Rejection of aluminum casting
Two sources of defects
The sources of spoilage in aluminum castings are two phenomena, which can act as a separately, and jointly:
- constant, progressive oxidation of molten aluminum and its saturation with hydrogen.
- Reducing the volume resistivity of aluminum during its transition from liquid to solid state.
Three types of solidification defects in castings
Figure 1 – Three types of solidification defects in aluminum castings:
gas porosity, shrinkage porosity, hot breaks and cracks [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, which are the reasons for the rejection of finished castings:
- pores;
- air saturation;
- inclusion;
- violation of tightness;
- surface defects;
- low strength;
- low plasticity.
Figure 2 – Sources of hydrogen in aluminum castings [1]
Figure 3 – Evolution of hydrogen in aluminum castings [1]
Figure 4 – Hydrogen porosity [1]
Measures to prevent defects
The following measures are taken to prevent or mitigate the effects of oxidation and hydrogenation:
- metal processing in a furnace and its degassing;
- strict control of melting and casting temperatures;
- 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. To do this, the following actions are taken:
- high quality of the original ingots;
- modern foundry equipment and casting technology;
- charge control (dry charge, fast melting);
- temperature control during melting and casting;
- purification of the melt and quality control of the melt;
- 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, leading to defects in foundry products:
- shells;
- shrinkage;
- air saturation;
- violation of tightness;
- low strength and ductility.
To prevent or weaken the effect of a decrease in the specific volume of aluminum during its solidification, the following measures are taken:
- optimal placement of the gating system;
- temperature control of the solidification process;
- grinding grain;
- 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 – Formation of macroporosity in aluminum castings [1]
Figure 5.2 – Example of macroporosity formation [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, as well as in aluminum-magnesium and copper-alloyed alloys: first, a dendritic structure is formed, 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 – Air entrapment with incorrect gating system [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.
Source:
Additional Information:
Castings Practice: The Ten Rules of Castings / John Campbell
Casting Defects Handbook: Aluminum & Aluminum Alloys / David Neff (Editor)
