In-depth microscopic characterisation of the weld faying interface revealing stress-induced metallurgical transformations during friction stir spot welding. (May 2021)

• Record Type: Journal Article
• Authors: Labus Zlatanovic, Danka; Balos, Sebastian; Bergmann, Jean Pierre; Rasche, Stefan; Zavašnik, Janez; Panchal, Vishal; Sidjanin, Leposava; Goel, Saurav
• Is Part Of: International journal of machine tools & manufacture. Volume 164(2021)
• Publication Date: 2021-05
• Subjects: Friction stir spot welding -- Delamination -- Weld faying interface -- Small punch test -- Dynamic precipitation; Machine-tools -- Periodicals; Manufacturing processes -- Periodicals; Machines-outils -- Périodiques; Fabrication -- Périodiques; Electronic journals; 621.902
• DOI: 10.1016/j.ijmachtools.2021.103716
• Languages: English
• ISSNs: 0890-6955

Abstract

Abstract: Friction stir spot welding (FSSW) is a solid-state welding process, wherein the properties of a weld joint are influenced by the state of friction and localised thermodynamic conditions at the tool-workpiece interface. An issue well-known about FSSW joints is their lack of reliability since they abruptly delaminate at the weld-faying interface (WFI). This study explores the origins of the delamination of multiple lap welded aluminium alloy (AA 5754-H111) sheets joined by FSSW at different rotational speeds typically used in industry. Experimental techniques such as the small punch test (SPT), Vickers hardness test, Scanning Electron Microscopy (SEM), Scanning Acoustic Microscope (SAM), Transmission Electron Microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDX) and Frequency-Modulated Kelvin Probe Force Microscopy (FM-KPFM) were employed. The experimental results revealed that a complex interplay of stress-assisted metallurgical transformations at the intersection of WFI and the recrystallised stir zone (RSZ) can trigger dynamic precipitation leading to the formation of Al3 Mg2 intermetallic phase, while metallic oxides and nanopits remain entrapped in the WFI. These metallurgical transformations surrounded by pits, precipitates and oxides induces process instability which in turn paves way for fast fracture to become responsible for delamination. Graphical abstract: Image 1 Highlights: A new approach is proposed to join multiple thin sheets of Al alloys using friction stir spot welding. Weld faying interface was comprehensively characterised using a wide range of microscopy techniques. Small punch tests and Vickers microhardness tests were performed to characterise the weld joints. An instance of dynamic precipitation was seen to result in the formation of Al3 Mg2 intermetallic brittle phase. The new experimental results revealed stress-induced metallurgical transformations in the welding faying interface.