Direct fabrication of unsupported inclined aluminum pillars based on uniform micro droplets deposition

https://doi.org/10.1016/j.ijmachtools.2017.01.001Get rights and content

Highlights

  • A method fabricating directly unsupported inclined aluminum pillars is proposed.

  • The correspondence between overlapping ratio and inclined angle is established.

  • Good metallurgical bonding occurs in the fusion interface between droplets.

Abstract

In order to investigate forming directly complex parts without support materials or structures by uniform micro droplets deposition technique, the present work focus on fabricating the unsupported inclined aluminum pillars through offset deposition. An experimental system is developed to produce and deposit uniform molten aluminum droplets. A model is introduced to describe the inclined angle of the droplet deposition at different offset ratios. A one dimensional heat transfer model is proposed to help select the initial temperature parameters of the impinging droplet and the previous solidified droplet to ensure that the fusion occurs. No melting, partial melting and excessive melting region at different offset ratios are determined. The correspondence between offset ratio and inclined angle is considered to be a simple cosine function, and the hypothesis is verified by experiments. The influence of deposition error on an inclined angle of pillars is studied. Internal microstructure of droplet fusion is observed in order to ensure good metallurgical bonding. All of these studies show the feasibility of fabricating directly unsupported inclined aluminum pillars in the limited angle range by using uniform micro droplets.

Introduction

Uniform molten metal droplets technique draws more and more attention due to its potential of fabricating rapidly metallic components [1], [2], [3]. In this technique, 3D object can be fabricated through a highly flexible "digital fabrication" process [4], [5]. At present, these parts are mostly two-dimensional structure [6], [7]. Complex parts are formed mainly by the powder [8] or using removable structure [9] as supports in additive manufacturing. However, when a part has micro slender pipe [10], [11] or porous features [12], it will be difficult to clear the support materials or structures. Therefore, it is necessary to develop a new direct forming method to fabricate complex structure. Characteristics of offset deposition between micro droplets can be to provide a potential method without support [2], [13], [14]. It provides a possibility for fabricating directly micro complicated metal structures, because the metal droplet diameter can be controlled in the range of 100–1000 µm [2], [6], [7], [14].

The key problem is to control spreading and fusion shape of deposited droplets at different offset ratios. Droplet fusion is a complicated fluid and thermal behavior, which includes impacting [15], spreading [16], re-melting [17], oscillation [18] and solidification [19] of metal droplets. Low Weber number deposition is a prerequisite to maintain the droplet spreading controllable [18]. Impinging velocities as an important parameter of Weber number strongly affect the final shapes of the pileup [20]. More complex inclined impact of a molten droplet deposited onto a substrate is predicted usually by numerical model [21], [22]. The impact fluid dynamics, cooling and subsequent solidification of the impinging droplet in the pile-up is strongly influenced by the geometry of the previous solidified droplet [20]. In addition, the final spreading shape depends mainly on spreading time and solidification time. A necessary condition, though not sufficient, for a droplet to take on a final spherical cap shape is that the liquid comes to rest before significant solidification takes place [18]. Good fusion is important for forming quality. To ensure good fusion, droplet temperature and substrate temperature need to be selected properly [14], [23]. Re-melting between impinging droplet and previous droplet could be considered as an one-dimensional Stefan problem with phase change [14], [19], [24], where the thermal and dynamic behaviors of molten metal droplets belong to non-isothermal deposition [25]. Less re-melting and poor metallurgical bonding under inappropriate droplet temperature or substrate temperature will result in micro-void and cold lap [5], [26].

The aim of the present work is to investigate fabricating directly unsupported inclined aluminum pillars by molten aluminum droplets under different fusion ratios and offset ratios. An experimental system of ejecting and depositing uniform molten aluminum alloy droplets is developed. A model is introduced to describe the inclined angle of the droplet deposition at different offset ratios. A one dimensional heat transfer model is proposed to select the initial temperature parameters of impinging droplet and previous solidified droplet to ensure that fusion occurs. The correspondence between offset ratio and inclined angle is verified by experiments. Internal microstructure of droplet fusion is observed in order to ensure good metallurgical bonding. The potential feasibility of producing complex 3D metal parts deposited by molten metal droplets has been demonstrated.

Section snippets

Experimental system and method

Schematic diagram of the experimental apparatus is shown in Fig. 1a, which consists mainly of a three-dimensional platform, a metal melting crucible, a protective gas system, a pulse generation system, a temperature detection system, and a substrate heating system. Numerical Control (NC) code generated by host computer is downloaded into Programmable Multi Axis Controller (PMAC), which controls three-dimensional platform to the corresponding positions, then triggers piezoelectric ceramic. The

Results and discussion

Capillarity force and viscous force are hampering the droplet spreading. At low Weber numbers We, the Ohnesorge number can be instead of the Reynolds number, because at low We the spreading process is driven by capillarity forces and is almost unaffected by the impact velocity [18]. We can be calculated by Eq. (2).We=ρU2rσwhere ρ is droplet density, U is impinging speed, r is the droplet radius and σ is surface tension. The Ohnesorge number Oh can be calculated by Eq. (3).Oh=μρσr

The impinging

Conclusions

  • (1)

    Uniform micro aluminum droplet deposition belongs to low Weber number deposition. The spreading process is driven by capillarity forces and is almost unaffected by the impact velocity.

  • (2)

    No melting, partial melting and excessive melting region at different offset ratios ξ are determined. In the constant fusion ratio δ, the bigger offset ratio ξ is, the smaller inclined angle θ is and the more difficult inclined angle θ is adjusted.

  • (3)

    The spherical cap of the top droplet could fit to offset deposition

Acknowledgments

This work was supported by National Natural Science Foundation of China (No. 51675436), Natural Science Basic Research Plan in Shanxi Province of China (No. 2015JQ5123), Programme of Introducing Talents of Discipline to Universities (Grant no. B08040).

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    1

    Daicong Zhang is a Ph.D. student at School of Mechanical Engineering, Northwestern Polytechnical University. His research interests include modeling of components, optimization of fabrication strategy and metal droplet deposition.

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    Lehua Qi is a professor from School of Mechanical Engineering at Northwestern Polytechnical University, and she also is the team member from Education Ministry Key Laboratory of Contemporary Design and Integrated Manufacturing Technology. Professor Qi received Ph.D. degree in System Engineering from Northwestern Polytechnical University. Her research interests include micro-manufacturing based on metal droplet generation technology, manufacturing technology and forming mechanism of metal matrix composites and image recognition for the C/C composites.

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    Jun Luo is an associate professor from School of Mechanical Engineering at Northwestern Polytechnical University. His research is in package technology, metal droplet disposition, jet breakup simulation, droplet charging and deflection, droplet based solid freeform fabrication.

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    Hao Yi is a Ph.D. student at School of Mechanical Engineering, Northwestern Polytechnical University. His research interests include dynamic behaviors of metal micro-droplet impacting, heat and mass transfer of impinging metal micro-droplets on solid surfaces and surface topography in metal micro-droplet non-isothermal deposition.

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    Xianghui Hou is an Associate Professor at Department of Mechanical, Materials, and Manufacturing Engineering, The University of Nottingham, University Park, Nottingham. His research interests include nano-composite coating, nano-film growth, etc.

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