Science in China Series E: Technological Sciences                                                                  www.sci...
micrometers ahead of a crack-tip[6,7]. However, at high              mm with L in the rolling direction. Boron nitride pow...
Figure 1 Microstructures of Al–Mg sheet alloys. (a) AT1; (b) AT2; (c) AT3. Arrows indicate intermetallic particles.element...
Figure 4 Photo of sample after biaxial stretch testing.Figure 3 Warm tensile testing results of AT2 samples after differen...
treatment. In contrast, for the AT2 and AT3 alloys, a                amount of coarse particles, the HIP process effective...
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!Improvement of warm formability of al mg sheet alloys containing coarse second-phase particles

  1. 1. Science in China Series E: Technological Sciences© 2009 SCIENCE IN CHINA PRESS SpringerImprovement of warm formability of Al–Mg sheetalloys containing coarse second-phase particlesHanLiang ZHU1†, Arne K DAHLE1 & Amit K GHOSH21 Materials Engineering, University of Queensland, Brisbane, QLD 4072, Australia;2 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI48109, USASeveral alloying elements involving Zr, Cu, Zn and Sc were added to Al–Mg sheet alloys in order toobtain an excellent combination of high strength and good high-temperature formability. Microstruc-tural examination showed that coarse intermetallic particles were formed in the microstructure andtheir amounts changed with variations of the alloying elements. During warm rolling of thermome-chanical treatments prior to warm deformation, the coarse particles initiated cracks, decreasing thewarm formability. For healing the crack damage and further improving the warm formability, a processof hot isothermal press was developed and optimized to the sheet alloys. With this process, the biaxialstretch formability at 350℃ was improved by 22% for an aluminum alloy containing a large amount ofcoarse particles.aluminum alloy, microstructure, second-phase particle, hot isothermal press, high-temperature formability1 Introduction strength Al–Mg alloys have received much attention[1 4]. ― A combination of high strength and good high tem-Reduction of vehicle weight is an important and realistic perature formability in aluminum alloys can be achievedmethod to reduce the CO2 emissions from cars. An ef- by refining the grain size and introducing the precipi-fective way to reduce the vehicle weight is the use of tates in the microstructure[5]. For these purposes, somealuminum alloys in automotive components. For exam- alloying elements such as Mn, Cr, Cu, Zn, Zr and Sc areple, the replacement of current steel construction by added to the aluminum alloys. However, when the con-aluminum sheet alloys for a typical automobile body-in- centrations of the alloying elements exceed their solidwhite could provide a mass reduction of 50%[1]. For solubilities in Al matrix, coarse intermetallic particlesthese structural applications in automobile, high strength can be formed in the microstructure. It is well estab-is an essential requirement to the aluminum alloys. lished that the coarse particles play a fundamental role inHowever, the increase of strength is always accompa- the ductile fracture of aluminum alloys[6]. They can nu-nied with a substantial decrease in ductility, and the am- cleate cracks and influence the growth and coalescencebient temperature ductility and formability of high- of the crack during deformation, greatly decreasing thestrength aluminum alloys are always very limited, caus- formability. The most common modes of nucleation areing the conventional manufacturing operations such as the complete or partial interfacial decohesion or therolling, forging or drawing to be difficult[1]. However, it fracture of second-phase particles. At ambient tempera-has also been documented that the poor room tempera- ture, some kinds of coarse particles like CuAl2 andture ductility of aluminum alloys can be improved by Al2CuMg can be fractured more than several hundredchanging the forming temperature to elevated tempera-tures[2]. In recent researches, material development and Received June 20, 2008; accepted August 21, 2008 doi: 10.1007/s11431-008-0275-6deformation behavior at elevated temperatures of high- † Corresponding author (email: Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45
  2. 2. micrometers ahead of a crack-tip[6,7]. However, at high mm with L in the rolling direction. Boron nitride powdertemperature, the stress undergone by the particles is was used as the lubricant and it was sprayed on themuch smaller and damage only nucleates by parti- blanks and baked to a dry condition. Forming tests werecle/matrix decohesion allowed by the accumulation of performed on a heated rectangular die-punch device,plastic strains along the interfaces[6,8]. Moreover, if the which was mounted on an Instron-1116 testing machinesecond-phase particles have a rounded shape or higher with 250 kN capacity. Biaxial stretch tests were con-fracture strength, cracks also usually initiate at parti- ducted at 350℃ and a fixed cross-head speed of 10cle/matrix interface[6,7]. mm/s. The depth of the formed part was calculated ac- The interface decohesion between particle and matrix cording to load vs punch displacement curves at peakcan also occur during heavy rolling of thermomechani- load, where necking occurred on the sheet. This partcal treatments in order to manufacture the sheet product. depth was used as a measure of biaxial formability.The pre-exhibiting interface defects can rapidly growand coalesce during the following forming deformation, 3 Results and discussionresulting in early failure of the sheet components. In aprevious study annealing treatment at high temperature Second-phase particles were formed in all the three alu-of 550oC was used to heal the particle/matrix interface[8]. minum alloys. The microstructure of the three alloys isHowever, it was found that defect-free interfaces might illustrated in Figure 1. It can be found that a largerbe obtained by long annealing time, but the possibility amount of second-phase particles are observed in theof loss of certain alloying elements due to evaporation microstructure of the AT2 alloy than in that of the AT1and/or grain coarsening was a practical problem[8]. In alloy due to the additions of Zr, Cu and Zn. The additionthis study, a process of hot isothermal press (HIP) was of 0.3 wt% Sc to the AT2 alloy results in a great increasedeveloped for the sheet alloys. It is supposed that under in the number and size of the coarse particles in the AT3the effect of high pressure the annealing time and tem- alloy (Figure 1(c)). Also, two kinds of second-phase par-perature can be dramatically decreased and the devel- ticles are observed: coarse cuboidal shape with a sizeoped HIP process should increase the high-temperature around 5 μm in the AT2 and 10 μm in the AT3 and fineformability. rounded shape with a size up to 2 μm in all the three alloys. The coarse particles with a cuboidal shape were2 Materials and experimental procedure identified as Ti-enriched Al3Zr in the AT2 alloy or Zn- and Mg-enriched Al3(Zr, Sc) phase in the AT3 alloy. TheThree Al–Mg sheet alloys with variations of the alloying fine rounded particles in the three alloys were found toelements were prepared in this study: be Fe-enriched intermetallic particles. Al-4.5Mg-0.9Mn-0.2Cr-0.2Fe-0.1Si (AT1), From Figure 1, it can also be found that the grain Al-3.5Mg-0.2Mn-0.2Cr-0.2Fe-0.1Si-0.2Zr-0.5Cu-0.7Zn structures have a great change due to the additions of the (AT2), AT2+0.3Sc (AT3), alloying elements. As shown in Figure 1(a), in the AT1all compositions are in wt%. The as-received (hot-rolled) alloy, most grains exhibit an equiaxed shape, indicatingalloy sheet has a thickness of 8.0 mm. In order to pro- a fully recrystallised state, whereas in the AT2 alloy withduce thin sheet products, they were submitted to the the additions of Zr, Cu and Zn (Figure 1(b)), the grainsthermomechanical treatments including warm rolling at are slightly elongated and have a smaller grain size than180℃ to a final thickness of 1.0 mm and annealing at alloy 1. With further adding Sc to the AT2 alloy, the450℃ for 60 min. For hot isothermal pressing, the an- grains in the AT3 alloy (Figure 1(c)) are severely elon-nealed sheets were then put into a designed tool that gated along the rolling direction, which is typical of de-could prevent the extension of the sheets under high formed grain structures of aluminum alloys[9]. Also, thepressure and the elevated temperature. grain thickness is much smaller in the AT3 alloy than Tensile specimens were cut along the rolling and those in the other two alloys. The additions of Zr, Cu, Zntransverse directions. Warm tensile tests were carried out and Sc to the Al–Mg alloy can form solid solutions inat 350℃ and 0.015 s−1 on an Instron 4505 testing ma- the α-Al matrix, resulting in a solute drag effect on grainchine. For the biaxial stretch tests, the sheets were cut to boundary migration during thermal exposures and de-rectangular blank samples of size of L×W =200 mm×140 formation. Also, when the concentrations of the alloying42 HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45
  3. 3. Figure 1 Microstructures of Al–Mg sheet alloys. (a) AT1; (b) AT2; (c) AT3. Arrows indicate intermetallic particles.elements exceed their solid solubilities in the alloy, theintermetallic particles can be formed, limiting the graingrowth during the thermal exposures. Both the solutes insolid solutions and particles can retard the recrystalliza-tion process during the thermomechanical treatments,resulting in refined grain size and elongated grain shape.Especially, Sc has a stronger anti-recrystallization effecton aluminum alloys[10] and a larger amount of coarseintermetallic particles promote the particle stimulatednucleation (PSN), resulting in much thinner elongatedgrains and fine grain zones around the particles[5]. Figure 2 SEM morphology of cracks around a second phase particle During warm rolling of thermomechanical treatments after thermomechnical treatments. Arrow shows the crack.the second-phase particles can nucleate cracks[6]. In thisstudy, Fe-rich intermetallic particles have a rounded the larger particles distort the elongated grains. Thus,shape, the Al3(Sc, Zr) and Al3Zr have a high fracture coarser second-phase particles are more harmful to thestrength and the rolling deformation also occurs at the deformation than the finer ones. The pre-exhibitingwarm temperature. According to a previous research[6], cracks can result in rapid growth and coalescence of theinterface decohesion should be the major mode of crack cracks during the subsequent warm deformation, result-nucleation. Figure 2 shows the SEM morphology of a ing in inferior warm formability.crack around a second-phase particle after the ther- A process of hot isothermal press (HIP) was devel-momechnical treatments. It is evident that cracks initiate oped for the sheet alloy to heal the pre-exhibiting cracksat the particle/matrix interface. Moreover, cracks usually prior to warm deformation. A special die was used tonucleate first on larger particles that contain larger very limit the extension of the sheet during the HIP process.small scale internal or interface defects. Larger particles In order to optimize the process, samples of AT2 werealso induce more rapid decohesion with increasing strain submitted to the HIP treatment at different conditionsbecause the constrained zone of plasticity at the inter- and then to warm tensile testing. Figure 3 illustrates theface becomes larger[8]. As shown in Figures 1(c) and 2, true stress-true strain curves of the AT2 samples. With- HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45 43
  4. 4. Figure 4 Photo of sample after biaxial stretch testing.Figure 3 Warm tensile testing results of AT2 samples after different HIPprocesses.out the HIP treatment, the recrystallized sheet exhibitedthe highest stress level and the lowest fracture strain,indicating the worst warm formability. In contrast, with5 T press for 60 min at 450℃, the fracture strain wassomewhat increased but the flow stress was greatly de-creased. With further increasing the press to 25 T, thefracture strain was increased by 17% compared to thesample without the HIP treatment. However, with fur-ther extending the HIP process and increasing the press,the fracture strain could not be further increased but thestress level was greatly decreased. Thus, the process of25 T at 450℃/60 min was an optimal condition for Figure 5 Biaxial stretch testing results of investigated aluminum sheet alloys.healing the pre-exhibiting crack damage in this study. The sheets of the three alloys were pressed under the two alloys. This can be explained as the following. Dur-optimal condition, and then submitted to biaxial stretchtesting. The biaxial stretch test can examine the overall ing the thermal exposure of the HIP process at 450℃ductility of the sheet, which always exhibits anisotropic for 60 min, recovery and recrystallization processes thatplasticity between the rolling and transverse direc- were not completed during the annealing process couldtions[11,12]. In the biaxial stretch tests, a rectangular continue to occur. Also, the growth of the recrystallisedcup-shaped part was produced. Figure 4 shows a photo grains could occur, resulting in grain coarsening. Theof the sample after the biaxial stretch test. The formabil- recovery and recrystallization processes should be bene-ity of the sheet was evaluated by part depth defined as ficial to the warm formability of the aluminum alloysthe maximum punch penetration before a crack initiated. whereas the grain coursing should have a negative effect.For comparison, the sheets without the HIP treatment On the other hand, the pre-exhibiting cracks were closedwere also tested under the same condition. The part and diffusion bonding could take place, healing thedepth of the three alloys with or without the HIP treat- pre-exhibiting crack damage, which should give a greatment is shown in Figure 5. For samples without the HIP contribution to the increase of the warm formability oftreatment, the AT1 alloy that contained only a few in- the aluminum alloys. For the AT1 alloy, the recrystal-termetallic particles obtained the largest part depth. This lized grains can rapidly grow with less restriction of theis easily understood that a smaller amount of solute ele- solute elements and second-phase particles. The benefitments and coarse particles gave this alloy a better ductil- of healing a smaller number of cracks cannot compen-ity. However, after the HIP treatment, the part depth de- sate the loss due to grain coarsening of the bulk grains,creased for the AT1 alloy, while it increased for the other resulting in a decrease in the part depth after the HIP44 HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45
  5. 5. treatment. In contrast, for the AT2 and AT3 alloys, a amount of coarse particles, the HIP process effectivelylarge amount of pre-exhibiting cracks were healed by the enhances their biaxial formability due to healing theHIP process and the recovery and recrystallization proc- crack and continuing the recovery and recrystallizationesses during the HIP process improved the grain struc- processes. Therefore, the HIP process can be used fortures for deformation. Also, the grain coarsening is lim- further improving the warm formability of aluminumited by the larger amount of solute elements and sec- sheet alloys containing large amount of intermetallicond-phase particles. Therefore, the part depth is greatly particles.increased. In addition, the increase in the part depth forthe AT2 alloy is larger than that for the AT3 alloy that 1 Mcnelley T R, Ishi K O, Zhilyaev A P, et al. Characteristics of thecontains much more and much coarser intermetallic par- transition from grain-boundary sliding to solute drag creep in super-ticles. The reason is that much larger amount of coarse plastic AA5083. Metal Mater Trans A, 2008, 39(1): 50―64 2 Li D, Ghosh A K. Tensile deformation behavior of aluminum alloysintermetallic particles still rapidly nucleate cracks during at warm forming temperatures. Mater Sci Eng A, 2003, 352(1-2):the warm deformation and thus limit the benefit of the 279―286HIP process. At the optimal HIP process, the warm 3 Li D, Ghosh A K. Biaxial warm forming behavior of aluminum sheetformability of the AT2 alloy has been improved by 22%. alloys. J Mater Process Technol, 2004, 145(3): 281―293Therefore, the HIP process can be applied as one step of 4 Taleff E M, Nevland P J, Krajewski P E. Tensile ductility of severalthe thermomechanical treatments before the forming commercial aluminum alloys at elevated temperatures. Metal Materoperations for further improving high temperature Trans A, 2001, 32(5): 1119―1130 5 Cahn R W, Haasen P. Physical Metallurgy. Amsterdam: Elsevierformability of aluminum sheet alloys containing a large Science B.V., 1996. 2106amount of coarse particles. 6 Lassance D, Fabregue D, Delannay F, et al. Micromechanics of room and high temperature fracture in 6xxx Al alloys. Prog Mater Sci, 2007,4 Summary 52(1): 62―129 7 Toda H, Kobayashi T, Takahashi A. Mechanical analysis of toughnessThe addition of Zr, Cu and Zn to an Al–Mg sheet alloy degradation due to premature fracture of course inclusions in wroughtresults in the formation of a large amount of coarse in- aluminium alloys. Mater Sci Eng A, 2000, 280(1): 69―75termetallic particles in the microstructure. Further add- 8 Bae D H, Ghosh A K. Cavity formation and early growth in a super-ing 0.3 wt% Sc greatly increases the number and size of plastic Al–Mg alloy. Acta Mater, 2002, 50(3): 511―523the intermetallic particles. During warm rolling of the 9 Liu J, Morris J G. Recrystallization microstructures and textures in AAthermomechanical treatments, the coarse particles can 5052 continuous cast and direct chill cast aluminum alloy. Mater Scinucleate cracks at the particle/matrix interface, decreas- Eng A, 2004, 385(1-2): 342―351 10 Røyset J, Ryum N. Scandium in aluminium alloys. Int Mater Rev,ing warm formability. In order to heal the pre-exhibiting 2005, 50(1): 19―44crack damage, a process of hot isothermal press was 11 Koike J, Ohyama R. Geometrical criterion for the activation of pris-developed and optimized. For the aluminum alloy that matic slip in AZ61 Mg alloy sheets deformed at room temperature.contains smaller amount of particles, the HIP treatment Acta Mater, 2005, 53(7): 1963―1972deteriorates its warm formability due to grain coarsening. 12 Hosford W F, Caddell R M. Metal Forming: Mechanics and Metal-However, for the aluminum alloys that contain larger lurgy. 2nd edition. Englewood Cliffs: PTR Prentice Hall, 1993. 309 HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45 45