SE1551305A1

SE1551305A1 - Friction surface stir process

Info

Publication number: SE1551305A1
Application number: SE1551305A
Authority: SE
Inventors: Scott M Maurer; Michael R Eller; Zhixian Li
Original assignee: Lockheed Corp
Priority date: 2013-03-12
Filing date: 2014-03-07
Publication date: 2015-10-09
Also published as: DE112014001336T5; AU2014249475A1; US20140261900A1; PH12015501979A1; CN105209212A; JP2016516583A; WO2014164318A1

Abstract

A process is described that employs what can be termed a friction surface stirring (FSS) process on the surface of a metal object. The FSS process occurs on some or the entire surface of the metal object, at a location(s) separate from a friction stir welded joint. The FSS process on the surface produces a corrosion resistant mechanical conversion "coating" on the object. The "coating" is formed by the thickness of the material of the object that has been FSS processed. In one exemplary application, the process can be applied to a metal strip that is later formed into a tube whereby the "coated" surface resides on the inside of the tube making it highly resistant to corrosive flow such as seawater.

Description

HSML Ref No. 20057.l65WOUl FRICTION SURFACE STIR PROCESS FieldThis disclosure relates to corrosion resistant metal objects, and to the use of friction surface stirring (FSS) to enhance the corrosion resistance of metal objects.

Background Most metals, even marine-grade metals, show evidence of corrosion in waterenvironments, including salt, brackish, and fresh water environments. Corrosion isespecially pronounced in cold, deep salt water. Over time, the corrosion can bedetrimental to long-term operational sustainment of the metal object that is exposed to thewater environment.

The use of friction stir welding (FSW) to join two metallic objects at a weld jointis known. When those objects are exposed to a water environment, it has been observedthat at the location of the FSW joint, there is little or no corrosion that occurs, whilesignificant corrosion occurs on the metal objects at locations outside of the FSW joint in the base metal alloy.

Summary A process is described that employs what can be terrned a friction surface stirring(FSS) process on the surface of a metal object. The FSS occurs on some or the entiresurface of the metal object, at a location(s) separate from a FSW welded joint. The FSSprocess on the surface produces a corrosion resistant, mechanical conversion “coating”on the object. The mechanical conversion “coating” is formed by the thickness of thematerial of the object that has been FSS processed. The mechanical conversion “coating”can be a portion of the thickness of the metal object or the entire thickness of the object.

FSS is similar to FSW in that a rotating tool is used to soften or plasticize the metal material. However, FSS occurs over the surface of the metal object, instead of at a l joint between two objects. The FSS process can use a conventional FSW tool used toform a FSW Weld joint or a conventional FSW tool can be scaled-up in size for use Withthe larger surfaces subject to FSS.

The FSS tool can be used in a number of stir paths. For example, the FSS toolcan be traversed along linear paths on the metal object stirring in one direction or stirringin 2 directions (i.e. back and forth). In another embodiment, the FSS tool can start in thecenter and Work its Way out in a spiral pattern. In another embodiment, the FSS tool cantravel in a square or rectangular pattern and Work its Way out or in on the metal object.Other travel paths are possible.

The FSS can occur prior to or after machining operations on the metal object.The metal object can have any shape or size, and can be a plate, a bar, a rod, a tube, orother shapes. The F SS can occur on any shape of surface, for example planar or ﬂatsurfaces, curved surfaces, or combinations of curved and flat.

The object subject to FSS can be formed from metal alloys including, but notlimited to, aluminum alloys (2xxx, 3xxx, 5xxx, 6xxx, and 7xxx series of alloys)especially marine-grade aluminum alloys (5xxx and 6xxx series), titanium alloys, steelalloys such as stainless steel, and others.

The resulting FSS mechanical conversion “coating” is signiﬁcantly thicker thanconventional anti-corrosion conversion coatings, for example 5-10 times thicker.Although these FSS coatings are thicker than conventional chemical conversion coatings,they are integral to the parent metals surrounding and undemeath the FSS coating stirzone. The parent metal and the FSS coating have very similar, if not identical, therrnalproperties. Therefore, the FSS coatings possess an advantage over conventionalsuperﬁcial coatings, i.e. there is no de-bonding issue from Which the conventional coatingprocesses usually suffer and the thicker FSS coatings yield significantly longer lifetimesin marine and other corrosive environments. The FSS mechanical conversion “coating”is environmentally friendly since separate coating materials are not used. Because theFSS process has dissolved or minimized most of the precipitates, the FSS mechanical conversion “coating” contains fewer and smaller precipitates and cleaner grain boundaries, Without impacting the thermal performance or other material properties of themetal object.

In one exemplary application, the FSS process can be used on an object that isintended for use in Water, including salt Water, brackish Water, and fresh Water. Forexample, but not limited to, the metal object can be an object used in an ocean therrnalenergy conversion plant, a desalination plant, or a marine vessel. During its intended use,the object can be disposed underneath the Water, disposed on the Water, disposed abovethe Water but exposed to the water (i.e. splashes, salt fog, or other marine layerenvironments), or a combination thereof. The FSS process can be employed on a portionor the entire area of the metal object that in use is exposed to the water and/or marineenvironment.

The FSS process, together With FSW, can be used to produce an underwaterstructure that is formed from a single metal material. For example, in an ocean therrnalenergy conversion (OTEC) system, the heat exchanger, including the shell, plates andtubing, can be formed entirely from an aluminum alloy, thereby eliminating the use ofdissimilar metals or galvanic coupling.

In one embodiment, a friction surface stir process includes using a friction stirWelding tool to friction surface stir at least a portion of a non-jointed or FSW-joinedsurface of a metal object. This embodiment can be used in any combination With any ofthe dependent claims contained herein, and the dependent claims can be used in anycombination.

In another embodiment, a process includes friction surface stirring a non-jointedsurface of a metal object using a friction stir Welding tool. This embodiment can be usedin any combination With any of the dependent claims contained herein and the dependentclaims can be used in any combination.

In another embodiment, a method of increasing corrosion resistance of a metalobject includes friction surface stirring at least a portion of a non-jointed or FSW-joinedsurface of the metal object using a friction stir Welding tool to produce a mechanical conversion coating. This embodiment can be used in any combination With any of the dependent claims contained herein and the dependent claims can be used in any combination.

Drawings Figures lA-D illustrate a portion of an object With a surface thereof undergoingFSS.

Figure 2 illustrates a portion of an object With a surface thereof undergoing FSSseparate from a FSW joint on the object.

Figures 3A-C are side views illustrating another example of FSS on an objecttogether With machining after FSS.

Figure 4 is an end view of a tube that has been processed by FSS showing theFSS mechanical conversion “coating”.

Figures 5A-B illustrate an example of FSS of the entire thickness of an object.

Figures 6A-C illustrate a process of forrning FSS tubes.

Figures 7A-B illustrate an altemative process of forrning FSS tubes.

Figures SA-C illustrate examples of different FSS tube shapes and FSS tubesurfaces that can be formed.

Figures 9A-C illustrate examples of FSS objects provided With different surface finishes.

Detailed Description The following description describes a process that employs a FSS process on thesurface of a metal object. The FSS occurs on some or the entire surface of the metalobject, through some portion of or the entire thickness of the object. The metal objectcan have one or more FSW Welded joints, or have no FSW Welded joints. The FSSprocess on the surface produces a corrosion resistant mechanical conversion “coating” onthe object Which Will be referred to hereinafter as just a “coating”. The “coating” isformed by the thickness of the material of the object that has been FSS processed, Whichis determined by the penetration depth of the rotating tool used in the F SS process.

The FSS process is similar to FSW in that a rotating tool is used to soften orplasticize the metal material. However, FSS occurs over the surface of the metal objectinstead of at a joint between two objects as with FSW, and is not used to join two objectstogether.

With reference now to Figures 1A-D, a portion of a metal object 10 thatundergoes FSS is illustrated. The object 10 includes a surface 12 which can be planar orcurved. A FSS tool 14 is used to perforrn FSS on the surface 12. In this example, theFSS tool 14 can be identical in construction and operation to a conventional FSW toolused to form a FSW weld joint, or the tool 14 can be similar to a conventional FSW toolbut scaled-up in size for use with the larger surface 12 that is subject to FSS.

As would be understood by persons or ordinary skill in the art, the FSS tool 14rotates at high speeds while in contact with the obj ect”s surface. The tool 14 softens orplasticizes the metal material to a depth deterrnined by the penetration depth of the toolinto the object”s surface 12. Once the tool passes the metal, it stirs the metal behind thepin tool and consolidates it under the tool shoulder. The resultant surface “coating” willconsist of the metal with very fine equiaxed grains. This operation happens all in thesolid state, since there is no melting occurring during the FSS process.

In this example, the F SS tool 14 is moved in the direction of travel 15 shown bythe arrow in Figure 1B along the surface 12 to produce a FSS zone 16 (the FSS zone 16 isillustrated in F igures 1B and 1D in dashed lines). As shown in Figure lC, after each pathis completed, the tool 14 is shifted in the direction of the arrow (or the object is shiftedrelative to the tool) to complete a new FSS path. This process is repeated for the entiresurface area of the object 10 except for the borders as indicated in Figure 1D, or just aportion of the surface area.

The FSS begins by plunging the FSS tool into the object in Figure 1A andtranslating “north” along the long axis of the object and stopping before the tool reachesthe end of the object. The FSS tool can then translate back to the original startingposition and shift over a sufficient distance to ensure that sufficient overlap of the FSSzones will be achieved. The FSS tool is then again translated “nort ” along the object, and the shifting operation repeated until the entire object is overlapped with FSS zones.

Altematively, the FSS tool can stop at the end of each path and then shift while the tool isstill applying load and spinning. The tool can then begin translating “south” along theobject while overlapping the previous FSS zone. The tool can continue welding back-and-forth while shifting at the end of each pass until the entire sheet is FSS, except theborders. Other tool travel patterns are possible including, but not limited to, square,rectangular, or spiral patterns.

It is to be noted that the FSS process is employed on the surface 12 at locationsseparate from any FSW joints. In the example illustrated in Figures 1A-D, the object 10does not include any FSW joints.

Figure 2 illustrates an embodiment where the object 10” is formed by two initiallyseparate portions 18a, 18b that have been joined together along a FSW weld zone or joint20 by a conventional FSW process. In this embodiment, the tool 14 is traversed acrossareas of the surface 12” to create the FSS zone(s) 16 at locations separate from the FSWzone 20.

Figures 3A-C show cross-sectional views of an object 30 that has been processedby FSS, with Figure 3A showing one FSS pass and Figure 3B showing multiple passes.The penetration depth of the FSS tool 14 deterrnines the resulting depth of the “coating”.With reference to Figure 3B, it can be seen that multiple passes of the FSS tool 14 havesufficient overlap that the resulting stir zones (or friction stir processed (FSP) zones) havea consistent depth “D” across the entire object 30 to form the resulting FSS “coating” 32.The FSS “coating” 32 provides a corrosion resistant barrier that is signiﬁcantly thickerthan conventional anti-corrosion conversion coatings, for example 5-10 times thicker.

After performing the FSS, the surfaces of the object can be machined, ﬂy-cut,sanded, ground and/or polished, if desired, for example to smooth the surface. In oneembodiment, Figure 3C illustrates that the top surface of the overlapped stir zones can bemachined, for example machining away a portion of the thickness using a suitable cuttingdevice such as a mill bit, ﬂy-cutter, router, etc. If crevice corrosion is not a concern, thenthe machining step can be skipped.

The FSS process can be performed on objects having any shape, and on object surfaces of any shape. Figure 4 illustrates a hollow, cylindrical object or a tube 40 with a 6 hollow interior space 42 and a Wall thickness T that extends from an interior surface 44 toan exterior surface 46. FSS is perforrned on the exterior surface 46 to a depth D to forrnthe FSS “coating” 48. FSS can also be performed on the interior surface 44 as well.

The FSS “coating” 32 can have generally a constant depth on the object or thedepth of the coating can vary. For example, with reference to Figures 5A and 5B, a sideview of an object 50 is illustrated, where the object 50 has been processed by FSSthrough the entire thickness or depth D of the object 50 which may be beneficial is someapplications. In one embodiment, a conventional FSW tool with the pin lengthcomparable to the object”s thickness can be used to achieve ﬁdll thickness FSS. Inanother embodiment illustrated in Figure 5B, the FSS tool 52 is a self-reacting FSS toolwith an upper shoulder 54, a lower shoulder 56, and an independent pin 58 extendingbetween the shoulders 54, 56. The pin 58 is exposed between the shoulders 54, 56 whichare spaced apart a distance approximately equal to the thickness of the object 50 to obtainfull thickness FSS processing.

Figures 6A-C illustrate a tube forrning process that employs F SS. Starting withFigure 6A, a plate 60, for example of an aluminum alloy, is fully FSS processed for theentire depth of the plate and if desired machined as discussed above. As shown in Figure6B, the plate 60 is then cut into strips 62a, 62b,. . .62n to remove the non-FSS processedborders 64. With reference to Figure 6C, each strip is then rolled into a tube 65 and theedges joined along the seam 64”.

The edges can be joined using any suitable joining process. In one embodiment,the edges can be j oined using a high-frequency resistance welding process known in theart. The result is a tube 65 that is FSS processed on both the intemal and extemalsurfaces. Alternatively, as shown in Figures 7A-B, the edges can be joined using aconventional FSW process with a FSW tool 66 to create a fully FSS and FSW tube 68that minimizes or eliminates corrosion on both intemal and extemal surfaces.Alternatively, the edges can be joined using a first type of process, for example a weldingprocess such as electro-resistance or laser welding, and then the joined edges can be FSWdown the seam to create a fully FSS and FSW tube that minimizes or eliminates corrosion on both internal and external surfaces. 7 Figures 8A-C illustrate examples of FSS tube shapes and FSS tube surfaces thatcan be formed using the processes and techniques described above. These examplesillustrate that the process described in Figures 6A-C and 7A-B can be used to forrn tubeshaving many different shapes and surface enhancements. The surface enhancements canbe added prior to or after cutting into strips. In addition, the surface enhancements canoccur on some or the entire exterior surface or on some or the entire interior surface ofthe resulting tube. However, the surface enhancements are not limited to use on tubesand can be provided on any metal object that is subject to FSS processing describedherein.

The surface enhancements can be intended to increase the thermal performance,such as the heat transfer, of the tubes or metal object, or enhance any other property. Thesurface enhancements can be formed in any manner including, but not limited to,machining, stamping, chemical etching, and the like.

Figure 8A shows a cylindrical tube 80. The exterior surface of the tube 80 is alsoprovided with grooves or corrugations 82 that have been machined into the metal afterthe metal is FSS processed.

Figure 8B shows a trapezoidal shaped tube 84, where some or the entire exteriorsurface is machined with grooves 86. In this embodiment, some or the entire interiorsurface is also machined with grooves 88.

Figure 8C shows a rectangular shaped tube 90 where some or the entire exteriorsurface is machined with grooves 92. In this embodiment, some or the entire interiorsurface is also machined with grooves 94.

Figures 9A-C illustrate examples of different surface ﬁnishes that can be providedon the surfaces (interior and/or exterior) of the tubes or other metal objects that have beenFSS processed. Figures 9A-C illustrate various embossed surface finishes that can beformed in any manner including, but not limited to, machining, stamping, chemicaletching, and the like.

The FSS process is particularly useful on objects that are used in marineapplications and in applications that encounter water, especially salt water. Exemplary applications include, but are not limited to, heat exchangers used in desalination plants or 8 OTEC plants, condensers in power plant systems, and other cooling and liquid-liquid orliquid-air therrnal duty exchange applications. The FSS process can also be beneficial forcomponents used on naval or other maritime vessels or aircraft, surface, air or undersea,for example hulls, decks, rotor components, etc.

The examples disclosed in this application are to be considered in all respects asillustrative and not limitative. The scope of the invention is indicated by the appendedclaims rather than by the foregoing description; and all changes Which come Within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims

1. A method of increasing corrosion resistance of a surface of a metal object,comprising:friction surface stirring at least a portion of a non-jointed surface of the metal object using a friction stir Welding tool to produce a mechanical conversion coating.

2. The method of claim 1, comprising surface stirring the entire surface area of themetal object that in its intended use is exposed to Water, a marine environment or corrosive environment.

3. The method of claim 1, further comprising after surface stirring, machining, ﬂy- cutting, sanding, grinding or polishing the surface stirred portion of the surface.

4. The method of claim 1, Wherein the metal object is formed from a single metalmaterial.

5. The method of claim 1, Wherein the non-jointed surface of the metal object is a substantially flat and planar surface.

6. The method of claim 1, Wherein the non-jointed surface of the metal object is a curved or non-flat surface.

7. The method of claim 1, Wherein the metal object has both curved and ﬂatsurfaces.

8. The method of claim 1, Wherein the metal object is a plate, bar, rod or tube.

9. The method of claim 1, Wherein the metal object is completely friction surface stirred and the friction surface stirring penetrates the entire thickness of the metal object.

10. The method of claim 4, Wherein the single metal material comprises an aluminum alloy, a titanium alloy, or Stainless steel.

11. The method of claim 10, Wherein the aluminum alloy comprises a marine-grade aluminum alloy,

12. The method of claim 2, Wherein the metal object is an object used in an oceantherrnal energy conversion plant, a desalination plant, or a component of a maritime vessel or aircraft.

13. The method of claim 2, Wherein the metal object is a heat exchanger used in an ocean thermal energy conversion plant.

14. The method of claim 9, Wherein the metal object is machined, stamped or processed to create surface enhancements on at least one surface thereof.

15. The method of claim 9 or 14, Wherein the metal object is cut into strips.

16. The method of claim 15, Wherein after cutting, each of the strips is formed into atubular object.

17. The method of claim 16, Wherein adjoining edges of the tubular object formed bythe strip are friction stir Welded down the seam to produce a completely friction surface stirred and friction stir Welded tube.

18. The method of claim 16, Wherein adjoining edges of the tubular object formed by the strip are joined along a seam. 11

19. The method of clairn 18, Wherein the joined edges are friction stir Welded alongthe seam to produce a completely friction surface stirred and friction stir Welded tubular object. 12