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Periodic Tendency

The following is a qualitative description of the periodic nature of friction stir technologies. The text is from a paper presented by Dr. Burford at the 9^th International Friction Stir Welding Symposium held at The Von Braun Center in Huntsville, AL from 15-17 May 2012. It is an expanded version of a portion of the discussion included in a paper given earlier at the 6th Annual Technical Review Meeting of the FAA Joint Advanced Materials & Structures (JAMS) Center of Excellence held during May 19–20, 2010.

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In FSW, the side of the weld tool probe is pressed against the workpiece in a manner similar to that of machining with the side of an end mill. However, unlike end mill machining, in FSW the tool design and process parameters are selected such that the displaced material is captured, reconstituted, and joined back to the original material – as opposed to removing it from the work zone in the form of “chips” as is done in machining. Consequently, there are both similarities as well as dramatic differences in the dynamic response of the respective tools used in end milling and FSW, respectively.

In machining, it is important to clear the cut metal (chips) from the tool at a sufficient rate to prevent clogging of tool features, namely the flutes, etc. In FSW the opposite is true. The features of a FSW tool, such as threads, grooves, etc., are expected to become impacted with metal and thereby maintain a full frontal engagement between the tool and the material of the workpiece. In machining, only the tool cutting edges are expected to be in contact with the material ahead of the cutting front. This full engagement between the FSW tool and workpiece leads to unique dynamic behavior not typically experienced in machining. Notwithstanding the differences in tool engagement with the workpiece, however, the two processes share similar behaviors because of the similarities in which the tools are dynamically loaded (i.e. by side loading caused by advancing a rotating tool through the workpiece).

Advanced control techniques in machining have been investigated for reducing chatter. For example, Zhang and Sims [6] assessed the ability of “piezoelectric active vibration damping” to arrest chaotic tool behavior. To reduce defect formation in FSW associated with chaotic tool motion, Arbegast, [7] Boldsaikhan et al., [8] and Jene et al. [9] have studied machine tool-workpiece interactions by monitoring force feedback signals. As these studies demonstrate, in both machining and FSW, process monitoring can serve as the basis for reducing chaotic tool behavior and, thereby, provides a means for improving part quality in FSW as it has in machining.

In FSW and under particular conditions, the tool tends to oscillate side-to-side (nominally transverse to applied loading vector) while under the local dynamic side loading conditions imposed on the tool at the tool-workpiece interface. [10] In machining, when the tool oscillates in a chaotic manner, a self-excited vibration phenomenon called “chatter” tends to form, leaving erratic markings on the newly cut surface. Similar chaotic oscillations in FSW tend to be associated with the formation of volumetric defects (voids) within the joint, resulting from the lack of consistency in the reconsolidation of material along the joint line. [11]

The advancing, rotating FSW tool presses against the material directly ahead of it, creating a shearing action that extends around the tool front. In a generalized manner, when the material directly in front of the tool is sufficiently heated under the pressure and shearing action imposed on it by the advancing, rotating FSW tool, thin layers of material are transported from the advancing side of the tool to the retreating side of the tool. This action is repeated as the material ahead of the tool is again heated and pressed against sufficiently to cause it to shear and be transported along the front of the advancing tool. Each time material is transported across the face of the tool, cooler material is again exposed to the leading face of the tool.

This sequence of events leads to a repeating process of heating and shearing followed by heating and shearing (heat – shear – heat – shear …). The new interface ahead of the tool is again pressed upon until it is sufficiently heated to move the next band of material along the tool front from the advancing side to the retreating side. This undulation in metal movement along the leading edge of the tool promotes an oscillatory or alternating pattern in both normal and shear forces acting on the tool surface, which in turn causes the tool to move in a periodic or oscillatory motion, nominally side-to-side, as the tool is advanced. This process is schematically depicted in Figure 1.

The schematic in Figure 1 depicts the idealized oscillation of a mid-plane cross-section of a weld tool as it advances. Tool rotation is counter-clockwise and the direction of travel is toward the top of the page. The reaction forces act on the tool in opposition to the tool motion. A periodic shearing and movement of metal along the leading edge of the tool – from the advancing side to the retreating side – results and the tool oscillates side-to-side (nominally) in response to the dynamic reaction forces acting on the leading edge of the FSW tool probe. Material flow and the associated resultant forces acting upon the tool are actually much more complex than idealized in the model shown in Figure 1. With the tool probe completely submerged in the work-piece, forces act on the probe from multiple directions in response to its dynamic loading environment, the resultant of which may be measured experimentally. [12] The full engagement of the rotating, advancing FSW tool further aggravates its tendency to oscillate in a chaotic manner. Adding to the complexity of FSW tool oscillatory motion is the spinning motion of the tool shoulder face on the surface of the work-piece. This tends to cause a wandering or walking motion of the end of the tool, which even further promotes chaotic tool behavior as the tool seeks (or seeks to establish) a center of rotation on the work-piece surface. Uniformity in FSW tool oscillations is dependent upon the periodicity (or lack thereof) in the material flow behavior around the tool front. It is anticipated that the lower the abruptness in the material heating and shearing cycle, the less likely the process will become chaotic in its behavior (action). Selection of tool features and process parameters are expected to contribute to the overall stability of the tool control process.

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Specifications

Aluminum Applications

• AWS D17.3: Specification for Friction Stir Welding of Aluminum Alloys for Aerospace Applications
  • 2016 – Rev 1
• ISO 25239 (2011): Friction stir welding — Aluminium
  • Part 1: Vocabulary
  • Part 2: Design of weld joints
  • Part 3: Qualification of welding operators
  • Part 4: Specification and qualification of welding procedures
  • Part 5: Quality and inspection requirements
• ISO 18785: Friction stir spot welding — Aluminium
  • Part 1: Vocabulary
  • Part 2: Design of weld joints
  • Part 3: Qualification of welding operators
  • Part 4: Specification and qualification of welding procedures
  • Part 5: Quality and inspection requirements

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Selected Bibliography