On the Optimisation of Superplastic Free Forming Test of an AZ31 Magnesium Alloy Sheet Franchitti S.1, Giuliano G.1, Palumbo G.2, Sorgente D.2, Tricarico L.2 1
Università degli Studi di Cassino, Dip. di Ing. Industriale, Cassino, Italy
URL: www.dii.unicas.it 2
e-mail:
[email protected];
[email protected]
Politecnico di Bari, Dip di Ing. Meccanica e Gestionale, Bari, Italy
URL: www.dimeg.poliba.it
e-mail: d.sorgente@ poliba.it
ABSTRACT: In this work, an original algorithm, proposed in [1-2], has been used to predict the variable pressure-time load profile during a superplastic forming process, in order to get best performances, in terms of ductility, on the material. The so obtained curve has been used in experimental free bulging tests on an AZ31 magnesium alloy sheet. Tests results, in particular the maximum bulge height achieved by the material, have been compared with numerical results and with constant-pressure experimental tests. Key words: Superplastic Forming, Magnesium Alloys, Process Optimisation
1 INTRODUCTION Superplasticity is the ability of certain materials to undergo extreme elongations at a proper temperature and at a controlled strain rate. Under these conditions those materials can be stretched to several times their original length. For deformation in uniaxial tension, elongation to failure above 200% is usually indicative of superplasticity, although several materials can attain extension greater than 1000%. It is important to note that such values are greater by one or two orders of magnitude compared to those of conventional metallic materials. Superplastic deformation is characterised by low flow stresses and by a high uniformity of plastic flow: these properties make the Superplastic Forming (SPF) process commercially interesting. SPF is typically used to produce sheet metal parts that would otherwise be fabricated from several smaller pieces. The part-count reduction results in weight and cost savings. Typical materials used in commercial superplastic forming applications include Titanium (Ti-6Al-4V) and Aluminium based (Al 7475, Al-Li) alloys. In last years the attention of the research has been focused on superplastic behaviour in Magnesium
(Mg) alloys. Such alloys are characterised by high strength to weight ratios, which makes them very attractive if compared to other metallic alloys. However, due to the Mg hexagonal closed packed crystal structure, they do not allow high plastic elongations to failure at room temperature. Therefore, in order to improve formability, the process temperature for magnesium alloys has to be increased. SPF requires a detailed design of the technological process in order to better exploit its peculiar potentialities; nowadays, the Finite Element (FE) method represents the mainly used technique to simulate the sheet metal forming processes. In order to perform the forming process of Mg alloy sheets leading to the best ductility under superplastic conditions, the selection of forming pressure versus time profiles is very critical because it determines, of course, the quality of the formed part and the production time. In general forming pressure profiles are usually based on trial and error method, in order to reduce the forming time and maintain the integrity of the formed part. In this context, computer simulations based on FE method are very useful in the process design stage because they provide detailed information during the deformation step and they are particularly important in the prediction of the forming behaviour of the material.
Each part’s geometry is unique and requires a unique pressure/time profile to maintain the appropriate strain rate, therefore in SPF applications the strain rate control has a very important role: an appropriate strain rate value, at which the material can experience the maximum elongation to failure, can be defined; a controlled strain rate during SPF can be induced by a variable load profile in which the forming pressure changes during the process in order to keep the strain rate value in the material around the optimal value. For this purpose an original algorithm, proposed in [1-2], has been used to predict the variable pressuretime load profile during the forming process of the AZ31 magnesium alloy. 2 EXPERIMENTAL TESTS 2.1 Experimental equipment A laboratory-scale blow forming equipment has been used for testing the superplastic material at elevate temperatures. In this equipment the undeformed sheet is interposed between two creep resistant steel tools (the die and the blankholder) and a gas pressure acts on the lower side of the sheet forcing it to expand in the die cavity. Forming tools are placed inside a three zones split furnace of an INSTRON 4485 universal testing machine. Thus die and blankholder dimensions have been designed to fit the internal furnace diameter (90mm). Circular drawbeads, which avoid both drawing during the test and gas leakages from the forming chamber, are located just after the corner radius of the cavity and specimens must have an approximated diameter of 70mm. The die has a circular geometry with an aperture radius of 17.5mm (a) and a die entry radius of 3.0mm. The temperature on the sheet and on tools has been measured through type J thermocouples. Equipment tools have been preheated (keeping the furnace temperature at the test temperature) for 2 hours before each series of tests to get a thermal steady state condition during the following tests. Before each test the specimen has been kept for 2 minutes (before applying the gas pressure) to get, also on it, a stable temperature. Argon (Purity: 99.990%) has been inflated in the forming chamber (situated in the blankholder) through a gas cylinder and an electronic pressure regulator which brings pressure to the desired value. A LabView® program
interfaced to the testing machine and to the pressure regulator allows to control the blankholder force upon the specimen and the pressure in the forming chamber. A little fan, just out of the furnace, is used to cool the upper part of the equipment to avoid excessive heating near the load cell of the INSTRON machine. The dome height (h) on the specimen has been measured by a magnetostrictive transducer mounted on the crosshead of the universal testing machine. The signal during the test has been acquired from the same LabView® program and saved on a ASCII file. The three zones of the furnace have been controlled to get a stable temperature on the sheet of about 520°C. Tests can be performed both with constant pressure and with arbitrary pressure profiles controlling the electronic pressure regulator with the LabView® program. Detailed information of the experimental equipment used to carry out the blow forming test are reported in [3-4]. The material used in this study is a cross-rolled AZ31 Magnesium alloy sheet with an initial thickness of 0.5mm. 2.2 Material characterisation The commercial FE code, used as computational tool to simulate the SPF process, was MSC.Marc®. The equation, used in the code, to describe the behaviour of the Mg alloy AZ31, relates the flow stress to the strain and the strain rate: σ = Kε n ε& m
(1)
where K, m and n values are constant. In order to obtain material constants of the constitutive equation, the data of free bulging tests at two different pressure values have been used: the first at a constant forming pressure of 0.16MPa and the second at a pressure of 0.29MPa. For each single pressure value, the time step taken for the sheet to pass through the normalized polar height H=0 to H=1 was measured (H=h/a). The strain rate sensitivity index, m, can be determined by adopting the analytical expression: ⎛p ⎞ ln⎜⎜ 1 ⎟⎟ p m= ⎝ 2⎠ (2) ⎛ t2 ⎞ ⎜ ⎟ ln⎜ ⎟ ⎝ t1 ⎠ where t1 e t2 are the forming times necessary to obtain the same dome geometry at the constant pressures of, respectively, p1 and p2.
Using an inverse method, the parameters n and K, which characterise equation (1), have been numerically determined, on the base of few experimental tests. Detailed information for the determination of the material characteristic parameters are reported in [5-6]. 3 NUMERICAL OPTIMISATION OF LOAD CURVE
2.0E-02 -4 -1 8.7x10 s 8.7x10-4s-1
-1
strain rate (s )
1.6E-02 1.2E-02
-4 -1
strain rate=8.7x10 s 0.18 0.16
pressure (MPa)
In order to improve material formability, original algorithms, capable to predict the optimal pressure profile, have been developed. These algorithms can be interfaced to commercial FE codes and their main output is the pressure value for each time instant of the forming pressure that can keep the strain rate in the sheet around its optimal value [1-2]. In this work, the optimisation has been run for the bulging test on an AZ31 magnesium alloy sheet. Starting from the results of numerical simulations of the constant pressure tests (0.16MPa and 0.26MPa), it can be noted that, during the forming process (0
