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SLM Process Parameters and Characteristics

In document State of the art (página 30-36)

SLM involves a series of process parameters such as laser power, scanning speed, hatch spac-ing, and layer thickness. These, among the intensive list presented as follows, are adjusted such that a single melt line can fuse completely with the neighbouring melt trajectories and the preceding layer [8]. The key parameters of SLM are presented in detail by Spears. In his work, the author divides these parameters into four categories: (1) laser and scanning param-eters (table 2.2) , (2) powder material properties (table 2.3) , (3) powder bed properties and recoat parameters (table 2.4) , and (4) build environment parameters (table 2.5). Of the 50 parameters listed, only twelve are directly modifiable during the process. [1].

Table 2.2: SLM process parameters related to Laser and Scanning [1]

Parameter Description Controlled or

predefined Laser and Scanning parameters

1. Average power Measure of total energy output of a laser Controlled

2. Mode Continuous or pulsed wave Predefined

3. Peak power Maximum power in a laser pulse Predefined 4. Pulse width Length of a laser pulse when operating in pulsed

mode

Predefined

5. Frequency Pulses per unit time Predefined

6. Wavelength Distance between crests in laser electromagnetic waves

Predefined

7. Polarization Orientation of electromagnetic waves in laser beam

Predefined

8. Beam quality

Related to intensity profile and

used to predict how well beam can be focused and determine minimum theoretical spot size (equal to 1 for a Gaussian)

Predefined

9. Intensity profile Determines energy input at a specific locations

Predefined

10. Spot size Length and width of elliptical spot (equal for circular spots)

Controlled

11. Scan velocity Velocity at which laser moves across build plate

Controlled

12. Scan spacing Distance between neighboring laser paths Controlled 13.Scan strategy Pattern in which the laser is scanned across the

powder bed (hatches, zig-zags, spirals, etc.)

Controlled

Laser, depending on its mode, may be categorized as of continuous wave (CW) or pulsed wave (PW). PW emission results in an intermittent release of energy during the process [43].

the resulting piece. Other important characteristics of the laser that may be enlisted are its wavelength, polarization and intensity profile.

Table 2.3: SLM process parameters related to Powder Material [1]

Parameter Description Controlled or

predefined Powder material properties

14. Theoretical density Material density, limits maximum density of final component

Predefined 15. Thermal conductivity Measure of material’s ability to conduct heat Predefined 16. Heat capacity Measure of energy required to raise the

temperature of the material

Predefined

17. Latent heat of fusion Energy required for solid-liquid and liquid-solid phase change

Predefined 18 Melting temperature Temperature at which material melts Predefined

19. Boiling temperature

Temperature at which material vaporizes;

may only be important in certain process conditions

Predefined

20. Melt pool viscosity Measure of resistance of melt to flow Predefined 21. Coefficient of thermal

expansion

Measure of volume change of material on heating or cooling

Predefined

22. Surface free energy Free energy required to form new unit area of solid-liquid interfacial surface

Predefined

Table 2.3 continued from previous page

Parameter Description Controlled or

predefined 23. Vapor pressure Measure of the tendency of material to

vaporize

Predefined 24. Heat (enthalpy) of

reaction

Energy associated with a chemical reaction of the material

Predefined

25. Material absorptivity

Measure of laser energy absorbed

by the material, as opposed to that which is transmitted or reflected

Predefined

26. Diffusivity Important for solid state sintering, not as critical for melting

Predefined 27. Solubility Solubility of solid material in liquid melt Predefined 28. Particle morphology Measures of shape of individual particles

and their distributions

Predefined 29. Surface roughness Arithmetic mean of the surface profile Predefined 30. Particle size

distribution

Distribution of particle sizes Predefined

31. Pollution

Ill-defined factor describing change in properties of powder due to reuse

as dust and other particles added to powder

Predefined

The interaction between the laser beam and powder is closely related to its thermo-dynamic properties. Theoretical density will be the target and limit of densification for a particular material. Thermal conductivity is an important factor as heat conduction occurs as previously discussed. Heat capacity is a quantification of the energy required to raise the temperature of the material (with melting temperature as target) and thus is closely related to the sufficient energy input which achieves melting. Powder starts from solid state, transitions to liquid when melt and solidifies back. Latent heat of fusion is a measurement of the energy required for such phase changes to occur. Boiling temperature along with vapor pressure are

as it influences the required energy input. Particle size distribution and morphology are con-sidered as relevant powder characteristics. Pollution of the powder occurs when reused and/or mistreated.

Table 2.4: SLM process parameters related to Powder Bed properties and Recoat parameters [1]

Parameter Description Controlled or

predefined Powder bed properties and recoat parameters

32. Density Measure of packing density of powder particles, influence heat balance

Predefined 33. Thermal

conductivity

Measure of powder bed’s ability to conduct heat

Predefined

34. Heat capacity Measure of energy required to raise the temperature of the powder bed

Predefined

35. Absorptivity Measure of laser energy absorbed,

dependent on Ab and state of powder bed

Predefined 36. Emissivity Ratio of energy radiated to that of black body Predefined 37. Deposition

system

Recoater velocity, pressure, recoater type, dosing parameters

Controlled

38. Layer thickness Height of a single powder layer, limiting resolution and impacting process speed

Controlled 39. Powder bed

temperature Temperature of the powder bed Controlled

Powder bed characteristics are related to, but different, to those of the powder material.

This difference depends upon the packing density (which is a function of particle size and shape distribution) of particles in the powder bed. Free space may significantly alter thermo-dynamic properties, such as thermal conductivity, heat capacity, absorptivity and emissivity, of the material. Moreover, layer thickness has been deemed to have a considerable influence on densification.In the same way, deposition system parameters, such as recoater velocity, pressure, recoating type and dosing parameters, are characteristics to take into account when producing an SLMed part.

Table 2.5: SLM process parameters related to Build Environment parameters [1]

Parameter Description Controlled or

predefined Build environment parameters

40. Shield Gas Usually Ar or N2, but may also be He Predefined

41. Oxygen level

Probably most important environmental parameter; oxygen can lead to oxide formation in metal, change wettability, and energy required for welding

Controlled

42. Shield gas molecular weight

Influences heat balance, diffusivity into/out of part Predefined 43. Shield gas

viscosity

May influence free surface activity of melt pool, convective heat balance

Predefined 44. Thermal

conductivity

Term in heat balance Predefined

45. Heat capacity of gas

Term in heat balance Predefined

46. Pressure Influence vaporization of metal as well as oxygen content

Controlled 47. Gas flow

velocity

Influences convective cooling, removal of condensate

Controlled

50. Surface free energy

Between liquid and surround gas influence melt pool shape

Predefined

The fourth category is related to build environment parameters. Shield gas, usually Ar,N2 or He, is used to prevent oxidation (minimize oxygen level) and other chemical reac-tions in the powder bed. Some of its important characteristics are its molecular weigh, viscos-ity, thermal conductivviscos-ity, heat capacity and convective heat transfer coefficient. Convection is deeply influenced by the gas flow velocity. Ambient pressure influences vaporization of metal as well as oxygen content.

To these 50 parameters, Galy remarks the importance of considering the following pa-rameters as well [9];

• Flowability of powder: poor flowability may lead to poor powder dispersion when dis-tributed

• Hydrometry of powder: high humidity may also lead to poor dispersion

In document State of the art (página 30-36)

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