Friction stir welding for the masses

Invented by TWI (formerly known as the welding institute) in 1991, friction stir welding (FSW) is an attractive technique for a number of reasons: it requires no melting (the weld is made in solid state) and produces high-quality welds on a wide variety of metals from aluminium through to magnesium, copper, steels and titanium alloys

The process involves a spinning tool, in contact with the material to be welded, applied with sufficient force to create frictional heat in the material (about 80 per cent 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.

However, its industrial take up has been limited to those sectors with sufficient capital to invest in the high technology costs. The need for purpose-built ‘FSW’ machines can make it difficult for product manufacturers to justify implementation of the technology.

An alternative approach is to adapt milling machines, and this is exactly what Staffordshire-based control and monitoring specialist Sigmapi Systems has made possible. Working with TWI, Applied Measurements and Suffolk Precision, the company’s LOWSTIR system enables users to convert an existing milling machine into a friction stir welder and automatically monitors the process through an easy-to-use operator interface to provide feedback on the quality of the weld produced.

“The ability to go wireless with FSW has provided a huge advantage over the previous technology”

Unlike other friction stir welders, the LOWSTIR friction stir welding system includes a unit that attaches to most standard milling machines via an ISO taper, making it an affordable option for smaller enterprises. It is supplied with software to calibrate the system, monitor the welding process and log welding parameters for later analysis.

An earlier version of this system, developed through the EU LOSTIR programme, used a bespoke sensing head incorporating the tool holder, electronics, a ceramic heat shield and 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 mains power supply.
But whilst this system has been used successfully for a number of years, the use of telemetry for the signal transmission means an

ntenna/receiver arrangement is necessary complete with cable connections. Additionally, the electronics that are mounted within the sensing head are subject to severe vibration and heat.

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 now been modified to use a battery power supply and a Bluetooth connection to transmit machining data to a notebook computer. This means that the sensitive electronics and rotating antenna for the telemetry transmission is no longer required, nor are the stationary receiver or the signal processing unit, power supply or associated trailing cables.

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

What was once a highly specialised measurement costing thousands is now a relatively affordable process

The system is now being trialled at BAE Systems’ advanced technology centre, where, according to engineer Andy Wescott, the ability to ‘go wireless’ has provided a huge advantage 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. Also, as the power supply is now built-in using a rechargeable battery, this makes it a very versatile system. What was once a highly specialised measurement using equipment costing hundreds of thousands of pounds, is now a relatively inexpensive process using equipment available in most engineering workshops, all driven by easy-to-use software.

A weld-monitoring system has been developed 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 to accommodate the requirements of the user. The data gathered can be directly linked to the acceptance or otherwise of the weld. In addition, the device has the capability to monitor two user-defined temperatures via thermocouples - one to be attached to the FSW tool, the second will serve as a safety cut out to protect the integral telemetry circuit - monitoring the temperature at the interface between the tool holder and the weld-monitoring system.

The information gathered by the LOWSTIR device is displayed to the operator using a notebook PC running NI Labview. The instrument panel displays real-time numerical values of forces, torque, the temperature adjacent to the system electronics and, if desired, 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 recording of data. Alternatively an automatic trigger facility exists for initiating the recording of data. 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.

design essentials

The key facts to take away from this article

• The LOWSTIR system can convert milling machines into welders
• Wireless functions allow the system to be produced at lower cost
• A weld-monitoring system measures force and torque on the tool
• The LOWSTIR system can correlate between process stages and recorded data