Method of arresting crack growth for application at a narrow working space

Abstract

The AA6063 alloy used in this work was in the T5 temper state with chemical composition as shown in Table 1. Table 1 Chemical Composition of AA6063, wt% Element Mg Si Fe Cu Zn Mn Ti Other Al 0.46 0.46 0.15 0.014 <0.005 0.03 0.012 0.2 Bal Welding of the 15mm thick AA6063-T5 plates was carried out in two weld passes from both surfaces of the plates in a butt joint configuration. A tool with a shoulder diameter of 20mm and tool pin length and diameter of 8mm was employed for both weld passes. The work-piece was overturned about the weld center-line (WCL) after the first weld pass and the second pass weld was then fabricated in the same welding direction resulting in an asymmetrical weld cross section, Fig. 1, where the welding direction was maintained longitudinal to the direction of extrusion of the plates. The tool rotation and transverse speeds were kept at 1800rpm and 600mm/min respectively for both passes and the tool axis was tilted by 3° with respect to the vertical axis. The cross section of the weld area, perpendicular to the weld direction was analyzed using optical microscopy after metallographic preparation and etching in Kroll's reagent. The grain size was estimated using the linear intercept method. 2.2 Hardness and Tensile Tests The hardness distribution and tensile properties were investigated through micro-indentation hardness tests and traction tensile tests respectively. To characterize the hardness distribution along the weld width and thickness, multiple indentations were made on the weld cross section along three lines, 2mm from either surface and along the mid-thickness, using a Vickers indenter under a 200g force for 5seconds. The conventional macro-tensile strength of the joint was investigated on pin-loaded specimens with gage lengths and thicknesses of 60mm and 5mm respectively. The specimens were extracted transverse to the weld direction at different depths across the weld thickness, coincident with the locations of hardness measurement (Fig. 2(a)). The local yield strength of the weld zones and base metal were then investigated through miniature specimens precisely extracted by electro-discharge machining (EDM) parallel to the weld direction; Figs. 2(a) and 2(b). Fig. 1 Fabrication of a two-pass friction stir welded joint. The work-piece was overturned about the weld center-line after the first weld pass and the second pass weld was then fabricated in the same welding direction.