ACCUMULATIVE ROLL BONDING TECHNOLOGY OF ALUMINUM ALLOYS. Stefano ARGENTERO

Transcription

1 Abstract ACCUMULATIVE ROLL BONDING TECHNOLOGY OF ALUMINUM ALLOYS Stefano ARGENTERO Centro Sviluppo Materiali S.p.A., Via di Castel Romano 100, The Accumulative Roll Bonding (ARB) is an experimental severe plastic deformation (SPD) rolling procedure aimed at refining the grain structure of suitable alloys so as to increase their tensile yield strength. The process consists in rolling a couple of overlapped sheets at a given temperature and thickness reduction ratio (e.g. 50%). At suitable ARB conditions a bonding interface forms between sheets during the deformation process, due to temperature and plastic strain. The product of the first rolling cycle is cut in two similar sheets which are again overlapped and rolled again by the same procedure as the first rolling cycle. Several ARB rolling cycles can be repeated on the same work-piece. A strain hardening aluminum alloy (AA5083) has been rolled up to ten times, after optimum process window identification, so as to produce a resulting sheet comprehensive of 1024 layers. The final sample has been machined for room temperature tensile, compression, bending tests and for OM and TEM (Transmission Electron Microscope) observation. The results shown an increase of RT yield tensile and compressive stress and ultimate tensile stress and a decrease in RT maximum strain. TEM observation has shown the formation of very well defined and thin cell boundaries and, at higher scale, the formation of deformation bands parallel to the rolling direction of the sheet. Keywords: Severe Plastic Deformation, Accumulative Roll Bonding. 1. INTRODUCTION The accumulative roll bonding (ARB) is an experimental severe plastic deformation (SPD) rolling procedure, which is not employed yet as industrial metal working process, according to the author s knowledge. It is aimed at refining the grain structure of suitable metal alloys and, therefore, increasing its yield strength. It is very well known that the flow stress of a polycrystalline metal is directly related to the grain size or, more generally, to any microstructural feature describing the mean distance between dislocation obstacle (grain boundaries, high angle cell boundaries, precipitates). In comparison with other standard rolling procedures, ARB allows higher deformation degree. The process consists in rolling, after heating at a suitable temperature, two overlapped sheets to a given thickness reduction ratio (50% for instance). At the suitable ARB conditions (i.e. temperature and rolling speed) a bonding interface forms between sheets during the deformation process, due to temperature and plastic strain effect. The product of the first rolling cycle is cut in two similar sheets which are again overlapped and rolled again by the same procedure as the first rolling cycle. In the past several trials have been performed in order to investigate the behavior of a wide class of alloys under ARB process. Examples of interesting references about accumulative roll bonding are reported in [1] and [2]. The reason for the choice of AA5083 as candidate material is related to its good strain hardening potential and to its low sensitivity to thermal treatment. In fact, other precipitation hardening alloys are not compatible with the present purpose of ARB (improvement of tensile yield strength) because of their high sensitivity to thermal treatment. For these alloys the strengthening mechanism rely on a fine dispersion of precipitates more than a strain hardening controlled microstructure.

2 2. EXPERIMENTAL PROCEDURE Samples in the form of sheets with different width and length, ranging from 30 mm to 60 mm and from 300 mm to 600 mm respectively, were cut from a 2 mm thickness AA5083 H111. The chemical composition of the parent delivered sheets is 0.04 % Si, 0.24% Fe, 0.61 % Mn, 4.4 % Mg, 0.02 % Ti, 0.07 % Cr and Aluminum. After cutting, the AA5083 H111 sheet samples were cleaned with acetone on both sides and then overlapped to form several couplings. The work-pieces were so constituted by couples of two AA5083 H111 sheets joint with aluminum rivets fixed at both ends of the stack. After such preliminary procedure, several work-pieces were put into an electric furnace for heating. The preliminary thermal cycle was necessary to heat the work-piece at the suitable temperature. A reference work-piece was endowed with thermocouple, in order control the actual temperature of the work-piece. At the stabilized temperature of 350 C, each work-piece was extracted from furnace and rolled with a reduction ratio of 50%. The elapsed time between the furnace extraction of the work-piece and the rolling was about 6 seconds. The rolls were made of HS steel and were 180 mm in diameter. The rolling speed was 15 m/min (Von-Mises equivalent strain rate of approximately 7 s -1 ). After the ARB cycle the sample was immediately quenched in water to avoid an excessive grain growth during the waiting for processing of other work-pieces. After quenching each sample was cut in the length and coupled again with another in order to repeat the procedure. Several work-pieces processed with ARB technique up to ten ARB cycles have been produced. During the ARB process development, depending on the process conditions employed during numerous trials, concerns arose which can be listed below: Fracture at the border of the sheets Lack of bonding between layers Lack of planarity and straightness The ARB parameters finally adopted are the results of a trial and error procedure which led to the solution of the above mentioned process concerns. ARB-processed sheets, with ARB cycles ranging from 1 to 5, were observed in section with optical microscope in order to assess the bonding quality between different layers of the sheet and to observe the resulting microstructure. For comparison, the same metallographic inspection has been performed on AA5083-H111 (as received condition). Samples for optical microscope inspection have been extracted from the as received and ARB sheets (different number of ARB cycle), molded with resin, polished with 3 and 1 micron diamond paste and then etched with Keller s Reagent. The surface observed were transverse with respect to the rolling direction for all samples (in both ARB processed and H111 samples). Furthermore, in order to have a feedback on the influence of the process on the material mechanical behavior, 2 Kg Vickers Hardness test, were performed on samples processed with a number of ARB cycles ranging from one to ten. The hardness tests were performed on the surface transverse with respect to the rolling direction. After ARB samples were examined with XRD in order to assess the texture of the resulting work-piece after different ARB cycles. TEM samples were extracted from ten cycles ARB-processed sheets and examined on the surface parallel to the rolling plane. Tensile tests were performed on sample extracted from ten cycles ARB-processed sheets as well as from original AA5083 H111 sheet, with tensile direction parallel to the rolling direction. Compression tests were performed on sample extracted from ten cycles ARB-processed AA5083 sheets in order to assess the bonding strength of the material behavior under compressive load. The compression axis was parallel to the rolling direction.

3 3. RESULTS The ARB process conditions above mentioned resulted in a successful work-piece production in terms of geometric smoothness and bonding strength between layers. Some cracks appeared on both sides of the rolled work-piece after six ARB cycles and have been removed to hinder their propagation in the following ARB cycles. The OM micrograph of the 5083-H111, on transverse plane with respect to rolling direction, is shown in Fig. 1 while those relative to ARB cycles from 1 to 5 are shown in Fig. 2 to Fig. 6 respectively (on transverse plane with respect to rolling direction). The microstructure of the ARB processed sample shows a more globularly shaped and smaller size microstructure with respect to the as received AA5083 H111 sample. Some interface resulting from the bonding of the layers are also shown in the OM micrographs relative to ARB processed samples. The OM metallography has also shown the absence of defect within thickness of the work-piece. The visible precipitates are Al 6 (Mn,Cr) as well documented in literature [3]. Fig. 1 OM metallography. H111 (AR condition). Fig. 2 OM metallography. 1 ARB cycle. Fig. 3 OM metallography. 2 ARB cycle. Fig. 4 OM metallography. 3 ARB cycle. Fig. 5 OM metallography. 4 ARB cycle. Fig. 6 OM metallography. 5 ARB cycle. Vickers hardness test (2 Kg) on ARB-processed work-pieces shown that the material strain hardens at the ARB cycles (Fig. 7). Vickers Hardness is increased from 84 HV to 106 HV after the first ARB pass and does not seem to increase further after successive ARB passes, suggesting the attainment of a saturated microstructural state.

4 Fig. 7 Hardness test on transversal section of AA5083 specimen after every each ARB cycle. The pole diagrams resulting from XRD on (111) crystallographic plane are shown in Fig. 8 and shows the development of an equilibrium texture after the first cycles. Fig. 8 Pole diagrams relative to (111) plane of as received (H111) and ARB-processed samples after different passes (1 ARB cycle, 3 ARB cycle, 7 ARB cycle).

5 Both tensile and compression test on 10 ARB cycles samples confirm the strain hardening behavior of the material after ARB processing and are shown in Fig. 9. After ten ARB cycles Yield Stress increased from 170 MPa to 320 MPa, while the Ultimate Tensile Stress increased from 320 MPa to 380 MPa. The elongation decreased from 20% to 11%. Fig. 9 Tensile and compression test on 5 pass ARB processed AA5083 and tensile test on as received AA5083 H111. TEM micrograph of 10 cycles ARB-processed AA5083 samples are shown in Fig. 10. It can be observed the development of bands and cells which, for a wide range of aluminum alloys, are well documented in literature ([4],[5],[6]). The well-defined cell structure comes from the high dislocation mobility of aluminum alloys and from the deformation conditions i.e. temperature of the ARB. Fig. 10 TEM micrograph of a section parallel to the rolling plane (10 ARB cycles).

6 The Low Energy Dislocation Structures ( L.E.D.S, [7]), developed during deformation cycles are in turns a dynamic recovered microstructures and derive from the high stacking fault energy of aluminum alloys which allows an easy cross slip of dislocations. 4. DISCUSSION The experimental activity has shown that the effect of the process parameters (temperature, rolling speed, work-piece thickness, rolls diameter) are relevant for both macroscopic and microscopic features of the workpiece. Higher temperature and rolling speed promote the bonding strength between layers as well as the ductility of the material but depress the strain hardening effect on the resulting work-piece. The deformation temperature and strain rate also control the size of cell and cell wall developed during deformation ([7]). Higher temperature and lower strain rate encourage the development of net and thin cell walls at the expense of Taylor Lattice ([8]). Higher thickness reduction ratio promotes the bonding strength of the work-piece and the occurrence of cracks in those regions subjected to tensile mean stress i.e. the sides of the work-piece. The thickness of the work-piece as well as the rolls diameters are relevant for its chilling degree during the deformation. In fact, having fixed other ARB process parameters, the thicker is the sample and the smaller is the roll diameter, the lower is the contact area between rolls and work-piece and, consequently, the chilling degree of the work-piece during the deformation. At the above mentioned ARB conditions, which represent a compromise conditions between the discussed instances, it is possible to increase the hardness of the AA5083 with respect to the H111 condition from 84 HV to 107 HV. ACKNOWLEDGMENTS This work has been granted by the Ministero della Difesa della Repubblica Italiana, 6 Reparto, within the framework of PNRM 13/05 Project. LITERATURE [1] LONG LI, KOTOBU NAGAI AND FUXING YIN. Progress in cold roll bonding of metals. Sci. Technol. Adv. Mater. 9 (2008), , 11 pp. [2] B.L. LI, N. TSUJI, N. KAMIKAWA. Microstructure homogeneity in various metallic materials heavily deformed by accumulative roll-bonding. Materials Science and Engineering A 423 (2006) [3] J.S. VETRANO, C.A. LAVENDER, M.T. SMITH AND S. M. BRUEMMER. effect of precipitate structure on hot deformation of Al-Mg-Mn alloys. Pacific Northwest Laboratory. Richland, WA [4] I.C. HSIAO, S.W. SU, and J.C. HUANG. Evolution of Texture and Grain Misorientation in an Al-Mg Alloy Exhibiting Low-Temperature Superplasticity. Metallurgical and materials transactions a volume 31a, September [5] X. HUANG, N. KAMIKAWA, N. HANSEN. Strengthening mechanisms in nanostructured aluminum. Materials Science and Engineering A (2008) [6] D. A. HUGHES, D. C. CHRZAN, Q. LIU, N. HANSEN. Scaling of Misorientation Angle Distributions. Physical review letters 23 November 1998 volume 81, number 21. [7] DORIS KUHLMANN-WILSDORF. LEDS: Properties and effects of low energy dislocation structures. Materials Science and Engineering. Volume 86, February 1987, Pages [8] D. A. HUGHES, A. GODFREY. Dislocation structures formed during hot and cold working. Proceedings Hot Deformation of Aluminum Alloys II. Edited by T. R. Bieler, L.A. Lalli, S.R. MacEwen. The Mineral, metals & materials Society, Pages