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Refill Friction Stir Spot Welding Parameter Development

Refill friction stir spot welding parameter development in transport industry aluminium alloys

TWI Core Research Project P31030



Refill friction stir spot welding (Refill FSSW) has several advantages over other mainstream single-point joining or fastening processes. As a solid-state process producing forged microstructures, refill FSSW joints generate minimal residual stresses/distortions, whilst completely avoiding the common defects associated with fusion welding, such as porosity and solidification cracking. Refill FSSW is also a zero-mass adding technology, as no material is added to the weld. The flush surface finish (Fig. 1D) is highly desirable for applications where aerodynamics and aesthetics are critical factors.

With early adopters appearing in the railcar and automotive production industries, current industrial research focuses on proving joint properties and further decreasing the cycle time to attract more large-scale, high rate producers, who may typically value marginal cycle time gains over quality and mass penalties.

This report provides the reader with an impartial gathering of core data on the performance and properties of refill FSSW welds in various transport-sector aluminium alloys.


  • Determine ‘weldability’ of common transport sector aluminium alloys (AA2024-T3, AA5754-H24, AA7075-T6)
  • Benchmark comparison (resistance spot welding and riveting) of performance and properties
  • Understand effect of interfacial aerospace sealants and process and performance
Figure 1. 
A) Refill FSSW head (on TWI’s robotic arm)
B) Schematic of refill FSSW process
C) Refill FSSW tool assembly
D) Near-flush surface finish of refill FSSW
Figure 1. A) Refill FSSW head (on TWI’s robotic arm) B) Schematic of refill FSSW process C) Refill FSSW tool assembly D) Near-flush surface finish of refill FSSW


  • Full-factorial design of experiments, varying key parameters, used to determine alloy ‘weldability’
  • Ranking considered i) cross-section integrity and ii) static mechanical properties to be of key interest
  • Best performing coupons selected for i) fatigue and ii) interfacial sealant trials
  • Fracture surfaces and failure modes examined using electron microscopy



  • All joints produced surpassed benchmark strength
  • Best performing process parameters achieved static strengths ~95% of an equivalent aluminium rivet
  • The use of interfacial sealant led to a 50% increase in static strength (vs. no sealant)
  • Average lap shear values returned low scatter and consistent failure modes
  • Fatigue test results were used to establish an equation for the S-N curve. Acceptable fitting coefficients were found for each test condition
  • Failure close to the first million cycles was observed for the specimens in the bare condition, for maximum loads of 20% of static strength
  • Interfacial sealants improved fatigue strengths by factors of 2 to 10 (depending on alloy), without compromising microstructural consolidation and integrity
Avatar Dr Pedro de Sousa Santos IEng Senior Project Leader for Advanced Manufacturing Technologies, FFP

Pedro is working as a senior project leader in TWI’s Friction and Forge Processes section, managing client relationships and the delivery of projects involving friction welding technologies, predominantly for the aerospace and space sectors. Due to his experience in engineering projects, Pedro is affiliated with the Engineering Council and The Welding Institute. As an active STEM ambassador, he frequently presents and supports workshops on engineering related topics to audiences ranging from Year one to sixth form students.