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Metal additive manufacturing is an invaluable part of robust and on-demand supply chains of the future. With its ability to consolidate material to high density into geometrically complex structures, selective laser melting (SLM) is a powerful tool for the fabrication of high strength, lightweight components. A limited pool of qualified alloys remains to be an impediment to greater adoption. Martensitic steels are a class of alloys that see widespread use in industries for tooling and highly stressed load bearing parts. However, their presence in the literature remains sparse in comparison to several workhorse steel alloys. In this work, we discuss the print optimization for this class of steels, including optimization of laser power, scanning speed, hatch distance and platform heating temperature. We investigate the general microstructural features of these steels, which is dominated by post-solidification displacive martensitic transformation and its response to cyclic thermal cycles in SLM. We demonstrate that martensitic high strength steels can be processed by SLM to have excellent mechanical properties in the as-printed state. Greater understanding of the SLM processing of this class of steels will enable the fabrication of stronger and lighter parts for increasingly demanding applications.

MATERIALS FOR HUMANITY (MH 21)

Untapped strength in metal additive manufacturing: characteristics and properties of martensitic steels by selective laser melting

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technical paper

316L stainless steel is being extensively employed as structural material in infrastructural, automotive, aerospace, and nuclear energy applications. However, one of the major drawbacks which limits its wider applications is its low yield strength (170-300 MPa in annealed state). We explored the strengthening of 316L stainless steel with dispersed ceramic particles through selective laser melting (SLM). The feedstock, powder mixture of 316L powder and ceramic particles, was mixed using low energy ball milling process. This method has a neglectable change on powder’s flowability and shape. Results showed that the ceramic particles were uniformly dispersed in the 316L matrix after SLM. In the meantime, the grain size of the 316L was significantly refined. Tensile tests showed that the yield strength of the strengthened 316L was increased about 40% (from 599 to 832 MPa). Meanwhile, the 316L stainless steel with dispersed ceramic particles has a good combination of high strength and ductility. The strength enhancement is primarily attributed to grain refinement and Orowan strengthening. These findings offer a new perspective on the strengthening of 316L stainless steel.

Strengthening of 316L stainless steel through selective laser melting and dispersed ceramic particles

γ-TiAl-based alloys have been studied for decades due to their attractive properties. The material has been successfully employed in the making of turbocharger for passenger vehicles and turbine blades for aircraft engines. However, the applications are still limited because of their poor formability. Electron beam powder bed fusion (EBPBF) is advanced in fabricating components with high complexity using the layer-wise building method. Studies have shown that γ-TiAl alloys can be successfully fabricated using EBPBF. In this work, a bimetal component consists of Ti-48Al-2Cr-2Nb (turbine blade) and Ti-6Al-4V (fir tree mount) was designed. To avoid the formation process of γ-TiAl, Ti-48Al-2Cr-2Nb was fabricated using EBM on a wrought Ti-6Al-4V substrate directly. Straight and curved diffusion interlayers were designed and achieved using different printing strategies. No defect was observed in both of the EBM built parts and the interlayer zones. The average tensile strength of the samples with curved interlayer reached 389 MPa, which was much higher than TiAl/Ti-6Al-4V brazing joints. The tensile test revealed that the fracture occurred in the interlayer zone that consisted of α2-Ti3Al and B2 phases. This work developed a promising application of γ-TiAl based alloys with less formation process.

Additive manufacturing of γ-TiAl and Ti-6Al-4V bimetal component using electron beam powder bed fusion

A microfluidic low pass filter LPF is a passive flow regulator fabricated by introducing the compliant element into the structure design , therefore controlling the flows with reduced needs from external facilities. Previous studies have reported such devices fabricated by soft lithography , However, soft lithography typically requires expertise, intensive laboring, and days of preparations which hindered the potential of a wider application and further commercialization. As the microfluidic market developed on a rapid basis recently the 3 D printing which emerged as a promising alternative to soft lithography due to its advantages such as rapid prototyping and computer-aided design process has received increasing attention in recent years . Besides, additive manufacturing also enables the modular paradigm where the individual elements could be harnessed in a plug n play way . In this work, we present a 3D printed design of microfluidic LPF, in which a modular assembly way is adopted . The characterization of the device is conducted and analyzed.
We demonstrate that this 3D printed LPF could work together with a PZT micropump to form a portable miniaturized mi microfluidic pumping system with stabilized flow output , which could be used in various applications in the future.

3D printed microfluidic low pass filter in modular paradigm

Electron Backscatter Diffraction (EBSD) in Scanning Electron Microscope (SEM) has been one of the most popular tools for characterizing microstructures of materials and in particular alloys made by additive manufacturing (AM) in recent years. This characterization technique can provide abundant microstructural data, including phase distribution, grain size, grain boundary character, crystallographic orientation, and texture. However, it has recently been noticed that there are several challenges in the characterization of AM alloys by EBSD, causing the misunderstanding of material microstructures. This presentation discusses two challenges associated with the grain size determination by EBSD.
Grain size is an important parameter used in understanding the relationship between the mechanical property and microstructure of a 3D-printed alloy, and is usually measured by optical microscope (OM) and EBSD. However, a common issue is that the grain size determined by EBSD sometimes is inconsistent with that from OM. Here We measured the grain size from the same areas in a Ti64 sample by OM and EBSD, respectively, and compared the results, in order to correlate two techniques. The result shows that when the grain size is greater than 10um, the data from OM and EBSD can completely match. When the grain size is in the range of 4~10um, there is a significant deviation between the size distributions. For fine grains, e.g. <4um, the EBSD measurement is more reliable. This indicates that OM and EBSD are equivalent for analyzing grains which sizes are bigger than 10um, but EBSD is more suitable for measuring fine grains due to the higher spatial resolution than OM. Further analysis reveals that the accuracy of grain size measurement especially for the fine grains is strongly related to the threshold angle of grain boundary used in the EBSD data process.
Step size is another important parameter to determine the grain size. Another usual challenge in the EBSD analysis is how to choose the step size to obtain a high-quality EBSD map with high spatial resolution and high hit rate [1]. As the typical AM alloy has two different microstructural features in terms of grain size, melt pools with large columnar grains and fine-grained regions between the melt pools, researchers like to choose a small step size to map the sample. This is a conventional EBSD strategy used in the characterization of heterogeneous microstructures in materials. However, the recent characterization in Al printed alloys shows that such a strategy doesn’t work well at low magnification even a very small step size is used. Normally, grains in melt pools are visualized and measured well, but the fine grains at the melt pool boundaries are invisible at the low magnification due to the low hit rate. To improve EBSD map quality and grain size measurement at the fine-grained regions, higher magnification with a smaller step size has been used, but this condition limits the size of the mapping area. Compromising the magnification and grain size determination, a new EBSD strategy that includes two steps of mapping AM samples is recommended, firstly scanning a big area to show the domain microstructures of melt pools, and secondly mapping with higher magnification to show the typical fine-grained region in detail.

Challenges in EBSD characterization of heterogeneous microstructure in additively manufactured alloys

In the powder bed fusion process, exposure to oxygen and its incorporation into the melt pool is inevitable. Although oxygen exposure is conventionally recognized as a source of problems, recent experiments show otherwise, where the deliberate oxidation or introduction of oxygen actually improves the process and part quality. Hence, the influence of oxygen on the powder bed fusion process warrants further investigation. Yet, in-situ observation of the oxygen incorporation and its influence on melt pool dynamics is experimentally inexpedient, while oxygen-related effects in numerical models remain lacking. This work aims to shed light on the effect of oxygen content on melt pool dynamics through the use of thermal-fluid flow simulations. Our simulations have captured the oxygen absorption mechanism during melt pool formation and reveal its influence on the Marangoni effect and the subsequent oxygen distribution in the melt pool. The qualitative results obtained agree with experimental observations.

Influence of Oxygen Content on Melt Pool Dynamics

With flexible robots' development, the requirements for highly stretchable sensors are getting higher and higher. At present, flexible sensors have been widely used in wearable electronic devices to obtain human physical parameters, such as wearable electronic devices, motion recognition, and healthcare. Though the measuring ability of sensitivity and range has been largely improved compared to the traditional ones, the hysteresis in the loading and unloading state has always limited these sensors' application, which negatively influences the sensors' reliability and accuracy. In this paper, through the combination of highly stretchable dielectric elastomer cross-linked with polar hydrophobic ionic liquid and polymerized under UV light, a highly sensitive and highly stretched sensor is prepared, reaching up to 10 times stretchability at most and maintaining a low hysteresis (2.5%). The investigated sensor showed a very short response time (100ms), adjustable conductivity. Combined with some of our structural innovations, we have used smart gloves, touch sensors, and other applications to prove this material's broad prospects in soft robots, motion detection, wearable devices, and medical care.

Near-Hysteresis Free Ionic Conductive Elastomers with Long-Term Stability for Artificial Skin Applications

DEM-CFD simulation on spattering and denudation of laser powder bed fusion additive manufacturing

In this modern society, plagued by climate change and uncertain energy supply, clean water and energy are precious resources. These resources conjoin in a term called water-energy nexus. The Water and Energy crisis grows increasingly dire, especially in developing countries. Highly efficient solar energy desalination is a green solution that produces potable water using solar energy without further processing. A novel fully ceramic structure of integrated solar absorber/thermal insulator/water transport based on the two-layered structure of modified graphene on silica (MG@Silica) and porous silica structure was proposed. The proposed highly efficient ceramic solar desalination device addresses the issues of reusability and sustainability. By utilizing 3D printing, nanopores and micropores can be tailored to effectively maximize capillary effects to increase the water uptake. As a result, an advanced complex structure can be realized for a high water evaporation rate. Altering the ceramic’s surface with modified graphene can allow excellent solar absorption, good resistance to photodegradation, and the ability to remove ions and bacteria. The resultant desalinated water from the device meets the World Health Organization (WHO) drinking water standards.

Fabrication of 3D-Printed Ceramic Structures for Portable Solar Desalination Devices

Electrospray of plant based hydrogel microbeads for lab-based meat culture

Polluted sea water is a common problem in coastal cities, but its decontamination remains a big challenge due to the existence of salt ions, which may invalidate many prevailing treatment methods originally developed for waste fresh water. This work presents the first attempt to decontaminate the polluted sea water by synergizing two separate processes: photocatalysis and ozonation (see Figure 1). For low cost and human safety, the ozone concentration is kept to be < 60 ppm in the pumping gas (0.026 ppm in solution). Mechanism studies and experimental comparisons show that the photocatalysis, the ozonation and the photocatalytic ozonation (PCO) all have lower efficiencies in sea water than that in fresh water, and the PCO is always more efficient than only the photocatalysis or only the ozonation. More specifically, the PCO has a reaction rate constant about 23 times higher than only the ozonation. In addition, the sea water shows a positive synergistic effect of photocatalysis and ozonation and reaches the maximum when the pumping gas has an ozone concentration of 50 ppm in gas. In contrast, the fresh water shows a negative synergistic effect. Mechanism study using experiments infers that the dissolved ozone can enhance and stabilize the photocurrent by rapidly scavenging the photoelectrons. This work may pave the way to practical applications of sea water decontamination with high efficiency and low cost.

This work is supported by Research Grants Council (RGC) of Hong Kong (152184/15E, 152127/17E, 152126/18E, 152219/19E and 152156/20E), the National Natural Science Foundation of China (Project No. N_PolyU511/20) and The Hong Kong Polytechnic University (1-ZE14 and 1-ZVGH).

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Untapped strength in metal additive manufacturing: characteristics and properties of martensitic steels by selective laser melting

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Strengthening of 316L stainless steel through selective laser melting and dispersed ceramic particles

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