Metal Droplet Deposition Research Articles

Formation mechanism and elimination method of joint defects in lattice structures printed by metal droplet deposition manufacturing

Benefiting from the dense inner microstructure and smooth bead-like surface morphology, metal droplet deposition manufacturing (MDDM) is advantageous in manufacturing small metallic lattice structures, which have great potential in advanced aerospace, energy, and robotics applications. However, the improper top-layer droplet distance leads to joint defects in the printed lattices and deteriorates the forming accuracy. Moreover, if the substrate temperature and inclined angle increase, joint defects may occur even if the top-layer droplets are at an ideal distance due to droplet oscillation. This work proposes a novel method to eliminate joint defects by re-placing the joint region droplets to an ideal center distance. To this end, the dynamics of top-layer droplets (impact, spread, recoil, and oscillation) were investigated by combining morphology analysis and high-speed photography (Ts = 673 K, θ = 45°). A theoretical model was established to obtain the parameter range of defect-free joints through a series of bifurcate lattice printing experiments. Then, the morphology and generation mechanism of cumulative joint defects in the lattices at higher temperatures and angles (Ts = 773 K, θ = 60°) were analyzed. A parameter window of joint morphology with different values of inclined angle, substrate temperature, and top-layer droplet distance was established for predicting potential defects in the printing trace. Next, a novel method was proposed to remove redundant droplets and re-place top-layer droplet coordinates into the ideal center distance. Furthermore, the printing strategy was expanded to multi-truss lattices using three-dimensional transformation matrixes. Finally, multiple cells bifurcate, triangular, and pyramid lattices were successfully printed with perfect joints. The height deviation of joint regions is <5 % of the droplet diameter, verifying the feasibility of the proposed strategy. This work represents a further step towards MDDM printing of high-quality truss lattice structures and other complex 3D architectures.

Suppression of the height deviation on metal bumps manufacturing by an ultrasonic vibration control

On-demand droplet printing based on piezoelectric micro-jet device (PMJD) is considered a flexible and high-precision method to generate metal droplets directly for flip-chip bonding in industrial electronics. However, the quality of flip-chip bonding is closely related to the height deviation of the solidified droplets (the metal bumps), which is influenced by the complicated hydrodynamics of impacting and oscillation of the droplet with oxide film. Here, the numerical and experimental investigations are first conducted to study the effect of the liquid bridge and deposition parameters on the height deviation of the solidified droplets. The rapid oxidation of the liquid bridge and under-oscillation during the deposition process are the main reasons for height deviation. In addition, the undamped oscillation with high-speed impact and instantaneous solidification also deteriorates the height deviation. To this end, an oscillation control strategy based on ultrasonic-assisted metal droplet deposition (UAMDD) is proposed and verified to be a reliable regulation strategy to suppress the height deviation of the printed bumps. The effective regulating range of height deviation is studied experimentally by changing the ultrasonic vibration amplitude. Finally, a 10 × 10 array composed of 100 solidified metal droplets is printed with the UAMDD, which has the height deviation of 554 ± 6 μm. And the dimensionless height deviation (Δh/h) of solidified bumps is stayed below 2.1 %.

Ultrasonic vibration micro-jet ejection for metal additive manufacture

In comparison to laser additive manufacturing, metal droplet deposition technology presents substantial cost benefits by obviating the necessity for costly metal powders and high-power lasers. Nonetheless, prevalent metal droplet deposition methodologies often grapple with challenges such as intricate drive architectures, substantial droplet volumes, and diminished jetting frequencies. This paper proposes an ultrasonic vibration micro-jet ejection technique and constructs a prototype for metal additive manufacturing. Utilizing ultrasonic vibration of a tool rod, the technique generates a high-frequency acoustic pressure field within the crucible, which facilitates the ejection of molten metal from the nozzle. The study explores two operational modes: continuous jet and on-demand jet. It was determined that the tool rod's amplitude must exceed a certain threshold to initiate metal ejection. As the amplitude further increases, the droplet jetting behavior transitions from uniform to non-uniform ejection states. To enhance formability quality, the study examined the influence of various process parameters on single-pass and three-dimensional forming. The examination of jetted parts revealed that both uniform and non-uniform jets provide satisfactory formability and mechanical performance. This research presents a new direction for achieving efficient metal droplet jetting forming.

Gas entrapment and pore formation in metal droplet-based 3D printing

Gas entrapment behavior during metal droplet deposition tends to form pore defects and discourages interfacial bonding between droplets, resulting in lower densities and poorer performance of printed components. Moreover, the gas entrapment process is influenced by the interaction of gas-liquid two-phase flow and non-isothermal solidification, which makes the interfacial behavior of droplets challenging to identify. Therefore, gaining insight into the gas entrapment behavior and pore formation mechanism at the bottom of molten droplets is essential. This paper established a model of two-dimensional axisymmetric numerical by combining the fluid volume tracking method and heat transfer solidification model. Single metal molten droplet deposition experiments verified the accuracy of the model. The evolution of solid phase fraction, velocity field, pressure field, and contact surface heat flux during the deposition of the molten droplet at low Weber numbers (0.1-10) was investigated comprehensively. The mechanisms of gas entrapment and pore formation were revealed. The effects of molten droplet impact velocity, contact angle, and surface tension on the gas film evolution process and pore morphology were further investigated. The results show that decreasing the droplet impact velocity causes capillary waves to contact the substrate surface and form local solidification, allowing the gas disk to evolve into central and two peripheral pores. Moreover, the pore breaks away from the substrate and remains inside the droplet when decreasing the droplet contact angle to 50°. Furthermore, the rate of gas disk retraction decreases as surface tension reduces, which may accentuate the resistance effect of the contact surface solidification layer on the gas disk retraction, thus changing the final morphology of the pores. These findings elucidate the mechanism of pore formation at the bottom of molten metal droplets and further understand the interfacial behavior of droplets in 3D-printed metal structures.

Investigations into wetting and spreading behaviors of impacting metal droplet under ultrasonic vibration control

Ultrasonic-assisted metal droplet deposition (UAMDD) is currently considered a promising technology in droplet-based 3D printing due to its capability to change the wetting and spreading behaviors at the droplet-substrate interface. However, the involved contact dynamics during impacting droplet deposition, particularly the complex physical interaction and metallurgical reaction of induced wetting-spreading-solidification by the external energy, remain unclear to date, which hinders the quantitative prediction and regulation of the microstructures and bonding property of the UAMDD bumps. Here, the wettability of the impacting metal droplet ejected by a piezoelectric micro-jet device (PMJD) on non-wetting and wetting ultrasonic vibration substrates is studied, and the corresponding spreading diameter, contact angle, and bonding strength are also discussed. For the non-wetting substrate, the wettability of the droplet can be significantly increased due to the extrusion of the vibration substrate and the momentum transfer layer at the droplet-substrate interface. And the wettability of the droplet on a wetting substrate is increased at a lower vibration amplitude, which is driven by the momentum transfer layer and the capillary waves at the liquid–vapor interface. Moreover, the effects of the ultrasonic amplitude on the droplet spreading are studied under the resonant frequency of 18.2–18.4 kHz. Compared to deposit droplets on a static substrate, such UAMDD has 31% and 2.1% increments in the spreading diameters for the non-wetting and wetting systems, and the corresponding adhesion tangential forces are increased by 3.85 and 5.59 times.

Investigation of the molten droplet deposition offset based on the simple-component pseudopotential model

Metal droplet deposition manufacturing is an additive manufacturing technique that relies on the accurate prediction of deposition position, but is influenced by the wetting degree of solidified surface and the substrate. In this paper, a modified solid discriminant function is proposed for the simple-component pseudopotential model in the lattice Boltzmann method. The results show that the wetting degree of the substrate and the pre-solidified droplet causes the post-deposited droplet to deviate from the initial deposition distance Li* under interfacial tension, producing an offset δL*. However, the solidification effect inhibits spreading and rebound motion, thus changing the actual δL*. Under the solidification effect, different wetting degrees show that the horizontal deviation is more influenced by the pre-solidified droplet than the substrate and can be linearly predicted. The deposition can be divided into two stages depending on the attraction and repulsion motions of the droplets. In the attraction stage, the droplets form a three-phase contact line with the solidification surface, generating the pressure drop with the negative horizontal component under capillary expansion. In the repulsion stage, the second contact line is formed on the substrate, generating a smaller pressure drop in the horizontal direction. The essence of the difference effects of the two surfaces lies in the contacting duration and direction. Based on the principle of deposition deviation proposed in this paper, it will help to optimize the process parameters and improve the molding accuracy.

Metal Droplet Deposition: From Foundation to Engineering Manufacturing

Metal droplet deposition is of fundamental importance in many industrial and manufacturing processes. An in‐depth understanding of the fundamental issues of metal droplet deposition is particularly crucial for developing droplet deposition‐based engineering manufacturing technologies. This review systematically introduces the evolution of metal droplet deposition stages and characteristic behaviors in terms of metal droplet deposition dynamics. The characteristic morphologies of single‐metal droplets after solidification, that is, gas entrapment, ripples, bulging edges, and splashing, are reviewed. The formation mechanisms and influencing factors of different droplet morphologies are summarized. Moreover, the deposition strategy of multiple metal droplets to promote good interfacial bonding is further concluded. Based on these theoretical foundations, three typical engineering manufacturing techniques based on metal droplet deposition, that is, thermal spraying, soldering, and droplet‐based 3D printing, are presented to elucidate further the advantages and potential of metal droplet deposition applied to engineering manufacturing. Finally, some existing problems and future research perspectives are discussed.

Interfacial bonding and corrosion behaviors of HVOF-sprayed Fe-based amorphous coating on 8090 Al-Li alloy

To solve corrosion problems of Al-Li alloys in marine environment, Fe-based amorphous coating was first deposited on 8090 Al-Li alloy by high velocity oxy-fuel spraying. The microstructure, interfacial bonding and corrosion behaviors of the Fe-based amorphous coating are investigated. It is found that the coating is mainly composed of amorphous splats, but a few crystalline phases can be observed in the splats or splat surfaces. Even so, the corrosion resistance of the coated 8090 alloy is 7 times higher than that of the bare 8090 alloy. Moreover, due to the partially melting and rapid cooling of substrate during metal droplet deposition, there is an amorphous transition zone at the AMC/8090 alloy interface, indicating the formation of localized metallurgical bonding. Owing to the metallurgical bonding interface and the high compactness of coating, the adhesion strength of Fe-based amorphous coating can reach 58.2 MPa. In 3.5 wt% NaCl solution, a bipolar passive film can be formed on the Fe-based amorphous coating, and its compactness decreases as the polarization potential increases. Because of the aggregation of Cr element and the uneven passive film, the crystallized splats and crystallized splat surfaces are eroded preferentially, leading to the formation of corrosion channels and the corrosion failure of coating. The present work provides a guidance for the design of corrosion-resistance coatings on Al-Li alloys.

Investigation of dimensional accuracy of metal droplet deposition under repulsion using a lattice Boltzmann approach

Purpose This paper aims to investigate the influence of droplet infiltration and sliding on the deposition size and make a uniform deposition by controlling the interaction between droplets, using the three-dimensional lattice Boltzmann method (LBM) based on the actual working condition. Design/methodology/approach D3Q19 Shan-Chen LB approach is developed and optimized based on the metal droplet deposition. The Carnahan-Starling equation of state and transition layers are introduced to maintain the greater stability and low pseudo velocities. In addition, an additional collision term is adopted to implement immersed moving boundary scheme to deal with no-slip boundaries on the front of the phase change. Findings The numerical results show that the new¬ incoming droplet wet and slide off the solidified surface and the rejection between droplets are the reasons for the deviation of the actual deposition length. The total length of the longitudinal section negatively correlates with the deposition distance. To improve the dimensional accuracy, the deposition distance and repulsion rate need to be guaranteed. The optimal deposition distance is found to have a negative linear correlation with wettability. Originality/value The numerical model developed in this paper will help predict the continuous metal droplet deposition and provide guidance for the selection of deposition distance.

A LATTICE-BOLTZMANN METHOD SIMULATION OF THE HORIZONTAL OFFSET IN OBLIQUE COLUMN DEPOSITION OF ALUMINUM DROPLETS

The metal droplet deposition manufacturing technology adopts a point-by-point stacking method, which provide an unsupported manufacturing method for oblique column deposition with high flexibility. In this paper, a lattice Boltzmann model is established for simulating the continuous deposition process of the oblique column, and the horizontal displacement of the droplet on the solidification surface is studied. According to the charging and discharging process of surface energy, the deposition process is divided into four stages: falling, rapid expansion, slow expansion, and rebound. The forces on the deposited droplet are analyzed by the trend of surface energy, the gravitational potential energy, the kinetic energy, and the viscous dissipation. The internal flow of droplet is sliding in the expansion stage and rolling in the rebound stage. The internal flow of the droplet shows sliding state in the expansion stage and rolling state in the rebound stage. The acceleration of the deviation mainly occurs in the expansion stage, while the deviation distance occurs in the rebound stage. Combined with the forces in the expansion stage, it is concluded that the main driving forces of displacement are gravity and capillary force. With the increase of the droplet axial distance, the acceleration in expansion stage is shortened, and the peak of velocity is increased, so that the horizontal deviation is first increased and then decreased. This staged feature stems from the competitive relationship between the acceleration period and the maximum speed in the deviate motion. Under different deposition heights and solid-liquid wettability, the deviation distance maintains the same trend. Under a certain axial distance, the deviate distance decreases with the increasing solid-liquid wettability, or the increasing deposition height. The evolution tendency of the horizontal deviation distance is fitted, and the scanning step is optimized to realize the uniform deposition of the inclined column whose inclination angle is consistent with the theoretical result.

Generation mechanism and suppression method of landing error of two successively deposited metal droplets caused by coalescence and solidification

Accurate deposition of metal droplets is crucial to limit geometrical defects in metal droplet-based 3D printing. However, unpredictable movements of the droplets displaced from the original landing location due to surface tension-driven flow before it is hindered by solidification, named landing error, lead to broken traces, irregular waved surfaces, or collapsed structures. In the present work, the effect of solidification at the early-time coalescence of two deposited metal droplets on the landing error was revealed. High-speed images were presented to observe coalescence dynamics. A volume of fluid (VOF) based numerical model was developed to analyze the generation mechanism of the landing error. An interesting phenomenon is observed that the second droplet absorbs more mass from the first droplet to form a bigger spherical crown with the increase of substrate temperature, resulting in the distinct final shapes. Inspired by this, a mass conservation model was proposed to predict the dependence of the landing error on substrate temperature. Metal droplets overlapping experiments were further conducted to verify the model and establish the relationship between the landing error and the substrate temperature. Predictions agreed well with experimental results. Finally, a strategy of increasing the thermal conductivity of the substrate to suppress the landing error at high substrate temperatures was proposed, which significantly expanded the processing window of the accurate deposition. This work can provide useful theoretical and experimental guidance for accurate printing of high-quality lines or patterns by utilizing metal droplets.

Suppression of gravity effects on metal droplet deposition manufacturing by an anti-gravity electric field

Droplet-based 3D printing is a promising technique for metal forming in space, owing to the advantages of dispensing without large-scale energy equipment or customized materials. However, the extreme environment in space, for example microgravity, would change the printing mechanisms significantly compared with those on the ground, which hinders the application of droplet-based 3D printing in space. To develop a droplet-based 3D printing technique suitable for space manufacturing, the droplet deposition behavior under microgravity should be physically simulated in normal gravity environment. Here, a novel experimental system comprising an anti-gravity electric field was developed to suppress the gravity effects on droplets deposition by manipulating the droplets to perpendicularly deposit on a vertical substrate. Parameter mappings of regulation voltage were established based on theoretical modeling and experimental results. The controllable region of flight trajectories locates in the top half of the electric field. The charging voltage shows better regulation accuracy than the deflection voltage. Under the manipulation of the electric field, droplets deposit on the substrate and solidify into regular morphologies. The solidified contour and adhesion force of the deposited droplets were measured. Finally, a freestanding pillar and a bending wall structure were fabricated. This work demonstrates the feasibility and effectiveness of the anti-gravity electric field in manipulating droplets to perpendicularly deposit on vertical surfaces against gravity, and forming 3D structures.

The Additive Manufacturing Process of Electric Power Fittings Fabricated by Metal Droplet Deposition

Metal droplet deposition is a kind of additive manufacturing (3D Printing) technique that fabricates near-net part through droplets deposition with lower cost and higher efficiency. This paper proposed a solution to problems of electric power fittings that large inventories, high procurement costs, low manufacturing efficiency and transportation cost. Using additive Manufacturing technique - metal droplet deposition, electric power fittings fabricated on power construction site. This paper describes the manufacturing process of typical thin-walled samples (the structure optimized based on additive manufacturing principle) and ball head rings of electric power fittings. Aiming at the integral AM forming for ball and ball socket electric power fitting workpiece, a novel easy removal forming support material (ceramics and gypsum mixed with UV cured resin) have been developed. Here this support material was used to fabricate nested integral workpieces. Dimensional accuracy and microstructure of the test pieces were analyzed. The error of the height and width of the forming workpiece is within 5%. No obvious overlap trace (such as overlap line and cracks) observed, and the internal microstructure is equiaxial crystal. The average density of the component is 99.51%, which measured by drainage method and 13.39% higher than the cast raw material.

Hole-defects in soluble core assisted aluminum droplet printing: Metallurgical mechanisms and elimination methods

Using droplet-based 3D printing to directly fabricate microwave devices with smooth inner surfaces remains an elusive goal, because of the naturally scalloped shapes of solidified droplets and the staircase effect in 3D printing. In order to address this issue, a novel manufacturing process involving deposition of molten metal droplets on soluble cores is proposed. However, complete elimination of hole-defects formed during this manufacturing process is very challenging. This paper presents the study on the transport phenomena among four neighboring droplets using both numerical modeling and experimental methods. The formation mechanisms of hole-defects and their elimination are also discussed. The results show that the hole-defects on the inner surfaces are caused by the incomplete fusion and filling of residual liquid metal between neighboring droplets. Furthermore, these defects can be effectively eliminated by selecting the proper temperatures of droplets and substrates. Finally, a modified parameter mapping of solidified morphologies about the inner surfaces is established to help with printing temperature selection.

Parametric mapping of linear deposition morphology in uniform metal droplet deposition technique

The parametric mapping of linear deposition morphology with different droplet spacing is first proposed, which provides a baseline for parameter selection in uniform metal droplet deposition technique. Experiments related to linear deposition are carried out with deviating droplet size and deposition placement. Deposition morphology is first categorized into inclined region, wave-shaped region and horizontal region. Horizontal region is further classified into overlapped sub-region, clustered sub-region and discrete sub-region. Mathematical models about classification of deposition morphology are established by these initial parameters: offset ratio, merge ratio, solidification angle, and maximum spreading diameter. After mathematical models are established, boundaries between regions are extracted, and then verified by CCD (Charge-coupled Device) photographs and experimental data. Based on the above models and experiment results, the parametric mapping is found and verified. The following conclusions can be drawn: uncontrollable wave-shaped and clustered regions are not suitable for forming parts, hence the selected process parameters should be avoided in the above regions; Controllable inclined, overlapped and discrete regions can be used to forming part, hence the process parameters should fall into these regions. Furthermore, some possible applications are explored corresponding to three controllable regions with the help of the parametric mapping.

Annular laser beam based direct metal deposition

The paper provides a description of a direct annular laser beam based metal deposition head, which makes use of axial material feeding and variation of the laser beam intensity distribution (LBID) on the surface of the workpiece. The application and advantages of the developed head are presented by processes of direct metal droplet, metal wire and powder deposition. Due to the achieved process symmetry, the stability and robustness of the processes are increased. This is the most clearly seen in the case of metal droplet deposition, which is an inherently unstable process. In the case of wire deposition, a precise pre-set of the proportion of energy input into the workpiece surface is enabled, which has been shown to be an important parameter affecting the stability and process outcome. In the case of laser powder deposition, the benefits of the developed head are reflected in achieved powder catchment efficiency greater than 80 % and in the head’s ability to influence the deposited layer properties by adjusting the LBID.

Morphology Analysis and Process Research on Novel Metal Fused-coating Additive Manufacturing

Existing metal additive manufacturing equipment has high capital costs and slow throughput printing. In this paper, a new metal fused-coating additive manufacturing (MFCAM) was proposed. Experiments of single-track formation were conducted using MFCAM to validate the feasibility. The low melting alloy was selected as the forming material. Then, the effect of process parameters such as the flow rate, deposition velocity and initial distance on the forming morphology. There is a strong coupling effect between the single track forming morphology. Through the analysis of influencing factors to improve the forming quality of specimens. The experimental results show that the twice as forming efficiency as the metal droplet deposition. Additionally, the forming morphology and quality were analyzed by confocal laser scanning microscope and X-ray. The results show that the metal fused-coating process can achieve good surface morphology and without internal tissue defect.

Experimental Investigation of the Overlap Process in Metal Droplet Successive Deposition and Solidification

This paper presents a systematic numerical and experimental investigation of the transient transport phenomenon during the overlap of successive aluminum droplets impinging onto a substrate surface. The 3D models based on a volume of fluid (VOF) method were developed to investigate the successive deposition of molten metal droplets on a horizontally fixed aluminum substrate surface. The numerical models are validated with experiments. The comparisons between numerical simulations and experimental findings show a good agreement. The effects of various parameters such as impact velocities, substrate temperatures and droplet diameters on the maximum spread factor during impacting and spreading with solidification of a molten droplet onto an aluminum substrate surface under different parameters were studied. Based on the above research, the relationships between external morphology and internal microstructure were proposed, which was further certified by investigating the overlap of successive droplets.

Research on the manufacturing of electrical power fittings based on metal droplet deposition

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. For larger inventories, higher purchase cost, and lower production efficiency of Electrical power fittings now; this paper puts forward an innovation method to manufacture Electrical power fittings using the process of metal droplet deposition, meeting the needs of electrical repairs, while reducing installation costs and achieving zero inventories. In accordance with the structure features, through reasonable selection of process parameters, the typical thin-walled parts and ball head rings of Electrical power fittings were realized; meanwhile, microstructure analysis and performance tests of the parts were carried out. The result showed that the structures were reasonable and the mechanical properties were excellent. The works should be helpful for the process optimization and non-destructive detection of drop-based rapid prototyping techniques.

Geometry control of closed contour forming in uniform micro metal droplet deposition manufacturing

Micro metal droplet deposition manufacturing shows great potential applications in many industrial areas such as micro circuits printing, thin-wall metal parts, porous metal parts, and heterogeneous material parts. However, excessive overlapping of metal droplets in corners deteriorates the quality of printed parts. To solve this problem, the droplet center-to-center distance must always keep uniform and be in an ideal range. First, reasons of excessive overlapping in corners are analyzed and a mathematical model is proposed. Then droplet center-to-center distance is optimized and compensated according to corner angle of contour lines and number total of droplets so that the distance between adjacent droplets is proper. The coordinate of rearranged droplets is obtained by calculation. To verify this method, uniform solder droplets were ejected and deposited on a flat substrate. A series of deposition experiments were carried out. Some triangle contours were formed. The results show that quality of the formed parts had been significantly improved by using the proposed method.