Silicon in master alloys

Silicon in master alloys

In the world of jewellery, there are several ways to classify alloys. One of the most common divides alloys between those suitable for mechanical processing and those suitable for investment casting. In this article we delve into the role of silicon in this division.

When we use a master alloy, we do it because we want to give the gold or silver, we intend to process some precise characteristics. Pure gold, for example, has a modest hardness (22HV) which makes it unsuitable for everyday use, which is why most master alloys are able to increase it, even significantly.

Increasing the hardness of pure gold is an operation that is most of the time necessary and without contraindications. Other advantages brought by master alloys, on the other hand, bring with them side effects, more or less important, which can affect the success of the processing; this is the case of some alloys designed for investment casting applications.

Let’s take a look specifically what are the characteristics required by an alloy for investment casting:

• Increased fluidity
  The fluidity is necessary to allow the liquid metal to adequately fill the refractory mold, without the need to apply excessive over-pressures after casting.
• Shine after casting
  The loss of lustre after casting is an effect of the reaction between the high temperature alloy and the refractory material. What is commonly called “gypsum” is a compound made up of 25-30% calcium sulphate hemihydrate (CaSO4 · 1⁄2 H2O) which act as a binder, and 70-75% of silica, in particular cristobalite (SiO2), which represents the actual refractory. When calcium sulphate is heated to high temperatures it forms decomposition products:2CaSO₄ 2CaO + 2SO₂ + O₂in particular sulphur dioxide and oxygen can lead to the formation of sulphides and oxides of copper and zinc, the formation of these compounds alters the colour and surface brilliance of the metal. For this reason, one of the most felt needs in the field of investment casting is to obtain a well deoxidized tree.

A special case is that in which there are pre-assembled wax stones, for this application it is even more important that the tree is properly deoxidized, because it is not possible to easily remove the oxidation under a stone, which would therefore be not very shiny and dull.

• Reduced loss
  High loss is another common problem in investment casting, almost always due to the evaporation of zinc, which has a boiling temperature of 907 ° C. The fact that the cylinders are allowed to cool slowly in air, before being immersed in water, accentuates this drawback, since it has been shown that zinc is able to evaporate even after the metal has solidified.

The most common solution
Among the various additives that we can add to gold and silver, there is one capable of bringing improvements in all these areas: silicon.

The presence of even small quantities of silicon in the alloy is able to significantly increase the fluidity of gold and silver alloys; furthermore, the solidification temperature is lowered, and this gives the liquid metal more time to fill the mold shape before solidifying.

Another characteristic of silicon is that of forming a thin film on the molten metal which limits the contact between it and the plaster wall, in this way the decomposition compounds of calcium sulphate do not interfere with the copper and zinc present in the alloys. indeed, the presence of zinc can increase the protective characteristics of silicon. The thin film formed by the zinc, then, is colourless and transparent, so it does not alter the natural colour of the alloy in which it is put.

Alloys with silicon also have a reduced and independent decrease in the amount of zinc present in the alloy, it seems that this is always due to the protective film generated by this element, which prevents the evaporation of low-melting elements.

All fun and games, then?
Not exactly. Silicon has indisputable advantages in casting, but it also has disadvantages:

• Crystalline grain
  Silicon significantly increases the size of the crystalline grain of metals, not only that: it also inhibits the effects of grain refiners, thus making their action less effective. For this reason, the resulting metal will have a coarse-grained structure, with all the disadvantages that this entails.

• Fragility
  Silicon is poorly soluble in the main metals used in jewellery, this leads it to segregate itself in the edges of the crystalline grain, in the form of impurities. The metal thus obtained is more brittle and subject to fractures.

These disadvantages make it exceedingly difficult to mechanically machine an alloy containing silicon. The metal can easily break or present cracks during rolling and in any case the aesthetic appearance will be easily compromised by the coarse grain size of its crystals.

For these reasons, most investment casting alloys are not suitable for mechanical machining.

On the contrary, however, some alloys for mechanical processing may be suitable for investment casting, if they meet certain requirements such as a not excessively high zinc content and good fluidity. The performances of this type of investment casting alloys are generally lower than those with silicon, but still valid for those looking for a universal product to be used for all processes.