Machinability of powder materials
After covering the powder layer that has 50m of thickness on the baseplate which has same composition with powder and processing the stainless steel, tool steel, titanium alloy and three kinds of aluminum alloys powders based on specific laser parameters.Experiments show that stainless steel, tool steel and titanium alloy powder are not completely melted and will form good metallurgical bonding with the baseplate (take stainless steel as an example), besides, the microscopic analysis proves that the molten powder will form compact metallurgical bonding with the baseplate.
However, the aluminum alloy powders will have serious spheroidizing crystallization phenomenon (take AlSi25 as an example). Some sphere has fallen off from the baseplate, while the sphere that have been formed have about 300m of the diameters, which are much bigger than the thickness of powder layer, thus it will be difficult to spread powder for the next layer and will cause difficulties in the process of spreading powder and even interrupt the modeling process. Therefore, in the model manufacturing process, the spheroidizing crystallization phenomenon should be avoided.
It can be discovered in the further experiment that higher laser power is used in melting aluminum alloy powders. Besides, the laser power will vary in melting different aluminum alloy powders. Laser power that can completely melt AlSi25, AlSi10Mg and AlMgSi0.5 powders are respectively as 110W, 180W and 220W.
The spread powder performance of powder material
The spread powder performance of powder material is very important, as the non-uniform distribution of powder will directly influence the dimension accuracy of models. Powder layer that used in titanium alloy, which has the interval distribution of argon atomization titanium alloy powder particles in the range of 25-45m, its powder particles are distributed in Guassian as well as has 50m layer thickness. It can be seen that titanium alloy powder layer is not distributed in totally even as large number of intervals are existed in it. However, the powder particles of stainless steel, tool steel and titanium alloy are distributed evenly.
Powder layer of aluminum alloy, which has 50m layer thickness, has poorer spread powder performance compared with the power of stainless steel, tool steel and titanium alloy. Its powder cannot distribute even as some powder will accumulate in a small scale, while no powder will distribute in the other area of the powder layer.
After making the model surface with the above six kinds of powders and measuring the surface roughness of models, we found that SLM model that made by stainless steel, tool steel and titanium alloy powders, of which the surface roughness is 50-80m. However, the surface roughness of SLM model that made by aluminum alloy powder is 80-120m.
Different kinds of metal powders will have different machinabilty in the laser machining process, which can be caused by the different performances of material thermophysical characteristics. As is same with the process when laser processing the solid materials, in the SLM process, laser power that used in melting metal powders not only depends on the output power of laser, but also depends on the material absorption amount of laser. Under the same condition of laser output power, it is especially important for the material to absorb laser. Steel has 30 percents of absorptivity of solid laser (which has the wave length of 1064nm), while aluminum alloy only has 5 percents. Therefore, we can see that very little energy is used in melting aluminum alloy powder in the SLM process, in which most of the laser power is all reflected. By the mutual effect between laser and powder, the temperature of powder will rise. Besides, the melting degree of powder depends on the laser power as well as the melting point, specific heat capacity and other thermal performances of materials. The energy that used in melting metal powder will be higher when the melting point is higher, the heat conductivity is better and the specific heat capacity is bigger.
The melting point of steel and aluminum are respectively as 1539 degrees Celsius and 660 degrees Celsius (which is a kind of important performance for aluminum and steel). Although aluminum has low melting point, the oxide skin (the melting point is 2652 degrees Celsius) on the surface of aluminum alloy powder that is generated in the powder manufacturing process will improve the melting point of aluminum. Steel and aluminum have almost the same specific heat capacity, but their thermal transmission performances are totally different. In the SLM process, laser will react with powder and form metal molten pool which will expand its range to next layer of powder and its surrounding solidified metals. As metal molten pool has high temperature, it will have convection, radiation, heat conduction and other thermal physical phenomenon in or around the molten pool. Among them, the energy loss that caused by convection and radiation can be negligible, but the effect caused by heat conduction cannot be ignored.
Therefore, the thermal physical performances of metal materials including its absorption of laser, thermal conductivity, melting point and specific heat capacity will have obvious influence on the machinability of powders, FEM calculation has proved this kind of influence; in the experiment process, appropriate equipment should be chosen to solve the problem caused by aluminum alloy powder as it has light weight, which will be easy to form powder accumulation and is more difficult to spread powder compared with other powders.
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