IIW White Paper

4 Needs and challenges in welding and joining technologies

of welding processes like electron beam (EB) welding and laser beam welding (LBW) which produce narrow weld metal and HAZ for joining these processes. Similarly, in the case of heat treatable Al alloys, use of the friction stir welding process can improve the properties of weld joints. Thus, one can observe a shift from conventional fusion welding processes to advanced and solid state processes in the case of these alloys developed by alloy and microstructural modification of the conventional structural materials. The testing and evaluation of weld joints of these materials would include structural integrity assessment and ageing management as these aspects are directly linked with safety of the plants or components using these alloys and conservation of the natural resources (by extending the life of the existing plants and components). Use of new structural materials like ceramics, composites and fibre-reinforced plastics necessitates joining them to themselves as well as to metals. None of the conventional welding processes are suitable for this purpose and one has to depend on processes like active brazing, adhesive joining, transient liquid phase ( TLP) bonding etc. Weldability issues involved in these joints are considerably different from those present in welding of metals and alloys. Similarly, tests, characterisation and evaluation conducted on these joints for understanding the performance also differ considerably from those employed for conventional weld joints. The emphasis here is on the nature of the bond interface and strength of the joint, effect of thermal cycling, etc. unlike susceptibility to cracking in the case of welding of metals and alloys. Joining of advanced materials like bio-materials, electronic or magnetic or optical materials etc., throws up challenges that are not familiar to a conventional welding engineer or a materials scientist. Emphasis in this case shifts to preserving special physical properties like bio-compatibility, conductivity, magnetism etc. by employing micro-joining techniques, soldering and brazing. The processes and alloys chosen for soldering of semiconductor materials like Si differ considerably from those used in conventional soldering. Procedures like metallisation with layers of active elements like Cr or Ti and noble metal gold, and subsequent soldering using In-Bi alloys have been developed for joining Si wafers. In the case of bio-materials, encapsulation of medication implants is an area where joining is a critical issue. Often, metals like Ti have to be joined to organic compounds like polyamide to produce a hermetically sealed joint. Low power laser beam joining techniques have been developed to produce bond width as low as 200 μm for these applications. Thus, one can see the science and technology of joining also has its share in the advances that take place in communication, electronics, information technology and biotechnology that have significantly improved the quality of life. Use of new joining techniques for new materials and applications brings to the fore the need for testing, characterisation and inspection of these joints. Often components or weld joints made of thesematerials and processes are so small that miniature specimens and testing devices have to be used for the determination of local properties and testing. Furthermore, these joints need to be characterised for their physical properties in addition to mechanical and chemical properties. Conventional non-destructive evaluation tools, developed and optimised for large metallic structures would be grossly inadequate in evaluating these micro-joints made from new materials and joining techniques. Hence, often new probes, techniques and standards have to be developed for their inspection. Furthermore, an increasing use of multi-material joints and life cycle extension techniques (including repair) of welded structures and components will represent key technologies for the future. This does not only require new interface design technologies where the assessment of the optimum weldability (joinability) and the evaluation of the service performance have to be linked much more closely, if both have to be assessed together. Careful evaluation of the interaction between selected materials, design and fabrication method, ( see Figure 4.1) , is one of the key factors towards a successful avoidance of failures of welded components in service. It is of particular interest to develop advanced test techniques to determine the local property gradient of such joints to understand the deformation behaviour and hence establish the design parameters better. Mechanical characterisation (including corrosion resistance) and flaw assessment of such hybrid joints are not a straightforward issue and still create challenges for the welding mechanics.

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Through Optimum Use and Innovation of Welding and Joining Technologies

Improving Global Quality of Life