TWI Technology Briefing 990-2011
By K A Beamish
FULL REPORT
Background
There is an increasing industry need to join dissimilar materials, for example, steel chassis members to aluminium body panels to reduce vehicle weight; composite to metal components for aircraft structures and a range of dissimilar coatings to substrates. A wide range of joining techniques can potentially be applied to dissimilar welding applications, with the choice of approach depending on the combination of materials to be welded and the geometry of the components/joints involved.
Adhesive technology is currently applied to many dissimilar material combinations as are mechanical fastenings. Fusion welding technologies are also used, but must be applied with care due to the potential formation of brittle intermetallic compounds during welding, which can result in poor mechanical performance. Friction welding technology is, in general, one of the more successful approaches that can be applied to join dissimilar metals. Many material combinations can be directly joined using friction welding, with still more possible using a mutually compatible interlayer. The relative success of friction welding is due mainly to its solid phase nature (which limits overall heat input and minimises material melting), and the hot forging action of the process (which helps to expel the hottest material as weld flash in many cases).
This report provides a general introduction to the subject of dissimilar materials joining and the potential for use of friction welding, and begins by assessing current Industry needs. An overview of available joining methods is given, identifying those most likely to be successful and explaining why others are less successful for certain material combinations. This report is intended as an overview, and as such does not provide a detailed review of the application of individual processes to specific applications. The review does, however, identify a number of representative dissimilar joining applications, of interest to a range of industry sectors, where friction welding techniques may be of benefit. This report also outlines a number of test cases the will be investigated in detail in the next stage of this programme.
Objectives
- Establish the current status of dissimilar materials joining, including consideration of the materials, applications and technologies commonly involved.
- Identify a number of representative dissimilar joining applications suitable for practical investigation using friction welding techniques, of relevance to a wide range of industry sectors.
Approach
Dissimilar materials joining presents specific challenges in terms of the selection of appropriate joining methods. These challenges include issues with metallurgical compatibility, differences in thermal expansion, heating and cooling rates and the potential for joint corrosion. A series of technology summaries giving the strengths and weaknesses of individual joining techniques with reference to dissimilar material applications is given in this report, alongside some industrial applications. Common joining techniques have been reviewed as have less common techniques which are directly applicable to the joining of dissimilar materials. Although not strictly a joining technology, coatings are included as they are a key enabling technology for many applications. Adhesive bonding is included as this is a familiar technique applicable to a diverse range of material combinations. Explosive welding is also discussed, and it is believed that this process is exclusively used to join dissimilar metallic material combinations. Common friction welding processes have also been reviewed. Arc and laser welding techniques may suffer from the formation of intermetallics within the weld, however, electron beam welding may be more successful due to the relatively high intensity of the heat source.
This project intends to assess the suitability of friction welding for joining a number of dissimilar materials combinations. In order to ensure that these case studies have current industry relevance discussions have been held with industry experts and member companies. The principle industry drivers are identified, and an overview of materials usage and joining techniques is given, followed by a discussion of key issues and future initiatives, including opportunities for applying friction welding. From these discussions five topics have been selected for practical investigation by friction welding; copper to aluminium, aluminium to CMn steel, titanium to stainless steel, steel to nickel and aluminium to silicon carbide. These practical trials will form the basis of future work in this programme.
Main conclusions
A review of joining technologies has been undertaken with respect to dissimilar materials joining across a range of industry sectors. The following conclusions can be drawn:
- Welding, mechanical fastening, adhesive bonding, brazing and coatings technologies are widely used for joining dissimilar materials across a range of scales in many industry sectors.
- Differences in material melting points and intermetallic formation present specific challenges for fusion welding techniques when joining dissimilar materials, although steps can be taken to improve joint quality in somecases.
- Niche joining techniques have emerged to serve the dissimilar joining needs of a small number of industries, amongst them explosive welding for producing clad plate and also specialist plastics joining techniques.
- Solid phase joining techniques, such as friction welding are often considered beneficial for the production of bi-metallic joints as large-scale melting does not occur in either material.
- A review of industry sector needs has highlighted the desire for structural joints between steels or aluminium alloys and more expensive functional materials such as titanium and nickel superalloys.
Recommendations
- Advice regarding appropriate joining methods should be sought at an early stage when designing with dissimilar material combinations, as many factors can impact joint performance.
- Extensive mechanical testing will be required to confirm joint performance prior to industrial use, particularly where the joint will be exposed to hostile environments or used in safety critical applications.
Member Report No. 990-2011
Industry survey of dissimilar materials joining and opportunities for friction welding
