Colin joined the Electron Beam Department of TWI in 1985 after gaining a degree in physics at the University of Nottingham. Since that time he has worked extensively on the research and development of high power electron beam equipment and was substantially involved in the special 100kW machine built for Framatome in 1990.
His flair for digital electronics and software has led to many important innovations in real-time seam tracking, control technology for high power switch mode power supplies and high frequency dot matrix electromagnetic deflection systems. Currently he leads four projects in TWI's Core Research Programme involving electron beam monitoring, real-time seam tracking, high voltage discharge control and computer aided field analysis.
Numerous factors, both electrical and mechanical, affect the performance of an electron beam welding machine. Colin Ribton explains how weld quality can be improved using process control.
Electron beams can be used to reproduce consistently high quality welds, but this demands precise control of a number of process variables. Recent advances in equipment design have benefited from development of multi-axis computer numerical control systems which allow a welding parameter set to be reproduced with ease. Equipment advances have concentrated upon generating higher power beams for thicker section applications. Inevitably, higher power beams are required in larger vacuum chambers for larger components. This trend has led to a wider application of in-chamber gun designs which mount the electron gun on a gantry. This enables the gun to be manipulated around the workpiece, in preference to putting the workpiece on a traverse and moving it in front of the gun.
In-chamber designs can allow the entire chamber length in any direction to be used for welding, and as a lower mass is being moved, can give a greater positional accuracy. The main asset of the electron beam process ( i.e. the deep and narrow fusion zone produced) imposes a high precision upon the beam to joint line alignment such that accurate manipulators are required. Control of the beam convergence angle at high powers, and beam deflection at all power levels, can yield very beneficial welding results and as a consequence the associated electrical parameters need to be programmable by the computer numerical control (CNC) system.
Advances in CNC during the past 15 years have allowed a larger number of parameters to be controlled from the 'part program' the prepared list of commands used to execute a particular welding job. On most machine tools these parameters are mechanical movement axes or spindle speeds, but inclusion of electrical axes for TWI electron beam welding machines has made possible control of accelerating voltage, beam current, focus position, beam deflection amplitude and beam deflection frequency.
The fully integrated system allows these parameters to be interpolated simultaneously with the mechanical axes. The initial and final parameter sets are defined within the part program by the machine operator. The CNC then linearly changes each parameter such that, as the programmed mechanical position is reached, the parameters all attain their final values. A feedrate parameter is used to control the rate of mechanical movement, and this indirectly controls the rate of change of the electrical axes.
Machine reproducibility and parameter measurement
There are a large number of factors which may affect the welding operation of an electron beam machine. The Table shows the electrical parameters for the TWI indirectly heated diode gun column and the accuracy of each parameter. In the case of the accelerating potential the DC level is calibrated by connection of the power supply to a resistive divider under no load conditions. Unfortunately the stray capacitance of such dividers prohibits measurement of the AC component of the output. Use of a capacitive-resistive divider within the power supply, which is continuously monitored during welding, allows some estimation of the accelerating potential ripple. The accelerating potential is the most difficult parameter to measure accurately, and although having a far-reaching effect on nearly all the other electrical welding parameters, it is frequently neglected in industrial equipment calibrations.
The beam current accuracy is readily measured, most simply by monitoring the earth return current to the power supply. However, this current includes many high frequency elements due to stray capacitance within the transformer windings and rectifier assembly. Although these may be removed by filtering, this can prevent measurement of any ripple present. A test procedure frequently used directs the beam into an isolated catcher (or 'Faraday cup') within the chamber. A resistor to earth allows the collected current to be monitored directly, and the ripple may be displayed on an oscilloscope. A more practical alternative used by TWI is based on a DC current transformer (DCCT) within the gun column. This sensor uses two toroidal saturable reactors of approximately 100mm diameter. The extent of magnetisation caused by the beam as it passes through the centre of the core is measured by control electronics and the beam current is measured to an accuracy of ±0.2mA. The circuit has a small signal bandwidth of l0kHz. The sensor head has no effect upon the beam itself and, as no current other than the beam passes through it, the measured signal is an accurate measurement of the beam current level and ripple. This device is used continuously throughout welding to monitor the beam current and produce the feedback signal for the control electronics.
The TWI designed 100kW indirectly heated diode gun does not exhibit beam current ripple. Most previous electron guns have been triode designs which operate in space charge limited mode, i.e. the beam current is controlled by a bias (or grid) potential. The TWI diode gun operates in temperature limited mode up to its maximum beam current output. In temperature limited mode the beam current produced is controlled by the temperature of the cathode. One of its advantages is that the beam current is independent of the accelerating potential whilst the gun is operating in this mode. Consequently the beam current is unaffected by accelerating potential ripple. (For triode guns the beam current and accelerating potential are related; beam current varies as accelerating potential. 3/2 As a result, any accelerating potential ripple is transferred into beam current ripple. Also, coupling between the high voltage transformer and the grid circuit can occur causing amplification of the ripple.)
Focus lens coil currents are produced by DC supplies in constant current operation. The current demand for each of the two lenses is generated by the CNC as a digital value. This is converted by each DC supply into a coil current which may be monitored by an in-line resistor. The power supplies were selected on the basis of their accuracy over the full working range and long term stability. The measured linearity of the focus supplies was better than 0.2%. Deflection coil currents are produced by a digital signal generator which controls the frequency of the deflection pattern to better than 0.002%. The frequency is programmed in steps of 1Hz from the CNC giving a resolution of 0.01%. The amplitude of the pattern is determined by a digital value which is used to control the output of current feedback amplifiers, providing an overall accuracy in the region of 1%.
Accurate establishment of electrical parameters does not unfortunately guarantee reliable welding performance. Very small changes in electron gun geometry can lead to shifts in beam focal position, or changes in beam intensity sufficient to change the welding performance of the machine. For example, the position of the cathode (the filament in a directly heated triode) within the electrode geometry is critical to a tolerance of ±0.02mm in a typical electron gun. It is a common cause of anxiety that welding conditions cannot be directly transferred between nominally identical machines, or reproduced by replacement gun electrode assemblies. Mechanical dimensions, especially cathode setback, the size of grid cup hole, and position of anode plate need to be closely controlled if machine performance consistency is to be guaranteed.
System structure and design
The design of the TWI machine centred upon full integration of the welding process control functions with the CNC system such that a welding procedure could be entirely defined within a part program. A second feature required from the CNC was the machine interlock system and diagnostic message display. The design stages followed were initially to produce a block diagram of the proposed system, showing the interconnection of the control units; next the control software was designed and the interface protocols specified; then the individual units were designed, many of these including local processing and control (
e.g. the column and chamber pumps sequencer control unit) which were kept independent of the CNC operation. This structure allows the autonomous control units to be modified or replaced without changing the CNC software or interface.
Electron beam welding can produce severe electrical noise, especially when the accelerating potential breaks down across the gun electrode gap. High currents are produced and the subsequent transients permeate throughout the electrical system and can be particularly harmful to digital microelectronic equipment operation. To avoid this problem, the CNC and all control units have been electrically isolated.
The power supply (a common source of transients) has been further isolated by using optical fibres for communication to the control electronics. The use of opto isolators imposes the condition that all parameters are digitally defined. In addition, this is desirable to reduce any possible drift effects which occur with analogue signals.
Selection of a CNC system has been on the basis of the number of digital ports and the number of simultaneously interpolating axes. The system is required to control the following axes in a continuous interpolating fashion:
- three linear movements;
- two rotational movements;
- beam current;
- two lens currents;
- vertical and horizontal deflection amplitudes;
- deflection frequency.
The following parameters need to be controlled in a step wise fashion:
- accelerating potential;
- deflection pattern select number;
- gun primary filament current.
In addition to the digital parameters a large number of output control lines are used to enable traverse drives, indicate machine status and communicate to the other control equipment. Input ports are used to monitor power supply parameters, machine interlocks, keyboard input, seam tracking correction, and general status flags from the individual control units. Altogether, approximately 150 digital input channels and 100 digital output channels are used for communication. This requirement, combined with the need for 11 simultaneously interpolating axes (six of these being open-loop electrical axes) limited the available choice of CNC. An AB8200AT controller, manufactured by Allen Bradley, was chosen for the TWI machine. Experience with this type of controller has proven them to be particularly immune to interference from high voltage transients.
Electron beam machines are generally complicated as a number of systems are working together - the pumping system and controls, the high voltage supply, the traverse and drives, and the welding process control units. As a fault in any one of these systems can affect welding it is necessary to have a diagnostic error system to allow rapid location and rectification of any failure. Approximately 100 system faults are detected by the CNC and an appropriate error message is displayed.
The consequence of error detection has been programmed for each case, but generally falls into two categories. The first is that the error, if detected whilst the beam is on, will cause a warning message to be displayed, but otherwise operation will be unaffected. If the same type of error is detected whilst the beam is off it will cause the CNC to enter an emergency stop state, where the power supply is safely shut down and program execution is halted. The CNC will only be released from the emergency stop state when the fault is cleared. The second type of reaction to the detection of an interlock is reserved for more serious faults which would cause catastrophic failure if machine operation continued.
In these cases the power supply is safely shut down immediately and the CNC enters the emergency stop state, displaying the appropriate diagnostic message. Personal safety interlocks are hardwired to the power supply and pumping system so that these inhibit the equipment regardless of the CNC operation. This is an important feature as the software can be modified on site, and any possible software failures should not lead to safety interlock violations.
Process control features
Beam current
Control of the beam current from triode guns is readily achieved as the beam current and grid voltage are approximately inversely proportional over sizeable ranges of grid voltage with a relatively small gradient. Indirectly heated diode guns exhibit an extremely steep non-linear curve of beam current against back bombardment potential (BBV). The characteristic is such that at the middle of the beam current range a 5% change in BBV causes a 74% change in beam current.
The thermal effect as the gun heats up can cause a runaway in beam current for a fixed value of BBV and the only way to control the gun is to use a closed loop feedback amplifier. The high gain transfer characteristic of the gun would normally indicate that a low gain feedback controller is required. However, the conflicting constraints of accurate current levels and low current drift demand a high gain feedback amplifier. The compromise solution has been to use a proportional integrating error amplifier which, although it has a low proportional gain, integrates the difference between the beam current demand and actual values such that the BBV is adjusted to reduce the difference to zero. This type of controller amplifier has been successfully applied to a number of diode gun systems (including the EU86 150kW out-of-vacuum electron beam system) and has been proven to be unaffected by electrical noise within the system. Thermal lag within the diode gun cathode structure leads to a beam current response time of approximately two seconds from zero to 100kW beam power. This time is much less for changes within the range, e.g. from 10kW to 50kW takes less than 0.5sec. These responses are not as fast as a triode gun, which can be made to pulse at extremely high frequencies, but are suitable for the vast majority of high power electron beam applications where the beam current is changed over seconds or tens of seconds to produce gradual changes in fusion zone depth.
Flashover
The major advantage of a diode over a triode gun is the beam current response to electrode flashovers. In triodes the breakdown of the small gap between bias cup and filament because of vapour in the gun region reduces the effective bias voltage to near zero and leads to a surge in beam current to the full capability of the gun. This high power pulse of beam current inevitably results in damage to the workpiece. In a diode gun flashover across the wider gap of the gun electrodes is less likely to occur, but if it does the beam current cannot exceed the set level and reduces as the accelerating potential collapses. Defects will only be produced by the liquid metal freezing and will normally be of voidal form. These may be prevented by re-establishing the accelerating potential so that the process is interrupted for insufficient time to allow the weld pool to solidify. An additional problem associated with flashover between gun electrodes arises because of the high current arc produced. This can have sufficient energy to cause local melting and evaporation of material on the gun or column wall, causing pitting on the surface which can lead to a general reduction in the ability of the electrode assembly to withstand high potential stress so that further electrical breakdown becomes more likely.
Power supply design has addressed these two problems by using a 100kW switch mode inverter design. The stored energy in this supply is less than 60 Joules and its operation can be disabled within approximately 30µsec of detection of an arc. This leads to a very low energy discharge which does not cause damage to the surface of the electrodes.
Application of the accelerating potential following gun filament servicing takes a substantial time with motor generator based power supplies. During this operation, usually called gun conditioning, many discharges occur as dust particles and microscopic surface irregularities cause high potential stress regions between the electrodes. The electrical breakdown either destroys the particles or dislodges them from the high stress region.
Typically, the machine operator will make repeated attempts at applying progressively higher potentials until a level some 10% above the working level is attained. With high stored energy power supplies this procedure becomes prolonged as each discharge can itself produce electrode surface imperfections. A low energy conditioning mode of operating the TWI switch mode supply makes possible automatic gun conditioning, such that the full accelerating potential can normally be established within two minutes. In this mode the absolute maximum output power of the supply is reduced to 55W, approximately 30W of which is dissipated in the potential divider at 150kV output. Using this technique to condition the electrodes is not only beneficial in terms of reduced time, but also prevents damage to electrodes during conditioning, which leads to a lower probability of electrical flashover during subsequent welding operations.
When flashovers occur during welding, the response of the power supply must be to recover the accelerating potential as soon as possible without the flashover restriking between the electrodes. The best method of achieving this has been found to be to hold the potential at zero for a short time, and then to control its recovery in a linear ramp.
By suitable selection of the hold-off time and the recovery rate, the probability of a subsequent flashover is minimised. Work is in hand to optimise these parameters to enable more efficient recovery to the working accelerating potential level. Repeated long term tests (2½ hours continuous operation at 75kW) with the present system have proven the viability of this technique.
Programmable deflection system
Electron beam deflection during welding has long been known to improve weld pool stability and bead appearance. Deflection amplitude, frequency and pattern have become an important part of the weld parameter set and consequently must be defined from within the CNC program. One of the outstanding problems for electron beam welding in thick sections of material is to be able to reduce beam penetration gradually ( i.e. 'fade-out') without producing gross defects. As the beam power and penetration are reduced the optimum deflection parameter set is likely to change. To tackle deep section fade-outs fully the control system must be able to vary the deflection amplitude and frequency in a fully interpolating fashion. The TWI system updates the parameters several times per second, allowing smooth changes of the deflection parameters as the beam power is changed under the full control of the CNC. A seam tracking control unit can be integrated with the deflection system to allow real-time correction of beam joint misalignment. Correction is achieved by either mechanical movement through the CNC or deflection offset, the latter only being suitable for relatively shallow weld penetrations. The choice of seam tracking system is dictated by the workpiece geometry and the weld parameters.
Work at TWI has also identified a number of deflection patterns which can be extremely beneficial in production of deep and narrow fusion zones. These patterns are generated by complex current waveforms which, although attainable by analogue signal electronics, can be digitally programmed by computer with more flexibility, allowing design of specialised patterns for particular applications. In addition to producing deep narrow fusion zones, other types of pattern can be used for surface heat treatment, cosmetic weld bead melting, and controlling weld cool down rate for particular material applications. Pattern design is carried out on a PC and stored in the deflection unit to be referenced by a pattern number in the CNC program.
Several welding studies are being carried out at TWI with the 100kW equipment - most of these taking full advantage of the integrated design to allow multi-axis programming of parameters. The gun, control system and power supply provide an integrated unit offering 100kW capability and fully automatic operation. Some two years ago a unit of a similar design was installed at the Framatome Technical Centre, Le Creusot, France and has been successfully applied to a wide range of materials and different workpiece geometries (see Connect April 1991).
