Untreated parts that still need to be sintered are commonly referred to as green parts. After removing the binder, including partial removal, parts are called brown parts. This naming convention has its origin in ceramic production. Sometimes, the fully sintered parts are referred to as white parts, however this is uncommon in powder metallurgy.
The binder is a multicomponent polymeric mixture that holds the powder together to keep the shape. Usually, this is a polymer or mix of polymers.
In metal injection molding (MIM), the mixture of the binder and the powder is referred to as feedstock. Due to certain similarities between the materials used in MIM and LMM, Incus decided to adopt this same naming convention.
Solvent and catalytic debinding
For some materials, full thermal debinding is not possible or not economical, this could be due to oxygen sensitivity or high sintering activity at low temperatures of the metal, or high thermal stability of the binder. In those cases, other methods can be used. In solvent debinding, the parts are placed in an appropriate solvent that dissolves a fraction of the binder. In some cases the temperature also needs to be slightly elevated, sometimes room temperature can suffice.
In catalytic debinding, the parts are heated to relatively low temperatures in an atmosphere that catalyzes the decomposition of the binder. Usually, this atmosphere contains vapourized acid, e.g. nitric acid. The solvent and catalytic debinding of LMM parts is possible as an alternative to a single thermal debinding approach. This reduces the load of organics introduced into the debinding furnace without heat treating the metal and enables the use of shorter thermal debinding cycles.
The theoretical density is the maximum achievable density of a material. In reality, imperfections affect the density. In powder metallurgy porosity has the biggest impact on density, and also affects most mechanical properties negatively. Above a certain threshold at around 96% of the theoretical density, the effect of porosity is mostly negligible. For ease of understanding, the percentage value of theoretical density is used instead of absolute density values, since even the density of a single alloy can vary heavily within the defined composition range.
In an oxidative atmosphere, oxidation can take place. This chemical reaction commonly occurs in connection with oxygen. An oxidative atmosphere can be useful in debinding, since the burning of the binder reduces the necessary temperature and energy. However, the metal powder will also be oxidized, and this metal oxide will inhibit sintering. Therefore, the oxide must be reduced, which is usually done using a reductive atmosphere. The most commonly used oxidative atmosphere is air, due to its ease of use and abundance.
Reduction is the opposite reaction of oxidation, and always occurs simultaneously. Therefore, a reductive atmosphere is composed of a gas, that can easily be oxidized, thus acting as a reducing agent.
The most commonly used reductive atmosphere contains hydrogen gas. Due to it being considered explosive, pure hydrogen requires proper equipment. However, by diluting the hydrogen using inert gas, these safety regulations can be avoided, but the reductivity of the gas mixture, generally known as forming gas, is also lowered.
Inert substances do not react. What is considered inert can vary by the given reference material. For example, nitrogen gas is mostly considered inert, but can form nitrides with certain alloys, e.g. titanium alloys or 316L stainless steel. Some generally considered inert atmospheres are noble gasses, like argon and helium.
Strength is the ability to withstand stresses without failing. For example, during tensile tests there a several commonly given values that characterize the material: Re, Rm and Rp0.2. Re is the yield strength or elastic limit. If this stress is exceeded, permanent plastic deformation takes place. Rm is the ultimate tensilte strength of the material before it breaks. Rp0.2 is the offset yield at 0.2% of plastic deformation. This is an arbitrarily chosen point of the stress/strain diagram that has proven useful for comparing metals.