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Research Papers

The Influence of Processing Parameters on the Mechanical Properties of Selectively Laser Melted Stainless Steel Microlattice Structures

[+] Author and Article Information
S. Tsopanos, S. McKown, Y. Shen, W. J. Cantwell, W. Brooks, C. J. Sutcliffe

Department of Engineering, University of Liverpool, The Quadrangle, Liverpool, L69 3GH, UK

R. A. W. Mines1

Department of Engineering, University of Liverpool, The Quadrangle, Liverpool, L69 3GH, UKr.mines@liv.ac.uk

1

Corresponding author.

J. Manuf. Sci. Eng 132(4), 041011 (Jul 23, 2010) (12 pages) doi:10.1115/1.4001743 History: Received January 15, 2009; Revised April 21, 2010; Published July 23, 2010; Online July 23, 2010

The rapid manufacturing process of selective laser melting has been used to produce a series of stainless steel 316L microlattice structures. Laser power and laser exposure time are the two processing parameters used for manufacturing the lattice structures and, therefore, control the quality and mechanical properties of microlattice parts. An evaluation of the lattice material was undertaken by manufacturing a range of struts, representative of the individual trusses of the microlattices, as well as, microlattice block structures. Low laser powers were shown to result in significantly lower strand strengths due to the presence of inclusions of unmelted powder in the strut cross-sections. Higher laser powers resulted in struts that were near to full density as the measured strengths were comparable to the bulk 316L values. Uniaxial compression tests on microlattice blocks highlighted the effect of manufacturing parameters on the mechanical properties of these structures and a linear relationship was found between the plateau stress and elastic modulus relative to the measured relative density.

Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) Schematic of the SLM process and (b) the MCP realizer II machine

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Figure 2

Stainless steel 316L powder (27) particle size distribution

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Figure 3

SLM 316L struts and microlattice blocks prior to removal from steel substrate but after the excess powder has been removed

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Figure 4

Geometry of the BCC unit-cell (a) and an example of a microlattice block (b)

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Figure 5

Detail of formation of 90 deg microstruts (a) schematic and (b) actual

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Figure 6

SEM and cross-section images of SLM struts: (a) first generation structure (70 W, 1000 μs), (b) second generation structure (140 W, 500 μs), (c) external detail second generation structure, and (d) cross sectional detail of second generation structure, 90 deg build angle

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Figure 7

Example of the three dimensional complexity that can be achieved using the SLM process

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Figure 8

Strut tensile test arrangement in the Instron 3342 screw-driven machine with a 50 N load cell and clip gauge

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Figure 9

SEM micrographs of the failure surfaces of struts with an exposure time of 1000 μs and different laser powers following uniaxial tensile loading, 90 deg build angle

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Figure 10

Uniaxial tensile behavior of the 316L lattice struts using crosshead displacement: (a) Comparison of compliance corrected stress-strain data for different build angles. LP=140 W and LX=500 μs. (b) Comparison of compliance corrected stress-strain data at different processing parameters for vertical (90 deg build angle) struts.

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Figure 11

Repeat strut tension tests with clip gauge: 90 deg build angle, not compliance corrected. LP=140 W and LX=500 μs.

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Figure 12

Compliance corrected stress-strain data with clip gauge for 90 deg angle (vertical). LP=140 W and LX=500 μs. E=140 GPa, yield stress=144 MPa (bulk yield stress=170 MPa(28)).

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Figure 13

Variation in relative density of the microlattice blocks with laser power for different exposure times

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Figure 14

Example of the uniaxial block compression test results at several levels of crush (LP=140 W, LX=500 μs)

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Figure 15

Detail from crushed blocks: (a) the upper node of lattice BCC 45% crushed, (b) lower node of lattice BCC 45% crushed, (c) the upper nodes of lattice BCC 75% crushed, and (d) lower nodes of lattice BCC 75% crushed. LP=140 W and LX=500 μs.

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Figure 16

Block stress-strain curves for laser exposure times of (a) 500 μs, (b) 750 μs, and (c) 1000 μs

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Figure 17

Mechanical properties of blocks as a function of relative density: (a) the variation in the yield stress and (b) the variation in the elastic modulus with relative density

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