Surface nanocrystallization induced by shot peening and its effect on corrosion resistance of 6061 aluminum alloy
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nxiang Huang, Hai Liu, Xiaoling Li, Mengtian Ni, and Chen Lu School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China (Received 21 July 2014; accepted 14 October 2014)
The effects of the different shot peening parameters on the 6061 alloy specimens’ surface have been investigated. It is found that the compressive residual stresses and the surface roughness of the surface layer for all shot-peened specimens are improved greatly. The maximum stress value and the maximum roughness are obtained by the shot flow rate of 5.5 lbs/min and air pressure of 20 psi. The microstructure observation results indicate that a nanostructured layer with an average grain size below 100 nm has been created on the top surface layer of each specimen. In the top surface nanostructured layer, the microhardness is enhanced. It is resulted from the grain refinement and the strain hardening. The results of electrochemical measurements, surface corrosion morphology observation, and EDS analysis indicate that the corrosion susceptibility of the 6061 alloy could be significantly enhanced by means of the shot-peening-induced surface nanocrystallization.
I. INTRODUCTION
It is well known that grain size refinement can effectively enhance the properties and performance of the materials. For instance, the materials processed by rapidly solidified powder metallurgy (RS/PM) and severe plastic deformation (SPD) obtain ultrafine grain size and exhibits excellent mechanical properties.1,2 At present, it is still a challenge to prepare bulk nanocrystalline materials for structural applications due to some difficulties associated to the complicated process, small sample size, and high processing cost. However, majority of failures in metallic materials such as fatigue fracture, fretting fatigue, wear, and corrosion are initiated from their surface layers. Therefore it would be considerably effective to adopt an approach to enhance the material properties by means of the surface nanocrystallization. Recently, shot peening is an interesting surface treatment method which is potentially very useful in industrial applications as it is simple, flexible, and low cost. In the process of conventional shot peening and modified shot peening, material surface is impacted by hundreds of thousands of pellets repeatedly, causing SPD on the sample surface. This technique has been successfully applied in achieving surface nanocrystallization in a variety of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.323 3002
J. Mater. Res., Vol. 29, No. 24, Dec 28, 2014
http://journals.cambridge.org
Downloaded: 12 Mar 2015
materials including pure metals,3 steels,4–7 Al,8 Mg,9 Ti10 alloys and composite.11 It induces grain refinement and residual compressive stress into the surface layer of bulk materials, improving properties and performance of the materials. The studies by other researchers indicate that the properties such as fatigue strength,12,13 fretting fatigue resistance,1
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