermodynamic calculations (using software like ThermoCalc, FactSage etc) to evaluate phase transformations and transition temperatures for the alloy of given chemical composition, as well as temperature- dependant specific heat and density. A "thermo-mechanical" block is based on the coupled structural mechanics and heat transfer problem. Commercial software packages like ANSYS, LS-DYNA, SYSWELD, etc can be used.

Using such two blocks it would be possible to realize a combined thermo- mechanics and metallurgy calculations providing the stress-strain state, temperature and phase distributions for the whole structure of welding joint at any time moment of the process. Based on this information, one could use different criteria to evaluate whether the welding will be successful for the given Ni base superalloy material, welding technology and geometry of finish welded product.

Next, a flow diagram of the claimed method for a welding process control of

Nickel-based superalloy products as shown in Fig. 3 will now explain with reference to Fig. 4 to 13.

The method for a welding process control of Nickel-based superalloy products comprises first step of providing the data, relating to geometry of welded products, including geometrical parameters of products, welded bead and its position in the welded products. This data could be accumulated in a CAD model of welded products including welded bead. Such example CAD model is shown in Fig. 4, comprising two different parts to be welded.

Further the method as shown in Fig. 3 comprises a step of selecting of a welding technology and welding equipment and subsequently providing of data, relating to parameters of the selected welding technology and welding equipment, including type and power of a welding source, power distribution, welding electrode feed rate. Such data could be obtained by respective scanning means and/or inputted by operator.

Next, the method comprises a step of generating, via a computing means, of a welding pattern basing on data relating to the geometry of welded products, the material of welded products and the parameters of the selected welding technology and welding equipment.

In the example of implementation shown in Fig.3, FactSage is used to calculate phase distribution for the given alloy composition, including the equilibrium temperatures of liquidus, solidus and solvus, which are corrected using Guliver-Scheil model to take into account the kinetics of cooling process. SYSWELD is used to find the evolution of temperature and stress-strain fields. It also uses the calculation of FactSage through temperature-dependant thermo-mechanical properties. As output, one has the distributions of stresses, strains, and temperatures at each time moment of the welding process. This information is used in the proposed post-processing tool to make the recommendation whether the welding could be successful or not.

The present invention provides for generation, via a calculation means, of a welding pattern basing on data relating to the 3D geometry of welded products, material of the welded products and the parameters of the selected welding technology and welding equipment, and performing a virtual welding in the form of a numerical simulation, e.g., by using a set of software modules, such as Thermocalc, FactSage, SYSWELD, ANSYS, LS-DYNA and the like.

The welding pattern comprises: 1) a geometrical CAD model (see Fig.4), obtained by respective software means for example: SolidWorks, Pro/ENGI EER, CATIA, NX etc.; 2) a finite element model based on the CAD model (see Fig. 5); 3) a welding technology and welding equipment model, defining a size and a form of welding pool, for example as a Goldak's double ellipsoid, 4) whole set of additional welding pattern parameters, comprising at least power of welding equipment, electrode movement speed, sizing of the a Goldak's double ellipsoid, a pre-heating temperature, a composition of products to be welded, thermal properties of welded materials (e.g. temperature-dependent specific heat, density etc.), temperature-dependent thermo- mechanical properties (e.g. linear expansion coefficients, elastic moduli and constitutive behaviors thereto.

After generating the welding pattern, the method of Fig. 3 comprises performing, via a computing means, a process of virtual welding using said welding pattern to determine the parameters of the stress-strain state, temperature field and phase distribution for each section of a welding bead or a heat-affected zone at any time moment of the welding process.

In the present application a virtual welding generally shall be considered as computer implemented numerical solving of a coupled thermo-mechanical problem including metallurgical/phase transformations.

The virtual welding process is performed by a computing means basing on said computed welding pattern to determine predictive parameters of the stress-strain state, temperature field and phase distribution for one or more sections, preferable for each sections, of the welding joint and heat-affected zone at any time moment of the welding process. The example results of such virtual welding implementing are shown in Fig. 6 to 9, comprising a temperature distribution at a certain time moment after onset of welding, a von Mises stress distribution at a certain time moment after onset of welding, a distribution of tensile stresses (a_yy) at a certain time moment after onset of welding and a distribution of plastic strains (von Mises) at a certain time moment after onset of welding.

The performing of the virtual welding will result in obtaining parameters of the stress-strain state, temperature and phase distribution for each section of a welding bead or a heat-affected zone at a given time moment (As shown in Fig. 6 to 9).

Said virtual welding is coupled with a determining of weldability properties of superalloys under given conditions.

The following criteria could be used as the main criteria for predicting weldability properties of superalloys:

a weld cracking criterion (also referred as solidification cracking), which occurs at any time moment of the welding process, when there are a) tension stresses, and/or b) extending motions above the critical value in two-phase region aft the welding source;

a faulty fusion criterion, which occurs if there are base metal areas in which temperature was never above the liquidus temperature during the whole welding process, and

a liquation cracking criterion (caused e.g. carbide liquation) which occurs in the heat-affected zone by passing eutectic reaction: γ + MeC→ γ + MeC + liquid, where γ is the metal phase, MeC is a carbide phase stable at given conditions (e.g. NbC). To evaluate the possibility of liquation cracking, it is necessary to calculate the rate of dissolution of (carbide) particles in a metal matrix.

The above set of criteria shall not be considered as limiting, because the skilled person basing on his experience could provide additional criteria for several cases.

Generally, all criteria shall be included in a predetermined interval, however, it is possible to provide respective determining behavior for each criterion, for example a faulty fusion criterion, i.e. that the presence of metal areas in which temperature was never above the liquidus temperature during the whole welding process, shall be placed to zero.

Having considered the set of criteria one could make a conclusion about satisfactory weldability or unsatisfactory weldability.

In the claimed method, with satisfactory weldability property a notification is provided about possibility of obtaining high-quality welded products and welding of products is performed using the welding parameters determined during the process of virtual welding according to said welding pattern. Said welding will result in high quality product manufactured in optimal welding condition and free of defects (as shown in Fig. 11). Methods for welding of superalloy products are not limited specifically for the invention, and are known in the art. In particular, such method of welding is submerged arc welding, gas-shielded tungsten-arc welding, gas-shielded metal arc welding (MIG / MMA) etc.

In the claimed method, with unsatisfactory weldability property, a warning and information is provided about the possibility of a welding defect and its location, and recommendations are provided for changing at least one welding parameter to correct said defect. The warning and information about the possibility of a welding defect and its location could be indicated using respective display means, as shown for example in Fig. 10 for an incomplete weld penetration occurred after welding)

Further, the method is explained in following non-limiting example.

EXAMPLE

The method for a welding process control of Nickel-based superalloy products of claimed invention was applied for welding two superalloy sheets having length of 95 mm, width of 75 mm and thickness of 3 mm. The sheets ware made from Ni base alloy Inconel 718, having following composition:

The CAD model as shown in Fig. 4 and the finite elements model as shown in Fig. 5 correspond to the welded product manufactured from said sheets.

A welding process shall be performed along an X axis of Fig 4, starting from distal end.

The TIG (tungsten inert gas) welding process with the following parameters was applied: energy per unit length 150 J/mm and welding speed of 5 mm/sec. The parameters of welding pool were: length 4 mm, width 3 mm, penetration 2 mm.

For given superalloy Fig. 6 to 9 comprise a temperature distribution at a certain time moment after onset of welding, a von Mises stress distribution at a certain time moment after onset of welding, a distribution of tensile stresses (oyy) at a certain time moment after onset of welding and a distribution of plastic strains (von Mises) at a certain time moment after onset of welding. Said figures show the state at the moment of 5 sec after welding start. Fig. 10 shows that there is an incomplete weld penetration. This defect is caused by insufficient heating in said welding joint portions, as can be seen from Fig. 1 1 , and could be eliminated by respective adjusting the power of welding equipment, for instance by increasing the energy per unit length to 240 J/mm and by keeping the welding speed of 5 mm/sec. After the adjustment of the welding parameter, the welding joint could be obtained without defect as shown in Fig. 12, the size of melt pool was sufficient to ensure very good fusion (Fig. 13). Thereafter, the sheets were welded in optimal conditions using established parameters to manufacture high quality welded products (not shown).

The given example has demonstrated that the used welding pattern and computer implemented virtual welding process provide reliable results, allowing predicting the quality of welded products of Nickel-based superalloys.

Using the invention one can make a conclusion whether the selected welding parameters (power of heat source and welding speed) are appropriate for the given alloy composition, welding technology and geometry of the product to be welded and geometry of welding joint, and subsequent realize the welding process in optimal conditions. The invention will bring substantial economic benefit since the number of expensive trials and rejection of welded products will be significantly reduced or even eliminated.

Claims

CLAIMS:

A method for a welding process control of Nickel-based superalloy products, the method comprising:

- selecting of a welding technology and welding equipment,

- performing, via a computing means, a process of virtual welding using said welding pattern to determine the parameters of the stress-strain state, temperature field and phase distribution for each section of a welding bead or a heat-affected zone at any time moment of the welding process,

- determining weldability properties of superalloys under given conditions at a given time moment, basing on the obtained parameters of the stress-strain state, temperature and phase distribution for each section of a welding bead or a heat- affected zone, and using the result of the determination to assess the quality of welding joint, wherein:

2. The method for a welding process control of Nickel-based superalloy products according to claim 1 , further comprising displaying of a determined weldability property on a display device.

3. The method for a welding process control of Nickel-based superalloy products according to claim 1 , wherein determining of weldability properties of superalloys may include determining of local weldability at each of the welding sections and determining of overall weldability of the products made of superalloy.

4. The method for a welding process control of Nickel-based superalloy products according to claim 1 , wherein weldability properties of superalloys are determined basing on at least one criterion selected from the group consisting of a weld cracking criterion caused by tension stress and/or extending motion above the critical value aft the welding source, a faulty fusion criterion, a liquation cracking criterion and a grain boundary at centerline criterion.

5. A computer-readable medium, carrying a computer program comprising instructions for implementing said method for a welding process control of Nickel based superalloy products according to any of claims 1 - 4.