OFF GAS REACTION PRODUCTS DURING WELDING

BACKGROUND 1. Field

[0001] The present invention relates generally to the field of metals joining, and more particularly to shielding gas additives and welding methods useful for cast components such as superalloy gas turbine engine airfoils.

2. Description of the Related Art

[0002] Welding of superalloys presents a variety of technical challenges because of the high strength (and corresponding low ductility) that these alloys are optimized to achieve. Heat sources such as lasers and arcs are being applied to build additively manufactured (AM) parts or repair damaged superalloy components. Unfortunately, these alloys are very prone to hot cracking during processing, i.e. during laser metal deposition (LMD) as well as following subsequent heat treatment. The hot cracking during welding at room temperature can occur within the solidifying weld pool and into the adjacent base metal heat affected zone, which structurally compromises the integrity of the AM or repaired component.

SUMMARY

[0003] In one aspect of the present invention, a shielding gas system for processing of a superalloy weld or fabrication deposit comprises: at least one weld shielding gas; and at least one shielding gaseous additive material comprising one or more constituents that, when projected to blanket a molten weld pool during weld processing, produce one or more off gas reaction by-product gases that are then removed from a melt pool.

[0004] In another aspect of the present invention, a method for the removal of off- gas reaction products during welding, comprises: preparing at least one weld shielding gas and at least one shielding gaseous additive material as a shielding gas system; forming an electrical arc between a consumable electrode and at least one workpiece metal producing a melt pool, wherein the electrode is fed through a welding torch, wherein the weld shielding gas and the at least one shielding gaseous additive material are fed through the welding torch directed towards and surrounding the melt pool, wherein at least one off-gas reaction product is produced in a reaction with the shielding gas system and with weldability-troublesome elements within the melt pool.

[0005] These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention is shown in more detail by help of figures. The figures show preferred configurations and do not limit the scope of the invention.

[0007] FIG. 1 is a schematic view of an apparatus in process of welding illustrating aspects of an embodiment and use of an exemplary embodiment of the present invention.

[0008] FIG. 2 is a schematic sectional view of an apparatus in process of welding illustrating aspects of an embodiment and use of an exemplary embodiment of the present invention.

DETAILED DESCRIPTION

[0009] In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.

[0010] Broadly, an embodiment of the present invention provides a shielding gas system for superalloy welding and additive processing, including constituents which react with a weld shielding gas when heated during weld processing creating one or more scavenging gas by-product gases or slags that are removed from a melt pool.

[0011] Superalloy materials are difficult to fabricate and repair due to their poor ductility up to near their high melting points and susceptibility to weld solidification cracking and strain age cracking. These materials can have melting point ranges of 1200 to l400°C and higher, and are used for components in the hot gas path in gas turbine engines. Weld metal solidification cracking, such as hot cracking, liquation cracking and microfissuring, is often caused by segregation of low melting point eutectic compositions to last to solidify grain boundaries. Sulfur (S), phosphorous (P) and boron (B) are particularly problematic in terms of welding of nickel based alloys and some stainless steels. These elements are sometimes called tramp elements. Such elements can form eutectics such as Ni-NiS (with a melting point of 903K), Ni-Ni30 (melting point of 1148K) and Ni-Ni2B (melting point of 1413K). During solidification such eutectics represent liquid films on grain boundaries that cannont sustain shrinkage strains and that thereby result in cracking. Methods for removing elements that contribute to cracking during welding is desired.

TABLE 1 [0012] Elements that are frequently associated with low melting point eutectics include S, P, and B. Cracking associated with such precipitates can be controlled through exercising care to remove contaminants from materials. For example, refining base metals to have low residual element content, or refining filler materials to have low residual element content can help control cracking. Use of fluxes to scavenge residuals from weld metal can help. Procedure modifications can include weaving of the molten puddle to avoid micro structural alignment where segregation is most pronounced, avoiding weld restraint associated with base metal configuration and weld preparation, or avoiding weld concavity. Table 1 above lists a series of commonly used alloys in industrial turbine engine components in a first column. For the high Mn steels, the range is generally up to 28 percent manganese. The second column lists the associated problematic elements during the welding process. Additionally, in the third column is listed elements that can also contribute to the weld metal solidification cracking.

[0013] There can be compromises sometimes required to provide enough boron in nickel based castings for mechanical property optimization, such as creep, but not too much boron as to adversely affect weldability. Sulfur and phosphorous are generally reduced to low levels but extended elimination becomes expensive.

[0014] The addition of welding gaseous additives allows for the elimination of weld solidification cracking. The welding gaseous additives can help with alloys, and specifically with nickel-based and stainless alloys that are made difficult to weld by the presence of sulfur and phosphorous, and occasionally boron.

[0015] A common welding process known in the art is gas metal arc welding (GMAW). GMAW has two main subsets known in the art as metal inert gas (MIG) and metal active gas (MAG). The gases utilized in MIG include the inert gases argon and helium and mixes of these elemental gases. The gases used for MAG include non-inert gases as well as mixed gases that may include argon and helium plus oxygen, carbon dioxide and hydrogen. Nitrogen is also added to some of these mixes. Nitrogen is not completely inert and nitrides and nitrogen alloying can result in changes that affect the weld metal, however, oxidation can be eliminated or reduced by the introduction of nitrogen.

[0016] As illustrated in FIGS. 1 through 2, a welding process 10 includes an electrical arc that forms between a non-consumable tungsten alloy electrode (for gas tungsten arc welding (GTAW)) or consumable electrode (for gas metal arc welding (GMAW)) 16 and a workpiece metal 22 or metals that heats the workpiece metal 22 or metals (and for GTAW, a separately fed filler wire or for GMAW, the electrode 16) causing them to melt into a molten pool referred to as a melt pool 12.

[0017] Shielding gases 20 used in welding, typically at least one inert gas in a MIG welding and other mixes of gases in MAG welding, provide an inert atmosphere above the molten or melt pool 12 to avoid atmospheric reactions such as oxidation, nitridation, and negative effects of moisture such as hydrogen cracking. The shielding gases 20 also provide control over the weld melt pool 12 shape. Additionally, the shielding gases 20 can also provide arc control, or stabilization.

[0018] Typically, a welding torch 28, having a handle 30, is directed toward the base metal 22. For GAMW, the electrode 16, typically similar or complimentary in composition to the workpiece, is fed through the welding torch 28 through a nozzle 26. The shielding gases 20 and the electrode 16 are sent through the welding torch 28. A shielding cup 18 or similar component can be used to optimize the distribution of the shielding gases 20.

[0019] At least one shielding gaseous additive material 24 can be added to produce a shielding gas system. The shielding gas system includes the shielding gases 20 and the at least one shielding gaseous additive material 24. The at least one shielding gaseous additive material 24 can be added through the same tool and in the same path as the shielding gases 20 as shown in FIG. 1. The at least one shielding gaseous additive material 24 can be directed in the vicinity of the melt pool 12 to react with specific elements associated with difficult or troublesome weldability. The reaction results in the reduction of the problematic elements from the melt pool 12 and the generation of an off-gas reaction product 14 that is dispersed into the atmosphere, or alternatively, solid slag product that floats to the surface of the melt pool 12 and is subsequently removed. The resultant melt pool 12 is thereby cleansed of the problematic elements and is a sound welding product. The at least one shielding gaseous additive material 24 can be added as an already premixed gas blend or by way of a gas mixer that mixes pure gas constituents before directing the blended shielding gas system to the welding torch 28. The at least one shielding gaseous additive material can be up to and including 25 percent by weight of the total combined shielding gas system.

[0020] An example of the shielding gas system with the at least one gaseous additive material 24 and the welding shielding gases 20 is with the use of nitrogen trifluoride, NF ₃ to an otherwise inert tungsten inert gas (TIG (also GTAW)), or MIG, or additive MAG (also GMAW) weld shielding gases. Additionally, the at least one gaseous additive material 24 can be used for other welding processes such as plasma arc welding (PAW), flux cored arc welding (FCAW), laser beam welding (LBW) and the like. In the example below, the problematic boron is removed from the weld pool. Nitrogen trifluoride is a relatively safe gas, and is highly reactive with boron and produces a gaseous reaction product.

[0021] NF ₃ + B BF ₃ + N

[0022] The reaction is driven because the heat of formation of BF ₃ is much more negative than that of NF _3. Nitrogen trifluoride is available commercially (ref. Air Products, Inc.). [0023] Nitrogen trifluoride can be used to react with copper as well and produces a gaseous reaction product as well as a second product that is solid at room temperature and can be removed from the surface of the weld as a slag.

[0024] 2 NF ₃ + Cu N ₂ F ₄ + CUF ₂

[0025] The reaction is driven at elevated temperatures such as temperatures that occur during welding.

[0026] Another embodiment can involve the use of hydrogen as the at least one gaseous additive material 24 to a TIG or a MIG or additive a MAG weld process. The target removal is for the problematic sulfur along with phosphorous from the weld melt pool 12. Hydrogen has a heat of formation near zero and is relatively reactive with sulfur and produces a gaseous product.

[0027] H ₂ + S H ₂ S

[0028] and,

[0029] XH ₂ + P + 20 ₂ H3PO4

[0030] Phosphoric acid is aqueous at room temperature and normal atmospheric pressure but gaseous near the high temperature location of welding. Hydrogen can be used in welding, however, the percentage of hydrogen is generally very limited to approximately five percent. Here, to promote these reactions a higher percentage mix of up to approximately 25 percent hydrogen is used in order to reduce the unwanted elements. Another embodiment of the at least one gaseous additive material 24 can be ammonia NH4, with potential reactions including:

[0031] NH4 + P + 0 ₂ aHPCri, bHzPCri, CH3PO4, dH ₂ S04 plus nitrogen gas

[0032] Ammonia’s heat of formation is just -45.9 kJ mole ¹ providing the chemical driving force to scavenge sulfur and phosphorous by way of such products. Oxygen additives or oxygen compound additives such as carbon monoxide or carbon dioxide may also be considered to remove sulfur by generating sulfur dioxide and sulfur trioxide. This particular additive may be limited by potential generation of undesirable reaction products such as metal oxides. However, such by-products could be safely controlled in glove box welding or welding with special respirators or during deposition in additive manufacturing equipment with filters to remove such products.

[0033] As illustrated above, it is not necessary for the reaction products to be gaseous at standard conditions of temperature and pressure (STP). Near the point of welding, the temperature will be very elevated and most products that are aqueous at STP will be vaporized. Additionally, if solid products result in the reaction, they are most often less dense than the metals being joined or deposited and will generally float to and segregate at the weld melt pool 12 surface where they can be readily removed as innocuous slag without detrimental effect on weldability.

[0034] While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims, and any and all equivalents thereof.