Marriage casting aluminum

Two marriages source

The sources of spoilage in aluminum castings are two phenomena, which can act as a separately, так и совместно:

  1. constant, progressive oxidation of molten aluminum and its saturation with hydrogen.
  2. Reducing the volume resistivity of aluminum during its transition from liquid to solid state.

Three types of solidification defects in castings

Figure 1 – Три типа дефектов затвердевания в алюминиевых отливках:
gas porosity, shrinkage porosity, горячие разрывы и трещины [1]

The oxidation of hydrogen and the saturation of

As a result of continuous oxidation of molten aluminum and its saturation with hydrogen in the aluminum casting defects occur following, которые являются причинами брака готовых отливок:

  • поры;
  • насыщение воздухом;
  • включения;
  • нарушение герметичности;
  • поверхностные дефекты;
  • низкая прочность;
  • low plasticity.

Figure 2 – Источники водорода в алюминиевых отливках [1]


Figure 3 – Выделение водорода в алюминиевых отливках [1]


Figure 4 – Водородная пористость [1]

Measures to prevent defects

Для предотвращения или ослабления влияния окисления и насыщения водородом принимают следующие меры:

  • обработку металла в печи и его дегазацию;
  • жесткий контроль температур плавления и литья;
  • melt filtration.

When aluminum transition from liquid to solid state hydrogen dissolved therein is released and in cooperation with the oxides creates problems with porosity in the finished castings.

The main object of the molten aluminum while providing high quality is to maintain the rate of oxidation of the melt within certain limits. Для этого предпринимаются следующие действия:

  • высокое качество исходных чушек;
  • современное литейное оборудование и технологии литья;
  • контроль загрузки шихты (сухая шихта, быстрое расплавление);
  • контроль температуры при плавлении и литье;
  • очистка расплава и контроль качества расплава;
  • security measures in the processing and transportation of the melt and casting.

Shrinkage

Because of the decrease in specific volume of aluminum following defects may occur when it solidifies, ведущие к браку литейной продукции:

  • раковины;
  • усадка;
  • насыщение воздухом;
  • нарушение герметичности;
  • low strength and ductility.

Для предотвращения или ослабления влияния уменьшения удельного объема алюминия при его затвердевании принимают следующие меры:

  • оптимальное размещение литниковой системы;
  • температурный контроль процесса затвердевания;
  • измельчение зерна;
  • modifiers alloy application.

Decrease in specific volume at the transition aluminum alloy from liquid to solid condition can lead to a decrease obema- depending on casting alloy - up 7 %. Under adverse conditions of this difference in volume can be the cause of the marriage of cast aluminum products - shrinkage cavities, pores or fractures.


Figure 5.1 – Образование макропористости в алюминиевых отливках [1]


Figure 5.2 – Пример образования макропористости [1]

For, to obtain a good casting must provide the possibility of additional admission of liquid metal to the shrinkable microstructure during the solidification process.

When injection molding is achieved by high melt pressure, and in gravity casting - due to the height of feeder heads.

Effect solidification type

Also important is the type of solidification. The aluminum-silicon alloys - eutectic silumins with silicon content of about 13 % upon solidification forms a solid shell immediately. According to another occurs in the solidification of hypoeutectic silumin, а также в алюминиево-магниевых сплавах и сплавах с легированием медью: сначала образуется дендритная структура, and then harden the remaining components with lower temperature curing.

Effect of casting system

In gravity casting, to which the, for example, chill casting, feeding the melt into the gating system to produce the most critical or "thicker" casting portion. Not controlled or turbulent filling of mold cavities has a negative effect on the quality of castings.


Figure 6 – Захват воздуха при неправильной литниковой системе [1]

gating system, which allows to control the solidification front movement from the bottom to form the entrance to the gating system is very useful for quality castings. A good system of injection mold filling begins with the lower part and always, the layers of the new hot metal "lay" at the bottom, already solidified layers.

casting this type of system can partially compensate for the negative effect, which has a volume reduction when it solidifies aluminum and at the same time to guide the molten metal into the mold in such a way, to avoid another of its oxidation due to turbulent flow.

A source:

  1. TALAT 3207