Surface morphology is a subset of analytical imaging to describe the macroscopic and microscopic appearance of the top surface friction stir welds
In some cases, high spatial resolution imaging made with sophisticated microscopes is used to produce images of products, samples and objects that cannot be seen with the naked eye.
FSW of steel
Friction stir welding (FSW) of Ni base alloy and of carbon steel was conducted during the development of WC-based composite tools for FSW of high-softening-temperature materials at the Prince Sattam Bin Abdulaziz University in Saudi Arabia and the Suez University in Egypt et al.
The FSW tool was made from WC-based composites with 5 wt% Ni and 2.5 wt% Y2O3.
A visually acceptable weld shows a homogeneous surface from the start to the stop. The annealing colors above the heat affected zone are superficial colorations that result from the application of heat. They can be used on some materials such as stainless steels and copper alloys to assess and monitor the heat input during and even after FSW.
If large surface breaking voids, as at the start of this weld, or small surface breaking voids are visible, as at near the middle of this weld, the weld does not meet the acceptance criteria described in ISO 25239-1:2020(en) — Part 5: Quality and inspection requirements, which is, strictly speaking, only applicable to aluminium.
Image processing with Fuzzy Select Tool (Magic Wand) in GIMP 2.10 (GNU Image Manipulation Program). This is a pixel-based graphics software that includes functions for image editing and digital painting of raster graphics. The programm can be downloaded and used free of charge. Several YouTube videos are available, which explain its main features.
When welding carbon steel the annealing colors are less visible as shown here on a weld that shows some surface anomalities at the start.
Flash generation during FSW of 7050-T651
A study on friction stir welding of 10mm thialuminum-scandium alloy 7050-T651 castings (Al–5.6Zn–2.5Mg–1.6Cu) alloyed with 0.12 wt.% scandium was conducted at University of Applied Sciences of Southern Switzerland. The welds were made at a travel speed of 140 mm/min. The pin length was approximately 9.5 mm and the rotation speed was 240 rpm. Weld A was made as cast with acceptable visual appearance.
Flash was expelled, while making weld A' in the artificially aged T651 condition.
The flash was different on the advancing and retreating side of the weld. Gerenerating excessive flash is not a problem per se, as the flash can easily be removed with a milling cutter in the FSW machine or a grinding wheel of a manually operated angle grinder. However, in some cases excessive flash might be an indication for undercut, i.e. the weld being thinner than the plates, or for the pin being damaged by contacting the backing bar.
Above photo shows the flash from a different point of view. Therefore, some FSW machines have several cameras installed, e.g. one in front of the FSW tool and two behind the FSW tool (looking at the weld from above and at an angle).
Significant deformation occured in the start region of the weld, so that the unwelded region is clearly visible in the area, which had not been stirred by the traversing pin.
Dissimilar materials: 3mm thick Al 7075-T6 to pure copper
More photos required, please
We are currently searching for photos of the surfaces of friction stir welds, ideally made at five different welding conditions:
- Far too cold (generating surface void and risk of shearing off the pin) - welding speed much too fast and rotation speed much too low
- Too cold (an internal void can be assumed from looking at the surface) - welding speed too fast and rotation speed too low
- Just right - optimised welding conditions to achieve high-strength welds
- Too hot (uneven surface) - welding speed too slow and rotation speed too high
- Far too hot (blisters and a lot of flash) - welding speed much too slow and rotation speed much too high
We want to use them to train an automated image processing system to detect wrong parameters and re-adjust them either during welding or before making the next weld.
- Mohamed M. Z. Ahmed, Waheed S. Barakat, Abdelkarim Y. A. Mohamed 3, Naser A. Alsaleh and Omayma A. Elkady: The Development of WC-Based Composite Tools for Friction Stir Welding of High-Softening Temperature Materials. Metals 2021, 11, 285, https://doi.org/10.3390/met11020285, CC BY 4.0.
- ISO 25239-5:2020(en) Friction stir welding — Aluminium — Part 5: Quality and inspection requirements.
- Photos and information provided by Christian S. Paglia, Director of the Institute of Materials and Construction, Supsi, Switzerland.
- Çakan, A., Ugurlu, M., Kaygusuz, E. (2019): Effect of weld parameters on the microstructure and mechanical properties of dissimilar friction stir joints between pure copper and the aluminum alloy AA7075-T6. Materials Testing 61(2):142-148 DOI: 10.3139/120.111297.