Research Papers

Experimental Study of Failure Modes and Scaling Effects in Micro-Incremental Forming

[+] Author and Article Information
A. J. Nelson

Department of Mechanical Engineering,
Northwestern University,
Evanston, IL 60201

N. V. Reddy

Department of Mechanical Engineering,
Indian Institute of Technology,
Kanpur 208016, India

Jian Cao

Department of Mechanical Engineering,
Northwestern University,
Evanston, IL 60201
e-mail: jcao@northwestern.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF Micro- AND Nano-Manufacturing. Manuscript received November 20, 2012; final manuscript received July 24, 2013; published online xx xx, xxxx. Assoc. Editor: Shiv G. Kapoor.

J. Micro Nano-Manuf 1(3), 031005 (Aug 13, 2013) (15 pages) Paper No: JMNM-12-1075; doi: 10.1115/1.4025098 History: Received November 20, 2012; Revised July 24, 2013

Incremental forming (IF) is a relatively new technique that uses a simple hemispherical ended tool moving along a predefined three-dimensional toolpath to deform a sheet of metal into the desired shape. The greater process flexibility and higher formability in IF have resulted in greater academic and industrial interest in this process as it can successfully produce ultrathin parts beyond the forming limit seen in conventional stamping and the process does not require any geometry-specific tooling. Another emerging paradigm in manufacturing has been the increasing application of forming in micromanufacturing. The above stated process characteristics of IF make it an ideal candidate for being incorporated into the micromanufacturing paradigm. This work investigates micro-IF to examine how forces and occurrence of sheet failure change when the geometric dimensions of incremental forming are scaled down. The development of a highly repeatable micro-IF experimental setup is described and experiments are performed to show that a previously unknown buckling mode of deformation exists in micro-incremental forming, that is linked to the material microstructure. The analysis provides guidelines for the design and understanding of the micro-incremental forming process.

Copyright © 2013 by ASME
Topics: Buckling , Failure , Shapes
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Fig. 1

Schematic of SPIF process

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Fig. 2

(a) Overall design of blank holding fixture, (b) sectional view of blank holding fixture, and (c) fabricated sheet holding fixture

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Fig. 4

(a) Procedure for making bolt clearance holes and (b) 250 μm tool used for μ-SPIF

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Fig. 5

Schematic of error resulting from inaccurate fabrication of sheet clamping setup. Bottom face of the top constraint plate is highlighted in red. The angled dashed line is an exaggeration of the nonparallelism that may have existed.

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Fig. 6

Schematic showing incorrectly shaped tool and ideal cross section of tool

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Fig. 7

Example of drift in force measurement

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Fig. 8

(a) Etched strain grid on sheet and (b) prefabricated strain grid adhered to sheet surface

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Fig. 9

(a) Strain circle grid created with laser ablation and (b) picosecond laser system used to create the strain grid

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Fig. 10

(a) Schematic of toolpaths used, (b) channels formed on sheet, and (c) close up view of failure location

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Fig. 11

(a) 55 deg cone, (b) freeform component, and (c) triangular pyramids, formed using μ-SPIF

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Fig. 12

Illustration of the cone component and variables listed in DOE factors

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Fig. 13

Forming forces for failure by tearing in μ-SPIF of the cone

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Fig. 14

Forming forces for failure by buckling in μ-SPIF of the cone

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Fig. 15

A formed sample showing the phenomenon of sheet buckling. When buckling was observed, no tearing of the part had yet occurred.

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Fig. 16

Surface roughness measurements for SS304 sheets (a) before and (b) after polishing

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Fig. 17

Forming forces for μ-SPIF of a 65 deg cone with an unpolished sheet

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Fig. 18

Forming forces for μ-SPIF of a 65 deg cone with a polished sheet

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Fig. 19

CAD model of fixture designed to ensure planarity of clamped sheet

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Fig. 20

Cone formed with new fixture

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Fig. 21

Failure locations and observable types for cone tests run with the tensioned sample constraint plate. The lighter lines represent visible tearing, while the “x” marks indicate the appearance of sheet buckling. The lines inside the circles indicate the rolling direction in which the grains are more elongated.

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Fig. 22

(a) Geometry of formed 55 deg wall angle cone, (b) normal contact between tool and sheet in SPIF, (c) buckling of sheet in the plane, and (d) eventual failure of the sheet due to buckling (the section indicates the part of the sheet in contact with the tool and section indicates the buckled zone of the sheet)

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Fig. 23

Illustration of the profile of the funnel shape

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Fig. 24

Fracture locations for μ-SPIF of funnel shapes. The lighter lines represent visible tearing. Note the absence of “x” marks indicating an absence of buckling.

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Fig. 25

Typical force curves for μ-SPIF of the funnel shape

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Fig. 26

Experimental setup for macro-SPIF

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Fig. 27

Comparison for failure depths for macro-SPIF and μ-SPIF

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Fig. 28

Schematic of the FEA model

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Fig. 29

(a) Undeformed and (b) deformed strain grids for the cone shape corresponding to case 15 in Table 2




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