DESC:METHOD FOR HIGH SPEED KEYHOLE PLASMA ARC WELDING OF THICK PLATES/PIPES

FIELD OF INVENTION

[001] The present invention pertains to fusion welding of thick plates in single pass. More particularly the invention relates to proper bead formation in keyhole plasma arc welding of thick plates in single pass.

BACKGROUND OF THE INVENTION

[002] Keyhole plasma arc welding is a high intensity arc welding process, which uses the immense power available in ‘Plasma – The Fourth Sate of Matter’ for melting and joining the materials. Keyhole plasma arc welding is a development from TIG welding process, which uses a non-consumable tungsten-based electrode to generate the welding arc. The arc is generated between the tungsten electrode and the base material and is used to generate the plasma of usually the inert gas supplied through the torch. Initially, a pilot arc is initiated between the tungsten electrode and the copper nozzle, which is used to ignite the main arc to generate the plasma. The plasma is a high intensity heat source and can reach a temperature in excess of 25000 °C. The plasma thus created is contained within a water cooled copper nozzle and is pushed through an orifice of around 3 mm provided in the torch. The required shielding for protecting the molten pool is provided by the gap supplied through the outer periphery of the torch. Since the high intensity plasma is pushed through a small orifice, the arc is cylindrical and provides higher power densities. High power density of the plasma arc provides the energy to penetrate deeply in any metallic materials by keyhole mode of welding, a high efficiency mode for welding of thick material in single pass. The base material to be welded is placed below the arc at a standoff distance of usually 5-6 mm from the copper nozzle and the base material melts and fuses to form the weld joint. In keyhole plasma arc welding, the arc generates a hole in the material, which is surrounded by a thin film of molten metal. As the arc and the keyhole is moved along the joint, the molten material flows through the periphery of the keyhole and solidifies at the rear end. The stability of the keyhole needs a balance between the vapour pressure generated by metal vapour and the surface tension force exerted by the thin molten film surrounding the keyhole. Single pass welding is beneficial especially, for welding of exotic high temperature materials to reduce the heat induced damage.

[003] In keyhole plasma arc welding, the weld bead formation depends on the parameters like welding current, plasma gas flow rate and the welding speed. In any fabrication welding, reducing the welding speed improves the productivity. For the given current, the speed of operation can be increased by using higher plasma gas flow rates. However, the higher plasma gas flow rates need very careful control of the current especially, the start current to ensure stable keyhole and proper bead formation. The same is explained in detail.

[004] European patent EP 0689896 A1/1996 discussed the methodologies that can be used to carry out keyhole plasma arc welding of thick pipes such as variation of plasma gas flow rate in cyclic fashion, two side welding from both top and bottom, varying direction of welding, etc. to achieve defect free keyhole plasma weld with required penetration.

[005] European patent EP 0824987 B1 relate to variants for achieving defect free keyhole plasma arc welding of thick pipes like carrying out simultaneous welding of internal and external peripheries, optimizing the overlap portion after making an initial layer welding.

[006] European patent EP 2781293 A1/2012, US patent US 9375802 B2/2016 and US 0312012 A1/2014 relate to controlling the pulsing parameters like peak current, base current and especially, the pulse frequency to achieve defect free keyhole plasma arc welding in both plates and pipes. Pulse frequency will have to be chosen in such a way to synchronise with the weld pool formed at the back face of the metal during welding.

[007] US patent US 5225654A/1993 describes the methodology to achieve defect free keyhole plasma arc welding in thick high Ni steels. The invention relates to maintaining a depth to width ratio of 0.74 in the weld to achieve a stable keyhole, use of proper plasma gas flow rate, proper closing of the keyhole by stopping the movement and supply of filler material simultaneously, etc. to ensure proper bead formation.

[008] US patent US 8324524 B2/2009 describes a methodology of supplying varying shielding gas to the same molten pool by providing an insert tip with a shield cap to the periphery of the tungsten electrode. This enables supply of different shielding gas in the centre and the periphery and the inventors have arrived at optimum shielding gas combination to achieve acceptable bead profile in welding of austenitic stainless steels. US patent US 0193116 A1/2013 describes a similar approach for welding of ferritic stainless steels of thickness = 3 mm.

[009] Japanese patent JP2011200923 describes a method in which the mode of welding is periodically shifted from non-keyhole mode to keyhole mode to achieve stable rear bead in plasma arc welding.

[0010] All the prior arts mentioned are discussing different techniques like simultaneous welding at both face and root sides, controlling the aspect ratio of the weld bead, controlling the pulsing parameters, periodic shifting from non-keyhole mode to keyhole mode of welding, etc. to ensure proper bead formation and defect free keyhole plasma arc welding of various steel based materials.

[0011] The present invention describes the methodology to achieve stable keyhole mode of welding for thick and dense materials like nickel based superalloys. Various novel features that characterizes the invention are pointed out in the detailed description of the invention.

OBJECTS OF THE INVENTION

[0012] An object of the present invention is to develop a method for keyhole plasma arc welding of thick plates/pipes.

[0013] Another object of the invention is to develop a method for achieving stable keyhole for high speed welding of thick plates/pipes.

[0014] A further object of the invention is to implement the method for welding of high density materials like nickel based superalloys, niobium and alloys thereof.

SUMMARY OF THE INVENTION

[0015] According to this invention, there is provided method for high speed keyhole plasma arc welding of thick plates/pipes comprising steps of: maintaining the plasma gas flow rates at > 2.5 lpm at low starting current, which is = 60% of the welding current with the ramping time of 0.3-0.8 seconds, which involves ramping up the current from the starting current to the welding current and ramping down to the end current for proper bead formation in the thick plates/pipes.

[0016] The welding current is = 100 ampere and thickness of the plates > 5 mm.

[0017] The arc welding of nickel based superalloys can be carried out by controlling the start current.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

[0018] Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings of the exemplary embodiments and wherein:

Fig. 1 shows: Current characteristics in keyhole plasma arc welding in accordance with the present invention.
Fig. 2 shows: Top weld bead with unstable keyhole in nickel based superalloy according to invention.
Fig. 3 shows: Good weld bead at the top surface of the thick material of present invention.

DETAIL DESCRIPTION OF THE PRESENT INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

[0019] The present invention is pertinent to a technology relating to method for high speed keyhole plasma arc welding of thick plates/pipes.

[0020] Keyhole plasma arc welding is a high productivity process employed for single pass welding of thick plates/pipes and is used in various industrial sectors. Additionally, this offers advantages like reduction in heat input and hence, narrow weld width and heat affected zones (HAZ), etc. Hence, the keyhole plasma arc welding process is beneficial especially, for welding heat sensitive materials. To carry out welding in keyhole mode, the welding current is = 100 A. Due to its inherent higher power densities, the process can be applied for welding of thick plates/pipes in the range of 10-12 mm thickness in single pass, which is extremely advantageous for many applications involving exotic materials like stainless steels, Cr-Mo steels, aluminium, titanium and nickel based superalloys. Nickel based superalloys are finding applications in defence, aerospace and power sectors and single pass welding of thick nickel based superalloys can have multifold benefits.

[0021] In the present invention, a suitable method has been introduced for autogenous keyhole plasma welding of high density materials of thickness > 5 mm. The welding power sources have the capabilities to ramp up (tup) the current from the starting current (Is) to the welding current and ramp down (tdown) to the end current (IE) in order to ensure proper bead formation without any defects at start and stop points of the weld. This is an essential feature especially, in welding of symmetrical components so as to ensure smooth finish. In keyhole plasma arc welding, for the given current and welding speed, the penetration capability can be increased by increasing the plasma gas flow rates as the increase of plasma gas flow rate increases the intensity of the arc. Such high plasma gas flow rates is essential to carry out the welding especially in high dense materials and thicker plates. In high dense materials like nickel based superalloys, niobium & its alloys, molybdenum & its alloys, etc., with higher plasma gas flow rates (> 2.5 lpm), the control of starting current is found to be very critical. The use of actual welding current (Iw) as starting current can cause cut through and the effect is same with the very low starting current. With plasma gas flow rates > 2.5 lpm and low starting current, as the current is ramped to the welding current, owing to very high density of the materials, the surface tension forces go high and the sudden change to keyhole mode, results in very unstable keyhole due to loss in balancing effect leading to improper bead formation. This effect is depicted in 10 mm thick nickel based superalloy (Fig.2).

[0022] To ensure defect free high speed keyhole plasma arc welding with high plasma gas flow rates, the starting current needs to be maintained at = 60% of the welding current with the ramping time in the range of 0.3-0.8 second.

[0023] This helps in avoiding the sudden change to keyhole mode of welding and ensures proper bead formation in 10 mm thick nickel based superalloy as illustrated in Fig.3. Hence, in high density materials, for thick plate welding in single pass by keyhole plasma arc welding, the starting current is to be maintained at values more than 60% of the welding current with the ramping time in the range of 0.3 – 0.8 s.

[0024] The proposed invention gives rise to various advantageous features, which can be listed hereinunder.

Advantageous Features
- Applicable to thick plates/pipes including nickel based superalloys
- Defect free welds with good bead appearance
- Sound welds with good mechanical properties in high density alloys
- Sound welds with good mechanical properties in single pass autogenous plasma arc welding of thick plates.

[0025] It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particulars claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should typically be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogues to “at least one of A, B and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A or B” will be understood to include the possibilities of “A” or “B” or “A and B”.

[0026] It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims: -
,CLAIMS:WE CLAIM: -

1. Method for high speed keyhole plasma arc welding of thick plates/pipes comprising steps of: maintaining the plasma gas flow rates at > 2.5 lpm at low starting current, which is = 60% of the welding current with the ramping time of 0.3-0.8 seconds, which involves ramping up the current from the starting current to the welding current and ramping down to the end current for proper bead formation in the thick plates/pipes.

2. Method for high speed keyhole plasma arc welding of thick plates/pipes as claimed in claim 1, wherein the welding current is = 100 ampere and thickness of the plates > 5 mm.

3. Method for high speed keyhole plasma arc welding of thick plates/pipes as claimed in any of the preceding claims, wherein the arc welding of nickel based superalloys can be carried out by controlling the start current.

4. Method for high speed keyhole plasma arc welding of thick plates/pipes as claimed in any of the preceding claims is associated with the advantageous features such as herein described.