Microstructure characterization of the stir zone of submerged friction stir processed aluminum alloy 2219

• Published in: Materials characterization 2013-08, Vol.82, p.97-102
• Main Authors: Feng, Xiuli, Liu, Huijie, Lippold, John C.
• Format: Article
• Language: English
• Subjects: ALUMINIUM ALLOYS; Aluminum base alloys; Al–Cu alloy; Applied sciences; Base metal; CROSS SECTIONS; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; FRICTION; Friction stir processing; GRAIN REFINEMENT; GRAIN SIZE; Hardness; MATERIALS SCIENCE; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; METALS; Metals. Metallurgy; MICROHARDNESS; Microstructure; OPTICAL MICROSCOPY; Phase diagrams and microstructures developed by solidification and solid-solid phase transformations; Physics; Precipitates; PRECIPITATION; SCANNING ELECTRON MICROSCOPY; Solidification; Submerged friction stir processing; TRANSMISSION ELECTRON MICROSCOPY
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2013.05.010

Aluminum alloy 2219-T6 was friction stir processed using a novel submerged processing technique to facilitate cooling. Processing was conducted at a constant tool traverse speed of 200mm/min and spindle rotation speeds in the range from 600 to 800rpm. The microstructural characteristics of the base metal and processed zone, including grain structure and precipitation behavior, were studied using optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Microhardness maps were constructed on polished cross sections of as-processed samples. The effect of tool rotation speed on the microstructure and hardness of the stir zone was investigated. The average grain size of the stir zone was much smaller than that of the base metal, but the hardness was also lower due to the formation of equilibrium θ precipitates from the base metal θ′ precipitates. Stir zone hardness was found to decrease with increasing rotation speed (heat input). The effect of processing conditions on strength (hardness) was rationalized based on the competition between grain refinement strengthening and softening due to precipitate overaging. •SZ grain size (~1μm) is reduced by over one order of magnitude relative to the BM.•Hardness in the SZ is lower than that of the precipitation strengthened BM.•Metastable θ′ in the base metal transforms to equilibrium θ in the stir zone.•Softening in the SZ results from a decrease of precipitation strengthening.