Creating an Affordable Option for Friction Stir Welding

Author(s):
David Johnson - Sigmapi Systems Ltd.

Industry:
Manufacturing, Construction

Products:
LabVIEW

The Challenge:
Creating an affordable option for friction stir welding in smaller enterprises.

The Solution:
Adapting milling machines with the LOWSTIR device to use a notebook PC running NI LabVIEW. With NI hardware, the LOWSTIR device can calibrate the machine, monitor the welding process and log welding parameters for analysis.

Friction-stir welding (FSW) is a welding process invented and experimentally proven by Wayne Thomas and a team of his colleagues at The Welding Institute UK in December 1991. The process involves a spinning tool in contact with material to be welded with sufficient down-force to create frictional heat in the material (about 80% of the material's melting point). This causes the material to become soft, allowing the spinning tool to create the weld.

FSW has significant advantages over other joining techniques including good mechanical properties, low distortion, and an ability to weld some materials that cannot be welded by other methods. Most current uses involve the joining of aluminium alloys, for applications including creating airframes, aircraft components, ship decking, structures, rail carriages, automotive components, bridge components, pressure vessels, and space launch systems. In addition to aluminium components, development of FSW has recently been reported for the joining of magnesium, copper, steels, and titanium alloys.

Industrial practicing of FSW has been limited to those industry sectors that have sufficient capital to invest in the high technology costs. The need for dedicated FSW machines can make it difficult for product manufacturers to justify implementation of the technology. An alternative approach is to adapt milling machines. Unlike other friction stir welders, the LOWSTIR friction stir welding system includes a unit that attaches to most standard milling machines, making it an affordable option for smaller enterprises. It includes software to calibrate the system, monitor the welding process and log welding parameters for analysis. The information gathered by the LOWSTIR device is displayed to the operator using a notebook PC running LabVIEW. The instrument panel displays real time numerical values of forces, torque, the temperature adjacent to the system electronics, and the tool temperature. The system also has the capability to add real-time event markers to allow correlation between process conditions/stages and the recorded data. The main display screen has buttons to start and stop data recording. Alternatively, an automatic trigger facility exists for initiating data recording.

LOWSTIR Project

Standard milling machines lack the monitoring capabilities required to ensure high-quality FSW joints. LOSTIR, the original project funded in part by the European Commission in 2005 developed a low-cost FSW monitoring system for retro-fitting to milling machines.

This process used a bespoke sensing head incorporating the tool holder, electronics, a ceramic heat shield, and a rotating antenna for transmission of the data to a stationery receiver mounted onto the frame of the machine. The receiver is connected via cables to a signal processing module, computer, and main power supply.

Whilst this system has been used successfully for a number of years, the use of telemetry for the signal transmission means an antenna/receiver arrangement is necessary, complete with cable connections. Additionally, the electronics mounted within the sensing head are subject to severe vibration and heat.

Bluetooth^® Data Transmission

To ensure the forces from the welding process do not affect the electronics and gain the benefit of a wireless system, the sensor head has been modified to use a battery power supply and a Bluetooth® connection to transmit machine data to a stationery receiver such as a notebook computer fitted with a standard Bluetooth® receiver. This modification means that the sensitive electronics and rotating antenna for the telemetry transmission are no longer required, nor are the stationery receiver or the signal processing unit, power supply or associated trailing cables.

The original design has been considerably revised to accommodate these changes, creating a smaller, lighter, and stiffer sensor head.

The LOWSTIR system now comprises of one hardware component and one software component and the FSW tool, with the wireless link designed to support two-way communications for calibration and data collection.

BAE’s Experience

Andy Wescott of the BAE Advanced Technology Centre says one of the biggest problems with welding aluminium is distortion. However, with the appropriate clamping in place and decreased heat generated through the process, this problem is largely overcome. A further benefit of using FSW is that high-strength aluminium alloys can be welded, a process not possible using traditional techniques. Also, recent trials have demonstrated that FSW can be used to repair localised damage to large panels. Rather than replacing a large area with a single sheet of material at the existing weld or joint, just the damaged area is repaired with a slug of new metal which is welded, leaving no step or change in the contour of the original material. This saves time and money compared to larger repairs requiring more specialised equipment or a return of equipment to workshops.

The ability to ‘go wireless’ is a huge leap over the previous technology. Not having to worry about trailing cables to the laptop reduces setup time and makes measurements during welding very straightforward. The Bluetooth® switch is a simple jack-plug arrangement, pull out the plug and the LOWSTIR device will ‘search’ for the laptop. Once finished, simply replace the plug. Because the power supply is now built-in using a rechargeable battery, the system is more versatile. What was once a highly specialised measurement using costly equipment, is now a relatively inexpensive process using equipment available in most engineering workshops, all driven by easy-to-use software.

Weld Monitoring System

A weld monitoring system has been developed with LabVIEW to accurately measure the vertical and horizontal forces and torque on the tool. The sensor is machined from one piece of high-grade stainless steel, heat-treated for maximum strength and stability. The sensor design allows for various taper sizes to be attached and accommodate the requirements of the user. The data gathered can be directly linked to the weld. In addition, the device has the capability to monitor two user-defined temperatures via NI thermocouple devices, one attached to the FSW tool, the second serving as a safety cut-out to protect the integral telemetry circuit, monitoring the temperature at the interface between the tool holder and the FSW monitoring system.

The display also shows the current captured data values for the weld in progress and indicates whether they are within the acceptable range for satisfactory welding. The display has a multi-graph facility where the user can select which sensor values are displayed.

Other advantages of the FSW techniqueare that there is no arc drawn, and there are no radiation, fume or spatter. Additionally it uses much less energy than large, traditional welding machines.

Author Information:
David Johnson
Sigmapi Systems Ltd.
Tel: +44 (0)1782 740134
dj@sigmapisystems.com

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