NOTICE: this is the author’s version of a work that was accepted for publication in International Journal of Material Forming. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in International Journal of Material Forming 04/2010; 3(Suppl 1):951-954. DOI:10.1007/s12289-010-0926-6
PRELIMINARY STUDIES ON SINGLE POINT INCREMENTAL FORMING FOR COMPOSITE MATERIALS M. Fiorotto1*, M. Sorgente1, G. Lucchetta1 1
University of Padova – Dept. of Innovation in Mechanics and Management – Italy
ABSTRACT: This paper presents a preliminary investigation on applicability of Single Point Incremental Forming techniques (SPIF) to composite laminates. Initially the possibility of quickly and economically forming sheet metal moulds with complex geometry was evaluated. Then experimental tests were conducted to achieve the direct incremental forming of composite laminates. Different types of diaphragm were considered to avoid wrinkling and to improve the composite adhesion. The first results showed how this automated and flexible process is particularly suited for forming composite/metal hybrid materials. These laminates, consisting of alternating layers of thin metal sheet and composite, are used for their excellent damage tolerance properties, such as fatigue and impact resistance. In order to evaluate the possibility of simultaneously forming aluminium and composite sheets, different configurations of Fiber Metal Laminates (FML) were tested. Finally, compression tests were carried out to compare ductile fracture limits of the aluminium sheet and the FML. The preliminary experimental investigations reported in this paper show how moulds with complex geometry can be realized by SPIF. Choosing sufficiently stiff diaphragm and using vacuum to conform the composite materials, wrinkle-free parts can be formed in an incremental fashion, even if problems of non-uniform resin distribution still need to be solved. Moreover, FML with different configurations can be formed successfully, significantly improving the mechanical properties of sheet metal structures. KEYWORDS: Single Point Incremental Forming (SPIF), Composite Materials, Mold, Fiber Metal Laminates (FMLs)
1 INTRODUCTION The aircraft industry is characterized by complex products requiring many different types of parts produced in small lots. As a result, aircraft manufacturers need to store many molds, dies and fixtures, each of which is used annually to fabricate a relatively small number of sheet metal and composite parts. Applications of composites in the aircraft industry have increased rapidly. Composites, in the context of this paper, refer to laminated layers of unidirectional fibers embedded in a polymer matrix. The collection of laminated layers shaped onto a mold prior to curing is called a lay-up. In a typical lay-up, the angular orientation of the fibers in each layer, or lamina is specified so that the resulting composite has certain desired mechanical properties. Different materials are not only used side by side, at the level of structural elements, but also in mixed or socalled hybrid materials. These combinations offer engineering materials with good mechanical properties. Fiber-metal laminates (FMLs), for example, are high performance laminated structures based on stacked arrangements of a composite material and an aluminum alloy. Currently, a glass fiber reinforced epoxy/aluminum alloy FML is being considered for use
in the manufacture of the upper fuselage of the A380 Airbus aircraft. Previous work on FMLs has shown that they combine the excellent durability and machinability common to many metals with the superior fatigue and fracture properties offered by many fiber-reinforced composites [1]. As the use of these materials increases, there is a corresponding need to improve the manufacturing processes employed in fabricating parts. The Single Point Incremental Forming (SPIF) technology has been introduced in the recent past to manufacture sheet metal products by using Computer Numerical Control machines [2-4]. The major advantage of incremental forming is represented by the possibility to manufacture sheet metal parts difficult to form with traditional processes in a rapid and economic way without expensive dies and long set-up times. In SPIF the tooling is usually a simple frame for the sheet metal clamping, while the deformation is realized by using a tool that is moved along a predefined path by a robot or a CNC machine. Although the process can be rather slow compared to the traditional stamping or drawing processes, single point incremental forming of metal sheets represents the best way to manufacture prototypes and complex components produced in small batches for aeronautical, automotive and medical applications. At the present time, researchers have obtained feasible
____________________ * Corresponding author: DIMEG, Via Venezia 1, 35131 – Padova – Italy - Phone: +39-049-8276823 – Fax: +39-049-8276816, e-mail:
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results and potential applications on sheet metal products. With the target of realizing advantages similar to SPIF of sheet metals, this paper presents some preliminary investigations on applicability of SPIF techniques to composite materials. words or groups of key words). Key words must be typed using 10 point normal font. After the key words, leave 24 pt vertical space before the main text.
composites requires autoclave processing. Within some geometric constraints, a SPIF is capable of forming a variety of part shapes. Shapes used to demonstrate the abilities of using SPIF for composite materials are shown on Fig. 2.
1 EXPERIMENTAL TESTS 1.1
EXPERIMENTAL EQUIPMENT
For the experiments, square sheets (350mm×350mm) were used. They were clamped on the frame and put on the machine table. The size of the unsupported region of the sheet is 300mm×300mm. The tools were manufactured in stainless steel with ball-shaped top. The experimental campaign was carried out on a TRAUB TVC 200 (Fig. 1). The part model was modeled in Pro Engineer Wildfire 2.0. This model was used to generate the tool-path and to output numerical control codes. In order to reduce friction between the surface of the sheet and the tool, water-miscible metalworking fluid was used in the percentage of 25% (Blasocut™ 2000). A constant lubrication is granted by a hydraulic system during the experiments. Each experiment was performed three times to verify the repeatability.
Figure 2: Some shapes were formed in composite sheet
2.3
FORMING WITH A DIAPHRAGM
A cone-shaped part was selected to test the formability of a composite laminate by using a sheet metal as a diaphragm. A laminate have been developed by bonding alternating three plies of fabric woven, kevlar/glass/kevlar, impregnated with a epoxy resin. The tool path is a spiral in a horizontal plane with overlapping passes (Fig. 3). The pitch between successive laps is 1 mm, feed rate is 1500 mm/min, spindle rate is 500 rpm and the outer diameter of the cone is 240 mm. The tool radius is 10 mm and the depth of the path below the initial plane of the sheet is 60 mm.
Figure 1: Setting of experimental equipment
2.2
FORMING SHEET METAL MOLDS BY SPIF
A square aluminium sheet (350mm×350mm×0,5mm) was clamped on the support frame and was formed by SPIF. The sheet was then used as a mold for composite parts. Epoxy resin was mixed with a catalyst and the mold was wetted out with the mixture. Three plies of woven fabric, kevlar/glass/kevlar, were placed over the mold and rolled down into the mold using plastic rollers. The woven kevlar and glass fabrics respectively weigh 130 and 200 g/m2. The material has been securely attached to the mold, so air was not trapped in between the woven fabric and the mold. Rollers were used to make sure the resin was between all the layers, fibers were wetted throughout the entire thickness of the laminate and all of the air pockets were removed. The work has been done quickly enough to complete the job before the resin started to cure. Curing of composites took place at ambient temperature. The manufacture of high performance components from advanced
Figure 3: Cone-shaped part used in the test
The mechanical feasibility has been evaluated by observing three events which reduce performances: mechanical failure of the diaphragm, surface quality degradation and wrinkling of the composite. The main problem is that the composite sheet cannot plastically deform at a local point. Also the tool removes resin from the surface of the composite. So it is necessary to use a diaphragm that prevents removal of resin and confers stiffness and shape to the part. During the process, the composite tends to adhere not uniformly to the diaphragm. Two different materials for the diaphragm were tested: aluminum 1050 and PVC. A lower support was used to push the composite against the diaphragm. Two different configurations were considered. In the first case a steel support, consisting of a basement, 6 threaded rods, 6 springs and a circular ring, was used. A
neoprene layer, clamped between the ring and the aluminum sheet, was tested to improve the adhesion of the composite (Fig. 4).
Figure 6: Composite sheet formed by SPIF
Figure 4: The support with a neoprene sheet
After a few laps, in the transition zone between the inclined wall and the corner radius of the PVC sheet, a crack opening, triggered by stretching mechanisms, caused the failure of the test. Instead the aluminum sheet and the composite laminate were successfully formed. Because of gravity the composite laminate was locally detached from the aluminum sheet and was deposited on the neoprene layer. Resin was accumulated in the bottom of the concave-shape part. A vacuum bag was therefore used to improve the adhesion between the composite sheet and the diaphragm, as shown in Figure 5. A nylon film was applied to the aluminum sheet as a solid barrier to prevent that the composite could stick to the mold. A peel ply was applied on the composite sheets. It is a tightly woven nylon fabric, impregnated with a release agent. It is highly stretchable so that it can conform to complex geometries. The peel ply sticks to the laminate but it can easily be removed. A layer of bleeder cloth was placed above the release film. Its purpose is to absorb the resin in excess. The bleeder also acts as a breather, providing a continuous air path for pulling the vacuum. Finally the bag was applied. A pump was used to create the vacuum (-0.3 bar). The vacuum pump includes a means of an air tight piercing of the bag where the vacuum is to be drawn, a trap so that resin cannot be drawn into the pump, a vacuum gauge, and a relief valve to regulate the level of vacuum being pulled. Wrinkle-free parts could be formed in an incremental fashion. Reduced non-uniform resin distribution was observed.
2.4
FIBER METAL LAMINATES
In order to evaluate the possibility of simultaneously forming aluminum and composite sheets, different configurations of Fiber Metal Laminates (FML) were tested. These laminates consist of alternating layers of thin metal sheet and composite layers (Fig. 7).
Figure 7: FML (three layers of metal alloy and two layers of composite)
A panel with two aluminum plates and a composite core has been tested. Composite laminates consist of two woven kevlar fabrics. The results showed that the FML has not survived the test. A crack opening and its propagation led to a premature collapse of the SPIFed part. The core material prevented the transfer of stress to the underlying aluminum sheet. The core material is incompressible and therefore only a small reduction in thickness occured for the bottom sheet. Formability was also limited by the development of wrinkles along the inclined wall of the SPIFed parts which were found to occur earlier than the circumferential crack that can also be seen in Figure 8.
Figure 5: Vacuum bag
Small quantity of resin was accumulated in the bottom of the part (Fig. 6).
Figure 8: Mechanical failure on formed FML
To improve the formability, two aluminum sheets were simultaneously formed without the composite laminates, in order to avoid one processing step. The results showed that two plates can be formed by SPIF without wrinkles or surface quality degradation (Fig. 9).
Figure 11: Deformed structures after the compression test
Figure 9: Two aluminum sheets simultaneously formed in an incremental fashion
2.5
COMPRESSION TEST
Axial compression tests were carried out to measure the plastic flow behavior and ductile fracture limits of the materials with different configurations. The experimental tests were carried out on a 100kN hydraulic press MTS 322 T-slot. An iron plate was fixed to the lower jaw and a square and flat head punch was used. A load sensor was used to detect the time evolution of the load transmitted to the component. The compression test was carried out with a constant speed of the punch of 5 mm/min. The aim of the test is to quantify the benefits in terms of stiffness and strength given by the addition of the composite material to the metal sheet. Three configurations have been tested: aluminum sheet, Al/composite laminate and Al/composite/Al laminate. The test was stopped after the load started to increase exponentially. Three structures respectively weight 105 g, 221.8 g and 380.6 g.
Figure 12: Load vs punch displacement in the compression test
3 CONCLUSIONS This paper has presented a first investigation of the application of SPIF to composite materials. SPIF can be used to quickly and economically form molds with complex geometry. Choosing an aluminum sheet as a diaphragm and using a vacuum bag to conform the composite materials, wrinkle-free parts have been formed in an incremental fashion even though resin tends to accumulate at the bottom of the bag. The problems of non-uniform resin distribution have to be solved in future works. SPIF can be used to deform in one step two aluminum plates for FMLs. Panels with aluminum sheets and a composite core cannot be formed without wrinkles as the composite core prevents the local transmission of load to the lower plate. Significant improvements in specific stiffness and strength were gained by using FML formed by SPIF.
REFERENCES Figure 10: Setting of the compression test
All structures plastically deformed without rupture as it can be seen in Figure 11. Specific stiffness and strength of FML with one layer of aluminum and one layer of composite respectively improves by 2% and 104% in regards to the values of the single aluminum sheet (Fig. 12). Specific stiffness of Al/composite/Al laminate decreases by 4% when compared to the Al/composite laminate. But the results show a consistent improvement (21%) in specific strength.
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