The process of Friction Stir Welding Steel has rapidly become the preferred method for the production of aluminum structures that require superior strength, minimal distortion, and exceptional performance in terms of fatigue and corrosion resistance.
These attributes are due to the fact that it is an elastomeric joining technique that creates a forged grain-fined microstructure instead of the cast microstructure that is produced by the traditional fusion welding method.
Although the technique was initially developed to join aluminum and similar materials that are difficult to weld like magnesium, the numerous advantages of this technique led to there was an enormous desire to adopt friction stir welding in steel.
The tools used to do this are durable and stiff even at temperatures of more than 1.000 C and are immune to corrosion, resistant to hot iron, and impervious to the heavy tension and bending forces that they encounter as they travel through steel.
Member firm Element Six has recently developed an FSW tool to meet the requirements and has been thoroughly tested for welding carbon steels 6 mm thick.
Test of the Tool:
A variety of tools from different production batches were tested under various operating conditions. These included:
- Regimen 1 The 2m welding is a series of 2 m welds
- Multiple tools plunge into hard, cold steel
Tool running indefinitely at temperatures that are high;
- Pipeline welding is a common process of repair welding, as well as assembly work.
- Regimen 2 to 5 m welds
Fewer plunges of the tool into hard, cold steel; The tool typically operates for 20 minutes at a temperature that is elevated;
The typical modular construction pressure vessels.
- Regimen 3: Less, but with longer, 20m welds
Three or two plunges of a tool into hard, cold steel;
- Tool operating at high temperature for up to an hour or more
The panel manufacturing process is typical for vessels. The welds were produced at a rate of 300mm/min using S355J2+N stainless steel.
In all conditions of service, the instruments were competent in producing constant flawless, defect-free welds. The tested tools reached a total length of 40m with no failures arising during the tests.
The testing is ongoing to determine the longevity of the tools and 60m of welding time is now being reached using the 6mm tools.
The Ability to Increase the Thickness:
A different set of tools was developed through Element Six capable of welding steel that is up to 12mm thick and is currently in the process of being tested. Single-pass and square butt welds have been made in various sheets of steel like S355, S460, and DH36 with these tools.
2D lap welding, which simulates the attachment of an affixed patch to a ship’s hull is also possible using the 12mm tool. A similar weld is illustrated below.
Mechanical Properties of Weld:
The finely grained microstructure observed in friction stir welding generally gives them tensile characteristics that are closer in relation to parent metals than usually the case for traditional welds.
In the case of steel, this advantage is enhanced by the reality that welding happens within the temperature range of transformation and the careful selection of welding parameters can give an effective level of control over the phase changes which occur when welding.
It is possible for instance, to create welding welds that are optimized for strength, toughness, or both, depending on the requirements of the service.
In general, it is observed that friction stir welds that are made of steel are proven to be more durable than the metal from which they were created.
If friction stirs welding is constructed between different kinds of steel, for instance, carbon steel and stainless steel, the failure is more likely to be in the less slender of the two metals that are parent and further away than the welding area.
A variety of samples were taken from a single-pass butt weld that was made of S355 steel of 6mm thickness using E6 tools that had completed more than 60m of welding. They were tested for mechanical properties.
The samples passed a face bend and root test, which indicates that the specimens had no significant flaws and sufficient flexibility in the weld zone.
The cross-weld tensile test sample was not ductile in the parent metal a distance away from the weld as well as HAZ. The microstructures that are fine, multiphase produced by steel FSW create welds that can be more durable than the metal that is used as the base.
We took samples from an FSW weld of six millimeters thick S355 steel made by a tool that produced more than 60m of the weld. The weld was then tested using Charpy impact tests.
The Charpy impact test was conducted at -20 C and is in compliance with BS EN ISO 148-1:2016. The samples were taken from the thickness of the mid-weld and then notched in the direction of welding the thickness of the weld metal centerline.
Strengths of impact between 49 and 57, and 51 Joules KV were measured on the 3 test samples with the median being 56J. This is a significant amount above the minimum required value of the parent metal for S355 at 20 C, which was 27 JKV.
Because FSW is in a solid state i.e. it does not melt, and therefore, it is less affected by the alloy composition of the steels to be welded.
This makes it possible to join different kinds of steel much more quickly than other methods, such as carbon and Duplex stainless steels as well as to join steels that have been difficult to weld using other methods. On the micrograph, it is apparent that there is no alloying taking place between these two steel grades. The joint is made solely by mechanical mixing.
There are no alloying elements eliminated from either of the two grades within the weld area nor is there any segregation of elements within this area.
Duplex stainless steel remained in the same phase balance as the parent metal originally however, it has a smaller grain size within the zone of the weld. Welding samples have passed both the face and root test for bend, and in tests of tensile, the sample failed when tested against the carbon steel of the parent from the weld.
Repair and Fabrication of the Underwater Environment:
Friction stir welding can be used underwater and produces high-quality welding without any of the problems that are associated with other techniques of underwater welding.
This technology is being developed as part of the RESURGAM (Robotic Survey Repair and Agile Manufacture) project that will bring together the creation of FSW technology for steel and the development of digital manufacturing methods for both the initial modular construction of ships on multiple sites and for the repair in the water of damaged vessels or marine structures.
The FSW component within the program will be looking at the development of a set of FSW tools that are able to join different kinds of steel, ranging between 2 and 12mm thicknesses, both in the air and underwater.
These tools will be followed by the creation of an adaptable FSW system that could be modified to fit existing CNC milling or machining machines as well as a robotic system that can be used underwater to perform in-place repairs.
Live Oil Pipeline Repair:
Additionally, in addition to working underwater, FSW has been shown to work under oil, allowing repairs to be made in live oil pipelines as well as tankers for the storage of fuel.
The application is currently being developed as part of the FSWBOT program, where the robotic system developed from a PIG is used to traverse the pipeline to fix corrosion issues without the requirement for the oil flow to be stopped.