Application of 3D DIC-Assisted Residual Stress Measurements for Friction Stir Welding Weld from Ultrafine-Grained Alumin
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dge of the residual stresses is very important in many industrial applications. One of the most prominent examples are welds, because during welding a material is subjected to a complex thermal treatment that results in high residual stresses. Frequently, residual stresses in welds are measured using the hole-drilling method.[1] In this work, we describe a modification of this technique based on deformation measurements close to the holes. Conventionally, such measurements are carried out using strain gauge rosettes; here, 3D digital image correlation (DIC) is used. Such a modification allows not only delivering an increased number of measurement points, but also reduction of the distance between the holes. A DIC-assisted method was applied to a weld made using friction stir welding (FSW),[2] which is one of the modern welding techniques that takes place in the solid
TOMASZ BRYNK is with the Faculty of Materials Science and Engineering, Warsaw University of Technology, Wołoska 141, 02-507 Warsaw, Poland and also with the SCK CEN, Boeretang 200, 2400 Mol, Belgium. MARTA ORŁOWSKA and MAŁGORZATA LEWANDOWSKA are with the Faculty of Materials Science and Engineering, Warsaw University of Technology. Manuscript submitted August 13, 2020 and accepted October 14, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
state. It can therefore be used for joining heat-sensitive materials, such as those that are ultrafine grained (UFG). UFG materials feature superior strength because of their reduced grain size, and thereby elevated surface area of grain boundaries, and increased density of dislocations. They are commonly produced using severe plastic deformation (SPD)[3] methods. One such method that makes it possible to produce plates suitable for FSW is incremental equal channel angular pressing (I-ECAP).[4] The weld investigated was selected in such a way that the influence of both the SPD processing and the welding method on the residual stresses could be investigated. As a base material for welding, commercially pure aluminum 1050 (99.50 wt pct) was chosen in the form of plates with dimensions of 3 9 62 9 100 mm3. Material annealed at 450 C for 2 hours has grain size at a level of about 12.9 lm and a fraction of high-angle grain boundaries (HAGBs) reaching 81.7 pct. After eight passes of I-ECAP with route C, the microstructure underwent a reduction in grain size to about 1 lm, resulting in a fraction of HAGBs of 53.2 pct (for details on the processing procedure and microstructure features, see[5]). A TEM micrograph of this sample is presented in Figure 1(a). Similar welding was performed using FSW with a tool having a shoulder diameter of 12 mm. A tapered, threaded pin with a length of 2.85 mm, diameter at the shoulder of 5 mm and diameter at the tip of 3.5 mm was used. A rotational speed of 800 rpm and linear speed of 400 mm/min were applied. The angle of inclination of the tool was 1.5 deg. The microstructure of the stir zone (SZ), presented in Figure 1(b), features an average grain size of 3.1 lm and a fraction of HAGBs of 75
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