Process Parameter Optimization for Selective Laser-Melted High-Nitrogen Steel and the Effects on Microstructure and Properties

Question

1. What are the benefits of metal additive manufacturing over traditional processing methods?

2. What is the purpose of using chromium nitride powder in stainless-steel alloy powder preparation?

3. What is the relationship between relative density and laser energy density in high-nitrogen steel specimens?

4. How does energy density affect nitrogen concentration and loss in high-nitrogen steel samples?

Accepted Answer

Metal additive manufacturing offers several benefits over traditional processing methods. Firstly, it provides high efficiency, allowing for faster production times and reduced waste. Secondly, it offers high accuracy, ensuring precise and consistent results. Additionally, metal additive manufacturing enables near final shaping in a single pass, reducing the need for multiple manufacturing steps. This method also allows for the creation of complex shapes and geometries that may be challenging or impossible to achieve with traditional methods. Overall, metal additive manufacturing offers improved productivity, quality, and design flexibility compared to conventional manufacturing techniques.

Accepted Answer

Chromium nitride powder with a purity level of at least 99.0% is added as a nitrogen-increasing agent to stainless-steel alloy powder. The purpose of using chromium nitride powder is to enhance the nitrogen content in the stainless-steel alloy powder, which can improve its mechanical properties and corrosion resistance. The chromium nitride powder is a non-spherical powder, while the stainless-steel alloy powder is spherical. The powder is completely mixed in a planetary ball mill over four hours at a speed of 400 rpm, resulting in a homogeneous mixture with improved properties.

Accepted Answer

The relationship between relative density and laser energy density in high-nitrogen steel specimens is characterized by an initial increase in relative density as laser energy density rises, followed by a decrease. The specimen's relative density spans from 93.3% to 98.8%, with the highest relative density of 98.8% observed at an energy density of 148.8J/mm3. This pattern is attributed to the temperature of the melt pool, which affects the presence of unfused flaws and gas pores. At lower energy densities, the temperature of the melt pool is low, resulting in more unfused flaws and slower gas escape, leading to fewer gas pores. As the energy density increases, the temperature of the melt pool rises, prolonging the presence of the melt pool and increasing the time for gas escape, thereby reducing the unfused flaws and gas pores. However, excessively high energy density can result in keyholes, spatters, and spheroidization. The microstructural defects of the samples under typical energy density were examined using a scanning electron microscope (SEM), revealing the existence of spheroidized, unmelted, and splashing particles at different energy densities. The size of spheroidized particles and unmelted particles decreases as energy density increases, while irregular pores, spheroidized particles, and splashes may form at excessively high energy densities.

Accepted Answer

The nitrogen content of the sample steadily drops and the nitrogen loss rate gradually rises as the energy density rises. It is hypothesized that the rise in energy density hastens nitrogen evaporation. When the energy density is low, the molten pool solidifies quickly, and the nitrogen produced cannot escape in a timely way. As the energy density rises, the energy absorbed by the powder increases, and the relative amount of existence time for the molten pool formation rises, encouraging nitrogen atoms to flee and escalating nitrogen loss. As a result, the sample's nitrogen concentration falls and nitrogen loss rises as energy density rises. The selective laser melting occurs at a pressure close to ambient air, and the nitrogen content in the micro melting pool exceeds its saturation solubility, resulting in nitrogen escape and nitrogen loss. The phase composition analysis shows that the high-nitrogen steel sample is predominantly made of austenite (a-Fe) and ferrite (g-Fe), and the austenite phase decreases as energy density increases, while the ferrite phase increases. The nitrogen loss is influenced by the energy density, phase composition, and nitrogen solubility in steel.

Accepted Answer

The relationship between energy density and microstructure in high-nitrogen steel specimens during Selective Laser Melting (SLM) is significant. As the energy density increases, the size of cellular grains tends to become larger, and the columnar grains progressively become coarser. This is particularly evident when the energy density reaches 182.3 J/mm 3, which exhibits a needle-like structure. The increase in energy density leads to a higher heat input from the laser, resulting in a higher temperature of the molten pool. This, in turn, favors grain development and coarsening. The research suggests that a higher energy density during SLM slows the cooling rate of the melt pool, reduces undercooling, and promotes the formation of crystal nuclei. The temperature gradient and heat diffusion power are highest at the interface between the molten pool and the formed metal. Grains tend to grow perpendicular to the molten pool's border, leading to the formation of columnar crystals that develop in the opposite direction of heat dissipation. Additionally, as the energy density rises, the molten pool's lifetime lengthens, and nitrogen escapes more frequently. This relationship between energy density and microstructure is crucial for optimizing the SLM process and achieving desired material properties in high-nitrogen steel specimens.

Accepted Answer

The impact of energy density on the mechanical properties of high-nitrogen steel is significant. As the energy density increases, the tensile strength and yield strength initially rise and subsequently fall. The optimal mechanical characteristics of high-nitrogen steel are achieved when the laser power is 250 W, the scanning speed is 700 mm/s, and the corresponding energy density is 148.8J/cm 3. At this energy density, the yield strength, tensile strength, and elongation are 921.9MPa, 1205.1 MPa, and 27%, respectively. When the energy density is low, the number of unfused defects decreases, leading to a slight improvement in mechanical properties. However, when the energy density is too high, the nitrogen loss rate increases, resulting in minimal improvement in mechanical properties. The fracture morphology of high-nitrogen steel specimens at various energy densities also shows different patterns, with fewer and shallower dimples at low energy density, more deeper dimples and fewer internal flaws at moderate energy density, and molten droplet splashes inside the sample at high energy density, resulting in a high surface glossiness and mostly brittle fracture surfaces.

Accepted Answer

High-nitrogen steel exhibits better corrosion resistance than 316L and comparable corrosion resistance to pure titanium in 0.9% NaCl solution, simulated plasma, and Hank's solution. It has the highest open circuit corrosion potential and maximum pitting potential in 0.9% NaCl solution, the highest corrosion current density in Hank's solution, and the highest slope in the impedance curve, indicating better corrosion resistance. The passivation range of high-nitrogen steel is shorter than pure titanium but longer than 316L, and it has the slowest corrosion rate among the three alloys.

Accepted Answer

Increasing energy density initially increases the relative density of high-nitrogen steel, reaching a maximum of 98.8% at 148.8 J/cm3. This is due to the reduction of unmelted particles and spheroidized flaws. However, beyond this point, the relative density starts to decrease. The optimal process parameters for achieving maximum density are 250 W laser power and 700 mm/s scanning speed. These parameters result in the highest density and improved mechanical properties of the high-nitrogen steel.

Accepted Answer

The mechanical characteristics of high-nitrogen steel at 250 W and 700 mm/s are the highest, with yield strength, tensile strength, and elongation values reaching 921.9 MPa, 1205.1 MPa, and 27%, respectively. This indicates that the steel exhibits excellent mechanical properties under these specific process parameters, making it a suitable material for applications requiring high strength and ductility. The rise and subsequent fall in mechanical characteristics as energy density increases suggest that there is an optimal energy density range for achieving the best mechanical properties in high-nitrogen steel. This information can be valuable for researchers and engineers working with high-nitrogen steel, as it provides insights into the material's behavior under different processing conditions and helps in optimizing its mechanical properties for specific applications.

Process Parameter Optimization for Selective Laser-Melted High-Nitrogen Steel and the Effects on Microstructure and Properties

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Most frequently asked questions

1. What are the benefits of metal additive manufacturing over traditional processing methods?

2. What is the purpose of using chromium nitride powder in stainless-steel alloy powder preparation?

3. What is the relationship between relative density and laser energy density in high-nitrogen steel specimens?

4. How does energy density affect nitrogen concentration and loss in high-nitrogen steel samples?

5. What is the relationship between energy density and microstructure in high-nitrogen steel specimens during SLM?

6. What is the impact of energy density on the mechanical properties of high-nitrogen steel?

7. What is the corrosion resistance of high-nitrogen steel compared to pure titanium and 316L in different solutions?

8. How does energy density affect high-nitrogen steel density?

9. What are the mechanical characteristics of high-nitrogen steel at 250 W and 700 mm/s?

References

Metal Additive Manufacturing: A Review

Overview: high-nitrogen alloying of stainless steels

An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part II.: EIS interpretation of corrosion behaviour

Fabrication of Metal and Alloy Components by Additive Manufacturing: Examples of 3D Materials Science

Effect of molten pool boundaries on the mechanical properties of selective laser melting parts

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