Aluminum alloy refining for gravity casting

The importance of refining process to gravity casting: Most of the gravity cast parts are industrial products which always requier strict mechanical and chemical property. In addition to using ready aluminum casting alloy, the refining process can't be overlooked before pouring in the gravity casting workshop. Aluminum casting alloy always has oxides and other impurity which degrade the casting quality level. Oxides and other impurity can form loose inner structure even shrinkage holes which can be vital for a casting which is applied with quite heavy load. Furthermore, the gravity casting process is preferred for fluid control system parts such as pump housing, oil cylinder head and valve body all of which require to be airtight or oiltight. In this case, a casting with inner impurity always fails.

The other fatal impurity should be eliminated is the hydrogen ion. The hydrogen ion is the main reason for the porosity of aluminium gravity casting. The porosity degrades the density of the cast products along with its strength. A casting without degassing can be found instensive pin holes on its machined surface. Therefore such casting always fail to meet surface finish requirment. The not degassed cast products also meet serious trouble in surface treatment such as mirror polishing, stove varnishing, powder coating and plating. The unreleased hydrogen can form bubble when it is heated during surface treatment, then most surface treatment fails because of the damage from the casting's inner body. So whenever you want to reduce further risk during post treatment, degassing ( espcically elimiating hydrogen ) is the necessay step we have to implement.

Two kinds of aluminum alloy refining means

As we all know, the electrolytic aluminum should be mixed with desired alloys to obtain specific characteristics and enough fluidity for casting. Normally the foundry bought casting aluminum alloy from the ingot foundry direct. In the casting workshop, the molten aluminum casting alloys are remelted and poured into casts and cooled. Molten aluminum should be further heated to remove oxides, impurities and other active metals such as sodium and magnesium, before casting. Chlorine may also be bubbled through the molten aluminum to further remove impurities. However the aluminum alloy ingot currently used in foundries are always well alloyed and refined. Therefore in morden casting workshop where qualified aluminum ingot is in use, cleaning impurity finally focuses on cleaning oxides and hydrogen ion.

Refining by degassing and slag cleaning agent: normally we use degassing and slag cleaning agent to reduce oxides and some undesired impurity. This process can be very rough since it is hard to quantify the agent to be used. It depends on various working situation in casting workshop. For small workshop, where central melting furnace is not used, small crucibles are used for melting along with holing, is such condition, it is really hard to decide when to add the cleaning agent, even forget to add any agent during the whole process.

In HH gravity casting workshop, continuous melting furnace with holding furnace are in use to provide melten aluminium casting alloy. Each large ladle of aluminium alloy out of the continuous melting furnace is cleaned by degassing and cleaning agent to remove the undesired oxides and other impurtiies. After cleaning, the molten alloy is transfered to holding furnace for further casting. The three steps of melting and alloying, refining and cleaning and holding and pouring were separated in order to achieve desired clean alloy for gravity casting.

Refining by Nitrogen or argon with slag cleaning agent: removing oxides and impurity by slag cleaning agent may be the only thing done for refining in small gravity casting workshops. However as we talked above, it is not enough to provide real clean melton aluminum casting alloy. In HH, the slag and oxide cleaning process is a aasistant process to Nitrogen and Argon refining. Nitrogen or Argon is blowed through into melton aluminium alloy by a graphite turning nozzle with certain volume and for certain time to remove the hydrogen ion. Then a hydrogen contenct testing machine is used to mearsure its hydrogen content in alloy. Along with Nitrogen or argon refining, slag cleaning agent is added into the melten alloy which is turned to even by the graphite nozzel till the dedecting machine indicates it reduces to desired content level. The combined refining and cleaning process used in HH Machinry is much more efficient to provide pure aluminum casting alloy.

aluminium gravity casting technique database of SXLH

  1. Gravity casting capacity of HH aluminium casting.
  2. Testing equipments for gravity casting
  3. Range of gravity casting made by HH aluminium casting
  4. Chemical and mechanical property of aluminium gravity casting
  5. China national designation of aluminium casting alloy
  6. Conversion sheet of aluminium casting alloy designation of various countries
  7. General tolerance of aluminium gravity casting

FAQ's of aluminium gravity casting

  1. What is aluminium gravity casting?
  2. What is the difference between green sand casting and gravity casting?
  3. How to choose aluminium gravity casting?
  4. How many types of aluminium gravity casting?