Journal of Materials Processing Technology 212 (2012) 19–26 Contents lists available at ScienceDirect Journal of Materials Processing Technology jou rnal h om epa g e: www.elsevier.com/locate/jmatprotec Welding residual stress reduction by scanning of a defocused beam F. Tölle a,∗ , A. Gumenyuk a , A. Backhaus b , S. Olschok b , M. Rethmeier a , U. Reisgen b a BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin, Germany b Welding and Joining Institute, RWTH Aachen University, Pontstrasse 49, 52062 Aachen, Germany a r t i c l e i n f o Article history: Received 6 May 2011 Received in revised form 26 July 2011 Accepted 31 July 2011 Available online 6 August 2011 Keywords: Residual stresses Stress reduction High energy beam welding Post-weld heat treatment Laser scanner optics a b s t r a c t The residual stresses in narrow electron or laser beam welds with high stress gradients are decreased without any contact surfaces or additional equipment by applying the welding beam after welding in a defocused mode for heating the material regions in a certain distance from the weld on both sides. In case of electron beam application, the beam is positioned and focused by the electromagnetic coil with high frequency. In case of laser beam application a laser scanner optics enables fast positioning by an optomechanic beam deﬂection, while defocusing of the laser beam is obtained by increasing the distance between scanner optics and workpiece. Dependent on the component geometry and on the beam power different process parameters are used. The adjustable process parameters are the radius and the power of the defocused beam and the transversal and longitudinal distances between the welding and the defocused beam. The mechanism and the inﬂuence of the process parameters are investigated by FEM-simulation and a number of experiments on a ferritic steel S355J2+N with 5 mm thickness. FEM- simulation is used to reduce the matrix of process parameters for the experiments. The best experimental result shows a stress reduction of about 70%. © 2011 Elsevier B.V. All rights reserved. 1. Introduction In welding, a small material region is melted and cooled down with high temperature gradients. As described by Nitschke-Pagel and Dilger (2006) in their review of the causes of welding residual stresses for diverse welding procedures and various materials, the heat-affected material shrinks during cooling and causes stresses. Especially due to high energy welding processes like electron or laser beam welding these stresses can reach the temperature and phase dependent local yield strength in longitudinal weld direction. Stresses ranging at this level will be relaxed by plastic deforma- tions as shown by Nitschke-Pagel and Dilger (2007a) in their review concerning welding residual stress assessment. The authors point out that residual stresses can inﬂuence the performance especially for high-strength materials. In consequence of stress relaxation or stress corrosion cracking these high stresses can limit service life depending on the tensile strength of materials as discussed by Nitschke-Pagel and Dilger (2007a) referring to a number of fatigue tests. Han et al. (2002) substantiated this inﬂuence of welding resid- ual stresses on the fatigue strength as well as on the component life and developed a model for quantitative prediction of residual stress relaxation under cyclic load. The inﬂuence of high tensile residual ∗ Corresponding author. Tel.: +49 30 8104 3101; fax: +49 30 8104 1557. E-mail address: ﬂorian.toelle@bam.de (F. Tölle). stresses on stress corrosion cracking was discussed by Sun (1993) and Brauss et al. (1998), too. A large number of processes for lowering the welding resid- ual stresses is available like stress relief annealing as mentioned and described by Nitschke-Pagel and Dilger (2007b) or the static low-stress-no-distortion-technique as developed and discussed by Guan et al. (2006) which works through generating a speciﬁed temperature proﬁle in the welded component using heating and cooling elements. But these methods are better applicable for wider welds such as TIG welds and for simple component geometries to provide the required contact surfaces for heating and cooling elements. They are also cost-intensive. The more ﬂexible dynam- ically controlled low-stress-no-distortion-technique invented and described by Guan et al. (1994) uses a trailing cooling jet for gen- erating the required temperature proﬁle. Nevertheless, van der Aa et al. (2007) proved that this method is only effective for cooling jet travel speeds of less than 8 mm/s. For faster welding processes like beam welding with ordinary welding speeds higher than 10 mm/s the penetration effect of this cooling is minimized. Other mechan- ical methods of stress reduction yield lower tensile stresses only in near-surface areas. Water jet peening is one of such methods. Mochizuki (2007) showed that this peening effect only intro- duces compressive stresses in the treated surface and enhances the fatigue strength due to the reduction of stress corrosion cracking. Beam welding processes offer the advantages of low compo- nent deformations, low heat input and remote application, which enable joining of complex component geometries. The previously 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.07.019