Views: 268 Author: Site Editor Publish Time: 2019-07-15 Origin: Site
The heat treatment technology is a great invention in die casting industry. It is prominent in different metal process and alloys. One good example is in aluminum die casting. Generally, the conventional pressure die casting is associated with lots of features. One of them is the fact that it may exhibit blister forming porosity for some metal alloys, for example the age-hardenable aluminum alloy.
However, the formation of pores in the casting may lead to a great loss. Therefore, there is need for a way of avoiding this. One of the ways is by treating the casting within a temperature range, usually high, which will enable the solid metal to be change entirely into solution. A cooling process is then introduced which is aimed at terminating the solution treatment. The cooling process quenches the casting to a temperature range below 100°C to enable artificial or natural ageing.
Basically, in the heat treatment technology of aluminum pressure die casting is aimed at achieving a level of solute element solution which will enable age-hardening while preventing pore expansion in the casting and subsequently avoiding any form of blistering of the casting.
High pressure die casting (HPDC) is a technology that is mostly used in the mass production of metal components whose required properties include smooth surface finishes and close dimensional tolerances. Conventional high pressure die casting parts are relatively porous which makes it a great disadvantage in the casting industry. It has been discovered that the presence of entrapped gases such as hydrogen, vapors or air formed during the die wall lubricant decomposition coupled with the shrinkage porosity during solidification is responsible for the internal pores.
Due to the expansion of the gases contained in the internal pores during the conventional solution treatment at high temperatures, usually at 500°C, castings made from high pressure die casting aluminum alloys are not considered to be amenable. Consequently, the internal pores containing gas or gas forming compounds result in the formation of surface blisters on the castings. This phenomenon relating to the internal pores may have adverse effects both on the mechanical properties and the dimensional stability of the affected high pressure die castings.
It is possible for high pressure aluminum die castings to be relatively free of pores and without any trace of blisters. This can be achieved through techniques that involve cost penalties. These include squeeze casting, pore-free die casting, vacuum die casting. The most frequently applied of all the techniques is the vacuum systems which aims at the reduction of pores which may be present in the casting.
So, far, there are three stages in the conventional heat treatment procedure for aluminum alloys. They are:
#1. Solution treatment which is done below the melting point of the alloy, relatively at high temperatures for over 8 hours aimed at dissolving the constituent alloys while homogenizing and modifying the fundamental structure.
#2. Rapid cooling or quenching. This involves the done in both hot or cold water, and aims at retaining the solute elements in the extremely saturated solid solution.
#3. Alloy ageing. This is done by holding the alloy at a certain temperature which may be at different times to achieve strengthening or hardening through precipitation.