The welding of aluminium and its alloys

Flash butt welding

9.6.1 Process principles

As the name suggests flash butt welding is capable of making butt joints in bar-like or tubular components, L, T and X-shaped extrusions, etc. The weld is a solid phase joint where the two ends of the component are forged together at high temperature, any molten metal being expelled from between the two faces (Fig. 9.5). The process takes place in two phases, a ‘flashing’ and an upsetting phase. The two components to be joined are clamped in electrodes, at least one of which is movable. A low-voltage, high - amperage current is applied without the two components being in contact. The parts are then brought together at a controlled rate, resulting in a series of brief short-circuits as the asperities on the faying faces melt and burn off. This continuous series of short-circuits raises the temperature of the ends and expels some of the molten metal, giving the ‘flashes’ that give the process its name.

The heating melts and plasticises the metal and, once sufficient heat has been built up, the ends of the components are forged together, forcing out any melted metal, oxides and contaminants and some of the plasticised material, forming a ‘flash’ or ‘upset’. The expulsion of contaminants and
oxides means that pre-weld cleanliness is not as important as the conven­tional fusion welding processes. The weld is consolidated by this forging action, giving a high-strength joint even in heat-treatable alloys. The forging action also eliminates any cast structure and reduces the width of the HAZ. A monitor chart from a typical weld sequence is illustrated in Fig. 9.6.

9.6.2 Welding machines

The basis of the flash welding machine is an AC transformer, the majority of production equipment being single phase machines. The electrodes or clamps are mounted on two rigid platens, at least one of which is movable and powered by a pneumatic or hydraulic system (Fig. 9.7). The capacity of the machine is limited by the current requirements of the joint and the upset pressure available. The power demanded of the transformer is based on the cross-sectional area of the faying faces as a critical current density is required. The varying electrical conductivity of the different alloys also has an effect on power requirements and the range of yield strengths place varying demands on the upset pressure mechanism. As an approximation a machine capable of flash butt welding 65 cm2 of steel can weld only some 35 cm2 of aluminium.

The current requirements for flash butt welding range from around 12500 to 15500A/cm2 during the upset phase of welding. Current

Flashing

requirements during the flashing stage will be some 30-50% less than the upset current. Voltages vary from 2 volts at a low cross-sectional area to 20 volts for the thicker sections. The lowest voltage possible should be used consistent with stable flashing for the best results.

9.6.3 Electrode clamps

For the welding of steel copper alloys are generally used for the manufac­ture of the electrode clamps. For aluminium, however, steel, sometimes copper plated, has been found to give better results, conducting less heat away from the weld, providing a longer life and more positive clamping. By drawing the weld back through one of the clamps fitted with a knife edge it is also possible to shear off the upset as part of the removal process. A broach may be inserted into the bore of hollow components to remove any internal flash. To achieve a clean cut and to prevent smearing of the upset during removal the cutting edges must be kept sharp. The clamps are machined to match the outside shape of the components and are split to enable rapid insertion. They are also designed to clamp around 80% of the circumference and to be of a sufficient length that slippage does not occur during upsetting.

To prevent crushing or deformation of hollow components removable inserts or backing devices may be used beneath the clamp area. Sufficient distance must be left between the ends of the inserts to ensure that they do not take part in the welding operation.

9.6.4 Quality control

Provided that the equipment is correctly set-up and maintained, flash butt welding is a trouble-free process. Alignment of the components is vital to achieve low rates of weld rejects. Failure to align the components can result in ‘shelving’ where one component rides up over its partner and in uneven flashing, producing lack of fusion defects. Insufficient heat and/or inade­quate upset may both result in lack of fusion type defects or oxide entrap­ment. Both of these defects can be readily detected by the use of a bend test such as those required by the procedure approval specification BS EN 288 Part 4 - see Chapter 10, Table 10.3. Bend testing is a relatively inex­pensive method of assuring weld quality. Those non-destructive test tech­niques that are commonly used for interrogating arc-welded butt joints, such as radiography or ultrasonic examination, are not suitable for flash butt welding and the engineer is forced to consider destructive tests. Bend testing of pre-production test pieces prior to the start and at the end of a production period of some 8 hours is one of the most cost-effective and easily performed techniques. When this testing is supplemented by in­process monitoring of the welding parameters (Fig. 9.6) then it is possible to demonstrate a 100% acceptable weld quality. While it is written for the control of steel flash butt welding the specification BS 4204 ‘Flash Butt Welding of Steel Tubes for Pressure Applications’ is an extremely useful reference, full of information that may be applied to aluminium alloys. It gives recommendations on equipment choice, welding sequence control, procedure approval testing and production control testing. In addition there is an example of a flash welding weld procedure record form and a list of information required on a weld procedure specification.