Applications

Welding

New Laser Welding Capability from Prima Power Laserdyne

While the beginning of the LASERDYNE® product line can be traced to laser welding, most associate LASERDYNE with laser cutting and drilling for aerospace applications. Over the 33 year history of what is now Prima Power Laserdyne, laser welding systems have been supplied for aerospace (engine and airframe), electronics, fluid couplings, and medical device applications using CO2, Nd:YAG, and, more recently, fiber laser sources.

The availability of the high power CW and QCW fiber laser with its kilowatt-level average power, 1 micrometer (µm) wavelength, and high brightness (beam quality) provides a laser source with new capability and flexibility. Compared to CO2 lasers for welding, it is well documented that the 1 µm wavelength of the fiber laser provides benefits in terms of simplified beam delivery using fiber optic cables instead of turning mirrors; greater absorption by metals, especially those which are good conductors of electricity such as aluminum and copper; and less absorption by the plasma plume that is formed above the weld pool. The higher brightness means that the laser beam can be focused to smaller sizes which, in turn, leads to higher power density. These factors contribute to deeper penetration and faster welding speed than available from previous sources of equivalent average power. They also mean more stable welding processes in a wider range of metals and alloys.

Over the past several months, Prima Power Laserdyne has made significant investment in developing process and system capability for welding 2D and 3D components with high power CW and QCW fiber lasers. Metals and alloys for which capability has been demonstrated include 304 stainless steel; titanium alloys including Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo; and nickel based high temperature alloys including Inconel 625, Inconel 718, Haynes 230, and Hastelloy X.

Welding trials using a range of laser parameters and shield gases were performed with high power fiber lasers. Metallography (cross-sections) and X-ray radiography were used to document the relationship between laser (spot size, laser power, etc.) and processing (shield gas type, gas flow rates, method of gas delivery, welding speed, focus position, etc.) parameters and the resulting weld geometry and structure. Results of the trials showed conditions that lead to weld porosity and those that give porosity-free welds. They also showed the relationship between laser and processing parameters on weld shape and profile.

The most comprehensive investigation has been around laser welding aerospace alloys. The major challenge of laser welding aerospace alloys is the stringent joint requirements, that is, no cracking or porosity and correct weld geometry for good mechanical properties at high temperatures. Both CW and QCW fiber lasers are capable of welding these alloys. However, the challenge lies in developing laser and processing parameters which will produce good quality welds. Laser and processing parameters have been developed to weld a range of aerospace alloys. The work has highlighted that no single parameter controls weld quality whereas it is a combination of both laser and processing parameters which has a significant effect on the weld quality. It has also shown that crack and porosity-free welds can be readily produced in a range of nickel and titanium based alloys.

Welding trials also have been performed with addition of filler material. Certain alloys and dissimilar material combinations require the addition of filler material to control the structure of the weld metal and avoid cracking to ensure the required mechanical properties. In other cases, filler metal is used to control the weld geometry, such as to create a slight convexity (reinforcement) of the weld fusion zone. Filler material is also used to compensate for poor fit up and mismatch during laser welding in butt joint configuration. Laser welding with the filler wire is a multi-parameter process and there are a number of laser and filler wire parameters which determine the quality of the resultant weld. All of the important parameters for adding filler material have been developed and optimized to produce good quality welds.

This work has also lead to new system features. One such development includes a focusing lens assembly with cross-jet that maintains the compact profile of the LASERDYNE third generation BeamDirector®, called BD3Y. The cross-jet provides a high velocity gas barrier that prevents metal sparks from the weld zone from contaminating the protective lens cover slide. Critical to the design is that the cross-jet also not contaminate or otherwise interfere with the welding shield gas. The cross-jet nozzle can be used with the entire range of shield gas delivery devices including welding shoe and coaxial gas nozzle tip. The shielding gas shoe provides a controlled atmosphere for the weld zone while it is molten and while it is cooling to a temperature at which it will no longer be compromised by the ambient atmosphere. This is particularly important for welding materials, such as titanium alloys, that have a strong affinity for oxygen and nitrogen in the ambient atmosphere. An important benefit of the design of the focusing lens/shield gas assemblies for laser welding is that they can be quickly changed in order to vary the focused spot size.

New capability for laser control, most notably laser power ramping and laser pulse shaping with sub-millisecond resolution, has been demonstrated using the LASERDYNE S94P control. This additional control leads to more consistent, higher quality welds and increases the flexibility of the LASERDYNE system in terms of materials to which they can be applied. By controlling the temperature profile during weld formation and during cooling of the weld and heat affected zone, pulse shaping has been shown to control the weld profile and structure. For example, providing a lower amplitude pulse after the initial portion is used to control cooling of the alloys that harden during rapid cooling. Another example, is the use of pulse shapes with a higher amplitude on the leading edge for materials that are reflective to the laser beam even at 1 micrometer wavelength. Rapidly heating the surface leads to increased absorption of the laser beam and a more consistent process.

These features have since been standardized and will be part of the entire product line of 3 to 7 axes systems including LASERDYNE 795 and LASERDYNE 430BD.