Analysis of the causes determining dimensional and geometrical errors in 316L and 17-4PH stainless steel parts fabricated by metal binder jetting

The International Journal of Advanced Manufacturing Technology (2024) 132:835–851 https://doi.org/10.1007/s00170-024-13437-7 ORIGINAL ARTICLE Analysis of the causes determining dimensional and geometrical errors in 316L and 17‑4PH stainless steel parts fabricated by metal binder jetting Marco Zago1 · Nora Lecis2 · Marco Mariani2 · Ilaria Cristofolini1 Received: 29 November 2023 / Accepted: 11 March 2024 / Published online: 16 March 2024 © The Author(s) 2024 Abstract This work aims at investigating the causes affecting the dimensional and geometrical accuracy of holes in metal binder jetting stainless steel parts. Parallelepiped samples with a through hole were produced using AISI 316L and 17-4PH powders, differing for diameter (3, 4, 5 mm), and position of the axes with respect to the building plane (6, 9, 12 mm distance). Dimensions and geometrical characteristics were measured at green and sintered state by a coordinate measuring machine, determining the dimensional change and the geometrical characteristics. As expected, the shrinkage of linear dimensions is anisotropic; moreover, change in volume and sintered density are significantly affected by the position in the printing chamber. Higher shrinkage was measured along building direction (Z) – 18.5 ÷ 19.5%, than in the building plane – 16.5 ÷ 17.5%, and slightly higher shrinkage – 0.5 ÷ 0.8% was measured along powder spreading direction (X) than binder injection direction (Y). A variation up to 3% in relative density of sintered parts depending on the position in the building plane was observed in 316L. The dimensional change of diameters generally confirmed the shrinkage predicted by the model previously developed—difference between real and expected dimensional changes lower than 3%, except for three geometries (4 ÷ 6%). The cylindricity form error of sintered parts was strongly underestimated by the prediction model (up to 0.15 mm), but underestimation was considerably reduced (generally lower than 0.05 mm) adding the cylindricity form error due to printing. Dimensional and geometrical accuracy of holes are strongly affected by shape distortion of the parallelepiped geometry, in turn due to layer shifting and inhomogeneous green density during printing, and to the effect of frictional forces with trays during sintering. Gravity load effect was also observed on the holes closest to the building plane. Future work will improve the reliability of the prediction model implementing the results of the present work. Keywords Additive manufacturing · Metal binder jetting · Shrinkage on sintering · Dimensional and geometrical accuracy and precision 1 Introduction Binder jetting (BJ) is an additive manufacturing (AM) technique with strong potential for manufacturing of small and medium batches. With respect to other AM processes, BJ building rate is higher than building rate of both material extrusion processes and powder-based techniques, as Laser * Ilaria Cristofolini 1 Department of Industrial Engineering, University of Trento, Via Sommarive 9, 38123 Trento, Italy 2 Department of Mechanical Engineering, Politecnico Di Milano, Via Privata La Masa 1, 20156 Milano, Italy Powder Bed Fusion (L-PBF) and Direct Energy Deposition (DED). Moreover, the printing operation does not require vacuum or inert gas atmosphere in most cases, neither support structures in the powder bed or high energy source [1]. The build-up process occurs at room temperature, so that residual stresses, which can lead to part cracking or failure, do not represent an issue [2, 3]. The competitiveness is emphasized by the possibility of using different metal feedstocks [2, 3]. The interest in metal binder jetting (MBJ) is rapidly growing within research community and companies. Nevertheless, controlling the dimensional and geometrical accuracy and precision of final products still represents major issue to be addressed, being size and shape determined by the different operations involved in BJ process— printing, curing, de-powdering, debinding, and sintering. Vol.:(0123456789) 836 The International Journal of Advanced Manufacturing Technology (2024) 132:835–851 The 3D CAD file is converted to a machine-readable format (STL, AMF, STEP), which is imported in the printing software. The main process parameters, layer thickness and binder saturation among them, are set up by the operator. A blade or roller determines height and homogeneity of powder layers, while the liquid binder injected by the printhead defines the section of the product, previously computed by the discretization algorithm. By repeating powder spreading and binder injection, the part is built up. The part is removed from the printing box after curing (70 ÷ 200 °C), which enhances binder cross-linking and increases the strength of the green part [4]. Manual or automatic de-powdering allows removing any extra-powder, prior to debinding and sintering. Debinding consists in a chemical and/or thermal treatment aimed at removing the organic binder agent, while sintering at high temperature activates the diffusion mechanisms, which strongly bond the powders and reduce porosity, determining the final structure and microstructure. The influence of materials characteristics and process conditions on the dimensional and geometrical precision has been widely investigated in recent studies, aiming at improving the quality of BJ parts [5]. Particle size distribution (PSD) strongly affects both packing and sintered density, and in turn variations in volume and dimensional changes. As reported by German et al., an optimum mixture of bimodal particles can improve the packing density and final sintered density, also reducing the sintering shrinkage [6], as experimentally verified in recent work using MBJ feedstocks [7, 8]. Green density is also affected by process parameters; decreasing layer thickness green density increases [9, 10], and proper binder saturation interval has to be identified to control densification and resulting geometrical characteristics [11–14]. The influence of binder distribution on densification of Al during sintering has also been studied by transmission synchrotron X-ray imaging in [15, 16], highlighting the need for both developing alternative binders, and properly developing binder deposition patterns. The role of particle size on densification of Ti6Al4V was investigated in [17], considering binder-induced powder aggregation. Build orientation [18, 19] and position in the building plane [20] also play significant role in determining geometrical precision. Lee et al. proposed a model for particle spreading based on discrete elements, highlighting that high rake velocity can determine particle segregation; bigger particles tend to segregate in front of the rake and PSD shifts towards finer region [21]. Sintering conditions, as isothermal sintering temperature, holding time, and sintering atmosphere, determine the final densification, and consequently the volumetri (...truncated)