Friction stir processing of AM60B magnesium alloy sheets

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1 Materials Science and Engineering A 462 (2007) Friction stir processing of AM60B magnesium alloy sheets P. Cavaliere, P.P. De Marco Department of Ingegneria dell Innovazione, Engineering Faculty, University of Lecce, I Lecce, Italy Received 30 August 2005; received in revised form 20 March 2006; accepted 11 April 2006 Abstract The mechanical and microstructural properties of AM60B magnesium alloy resulting from the friction stir processing (FSP) were analysed in the present study. The sheets were produced by high-pressure die casting (HPDC) into the form of trial sheets 2.5 mm thickness and then friction-stir processed. The tensile mechanical properties were evaluated at room temperature in the longitudinal direction respect to the processing one. Tensile tests were also performed at higher temperatures and different strain rates in the nugget zone parallel to the processing direction, in order to analyse the superplastic properties of the recrystallized material and to observe the differences with the parent material as a function of the strong grain refinement due to the friction stir process. The dynamic recrystallized structure of the material was observed by employing optical and electron microscopy. The high temperature behaviour of the material was studied in the parallel direction, by means of tensile tests in the temperature and strain rate ranges of C and 10 2 to 10 4 s 1, respectively. The deformation behaviour in the high temperature regime ( C) is related to the grain boundary sliding (GBS) acting in the material parallel to the tensile direction, differing strongly from the lower temperature one in which the deformation is strongly linked to grain triple junctions fracture Elsevier B.V. All rights reserved. Keywords: Friction stir processing; Grain refinement; Superplasticity 1. Introduction Magnesium alloys are strategic materials for the application in the transportation industry because of the very low density and the high room temperature strength and rigidity [1 4]. However, the use of magnesium alloys is still strongly limited because of their poor formability at near room temperature. Many results have been published on the physical and mechanical properties of AM60 magnesium alloy [5,6]. Many researchers demonstrated the existence of superplastic behaviour of many magnesium alloys and composites [3,5 9]. It is clear that it is possible to achieve superplasticity at high strain rates, in conventional alloys, by making a strong reduction in grain size; this can be obtained by using a process such as equal-channel angular pressing (ECAP), high-pressure torsion or friction stir processing (FSP) in which the samples are subjected to a severe plastic deformation leading to a strong grain refinement [10,11]. In friction stir processing, a rotating tool, with a specially designed rotating probe, travels down the surfaces of metal plates, by producing a highly plastically deformed zone through Corresponding author. Tel.: ; fax: address: pasquale.cavaliere@unile.it (P. Cavaliere). the associated stirring action. The localized thermo-mechanical affected zone (TMAZ) is produced by friction between the tool shoulder and the plate top surface, as well as plastic deformation of the material in contact with the tool [12]. The probe is typically slightly shorter than the thickness of the work piece and its diameter is typically the thickness of the work piece [13]. FSP is a solid-state process and therefore solidification structure is absent and the problem related to the presence of brittle inter-dendritic and eutectic phases is eliminated [14]. The typical nugget stirred zone and TMAZ are surrounded by a heat-affected zone (HAZ); the process results in the obtaining of a very fine and equiaxed grain structure in the weld nugget obtained through a continuous dynamical recrystallization process causing a higher mechanical strength and ductility, the strong grain refinement produced by the process lead to the possibility to exhibit superplastic properties [15 17]. Recently interesting results on multi-step FSP, performed on AZ91D alloy, have been presented, showing the strong improving in formability properties due to the stirring action [18]. 2. Experimental procedure The AM60 magnesium alloy was supplied by Norsk Hydro Ltd. (Norway) in the high-pressure die casting (HPDC) state /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.msea

2 394 P. Cavaliere, P.P. De Marco / Materials Science and Engineering A 462 (2007) under the form sheets 2.5 mm thickness. The sheets were cut and subjected to FSP by employing a threaded steel tool with a rotating speed of 700 RPM and a travelling speed of 2.5 mm s 1. The tool was rotated in the clockwise direction while the specimens, fixed to the backing plate, were moved. The nib was 2.5 mm in diameter and 2.4 mm long, and a 20 mm diameter shoulder was machined perpendicular to the axis of the tool; the tilt angle of the tool was 3. Many tensile specimens, obtained by EDM, were cut from the nugget zone parallel to the processing direction. Before testing, the surfaces of the specimens were mechanically polished in order to eliminate all the possible surface defects effects. The gauge length dimensions were 40 mm in length, 4 mm in wideness and 2 mm in thickness; the participation of unaffected base material in the cross-section was less then 5%. The tensile tests were carried out using a LLOYD Instruments LR5K testing machine equipped with a resistance furnace. Tensile tests were performed in order to evaluate the mechanical properties of the material obtained by FSP at room temperature and at higher temperatures ( C) and different strain rates (10 1 to 10 4 s 1 ). The strain rate sensitivity coefficient (m) of the material was calculated by interpolating the data obtained by tensile tests at an equivalent strain of 0.2 from the slope of the double logarithmic plot of flow stress versus strain rate. The cubic interpolation was applied between σ and ε logarithmic values. Surfaces were prepared by standard metallographic techniques and etched with acetic-picral etchant (10 ml acetic acid, 4.2 g picric acid, 10 ml water and 70 ml ethanol) for microstructural observations. 3. Results and discussion The as received material (Fig. 1a) showed a classical cast structure with floating crystals, embedded in the eutectic phase, formed by the very fast cooling. Optical microscopy was performed on the transverse crosssections of the specimens. The FSP AM60 HPDC magnesium alloy revealed the classical formation of the elliptical onion structure in the centre of the specimen. This is a structure characterized by reasonably fine and equiaxed recrystallized grains (Fig. 1b). The higher temperature and severe plastic deformation results in grains recrystallized with a strongly different structure respect to the as-cast as received material. A statistical analysis on 200 grains led to a measurement of a mean grain size of 2.5 m with a standard deviation of 0.2. At a distance of 4 mm from the centre, many of the prior grains of the parent material started to appear. This region corresponds to the heataffected zone (Fig. 1c) where the hardness is low compared to the base metal. The hardness drops here because the precipitates are coarsened [19]. In the region adjacent to the nugget, i.e. TMAZ, a partial recrystallization is observed (Fig. 1d) because the temperature derived from the friction stir processing is not high enough and the deformation is not so severe to cause complete recrystallization in the material. The room temperature tensile properties of the FSP material compared with those of the HPDC one are reported in Table 1. The FSP produces a strong increase in mechanical strength respect to the base material together with an increase in the elongation to failure. Such behaviour was due to the consolidation of voids, caused by the HPDC, produced by the stirring Fig. 1. Microstructure behaviour due to FSP: (a) parent material, (b) stirred recrystallized zone, (c) HAZ and (d) TMAZ.

3 P. Cavaliere, P.P. De Marco / Materials Science and Engineering A 462 (2007) Table 1 Room-temperature tensile properties of the studied material in both HPDC and FSP conditions Material Yield strength (MPa) Ultimate tensile strength (MPa) Elongation (%) AM60B FSP AM60B HPDC action. In Fig. 2, a typical fracture surface of the base material is shown. The material exhibits a ductile behaviour demonstrated by the presence of fine dimples but the surfaces showed a presence of very large voids produced during the casting process. The increase in tensile strength after FSP is principally due to the voids closure produced by the stirring action (Fig. 3). The true stress versus true strain curves obtained at temperatures and strain rates showed a net strong initial strain hardening up to a peak. At the lower strain rate investigated, the peak was followed by a strong flow softening at all investigated temperatures. By increasing the temperature and decreasing the strain rate, the stress peak shifts to higher strains. Fig. 3. Fracture surface of the FSP material revealing (a) the disappearance of casting defects and (b) the presence of very fine dimples. Fig. 2. Fracture surfaces of AM60B HPDC material revealing (a) presence of large voids (b) due to the presence of oxide. Fig. 4 summarizes the flow stress behaviour of the material as a function of strain rate at all the testing temperatures of the present study, under the form of a double logarithmic plot. The value used in the plot is relative to a true strain of 0.2. A superplastic typical sigmoidal behaviour of the flow stress with the initial strain rate for all the investigated temperature, identifying three different regions of superplastic deformation, was recognized. Furthermore, an increase in flow stress with increasing strain rate and decreasing temperature was observed for all the studied conditions, the sigmoidal behaviour resulting more markedly at the higher temperatures. The variation of the total elongation to failure as a function of the different initial strain rate is shown in Fig. 5. It can be seen that the tensile ductility decreases as the strain rate is increased for all the temperatures investigated reaching good levels of deformation in the high strain rate regime for the higher temperatures investigated. The fine and stable grain structure leads to the exhibition of exceptional ductility in respect to the FSP unmodified alloy. As it can be seen in Fig. 6, the strain rate sensitivity, calculated at a true strain of 1, is high at temperatures ranging from 225 to 300 C. In the higher temperature range ( C) the maximum value shifts from the lower to the

4 396 P. Cavaliere, P.P. De Marco / Materials Science and Engineering A 462 (2007) Fig. 4. Flow stress vs. strain rate for all the studied temperatures of the hot tensile tested samples: ( ) 175 C, ( ) 200 C, ( ) 225 C, ( ) 250 C, 275 C, ( ) 300 C. Fig. 6. Strain rate sensitivity values in the temperature range C: ( ) 300 C, ( ) 275 C, ( ) 250 C, ( ) 225 C. intermediate strain rates reaching values close to 0.71 at 300 C, 10 3 s 1 and drops to very small values in the low strain rate range for all the investigated temperatures. In the high temperature range, the maximum strain rate sensitivity corresponds to the second region of sigmoidal variation of flow stress. High m values, calculated in this region, demonstrated that the deformation behaviour in the high temperature regime ( C) is related to the grain boundary sliding (GBS) acting in the material (Fig. 7a) parallel to the tensile direction, differing strongly from the lower temperature one (lower m values) in which the deformation is strongly linked to eutectic phase fracture (Fig. 7b), in particular the drop in strain rate sensitivity at 225 C and low strain rates can be due to the sliding of eutectic phase. Fig. 5. Ductility values of the FSP material in all the temperature and strain rate conditions: ( ) 300 C, ( ) 275 C, ( ) 250 C, ( ) 225 C, ( ) 200 C, ( ) 175 C. Fig. 7. Field-emission-gun scanning electron microscopy image of the microstructure (in a direction parallel to the loading axis) revealing (a) grain boundary sliding at 300 C, 10 3 s 1 differing from, (b) the deformation behaviour at lower temperatures, 200 C, 10 3 s 1 and (c) in which cracking of eutectic phase was observed.

5 P. Cavaliere, P.P. De Marco / Materials Science and Engineering A 462 (2007) Conclusions The AM60B magnesium alloy was successfully friction stir processed and the material exhibited improved room temperature tensile properties respect to the as received high-pressure die cast state because of the consolidation of casting defects as a consequence of the stirring action. The high temperature tensile tests ( C) performed at different strain rates (10 4 to 10 2 s 1 ) revealed a typical sigmoidal behaviour of the flow stress with the initial strain rate for all the investigated temperatures, identifying three different regions of superplastic deformation. The tensile ductility decreases as increasing the strain rate for all the temperatures investigated reaching good levels of deformation in the high strain rate regime for the higher temperatures investigated. The fine and stable grain structure leads to the exhibition of exceptional ductility with respect to the unmodified alloy in friction stir processed condition. SEM observations showed a regime of grain boundary sliding in the regions of higher strain rate sensitivity calculated for the material in FSP condition. References [1] T.G. Langdon, Mater. Trans. 40 (1999) [2] B.L. Mordike, T. Ebert, Mater. Sci. Eng. A 302 (2001) [3] K. Máthis, Z. Trojanová, P. Lukáč, C.H. Cáceres, J. Lendvai, J. Alloys Compd. 378 (2004) [4] C.H. Caceres, J.R. Griffiths, A.R. Pakdel, J.C. Davidson, Mater. Sci. Eng. A 402 (2005) [5] A. Rudajevová, M. Stanek, P. Lukáč, Mater. Sci. Eng. A 341 (2003) [6] H. Wetanabe, T. Mukai, M. Kohzu, S. Tanabe, K. Higashi, Mater. Trans. 40 (1999) [7] M. Mabuki, K. Ameyama, H. Iwasaki, K. Higashi, Acta Mater. 47 (1999) [8] A. Bussiba, A. Ben Artzy, A. Shtechman, S. Ifergan, M. Kupiec, Mater. Sci. Eng. A302 (2001) [9] A. Galiyev, R. Kaibyshev, Scripta Mater. 51 (2004) [10] I. Charit, R.S. Mishra, Mater. Sci. Eng. A 359 (2003) [11] Z.Y. Ma, R.S. Mishra, M.W. Mahoney, Scripta Mater. 50 (2004) [12] M. Guerra, C. Schmidt, J.C. McClure, L.E. Murr, A.C. Nunes, Mater. Charact. 49 (2003) [13] P. Ulysse, Inter. J. Mach. Tools Manuf. 42 (2002) [14] C.G. Rhodes, M.W. Mahoney, W.H. Bingel, Scripta Mater. 36 (1997) [15] H.G. Salem, A.P. Reynolds, J.S. Lyons, Scripta Mater. 46 (2002) [16] C.G. Rhodes, M.W. Mahoney, W.H. Bingel, M. Calabrese, Scripta Mater. 48 (2003) [17] Y.S. Sato, M. Urata, H. Kokawa, K. Ikeda, Mater. Sci. Eng. A 354 (2003) [18] Y.S. Sato, S.H.C. Park, A. Matsunaga, A. Honda, H. Kokawa, J. Mater. Sci. 40 (3) (2005) [19] K.V. Jata, K.K. Sankaran, J. Ruschau, Metall. Trans. A 31 (2000)