Effect of Grain Size Refinement and Precipitation Reactions

Effect of grain size refinement and precipitation reactions on strengthening in friction stir processed Al–Cu alloys Xiuli Feng, a,b Huijie Liu a,⇑ and Sudarsanam Suresh Babu b a State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People’s Republic of China b Department of Materials Science and Engineering, The Ohio State University, OH 43221, USA Received 4 August 2011; revised 3 September 2011; accepted 6 September 2011 Available online 10 September 2011 Strengthening by grain size refinement and precipitation in friction stir processing (FSP) of Al–Cu alloys was investigated. Inter- estingly, even with significant grain refinement (<1 lm), there was a substantial reduction in microhardness. This is attributed to the loss of precipitation hardening due to the formation of equilibrium h in lieu of metastable h 0 and h 00 precipitates. Theoretical calcu- lations suggest that the average grain size in the FSP zone must be smaller than 120 nm to compensate for this loss. 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Friction stir processing; Grain refining; Precipitation; Microhardness Fric tion stir pro ces sing (FSP) was developed based on friction stir welding (FSW), in which a rotating tool is plunged int o a fixed wor kpiece and traversed along the line of interest [1,2]. Since its development, FSP has been applied to modify casting microstru ctures [3], achieve superplasticity [4] and fabricate surface com- posites [5]. Cr iti cal pr ocess para met ers in FSP include the tool geometry, rotation speed, traverse speed and exter- nal cooling conditions. The grain size can be controlled by changing these parameters. For example, researchers have used FSP to obtain ultrafine-grained and nanocrys- talline aluminum alloys [6–10]. The grain size reported in the literature ranges from 0.02 to 0.7 lm. Grain refineme nt is an effective means to improve the mecha nical properties, e.g. yield streng th, of friction stir pro ces sed mat eria l. Thi s app roa ch has been demon- strated in FSP of 1050 Al alloy [10]. However, in age- hardenable aluminum alloys, precipitation strengthening is the dominant mechanism [11]. Dixit et al. [12] calcu- lated the contribution of precipitation strengthening for 705 0 an d 7055 Al in peak-a ged co nd it io n to be 100 MPa for both alloys. In contrast, the strengthening contribution from a matrix with an average grain size of 1.5 lm is only 69 MPa. However, during FSW/FSP, coarsening, dissolution of streng thenin g precipitates, as well as reprec ipitati on of metast able and/or equili brium phases are expec ted. These microstructu re evolu tions may eventually lead to softening [13–15]. The above research also suggests that it is possible to offset the softening by greatly reducing the gr ain size to finer than 1.5 lm. Approaches to combine the strengthening due to ultrafine grain sizes and precipitation have not been thoroughly explored. The goal of the current research is to analyze the strengthening due to precipitation and grain refinement (<1.5 lm) in age-hardenable aluminum alloys. To refine the grain size below 1.5 lm, FSP was perfor med under water at different traver se speeds . The 2.5 mm thick age-hard enable commerci al Al–Cu alloy (2219-T6 Al) plates were fixed in a tank filled with water at 298 K. A pinless tool with a shoulder diameter of 22 mm was uti li zed. FSP wasperfo rme d at a tool rotation speed (x) of 1000 rpm and a traverse speed ( v) of 200–400 mm min 1 . Displacement control was used to maintain the plunge depth at 0.5 mm into the top sheet surface. The tilt angle of the rotating tool with respect to the Z-a xi s of the FSW ma chinewas 2.5. The dir ecti ons of the FSP geometr y are referr ed to LD,TD and ND (lo n- gitudinal, transv erse and normal direct ions,respectively). The friction stir processed plates were cross-sectioned perpendicular to the LD, and then ground and polished to 1 lm. Vickers micr oha rdn ess was mea sur ed (load 50 g, dwell time 10 s, center-to-center spacing 120 lm) using a Leco AMH-43 microhardness machine. Electron back-scattered diffraction (EBSD) analysis (step size of 0.2 lm) was perfor med usi ng a Quanta 200 opera ted at 20 kV to anal yze the gr ai n st ruct ur e and textur e 1359-64 62/$ - see front matte r 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.09.009 ⇑ Corresponding author. e-mail addresses: [email protected]; alimse@ 163.com Available online at www.sciencedirect.com Scripta Materialia 65 (2011) 1057–1060 www.elsevier.com/locate/scriptamat

Effect of Grain Size Refinement and Precipitation Reactions

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