{"id":4476,"date":"2020-09-03T10:05:18","date_gmt":"2020-09-03T10:05:18","guid":{"rendered":"http:\/\/www.alalloycasting.com\/?p=4476"},"modified":"2026-06-01T02:07:05","modified_gmt":"2026-06-01T02:07:05","slug":"degassing-molten-aluminum","status":"publish","type":"post","link":"https:\/\/www.alalloycasting.com\/ar\/degassing-molten-aluminum\/","title":{"rendered":"Degassing Molten Aluminum"},"content":{"rendered":"

Degassing molten aluminum removes dissolved hydrogen and nonmetallic inclusions\u2014the two primary contaminants that cause porosity, reduce tensile strength, and compromise fatigue life in finished castings. Rotary inert-gas degassing is the most widely adopted industrial method, achieving hydrogen levels below 0.10 mL\/100 g Al in properly optimized systems. This guide covers the metallurgical fundamentals, process variables, equipment selection, and quality benchmarks that determine degassing effectiveness across foundry and continuous casting operations.<\/p>\n

\"Degassing
Degassing Molten Aluminum<\/em><\/figcaption><\/figure>\n

Why Does Hydrogen Dissolve in Aluminum During Melting?<\/h2>\n

Aluminum’s affinity for hydrogen increases sharply above its liquidus temperature (660 \u00b0C for pure Al). At 750 \u00b0C, the equilibrium solubility of hydrogen in liquid aluminum reaches approximately 0.69 mL\/100 g, compared to only 0.036 mL\/100 g in the solid state\u2014a ratio of roughly 19:1. This dramatic drop during solidification forces hydrogen out of solution, nucleating gas pores throughout the casting matrix.<\/p>\n

The primary hydrogen source is moisture. Water vapor in the furnace atmosphere, damp charge materials, wet flux, and even humid ambient air dissociate at the melt surface according to:<\/p>\n

2Al + 3H\u2082O \u2192 Al\u2082O\u2083 + 6H (dissolved)<\/strong><\/p>\n

Hydrocarbons from lubricants, coatings, and combustion products contribute additional hydrogen. Controlling these upstream sources is important but insufficient on its own\u2014active degassing of the melt remains essential.<\/p>\n

\n\n\n\n\n\n\n\n\n
Hydrogen Source<\/strong><\/th>\nTypical Contribution<\/strong><\/th>\nMitigation<\/strong><\/th>\n<\/tr>\n<\/thead>\n
Atmospheric moisture<\/td>\n40\u201355%<\/td>\nDry furnace atmosphere, cover gas<\/td>\n<\/tr>\n
Charge material (returns, scrap)<\/td>\n20\u201330%<\/td>\nPreheat to 200\u2013250 \u00b0C<\/td>\n<\/tr>\n
Fluxes and grain refiners<\/td>\n10\u201315%<\/td>\nStore in sealed, heated containers<\/td>\n<\/tr>\n
Refractory \/ crucible interaction<\/td>\n5\u201310%<\/td>\nUse low-porosity refractories<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

What Are the Effects of Inclusions in Molten Aluminum?<\/h2>\n

Beyond hydrogen, nonmetallic inclusions\u2014primarily alumina (Al\u2082O\u2083), spinels (MgAl\u2082O\u2084), carbides (Al\u2084C\u2083), and various borides\u2014degrade casting integrity. These particles originate from oxidation of the melt surface, erosion of refractory linings, reactions with cover fluxes, and carryover of dross during transfer.<\/p>\n

Inclusions as small as 10\u201320 \u00b5m act as stress concentrators and crack initiation sites. In aerospace-grade castings, inclusion content is frequently specified below 0.5 mm\u00b2\/kg using PoDFA (Porous Disc Filtration Apparatus) analysis. Automotive structural parts typically require values below 1.0 mm\u00b2\/kg.<\/p>\n

\n\n\n\n\n\n\n\n\n
Inclusion Type<\/strong><\/th>\nSource<\/strong><\/th>\nSize Range<\/strong><\/th>\nPrimary Effect on Casting<\/strong><\/th>\n<\/tr>\n<\/thead>\n
Al\u2082O\u2083 films \/ particles<\/td>\nMelt surface oxidation, transfer turbulence<\/td>\n1\u2013500 \u00b5m<\/td>\nReduced ductility, leak paths<\/td>\n<\/tr>\n
MgAl\u2082O\u2084 spinel<\/td>\nMg-containing alloys (e.g., 5xxx, A356)<\/td>\n1\u201350 \u00b5m<\/td>\nHard spots, machining tool wear<\/td>\n<\/tr>\n
Al\u2084C\u2083<\/td>\nCarbon-based refractories, SiC particles<\/td>\n5\u201330 \u00b5m<\/td>\nPitting corrosion, surface defects<\/td>\n<\/tr>\n
TiB\u2082 agglomerates<\/td>\nGrain refiner overuse or poor distribution<\/td>\n10\u2013100 \u00b5m clusters<\/td>\nLocalized embrittlement<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

The critical point often underappreciated in production settings: degassing and inclusion removal are coupled processes. Inert gas bubbles collect suspended inclusions by flotation as they rise through the melt, so a well-designed degassing step simultaneously addresses both contaminants.<\/p>\n

How Does Rotary Degassing of Aluminum Work?<\/h2>\n

Rotary degassing introduces an inert gas\u2014typically argon or nitrogen, sometimes blended with 1\u20135% chlorine\u2014through a high-speed rotating impeller submerged in the melt. The impeller shears the gas stream into fine, uniformly dispersed bubbles (ideally 2\u20135 mm diameter). Degassing efficiency depends on three interconnected mechanisms:<\/p>\n

    \n
  1. Diffusion-driven hydrogen transfer.<\/strong>\u00a0Dissolved hydrogen migrates across the diffusion boundary layer into the low-partial-pressure interior of each inert bubble. Smaller bubbles provide a larger total surface area per unit volume of gas, increasing the mass transfer coefficient.<\/li>\n
  2. Bubble residence time.<\/strong>\u00a0Finer bubbles rise more slowly, extending contact time with the melt. A 3 mm bubble rises at roughly 0.25 m\/s versus 0.45 m\/s for an 8 mm bubble\u2014nearly double the residence time in a typical 600 mm deep crucible.<\/li>\n
  3. Melt circulation.<\/strong>\u00a0The rotor creates controlled vortex flow that continuously brings untreated metal into contact with the bubble swarm, preventing stagnant zones.<\/li>\n<\/ol>\n

    For inclusion removal, the small bubbles physically attach to oxide films and particles via surface tension forces and carry them to the surface, where they report to the dross layer.<\/p>\n

    What Process Parameters Control Degassing Effectiveness?<\/h3>\n

    Achieving repeatable results requires tight control of several interrelated variables:<\/p>\n

    \n\n\n\n\n\n\n\n\n\n
    \u0627\u0644\u0645\u0639\u0644\u0645\u0629<\/strong><\/th>\nRecommended Range<\/strong><\/th>\nEffect of Deviation<\/strong><\/th>\n<\/tr>\n<\/thead>\n
    Rotor speed<\/td>\n350\u2013500 RPM (varies by rotor diameter)<\/td>\nToo low: coarse bubbles, poor dispersion. Too high: excessive vortex, surface turbulence re-entrains oxide<\/td>\n<\/tr>\n
    Gas flow rate<\/td>\n5\u201315 L\/min (ladle), 15\u201340 L\/min (in-line)<\/td>\nExcess gas wastes purge media and increases turbulence without proportional H\u2082 removal<\/td>\n<\/tr>\n
    Treatment time<\/td>\n8\u201315 min per 500 kg (batch)<\/td>\nUnder-treatment leaves residual H\u2082 above spec; over-treatment adds cost and temperature loss<\/td>\n<\/tr>\n
    Melt temperature<\/td>\n700\u2013740 \u00b0C (alloy dependent)<\/td>\nHigher temperatures increase H\u2082 solubility and reabsorption risk<\/td>\n<\/tr>\n
    Rotor submergence depth<\/td>\n50\u2013100 mm above crucible floor<\/td>\nShallow placement allows gas short-circuiting to surface<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n

    A common operational error is compensating for a worn rotor by increasing gas flow. This produces larger bubbles, worsens surface turbulence, and often\u00a0increases<\/em>\u00a0oxide content despite appearing to maintain gas throughput. Rotor condition should be inspected on a fixed schedule\u2014graphite rotors typically require replacement after 60\u201380 operating hours in standard aluminum alloys.<\/p>\n

    \"Preliminary
    Preliminary oxidation of graphite rotor<\/em><\/figcaption><\/figure>\n

    How to Measure Hydrogen Content After Degassing?<\/h2>\n

    Quantitative verification is non-negotiable. The two most common methods in production environments are:<\/p>\n