As well as being highly important as investments, commodities and in jewellery, precious metals and their alloys are highly valued materials for high-performance applications. These applications range from chemical, pharmaceutical and medical device industries through to electronics, renewable energy and automotive.
Precious metals include gold, silver and the platinum group metals, or PGMs. The PGMs consist of platinum, palladium, ruthenium, rhodium, osmium and iridium. Alloys of these metals are particularly useful in applications where high electrical conductivity, resistance to high temperatures, oxidation resistance and good mechanical properties are required.
Precious metal products are generally manufactured via melting followed by mechanical processing.
The mechanical techniques employed for processing precious metals are similar to that of other metals and are divided into casting, forming, cutting and joining.
Casting is the process of introducing molten metal into a mould and allowing the metal to cool and solidify in order to take the shape of the mould. The various types of casting process include
Casting is usually used to produce ingots or shapes. Ingots are simple shapes indented for further processing, whereas shapes are generally more complex geometries requiring little further processing.
PGMs and PGM alloys typically have high melting temperatures and are difficult to melt. For that reason, high-frequency induction heating is often used for melting, as well as electron beam heating and arc melting.
Induction heating is the quickest and most effective method for larger quantities of PGMs (greater than 1 kg). For PGMs with higher melting temperatures, arc welding is used for smaller amounts (less than 1 kg) and electron beam melting for larger quantities.
When casting precious metals, there are some defects which can arise that are important to look out for.
The appearance of pores is the most common defect that can appear when casting. Types of porosity defect include gas porosity, shrinkage porosity, inclusion porosity and oxide inclusion porosity.
Figure 1. Examples of porosity defects in precious metals. Upper left: gas porosity, upper right: shrinkage porosity, lower left: inclusion porosity, lower right: oxide inclusion porosity. Credit: Heraeus.
2. Hard spots
Another defect which can appear in the casting process is the appearance of hard spots. These are inclusions that have a higher hardness than the surrounding metal and usually become visible during the polishing stage. Hard spots can be very difficult to repair.
Figure 2. Examples of hard spots in precious metals. Upper left: hard spots in white gold, upper right: hard spots caused by grain refiners, bottom: hard spots caused by contaminated gold. Credit: Heraeus.
3. Rough surface
Improper casting can also result in excessive surface roughness.
Figure 3. Examples of rough surface defects in precious metals. Upper left: dendritic surface, Upper right: sandy surface, Bottom: water mark. Credit: Heraeus.
Cracking is another defect which can occur in the process of casting precious metals. Cracks appear most commonly during the de-investing process.
Figure 4. Examples of cracking in precious metals. Upper left: cracking caused by silicon, upper right: cracking in red gold, bottom: cracking caused by impurity inclusions. Credit: Heraeus.
Discolouration of precious metals is particularly troublesome in jewellery making.
Figure 5. Examples of discolouration in precious metals. Left: red stain or fire stain, right: tarnishing. Credit: Heraeus.
Forming of precious metals is the process of using plastic deformation to reshape the object and is often done in the presence of heat. In forming processes, no material is added or removed, in contrast to cutting. Some common forming processes include:
Most forging processes, when performed cold, increase the strength of the metal via the modification of the metal microstructure. This is known as work hardening. When the forging processes are performed at elevated temperatures, the work hardening is limited.
Forming can induce defects in precious metals, depending on the forming process.
Cutting is a mechanical process used to shape metals by removing unwanted material. Many precious metals, and particularly their alloys, have high hardness and require cutting tools tipped with diamond or carbide.
The cutting processes used on precious metals include:
For the harder precious metals and their alloys, the most significant effect during cutting and machining is the wear sustained by the cutting equipment, as opposed to the precious metal parts.
Common joining processes used for precious metals include:
The various joining processes enable further applications of precious metals by combining their high-performance properties with the properties of other materials.
Thanks to their high-performance, such as excellent heat and corrosion resistance, high electrical conductivity and hardness, precious metals and their alloys are found in almost every industrial product. Some of the industries for which precious metals in most demand include
For industrial applications, gold and silver typically come in granular form. Other precious metals, including the PGMs platinum, palladium, rhodium, ruthenium and iridium come in sponge or powder form. Sponge is a form of powder with multiple grain sizes.
Figure 6. Top: gold in granular form, bottom: PGM powder. Credit: Heraeus.
When it comes to sourcing precious metals, quality is of huge importance. Defects can play an important role in the performance of finished parts. Because of this, it’s important to choose a supplier of precious metals, which can deliver high purity. One of the leading precious metals suppliers today is Heraeus.
Heraeus provides precious comprehensive expertise in the precious metal loop – from trading to precious metal products to recycling. The product range varies from contact products, PGM-fabrications and semi-finished materials to high-tech materials and compounds. Their fields of application are sliding and switching contacts, spark plugs, bushings, glass tools, labware, wires and materials for probe needles as well as master alloys, TC-balls, pen points and irradiation sources.
A globally leading technology group, Heraeus is headquartered in Hanau, Germany. Founded in 1851, it is a family-owned portfolio company which traces its roots back to a pharmacy opened by the family in 1660. Today, Heraeus combines businesses in the environmental, energy, electronics, health, mobility and industrial applications sectors.
The Global Business Unit Heraeus Precious Metals is a leading provider of precious metal services and products. We combine all activities related to our comprehensive expertise in the precious metal loop – from trading to precious metal products to recycling.
The business line Functional Materials is an expert for mechanical processing of precious metals and their alloys. The product range varies from contact products, PGM-fabrications and semi-finished materials to high-tech materials and compounds. Their fields of application are sliding and switching contacts, spark plugs, bushings, glass tools, labware, wires and materials for probe needles as well as master alloys, TC-balls, pen points and irradiation sources.