A Review on Metal Binder Jetting 3D Printing

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Introduction
Additive Manufacturing, more commonly referred to as 3D printing, is an innovative manufacturing technique that constructs three-dimensional components in a sequential manner, using a digital model as its primary source.Traditional subtractive manufacturing processes, such as cutting, drilling, or machining material away from a solid block, are contrasted with additive manufacturing methods, which entail adding material on top of existing material in successive layers in order to construct the finished object.According to ASTM standards [1], the vast majority of 3D printing or Additive manufacturing (AM) processes can be divided into seven categories based on the methods they employ, the materials they utilize, and the specific techniques they employ.BJ is one of the most recent developments in additive manufacturing [2].In this process, a liquid binder is deposited selectively onto a powdered material, therefore bonding the powdered material's particles together to generate green parts.These green parts are subsequently sintered to produce the finished product [3].The BJ technology has the potential to produce complicated shapes and parts without the requirement for elaborate support systems.The schematic picture of binder jetting as shown in Fig. 1 and the process consists of the following individual actions.The construction platform is covered in powder, which is distributed using a roller.When it is necessary, the print head will deposit the binder glue on top of the powder.Layer thickness in a model reduces the height of the construction platform [4].A second coating of powder is applied on top of the first.The powder and liquid come together to form the required part.The area around the object is still covered with loose powder.The procedure is carried many numerous times until the finished product has been produced in its whole.Depending on the type of material used, the intended application of the part, and the desired properties of the final product, specific post-processing processes will be required following the printing of a green part.It is a versatile process, and the post-processing phases can vary significantly depending on the circumstances [5].The use of binder jetting to fabricate metals is a novel technique to additive manufacturing that possesses its own set of benefits, including other methods of metal additive manufacturing, such as those relying on lasers, often move at a slower pace than this one does.It is also able to construct bigger metal pieces without the need for a build chamber, which restricts the amount of area available.In addition to this, it makes it possible to create intricate geometries and interior structures, both of which would be difficult to accomplish with more conventional production techniques.Moreover, a variety of metal powders may be employed, which opens the door to the creation of a wide variety of materials and alloys [6].This study focuses at the effectiveness of the binder jetting technique in building complex geometries out of some of the most important advanced metals, including titanium, Inconel and stainless steel.

Titanium based powders
Evan Wheat et al., [7] evaluated the quality of the binder jetting additively manufactured green and sintered parts, including the bulk density, relative density, particle size, and pore size, as well as the size of the sinter neck.They concluded that computed tomography (CT) is a practical method for evaluating BJAM components.The results of the CT investigation showed bulk and per layer properties in both the green and sintered stages of the material.They additionally observed that the bulk density of the sintered pieces was increased due to the incorporation of finer particles.Binder-jetting additive manufacturing of titanium structures for orthopedic purposes has been explored by EsmatSheydaeian et al., [8] who investigated the impact of varying across layer thickness configuration.Within each titanium sample, two layers are carefully controlled, with thicknesses of 80 and 150 µm.The results showed that porous cellular Ti structures with variable porosity could be successfully manufactured, with the same mechanical strength behavior under compression stress being achieved.The connection between theory of sintering and process results was the main topic of another work by evan et al [9].Furthermore, the current study provides further insights into the subject matter, building upon the authors' previous publication [7] which focused on the characterisation of sintered structures in commercially pure titanium components produced by powder bed binder jetting additive manufacturing.The study conducted by Abdolreza Simch et al. [10] examined the microstructural features and mechanical properties of the Ti-6Al-4V alloy.The investigators specifically explored the impact of powder particle size and 3D printing parameters on these characteristics.Large and inter-aggregate pores are shown and described to have formed during binder jetting.It has been developed that highdensity Ti-6Al-4V parts may be successfully fabricated (≥96% PFD) with a microstructure that is equivalent to that of metal injection molded (MIM) titanium parts.The results for tensile strength and elongation come within the range of 880±50 MPa and 6±2%, which is on par with the tensile strength of metal injection molded components and superior than the ductility of the laser based powder bed fusion methods.

Nickel based powders
Binder jetting 3D printing of Inconel 718 components has been studied by Peeyush Nandwana et al., [11], who reported on the influence of particle feedstock on sintering kinetics and provided empirical models for solid-state and supersolidus liquid phase sintering (SLPS).They identified that the SLPS is a viable method for achieving full densification of Inconel 718 within an appropriate period of time.Properties of binder jet printed and heat-treated samples were studied by Amir Mostafae et al., [12] whose examined the impact of powders produced from various atomization processes.Samples were binder jet printed with either air-melted gas atomized (GA) or water-atomized (WA) powders of nickel-based alloy 625, and their microstructural development and mechanical characteristics were thencompared in greatdetail.As a result of changes in powder shape and chemistry, GA printed samples obtainedgreater sintering density (99.2% vs. 95.0%)than WA samples.Whencomparing samples from GA and WA at theirdensest, the grain sizes were 89±21 µm and 88±26 µm, respectively.Fine and equiaxed grains, near-full density, and a homogenous distribution of composition and precipitate phases are all correlated to the exceptional GA sampleresults, whichinclude a good balance between tensilestrength and ductility.In the presentwork, Pablo D. Enrique et al. [13] examined the surface roughness and near-surface porosities of Inconel 625 components produced using binder-jetting additive manufacturing.They used an electrosparkdeposition (ESD) technology for their investigation (Fig. 2).The phenomenon of localized surface melting and materialtransfer from the electrostaticdischarge (ESD) electrode leads to the formation of a zone near the surface characterized by higher density (increasing from 62.9% to 99.2%) and enhanced hardness (rising from 109 HV to 962 HV).

Stainless steel powders
Binder jetting-additive manufacturing of SS 316L was investigated by Saereh Mirzababae et al. [14] for its processes, theories, and uses.This literature review of binder jetting covers a wide range of topics, including powder characteristics (shape and size), binder properties (binder chemistry and droplet formation mechanism), printing process parameters (layer thickness, binder saturation, drying time), and post-processing sintering mechanism and densification processes (Fig. 3).In addition, they highlight and outline the most important aspects of 316L binder jet manufacturing, including feedstock selection, printing parameters, sintering temperature, sintering time, atmosphere, and heating rate.In order to enhance the final density and surface polish of BJ printed materials, Truong Do et al. [15] worked on modifying the current binder jetting (BJ) technique.Using a modified version of the normal BJ method, almost completely dense components were produced.Using a combination of SS powders in two sizes was shown to increase packing density.Surface roughness is reduced because tiny stainless steel particlesfill in cracks between larger ones.As a result, not only have distortion and shrinkage been minimized, but the surface polish has also been enhanced, in the finished components.Powder spaceholders(PSH) made from 30 m equal-sized PMMA powders were used in an investigation by Ganesh Kumar et al. [16] into the fabrication of high porosity 316L stainless steel (SS316L) with total porosity (40-60%) and pore openness index (0.87 to 1).The porosity of binder jet parts has been systematically investigated from two distinct perspectives: (1) the processing approach, in which the isothermal sintering temperature and binder volume were varied to achievedifferent binder saturation rates (55%, 100%, and 150%); and (2) the feedstock modification approach, in which PSH (30 vol.%PMMA) was added to pure SS316L feedstock.

Conclusions
The metal binder jetting technique used in 3D printing has manysignificantbenefits.This study examines the binder jetting 3D printing process, focusing on its ability to fabricateintricate structures with exceptionalprecision and accuracy using key metalssuch as titanium, Inconel, and stainless steel.Thesemetalsfind extensive use in severalsectorssuch as defense, aerospace, biomedical, and industrialdomains.

Fig. 2
Fig.2SEM micrograph of a cross-sectioned BJAM part after sintering a) within the mass and b) near the surface.[13]