Heat treatment of aluminum castings

Thermal aluminum cast parts makes it possible to improve their mechanical properties or corrosion resistance.

Types of heat treatment of castings

В зависимости от метода литья и применяемого сплава к алюминиевой отливке могут применяться следующие виды термической обработки:

  • отжиг для снятия остаточных напряжений;
  • стабилизирующий отпуск;
  • гомогенизация;
  • полный отжиг;
  • hardening of aging.

The most important type of heat treatment for aluminum castings is aging, usually – artificial aging.

Principles hardening aluminum alloys aging

A prerequisite for, hardening to occur in the aluminum alloy aging, It is to reduce the solubility of aluminum in solid solution of one or more alloy components with decreasing temperature,.

Usually, процесс искусственного старения включает три стадии:

  1. При нагреве на твердый раствор (нагреве под закалку) происходит растворение в твердом растворе алюминия достаточного количества компонентов сплава, which provide a hardening aging.
  2. With sufficiently rapid cooling during quenching, these components remain in the solid solution. In this state of the casting are relatively mild.
  3. With aging - for cast products, primarily, artificial - is an allocation of these dissolved components in the form of submicroscopic smallest phase, which give rise to the hardness and strength of the aluminum alloy. These tiny phase, which is scientifically called "coherent or semicoherent phases", represent obstacles for dislocation motion in the metal, and therefore strengthens the previously soft metal.

Thermally hardenable aluminum casting alloys

Термической обработке старением поддаются следующие типы алюминиевых литейных сплавов:

  • алюминий-медь;
  • алюминий-медь-магний;
  • aluminum-silicon- magnesium;
  • алюминий-магний-кремний;
  • aluminum-zinc-magnesium.

Нагрев на твердый раствор (нагрев под закалку)

For, to introduce a reinforcement of the alloy components in solid solution so quickly, as possible, the heating temperature for a solid solution should be as high, no closer than 15 ° C of the liquidus temperature, to avoid local submelting. For this reason, often casting aluminum alloys, containing copper, подвергают нагреву на твердый раствор в два этапа: сначала до 480 ° C, then to 520 ° C.

The duration of heating for hardening depends on the wall thickness of the casting and casting method. Compared with casting in sand molds, chill casting requires a shorter heating for quenching dissolution hardening alloying components due to a milled microstructure. Basically, heating about one hour is usually sufficient. Longer heating for quenching - up 12 hours - used, for example, alloys of aluminum-silicon-magnesium for spheronization or rounding silicon eutectic, чтобы повысить пластические свойства сплава (относительное удлинение).

During the heating for hardening strength castings it remains very low. It is therefore necessary to take measures to prevent their bending and buckling. For this large and complex castings secured in specially adapted.

Hardening of aluminum castings

Hot casting must be cooled in water as soon as possible - for 5-20 seconds depending on the thickness of its walls, to suppress any unwanted and premature allocation of dissolved alloy components.

After hardening of the casting have a high plasticity. Quenching the casting during hardening can lead to the formation of high internal stress and warpage articles. Эту проблему решают выбором закалочной среды и способа ее подачи: погружение в воду или подача воды через спрейеры – the water is cold, hot water or oil.

Anyway, any actions by editing the product must be carried out after hardening, but before aging.

aging operation

aging operation leads to a considerable increase in hardness and strength of the mold structure by allocating smallest hardening phases. Only after this operation, the product receives its desired properties, its final shape and dimensions.

Conventional alloys are subjected to artificial aging. Artificial aging - it is aging at elevated temperatures, unlike natural aging, которое проводят при температуре окружающей среды («комнатной» температуре).

aging temperature and duration may vary depending on the desired properties of the aluminum alloy casting. for instance, Mechanical properties can be "adjusted" because, to obtain a high hardness and strength, но за счет более низкой пластичности (относительного удлинения) материала отливки. And vice versa, можно достичь более высокой пластичности (относительного удлинения), but at the expense of hardness and strength. When selecting the aging temperature and duration of exposure of its particular alloy used diagram aging, которые разработаны для многих сплавов (рисунки 1-4).

1-starenie-alyuminiyaFigure 1 - Dependence of the yield strength of the casting aluminum alloy
Al Si10Mg on its temperature artificial aging

2-starenie-alyuminiyaFigure 2 – Relative elongation casting aluminum alloy Al Si10Mg on its temperature artificial aging

3-starenie-alyuminiyaFigure 3 – The dependence of the strength of the casting aluminum alloy
Al Si10Mg on its temperature artificial aging

4-starenie-alyuminiyaFigure 4 – The hardness of the casting aluminum alloy Al Si10Mg
on the temperature of artificial aging

В сплавах алюминий-кремний-магний имеется дополнительная возможность влиять на уровень предела прочности и относительного удлинения материала отливки путем изменения содержания магния в комбинации с варьированием параметров термической обработки (рисунок 5).

5-starenie-alyuminiyaFigure 5 - Influence of the magnesium content of the alloy Al Si7 the tensile strength in the cast and aged states

Typical parameters for the artificial aging of any type of cast aluminum alloys are shown in Figure 6.

6-starenie-alyuminiyaFigure 6 - Parameters of artificial aging of cast aluminum alloys

A source: Aleris International, 2014