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Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45

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Improvement of warm formability of Al–Mg sheet alloys containing coarse second-phase particles

HanLiang ZHU1†, Arne K DAHLE1 & Amit K GHOSH2 1 Materials Engineering, University of Queensland, Brisbane, QLD 4072, Australia; 2 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI48109, USA

Several alloying elements involving Zr, Cu, Zn and Sc were added to Al–Mg sheet alloys in order to obtain an excellent combination of high strength and good high-temperature formability. Microstruc-tural examination showed that coarse intermetallic particles were formed in the microstructure and their 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 the warm formability. For healing the crack damage and further improving the warm formability, a process of hot isothermal press was developed and optimized to the sheet alloys. With this process, the biaxial stretch formability at 350℃ was improved by 22% for an aluminum alloy containing a large amount of coarse particles.

aluminum alloy, microstructure, second-phase particle, hot isothermal press, high-temperature formability

1 Introduction

Reduction of vehicle weight is an important and realistic method to reduce the CO2 emissions from cars. An ef-fective way to reduce the vehicle weight is the use of aluminum alloys in automotive components. For exam-ple, the replacement of current steel construction by aluminum sheet alloys for a typical automobile body-in-

white could provide a mass reduction of 50%[1]. For these structural applications in automobile, high strength is an essential requirement to the aluminum alloys. However, the increase of strength is always accompa-nied with a substantial decrease in ductility, and the am-bient temperature ductility and formability of high-

strength aluminum alloys are always very limited, caus-ing the conventional manufacturing operations such as rolling, forging or drawing to be difficult[1]. However, it has also been documented that the poor room tempera-ture ductility of aluminum alloys can be improved by changing the forming temperature to elevated tempera-tures[2]. In recent researches, material development and deformation behavior at elevated temperatures of high-

strength Al–Mg alloys have received much attention[1―4]. A combination of high strength and good high tem-

perature formability in aluminum alloys can be achieved by refining the grain size and introducing the precipi-tates in the microstructure[5]. For these purposes, some alloying elements such as Mn, Cr, Cu, Zn, Zr and Sc are added to the aluminum alloys. However, when the con-centrations of the alloying elements exceed their solid solubilities in Al matrix, coarse intermetallic particles can be formed in the microstructure. It is well estab-lished that the coarse particles play a fundamental role in the ductile fracture of aluminum alloys[6]. They can nu-cleate cracks and influence the growth and coalescence of the crack during deformation, greatly decreasing the formability. The most common modes of nucleation are the complete or partial interfacial decohesion or the fracture of second-phase particles. At ambient tempera-ture, some kinds of coarse particles like CuAl2 and Al2CuMg can be fractured more than several hundred Received June 20, 2008; accepted August 21, 2008 doi: 10.1007/s11431-008-0275-6 †Corresponding author (email: h.zhu1@uq.edu.au)

42 HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45

micrometers ahead of a crack-tip[6,7]. However, at high temperature, the stress undergone by the particles is much smaller and damage only nucleates by parti-cle/matrix decohesion allowed by the accumulation of plastic strains along the interfaces[6,8]. Moreover, if the second-phase particles have a rounded shape or higher fracture strength, cracks also usually initiate at parti-cle/matrix interface[6,7].

The interface decohesion between particle and matrix can also occur during heavy rolling of thermomechani-cal treatments in order to manufacture the sheet product. The pre-exhibiting interface defects can rapidly grow and coalesce during the following forming deformation, resulting in early failure of the sheet components. In a previous study annealing treatment at high temperature of 550oC was used to heal the particle/matrix interface[8]. However, it was found that defect-free interfaces might be obtained by long annealing time, but the possibility of loss of certain alloying elements due to evaporation and/or grain coarsening was a practical problem[8]. In this study, a process of hot isothermal press (HIP) was developed for the sheet alloys. It is supposed that under the effect of high pressure the annealing time and tem-perature can be dramatically decreased and the devel-oped HIP process should increase the high-temperature formability.

2 Materials and experimental procedure

Three Al–Mg sheet alloys with variations of the alloying elements were prepared in this study:

Al-4.5Mg-0.9Mn-0.2Cr-0.2Fe-0.1Si (AT1), Al-3.5Mg-0.2Mn-0.2Cr-0.2Fe-0.1Si-0.2Zr-0.5Cu-0.7Zn

(AT2), AT2+0.3Sc (AT3), all compositions are in wt%. The as-received (hot-rolled) alloy sheet has a thickness of 8.0 mm. In order to pro-duce thin sheet products, they were submitted to the thermomechanical treatments including warm rolling at 180℃ to a final thickness of 1.0 mm and annealing at 450℃ for 60 min. For hot isothermal pressing, the an-nealed sheets were then put into a designed tool that could prevent the extension of the sheets under high pressure and the elevated temperature.

Tensile specimens were cut along the rolling and transverse directions. Warm tensile tests were carried out at 350℃ and 0.015 s−1 on an Instron 4505 testing ma-chine. For the biaxial stretch tests, the sheets were cut to rectangular blank samples of size of L×W =200 mm×140

mm with L in the rolling direction. Boron nitride powder was used as the lubricant and it was sprayed on the blanks and baked to a dry condition. Forming tests were performed on a heated rectangular die-punch device, which was mounted on an Instron-1116 testing machine with 250 kN capacity. Biaxial stretch tests were con-ducted at 350℃ and a fixed cross-head speed of 10 mm/s. The depth of the formed part was calculated ac-cording to load vs punch displacement curves at peak load, where necking occurred on the sheet. This part depth was used as a measure of biaxial formability.

3 Results and discussion

Second-phase particles were formed in all the three alu-minum alloys. The microstructure of the three alloys is illustrated in Figure 1. It can be found that a larger amount of second-phase particles are observed in the microstructure of the AT2 alloy than in that of the AT1 alloy due to the additions of Zr, Cu and Zn. The addition of 0.3 wt% Sc to the AT2 alloy results in a great increase in the number and size of the coarse particles in the AT3 alloy (Figure 1(c)). Also, two kinds of second-phase par-ticles are observed: coarse cuboidal shape with a size around 5 μm in the AT2 and 10 μm in the AT3 and fine rounded shape with a size up to 2 μm in all the three alloys. The coarse particles with a cuboidal shape were identified as Ti-enriched Al3Zr in the AT2 alloy or Zn- and Mg-enriched Al3(Zr, Sc) phase in the AT3 alloy. The fine rounded particles in the three alloys were found to be Fe-enriched intermetallic particles.

From Figure 1, it can also be found that the grain structures have a great change due to the additions of the alloying elements. As shown in Figure 1(a), in the AT1 alloy, most grains exhibit an equiaxed shape, indicating a fully recrystallised state, whereas in the AT2 alloy with the additions of Zr, Cu and Zn (Figure 1(b)), the grains are slightly elongated and have a smaller grain size than alloy 1. With further adding Sc to the AT2 alloy, the grains in the AT3 alloy (Figure 1(c)) are severely elon-gated along the rolling direction, which is typical of de-formed grain structures of aluminum alloys[9]. Also, the grain thickness is much smaller in the AT3 alloy than those in the other two alloys. The additions of Zr, Cu, Zn and Sc to the Al–Mg alloy can form solid solutions in the α-Al matrix, resulting in a solute drag effect on grain boundary migration during thermal exposures and de-formation. Also, when the concentrations of the alloying

HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45 43

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, the intermetallic particles can be formed, limiting the grain growth during the thermal exposures. Both the solutes in solid 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 effect on aluminum alloys[10] and a larger amount of coarse intermetallic particles promote the particle stimulated nucleation (PSN), resulting in much thinner elongated grains and fine grain zones around the particles[5].

During warm rolling of thermomechanical treatments the second-phase particles can nucleate cracks[6]. In this study, Fe-rich intermetallic particles have a rounded shape, the Al3(Sc, Zr) and Al3Zr have a high fracture strength and the rolling deformation also occurs at the warm temperature. According to a previous research[6], interface decohesion should be the major mode of crack nucleation. Figure 2 shows the SEM morphology of a crack around a second-phase particle after the ther-momechnical treatments. It is evident that cracks initiate at the particle/matrix interface. Moreover, cracks usually nucleate first on larger particles that contain larger very small scale internal or interface defects. Larger particles also induce more rapid decohesion with increasing strain because the constrained zone of plasticity at the inter-face becomes larger[8]. As shown in Figures 1(c) and 2,

Figure 2 SEM morphology of cracks around a second phase particle after thermomechnical treatments. Arrow shows the crack.

the larger particles distort the elongated grains. Thus, coarser second-phase particles are more harmful to the deformation than the finer ones. The pre-exhibiting cracks can result in rapid growth and coalescence of the cracks during the subsequent warm deformation, result-ing in inferior warm formability.

A process of hot isothermal press (HIP) was devel-oped for the sheet alloy to heal the pre-exhibiting cracks prior to warm deformation. A special die was used to limit the extension of the sheet during the HIP process. In order to optimize the process, samples of AT2 were submitted to the HIP treatment at different conditions and then to warm tensile testing. Figure 3 illustrates the true stress-true strain curves of the AT2 samples. With-

44 HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45

Figure 3 Warm tensile testing results of AT2 samples after different HIP processes. out the HIP treatment, the recrystallized sheet exhibited the highest stress level and the lowest fracture strain, indicating the worst warm formability. In contrast, with 5 T press for 60 min at 450℃, the fracture strain was somewhat increased but the flow stress was greatly de-creased. With further increasing the press to 25 T, the fracture strain was increased by 17% compared to the sample 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 the stress level was greatly decreased. Thus, the process of 25 T at 450℃/60 min was an optimal condition for healing the pre-exhibiting crack damage in this study.

The sheets of the three alloys were pressed under the optimal condition, and then submitted to biaxial stretch testing. The biaxial stretch test can examine the overall ductility of the sheet, which always exhibits anisotropic plasticity between the rolling and transverse direc-tions[11,12]. In the biaxial stretch tests, a rectangular cup-shaped part was produced. Figure 4 shows a photo of the sample after the biaxial stretch test. The formabil-ity of the sheet was evaluated by part depth defined as the maximum punch penetration before a crack initiated. For comparison, the sheets without the HIP treatment were also tested under the same condition. The part depth of the three alloys with or without the HIP treat-ment is shown in Figure 5. For samples without the HIP treatment, the AT1 alloy that contained only a few in-termetallic particles obtained the largest part depth. This is easily understood that a smaller amount of solute ele-ments and coarse particles gave this alloy a better ductil-ity. However, after the HIP treatment, the part depth de-creased for the AT1 alloy, while it increased for the other

Figure 4 Photo of sample after biaxial stretch testing.

Figure 5 Biaxial stretch testing results of investigated aluminum sheet alloys.

two alloys. This can be explained as the following. Dur-ing the thermal exposure of the HIP process at 450℃ for 60 min, recovery and recrystallization processes that were not completed during the annealing process could continue to occur. Also, the growth of the recrystallised grains could occur, resulting in grain coarsening. The recovery and recrystallization processes should be bene-ficial to the warm formability of the aluminum alloys whereas the grain coursing should have a negative effect. On the other hand, the pre-exhibiting cracks were closed and diffusion bonding could take place, healing the pre-exhibiting crack damage, which should give a great contribution to the increase of the warm formability of the aluminum alloys. For the AT1 alloy, the recrystal-lized grains can rapidly grow with less restriction of the solute elements and second-phase particles. The benefit of healing a smaller number of cracks cannot compen-sate the loss due to grain coarsening of the bulk grains, resulting in a decrease in the part depth after the HIP

HanLiang ZHU et al. Sci China Ser E-Tech Sci | Jan. 2009 | vol. 52 | no. 1 | 41-45 45

treatment. In contrast, for the AT2 and AT3 alloys, a large amount of pre-exhibiting cracks were healed by the HIP process and the recovery and recrystallization proc-esses during the HIP process improved the grain struc-tures for deformation. Also, the grain coarsening is lim-ited by the larger amount of solute elements and sec-ond-phase particles. Therefore, the part depth is greatly increased. In addition, the increase in the part depth for the AT2 alloy is larger than that for the AT3 alloy that contains much more and much coarser intermetallic par-ticles. The reason is that much larger amount of coarse intermetallic particles still rapidly nucleate cracks during the warm deformation and thus limit the benefit of the HIP process. At the optimal HIP process, the warm formability of the AT2 alloy has been improved by 22%. Therefore, the HIP process can be applied as one step of the thermomechanical treatments before the forming operations for further improving high temperature formability of aluminum sheet alloys containing a large amount of coarse particles.

4 Summary

The addition of Zr, Cu and Zn to an Al–Mg sheet alloy results in the formation of a large amount of coarse in-termetallic particles in the microstructure. Further add-ing 0.3 wt% Sc greatly increases the number and size of the intermetallic particles. During warm rolling of the thermomechanical treatments, the coarse particles can nucleate cracks at the particle/matrix interface, decreas-ing warm formability. In order to heal the pre-exhibiting crack damage, a process of hot isothermal press was developed and optimized. For the aluminum alloy that contains smaller amount of particles, the HIP treatment deteriorates its warm formability due to grain coarsening. However, for the aluminum alloys that contain larger

amount of coarse particles, the HIP process effectively enhances their biaxial formability due to healing the crack and continuing the recovery and recrystallization processes. Therefore, the HIP process can be used for further improving the warm formability of aluminum sheet alloys containing large amount of intermetallic particles.

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