The Benefits of Pre-Heating for Friction Welding

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Project Code: 30184

Objectives

To critically review the use of pre-heating in welding processes, compare and contrast pre-heating techniques and identify those suitable for deployment in friction welding.
To characterise the benefits of pre-heat in rotary and linear friction welding in a range of high performance materials.
To determine the improvement provided by pre-heat in terms of maximum permissible weld area, control of cooling rate and achievable weld properties. Process economics will also be considered.

Project Outline

Preheating is frequently used when arc welding steels to reduce the cooling rate of the weld and base material to a) reduce the incidence of hydrogen cracking in carbon-manganese steels, b) prevent over hardening of medium carbon and low alloy steels, c) reduce residual stresses and minimise distortion and d) control weld microstructure.

Preheating is used in flash butt welding to extend the capacity of particular machines and permit the joining of larger cross sections than otherwise would be possible. In this instance the raised workpiece temperature also makes the flashing process easier to start and maintain and reduces the temperature gradient through the workpieces.

Preheating may be required when brazing to reduce the necessary weld energy (heating) and reduce the time required to complete the joint.

Conversely, preheating is rarely used in friction welding at the present time. A preheat effect can be achieved by the friction process itself (by applying a low pressure for a brief period at the initiation of the weld cycle). This approach however assumes sufficient additional energy supply and an adequate machine rating, and as such is not useful for extending the range of current equipment.

Process economics, and the desire to upscale manufacturing without increasing machine capacity, points to the adoption of external preheating for rotary friction welding. The expansion of linear friction welding (to include welding of steels and super alloys on stored energy equipment) has also sparked interest in the application of preheat.

Changes in material usage and a wider appreciation of friction welding has led to growing interest in its application to higher strength, high hardenability steels, which will likely require preheating to achieve acceptable weld properties. In addition existing (rotary) friction welding users are considering applying linear friction welding to circular cross sections because of the more uniform interface temperature produced. Preheating in friction welding may also facilitate the joining of dissimilar material combinations, particularly where a large difference in melting point exists between the two different metals concerned.

This project will review available preheat techniques and identify materials/applications where the introduction of preheat to friction welding operations will be most beneficial. Experimental trials will characterise the benefits of preheating and produce industry appropriate demonstrators and performance data. Technical activities will be complemented by an economic assessment and cost-benefit analysis.

Relevant Industry Sectors

Industry Need

Friction welding is an established method for high volume manufacture of a range of components in the automotive, construction (yellow goods), oil and gas, aerospace, and power sectors. Friction welding processes produce high quality solid phase welds and allow high productivity automated manufacturing. These processes are however usually limited to the welding of relatively small items / weld areas. Whilst some examples of large scale friction welding do exist (most notably in aero-engine manufacture) equipment costs for the friction welding of large parts can be significant.

Based on the current successful deployment of friction technologies, many companies have an interest in extending the achievable weld area of existing equipment and/or permitting the welding of higher strength materials on lower capacity equipment. Interest in the aerospace sector includes linear friction welding (LFW), where a desire to extend machine capability also exists.

The construction sector is looking for high quality joining methods to allow larger section reduced weight materials to be used, to replace current heavy section forgings and castings. In the oil and gas sector interest exists in the application of friction welding to higher strength steels and larger sections (e.g. chain links). Friction welding would be appealing in such cases, however the rapid heating & cooling rates involved can be problematic for certain material grades.