Solid State Manufacturing Technologies

Friction stir processing is an emerging processing technique based on the principles of friction stir welding. Friction stir welding is a relatively new joining process, developed initially for aluminum alloys, by The Welding Institute (TWI) of UK (Thomas et al., 1991). It is a solid-state joining technique that is energy efficient, environment friendly, and versatile. It is being touted as the most significant development in metal joining in a decade. The basic concept of friction stir processing is remarkably simple. A rotating tool with pin and shoulder is inserted in the material to be joined, and traversed along the line of interest (Figure 1). The heating is localized and generated by friction between the tool and the work piece, with additional adiabatic heating from metal deformation. A processed zone is produced by movement of material from the front of the pin to the back of the pin. As mentioned later, the pin and shoulder of the tool can be modified in a number to ways to influence material flow and microstructural evolution. The following unique features of friction stirring can be utilized to develop new processes:

• Low amount of heat generated,
• Extensive plastic flow of material,
• Very fine grain size in the stirred region,
• Random misorientation of grain boundaries in stirred region,
• Mechanical mixing of the surface layer,
• Large forging pressure, and
• Controlled flow of material.

In this brief overview of friction stir technologies, an outline of several new concepts developed in the last few years are presented, which make friction stir processing (FSP) a generic tool for localized microstructural modification and manufacturing.

NSF I/UCRC for Friction Stir Processing: A five-university [Brigham Young University (lead institution), South Dakota School of Mines and Technology, University of South Carolina, University of North Texas and Wichita State University] Industry/University Cooperative Research Center on Friction Stir Welding and Processing has started from October 2004. Please contact Rajiv Mishra for more details about the UNT site.

Welding

As mentioned earlier, FSW was the parent technology invented by Thomas et al. in 1991. In the last ten years, this technology has taken off in a number of applications due to a number of benefits (Table 1). Apart from these advantages, it can be considered as enabling technology for joining of high strength aluminum alloys which are classified as unweldable by fusion welding techniques. The joint efficiency of 75-96% has been reported for 7XXX and 2XXX alloys (Table 2). The research and development activities in FSW have reached a critical level and TWI organizes one international conference every year from 1999 and a number of other conferences are organized every year by various materials related societies around the world. The developments in FSW have been periodically reviewed in the last few years and therefore only limited information is presented here. The readers will find the conference proceedings from the TWI, ASM and TMS particularly informative.

Table 1. Benefits of Friction Stir Welding

Table 2. Friction stir weld joint efficiency for various aluminum alloys.

The material flow and microstructural development during FSW depend on the tool design and process parameters. In the beginning, the tool was quite simple with a cylindrical pin and shoulder (Figure 3a). With the simple tool design material flow and mixing was limited and in turn the welding speeds were relatively low. The tool design has progressively become more complicated and its design features have evolved to move and mix material more efficiently. Figure 3 shows a few pin and shoulder designs. With these design innovations from TWI, the welding speeds of more than 1 m/min are currently being commercially practiced for low strength aluminum alloys.

Superplasticity

Friction stirring results in very fine grain microstructure in the stirred region (often referred to as ‘nugget’). The grain refinement results from intense plastic deformation associated with the movement of material from the front to the back of the rotating pin. The fact that friction stirring of material leads to very fine grain size can be used to obtain very fine grained microstructure in desired regions to develop new concepts of superplasticity, such as selective superplastic forming and thick plate superplastic forming. In addition, the grain boundary misorientation distribution of friction stirred zone shows predominantly high angle grain boundaries in a FSP 7075-Al alloy. The combination of very fine grain size and high angle grain boundaries is ideal for superplasticity. Table 3 summarizes some examples of high strain rate superplasticity in FSP aluminum alloys. It shows the possibility of using a simple FSP to produce a microstructure conducive for high strain rate superplasticity in commercial aluminum alloys. Based on these results, a three-step manufacturing of components can be envisaged: Cast + Friction Stir Process + Superplastic forge or form.

Table 3. Superplastic ductility at high strain rates in FSP aluminum alloys.

Casting Modification

Casting is a very widely used manufacturing technique because of its unique ability to produce complex shaped part at low cost. However, its performance is limited by many metallurgical features, such as, dendritic porosity, particulate oxides/inclusions, secondary dendritic arm spacing (SDAS), and iron-phase intermetallics. FSP provides an unique opportunity to embed ‘wrought’ microstructure in ‘cast’ component by localized modification. Such approach for components requiring higher performance would lead to the best combination, low overall cost due to casting and higher performance in localized areas due to wrought microstructure. Table 4 gives an example of localized property improvement in A356 aluminum alloy. FSP results in dramatic improvement of ductility and strength. Testing of multiple specimens showed very significant improvement in Weibull modulus and quality index (often used for castings).

Table 4. Tensile properties of as-cast and FSP A356 at room temperature using mini-tensile specimens.

Channeling

Friction stir channeling (FSC) is a new concept to produce integral channels in metallic materials. There are many applications where heat exchange is needed or desirable. The conventional approaches of building heat exchangers or incorporating fluid channels can be broadly divided in two groups. The first group would consist of use of tubes and joining processes, whereas the second group would involve machining of channels and joining of several pieces. FSC concept on the other hand can be used to create integral channels in a solid plate in one step. The shape and size of the channel can be controlled by the tool design as well as process parameters.

Microforming

The trend of producing more compact/integrated systems demands miniaturization of the components involved. At the same time, the system should be capable of performing at par or sometimes better than those macro-systems conventionally available. Micro-Electromechanical-Systems (MEMS) are good example of such a push. In addition, requirements of miniaturization can be seen in a very diverse consumer product sectors, be it smaller and smaller cell phones and consumer electronics, to biomedical implants and tiny cameras for medical applications, to microturbines and so on. Components employed in MEMS and similar devices are generally made with traditional techniques such as etching, photolithography, electroless and electrochemical deposition, and micromachining. Conventional forming of macrocomponents, such as forging, is widely used because it can produce large volume of components in cost-efficient manner. Microforming has been difficult because of frictional effects associated with metal forming processing. For microcomponents the surface area/volume ratio is large and new concepts are needed to extend forming processes to micro-levels. Combination of FSP and superplasticity can be enabling technology for manufacturing of metallic microcomponents by replication. Figure 5 shows the enhanced replication of superplastic FSP 7075 Al alloy using a channel die of 1 mm length, 0.2 mm width and 0.5 mm depth. The better formability of superplastic material is quite evident. This technique will allow fabrication of microcomponents from common engineering alloys.

Powder Processing

Powder metallurgy is used to make alloys and composites of non-equilibrium compositions. The processing steps often involve powder compaction and further thermomechanical processing. For aluminum alloys, three microstructural features are very important; prior-particle boundaries, microstructural inhomogeneity, and size of primary intermetallic particles. Breakage of the aluminum oxide film on prior-particle boundaries by extrusion or forging is critical for ductility, fatigue and fracture toughness. Because of the material flow pattern, some microstructural inhomogeneity can not be eliminated in forging and extrusion. Friction stir processing provides opportunity to homogenize microstructure for subsequent forming operations or produce selectively reinforced regions. Because of the severe plastic deformation associated with friction stirring, the prior-particle boundaries and any powder scale microstructural or chemical inhomogeneity are eliminated. Using this approach P/M aluminum alloy with ~700 MPa strength and >10% ductility has been obtained.

Enhanced Low-Temperature Formability

Manufacturing of components from thick plates usually is done by joining, as bending and shaping is difficult because of limited ductility. In as FSP condition, aluminum alloys exhibit very high ductility. Mahoney et al. (2003) have used this to bend 1” thick 2519 Al plate with just a partial FSP layer on the tensile side. This would give designers added flexibility of shaping sheets and plates with localized enhanced formability. With low heat input from FSP, the region of reduced strength is very limited. In applications where some of the welded joints are replaced by bends, such design can lead to significantly higher performance.