A wide range of materials for metal injection moulding is available and still more are being developed. Low alloy steels and stainless steels, however, represent by far the greatest part of metal injection moulding materials produced today.

The densities of metal injection moulded alloys are usually 96% of full density or higher. The microstructures are isotropic (i.e. equal material properties in all directions) and free from nonmetallic impurities. Residual pores are isolated, very fine and spherical. This is the reason why metal injection moulded materials generally have much higher strength properties than cast or wrought alloys of the same composition.

The metal powders used in metal injection moulding are usually at least one order of magnitude finer than the powders used in die compaction.

Low alloy steels are often based on carbonyl iron powder which is composed of spherical particles with particle sizes between 1 and 10 µm. Alloys are formed by mixing the base powder with carbonyl nickel and other alloy constituents.

These low alloy steels are often quench-and-temper heat treated or case hardened after sintering and attain high hardness and strength levels combined with a high ductility and fatigue strength which is favourable for many applications where mechanical strength is the main requirement. MIM-4140 and MIM-4340 are the standard grades for this class of metal injection moulding alloys.

High alloy materials

High alloy materials such as stainless steels (MIM-316L) are usually made from gas or water atomised alloy powders with particle sizes of less than 40µm. While gas atomised particles are spherical, water atomised powders have irregular particle shapes.

There is no general rule whether spherical or irregular powders are better suited for metal injection moulding; some manufacturers even prefer mixtures of both types of powder. Further, it is reported that metal injection moulded stainless steel made from a mixture of carbonyl iron and a master alloy powder is comparable in corrosion resistance and even superior in surface quality to the same alloy made from a prealloyed powder.

Parts consumers can rely on specifications of standard mechanical properties for frequently used metal injection moulded materials. The hardness or ductility level, respectively, can be varied continuously depending on the requirements of the application.

Nickel-free alloys

There is an increasing demand for nickel-free alloys in applications where metal injection moulded parts are in direct contact with human skin or tissue, as harmful allergic reactions could be caused by nickel.

Examples are found in the jewellery and watch making industry, as well as in binocular frames, orthodontic brackets, and medi-cal technology.

In addition to excellent corrosion resistance, these applications often demand high strength. Various alloys have been designed which fulfill the requirements of sintering technology and at the same time provide the material properties required by the application.

Special steels

The wide range of ferrous metal injection moulded alloys used for structural components also includes hardenable stainless steels like the precipitation hardening MIM-17-4 PH and iron-chromium-carbon alloys with 13% and 17% chromium. Tool steels and high-speed steels also manufactured by metal injection moulding as well.

Many soft magnetic alloys like iron-phosphorus, iron-silicon, iron-nickel and iron-cobalt alloys, Invar and Kovar are also available in high quality as metal injection moulding materials.

Non-ferrous materials

Many non-ferrous materials are also produced by metal injection moulding. Particularly attractive due to its high strength, light weight, corrosion resistance, and potential cost savings is titanium.

Some specialised metal injection moulding producers are able to manufacture parts from titanium alloys such as Ti6Al4V or Ti6Al7Nb with an oxygen content below 2000 ppm. Titanium parts are used in medical and dental applications and in jewellery or as watch parts.

Even copper base alloys and aluminium parts have started series production. The high thermal conductivity of aluminium is the reason to use it for heat sinks.

Heavy metals and tungsten alloys

Heavy metals and alloys such as tungsten, cobalt and nickel based high temperature alloys are also produced by metal injection moulding, as are tungsten carbide-cobalt cemented carbides for cutting tools and wear parts.

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