THEMATIC ISSUE: FLEXIBLE FORMING - INCREMENTAL SHEET FORMING & ROLL FORMING Towards accuracy improvement in single point incremental forming of shallow parts formed under laser assisted conditions Amirahmad Mohammadi & Hans Vanhove & Albert Van Bael & Joost R. Duflou Received: 9 September 2014 /Accepted: 30 November 2014 # Springer-Verlag France 2014 Abstract Single point incrementally formed parts with a low wall angle geometry typically exhibit a manufactured geom- etry that significantly deviates from the design surface due to accumulated unwanted bulging deformation. Development of the bulge on the bottom of the part might result in wrinkling of the sheet at the bulged region which leads to higher forming forces and can even cease the forming process. In this study, the geometric inaccuracy of low angled parts is investigated by means of both Finite element analysis and an experimental campaign on a conical geometry. Deformation mechanisms in shallow sloped parts have been studied in detail and the tool- sheet contact area has been characterized both for low and high angled geometries. In a second phase, the laser assisted single point incremental forming process and its potential for improving accuracy are investigated. To obtain suitable pro- cess parameters for a warm forming condition, a transient heat transfer analysis is developed to simulate the laser movement on the conical geometry. Based on the simulated and experi- mentally determined tool-sheet contact zone, different laser spot positioning strategies have been used while the accuracy of the part and forming forces were measured. It has been observed that overforming of the cone wall is due to the continuous deformation of the sheet outside the contact zone which changes into underforming upon laser treatment. By selection of a proper laser positioning strategy a reduction of 42 % in bulge height is observed. This shows its effect in reducing radial forming forces. Keywords Single Point Incremental Forming . Accuracy . Laser . Contact zone . FE modeling Introduction Most accuracy problems in SPIF arise from the lack of a die to support the metal sheet during the forming process, allowing unwanted plastic deformation outside of the tool-sheet contact zone to occur, which often results in excessively high dimen- sional inaccuracy. To overcome this problem, the metal sheet should be retained outside of the forming zone. It has been shown that a geometry dedicated backing plate can reduce overbending of the sheet at initial stages of the forming by supporting the perimeter of the workpiece [1]. Additionally, intermediate backing plates can be utilized to support strategic zones during forming, acting as partial dies. Yet, this compro- mises the flexibility of the process. However, the effect of using backing plates can also be achieved through localized in process modification of the material properties by laser scan- ning. It has been shown that in-process generation of selec- tively hardened bands at strategic locations in the forming geometry, such as areas with discontinuously changing draw- ing angles, has the same function as an intermediate backing plate and can thereby control subsequent unwanted plastic deformation of hardenable sheet metals [2]. Redistribution and releasing of residual stresses after unclamping and trimming of the SPIF formed parts might also induce inaccuracies. This type of inaccuracies is more severe for high strength materials and shallow wall angled parts which typically show lower geometrical stiffness. To preserve the accuracy after trimming, SPIF parts should be stress-relief annealed before unclamping [3]. Recently a new approach for stress relief annealing of SPIF formed parts using a defocused laser beam has been proposed, which additionally allows for local stress release and can be used on large scale parts without A. Mohammadi (*) : H. Vanhove : J. R. Duflou Department of Mechanical Engineering, KU Leuven, Leuven, Belgium e-mail: Amirahmad.Mohammadi@kuleuven.be A. Van Bael Department of Metallurgy and Materials Engineering, KU Leuven, Leuven, Belgium Int J Mater Form DOI 10.1007/s12289-014-1203-x