Materials Science and Engineering A 445–446 (2007) 501–508 Quality assessment of friction stir welding joints via an analytical thermal model, iSTIR Pedro Vilac ¸a a,∗ , Lu´ ısa Quintino a , Jorge F. dos Santos b , Rudolf Zettler b , Sharham Sheikhi b a Universidade T´ ecnica de Lisboa, Instituto Superior T´ ecnico, Sec¸ c˜ ao de Tecnologia Mecˆ anica, Lisboa, Portugal b GKSS Forschungszentrum, Insitute for Materials Research, Geesthacht, Germany Received 19 April 2006; received in revised form 20 September 2006; accepted 25 September 2006 Abstract An inverse engineering approach has been implemented in order to demonstrate the feasibility of using the analytical thermal code iSTIR to establish correlations between the parameters of friction stir welding (FSW) and the properties of the resulting joints. The fundamentals of the iSTIR code are reviewed, and applications to two examples are presented. The components of the force and the torque applied by the tool to the parts and the temperature field were measured. The mechanical data allowed calculation of the mechanical power delivered by the tool into the weld joint, and the thermal field was used as an input for the iSTIR code, resulting in calculations of the heat power dissipated during the stationary state of the welding process. Correlations were then established between the thermal efficiency and the FSW parameters, allowing a comparison of the mechanical properties with the process parameters. The results show maximum thermal efficiency for intermediate FSW conditions, and that the evolution of the thermal efficiency is in accordance with the evolution of the assessed mechanical properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Friction stir welding; Analytical thermal model; Experimental measurements; Thermal efficiency; Mechanical efficiency 1. Introduction Friction stir welding (FSW) [1] is a method that produces better quality welds than other conventional processes, for all aluminium alloys [2,3] and most other metallic structural mate- rials, with the exception of some very hard metals [4,5]. This has led to a rapid implementation of the process in industry, aimed at higher production efficiency and component performance. The transport industries were the first to adopt this process [6], but nowadays FSW can easily be found in other industrial fields [7]. New tool geometries and important process developments are being accomplished on the basis of continuous research into the complex coupled phenomena related to the viscoplastic flow of the material [8], the development of the temperature profile, and the evolution of the metallurgy [9]. Moreover, easy, ready-to- use decision tools capable of anticipating the influence that tool design and process parameters have on the ultimate properties of joints play an important role in accomplishing transfer of the ∗ Corresponding author. Tel.: +351 218419002; fax: +351 218419058. E-mail address: pedro.vilaca@ist.utl.pt (P. Vilac ¸a). FSW process into industry. Similarly to the classical approach implemented in the case of conventional welding processes [10], such decision tools may be constructed through the establish- ment of correlations between FSW parameters and the heat input in the process. This procedure is valid if the thermal efficiency of the FSW process shows qualitatively the same trend as the most significant properties of the weld joint, for example the minimum hardness, ultimate tensile strength, toughness, fatigue properties and corrosion resistance. Although significant efforts have been allocated to the devel- opment of numerical solutions for modelling the features of FSW, the complex coupled phenomena of the FSW process have not allowed the establishment of a reliable commercial code that enables welding engineers to design and develop applications of FSW. Approaches to the computational modelling of FSW are typically based on continuum solid mechanics, usually with use of the finite-element method, and continuum fluid dynamics, usually with use of the finite-volume method [11]. This paper demonstrates the feasibility of using the analytical code iSTIR [12] for the establishment of correlations between FSW parameters and thermal efficiency. An inverse engineering approach based on an experimental/analytical modelling proce- dure, represented in Fig. 1, has been implemented. 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.091