3D Printing of Metals: Laser Additive Manufacturing (LAM) via Selective Laser Melting (SLM)  and Laser Metal Deposition (LMD)

Additive Manufacturing (AM) is a revolutinary metallurgical production method capable of producing highly complex parts directly from a computer file and raw material.

Its high potential lies in its ability to manufacture customised products with individualisation, complexity and weight reduction for free. 

For metallurgical 3D priniting the Laser-assisted Additive Manufacturing (LAM) methods prevail.

 

 

Two main methods are used in the field of Laser Additive Manufacturing:

 

Laser Additive Manufacturing (LAM):

Selective Laser Melting (SLM) & Laser Metal Deposition

 

Laser Additive Manufacturing is a technique that enables producing complex parts and products directly on the basis of a digital computer model file and raw material powders. Its extremely high potential for manufacturing and metallurgy lies in its unique ability to manufacture customised products with individualisation, complexity and weight reduction for free. Up to now, this potential cannot be fully realised because no materials tailor-made for these unique processes exist. Our interest lies in merging expertise on the process design side with advanced materials design and characterization to develop novel tailor-made materials suitable for and exploiting the unique characteristics of Laser Additive Manufacturing, together with their respective optimized manufacturing technology. As a result, new materials will be available for the field of Laser Additive Manufacturing that can be processed without costly post treatments and that exceed the properties of their conventional counterparts. At the same time, Laser Additive Manufacturing enables a new and accelerated approach to rapid alloy design.

Principle of Selective Laser Melting (SLM)
Principle of Selective Laser Melting (SLM)

 

Selective Laser Melting (SLM) uses a laser to melt powdered metal in chamber of inert gas or vacuum. 
When a layer is finished, the powder bed is moved downward, and an automated roller adds a new metal poweder layer which is melted to form the next cross section of the model. 
SLM is a metal additive manufacturing technique similar to Selective Laser Sintering (SLS)
A main difference between them being that SLS sinters the material, i.e heating to below the melting point until the particles merge with one another, while SLM melts the material, creating a melt pool in which material is consolidated before cooling to form a solid structure.
Also, in SLM each new layer synthesis leads to a heat treatment of the already solidified layer beneath it.

 

 

 

Laser additive manufacturing production of oxide- and nitride-dispersion-strengthened materials through atmospheric reactions in liquid metal deposition

Despite being extremely attractive compounds for strengthening, oxides and nitride particles have found only limited use in metallic materials design, as obtaining appropriate size and dispersion up to nownecessitates production
by time- and cost-intensive powder metallurgy processes. Here we present an alternative production method, based on the oxide and nitride formation during liquid-metal-deposition procedures in oxygen and/or nitrogen containing atmospheres. Rapid solidification of the small liquid zone suppresses floatation and agglomeration of particles, while subsequent thermo-mechanical treatments densify the material and aids particle dispersion. The in-situ particle formation coupled to the high deposition rates ensures a drastically shortened production chain. The feasibility of the method is exemplarily demonstrated on austenitic stainless steel and commercially available deposition techniques as used in additive manufacturing, performed without shielding gas but instead at air. Evenwithout substantial optimisation of processes and material, N2 vol.% of hard and stable Cr2N particles with sizes down to 80 nm could be evenly dispersed, resulting in pronounced strengthening at both roomtemperature and 700°C without significant loss in ductility.

 

Efficient additive manufacturing production of oxide- and nitride-dispersion-strengthened materials through atmospheric reactions in liquid metal deposition
Materials and Design 111 (2016) 60-69
Materials and Design 111 (2016) 60 Addit[...]
PDF-Dokument [4.0 MB]
Laser Additive Manufacturing of dispersion strengthened alloys Laser Additive Manufacturing of dispersion strengthened alloys

 

 

 

Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting

 

Materials produced by selective laser melting (SLM) experience a thermal history that is markedly different from that encountered by conventionally produced materials. In particular, a very high cooling rate from the melt is combined with cyclical reheating upon deposition of subsequent layers.
Using atom-probe tomography (APT), we investigated how this  conventional thermal history influences the phase-transformation behavior of maraging steels (Fe–18Ni–9Co–3.4Mo–1.2Ti) produced by SLM. We found that despite the “intrinsic heat treatment” and the known propensity of maraging steels for rapid clustering and precipitation, the material does not show any sign of phase
transformation in the as-produced state. Upon aging, three different types of precipitates, namely (Fe,Ni,Co)3(Ti,Mo), (Fe,Ni,Co)3(Mo,Ti), and (Fe,Ni,Co)7Mo6 (l phase), were observed as well as martensite-to-austenite reversion around regions of the retained austenite. The concentration of the
newly formed phases as quantified by APT closely matches thermodynamic equilibrium calculations.

 

Maraging Steel by Laser Additive Manufacturing Maraging Steel synthesized by Laser Additive Manufacturing and probed by Atom Probe Tomography
Maraging Steel by Laser Additive Manufacturing Atom probe tomography Atom probe tomography: Nanostructure of Maraging Steel produced by Laser Additive Manufacturing, here: LMD (J. Mater. Res., Vol. 29, No. 17, Sep 14, 2014)
Eric A. Jägle, Pyuck-Pa Choi, Jan Van Humbeeck and Dierk Raabe (2014). Precipitation and austenite reversion behavior of a maraging steel produced by selective laser melting. Journal of Materials Research, 29, pp 2072-2079. doi:10.1557/jmr.2014.204.
Jägle et al. (2014) 29 17 Laser additive[...]
PDF-Dokument [489.4 KB]

Comparison of Maraging Steel Micro- and Nanostructure Produced Conventionally and by Laser Additive Manufacturing

Laser Additive Maufacturing Maraging Steel Laser Additive Maufacturing Maraging Steel

Maraging steels are used to produce tools by Additive Manufacturing (AM) methods such as Laser Metal Deposition (LMD) and Selective Laser Melting (SLM). Although it is well established that dense parts can be produced by AM, the influence of the AM process on the microstructure - in particular the content of retained and reversed austenite as well as the nanostructure, especially the precipitate density and chemistry, are not yet explored. In this specific project, we study these features using microhardness measurements, Optical Microscopy, Electron Backscatter Diffraction (EBSD), Energy Dispersive Spectroscopy (EDS), and Atom Probe Tomography (APT) in the as-produced state and during ageing heat treatment. We find that due to microsegregation, retained austenite exists in the as-LMD- and as-SLM-produced states but not in the conventionally-produced material. The hardness in the as-LMD-produced state is higher than in the conventionally and SLM-produced materials, however, not in the uppermost layers. By APT, it is confirmed that this is due to early stages of precipitation induced by the cyclic re-heating upon further deposition—i.e., the intrinsic
heat treatment associated with LMD. In the peak-aged state, which is reached after a similar time in all materials, the hardness of SLM- and LMD-produced material is slightly lower than in conventionally-produced material due to the presence of retained austenite and reversed austenite formed during ageing.

 

 

Acta Mat. 2011, 59, p. 364