Metal Droplet Deposition Research Articles

Numerical and experimental investigation of molten metal droplet deposition applied to rapid prototyping

Rapid prototyping based on molten metal droplets deposition is an additive process in which parts are produced from molten materials in a single operation without the use of any mold or other tooling. Near-net shaped parts are fabricated by sequentially depositing molten droplets layer by layer. This paper presents a systematic numerical and experimental investigation of the transient transport phenomenon during the droplets impinging onto a substrate surface. The 3D models based on a volume of fluid (VOF) method were developed to investigate the deposition of molten metal droplets on a horizontally aluminum substrate surface. Based on the above research, a semiquantitative relationship between external morphology and internal microstructure was proposed, which was further certified by investigating the piled vertical columns and the three-dimensional parts. The works should be helpful for the process optimization and non-destructive detection of drop-based rapid prototyping techniques.

Investigation of molten metal droplet deposition and solidification for 3D printing techniques

This study investigated the transient transport phenomenon during the pile up of molten lead-free solder via the inkjet printing method. With regard to the droplet impact velocity, the distance from nozzle to substrate can be controlled by using the pulse voltage and distance control apparatus. A high-speed digital camera was used to record the solder impact and examine the accuracy of the pile up. These impact conditions correspond to We = 2.1–15.1 and Oh = 5.4 × 10−3–3.8 × 10−3. The effects of impact velocity and relative distance between two types of molten droplets on the shape of the impact mode are examined. The results show that the optimal parameters of the distance from nozzle to substrate and the spreading factor in this experiment are 0.5 mm and 1.33. The diameter, volume and velocity of the inkjet solder droplet are around 37–65 μm, 25–144 picoliters, and 2.0–3.7 m s−1, respectively. The vertical and inclined column structures of molten lead-free solder can be fabricated using piezoelectric ink-jet printing systems. The end-shapes of the 3D micro structure have been found to be dependent upon the distance from nozzle to substrate and the impact velocity of the molten lead-free solder droplet.

Building of nested components by a double-nozzle droplet deposition process

According to the nested components jointed with multiple parts,a double-nozzle droplet deposition process was put forward in this paper, and the experimental system was developed. Through the research on the properties of support materials and the process of double-nozzle droplet deposition, the linkage control of the metal droplet deposition and the support material extrusion was realized, and a nested component with complex construction was fabricated directly. Compared with the traditional forming processes, this double-nozzle deposition process has the advantages of short cycle, low cost and so on. It can provide an approach way to build the nested parts.

Impact-driven ejection of micro metal droplets on-demand

On-demand metal droplet deposition will be a cornerstone technology in 3D metal printing. However, suitable small nozzles are hardly available, limiting the resolution and surface finish of final products. Here, the ejection of record-small metal droplets with a diameter of only 0.55±0.07 times the nozzle diameter was demonstrated. To this end, a novel metal drop-on-demand (DoD) generator for high-temperature metal processing was designed and manufactured. A metal rod was utilized to transfer a vibration pulse, which was required to eject a liquid droplet, from a low-temperature region to the high-temperature liquid metal close to the nozzle. The influence of the pulse characteristics on the droplet ejection regime was studied experimentally and numerically. A 2D axisymmetric numerical model revealed that the shorter pulses allow reducing the droplet size, with the pulse duration of 13μs resulting in the smallest feasible droplets. A novel method to create such short pulses, by impacting the metal-ring connected rod with a solid impactor was manufactured and tested, and the benefits of this method over more the spring-type pulse transfer was experimentally confirmed. This research provides a feasible way to achieve ejection of the small metal droplet on-demand.

Influence of Interfacial Bonding between Metal Droplets on Tensile Properties of 7075 Aluminum Billets by Additive Manufacturing Technique

7075 aluminum billets were fabricated by micro droplet deposition manufacturing technique, and the influence of interfacial bonding between metal droplets on the tensile properties was studied. Three sets of samples were manufactured under different temperature conditions, and their mechanical properties were compared. The results show that the temperature of the metal droplets and substrate significantly affect the tensile strength of the sample. Moreover, with proper temperature setting, the 7075 aluminum billets manufactured by micro metal droplet deposition could achieve very good mechanical properties with a tensile strength of 373 MPa and an elongation of 9.95%, which are very similar to those of an extruded sample. Moreover, a metallurgical bonding diagram based on numerical calculations of interfacial temperature was established to predict the interfacial bonding state. In addition, the fracture morphologies of these specimens were observed. It is indicated that there was a significant transformation of failure mechanism with the improvement of metallurgical bonding, which agreed well with the numerical results.

Transient behaviour of deposition of liquid metal droplets on a solid substrate

This paper investigates the mechanisms that contribute to the spreading of liquid metal macro-drop deposited during Stationary Pulsed Gas Metal Arc Welding on an initially cold solid workpiece. Surface tension and inertial effects take an important part in the behaviour of the liquid metal macro-drop, but in this configuration the influence of energetic effects can also be significant. The experimental results are discussed in the light of dimensional analysis in order to appreciate the influence of the process parameters and the physical mechanisms involved on the spreading of a macro-drop. A law is established to model forced non-isothermal spreading.

The fusion process of successive droplets impinging onto a substrate surface

Successive deposition of uniform metal droplet is a new kind of 3D printing and rapid prototyping technology. This paper presents a systematic numerical investigation of the transient transport phenomenon during the fusion of successive droplets impinging onto a substrate surface. The physical mechanisms of the fusion process, including the bulk liquid, capillarity effects at the liquid–solid interface, heat transfer, and solidification, are identified and quantified numerically. The 3D models based on a volume of fluid method were developed to investigate the successive deposition of molten metal droplets on a horizontally aluminum substrate surface. The numerical models are validated with experiments. The comparison between numerical simulations and experimental findings shows a good agreement. The effects of relative distances between two successive molten droplets on the end-shapes of impact regime are examined. This investigation is essential to implement effective process control in metal micro-droplet deposition manufacture.

Development of a Facility for Molten Metal Micro-droplets Generation. Application to Microfabrication by Deposition

The development of a facility for the generation and deposition of molten metal droplets of small size (on the order of 1 mm) is described. The facility will allow accurately synchronize the instants at which the micro droplets are generated and fall in the vertical deposition direction (Z axis) with the movement (X and Y axes) of the substrate located below the micro-droplet generator. A great challenge of this type of processes is to find optimal operating conditions that allow us to manufacture with sufficient accuracy tridimensional metallic micro structures, more or less complex, which can be of interest in the electronic or additive manufacturing industry. These optimal conditions are affected by many factors including the oxidation phenomena that usually occur in many liquid metals. These oxidation phenomena tend to form an oxide layer on the molten metal surface exposed to the ambient air, which may greatly modify the viscosity, wetting capability or surface tension of the liquid droplet. A detailed analysis of the effect of the oxygen content in the atmosphere where the experiments are made on the outcome of the droplet impact on a solid surface is carried out in this work. The results of this analysis show a strong influence of the surface oxidation on the droplet dynamics before and after the impact.

Remelting and bonding of deposited aluminum alloy droplets under different droplet and substrate temperatures in metal droplet deposition manufacture

Micro-void and cold lap are the most common defects of 3D components in aluminum alloy droplet deposition manufacture, which have been found to be associated with the poor remelting and metallurgical bonding between droplets under inappropriate droplet and substrate temperatures. To address this problem, the appropriate temperature condition for achieving good metallurgical bonding between droplets was analyzed and the relationships among surface temperature (Tsurf), substrate temperature (Tb), droplets temperature (Tdrop), droplets deposition frequency (f) and height of deposition layer (h) were estiblished by one-dimensional (1D) heat transfer analytical model. On this basis, a three-dimensional (3D) transient finite element mode was developed to simulate the thermal behavior during metal droplet deposition component using element birth and death technology. The distribution and variation of surface temperature of previously deposited droplets (Tsurf) were predicted and analyzed under different process parameters in fabricating aluminum alloy 3D components. And then, a series of deposition experiments were conducted with different droplet temperatures (Tdrop) and substrate temperatures (Tb). The experimental results show that the good remelting and metallurgical bonding between droplets were obtained with the appropriate combination of Tdrop (700°C) and Tb (450°C). The micro-void and cold laps were not observed in the interior of formed samples, indicating that experimental results basically agree with the theoretical analysis and simulation results. The work provides a useful theoretical and experimental guide for metal droplets deposition manufacture.

3D numerical simulation of successive deposition of uniform molten Al droplets on a moving substrate and experimental validation

The deposition of uniform metal droplets has attracted extensive interest for potential application in rapid prototyping and manufacturing. In this paper, a three-dimensional model based on a volume of fluid (VOF) method was developed to investigate the successive deposition of molten Al droplets on a horizontally moving substrate. In this model, protecting gas surrounding the droplets was treated as a void region, and then the droplets were considered as a single-phase fluid. The spreading, cooling and solidification processes of droplets were investigated under different substrate velocities. The simulated final morphology of droplets agreed well with experiments carried out under the same conditions. It was found that the molten Al droplets solidified in a layer by layer mode due to the high heat conductivity. A series of L-shaped ridges appeared on the surface of solidified droplets, which resulted from the integrated effects from solidification and oscillation consisting of alternately spreading and recoiling of droplets.

Predication and measurement of deflected trajectory and temperature history of uniform metal droplets in microstructures fabrication

The deposition of metal droplets has been attracted wide interests for potential applying in microstructures fabrication. A uniform droplet generator with image acquiring system and droplet temperature measuring system was established. A coupling analysis method on combination of droplets dynamic and thermal models was proposed. Effect of experimental parameters on deflected trajectory and the temperature history of the uniform droplets was investigated. The measured trajectory and temperature history agreed well with the predicted results, indicating the validity of the proposed analysis method. The predicted results with different experimental parameters showed that the deflected trajectories and thermal histories of the droplets with diameters close to 100 μm or less were strongly dependent on their sizes. Small droplets had a narrow deposition length for obtaining parts with good metallurgical bonding. The predicted results also showed that it was effective in controlling the deflected trajectories and thermal histories of droplets by adjusting the pressure when it was low. At last, micro alloy tubes with good metallurgical bonding were obtained using 50 μm diameter nozzle when the experimental parameters were chosen according to theoretical models. The deposited results show that proper parameters for microstructures deposition can be obtained according to the proposed analysis method.

Experimental Research of Process Parameters in Metal Droplet Deposition Manufacturing

In the droplet-based manufacturing process, the accuracy and density of forming parts are determined by experimental parameters such as the velocity and temperature of substrate, droplets spraying frequency, line deposition space and layer thickness etc. Using Sn60-Pb40, the droplets deposition experiment was conducted to investigate the effect of the parameters on the accuracy and density of formed lines, layers and solids on the experimental system. The experiment results showed that the high quality lines were obtained when the overlapping ratio of the adjacent droplets was controlled around 30% by the substrate velocity and droplet spraying frequency, meanwhile，the temperatures of the substrate and droplet were maintained at 270°C and 140°C, respectively. When the lines deposited space was about 90% of droplet diameter and the layer thickness was about 80% of droplet diameter, good deposition results of layers and solids can be obtained. This work has offered experimental guide for metal droplet deposition manufacturing.

A 3D computational model of heat transfer coupled to phase change in multilayer materials with random thermal contact resistance

An improved understanding of the heat transfer in materials consisting of two layers (splat and substrate) is essential for many industrial applications. We are interested in the deposition, rapid cooling and solidification of metal droplets (known as splats) brought into contact with a cold substrate. We therefore need to understand the temperature history in both the splat and the substrate, including phase change phenomena. A new model of the thermal contact resistance based on a random distribution of contact points, rather than the uniform distribution commonly used, is presented in this paper. Phase change has been also considered, using the enthalpy-porosity formulation. Simulations have been conducted with the commercial package CFX-4. The computational results for the cooling rate of the splat obtained using the random contact distribution model are in good agreement with available experimental results. In addition, results obtained from the random model provide information on the inhomogeneity affecting the temperature at the interface between the splat and the substrate.

Building three‐dimensional objects by deposition of molten metal droplets

PurposeThe purpose of this paper is to determine conditions under which good metallurgical bonding was achieved in 3D objects formed by depositing tin droplets layer by layer.Design/methodology/approachMolten tin droplets (0.18‐0.75 mm diameter) were deposited using a pneumatic droplet generator on an aluminum substrate. The primary parameters varied in experiments were those found to most affect bonding between droplets on different layers: droplet temperature (varied from 250 to 325°C) and substrate temperature (varied from 100 to 190°C). Droplet generation frequency was kept low enough (1‐10 Hz) that each layer of droplets solidified and cooled down before another molten droplet impinged on it.FindingsIn this paper, a one dimensional heat transfer model was used to predict the minimum droplet and substrate temperatures required to remelt a thin layer of the substrate and ensure good bonding of impinging droplets. Cross‐sections through samples confirmed that increasing either the droplet temperature or the substrate temperature to the predicted remelting region produces good bonding between deposition layers.Originality/valueThis paper used a practical model to provide reasonable prediction of conditions for droplet fusion which is essential to droplet‐based manufacturing. The feasibility of fabricating 3D metal objects by deposition of molten metal droplets has been well demonstrated.

Research on lateral instability of the uniform-charged droplet stream during droplet-based freeform fabrication

A droplet initial displacement assumption was made and a dynamic model of the charged droplets was developed to understand the lateral instability of the uniform-charged droplet stream for freeform fabrication. The distribution of charged droplets depending on deposition distance was obtained based on this dynamic model after the droplet charge was calculated by the FEM. Then, a droplet charging and depositing apparatus is set up to generate the uniform-charged droplet stream. The droplet charge was measured and the droplet distributions at different distances were collected in the experiment. The results show that the theoretical radius of droplet distribution area agrees well with the mean experimental value at the deposition distance of 50 cm. The theoretical trend of droplet stream dispersion fits well with experimental result. The maximum accurate deposition distance within which the radius of distribution area is less than one droplet diameter is about 27 cm according to the prediction of droplet distribution, which provides the basis for further accurate metal droplet deposition.

Drop-on-demand deposition of superheated metal droplets for selective infiltration manufacturing

Abstract The deposition of metal droplets is of great engineering interest because of its practical applications to such various fields as spray forming, surface treatments, and solid freeform fabrication (SFF), etc. The objective of this work is to develop a new drop-on-demand generator of superheated metal droplets that is suitable for the selective infiltration manufacturing (SIM), a new SFF process for the direct fabrication of metal parts. The developed generator comprises a solenoid vibrator, a micro-drilled 130 μm diameter nozzle, an impulse-transmitting rod, and a tubular heating element. A parametric study showed that the diameter of droplets was 301 ± 10 μm when the superheating temperature of Sn–37Pb wt% was set at 260 °C, 299 ± 12 μm at 290 °C, and 301 ± 12 μm at 320 °C. Experiments showed that the developed generator is able to deposit superheated metal droplets according to the frequency of the vibrator.

Parametric study of metal droplet deposition and solidification process including contact resistance and undercooling effects

The effects of process parameters on metal droplet spreading and solidification are investigated numerically. The parameters include the impinging velocity, contact resistance between the droplet and substrate, and degree of undercooling associated with rapid phase change. A simplified model is adopted to describe the splat size evolution and energy equations for the liquid, solidified layer, and substrate are simultaneously solved to analyze heat transfer processes during solidification. The energy equations are coupled by boundary conditions such as contact resistance and undercooling in a regularized calculation domain formed by means of algebraic grid generation. The results reveal that impinging velocity and contact resistance have strong effects on both final splat size and solidification time. The effects of undercooling are not significant unless the nucleation temperature is low in relation to the initial liquid superheating.

Charged Molten Metal Droplet Deposition as a Direct Write Technology

ABSTRACTThe formation of highly uniform charged molten metal droplets from capillary stream breakup has recently attracted significant industrial and academic interest for applications requiring high-speed and high-precision deposition of molten metal droplets such as direct write technologies. Exploitation of the high droplet production rates intrinsic to the phenomenon of capillary stream break-up and the unparalleled uniformity of droplet sizes and speeds attained with proper applied forcing to the capillary stream make many new applications related to the manufacture of electronic packages, circuit board printing and rapid prototyping of structural components feasible. Recent research results have increased the stream stability with novel acoustic excitation methods and enable ultra-precise charged droplet deflection. Unlike other modes of droplet generation such as Drop-on-Demand, droplets can be generated at rates typically on the order of 10,000 to 20,000 droplets per second (depending on droplet diameter and stream speed) and can be electrostatically charged and deflected onto a substrate with a measured accuracy of ±12.5 µm. Droplets are charged on a drop-to-drop basis, enabling the direct writing of fine details at high speed. New results are presented in which fine detailed patterns are “printed” with individual molten metal solder balls, and issues relevant to the attainment of high quality printed artifacts are investigated.

Molten droplet solidification and substrate remelting in microcasting Part I: numerical modeling and experimental verification

This paper presents a numerical model of a molten metal droplet impinging, solidifying and bonding to a solid substrate. The physical and numerical model includes dissimilar materials, multi-dimensional axisymmetric heat transfer, tracking of solid/liquid interfaces during remelting and solidification, and coupled treatment of the continuous droplet/substrate region. The numerical model solves for the evolution of the temperature distribution in the droplet and substrate, predicts the position of the remelting and solidification fronts, and accounts for convective motion. The effect of the convection induced by the droplet spreading is modeled through a time-dependent effective thermal conductivity. High-speed filming of the molten droplet impinging and spreading on the substrate is performed to obtain the required parameters to determine this time dependent effective conductivity. The accuracy of the model is investigated with experimental techniques. This research is directly related to the development of microcasting Shape Deposition Manufacturing (SDM) which is a process for automatically fabricating complex multi-material objects by sequentially depositing material layers. Microcasting is a molten metal droplet deposition process in SDM, which is able to create fully dense metal layers with controlled microstructure. Important issues in the production of high quality objects manufactured with microcasting SDM are: attainment of interlayer metallurgical bonding through substrate remelting, control of both substrate and droplet cooling rates, and minimization of residual thermal stresses. To validate experimentally the numerical modeling approach, predicted cooling rates are compared with thermocouple measurements and substrate remelting depths are verified through optical metallographic techniques.

A comparison of models, numerical simulation, and experimental results in droplet deposition processes

The deformation and solidification of a droplet impinging on a flat surface has been studied extensively because of the wide variety of applications pertinent to this process. In the present work, predictions of the solidification and deformation behavior of the droplet deposition process obtained with a semi-analytical metal-droplet deposition model and by numerical simulation are compared with available experimental results. Cases with and without solidification are considered. The results provide some insight concerning the validity of the assumptions underlying the various approaches. In general the numerical simulation approach yields very good predictions of the splat deformation and solidification process parameters such as maximum splat radius and solidification time. On the other hand, the semi-analytical approach provides a very good qualitative evaluation of the solidification process.