The effect of aluminium alloy liquid cleanliness on the die casting process and product quality

The cleanliness of the alloy fluid is the permissible level of oxidation inclusions, non-metallic inclusions and alloy gases in the alloy fluid. Excessive inclusions in the alloy can affect the die casting process and the quality of the product. Specifically, there are several aspects:

The fluidity of the alloy liquid is reduced, the ability to fill the mould, the ability to compensate for shrinkage is reduced, thus affecting the pressure transfer, resulting in poorly formed products and loose organisation.

Inclusion of missing material in the product gate, impurities in the machined surface chipping or less material depression.

Disruption of the continuity of the product tissue, reducing the mechanical properties (strength and elongation) of the product.

Increased risk of product leakage and, in severe cases, cracking or fracture due to stress concentration in the product under load.

Formation of pitting or mesh-like pinholes on the machined surface of the product.

Due to the above problems, when melting die casting alloys, sufficient attention should be paid to the quality of the raw alloy, the effect of slagging and de-gassing and final refining to ensure that the quality of the alloy liquid meets the product quality requirements.

I. The generation of oxidation inclusions and gases in the melting process of aluminum alloy.

When the high temperature aluminum block or aluminum alloy liquid and the gas in the furnace hall N?, O?, H?O, CO?, CO, H?, CmHn, etc. come into contact, a series of physical and chemical processes such as chemistry, chemistry division, dissolution, diffusion, etc. occur at the contact interface, all these processes will survive Al?O? (γ), this γ-crystal-like Al?O? is chemically stable and does not decompose in the aluminum liquid. It is the main oxidation inclusions in aluminium die-cast parts. The main reaction equations for its formation are as follows:

Al(solid) + 3H?O(vapour) → Al(OH)?(solid) + 3/2H?(vapour). When the temperature is below 250°C, the reaction occurs when the ingot comes into contact with the water vapour of the air, and the resulting Al(OH)? is a white powder with a loose organisation, which does not protect the ingot and is commonly known as "aluminium rust".

2Al(OH)? → Al?O? + 3H?O. When the temperature is higher than 400°C, this aluminium rust Al(OH)? decomposes and produces a loose Al?O?, which adsorbs water vapour and hydrogen, increasing the gas content and oxidation inclusions in the aluminium solution.

Al (liquid) + 3/4O? (gas) → 1/2Al?O? (γ-crystal), this Al ?O? crystal is a kind of oxide film, which has a protective effect on the alloy liquid, but should not be stirred and destroyed more, otherwise it will also sink into the alloy liquid to form impurities (its specific gravity is 3.5 g/㎝3, larger than the alloy liquid).

Al (liquid) + 3/2 H?O (gas) → 1/2Al?O? + 3H, this hydrogen dissolves into the alloying liquid in an atomic state, increasing the amount of gas contained in the alloying liquid.

4 m /3Al (liquid) + CmHn → m/3Al4C? + n/2H?, with an oil-stained retort increasing the hydrogen content of the alloy (H? dissociates into the atomic state H, which dissolves in the aluminium liquid)

H? (gas) → 2H, dissolved in aluminium liquid

Control the alloy composition and non-metallic inclusions to avoid the formation of coarse brittle compounds [such as (FeMn)Al6] and non-metallic inclusions calcium silicide and calcium nitride, calcium phosphide, causing specific gravity bias, formation of hard mass points and increased alloy gas absorption.

II. Process points for reducing inclusions and gas content in alloy liquids and products

Aluminium alloy ingots are kept dry and free from aluminium rust.

Aluminum alloy ingots are accepted with reference to GBT8733-2016 standard.

When formulating alloy ingots, use high grade crystalline silicon with low calcium content to reduce air absorption and hard point formation in the alloy liquid, in principle, calcium impurities in the alloy liquid <0.03%.

Slag bag, overflow channel, exhaust channel material should not be used directly as a reheat material to join the aluminium liquid, it needs to be melted separately, de-slagged, de-gassed and refined, and adjusted composition qualified after pouring into the aluminium ingot spare.

The direct use of reheat material should be free of oil and moisture (composition of qualified material shanks, scrap products).

The proportion of the reclaim material should not exceed 40%, otherwise some easily burned elements will lead to unqualified composition, or too many oxides in the alloying liquid will not be easily refined and excluded.

Melting furnace to maintain the chamber alloy liquid refining, the current common way is refining agent + inert gas (nitrogen or argon), the way belongs to the adsorption refining, adsorption refining is the disadvantage of inert gas in the alloy liquid floating up, only the gas in contact with the impurity interface can play a role in removing inclusions refining, no inert gas through the interface will not be able to achieve the effect of refining to remove oxidation inclusions, which requires the operator to work carefully to ensure that a maximum number of impurities interface gas through (molecular gas can only be attached to the oxidation inclusions interface, de-slagging and de-gassing completed simultaneously)

Strict control of the inert gas pressure to ensure that the gas rises to the surface of the alloy to produce a ≯ 50㎝2 wave of liquid metal, otherwise it will increase the oxidation of the alloy.

If inert gas is used for refining with nitrogen, the temperature of the alloy liquid is in principle no greater than 720°C, otherwise the nitrogen reacts with the aluminium liquid and generates AlN crystals, increasing impurities and losses.

Long time (greater than 72 hours) insulation or insulation temperature is too high (greater than 760 ℃, more than 1 hour), in principle, the furnace alloy can only be poured into ingots, according to the furnace material used, can not be directly used for die-casting production, insulation time of 4 ~ 6 hours or more, need to re-refining before use. Because long time heat preservation and high temperature for the alloy liquid natural gas absorption to create the conditions, resulting in the alloy gas content increased.

When pouring and taking soup, reduce the stirring of the alloy liquid surface as much as possible, avoid the liquid surface to take soup smoothly and reduce the repetitive oxidation of the alloy liquid.

Turn the water bag (pot), insulation furnace wall, take soup spoon to keep clean, reduce the adhesion of oxide inclusions accumulation.

Three, die-casting alloy liquid quality standards and determination methods

For the general casting industry, it is impossible to achieve absolutely no impurities and absolutely no gas in the alloy liquid (the cost is too high and unnecessary), but it can be different according to the requirements of product quality and the different casting processes. For metal casting and general products, its gas content and inclusions requirements can be relaxed some, because its solidification speed is fast, gas and inclusions are not easy to gather and grow; for sand casting and airtight requirements of the product, the requirements to be slightly higher. At present the existing die-casting production enterprises, the usual practice is: 1:

1. The determination method and standard of gas content

Determine the amount of gas in the alloy liquid through the equivalent density:

D = (D1-D2)/D1×100%, where D - equivalent density, D1 - density of solidification under conventional air pressure, D2 - density of solidification under vacuum. Standard: D < 2% is sufficient to achieve a gas content of 0.2 ml/100 g or less.

2. Determine the slag content of the alloy by means of K-mode

Standard: K＜0.4 can be

3. Using the testing method of aluminium melt slagging tester to detect and determine

This instrument is suitable for the inspection of the cleanliness of the alloy liquid, such as the residue of the slag remover, the determination of the amount of finer grains such as Ti-B and the confirmation of the effect of the addition of Ca, Na, Sr and Sb.