BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pre-welding analytical methods as well as methods and lasers for welding metals and metal alloys, and particularly titanium or titanium alloys, with a laser independent of any cover gas. Specifically, the present invention relates to the pre-determination of the spectrum of the electronic transition of metal vapor in a welding process and providing a narrow bandwidth fiber laser that eliminates optical interaction between the laser beam with the spectrum of the electronic transition of metal and metal alloys.

2. Description of the Related Art

Laser beam welding is a welding technique used to join multiple pieces of metal through the use of a laser. However, weld defects such as solidification cracks, porosity, material loss (spattering), bumps and dents (undercut), lack of penetration, coloration (oxidation) etc. might be unacceptable in some applications for functional or aesthetic reasons. Two of the Applicants of the present application have previous experience creating welding solutions with respect to complex welding problems. PCT/US2010/050626 (hereinafter the '626 app) which was published on 5 Apr. 2012 as WO 2012/044285 A1 and whose contents is incorporated by reference in its entirety concerns the use of fiber lasers for welding titanium in combination with a cover gas. This application describes a method and a system for reducing contamination in the resulting plasma of a weld produced by a fiber laser. According to this application, the contamination in a weld can be reduced 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 a laser beam with a so-called inert cover or shielding gas, preferably argon. It was postulated in this application that soot was produced by the interference of the fiber laser beam emission with the cover gas as the prior art use of a Nd:YAG laser in combination with an argon shielding (cover) gas produced limited, if any, soot.

In particular, the '626 app was directed to solving the problem that using an off-the-shelf fiber laser results in poor optical weld quality compared to a weld with a Nd:YAG laser (FIG. 1).

The '626 app provided that this effect was caused by interactions of the laser light with the electronic transitions of the cover gas, i.e. Argon at 1050.65 and 1067.36 nm or He at 1066.76 or 1082.91 nm. The '626 app further taught changing the center wavelength and reducing the emission linewidth of the laser from 1070±5 nm to 1064±0.5 nm would avoid the interaction with the electronic levels of the cover gas and thereby secure a weld with good visual quality (FIG. 2).

The '626 app further disclosed that the laser emission wavelength could additionally be tuned to minimize the interaction of the laser beam with the electronic levels of the ions of the weld material to further improve the visual aspect of the weld. One of disadvantages of the solution proposed in '626 app is that the fiber laser systems works in a specific optimal configuration, including laser spectrum, which takes into account only effects of the interaction of the laser light with the electronic transitions of the cover gas (e.g. argon) as well as with the electronic levels of the ions of the weld material.

The present application sets forth the discovery that the '626 app is not completely correct, that the soot or other detritus which caused coloration (resulting from oxidation, particle deposition and similar effects or their combinations) of the weld was not created by the interaction of the ions of the weld material or by the interaction of the spectrum of the laser with that of the cover gas. Nevertheless, the solution set forth herein below still requires the narrowband fiber lasers disclosed in the '626 app.

The demands of providing narrowband, stable lasers equipped to provide sufficient power that are cost-efficient and not complicated has lead one of the Applicants to develop multimode lasers that meet these difficult requirements. As provided in PCT/US2014/40454 which was published as WO 2014/197509 A1 on 11 Dec. 2014, the contents of which are incorporated by reference in its entirety, non-linear effects can cause unwanted spectral broadening of the laser emission spectrum where higher power densities are pursued.

The degree to which such lasers could be adapted had yet to be explored not to mention whether the quality of light, as opposed to that provided by a single mode laser, would be sufficient for welding.

Accordingly, there is a need for an improved system and method for pre-selecting a laser spectrum to for welding a metal/alloy as well as providing a related welding process and fiber lasers that can implement same.

SUMMARY OF THE INVENTION

The present invention provides a method of pre-selecting the parameters of a metal/alloy welding process independent of the spectral properties of a cover gas. This method includes determining the spectrum of an electronic transition of the metal/alloy vapor by analyzing an emission spectrum of a metal/alloy welding process using a broad bandwidth energy source; and selecting a laser wavelength and linewidth outside the spectrum of any electronic transition of the metal/alloy vapor. It follows that under certain circumstances that electronic transition spectra may be available in the literature and the first experimental step set forth above may not be necessary.

According to the present invention, a method of pre-selecting the parameters of a metal/alloy welding process independent of the spectral properties of a cover gas comprise: determining the spectrum of an electronic transition of the metal/alloy vapor by analyzing an emission spectrum of a metal/alloy welding process using a broad bandwidth energy source or obtaining this information from literature; and selecting a laser wavelength and linewidth outside the spectrum of any electronic transition of the metal/alloy vapor. However, if selecting a laser wavelength and linewidth outside the spectrum of any electronic transition of the metal/alloy vapor is not possible, laser wavelength and linewidth is modified that way that interactions with lines of high oscillator strength are minimized. It should be noted that high oscillator strength generally means that the probability for the absorption of a photon of a certain wavelength is high

A metal/alloy system that can immediately benefit from the present invention is titanium or a titanium alloy which can be used e.g. in medical applications for manufacturing medical devices requiring clean welds. As provided herein below, avoidance of the spectrum of titanium vapor in a plasma plume results in a weld substantially free of detritus. While the '626 app spoke of minimizing soot, Applicants recognize that soot and other contaminants may be causing weld coloration (resulting from oxidation, particle deposition and similar effects or their combinations), hence use of the term detritus in lieu of soot.

The present invention further provides a method of creating a metal/alloy weld in a workpiece independent of the spectrum of a cover gas. This method includes selecting one of a number of spectral bandwidths that avoids the spectrum of an electronic transition of a metal/alloy vapor. In particular, it is preferred that a method of creating a metal/alloy weld in a workpiece independent of the spectrum of a cover gas, the method comprises following steps: selecting one of a number of spectral bandwidths that avoids the spectrum of an electronic transition of a metal/alloy vapor; providing a spectrally-stable laser with power sufficient to create a weld pool with one of said spectral bandwidths; and subjecting the workpiece with an output from said laser sufficient to create a weld. It is clear that this requires the practice of the abovementioned pre-selection analysis.

To implement the weld, one must provide a spectrally-stable laser with power sufficient to create a weld pool with one of said spectral bandwidths with an output power sufficient to create the weld.

It is especially preferred that the metal/alloy is titanium or a titanium alloy.

In another preferred embodiment of the method of creating a metal/alloy weld, the weld is characterized by being substantially free of coloration from plasma detritus.

It is further preferred that the weld track is characterized by being substantially free of geometrical height or width fluctuations. Weld tracks possess more or less periodic height fluctuations along the laser weld line. These fluctuations are induced by fluctuations of the heat flow during the solidification of material strongly influenced by the fluctuations of absorbed laser light.

In yet another preferred embodiments of the method of creating a metal/alloy weld, the real time detected signals from the welding process are substantially free of absorption based disturbances. Signal detection during welding processes is perturbed by signals coming from the metal vapor plume laser light interaction. Therefore reducing the absorption in the vapor plume will increase substantially the signal detection quality. The latter by reducing the strength or even the presence of the signal.

While the experiments performed to date concern use of titanium and titanium alloys, the present invention is not so limited to these materials only. It is contemplated that the physics of the titanium plumes described herein are applicable for other metal/alloy materials or systems.

The focus of the present invention is to avoid creation of welds that suffer from coloration, such coloration due to detritus thrown off from the plasma. As the experimental results clearly provide, manipulation of the spectrum of the laser so as to avoid the metal/alloy vapor results in a plume geometry is different than where there is interference.

The present invention also provides a narrowband fiber laser, especially a multimode fiber laser with stable narrowband beam emission, which is customized to produce laser beam with necessary parameters. This fiber laser includes fiber Bragg gratings pre-selected to emit an output in a pre-determined narrow spectral bandwidth, the narrow bandwidth corresponding to a spectral bandwidth that avoids the spectrum of an electronic transition of metal/alloy vapor—determined as provided hereinabove—an active fiber configured to suppress non-linearities; and diode lasers configured to pump the active fiber so it can laser in the narrow bandwidth. Suppression of the non-linearities is an important aspect of the present invention as the laser must reliably emit in a narrow bandwidth and the prior art single mode lasers configured for this required additional componentry to accomplish this. As mentioned above, a cost-efficient and uncomplicated example of such narrowband fiber laser system can be found in the international application PCT/US2014/40454 which was published as WO 2014/197509 A1 on 11 Dec. 2014 and which content is explicitly incorporated into the present application by reference in order to avoid repetitions.

Just in summary, PCT/US2014/40454 discloses an oscillator having a Fabry-Perot configuration. In particular, the disclosed oscillator is configured with a multiple transverse mode or simply multimode (“MM”) step-index active fiber. The opposite ends of the active fiber are coupled to respective MM passive fibers. The resonant cavity is defined between two MM fiber Bragg gratings written in respective cores of MM passive fibers. As known, one of the main reasons for line broadening in single mode (SM) fiber lasers is the presence of so-called nonlinear effects (“NLE”), such as Raman scattering and four wave mixing, which become more pronounced with the power increase. The threshold for NLE can be lowered by increasing a core diameter and cavity length. Hence MM fibers characterized by large diameter cores and smaller cavity lengths are characterized by relatively narrow spectral linewidth.

Also in this case, it is preferred that the metal/alloy is titanium or a titanium alloy.

The preferred embodiment for the laser for the present invention is a multimode laser with a large mode area active fiber so as to minimize the non-linearities and provide a simple oscillator that can provide an efficient and economic system for welding using a narrow bandwidth output.

It is further preferred that the laser is configured to provide a multi-mode output.

In another embodiment for the laser according to the present invention it is preferred that a central wavelength of the narrow spectral bandwidth is in a range from 1020 nm to 1090 nm.

In yet another embodiment it is preferred that a central wavelength of the narrow spectral bandwidth is in a range from 1400 nm to 2100 nm.

