Laser additive manufacturing is attractive for the production of complex, three-dimensional parts from metallic powder using a computer-aided design model
. The approach enables the digital control ...of the processing parameters and thus the resulting alloy's microstructure, for example, by using high cooling rates and cyclic re-heating
. We recently showed that this cyclic re-heating, the so-called intrinsic heat treatment, can trigger nickel-aluminium precipitation in an iron-nickel-aluminium alloy in situ during laser additive manufacturing
. Here we report a Fe19Ni5Ti (weight per cent) steel tailor-designed for laser additive manufacturing. This steel is hardened in situ by nickel-titanium nanoprecipitation, and martensite is also formed in situ, starting at a readily accessible temperature of 200 degrees Celsius. Local control of both the nanoprecipitation and the martensitic transformation during the fabrication leads to complex microstructure hierarchies across multiple length scales, from approximately 100-micrometre-thick layers down to nanoscale precipitates. Inspired by ancient Damascus steels
-which have hard and soft layers, originally introduced via the folding and forging techniques of skilled blacksmiths-we produced a material consisting of alternating soft and hard layers. Our material has a tensile strength of 1,300 megapascals and 10 per cent elongation, showing superior mechanical properties to those of ancient Damascus steel
. The principles of in situ precipitation strengthening and local microstructure control used here can be applied to a wide range of precipitation-hardened alloys and different additive manufacturing processes.
The design of advanced alloys specifically tailored to additive manufacturing processes is a research field that is attracting ever-increasing attention. Laser powder-bed fusion (LPBF) commonly uses ...pre-alloyed, fine powders (diameter usually 15–45 µm) to produce fully dense metallic parts. The availability of such fine, pre-alloyed powders reduces the iteration speed of alloy development for LPBF and renders it quite costly. Here, we overcome these drawbacks by performing in-situ alloying in LPBF starting with pure elemental powder mixtures avoiding the use of costly pre-alloyed powders. Pure iron, chromium, and nickel powder mixtures were used to perform in-situ alloying to manufacture 304 L stainless steel cube-shaped samples. Process parameters including scanning speed, laser power, beam diameter, and layer thickness were varied aiming at obtaining a chemically homogeneous alloy. The scientific questions focused on in this work are: which process parameters are required for producing such samples (in part already known in the state of the art), and why are these parameters conducive to homogeneity? Analytical modelling of the melt pool geometry and temperature field suggests that the residence time in the liquid state is the most important parameter controlling the chemical homogeneity of the parts. Results show that in-situ alloying can be successfully employed to enable faster and cost-efficient rapid alloy development.
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Laser Additive Manufacturing (LAM) of light metals such as high-strength Al-based alloys offers tremendous potential for e.g. weight reduction and associated reduced fuel consumptions ...for the transportation industry. Typically, commercial Sc-containing alloys, such as Scalmalloy®, rely on precipitation hardening to increase their strength. Conventional processing involves controlled ageing during which ordered and coherent Al3Sc precipitates form from a Sc-supersaturated solid solution. Here we show how the intrinsic heat treatment (IHT) of directed energy deposition (DED) can be used to trigger the precipitation of Al3Sc already during the LAM process. High number densities of 1023 nano-precipitates per m3 can be realized through solid-state phase transformation from the supersaturated Al-Sc matrix that results from the fast cooling rate in LAM. Yet, the IHT causes precipitates to coarsen, hence reducing their strengthening effect. We implement alternative solidification conditions to exploit the IHT to form a Zr-rich shell around the Al3Sc precipitates that prevents coarsening. Our approach is applicable to a wide range of precipitation-hardened alloys to trigger in-situ precipitation during LAM.
Atom-probe tomography is a materials characterization method ideally suited for the investigation of clustering and precipitation phenomena. To distinguish the clusters from the surrounding matrix, ...the maximum separation algorithm is widely employed. However, the results of the cluster analysis strongly depend on the parameters used in the algorithm and hence, a wrong choice of parameters leads to erroneous results, e.g., for the cluster number density, concentration, and size. Here, a new method to determine the optimum value of the parameter d
max is proposed, which relies only on information contained in the measured atom-probe data set. Atom-probe simulations are employed to verify the method and to determine the sensitivity of the maximum separation algorithm to other input parameters. In addition, simulations are used to assess the accuracy of cluster analysis in the presence of trajectory aberrations caused by the local magnification effect. In the case of Cu-rich precipitates (Cu concentration 40–60 at% and radius 0.25–1.0 nm) in a bcc Fe–Si–Cu matrix, it is shown that the error in concentration is below 10 at% and the error in radius is <0.15 nm for all simulated conditions, provided that the correct value for d
max, as determined with the newly proposed method, is employed.
Alloys manufactured by laser powder-bed fusion have intrinsic and hierarchical microstructural features inherited from the fast solidification (up to 104 K/s) and subsequent thermal cycles. This ...creates epitaxed grains, dislocation cell structures, and second-phase oxide nanoparticles. Epitaxed grains follow a pattern where finer grains are found in the melt pool centerline along the laser track. Upon further annealing, this characteristic microstructure has pronounced consequences on the recrystallization mechanisms and thus on grain topology. By changing the scanning strategy, we control the emerging grain patterns in a representative alloy (AISI 316L austenitic stainless steel) by creating linear strings for unidirectional scans, while a chessboard grain pattern arises by applying a 90°-rotation between layers. Upon post-processing annealing (at 1150 °C from 15 min to 8 h), we study the relationship between the as-built and recrystallized microstructures. Recrystallization starts with fine nuclei in regions with high dislocation density along the melt pool centerlines, resulting in early-stage linear impingement (linearly clustered nucleation), as revealed by microstructural path analysis. Recrystallization is sluggish, due to dynamic Zener-Smith pinning. This effect leads to jerky boundary motion due to periodic pinning and depinning from oxide particles, caused by their gradual coarsening. Lower nuclei number density slows kinetics for the case of unidirectional scanning, while twinning aids in the nucleation of grains with mobile grain boundaries. Our findings show that changes in the laser scanning strategy are a suitable design tool for tailoring recrystallization and thus microstructure.
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