Experimental and Theoretical Work – Hybrid Welding, Laser Beam Splitting and Toe Wetting

Work was conducted to determine the minimum laser power requirement and power density to achieve effective wetting at the weld toe. As such, work with the scanning head was discontinued, and dual laser beam studies have also been concluded in favor of an offset (with respect to the weld centerline) single-beam approach. The use of GMAW-P alone for welding sheet metal is constrained by the minimum effective bead size that can be maintained without the occurrence of the bead humping phenomenon. Introduction of the additional heat input from the laser beam allows a reduction in this effective minimum bead size that also allows considerable increase in welding TS.

Mechanisms in operation and controlling the tendency or otherwise for bead humping to occur are the arc force, bead size, TS, WFS, laser power, laser focus, and power density. The weld bead is prone to a humping discontinuity when the toe angle between the weld and the base material becomes less than 90 degrees.

HSV was used to show the interaction of the laser spot and the GMAW-P weld pool, Figure 5.5. The laser is set up to lead the front edge of the GMAW-P weld pool by 2.0 mm, with a tolerance of0.5 mm. This allows for a stable interaction and focus height for the laser. It is also routinely observed that if the laser is focused at a point on the GMAW-P weld pool surface that significant spatter is generated resulting in a weld bead size that is too small because of volume loss of weld metal.

Figure 5.5 Setup for HSV of the CO2Laser/GMAW-P Hybrid Welding Process

Figure 5.6 Adjustable Beam Distance for CO2Laser Split Optics showing 3.2-,

6.4- and 9.5-mm Dual Spot Spacing

The experimental work conducted is summarized below for scanning and dual-spot Nd:YAG, and both single- and dual-spot CO2lasers, Figure 5.6, used with GMAW-P. A fully integrated system was completed and used for controlling and recording welding parameters, initiating the GMAW and laser welding power, and the fixture motion.

5.4.1 Scanning Laser Nd:YAG and Dual Spot

The Trumpf PFO system is limited to 1-kW laser power, and thus offers limited opportunity to increase welding speed and productivity. As such, a dual-spot technique was used with 4 kW of Nd:YAG LBW power. Beam splitting was accomplished using commercially available split optics, but using each of the two beams on the same weld bead.

Scanning the laser beam using the Trumpf PFO is limited to 1-kW laser power. Trials were conducted to maximize the bead wetting and to increase productivity by

increasing the welding speed achievable. Laser/GMAW hybrid welding was also conducted with single and dual beams (split optics), Figure 5.7 using Nd:YAG laser power up to 4 kW, Figure 5.8.

Figure 5.7 100-W Dual Beam Nd:YAG Laser Burn Pattern on Thermographic Paper to Illustrate TEM 01 Laser Mode

Figure 5.8 Fibre-Delivered Nd:YAG Laser and ESAB GMAW-P Equipment Setup on a Lap Joint for Single- and Dual-Spot Welding Trials

5.4.2 CO2Dual Beam

A 4-kW CO2laser was used for dual-spot welding, and the productivity evaluated based on the 2 kW available for each laser spot, Figure 5.9, and the available heat input to increase the wetting angle. The coating on the Perspex was applied only to highlight the laser ‘burn pattern’, not for laser absorption.

Figure 5.9 CO2Laser Burn Pattern on Perspex for Dual Beam in TEM 01 Mode

Work was then conducted with a single-beam approach, the laser beam being offset to the lower sheet in a joint. Work was carried out to establish the minimum laser power requirement to achieve effective wetting at the weld toe. Laser/GMAW-P hybrid welding was conducted with CO2laser power up to 4 kW, with CO2LBW/GMAW-P trials carried out with an offset (with respect to the weld centerline) single-beam approach.

5.4.3 CO2Offset Beam

The beam was offset onto the lower ligament of the joint to increase the wetting of the lower weld toe that was considered the limiting factor in making a continuous high- speed fillet weld. Defocusing the laser spot is a balance of sufficient power density with the beam width. The beam width and beam position relative to the lower GMAW-P weld toe were considered important and were investigated in this work. The same approach was applied to a T-butt weld. The offset and defocused CO2laser spot was used in GMAW-P/LBW hybrid welding trials on sheet metal joints

5.5 Calorimetry

Calorimetry was performed for GMAW-P, and LBW/GMAW-P. The technique employed was that established by Smartt et al. at INEL, involving recording weight change due to liquid nitrogen boil-off with time. Calorimetry was conducted for GMAW-P, and CO2laser/GMAW-P welding., with the equipment set-up integrated into the data acquisition system, Figure 5.10.

Figure 5.10 Control Block Diagram of the Calorimetry Experiments

5.6 Experimental Procedure for HSV

The following sequence was used to setup and record HSV images on the Kodak HSV system:

1. Depth of focus – extension to lens/aperture.

2. Recorded field of view and calibrated same in mm – used 25- to 40-mm field of view.

3. Aiming point behind the arc to get a good view of the hump formation when using high-speed GMAW-P and LBW/GMAW-P.

4. Calibration of light source – exposure to set light levels across diode array in the Kodak camera.

5. Focus with band-pass filter when arc is running.

6. A fiberoptic lightbox was used to illuminate the area of interest with respect to the GMAW torch for field of view, preliminary focusing and calibration image. Figure 5.11 shows the wire tip, and metal scale indicating a 27-mm field of view.

7. A frame rate of 250 fps provided 2 s of recording time on the Kodak camera. This allowed recording of a 150-mm weld length at a TS of 3 m/min.

Figure 5.11 Calibration Image for HSV Using a Millimeter Scale

HSV was conducted for both BOP welds and fillet welds in lap joints. The frame rate was selected to record the weld pool dynamics during hump formation, and

particularly to record the event which causes the end of one hump and the beginning of the next. This key even between humps has not previously been documented in a way that allows it to be definitively seen. Being able to view and interpret this event is key to the understanding of the humping discontinuity.

5.7 Accuracy and Inaccuracy of Methods, Verification, and Repeatability