A model of material flow during friction stir welding

doi:10.1016/j.matchar.2007.10.002

Materials Characterization

Volume 59, Issue 9, September 2008, Pages 1206-1214

https://doi.org/10.1016/j.matchar.2007.10.002 Get rights and content

Abstract

Tin plated 6061-T6 aluminum extrusions were friction stir welded in a 90° butt-weld configuration. A banded microstructure of interleaved layers of particle-rich and particle-poor material comprised the weld nugget. Scanning and transmission electron microscopy revealed the strong presence of tin within the particle-rich bands, but TEM foils taken from the TMAZ, HAZ and base material showed no indication of Sn-containing phases. Since tin is limited to the surface of the pre-weld extrusions, surface material flowed into the nugget region, forming the particle-rich bands. Similarly, the particle-poor bands with no tin originated from within the thickness of the extrusions. A model of material flow during friction stir welding is proposed for which the weld nugget forms as surface material extrudes from the retreating side into a plasticized zone surrounding the FSW pin. The extruded column buckles between the extrusion force driving the material into the zone and the drag force of the in-situ material resisting its entry. A banded microstructure of interleaved surface material and in-situ material, therefore, develops. The model successfully describes several of the experimentally observed weld characteristics, but the model is limited to specific conditions of material flow and assumptions regarding steady-state.

Introduction

Invented in 1991 by The Welding Institute, Friction Stir Welding (FSW) is a novel solid-state joining process that is gaining popularity in the manufacturing sector [1], [2]. FSW utilizes a rotating tool design to induce plastic flow in the base metals and essentially “stirs” the pieces together. During the welding process, a pin, attached to the primary tool, is inserted into the joint with the shoulder of the rotating tool abutting the base metals. As the tool traverses the joint, the rotation of the shoulder under the influence of an applied load heats the metal surrounding the joint and with the rotating action of the pin induces metal from each workpiece to flow and form the weld. The microstructure resulting from the influence of plastic deformation and elevated temperature is characterized by a central weld nugget surrounded by a thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ). The welded joint is fundamentally defect-free and displays excellent mechanical properties when compared to conventional fusion welds [3], [4], [5], [6]. Over the last fifteen years, numerous investigations have sought to characterize the principles of FSW and to model the microstructural evolution. The current status of FSW research has been well summarized by Mishra and Ma [7].

The flow of material during FSW is a complex process that is not fully understood despite numerous investigations and models. Several studies have compared material flow during FSW with wrought metal processes and have modeled weld nugget development as an extrusion process [7], [8]. In particular, Krishnan [9] and Sutton et al. [10] hypothesized that the nugget forms as a volume of material from the weld surface extrudes into the joint during each revolution of the tool. Seidel and Reynolds utilized fluid mechanics to create a two-dimensional model of FSW as a non-Newtonian fluid flowing around a rotating cylinder [11]. Though each of these models predict some characteristics of the weld nugget, there are limitations. For example, the models do not adequately describe the formation of the banded microstructure of particle-rich and particle-poor regions commonly observed during FSW [9], [12], [13], [14]. The following study investigates friction stir weld nugget development during the welding of 6061-T6 extrusions that were plated with tin to facilitate the tracking of material flow. A model is proposed in which surface material from the retreating side extrudes into the plasticized material zone within the weld thickness. As the extrusion enters the plasticized region, the column buckles under the drag force, resulting in an interleaved structure of surface material and in-situ material. The model further predicts that the extrusion column exhibits a higher particle density than the plasticized zone, thus leading to the non-uniform particle distribution in the weld nugget.

Section snippets

Experimental Procedure

Aluminum 6061-T6 extrusions produced in accordance with ASTM B 317 with a thickness of 6.35 mm and a width of 195.0 mm were obtained and welded in the configuration represented in Fig. 1. As shown in the diagram, the longitudinal grain directions of the individual extrusions are welded perpendicular to one another, such that FSW occurs along the L-direction of the advancing side and along the LT-direction of the retreating side. This configuration was selected in cooperation with the welder,

Results and Discussion

The investigated 6061 alloy belongs to the age-hardenable group of aluminum alloys. Its microstructure well away from the weld is composed of equiaxed grains (approximately 50 μm in diameter) with the dispersed strengthening phase, Mg₂Si, in the form of coherent rods, as shown in Fig. 2. Fig. 3 shows the microstructure of a typical FSW nugget produced during this study. The complex flow pattern of the FSW process is clearly evident, and the weld nugget is primarily characterized by the banded

Conditions for Extrusion

Consider the cross-sectional profile of the FSW process for a typical butt weld configuration. As the tool and pin rotate, the flow stress in a region surrounding the weld joint is exceeded, facilitating solid-state material flow. The shoulder of the rotating tool plasticizes a thin, cylindrical volume of material on the surface, and the pin plasticizes a volume of material within the joint thickness. The net effect is a flow-capable region with a cross-sectional shape resembling that of the

Conclusions

Tin plated 6061-T6 aluminum extrusions were friction stir welded in a 90° butt-weld configuration. The weld nugget exhibited a banded microstructure consisting of alternating layers of material “rich” in secondary phase particles and material “poor” in the particles. Transmission electron microscopy revealed that the particle-rich bands contained a strong presence of tin, while TEM foils taken from the TMAZ, HAZ and base material thickness did not indicate any Sn-containing phases. Since tin is

Acknowledgements

The authors would like to specifically thank the Philip and Elaina Hampton Fund for Faculty International Initiatives and the Polish Ministry of Science and Higher Education (Grant No. N 507 094 32/2648) for making this research collaboration a reality.

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Citation Excerpt :
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A model of material flow during friction stir welding

Abstract

Introduction

Section snippets

Experimental Procedure

Results and Discussion

Conditions for Extrusion

Conclusions

Acknowledgements

Mat Sci Eng A

Mater Sci Eng R Rep

Mat Sci Eng A

Mater Sci Eng A

Calphad

J Mater Process Technol

TWI Bulletin