Material

Material properties for a specific application are constrained by the application

requirements. Once a material is selected for processing by SLM, material characterization is

performed to check its physical properties and chemical composition. Physical properties like

density, thermal capacity (specific heat capacity), latent heat of fusion and melting

temperature determine the melt enthalpy of the metal. The melt enthalpy is the amount of

energy needed to melt the powder completely and it determines the heat balance (Van Elsen,

2007). The thermal conductivity is the main driving factor in the heat balance and the thermal

expansion coefficient determines the stresses introduced during re-solidification of the molten

metal. These material properties are determined after the material selection phase. The

chemical composition of the metal defines its alloying behaviour in a multi component

Absorption of incident radiation by a powder bed is quite different from bulk metal (Tolochko et al, 1997). Multiple interactions of the incident radiations in the small pores present in the powder bed increase its laser absorption. Powder absorption depends on compaction, (nature of) material (intrinsic material absorption characteristic i.e. copper is more reflective than stainless steel) and wavelength of incident radiations. This is explained in more detail in Chapter 5.

A majority of the SFF technologies use metal in powder form. Some of the properties associated with the powder material are detailed below:

Particle Size Distribution (PSD)

Particle Size Distribution (PSD) shows the frequency of a particle size in a sample of powder material. PSD affects the important powder characteristics of flowability and compaction (German, 1989). A wider PSD is known to increase the powder bed density of the material, as wider PSD ensures that there is enough smaller particles to fill the gaps between larger particles in the powder bed. This high compaction of the powder bed in turn increases the sintered density of the material (Zhu et al, 2007). The ratio of larger to smaller particles in the powder can influence the flowability of the particles in layer based SFF processes (Other factors such as humidity and shape of the particles can also influence flowability). If the powder contains a large number of smaller particles, the chances of agglomeration are increased. This is due to the Van der Wall forces being more pronounced in smaller particles (small particles have high surface area to volume ratio) thus making the deposition process more difficult (Simchi, 2004) (Boivie, 2001). Packing density and flowability of the powder is further discussed in detail in Chapter 6. On the other hand, a very narrow PSD can improve the consistency of the melt but reduces the packing density of the material (Lang et al, 2000).

Apart from this, PSD can also influence the part quality. A greater number of smaller particles

reduces the energy required to melt them and also improves the part surface roughness

(Karapatis et al, 1998) (Sears, 1999) (Syvanen et al, 2000) (Lu et al, 2001). If the powder

consists of smaller particles, thinner layers can be deposited (depends on the flowability of the

powder). According to Mazumder et al (2000), with thicker layers, the laser beam has a larger

distance to diverge and forms a melt pool which is larger at the bottom as compared to the top.

By reducing the particle size, the layer thickness can be reduced, thus reducing the side surface roughness of the part.

Based on the above discussion, selection of a specific PSD powder is very important to the process and final quality of the parts. To some extent there is a compromise between the percentage of smaller and larger particles. If a lot of smaller particles are selected, the surface roughness is reduced and so is the layer thickness thus improving the part quality, but there is more chance of agglomeration and problems in the deposition process. If the powder consists of a very high percentage of larger particles, the flowability is improved but the powder bed density is reduced which could reduce the final density of the part. Therefore, a powder should consist of adequate amount of smaller particles to fill the gaps and increase the powder bed density but not influence the powder flowability and deposition (Zhu et al, 2007).

Particle morphology

Powder morphology is the shape of the particles in the powder. The powder morphology is directly influenced by the preparation method i.e. milled, water atomized or gas atomized. The gas atomization technique is found to produce more spherical particles as compared to the other two. Spherical particles are known to improve powder flowability and the quality of the layer and final product in SFF technologies (Niu and Chang, 1999). A non-spherical powder has been observed to have less compaction and hence increased porosity in the parts.

Density

Densities of powder are of two types i.e. individual particle density and the packing density (apparent and tap densities) of the powder as a whole. The individual particle density depends upon the type of material and is an intrinsic material property, while the packing density can be a loosely packed density or tapped (compacted) density of the powder depending on the compaction method.

Thermal conductivity

The thermal conductivity is the material’s ability to conduct heat across individual

particle(s) and changes with the temperature of the particles. The thermal conductivity of the

the particles, which in turn depends upon the compaction of the powder. The higher the compaction the more will be the contact points and thus higher heat transfer will take place across the powder layer.

Specific heat capacity

Specific heat capacity or specific heat is the amount of heat energy required to raise a unit quantity of material by one degree in temperature. The specific heat capacity of the material affects the heat balance (see section 2.3.3.3).

Latent heat of fusion

The latent heat of fusion is defined as the amount of energy required to change the state of unit mass of material from solid to liquid without rise in temperature. It is also used in heat balance (see section 2.3.3.3)

Melting point

The melting point of a material is the temperature at which the material in solid state will change into liquid phase.

Evaporation point

Similar to melting point, the evaporation point is the temperature at which the material will change its phase from liquid to gas.

Viscosity

Viscosity is resistance of the material to flow in the molten state. The viscosity of the material in molten state changes with temperature and influences the part quality (discussed in detail in section 2.3.3.3)

Surface free energy

The surface free energy quantifies the disruption of chemical bonds when a surface is

transformed/ changed (Van Elsen, 2007) the surface free energies have effect on the wetting

and balling behaviour of the molten material (Kruth et al, 2003). Both of these phenomena are

explained in detail in section 2.3.3.3.

Absorption (percentage)

The absorption is the percentage of the incident radiation which is absorbed by a

material. It is also known as laser/energy coupling (further discussed in section 2.2.3.1)