Effect of Low Feed Rate FSP on Microstructure and Mechanical Properties of Extruded Cast 2285 Aluminum Alloy

Transcription

1 614 J. Mater. Sci. Technol., Vol.23 No.5, 2007 Effect of Low Feed Rate FSP on Microstructure and Mechanical Properties of Extruded Cast 2285 Aluminum Alloy L.Karthikeyan 1), V.S.Senthilkumar 2), D.Viswanathan 2) and S.Natarajan 2) 1) Department of Mechanical Engineering, Panimalar Engineering College, Chennai-103, TamilNadu, India 2) Department of Mechanical Engineering, College of Engineering, Anna University, Chennai-025, TamilNadu, India [Manuscript received August 14, 2006, in revised form February 8, 2007] Friction stir processing (FSP), a variation of FSW (friction stir welding) is an emerging surface engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving the mechanical properties of material. FSP can also produce fine grained microstructures through the thickness to impart superplasticity. The technology involves plunging a rapidly rotating, non consumable tool, comprising a profiled pin and larger diameter shoulder, into the surface and then traversing the tool across the surface. The pin and the shoulder friction heat the surface which alters the grain structure in the processed area thereby improving the mechanical properties. This paper presents the effects of FSP on microstructure and mechanical properties of extruded cast 2285 aluminum alloy at three different feed rates viz. 10, 12 and 15 mm/min. With the increase in the feed speed the material was observed to have increased impact strength. FSP also increased the tensile and yield strengths with increases in hardness and ductility values also. The observation has been listed in detail and pictorially represented. KEY WORDS: Friction stir processing; Feed rate; Fine grain microstructure 1. Introduction and Background Friction stir welding (FSW), a solid state joining process invented at The Welding Institute (TWI), UK in 1991, is a viable technique for joining Al alloys, which are difficult to fusion weld [1]. FSP (friction stir processing), a variation of FSW uses the same tooling, has been proven effective in selectively modifying the microstructure of specific areas to improve local properties [1,4]. To friction process a location within a plate or sheet, a specially designed cylindrical tool is rotated and plunged into the selected area. The tool has a small diameter pin with a concentric larger diameter shoulder. The rotating pin contacts the surface [3], as it descends to the part, friction heats the surface. When shoulder contacts the surface, its rotation causes additional frictional heat and plasticizes a larger cylindrical metal column around the inserted pin. The area to be processed and the tool are moved relative to each other such that the tool traverses, with overlapping passes, until the entire selected area is processed to a fine grain size and the material is transported from the leading to the trailing face of the pin. As the processed zone cools without solidification due to absence of any liquid, it forms a defect free recrystallized fine grain microstructure [2]. The process as described above is illustrated by steps in Fig.1(a) (d) [1]. 2. Experimental The base material for this investigation was extruded cast 2285 aluminum with composition as given in Table 1. Procured material was cut into pieces with the size of 60 mm 30 mm 5 mm for processing. To whom correspondence should be addressed, vssk70@yahoo.com, lkarthikeyan@yahoo.com. A milling machine was used for friction stir processing (FSP) of 2285 aluminum alloy. The machine has a maximum speed of 2000 r/min and 10 Horse Power. The tool material used was HSS for both shoulder and pin. The pin was 4 mm long and the shoulder diameter was 16 mm. The shoulder was flat without any scrolls underside. The rotating pin was inserted into an initially predrilled hole of 4.5 mm long. Tool tilt angle was 2. Processing began at a spindle speed of 800 r/min and travel rate of 10 mm/min. The speed was increased by 100 r/min every pass up to a final speed of 1200 r/min. The same step was repeated for different feed rates. The plates were then subjected to mechanical testing and microstructure observation. The workpieces were swabbed with soft cotton for 15 s by using an etchant hydrofluoric acid (0.5 ml, 99.5 ml H 2 O). The microstructural observation was made under a LEICA optical microscope. 3. Results and Discussion 3.1 Mechanical property Table 2 shows the hardness and mechanical properties of the cast Al alloy and friction stir processed samples corresponding to different feed rates. The results tabulated above are described graphically in detail in Fig.2(a) (e) with property variations (along ordinate) to different feed rates (along abscissa). Figure 2(a) depicts the result of hardness tests graphically. They show an improvement between extruded cast Al alloy and FSP microstructures as consistent with previous studies [1]. They are approximately equal between different travel rates though the material becomes soft as processing is done with higher travel rates. Figure 2(b) shows the impact test results conducted on the processed alloy. It illustrates an enormous improvement in resistance to fracture. The values keep progressively increasing with higher

2 J. Mater. Sci. Technol., Vol.23 No.5, Table 1 Composition of Al 2285 alloy, in wt pct Cu Mg Si Fe Mn Ni Zn Ti Pb Sn Al rest Fig.1 FSP process: (a) rotating tool, (b) pin is plunged into workpiece, (c) shoulder touches the surface, (d) tool traverses the plate until a fully recrystallized fine grain microstructure is obtained Table 2 Mechanical property observations Brinell Impact test Ultimate tensile Elongation Yield stress hardness (Charpy)/(J/mm 2 ) /(N/mm 2 ) /% /(N/mm 2 ) Extruded cast Al alloy Feed at 10 mm/min Feed at 12 mm/min Feed at 15 mm/min Fig.2 (a) Relationship between feed rate and Brinell hardness, (b) relationship between feed rate and impact energy

3 616 J. Mater. Sci. Technol., Vol.23 No.5, 2007 Fig.3 (a) Relationship between feed rate and tensile stress, (b) relationship between feed rate and elongation Fig.4 Relationship between feed rate and yield stress Fig.5 (a) Optical micrograph of microstructure of parent metal, (b) optical micrograph of microstructure of parent metal travel speed. This proves that, if FSP is applied at specific important locations, property improvements will be obtained enhancing the service life or performance of a structure. The ultimate tensile stress values are graphically illustrated in Fig.2(a). The value for extruded cast Al alloy was 160 N/mm 2. Comparatively the processed alloy had an increase of 20% 50%. As shown in previous research the tensile stress value peaked at an intermediate travel speed [6]. Figure 3(b) shows the elongation of the processed alloy and is found to be high in all cases. The base alloy showed an elongation of 2% to failure, due to casting defects & inhomogeneous microstructure. The elongation peaked at 10 mm/min and approximately equal at travel rate of 12 mm/min. At a feed rate of 15 mm/min the ductility dropped to 7.5% as expected from literature [5]. Figure 4 shows the yield stress graph plotted for different values of travel rate. The values though show an increasing trend compared with the parent alloy, and it gets progressively decreased as the alloy is processed with higher travel speeds. 3.2 Microstructure observation Typically, aluminum alloy castings contain porosity, segregated chemistries and inhomogeneous microstructures. Figure 5(a) is an optical micrograph of the starting metal and the arrow points out the porosity present. This undesirable feature leads to property degradation. Figure 5(b) shows the optical

4 J. Mater. Sci. Technol., Vol.23 No.5, Fig.6 (a) Stereo-optical micrograph of friction stir processed material at feed 10 mm/min, (b) at feed 12 mm/min, (c) at feed 15 mm/min, (d) optical micrograph of nugget zone at feed 10 mm/min, (e) at feed 12 mm/min, (f) at feed 15 mm/min Fig.7 Optical micrographs of (a), (b), (c) and (d) portions of nugget zone of processed material at 10 mm/min

5 618 J. Mater. Sci. Technol., Vol.23 No.5, 2007 Fig.8 Optical micrographs of (a), (b), (c) and (d) portions of nugget zone of processed material at 15 mm/min micrograph of the grain structure of the parent metal. FSP results in significant microstructural evolution within and around the stirred zone. This leads to substantial change in the post processed properties. FSP results in plastic deformation around rotating tool and friction between the tool and workpiece. The contribution of intense plastic deformation and high temperature exposure within the stirred zone during FSP results in recrystallization and development of texture within the stirred zone and precipitate dissolution and coarsening within and around the stirred zone. Following FSP, the porosity is eliminated in the processed area and a microstructure of fully recrystallized fine grains is created. The presence of equiaxed recrystallized grains as compared with parent material can be seen in the stereo-micrograph. Figure 6(a) (c) depicting the parent and processed material for different feed rates. The nugget zone (the area where tool pin and shoulder traverses on the processed alloy) having extremely refined grains due to friction stir process. The nugget zone has a basin shape in all the processing feeds of 10, 12 and 15 mm/min as can be seen in the Fig.6(d) (f). The optical micrographs of the left, right, top & bottom portions of the nugget zone for feed rates 10 and 15mm/min are depicted in Figs.7(a) and 8. It can be seen that in both the cases the grain size at the bottom of nugget is more refined than the grains at the top. It can be attributed to the fact that the bottom portion experiences a shorter thermal cycle and lesser peak temperature compared to the top portion. The micrographs Figs.7 and 8 exposes the left and right portion of the nugget zone as well as the heat affected zones of processed alloy and the grains there can be seen as smaller than the parent metal. 4. Conclusions (1) FSP is a viable method for altering the microstructure of extruded cast aluminum 2285 alloy (2) The defects present in cast aluminum alloys are eliminated on processing the alloy by the FSP process. (3) FSP alters the microstructure of extruded cast 2285 Al alloy so that property improvements are obtained. (4) With the increase in the feed rate the ultimate tensile and yield strengths show a slight decrease though the hardness value remains the same. The energy required to break the specimen also increases with the increase in the feed rate. The increase in ductility also shows a slight decrease as the feed rate increases. REFERENCES [1 ] M.W.Mahoney and S.P.Lynch: Friction Stir Processing, MW Mahoney, Manager Senior Scientist, Rockwell Scientific Co. LLC, Ca91360, USA, mmahoney@rwsc.com [2 ] P.Cavaliere: Mechanical Properties of Friction Stir Processed 2618/Al 2 O 3 /20p Metal Matrix Composite, Elsevier-Composites Part A-applied Science and Manufacturing, 21 March [3 ] C.J.Sterling. T.W.Nelson, C.D.Sorensen and M.Posada: Effect of Friction Stir Processing on the Microstructure and Mechanical Properties of Fusion Welded 304L Stainless Steel, Dept. of Mechanical Engineering, Brigham Young University, USA. [4 ] Z.Y.Ma, S.R.Sharma and R.S.Mishra: Scripta Materialia, 2006, 54(9), [5 ] P.B.Berbon, W.H.Bingel, R.S.Mishra, C.C.Bampton and M.W.Mahorey: Scripta Materialia, 2001, 44(1), 61. [6 ] Terry Khaled: An Outsider Looks at Friction Stir processing, Chief Scientific/Technical Advisor, Metallurgy, Federal Aviation Administration, 3960 Paramount Boulevard, Lakewood, CA 90712, (562) , terry.khaled@faa.gov, Feb 2005.