Welding & Metal Fabrication: Solid State Welding

Solid state welding is a group of welding processes which produces coalescence at temperatures essentially below the melting point of the base materials being joined, without the addition of brazing filler metal.

Friction stir welding is used particularly for joining aluminum alloys in shipbuilding and marine industries, aerospace, automotive and rail industry. Furthermore, the technology provides significant advantage to the aluminum extrusion industry. Automotive suppliers are already using the technique for wheel rims and suspensions arms. Fuel tanks joined by FSW have already has been launched in spacecraft, and many other space advances are under development. Aluminum panels for high speed ferries and panels for rail vehicles are also produced. Friction stir welding is making an impact as a material processing technique and the prognosis for the successful welding of steel products by FSW looks promising.

Friction stir welding (FSW) is a relatively new joining process that has been demonstrated in a variety of metals, such as steel, titanium, lead, copper, and aluminum. The process is especially advantageous for joining aluminum and has been exploited commercially around the world in several industries. The unique properties of friction stir welds make possible some completely new structural designs with significant impact to ship design and construction.
Friction-stir processing (FSP) is an emerging surface-engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving strength and ductility, increase resistance to corrosion and fatigue, enhance deformability, and improve other properties.

In conventional rotary FSW, the relative velocity of the tool increases from zero at the probe centre to maximum velocity at the outer diameter of the shoulder. The dual-rotation technique can significantly modify the velocity gradient between the probe centre and the shoulder diameter. This technique provides a differential in rotation speed and the option for rotation in opposite directions. This technique can provide for a more optimized rotational speed for both probe and shoulder.

The earliest reference to the use of frictional heat for solid-phase welding and forming appeared over a century ago in a United States patent. A period of fifty years then passed before any significant advancement in friction technology took-place namely a British patent in 1941 that introduced what is now known as friction surfacing. Yet another fifty years went by before friction stir welding (FSW) was invented at TWI. This comparatively recent innovation has permitted friction technology to be used to produce continuous welded seams for plate fabrication, particularly in light alloys.

In this article, Friction Stir Welding trials which were carried out on a modified vertical heavy duty-milling machine are analyzed. Unlike aluminum and most non-ferrous materials, which show little or no visible change during welding owing to increase in temperature, a color change was distinctive in the FSW of both grades of steel, which gave approximate indications of temperature. The tool shoulder reached a bright orange color which indicated an approximate temperature of over 1000°C, within a few seconds of making contact. Also as the tool travels along the seam, the ensuing weld track behind the trailing edge of the rotating tool appeared orange/bright red (900-1000°C).

The article describes mechanical properties, weld integrity and microstructure of a low carbon steel and a low carbon 12% chromium alloy steel. In general, mechanical testing and metallographic examinations showed good properties and reasonably uniform shaped double-sided weld profiles with no evidence of buried defects.

In friction joining and forming, the process is akin to find layer of high viscosity between solid components in relative motion and under significant compressive loading. The thixotropic properties and fluid flow features that occur in conventional friction welding have been reported, along with friction induced ‘third body’ conditions and superplasticity that occurs as result of extreme plastic deformation. Thus the science of these processes, in some respects, is probably closely allied to that of rheology.

Friction Stir Welding (FSW) is a solid state welding process where a machine rotates, plunges, and then traverses a special shaped FSW Tool along a joint to form a weld. The rotation action and the specific geometry of the FSW tool generates friction and mechanical working of the material which in turn generate the heat and the mixing necessary to transport material from one side of the joint line to the other. The process has numerous advantages over other joining technologies and can be used to weld numerous materials including, but not limited to aluminum, bronze, copper, titanium, steel, magnesium, and plastic.

The skew-stir™ technique provides an easier material flow path than conventional FSW and a weld nugget region of width greater than the diameter of the probe. In addition the skew action provides an orbital forging action at the root of the weld, which improves weld quality in this region. The use of the Re-stir™ welding technique provides a cyclic and essentially symmetrical welding and processing treatment. Most problems associated with the inherent asymmetry of conventional rotary FSW are avoided.

Solid state welding is a group of welding processes which produces coalescence at temperatures essentially below the melting point of the base materials being joined, without the addition of brazing filler metal.