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Research Papers

Laser Joining of Continuous Glass Fiber Composite Preforms

[+] Author and Article Information
Huade Tan

Graduate Research Assistant
e-mail: ht2288@columbia.edu

Y. Lawrence Yao

Professor
Fellow ASME
e-mail: yly1@columbia.edu
Department of Mechanical Engineering,
Manufacturing Research Laboratory,
Columbia University,
NY 10027

Contributed by the Manufacturing Engineering Division of ASME for publication in the Journal of Manufacturing Science and Engineering. Manuscript received April 12, 2012; final manuscript received December 26, 2012; published online January 22, 2013. Assoc. Editor: Wei Li.

J. Manuf. Sci. Eng 135(1), 011010 (Jan 22, 2013) (11 pages) Paper No: MANU-12-1109; doi: 10.1115/1.4023270 History: Received April 12, 2012; Revised December 26, 2012

A laser fusion joining method is investigated for the purpose of through thickness strengthening of glass fiber reinforced laminate composites. Laser fusion joining is evaluated as a potential process to replace mechanical reinforcements used in conventional laminate composite fabrication. A two step laser process is developed to form fusion bonds between fibers within a single bundle and between adjacent fiber bundles. Coupled heat transfer and viscous flow modeling is carried out to investigate the temperature and dynamics of the joining process under three experimentally observed conditions. Linear elastic finite element analysis is used to investigate the effect of joint morphology on stress concentrations and strength. Joint strength is found to be a function of the fiber contact angle and packing density at the joint interface. Tensile tests show that laser joined fiber bundle strength is on the same order of magnitude as the raw fiber bundles. The challenges to laser processing of three dimensional fiber reinforcements in laminate composite fabrication are discussed.

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Figures

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Fig. 1

Finite element model schematic of the outlining the Cartesian, material, and spherical coordinate axes defined with respect to an ideal spherical joint interface. For computational simplicity, one eighth of the joint geometry is modeled with symmetric boundary conditions on each of the dividing planes.

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Fig. 3

(a) An optical micrograph of melt pools achieved by scanning the laser along the fiber bundle from one free end. On all samples the laser is scanned from right to left. Melt beads are arranged in increasing order of scan length from top to bottom. (b) Simulation output of the bead formation process depicted in order of increasing scanning length. Simulation snap shots are taken at 0.5, 3, 6 and 9 s from top to bottom.

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Fig. 4

(a) Measurement results of the melt bead diameter obtained from microscopy images post processing versus scan distance plotted as markers with error bars signifying one standard deviation. Numerical results of the bead diameter versus scan distance are plotted in a solid line. (b) Temperature output plotted as a function of time. Note, given a scan speed of 0.5 mm/s, the scanning length of 8 mm corresponds to the 16 s. At this stable scanning speed, relative motion between the bead and the laser spot is zero.

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Fig. 2

(a) The laser fusion joining process schematic is presented showing the profile of a laser source scanning along a fiber bundle to initiate and propagate the melt bead formation process within a single bundle. (b) Joining is achieved between two preformed melt beads by irradiating both beads at the contacting point between the two bundles. A spherical bead is achievable if no mechanical constrains are applied on the melt pool during the joining process. Mechanical constraints or tension applied on the melt during the joining processes was found to result in ellipsoidal bead morphologies and tended to compact fibers in radial direction.

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Fig. 6

(a) Melt separation distance measured from optical microscopy is plotted against laser exposure time as markers with error bars signifying one standard deviation. Simulation results of the separation distance between melt pools are plotted as a function of processing time in a solid line. (b) The maximum simulation melt temperature is plotted as a function of processing time. A very sudden separation behavior of the melt is captured in this model at time t = 0.4 s, as is characteristic of the void formation process observed during joining experiments. A stable melt temperature and separation distance is observed after the initial melt formation and separation instability.

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Fig. 7

(a) An optical micrograph is presented showing the joint morphologies obtained by irradiating two bundles at the point of contact between two bundles with preformed bead ends. All joint morphologies depicted are produced with similar processing times and initial bead volumes, showing good consistency and repeatability of the joining process. (b) Simulation output of the joint formation process arranged in order of increasing time: 0.1, 0.4, 1.0, and 2.0 s from top to bottom.

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Fig. 8

An optical micrograph of joint morphologies obtained from stretching the joint to various lengths during cooling. If given the same starting melt volume, the joint diameter decreases relative to the final joint length. The morphology of the melt volume progresses from an ellipsoidal profile to a conically tapered profile when stretched. The fibers at the joint interface were also observed to compact in the radial direction when stretched.

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Fig. 10

The morphology and temperature simulation results of the joint formation process are plotted as a function of the simulation time. The joint radius, measured from the center of the computation domain is plotted in a solid line and the temperature history of the melt is plotted in a dotted line. The joint formation process is observed to occur in two well defined phases. The temperature of the melt is observed to increase monotonically even after the joint radius reaches a maximum.

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Fig. 11

The stress state along the constant radius of a spherical joint 1.2 times the diameter of a cylindrical bundle is plotted in spherical coordinates along φ=0 as a function of the contact angle θ depicted in Fig. 1. For a spherical joint diameter 1.2 times the bundle diameter, the maximum contact angle between the joint and bundle is defined at θ=θmax=56.4 deg. Shear stress components (Sθθ and SRθ) at edge of the joint–bundle interface (θ=θmax) are observed to exceed the maximum axial stress SRR at θ=0 deg.

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Fig. 12

Finite element results of the idealized joint morphology, depicted in Fig. 1, are plotted as a function of the maximum contact angle θmax defined by the diameter of the melt bead. The stress concentration factor K, defined in Eq. (10), is the ratio of the maximum principal stress at θmax to the axial stress at θmax=0.

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Fig. 14

(a) An SEM image of the fiber fracture surface at periphery of a joint (near θ=θmax). A concave, angled fracture surface observed in the fracture surface is indicative of a shear dominant failure mechanism in this region of the joint. Note that fibers fracture near the surface of the joint in this region and the fracture surfaces are aligned in a common direction toward the center of the joint. (b) An SEM image of a fiber fracture surface found near the center of a joint. The flat fracture surface observed in this figure is indicative of a brittle fracture due to axial tension. Fibers at the center of the joint tended to exhibit the same fracture behavior and to fracture some distance away from the joint interface.

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Fig. 5

(a) An optical micrograph of the melt morphologies of single bundle specimens irradiated at the center of the bundle while the ends were mechanically constrained. Melt samples are arranged in increasing order of laser exposure time from top to bottom. (b) Flow simulation output of the bundle separation process arranged in order of simulation time: 0.05, 0.20, 0.40, 0.45, and 2.0 s from top to bottom.

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Fig. 9

(a) An SEM image of a joint interface after stretching and fracture showing high fiber packing density. (b) An SEM image of a fractured non stretched joint interface showing low fiber packing density. Fiber density is observed to be controlled by bundle confinement and stretching during the joining process. Low fiber packing density is found to be a source of added stress concentrations resulting in lower loading limits during tensile tests.

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Fig. 13

(a) Typical load displacement output from single bundle tensile tests are plotted comparing the critical load achieved from a stock fiber bundle and a joined fiber bundle. Progressive failure is observed in stock fiber bundles after the critical load is reached, characterized by the sequential fracture of individual fibers. A more rapid fracture process is observed in joined samples. (b) Compiled tensile test results are plotted comparing the critical load achieved versus measured bundle diameter. Average critical load values are denoted by markers with error bars depicting the first standard deviation.

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