In the metal laser 3D printing forming technology, due to the inherent spheroidization effect and step effect of laser layer-by-layer processing of metal powder materials, even the highest precision SLM technology is used.
1 How SLM works
Selective Laser Melting (SLM) was first proposed by the Frooholhol Institute of Germany in 1995 and works in a similar way to SLS. SLM converts the energy of the laser into heat to shape the metal powder. The main difference is that the SLS is not completely melted during the manufacturing process, and the SLM is heated during the manufacturing process until it is completely melted.
SLM working principle diagram (Source: additively.com)
The SLM workflow is that the printer controls the laser to selectively illuminate the powder over the laid powder, and the metal powder is heated to complete melting to form. The piston then lowers the table by a unit height, a new layer of powder is spread over the formed current layer, and the device transfers the data of the new layer section for laser melting to bond with the previous layer. Cycle through layers until the entire object is formed. The entire process of the SLM is carried out in an inert gas-protected processing chamber to avoid oxidation of the metal at high temperatures.
SLM technology printing process
The laser melts the powder according to the contour of the current thin layer
A new layer of powder is spread over the current layer and melted layer by layer.
Obtain the final product
The difference between SLM and SLS:
SLS is laser sintering. The metal material used is a mixed powder with a low melting point metal or polymer material. During the processing, the low melting point material melts but the high melting point metal powder does not melt. First, the lamp is heated to a temperature exceeding the crystallization temperature by a lamp heating or a metal plate heat radiation, which is about 170 degrees Celsius. The formed material is bonded by the melted material, so the pores exist in the solid body, and the mechanical properties are poor. When some parts are used, they are subjected to high temperature remelting.
SLM is a selective laser melting. As the name implies, the laser is used to completely melt the powder during the processing. No adhesive is needed, and the precision and mechanical properties of the molding are better than those of SLS. However, because the SLM has no thermal field, it needs to heat the metal from a normal temperature of 20 degrees Celsius to a melting point of thousands of degrees. This process consumes a huge amount of energy.
2 Advantages & Technical Limits
The main advantages of SLM:
SLM molded metal parts have a high density of over 90%;
Mechanical properties such as tensile strength are superior to castings and even reach the level of forgings. Micro Vickers hardness can be higher than forgings;
Due to the complete melting during the printing process, the dimensional accuracy is higher;
A large amount of material can be saved compared to conventional material reduction.
SLM technology limitations:
The molding speed is low, and in order to improve the processing accuracy, a thinner processing layer thickness is required. It takes a long time to process small-volume parts, so it is difficult to apply to large-scale manufacturing;
Equipment stability and repeatability also need to be improved;
Surface roughness needs to be improved;
The whole set of equipment is expensive, the molten metal powder needs a laser with a higher power than the SLS, and the energy consumption is higher;
The SLM technology is more complicated and requires a supporting structure, and many factors are considered. Therefore, it is mostly used in industrial grade additive manufacturing .
During the SLM process, the metal melts and solidifies instantaneously (cooling rate is about 10000 K/s), the temperature gradient is large, and great residual stress is generated. If the rigidity of the substrate is insufficient, the substrate is deformed. Therefore, the substrate must have sufficient rigidity to resist the effects of residual stress. Stress relief annealing eliminates most of the residual stress.
Excessive residual stress of the workpiece, deformation of the substrate caused by insufficient rigidity of the substrate
3 SLM application
SLM material
Powder materials that can be used in SLM technology are mainly classified into three types, namely, mixed powder, alloy powder, and elemental metal powder.
1. Mix the powder. The mixed powder is a mixture of a certain proportion of different powders. Existing research shows that the mechanical properties of components formed by SLM are affected by density and molding uniformity, and the density of mixed powders needs to be improved.
2. Pre-alloyed powder. According to different compositions, the prealloyed powder can be divided into nickel-based, cobalt-based, titanium-based, iron-based, tungsten-based, copper-based, etc., studies have shown that the pre-alloyed powder material can produce components with a density of more than 95%;
3. Elemental metal powder. Generally, the elemental metal powder is mainly titanium metal, and its formability is good, and the density can reach 98%.
Metal powder for 3D printing
At present, SLM technology is mainly used in the industrial field, and has outstanding technical advantages in the fields of complex molds, personalized medical parts, aerospace and automobiles.
Aerospace
In the process of the development of the manned spacecraft SuperDraco by SpaceX, SpaceX, the engine of the manned spacecraft was built using SLM technology. The structure of the SuperDraco engine's cooling channels, spray heads, throttles, etc. is very complex, and 3D printing solves the problem of manufacturing complex structures. The strength, toughness, and breaking strength of the parts manufactured by SLM can meet all the stringent requirements, enabling SuperDraco to work in high temperature and high pressure environments.
SpaceX's manned spacecraft engine built with SLM technology (Source: SpaceX)
Titanium alloy blades printed with SLM technology (Source: SLM Solutions)
Aerospace parts printed using SLM technology
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