Friction Stir Processing, Friction Stir Welding and Friction Stir Spot Welding are three novel joining processes that are making significant inroads in a variety of industries. They are focused on a solid-state interaction that uses the mechanical energy of a rotating tool to heat, plasticize and disrupt an oxide layer that covers the interfaces of a joint.

All three of the friction-stir processes can use the same tool, the same machinery and share the same basic process variables. They differ in nature mainly by the selection of parameters, the joint configurations and inherent defects to each joint type. FSP and FSW use a tool that is plunged into the material and translated across the surface of one or more sheets in either the lap or butt configuration. FSSW is used to join a material at a single point in a lap configuration. Under optimal operating conditions, a stir zone is developed around the periphery of the tool without any transverse movement of the working area. The processing breaks up the oxide layer, mixes the material and creates a metallurgical bond.

Since its invention in 1991 at The Welding Institute, FSW has undergone significant development and gained great acceptance. It is now commonly used for joining low melting-temperature alloys such as aluminum, although ongoing research on welding iron, Inconel and NiAl alloys has been reported in the literature. This process is particularly suitable for joining dissimilar, precipitation hardened and metal-matrix materials that would otherwise be considered unweldable using conventional welding methods. The solid-state welding process has now been implemented into the aerospace, rail, shipbuilding and automotive industries.

How it Works

In all three variants of friction-stir processing, a rotating tool is plunged into a material to create a heat and a shear layer near the periphery of the tool. The shear-layer thickness and velocity are a function of the rotational speed, travel speed and contact pressure of the tool shoulder. Traversing the tool across the surface causes the material in front of the tool to displace behind it. If a critical travel speed is met, the shear-layer thickness is reduced such that the shear strength of the layer becomes less than the bond strength and may further result in the velocities of the shear layer and the tool being equal.

If the rotational speed and contact pressure at the shoulder are at the proper relationship with the travel speed, a temperature gradient will decrease from the outside of the shoulder to the edge of the pin. The shear strength of the material should decrease proportionally to the temperature gradient such that bond and shear strength at the interface are equal. If the shear-strength gradient is correct, there will be a quasi-symmetrical volume of material that deforms around the periphery of the tool. For a given travel speed, materials with a high coefficient of friction will require a lower rotational speed because the heat generated at the interface will be greater. The greater heat will cause the base material to soften, making deformation away from the interface easier, which creates a larger stir zone without a gross melt layer.

Defects

Defects such as worm holes, surface galling, lack of fusion, scalloping and liquation cracking can occur outside the operating window. Defects can originate as a result of improper flow or geometry. Geometric defects occur when either the tool geometry is insufficient to penetrate the entire thickness of the material or the seam is not properly tracked by the tool during welding. It is believed that the optimum welding parameters provide conditions where there is balance between sticking and slipping of the material around the pin. The material produces almost a whipping flow. Flow-related defects occur when welding is conducted outside the proper processing window, such as when the welding conditions are either too hot or too cold.  

Cold-Processing Defects
Under cold-processing conditions with slip, the resulting defect is either a surface defect or wormhole. The loss of material on the retreating side of the stir zone (below the pin) or the retreating side close to the shoulder surface results in a wormhole – lack of surface fill or volumetric defect on the advancing side. The appearance of these defects is a direct result of the lifting of the sheet, or sheet separation, and can be observed as a flash or raised crown depending on the amount of the forging force.  

Hot-Processing Defects
During the welding of sheet material in a lap configuration under hot conditions, the material in the thermal mechanically affected zone on the retreating side is forced upwards, causing the thinning of sheets. A critical total mass of material must move from the front to the rear of the tool through the extrusion zone to maintain mass balance and prevent void formations.

Heat Sources

Localized heating from the shoulder allows for a material that is both in contact and in close proximity with the pin to be deformed and transported around it. There are two main heat sources intrinsic to FSW:

• Friction from the contact between the tool and the base material
• Deformation around the tool

Unlike conventional arc-welding processes, the heat generation cannot be considered as axis-symmetric. Consideration of both the rotational and translational velocities leads to the definition of the retreating side (RS) and advancing side (AS) in a weld. The AS has resultant vectors from the tool rotation and advancement of the material that point in opposite directions, and on the RS they point in the same direction. As expected, each side would have a different thermal experience.

The friction condition between the tool and workpiece interface, along with the material flow within the weld, dictate how much heat is produced. Interface heat is generated by friction, while the flowing material dissipates heat primarily from plastic deformation. It is the contact condition, such as when there is sliding, sticking or partial sliding/sticking, which determines the heat generation. Tool design, welding parameters and alloy type all contribute to the contact condition during FSW. Relative velocity differences between the workpiece and the tool increase as the distance from the tool increases. More heat is generated further away from the center of the tool.

The same can be said about the heat-generation rate below the pin. The nonuniform heating is a result of a difference in relative velocities along different angular locations on the pin surface. Local differences in heat generation do not lead to large variations in local temperatures because of the rapid recirculation of plasticized material along the pin surface.

Weld Characterization

Finished welds have different regions that can be characterized as a function of the different processing conditions during welding. These regions include the stir zone (SZ), the thermal mechanically affected zone (TMAZ), the heat-affected zone (HAZ) and the base material (BM). The SZ is characterized by the most mechanical working caused by sticking and direct contact with the pin. TMAZ experiences both thermal and mechanical effects (i.e. heating of the material and plastic deformation caused by the tool). Yet, the tool has no direct contact with this region. The HAZ is the region in which the process heat causes microstructural changes, such as grain coarsening, along with the formation and dissolution of precipitates. Mechanical properties may be deteriorated in this region.

Material flow during FSW has been studied using pre-embedded tungsten markers that can later be observed using radiographic methods. Realizing that several velocity fields exist during welding, their combined input would cause the material to not only flow from the front of the pin to the rear but also with a vertical displacement component. The material in close proximity to the pin becomes more plasticized at higher temperatures. When this material is swept around the pin to a colder region, it forms a stronger solid. While this hot material yields, the adjacent material is not strained.

When both the bulk material and the high-temperature material cool down, the material next to the pin then becomes sticky attempting to drag the bulk material along with it, resulting in a period of high strain. Since new material is continuously entering into the stir zone, heat generation is not always constant, resulting in asymmetric flow. The hot-cold-hot cycle repeats, giving way to the oscillating stir zone. This variation in temperature results in a variation of viscosity. As the viscosity decreases, so will the extent of sticking of the material to the pin.

Tool shoulder-to-pin diameter ratios play an important role in SZ development. Tools with shoulder-to-pin diameter ratios close to 1 do not produce a TMAZ and do not adequately preheat and soften the material before the tool advanced. Tools with very large shoulder-to-pin diameter ratios may preheat and soften too much of the material, resulting in the material not sticking to the pin. As an empirical suggestion, tools with shoulder-to-pin diameter ratios close to 2 or 3 are more favorable than those outside this range to induce plastic flow around the pin, provided there is sufficient heat input. Without sufficient heat input, inadequate plastic flow occurs with the formation of voids.

A friction system will stick or seize when the contact pressure is high and the coefficient of friction is low because the contact becomes plastic. Seizure occurs when the strength properties vary little with depth. A flowing surface will not cause any large-scale mass flow if the velocity gradient is on the order of 10¹⁰ -10¹¹ poise per micron. Seizure may take place if the surface temperature gradient is less than or equal to a critical value.

The critical temperature gradient is that at and below which mechanical momentum can be transferred between adjacent layers. The system will also seize if, while in the steady state, deceleration or surface cooling lowers the surface temperature so that the temperature gradient falls below the critical value. When the sliding speed increases, surface temperatures at both local and bulk contact areas are raised by frictional heating. The heat is conducted into the bodies, causing their temperature to rise, giving thermal stresses that superimpose on the mechanical ones, causing thermo-mechanical wear. The temperature rise depends on the mechanical and thermal properties of the materials, on the load acting across the interface and on the velocity of sliding.

Applications and Future Markets

FSW offers users a number of advantages over many traditional arc-welding procedures. By eliminating melting of the target materials, a high-quality weld can be formed with minimal distortion and filler metals and potentially an elimination of a variety of toxic fumes and output. In addition, with the elimination of many filler materials, the process can be considered environmentally friendly and energy-efficient.

Until recently, applications for these processes were limited to aluminum and other nonferrous low melting-point alloys. In addition, remote and on-site applications were difficult because of the complexities and size of the equipment and tooling. However, recent work at Oak Ridge National Laboratory and other research institutions is leading the way in formulating new tool compositions that allow for FSW of higher-strength and higher melting-point materials. Portable equipment, including robotic “pigs,” allows FSW to be used in the lucrative and expanding pipeline segments.

One of the benefits of this process is the ability to join dissimilar metals. As such, a number of potential applications are possible. Currently, this facet is being utilized by the aerospace industry. Another high-growth industry that may benefit from this technology is health care, specifically biomaterials. Many biomaterials feature a combination of unique, low-melting intermetallics such as nickel- titanium, cobalt-chromium, Ti-6Al-4V and alpha/beta titanium mixes. Typically, these materials are joined using diffusion bonding, an expensive and time-consuming process that does not guarantee a homogenous microstructure around the joining area. FSW may allow these materials to be joined in a fast, efficient manner that will provide manufacturers and end-users a cheaper and more reliable product. Other areas that are beginning to look into FSW include the nuclear and automotive industries.

Conclusion

FSW and its sister processes are opening many possibilities for the future of joining ferrous and nonferrous metals. Significant opportunities exist to reduce energy requirements and offer customers a weld with a minimal heat-affected zone and homogenous microstructure. Recent advances allow this process to move from the laboratory to the field and offer users an interesting and innovative tool for a number of new applications. IH  

For more information:  Contact Matthew Karsh, materials engineer, Denver Welding & Research, 1500 Illinois St., Golden, CO 80401; tel: 917-306-3775; e-mail: matt@denverweld.com; web: www.denverweldingandresearch.com. The author, Scott Gordon, is the president of Denver Welding & Research.