Laser Forming of Fibre Metal Laminates

Transcription

1 Lasers in Eng., Vol. 15, pp Reprints available directly from the publisher Photocopying permitted by license only 2005 Old City Publishing, Inc. Published by license under the OCP Science imprint, a member of the Old City Publishing Group Laser Forming of Fibre Metal Laminates S. P. EDWARDSON 1*, P. FRENCH 1, G. DEARDEN 1 K. G. WATKINS 1, W. J. CANTWELL 2 1Laser Group, 2 Impact Research Centre, Department of Engineering University of Liverpool, Liverpool, L69 3GH, UK The laser forming process has been shown to be a viable method of shaping metallic components, as a means of rapid prototyping and of adjusting and aligning. Although the process does compete with conventional forming processes, applications are being discovered where laser forming alone can achieve the desired results. The application reported in this work demonstrates how the process can be used to form recently developed high strength fibre metal laminate materials. These materials due to their construction and high strength are difficult to form once constructed using conventional techniques. Fibre metal laminates are of particular interest to the aerospace industry, where the high strength yet lightweight construction of parts made with these materials offers significant weight reductions and hence a reduction in operational costs of new large commercial aircraft such as the Airbus A380. In addition a more recent application under investigation for these materials is in the construction of street furniture (e.g. litter bins) and airline cargo containers utilising their excellent blast resistance capabilities to save lives in the event of terrorism. Keywords: Laser Forming, Bending, Fibre Metal Laminates, Metal Laminate Composite, FML, MLC, GLARE 1. INTRODUCTION This investigation is primarily concerned with the process of laser forming or laser bending of metal sheet material with a high power laser beam. Laser forming has become a viable process for the shaping of *Corresponding author: s.p.edwardson@liverpool.ac.uk 233

2 234 EDWARDSON, et al. metallic components, as a means of rapid prototyping and of adjusting and aligning. The laser forming process is of significant value to industries that previously relied on expensive stamping dies and presses for prototype evaluations. Relevant industry sectors include aerospace, automotive, shipbuilding and microelectronics. In contrast with conventional forming techniques, this method requires no mechanical contact and thus promotes the idea of Virtual Tooling. It also offers many of the advantages of process flexibility associated with other laser manufacturing techniques, such as laser cutting and marking [1]. Laser forming can produce metallic, predetermined shapes with minimal distortion. The process is similar to the well established torch flame bending used on large sheet material in the ship building industry but a great deal more control of the final product can be achieved [2]. The laser forming process is realised by introducing thermal stresses without melting into the surface of a work piece as a de-focused laser beam is passed over it. These internal stresses induce plastic strains, bending or shortening the material, or result in a local elastic plastic buckling of the work piece depending on the mechanism active [3]. The range of materials that can be laser formed is considerable. As there is only localised heating involved below the melting temperature good metallurgical properties can be retained in the irradiated area [4, 5]. Materials of particular interest are specialist high strength alloys [6]. These include titanium and aluminium alloys. These materials are widely used in the aerospace industry where the implementation of laser forming as a replacement of existing manufacturing processes is under investigation [7, 8, 9, 10] as well as other industry areas [11, 12, 13, 14]. The application reported in this work demonstrates how the laser forming process can be used to form recently developed high strength Fibre Metal Laminate materials. These materials, due to their construction and high strength, are difficult to form once constructed using conventional techniques. Fibre Metal Laminates (FML), sometimes referred to as Metal Laminate Composite materials (MLC), are of particular interest to the aerospace industry, were the high strength yet lightweight construction of parts made with these materials offers significant weight reductions and hence a reduction in operational costs of new large commercial aircraft such as the Airbus A380 and the proposed ultra-efficient Boeing 7E7. Other industries where these materials are of interest include automotive, in particular the high performance sports car and racing sectors such as Formula One. A more recent application under investigation for these materials is in the construction of street furniture (e.g. litter bins) and airline

3 LASER FORMING OF FIBRE METAL LAMINATES 235 cargo containers utilising their excellent blast resistance capabilities to save lives in the event of terrorism. 2. MATERIALS The materials investigated in this study are composite laminates or layered structures. FML or MLC consist of thin sheets of aluminium bonded and alternated with thin sheets of fibre reinforced composite (figure 1). The first FML was ARALL (Aramid Reinforced ALuminium Laminates) developed at the Delft University of Technology, a combination of aluminium and aramid/epoxy [15]. Although the material showed promise, adoption by the aerospace industry for which it was developed was slow. With the development of GLARE (GLAss REinforced), an aluminium glass fibre laminate, a commercial breakthrough came when Airbus decided to use the material on its 650 seat A380. GLARE was intended to be an alternative to aluminium in aircraft structures. Research has shown it has benefits over both aluminium and glass fibre reinforced composites, especially in fatigue and impact. By the selection of different types of laminate components, together with the possibility to vary the volume fraction of the composite and fibre orientation, a wide range of material properties of the resultant product can be produced [15, 16]. The FML materials used in this investigation have been developed in the Materials Science Division of the University of Liverpool, work is ongoing to develop FIGURE 1 Schematic of the FML lay-ups used in the investigation.

4 236 EDWARDSON, et al. materials (or material combinations) that require much shorter manufacturing times and have superior impact resistance. Four types of materials were investigated of different lay-ups or construction, a schematic of the construction, lay-ups and nomenclature for the FML used is shown in figure 1. The first material used was a laminate of 0.3mm aluminium 2024 alloy and a glass reinforced polyamide. The polymer is a thermoplastic which has a melting point of approximately 280ºC. The second material was a laminate of 0.3 mm aluminium 2024 and a self-reinforced polypropylene; this polymer is also a thermoplastic which has a lower melting point of approximately 165ºC. The third material was a laminate of 0.3mm aluminium 2024 and a glass fibre reinforced polypropylene, the polymer here has a similar melting point to the previous material. Unlike the other materials used where the fibre orientations are orthogonal and bi-directional, it was possible with this last material to set the fibre orientations as bi-directional (standard) or in a single direction so as to investigate the effect of material anisotropy. A fourth GLARE thermosetting type material was also investigated after work was completed on the previous three material combinations. This material was a 2/1 lay-up combination of 0.9mm Al2024 and glass fibre reinforced epoxy. The reasons for investigating this lay up combination involving a thermosetting polymer will be discussed later. FIGURE 2 4/3 Polyamide based FML as Received Section.

5 LASER FORMING OF FIBRE METAL LAMINATES 237 The materials were manufactured using Teflon coated steel moulds where the laminates are laid up, using a polypropylene bonding interlayer to bond the pre-preg composite material to the aluminium. The moulds were then heated and a pressure applied to the upper surface to melt the bonding layers. A mounted and polished section of a 4/3 glass reinforced polyamide based FML is shown in figure 2. FML materials can be formed conventionally into components. However due to their high strength and laminated construction, difficulties can arise such as a limited minimum bend radius and the need for a metal layer within the laminate in order to deform it. The materials have considerable anisotropy and its axes change direction as any forming operation proceeds. Also, in laminated parts the layers can slip over one another. Another factor is the large residual stresses that remain between each layer after manufacturing, this can produce considerable distortion in a formed part. The aim of this study was to demonstrate the potential of laser forming as a manufacturing tool for FML materials, either as a means of direct manufacture or a means of alignment and distortion removal. 3. EXPERIMENTAL This investigation consists of an initial feasibility study to determine whether or not laser forming could be used to bend FML materials, a more detailed study of the laser forming characteristics and an investigation into the implications of laser forming on the material structure, including thermocouple analysis. A coupon size of 80x40mm was used throughout these studies and the bend line was always across the shortest width (i.e. a 40mm long bend) in the middle of the coupon. Energy parameters consistent with the temperature gradient mechanism (TGM) were used throughout this study. This mechanism produces a bend towards the laser [3]. An additional investigation was also performed to demonstrate the capability of the process to form larger more complex structures from 200x100mm coupons of FMLs. 3.1 Experimental Set-up An Electrox 1.5 kw CO 2 laser, wavelength 10.6µm, operated in a continuous wave mode was used for the forming process. The laser beam was fed via turning mirrors to a set of X-Y-Z CNC tables for beam manipulation. The FML coupons were guillotine cut to the correct dimensions

6 238 EDWARDSON, et al. and the upper surfaces were cleaned with acetone. They were then sprayed with graphite in order to increase the absorption of the 10.6µm radiation. The coupons were clamped 30mm from the scan line along one edge during processing using an aluminium clamp as can be seen in figure 3. FIGURE 3 Experimental Set-up. A laser range finder was used to record height data either side of the irradiation line and hence determine the bend angle after each pass. The sensor can be seen to the right of the processing head in figure 3. Combined with Galil software/pc based control of the CNC axes and shutter, the system was fully automated, producing bend angle output to file after each pass. The thermocouple analysis of a multi-pass strategy using a single Al foil was performed using K type thermocouples, which have a range of -200 to 1370ºC. The thermocouples were attached to the bottom surface of the foil on the centreline of the laser irradiation line using adhesive pads. An Agilent 34970A data acquisition unit was used to record the temperature data from the thermocouples, acquiring data at up to 250 readings per second. In order to verify the material integrity, the

7 LASER FORMING OF FIBRE METAL LAMINATES 239 laser formed samples were cut using a band saw, mounted, polished and photographed using an optical microscope. The materials could not be etched due to the presence of the composite. 4. RESULTS & DISCUSSION 4.1 Feasibility Study The initial concept for the use of laser forming with FML materials was specifically based around the development of new thermoplastic based fibre reinforced composites with high melting temperatures. In that it was thought that it may be possible to form the material as if it were the equivalent thickness of metallic solid, generating a semi-uniform thermal gradient through the thickness (TGM) [3], this can be seen in figure 4. FIGURE 4 Treating the section as a metallic solid results in a buckling of the Upper Layer due to non- TGM parameters and excessive heating. Initial tests on 1.38mm thick 2/1 glass reinforced polyamide based FML using 800W, a 5mm beam diameter and 80mm/s processing speed produced excessive melting of the upper 0.3mm Al 2024 layer, with little or no heat transfer into the lower layers. This led to the conclusion that due to the extreme differences in thermal properties between each of the layers in an FML lay-up it is not possible to set-up a uniform thermal gradient through

8 240 EDWARDSON, et al. the thickness. However, as the TGM causes a plastic compression just of the upper surface layers of a solid metallic section, it was thought that by forming by TGM the upper aluminium layer alone, a moment could be generated sufficient to bend the material section (figure 5). FIGURE 5 Laser forming the upper layer alone results in a positive bend, no melting and no obvious damage. Initial tests of this theory found that it was difficult to set up the TGM across the 0.3mm thickness. The material tended to buckle (Buckling Mechanism [3] ) and as the upper layer was constrained the buckle tended to be away from the laser and hence delamination occurs. Figure 4 shows the results of a 2/1 glass reinforced polyamide based FML processed at 300W, 3mm beam diameter and 90mm/s. By tuning the processing parameters for a high thermal gradient across the thickness of the upper aluminium layer, it was possible to produce a significant bend in the 1.38mm 2/1 glass reinforced polyamide based FML without any melting or damage (figure 5). Figure 6 shows the results of increasing numbers of passes using two different processing parameters, hence demonstrating the feasibility of using laser forming to bend FML materials. Additionally, what can be noted from this is the relatively small energy input required to bend the material. 4.2 Laser Forming Characteristics of FML Materials A number of studies were conducted in order to determine the laser forming characteristics of laminated materials. The first study was a repeatability test in

9 LASER FORMING OF FIBRE METAL LAMINATES 241 FIGURE 6 Laser Forming of 1.38mm 2/1 Glass Reinforced Polyamide based FML. order to confirm the initial results. Figure 7 shows the results of laser forming three 2/1 glass reinforced polyamide based FML coupons at 200W, 90mm/s and a 2.5mm beam diameter. The results show a good repeatability of the process. In addition it can be seen that there is a consistent linear increase in bend angle with increasing number of passes up to 20 passes. A study was also conducted to determine the effect of increasing numbers of layers for each of the thermoplastic based materials tested, using the same energy input parameters for each lay-up. In order to determine the FIGURE 7 Repeatability Test, 1.38mm 2/1 Glass Reinforced Polyamide based FML.

10 242 EDWARDSON, et al. effect of the bottom layer on achievable bend angle when forming 2/1 structures it was possible to manufacture 1/1 lay-ups (figure 1) without a bottom aluminium layer. Figure 8 shows the results for glass reinforced polyamide based FML. A comparison was also made with the laser forming of a single 0.3mm Al 2024 foil at the same energy input parameters. The thickness of the 3/2 and 4/3 lay-ups are 2.35mm and 3.1mm respectively. FIGURE 8 The Effect of Increasing No. of Layers on the Laser Forming of Glass Reinforced Polyamide based FML. It can be seen in figure 8 that as would be expected the achievable bend angle falls with increasing number of layers, for the 3/2 lay-up the maximum bend angle after 10 passes is 2º. This is consistent with the increase in material strength and increasing ratio of depth of material to available depth of plasticized zone. Hence, the moment generated in the upper surface produces less overall bend. The limiting effect of the lower layers can clearly be seen when comparing the 1/1 and 2/1 forming results. The results of this study for the second material type, a self-reinforced Polypropylene are shown in figure 9. Here a slightly higher energy fluence was used; 150W, 1.5mm beam diameter and 90mm/s. It can be seen that in this material using these energy input parameters a considerable bend angle can be formed after 10 passes in the 2/1 lay-up, of 20.6º, and in the 3/2 and 4/3 lay-ups more forming is seen

11 LASER FORMING OF FIBRE METAL LAMINATES 243 FIGURE 9 The Effect of Increasing No. of Layers on the Laser Forming of Self-Reinforced Polypropylene based FML. when compared to the forming of glass fibre reinforced polyamide based FML (figure 8). This bend angle increase may be due to the optimisation of the forming parameters and/or an indication of a difference in strength between the materials and hence formability. Figure 10 shows results for the study on glass-reinforced polypropylene FIGURE 10 The Effect of Increasing No. of Layers on the Laser Forming of Glass-Reinforced Polypropylene based FML.

12 244 EDWARDSON, et al. based FML. The fibre orientations for this study were as standard, bidirectional and orthogonal. As can be seen a comparison was made with the single 0.3mm Al 2024 foil, a 1/1 lay-up and the other standard configurations. As with figures 8 & 9 the results shown in figure 10 show the decrease in achievable bend angle with increasing number of layers used. For the same energy parameters used in figure 8 there is less overall forming indicating an increase in material strength between the selfreinforced polypropylene and the glass fibre reinforced polypropylene based FMLs. It can also be seen in figures 8, 9, and 10 that the bend angle rate per pass falls off with increasing number of passes. This may be due to a combination of the factors that influence the laser forming of solid metallic components such as strain hardening, plus an indication of a mechanical limit where the non-uniformity of the mechanical properties through the material thickness allows for a certain amount of distortion, before the bending strength of the material increases as the lower layers are placed under increasing tensile load. Figure 11 shows results for the effect of fibre reinforcement orientation on achievable bend angle using a 2/1 glass fibre reinforced polypropylene based FML. The fibre orientations are reported relative to the scan line direction. As can be seen in figure 11, the orientation of the reinforcement fibres has a large effect on the achievable bend angle and hence the bending strength of the material. The largest bend angle 21.2º is formed after 10 passes with the fibres FIGURE 11 The Effect of Fibre Orientation on the Laser Forming of Glass-Reinforced Polypropylene based FML.

13 LASER FORMING OF FIBRE METAL LAMINATES 245 parallel to the bending line thus offering little or no additional bending strength to the material. This study gives an insight into the effect of material anisotropy on the laser forming process. This effect could be used to improve the formability of a material in a particular orientation. The results of all these studies show that the effectiveness of laser forming to produce sharp single bends in these materials decreases with increasing number of layers. However there is sufficient available distortion per scan line even in 4/3 lay-ups for multiple scan line large radii bends and even the capability to use the process to align and remove distortion postconventional forming. It can be seen that the 2/1 lay-up shows the best potential for the use of laser forming as a direct manufacturing tool. 4.3 Implications of Laser Forming on Material Integrity It has been shown that laser forming can be used to produce significant bends in FML materials, in particular 2/1 lay-ups. It is necessary to determine what effect this process has on the material integrity, in particular the effect on the thermoplastic composite material which has a relatively low melting temperature. It has been described earlier that the approach taken to laser form these laminated structures relies on forming the top thin layer and as such a very small energy input is used. In order to determine how much heat is transmitted through the upper aluminium layer to the composite material a thermocouple study was performed. A thermocouple was mounted on the bottom surface of a 0.3mm Aluminium foil under the scan line, the foil was then processed using the empirically determine energy parameters. Figure 12 shows the thermocouple results for 6 passes at 200W, 90mm/s and a 2.5mm beam diameter. As can be seen in figure 12 the peak temperature seen at the bottom surface during forming is approximately 65ºC. Due to the thin section and high thermal conductivity of the aluminium the heat is rapidly dissipated after each pass. In addition it can be seen that thermal equilibrium is reached after the second pass with no further increase in peak temperature for subsequent passes. At the temperatures recorded on the bottom surface and hence the temperature experienced by the first composite layer, there should be little or no effect on the structure of the composite. This is backed up by optical microscopy of the irradiated area, an example of which is seen in figure 13. This shows the irradiated zone of a 2/1 polyamide based FML after 5 passes at 200W, 90mm/s and a beam diameter of 2.5mm. It can be seen that the composite layer appears undamaged, with no delamination and no

14 246 EDWARDSON, et al. FIGURE 12 Thermocouple Output for a 0.3mm Al 2024 Foil. FIGURE 13 2/1 Polyamide FML after 5 passes, 200W, 90mm/s, 2.5mmØ. reduction in the distance between the upper and lower aluminium layers, all other samples processed at optimum parameters are consistent with this result shown in figure 13. It can be also noted from figure 13 that a thickening is present in the irradiated zone of the upper layer. This is perhaps consistent with the TGM theory [3] in that as the material is

15 LASER FORMING OF FIBRE METAL LAMINATES 247 shortened laterally in the upper surface layers, to account for the volume of material, there is a thickening of the section. The effect observed in the FML samples is very pronounced when compared to laser formed solid metallic components, this could be due to the thin section of the upper layer or a unique effect due to the constraints of the lower layers. Further study would reveal this. An effect on the FML structure when processing with non-optimum FIGURE 14 Upper layer cracked due to non-optimum excessive heating. FIGURE 15 Delamination due to failure in bonding layer.

16 248 EDWARDSON, et al. (TGM) parameters is shown in figure 14. This 4/3 polyamide based FML was processed using 5 passes at 300W, 90mm/s and a 3mm beam diameter. It can be seen that the upper layer is cracked, this is thought to be due to excessive heating through the section leading to a sufficient reduction in yield stress and hence ultimate tensile strength, such that due to the limiting strength of the lower layers, the generated compression of the upper aluminium layer in the irradiated zone is more than can be carried by the Al2024 at that temperature and thus a crack forms. Figure 15 shows how the laser forming process on laminated materials relies on the ability to transmit the generated moment through to the lower layers. In other words the process relies on the strength of the bonds between the layers. In figure 15 it can be seen that bonding between the upper and the composite layers has failed and delamination has occurred. On closer inspection it was discovered that the polypropylene interlayer had folded back on itself in the mould prior to curing for this sample and thus an incomplete bond was formed. 4.4 Laser Forming of More Complex FML Components It has been demonstrated in the previous sections that it is possible to laser form Fibre Metal Laminate materials. For 2/1 lay-ups a considerable single bend angle is possible, for 3/2 and 4/3 lay-ups bend angles of only a few degrees are possible in a reasonable number of passes. This limits the manufacturing capability of the laser forming of FML components. However it is possible to form large radii bends using a series of stepped single bends of no more than a few degrees each. An example of this strategy is shown in figure 16; this part-cylinder was formed from a 200x100mm 2/1 glass reinforced polyamide based FML coupon, using 12 scan lines at 10mm intervals, using just 2 passes per line at 150W, 90mm/s and a 1.5mm beam diameter. From figure 6, 2 passes at these energy parameters results in a bend angle of ~2.8º. Considerably more forming has been achieved in coupon shown in figure 17 using the same strategy on a self reinforced polypropylene based FML. It can be seen in figure 9 that these parameters give ~5º after 2 passes per line. As discussed earlier there is sufficient available distortion per scan line even in 4/3 lay-ups for this multi-line strategy and even the capability to use the process to align and remove distortion post conventional forming. It can be seen however that the 2/1 lay-up shows the best potential for the use of laser forming as a direct manufacturing tool. As mentioned, there is a requirement for a metal layer to be within the FML to form the material

17 LASER FORMING OF FIBRE METAL LAMINATES 249 FIGURE x100mm Part-Cylinder formed from Glass Reinforced Polyamide based FML. FIGURE x80mm Part-Cylinder formed from 2/1 Self Reinforced Polypropylene based FML. successfully i.e. a minimum of a 3/2 lay up [16], laser forming therefore offers a useful tool to produce bends in 2/1 lay-up FMLs. It may also be possible to use a 3D laser forming approach [8] to form FML materials. However as can be seen in figure 10, the inherent effect of material anisotropy may add an unwanted additional complication to a 3D problem. Work is ongoing, however, by a number of research groups [8, 13] including

18 250 EDWARDSON, et al. the University of Liverpool on systems that use predictive and adaptive approaches to 3D laser forming independent of residual stress history and non-uniform material behaviour. 4.5 Laser forming of thermosetting based FML materials After the success of laser forming the thermoplastic based FMLs it was decided to verify the results using a GLARE type material (GLARE 3) [16], this material is a laminate of 2024 aluminium and glass fibre reinforced epoxy, a thermosetting material. As shown earlier, the laser forming process when applied to laminate structures relies on the bending of the upper layer alone, therefore in order to aid the process for this study, it was decided to increase the thickness of the Al2024 metal layers to 0.9mm, improving the laser formability of the material as it were by increasing the achievable moment. The material investigated was a 2/1 lay-up of glass fibre reinforced epoxy and 0.9mm Al2024. The fibre directions were bi-directional and orthogonal. The samples were again 40x80mm. As with the work presented earlier on the thermoplastic FMLs an initial feasibility study was performed. The first energy parameters investigated were taken from the successful work on the other materials, 150W, 1.5mm beam diameter and a speed of 90mm/s. As may be expected there was little FIGURE 18 Laser forming 2/1 GLARE type materials, initial feasibility test.

19 LASER FORMING OF FIBRE METAL LAMINATES 251 or no forming, perhaps due to the increased thickness and hence strength of the upper layer. It was therefore decided to increase the laser power to 300W and leave the other parameters the same, this produced a usable result. This result and the result of a repeatability test can be seen in figure 18. Here it can be seen that it is possible to laser form this thermosetting GLARE type material to the same extent as the thermoplastic FMLs. A limit can be seen in the data at approximately 6 which corresponds to a point were some delamination occurs, this is consistent with the problem of a minimum achievable bend radius for these materials [16]. In addition the energy parameters used caused some surface melting, therefore it was decided to increase the beam diameter to 3mm for a further study to establish optimum processing parameters where delamination does not occur. The result of this study at various processing speeds can be seen in figure 19. It can be seen here that the rate of forming per pass is governed by the energy input, the slower the traverse speed for the same power and spot size, FIGURE 19 Laser forming 2/1 GLARE type materials at various processing speeds. the higher the energy input and hence the higher the bend angle per pass. At 40mm/s it can be seen that the bend angle data is reasonably linear until approximately 5. At this point (pass 5) there is a significant increase in bend angle rate per pass, as with the previous study (figure 18) this also corresponds to a point where some delamination of the upper layer away

20 252 EDWARDSON, et al. from the lower layers can be observed in the sample. In this case the delamination appears to reduce the bending stiffness of the section as seen by the upper layer, thus allowing more deformation to occur for the given energy parameters. At approximately 13 (pass 7) the sample fails. The upper layer completely delaminated on the free end of the plate (the other end was still attached and in the edge clamp), the lower layers sprung back flat, as they were only elastically constrained and the upper Al2024 layer remained bent. Although this result demonstrates that it is possible to damage the material using laser forming, it can also be seen that as with the previous sections there is sufficient available distortion per scan line for multiple scan line large radii bends and even the capability to use the process to align and remove distortion post-conventional forming. This was observed at 60mm/s, that providing the plate was not bent to more than 5 in a single location, no delamination or damage occurred. FIGURE 20 Laser forming a multiple scan line large radii bend, 2/1 GLARE type material, 240x80mm. In order to demonstrate the concept of multiple scan line large radii bends in this material, a demonstration part was produced. As with the previous section a 240x80mm coupon was used and processed using 300W, 3mm beam diameter, a processing speed of 40mm/s. 21 lines, 10mm step between the lines and 2 pass per line were used. From figure 19 it can be seen that these energy

21 LASER FORMING OF FIBRE METAL LAMINATES 253 parameters would give approximately 2.5 per line, well below the 5 damage threshold. The completed demonstration part is shown in figure 20, it can be seen that this technique employing a number of smaller bends can produce a large overall distortion. One possible drawback with this technique however, is the fact that only the upper metal layer is plastically deformed, the lower layers are merely elastically constrained at such a large bending radius, thus adding possible un-wanted residual stresses between the layers. A solution to this issue and to the problem of a minimum bend radius is the (laser) pre-forming of each metal layers to a near required shape prior to bonding, then final alignment and adjustment with a laser forming technique. 5. CONCLUSIONS It has been shown that it is possible to laser form Fibre Metal Laminate materials without damage to the material or structure. The process is realised by laser forming by the TGM the upper aluminium layer alone. The results have shown that the effectiveness of laser forming to produce sharp single bends in these materials decreases with increasing number of layers. However it was shown that there is sufficient available distortion per scan line even in 4/3 lay-ups for multiple scan line large radii bends and even the capability to use the process to align and remove distortion post-conventional forming. It has been shown that the 2/1 lay-up shows the best potential for the use of laser forming as a direct manufacturing tool. As it is a requirement that a metal layer needs to be within the material to conventionally form the material successfully (i.e. a minimum of a 3/2 laminate), laser forming offers a useful tool to produce bends in 2/1 FML materials. An insight into the effect of material anisotropy on the laser forming process was also presented. This effect could be used to improve the formability of a material in a particular orientation. It was also shown that the technique could be used to form thermosetting GLARE type materials. A large part-cylinder was formed using a series of small bends. An improvement to the laser formability of the material was made by increasing the thickness of the metal layers, so as to increase the moment generated by laser forming the upper layer alone. Further study is planned on using laser forming to pre-bend the metal layers prior to curing to achieve larger available deformation and possibly more complex 3D structures.

22 254 EDWARDSON, et al. ACKNOWLEDGEMENTS The authors acknowledge the contributions to this study by Mr Jonathan Howard and Miss Heather Tjia. Thanks are also given to Derek Riley of BP Amoco for the kind donation of the self reinforced polypropylene. The authors are also grateful for the funding provided by the EPSRC. REFERENCES [1] Magee, J., Watkins, K. G., Steen, W. M. Advances in Laser Forming. Journal of Laser Applications. Vol.10 No. 6: December, 1998; pp [2] Moshaiov, A., Vorus, W. The Mechanics of the Flame Bending Process, Theory and Applications. Journal of Ship Research. Vol. 31 No. 4: December, 1987; pp [3] Vollertsen, F. Forming, Sintering and Rapid Prototyping. Handbook of the Eurolaser Academy, Vol. 2. Schuöcker, D (Editor), Chapman & Hall, 1998: [4] Maher, W., Tong, K., Bampton, C., Bright, M., Wooten, J., Rhodes, C. Laser Forming of Titanium and Other Metals is Useable Within Metallurgical Constraints. Proceedings of the 17 th International Congress on Applications of Lasers & Electro-Optics (ICALEO 98), Orlando, Fl, November 16-19, Section E, 1998; pp [5] Shackel, J., Sidhu, J., Prangnell, P. B. The Metallurgical Implications of Laser Forming Ti- 6AL-4V Sheet. Proceedings of the 20 th International Congress on Applications of Lasers & Electro-Optics (ICALEO 2001), Jacksonville, FL, October 2-5, [6] Magee, J., Watkins, K. G., Steen, W. M., Laser Bending of High Strength Alloys. Journal of Laser Applications, Vol.10 No. 4: 1998; pp [7] Magee, J., Watkins, K. G., Steen, W. M., Cooke, R. L., Sidhu, J. Development of an Integrated Laser Forming Demonstrator System for the Aerospace Industry. Proceedings of ICALEO 98, Section E, 1998; pp [8] Edwardson, S. P., Watkins, K. G., Dearden, G., Magee, J. Generation of 3D Shapes Using a Laser Forming Technique. Proceedings of the 20 th International Congress on Applications of Lasers & Electro-Optics (ICALEO 2001), Jacksonville, FL, October 2-5, [9] Watkins, K. G., Edwardson, S. P., Magee, J., Dearden, G., French, P., Cooke, R. L., Sidhu, J., Calder, N. Laser Forming of Aerospace Alloys. Proceedings of the SAE Aerospace Manufacturing Technology Conference, 2001, Aerospace Congress, Seattle, 2001: Paper No [10] Edwardson, S.P., Watkins, K.G., Dearden, G., French, P., Magee, J. Strain Gauge Analysis of Laser Forming, Proceedings of the 21 st International Congress on Applications of Lasers & Electro-Optics (ICALEO 2002), Scottsdale, Arizona, October 14-17, [11] Blake, R.J., Pearson, R.M., Revell, A.B. Laser Thermal Forming of Sheet Metal Parts Using Desktop Laser Systems. Proceedings of the 16 th International Congress on Applications of Lasers & Electro-Optics (ICALEO 97), Section E, 1997; pp [12] Hoving, W. Accurate Manipulation Using Laser Technology, Proc. of the 3 rd International Conference on Laser Assisted Net-shape Engineering (LANE 2001), Erlangen, Germany, August 2001, Eds. M. Geiger & A. Otto, Meisenbach, Bamberg, Germany; pp , ISBN

23 LASER FORMING OF FIBRE METAL LAMINATES 255 [13] Reutzel, E. W., Martukanitz, R.P., Michaleris, P., Zhang, L., Savitz, A.J., Magnusen, J.P., Aburdene, J.U., Gombotz, K.J. Development of a System for the Laser Assisted Forming of Plate. Proceedings of the 20 th International Congress on Applications of Lasers & Electro-Optics (ICALEO 2001), Jacksonville, FL, October 2-5, [14] Thompson, G., Pridham M. A Feedback Control System for Laser Forming. Mechatronics. Vol.7 No.5: 1997; pp [15] Vlot, A., Gunnink, J. W. Fibre Metal Laminates: An Introduction. Kluwer Academic Publishers, 2001, ISBN [16] Edwardson, S. P., French, P., Dearden, G., Watkins, K.G., Cantwell, W. J. Laser Forming of Metal Laminate Composite Materials, Proceedings of the 22 nd International Congress on Applications of Lasers & Electro-Optics (ICALEO 2003), Jacksonville, Florida, October 13-16, 2003.