Research Papers

Beneficial Interface Geometry for Laser Joining of NiTi to Stainless Steel Wires

[+] Author and Article Information
Grant Brandal

e-mail: gbb2114@columbia.edu

Gen Satoh

e-mail: gs2358@columbia.edu

Y. Lawrence Yao

e-mail: yly1@columbia.edu
Mechanical Engineering Department,
Columbia University,
New York, NY 10027

Syed Naveed

Boston Scientific Corporation,
Marlborough, MA 01752
e-mail: syed.naveed@bsci.com

Manuscript received April 15, 2013; final manuscript received September 12, 2013; published online November 5, 2013. Assoc. Editor: Yung Shin.

J. Manuf. Sci. Eng 135(6), 061006 (Nov 05, 2013) (10 pages) Paper No: MANU-13-1165; doi: 10.1115/1.4025495 History: Received April 15, 2013; Revised September 12, 2013

Joining the dissimilar metal pair of NiTi to stainless steel is of great interest for implantable medical applications. Formation of brittle intermetallic phases requires that the joining processes used for this dissimilar pair limits the amount of over-melting and mixing along the interface. Thus, because of its ability to precisely control heat input, laser joining is a preferred method. This study explores a method of using a cup and cone interfacial geometry, with no filler material, to increase the tensile strength of the joint. Not only does the cup and cone geometry increase the surface area of the interface, but it also introduces a shear stress component, which is shown to be beneficial to tensile strength of the wire as well. The fracture strength for various cone apex angles and laser powers is determined. Compositional profiles of the interfaces are analyzed. A numerical model is used for explanation of the processing parameters.

Copyright © 2013 by ASME
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Fig. 1

Ternary phase diagram for Fe–Ni–Ti at 1173 K [13]

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Fig. 2

Schematic diagram describing the geometry used. Point 1 is at the apex of the cone and point 2 is on the outside edge

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Fig. 3

Mode I stress intensity (KI) and Mode II stress intensity (KII) versus angle of interface, for a constant uniaxial load. The interfacial area is superimposed over-top, indicating that as the cones become sharper, the area increase results in a decrease of stress intensity.

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Fig. 4

Longitudinal sectioned time snapshot of thermal accumulation in 90 deg cones. Corresponding times are: (a) 1 s, (b) 4 s, and (c) 6.8 s (laser has shut off). Laser location indicated by white dotted line. Power: 15 W, angular velocity: 3000 deg/s.

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Fig. 5

Uniformity along line segment 1–2 on the interface. As the angular velocity is increased, the thermal distribution becomes more even and the average temperature rises.

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Fig. 6

Time history of points 1 and 2 along the interface from Fig. 2, compared to points at the top and bottom of the interface of wires with flat interfaces. Temperature difference between the 2 points on the rotated wires throughout the laser scan is minimal. Laser power is 15 W for the rotated wires, and 35 W for the flat interfaces.

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Fig. 7

Images of four different combinations of power and cone angle. The outer surface near the joint of the 90 deg wires experienced more deformation. Rotational velocity is held constant at 3000 deg/s

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Fig. 8

Longitudinal section image of a 120 deg cone processed at a power of 15 W. The arrows indicate the paths of EDX line scans.

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Fig. 9

Longitudinal section image of a 120 deg cone processed at a power of 17 W. Excessive melting and deformation is present on the outer surface.

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Fig. 10

EDX line scan across line II, indicating the mixing occurring in the joint. Sample irradiated at 13 W.

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Fig. 11

EDX line scan across region II indicated in Fig. 8. Gradual decrease of concentration across the width of the interface is indicative of diffusion. Sample irradiated at 15 W.

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Fig. 12

EDX compositional map, corresponding to the longitudinal sectioned image of Fig. 8, showing iron (red) left of the mixed region; nickel (green) and titanium (blue) are to the right of the mixed region. Laser power is 15 W. A mixing region is observed along the interface, the width of which decreases toward the center of the wire. This indicates that the lighter colored regions along the interface of the longitudinal sectioned images are indeed the mixed regions.

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Fig. 13

Increase of load at fracture with increasing laser power input. Standard error for each level is indicated. Note that the 90 deg wires are stronger than the 120 deg wires at every power except for 17 W.

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Fig. 14

Stress at fracture over a range of power levels. The 90 deg wires are consistently stronger than the 120 deg wires, which is consistent with predictions.

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Fig. 15

Comparison of fracture strength between two different angles. The percentage scale on the left is how much stronger the 90 deg joint is compared to the 120 deg joint, for given power and angular velocities. As the power is increased, the difference between the two geometries reduces. This graph does not indicate which parameters gave the best overall results, but simply indicates the difference between the two angles.

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Fig. 16

Maximum average strength at fracture achieved for each wire geometry. Standard error is indicated. The 180 deg samples are the nonrotated, flat interfaces. Both of the conical wires have much higher fracture strengths.

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Fig. 17

Fracture surface indicating brittle transgranular fracture. 90 deg interface, 17 W, 3000 deg/s. Fractured at 200 MPa, along a surface not corresponding to the material interface.

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Fig. 18

Fracture surface of a sample that broke at 393 MPa, which was the highest strength achieved. The original cup and cone geometry was intact after fracture. Quasi-cleavage is apparent, indicating a better joint than Fig. 17.




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