Research Papers

Laser Autogenous Brazing of Biocompatible, Dissimilar Metals in Tubular Geometries

[+] Author and Article Information
Gen Satoh

Department of Mechanical Engineering,
Columbia University,
New York, NY 10023
e-mail: gs2385@columbia.edu

Grant Brandal

Department of Mechanical Engineering,
Columbia University,
500 West 120th Street,
Mudd Rm 220,
New York, NY 10023
e-mail: gbb2114@columbia.edu

Syed Naveed

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

Y. Lawrence Yao

Fellow ASME
Department of Mechanical Engineering,
Columbia University,
New York, NY 10023
e-mail: yly1@columbia.edu

1Corresponding author.

Manuscript received December 3, 2015; final manuscript received October 10, 2016; published online November 10, 2016. Assoc. Editor: Hongqiang Chen.

J. Manuf. Sci. Eng 139(4), 041016 (Nov 10, 2016) (14 pages) Paper No: MANU-15-1636; doi: 10.1115/1.4035034 History: Received December 03, 2015; Revised October 10, 2016

The successful joining of dissimilar metal tubes would enable the selective use of the unique properties exhibited by biocompatible materials such as stainless steel and shape memory materials, such as NiTi, to locally tailor the properties of implantable medical devices. The lack of robust joining processes for the dissimilar metal pairs found within these devices, however, is an obstacle to their development and manufacture. Traditional joining methods suffer from weak joints due to the formation of brittle intermetallics or use filler materials that are unsuitable for use within the human body. This study investigates a new process, Laser Autogenous Brazing, that utilizes a thermal accumulation mechanism to form joints between dissimilar metals without filler materials. This process has been shown to produce robust joints between wire specimens but requires additional considerations when applied to tubular parts. The strength, composition, and microstructure of the resultant joints between NiTi and stainless steel are investigated and the effects of laser parameters on the thermal profile and joining mechanism are studied through experiments and numerical simulations.

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Fig. 1

Schematic diagram of laser processing setup for tube joining. Note helical laser scan path to promote temperature uniformity at the joint interface.

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Fig. 2

Optical micrograph of typical joint produced between NiTi and stainless steel tubes through the autogenous laser brazing process. Note discoloration on outer surface of NiTi tube due to oxidation.

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Fig. 3

EDS maps of tube wall after cross sectioning on YZ-plane for laser powers of (a) 13 W and (b) 15.2 W. Note greater deformation for higher power processing. Red on the rectangular right-hand side represents Fe, while the deformed left-hand side has blue and green for Ti and Ni, respectively.

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Fig. 4

EDS line scans across joint interface for samples irradiated at (a) 13 and (b) 15.2 W. Scanning was performed in the middle of the joint, on line II indicated in Figs. 3(a) and 3(b).

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Fig. 5

Joint width versus laser power as measured from EDS line scans. Line numbers correspond to those indicated in Figs. 3(a)3(b).

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Fig. 6

Thermal contours during laser processing from numerical simulation. Simulated geometry cross sectioned along YZ-plane for clarity. Note sharp drop in temperature across interface prior to joining.

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Fig. 7

Simulated temperature time histories from nodes at different distances away from the joint interface along the irradiated tube surface. Oscillation in temperature due to rotation of sample under laser beam.

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Fig. 8

Optical micrographs of typical tube wall cross section after mechanical polishing and chemical etching. Images taken (a) 2.5 mm away, (b) 1.2 mm away, and (c) at the dissimilar metal interface along the NiTi tube wall.

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Fig. 9

Average grain size as a function of distance from joint interface as measured through line-intersect method on samples joined at two separate laser powers. Note decrease in grain size at interface for lower power.

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Fig. 10

Grain size as predicted by the Monte Carlo simulation and as measured experimentally for sample processed at 13 W power level. Note close agreement in grain size over laser scanned region.

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Fig. 11

(a) EBSD grain map with euler angle coloring. Note large grains on NiTi (left) side of joint, equiaxed grains on SS (right) side of joint, and columnar grains oriented along axial direction of tubes in the joint region. (b) Phase map of region shown in (a). Note formation of multiple phases in single grain regions.

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Fig. 12

(a) Grain map and (b) phase map of joint region for sample irradiated at laser power of 15.2 W. Note significantly greater joint width than observed in Fig. 11 and equiaxed grain geometry.

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Fig. 13

Joint strength versus laser power. Note sharp drop in joint strength at 14 W.

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Fig. 14

Optical micrographs of fracture surface for sample processed at 13 W laser power. Note joining across entire NiTi tube wall (left) and a portion of the thicker SS tube wall (right) as indicated by tube and joint inner diameters (I.D.) and outer diameters (O.D.).

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Fig. 15

Projected area of fracture surface as a function of laser power. Note small fracture area for low power irradiation and overmelting resulting in larger projected areas for high power irradiation.

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Fig. 16

Modulus (a) map and (b) line profiles of a localized region around the joint for a sample irradiated at a laser power of 13 W. Note similarity between modulus of stainless steel base material and intermetallics within the joint.

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Fig. 17

Hardness (a) map and (b) line profiles of a localized region around the joint for a sample irradiated at a laser power of 13 W. Note intermetallics are significantly harder than either of the base materials and gradual change from level of base NiTi to that of intermetallics and steep gradient on stainless steel side.

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Fig. 18

Modulus (a) map and (b) line profile for a sample irradiated at a laser power of 15.2 W. Note the large joint width when compared to that shown in Fig. 16. Modulus of intermetallics is again similar to that of the stainless steel. Gradient is observed toward NiTi side of joint.

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Fig. 19

Scanning electron microscope image of indentation performed within joint of sample irradiated at laser power level of 13 W. Note primary radial crack extending upward from one corner of indentation. Significantly smaller cracks are observed on the other two corners.

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Fig. 20

DIC optical micrograph of indentation shown in Fig. 21. Note location of indentation within joint and proximity of lower corners to base materials which blunt crack growth.

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Fig. 21

SEM image of indentations performed within joint for sample irradiated at laser power of 15.2 W. Note lack of primary radial cracks and single secondary radial crack initiated at edge of indentation impressions (inset).

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Fig. 22

Modulus over hardness maps for the samples shown in Figs. 16 and 18 above. Note the uniformity of the E/H ratio in the NiTi and joint regions. This suggests that the residual stress field surrounding the indenter, which is responsible for radial crack growth, is the same between the two materials.

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Fig. 23

Representative fracture surface for sample irradiated at low laser power of 13 W. Surface shows features indicative of quasi-cleavage fracture.

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Fig. 24

Representative fracture surface for sample irradiated at high laser power of 15.2 W. Note surface morphology indicative of transgranular fracture. Multiple grains are observed in the in-plane direction.

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Fig. 25

Hardness (a) map and (b) line profile for a sample irradiated at a laser power of 15.2 W. Hardness line profiles show greater uniformity of hardness across joint than that observed for samples irradiated at lower power level of 13 W (Fig. 17).




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