Laser additive manufacturing technology has the advantage of quickly forming complex-shaped parts and has received widespread attention in recent years.
This article introduces two laser additive manufacturing technologies, directed energy deposition (DED) and selective laser melting (SLM), and summarizes the research progress in laser additive manufacturing of 316L stainless steel from the perspectives of common defects, structure and texture and mechanical properties. .
It analyzes the existing problems in laser additive manufacturing of 316L stainless steel and looks forward to its development prospects.
1. Laser Additive Manufacturing Process
Laser additive manufacturing includes two technologies, directed energy deposition (DED) and selective laser melting (SLM).
Both use high-energy laser beams as heat sources to locally melt metal powder and form a molten pool; when the laser beam moves away, the molten pool quickly solidifies. However, the working principles of DED and SLM technologies are different.
DED, also known as near-net shape laser engineering, laser metal deposition or rapid laser forming, is a typical laser additive manufacturing technology for coaxial powder delivery.
The metal powder inside the powder drum enters the weld pool through the conveying tube and specially designed nozzle. The laser beam moves along a predetermined trajectory under computer control to melt the metal powder and form a thin layer.
After this, the deposition head moves upwards, continuing to deposit the next layer, repeating this process layer by layer until the desired part is produced.
In addition to directly forming metal parts, DED technology can also be used to prepare coatings, repair damaged components, and prepare functionally graded materials. SLM is a typical powder bed laser additive manufacturing technology.
The metal powder is not sprayed from the nozzle, but is pre-spread evenly on the powder bed. The laser beam selectively melts the powder bed according to a predetermined path.
After a layer is formed, the powder bed moves down a certain distance and is selectively repulverized and melted, repeating this process layer by layer until the desired part is produced.
To prevent oxidation of 316L stainless steel parts, the DED and SLM forming processes need to be carried out under inert gas protection.
In addition to different operating principles, Directed Energy Deposition (DED) and Selective Laser Melting (SLM) also have significantly different process parameters. The laser beam diameter in DED typically ranges between 600 and 1300 μm, while in SLM it is significantly smaller, usually between 15 and 80 μm.
At present, the 316L stainless steel powder used for laser additive manufacturing is often prepared by atomization.
Considering the beam diameters of DED and SLM, the particle size of 316L stainless steel powder for DED is usually between 45 and 180 μm, while for SLM, it is normally between 5 and 63 μm.
During the DED process, the laser power (P) can reach 200 to 720W, but the scanning speed (v) is often less than 10mm·s -1 .
Consequently, the energy density of the line ( Ei =P/v) is extremely high, reaching tens or hundreds of joules per millimeter. Higher line power density leads to stronger laser penetration, so the layer thickness in DED is generally set between 254 and 500 μm, with a scanning range typically set between 350 and 500 μm.
In contrast, SLM often uses lower laser power (60 to 380 W) and higher scanning speed (30 to 7000 mm s -1 ), resulting in a very low line power density, typically between 0.01 and 0.5 J mm -1 .
To ensure the absence of insufficient fusion defects, both the scanning range and layer thickness in SLM must be small enough, generally between 20 to 300 μm and 10 to 60 μm, respectively.
During DED formation, the temperature gradient and cooling rate can reach up to 10 2 to 10 3 K·mm -1 and 10 3 to 10 4 K·s -1 respectively.
As the line energy density in SLM is lower, the temperature gradient within the weld pool is even greater, up to 10 3 to 10 5 K·mm-1, and the cooling rate is faster, reaching 10 4 to 10 7 K·s -1 .
2. Common Defects
Compared to traditional metallurgical techniques, laser additive manufacturing technology has significant advantages.
However, if the process parameters are chosen incorrectly, various defects can be introduced during the forming process, such as porosity, lack of fusion and cracking. These defects can significantly decrease the mechanical properties of 316L stainless steel.
Therefore, how to reduce or eliminate these defects is a crucial question in laser additive manufacturing.
Porosity and lack of fusion are the two most common defects in laser additive manufacturing of 316L stainless steel. The porosity normally appears spherical, mainly originating from gases in the 316L stainless steel powder.
The pores within the powder cannot be completely expelled during the rapid solidification of the weld pool and remain within the component. Inert gases can also be drawn into the weld pool and form pores.
Furthermore, during the laser additive manufacturing process, the highest temperature occurs at the surface of the part. The heat is conducted inward, forming a wide, shallow melting pool.
However, when the laser power is extremely high or the laser scanning speed is very low, that is, the laser line energy density is very high, the weld pool formation will switch from a laser conduction mode to heat to a deep melting mode, forming a narrow mode and deep melting pool channel.
This channel is very unstable and can easily form pores at the bottom of the weld pool. It is generally believed that when the width to depth ratio of the weld pool falls below a certain critical value, the deep melting mode is triggered.
To control the porosity rate of 316L stainless steel in laser additive manufacturing, the gas content of 316L stainless steel powder must be strictly controlled and the process parameters must be optimized to avoid porosity caused by deep melting mode.
In contrast to the mechanism of porosity formation, the lack of fusion defects typically originates from insufficient laser energy density, leading to inadequate fusion depth, resulting in large, irregularly shaped voids between layers. This type of defect can usually be resolved by reducing the layer thickness.
Balification is one of the common defects in laser additive manufacturing of 316L stainless steel. This phenomenon refers to the inability of molten metal droplets to form a continuous molten line, instead creating a teardrop-shaped surface on the formed part; the cumulative effect of marking can significantly reduce part accuracy.
This effect is often caused by an excessive oxygen content in the formation cavity, which results in oxidation of the surface of the metal droplets, preventing their fusion. Therefore, it is crucial to strictly control the oxygen content in the forming cavity during the forming process.
316L stainless steel is one of the suitable metals for additive manufacturing, but there have been reports of cracking defects in laser additive manufacturing of 316L stainless steel.
Thermal cracking (or solidification cracking) is one of the important cracking mechanisms, often occurring in the final stage of rapid solidification of the molten pool.
At this point, the proportion of solid phase is large, the molten pool is occupied by a cellular substructure, and there is a liquid phase film at the boundary of the cellular substructure.
The strength of this structure is extremely low, which makes it prone to cracking under tensile stress, and at this time, it is difficult for liquid to flow to fill the crack area, eventually forming a thermal crack at the grain boundary.
Common defects and their formation mechanisms in laser additive manufacturing of 316L stainless steel are shown in Table 1.
Table 1: Common defects and their formation mechanisms in laser additive manufacturing of 316L stainless steel.
| Defect name | Training Mechanism |
| Pore | There are pores inside the powder; inert shielding gas cannot be expelled from the weld pool; inappropriate selection of process parameters results in excessive laser energy density, triggering a deep melting mode. |
| Incomplete merger | The laser energy density is insufficient, resulting in inadequate melting depth. |
| Spheroidization | Excessive oxygen content prevents the metal droplets from merging to form a continuous molten line. |
| Crack | During the final stage of solidification, the liquid phase film at the boundary of the cellular substructure cracks under tensile stress. |
3. Microstructure
Due to the high temperature gradient and cooling rate, the solidification process of laser additive manufacturing exhibits a rapid quenching effect.
316L stainless steel prepared in this way has an extremely unbalanced structure that traditional casting methods cannot achieve.
It typically forms a columnar crystalline structure, with numerous tiny cellular substructures within the columnar grains, as depicted in Figure 1.
During the solidification process (including casting, welding, laser additive manufacturing, etc.), the temperature gradient G in the liquid phase at the leading edge of the solid/liquid interface and the growth rate V of the solidification front together determine the morphology and the size of the grain and its internal substructure.
The lower the G/V, the easier it is to form an equiaxed grain structure; conversely, a columnar grain structure is more likely to form. Temperature gradients and growth rates vary across the weld pool.
Typically, the bottom of the weld pool has a large temperature gradient and a small growth rate, promoting the formation of columnar crystals, while the top of the weld pool has a small temperature gradient and a high growth rate, facilitating the formation of columnar crystals. formation of equiaxed crystals.
Laser additive manufacturing uses a layer-by-layer deposition method. To ensure sufficient interlayer bonding, some of the material from the previous layer will be remelted, so the equiaxed crystalline structure at the top of the weld pool often does not exist, while the columnar crystals at the bottom of the weld pool extend layer by layer. through epitaxial growth.
Furthermore, unlike the dendritic structure of cast 316L stainless steel, the weld pool from laser additive manufacturing cools extremely quickly during solidification, limiting the formation and growth of secondary dendrites.
Therefore, the solid/liquid interface of 316L stainless steel molten pool during laser additive manufacturing generally advances through cell growth.
During solidification, DED-formed 316L stainless steel typically forms a primary austenite structure, and in the cell walls of the cellular substructure, there is a pronounced segregation of ferrite stabilizing elements such as chromium and molybdenum, thus promoting the formation of a small amount of ferrite.
Compared to DED, SLM cools faster, the element segregation effect is greatly reduced, generally not enough to form stable ferrite, so the 316L stainless steel formed by SLM generally exhibits a single-phase austenite structure, without ferrite formation.
In addition to the segregation of ferrite stabilizing elements such as chromium and molybdenum into the cell walls of the cell substructure, the slight difference in orientation between adjacent cell substructures leads to a large number of dislocations clustered in the cell walls, while the dislocation density within the cell substructure is relatively low, forming a typical displacement cell.
In addition, a certain crystallographic texture is formed on 316L stainless steel through laser additive manufacturing. Throughout the process, the direction of heat flow varies within the weld pool, but in general, it opposes the direction of formation.
The structure of 316L stainless steel formed by laser additive manufacturing consists mainly of austenite with a face-centered cubic structure.
Because the <100> direction is the fastest growth direction for cubic crystals, 316L stainless steel typically forms a <100> fibrous texture along the forming direction during laser additive manufacturing. Adjusting process parameters, such as laser scanning strategies, can effectively control texture formation.
4. Mechanical Properties
4.1 Tensile Properties
The yield strength and tensile strength of 316L stainless steel produced through laser additive manufacturing typically range from 300 to 600 MPa and 400 to 800 MPa, respectively, significantly higher than the yield strength (200 to 300 MPa) and the tensile strength (500 to 600 MPa) of 316L stainless steel prepared using traditional methods.
The ultra-high yield strength of laser additively manufactured 316L stainless steel is attributed to its multi-scale structural organization, such as fine grains (approximately 0.2 mm in size), cellular substructures (less than 1 μm in diameter), small high-density structures. angular grain boundaries (up to 41%), dislocation networks (on the scale of hundreds of nanometers), precipitated phases (10 to 150 nm in size), and localized element segregation (less than 1 nm in range).
This heterogeneous multi-scale organization also contributes to the continuous and stable post-yield hardening of 316L stainless steel.
Furthermore, similar to 316L stainless steel prepared by traditional processes, 316L stainless steel manufactured with laser additive also exhibits dynamic Hall-Petch effect due to the formation of nanotwins aiding deformation during the tensile plastic deformation process, which helps to increase the work hardening effect, thereby achieving high tensile strength and ultra-high elongation after fracture.
The post-fracture elongation rate of laser additive manufactured 316L stainless steel is closely correlated with the porosity within the material.
316L stainless steel formed by laser additive manufacturing typically exhibits columnar crystalline structures and forms certain crystallographic textures, making the tensile properties of the formed stainless steel anisotropic. By adjusting the scanning strategies, one can effectively reduce the crystallographic texture, making the yield strength isotropic.
However, columnar crystalline structures still lead to varying levels of work hardening in different directions during the tensile process of 316L stainless steel formed by laser additive manufacturing, resulting in significant differences in tensile strength and elongation upon fracture in different directions. .
Furthermore, traditional manufacturing processes of 316L stainless steel can trigger martensitic phase transformation during plastic deformation, but no deformation-induced martensitic transformation has been found in current studies on the plastic deformation of 316L stainless steel formed by laser additive manufacturing. .
4.2 Fatigue performance
The fatigue performance of 316L stainless steel formed by laser additive manufacturing is influenced by several factors, including microstructure, internal defects, surface roughness and loading direction.
The fine-cell substructure within the 316L stainless steel structure formed by laser additive manufacturing significantly prevents dislocation slip and crack nucleation, greatly improving the fatigue performance of 316L stainless steel.
After laser additive manufacturing, post-processing heat treatment is generally required for 316L stainless steel parts, during which the microstructure of 316L stainless steel may change, affecting its fatigue performance.
Studies show that after stress relieving annealing at 470℃, the cellular substructure of 316L stainless steel formed by laser additive manufacturing does not change significantly, therefore, low-temperature stress relieving annealing does not greatly affect its fatigue resistance.
However, when the heat treatment temperature is high enough, it can affect the cellular substructure of 316L stainless steel formed by laser additive manufacturing, thereby affecting its fatigue performance.
Internal defects and substantial surface roughness significantly degrade the fatigue performance of 316L stainless steel formed by laser additive manufacturing. Studies indicate that internal imperfections (such as voids and unmelted powder) and rough surfaces lead to localized stress concentration in 316L stainless steel.
These areas of stress concentration tend to be the initial sites for the nucleation of fatigue cracks, thus promoting fatigue failure. Furthermore, the loading direction markedly affects the fatigue performance of 316L stainless steel formed by laser additive manufacturing.
The fatigue strength of laser-formed 316L stainless steel is highest when the loading direction is perpendicular to the forming direction, lowest when they are parallel, and lowest at an angle of 45 degrees.
However, current research into the propagation mechanisms of fatigue cracks in laser-formed 316L stainless steel is in its infancy and many mechanisms remain unclear or even contradictory.
5. Conclusion
316L stainless steel, with its exceptional combined mechanical and corrosion resistance properties, is among the most widely used stainless steel materials. Traditional casting methods produce 316L stainless steel with coarse grains and low strength.
Thermal mechanical processing significantly refines the grain and introduces high-density dislocations, increasing the strength of 316L stainless steel.
However, this procedure is complex and normally used for parts with simple shapes.
Laser additive manufacturing technology, characterized by its layer-by-layer deposition and rapid solidification, enables rapid molding of complex parts, imparting unique organizational characteristics such as small grains, internal cellular substructures, high-density small-angle grain boundaries, and high -density dislocations, which are unmatched by traditional metallurgical methods.
Laser additive manufacturing of 316L stainless steel produces superior strength and plasticity compared to 316L stainless steel prepared by traditional metallurgical methods.
However, laser additive manufacturing technology is still in the early stages of research and application. Future studies should further explore the microstructures and mechanical behaviors of 316L stainless steel molded by laser additive manufacturing, along with an in-depth investigation of the impact of process parameters on structure and performance.
Precise control of the manufacturing process will provide more technical support for your wide industrial application.
Furthermore, controlling the crystallographic texture to improve the performance anisotropy of 316L stainless steel molded by laser additive manufacturing and unraveling its fatigue crack propagation patterns are essential areas of future research.
