3D LASER FORMING OF SADDLE SHAPES

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1 Reprinted from: S P Edwardson, K G Watkins, G Dearden, J Magee 3-D Laser Forming of Saddle Shapes Proc 3rd International Conference on Laser Asssisted Net Shaping (LANE 2001) Erlangen, August, 2001 pp Meisenbach-Verlag Bamberg ISBN D LASER FORMING OF SADDLE SHAPES S. P. Edwardson, K. G. Watkins, G. Dearden, J. Magee* Laser Group, Department of Engineering, The University of Liverpool, UK (*Currently at: National Centre for Laser Applications, NUI Galway, Ireland) Abstract There has been a considerable amount of work carried out on two-dimensional laser forming, using multi-pass straight line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys. However in order to advance the process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale 3D laser forming. The objective of this investigation is to establish rules for the positioning and sequencing of the irradiation lines required for the controlled 3D-laser forming of a symmetrical saddle shape from rectangular sheet material. The saddle shape was chosen due to its more complex 3D geometry, the double curvature providing a useful case study with which to build up the design rules for such 3D shapes. The investigation consisted of the laser forming of 400mm x 200mm 1.5mm gauge Mild Steel CR4 sheet using a CO 2 laser source. The scan strategies tested consist of straight and radial lines and concentric circular patterns. The active laser forming mechanism used varied from the temperature gradient to the upsetting mechanism depending on beam parameters and traverse speed used. The final geometries of the parts formed were verified using a coordinate measuring machine and are presented. It was possible to produce a controlled repeatable saddle shape using a concentric race-track strategy, employing the upsetting mechanism at the centre of the sheet, thus shortening the material at the centre and the plate naturally saddles. However the results of the investigation show that the problem of 3D laser forming is extremely complex. It was found that once a successful scan strategy was discovered symmetry was difficult to achieve due to the asymmetrical nature of the forming process itself, in that it was not possible to form the whole sheet at the same time. Keywords: Laser Techniques, Forming Methods, Laser Forming, Bending, 3D Shapes, Saddle, Double Curvature

2 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 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, and microelectronics. In contrast with conventional forming techniques this method requires no mechanical contact and hence 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 into the surface of a work piece. 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 metals that can be laser formed is considerable. As there is only localised heating involved below the melting temperature the bulk properties are not altered and good metallurgy is retained in the irradiated area [4]. Materials of particular interest are specialist high strength alloys [5]. These include titanium and aluminium alloys. These materials are widely used in the aerospace industry where the implementation of laser bending as a replacement of existing manufacturing processes is under investigation [6] as well as other industry areas [7]. There are numerous application areas of the laser forming process in use today that are considered cost effective in the design and/or manufacturing process, these include: Rapid prototyping of irregular shapes for turbine blade applications Non-contact forming for installation and adjustment of non-accessible parts Automotive shapes for prototype and validation testing Aerospace shapes for precision shaping of tanks and pressure vessels Unbending techniques for repairs and alignment applications Tube and pipe precision forming. Final configuration production parts for small quantities of parts. Rapid prototyping of parts prior to final production. 2 Experimental There has been a considerable amount of work carried out on two-dimensional laser forming, using multi-pass straight line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys [1,3-8] and some 3D work [9-12]. However in order to advance the process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale 3D laser forming. The investigation consisted of the laser forming of 400mm x 200mm 1.5mm gauge Mild Steel CR4 (Cold rolled) sheet using a CO 2 laser source. The objective being to establish rules for the positioning and sequencing of the irradiation lines required for the controlled 3D-

3 laser forming of a symmetrical saddle shape from the rectangular sheet material (Fig. 1). The saddle shape was chosen due to its more complex 3D geometry, the double curvature (i.e. positive to negative curvature) providing a useful case study with which to build up the design rules for such 3D shapes. 2.1 Experimental Set-up An Electrox 1.5 kw CO 2 laser, wavelength 10.6µm, run in continuous wave mode was used for the forming process. The laser beam Fig. 1: The Ideal Saddle shape was fed via turning mirrors to a height adjustable work bed mounted on a set of X-Y CNC tables. The 400mm x 200mm samples were held in place by a simple pair of stops that held the plate in position in the X-Y directions but allowed for Z-axis deformation as the part was processed (Fig. 2.). The steel work bed acted as an effective heat sink for the plate being formed, no aditional cooling was used. The samples were laser cut to the correct dimensions in order reduce any pre-stressing that might influence the forming results. To prepare them for forming they were first cleaned with acetone in order to remove the oil that protected them from oxidation. They were then spayed with graphite in order to increase the absorption of the 10.6µm radiation to around 80%. The graphite was sprayed using a hand held can, making it very difficult to achieve an even coverage of such a large plate. If too thick the graphite would simply burn off and if too thin the absorption would be poor. Fig. 2: Experimental Set-Up The scan strategies were developed via a purely empirical method. By simply taking an initial concept it was possible to develop it on a scan-by-scan basis. As there are many process variables in laser forming it was decided to hold as many as constant as possible and simply vary the scan speed and scan pattern to achieve the desired result, a distortion free and smooth contoured symmetrical saddle shape (Fig.1) with the minimum heat input. The laser power was held at 800W at the output window of the laser, however a power puck test at the processing head revealed a 5% loss from the turning mirrors, the actual power at the work surface was 760W. A beam diameter of 7mm was used, a beam of this size produces a large radii un-faceted or non-humped bend which was desirable in this case for a smooth contour.

4 The geometries of the parts formed were verified using a Mitutoyo TM Co-ordinate Measuring Machine (CMM). This system employs a micro-switched stylus on a 3 axis gantry to give height data for a known grid density, thus allowing contours to be plotted. 3 Irradiation Strategy Development & Results The scan patterns used to laser form the plates into saddles were arrived at after considering how the various mechanisms at work during laser forming may be used to form a saddle shape. The essential characteristics of forming a saddle from a flat rectangular piece of material are a shortening of both the diagonals and the length and width of the rectangle to give rise to the contours of a saddle. Due to this all the patterns must be symmetrical along the length and width of the plate and have their centre at the centre point of the rectangular plates. Therefore the initial concept then was to shorten the centre of the plate using a relatively slow processing speed to employ the upsetting mechanism and the geometry of the plate itself would produce the saddle shape. 3.1 Strategy 1 The concept behind this strategy (Fig. 3) was to shorten the plate along both its length and diagonals to form the saddle without using a basic X shape that might give rise to faceting or folding effects on the curved surface. The centre line was irradiated first and then the arcs in opposite directions. A not to scale schematic of the strategy is given in Fig. 3. Fig. 3: Scan Strategy 1, Speed 15mm/s Fig. 4: 3D Contour Plot Strategy 1 Fig. 5: Contour Plot Strategy 1 Fig. 6: Contour Plot Strategy 1 (end view)

5 All of the given contour plots are in the as formed orientation, with the laser direction being vertically down in the Z plane. Fig. 4 shows that a saddle shape has been formed, however Fig. 5 and 6 show that the saddle is distorted with little or no forming along the shorter edges. A visual inspection of the saddle showed faceting or folding effects around the centre of the sample and minimal curving of the short edges. This strategy was a success as an attempt to shorten the length and width of the plate with the minimum of heat input to avoid distortion of the shape. Smooth curvature of the sample is prevented however due to the fact the that the three irradiation lines pass too close to each other in the centre of the plate, giving rise to the faceting effects in this area. These effects only became noticeable when the sample had been measured by the CMM, in Fig. 6 the crease down the centre of the plate is evident. It was thought a faster traverse speed might aid in avoiding this effect as opposed to changing the placement of the arcs, also the plate suffers from a lack of curvature along its short edges, which is why scan strategy 2 was developed based on strategy Strategy 2 This strategy was a development of strategy 1. This aimed to achieve the same effect as the previous but with additional forming of the shorter edges (Fig. 7). An attempt is made to avoid the faceting effects by increasing the speed. As with the previous strategy the centre line was irradiated first, then the longer arcs and then the shorter arcs both in opposite directions. Fig. 7: Strategy 2, Speed 20mm/s Fig.8: 3D Contour plot Strategy 2 Fig. 9: Contour Plot Strategy 2 Fig. 10: Contour Plot Strategy 2 (end view)

6 Fig. 8 and 9 show that this strategy has successfully produced a reasonably symmetrical saddle shape if a little shallow. Forming of the shorter sides can also be seen. There was still some evidence of faceting and a lack of smooth contours at the centre of the plate however, and the saddle was somewhat twisted (Fig 10). There was some success in gaining more curvature along the shorter edges of the plate (Fig 10). The additional two arcs near the ends of the plate were added and the centre line shortened in an attempt to achieve this. The increase in speed did reduce the faceting effects but the drop in heat input to the plate reduced the overall amount of forming achieved. It was thought that a further increase in speed would eliminate the faceting but more irradiation lines would be required in order to maintain the amount of forming and to even out the heat input in order to avoid distortion. This lead to the development of strategy Strategy 3 The concept behind this strategy was to shorten the plate along its width and length. Curved lines were used in an attempt to avoid forming a box shape. This strategy used a concentric square circular pattern, the inner square was irradiated clockwise, the middle anticlockwise and the outer clockwise again to even out any distortion (Fig. 11). Fig. 11: Strategy 3, Speed 30 mm/s Fig. 12: 3D Contour Plot Strategy 3 Fig. 13: Contour Plot Strategy 3 Fig. 14: Contour Plot Strategy 3(end view) Fig. 12 and 13 show that this strategy has also produced a reasonably symmetrical if shallow saddle shape. Fig. 14, however, shows that the shorter edges have little or no forming

7 but the contours leading up to the edge do suggest the formation of the saddle shape. The faceting effects were eliminated with the increase in speed but the further reduction in heat input into the plate resulted in less forming and a loss of the smooth contours. However an attempt to increase the magnitude of forming by slowing the speed down using this strategy resulted in a distorted sample. It was concluded that a more subtle approach was required. It was thought that more irradiation lines in the areas where the magnitude of forming was a problem, namely the shorter sides, were required and this lead to the development of strategy Strategy 4 This strategy was developed to increase the depth of curvature along the shorter edges (Fig. 15). The concentric arcs at both of the shorter ends were designed to accentuate the contours required in those areas. Straight lines were used along the longer edges as it was thought arced lines were not influencing the final geometry. This strategy demonstrated the influence of the sequence of irradiation lines within a pattern. The pattern is concentric and circular and was initially executed processing from the centre to the periphery. This produced a shape with its highest point at the centre, not a saddle shape. However reversing the sequence, by irradiating firstly the arcs at alternating ends and then processing towards the centre using clockwise and anticlockwise concentric squares, produced the results below. Fig. 15: Strategy 4, Speed 30mm/s Fig. 16: 3D Contour Plot Strategy 4 Fig. 17: Contour Plot Strategy 4 Fig.18: Contour Plot Strategy 4 (end view) Fig. 16 and 17 show that a saddle shape has been formed. The influence of the concentric arcs are clear along the shorter sides with the contour lines echoing the irradiation strategy there.

8 The use of straight lines in the X direction produced the same if not more depth of curvature along the longest side as the arced lines. Fig. 18 shows that one side of the saddle is higher than the other but only by approximately 1mm. However, as can be seen on the right hand side of Fig. 17, this seems to pull the rest of the geometry out of shape. This distortion could be due to not centring the plate correctly or a pre-stressing of the plate. This second point can have a large influence on the repeatability of laser forming in that it is not always possible to know the stress history of a sample. Also, symmetry is hindered due to the asymmetric nature of the laser forming process itself, in that it is impossible to form the whole plate at once. To develop this strategy further it was thought that the sharp corners should be avoided, as these can cause distortion, due to a hot spot were the laser dwells as the table changes direction. Also working too close the edge of the plate should be avoided as this appears to influence the generation of a smooth curve along the edge. These points were taken into account when developing strategy Strategy 5 This strategy was a development of all the previous attempts. It was designed to shorten the plate across its length and width in order to give a smooth contoured saddle. The concentric circular pattern was found to work best when processing from the centre to the periphery. The inner circle was processed clockwise then each subsequent outer loop in the opposing direction (Fig. 19). The start points of the loops were spread evenly around the plate as dwell points occur due to a mechanical delay between the shutter opening and closing and table movement. It was found that this strategy allowed slowing of the processing speed in order to increase the magnitude of forming. Fig. 19: Strategy 5, Speed 20mm/s Fig. 20: 3D Contour Plot Strategy 5 Fig. 21: Contour Plot Strategy 5 Fig.22: Contour Plot Strategy 5 (end view)

9 This race track strategy successfully produced a very symmetrical saddle shape. Figs. 20, 21 and 22 confirm this. The concentric pattern appears to stabilise the saddle shape, even when processing at slower speeds for additional forming. The repeatability of this strategy was also very good. Samples processed at the same parameters are within a 2mm tolerance. It was also found that further forming could be achieved with this strategy for a given speed if the plate was supported centrally above the base plate and allowed to form freely without being hindered by its own weight. Due to the limitations of the set-up no account was taken of the beam parameters changing as the sample was distorting in the Z-axis. Online beam control with height sensing is a requirement in such forming operations. Development of an online monitoring system with predictive distortion correction abilities is also a requirement if any 3D laser forming operation is to be used in a manufacturing environment. A foreseeable problem with a system such as this which makes online distortion correction difficult is that, as with strategies used in this investigation, the final geometry of the part is not reached until sometime after processing has stopped, when the plate has cooled somewhat and the elastic stresses have been released leaving a plastically formed part. The results of this investigation show that the problem of 3D laser forming is extremely complex. The active forming mechanisms used in all of the strategies attempted are a combination of the upsetting and the temperature gradient mechanism (TGM). Inspection of the heat affected zone on samples processed at speeds as low as 20mm/s show evidence of a steep thermal gradient through the thickness of the material more consistent with the TGM than the upsetting mechanism. In strategy 5 the straight irradiation lines in the X-axis provide a longitudinal shrinkage of the plate that causes the longer sides to curve downwards (negative curvature) and as a result of this and the transverse shrinkage due to the semicircular irradiation lines, the shorter sides curve upwards (positive curvature). The direction and relative magnitude of the forming of each side is dependent on the length to width ratio of the sample used. Providing the scan strategy is resized accordingly and beam parameters tuned a saddle shape could be produced in any size of sheet material. 4 Conclusions It is possible to accurately produce a saddle shape from rectangular sheet Mild Steel CR4 using a concentric race track strategy. The processing parameters were 800W, 7mm diameter beam, graphite sprayed and a speed of 20mm/s. The essential characteristics of forming a saddle shape from a flat rectangular piece of material are a shortening of both the diagonals and the length and width of the rectangle to give rise to the contours of a saddle. Any strategy to form a saddle shape must be symmetrical along the length and width of the plate and have a centre at the centre point of the rectangular plate. Symmetrical laser forming is hindered due to the asymmetric nature of the laser forming process itself, in that it is not possible to form the whole plate at once. A solution to this may be scanning optics. Any pre-stressing of a work piece is a large factor in the magnitude of forming and any distortion of the final part.

10 Development of an online monitoring system with predictive distortion correction abilities is a requirement if any 3D laser forming operation is required to be used in a manufacturing environment. Future work will include the investigation of other 3D shapes and the development of an online monitoring system. 5 Acknowledgements The authors gratefully acknowledge the assistance given by Prof. L. Cooke, Dr. J. Sidhu and Dr. N. Calder of BAE Systems and G. Trowbridge of Rolls-Royce during this investigation. 6 References [1] Magee, J.; Watkins, K. G.; Steen, W. M.: Advances in Laser Forming. In: Journal of Laser Applications, December 1998, Vol. 10 No. 6, [2] Moshaiov, A.; Vorus, W.; The Mechanics of the Flame Bending Process: Theory and Applications. In: Journal of Ship Research, Dec. 1987, Vol. 31, No. 4, [3] Vollertsen, F.: Forming, Sintering and Rapid Prototyping. In: Handbook of the Eurolaser Academy Vol. 2, Schuöcker, D (Ed.), 1998, Chapman & Hall, [4] Maher, W. et al: Laser Forming of Titanium and Other Metals is Useable Within Metallurgical Constraints. In: Proc. of ICALEO 1998, Section E, [5] Magee, J.; Watkins, K. G.; Steen, W. M.: Laser Bending of High Strength Alloys. In: Journal of Laser Applications, 1998, Vol.10, No. 4, [6] 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. In: Proc. of ICALEO 1998, Section E, [7] Blake, R.J. et al: Laser Thermal Forming of Sheet Metal Parts Using Desktop Laser Systems. In: Proc. of ICALEO 1997, Section E, [8] Sprenger, A.; Vollertsen, F.; Steen, W. M.; Watkins, K. G.; Influence of Strain Hardening on Laser Bending. In: Manufacturing Systems 24, 1995, [9] Hennige, T.: Laser forming of spatially curved parts. In: Geiger, M; Vollersten, F. (Ed.): Laser Assisted Net Shape Engineering, Proc. of the LANE 97, Sept , 1997, Erlangen, Germany, [10] Vollerstsen F.: Applications of lasers for flexible shaping processes. In: Proc of the 12 th International Congress (LASER 95), Meisenbach, 1995, [11] Magee, J.; Watkins, K. G.; Hennige, T.: Symmetrical Laser Forming. In: Proc. of ICALEO 1999, Section F, [12] Edwardson S. P.: Laser Forming Dish Shapes A 3D Case Study. M.Sc.(Eng.) Thesis 1999, The University of Liverpool.