0
Research Papers

The Effects of Laser Forming on NiTi Superelastic Shape Memory Alloys

[+] Author and Article Information
Andrew J. Birnbaum, Y. Lawrence Yao

Department of Mechanical Engineering, Columbia University, New York, NY 10027

J. Manuf. Sci. Eng 132(4), 041002 (Jul 21, 2010) (8 pages) doi:10.1115/1.4000309 History: Received November 10, 2006; Revised December 15, 2008; Published July 21, 2010; Online July 21, 2010

This work focuses on application of the laser forming process to NiTi shape memory alloys. While all NiTi shape memory alloys exhibit both superelasticity and the shape memory effect, this study is restricted to a temperature range over which only the superelastic effect will be active. Specifically, this work addresses laser forming induced macroscopic bending deformations, postprocess residual stress distributions, and changes in microstructure. Like traditional ferrous alloys, the laser forming process may be used as a means for imparting desired permanent deformations in superelastic NiTi alloys. However, this process, when applied to a shape memory alloy also has great potential as a means for shape setting “memorized” geometric configurations while preserving optimal shape memory behavior. Laser forming may be used as a monolithic process, which imparts desired deformation while maintaining desired material behavior. Characterization of the residual stress field, plastic deformation, and phase transformation is carried out numerically and is then subsequently validated via experimental results.

Copyright © 2010 by American Society of Mechanical Engineers
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Figure 2

Temperature dependence of flow stress and critical stress required for phase transformation. Note the cross-over phenomenon occurring just below 1000 K, where the flow stress becomes lower than the stress required for phase transformation. Flow stress at room temperature was obtained from the tensile test performed (Fig. 6). Flow stress variation with temperature, as well as phase transformation stress, was obtained from literature (15).

Grahic Jump Location
Figure 3

Schematic of laser-specimen experimental setup

Grahic Jump Location
Figure 4

As received, austenitic grain structure and average grain size ∼50 μm. Note the smooth equiaxed grain structure.

Grahic Jump Location
Figure 5

As received, austenitic X-ray diffraction spectrum at room temperature (A—austenite and M—martensite)

Grahic Jump Location
Figure 6

Micrograph of through thickness cross section of five scan specimen (medium magnification and top surface). Note the transition to SIM as the top surface is approached (P=250 W, v=15 mm/s, and d=7 mm).

Grahic Jump Location
Figure 9

X-ray diffraction spectra over successive scans (P=250 W, v=15 mm/s, and d=7 mm) revealing an increase in martensitic content at the expense of the parent austenitic matrix (untreated surface). The [110] positive peak shift is due to tensile residual stress (Y-direction) induced by the LF process. (A—austenite and M—martensite).

Grahic Jump Location
Figure 10

X-ray diffraction spectra over successive scans (P=250 W, v=15 mm/s, and d=7 mm) revealing an increase in martensitic content at the expense of the parent austenitic matrix (irradiated surface). Note the presence of R-phase in the fourth scan (A—austenite and M—martensite). Also note the negative peak shift from the baseline corresponding to compressive residual stress (Y-direction) in the irradiated surface.

Grahic Jump Location
Figure 15

Martensitic volume fraction distribution perpendicular to laser scan on irradiated surface for three distinct temperature gradients

Grahic Jump Location
Figure 16

Average bending angle and martensitic volume fraction as a function of laser scans for selected temperature gradients

Grahic Jump Location
Figure 17

Temperature and plastic strain time history at a representative point on the laser scan path. Laser arrives at t=2.5 s (P=250 W, v=15 mm/s, and d=7 mm).

Grahic Jump Location
Figure 14

Numerically predicted changes in martensitic volume fraction as a function of the number of laser scans for the top and bottom surfaces (P=250 W, v=15 mm/s, and d=7 mm)

Grahic Jump Location
Figure 1

Characteristic constitutive response of NiTi superelastic shape memory alloy in Af<Toperating<Md(15)

Grahic Jump Location
Figure 7

Micrograph of through thickness cross section of five scan specimen (high magnification and top surface). Note the presence of grains of varying extent of transformation (P=250 W, v=15 mm/s, and d=7 mm).

Grahic Jump Location
Figure 8

Micrograph of through thickness cross section of five scan specimen, P=250 W, v=15 mm/s, and d=7 mm (high magnification and bottom surface). Note the presence of SIM.

Grahic Jump Location
Figure 11

Numerical and experimental average bending angle, P=250 W, v=15 mm/s, and d=7 mm

Grahic Jump Location
Figure 12

Bending angle distribution along laser scan path for varying numbers of laser scans. P=250 W, v=15 mm/s, and d=7 mm.

Grahic Jump Location
Figure 13

Contour plot of martensitic volume fraction (P=250 W, v=15 mm/s, and d=7 mm)

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In