Research on Metal Powder Melting Technology

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A new technology of rapid prototyping metal powders through the selective laser melting (SLM) process was discovered in recent years, thus made the directly forming of nearly full density parts possible. SLM technology can simplify the over-complicated producing process caused by selective laser sintering (SLS) technology. Main producing methods through SLS technology are as follows:

a. Investment casting: first, use SLS technology to shape up polymer prototype parts, then use the prototype’s thermal degradation for casting.

b. Sand casting: use coated sand to form the cavity of molded parts and sand core (that is manufacturing sand model directly) and then followed casting.

c. Selective laser sintering (SLS) indirectly shaping prototypes: first get the mixture of polymer and metal powder or polymer-coated metal powder smelt through SLS, then shape through degreasing, high-temperature sintering, impregnating and other processes.

d. Selective laser sintering (SLS) directly shaping prototypes: first, mix low-melting-point metal powders with high-melting-point metal powders, wherein the former works as a binder in the forming process. Then use SLS technology to shape the metal parts.

Finally, those metal parts still need post-processing, including dipping into low melting point metals; high temperature sintering and hot isotactic pressing. Thus, their mechanical properties are much more easily influenced by numerous factors during these methods’ low-type processes.

SLM is a type of technology that first using the laser beam to melt the powders  under heat, then cooling and solidifying before molding. To melt them, the required laser energy density shall exceed 106W/cm². After the complete melt under laser action and cooling down, it can be achieved with metal welding metallurgy synthetic. It is just through these processes that SLM technology can form a three-dimensional layer cumulatively.

Currently, SLM technology internationally is applied in manufacturing molds, tools, bio-implants and other metal parts. However, the technology still not matures, and the application area is also not wide. SLM technology research mainly focused on the following aspects: molding equipment, metal powder, molding process molding mechanism, molding performance, simulation of molding process and the application of molding parts.

Metal powders are widely for SLM technology. The cost for designing suitable material and preparing the powder for SLM molding by one’s own is relatively high. Therefore, those used for SLM technology mainly are commercial. We can study their formability to set up powder standards for the technology. Powders suitable for SLM can fall into three kinds: mixed powders, pre-alloy powders, and elemental metal powders.

Mixing certain ingredients evenly together, we can get the powders we want, but still the laser spot size’s requirement on powder particle size has to be taken into consideration. The second kind of powder, pre-alloy powders by name, according to the main component can be divided into iron-based, nickel-based, titanium-based, cobalt-based, aluminum, copper, tungsten powder and so on. The last kind of powders mainly refers to titanium powders. The experiments showed that: titanium powders function better in molding with the molded parts’ relative density up to 98%.

In summary, elemental metal powders and pre-alloyed powders are applied widely in SLM technology studies, as products made with them proved to have better composition distribution and mechanical properties, thus molding researches shall mainly focus on the process optimization of these two kinds of powders to improve products’ density.