February 2021 Volume 3

FORGING RESEARCH

Friction extrusion, for wires, rods, and bars along with shear assisted processing and extrusion (ShAPE) for hollow cross sections, are solid-state methods of consolidating powder directly into extruded products. In these processes, a rotating die is forced against a powder bed, and the material extrudes through the die face as seen in Figure 2 for friction extrusion of Al-TM powder. Friction extrusion offers several advantages, including reduced processing time from powder

to rod or bar, in comparison to traditional extrusion routes [12]. This process eliminates compaction, canning, degassing, preheating the billet, and the need to remove a can, significantly reducing production time. In addition, extensive refinement of the aluminum matrix and second phases strengthen particles due to severe plastic deformation results in greatly enhanced ductility [12].

Figure 2. Friction extrusion of 1” diameter bar, in a single step, directly from Al-TM powder

These novel solid-state consolidation routes open an opportunity for the forging industry to take advantage of high-performance powder based aluminum alloys and would also apply to other powder matrix material systems. Unlike AMwire or powder bed processes, ShAPE and AFSD products tend to produce material in a bulk form. Rather than fine details and complex shapes, the goal is high-quality material produced under conditions of severe plastic deformation. Product is fine grained and fully densified yielding exceptional metallurgical performance. Material from the friction consolidation process is ready for additional forging work to retain and improve properties, create a more nearly net result, and reduce subsequent machining operations. To enable use of these consolidation routes for nonferrous alloys, further research isunderwayatColoradoSchool ofMinesMetallurgy and Materials Engineering Department, supported by FIA and FIERF, and the Center for Advanced Non-Ferrous Structural Alloys (CANFSA) [27]. Strain, strain rate, and temperature effects on thermal stability of Al-TM alloys are of primary interest to this project. As previously mentioned, Al-TMalloys are of interest due to the high-temperature mechanical properties. One method to address thermal limits of aluminum is to alloy with several transition metals that have low diffusion coefficients, including; Fe, Cr, Ti, V, Nb, and Mo [9]. These elements diffuse slower at elevated temperatures, coarsening of particles on grain boundaries or within the grain occurs at reduced rates. Stabilized

phases continue to contribute to strengthening by pinning grain boundaries thus reducing precipitate growth and inhibit dislocation movement by Orowan strengthening [9]. Powder aluminum alloys are produced through gas atomization, this allows for a great flexibility in composition and range of powder size from approximately 10 to 1000 µm [28,29]. Gas atomization of the liquidmelt forms very small powder particles, and these particles undergo very fast cooling rates in comparison to casting; thus the term rapid solidification [29]. Al-TM alloys produced through rapid solidification have been reported with grain size of 4-5µm [12], with further refinement achieved in consolidation [12,30]. Via the gas atomization process, it is also possible to produce alloys with alloying element concentrations greater than the solid state equilibrium [9]. These supersaturated solid solutions increase the fraction of second phase formed on the dissolution of the saturated phase [9], thus increasing the Orowan strengthening contribution. Limited solubility at elevated temperature prevents coarsening of second phases. In high-temperature aluminum alloys similar to Al-TM, the fine second phase particles are typically metastable and formed during rapid solidification [9]. These metastable strengthening phases have been shown to transform to more stable equilibrium phases in the temperature range 400-500°C [9,31,32]: thus, consolidation of powder should be performed at low enough temperatures to retain these phases.

FIA MAGAZINE | FEBRUARY 2021 80