Scanning the field - electron beam deflection technology reviewed

TWI Bulletin, September/October 2001

Olivier Nello joined TWI in 1997 after completing his National Service in Africa as tutor in Electronics at the Ecole Africaine de la Meteorologie et de l'Aviation Civile. He gained an MSc in Electronics at the Pierre et Marie Curie University in Paris and is currently studying for an MBA at the Open University Business School. He has worked in the Electron Beam and Friction and Forge Processes department for four years, mainly involved in equipment development and research such as beam deflection and beam characterisation.

Having studied metallurgy and materials science at Cambridge Bruce Dance graduated in 1986 and joined TWI the following year. As principal research metallurgist now in the Electron Beam group he has worked on a wide range of EB welding and processing related subjects such as surface treatment, demagnetisation, EB weld distortion, and the development and application of EB deflection technology.

The need to improve manufacturing efficiency and effectiveness in the aerospace and other industries has led TWI to examine a range of new electron beam (EB) welding and texturing techniques, based around the concept of beam deflection. Some of these techniques, as Olivier Nello and Bruce Dance report, can have very beneficial effects on the fusion zone shape and bead appearance of the weld and on the surface texture of a material.

As conventionally applied, the technique can make welds with very low heat inputs, at relatively high weld speeds. What is less well known is the potential of the control and manipulation of the EB heat source for other materials processing tasks, as well as advanced welding techniques.

In practice the EB heat source can be split using high band width beam deflection, so that a portion of the beam's energy can make the weld in the normal way, and the remainder can be timeshared between locations elsewhere on the workpiece. The beam is cycled between the different locations hundreds of times a second, so the weld pool does not usually have time to react to the changing heat source.

Deflection equipment

To manipulate the EB in this way, a programmable deflection system ( Fig.1) has been designed and built at TWI with the capability to both rapidly deflect the beam in X and Y axes, as well as manipulate the focus of the beam (Z axis) with similar speed.

Taking into account that a deflection system could be used for a large range of different applications, this equipment was designed for ease of use and rapid fitment. Commercial products such as computers and function generators have therefore been used.

The programmable deflection system can be used to allow developments of specialised deflection patterns. A deflection pattern consists of a number of dots defining a path along which the beam will be deflected and defocused. Each dot is represented by three co-ordinates (x, y, z).

Currently, a five axis programmable deflection system is under development to allow for improved control of the EB heat source. With this new system, it will be possible to deflect the beam along the four axes as well as change its focal position.

Deflection patterns are designed on a PC using a tailored visual basic program ( Fig.2) and then downloaded into the deflection unit. Modifications or new deflection patterns can be designed and tested in just a few minutes.

The deflection pattern may consist of up to 16000 discrete dots, which may be cycled through at a range of frequencies. Both the frequency and amplitude of the pattern may be continuously varied via the front panel controls of the function generators or from the computer.

Using this equipment, the EB may be deflected at up to 1 km/sec and at a deflection angle of up to 17° with an accelerating voltage of 150kV.

This programmable deflection system can be tailored to fit into most EB welding machines and the means of controlling frequency, amplitude and pattern can be fully automatic, via the CNC of the EB welding machine.

Thermal control during welding

In the aerospace industry, the global drive to reduce manufacturing, operating and maintenance costs has led manufacturers to examine new aeroengine designs operating at higher temperature and stress levels. The ability to use higher temperature materials in the hot stage of the engine could significantly extend engine life and contribute to improvements in specific fuel consumption.

One route to achieving this is to use the newer nickel based superalloys in the hot parts of the engine, but many of these alloys are restricted in their application because they are known to be difficult to weld reliably.

However, a new welding technique achieved via the use of the programmable deflection system has enabled welding of these materials, previously considered near impossible.

This welding technique is based around beam deflection during the welding process, to modify and control the thermal profile of the process as the joint is made. The beam is deflected from the main weld seam to a series of discrete points around the weld, providing in-situ heating and dynamic stress control ( Fig.3).

The EB is manipulated within the weldpool in the usual way and within two raster zones ahead of the weldpool to each side of the joint. The defocusing system, which is synchronised with the deflection system, reduces the power density distribution of the raster zones to avoid melting the surface of the material.

Welding trials showed that crack-free welds of satisfactory bead appearance could be achieved with a beam focal spot size of less than 0.5mm ( Fig.4).

Manipulating the EB so that it effectively becomes a multiple heat source can allow a radical change in the application of the welding process to these materials that would be likely to suffer cracking or other welding difficulties when welded in a more conventional fashion.

The application of multiple heat sources to the welding process allows the solidification rate of the weld metal to be controlled, as well as some of the stresses around the solidifying and cooling weld metal. Welds made in this way are frequently made at lower weld speeds than would normally be the case for EB welds, and are made with a higher heat input per unit length. Despite this, levels of distortion in such welds are frequently relatively low.

At its simplest, the special welding procedure can consist of a simple travelling preheat zone ahead of the weldpool. At its most complex, a large change in the stresses around the weld can also be achieved, and welds with microstructures not normally found in welds of low distortion are made.

Integral cosmetic pass

Various welding problems have been reported on the assembly of stator vanes of aero engines. The two key problems have been identified as weld cratering and weld spatter. The former gives both imperfections to the weld as well as gross defects within the weld itself and has been most troublesome. The latter requires that some time is spent either setting the work up to avoid its occurrence or cleaning it after the welding process.

One method that could improve matters is the use of the programmable deflection system to improve the cosmetic appearance of the welds by making a simultaneous cosmetic pass. The integral cosmetic pass is achieved by deflecting the EB periodically from the weldpool to a raster zone located behind the weldpool ( Fig.5).

Over the raster zone, the beam is softened, or in other words, reduced in time-averaged power density, by both the defocusing action and the deflection unit, which scans the beam at very high speeds, approximately one km/sec. This soft beam striking the still hot workpiece surface behind the weldpool is easily able to melt the surface in a controlled fashion and thus make a cosmetic pass with a fraction of the usual heat input required.

When compared with separate dressing operations, distortion and production cycle can both be reduced by the use of such a welding technique. Welding trials demonstrated that welds of satisfactory bead appearance could be achieved with a beam focal spot size of less than 0.5mm diameter ( Fig.6).

Careful development and control of the welding procedures is required for optimum results. Welding trials demonstrated that small changes in the distribution of the EB heat source were responsible for significant changes in top bead appearance.

Texturing

More recently, the development and manufacture of the programmable deflection system has led TWI to examine a range of new texturing techniques. These can be used as pre-treatment operations to make novel surface textures with re-entrant features for applications such as lubricant retention and enhanced bonding ( Fig.7).

New control techniques have been developed that allow previously unattainable results to be obtained in a wide variety of materials.

Holes are formed with controlled profiles at rates of hundreds or thousands per second, in an inherently clean vacuum environment. The speed and hole formation characteristics of EB texturing are unmatched by any other process.

Tests carried out at TWI have shown that, after a relatively short period of development, convoluted textures containing re-entrant features can be made on flat, angled, and curved surfaces at a range of different gun-to-work distances, using beam powers of just 2-3kW.

Commercial exploitation of this technology is now close to hand, and significant further developments in this area are anticipated.

Summary

The practical application of these techniques requires the fitment of a programmable deflection and defocusing system to an otherwise standard EB welding machine.

To date these techniques have been demonstrated for:

• Successful EB welding of crack sensitive Ni-based superalloys
• EB welding with an integrated cosmetic pass, giving excellent appearance, integrity and low distortion
• EB texturing giving unique surfaces in a variety of materials

The full set of applications has not yet been realised and it is believed that further advanced EB welding and processing techniques will be based on the use of advanced beam deflection technology.

References