The Influence of Layer Thickness on the Microstructure and Mechanical Properties of M300 Maraging Steel Additively Manufactured by LENS® Technology

This work investigates the effect of layer thickness on the microstructure and mechanical properties of M300 maraging steel produced by Laser Engineered Net Shaping (LENS®) technique. The microstructure was characterized using light microscopy (LM) and scanning electron microscopy (SEM). The mechanical properties were characterized by tensile tests and microhardness measurements. The porosity and mechanical properties were found to be highly dependent on the layer thickness. Increasing the layer thickness increased the porosity of the manufactured parts while degrading their mechanical properties. Moreover, etched samples revealed a fine cellular dendritic microstructure; decreasing the layer thickness caused the microstructure to become fine-grained. Tests showed that for samples manufactured with the chosen laser power, a layer thickness of more than 0.75 mm is too high to maintain the structural integrity of the deposited material.

Improvement of corrosion resistance of maraging steel manufactured by selective laser melting through Intercritical heat-treatment

Existing studies suggest that martensite-to-austenite reversion can increase the overall mechanical strength of maraging steel. Their effect on corrosion properties, however, is unclear. Selective laser melted (SLM) specimens were tempered near austenite finish temperatures to investigate the electrochemical effect of reversed austenite. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS) were used to characterize their microstructure. To define and test pitting performance, potentiodynamic polarization and open-circuit potential were performed in a 3.5 wt. % NaCl solution. The reversed austenite precipitated mainly along the martensite lath boundaries during the Intercritical heat treatment at 720°C. The nucleation of reversed austenite is allowed by the local Ni enrichment caused by the dissolution of intermetallic particles. As a result, the tempered 720°C specimens reported a higher pitting potential, lowest corrosion current density, and lowest corrosion rate than the as-printed, aged, and homogenized specimens. No investigations have been performed to date that demonstrate the impact of austenite reversion on the corrosion susceptibility of SLM maraging steel. Other than being nobler, austenite is lighter than martensite due to reduced precipitant density, accounting for fewer galvanic cells and a lower corrosion rate.

On the Chemical Heterogeneity of Austenite in Maraging Steel

The change of Ni-, Cr-, Cu- contents in maraging steel composition occurring on heating in the subcritical and intercritical interval has been studied by the X-ray spectral microanalysis. Heating in the temperature range from 490 to 550C has resulted in increasing of Ni- Cu- concentrations in the 1iquation austenite when the latter is present in the steel structure as a consequence of several reasons (the large ingot, low level of forging reduction ratio, etc.). The significant enrichment of surface layers of austenite inclusions may probably occur if there are great differences between interphase and intraphase diffusion rates. By varying the thermal treatment and thus the Ni-diffusion in austenite it is possible to create austenite layers with different Ni-contents within a grain or massive martensite and it is also possible to control the material properties.

Improving Fatigue Limit and Rendering Defects Harmless through Laser Peening in Additive-Manufactured Maraging Steel

Additive-manufactured metals have a low fatigue limit due to the defects formed during the manufacturing process. Surface defects, in particular, considerably degrade the fatigue limit. In order to expand the application range of additive-manufactured metals, it is necessary to improve the fatigue limit and render the surface defects harmless. This study aims to investigate the effect of laser peening (LP) on the fatigue strength of additive-manufactured maraging steel with crack-like surface defects. Semicircular surface slits with depths of 0.2 and 0.6 mm are introduced on the specimen surface, and plane bending-fatigue tests are performed. On LP application, compressive residual stress is introduced from the specimen surface to a depth of 0.7 mm and the fatigue limit increases by 114%. In a specimen with a 0.2 mm deep slit, LP results in a high-fatigue-limit equivalent to that of a smooth specimen. Therefore, a semicircular slit with a depth of 0.2 mm can be rendered harmless by LP in terms of the fatigue limit. The defect size of a 0.2 mm deep semicircular slit is greater than that of the largest defect induced by additive manufacturing (AM). Thus, the LP process can contribute to improving the reliability of additive-manufactured metals. Compressive residual stress is the dominant factor in improving fatigue strength and rendering surface defects harmless. Moreover, the trend of the defect size that can be rendered harmless, estimated based on fracture mechanics, is consistent with the experimental results.

High-Resolution Microstructure Characterization of Additively Manufactured X5CrNiCuNb17-4 Maraging Steel during Ex and In Situ Thermal Treatment

Powder and selective laser melting (SLM) additively manufactured parts of X5CrNiCuNb17-4 maraging steel were systematically investigated by electron microscopy to understand the relationship between the properties of the powder grains and the microstructure of the printed parts. We prove that satellites, irregularities and superficial oxidation of powder particles can be transformed into an advantage through the formation of nanoscale (AlMnSiTiCr) oxides in the matrix during the printing process. The nano-oxides showed extensive stability in terms of size, spherical morphology, chemical composition and crystallographic disorder upon in situ heating in the scanning transmission electron microscope up to 950 °C. Their presence thus indicates a potential for oxide-dispersive strengthening of this steel, which may be beneficial for creep resistance at elevated temperatures. The nucleation of copper clusters and their evolution into nanoparticles, and the precipitation of Ni and Cr particles upon in situ heating, have been systematically documented as well.

Resistance spot welding of additively manufactured maraging steels Part I: Nugget formation

Application of additively manufactured steels is unavoidably involved in the resistance spot welding with conventionally manufactured steels. However, the microstructural evolution of an additive manufactured steel at high temperatures is still unknown, especially for the rapid solidification process. This paper investigated the microstructural evolution of a selective laser melted maraging steel during the rapid solidification process via resistance spot welding. Asymmetrical fusion zone with boat shape was found in the spot weld due to the rougher surface and larger electrical resistance of maraging steel via selective laser melting process. The rapid expansion of fusion zone at end of welding process was caused by the carbide formation at the heat-affected zone of maraging steel via selective laser melting process. Besides, printing orientation affected the surface roughness of a selective laser melted maraging steel and subsequently significantly influence the early stage of formation of fusion zone of additively manufactured maraging steel. We expect that our findings will pave the way to the future application of additively manufactured steels in the industries.