SLM: general process features and most common defects

The research on Selective Laser Melting (SLM) is mainly focused on the production of stainless steels, titanium, aluminum and nickel alloys. The technique find application in all the industrial fields in which the production of specially designed high-tech components is required; some examples are dental and medical applications, heat exchangers, light structures for the robotic and aeronautical field, components for aircraft engines as rotors and gas turbine blades [137].

The SLM technique allows to produce a tridimensional object through the deposition and consolidation of consecutive powder layers (see the scheme in figure 1.37). The first powder layer is spread out on a metallic support plate, then a high energy laser beam scans the powder bed and causes the selective fusion of some areas of it. At each subsequent step, a thin layer of powders is spread out on the previous deposited powders and then is scanned along the current section profile by the laser beam. The laser beam irradiation causes both the local melting of the powder and the partial remelt of the adjacent material and the previously deposited layer; then the consolidation of the added material and the formation of strongly bond with the previous layer occur through the solidification of the molten pool when the laser beam moves out [138]. The deposition and laser scan steps are repeated until the object, which at the end of the manufacturing process is buried in the loose powders, is complete. At this point, it is necessary to remove the unused powder and detach the piece from the support plate, this is usually done with electrical discharge machining (EDM).

Figure 1.37. Scheme of the SLM process.

The general production procedure described above determines the typical laser related features in the parts produced through SLM. These characteristic features are the “laser tracks”, visible on the plane of the deposited layers and that keep track of the path followed by the laser beam during the scan of each layer, and the so-called “melt pools”, visible on the planes perpendicular to the deposition plane and that are due to the formation of the molten pools and the penetration of these into the underlying layers. The boundaries between laser tracks and melt pools become clearly visible at the optical microscope after chemical etching, some examples of them are shown in figure 1.38. These boundaries, sometimes referred as track-track and layer-layer molten pool boundaries (MPB), can affect the plastic deformation and the fracture mode of the sample and, therefore, represent a source of anisotropy in the mechanical behavior [139].

Figure 1.38. Optical micrographs showing the laser tracks on the horizontal plane and the arc-shape melt pool boundaries on the vertical plane of a SLM Inconel 718 alloy. From: [140].

Actually, the products obtained by SLM technique are inevitably characterized by a certain anisotropy deriving from the asymmetric process conditions between the Building Direction (BD), along which the forming piece grows through the

progressive addition of the layers (BD is indicated in figures 1.37 and 1.38) and the directions laying on the deposition plane, i.e. orthogonal to BD. The production of a metallic component through SLM technique is a complex process characterized by a large number of parameters whose optimization determines the final quality of the product. The main SLM process parameters are listed below:

• laser radiation parameters:

o radiation wavelength o laser power

o wave form (continuous or pulsed)

• laser scan parameters:

o scan speed of the beam

o hatching distance between consecutive scanning lines o scanning strategy:

▪ scanning path (unidirectional, bidirectional, spiral, etc.)

▪ scanning scheme (parallel stripes, square areas, etc.)

▪ scanning orientation between layers (rotation of an angle of 0°, 45°, 67°, 90° or 180°)

▪ eventual scanning repetition

• system parameters:

o powder layer thickness

o chamber atmosphere (nitrogen or argon inert gas to avoid oxidation) o eventual pre-heating of the support plate

• material parameters:

o physical properties: elastic modulus and density

o thermal properties: heat capacity, latent heat of fusion, thermal conductivity, Coefficient of Thermal Expansion (CTE)

o mechanical and thermomechanical properties: Ultimate Tensile Strength (UTS), Yield Strength (YS), fracture toughness, thermal shock resistance

• powder parameters:

o absorbance to the laser radiation o particle shape

o size distribution o flowability

The amount of energy absorbed by the powders depends firstly on their absorbance, which can also be much greater with respect to the bulk material for the same laser wavelength due to multiple reflections and absorptions of the radiation on the surface of the powders [132]. Furthermore, the absorbed energy is also related to the imposed Volumetric Energy Density (VED) given by equation 1.10:

𝑉𝐸𝐷 = ^𝑃

𝑣ℎ𝑑𝑑 eq. 1.10

where 𝑃 is the laser power, 𝑣 the scan speed, ℎ_𝑑 the hatching distance and 𝑑 the powder layer thickness. A schematic representation of these parameters is shown in figure 1.39.

Figure 1.39. Scheme showing the main SLM process parameters which determine the Volumetric Energy Density (VED), i.e. laser power 𝑃, scan speed 𝑣, hatching distance ℎ𝑑 and layer thickness

𝑑. From: [137].

These process parameters have to be optimized with the aim to obtain a suitable amount of energy absorbed by the powder bed [141]. Actually, a too low absorbed energy determines the formation of defects and porosity due to lack of fusion, which can be due to an insufficient penetration of the molten pool into the substrate or to an excessive hatching distance that doesn’t allow the overlap between the subsequent laser tracks. Furthermore, a low absorbed energy favors the balling phenomenon [133] [142] [143], which is due to a poor wettability of the liquid formed by melting of the powders on the platform or on the previously deposited material surface. In these conditions, the liquid surface tension causes the formation of spherical drops resulting in a poor consolidation of the added material and formation of voids.

Figure 1.40. SEM micrograph showing some examples of gas porosities and lack of fusion porosities on a SLM Inconel 718 alloy. From: [144].

On the other hand, also an excessive absorbed energy can lead to the formation of defects as melt pools with keyhole shape [143], caused by too deep penetration, porosities due to the trapping of bubbles formed due to vaporizing of the too overheated material (see figures 1.40 and 1.41) and formation of cracks in the solidifying molten pool [145]. Trapped gas can also derive from not well packed powders or from porosities already present in the initial powder feedstock [146].

Figure 1.41. Examples of typical defects of the SLM products: keyhole geometry and porosity (left) and balling (right). From: [143].

Further defects that can be found in the components produced by SLM are cracks and delamination between layers due to thermal stresses developed in the material and caused by the high temperature gradients and solidification shrinkages which occur during all the laser-based processes, in particular SLM [147]. Internal stresses and cracks can also be due to phase transformations that occurs during the heating and cooling cycles at which the material is submitted during the SLM process [148]. The thermal stresses can be reduced through the pre-heating of the support plate, which allows to reduce the thermal gradients arising during the process, or by performing a second scan at lower laser power and higher scan speed after the deposition of each layer in order to induce stress release [147] [149].

The choice of the scan strategy has also an important role in the formation of residual stresses and the anisotropy of the produced sample [150]. The correct choice of the scan strategy allows to balance the energy input applied on the powder layer and to accommodate the introduced thermal stresses. It also allows to avoid to accumulate defects systematically in the same zones, typically at the beginning and at the end of the scan lines [146]. Furthermore, the scan strategy affects the grain structure, the material texture and the crack formation because it determines the direction and the intensity of the heat fluxes arising during the process [151].

Therefore, it could be even convenient to vary its effect depending on the size and the geometry of the produced part [152].

During the SLM process, the extremely fast solidification of the molten pools generates the formation of non-equilibrium microstructures characterized by the presence of second metastable phases. The cooling rates and thermal gradients have also an important effect on the size and the directionality of the grains and, therefore, they strongly affect the mechanical properties and the anisotropy level of the as-built material. It is very hard to correctly predict the microstructure arising

from the SLM process because of the complex solidification conditions, but also for the remelting, which occurs in the overlap region between laser scans and between consecutive layers, and the heterogeneous thermal histories that each portion of the material undergoes. Actually, whenever a new layer is added to the stack, the heat generated during the laser scan is partially transferred to the substrate by conduction, therefore the previously deposited material is submitted to a sequence of thermal cycles of decreasing intensity [148] which can cause microstructural alterations [153].