Peak Stir Zone Temperatures during Friction Stir Processing

Abstract

The stir zone (SZ) temperature cycle was measured during the friction stir processing (FSP) of NiAl bronze plates. The FSP was conducted using a tool design with a smooth concave shoulder and a 12.7-mm step-spiral pin. Temperature sensing was accomplished using sheathed thermocouples embedded in the tool path within the plates, while simultaneous optical pyrometry measurements of surface temperatures were also obtained. Peak SZ temperatures were 990°C to 1015°C (0.90 to 0.97 T Melt ) and were not affected by preheating to 400°C, although the dwell time above 900°C was increased by the preheating. Thermocouple data suggested little variation in peak temperature across the SZ, although thermocouples initially located on the advancing sides and at the centerlines of the tool traverses were displaced to the retreating sides, precluding direct assessment of the temperature variation across the SZ. Microstructure-based estimates of local peak SZ temperatures have been made on these and on other similarly processed materials. Altogether, the peak-temperature determinations from these different measurement techniques are in close agreement.

The Effect of Concurrent Straining on Phase Transformations in NiAl Bronze During the Friction Stir Processing Thermomechanical Cycle

“…Details of the thermocouple placement, time constant values, temperature measurement procedures, and measurement repeatability were also described in a previous publication. [5] Briefly, three sheathed and grounded type K thermocouples of 1.6 mm in diameter were placed so that the thermocouple tips were 6.35 mm (one-half of the pin length) beneath the plate surface at a location corresponding approximately to the center of the tool traverse. One thermocouple was placed along the centerline of the intended tool path, while the other two were placed in a staggered arrangement along the intended tool path and offset by 2.9 mm to either side of the centerline.…”

“…In an earlier work, [5,22] the relationship between the equilibrium volume fraction of the b phase as a function of temperature in the two-phase field in the same alloy was established by annealing experiments involving prolonged heating at temperatures varying from 1033 K to 1273 K (760°C to 1000°C) that was followed by water quenching, and the relationship between the temperature, T, and V b was found to be…”

“…[4] Direct measurement of the temperature cycle by thermocouples embedded along the tool path within the tool profile in thick plates of as-cast NiAl bronze showed that peak stir zone temperatures are 0.92 to 0.97 T melt during FSP of this material. [5] These direct measurements were conducted during single-pass FSP runs using a Densimet 176 tool with a shoulder diameter of 28.6 mm. The pin was in the shape of a truncated cone 12.7 mm in length, having a base diameter of 15 mm, a tip diameter of 6.3 mm, and a stepped-spiral feature on its surface.…”

“…Measured dwell times above T % 1173 K (900°C) (T % 0.9 T melt ) varied from 2.2 to 8.7 seconds for such processing conditions. [5] The as-cast NiAl bronze material of interest in these investigations exhibits a characteristic series of phase transformations [5,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] that commence as the temperature increases above T % 0.8 T melt (T % 1073 K (800°C)), so dwell times at such temperatures as well as the local peak temperature will be factors determining the extent to which such transformations attain equilibrium. A procedure involving quantitative analysis of stir zone microstructures resulting from FSP-induced transformations was used to assess local peak temperatures in this alloy.…”

“…The stir zone measurements were in good agreement with the thermocouple data despite the very brief dwells typical of the FSP runs, and it was suggested in an earlier investigation that the concurrent straining during FSP thermomechanical cycles serves to accelerate the phase transformations that occur upon heating of this alloy. [5,20,22] Indeed, concurrent straining during hot torsion of a pearlitic eutectoid steel was shown to accelerate the spheroidization rate by a factor of %10 4 . [23][24][25] Such a reaction is controlled by interstitial C diffusion, while all of the transformations in NiAl bronze involve diffusion of substitutional elements; thus, a less remarkable acceleration of the phase transformations might be expected.…”