The welding machine is the Pro-beam K2000 electron beam welder equipped with a 500 mA 80 kV that can be used at accelerating voltages between 60 kV and 80 kV. The beam produced by a triode is focused on the work piece by a magnetic coil.
The emitting source is a 0.5 mm thick, 3.5 x 3.5 mm square tungsten electrode that is positioned into the cathode holder using the Pro-beam precision mounting equipment.
The gun is placed on a 5 axes gantry inside the vacuum chamber facing forward. The work distance between gun and work piece is typically 400 mm (480 mm between work piece and center of magnetic lens).
Before a new weld, a new cathode is installed, peaked and the centering/stigmatization tests carried out.
The K2000 was updated to the new multimode controller and the EB vision software which allows the work described in this paper.
2.1. Cleaning
The parts are either degreased using a degreaser followed by drying with isopropanol/acetone. The magnetic cleaning is carried out to a level of 2 Gauss prior fabrication. Gauss measurement is carried out by Hall effect probe.
2.1. Setup circular component
A forged shell 1800 mm OD, 90 mm thick is placed on blocks of conducting material on the 20 tons capacity turn table (Fig. 2). The cylinders are setup using a dial test indicator (DTI) ensuring they are correctly centered. Moreover, the DTI is used to ensure the weld face is flat and perpendicular to the incident beam. The welding technician setting up the part stopped improving the setup at 0.5 mm on circumference and 0.2 mm on flatness. A large circular impingement block (aimed at capturing excess beam power) is installed such that it is level with the joint line (roughly 500 mm joint line). The dome to be welded to it is next positioned over it (in this case craned down using 3 lifting lugs).
2.1. Beam angle test
This test allows the operator to determine whether something has been setup wrong. It is mainly aimed at testing the gun or beam angle relative to the work piece.
Two blocks are setup at a distance D from one another. In this case D is equivalent to the thickness of the testpiece(Fig. 3). The test is done in 2 perpendicular orientations if necessary. In the case of the K2000 there is the added option of moving the gantry back.
2.2. Working distance test/monitoring
ASME IX authorizes a maximum of 5% work distance variation[1], this leaves the necessity to ensure the part has correct ovality and is properly centered. The maximum ovality of a component per ASME VIII is 1% [2]. For a nominal 1800 mm outer diameter vessel, this means a maximum diameter of 1809 mm and a minimum of 1791 mm. Considering a perfectly centered part, there could have a 9 mm working distance variation between the gun and the part which is very tolerable. It is then important to ensure the part is correctly centered.
Nuclear AMRC defined a test that aims at assessing the distance between the gun and the part (Fig. 4).
Considering the gun was calibrated at the correct working distance, WD (= 420 mm) during setup, to find WD2 the actual distance from the joint line the gun is displaced in the Z direction by a value D. A seam tracking scan is made to assess whether the gun is further or closer to the part.
In Fig. 4, the same joint line will be perceived as being higher or lower than it is in the scan, returning a value D2. D2 can then be used to calculate WD2 through the following formula:
$$\frac{D}{{WD}_{2}}=\frac{{D}_{2}}{WD}$$
The result of the test can either instruct the operator to re-centre the part or to carry out the welding with corrections. In the case of a circular component, the test described here uses the seam tracking discussed below to capture the working distance of the part at 174 mm intervals.
The test is carried out on a centered part and the same part that has been moved off centre by 25 mm (~ 5% of working distance magnetic lens to work piece). The work offset chosen is D = 40 mm.
2.2. Seam tracking
A vessel of diameter 1800 mm will have a circumference of 5600 mm. Attempting a scan for a component of this length has shown to crash the system by the sheer size of the data collected.
The programming effort here is instead of scanning the full component, to carry a short scan at a predefined distance.
In the present case, the seam tracker tracks the joint line at 32 positions around the vessel, leaving 174 mm between each correction.
Each scan is 1 mm long and has a works within a 10 to 20 mm height envelope to ensure the joint line is fully captured.
Nuclear AMRC uses mainly two weld preparations in thick section welding. Figure 5 presents both. The first weld prep consists of a joint line between two dissimilar thickness. The bottom section being thicker to allow a support of the melt pool. However, there is no direct view of the joint but rather the edge and the weld becomes dissimilar thickness effectively. The second weld prep add an extra 2 mm step at the front to allow to solve these two issues.
Once the scan is made, it is possible to correct the positioning of the beam to ensure the joint line is tracked for the full duration of the weld. The (a) shows the case of a fully corrected section with no error. Errors can occur when the joint line is too faint or several features akin to joint lines are present, as is the case for example (b). It is then further necessary to calibrate the seam tracker. In this case the shadow of the step was recognized as a potential joint line and the operator selected the correct one instead.
The seam tracker automatically saves the captured images of the seam-tracking operation, including the success/failure overlay.
For long components, the benefits of using several short scans over one large one are in cases of failure one can just repeat the last scan instead of repeating a complete scan. In complex components it is likely that settings need changing at every scan such that one setting does not fit a full 5600 mm assembly.
Saw marks or turning marks also have tendency to be picked up as a potential joint line, disrupting the seam tracking process. Using the electron optics at high power (50 mA) over the joint line area has a cleaning effect that can remove these marks improving the situation greatly.
2.3. Differentiating between geometrical or magnetic misalignment
The seam tracking outputs values. This is the task of the operator to decide what to do with those. At Nuclear AMRC, the program automatically warns the user if a value has been reached such that the angles discussed in Table 1 are reached. In Fig. 6, one can see depending on the type of misalignment, mechanical or magnetic, the operator can choose whether to use a correction using the mechanical axes or the magnetic axes.
For the magnetic axes only:
Using both axes:
When attempting a double correction, it has shown useful to carry out a beam angle test to ensure the angle has been set correctly.
2.3. Gap monitoring
It is possible to track the joint opening using the seam tracking data. A python code is used to read the data and carry out the measurement of the gap. For this, it is necessary to know the dimensions of the scan in pixel/mm. The data can then be applied to either correct the beam parameters or warn the operator of an anomaly (i.e., if the gap is above what was qualified).
2.3. Simulation
A simulation is then completed using electron optics (Fig. 7). The parameters represented during the simulation are gun/component movement. Parameters not represented during the simulation are the beam current/beam focal value and beam oscillation. The simulation can be setup at welding speed or for convenience, lower or faster.
During the simulation, it is ensured that the crosshair is kept at the required position, here centered on the joint line proving that the seam tracker has completed the corrections properly. The simulation allows the operator to see any error related to setup prior to welding. Should the crosshair move away from the joint line between the points, the seam tracker can be setup at a shorter interval. A similar feature provided by Pro-beam is the “scanning like welding” feature that captures the full length of the joint and the operator can ensure the joint is below a level of deflection.
2.3. Tacking
Welding a non-secure component has shown to create large gaps often in the opposite direction of the solidifying weld.
The assembly joint line need to be tacked to ensure the weld is successful. In general, 16 tacks are carried out. In this case, the tacks are 174 mm long, 10 mm deep and are carried out every 348 mm such that all tacks fall between 2 seam tracking points. The tacks are welded following a sequence developed at Nuclear AMRC to ensure no gaping occurs during tacking process. The sequence is shown Fig. 8.
The sequence was optimized to minimise gapping. The low depth of the welds was selected such that there would not impart any solidification based stresses high enough to move the component. Similarly, the sequence was balanced to prevent the same issue.
2.3. Welding
The welding parameters are shown elsewhere [3].
2.3. Data logging
The data is automatically captured by the multimode. The EB vision software allows to visualize the data, but it is also possible to view as CSV files.