Electric heating – Metal heating – By arc
Reexamination Certificate
2001-11-28
2004-11-16
Dunn, Tom (Department: 1725)
Electric heating
Metal heating
By arc
Reexamination Certificate
active
06818857
ABSTRACT:
BACKGROUND OF THE INVENTION
For the past 2-3 decades, the production of welded metal bellows has been a very labor-intensive task. Typically, a number of stamped circular metal diaphragms about 0.004″ thick are loaded in pairs into a clamping mechanism, and the clamp closed The operator then welds the inside diameter (ID) of the diaphragms using conventional welding methods such as a tungsten-inert-gas (TIG) welding torch, plasma, electron beam, NMG, gas, and/or GTAW. After a number of these welded pairs of diaphragms are made, for example via TIG, they are loaded onto an arbor between copper heat dispersing rings, and end plates clamped against the assembly to hold the metal diaphragms in tight contact with their neighboring part. The assembly is then mounted in a lathe, and rotated at a slow constant speed under a second TIG welding torch. The operator then adjusts the position of the welding torch so as to bring the welding tip in close proximity to the first seam of two of the diaphragms. An arc is struck between the torch tip and the seam, and the operator carefully follows, utilizing a microscope, any deviations in the seam path with the torch tip. By following the deviations in the seam path, the operator can maintain an arc that is fairly well centered on the diaphragm seam. After completing a weld, the TIG torch is moved to the next seam location and the procedure is repeated. This has been the standard method of producing welded metal bellows since their inception in the last 2-3 decades.
This method can be very tiring for the operator, who may be required to weld many convolutions to produce a single bellows capsule. Welding speeds using this method depends very much on operator skill level, with the top speed for an experienced welder being limited by the manual adjustments that have to be made during the weld sequence. Typical welding speeds thus vary from 10″ to 12″ per minute for a typical 0.008″ thick diaphragm pair. Accordingly, the entire welding sequence from first weld to last weld may require a skilled operator an hour or more, and an unskilled or semi-skilled operator considerably more time. Most, if not all, of the welding is done while the operator monitors the operation through a microscope. Weld gas such as argon is directed to pour over the weld area during the weld sequence, potentially exposing the operator to the long term effects of mild oxygen starvation. The weld gas is allowed to flow continuously since the weld area should be open for operator access. Due to the small diaphragm dimensions, it is difficult to cost effectively automate this process, while maintaining the consistency, weld integrity, and reliability obtained with skilled welding operators.
There have been attempts by laser manufacturers and welded bellows manufacturers to utilize a laser in this area of processing. A number of bellows manufacturers have been involved in some experiments with laser welding. However, the reliability and repeatability of laser edge-welding methods has not been sufficient, and, therefore, production of laser welded bellows has been limited to prototype and test production quantities.
One major drawback of using a laser beam for this type of welding process is that the laser beam dimensions are typically very small. The laser beams also typically have very high power densities. In fact, power densities as high as several Mwatts/cm
2
are far hotter than any TIG torches presently used for this type of work, and can instantly melt most materials put in range of its focus. Consequently, welding fixtures used for laser processing should be made with extreme precision. Deviations in the weld seam can occur because of, for example, machining tolerances. These deviations can cause the weld seam to wander from side to side as the bellows assembly is rotated under the TIG torch. The manual methods of welding bellows diaphragms relies heavily on operator skills to correct for any deviation in the weld seam. It requires the skills of a seasoned operator to produce consistently good welds. Upon completion of the welding, it is common practice to inspect each weld bead. Such inspection is also a painstaking exercise that is currently undertaken manually. An operator utilizes a microscope to examine each weld bead as it leaves the weld station. If any poor or below standard welds are used in any bellows assembly, early failure during exercising will often occur. Any incomplete welds can result in vacuum leaks, and usually the scrapping of the complete bellows.
Another issue associated with laser processing is known in the industry as ‘spatial mode hopping’. When a laser is operating, a profile of the energy distribution within the beam may indicate that the beam is hottest in the central area. However, adjustment of the laser resonator mirrors or a change in the pump power to the arclamp can change the spatial distribution, so that the hottest part of the beam may appear to move around within the beam cross-section. In practice, the spatial mode of a high-power laser system frequently changes as the laser power is programmed to increase, or decrease, at the start, or end, of the weld sequence. With a static beam, these variations in power density within the beam cross-section are often reflected in the molten material of the weld pool. Because of the small size of the weld pool, and the motion of the seam away from the molten area, rapid hardening of the molten material occurs upon leaving the weld pool. Thus any asymmetric melt due to variations in power density within the laser beam can be preserved in the weld, resulting in a lopsided appearance of the weld bead. This asymmetry can give rise to unwelded sections, thin or broken weld beads, and/or non-uniform stress characteristics. Often, this results in cracking during test cycling of the finished products, rendering them useless. One way to potentially overcome this problem would be to use a laser with a constant low order mode profile. That is, use a laser which can maintain a closely controlled power distribution profile across the diameter of the laser beam throughout the range of laser output power. However, a constant power distribution across the diameter of the laser beam is difficult, if not impossible, to achieve, and even more difficult to maintain.
As mentioned, deviations in the weld seam can occur, for example, due to machining tolerances. There have been many methods investigated to maintain the precise alignment required between the focused laser beam and the weld seam. Some of these methods involve magnified vision systems, tactile sensors, magnetic and capacitive proximity sensors, and digital signal processing (DSP) techniques. The method disclosed by Chang in U.S. Pat. No. 6,040,550 uses a magnified vision system and computer to locate the center of the weld seam. The laser beam position is then controlled with mirrors in an effort to follow any meandering in the weld seam path. However, this system is vulnerable to vibration, and can quickly lose accuracy of alignment. The Chang system has no compensation for thermal effects such that the system alignment can drift throughout the day with temperature, adversely affecting performance. In fact, many of the prior methods are difficult to design, expensive, and require careful setting up and maintenance. In addition, the prior methods can be prone to problems due to vibration, temperature and/or radio frequency interference (RFD) from the laser systems.
BRIEF SUMMARY OF THE INVENTION
The subject invention relates to a method and apparatus for welding. The subject invention can be used in the manufacturing of welded metal bellows. The subject method can allow higher manufacturing throughput of finished product, with consistently better quality and less operator attention than the present methods. In addition, the subject technique can reduce the volume of weld shielding gas which is consumed during the welding process, and Java reduce contamination by automation of parts handling equipment. The higher production methods
Cho Heung Ki
York Stanley J.
Dunn Tom
Johnson Jonathan
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