Large Strain Extrusion Machining of AA7050

Daniel R Klenosky, Purdue University

Abstract

Aluminum has remained among the most commonly-used materials for aerospace applications for the past 100 years. It is advantageous due to its relatively high strength, low weight, ease of forming, electrical and thermal conductivity, and low cost. Alloy 7050 is among highest strength aerospace-grade aluminum alloys and can be formed into a wide variety of shapes via extrusion, rolling, forging, and other processes. Unfortunately, consistent and homogenous process control can be difficult to achieve during these bulk forming operations due to the complexity of the Al-Zn-Mg-Cu alloy system. Poor process control can result in defects and/or inhomogeneous microstructures and properties, during either deformation or during post-deformation heat treatment. It is therefore desirable to learn more about the nature and mechanisms of microstructural evolution during bulk deformation processes, and it may also be desirable to seek alternative production methods where possible. Large strain extrusion machining (LSEM) is a simultaneous cutting-extrusion process that has been applied to AA7050 in this work. It is a single-step deformation process that is used both as a tool to explore deformation in general, and also as a potential alternative method to produce AA7050-T7 plate, one of the most commonly used forms of the material. Experiments in this work include LSEM starting from different initial microstructural conditions and different deformation conditions. Specifically, hardened AA7050-T7451 material, which is typically impossible to process due to its high strength and low workability, has been processed via LSEM at an effective strain larger than 1 to create continuous AA7050 plate with very smooth surfaces in a single deformation step. Room temperature LSEM has also been applied in this work to produce plate from O-tempered, as-cast, and as-homogenized AA7050. Results from experiments with these softer materials show the limitations of the LSEM process. Sticking friction, especially at slow cutting speeds, causes problems that lead to built-up edges at the strip-tool interfaces and poor surface quality. While continuous strips with good surface quality is possible for both annealed and as-homogenized material, careful balance must be maintained between cutting quickly enough to limit sticking friction but slowly enough to limit strain rate and adiabatic heating. After proper balance and control of these considerations was achieved, LSEM plate created from as-homogenized material was artificially aged successfully to create a product with properties similar to conventionally-rolled AA7050-T7451 plate.

Degree

Ph.D.

Advisors

Johnson, Purdue University.

Subject Area

Materials science

COinS