FORMABILITY OF AZ31 MAGNESIUM ALLOY IN WARM INCREMENTAL FORMING PROCESS G. Ambrogio1, S. Bruschi2, A. Ghiotti3, L. Filice1,* 1
DipMec University of Calabria, Rende (CS) – Italy 2 DIMS University of Trento, (TN) – Italy 3 DIMEG University of Padua, (PD) – Italy
ABSTRACT: A number of scientists are currently involved in Incremental Forming investigation, in order to better understand the process and to extend its applicability to industrial practice. If flexibility is probably the main advantage deriving from Incremental Forming use, today a new point of view is given by the possibility to use the process instead of a sequence of operations, allowing also a better sustainability. Formability enhancing is one of the keyfactors of this success and for this reason it is widely investigated, also on lightweight materials, for which the use of a preliminary heating is required. In the paper the formability of Magnesium AZ31 is investigated in warm Incremental Forming, focusing the attention on the tools currently utilised for describing material formability. It is shown that Forming Limit Curves fail in this goal while a study of fracture, exploiting results of conventional tensile tests, supplies more appreciable results. KEYWORDS: AZ31, Formability, Incremental Forming
1 INTRODUCTION It is well known that Incremental Forming process allows a higher formability as compared to the traditional stamping processes due to the possibility to extend and control material forming “under necking conditions” [1-2]. This capability is surely more interesting when materials characterized by a low formability are processed. Among these, Magnesium is increasing its importance in the last years due to the need of lightweight materials in some applications and its high performance vs. weight ratio [3]. Reduction of pollution is just one of the reasons why the use of lightweight materials is today strongly encouraged. What is more, the possibility to obtain one component by a single process instead of a proper sequence, is today regarded as very interesting with the aim to reduce the environmental cost of the operations leading the processes more sustainable. However, Magnesium forming at room temperature is not possible, thus a new scenario is today given by the warm processes, in which an interesting level of formability is allowed thanks to material heating. Some preliminary tests in this way showed a suitable material behaviour [4-5]. The equipment to work Magnesium in warm conditions is not very expensive and thus interesting also from an industrial point of view.
In the study here addressed, formability is approached deriving FLC conditions at fracture directly from uniaxial tensile tests in different cases in order to build modified FLDs in which necking occurrence is obviously overcome. Thus, the strain occurring in Incremental Forming is considered in order to investigate the process feasibility. This analysis tool may be used in two different ways: firstly as verifying technique, highlighting the points in which material breaking is possible, and giving an advise on the process robustness; secondly, as design technique, investigating the optimal process parameters (for instance temperature) that allow the higher formability, directly using cheap and fast tensile tests. All these aspects are introduced and discussed in the paper.
2 WARM SINGLE POINT INCREMENTAL FORMING ON MAGNESIUM ALLOY First of all, Warm Incremental Sheet Forming required a carefully attention to the equipment design in order to ensure a more homogeneous temperature distribution on the sheet. According to that, the equipment shown in Figure 1 was developed. It allows to clamp circular and square blanks by means of a proper blankholder. A backing plate was included into the device in order to
____________________ * Corresponding author: P. Bucci Street – 87036 Rende (CS) - Italy,
[email protected].
reduce the bending effect at the die corner; the final working area was so fixed to 100 mm diameter. A particular attention was paid to design the heating and insulation system, in order to achieve an effective thermal control on the sheet and to avoid thermal gradients. This aim was pursued through numerical simulations and tests. The final solution consists of a heater band placed along the external surface of the die, governed by a PID controller with three thermocouples placed at different radii on the specimen. All over the tests, the differences among the measured data were always lower than 5°C. The tests were carried out on AZ31–O sheets and the target geometry was a truncated cone and pyramid with a major dimensions (L0, D0) equal to 100 mm and a depth (H) equal to 40 mm; the value of the minor side depended on the value of the wall inclination angle α. All the experiments were carried out on a 3-axis CNC milling machine; a proper cooling system was necessary to avoid heat transfer to the machine. MoS2 was utilized to lubricate the interface between punch and sheet.
Figure 1: Warm Incremental Sheet Forming equipment
In order to determine the formability limits of AZ31 in Incremental Forming, an experimental campaign was carried out. Starting from the available base of knowledge, three main process parameters were taken into account namely: the punch diameter (Dp), the tool depth step (p) and the sheet temperature (T). Three levels for each of them were considered. In particular, as concerns sheet temperature, the considered range was 200-300°C, which corresponds to the maximum formability conditions [6]. On the other hand, for sake of simplicity, sheet thickness was fixed equal to 1 mm all over the tests. Furthermore, since the aim of the research was to assess failure conditions, the wall inclination angle was increased by 5° up to reach the critical value (αmax) for each set of parameters. Table 1 shows the experimental plan ranges taking into account during the experimental campaign. After each test, the major and the minor strains were measured using an optical system based on the deformation of circles previously marked on the sheet.
Table 1: Process parameters
Parameter Thickness Depth step Tool diameter Temperature Wall angle Tool speed Feed
Value 1.0 mm [0.3 – 1.0]mm [12.0 – 18.0] mm [200 – 300] °C [45 – 60] ° 100 rpm 300 mm/min
As expected, a strong influence was played by the process temperature and the depth step: more in detail, increasing the first and reducing the second, major strain value increases too. The maximum α value was 60° at 300°C, which corresponds to a major strain 1.2 approximately.
3 METHODOLOGIES TO MODEL MATERIAL FORMABILITY Traditional manufacturing science introduced some methodologies for modelling material formability in sheet metal forming. Today the use of curves in the strain domain seems to be the better way since no reliable criteria have been proposed and tested in this field, thus a meso-scale approach development is today a work in progress while macro-scale ones are more reliable. Among the latter approaches, Nakazima test is regarded as one of the most interesting since it well represents the failure conditions in stamping. However, it does not work very well in Single Point Incremental Forming when strain vector sometimes reaches high modules before material fracture, well beyond the Forming Limit Curves. A typical objection is that Nakazima test supplies curves at necking while in Incremental Forming formability is measured at failure and this is of course true. But it is also true that in stamping processed necking and failure are very close in terms of strain vector module. This is different in Incremental Forming in which it is possible to generate a diffuse necking that strongly increases the amount of strain allowed by the material before rupture. For the above considerations, a question seems to be strategic: does exist an “independent” test able to predict material behaviour in Incremental Forming processes? In order to build another brick of knowledge in this field, two conventional tests are reported in the following. As it will be shown, tensile tests, analysing the specimen at fracture, supply suitable indications. 3.1 NAKAZIMA TEST: FLC The formability of AZ31 was evaluated by performing Nakazima-based tests, using an equipment consisting of a die, a blank-holder and an hemispherical punch (Figure 2). In order to perform the formability tests at 200° and 300°C the testing equipment was installed on the a
testing machine and placed inside the heating furnace. Actually, the latter ensures the temperature conditions during the test; in fact, the testing temperature was imposed by the furnace control system and the whole equipment (die, blank holder and punch) was heated.
calculate the specimen true strain at any time increment, five points are automatically chosen in the gauge section and their major strains calculated by the ARAMIS™ software from their relative displacements at each time increment. Strains were recorded both at necking (calculated where the force-stroke curve reaches its maximum) and at failure.
Inductor
Specimen Aramis
Figure 3: Uni-axial tensile test equipment and ARAMIS™ measures Figure 2: Nakazima-test equipment
A punch speed of 0.1 mm/s was imposed. According to literature review [7], specimens having varying ratio between major and minor base were used in order to cover the whole minor strain vs. major strain diagram. In particular, for each investigated condition, different rectangular specimens were chosen with dimensions varying from 100x12.5 to 100x100 mm2. Naturally, in order to measure the strain distribution after forming, the sheets were previously electro-chemically meshed. The contact surfaces were lubricated using a MoS2 based grease in order to prevent the blank-punch sticking during forming. During the Nakazima test, each specimen was formed up to the local necking, avoiding the fracture conditions. After that the minor-major strains were measured by using an optical deformation analysis system, by which the forming limit curves can be built summarizing all the measurement conditions. The forming limit curves (FLCs) obtained at 200°C and 300°C are shown directly in the next Figures 4 and 5, as continuous lines.
The results of Nakazima test (FLC) and Tensile Test at necking and fracture are reported in the next Figures 4 and 5, representative of measures at 200°C and 300°C respectively.
Figure 4: Formability at 200°C
3.2 TENSILE TEST The uniaxial tensile tests were performed on a 50 kN MTS™ hydraulic testing machine. The specimens were heated up to 200° and 300°C by an high frequency inductive heating system and their temperature was controlled through a K-type thermocouple spot-welded in the middle of the specimen gauge length. Specimen strains during deformation were automatically calculated thanks to the software implemented into the ARAMIS™ optical measurement system (Figure 3 on the left). The optical system measures in-line the specimen deformation by following the displacements of the points of a stochastic pattern previously sprayed on the specimen surface (Figure 3 on the right). In order to
Figure 5: Formability at 300°C
4 DISCUSSION OF RESULTS
5 CONCLUSIONS
The Figures above supply a qualitative result since a certain scattering in the data collecting was recognised and, of course, a statistical interpretation of the information could give more assessed evidences. However a conclusion can be derived, taking into account the measures obtained by the SPIF campaign on cone and square base pyramid geometries. In the next Figures 6 and 7, the minor strain-major strain combinations at fracture for SPIF specimens are reported, using the measures obtained by the experimental campaign previously discussed. The basic idea is to draw the constant thickness curve (dotted line) [8,9] starting from the value observed in the failure region in tensile specimens. In other words, it is assumed that material breaking does not significantly depend on the process conditions but there is a thinning threshold that cannot be overcome. This result matches the one presented by other authors that utilised different indicators to describe material formability limits [10].
Formability in Single Point Incremental Forming is up to now an open point for any researcher in this field. The development of reliable methodologies to predict material failure in the process design is today very impellent. This paper opens a new possibility utilising simple uni-axial tensile tests, analysing the specimens at fracture, in order to define the true limits of material at the different temperatures when they are worked by the SPIF process. The results are very encouraging as discussed in the paper.
Figure 6: Forming limit curve for SPIF - 200°C
Figure 7: Forming limit curve for SPIF - 300°C
ACKNOWLEDGEMENT The authors would like to thank prof. A. Forcellese and the manufacturing team of the University of Ancona for their support in developing the experiments of this paper.
REFERENCES [1] H. Iseki: An experimental and theoretical study of a forming curve in incremental bulging of sheet metal using a spherical roller. Journal of Material Processing Technology, 111:150-154, 2001. [2] J. Jeswiet, F. Micari, G. Hirt, A. Bramley, J. Duflou, J. Allwood: Asymmetric Single Point Incremental Forming of Sheet Metal. Annals of the CIRP, 54/1, 2005. [3] M. Kleiner, M. Geiger, A. Klaus: Manufacturing of Lightweight Components by Metal Forming. Annals of the CIRP, 52/2: 521- 542, 2003. [4] S. Lee, Y.H., Chen, J.Y. Wang: Isothermal sheet formability of magnesium alloy AZ31 and AZ61. Journal of Material Processing Technology, 124: 1924, 2004. [5] A. Forcellese, M. El Mehtedi, M. Simoncini, S. Spigarelli: Formability and microstructure of AZ31 magnesium alloy sheets, Key Engineering Materials, 344: 31-38, 2007. [6] G. Ambrogio, L. Filice, G.L. Manco : Warm incremental forming of magnesium alloy AZ31, Annals of the CIRP, 57/1/2008: 257-260. [7] K. Lange: Metal Forming Handbook. SME edited, 2004. [8] L. Filice, L. Fratini, F. Micari: Analysis of material formability in incremental forming. Annals of the CIRP, 51/1:199-202, 2002. [9] M. Bambach, M. Todorova, G. Hirt: Experimental and numerical analysis of forming limits in CNC Incremental Sheet Forming. In: Proc. of Shemet 2007 Conference, 511-518, 2007. [10] G. Hussain, L. Gao, N. Hayat, Xu. Ziran: A new formability indicator in single point incremental forming, Journal of Material Processing Technology, doi:10.1016/j.jmatprotec.2008.11.024.
