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Research Papers

Toward Automation of Friction Stir Welding Through Temperature Measurement and Closed-Loop Control

[+] Author and Article Information
Axel Fehrenbacher, Neil A. Duffie, Nicola J. Ferrier, Frank E. Pfefferkorn

Department of Mechanical Engineering,  University of Wisconsin—Madison, 1513 University Avenue, Madison, WI 53706

Michael R. Zinn1

Department of Mechanical Engineering,  University of Wisconsin—Madison, 1513 University Avenue, Madison, WI 53706mzinn@wisc.edu

1

Corresponding author.

J. Manuf. Sci. Eng 133(5), 051008 (Oct 12, 2011) (12 pages) doi:10.1115/1.4005034 History: Received July 05, 2010; Revised September 02, 2011; Published October 12, 2011; Online October 12, 2011

The objectives of this work are to determine an accurate temperature feedback strategy and to develop a closed-loop feedback control system for temperature in friction stir welding (FSW). FSW is a novel joining technology enabling welds with excellent metallurgical and mechanical properties, as well as significant energy consumption and cost savings. However, numerous parameter and condition variations are present in the FSW production environment that can adversely affect weld quality, which has made extensive automation of this process impossible to date. To enable large scale automation while maintaining weld quality, techniques to control the FSW process in the presence of unknown disturbances must be developed. One process variable that must be controlled to maintain uniform weld quality under the inherent workpiece variability (thermal constraints, material properties, geometry, etc.) is the weld zone temperature. Our hypothesis is that the weld zone temperature can be controlled, which can help in controlling the weld quality. A wireless data acquisition system was built to measure temperatures at the tool-workpiece interface. A thermocouple was placed in a through hole right at the interface of tool and workpiece so that the tip is in contact with the workpiece material. This measurement strategy reveals temperature variations within a single rotation of the tool in real time. In order to automate the system, a first order process model with transport delay was experimentally developed that captures the physics between spindle speed and measured interface temperature. The model has a time constant of 110 ms and a delay time of 85 ms. Using this temperature measurement technique, a closed-loop temperature control system with a bandwidth of 0.3 Hz was developed. Interface temperatures in the range from 555 °C to 575 °C were commanded to an integral controller, which regulated the spindle speed between 850 rpm and 1250 rpm to adjust the heat generation and achieve the desired interface temperatures in 6061-T6 aluminum. To simulate changes in thermal boundary conditions, backing plates of different thermal diffusivities were found to effectively alter the heat flow, hence, weld zone temperature. The integral controller that manipulates spindle speed is applied when welding during these intentionally introduced weld disturbances. The measured temperature stayed within ±5 °C after introducing the disturbance, compared to a 50 °C change in temperature when no control was applied.

Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of the FSW process

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Figure 2

(a) Changes in workpiece geometry and (b) changes in workpiece properties

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Figure 3

(a) Schematic of change in thermal resistance and capacitance and (b) schematic of welding around a workpiece corner

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Figure 4

Main components of wireless DAQ system

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Figure 5

(a) Schematic of thermocouple location on the FSW tool (not to scale, bottom view, the pin diameter tapers from 7 mm to 5.5 mm) and (b) schematic of through hole location for thermocouple on the FSW tool (not to scale, section view)

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Figure 6

Experimental setup on CNC mill

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Figure 7

(a) Photograph of instrumented tool holder with (b) close-up of FSW tool

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Figure 8

Schematic of different backing plates

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Figure 9

Generalized block diagram of entire control system

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Figure 10

FFT of interface temperature measurements during FSW traverse. The spindle speed was changed at a frequency of 1 Hz and amplitude of 50 rpm around a nominal speed of 1000 rpm. The travel speed was constant (200 mm/min), the workpiece was 6061-T6.

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Figure 11

Frequency response plot of physical process relating spindle speed and interface temperature. The first order model has a break point of ωB  = 1.45 Hz.

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Figure 12

Frequency response plots of open-loop (OL) physical process (uncompensated system) and of OL physical process with integral controller (compensated system). Gain and phase margins are GM = 19 dB and PM = 75 deg, respectively. The cross-over frequency of the OL compensated system is ωC  = 0.22 Hz.

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Figure 13

Simulated and experimental frequency response plot of closed-loop compensated system, showing bandwidth ωB  = 0.32 Hz

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Figure 14

Simulated step response of closed-loop compensated system, showing time constant τ = 0.75 s and 10–90% rise time Tr,1  = 1.1 s

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Figure 15

Measured interface temperature, axial welding force and spindle power vs. time for a weld under nominal conditions. (a) Pin first contacts, (b) Shoulder first contacts, (c) Plunge finished, 3 s dwell starts, (d) Dwell finished, traverse starts, and (e) Traverse finished, tool retracts. The average temperature Tavg , force Fz,avg , and power Qz,avg are shown for the middle 75% of the weld traverse.

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Figure 16

Measured interface temperature versus angular position of FSW tool during a weld under nominal conditions. The solidus temperature is 582 °C.

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Figure 17

Average interface temperature during weld traverse versus spindle speed. For all tests, the travel speed was 200 mm/min and the plunge depth was 3.2 mm.

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Figure 18

Step commands in desired interface temperature, measured interface temperature, and commanded spindle speed using integral compensation

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Figure 19

Sinusoidal commands in desired interface temperature, measured interface temperature, and commanded spindle speed using integral compensation

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Figure 20

Measured interface temperature, commanded spindle speed, measured spindle power, and measured axial force when welding across a titanium and copper backing plate

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Figure 21

Measured interface temperature, commanded spindle speed, measured spindle power, and measured axial force when welding across a copper and titanium backing plate

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