Effects of individual additions

The effects of the elements most commonly used as small additions to carat gold alloys are discussed in the following paragraphs.

Zinc

Some quantity of zinc can be alloyed in yellow gold without changing the microstructure. The specific amount of zinc which can be tolerated depends on fineness (caratage) and the silver to copper ratio. Gold can dissolve approximately 3%

zinc (by mass) without any change in microstructure. Higher concentrations cause the formation of new phases, including intermetallics; a detrimental influence on properties can be expected.

In 14 and 18 carat alloys, higher concentrations of zinc are possible due to the higher solubility of zinc in silver and copper. However, zinc is usually a small addition in casting alloys with some beneficial effects. The recommended upper limit of addition is approximately 2% in 14 and 18 carat yellow gold. In low carat golds (8-10 carat) and in nickel white golds, zinc is a standard alloying element and is often present in higher amounts.

Influence of zinc on solidification range of 14ct alloy

Influence of zinc on the solidification range

Effect of grain size on porosity 14 ct yellow gold

Porosity mean value Porosity maximum

Figure 3.3.1

The effects of zinc additions in 14 and 18 carat yellow gold are:

a) Reduction in liquidus and solidus temperatures:

The influence of zinc is shown in Figures 3.3.2 & 3.3.3, whereby silver is substituted by zinc and gold and copper are kept constant. Substituting copper by zinc might lead to a somewhat different effect; it will certainly bleach the colour.

b) Increase in form-filling:

The influence of zinc additions up to 2% on the form-filling of 14 and 18 ct yellow gold is shown in Figure 3.3.4. All the tests were performed with a test grid under constant conditions. The beneficial effect is remarkable.

c) Reduction in surface roughness:

The surface of cast items is much smoother if the alloy contains up to 2% zinc.

The effect is more pronounced with heavy parts. A roughness reduction to approximately a third can be achieved.

Both increased form-filling and reduced roughness can be related to the effect of zinc on reducing the interfacial tension, which, in turn, improves the wetting of the investment with the melt and reduces capillary forces. Thus, the melt can more easily fill thin cavities and reproduce the smooth surface of the pattern. This avoids the formation of a dendritic surface structure.

d) Reduction of reaction with investment and gas porosity:

Small zinc additions have proven able to reduce the reaction of the melt with the investment and in this way decrease the incidence of gas porosity. The reason for this is not quite clear. Probably, the formation of a dense layer of zinc oxide at the surface of the solidifying melt prevents the interaction of melt with the investment.

Tensile tests on cast samples has shown that a small addition of zinc improves the elongation (ductility) by reducing the porosity, Figure 3.3.5. There is also an increase in tensile strength, indicating that the effect is really related to physical integrity of the samples.

However, it should be recognised that zinc additions higher than the recommended value (approximately 2-3%) might have an adverse effect, e.g.

increase the reaction with investment and, therefore, increase gas porosity.

e) Brighten the surface in the as-cast state:

Zinc has a stronger affinity to oxygen. During the cooling period of a casting, a thin, relatively dense layer of almost colourless zinc oxide is formed on the surface, inhibiting the formation of a thick voluminous black scale of copper oxide, Figure 3.3.6. The pieces have a bright yellow appearance. Pickling removes the zinc oxide layer easily without de-coloration of the surface.

Influence of zinc addition on elongation

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 on formfilling of a grid

0.0 0.5 1.0 1.5 2.0 Formfilling (%) ^{14 ct}_{18 ct}

Figure 3.3.4

Figure 3.3.6

without Zn 2% Zn

Influence of zinc on surface oxidation (14 ct)

Zinc has a high vapour pressure; it boils at 907°C (1665°F) at atmospheric pressure.

Thus, adding pure zinc to the melt is difficult. A great deal of zinc evaporates (in air, with formation of white fumes of zinc oxide). This effect can be reduced by wrapping the zinc in copper foil and immersing it very fast into the melt.The better way is the use of brass as a master alloy. Alloyed already with copper, the vapour pressure of zinc is significantly reduced. Use of brass with 70% copper or higher is recommended.

(Note: Brass with 60% copper and less often contains lead (as an alloying element) and some other impurities. Lead contamination in gold is undesirable). Once zinc is alloyed in yellow gold alloy, the alloy is stable. Significant loss by evaporation is prevented if the level of approximately 2% is not exceeded. Even a moderate vacuum can be applied.

However, melting in air causes formation of zinc oxide and therefore reduces the zinc concentration in the alloy. The main defects arising are inclusions of zinc oxide in the alloy. For example, surface defects can be generated, Figure 3.3.7. This defect is mainly caused by remelting of dirty material, e.g recycled sprues.

Higher zinc concentrations (exceeding the recommended level) increase the reaction of the melt with the investment. The result is a bad surface and more gas porosity.

Silicon

Silicon lies on the border between being a beneficial addition and deleterious impurity. It has several merits: silicon increases the melt fluidity and form-filling. Its effect is more pronounced than that of zinc. In yellow carat gold alloys, silicon produces a clean, yellow surface without the dark scale of copper oxide. The reason for this effect is the same as for zinc. A thin colourless, dense layer of its oxide, silica, forms at the expense of copper oxide.

The high affinity of silicon for oxygen makes silicon a strong deoxidiser. However, this use is not essential in yellow gold alloys.

Silicon additions have some disadvantages:

a) Embrittlement

The disadvantage of silicon additions is rooted in its limited solubility, particularly in high carat jewellery alloys.

Its solubility depends mainly on the copper content of the alloy, as it is insoluble in gold and silver and forms low melting eutectics (gold-silicon 363°C/685°F, silver-silicon 835°C/1535°F). In copper, silicon is soluble up to almost 5%.

If the solubility of silicon is exceeded, a low melting eutectic is formed in the jewellery alloy, causing embrittlement and cracks. Most critical are silver-rich, high carat alloys.

For an 18 ct alloy of composition gold 75% - silver 4.5% - copper 18% - zinc 2.5%, the critical silicon concentration is 0.05%. Higher silicon additions can cause embrittlement. 14 carat alloys can tolerate approximately 0.1% and 10 ct alloys ~0.3% silicon.

Figure 3.3.7 Inclusions of zinc oxide

The allowable silicon addition has to be determined for any given alloy composition. It decreases with increasing (gold + silver) content and should not be used in high (21/22) carat golds.

b) Grain coarsening effect

Another disadvantage of silicon is its pronounced grain coarsening effect.

It causes an extremely coarse grain, even at a very low concentration. The main consequence is a tendency to intergranular cracking. The appearance of the defect is very similar to that in Figure 3.1.10.

Inclusions of silicon dioxide occur in castings, especially where remelting dirty material such as recycled scrap is done.

Grain refiners

To compensate for the undesirable grain coarsening effect of silicon and to refine the grain size of jewellery alloys in general, many attempts to apply grain refiners have been made.

Published work has shown that additions (and combinations) of: iridium, ruthenium, zirconium, cobalt, boron, yttrium, (zirconium + boron), (cobalt + boron) and barium were effective as grain refiners. In almost all cases, the additions were in the range of 0.005 to 0.05% weight. Cobalt was added up to 0.2%. They all act in a similar way: they form very fine dispersed nuclei as starting points for the formation of grains at solidification. The mechanism of nucleation can be different. In all cases, small concentrations are effective.

In Table 19 the grain refiners are classified in terms of application and working mechanism.

Frequently used grain refiners are the high melting platinum group metals, iridium and ruthenium, with limited solubility in gold alloys, and also some very reactive elements. In the latter case, intermetallic compounds or even oxides or nitrides form the effective nuclei.

Type Field of appliance Working mechanism Examples

1 Casting

1a High melting temperature, limited iridium, ruthenium,

solubility in the alloy (cobalt)

1b High reactivity with oxygen, low rare earth (yttrium),

solubility, formation of fine dispersed boron, barium, (calcium) intermetallics or oxides

1c Probably formation of intermetallic e.g. zirconium/boron

compounds cobalt/boron 2 Soft annealing Formation of fine dispersed Cobalt,

segregations at annealing temperature All type 1 additions can also decrease grain size on soft annealing

Table 19 Grain refiners in yellow gold alloys

Iridium

Iridium is the most frequently used addition for grain refining of gold alloys. Investigations have mainly been performed with 14 and lower carat alloys.

In high carat alloys, the effect would be expected to be less pronounced, due to the extremely small solubility of iridium in gold and silver. In contrast, iridium is miscible in copper, more than 10% (by weight), forming a homogeneous solid solution.

Discrepancies exist between observations by different investigators. Some have found a grain refining effect in the range of 50 ppm iridium, whilst others could not demonstrate such an effect even with concentrations of 0.1% and higher. The grain refining effect of iridium in silicon-containing alloys is uncertain.

The main causes of these different results are variations in alloying and melting techniques as well as the basic composition of the alloys studied.

A master alloy has to be used to achieve good results. Preferably, a master alloy with copper should be used, where the iridium concentration should not be too high (<2%). Extreme care has to be taken to obtain a homogeneous iridium distribution in the carat gold alloy. An addition of approximately 0.01% iridium in the alloy is usually sufficient. A higher concentration has an adverse effect.

Use of iridium can lead to two types of defect:

- insufficient and inhomogeneous grain refining effect.

- segregation and hard spots.

Inhomogeneously distributed grain refiner or a too high concentration leads to hard inclusions causing polishing problems (‘comet tails’) and, in extreme cases, cracks, Figure 3.3.8.

Ruthenium

Ruthenium is another addition used for grain refining in carat gold alloys. The effect of ruthenium as a grain refiner is similar to that of iridium. It shows a remarkable refining influence in the concentration range of 0.001 to 0.01%. Again, a higher concentration is detrimental due to the formation of coarse particles. Obtaining a homogeneous distribution of ruthenium in yellow gold is difficult, and so iridium is preferred.

Figure 3.3.8 Iridium clusters at surface

4 EQUIPMENT

At least six steps of the investment casting process require specific equipment.

That is:

• the vulcaniser (vulcanising press), to make the rubber moulds,

• the wax injector, to make the wax patterns,

• the investment mixer, to make the investment slurry,

• a dry or steam dewaxer,

• the burnout oven,

• a melting/casting machine.

A wet sand or grit blasting machine can be added to this list, to complete the removal of the investment from the cast tree. Also, other less costly equipment, which is not strictly required but can simplify some steps of the process, can be considered.

As remarked in the introduction, we should select qualified producers and suppliers who have a technical knowledge of the process as well as give good after-sales service for the supplied equipment. These are essential.

First of all, we should establish our goals (our production requirements) and what we need to attain them. This decision can be taken only by the goldsmith.

Otherwise, there is always the risk that vital decisions on equipment for our workshop could depend heavily on the ‘opinions’ of the local vendor. Frequently, a production line made from poorly matched equipment will be the result of such policy.

There are many trade fairs around the world, where you can obtain up-to-date information on production equipment from a range of manufacturers. Examples include:

• VicenzaOro, each year in January and June, in Italy

• Basel Fair, March/April in Switzerland

• Inhorgenta, February, Munich, Germany

• Hong Kong Trade Fair, September, Hong Kong

• Las Vegas Trade Fair, June, USA

• “Catalog In Motion”. This fair is held in February each year in Tucson, Arizona, USA.

At such events, we can see and try equipment, compare different manufacturer’s products and technical production problems and requirements can be discussed with leading experts in the field.

When you have clarified your ideas and established your objectives, it is useful to prepare an ‘evaluation chart’ for each type of equipment. In this chart you will record a description of the different technical features of the equipment, along with competence and efficiency of the producer or vendor. In this way you will be able to make a more objective final choice, especially if you also make an evaluation of costs versus benefits and of the return on investment. So many unpleasant surprises will be avoided or at least mitigated.

In addition to these general rules, we recommend that you add the following guidelines:

• Never buy equipment without seeing it and without discussing every detail. You should test it and evaluate ease of use, controls, possibility of programming, ease of servicing.

• Find out if the producer himself has developed and built the equipment or if the offered equipment is a cheap copy of something developed by someone else.

• Discuss the offered guarantee in depth.

• Find out how many pieces of the offered equipment have been sold and to what company. Talk to some of their customers about their experiences.

• Ensure the producer will offer adequate training of your staff and will commission the equipment in your factory so that it operates at the agreed specification

• Never choose equipment on the basis of price only. ‘Cheap’ is rarely good economy!

Whatever equipment you buy, you should schedule the required servicing, i.e. what tests should be done and when, and who should do them. All servicing operations will be recorded on suitable charts. For example, for a melting/casting machine, the cooling system should be checked daily, the filters of the exhaustion fixture weekly, the oil of vacuum pumps monthly, etc.

Now we will discuss the specific equipment.

4.1 VULCANISERS

Apparently, a vulcaniser is a very simple item of equipment. It is basically a simple screw press, with two heated platens and a temperature controller. However, in practical use even such a simple piece of equipment can give unpleasant surprises. If the temperature control system is crude and unreliable, it will be hard to obtain good quality rubber moulds, with all consequent problems.

The characteristics to consider here are:

• the operating temperature range,

• the size of the heating platens,

• the maximum opening between the platens (i.e. the maximum thickness of the rubber mould).

We should keep in our mind that several modern silicone rubbers don’t tolerate temperature inaccuracies larger than 1°C (1.8°F). Other factors include uniformity of temperature across the platens and the control system. An accurate temperature control, without wide swings of temperature, is fundamental for a good vulcaniser.

It should be possible to check the calibration and the correct operation of the temperature controller. For this purpose, in many vulcanisers there are proper holes in the platens, where a calibrated thermocouple or other suitable device can be inserted to check temperature level and distribution at different points of the platens.

The approximate cost of a vulcaniser can range from about 350 Euros/US $ for a very basic model to 650 Euros/US$ for a more sophisticated one with electronic temperature control, Figure 4.1.1. A multiple vulcaniser, with multiple temperature control and digital temperature display, can cost up to about 2,200 Euros/US$.

Figure 4.1.1 Vulcanizer with digital temperature control

In document 116860058 Handbook on Investment Casting Gold Jewellery (Page 67-76)