Authors: Dang Khoi Le, Shinichi Tashiro, Kieu Anh Duong Nguyen, Quang Ngoc Trinh, Tetsuo Suga, Naoki Sawamura, Kazuhiro Fukuda, Shuji Sasakura, Patricio Fernando Mendez, Anthony B. Murphy, Van Hanh Bui, Manabu Tanaka

DOI: 10.1007/s00170-026-17867-3

A team of welding researchers at the Joining and Welding Research Institute, The University of Osaka, working with Hanoi University of Science and Technology, Kobe Steel, the University of Alberta and CSIRO, has quantified exactly how three common flux ingredients — TiO₂, SiO₂ and CaF₂ — change the way molten metal detaches from a rutile-type flux-cored arc welding (FCAW) wire. The open-access paper appeared in The International Journal of Advanced Manufacturing Technology in 2026 and gives consumable manufacturers their first systematic, high-speed-video-backed map of how flux chemistry governs arc stability, spatter and fume.

Droplet diameter (DD) at the wire tip directly drives arc stability, spatter, bead appearance and even fume formation. Smaller droplets mean calmer spray transfer, lower spatter and lower fume; larger globular droplets mean the opposite. Yet, despite decades of FCAW use, most public studies have looked at deposition rate, penetration or slag peel, not at the metal transfer frequency (MTF) and DD as quantitative functions of flux chemistry. That gap is what the Osaka-led team set out to close.

The researchers built seven prototype 1.2 mm rutile flux-cored wires (AWS A5.20 E71T-1C) on a common formulation:

• Wire 1 (standard): 40.2% TiO₂, 4.5% SiO₂, no CaF₂
• Wires 2–3: reduced TiO₂ (29.7% and 20.1%)
• Wires 4–5: raised SiO₂ (7.2% and 9.9%)
• Wires 6–7: added CaF₂ (4.9% and 9.7%)

Bead-on-plate welds were run on SS400 mild steel with an OTC Daihen DP-350 power source in DCEP, 20 mm CTWD, 5 mm/s travel speed, and Ar-20%CO₂ shielding at 20 L/min. Each wire was welded at three currents — 220 A, 250 A and 280 A — with arc voltage adjusted between 27 V and 32 V to hold a stable arc length. A Memrecam high-speed video camera using the shadowgraph method captured every droplet detachment, and the frames were post-processed to count MTF and measure DD.

1. Iron content controls wire feed speed, not the oxides. WFS tracked the iron powder fraction in the flux almost linearly and was essentially independent of TiO₂, SiO₂ or CaF₂. More iron means a more conductive sheath, lower resistance heating and a higher WFS at a given current setpoint.

2. CaF₂ craters the transfer frequency. Adding calcium fluoride cut MTF at 220 A from 86.9 Hz ± 6.3 Hz on the standard wire to just 34.0 Hz ± 7.0 Hz at 9.7% CaF₂. Two mechanisms are responsible: vaporizing CaF₂ generates a strong recoil pressure that shoves the droplet back up the wire, and the high specific heat of CaF₂ plasma constricts the arc, raising arc pressure under the droplet. Both forces fight detachment. The effect weakens at 280 A, where higher electromagnetic pinch force dominates.

3. TiO₂ gives a small, steady boost. Raising TiO₂ nudged MTF from 162.5 Hz to 170.3 Hz at 280 A — a modest but consistent shift, attributed to lower viscosity and more stable slag behavior.

4. SiO₂ produces the biggest positive jump. Increasing SiO₂ from 4.5% to 9.9% lifted MTF at 280 A from 170.3 Hz to 222.9 Hz — a 31% increase. The mechanism is slag-metal chemistry: SiO₂, being less basic than TiO₂, transfers more oxygen into the droplet, lowering surface tension from roughly 0.96 N/m (TiO₂ case) to about 0.88 N/m. Lower surface tension lets the droplet neck and detach faster, producing smaller droplets.

5. Droplet diameter is set by MTF, not WFS. Across all seven wires, DD scaled inversely with MTF and was largely insensitive to wire feed speed. In other words, chemistry — not how fast the wire is pushed in — determines the size of the droplet that hits the pool.

For consumable designers the recipe is now quantitative: fluorides buy low diffusible hydrogen but they coarsen transfer and raise spatter, especially at low currents; silica refines droplets and stabilizes spray transfer but must be balanced against slag fluidity and oxygen pickup; titania is a mild tuning knob. For welders and fabricators, the data explains why some rutile wires run "buttery" at low heat input while others pop and spatter — and why wire choice matters more than voltage trim when chasing a calm arc.

The authors conclude that the findings give manufacturers direct, physics-based guidance for designing the next generation of rutile FCAW wires that combine stable spray transfer, low spatter, low fume and controlled hydrogen — all of which translate into higher productivity and better weld quality on the shop floor.

The article is open access under a CC BY-NC-ND 4.0 license; the full PDF and bibliographic record are available from the University of Osaka archive at https://hdl.handle.net/11094/104797.