Ultrafine-Grained Al-Mg-Sc Alloy via Friction-Stir Processing

NILESH KUMAR 0 RAJIV S. MISHRA 0 0 NILESH KUMAR, Postdoctoral Research Associate , and RAJIV S. MISHRA, Professor, are with the Department of Materials Science and Engineering, Center for Friction Stir Processing, University of North Texas , Denton, TX 76203 . Contact Friction-stir processing (FSP) of twin-roll cast (TRC) Al-Mg-Sc alloy resulted into ultrafinegrained microstructure. The alloy was processed in as-received and aged (563 K [290 C], 22 hours) conditions and at three different tool rotation rates: 800, 400, and 325 rpm. The microstructural features were characterized using electron backscattered diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The grain size varied from 0.89 lm to 0.39 lm depending on the processing and initial thermo-mechanical conditions of the alloy. The TRC alloy processed at 325 rpm in aged condition had all the grains less than 1 lm, and 95 pct of grains had high-angle grain boundaries (HAGBs). In all the cases, the fraction of HAGBs were more than 80 pct. The variation of misorientation angle distribution was similar to the theoretical MacKenzie distribution for cubic crystal materials. Grain size analysis at different sections and locations on the transverse section of the dynamically recrystallized zone showed a homogeneous and equiaxed microstructure. The average dispersoid (Al3(Sc,Zr)) size was ~8.0 nm in diameter obtained using high-resolution TEM. Grain size reduction was observed with increase in Zener-Hollomon parameter. It was shown that under the current microstructural and deformation conditions, dynamic recrystallization via particlestimulated nucleation might not be possible during FSP. - involved because of frictional and adiabatic heating during processing of a material. Also, in comparison to other techniques, this is a relatively less explored technique, and efforts are being made to process UFG materials using FSP and understand their mechanical properties. FSP has emerged as a generic microstructure modification tool in the last decade. All the initial attempts to refine grain size by FSP were limited to fine-grained microstructure (1 to 10 lm).[15] Mishra and Ma[15] have tabulated the grain sizes obtained by FSP or friction-stir welding (FSW) under various processing conditions. It is clear from the experimental conditions and resulting grain size that an external cooling medium or a special design of tool was required to obtain UFG microstructure. Of late, there have been some efforts to obtain UFG microstructure by changing the processing parameters such as tool rotational rate (x) at constant tool traverse velocity (m) or the ratio x/m.[12,16,17] Su et al.[18] and Rhodes et al.[19] demonstrated the possibility of achieving grains as small as 25 to 100 nm by employing special cooling arrangement during FSP. But as mentioned earlier, most of the mean grain sizes tabulated by Mishra and Ma were larger than 1 lm under ordinary processing conditions.[15] Hence, these observations are indicative of coarsening of grains during FSP/FSW. In fact, Rhodes et al.[19] obtained 2 to 5 lm grain size after heating the samples having initial grain sizes in the range 25 to 100 nm at 523 K to 623 K (250 C to 350 C) for 1 to 4 minutes. Being a high-temperature process, nanosized grains formed during FSP grow very rapidly under the influence of thermal cycle when no forced cooling is imposed. Due to this, the real potential of FSP for microstructural refinement cannot be tapped. Use of external cooling media necessitates extra fixtures during processing, and their use may not be feasible in every condition. The presence of precipitates or dispersoids is known to inhibit the grain growth via Zener pinning. Hence, the untapped potential of FSP can be exploited not only by controlling the processing parameters but also by using a material that either contains thermally stable precipitates or dispersoids, or can precipitate out such particles during processing thereby retarding the uncontrolled grain growth during processing. Hence, UFG microstructure may be obtained by controlling the grain growth by the use of precipitates or dispersoids during FSP. The current research deals with this. A newly developed Al-Mg-Sc alloy was processed using FSP to obtain UFG microstructure. The characteristics of UFG microstructure obtained via FSP are entirely different from those obtained by conventional SPD techniques. A microstructure with equiaxed, homogeneous, and a completely random distribution of grains was obtained. II. EXPERIMENTAL A twin-roll cast (TRC) Al-Mg-Sc alloy sheet (~3.75 mm thickness) was processed using FSP. The nominal composition of the alloy was Al-4Mg-0.8Sc0.08Zr, wt pct. The alloy was processed in two different thermomechanical conditionsas-received (AR) and AR + aged. The aging was carried out at 563 K (290 C) for 22 hours. The material processed in the AR condition will be referred to as AR + FSP (x) and the one in aged condition as aged + FSP (x). Here, x stands for tool rotation rate (revolution per minute, rpm). x will be replaced with appropriate value while making reference to a particular tool rotation rate. A tool steel tool was used to process the material. The geometrical details of the tool are provided in Table I. Three different tool rotational rates were used800 rpm, 400 rpm, and 325 rpmto process the material. In each case, other processing parameters such as tool traverse speed, tool tilt angle, and plunge depth were kept constant. The tool traverse speed, tool tilt angle, and plunge depth were 3.4 mm/s (8 ipm), 2.5 deg, and 2.5 mm (0.097), respectively. FSP resulted in the grain refinement. The microstructural information such as grain size, its distribution, misorientation angle distribution, etc. were obtained using electron backscatter diffraction (EBSD) using an HKL EBSD system fitted on FEI Helios NanoLab 600 FIB/FESEM (FEI Company, Hillsboro, OR). Each sample was mechanically polished using water-based diamond suspension up to 1 lm grit size and final polishing on 0.02 lm using colloidal silica suspension. EBSD was carried out in as-polished condition. In AR and FSP conditions, a step-size of 1 lm and 0.1 lm, respectively, were chosen. The primary Al3(Sc,Zr) dispersoids in as-received condition and after FSP were characterized using a scanning electron microscope (SEM). Transmission electron microscope (TEM) was used to characterize secondary nanosized Al3(Sc,Zr) dispersoids. A 2-mm disk was thinned down to 80 lm followed by electropolishing in a twin-jet polisher. Electropolishing was carried out at 30 V and 243 K ( 30 C). A mixture of CH3OH and 20 pct HNO3 was used as an electrolyte. A. Grain Refinement The EBSD micrograph for AR and grain size distribution (GSD) histogram and cumulative GSD curve for AR and AR + FSP (325 rpm) TRC alloy are shown in Figure 1. The thick dark lines in the micrograph (Figure 1(a)) represent high-angle gra (...truncated)