Binder jet 3D printing—Process parameters, materials, properties, modeling, and challenges

In-situ alloying in laser-based additive manufacturing processes: A critical review

Additive Manufacturing (AM), or 3D printing, processes depend on a user-defined set of optimized process parameters to create a component. Monitoring and control of AM processes in real-time can help achieve process stability and repeatability to produce high quality parts. By applying in-situ monitoring methods to the AM process, defects in the printed parts can be detected. In this review, application of both imaging and acoustic methods for the detection of sub-surface and internal defects is discussed. Imaging methods consist of visual and thermal monitoring techniques, such as optical cameras, infrared (IR) cameras, and X-ray imaging. Many studies have been conducted that prove the reliability of imaging methods in monitoring the printing process and build area, as well as detecting defects. Acoustic methods rely on acoustic sensing technologies and signal processing methods to acquire and analyze acoustic signals, respectively. Raw acoustic emission signals can correlate to particular defect mechanisms using methods of feature extraction. In this review, representation and analysis of the acquired in-situ data from both imaging and acoustic methods is discussed, as well as the means of data processing. Ex-situ testing techniques are introduced as methods for verification of results gained from in-situ monitoring data. 

In-situ Monitoring of Sub-surface and Internal Defects in Additive Manufacturing: A Review

Pharmaceutical applications of powder-based binder jet 3D printing process - A review.

In this study, the influence of sintering temperature and hold time on densification, mechanical properties, and microstructural characteristics of 17–4 PH stainless steel samples produced by binder jet 3D printing (BJ3DP) was investigated. An ultimate tensile strength (UTS) of 1050 MPa, an elongation at break (A) of >4% and a hardness of ≥330 HV10 through sintering at 1300 °C for 90 min were achieved. The tensile properties are predominantly influenced by porosity as opposed to the grain size or the volume fraction of δ-ferrite. A layered porosity appears in the sintered parts, which is attributed to the layer-wise BJ3DP process. Shrinkage in z-direction is about 2% higher than in x- and y-direction and is therefore anisotropic. Fluctuations in green density of up to 1.2% affect sintered density throughout the study and are compensated best by extended hold times. Microstructural analyses show the expected grain growth and increase in δ-ferrite fraction by increasing sintering temperature and more extensive hold times, while the comparison of fringe and core microstructures reveals little differences of the grain size but significant differences of the fraction of δ-ferrite and porosity.

The effects of sintering temperature and hold time on densification, mechanical properties and microstructural characteristics of binder jet 3D printed 17-4 PH stainless steel

Several additive manufacturing (AM) methods use powder feed materials. Selective laser sintering is an example of a versatile AM method, using feed material in powder form, capable of producing polymer and metallic parts. In the variations of this technique, a laser spot or an electron beam is used to locally sinter or melt a packed powder bed. After the completion of sintering on each layer, further powder is added on top of the existing bed so that the next layer may be joined. A major challenge in this method is controlling the porosity of the powder bed so that the final part has uniform and maximum density. Uniformity in the packing of bed from one layer to the other is important for optimizing the processing parameters. This review is focused on considering the packing characteristics of polydisperse hard particle beds and the determination of the expected density achievable for a given particle size and shape distribution. Models are presented for discrete mixtures as well as continuous distributions. The effect of the initial configuration of a particle bed on its ability to form a highly dense packing is also discussed. Blending of different particle sizes and shapes can be used to substantially increase the packing density, but can also lead to separation or segregation of the bed. Through appropriate control of the particle shape and use of wide distributions, packing densities close to 100 % can theoretically be achieved, but practicality and various effects that appear at small size scales prevent from achieving such high packing densities. Recent advancements have reduced the dependence of AM part quality on the density of the packed particle bed but the packing is still important for considerations such as thermal conductivity of the bed and absorption of laser power in the bed. Improved knowledge of packed bed characteristics can be helpful in developing AM methods for novel material systems.

Effect of particle size distribution on the packing of powder beds: A critical discussion relevant to additive manufacturing

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Effect of particle size distribution on the packing of powder beds: A critical discussion relevant to additive manufacturing