New developments in advanced welding

Quality assurance

7.6.1 Beam measurement

For a full exploitation of the advantages of the electron beam a welding, tool knowledge of beam properties is necessary. The processes that occur in electron beam welding are very complex and are characterised by a great many different parameters such as accelerating voltage, beam current, focus position and power density distribution. To determine the beam parameters and to facilitate the parameter transfer between different electron beam units a number of beam-diagnostic systems, which apply different measurement principles, are currently under development (Elmer and Teruya, 2001; Akopiants, 2002; Bach et al., 2002). The DIABEAM (Dilthey et al., 1992, 1997, 2001) has been developed in the ISF-Welding Institute, RWTH-Aachen University. This system may be employed in almost all existing electron beam units and allows signal acquisition of the electron beam up to a power of 100 kW. The DIABEAM measurement system was developed for easy determination of the focus position with slit measuring or a rotating wire, eliminating the need for complex and cost-intensive welding tests. The measurement and the three-dimensional display of the power density distribution across the beam diameter can be made by means of the apertured diaphragm. The other purpose of beam diagnostics, by in-time identification of variations of beam characteristics, is the prevention of negative influence, caused for instance by cathode adjustment, cathode distortion or variation of the vacuum level, on the welding result. As a result of the three measuring processes (hole, slot and rotating wire), the DIABEAM beam diagnosis system is suitable for a broad range of applications, especially for analysing and quality assurance of the beam.

As part of a common European research project, 12 different electron beam welding machines all over Europe have been measured (Dilthey and Weiser, 1994). A general dependence between electron beam parameters and welding results has been established and the power density, necessary for the formation of a vapour capillary, was experimentally determined.

The DIABEAM system deflects the measured beam over a combined double slit-hole sensor using a deflection unit or measures the undeflected beam with a newly developed rotating wire sensor device. The sensor transmits the signals via an amplifier integrated into the sensor case to a transient storage card (maximum scanning rate 40 MHz). In the measurement mode the beam is deflected with a very high deflection velocity of 200-900 m/s, depending on the beam power, by a computer controlled function generator with amplifier, which prevents destruction of the sensor.

The deflection is effected by a special coil, which enables a maximum deflection angle of ± 8. Each time the beam passes over the sensor, the measured data are read in by the transient storage card and displayed online on the computer monitor making correction adjustments possible.

To understand the results of measurements delivered by the DIABEAM system it is necessary to know the different definitions of beam diameters used by DIABEAM. There are two alternative diameter definitions that use interpretation of the width of the measured electrical signal or statistical evaluation of obtained power density distribution for the correspondent diameter definition. Figure 7.19 shows the layout of the DIABEAM system adapted to an EB welding equipment measured signal interpretation. As the electron density in the beam is proportional to the measured signal amplitude the most straightforward way to define beam diameters is illustrated in Fig. 7.20(a) for the one-dimensional rotational symmetrical case.

The diameter is defined as the width of the distribution for a given fraction of the maximum signal amplitude. In a two-dimensional case the contour plot of the beam may be used for the diameter definition, Fig. 7.20. The contour lines correspond to a given fraction of the maximum signal amplitude. The beam area including all amplitudes which exceed a given amplitude fraction defines the contour area Ax. The diameter dx (where x is the fraction of the amplitude in %) is defined as the diameter of the corresponding circle having the area Ax:

[7.1]

Interpretation of power density distribution

In the case of a two-dimensional measurement (hole measurement) there is another possible way to define beam diameters. Instead of using the decrease of the signal amplitude, one can use a definite fraction of the power density integral over an area to the total beam power. In other words, the power density flux which crosses the beam area should constitute X % of the total beam power. Figure 7.20(b), illustrates the above definition in a one­dimensional projection of a measured two-dimensional power density distribution.

In the simplified rotational symmetrical case it can be written:

dpx

Px = f 2 p(r)2pdr ^0

where p (r) is the two-dimensional power distribution function, dPX is the beam diameter and PX is the beam power included in the area with diameter

dPX.

The diameter dPX is once again defined as the diameter of a circle with equivalent area APX.

In the general case one can write:

Pt - = X = f p(x, y )dA [7.3]

Ptotal JApx

and

dpx = 2 Щх [7.4]

In DIABEAM the diameter based on the power definition is called the dPX diameter (e. g. dP50), where X is the fraction of total power in %. For all DIABEAM measurements five different contour values are evaluated according to one of the two described methods.

7.6.2 Sensor systems

Scanning can alternatively be carried out via a slit - or apertured diaphragm or via a rotating tungsten wire. Slit measurement with slit widths of 20 mm is a comparatively fast and simple determination of a signal which is proportional to the beam intensity and also the determination of beam diameter. The principle of slit measurement with the appropriate voltage signal over the beam cross-section is depicted in Fig. 7.21.

With the slit measuring process, the core and edge areas of the beam can be compared by means of five different selectable diameters. Measurement of the beam diameter under varying working distances enables the caustic curve of the beam to be determined and displayed. In this way, the beam
aperture can be determined precisely, thereby simplifying considerably the welder’s selection of electrical and geometric parameters.

The application of a double-slit sensor enables online measurement of the deflection speed and increases the precision of the measurement. Here, the beam is, at the start of the measurement, deflected from its neutral position transversely over both slit sensors; there is no deflection in the longitudinal direction. The deflection speed is determined by the measured difference in time of the signals arriving at the first and at the second slit.

The apertured diaphragm measuring process enables detailed assessment of the electron beam. The high local and temporal resolution (up to 400lines per mm and 40 M samples/s) of the measuring system enables the characteristics of the beam to be assessed with regard to changes in it in relation to certain parameters. These parameters may be electrical or non-electrical and include cathode adjustment, cathode deformation, vacuum and working distance. The apertured diaphragm measurement is carried out by deflecting the beam via the hole on a line-by-line basis. During fast deflection, recording of the signal (i. e. the current of electrons conducted from the hole) by a transient storage card is initiated by the DIABEAM hardware, Fig. 7.22(a). The beam is, in accordance with the parallel arrow lines (shown in the figure), deflected over the sensor in the X-direction. Between the individual parallel scans a static deflection in the 7-direction is carried out in every case. In general, 50passes are carried out. Thus the power density distribution in the beam may be drawn up in a three-dimensional representation, as shown in Fig. 7.22(b).

The rotating wire sensor is also used in beam diagnostics. This new measuring variation has been developed to investigate whether and to what degree metal ions influence the power density distribution. Here a rotating tungsten wire with a diameter between 0.1 mm and 5 mm and at a speed of

Electron

beam

7.22 Type of beam deflection pattern for hole measurements (a) and corresponding 3D-representation (b).

up to 1.000 s-1 moves through the beam; the principle is illustrated in Fig. 7.23. Beam deflection is unnecessary here. The tungsten wire is coupled to a solid copper plate in order to increase heat dissipation and the current derived from the wire measured in the form of a voltage signal. The measuring principle is similar to the slit measurement principle except that the diameter of the tungsten wire is smaller than is the diameter of the beam. In principle both methods (the rotating sensor and slit measurement) can be used to carry out the same type of beam diagnostics. An advantage of the rotating wire method is that it is possible to measure the non-vacuum electron beam; this rapidly spreads out in the atmosphere due to the scattering of the beam electrons on contact with air molecules (see Section 7.5.1).

7.2 Applications

Because of the great many materials that can be welded with the electron beam, such as tungsten, titanium, tantalum, copper, high-temperature steels, aluminium and gold as well as the large range of thicknesses that can be worked on, the method has a wide variety of applications. Such applications range between the micro-welding of sheets with thicknesses of less than 0.1mm (here low and extremely precise heat input is important) and thick plate applications.

In heavy plate welding, the advantages of the deep penetration effect and the consequent joining of large cross-sections in one working step using a high welding speed, low heat input and small weld width become obvious. With modern welding equipment, wall thicknesses of more than 300mm (aluminium alloys) and of more than 150 mm (low and high-alloy steel materials with length-to-width ratios of approximately 50:1) are joined quickly and precisely in one pass and without filler metal.

Listed below are some industrial applications where electron beam welding is an established tool.

• Reactor construction and chemical apparatus engineering: welding of high - alloy materials, welding of materials with high affinity for oxygen, production of fuel elements and of circumferential welds of thick-walled pressure vessels and pipes;

• Pipeline industry;

• Turbine manufacturing: production of guide blades and distributors;

• Aircraft construction: welding of structural/load-bearing parts made of titanium and aluminium alloys and of landing gears made of high strength steels;

• Automobile industry: welding of driving gears, pistons, valves, axle frames and steering columns;

• Electronics industry;

• Tool manufacturing, e. g. manufacturing of bimetal saw bands;

• Surface treatment;

• Material remelting;

• Electron beam drilling with up to 3000 drills per second (Dilthey, 1994).

Because of the numerous advantages (such as minimum workpiece heating by high power methods, small beam diameters and high welding speeds) electron beam welding is being increasingly applied in industrial practice. Low heat input also allows the welding of readily machined parts. The economic profitability of electron beam welding is partly due to the high welding speeds and therefore the short cycle times and partly due to the high quality. The clean vacuum process without the presence of oxygen together with constant process parameters make the weld seams easy to reproduce.

There are also some disadvantages. The workpieces to be welded must be electrically conductive and there is the risk of hardening and cracking due to high cooling rates; this restricts the range of materials that can be worked by the process. Investment costs are high because the beam deflection is carried out using magnetic fields and the whole process needs to be shielded because of the development of X-ray radiation during welding.

7.3 References

Akopiants K. S., (2002), ‘System of diagnostics of electron beam in installations for electron beam welding’, The Paton Welding Journal, 10, 27-30 Bach F. W., et al. (2002), ‘Non vacuum electron beam welding of light sheet metals and steel sheets’, IIW Document Nr. IV-823-02 Behr W., (2003), Elektronenstrahlschweifien an Atmosphare, Aachen, Shaker Verlag Carslaw H. S. and Jaeger J. C., (1967), Conduction of Heat in Solids, Oxford, Clarendon Press

Dilthey U., (1994), Schweifitechnische Fertigungsverfahren, Dusseldorf, VDI-Verlag Dilthey U. and Behr W., (2000), ‘Elektronenstrahlschweifien an Atmosphare’, Schweifien und Schneiden, 8, 461-85 Dilthey U. and Weiser J., (1994), ‘Analysis of beam/workpiece interaction applied to electron beam welding for industrial application’, Final report: BREU-0134-CT90 Dilthey U., Ahmadian M. and Weiser J., (1992), ‘Strahlvermessungssystem zur Qualitatssicherung beim Elektronenstrahlschweifien’, Schweifien und Schneiden 44 (4), 191-4

Dilthey U., Bohm S., Dobner M. and Trager G., (1997), ‘Comparability and replication of the electron beam welding technology using new tools of the DIABEAM measurement device’, EBT ’97: 5. Internat. Conf. on Electron Beam Technologies, 76-83, Vama, Bulgaria

Dilthey U., Brandenburg A., Moller M. and Smolka G., (2000a), ‘Joining of miniature components’, Welding and Cutting, 52 (7), E143-E148 Dilthey U., Smolka G., Lugscheider E. and Lake M., (2000b), ‘Electron-beam-induced phase generation at solder systems applied with high-performance cathode sputtering’, VTE, 13 (1), E9-E15

Dilthey U., Goumeniouk A., Bohm S. and Welters T., (2001), ‘Electron beam diagnostics: a new release of the DIABEAM system’, Vacuum, 62, 77-85 Draugelates U., Bouaifi B. and Ouaissa B., (2000), ‘Hochgeschwindigkeits- ElektronenstrahlschweiBen von Aluminiumlegierungen unter Atmospharendruck’, Schweifien und Schneiden, 52, 333-8 Elmer J. W. and Teruya A. T., (2001), ‘An enhanced Faraday cup for rapid determination of power density distribution in electron beams’, Welding Journal, 80, 288-95 Janssen W., (1991), Verbesserung des Elektronenstrahlschweifiens mit Hilfe der flexiblen Doppelfokussierung, Aachen, VDI Verlag Schultz H., (2000), Elektronenstrahlschweifien, Dusseldorf, Deutscher Verlag fur SchweiBtechnik (DVS)

Schiller S., et al. (1977), Elektronenstrahltechnologie, Stuttgart, Wissenschaftliche Verlagsgesellschaft

von Dobeneck D., Lower T. and Adam V., (2000), Elektronenstrahlschweifien - Das Verfahren und seine industrielle Anwendung fUr hochste Produktivitat, Verlag Moderne Industrie, Landsberg