Richardson Richardson's blog : Types of defects in the SLM process, their causes, and their adverse effects _ component
Original Title: Types, Causes and Adverse Effects of Defects in SLM Process
Defects in SLM process can be divided into defects in powder spreading process
and defects in printing process. Defects in powder spreading process include
irregular powder layer, and defects in printing process include splash,
spheroidization, pores, poor surface quality, cracks, geometric deformation,
etc. Dr. Cao Longchao from Huazhong University of Science and Technology
summarized the common defects and their formation mechanism in SLM manufacturing
reported in the literature. The different defects and their formation process
are introduced below, and the impact of different defects on the application of
printing components is summarized. Defect is one of the biggest bottlenecks for
the wide application of SLM technology in industry. 1. Splash Splash is one of
the most common defects in SLM process, which can directly affect the
interaction between laser and material and lead to other defects. The study
shows that the spatter is mainly caused by the lateral shielding gas flow, the
fluctuation of the molten pool and the recoil pressure. The splashing on the
powder will form large metal particles, and then produce underfusion and
porosity defects, which are harmful to the tensile strength and fatigue
performance of the component. In addition, the splashing on the surface of the
solidified layer will affect the next layer of powder, resulting in the next
layer of powder being uneven and uneven, and even damaging the scraper. Physical
process of interaction between laser molten pool and laser beam In order to keep
the metal powder bed from being contaminated by spatter, a high-speed protective
gas flow can be used to remove the spatter. However,3d titanium wire,
excessive airflow will affect the surface quality of the powder layer, and how
to optimize the airflow technology has become the direction of
manufacturers'continuous efforts. 2. Spheroidization Expand the full text
Spheroidization is a unique metallurgical defect in the manufacturing process of
metal-based powder bed. Spheroidization occurs when the liquid metal solidifies
into a spherical shape under the action of surface tension. If the laser beam
energy density is too high or too low, it will lead to this phenomenon. If the
energy is too low, the metal powder will not be completely melted, which will
lead to spheroidization. If the energy is too high, the liquid metal will splash
on the unmelted metal powder, which will also lead to spheroidization.
Spheroidization will affect the quality of the next layer of powder,titanium filler
rod, affect the surface quality of components, and also lead to defects such
as poor fusion and slag inclusion. Further, spheroidization reduces the tensile
strength and fatigue resistance of the member. To reduce and avoid
spheroidization, it is necessary to optimize the printing process and find the
optimal parameter combination. Defects in 3D printed Ti6Al4V alloy and examples
of their existence as fatigue cracks 3. Air Holes Porosity is the main defect
type of SLM components, and it is one of the defects that have the greatest
impact on the mechanical properties of SLM components, and it is also one of the
focuses of industry and academia. In the process of SLM, the rapid melting and
solidification of the material and the violent fluctuation of the molten pool
will lead to the formation of pores. The size, number, morphology and location
of the pores have an important impact on the mechanical properties of the
component. The higher porosity will shorten the fatigue life of the molded
parts, and the pores near the surface have a greater impact on the fatigue
properties of the molded parts than those at other locations. According to the
formation mechanism of stomata, it can be divided into raw material-related
stomata and laser-induced stomata. The process of pore formation is very complex
and closely related to the process parameters. Reasonable printing process and
uninterrupted process optimization for material use times are still important
conditions to avoid pores. 4. Surface quality With the development of SLM
technology, the types of printable materials are more and more abundant, ti6al4v
,ti6al4v
eli, and the mechanical properties of components have been greatly improved,
but the relatively poor surface quality is still one of the main obstacles
limiting the development and industrial application of SLM. The surface
roughness of the intermediate layer will affect the quality of the next layer,
leading to internal defects; the surface roughness of the forming surface will
seriously affect the fatigue performance of components. SEM and reconstructed
images of the upper surface at different laser powers The surface quality is
closely related to the flow behavior of the molten pool.The surface quality can
be effectively improved by adjusting the process parameters such as laser power,
scanning speed and powder layer thickness.The surface quality can also be
improved by post-treatment, but it will increase the cost and reduce the
efficiency.Therefore, it is necessary to explore new methods to improve the
surface quality of components. 5. Crack The formation of cracks in SLM process
is related to temperature distribution, residual stress and poor fusion. The
cracks caused by residual stress can be divided into solidification cracks and
liquefaction cracks, which are related to materials. Solidification cracks are
caused by the large temperature gradient between the molten pool and the
solidified metal, resulting in large deformation of the molten pool. However,
the insufficient fluidity of the liquid can not compensate for the deformation
of the molten pool; Liquefaction cracking occurs in the partially melted zone
and is related to the extent of liquefaction, grain structure, thermal
elongation, shrinkage and confinement of the metal. Cracks inside a 3D printed
metal turbine blade In addition, underfusion caused by poor fusion is also a
common type of crack in SLM components, which has a fatal impact on the
mechanical behavior and fatigue life of SLM components. Underfusion cracks often
occur between adjacent scanning paths or between deposition layers, which are
mainly caused by incomplete melting of metal powder, and may lead to
delamination defects when the cracks are serious. An example of a layere defect
Cracks have a fatal impact on SLM components, and it is an important challenge
for academia and industry to reduce crack defects of SLM components. Alloying
and adjusting process parameters are two possible ways to improve microcracks,
but the success rate is still limited. At present, there is little research on
the cracks of SLM components, and it is urgent to further study. 6. Geometric
deformation Due to the geometric characteristics of the component, heat
accumulation, stress concentration and other reasons, different degrees of
geometric defects will be formed in the SLM process. The lighter degree may
cause deformation, resulting in dimensional errors, and the serious degree may
lead to incomplete structure, or even failure of the processing process. The
geometric accuracy of SLM components will affect the assembly and use
performance of the components. It has been found that different scanning methods
will have an important impact on the temperature field, and the temperature
field will affect the distribution of residual stress and the geometric
deformation of the component; the laser spot, power and scanning speed will
affect the dimensional accuracy and surface quality, and improving the geometric
accuracy of the component will greatly promote the industrial application of SLM
technology. 7. The powder layer is irregular In the SLM process, the filling
quality of the powder will affect the thermal conductivity of the powder bed and
its absorptivity to the laser. The filling density of powder layer will affect
the thermal conductivity of powder bed and the flow of molten pool, the
thickness of powder layer will affect the stability of molten pool, the melting
state and the internal defects of components, and the irregular surface of power
layer will also cause the quality problems of components. At the same time, SLM
is formed layer by layer, and a component has to be manufactured through
multiple layers, so each layer of powder is very important to the quality of the
component. Powder spreading is the key step of SLM manufacturing, the quality of
the powder layer is directly related to the absorption of the material to the
laser, the thickness of the powder layer will affect the stability of the molten
pool, too thick powder layer will cause defects such as lack of fusion, pores
and so on,titanium
plate gr7, too thin powder layer will reduce the manufacturing efficiency,
therefore, strictly controlling the quality of powder spreading is of great
significance to improve the quality of SLM components. Return to Sohu to see
more Responsible Editor:. yunchtitanium.com
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