Success in sintering

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Stephen Dyson, special operations manager at Protolabs, on how direct metal laser sintering is evolving to become an important additive manufacturing tool

Additive manufacturing, the gradual build-up of source material to create a final product, was the primary means of producing components in 3D printing at the inception of the process. The resulting structures were usually created from waxy polymers and were used solely as models to check the details on final designs, rather than being used as actual components.

It didn’t take long however for the industry to recognise the manufacturing potential of 3D printing, and investment in research soon generated new methods and materials that were robust enough to make usable components, and a whole new means of manufacturing came into being.

Among the new materials that could be laid up in additive form were metal particulates, composed of very small, spherical feedstock of consistent size. When delivered from within a polymeric medium, these particulates created a flow of material that could clump together and be used to form a structure, prior to a secondary process being used to finalise the product.

Materials such as this could be made into solid items using the well-defined process of sintering, leading to final products that could either be used as they are or further worked on using standard machine tools. The sintering process is carried out at an elevated temperature, though well below the melting temperature of the metal feedstock, which effectively burns off the remnant polymer material. This is still a standard means of laying up 3D designs in an additive way, and feedstock is now available in the form of many different pure metals and alloys, making it a highly versatile practice. But engineers are people who love to tinker with materials and processes, and it is in this tinkering that they have since faced an enormous issue with 3D printing aluminium and other fast-oxidising metals and alloys.

Aluminium, magnesium, and even titanium are excellent metals with a high strength-to-weight ratio that find extensive use in machined components. However, these fast oxidising materials cannot be removed from their ore, itself an oxide, by simple melting as with metals such as iron, as exposure to air in a removal process will make them return to the oxide form. Typically removed from their oxide by electrolysis, they have been difficult to use for 3D printing as they cannot be sintered in normal ways. However, new processes involving lasers have been developed to make these materials usable in 3D processes.

The use of lasers to rapidly heat a very thin layer of printed material has begun a revolution in the manufacture of components out of lightweight materials. This new, dual-part process has been developed explicitly to minimise the effects of oxidation by reducing the heating of thin layers of aluminium powder.

While still in its development phase, DMLS has the potential to create complex products quickly and out of materials which are traditionally very difficult to use.

Termed direct metal laser sintering (DMLS), this fast technology is suitable for producing new parts and components, and for repairing or re-manufacturing applications, and can be deployed by the same industries that currently use additive and 3D printing methods, such as the aerospace, automotive, medical, and defence industries. Using selective laser sintering, thin layers of atomised fine metal powder are evenly distributed using a coating mechanism onto a metal substrate plate that is fixed to an indexing table moving in the vertical (z) axis. This takes place inside a chamber containing a tightly controlled atmosphere of inert gas, such as argon or nitrogen, to prevent fast oxidation of the substrate.

Once each layer has been distributed, each 2D slice of the part geometry is fused by selectively melting the powder. This is accomplished with a high-power laser beam, usually from an ytterbium fibre laser. The laser energy is intense enough to permit full melting in a localised area and then the subsequent mass transport of particles to form solid metal, but without problems associated with oxide build-up. The process is repeated layer after layer until the part is complete. At this point, the component is effectively finished from a printing point of view and robust enough to allow further processing such as drilling, tapping, and machining to a high surface finish.

While still in its development phase, DMLS has the potential to create complex products quickly and out of materials which are traditionally very difficult to use in rapid prototyping applications. However, the need to include these materials in the repertoire of printable materials has driven the research that has made 3D printing aluminium and allied materials as effective as 3D printing stock materials.

With DMLS still effectively in development, is what we are seeing now likely to be significantly different from the future? Certainly, advances in laser technology are likely to have an impact from a couple of points of view. Firstly, beam intensity is a controlling factor and this could be increased to not only speed the process up, but offer the possibility of increased depth of penetration and a resulting component that has much greater density. Further advances may come from using a set up with multiple lasers of different intensity, so that some parts of a component might be heated at different rates, leading to differences in density in specific areas.

The beauty of using lasers is that it is a technology that has many different elements to how it works and operates, and changes to these can have a profound effect on the final product. And knowing how engineers like to tinker, there is little doubt that enhancements to the DMLS process are not far away.

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