Laser Transmission Welding of Thermoplastics—Part I: Temperature and Pressure Modeling

[+] Author and Article Information
James D. Van de Ven

Mechanical Engineering Department, University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455vandeven@me.umn.edu

Arthur G. Erdman

Mechanical Engineering Department, University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455agerdman@me.umn.edu

J. Manuf. Sci. Eng 129(5), 849-858 (Apr 17, 2007) (10 pages) doi:10.1115/1.2752527 History: Received May 31, 2006; Revised April 17, 2007

This paper discusses the development of a model of laser transmission welding that can be used as an analytical design tool. Currently the majority of laser transmission welding (LTW) applications rely on trial and error to develop appropriate process parameters. A more rigorous design approach is not commonly used primarily due to the complexity of laser welding, where small material or process parameter changes can greatly affect the weld quality. The model developed in this paper also enables optimizing operating parameters while providing monetary and time saving benefits. The model is created from first principles of heat transfer and utilizes contact conduction that is a function of temperature and pressure, Gaussian laser distribution, and many material properties that vary with temperature including the absorption coefficient. The model is demonstrated through a design example of a joint between two polyvinyl chloride parts. The model is then validated with samples welded with a diode laser system using the operating parameters developed in a design example. Using the weld width as the primary output, the error between the model and the experimental results is 4.3%, demonstrating the accuracy of the model.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Power intensity of the laser beam computed by scanning over a 1‐mm-diameter hole

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Figure 2

Average power intensity of the laser beam as a function of distance from the center of the beam. The slightly jagged (blue) line is the averaged power intensity and the smoother (dashed green) line is a Gaussian curve fit to the data. The equation of the Gaussian fit line is: I=9795.7e−87000r2, where I is the power intensity (W∕m2) and r is the radial distance from the center of the beam (m). Note that these values need to be divided by the transmission index of the optical density filter to yield the actual beam intensity.

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Figure 3

Heat transfer components of the two-dimensional numerical model. Material B is the “transparent” part and Material C is the “absorbing“ part. In this view, the laser beam is collinear with the positive z axis and is traveling into the paper in the direction of the positive y axis (not labeled).

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Figure 4

Laser spot approaching the nodes on the surface of the x‐y plane. This diagram is formed by rotating Fig. 3+90deg about the x axis. For reference, the z axis is pointing out of the paper. Using this diagram the number of nodes along the x axis that are within the laser beam can be calculated.

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Figure 5

A volume element contained within the interior of one of the two parts. The four surrounding nodes are labeled to aid in the heat transfer equations. Note that the mesh is square allowing Δx=Δz.

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Figure 6

Absorption coefficient of clear PVC as a function of temperature. Note that the values above 550K are predicted values.

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Figure 7

Elastic modulus of PVC from two different sources and the values fit for the model

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Figure 8

The four output plots of the model for the design solution. The plot in the upper left shows the temperature of the “transmissive” part at the interface of the two materials as a function of time and width. The plot in the upper right is a zoomed contour plot of the interface zone at the time step that the maximum temperature occurs. Note that in this case, the interface occurs at a depth=3.2×10−3m. As can be seen, this maximum temperature typically occurs slightly below the surface of the “absorbing” part. The plot in the lower left shows the internal pressure as a function of time and width. Finally, the plot in the lower right shows the gap size along the width as a function of time. Note that the width dimension is along the x axis (Fig.  12) with a mirror boundary condition at x=0.

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Figure 9

Zoomed contour plot of version 4 of the convergence study. This frame coincides with the time that the maximum weld width is achieved. The interface between the two materials occurs at depth=3.2×10−3m.

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Figure 10

Solid model of the T-joint geometry with the associated dimensions in millimeters. Note the “transparent” part forms the top of the T, while the “absorptive” part forms the stem of the T.





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