Research Papers

Effect of Particle Size Distribution on Powder Packing and Sintering in Binder Jetting Additive Manufacturing of Metals

[+] Author and Article Information
Yun Bai, Grady Wagner

Design, Research, and Education for Additive
Manufacturing Systems Laboratory,
Department of Mechanical Engineering,
Virginia Tech,
Blacksburg, VA 24061

Christopher B. Williams

Design, Research, and Education for Additive
Manufacturing Systems Laboratory,
Department of Mechanical Engineering,
Virginia Tech,
Blacksburg, VA 24061
e-mail: cbwill@vt.edu

Manuscript received December 14, 2016; final manuscript received April 19, 2017; published online June 1, 2017. Assoc. Editor: Zhijian J. Pei.

J. Manuf. Sci. Eng 139(8), 081019 (Jun 01, 2017) (6 pages) Paper No: MANU-16-1646; doi: 10.1115/1.4036640 History: Received December 14, 2016; Revised April 19, 2017

The binder jetting additive manufacturing (AM) process provides an economical and scalable means of fabricating complex parts from a wide variety of materials. While it is often used to fabricate metal parts, it is typically challenging to fabricate full density parts without large degree of sintering shrinkage. This can be attributed to the inherently low green density and the constraint on powder particle size imposed by challenges in recoating fine powders. To address this issue, the authors explored the use of bimodal powder mixtures in the context of binder jetting of copper. A variety of bimodal powder mixtures of various particle diameters and mixing ratios were printed and sintered to study the impact of bimodal mixtures on the parts' density and shrinkage. It was discovered that, compared to parts printed with monosized fine powders, the use of bimodal powder mixtures improves the powder's packing density (8.2%) and flowability (10.5%), and increases the sintered density (4.0%) while also reducing the sintering shrinkage (6.4%).

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Fig. 1

Green part printing process in binder jetting (see figure online for color)

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Fig. 2

Comparison of apparent density, tap density, and green density for different powders (see figure online for color)

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Fig. 3

Sintered density and volumetric shrinkage of 5 μm powder and its bimodal mixtures, sintered at 1080 °C for 2 h

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Fig. 4

Sintered density and densification comparison under various sintering conditions (error bars are constructed with the min and max of the data)

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Fig. 5

Sintered density of loosely packed powders in crucible with different heating rates

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Fig. 6

Surface microstructure of bimodal powder parts sintered at 1060 °C for 120 min: (a) 75 + 15 μm powder (800×) and (b) 30 + 5 μm powder (2000×)

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Fig. 7

Optical microscopy of 5 μm powder (left) and 30(73%) + 5 μm powder (right), sintered under the conditions in Sec. 3.2

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Fig. 8

An illustration of the relationship between the constituent powder shrinkage and the design of powder mixtures for maximum sintered density (the separation line is only representative in this figure and needs to be determined for each powder mixture based on particle size ratios)




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