Macroscale Part Research Articles

Effects of binder droplet size and powder particle size on binder jetting part properties

PurposeIn binder jetting, the interaction between the liquid binder droplets and the powder particles defines the shape of the printed primitives. The purpose of this study is to explore the interaction of the relative size of powder particles and binder droplets and the subsequent effects on macro-scale part properties.Design/methodology/approachThe effects of different particle size distribution (5–25 µm and 15–45 µm) of stainless steel 316 L powders and droplet sizes (10 and 30 pL) on part density, shrinkage, mechanical strength, pore morphology and distribution are investigated. Experimental samples were fabricated in two different layer thicknesses (50 and 100 µm).FindingsWhile 15–45 µm samples demonstrated higher green density (53.10 ± 0.25%) than 5–25 µm samples (50.31 ± 1.06%), higher sintered densities were achieved in 5–25 µm samples (70.60 ± 6.18%) compared to 15–45 µm samples (65.23 ± 3.24%). Samples of 5–25 µm also demonstrated superior ultimate tensile strength (94.66 ± 25.92 MPa) compared to 15–45 µm samples (39.34 ± 7.33 MPa). Droplet size effects were found to be negligible on both green and sintered densities; however, specimens printed with 10-pL droplets had higher ultimate tensile strength (79.70 ± 42.31 MPa) compared to those made from 30-pL droplets (54.29 ± 23.35 MPa).Originality/valueTo the best of the authors’ knowledge, this paper details the first report of the combined effects of different particle size distribution with different binder droplet sizes on the part macro-scale properties. The results can inform appropriate process parameters to achieve desired final part properties.

Hopping Light Vat Photopolymerization for Multiscale Fabrication.

3D objects with features spanning from microscale to macroscale have various applications. However, the fabrication of such objects presents challenges to additive manufacturing (AM) due to the tradeoffs among manufacturable feature resolution, maximum build area, and printing speed. This paper presents a projection-based AM process called hopping light vat photopolymerization (HL-VPP) to address this critical barrier. The key idea of HL-VPP is to synchronize linear scanning projection with a galvo mirror's rotation. The projector moves continuously at a constant speed while periodically rotating a one-axis galvo mirror to compensate for the projector's linear movement so synchronized hopping motion can be achieved. By this means, HL-VPP can simultaneously achieve large-area (over 200mm), fast-speed (scanning speed of 13.5mm s-1 ), and high-resolution (10µm pixel size) fabrication. The distinguishing characteristic of HL-VPP is that it allows for hundreds of times lower refresh rates without motion blur. Thus, HL-VPP decouples the fabrication efficiency limit imposed by the refresh rate and will enable super-fast curing in the future. This work will significantly advance VPP's use in applications that require macroscale part size with microscale features. The process has been verified by fabricating multiple multiscale objects, including microgrids and biomimetic structures.

TOPOLOGY OPTIMIZATION OF ADDITIVE MANUFACTURING COMPONENTS WITH LATTICE STRUCTURES FOR MULTIPHYSICS APPLICATIONS

This paper aims to optimize additive manufacturing parts containing lattice structures by means of topology optimization. The proposed method relies on numerical homogenization to model the behavior of the mesoscale lattices inside the macroscale component with an equivalent material model. While the topology of the unit cell is fixed a priori, the material constitutive model is considered to be a function of multiple size parameters of the lattice struts and the orientation of the lattice cell. Both the size parameters and the orientation are considered as design fields in the design domain of the optimization in order to optimize the distribution of orthotropic lattice structures while adapting their orientation to the macroscale loads. In addition, the shape of the macroscale part is described by a separate level set function. Besides avoiding the appearance of degenerate lattice structures, the strict control over the macroscale shape permits the inclusion of different manufacturing constraints. The design fields describing the lattice geometry and macroscale shape are parametrized explicitly by filtering sets of independent optimization variables. This explicit design parametrization allows one to simultaneously optimize the shape of the macroscale part and the lattice structures inside the part. Moreover, the optimization problem is expressed and solved as a standard nonlinear programming problem, which allows one to easily adopt the formulation for different design problems. This interesting feature is illustrated in the numerical examples which deal with different physical problems, and several objective and constraint functions.

A Low-Cost Electrochemical Metal 3D Printer Based on a Microfluidic System for Printing Mesoscale Objects

For the additive manufacturing (AM) of metal objects, the powder-based fusion (PBF) method is routinely utilized to fabricate macroscale parts. On the other hand, electrochemical additive manufacturing (ECAM), in which metallic structures are deposited through the electrochemical reduction of metal ions, is a promising technique for producing micro- and nanoscale objects. However, a gap exists in terms of fabricating mesoscale objects within the current AM techniques. The PBF method is limited by fabrication precision due to pronounced residual stresses, and most current ECAM systems are difficult to scale up to print mesoscale objects. In the present paper, the novel design of a low-cost ECAM 3D printer based on a microfluidic system is proposed for fabricating mesoscale metal parts. The meniscus-guided electrodeposition approach is utilized, in which a meniscus is formed between the print head and substrate, and electrodeposition is confined within the meniscus. A 3D object is fabricated by the meniscus moving with the print head according to the programmed pattern and the material subsequently being deposited at the designated locations. The key to the proposed design is to maintain a mesoscale meniscus, which normally cannot be sustained by the electrolyte surface tension with a print nozzle having a mesoscale diameter. Therefore, a microfluidic system, called the fountain pen feed system, constituting a semi-open main channel and comb structure, was designed to maintain a mesoscale meniscus throughout the printing process. Two materials, copper and nickel, with various geometric shapes were attempted to print by the proposed ECAM system, and, during the printing process, both fluid leaking and meniscus breaking were completely prevented. Free standing tilted copper pillars with controlled angles were printed to show the ability of the proposed design in fabricating 3D structures. A copper circuit was also printed on a non-conductive substrate to demonstrate a possible application of the proposed ECAM system in the fabrication of functional electronics.

Die-less fabrication of miniaturized parts through single point incremental micro-forming

In recent years, demand for miniaturized products has significantly increased in biomedical and micro-electronics industries. In this article, a relatively new sheet forming technique named incremental sheet micro-forming (μISF) is comprehensively investigated to address the emerging need for micro-manufacturing. Using μISF, a metal foil is deformed into complex three dimensional shape by navigating a hemispherical-tip tool over it in a pre-defined toolpath. Experiments are conducted using 50 μm thick AL 1100-O and AL 5052-H19 foils on a table-top numeric controlled machine coupled with force transducer. Measured force data reveal a cyclic nature in axial direction with fluctuations close to 13% that is unprecedented in macro-scale metal parts. This fluctuation can be attributed to either heterogeneous material deformation due to high grain size to thickness ratio or planar anisotropy. The subsequent analysis provides insight into extent of geometric accuracy and enhanced material formability obtained in μISF process. Also, a novel defect is detected due to feature twisting that is never observed in macro-scale parts while forming pyramid shaped geometry. The developed setup is proved to obtain reliable and repeatable results and validates that μISF can provide good alternative for flexible manufacturing of miniaturized products.

Predictive Model for Thermal and Stress Field in Selective Laser Melting Process—Part II

Finite Element Analysis (FEA) is used to predict the transient thermal cycle and optimize process parameters to analyze these effects on deformation and residual stresses. However, the process of predicting the thermal history in this process with the FEA method is usually time-consuming, especially for large-scale parts. In this paper, an effective predictive model of part deformation and residual stress was developed for accurately predicting deformation and residual stresses in large-scale parts. An equivalent body heat flux proposed from the single layer laser scan model was imported as the thermal load to the layer by layer model. The hatched layer is then heated up by the equivalent body heat flux and used as a basic unit element to build up the macroscale part. The thermal history and residual stress fields of two solid parts with different support structures during the SLM process were simulated. Layer heat source method has the capability for fast temperature prediction in the SLM process, while sacrificing modeling details for the computational time-saving purpose. Thus numerical modeling in this work can be a very useful tool for the parametric study of process parameters, residual stresses and deformations.

Buckling, build orientation, and scaling effects in 3D printed lattices

Cellular lightweight structures offer superior mechanical properties when compared to their bulk counterparts. Fabricating them, however, remains a challenge as even the most advanced processes exhibit considerable limitations. Material jetting, also referred to as (inkjet) 3D printing, offers multi-material capability, which, in combination with soluble support material, makes it a capable alternative to conventional processes. However, while the process is well investigated on a macro scale, these findings do not apply to small feature sizes. Here, we study the impact of buckling, build-orientation, and scaling on the mechanical properties at scales that approach the printers’ limits. Strong interaction effects and large discrepancies from macro scale parts are found. Specifically, the findings show that complete stress–strain curves can be accurately predicted when these effects are considered, whereas conventional material models tend to overestimate the mechanical properties of bend- and stretch-dominated lattices by up to an order of magnitude. This is particularly important for safety-critical and highly efficient parts. Further, the findings might help to establish the material jetting process in the field of cellular structures, which has to date been dominated by single material processes.

A scalable predictive model and validation for residual stress and distortion in selective laser melting

The unique thermal cycle of rapid melting, solidification, and melt-back in selective laser melting (SLM) induces steep residual stress gradients which lead to part distortion. However, it is extremely difficult to predict residual stress in SLM due to complex multi-physics phenomena and scale-up of millions of laser scans. This paper has developed a geometry scalable predictive model across microscale laser scan, mesoscale layer hatch, and macroscale part build-up to fast predict residual stress in different scanning strategies. The predictions were validated using the L-shaped bar and bridge structures. The geometry scalability law provides an effective tool for optimizing part designs.

Efficient predictive model of part distortion and residual stress in selective laser melting

Selective laser melting (SLM) is a promising technology to manufacture functional (end-use) metal parts with complex geometry directly from CAD data. The process induced high tensile residual stress and part distortion due to the non-uniform heat input during a SLM process would detrimentally affect the part performance. However, it is extremely challenging to predict distortion of a practical SLMed part if each single track is taken into account by using the conventional modeling methods The complex multiphysics phenomenon such as fluid flow in the melt pool, phase transformation during cooling, and resulted anisotropic properties further complicate this issue. In this study, a temperature-thread multiscale modeling approach has been developed to effectively predict residual stress and part distortion of a twin cantilever. An equivalent body heat flux has been proposed from the microscale laser scan model and imported as the “temperature-thread” to the subsequent mesoscale layer hatch model. The hatched layer is then heated up by the equivalent body heat flux and used as a basic unit to build up the macroscale part in a layer by layer fashion. The thermal history and residual stress fields of the twin cantilever during the SLM process were simulated. The predicted cantilever distortion agrees with the measured data with a reasonable accuracy.

Prediction of Residual Stress and Part Distortion in Selective Laser Melting

Selective laser melting (SLM) is widely used to make functional metal parts. The high-temperature process will produce large tensile residual stress (RS) which leads to part distortion and poor product performance. Traditional modeling approaches are not practical to predict residual stress and part distortion due to the exceedingly high computational cost. In this study, two efficient multiscale modeling methods have been developed to across microscale laser scan, mesoscale layer hatch, and macroscale part buildup for fast prediction of residual stress and part distortion. A concept of equivalent heat source has been developed from the microscale laser scan model. In the “stress-thread” method, the local residual stress field was predicted by the mesoscale layer hatch model using the equivalent heat source, then the residual stress field is imported, i.e., “stress-thread”, to the macroscale part buildup model to predict residual stress and part distortion. In the temperature-thread method, the powder–liquid–solid material transition has been incorporated. A body heat flux obtained from the microscale laser scan model is applied, i.e., “temperature-thread”, to the hatch layer. Then multiple hatches are sequentially “deposited” in the macroscale part buildup model with different scanning strategies. The predicted part distortions by both methods were compared and compared with the experimental data.

A multiscale modeling approach for fast prediction of part distortion in selective laser melting

Selective laser melting (SLM) is a powder bed based additive manufacturing process. It is widely used to make functional parts in a layer upon layer fashion. The severe temperature gradients produce large tensile residual stress which leads to part distortion and negatively affect product performance. Due to the complex coupling multi-scale mechanisms, it is a great challenge to predict part distortion since traditional modeling approaches demand an exceedingly long computational time. This study has developed a practical multi-scale modeling methodology for fast prediction of part distortion by integrating a micro-scale laser scan model, a meso-scale layer hatch model, and a macro-scale part model. A concept of equivalent heat source has been developed for the micro-scale laser scan model. Local residual stress field was predicted in the meso-scale layer hatch model using the equivalent heat source. The residual stress field was then imported to the macro-scale model to predict part distortion and residual stress. The predicted part distortions were validated with the experimental data with four different scanning strategies.

The development of a microscale strain measurement system applied to sheet bulge hydroforming

Micro and multiscale sheet metal forming processes represent new and attractive solutions to many manufacturing problems. However, evaluating the strains in these products is a difficult endeavor. Larger organizations are utilizing commercially available microscale digital image correlation systems to measure the strains in microscale parts or on macroscale parts with critical microscale features. The cost of these strain measurement systems is preventing smaller research and development organizations from entering this challenging area or they are forgoing the ability to determine strains. The present paper describes the development of a method for creating microscale grids and measuring strains on microscale parts or microscale locations on larger parts. The method developed was able to measure true strains up to 0.618 for square grids that are 127μm measured from center-to-center. Microscale strains resulting from sheet bulge hydroforming experiments using 11mm, 5mm, and 1mm diameter dies were evaluated and material properties of the sheet metal were estimated based upon the strains measured in conjunction with FEA simulations and compared to analytical solutions and microscale tension tests. The material properties determined using the strains and FEM approach were consistent with the other methods.

A structured approach to integrate MEMS and Precision Engineering methods

Two different kinds of manufacturing technologies are used to produce micro systems or micro structures on macro-scale parts. MEMS (micro-electro-mechanical systems) are produced by using bottom up techniques such as lithography, etching and bonding, which were adopted from well established microelectronic manufacturing processes. Precision Engineering systems apply a top-down approach and use very precise versions of conventional manufacturing techniques such as milling, turning or powder injection molding to produce micro-mechanical parts. At the present time, both technologies face the challenge of long iterative product development cycles before the functionality of the product and the set-up of the production system can be verified. A structured, systematic, and rational design approach including the verification of both the product's functionality and its efficient manufacturability is necessary to enable the fast development and cost-effective production of so-called killer applications. Even though both approaches offer complementary advantages concerning the range of materials, design flexibility and the capability to produce parts in small and large volumes, there have been no attempts to integrate the two technologies up to now. Through the framework of axiomatic design, this paper proposes a structured approach to derive and simultaneously verify micro systems and the necessary manufacturing processes by applying and integrating both Precision Engineering and MEMS techniques.

Validation of Multi-scale Model Describing Microstructure Evolution in Steels

Properties of deformed steels depend on various microstructure parameters such as distribution of grain size and precipitates. Strain, strain rate and temperature inhomogeneities make quantitative prediction of microstructure difficult but the Finite Element method is able to model these inhomogeneities. Different scales of phenomena occurring in deformed materials are another difficulty in modelling. Microstructure evolution can be described by more realistic methods (e.g. Cellular Automata CA, Monte Carlo), which, on the other hand, are unable to simulate larger samples. Therefore, development of the methods capable of spanning multiple scales became a current challenge. CAFE modelling, which couples FE and CA methods, is the objective of the paper. The model consists of two layers. The micro-scale layer, simulated by CA, represents microstructure evolution including nucleation and growth of the grains. Evolution of a dislocation density is described for every grain separately by solving differential equation. The FE thermal-mechanical model is used as a macro-scale part. Multistage plane strain compression tests for niobium steel are considered. Distributions of initial and final grain size are measured during the tests. The results from the CAFE model are compared to the measurements and to the predictions by a conventional model. The comparisons confirm the capability of the CAFE method to predict flow stress, recrystallized fraction and grain size distribution. Conventional approach gives a good agreement with experiments for an average grain size only.

Validation of FE simulation based on surface roughness model in micro-deep drawing

Metal forming technology has been paid great attention as one of the most economical mass production methods for sub-millimeter-scale microparts as well as macro-scale parts. Although tools and dies are essential for the manufacture of microformed parts, the fundamental knowledge and technological data on their fabrication have not been accumulated. Therefore, establishing design manuals and codes for tools, dies and blank materials for microforming is important for realizing high-precision forming. Particularly for miniaturization in sheet metal forming, the surface roughness of tools and blanks are thought to largely affect the processing characteristics and accuracy of products. From the standpoint of the design of tools and blank materials, this study was focused on the surface roughness of tools and blanks in the micro-deep-drawing process, and aimed at clarifying the effect of surface roughness on microforming characteristics. In our previous study, by finite element (FE) simulation considering surface roughness, the effect of tool surface asperity on the drawn cup surface quality in the two-stage micro-deep-drawing process was simulated. In this study, to verify the validity of the FE model of surface roughness in the micro-deep-drawing process, a new high-precision sequential blanking and drawing setup was developed for the experiment and a microcup with 500 μm diameter was fabricated from stainless steel (SUS304) ultrathin foil of 20 μm thickness. For the evaluation of the drawn cup, the cup geometry, thickness strain distribution, and surface roughness were measured. By comparisons of the FE simulation and the experimental results, the validation of the surface roughness model was studied, and the notable influential factors at the microscale were discussed.

Method for polymer hot embossing process development

Molding technologies associated with fabricating macro scale polymer components such as injection molding and hot embossing have been adapted with considerable success for fabrication of polymer microparts. While the basic principles of the process remain the same, the precision with which the processing parameters need to be controlled especially in the case of molding high aspect ratio (HAR) polymer microparts into polymer sheets is much greater than in the case of macro scale parts. It is seen that the bulk effects of the mold insert fixture and molding machine have a dominant influence on the molding parameters and that differences in material parameters such as the glass transition temperature (Tg) of polymer sheets are critical for the success and typically differ from sheet to sheet. This makes it very challenging to establish standard processing parameters for hot embossing of sheet polymers. In the course of this paper, a methodology for developing a hot embossing process for HAR microstructures based on known material properties and considering the cumulative behavior of mold, material, and machine will be presented. Using this method force–temperature–deflection curves were measured with the intent of fine tuning the hot embossing process. Tests were carried out for different materials using a dummy mold insert yielding information that could be directly transferred to the actual mold insert with minimum development time and no risk of damage to the actual microstructures.