Patent Application
20140305910
1. Field of the Invention
The present invention relates to a method and system for welding titanium or titanium alloys with a fiber laser and using an inert cover gas. Specifically, the present invention relates to the reduction of contamination in a weld by narrowing the bandwidth of a fiber laser that is acting on a titanium or titanium alloy workpiece thus reducing or eliminating optical interaction between the laser beam with the cover gas.
2. Description of the Related Art
Recent technological developments in fiber laser technology have expanded the capabilities for laser welding. In addition, these developments have changed the cost dynamics of laser welding in many ways. Now, new compact and air cooled pulse mode fiber lasers are available with pulse durations up to 20 ms and a peak power level of up to 6 kW. Unfortunately, using these standard broad band fiber lasers for specific application involving titanium or titanium alloys and inert cover gas led to unintended consequences and as will be discussed the degradation of weld quality.
In general, the major advantage of laser beam welding is the lower heat input into the part that minimizes welding stresses and reduces distortion. This is achieved by the high energy density of the beam which produces a small fusion zone with high welding speeds. Titanium materials can be welded without a filler metal and with the use of an effective shielding gas to avoid contamination. Also, laser welding has the advantage of high productivity and ease of automation that helps reduce manufacturing costs. All of these benefits have increased the use of laser welding in such areas as medical device manufacturing.
Titanium is an excellent corrosion resistant and biocompatible material that is used widely in precision or aesthetic oriented applications such as those in the medical devices industry. However, titanium can easily react with low density elements like oxygen, and nitrogen, which requires that the welding region be thoroughly shielded by an inert gas to avoid contamination from air, moisture, and/or other soot-forming hydro-carbonaceous matter during the welding process.
Lasers offer a wide range of welding capabilities such as spot, conduction, penetration and hybrid welding. Laser welding is performed virtually in every industry as it offers numerous benefits such as high welding speeds, short weld cycles, low heat input, low heat affected zone and minimal distortion.
A fiber laser is a laser in which the active gain medium is an optical fiber doped typically with rare-earth elements such as erbium or ytterbium (among others).
The advantages of fiber lasers over other types of lasers include the very high efficiencies typically over 30% of a wall plug. Further, it should be noted that fiber lasers are compact, compared to rod or gas type lasers of comparable power. The fiber's waveguiding properties eliminate thermal distortion of the optical path, producing a diffraction-limited, high-quality, optical beam. Fiber lasers also exhibit high vibrational stability and no maintenance during operation.
The good beam quality of fiber lasers coupled with high CW powers offers deep penetration welding, as well as shallow conduction mode welding but can yield difficulties in soot generation. Modulating these CW lasers offers pulsed laser capabilities with high peak and low average power for low heat input applications. High modulation frequencies on high power lasers up to 10 kW offer very high throughput in pulsed applications. The fiber delivery offers flexibility to integrate into conventional weld head, galvo heads, robotic are and remote welding systems. Whatever the beam delivery employed, fiber lasers offer unparallel performance.
Typical spot welding applications include galvo-based beam delivery that delivers high speed welding of products such razor blades, and HDD flexures that take advantage of the pulsed capabilities of the fiber lasers. Fiber lasers can be focused to small spot with extremely long focal lengths, thus remote laser welding capabilities with fiber lasers are highly enhanced. The benefit of large stand off in the order of 1-2 meters increases the work area multifold over conventional robotic systems. Such remote welding stations equipped with the fiber lasers include welding door panels, multiple welding of spot and lap welds all over the auto body frame.
The laser beam quality of the fiber laser (ability to focus to the smallest spot diameter) is superior to other solid state high power lasers (for example Nd:YAG). Also, because of the better beam quality, the standoff distances of the focusing optics to the workpiece can be extended to achieve a clearance of two to four feet for some applications. These are some of the advantages in transferring welding applications to fiber lasers.
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Accordingly, there is a need for an improved system and method for utilizing a fiber laser to affect a titanium or titanium alloy weld in the presence of an inert gas so as to reduce the effects of a higher plasma intensity and resulting contaminant content of the plasma.
An aspect of the present invention is to provide an improved system and method for utilizing a fiber laser to affect a titanium or titanium alloy weld in the presence of an inert gas so as to reduce the contaminant content of the plasma formed during the welding process.
Another aspect of the present invention is to provide a fiber laser having a narrow emission bandwidth at the selected wavelength to reduce the interaction of the laser beam with the cover gas and the weld plasma.
The present invention relates to a system and method for reducing contamination in the plasma of a weld produced by a fiber laser. The invention establishes the fiber laser in an optimal configuration for applying a high density beam, in a small fusion zone, to a weld material, additionally preferred with a top-hat beam profile having a square temporal shape. The beam from the fiber laser is applied at selected wavelength (1064 nm as an example) in a narrow bandwidth (of +/−0.5 nm as an example). An inert shielding gas is utilized in a cross-flow or glove box configuration around the welding region to prevent contamination. The inert shielding gas may be technically pure Argon, Helium, Neon, or Krypton with recognition of price limitations, or may be a mixture. The method is optimized by maintaining a numerical aperture for the high density beam, a set of beam divergence properties, and calculating an optimal focus spot size while maintaining a narrow band width. The system can utilize a single laser input, or can utilize multiple lasers joined by coupling means and utilizing a switch to select one or more of the fiber lasers.
The method comprises a number of steps that begin with establishing the fiber laser in an optimal configuration for applying a high density beam of laser light, in a small fusion zone, to a metallic weld material that does not have a filler metal such as titanium, titanium alloy, and optionally nickel or copper. The beam S of laser light is applied in a narrow bandwidth, to the weld material to form a weld in the welding region. The optimal narrow bandwidth for the present embodiment application is at 1064 nm+/−0.5 nm, and particularly avoids the spectral lines for argon at 1050.65 and 1067.36 nm, for helium at 1066.76 nm or 1082.91 nm. An inert shielding gas, such as argon, helium, krypton, or neon is utilized in a cross-flow or glove box configuration around the welding region to prevent contamination. Krypton is disfavored due to its high commercial cost, as well as helium, which is used in low quantities as a mixture in some applications to enable post weld leak inspection. Both gases also have higher conductivity properties.
The present system and method also optionally involves the creation of a specific narrow bandwidth fiber laser to minimize an interaction with spectral lines of the cover gas and to minimize an interaction with the spectra of metal weld ions during welding. More than one spectral line may need to be avoided. Where a gas mixture is used as the cover gas or a metal alloy is used several spectral lines must be avoided. Spectral interaction with the narrow bandwidth fiber laser must be avoided.
The method further comprises the steps of determining and maintaining a suitable numerical aperture for the high density beam; establishing and maintaining a set of beam divergence properties; and, calculating an optimal focus spot size.
The optimal configuration for applying the high density beam of laser light, in the small fusion zone, is determined by selecting one or more characteristics from a list which includes: beam delivery; numerical aperture; spot size; spatial profile; temporal profile; pulse duration; pulse repetition rate; weld spots overlap; weld fixture; and the speed of the weld generation. If applied properly, the optimized configuration produces a bright, shiny, and oxide-free surface finish without soot.
The system of the present invention for laser beam welding comprises an air cooled pulse mode fiber laser placed in an optimal configuration for applying a high density beam of laser light in a narrow bandwidth of 1064 nm+/−0.5 nm to a welding region having argon as a cover gas. The welding region further comprises: a workpiece to be welded; a small fusion zone within which said weld is to be applied; and, a weld material for forming the weld in the small fusion zone located on the workpiece. The weld material includes: titanium; a titanium alloy; or, optionally, other non-ferrous metal without a filler metal, and applied at a material thickness that is optimum with the spot size used in order to avoid any influences in the weld finish. Experiments were performed with titanium material having a thickness of 300 microns while using a spot size of 600 microns. Optionally, an inert shielding gas such as industrial quality argon or helium is applied to the welding region in a cross-flow so to prevent contamination of the weld.
The system can utilize a single laser input, or can utilize multiple lasers joined by coupling means and utilizing a switch to select one or more of the fiber lasers.
The above, and other aspects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying, drawings, in which like reference numerals designate the same elements.
Reference will now be made in detail to several embodiments of the invention that are illustrated in the accompanying drawings. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form.
As noted earlier during simple substitution there was a surprisingly high soot generation when yielding in an Argon (Ar) cover gas and substituting a solid state Nd:YAG laser at 1064 nm with a conventional fiber laser at 1070 nm using all other similar factors. This was unacceptable and during development of the present invention the inventors determined that the fiber laser-matter interaction was fundamentally different from Nd:YAG laser-matter interactions because the resulting plasma with the fiber laser indicated a much larger and brighter white/blue spectral signal when compared with a smaller less intense white/yellow spectral signal produced with Nd:YAG lasers.
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The beam 22 is directed into a welding region 30 having a small fusion zone 26 which allows a weld material 28, such as titanium, to form a weld on a workpiece 24. The weld may move as either the workpiece or the laser moves in use, or both.
An inert argon shielding gas is applied in a cross-flow region 32 (or all components can be placed into glove box with a controlled environment) which serves as a cover gas in the welding region 30 to prevent contamination of the weld. Commercial grade argon with purity 99.996% is optimally used. The gas shielding flow, orientation, and angle are optimized to obtain a bright, shiny and oxide free surface finish.
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An inert shielding gas, preferably argon, is applied in a cross-flow region 78 (or all components can be placed into glove box with a controlled environment) which serves as a cover gas in the welding region 76 to prevent contamination of the weld.
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From step 102, the flow advances to step 104 where a weld material, such as titanium, is placed in position on a workpiece. Step 104 can be performed prior to steps 100 and 102; the importance of the step lies in its being performed prior to generation of the laser beam.
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From step 110, the flow advances to step 112 where the fiber laser generates a beam within a narrow bandwidth, here designated as 1064 nm+/−0.5 nm due to the selected argon cover gas that allows the plasma of the weld to form without the contaminants that are found to form when outside the optimal range. The laser beam is then directed, at step 114, into a small fusion zone where the weld material is located on the workpiece. As the beam is working on the weld material, the system queries at step 116 as whether or not an inert gas is applied in a cross-flow around the fusion zone. If the response to the query is “YES,” then an inert gas designated for the system to avoid spectral interference such as argon is applied in a cross-flow across the wok zone at step 118 (or all components can be placed into a controlled environment, e.g. a glove box (see below at step 120)). However, if the response to the query at step 116 is “NO,” then the flow advances to step 120 where all the components are placed inside a controlled inert gas atmosphere.
From step 118, and from step 120, the flow advances to step 122 where the weld is formed on the workpiece. After the weld has been formed, the method queries as to whether or not another weld is to be performed. If the response to the query is “YES,” then the flow returns to step 104 where the weld material is placed in position for the next weld formation. If, however, the response to the query at step 124 is “NO,” then the flow advances to step 126 where the process is terminated.
The disclosed method was tested and produced welding results that confirmed that the root cause of the stronger plasma behavior and higher soot generation with the 1070 nm fiber laser was due to the interaction between the laser wavelength output and the argon absorption/emission line at 1067.35 nm. As a result, using the discovery, and the proposed systems and methods an improved behavior results with minimum soot generation similar to the Nd:YAG laser. The superior weld was the narrow bandwidth fiber laser at 1064 nm. This avoided additional interactions between the laser beam, the cover gas and the weld material (titanium). Following the method steps will allow the same benefits for other cover gases and weld materials.
Having described at least one of the preferred embodiments of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes, modifications, and adaptations may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.
