Research Papers

Strength and Phase Identification of Autogenous Laser Brazed Dissimilar Metal Microjoints

[+] Author and Article Information
Gen Satoh, Y. Lawrence Yao

Manufacturing Research Lab,
Columbia University,
New York, NY 10027

Caian Qiu

Champaign Simulation Center,
Champaign, IL 61820

Syed Naveed

Boston Scientific Corporation,
Marlborough, MA 01752

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received January 26, 2014; final manuscript received September 17, 2014; published online November 26, 2014. Assoc. Editor: Wei Li.

J. Manuf. Sci. Eng 137(1), 011012 (Feb 01, 2015) (12 pages) Paper No: MANU-14-1035; doi: 10.1115/1.4028778 History: Received January 26, 2014; Revised September 17, 2014; Online November 26, 2014

The continued advancement of implantable medical devices has resulted in the need to join a variety of dissimilar, biocompatible metal pairs to enable selective use of their unique properties. Typical materials used in implantable medical devices include stainless steel (SS), titanium, platinum (Pt), as well as shape memory materials such as NiTi. Joining these dissimilar metal pairs, however, often results in excessive formation of brittle intermetallics, which significantly reduce the strength of the joints. The use of filler materials to combat the formation of intermetallics, however, results in reduced biocompatibility. Autogenous laser brazing is a novel process that is able to form thin, localized joints between dissimilar metal pairs without filler materials. In this study, the formation of autogenous laser brazed joints between NiTi and SS wires is investigated through experiments and numerical simulations. The strength, composition, microstructure, and phase formation of the resultant joints are investigated as a function of processing parameters and thermal, fluid flow, and phase prediction simulations are used to aid in understanding the joint formation mechanism.

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Fig. 1

Schematic diagram of proposed autogenous laser brazing process for (a) wire–wire (∼400 μ in diameter). Wire–wire process utilizes a Gaussian laser intensity distribution.

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Fig. 2

Optical micrograph of dissimilar metal joint between NiTi and SS observed from the (a) top, (b) side, and (c) bottom of the joint. Sample irradiated on NiTi-side of joint. Note complete joining around circumference of wires.

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Fig. 3

Optical micrograph of joint cross section along the YZ-plane. Note wider joint width at top, irradiated surface and nearly uniform joint width along the remainder of the joint. Evidence of directional solidification is also observed in the top portion of the joint.

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Fig. 4

EDX line scan composition profiles for scans: (a) i, (b) ii, and (c) iii from Fig. 3. Note change in width of mixed zone (joint width) at different locations along wire thickness.

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Fig. 5

EDS Map of region 1 in Fig. 3 with the left side showing the composition of NiTi and the right showing the composition of Stainless Steel. Note transport of Fe-rich material along NiTi wire surface.

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Fig. 6

Thermal model of autogenous laser brazing process showing thermal accumulation at joint interface as laser beam approaches. “▼” symbol indicates position of laser beam. (a) Equilibrium temperature distribution far from interface. (b) Beginning of thermal accumulation at interface. (c) Melting of interface.

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Fig. 7

Simulated temperature–time profiles for points at different distances from the joint interface. Note higher peak temperatures for points located closer to the interface indicating thermal accumulation.

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Fig. 8

Color contour map of mass fraction of Fe in NiTi–Fe joint formed by autogenous laser brazing as predicted by numerical simulation. Fluid flow is driven by Marangoni convection with a positive surface tension temperature coefficient. Resultant composition profile matches that observed experimentally as shown in Fig. 5. Right side shows pure Fe, left side shows NiTi, and gradient at upper surface shows mixing of materials due to Marangoni convection.

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Fig. 9

(a) Representative joint width versus laser power at constant scan speed. (b) Representative joint width versus scan speed at constant laser power. Note excessive melting of the top of the joint in (a) at extreme power levels due to different total energy input and thermal profile.

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Fig. 10

EBSD map of interface on YZ-plane cross section of sample joined at 8.5 W and 1.75 mm/s scan speed. Note large grains on left and right and a layer of smaller grains at the center. Coloring is based on Euler angles with red, green, and blue representing the (001), (101), and (111) normal orientations for cubic crystals and the (0001), (2110), and (1010) normal orientations for hexagonal crystals. No specific orientation relationship is observed between the base metal grains and those within the joint. Color image available when viewed online.

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Fig. 12

Phase development curves as predicted by CALPHAD simulation for compositions of (a) 48 at. % Ni, 37 at. % Ti, 14 at. % Fe, 1 at. % Cr and (b) 34 at. % Ni, 35 at. % Ti, 23 at. % Fe, and 8 at. % Cr. Note initial formation of TiFe2 from liquid in both cases followed by formation of Ni3Ti prior to complete solidification.

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Fig. 14

Typical load-displacement curve for joint formed using autogenous laser brazing process. Appreciable plastic deformation is observed after yield, which occurs at the same load observed in the base SS material as shown in Fig. 13. Total plastic extension is lower than observed in the original base material but is for a shorter length of SS wire. Note initial shallow slope region due to straightening of the wire by the Bollard gripping apparatus.

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Fig. 11

EBSD/EDS phase identification map of region shown in Fig. 11. Joint interface is roughly 5 μm in width and consists of primarily TiFe2 and TiNi3. Note layer of FeTi at NiTi-interface boundary.

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Fig. 13

Load-displacement curves for base materials NiTi and SS. Note load plateau in NiTi curve indicating phase transformation accommodated deformation (i.e., superelasticity).

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Fig. 15

Load at fracture as a function of laser power at constant scan speed. Peak load is observed at same laser power that results in most uniform joint width (Fig. 9(a)).

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Fig. 16

Fraction of cross-sectional area joined versus laser power for samples joined at a scan speed of 2 mm/s. Note close resemblance to joint strengths shown in Fig. 15 both in magnitude and standard deviation.

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Fig. 17

Load at fracture as a function of scan speed for a constant laser power. Decreasing trend caused by lower energy input at higher scan speeds.

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Fig. 18

Fraction of wire cross-sectional area joined at interface as a function of laser scan speed for samples irradiated with 8.5 W laser power. Note close resemblance to load at fracture profile as a function of scan speed shown in Fig. 17.

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Fig. 19

Representative EDS maps of fracture surfaces on (a) NiTi and (b) SS sides of joint. Red, green, and blue, represent Fe, Ni, and Ti. Majority of fracture surfaces show Ni- and Ti-rich composition with patches of Fe-rich areas.

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Fig. 20

SEM of fracture surface for sample irradiated on SS suggesting quasi-cleavage fracture



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