Electric heating – Metal heating – By arc
Reexamination Certificate
1998-10-30
2001-12-11
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121630, C219S121640, C219S130010, C219S124340, C219S130210
Reexamination Certificate
active
06329635
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to improved methods for weld monitoring and for monitoring laser heat treating processes.
DESCRIPTION OF THE RELATED ART
The use of laser beam welding in industrial processes has increased significantly in recent years. Compared to conventional arc-welding, laser beam welding allows higher process speeds, better precision, and smaller heat affected zones. Although cost has traditionally been a limitation to the installation of laser welding stations on the factory floor, consideration of all manufacturing costs and production requirements usually reveals that laser welding is competitive and in many cases better. The continuing increase in power and decrease in price of new laser systems enhance the competitive advantage of laser welding. As the use of laser welding technology increases, so does the need for reliable methods for process monitoring.
There are several characteristic signals associated with the laser beam welding process. If a laser beam of sufficient irradiance to melt the metal is focused at or into the surface of the work piece, a keyhole is formed. This keyhole within the molten metal is sustained by the evaporation of metal from the weld pool. The outward flow of this metallic vapor from the keyhole produces acoustic waves. Plasma is formed within and above the keyhole by ionization of the shielding gas and the metallic vapor. The primary signal from the welding process is the infrared emissions from the weld pool. Secondary signals include the radiation from the plasma (primarily in the visible and ultraviolet region) and the acoustic waves. Most weld monitoring methods sense at least one of these signals.
Variations in the plasma or acoustic emissions may indicate changes in weld quality, but monitoring these secondary signals tends to be difficult, complex, and expensive to implement. On the other hand, primary infrared emissions simply indicate the heat content of the weld. For example, deeper penetration tends to correlate with increased local heat input (caused by higher laser beam irradiances or slower travel speeds). Greater heat input results in higher temperatures and increased infrared emissions. The temperature may be monitored by a pyrometer, but this is difficult due to the slow response time of the system and the presence of an intense thermal signal from the plasma inside the keyhole. A better indicator is the infrared energy emitted by the weld, including both the contributions from the hot molten metal and the keyhole plasma. The ultraviolet and visible contributions may be minimized by selecting an infrared detector having a maximum sensitivity in the range of the molten metal emissions.
Using Wien's law (&lgr;
max
=2898/T), the wavelength (&mgr;m) with the maximum radiant energy may be estimated at a specific temperature (K). For the case of steel, which is liquid from about 1800 to 3100 K, the wavelength of interest for weld monitoring would be in the near infrared (from 0.9 to 1.6 &mgr;m). Due to the higher temperature and characteristic emissions of the keyhole plasma, the majority of the radiant energy is at shorter wavelengths in the visible and ultraviolet range.
U.S. Pat. No. 5,674,415 issued Oct. 7, 1997 to Keng H. Leong and Boyd V. Hunter, and assigned to the present assignee, discloses a low cost, robust, and rugged weld monitoring device. The robustness of the device is related to the ease of data analysis and the minimization of mistagged welds. The rugged weld monitor operates reliably in the harsh manufacturing environment. An infrared signature emitted by a hot weld surface during welding is detected and this signature is compared with an infrared signature emitted by the weld surface during steady state conditions. The result is correlated with weld penetration. The signal processing is simpler than for either UV or acoustic techniques. Changes in the weld process, such as changes in the transmitted laser beam power, quality or positioning of the laser beam, change the resulting weld surface features and temperature of the weld surface, thereby resulting in a change in the direction and amount of infrared emissions. This change in emissions is monitored by an IR sensitive detecting apparatus that is sensitive to the appropriate wavelength region for the hot weld surface. The weld monitor output is represented in an intuitive graphical format, in which the data is represented by a plot of voltage versus time. The infrared weld signal is monitored by one sensor integrated into the beam delivery optics, which greatly simplifies collection and analysis of the data. The infrared detector collects emissions from directly above the weld, and does not require sensors beneath the weld. Integration of the detector into the beam delivery optics makes the system very compact and less susceptible to bumping or misalignment. This monitoring concept has been incorporated into transmissive or reflective optics on both CO
2
and Nd:YAG lasers.
It is an object of the present invention to provide an improved method for weld monitoring.
It is an object of the present invention to provide an improved method for monitoring a laser heat treating process.
It is another object of the present invention to provide an improved weld monitoring method that provides real time monitoring of an infrared (IR) signature of a weld to identify a predetermined weld parameter, such as, surface weld quality, partial and full penetration.
It is another object of the present invention to provide an improved weld monitoring method that provides real time monitoring of an infrared (IR) signature of a weld to identify a predetermined weld parameter, such as, workpiece misalignment.
It is another object of the present invention to provide an improved weld monitoring method that provides real time monitoring of an infrared (IR) signature of a weld to identify a predetermined weld parameter, such as, workpiece contamination.
It is another object of the present invention to provide improved weld monitoring and laser heat treatment monitoring methods that utilize an infrared (IR) detector and that provide reliable and effective operation.
It is another object of the present invention to provide such an improved weld monitoring and laser heat treatment monitoring methods that overcome many of the disadvantages of prior art arrangements.
SUMMARY OF THE INVENTION
In brief, these and other objects of the invention are provided by improved methods for weld monitoring and improved methods for laser heat treatment monitoring.
In the method for weld monitoring, an infrared (IR) signature emitted by a hot weld surface during welding is detected. The detected infrared signature is compared with a steady state infrared signature signal. The compared results are correlated with a predetermined weld parameter. The predetermined weld parameter includes at least one of a full penetration weld, a workpiece misalignment, and a workpiece contamination. In the method for monitoring a laser heat treating process, an infrared (IR) energy signal emitted by a workpiece surface during the laser heat treating process is detected. The detected energy signal is compared with a predefined voltage range. The compared results are correlated to identify a potential defect.
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patent: 4375026 (1983-02-01), Kearney
patent: 4854724 (1989-08-01), Adams et al.
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Product Brochure entitled “Modular Process Control system (MPC)”, dated 1993, published by Fraunhofer, Aachen, Germany.
Product Brochure entitled “SynchroVision—For Pulsed Laser Welding”, undated, published by Control Vision, Idaho Falls, ID.
Nava-Rudiger, and Houlot, M. (1997). Integration of real time quality control systems in a welding process. J. Laser Appl., 9, pp. 95-102.
Olsen, F.O., Jorgensen, H., Bagger, C., Kristensen,
Leong Keng H.
Sanders Paul G.
Elve M. Alexandra
Pennington Joan
The University of Chicago
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