February 2021 Volume 3

FORGING RESEARCH

Solid-State Processing Pathways for High Performance Aluminum Powder Alloys S. Shirley 1 , A.J. Clarke 1 3 , R. Mayer 2 , S. Whalen 3 , T. Pelletiers 4 , J. K. Yoder 5 , H. Z. Yu 5 , B.J. Phillips 6 , J.B. Jordon 6 , P.G. Allison 6 , K.D. Clarke 1 3 1 Colorado School of Mines 2 Queen City Forge 3 Pacific Northwest National Laboratories 4 Kymera International 5 Virginia Tech 6 University of Alabama

As the aerospace and automotive industries continue to push the boundaries of fuel efficiency, the demand for lightweight, high performance materials increases. Titanium and aluminum alloys are the primary metals used to meet the requirements of critical lightweight applications. In the commercial sector, the Boeing 747 is 68% aluminum and 4% titanium; while fighter jets are 50% aluminum alloys and 13% titanium by weight [1]. Clearly, titanium is utilized more broadly in specific applications where material cost can be justified. Over the operating lifetime of a fighter jet, material cost is approximately 2%, whereas 50% of the cost is operational, which is primarily due to fuel [1]. To reduce operational costs, switching titanium parts to less dense aluminum alloys will reduce the mass of the plane and increase fuel efficiency. Savings in fuel due to lightweight materials has become a critical factor in the automotive market as well in order to reduce CO2 emissions [2]. With automotive and aerospace making up some of the largest markets to the forging industry [3], it is critical to continue developing production and processing pathways to meet demand for high- performance materials. Over the last few decades, research work has focused on developing high-temperature aluminum alloys. Promising developments in high-temperature alloys have been achieved through alloying and rapid solidification of powder production[4,5]. One of the aforementioned alloys, aluminum with transition metal alloying elements (Al-TM), has been developed to retain high temperature

mechanical properties [6]. The retention of ultimate tensile strength (UTS) in Al-TM alloys at elevated temperatures, up to ~350°C [7], is due to the reduced solubility of alloying elements, leading to retention of strengthening precipitates at elevated temperatures [8]. The fine precipitates are typically metastable and formed during rapid solidification [9] eliminating production in a bulk form such as casting. Consolidation of powders to a bulk form must therefore be conducted via a solid-state process at relatively low temperatures to retain mechanical properties. Given the need to consolidate powders to access some high performance alloys, the economic consolidation of powder alloys is critical to enable the forging industry to take advantage of powder alloys. While consolidating powder into a bulk form, it is desirable to retain or improve the inherent mechanical properties of the alloy. For example, the primary strengthening precipitates in the Al-TM alloy coarsen and transform in the temperature range of 430°- 550°C, requiring a lower temperature solid-state consolidation route. Currently, densification routes such as extrusion [10] could be used to produce bulk product to serve as feed stock for forging. Typical canned extrusion of powders requires multiple steps to achieve a consolidated bar from powder. Powder is loaded into a can, normally vibrated to increase the packing density, and the can is sealed under vacuum to isolate the powder from the atmosphere [11]. The canned powder is heated and extruded through the die, with the

FIA MAGAZINE | FEBRUARY 2021 78