Temperature measurement: an essential element for a quality friction stir welding (FSW) operation
Friction stir welding (FSW) is a solid state welding technique, which means that the melting point of the material is not reached. Therefore, the welding temperature is the most important parameter to control in order to achieve a good FSW weld.
Although there are currently tools and procedures for measuring welding temperature, they are not always reliable, which can have an impact on the quality of the part welded by FSW.
As a result of these observations and after many years of study and measurement, Stirweld has developed and commercialised a brand new welding temperature measurement tool, allowing 100% quality control on friction stir welding (FSW) applications.
Stirweld FSW smart tool holder
Defects encountered depending on the FSW parameters applied
In friction stir welding, there are different types of defects. The most common defects encountered in friction stir welding are related to the inadequate definition of welding parameters. Indeed, a good quality weld depends on :
The choice of the FSW tool according to the application.
The speed of rotation of the tool (in RPM).
The feed speed (in millimetres per minute).
The force (in kN).
If these 4 parameters are correctly set, FSW defects are avoided. Then comes the welding temperature. In friction stir welding, the weld temperature depends on two of these four parameters: the rotation speed and the feed rate. The faster the tool rotates and the slower the feed, the hotter the weld. Conversely, the slower the tool turns and the faster the feed rate, the cooler the weld.
In friction stir welding, there are different types of defects depending on the heat supplied during the welding operation (by the rotation speed and the feed speed):
- On a too hot weld: appearance of wormhole defects on the surface or on the weld. This defect is due to too much friction. The melting point of the material is exceeded because of the speed of rotation, which makes the material liquid, leaving wormholes (in the form of a groove).
- On a too cold weld: appearance of porosity contiguous to the root of the weld. This defect is due to a lack of mixing of the material, as a result of a too low rotation speed and a too fast feed speed.
To prevent these defects, the solution is to find the right ratio between rotation speed and feed speed to be in the right operating window for a quality welding job.
As the graph below shows, a good weld (which is a defect-free weld) is the result of a ratio of good parameters used. This is referred to as the parameters defined in a good operating window (rotation speed, welding speed, tool geometry and force control).
Temperature mapping (°C)
- Welding too hot (orange zone): corresponds to the operating window of a too high rotational speed to low welding speed ratio. This leads to a risk of porosity and/or wormholes.
- Welding too cold (blue zone): corresponds to the operating window of a ratio of too low rotation speed to too high welding speed. This can cause porosity at the root of the weld.
- Flawless weld (green zone): corresponds to the optimal operating window of the process
How to measure FSW temperature with existing tools?
There are currently processes and tools for measuring welding temperature. However, each of these tools and procedures is not very reliable. Indeed, we will see that they do not accurately measure the temperature of the heat generated by the friction between the tool shoulder and the heat generated on the welded part, whereas these two elements are crucial for the quality control of FSW welds.
FRICTION STIR WELDING TEMPERATURE MEASUREMENT BY THERMAL OR INFRARED CAMERAS.
Thermal cameras and infrared cameras are non-contact temperature measurement tools used in a variety of situations.
As explained above, the heat generated during friction stir welding is located under the tool shoulder, which is invisible in the field of view of a thermal camera. The measuring field is therefore placed behind the friction zone, a few millimetres from the melting point.
As aluminium is a very good conductor of heat, it diffuses the heat caused by the friction to the rest of the welded part almost instantaneously. A few millimetres are enough to lose a lot of heat (in the range of 150 – 200 °C).
In addition, this measuring principle consists of measuring the amount of infrared emitted by the object according to its reflectivity. Aluminium is also a highly reflective material. The radiation caused by heat is perfectly reflected and makes the measurement completely inaccurate.
FSW TEMPERATURE MEASUREMENT BY THERMOCOUPLE.
The thermocouple is a thermal measuring instrument. One side of the thermocouple is welded with metal wires and brought into contact with the element whose temperature is to be measured. The other side is held at a reference temperature (20°C).
To measure the temperature of a weld, the thermocouple is then placed inside the workpiece – this requires the workpiece to be drilled and the thermocouple inserted. It should be noted that not all applications can be drilled. Furthermore, the drilling operation adds cost to the manufacturing operation.
Secondly, as with thermal camera or infrared temperature measurement, the thermocouple is placed a few centimetres away from the weld, which makes the measurement imperfect. It should also be noted that the heat released is not spread evenly depending on the type of aluminium used and its diameter.
Stirweld’s smart tool holder: the new temperature measurement tool for FSW equipment
The very first temperature measurement tool on FSW equipment marketed by Stirweld.
For several years, Stirweld has been investigating the possibilities of new tools to directly measure the temperature of friction stir welding.
Based on thousands of measurements, Stirweld has developed an algorithm, integrated in a tool holder, to measure the temperature of the welded part via the FSW tool. These measurements are based on thousands of calculations made on different grades of aluminium or different FSW tool sizes.
As the calculation of the workpiece temperature is done directly on the friction point, it allows to measure at the right place, without reflectivity problems. Also, this measurement offers excellent repeatability as the tool measures the temperature with an accuracy of +/-2.7°C. In today’s FSW market, this corresponds to an unprecedented measurement accuracy and therefore provides unparalleled reliability in welding temperature measurement.
Stirweld FSW smart tool holder for temperature measurement
Applications of the temperature measurement tool on FSW equipment.
100% QUALITY CONTROL
The smart tool holder provides instant knowledge of the welding temperatures in order to be in the right operating window and to obtain a quality FSW weld.
Also, now we know the ideal temperature for each application (and therefore the correct rotation and feed speeds to be applied) in order to be as efficient as possible and to have 100% control over the quality of the live welds.
On the graph below, we can see that the optimum FSW welding temperature for 6060 T6 aluminium is 450°C for a welding speed of 3m/min. These parameters should therefore be adopted to eliminate any risk of defects for this type of aluminium.
Optimization of welding speed according to optimum process temperature
OPTIMISATION OF WELDING PARAMETERS
The other advantage of knowing the right operating window in real time is to be able to determine the best ratio between feed rate and rotation speed.
In the industrial sector, the objective is to have the best possible productivity while having the least loss in terms of cost, time and parts. The tool holder with temperature measurement makes it possible to quickly find the right application window to weld as quickly as possible, without defects.
If the melting temperature is exceeded during the welding operation due to high rotation speed and low feed rate then we will end up with porosities at the top of the weld. On the other hand, if the temperature decreases too much due to a low rotation speed and a high feed rate, we will end up with porosities at the bottom of our weld.
The intelligent tool holder we have developed at Stirweld has been designed by friction stir welding specialists. The tool is designed for demanding FSW components such as space, aerospace and automotive applications.
It is possible to add this temperature measurement option to all our hybrid FSW heads and spindles, designed to be mounted on machining centres or robots.