Development of ultrafine grained Al 7075 by cryogenic temperature large strain extrusion machining
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ARTICLE Development of ultrafine grained Al 7075 by cryogenic temperature large strain extrusion machining Xiaolong Yin, Yunyun Pi, Jiayang Zhang, and Wenjun Denga) School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou 510641, China (Received 5 April 2018; accepted 14 August 2018)
Large strain extrusion machining (LSEM) emerges as an innovative severe plastic deformation method of fabricating ultrafine grained materials. However, substantial heat generated during LSEM would sacrifice the mechanical properties of materials. Cryogenic temperature (CT) LSEM is put forward to overcome this shortcoming. The Al 7075 was processed by cryogenic and room temperature (RT) LSEM to investigate their comparative effects on mechanical and microstructural properties. Results indicate that the chip morphology of CT LSEM is featured with better integrity. Grains are refined to less than 200 nm by CT LSEM. A more complicated microstructure with high dislocation density is observed in the CT LSEM specimens. The hardness of cryogenic and RT LSEM specimens increases with the compression ratio and reaches the highest values of 187HV and 170HV, respectively. Dislocation strengthening is the main contributor, accounting for the higher hardness of CT LSEM specimens.
I. INTRODUCTION
The Al 7 series alloys have extensively been applied in automobile and aerospace structures and other high strength components owing to their high strength-toweight ratio, and high thermal and electrical conductivity.1–3 It is well known that refining the grain structure to ultrafine grained (UFG) regime can further enhance the mechanical properties of these Al alloys. Consequently, preparation of UFG Al alloys has raised increasing attention recently. Severe plastic deformation (SPD) is the most frequently used technique for the production of UFG materials. However, preparation of UFG Al 7 alloys with conventional SPD methods requires large plastic strains and exhibits limited success by virtue of the formation of metastable precipitates that restrict the formability of the alloys during processing at room temperature (RT).4 One alternative way to increase the deformability is increasing the process temperature, and homogeneous structures can be produced. However, elevated temperature will probably give rise to overaging and dynamic recovery, thus sacrificing the mechanical properties of the alloys.5,6 Furthermore, large amounts of heat generated during the SPD process at RT will also exert negative impact on the material. To obviate this difficulty, deformation at cryogenic temperature (CT) has been identified as a promising route of producing UFG Al alloys and eliminating the temperature-induced effects simultaneously. a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2018.313 J. Mater. Res., 2018
To date, cryogenic SPD methods are widely used to achieve UFG materials in various pure metal and alloys. Li et al.7 studied the microstructure refinement mechanics of pure Cu with
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