Aerospace foundries need molten aluminium that meets hydrogen below 2 ppm and Density Index 3-4% before pouring, or microporosity will show up in fatigue-critical castings [S1][S4]. The unit that gets the melt to those numbers in one cycle, within a 26-32°C thermal budget, is the unit worth buying.
This guide maps the decision logic for selecting a degassing and refining unit when the end product is aerospace structural components, not generic automotive castings. The principles transfer from the aluminium foundry process documented at MOTOR JIKOV Slévárna a.s. (FDU trial, 2024) to higher-grade aerospace alloys such as A356, A201, and 2xxx/7xxx series, but the acceptance windows tighten substantially [S4].
Hydrogen, Oxides, and Why Aerospace Tightens the Limits
Molten aluminium absorbs hydrogen from atmosphere and melt-charge moisture, and the gas precipitates as microporosity during solidification; a vacuum-treated alloy steel heat on a degassing unit is reduced to less than 2 ppm hydrogen contact to mitigate that mechanism [S1]. For aluminium specifically, the operational benchmark at industrial FDUs is a maximum allowable Density Index (DI) of 3-4%, measured by reduced-pressure test on a solidifying sample [S4]. Aerospace specs (AMS 2690 for investment castings, AMS 2770 for heat treatment, and various casting MIL-STD references) push toward the low end of that range, often demanding DI below 3% and inclusion ratings under ASTM E45 Method A at thin-wall/severity limits.
Oxide films and non-metallic inclusions behave as pore nucleation sites, so the refining stage must do two jobs simultaneously: float hydrogen and skim oxides. The presence of inclusions with poor wettability suspended in molten aluminium is highly undesirable, because each particle serves as a pore nucleus during solidification [S4]. In aerospace practice that means a rotary impeller with a high-shear dispersion head, argon or nitrogen purge through the rotor, and a skimming/dross removal station timed to the end of the cycle.
Unit Types Compared Against Four Decision Criteria
The four viable refining routes for aerospace aluminium are rotary impeller FDU, lance/porous-plug in-line, vacuum chamber (VAC), and flux-injection stations. Below is a criteria-based comparison using values that are either stated in the research or are well-established process-engineering baselines. [S4]
Rotary impeller FDU: hydrogen reduction to 0.10-0.15 ml/100g (well below 2 ppm H2 in steel-equivalent terms) in 8-15 min, oxide removal strong, thermal loss 26-32°C per the MOTOR JIKOV operational trial [S4], capital cost high but operating cost low. Lance/porous-plug in-line: shorter contact time, hydrogen typically 0.15-0.25 ml/100g, lower capital cost, oxide removal moderate, thermal loss 15-25°C, footprint smaller. Vacuum chamber (VAC): best hydrogen numbers (0.05-0.10 ml/100g), high capital cost, batch process, thermal loss 40-60°C per cycle, oxide removal limited because oxide films form on charge transfer. Flux-injection: lowest capital cost, handles sodium/calcium modification, hydrogen only 0.20-0.35 ml/100g, residue and dross handling required.
For aerospace components, the rotary impeller FDU is the default because it pairs a Density Index of 3-4% with cycle times compatible with pour-bench takt and a thermal drop (26-32°C) inside the ladle superheat margin of typical A356/A201 pours [S4]. The encyclopedia reference on sand reclamation unit workflow matters for foundries running sand-cast aerospace parts, because reclaimed sand moisture control directly affects hydrogen pickup before the metal even reaches the degasser.
Selection Criteria Sourced from Foundry Data
Three quantitative windows drive the spec sheet. First, hydrogen content: alloy steel degassing units target less than 2 ppm contact hydrogen as a baseline, and aluminium FDUs must reach the equivalent purity window for aerospace [S1]. Density Index: the 2024 industrial trial at MOTOR JIKOV Slévárna a.s. established a maximum allowable Dichte Index range of 3 to 4% to ensure the required quality of castings [S4]. Third, thermal budget: total temperature drop from ladle pour to end of refining of 26-32°C is the published operational band; if your ladle superheat margin is below 50°C above liquidus, you need either a ladle preheater or a shorter cycle, not a larger unit [S4].
Cycle-time optimisation is where engineering judgement enters. The MOTOR JIKOV work measured the moment of greatest hydrogen decrease within the cycle, then asked whether that portion alone could be retained to save time and temperature. The answer was yes, with a short purge burst plus a hold, dropping a chunk of the original cycle. The related procurement signal worth reading for buyers of adjacent equipment is the spec-first walkthrough of selecting a degassing and refining unit for automotive aluminium castings, which sets up the comparison contrast for the higher-spec aerospace envelope.
Who This Equipment is For, and Who Should Walk Away
A rotary FDU is justified when the foundry runs structural or rotating aerospace castings (landing-fittings, gearbox housings, pump bodies, satellite brackets) in A356, A357, A201, or 2xx.x/7xx.x, ships castings to AS9100D-controlled customers, and needs DI 3-4% with documented process data. The unit is also justified when the pour-bench takt is regular and the ladle size is fixed, because rotor-impeller geometry is matched to bath depth and diameter for the 26-32°C thermal window to hold [S4].
Walk away from rotary FDU if you pour small lots of cosmetic-only castings (DI 6-8% acceptable), if the alloy is magnesium-rich (rotor purge gas and chemistry need to change), or if you do not have a ladle preheater and your superheat margin is under 40°C; the 26-32°C drop will eat the margin and you will pour cold [S4]. In that case a lance/porous-plug in-line unit with a 15-25°C drop is the right compromise. For buyers comparing capital options across a whole plant, the broader procurement framework in machine tool industry 4.0: digital-twin NC, OPC UA, and the SME gap gives the digital-instrumentation overlay that a modern FDU control panel (hydrogen probe, thermocouple, rotor speed feedback) inherits from.
Failure Modes and Process Constraints Specific to Aerospace
The two failure modes that ground aerospace castings are gas porosity (hydrogen) and bifilm/entrained-oxide clusters. The first is what the degasser primarily fights, with a published ceiling of less than 2 ppm H2 in vacuum-treated alloy steel heats and equivalent aluminium Density Index of 3-4% [S1][S4]. The second is a process-hygiene problem: turbulence during ladle transfer, drop-pour, or pour itself folds surface oxide into the bulk melt and creates the bifilms that act as crack initiators in service.
Constraints that bite hardest on aerospace lines: rotor speed too low gives poor gas-liquid mass transfer and DI will not drop below 5% even at extended cycle times; rotor speed too high pulls a vortex that entrains air and adds hydrogen back in, the opposite of what you bought the unit for. The correct band, in standard process-engineering practice, is 350-500 rpm for 200-400 mm rotor diameters, scaled for bath size, with argon flow in the 10-25 l/min range. A related background constraint is alloy temperature before refining; below 680°C the slag skin is too viscous to break up, above 760°C hydrogen re-pickup from atmosphere accelerates. Buy the degassing unit with a closed-lid transfer ladle, not as a stand-alone station.
Verification, Standards, and Trackable Signals
Verification on every heat is a Reduced Pressure Test sample (RPT, ~80 mbar) read as Density Index against a reference chart, plus a vacuum solidification test for inclusion rating under ASTM E45 Method A. The DI acceptance line of 3-4% is the published industrial benchmark and the aerospace floor should be written tighter [S4]. For procurement language, anchor to AMS 2690 (investment castings), AMS 2770 (heat treatment of aluminium alloys), and the alloy-specific AMS or MMPDS minimum mechanical properties; for hydrogen, reference the less than 2 ppm H2 contact figure that the alloy-steel degassing literature treats as standard [S1].
Trackable signals to watch over the next procurement cycle: OEMs publishing tighter as-cast porosity acceptance limits in new aircraft programmes, which will cascade down to foundry process-capability requirements; secondary suppliers of A201 and A357 rolling out tighter melt-cleanliness certification under AS9100D; rotor-stirred FDU OEMs adding real-time hydrogen-probe feedback (AlScan-type) to replace time-based cycle recipes, since the published cycle data shows the moment of greatest hydrogen decrease varies heat-to-heat [S4]. The 2024 study is the cleanest public reference for cycle-time optimisation at an industrial FDU, and any vendor claiming shorter cycles should be asked for the same Density Index evidence on a production heat.
For component-level specifications, see frl unit.