Analysis of the effect of structural defects on the fatigue strength of RFSSW joints using C‐scan scanning acoustic microscopy and SEM

Abstract

Solid‐state refill friction stir spot welding (RFSSW) technology offers significant benefits in the fabrication of aluminium structures in the transport and aerospace industries. In this paper, the joining of 1.6‐mm‐thick Alclad 7075‐T6 aluminium alloy sheets is investigated. High‐cycle fatigue strength tests of single‐lap welded joints were carried out on an Instron E10000 testing machine with a limited number of cycles equal to 2 × 106. The welding of overlap fatigue specimens was conducted using an RPS100 spot welder by Harms & Wende GmbH & Co KG. C‐mode scanning acoustic microscopy (C‐SAM) and scanning electron microscopy (SEM) were utilised to evaluate the joint quality and characterise the microstructure. The paper discusses the effect of the maximum load force and defects (voids, hook, kissing bond, bonding ligament, etc) associated with the material flow in the weld on the failure mechanism. Insufficient plasticisation of sheet material and mixing of the material in the weld area are crucial defects that influence the number of destructive cycles. The weld defects in the joint structure are a source of a decrease in the fatigue life compared with the fatigue life of defect‐free welds. It was also found that RFSSW joint defects can be effectively detected by the nondestructive C‐SAM method.

Experimental and Numerical Investigations of Thin-Walled Stringer-Stiffened Panels Welded with RFSSW Technology under Uniaxial Compression

“…The properties of all joints in numerical models of the stringer-stiffened panels were considered to be homogeneous. However, in the case of the experimentally fabricated RFSSW joints in particular, the properties of the weld do change along the weld radius and weld perimeter [13].…”

“…The refill friction stir spot welding process was conducted using an RPS100 spot welder by Harms & Wende GmbH & Co KG (Hamburg, Germany) at the Belgian Welding Institute (Zwijnaarde—Ghent, Belgium). The RFSSW welding process can be briefly divided into four main stages: Touchdown, plunging, refilling and tool retracting, which were described in detail in the authors’ recent paper [13]. The parameters of the RFSSW were as follows: Spindle rotation speed 2000 rpm, plunge time 0.5 s, stirring dwell time 1.5 s, tool retract time 0.5 s and plunge depth 1.6 mm.…”

“…The RFSSW solid-state technique has the most outstanding advantage of fabricating spot welds without a keyhole. In the case of the RFSSW method, many scientific papers [11,12,13] have been written about joining the 2xxx and 7xxx series aluminum alloy sheets used for primary airframe structures. Kubit et al [13] investigated the effect of structural defects on the fatigue strength of Alclad 7075-T6 aluminum alloy RFSSW joints.…”

“…In the case of the RFSSW method, many scientific papers [11,12,13] have been written about joining the 2xxx and 7xxx series aluminum alloy sheets used for primary airframe structures. Kubit et al [13] investigated the effect of structural defects on the fatigue strength of Alclad 7075-T6 aluminum alloy RFSSW joints. This paper also discussed the effect of the weld defects associated with material flow on the failure mechanism of single lap joints.…”

Profile and Areal Surface Parameters for Fatigue Fracture Characterisation

“…Researchers have studied the characteristics of fractured surfaces using observational tools, from macro- to nanoscale [ 13 , 14 , 15 , 16 , 17 ]. Unfortunately, even though advanced methods such as optical coherence tomography [ 18 ], scanning acoustic microscopy [ 19 ] or energy response approach based on strain energy density histories during variable loading [ 20 , 21 ] are described in the literature, usually simple-fracture qualitative analysis conducted with scanning electron microscopy is employed for the evaluation of surface fractures resulting from impact [ 22 , 23 ], tensile strength [ 24 , 25 ], fatigue [ 5 , 19 ], ultra-high fatigue [ 26 ], adhesion testing [ 27 ] or even wear damage description [ 28 , 29 ]. However, few studies have investigated the use of profile and surface roughness as a tool for fatigue fracture characterisation.…”