Utilisation of welding flash friction

Ryszard Michalski, The Institute of Welding, Gliwice, Poland

Abstract Examples of the beneficial uses of flash are given and discussed.

Introduction One of the characteristic features of friction welding is the formation of the 'flash' during the friction period as a result of the heated-up layers of metal being forced outside the contact surface. This is a continuous process and its volume depends on that of heat evolution. This in turn is dependent on the frequency of the relative rotary motion of the parts being friction welded and on the force exerted on the surfaces being joined and subjected to friction.

The final shape and size of the metal flash depends on the shape of the elements being welded, the type and quality of the metals being welded and the welding process parameters.

As can be seen in Fig 1, the metal flash changes the geometry of the welded elements in the actual joining zone and is detrimental to surface appearance. In the case of pipes, the bore may be reduced, whilst notches may reduce fatigue strength of the joint. In view of these drawbacks, the metal flash is generally removed by the turning process.

Processes Upset formation But studies at the Institute of Welding, Gliwice, Poland have shown that the flash can be used for a variety of purposes: for example, Fig 2 shows the formation of an upset in the welding zone. Here, the welding equipment should be equipped with suitable stoppers, known as keep plates, which progressively press harder against the flash, increasing the welding surface in the final stage of the process. The technique can be used for making various stepped shafts of different diameters, so that the flash may be used as a disc, flange or cam: a preferable method of producing such a part than machining.

Strengthening T-joints Metal flash can also be used to increase the strength of a T-joint, especially when one of the sections is made of sheet or strip. This is achieved by increasing the weld surface, as shown in Fig 3. A practical example is the three-membered, forged and welded clamping ring used in the mining industry on double-chain, push-plate conveyors for joining the scrapers to the chains (Fig 4). The friction welded surface is increased by about 35% in this example.

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Examples of macrostructures of friction welded joints in elements made of constructional steels. The flash is indicated by arrows.

Two.frictional welded shafts (40mm diameter), with an upset formed by the flash. (a) crude; (b) after machining.

(a) to (d). Examples of joint macrostructures.

262 MATERIALS & DESIGN Vol. 10 No. 5 SEPTEMBER/OCTOBER 1989

Fig4 A three.membered forged and welded clamping ring used in the mining industry for joining the scrapers to chains in two-chain push.plate conveyors. 1 - shank (die forging), 2, 3 and 4 - links with M24 thread

r~g5 Example of the shape of the end of a 40ram diameter shaft made from carbon steel: (a) outside appearance; (b) macrostructure.

~ 6 Macrostructure of the closure of the bore of a 45 /26mm tube by means of metal flash formed by: (a) friction welding; (b); friction forming. In both cases, the outer flash has been removed by turning.

Fig7 Typical macrostructures o f joints clamped by metal flash produced during friction welding and forming operations.

Friction forming Flash can also be utilised for increasing the end diameter of a shaft over a section up to 12mm long (Fig 5). In this case a welded joint is not intended and the surfaces are subsequently separated; hence the process is known as friction forming. To obtain upsets of other shape, the forming cycle can be repeated two or three times under the s ame or different welding conditions.

Closing tube bores Metal flash can be used for closing the bore of a thick- walled cylinder in a direction perpendicular to its axis, as shown in Fig 6. Investigations have been conducted on cylinders with outside diameters between 30 and 60mm. Two cylindrical tubes with mandrels positioned inside are friction welded or friction formed. In the first case, the bore is closed at the welding point of the two mandrels, while in the other cases, two closed ends of separate tubes are obtained. The method is a simple and inexpensive method of manufacturing cylindrical elements with bores closed perpendicular to the axis of the symmetry. When

closing the tube bore, the outer flash might be shaped simultaneously.

Clamping Finally, metal flash may be used for clamping elements mounted on shafts or placed inside cylindrical objects, as shown in Fig 7. The joints thus obtained have been found to exhibit good mechanical strength, especially in torsion, provided that the metal flash is pressed very strongly against the element being clamped.

Equipment considerations The working surfaces of keep plates and mandrels may be flat, convex or concave and may either be designed to remain stationary or to move through a predetermined distance at a fixed rate, either in the direction of flash formation or opposite to that direction. One limitation of the process is that, compared to traditional frictional welding techniques, much greater frictional and upsetting forces must be applied, because the friction surface increases as the process progresses.

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