Abstract: Additive Manufacturing (AM) technologies theoretically allow the production of complex products without any geometrical restriction. Nevertheless, production process delineates some limitations on the resulting dimensional and geometrical precision. This is a critical issue mainly for Metal Binder Jetting (MBJ) process, on the reason of anisotropic dimensional change and distortion on sintering. Literature reports fairly reliable models for predicting the deformation on sintering. However, the application of such methods might be time consuming from industrial perspective, because of the extensive experimental analysis required to assemble a robust material database. For that reason, this work aims at proposing an alternative approach for compensating dimensional and geometrical change on sintering. Two complex geometries, having similar geometrical features with different sizes, were printed and measured by a coordinate measuring machine before and after sintering process. The analysis of cylindricity form errors reveals an excellent geometrical stability of smaller geometry. Therefore, dimensional change along printing direction was derived in order to obtain a precise scaling factor for improving the dimensional and geometrical precision. By contrast, bigger samples encountered a dramatic distortion, which required a complete redesign. The shape of the distorted cylinder was approximated with an ellipse and a corrective function has been proposed for compensating green geometry.
Keywords: Additive manufacturing | Binder jetting | Design for AM | Distortion
Abstract: The anisotropy of dimensional change in compaction plane of rings made of three low alloyed steels was investigated as a function of green density and geometry. Increasing green density and (D ext–D int)/H ratio, the anisotropy of both shrinkage and swelling increases. A correlation with springback during ejection of the rings from die cavity after cold compaction was found. The ratio between the dimensional changes of diameters, as a function of the ratio between springback of diameters, describes a linear correlation intersecting point (1,1), representative of isotropic behaviour. This correlation confirms the hypothesis of an effect of micropores, generated in the green parts during ejection from die cavity, on dimensional change anisotropy. An analytical correlation was determined for the anisotropy of dimensional change in the compaction plane as a function of green density and geometrical parameter, which can be implemented in the design methodology accounting for the anisotropic dimensional change previously proposed.
Abstract: Metal binder jetting (MBJ) is an additive manufacturing (AM) technology split into two process steps: printing and sintering. Firstly, product is built up layer-by-layer by the selective deposition of a binder agent on a powder bed. Secondly, a thermal treatment (sintering) consolidates the metal structure. MBJ is currently becoming more and more attractive on the reason of high potential scalability, cost-effective production and wide range of available material feedstocks. However, the transition towards industrial scale production is restrained by the critical control of dimensional and geometrical precision of parts after sintering operation. In fact, product geometry is affected by anisotropic dimensional change or even shape distortion. This study aims at investigating the dimensional and geometrical precision of through holes. Three sample geometries were designed, having a through hole with axis perpendicular to the building direction and located at different levels along sample height. Samples were measured by a coordinate measuring machine before and after sintering, in order to assess the shrinkage and any shape change. Results highlight the inhomogeneous volumetric and linear shrinkage of the three geometries, which is influenced by the printing position in the building plane. A macroscopic deformation of parallelepiped geometry was also evidenced, caused by the superposition of layer shifting originated on printing, and by the frictional forces between sample surface and alumina support during sintering. Such distortion significantly affects the shrinkage and form error of holes.
Keywords: Design for AM | Dimensional and Geometrical Precision | Metal Binder Jetting | Shape Distortion
Abstract: Recent research has been focused on the binder jetting (BJ) additive manufacturing technique due to the high potential possibilities in industrial applications. The actual limitation of BJ process can be attributed to the difficult control of the product quality. In fact, a high dimensional variation occurs on sintering, which can detrimentally affect dimensional and geometrical precision, when not properly considered in the design step. This paper aims at investigating the influence of sintering on the dimensional change of through holes, with different diameter size and different axis orientation with respect to the building direction. Samples were measured in the green and sintered state by means of a coordinate measuring machine in order to calculate the diameter shrinkage. The empirical data were successfully compared with the prevision of an analytical model demonstrating that diameter shrinkage is influenced by: the anisotropic dimensional change, the axis orientation and the position of the two diametral opposite points used to identify the diameter. A deep analysis of the results showed a non-negligible effect of the gravity-induced load and of the inhomogeneous shrinkage on sample geometry. This study highlighted that the analytical model may serve as a basis in the design step for improving the dimensional quality of BJ product.
Keywords: Binder jetting | Design for additive manufacturing | Dimensional and geometrical precision
Abstract: The anisotropic dimensional changes during sintering were investigated for rings made of eight different materials with different green densities and H/(Dext −Dint) ratio. Dimensional changes are affected by green density, as shown in previous works, while the geometrical parameter does not display a clear influence. The anisotropy parameter K defined in a previous work does not describe anisotropy of dimensional change unambiguously, due to the anisotropy of shrinkage/swelling in the compaction plane. A new anisotropy parameter (K 3D) was therefore defined considering the dimensional changes of internal diameter, external diameter and height. This parameter displays an unambiguous dependence on the equivalent isotropic dimensional change and will be used in further work to develop a predictive model for the prediction of the anisotropic dimensional change during sintering of parts with different green densities and geometry.
Abstract: The influence of compaction strategy on compressibility and densification of metal powders has been extensively studied in previous work; effective compaction mechanics relationships and a densification model have been derived on experimental basis. Nevertheless, such studies also highlighted the need for further investigation concerning filling step, playing major role in obtaining high density, homogeneously distributed. This work focuses on filling step, considering the influence of both geometry, and filling strategy. Ring shaped parts with different height to thickness ratios (H/T) have been produced, also varying filling parameters as filling shoe speed, suction speed, and number of shakes of the filling shoe. Filling density was derived, as a function of above parameters, also highlighting the most critical parameters affecting filling density. Moreover, green density was measured in different points, referring to filling shoe movement, aiming at identifying the effect of filling strategy on flatness and parallelism of planes resulting from compaction.
Abstract: Anisotropic dimensional change on sintering has been studied in depth in previous work, as affected by powder mix, geometry, sintering conditions etc. Previous results also revealed anisotropic dimensional changes in the compaction plane, to be necessarily considered when designing precise and accurate parts. Anisotropy in the compaction plane is expected to be markedly affected by inhomogeneities in green density distribution, in turn related to compaction strategy. Aiming at investigating in depth the influence of the compaction strategy, compaction speed, hold down force, and hold down time were selected as parameters. Full factorial testing was performed, three levels for each parameter, and three different materials were considered. Dimensional changes were measured, and the effect of the selected parameters was analyzed, also referring to the different sintering mechanisms occurring in the different materials. As a result, relationships describing the effect of the compaction parameters on the dimensional changes are proposed.
Abstract: The interest of composite extrusion modelling (CEM) for additive manufacturing of metal components is growing up due to the low cost of this process and to the possibility of using commercial MIM feedstock. In a previous study, the successive stages of the processing route of simple copper parts have been optimised with regard to the final weight density and surface roughness. This process can be applied to the fabrication of parts with porous architecture, to be used as heat sinks in power electronics. For optimal properties of these devices, the porous components should be directly printed upon Direct Bonded Copper (DBC) substrates. As a first step toward this objective, cubic samples have been printed upon plain copper or DBC plates and the shear strength of these assemblies has been measured. In addition, porous parts including regularly distributed channels have been fabricated and their efficiency as heat sinks has been assessed.
Abstract: Additive manufacturing allows high complexity of manufactured structures, permitting entirely new design capabilities. In the context of complex design, lattice structures hold the most promise for high complexity, tailorable and ultra-lightweight structures. These unique structures are suitable for various applications including light-weighting, energy absorption, vibration isolation, thermal management amongst many others. This new complexity leads to new manufacturing quality control and metrology challenges. Traditional metrology tools cannot access the entire structure, and the only reliable method to inspect the inner details of these structures is by X-ray computed tomography (CT). This work highlights the challenges of this process, demonstrating a novel workflow for dimensional metrology of coupon lattice samples—using a combination of surface and internal metrology using tactile probe and CT. This dual combined approach uses traditional surface coordinate measurement on exterior accessible surfaces, which is followed by internal lattice measurements. The results show a clear method and workflow for combining these technologies for a holistic dimensional inspection. The confidence gained by inspection of such lattice coupons will support the application of these lattices in end-use parts.
Keywords: calibration | laser powder bed fusion | lattice structures | metal additive manufacturing | metrology | X-ray tomography
Abstract: Dimensional and geometrical precision of parts produced by binder jetting is a crucial issue to be considered aiming at promoting the transition to industrial production. The influence of both the printing and the sintering processes has to be evaluated, and the high shrinkage has to be considered. The task is further complicated by the anisotropy of dimensional change on sintering. The aim of this work is to investigate the dimensional and geometrical precision of cylindrical holes, as affected by the anisotropy of dimensional change on sintering after binder jetting. AISI 316 L powder was used to produce five different geometries, characterized by four holes with different orientation with respect to the printing direction. The geometrical features were measured both in the green and in the sintered state with a coordinate measuring machine, and the dimensional changes, as well as the geometrical variations, were calculated. According to the author's previous experience, a theoretical model has been defined, which aims at predicting the geometry of the holes as derived by the anisotropic dimensional change on sintering. The expected dimensional change of hole diameters, the variation of cylindricity, and the variation of the axis inclination were calculated by the model and compared to those derived from measurement. Good agreement between predicted and measured results has been observed, providing that the influence of printing process parameters is considered.
Keywords: Binder jetting | Dimensional and geometrical precision | Metal additive manufacturing | Shrinkage on sintering
Abstract: The precision of parts produced by Powder Metallurgy (PM) strongly depends on the careful design of PM process parameters. Among them, high sintering temperature is generally considered as detrimental for dimensional and geometrical precision, and therefore neglected in industrial production. Nevertheless, high sintering temperature would strongly improve mechanical characteristics of PM parts, so that the real influence of high sintering temperature on dimensional and geometrical precision is of great interest for PM companies. This study investigates the influence of sintering temperature (up to 1350 °C) on dimensional and geometrical precision of real parts. Dimensional changes on sintering and the effect of sintering temperature have been evaluated. Geometrical characteristics have been measured both in the green and in the sintered state, and the real influence of sintering temperature has been highlighted. As a conclusion, it has been demonstrated that the larger shrinkage due to the high sintering temperature is not detrimental with respect to the dimensional precision, being it reliably predictable. Moreover, the influence on geometrical characteristics is unexpectedly low. The encouraging results of this study convinced the main PM companies in Europe to further investigate the influence of high sintering temperature, as partners in a Club Project within the European Powder Metallurgy Association (EPMA).
Keywords: Design for powder metallurgy | Dimensional and geometrical precision | High temperature sintering process | Precision engineering | Product development
Abstract: Powder behavior during compaction has been studied in depth during the long-lasting cooperation between Sacmi Imola s.c and the powder metallurgy research group at the University of Trento. The strong influence of many different parameters on compressibility and densification of powders has been highlighted, and synergistic effects have been observed. The experimental data continuously recorded by industrial presses served as a basis to derive compaction mechanics relationships and the densification model. This work is focused on the densification equation. The coefficients in the densification equation are critically evaluated as a function of the variables considered, using data coming from different materials, with different particle size, forming different geometries through different compaction strategies. The goal is to identify the relationships describing the variation of coefficients as a function of such variables. As a consequence, it will be possible to distinguish the relative weight of the variables governing the densification process.
Abstract: The mechanics relationships describing powder behaviour in uniaxial cold compaction have been derived in previous work on the basis of experimental data, as affected by many different variables in the compaction process. The influence of geometry, chemical composition, compaction strategy etc. were investigated in depth. This work focuses on warm die compaction, which stands for an interesting opportunity to increase density, also obtaining more homogeneous density distribution. Cylindrical specimens have been produced using a commercial diffusion bonded low alloy steel powder, to which different lubricants, in different amounts, have been added. The influence of type and amount of lubricant has been directly highlighted by the comparison of ejection force and energy. Axial and radial spring-back have been also evaluated. A deeper knowledge of the behaviour of the different powder mixes in warm die compaction has been obtained comparing the derived friction coefficients and constitutive models.
Abstract: Powder behavior in uniaxial cold compaction has been extensively investigated in previous work. The constitutive model of different powder mixes has been derived, and the influence of several variables, such as geometry, chemical composition, lubricant type and amount etc., has been studied in depth. This work focuses on the influence of warm die compaction. A commercial diffusion bonded low alloy steel powder, added with 0.6% wt. lubricant, has been used producing cylindrical specimens with two different H/D ratios, both in cold and in warm die compaction. Concerning warm die compaction, two different lubricants have been added. The constitutive model and the densification curves have been derived for all the powder mixes using the data recorded by the press, in terms of forces and displacements. Comparing ejection force and energy, the influence of warm die compaction, type of lubricant, and height of the specimens have been highlighted.
Abstract: The effectiveness of powder metallurgy as net-shape/near net-shape manufacturing technology is determined by the possibility of obtaining complex parts matching the required narrow tolerances. Sintering process determines change in volume of the green, and the related dimensional changes are significantly anisotropic. Anisotropy is affected by several variables, such as material, compaction and sintering parameters, geometry, whose influence is difficult to be distinguished and determined. Anisotropic dimensional change on sintering has been investigated in depth using an experimental approach, relating measurements results to the mechanisms responsible for the phenomenon. Main results concerning the influence of different variables are briefly presented in this work. Such results served as the basis for the development and further improvement of a design method, aimed at predicting anisotropic dimensional change. Main steps of the design method are presented and an example of application to a real part is described. Strong agreement between predicted and real dimensional changes has been observed, and compared to the attainable dimensional tolerances.
Keywords: Design for powder metallurgy | Precision engineering | Product development
Abstract: Anisotropic dimensional change on sintering may strongly affect the precision of parts produced by press and sinter. In previous work a design procedure accounting for anisotropic dimensional change of axi-symmetric parts (disks and rings) has been developed on the basis of experimental data. In this work the procedure has been applied to predict the anisotropic dimensional change of real parts produced in industrial conditions, providing that coaxial rings were identified in the geometry of the actual parts. Parts were highly different for material, complexity of geometry, green density and process conditions. Parts were measured in the green and sintered state and the measured dimensional changes were compared to the predicted ones, finding a good agreement. The procedure was also adapted to predict dimensional change of an oval feature, and highly satisfactory results were obtained.
Keywords: Anisotropy | Dimensional change | Powder metallurgy | Precision of PM parts
Abstract: AISI 316L powder mix was sieved in three particle sizes and successively compacted on rings with two different H/(D ext -D int ) ratios. The height and diameters were measured by a CMM at the green and at the sintered state. The shrinkage was calculated to study the influence of particle size and geometry on the anisotropy of dimensional change. As expected, the volumetric shrinkage increases moving from coarser to finer particle size, and the measurements confirmed the anisotropy of the dimensional variation. The measurements revealed that the lowest rings shrink more than the taller rings. The reference to the radial and axial stress during prior cold compaction shows an interesting correlation, in which shrinkage increases on increasing the stress. The further investigation of the deviatoric stress and the deviatoric shrinkage provided a possible explanation of the anisotropy of dimensional change on sintering as a function of the inhomogeneity of the compaction stress.
Abstract: Compaction mechanics relationships describing the behaviour of AISI 316L powder mixes during uniaxial cold compaction were derived in previous work by continuously recording the data of an industrial press. The reliability of the relationships depends on the accuracy in identifying the threshold of elasto-plastic transition, what is a difficult task due to the peculiar characteristics of powder mixes. The influence of particle size on the transition threshold has been investigated in the present work. Standard commercial AISI 316L powder was sieved in three particle sizes (fine - intermediate - coarse), 1% Acrawax was added as a lubricant and ring-shaped parts were produced by uniaxial cold compaction. Processing the data recorded during compaction, the mean relative density corresponding to the beginning of prevailing plastic deformation has been identified for all the powder mixes by means of a recursive procedure. The mechanisms responsible for the different transition thresholds have been studied in depth.
Abstract: Several models have been proposed in literature for decades to describe the relationships between the compaction pressure and green density. Some of them are almost empirical, others are based on a theoretical approach starting from the phenomena occurring during cold compaction. The authors of the present paper have recently (2018) proposed a model based on the investigation of the deformation experienced by the powder mix when subject to the application of the compaction pressure. In this work, the model is compared with the three most recently published ones: the Aryanpour and Farzaneh model (2015), evaluating the contribution of rearrangement; the Parilak et al. model (2017) that was validated with experimental data relevant to 205 powder mixes, and the Montes et al. equation (2018) that was developed considering the local stress and strain in the interparticle contacts. Compaction experiments were carried out on a commercial AISI 316L stainless steel powder mix, sieved in different particle size ranges, and the data collected by the compaction press control unit were used.
Abstract: The influence of the microstructure of uniaxial cold-compacted green iron on the sintering shrinkage was investigated. Pores in the green parts are very slightly oriented, while the dimension of the interparticle contact areas is anisotropic. A large and anisotropic anisothermal shrinkage in alpha iron was measured, greater than isothermal shrinkage at the 1120°C. The results were interpreted on the basis of the geometrical and of the structural activity, and the effective diffusivity responsible for neck growth was determined.
Abstract: Sintering shrinkage of prior cold compacted iron rings with different geometry (height to wall thickness ratio) and green density in the 6.5–7.3 g/cm3 range was investigated. It displays a minimum at an intermediate green density. Axial, tangential and radial shrinkages are different, due to the gradients of green density along the axial and the radial directions. Therefore, the effect of height on shrinkage and its anisotropy is the result of their effect on the stress distribution in the green parts during cold compaction, and the resulting green density and deformation experienced by the powder. Anisotropy decreases on increasing shrinkage.
Keywords: geometry | green density | Shrinkage anisotropy
Abstract: In previous work the data recorded by an industrial press (forces and displacements) were extensively used to describe powder behavior during uniaxial cold compaction. Satisfactory models describing densification were proposed, as the result of the axial and radial stresses acting on the powder column. The recorded data are very precise from an industrial perspective, mainly in the last stage of compaction, directly related to final green density. Nevertheless, they are slightly scattered in the first stage, which is interesting from a scientific perspective, as the step where rearrangement occurs. To overcome this limit, this work proposes a methodology to analyze the signal of forces in the frequency domain to increase the signal to noise ratio. The signal was decomposed through the Fourier transformation, and the noise was smoothed by a low-pass filter specifically designed for the press. Reliability of the data and effectiveness of the derived relationships result significantly improved.
Keywords: Compaction mechanics | Signal analysis | Smoothing operation
Abstract: In previous work the anisotropic dimensional change on sintering has been investigated in depth. An anisotropy parameter has been identified, depending both on geometry and on sintering conditions, and it has been used to define a model for the anisotropic behaviour. A design procedure accounting for anisotropic dimensional changes has been proposed. This work summarizes the main results obtained within the Design for Sintering Club Project, aimed at validating and enlarging the aforementioned design procedure by the application on real industrial parts. Project partners provided axi-symmetric parts, which were measured both in the green state and after sintering in standard industrial conditions. The real dimensional changes were compared to the dimensional changes predicted by means of the design procedure based on the anisotropy model. The results, also compared to the attainable dimensional tolerances, allowed validating the design procedure, and showing directions to further improvement.
Keywords: Anisotropy | Design for sintering | Dimensional change
Abstract: The sintering shrinkage of AISI316L rings with different height was correlated to the stress field applied to the powder mix dunng prior cold compaction. Both the stress field and the sintering shrinkage are isotropic in the compaction plane, while anisotropic along the axial direction with respect to the radial and tangential directions. A clear correlation between shrinkage and compaction stress was individuated, both from the mean analysis and from the local analysis; the higher the compaction stress, the higher the sintering shrinkage Such a correlation was used to predict the axial shrinkage theoretically. The anisotropy of shrinkage was correlated with the deviatonc component of the stress field.
Abstract: Compaction mechanics has been investigated in previous work using the data continuously recorded by an industrial press, aiming at obtaining the constitutive model of powder mixes actually representing powder behaviour during uniaxial cold compaction. The influence of geometry and powder mix has been previously investigated. In this work the same experimental approach was followed, aiming at highlighting the influence of the lubricant admixed to the metal powder. Two different lubricants, in two different amounts have been added to a commercial diffusion bonded low alloy steel powder. Different compaction strategies have been used to produce ring shaped parts, characterized by different H/(Dext-Dmt) ratios (0.5, 1,1.5, 2) and different green densities (6.7, 6.9, 7.1 g/cm3). The recorded data have been used to derive the mechanics relationships governing densification for each powder mix. The results have been compared to highlight the innuence of lubricant type and amount.
Abstract: Sintering shrinkage of uniaxially cold compacted axi-symmetrical parts is anisotropic. Not only is the shrinkage of height (parallel to the compaction direction) different from that of the diameters (in the compaction plane), but even the dimensional changes of the inner and of the outer diameters differ significantly. This behaviour has been investigated on iron rings with different geometry and green density. In the compaction plane radial and tangential shrinkages are different, and tangential shrinkage significantly changes along the wall thickness. This is due to the distribution of radial and tangential stresses during cold compaction that in turn depends on the axial stress. The stress field and its dependence on both green density and geometry were hypothesized to propose a working direction to interpret the anisotropy of sintering shrinkage in real parts.
Keywords: Geometry | Green density | Sintering shrinkage
Abstract: In previous works the deformation of the powder column during uniaxial cold compaction was studied by means of experimental data, also deriving relationships with densification and the axial and radial stresses distribution. Cylindrical specimens were considered in all the previous works. Aiming at enlarging the field of application of the results, obtained by the data continuously recorded by an industrial press without any additional device, the influence of geometry is investigated in this work. Rings characterised by different H/T ratios (Height on Thickness equal to 1, 2, 3, 4), considerably higher than in the previous works, were produced using different compaction strategies. Densification curves are derived, where density is reported as a function of the pressure applied by the upper punch, also highlighting density gradients. The deformation of the powder column is derived from experimental data and related to the densification. The results are compared with those obtained investigating cylindrical specimens.
Abstract: The study deals with the investigation of a sol-gel/nanoclay composite coating as a protective layer for the corrosion protection of hot dip galvanized (HDG) steel. Sol-gel coatings prepared from glycidoxypropyltrimethoxysilane, tetraethoxysilane and methyltriethoxysilane with 2. wt.% of sodium montmorillonite were investigated. The effect of different curing times (2, 5, 10, 15. min) and different curing temperatures (90 °C, 150 °C, 180 °C) on the protection properties of the hybrid layer was investigated. ToF-SIMS analyses and FT-IR measurements highlighted that the higher is the curing temperature, the higher is the densification of the sol-gel network. FT-IR spectroscopy measurements underlined that the Si-O-Si/Si-OH ratio rises by increasing curing time or temperature. ToF-SIMS analyses indicate a lessening in the coating thickness by increasing the curing temperature. The densification of the network through formation of siloxane bonds was found to be responsible for the improved protection offered by the sol-gel layer which leads to a remarkable drop of the cathodic current density and an increase of the resistance associated to the sol-gel film.
Keywords: Corrosion | EIS | FT-IR | HDG | Sol-gel
Tieniti in contatto con
Per qualunque informazione non esitare a
contattare la Segreteria ADM tramite le modalità
previste nella sezione Contatti.
Se vuoi diventare membro dell'Associazione ADM, visiona
la pagina "Diventa Socio".
Iscriviti e sarai informato su tutte le
novità riguardanti l'Associazione