Today, a large number of different steels are being processed by Additive Manufacturing (AM) methods. The different matrix microstructure components and phases (austenite, ferrite, martensite) and the various precipitation phases (intermetallic precipitates, carbides) lend a huge variability in microstructure and properties to this class of alloys. This is true for AM-produced steels just as it is for conventionally-produced steels. However, steels are subjected during AM processing to time-temperature profiles which are very different from the ones encountered in conventional process routes, and hence the resulting microstructures differ strongly as well. This includes a very fine and highly morphologically and crystallographically textured microstructure as a result of high solidification rates as well as non-equilibrium phases in the as-processed state. Such a microstructure, in turn, necessitates additional or adapted post-AM heat treatments and alloy design adjustments. In this review, we give an overview over the different kinds of steels in use in fusion-based AM processes and present their microstructures, their mechanical and corrosion properties, their heat treatments and their intended applications. This includes austenitic, duplex, martensitic and precipitation-hardening stainless steels, TRIP/TWIP steels, maraging and carbon-bearing tool steels and ODS steels. We identify areas with missing information in the literature and assess which properties of AM steels exceed those of conventionally-produced ones, or, conversely, which properties fall behind. We close our review with a short summary of iron-base alloys with functional properties and their application perspectives in Additive Manufacturing.

Steels in additive manufacturing: A review of their microstructure and properties

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. Here, 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.

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

Causes of failure and repairing options for dies and molds: A review

Microstructure, microhardness and corrosion resistance of remelted TiG2 and Ti6Al4V by a high power diode laser

The authors uses laser additive manufacturing (LAM) as a combinatorial method for synthesizing microstructurally and compositionally piecewise graded bulk alloys. Authors fabricate blocks consisting of a sequence of ≈500 μm thick tool steel layers, each with different chemical composition, by laser metal deposition where alloy powders are deposited layer-wise on a substrate. The reference materials are a Cr–Mo–V hot working tool steel and a Ni-based maraging steel. The layers between them consist of corresponding blends of the two materials with varying composition from layer to layer (alloy volume fractions 80:20, 60:40, 40:60, and 20:80). The bulk alloy is hot rolled and heat treated. Subsequently each layer is characterized for microstructure, chemical composition and mechanical properties using electron back scatter diffraction, tensile testing, and indentation. The approach is an efficient high-throughput method enabling rapid probing of novel compositional alloy blends. It can be applied for finding new alloys both, by LAM and for LAM. For the tool steel blends synthesized here, authors observe that the Cr–Mo–V tool steel, when mixed with the Ni-base maraging steel, can be continuously tuned for a strength-ductility profile in the range of 800–1650 MPa strength and 15–25% tensile elongation.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/srin.201600416

Combinatorial Alloy Design by Laser Additive Manufacturing

A high power solid Nd-YAG laser, operating in continuous mode, was used to re-melt the surface of a maraging steel, 14 Ni (200 grade), in different heat treatment conditions. Processing parameters were optimized for the employment of this technique as a repairing procedure of damaged surfaces to increase the useful life of tools made in maraging steels. Different zones can be found in laser-heated material. In addition, the absence of austenite reversion, consistent with the low nickel content of this steel, was found with the exception of a few very small pools of austenite found in a narrow layer in the heat-affected zone of samples corresponding to one of the studied heat treatment conditions of steel. Laser surface melting (LSM) combined with aging heat treatment causes a significant rise in the hardness (from 325 to more than 520 HV0.1) and in the wear resistance of the surface layer (with a decrease in the volume of lost material from 0.52 mm3 in the steel in the laser surface melted condition to 0.28 mm3 in the steel in laser surface melted and finally peak age hardening condition).

Laser surface melting: A suitable technique to repair damaged surfaces made in 14 Ni (200 grade) maraging steel

Summary


We describe the microstructure of Nd:YAG continuous wave laser surface melted high-speed steel, namely AISI M2, treated with different laser scanning speeds and beam diameters on its surface. Microstructural characterization of the remelted surface layer was performed using light optical and scanning electron microscopy and X-ray diffraction. The combination of the three techniques provided new insights into the substantial changes induced by laser surface melting of the steel surface layer. The advantage of the method is that it avoids the difficult and tedious work of preparing samples of this hard material for transmission electron microscopy, which is the technique normally used to study these fine microstructures. A melted zone with a dendritic structure and a partially melted zone with a heterogeneous cellular structure were observed. M2C carbides with different morphologies were identified in the resolidified surface layer after laser melting.

Microstructural characterization of laser surface melted AISI M2 tool steel

The tools and dies surface may suffer from severe damage in service due to sharp working conditions, such as extreme thermomechanical loading and abrasive environments. The steel and high strength alloys used for these tools are usually very expensive; hence, those in charge of surface repairing should welcome laser processes in order to increase the service life of the tool and shorten the time required to amortize the investment made. In this paper, a high power laser has been used to modify the microstructure as well as the surface properties of low nickel (14 wt.%) maraging steel by means of the laser melt injection process. The power of TiN was injected into the laser-melted steel. The microstructure, hardness and wear resistance of the metal matrix composite ‘new surface’, followed by peak-age hardening, have been studied. Laser parameters were optimized to obtain a defect-free material. Laser surface processing impairs the surface properties of the maraging steel as hardness decreases in the heated regions; thus, a final age hardening treatment is necessary to obtain the required high strength typical of maraging steels. Copyright © 2014 John Wiley & Sons, Ltd.

A study of laser melt injection of TiN particles to repair maraging tool steels

We applied a laser-surface melting treatment to AISI M2 high-speed steel – hardened and tempered – and studied the resulting surface characteristics (microstructure) and mechanical behaviour (hardness and wear performance). The steel was treated using a Nd:YAG continuous-wave laser with different operation conditions. The influence of the laser processing parameters on the individual beads and on the characteristics of the molten surface layer obtained using the multipass system with 50% overlap was studied. The microstructure characterization for all conditions is formed of MC- and M2C-type carbides, martensite, and retained austenite; the quantities of this phase depend on the operating conditions. It has been determined that low levels of power density and high scanning speeds of the beam lead to greater homogeneity in the microstructure with high hardness values and wear resistance.

Modification of AISI M2 high-speed tool steels after laser-surface melting under different operating conditions

We applied a laser surface melting treatment to AISI M2 high-speed steel and studied the resulting surface characteristics (microstructure) and mechanical behavior (hardness and wear performance). The steel was treated using an Nd:YAG continuous-wave laser with different scanning speeds and power densities. The influence of the laser processing parameters on the melted surface layer was studied. The microstructure was characterized using optical microscopy, scanning electron microscopy and X-ray diffraction. We also assessed the effects of overlapping on the microstructure and the mechanical properties of the wear-resistant surface. Copyright © 2010 John Wiley & Sons, Ltd.

G. Castro

Papers

Laser surface melting: A suitable technique to repair damaged surfaces made in 14 Ni (200 grade) maraging steel

Microstructural characterization of laser surface melted AISI M2 tool steel

A study of laser melt injection of TiN particles to repair maraging tool steels

Modification of AISI M2 high-speed tool steels after laser-surface melting under different operating conditions

Microstructure and mechanical behavior of laser surface melted AISI M2 high‐speed steel