WO1997019781A1

WO1997019781A1 - Gmaw welding of titanium and titanium alloys

Info

Publication number: WO1997019781A1
Application number: PCT/US1996/018994
Authority: WO
Inventors: Thomas David Erichsen; Thomas James Dorsch
Original assignee: FMC Corp
Current assignee: FMC Corp
Priority date: 1995-12-01
Filing date: 1996-11-26
Publication date: 1997-06-05
Anticipated expiration: 1998-06-01
Also published as: AU1084797A

Abstract

Method of welding titanium using gas metal arc welding procedure where the shielding for the weld included torch shielding gas only and no trailing or backup shielding. The technique used here is a departure from known titanium welding specifications wherein extensive shielding is required.

Description

GMAW Welding of Titanium and Titanium Alloys

BACKGROUND OF THE INVENTION

This invention has to do with a heretofore unpracticed method of welding titanium and titanium alloys. All welding of titanium or titanium alloys, wherein significant heat is generated, requires that secondary shielding of the weld and the heat affected zone be maintained until temperatures in the zone drop below a critical level. No secondary or back up shielding is needed to practice the method disclosed herein.

DESCRIPTION OF THE PRIOR ART

In prior art titanium and titanium alloy (used interchangeably in this disclosure) welding situations it has been commonly thought that special steps and precautions are necessary to make strong welds in titanium alloys. For instance, current industry practice when using an arc welding process, specifically gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW), with titanium is to weld using three inert gas shielding systems. A primary gas shield is incoφorated within the welding torch and is used to prevent contamination by the atmosphere of the weld metal. A secondary shield, often a trailing gas shield, a cumbersome enclosure of significant length containing an inert gas, is used to shield the weld and heat affected zone metal as it cools. This secondary shield will trail the torch. A third shield, a back up shield, will shield the root side of the weld. This shield, most often a backing bar with a channel under the weld root to contain the inert gas, is also cumbersome to manipulate as it has to be long enough to allow weld cooling while the weld is still shielded by inert gas; however, in some cases where open root welding is performed, a back up shield may be used to obtain a desired weld profile.

Nontechnical publications of welding processes that just generally survey the various arc welding processes may not be specific in reciting that secondary shielding is necessary. They do however point out that welding titanium may be difficult due to the fact that titanium has a strong affinity for hydrogen, oxygen, and nitrogen gases. Exposure to these gases when the titanium is at temperatures of above 540° C. (1000° F.) will tend to embrittle the material. For this reason it is common knowledge in the industry, and well documented, that to avoid such detrimental embrittlement, extensive shielding using an inert gas such as helium or argon contained proximate the heat affected zone and the weld itself is necessary. Although not directly pertaining to welding it is instructive to appreciate that casting of titanium must be done in a vacuum furnace because of the highly reactive nature of titanium in the presence of oxygen. This seems to reinforce the conventional wisdom and all the welding specifications that require extensive shielding of titanium welds as they are being made.

It is known that titanium alloy can be welded using fluxes rather than shielding gases. This technique has been observed by one of the inventors hereof, Thomas James Dorsch, in a trip in 1995 to the Ukraine. No performance data on this weld shielding technique, or the resulting welds, was made available to Mr. Dorsch. In any event, this flux shielded method of welding titanium alloys is not at all similar to the gas shielded metal arc welding technique disclosed herein.

It is known that in one manufacturing plant visited by Mr. Erichsen, one of the inventors herein, baskets constructed of thin sections of titanium material are welded together using a GMAW technique without a secondary shielding gas. In this situation the welds are very small and for the particular application, mechanical properties are not of primary consideration, and any embrittlement of the weld would be acceptable. It should also be noted that at the same manufacturing plant, welds were being made in more critical titanium components using the fully shielded technique with primary, secondary, and back up shielding devices. Another example of titanium welding using only the primary shielding was observed by one of the inventors wherein it was used to join sections of titanium scrap to be vacuum arc remelted. In this situation, embrittled welds with impaired mechanical properties are acceptable since the primary concern is to provide a path for electrical conductivity. It has been industry belief that extensive shielding of titanium welding is necessary. For instance in a publication copyrighted in 1982, Welding Handbook, Seventh Edition, Volume 4, Metals and Their Weldability, there is a statement that:

"Because of the sensitivity of titanium to embrittlement by oxygen, nitrogen, and hydrogen, the entire component or that portion to be heated above about 500° F. must be protected from atmospheric contamination. Protection or shielding is commonly provided by high-priority inert gas cover in the open..."

It is not known if earlier versions of this publication had the same warning years before the 1982 version.

Bringing this same admonition up-to-date, a current publication, the Materials Properties Handbook: Titanium Alloys published by ASM International/The Materials Information Society states in its "Technical Note 10: Welding and Brazing" that: "...because titanium and titanium alloys are extremely reactive above 540° C. (1000° F.), precautions must be taken to shield the joint from air." They further state that, "For successful arc welding of titanium and titanium alloys, complete shielding of the weld is necessary because of the high reactivity of titanium to oxygen and nitrogen at welding temperatures. Shielding is required for weldment areas that exceed about 540° C. (1000° F.) in air."

One theory or justification for full secondary shielding of titanium welds is that when gas metal arc welding of titanium is being performed the droplets of filler material being transferred across the arc are exposed to a high temperature. This high temperature and fine particle size of the droplets of filler makes the filler metal highly susceptible to contamination by impurities in the arc atmosphere. Furthermore, the deposited metal reacts with oxygen and nitrogen in the atmosphere during cooling much more readily than steel or aluminum.

The appearance of a titanium weld has been one criteria for acceptance. A weld was required to have a clean surface, free of deposits and discoloration. It would have a light golden sheen just a shade or so more yellow or gold than the base material. Military Standard 2219 at 5.4.4.3 addressing "Discoloration of Titanium" states that, "Contamination resulting in discoloration of weld bead and the adjacent area is not acceptable except as follows: [goes on to specify when colors from bright silver to blue and gray will be acceptable or cause for rejection.] In the discovery presented here, however, the appearance of the weld is not within the normal acceptance range. Welds made without the industry standard shielding are much darker, indicating a certain degree of oxidation on the surface of the weld.

SUMMARY OF THE INVENTION This invention is the discovery that perfectly good welds, in situations where significant weld heat and a significant heat affected zone are created, for instance multiple pass welds on structures thicker than .125 inches, can be made in/on/to titanium alloys using gas metal arc welding techniques without resorting to the cumbersome and expensive secondary shielding now practiced and thought to be required by the industry.

Such an acceptable weld is formed using either inert gas tungsten-arc (GTAW) welding techniques or, preferable to the inventors, inert gas metal arc (GMAW) welding techniques.

No special preparation of the titanium to be welded is needed although proper cleaning of the titanium work piece prior to welding is expected to improve the overall acceptability of a titanium weld. A conventional "MIG" (also known as GMAW) welder feeding a consumable electrode, for instance ER-Ti-5-1, is used with an inert shielding gas such as helium or argon. The only shielding provided by the gas is in the vicinity of the torch tip where a conical curtain of shielding gas is delivered to the weld zone proximate to the tip of the torch.

No further shielding is provided. No gas filled extended shield trails the torch. No back up shield on the back side of the weld is used.

The weld is made in the same way a weld is made when welding steel. An arc is struck and a weld pool is formed. The torch progresses along the weld line at the feed rate necessary to get good penetration and weld shape.

The object of this invention is to produce significantly large welds in/on/to titanium, that are acceptable welds in performance, without using secondary gas shielding after the weld has been made and while the weld bead and the heat affected zone proximate the weld is cooling.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be further understood by reference to the appended figures wherein:

Figure 1 is a representation of a prior art secondary welding shield for use in welding titanium;

Figure 2 is a photo of an etched sample of welded titanium using the technique disclosed herein;

DESCRIPTION OF THE PREFERRED EMBODIMENT

Looking at Figure 1 the embodiment of the prior art, in this case, a shielding gas distributor or container is shown. This trailing shield 10 includes an opening for receiving a conventional MIG welding torch. The secondary shield would be clamped by means of nozzle clamp 12 secured by fasteners 26 to the torch and travel with the torch when welding titanium. The primary shielding is provided by the inert gas delivery system of the MIG torch. The gas is delivered around the tip of the torch to surround the consumable electrode with a cone of inert gas. This style of torch is well known and commonly used in welding steel or aluminum. Unique in the welding of titanium is the requirement to shield the completed weld from oxygen, nitrogen, and hydrogen while the weld and the surrounding heat affected zone cools from its coalescence state to a solid state below 1000° F. This secondary shielding is done by flooding the zone of the weld with an inert gas such as helium or argon, or a combination of helium and argon. The secondary shield container 10 will contain the shielding gas supplied by an inert gas feeder line 24 in the heat affected zone. The trailing shield housing 14 may be filled with tightly packed stainless steel wool contained in the housing by a perforated plate 18 having inert gas delivery holes 20. The same shielding gas as is used in the primary shielding of the tip of the torch. The shielding enclosure 14 may be attached to the torch, with the torch exiting the lower end of the nozzle clamp 22, or may be independent of the torch. The critical thing is that the torch and the shielding enclosure are close together as the weld is being made such that no air can contact the weld as it is being made or for a period of more than a number of seconds after the welded titanium has coalesced.

Figure 2 is a photo of an etched sample showing the crystalline structure of a weld made in the improved method disclosed in this specification. A comparison of coupons with a coupon of a conventionally shielded titanium weldment shows no functional difference in the welds. The shielded weld, however, is much more expensive to make as the secondary shielding is cumbersome and a severe limitation on the flexibility of torch control which is available when the torch is not burdened with the secondary shield. Although not a significant cost, there is a savings in inert gas costs associated with this invention as only a small volume of inert gas is required to shield the welding tip. The need for flooding the secondary shields is not a part of the single shielded titanium weld.

It is seen here that the single shielded weld is at least as strong as the weld performed to Military Standard 2219. The inventors have found several instances wherein the titanium weld, using the single shield process described herein, is as good as and arguably superior to the welds made that meet the industry standard specifications and requirements.

The only observable difference between the conventional specification weld and the single shielded welding technique disclosed herein is that the new technique provides for welds that are discolored on the surface of the weld. The weld appears dark and burned. This may be from surface oxidation; however, this surface discoloration has not caused any detriment in weld quality.

In summary what has been provided and shown herein is a method of welding titanium alloys wherein a significant heat affected zone is generated by the welding process, such as in welding titanium plate or titanium work pieces above, but not limited to, .125 inches thick. This is done by feeding a consumable titanium alloy welding electrode through the tip of a welding torch, said welding torch including an inert gas delivery means for delivering an inert gas which forms a conical shield of inert gas directly proximate said consumable titanium alloy welding electrode. As is usual practice an arc is created between said consumable titanium alloy welding electrode and said titanium alloy to generate heat sufficient to produce coalescence of said titanium alloy directly proximate said inert gas conical shield. Finally the inert gas conical shield is removed from the area of titanium coalescence before said area of titanium coalescence and the resulting proximate heat affected zone has cooled sufficiently to preclude oxidation of titanium material in the area of coalescence. Although the invention herein appears simple, it represents a significant departure from the teachings in the art — requiring secondary shielding until the weld is cool — and would not be apparent from a study of the art. It represents a new technique of welding titanium that wasn't taught in the prior art. Thus, the following claims are meant to broadly claim the method of welding titanium using a technique heretofore unknown and taught away from by the welding industry.

Claims

WHAT IS CLAIMED IS:

1 . A method of welding titanium alloys wherein mechanical properties of the weld are a primary consideration and embrittlement in the welded zone is unacceptable and where a significant heat affected zone is generated by the welding process comprising: feeding a consumable titanium alloy welding electrode through the tip of a welding torch, said welding torch including an inert gas delivery means for delivering an inert gas which forms a conical shield of inert gas directly proximate said consumable titanium alloy welding electrode; creating an arc between said consumable titanium alloy welding electrode and said titanium alloy to generate heat sufficient to produce coalescence of said titanium alloy directly proximate said inert gas conical shield; removing said inert gas conical shield from said area of titanium coalescence before said area of titanium coalescence and the resulting proximate heat affected zone has cooled sufficiently to preclude oxidation of titanium material in the area of coalescence.

2. The method in accordance with Claim 1 wherein said oxidized weld surface has a hardness insignificantly greater than the weld surface of a titanium weld having an unoxidized weld surface.

3. An improved titanium weld having the property of an oxidized weld surface made by the process of: feeding a consumable titanium alloy welding electrode through the tip of a welding torch, said welding torch including an inert gas delivery means for delivering an inert gas as a shield directly proximate said consumable titanium alloy welding electrode; creating an arc between said consumable titanium alloy welding electrode and said titanium alloy to generate heat sufficient to produce coalescence of said titanium alloy directly proximate said inert gas conical shield; removing said inert gas shield from the area of titanium coalescence before said area of titanium coalescence and the resulting proximate heat affected zone has cooled sufficiently to preclude oxidation of titanium material in the area of coalescence.

4. The product in accordance with Claim 3 wherein said titanium weld has an oxidized surface layer causing moderate discoloration of said weld without significant slag buildup.

5. The product in accordance with Claim 4 wherein said level of oxidation is on the surface of said titanium weld.

6. A method of welding titanium with a metal arc gas welding torch comprising confining shielding gas of said welding torch to the weld area immediately adjacent the weld location.

7. The method in accordance with Claim 6 further comprising the step of removing any secondary gas shield containment device from said metal arc welding torch prior to welding said titanium.

8. The method in accordance with Claim 7 wherein production welds are made in titanium without the use of said secondary gas shield containment device.

9. A method of welding titanium alloys for the purpose of forming a permanent bond between two or more titanium alloy components which meets published welding specifications pertaining to weld strength and ductility for acceptable welds in titanium alloys comprising the steps of: feeding a consumable titanium alloy welding electrode through the tip of a welding torch, said welding torch including an inert gas delivery means for delivering an inert gas which forms a conical shield of inert gas directly proximate said consumable titanium alloy welding electrode; creating an arc between said consumable titanium alloy welding electrode and said titanium alloy to generate heat sufficient to produce coalescence of said titanium alloy directly proximate said inert gas conical shield; removing said inert gas conical shield from said area of titanium coalescence before said area of titanium coalescence and the resulting proximate heat affected zone has cooled sufficiently to preclude oxidation of titanium material in the area of coalescence. -9-

10. The method in accordance with Claim 9 wherein said published welding specification includes Military Standard 2219 with the exception of 5.4.4.3 pertaining to the color of the weld.

1 1. The method in accordance with Claim 1 wherein said weld formed by the method has a dark appearance.

12. The method in accordance with Claim 1 1 wherein said weld formed by the method has a burned appearance caused by surface oxidation.

13. A method of welding titanium structures having a thickness greater than .125 inches and meeting published welding specifications pertaining to weld strength and ductility for acceptable welds in titanium alloys comprising the steps of; feeding a consumable titanium alloy welding electrode through the tip of a welding torch, said welding torch including an inert gas delivery means for delivering an inert gas which forms a conical shield of inert gas directly proximate said consumable titanium alloy welding electrode; creating an arc between said consumable titanium alloy welding electrode and said titanium alloy to generate heat sufficient to produce coalescence of said titanium alloy directly proximate said inert gas conical shield; removing said inert gas conical shield from said area of titanium coalescence before said area of titanium coalescence and the resulting proximate heat affected zone has cooled sufficiently to preclude oxidation of titanium material in the area of coalescence.

14. The method as disclosed in Claim 13 wherein said weld meets or exceeds weld ductility standards for acceptable welds.

15. The method in accordance with Claim 14 wherein said weld would be rejectable under published welding standards for titanium welds based on the surface color of the weld.

16. The method of Claim 15 wherein said weld surface color is dark and said weld appears burned due to surface oxidation.