The Effect of Friction Stir Processing and Subsequent Rolling on ... - TMS

Friction Stir Welding and Processing III Edited by K.V. Jata, M.W. Mahoney, R.S. Mishra, and T.J. Lienert TMS (The Minerals, Metals & Materials Society), 2005

THE EFFECT OF FRICTION STIR PROCESSING AND SUBSEQUENT ROLLING ON THE SUPERPLASTIC BEHAVIOUR OF ALUMINIUM ALLOYS Stavros Katsas, Graham Todd, Martin Jackson, Richard Dashwood, Roger Grimes Imperial College London, Department of Materials, South Kensington Campus, London SW7 2AZ, UK Keywords: Friction Stir Processing-Welding, Rolling, Superplasticity, Al-Mg-Zr alloys Abstract Friction stir processing (FSP) can dramatically reduce grain size conferring excellent superplastic behaviour in certain aluminium alloys. FSP of thick plate followed by rolling to sheet could potentially be used as a method to improve performance of established superplastic alloys or to induce superplastic behaviour in alloys not normally associated with this phenomenon. An extruded Al-Mg-Zr alloy was FSP’ed prior to rolling to sheet. The development of microstructure and superplastic behaviour was characterised using a combination of light, scanning, orientation imaging, and transmission microscopy, coupled with hot uniaxial tensile testing. FSP transformed the coarse, highly textured extruded, structure into a very fine (approximately 600nm) randomly orientated, equiaxed material. The structure of the material after rolling and thermal treatment was complex and in certain cases significant grain coarsening resulted. A detailed investigation of factors responsible for this will be described. Introduction Work has been carried out in Imperial College towards the development of an Al-4Mg-1Zr alloy with enhanced superplastic properties for automotive applications [1, 2]. The alloy is produced with a simple process (particulate casting-hot extrusion-cold rolling) and the resulting sheet exhibits elongations >600% at a strain rate of 10-2s-1. However, recognising that sheet derived from an extrusion by rolling is likely to be narrower than that required for the forming of a commercial car panel, alternative routes are being investigated. Recently, it was demonstrated that if the extruded Al-4Mg-1Zr alloy was friction stir processed, excellent superplastic performance resulted with ductilities up to 1280% [3] and exhibited superior performance than sheet rolled from the same extrusion. Based on these findings, the possibility of employing friction stir welding (FSW) has been considered for the joining of two or more blanks, which would subsequently be cold rolled to produce wider sheet required by the industry. To assess the potential of this route and the superplastic properties of the end product, Al-4Mg-0.75Zr and Al-5Mg-1Zr alloys (part of the same development programme with equivalent SPF properties) were friction stir welded and subsequently rolled. As no previous records of a rolled FSW are available, the microstructural changes introduced by rolling and the effect the additional deformation has on the superplastic properties are reported in this paper. Experimental The processing parameters of the examined material are summarised in table 1. The microstructure in both the FSW and parent metal was characterised prior to and post rolling using light, scanning, orientation mapping (EBSD) and transmission electron microscopy (TEM). The superplastic performance was assessed by hot uniaxial tensile testing using conditions determined in previous work [1]. 291

Table 1: Processing parameters and rolling schedule of the as extruded Al-4Mg-0.75Zr and Al5Mg-1Zr. Alloy

Extrusion Temp ( oC )

Extrusion Size & Ratio

Al-4Mg-0.75Zr

550

20mm x 10mm 5:1

525

40mm x 18mm 10:1

Al-5Mg-1Zr

Heat-Treatment part of the alloy for 100 hrs at 360°C before rolling

none

Rolling (Reduction) cold (82%) partially hot rolled at 500°C to meet welding jig requirements. Hot & cold rolled after welding (85%)

Results and Discussion As-Extruded Al-4Mg-0.75Zr & Al-5Mg-1Zr Material Before investigating the friction stir welded microstructure, an introduction to the asextruded parent metal alloys is necessary. Using a commercial particulate casting process, developed by the Aluminium Powder Company (ALPOCO) and the high solidification rates it employs (102-103 oC/s-1), Zr is largely retained in supersaturated solid solution. The resultant particles are consolidated using hot extrusion before rolling to sheet. During earlier phases of this development it had been demonstrated that when the Zr content was 500°C, heat treatment did not improve superplastic performance. Thus, the heat treatment schedule of 100 hours at 360°C was applied to part of the Al-4Mg-0.75%Zr material but not the Al-5Mg-1%Zr material. Extruding at 550°C resulted in a partially recrystallised state; fine submicron grains and coarser (2-3µm) recrystallised grains coexist with elongated ones (Figs.1a and 1b). The average grain size, calculated using the linear intercept method, was approximately 2µm. Comparing the process with conventionally cast Zr containing aluminium alloys where the precipitation of Zr from solid solution is achieved with thermal treatments, precipitation in this alloy was shown to take place during extrusion [1,2]. In the examined samples, two forms of precipitation were observed; (i) rod semi-coherent fan arrangements of Al3Zr particles, triggered by enhanced grain boundary migration due to a combination of heat and strain during extrusion (Fig.1c); and (ii) spherical-coherent (550nm) particles, as a result of the elevated extrusion temperature only. Extensive TEM observations showed the former to be the predominant precipitation morphology. The coarser particles (>10nm) of the latter precipitation morphology were generally observed at grain and subgrain boundaries. In Fig. 1a and b, apart from the fine submicron grains, coarser grains were also observed. A TEM study focused on such areas reveals the presence of larger Al3Zr particles at the expense of the fine-spherical. Examination of the heat treated microstructure (100hrs at 360°C), illustrated that further precipitation and dispersion had taken place. Transformation from (coherent) cubic to (incoherent) tetragonal Al3Zr also occurred, as needle-shaped precipitates were observed at grain boundaries (Fig. 1d). Effect of Friction Stir Welding After extrusion two blanks from each alloy were friction stir welded producing a sample of approx. 40mm width by 10mm thick. The welding process introduces large amounts of deformation and heat due to friction between the tool and the workpiece. The resultant microstructure is shown in Fig. 2 and it is clear that dynamic recrystallisation has occurred. Such observations have been widely reported in the literature for several conventional aluminium alloys during FSW [5-10]. In Fig.2a, a grain size of ~1µm is observed coupled with a high dislocation density. Due to the deformation during friction stirring, the fan arrangements of Al3Zr particles were broken up into smaller constituents. One must be careful, however, because as the thermal treatment study showed (Fig. 1d), raising the temperature will unavoidably coarsen and 292

transform some of the existing Al3Zr to tetragonal. Furthermore, while examining the asextruded microstructure with TEM, there was also evidence that FSW had redistributed the precipitates and contributed towards a more uniform Al3Zr dispersion (Fig. 2b). When the welded material was subjected to the Zr precipitation thermal treatment that improves the SPF ductility of the parent metal, significant Al3Zr coarsening occurred, with particles in the range of 300nm being observed. Taking into account the higher dislocation density seen in the weld even after the thermal treatment, it is suggested that the more uniform Al3Zr distribution effectively blocks dislocation movement. However, the particle coarsening also implies that a high dislocation density in the alloy will offer paths for the slow diffusing Zr during the thermal treatment.

Figure 1: a) Bright field TEM micrograph of the as extruded Al-4Mg-0.75Zr (section parallel to the extrusion direction); b) EBSD map of the as extruded Al-4Mg-0.75Zr (all sections) with {111} pole figure; c) Bright field micrograph of the Al3Zr fan-shaped arrangements in the extruded Al-4Mg0.75Zr; and d) Bright field micrograph, showing Al3Zr coarsening and needle-shaped precipitates at grain boundaries after 100hrs at 360oC.

Figure 2: a) Low and b) Higher magnification bright field micrograph of the as extruded and FSW’ed Al-4Mg-0.75Zr, showing also the Al3Zr distribution.

Superplastic Performance after Rolling Detailed descriptions of the superplastic capability of the Al-4Mg-1Zr alloy in both as extruded [3] and rolled variants has been described elsewhere [1, 2]. A brief reference to these earlier results is, nevertheless, necessary for the explanation of the performance of the present material after rolling. Ma et al [3] reported that the parent metal (Al-4Mg-1Zr) exhibited an optimum SPF ductility of 1015% at 580oC and at a strain rate of 10-2s-1. After FSW a ductility of 1280% at higher strain rate (10-1s-1) and lower temperature (525oC) was 293

achieved. This difference in the optimum conditions probably derives from the minimum energy requirement (either as temperature or as dislocations) to initiate dynamic recrystallisation that will refine the grain size. Referring to Figs. 1a and 1b, it can be seen that after hot extruding, the cast structure partially recrystallised and so the stored energy in the form of dislocations is limited compared to the weld. Therefore to achieve the parent metal optimum SPF conditions a higher heat input will be required to initiate dynamic recrystallisation, as opposed to the weld region where the dislocation density is higher and an equiaxed structure already exists. Similarly, the rolled Al-4Mg-1Zr has optimum ductility (620%) at an even lower temperature (500oC) than the friction stir processed material [1-3]. In Fig. 3a, the SPF ductilities of the Al-4Mg-0.75Zr and the Al-5Mg-1Zr alloys are displayed for both parent and weld metal after rolling. While, material rolled from the parent metal exhibited superplastic behaviour, none of the material rolled from the weld region was superplastic. In Fig. 3b, the interface ductility for the cold rolled Al-4Mg-0.75Zr is examined, showing the SPF deterioration during the transition from the parent metal to the weld. It is also interesting to note that the hot rolled Al-5Mg-1Zr weld exhibits better ductility than the cold rolled. According to the above energy requirement assumptions, it is probable that the hot rolled material would exhibit better SPF properties under different conditions but lack of material did not allow further testing. It should be noted that direct comparison between the ductilities quoted from Ma et al. [3] and figure 3 is not possible as these were achieved with significantly different geometry specimens. (a)

(b)

Figure 3: a) Superplastic ductility tests performed at the optimum SPF conditions (500oC-10-2s-1 [1]) for parent metal and FSW from cold rolled Al-4Mg-0.75Zr and cold and hot rolled Al-5Mg-1Zr, b) Comparison of the superplastic performance in transition from the parent metal to the FSW for the cold rolled Al-4Mg-0.75Zr.

Microstructural Characterization after Rolling To explain the results of Fig. 3, orientation mapping and TEM as well as x-ray diffraction analysis were performed. Samples from rolled parent metal and weld for all three conditions described in Fig. 3a were examined after simulating the thermal cycle of the SPF test (20 minutes at 500oC + water quenching). Examination of the microstructure of the Al-4Mg-0.75Zr alloy after rolling and annealing at the forming temperature, revealed significant grain growth in the weld area (elongation 28%-Fig. 3a) while in the parent metal the microstructure remained stable (elongation 400%-Fig. 3a). In the parent metal regime, the microstructure was similar, in terms of grain size, to that seen in the friction stir welded material (Figs. 2a and 4a). The fan-shaped arrangements of Al3Zr are dispersed during rolling into smaller arrangements. Additionally, in Fig. 4a, despite the high temperature treatment, a high dislocation density and well defined subgrains could be observed. This can be attributed to the presence of the cubic Al3Zr particles, which have managed to pin the dislocations and hinder any changes. Selected area diffraction patterns (SADPs) showed that the Al3Zr succeeded in the task, even though coarse tetragonal particles were also present, which could result in particle stimulated 294

nucleation (PSN). The EBSD map and the corresponding {111} pole figure in Fig.4b show that after annealing the parent metal had not fully recrystallised and maintained the rolled texture, thus consisting of elongated grains with a cellular substructure. In Fig. 4c, a bright field micrograph taken from the interface of the parent metal and the weld revealed the significant difference in grain size. In FSW region (Fig. 4d and 4e), grain growth has consumed the dislocations. It seems that a possible explanation for the microstructural differences between parent metal and weld is that the combination of high temperature and strain developed during FSW (of the as extruded material) provide an increased susceptibility to the tetragonal transformation. A factor contributing towards this may also be that the high dislocation density offers coarsening paths for the slow diffusing Zr to coalesce into larger particles during forming.

c

Figure 4: a) Bright field micrograph and b) corresponding EBSD map with {111} pole figure of the parent metal (Al-4Mg-0.75Zr) after rolling and annealing for 15min at 500oC, c) Bright field micrograph of the interface of the parent metal and the weld and d) Bright field micrograph and e) corresponding EBSD map with {111} pole figure of the weld (Al-4Mg-0.75Zr) after rolling and annealing for 15min at 500oC.

To quantify the effect of FSW and subsequent rolling on the Al3Zr morphology, particle analysis was performed by measuring their diameter from TEM micrographs and also by using x-ray diffraction [11]. The average particle size derived by measuring the precipitates (Fig. 5b) was 12nm and 14nm for the as-extruded parent metal and weld respectively. After cold rolling and annealing at the forming temperature, this pattern was repeated, albeit the particle sizes increased to 22nm in the parent metal and 24nm in the weld. Fig. 5a, illustrates that subsequent rolling and annealing shifts the maximum peak in larger particle sizes. Dark field TEM was employed to determine the transition range to tetragonal phase. This was found to be close to 100nm. Despite the peak shifting to larger sizes (Fig.5a), tetragonal particles were only found post rolling. The results from the two methods employed are compared in Fig.5b. Despite the differences between the two, an increasing trend was recorded from the parent metal to the weld and from the extruded to the rolled-annealed material and in both cases the particle size doubled after rolling and annealing. Grain growth phenomena, like those seen in the weld nugget after rolling, have also been reported for a 7010 aluminium friction stir welded alloy [12]. There, the available secondphase particles, including the Al3Zr coming from the minor zirconium additions, were not sufficient to pin the structure during subsequent solution treatment. The explanation for the

295

growth in the weld region of the Al-Mg-Zr alloys, despite the high zirconium content, is probably similar and is a combination of stored energy and Al3Zr coarsening. (a)

(b)

Figure 5: a) Distribution and average size of cubic Al3Zr measured using quantitative TEM in the Al-4Mg-0.75Zr alloy and b) Comparison of average cubic Al3Zr as calculated from the two methods (Quantitative TEM – Fig.5a and XRD analysis) in the Al-4Mg-0.75Zr alloy.

Material from the weld region from both alloys subjected to the SPF thermal cycle prior to rolling exhibited a very fine and stable submicron grain structure. After rolling, however, and applying the simulation of the SPF thermal cycle, the grain size increased from ~1µm to 30µm. Obviously rolling will increase the dislocation density and the grain boundary area. According to Ma et al [13] the difference in the ductility of the as extruded parent metal and friction stir processed region can be attributed to the higher percentage of high angle grain boundaries in the FSP microstructure. Fig. 1b supports this claim but it also shows significant recrystallisation has occurred in the extrusion. When this material was rolled, the resulting microstructure consisted of elongated grains and a cellular substructure denser than that in the extrusion. It is assumed that the combination of high temperature and strain during superplastic forming in the parent metal triggers dynamic recrystallisation giving an equiaxed grain structure and good superplastic properties, nevertheless lower than the as extruded and friction stir processed region. The implication is that an equiaxed microstructure, even strain free, will give superior SPF performance compared with an equiaxed microstructure produced via dynamic recrystallisation during SPForming. In the latter case low angle grain boundaries which will not participate in grain boundary sliding (GBS) will unavoidably be present. Based on this assumption the lower ductility in the as rolled material might be explained. 296

The friction stir welded material, prior to rolling, possessed a fine, equiaxed, high angle grain boundary microstructure. It should be noted that the FSW grain structure, resulting from dynamic recrystallisation, has a higher dislocation density than the parent metal (Figs. 1 and 2). After rolling, the grains will also be elongated but smaller than that of the parent metal. Taking into account that the dislocation density in the weld area is greater (FSW+cold rolling) and the grain size is smaller, the subgrain size should also be smaller. Therefore the superplastic ductility after dynamic recrystallisation occurs should be improved. The results of Fig. 3, clearly show the opposite. The above hypothesis would probably apply, had no particle coarsening taken place. Even though FSW does not result in significant particle coarsening, it appears that the higher dislocation density has a significant influence on the performance. After coarsening during annealing at the forming temperature (Fig. 3), the Zener pinning capability is reduced and grain coarsening resulted. In the light of the somewhat surprising obtained after cold rolling the FSW material, limited tests were performed on hot rolled Al-5Mg-1Zr. In Fig. 6a and 6c, grains and their corresponding SADPs are shown from the parent metal of cold rolled Al-4Mg-0.75Zr and hot rolled Al-5Mg-1Zr. In both alloys a cellular substructure was preserved after annealing by the fine Al3Zr even though a small number of tetragonal particles were present. Figs 6b and d demonstrate that passing from parent metal to weld, the volume fraction of tetragonal particles increased. The main difference between the cold rolled Al-4Mg-0.75Zr weld and the hot rolled Al-5Mg-1Zr weld was that in the former all the grains have grown with a resulting average size of ~30µm, while in the latter the majority of the microstructure was fine apart from pockets of individual coarse grains (15-20µm) (Fig. 6d). The limited grain growth in the Al-5Mg-1Zr hot rolled at 500oC was probably a consequence of the lower dislocation density generated by the hot rolling so that only few grains coarsened. In terms of superplastic ductility, this translated into an increase from the 80% of the cold rolled to 140% in the hot rolled.

Figure 6: a) Bright field TEM micrograph of the Al-4Mg-0.75Zr parent metal after cold rolling and annealing, b) Dark field micrograph of the Al-4Mg-0.75Zr weld after cold rolling and annealing, c) Bright field micrograph of the Al-5Mg-1Zr parent metal after hot rolling at 500oC and annealing and d) Bright field micrograph of the Al-5Mg-1Zr weld after hot rolling at 500oC and annealing.

297

Conclusions This paper describes preliminary investigation of the influence of rolling on the superplastic performance of friction stir welded material. The explanations proposed are therefore based on limited data and are tentative. Nevertheless, the following tentative conclusions are drawn: • Al-Mg-Zr alloys can exhibit very good superplastic behaviour in the as extruded condition even with relatively modest extrusion ratios. • The as-extruded microstructure contained a significant proportion of recrystallised grains. • As expected, the microstructure of the friction stir welded region comprised of dynamically recrystallised equiaxed fine grains. • In contrast with the extruded parent metal, cold rolling of the friction stir weld resulted in total loss of the superplastic capability. • Simulation of the SPF thermal cycle on cold rolled welded material resulted in major coarsening of the Al3Zr particles but little change in the cold rolled parent metal. • Simulation of the SPF thermal cycle on cold rolled welded material resulted in major grain growth but had very little effect on the cold rolled grain structure of the parent metal. • Hot rolling of the welded material greatly reduced grain coarsening when subjected to the simulated SPF thermal cycle and resulted in significantly improved ductility on hot tensile testing. • Hot rolling of the parent metal resulted in greatly reduced ductility on hot tensile testing compared with the cold rolled parent metal. • It is suggested that the observed coarsening is the result of a higher dislocation density in the weld compared with that in the parent metal. This implies that there is an upper limit to the energy/dislocation density above which the Al3Zr size and distribution cannot effectively stabilise the microstructure at SPF temperatures. Acknowledgements Friction stir welding of the Al-4Mg-0.75Zr by Rockwell Scientific (Dr. M.W. Mahoney), and of the Al-5Mg-1Zr by TWI Ltd (N. Dodsworth) is gratefully acknowledged. References 1. R. Grimes, R.J. Dashwood, A.W. Harrison, H.M. Flower: Mater. Sci. Technol., 16 (2000) p.1334. 2. R.J. Dashwood, R. Grimes, A.W. Harrison, H.M. Flower: Mater. Sci. Forum, 357-359 (2001) p. 339. 3. Z.Y. Ma, R.S. Mishra, M.W. Mahoney and R. Grimes: Mat. Sci. Eng. A, 351 (2003) p.148. 4. A.W. Harrison, ‘High Strain Rate Superplasticity of Aluminium-Magnesium alloys’ (PhD Thesis, Department of Materials-Imperial College London, 2003). 5. C.G. Rhodes, M.W. Mahoney, W.H. Bingel, R.A. Spurling, C.C. Bampton: Scripta Mater 36 (1997) p.69. 6. G. Liu, L.E. Murr, C-S. Niou, J.C. McClure, F.R. Vega: Scripta Mater 37 (1997) p.355. 7. L.E. Murr, G. Liu, J.C. McClure: J Mater Sci 33 (1988) p.1243. 8. M.W. Mahoney, C.G. Rhodes, J.G. Flintoff, R.A. Spurling, W.H. Bingel.: Metall Mater Trans 29A (1998) p.1955. 9. Y.S. Sato, H. Kokawa, M. Enomoto, S. Jogan: Metall Mater Trans 30A (1999) p. 2429. 10. K.V. Jata, K.K Sankaran, J.J. Ruschau: Metall Mater Trans 31A (2000) p. 2181. 11. B.D. Cullity, Elements of X-Ray Diffraction, 2nd edition, Addison-Wesley Publishing Company, 1978, p.411. 12. K.A. Hassan, A.F. Norman, D.A. Price, P.B. Prangnell: Acta Mater. 51 (2003) p.1923. 13. Z.Y. Ma, R.S. Mishra, M.W. Mahoney, R.Grimes, ‘Effect of friction stir processing on the kinetics of superplastic deformation in an Al-Mg-Zr alloy’, Mat.Sci.Eng. A (2004)- to be published. 298