1.metal that does not melt at high temperatures
First of all, 3D printing cannot be achieved from metals that do not melt at high temperatures. 3D printing metal is fundamentally based on heating metal powder or wire under a heat source (like a laser or electron beam), then layer stacking forms a three-dimensional solid. Consequently, the metal cannot be worked by 3D printing technology if it cannot melt at high temperatures. For instance, 3D printing is not feasible for some high melting point metals such tungsten, rhenium, etc. whose melting points greatly exceed the greatest temperature presently attainable by 3D printing technology.
2 .Reactive metals as well as flammable and explosive ones
Furthermore unfit for 3D printing are several reactive metals as well as combustible and explosive ones. These metals oxidize easily in air or react with moisture to produce oxides or hydrides, which might influence material characteristics and printing quality. For instance, although aluminum inhibits this procedure, titanium and aluminum alloys are prone to produce oxide layers during laser sintering and need to be removed under high-temperature protection. Furthermore, active metal powders like titanium and aluminum have explosive characteristics expressed as tiny particles, which raises processing safety hazards. Therefore, even if 3D printing technology can theoretically be used to process these metals, extra safety precautions must be taken in practical operation, such as using inert gas to protect the atmosphere and controlling dust concentration, so increasing the difficulty and cost of processing.
3.low melting point volatile metals
Furthermore unfit for 3D printing are low melting point and volatile metals. throughout the heating process, these metals are prone to volatilization, or liquefaction, which causes great material loss throughout the printing process and challenges in creating stable three-dimensional structures. Mercury, for instance, is a low melting point metal whose melting point is just -38.83 °C, well below ambient temperature, hence cannot be 3D printed. Similarly, some alloys including volatile elements are not fit for 3D printing since volatile components are lost during heating, resulting in changes in alloy composition and hence influencing the performance of the final product.
4.Restraints on Material Attributes
Apart from the previously mentioned elements, some properties of metal materials can restrict their capacity for 3D printing. For instance, some metals have a great degree of brittleness, which makes them prone to cracking and fracture during processing; other metals have a high degree of viscosity, therefore it is difficult to exactly manage in a molten condition. These characteristics will make 3D printing more difficult and expensive, and even cause components that satisfy the criteria to fail to be able to be effectively produced.
5.Technical restrictions and fixes
Notwithstanding the above described constraints, engineers and researchers have been working to overcome them. The spectrum of metals that can be 3D printed can be enlarged, for instance, by bettering printing equipment and process parameters, raising printing temperature and speed, and employing unique powder preparation and processing procedures. Furthermore growing regularly are certain new 3D printing technologies like direct energy deposition and adhesive spraying, which offer processing options for more varieties of metals.
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