Annals of the CIRP Vol. 56/1/2007 -289- doi:10.1016/j.cirp.2007.05.067

Investigation of Post-Superplastic Forming Properties of AZ31 Magnesium Alloy

M.K. Khraisheh1, F.K. Abu-Farha1, K.J. Weinmann2 (1)1Center for Manufacturing and Mechanical Engineering Dept., University of Kentucky, Lexington, KY, USA

2Department of Mechanical Engineering, University of California, Berkeley, CA, USA

AbstractIn the metal forming industry, most of the efforts are directed towards materials and process development, with little attention paid to the properties of the formed components. In Superplastic Forming (SPF), the issueof post-forming properties is particularly important because of the large plastic deformation, significant microstructural changes, and exposure to elevated temperatures for prolonged periods of time. In this work, a detailed experimental study on the mechanical and microstructural properties of superplastically-formed AZ31magnesium alloy is presented. The results clearly show the necessity to integrate post-superplastic forming analysis with material and process development for SPF optimization.

Keywords:Deformation, Magnesium, Superplastic Forming

1 INTRODUCTION Superplastic Forming (SPF) is becoming a familiar term in the metal forming industry, a fact embodied by the increasing number of aerospace and automotive partsformed using SPF. The increasing demand for lightweightalloys and the inability of conventional forming techniques to effectively form these alloys uniquely position SPF tobecome the process of choices in the future. Most of the activities in the field of SPF are focused on the material/process level, with very limited attention given to the properties of the components formed using SPF (i.e. post-superplastic forming properties). The main characteristics of SPF make the issue of post-SPFproperties particularly important. Generally, higher strain values and better deformationuniformity are often the criteria for selecting the optimumprocess parameters and evaluating the various proposed optimization practices. This could be misleading since maximum ductility does not necessarily produce the best mechanical properties in a formed component. Prolongedexposure to elevated temperatures, large plastic strains and the corresponding microstructural changes, are all factors that might deteriorate the mechanical properties of superplastically-formed materials, and their effects need to be investigated and quantified in details.There are few available studies on post-superplastic forming (post-SPF) that focus on Aluminum or Titaniumalloys [1-6]. In general, these studies are limited to anarrow range of temperatures, strains and strain rates, and do not investigate this important subject in a systematic way. In this work, a systematic approach for evaluating the mechanical and microstructural post-superplastic forming properties of the AZ31 magnesium alloy is presented. Magnesium alloys are receiving increasing interest from the industry, and there is no available study in the literature on the post-superplastic forming properties of magnesium alloys. Specimens, which were machined from superplastically-formedcomponents under uniaxial and biaxial loading conditions, are tested at room temperature to assess the changes in yield strength, ultimate tensile strength and tensile ductility, with reference to the properties of the alloy in the as received condition. These changes in the mechanical

properties are thereafter correlated to the microstructural changes in the material; cavitation and grain-growth.

2 EXPERIMENTS

2.1 Superplastic Forming at 400 C The material used in this study is the commercial AZ31-H24 magnesium alloy, received in 3.22mm thick sheets.This alloy exhibits superplastic behavior at temperatures higher than 325 C and at strain rates below 10-3 s-1 withthe maximum ductility achieved at about 400 C [7, 8]. Tensile test specimens (38x16mm) were machined alongthe rolling direction of the sheet and constant true strain rate uniaxial tensile tests were carried out at 400 C. Heating to the forming temperature took about 35 min, followed by 30 minutes dwell time to achieve thermal equilibrium. The tests were stopped when the specimensreached a certain pre-assigned true strain value, and thespecimens were then cooled down to room temperature maintaining almost no load. Four different strain rates (10-3, 5x10-4, 2x10-4 and 10-4 s-1) and six true strain values (30, 50, 70, 90, 110 and 130%) were covered, whereeach combination was repeated at least twice forrepeatability assurance. Stress-strain curves of the specimens strained at 2x10-4 s-1 to different strain values are shown in Figure 1. The results shown in Figure 1 clearly indicate that the tests are repeatable and that the experimental setup and conditions are well controlled.

2.2 Post-Superplastic Forming (Post-SPF) Tests at Room Temperature

To evaluate deformation uniformity, width and thicknessdistributions along the gauge section of each specimen were measured and recorded. Thereafter, the deformed specimens were machined along the sides to produce auniform width. Thickness, on the other hand, was not altered to avoid distorting the specimens. The machined specimens were then tested again in simple tension atroom temperature at a constant speed of 1.5mm/min toevaluate the post-SPF mechanical properties.

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Figure 1: Stress-strain curves of specimens stretched to various strains at 2x10-4 s-1.

3 RESULTS

3.1 Deformation UniformityHaving recorded the width and thickness along each specimen, the effect of forming strain rate on theuniformity of deformation can be evaluated asdeformation progresses. An example is shown in Figure2, where the thinning ratio percentage (defined as the percentage ratio between the thickness after deformationand the initial sheet thickness) is plotted along the gauge length for specimens deformed to a true strain value of 110% at different strain rates. Forming at the high strain rate of 10-3 s-1, not only resulted in severe thinning, but also yielded the largest thickness variation along the deformed specimen at about 25%. This variation drops to about 4% for the lowest strain rate of 10-4 s-1.

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Figure 2: Thickness distribution along specimens strained to 110% at various strain rates.

By combining the results of maximum thinning at various strain levels for all the strain rates covered, a deformation map as the one shown in Figure 3 can be generated. Each color in this map corresponds to a certain thinningratio band, which is set to 2.5% wide. To highlight the importance of such a plot, consider for instance the true strain of 50%. By referring to the map, it is noticed that all the strain rates share the same color strip (dark blue). This means that up to that strain level, changing the rate of deformation would not yield any improvement in terms of deformation uniformity. Nevertheless, as the straining level increases, color variation along any vertical line (i.e. constant strain) is noticed, always in favor of the lowerstrain rate. This color variation keeps increasing withstrain, giving a quantitative measure of deformation uniformity as a function of strain and strain rate. If a strain level of 130% is desired, the map in Figure 3 indicates that the thinning ratio percentage is around 55% when

deformation takes place at a low strain rate of 10-4 s-1 and around 37.5% when deformation takes place at the high strain rate of 10-3 s-1. Practically, such a map would be used in a reversed way by specifying the lowestacceptable thinning level, then selecting strain rate for a give desired strain value. If a part, for example, is to beformed with no less than 60% thinning at the most critical region with 110% true strain, Figure 3 indicates thatforming at 2x10-4 s-1 or slower would guarantee that.

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Figure 3: Maximum thinning ratio percentage for various strain rates at different deformation stages.

3.2 Post-SPF Mechanical PropertiesThe mechanical properties of the superplastically-formedspecimens were determined from room temperaturetensile tests. The goal is to arrive at a quantitativeassessment of the changes in the yield strength, tensile strength and room temperature ductility. For the different combinations of strain rates and superplastic strain values used in this study, post-superplastic forming properties were compared to reference values, corresponding to theproperties of the as received material. A 3D plot of the post-SPF room temperature ductility is shown in Figure 4.In this figure, the percentage ratio between the fracture strain of the post-SPF specimen (H) and that of the as-received material (Hf) is plotted against the amount of superplastic strain for different strain rates. The trend issomewhat expected; the higher the superplastic strain, the lower the post-SPF ductility is. Also, the effect of strain rate is significant, particularly at high superplasticstrains, where the lower strain rate gives higher ductility.In fact, specimens stretched at 10-3 s-1 fractured at about 130% true strain, and therefore the post-SPF ductility wasset to zero. An interesting observation from Figure 4 is the high strainratios (>100%) achieved at low superplastic strains for all the strain rates.

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112.5-120105-112.597.5-10590-97.582.5-9075-82.567.5-7560-67.552.5-6045-52.537.5-4530-37.522.5-3015-22.57.5-150-7.5

Figure 4: Post-superplastic forming map of room temperature tensile ductility.

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These ratios imply that the post-SPF ductility is initiallyenhanced, and then gradually decreases as higher strainsare achieved. An explanation of this behavior can be given from a heating cycle analysis, in which tensile specimens were heated to 400 C and then cooled downto the ambient temperature without straining. Room temperature tensile tests of these specimens revealed about 23 % increase in ductility over the as receivedmaterial. It is believed that these changes are caused bythe associated microstructural changes due to heating, as will be further discussed in later sections. The post-SPF tensile strength is shown in Figure 5. Similar to Figure 4, the ultimate tensile strength of the post-SPF material is inversely proportional to the amount of superplastic strain, but with less dependency on strain rate. The heating cycle analysis discussed above concluded that heating was responsible for about 7% drop in the ultimate tensile strength.

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87.5-9085-87.582.5-8580-82.577.5-8075-77.572.5-7570-72.567.5-7065-67.562.5-6560-62.557.5-6055-57.552.5-5550-52.5

Figure 5: Post-superplastic forming map of room temperature ultimate tensile strength.

The post-SPF yield strength results were different from the ductility and tensile strength results. Neither superplastic strain nor strain rate showed any significantimpact on the yield strength of the post-SPF material, as shown in Figure 6. Yield strength of the post-SPF material varies between 58% and 62% of the yield strength of the as-received material, regardless of strain or strain rate. This large drop in the yield strength is explained from the heating cycle analysis, discussed in the next section(section 3.3).

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Figure 6: Post superplastic forming room temperature yield strength.

3.3 Post-SPF Mechanical Properties vs.Microstructural Changes

The results discussed in the previous section suggest that both heat and strain affect post-SPF properties. The keyfor understanding these effects is to investigate the microstructural changes associated with them separately.

Grain-growth dependence on both heat and strain isdifferent for different superplastic materials. For the AZ31Mg alloy, an investigation revealed a strongerdependency of grain growth on heat compared to strain. Figure 7 shows the static grain growth (only heating without straining) curve at 400 C, which clearly indicates that most of the grain growth takes place within the first 65 minutes of heating (from 4.5 to 8 microns). Note that this time is almost the same as the total heating time prior to straining in the tensile tests (section 2.1). This means that at the threshold of straining, the actual grain size is about 8m, and not the initial grain size of 4.5m for the as received material. The results of the heating cycle investigation concluded that heating the sample at 400 qC for 65 minutes and then cooling to room temperature caused the grains to grow to about 8m. Room temperature tensile tests of the material that was subjected to this cycle recorded about38% drop in the yield strength as shown in Figure 6. Such result is consistent with the Hall-Petch relation.

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Figure 7: Static grain growth at 400 C. Grain growth analysis explains the post-SPF results of theyield strength, but does not explain the reduction inductility and tensile strength as a function of superplasticstrain. Examining the cavitation behavior of the material provided the explanation. Cavitation during SPF is temperature and strain dependent, and does not dependon heating time. The growth of voids (or cavities) duringsuperplastic deformation at 400 C is shown in Figure 8. The area fraction of voids was measured by an optical microscope using specialized visualization software.Interestingly, by plotting the post-SPF ductility on thesame graph, one can clearly observe the correlationbetween the two quantities. Escalation of cavitation in the material, especially after a superplastic strain of 50%, causes the deterioration of post-SPF ductility. The same applies to the effect of cavitation on the tensile strength, presented in Figure 5.

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Figure 8: Cavitation vs. post-SPF ductility

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In conclusion, these results indicate that cavitation is directly responsible for the deterioration of post-SPFmechanical properties that are strain dependent, namelyductility and tensile strength. Yield strength, on the other hand, is not strain dependent, but is rather associated with heat and heating time, which directly control the grain-growth in the material.

4 POST-SPF INVESTIGATION FOLLOWING BIAXIALSTRETCHING

During actual superplastic forming, biaxial stretching is the dominant loading condition. For this reason, in this section, the post-SPF properties of superplasticallydeformed materials under biaxial stretching areevaluated. Circular disks, 80mm in diameter, were cut from 1.65mm thick sheets of the same alloy. The sheets were superplastically-bulged using pressurized argon gasinto various cylindrical dies, at 400 C and constant effective strain rate of 2x10-4 s-1. The dies are all 63.5mm in diameter, but have different depths; 12.5, 19 and 25.5mm. Forming pressure profiles were generated fromFinite Element Analysis to ensure constant effective strain rate. More details on the superplastic bulge forming setup, the FE simulations and the generated pressure-time profiles are available in [8]. Cups were formed to the three different heights. Out of the flat bottom part of each cup, a 12.5x9.5mm tensile specimen was machined (milling) along the rolling direction of the sheet for post-SPF testing, as shown in Figure 9. Thickness at the gauge section of each machined specimen provided a mean to estimate the superplastic thickness strainachieved during SPF as also shown in Figure 9.

Figure 9: Tensile specimens machined out of the formed cups and their corresponding thickness strains

The machined specimens were then subjected to uniaxialtensile tests at room temperature to evaluate the post-SPF properties. Each test was repeated three timesunder the same conditions to ensure the accuracy of the results. Post-SPF results of the biaxially deformed specimens are summarized in Figure 10, and show the same behavior as in the uniaxial SPF case.Ductility enhancement due to heating is also observed in this case at low strain values. The tensile strength is lower for the heated specimens, and similarly decreases with superplastic strain. Note that large strains were not achieved in the cups, and therefore sharp drops in post-SPF ductility and ultimate strength were not observed. Finally, no effect of superplastic strain on the post-SPFyield strength was observed, supporting the conclusions discussed in section 3.3 that post-SPF drop in the yieldstrength is related to grain growth, which largely depends on heating that takes place prior to deformation. Theinteraction between the microstructural changes and the post-SPF mechanical properties for the biaxial case under wide range of strains and strain rates need to be further investigated.

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5 SUMMARYThe mechanical properties of superplastically deformedAZ31 Mg alloy are significantly affected by processparameters such as temperature, strain and strain rate. The change in the yield strength was found to be related to grain growth while the changes in the tensile strength and ductility were found to be related to the cavitationdeveloped during deformation. In addition, the studyprovides new and unique quantitative maps describing the effects of various process parameters on deformation uniformity. These important findings can be very useful for the optimizing the superplastic forming process.

6 ACKNOWLEDGMENTSThe support of the National Science Foundation,CAREER Award # DMI-0238712, is acknowledged.

7 REFERENCES [1] Wisbey, A., Kearns, M., Patridge P., Bowen, A.,

1993, Superplastic Deformation and Post-formedMechanical Properties of High TemperatureTitanium Alloy IMI834, Mat Sci Tech, 9/11: 987-993.

[2] Cope, M., Evetts, D., Ridley, N., 1987, Post-FormingTensile Properties of Superplastic Ti-6Al-4V Alloy,Mat Sci Tech, 3/6:455-461.

[3] Duffy, L., Hawkyard, J., Ridley, N., 1988, Post-Forming Tensile Properties of Superplastically Bulge Formed High Strength Ti-Al-Mo-Sn-Si Alloy, Mat SciTech, 4/8:707-712.

[4] Miyagi, Y., Hino, M., Eto, T., Hirose, Y., 1987, VoidFormation and its Effect on Post-FormedMechanical Properties in Superplastic AA7475 Alloy,Kobelco Technology Review, 2:45-48.

[5] Shakesheff, A., 1985, The Effect of Superplastic Deformation on the Post-Formed Mechanical Properties of the Commercially Produced Supral Alloys, Proceedings of the Superplasticity in Aerospace Aluminium, Cranfield, England, 36-54.

[6] Dunford, D., Wisbey, A., Partridge, P., 1991, Effect of Superplastic Deformation on Microstructure, Texture, and Tensile Properties of Ti-6Al-4V, Mat Sci Tech, 7/1:62-70.

[7] Abu-Farha, F., Khraisheh, M., 2006, On theSuperplastic Forming of the AZ31 Magnesium Alloy,Proceedings of 7th Int. Conference on Magnesium,Dresden, Germany, Wiley-VCH Verlag, 399-405.

[8] Khraisheh, M., Abu-Farha, F., Nazzal, M., Weinmann, K., 2006, Combined Mechanics-Materials Based Optimization of Superplastic Forming of Magnesium AZ31 Alloy, Annals of the CIRP, 55/1:233-236.

H(mm)

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12.5 86 15

19 63 46

25.5 47 76