Laser weld penetration estimation using temperature measurements Page: 4 of 27
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1. Introduction
Laser beam welding with its precision, low heat effects and high speed has gained increas-
ing acceptance in the industry. This joining process is well suited for high volume automated
manufacturing. However, there is a lack of on-line weld monitoring methods to ensure weld
quality. In some critical applications, such as airbag canisters, there is a need for 100% weld
quality assurance. In many applications, partial weld penetration is required for joining and it is
important to determine the extent of penetration in such applications. Currently, statistical pro-
cess control with destructive testing is carried out to ensure weld penetration.
This paper proposes a model-based technique for weld penetration depth monitoring. It
builds on an analytical model relating penetration depth to laser power, weld bead width on the
top surface, and welding speed for on-line penetration depth estimation (Lankalapalli et al 1996).
In this model, the absorbed laser power is based on the power meter reading, assuming 100%
power absorption, to obtain penetration depth estimates. This assumption would lead to errors in
depth estimates because the power absorbed is usually unknown. The power meter cannot
account for the power losses along the path to the workpiece such as absorption by the beam
delivery optics and plasma, and reflection by the workpiece. This problem is worsened by the
lack of consistent and accurate commercially available power meters for measuring high power
CO2 laser beams (Leong et al, 1994). In this paper, the problems associated with power mea-
surement are eliminated by measuring temperature on the bottom surface of the workpiece and
relating it to the penetration depth. For this purpose, the depth estimation model proposed by
Lankalapalli et al (1996) is extended to construct a relationship between penetration depth, weld
bead width, welding speed, and temperature distribution.
1.1 Modeling of laser welding
Various mathematical models of the laser welding process have been reported since the
early 1970s. These models include simple moving line heat source models (Arata and Miya-
moto, 1972; Swift-Hook and Gick, 1973; Steen et al, 1988) and complex models which account
for fluid dynamics and laser-plasma interaction (Dowden et al, 1983, 1985; Ducharme et al,.2
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Lankalapalli, Kishore N.; Tu, Jay F.; Leong, Keng H. & Gartner, Mark. Laser weld penetration estimation using temperature measurements, report, October 1, 1997; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc691181/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.