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MATERIALS ENGINEERING

SEMINAR

“Microstructure Evolution in 6XXX and 7XXX Aluminum Alloys During Extrusion and Related Heat Treatments”

By

Yiwei Sun

Purdue MSE Ph.D. Final Exam

Advisors: Professors David Johnson and Kevin Trumble

ABSTRACT

AA6XXX (Al-Mg-Si) and AA7XXX (Al-Zn-Cu-Mg) aluminum alloys are precipitation strengthened alloys with applications in construction, automotive, and aerospace industries. Manufacture procedures for these alloys include casting, homogenization treatment, deformation processing, solutionization treatment and age hardening. The present work focuses on the microstructure development during hot deformation processing (especially extrusion) as well as the related heat treatments including the pre-deformation homogenization and the post-deformation solutionization, with emphasis on phase constituents and grain structure.

Extrusion is a shaping process in which the material is pushed through a die forming rods, tubes or complex profiles. For the mid and high strength alloys such as 6061 and 7050, recrystallization is undesired as it is detrimental to the mechanical properties. However, deformation involved in extrusion is highly heterogeneous, making it challenging to completely control the grain structure especially in the surface region. In the current work, a small scale extrusion setup is created, which enables more convenient examination and faster quenching of extrudate and billet discards compared with industrial extrusion. In this way, the dynamic and static microstructure evolution processes are studied separately for a clearer understanding in recrystallization and grain growth behavior of the alloy. On the other hand, recrystallization is significantly influenced by the small concentration of Zr in the alloy, which forms Al₃Zr nano-particles to inhibit recrystallization. However, the size, number, spatial distribution and coherency of Al₃Zr determine how effective Zr is in grain structure control. Therefore, the role of Zr in grain structure development is also investigated. The current work aims to provide deeper scientific understandings in recrystallization mechanisms and grain growth behavior associated with extrusion and heat treatments and contributes to future optimization of final product grain structure.

Date: Thursday, November 16, 2017

Time: 9:30 A.M.

Place: ARMS 1021