13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Effect of welding defects on plastic behaviour and fatigue lifetime of friction stir welded Al-Cu-Li alloy Thomas Le Jolu1,2, Thilo F. Morgeneyer1,*, Anne Denquin3, Anne-Françoise Gourgues-Lorenzon1 1 MINES ParisTech, Centre des Matériaux, UMR CNRS 7633, BP 87, 91003 Evry cedex, France 2 Now at CEA Saclay, 91191 Gif-sur-Yvette cedex, France 3 Onera BP 72-29 Avenue de la Division Leclerc 92322 CHATILLON cedex, France * Corresponding author: thilo.morgeneyer@mines-paristech.fr Abstract The effects of joint line remnant (JLR), kissing bond (KB), and clearance between the sheets (Gap) on tensile and fatigue properties of 2198-T851 friction stir welds have been quantitatively evaluated with respect to a reference weld made using one single sheet. The JLR has no significant influence in the investigated conditions. KB and Gap-induced defects do not significantly influence plastic yield but may induce premature crack initiation by ductile tearing and intergranular decohesions respectively. A critical value for KB opening (280 MPa), a threshold value for fatigue crack propagation from the KB (1 MPa√m) and crack growth rates consistent with literature data have been determined. Keywords Friction stir welds, Welding defects, Tensile properties, Fatigue resistance 1. Introduction Friction stir welding (FSW) of so-called “non-weldable” Al-Cu alloys has been developed to substitute riveting in lightweight structures [1]. It allows avoiding hot cracking and limiting component distortion. Internal flaws that are difficult to detect by using non-destructive evaluation (NDE) may appear under certain processing conditions. After welding, the natural oxide layer at butt surfaces may yield a defect [2-6] sometimes called “joint line remnant” (JLR). If the JLR is connected to the weld root and induces fracture after severe bending of the weld (e.g. [3, 7-8]), it is often referred to as a “kissing bond” (KB). In butt-welding conditions, a “Gap” might be left between the two sheets, leading to poor mixing of matter during FSW. The impact of such defects on tensile and fatigue properties has only been scarcely investigated [9-10]. The purpose of the present study is to compare the effects of JLR, KB and Gap-induced features on the tensile behaviour and fatigue lifetime of FSW joints of an Al-Cu-Li alloy dedicated to aircraft applications. 2. Experimental details 2.1. Materials and welding conditions A 3.1-mm-thick sheet of AA2198-T851 with chemical composition Al-3.20 Cu - 0.98 Li - 0.31 Ag - 0.31 Mg - 0.11 Zr - 0.04 Fe - 0.03 Si (wt %) was considered. The sheet had been aged at 155°C for 16h. Pancake-shaped grains (10 - 20 µm in thickness) were observed in this material (Fig. 1). The hardness of this material was around 150±5 HV0.1, except at mid-thickness (137±5 HV0.1). FSW butt joints were realised along the rolling direction using 500 mm (RD) x 150 mm (TD) coupons. Optimised welding parameters allowed minimising external defects such as flashes. Except for Gap bearing welds, the welding machine was displacement-controlled with a travelling speed of 480 mm.min-1, a rotational speed of 1200 rpm, a conventional retractable tool with 13 mm in shoulder diameter and 4.2 mm in pin diameter. “Sound” welds were made by moving the tool into a single sheet, to avoid any native oxides at edges before welding. “JLR bearing” welds resulted from natural oxidation of the coupons before welding. “KB bearing” welds were fabricated
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