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SLM Selective Laser Melting: 3D Printing Technology for Powder Metallurgy Prototype

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-09-11 08:55:00
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How SLM works

Selective laser melting (SLM) was first proposed by the Froounholfer institute in Germany in 1995, and its operation principle is similar to SLS. SLM converts laser energy into heat to  powder metallurgy. The main difference is metal powder does not completely melt in SLS process, while in the manufacturing process of SLM, metal powder is heated to form after complete melting.

The SLM process is: the printer controls a laser to selectively illuminate the laid powder, and the metal powder is heated until completely melted and formed. The piston then lowers workbench height by one unit, and a new layer of powder is spread over the existing layer. The equipment transfers new layer data to laser melt and bond with previous section. This process is repeated layer by layer until the whole part finished.



The difference between SLM and SLS

SLS is laser sintering, its material is mixture powder by low melting point metal or polymer material, the low melting point material melting but high melting point powder is not melting. The powder is heated above crystallization temperature, which is about 170 degrees Celsius, either by heating a lamp or by radiating heat from a metal plate.

It uses melted materials to form parts, so there are pores inside, and the mechanical properties are poor. Some parts have to be remelted at high temperature to be used.

SLM is selective laser melting, which, as its name implies, means the powder is completely melted, and binder is needed. Its precision and mechanical properties are better than SLS. However, because SLM has no hot field, it needs to heat metals from room temperature of 20 degrees Celsius to melting point of 1000 degrees Celsius, which requires huge energy consumption.

Main advantages of SLM

SLM parts with high density, up to 90%;

Mechanical properties such as tensile strength are better than castings and even reach the level of forgings. Micro vickers hardness can be higher than forgings;

Due to complete melting in process, the dimensional accuracy is higher;

Compared with traditional reduction material manufacturing, it can save a lot of materials

SLM technical limitations

Powder metallurgy speed is low, in order to improve machining accuracy, need to use a thinner machining layer. It takes a long time to process small parts, so it is difficult to be used in large-scale manufacturing.

Equipment stability and repeatability need to be improved.

Surface roughness needs to be improved.

The whole equipment is expensive, melting metal powder requires more powerful laser than SLS, and energy consumption is high;

SLM technology is complicated and needs to be supported, so there are many factors to be considered. Therefore, it is widely used in industrial grade additive manufacturing.

In process of SLM, temperature gradient of instant melt and solid (cooling rate is about 10000K/s) is very large, resulting in great residual stress. If rigid not enough, the substrate will deform. Therefore, the substrate must be rigid enough to resist residual stress. De-stressing annealing can remove most residual stress.

SLM material

The materials for SLM technology are mainly divided into three categories, mixed powder, pre-alloy powder and simple metal powder.

1. Mixed powder. Mixing different powders in a given proportion. Existing studies show that the mechanical properties of SLM components are affected by density and forming uniformity.

2. Pre-alloy powder. According to the composition, pre-alloy powder can be divided into nickel base, cobalt base, titanium base, iron base, tungsten base, copper base and so on.

3. Simple metal powder. The main simple metal powder is titanium, which has good formability and density up to 98%. Currently, SLM technology is mainly applied in industrial fields, with outstanding technical advantages in the fields of complex molds, personalized medical parts, aerospace and automobiles.

Keywords in the article: Powder Metallurgy SLM
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