Residual Stress Distributions in Bi-Metal (Ferritic to Austenitic Steel) Joints Made by Laser Welding Ruiz-Hervias Jesus 1,a , Iordachescu Mihaela 1,b , Luzin Vladimir 2,c , Law Michael 2,d , Iordachescu Danut 3,e , Ocaña Jose-Luis 3,f 1 Materials Science Dpt., E.T.S.I. Caminos, Universidad Politécnica de Madrid, Spain 2 Bragg Institute, Australian Nuclear Science and Technology Organisation, Australia 3 Laser Centre UPM, Universidad Politécnica de Madrid, Spain a jr@mater.upm.es, b miordachescu@mater.upm.es, c vll@ansto.gov.au, d michael.law@ansto.gov.au, e danut.iordachescu@upm.es, f jlocana@etsii.upm.es Keywords: autogenous laser welding, bi-metallic joint, microstructure, residual stresses Abstract. In this work, autogenous laser welding was used to join thin plates of low carbon ferritic and austenitic stainless steel. Due to the differences in the thermo-physical properties of base metals, this kind of welds exhibit a complex microstructure, which frequently leads to an overall loss of joint quality. Four welded samples were prepared by using different sets of processing parameters, with the aim of minimizing the induced residual stress field. Microstructural characterization and residual strain scanning (by neutron diffraction) were used to assess the joints’ features. Introduction Residual stress develops in fusion welding due to solidification and solid-state transformations into the weld pool and adjacent heat affected zone (HAZ). This may result on severe degradation of the structural integrity and performance of welded components [1, 2]. Therefore, mitigation of residual stresses still remains an important issue in welding. Various techniques, i.e. mechanical and thermal stressing of the welded joints, either during or after welding, have been developed to alleviate and mitigate the residual stresses [3, 4]. Joining of dissimilar metals is generally more challenging than joining similar ones because of the difference in the physical, mechanical and metallurgical properties of base materials. Welding of ferritic to austenitic stainless steels is considered to be a major issue. The transformation strains in solid-state phase, such as austenite to martensite or ferrite during cooling and different thermal expansion coefficients result in the formation of significant residual stress. This, together with expected differences in microstructure (formation of hard zones close to the weld interface and relatively soft ones adjacent to the hard zone) may lead to crack formation at the interface and eventually to failure in service [5]. Among the available welding techniques for dissimilar metals joining, autogenous laser welding has received increasing attention due to its ability to control the location of the focused beam with respect to the joint [6]. Thus, using an appropriate high power density control, welding based on the keyhole principle ensures a reduced energy transfer to the base materials. This in turn favours the formation of a narrow weld region with low residual stresses and small distortions. The present investigation addresses the influence of laser welding processing parameters used for joining dissimilar metals (ferritic to austenitic steel), on the induced residual stress field. Different sets of parameters were used to engineer the base metals apportionment at joint formation, namely distinct dilution rates. Microstructure characterization and residual strain scanning, carried out by neutron diffraction, were used to assess the joints. Through-thickness residual stress maps were determined in the autogenous laser welded samples of dissimilar steels with high spatial resolution. As a result, an appropriate set of processing parameters, able to minimize the local tensile residual stress associated to the welding process, was found. Materials Science Forum Vol. 772 (2014) pp 181-185 © (2014) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.772.181 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 137.157.8.253, Australian Nuclear Science and Technology Organisation, Kirrawee, Australia-15/11/13,01:33:36)