Origins of Microtexture Development in Friction Stir Welds of Aluminum

Transcription

1 Origins of Microtexture Development in Friction Stir Welds of Aluminum David P. Field, TexSEM Laboratories Tracey W. Nelson, Brigham Young University Kumar V. Jata, WPAFB Acknowledgments: Kevin Colligan, Boeing Burke Hunsaker, BYU Matt Nowell, Ron Witt, TSL Aeromat 2000, Bellevue, WA, June26-29, 2000

2 Outline of Presentation Introduction Description of the experiments and analyses Discussion of texture in Al deformed by shear deformation Results of experiments Discussion and conclusions

3 Introduction Motivation for the study Determine the extent of texture gradients in friction stir welds in Al and investigate their effect on corrosion, yield, fatigue, fracture, and general GB integrity. Investigate how material flows during stir welding and the effect of tooling and weld parameters on structure evolution (will not be addressed in this discussion.

4 Experimental Details Three alloys analyzed, 1100, 6061, and C458 (Al-1.8Li-2.69Cu- 0.3Mg-0.3Mn-0.08Zr [weight percent]). All plates were approximately 6 mm thick. First set of analyses involved post-mortem interrogation of weld microstructure of 1100 and C458 alloys. Welds performed using tool with 19 mm shoulder and 6.35 mm pin at 700 rpm and 18 cm/min (hot weld conditions). Second experiment was an interrupted weld pass through 6061 (specimen supplied by K. Colligan). Microstructure interrogation primarily by orientation imaging.

5 Sections for OIM Analysis 1. Cross-section of weld 2. Plan view section near surface (directly under the shoulder). 3. Plan view section near the through-thickness mid-plane of the weld. 4. Plan view section near the bottom surface (between the end of the pin and the metal surface).

6 Optical micrographs of a cross section through a weld in 1100 Al. Showing the appearance of the weld nugget and deformation of the TMAZ. A closeup view of the grain structure within the weld is also shown.

7 Base Metal Orientation Color Key Weld Nugget Transition Region Cross-Section of Weld

8 Idealized Shear Texture Components in Aluminum SPN SD Partial A Fiber Partial B Fiber C Texture

9 Cross section of weld through 1100Al ND TD Orientation Color Key

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11 Tool Bottom Tool Edge The textures of the tool bottom and the edge of the tool are shown above. The tool edge shows a classical shear texture with the shear plane normal lying mutually orthogonal to the tool axis and feed direction. (The right edge has the shear plane normal oriented perpendicular to that on the left edge!) The texture near the bottom of the tool shows a (111) fiber indicative of shearing with the shear plane normal aligned with the axis of the tool and the shear direction being random.

12 Thetexturenearthetopoftheweld(directly underneath the tool shoulder) is represented by the set of pole figures at the right. They show a combination of a (111) shearing texture and a (200) compression texture. There also exists a strong texture gradient across the width of the weld as indicated on the following page. The pole figures shown on the following page represent textures in the image above taken in 10 equal increments across the width of the image from left to right.

13 Texture gradient just under the tool shoulder from the trailing to leading side of the weld as determined from the throughthickness cross-section. A more accurate determination can be made from plan view sections.

14 1100 Al - Directly under the tool shoulder ND TD

15 1100 Al - Directly under the tool shoulder

16 Taylor Factor The Taylor factor, M, is the ratio of accumulated slip to macroscopic deformation. It is given by: γ M = i i ε where γ is the amount of slip on slip system i, and ε is the macroscopic strain imposed on the material.

17 Taylor Factor Map of Region Under Tool Shoulder σ σ Gray-scale shading shows the susceptibility of the material to plastic flow from soft material (dark) to hard material (light) for an assumed tensile deformation as indicated.

18 1100 Al mid-thickness ND TD

19 1100 Al mid-thickness

20 Taylor Factor Map of Mid-Thickness Region σ σ Gray-scale shading shows the susceptibility of the material to plastic flow from soft material (dark) to hard material (light) for an assumed tensile deformation as indicated.

21 1100 Al under pin ND TD

22 1100 Al under pin

23 Taylor Factor Map of Region Under Pin σ σ Gray-scale shading shows the susceptibility of the material to plastic flow from soft material (dark) to hard material (light) for an assumed tensile deformation as indicated.

24 C458 Transition Region

25 Taylor Factor Map of TMAZ-Stir Zone Transition Region σ σ Gray-scale shading shows the susceptibility of the material to plastic flow from soft material (dark) to hard material (light) for an assumed tensile deformation as indicated.

26 Alloy C458 TMAZ-weld transition region. Orientations along two distorted grains are shown. Depending upon crystallite lattice orientation, the lattice will rotate as a rigid body with the grain, or will accommodate deformation through local compatibility strains, resulting in a spreading of the texture, but maintaining the nominal component.

27 Schematic of Interrupted FSW Experiment Weld tool retracted at a speed so as not to disturb thread tracks in the Al plates being welded. Direction of tool travel Welded Region Analysis Plane

28 Orientation Color Key Collage of orientation images showing orientations and grain morphology in front of, and behind, the weld tool. Colors indicate crystallite lattice planes oriented in the plane of the image. (6061 Al)

29 Structure after weld Images obtained several mm in front of, and behind, the weld tool. Structure before weld

30 Leading Edge Teeth Green Region C Texture SPN Blue Region SD Base metal texture consists of the typical cube and goss recrystallization components. The bottom PF shows the transition near the top of the teeth (as they just hit the base metal. As the metal channels deep between the teeth, two C components are formed, the stronger of which aligns with the upper tooth surface, and the weaker with the lower tooth surface.

31 Under the Pin ND Orientation Color Key SPN The deformed region under the pin shows a well-defined spatial gradient in crystallographic texture, and overall, a reasonably well developed partial B fiber. The shear plane normal is in the direction indicated this corresponds to the banding seen in the structure. Partial B Fiber SD SPN

32 Trailing Edge Teeth SPN SD Shear bands emanating from directly behind the pin. The strong band shows a typical shear texture after the orientations were rotated 90 degrees about the axis along which the tool travels. The direction of shear is normal to the image.

33 Deformation bands are formed during the welding process. Analysis of the welded aluminum reveals bands of small and large grain sizes. These are likely the result of recrystallization in the deformation bands and surrounding material.

34 Discussion and Conclusions Strong texture gradients exist in stir welds in Al, not only from the transition region to the weld nugget, but within the weld region itself. The primary identifiable texture components through the weld region are associated with shear deformations. The plane and direction of shear can be identified assuming that the deformation texture components survive the dynamic recrystallization process. When used in conjunction with polycrystalline plasticity models, this will enable determination of the flow field suffered by the metal during the welding process. Regions where local deformation may occur exist in bands along the weld direction and can be identified using Taylor factor analysis. Heterogeneity in corrosion, fracture and fatigue properties due to differences in crystallographic texture and GB structure from point to point within the weld will exist. Identifying these differences is a subject of continuing research.

35 Schematic of 111 plane etracees Weld Direction a AS Weld Direction ST View Weld Direction Top Surface LT View