Microstructure in metal components built using SLM

Short laser/powder interaction times characterise selective laser melting on powder bed. These localised and short-lived high temperature variations significantly affect the microstructure at the local (melt pool) scale. In turn, the additive character of the process and unique solidification conditions generate distinctive morphological and crystallographic textures at the component scale.

Generally, materials processed by SLM exhibit a very fine, non-equilibrium structure [1][2]. What happens in the melt pool (melt flows,…), its stability and its dimension determine to a large extent the porosity [link] and the surface micro-roughness (such as orange-peel aspect for instance) of the final product (the macro-roughness is combined with the digitalisation, or staircase, effect). In addition, experimental results [3] show that the high thermal gradients occurring during SLM lead to a very fine microstructure with submicron-sized cells. 

Typical melt pool in SLMed metal component - as-built condition - ref [7]

This means the SLM products can have a high hardness even without the application of a precipitation hardening treatment (such as AlSi10Mg [1] or Ti64 [4]).

Melt pool

Generally, rapid solidification gives rise to the occurrence of segregation phenomena (enrichment of a materials constituent at a free surface or interface) and the presence of non-equilibrium phases []. Segregation, in addition to heat build up generated in the component during scanning and partial remelting of the subsequent layers, leads to the precipitation of intermetallic phases. This is what makes the melt pool boundaries visible upon etching (in as-built condition). Melt pools’ cross-sections are typically half-cylindrical in shape: due to the moving heat source, the melt pool is not circular, but elongated. Their size is dependent of the laser processing parameters (power, scan speed, layer thickness, etc) [5][6]. High-energy density of the laser gives rise to a directional heat transfer and as a result can also give rise to a directional solidification [7]. As a result, the scanning strategy is expected to have an influence on directionality of the solidification.

Schematic diagram of the crystallization solidification of molten pools during SLM process: (a) single molten pool; (b) “layer–layer” MPBs; (c) “track–track” MPBs.The arrows represent the grain orientations. - ref[7]

Surface temperature distribution at various scanning velocities. The broken-line circles on diagrams are projections of the laser beam. ref [5, 6]

Microtexture

The texture is mainly dependent on the local heat fluxes and to a lesser extent on the substrate crystallography. A closer look into the microstructure of aluminium alloy samples for instance [1] reveals the presence of a very fine cellular–dendritic solidification structures. Three different zones can be distinguished across the melt pool. Two zones show fine and coarse cellular structures respectively; they are inside the melt pool. The heat affected zone (HAZ) is defined around the melt pool in the previous layer.

Microstructure across a melt pool in aluminium alloy. ref[1]

Crystallographic texture

The directional solidification in the melt pool causes not only a morphological texture but a crystallographic texture as well. It is seen that grains are growing perpendicular to the melt pool border towards the centre of the melt pool. 

In Ti6Al4V [1] and AlSi10 [4] for instance, the growth direction of the grains in the substrate plane is perpendicular to the isotherms of the melt pool. In other words, elongated grains are observed to be parallel and aligned with the local conductive heat transfer [8]. 

Since a relatively small melt pool is created on top of a large already consolidated block of material, the heat flows away radially and grains grow towards the centre of the melt pool. This creates a morphological (dendrites, cells, etc…) as well as crystallographic (grains) texture in the parts. 

Crystal orientation maps in two perpendicular views for an AlSi10Mg SLM part. The scanning and building direction is indicated by the black arrows on the pictures and some of the melt pool borders are pointed out by the dashed lines. ref[1]

Macrostructure

The macrostructure is determined by the way in which the different individual tracks are combined. In other words, it is defined by the scanning strategy across a certain layer of the product. 

Across few layers, and due to partial remelting of the previous layers, elongated grains of several can grow hundred micrometers across successive layers [1], [4], [9]. It is interesting to note that the extent of the morphological and crystal texture could be adjusted by varying the hatch spacing and/or the layer thickness since these parameters directly determine the amount of partial remelting of the neighbouring tracks.

To sum up...

• The distinctive process of adding material track after track and layer after layer together with the fast and directional cooling rates during selective laser melting creates a unique structure. 
  
• The microstructure inside a scan track is dependent on the value and direction of the thermal gradients at the border of the moving melt pool and is repeated in a similar way for every scan track. The specific SLM process conditions, such as short interaction times, high temperature gradients and the highly localized melting process, lead very fine material microstructures, hence great hardness values.
  
• Non-equilibrium phases of commercially available pre-alloyed powders are also accessible. 
  
• Grains (crystallographic texture) can grow along the melt pool thermal gradients enabling potential control of the grain growth direction and general mechanical properties. 
  
• Considering the full component macrostructure, a morphological and crystallographic texture is present and driven by the scanning strategy.
  

References
[1]      L. Thijs, K. Kempen, J. P. Kruth, and J. Van Humbeeck, “Fine-structured aluminium products with controllable texture by SLM of pre-alloyed AlSi10Mg powder,” Acta Mater., vol. 61, pp. 1809–1819, 2013.
[2]      I. Yadroitsev, P. Krakhmalev, I. Yadroitsava, S. Johansson, and I. Smurov, “Energy input effect on morphology and microstructure of selective laser melting single track from metallic powder,” J. Mater. Process. Technol., vol. 213, no. 4, pp. 606–613, Apr. 2013.
[3]      I. Yadroitsev, P. Krakhmalev, and I. Yadroitsava, “Selective laser melting of Ti6Al4V alloy for biomedical applications : Temperature monitoring and microstructural evolution,” J. Alloys Compd., vol. 583, pp. 404–409, 2014.
[4]      L. Thijs, F. Verhaeghe, T. Craeghs, J. Van Humbeeck, and J. P. Kruth, “A study of the microstructural evolution during selective laser melting of Ti-6Al-4V,” Acta Mater., vol. 58, no. 9, pp. 3303–3312, May 2010.
[5]      a. V. Gusarov, I. Yadroitsev, P. Bertrand, and I. Smurov, “Heat transfer modelling and stability analysis of selective laser melting,” Appl. Surf. Sci., vol. 254, no. 4, pp. 975–979, Dec. 2007.
[6]      a. V. Gusarov, I. Yadroitsev, P. Bertrand, and I. Smurov, “Model of Radiation and Heat Transfer in Laser-Powder Interaction Zone at Selective Laser Melting,” J. Heat Transfer, vol. 131, no. 7, p. 072101, 2009.
[7]      W. Shifeng, L. Shuai, W. Qingsong, C. Yan, Z. Sheng, and S. Yusheng, “Journal of Materials Processing Technology Effect of molten pool boundaries on the mechanical properties of selective laser melting parts,” J. Mater. Process. Tech., vol. 214, no. 11, pp. 2660–2667, 2014.
[8]      J. Gockel, J. Fox, J. Beuth, and R. Hafley, “Integrated melt pool and microstructure control for Ti – 6Al – 4V thin wall additive manufacturing,” vol. 000, no. 000, pp. 1–5, 2014.
[9]      E. Chlebus, T. Kurzynowski, and B. Dyba, “Microstructure and mechanical behaviour of Ti ― 6Al ― 7Nb alloy produced by selective laser melting,” vol. 62, pp. 3–10, 2011.