For the past few decades, materials have been undergoing revolutionary, not just evolutionary, changes. Directionally-solidified eutectic and single-crystal superalloys; intermetallics and long range-ordered alloys; toughened ceramics; polymer-, metal-, and ceramic-matrix composites; compound and amorphous semiconductors; high-temperature superconductors; and now nanocrystalline materials, carbon nanotubes, and nanocomposites. Unfortunately, processes for joining these material have not undergone comparable change. In fact, one could argue that little has changed, in any fundamental way, in two millennia. We still mechanically fasten, adhesively bond, or weld (or braze or solder) materials into parts, devices, or structural elements, and these into assemblies, packages, or structural systems much as we have for generations. This is clearly not the case with materials. Beyond revolutionary changes in compositional control and microstructural development, there has been a paradigm shift to where materials are beginning to be designed and synthesized at the atomic scale, from first principles of material science, atom by atom. As part of this unparalleled change, the boundaries between materials and their structure have become blurred to the point that it often neither possible nor meaningful to distinguish between the materials and the functional entity. The best, but not only, example is in solid-state microelectronics, where junctions between or among p-type and n-type extrinsic semiconductors are synthesized at the same time as the materials comprising the junctions. Material synthesis, functional device synthesis, and functional system synthesis occur simultaneously and seamlessly.
But, joining hasn’t fundamentally changed. It continues to be the last step in product manufacturing; too frequently an after-thought, and almost always a ‘necessary evil’ or ‘means to an end’ that is expected to detract from rather than add to a material’s properties or a system’s performance. As such, it has remained a ‘secondary process’; not because it is of secondary importance in achieving the end goals, but because it is performed after all primary processes to produce the material and to produce shapes or forms of and conditions in components or structural elements have been completed.
As we enter the new ages of information and biotechnology, both of which are enabled in large part by nanotechnology, it is becoming clear that joining, always a key process in the creation of useful products (since few useful products are made from single materials or single parts), must undergo a fundamental and dramatic change. Surely just evolutionary change will not be enough, as that type and rate of change has not even allowed joining to keep pace with the revolutionary changes that have taken place in materials. And arguably, not just revolutionary in the rate of change, as such a change in rate of development will make it tough, if not impossible, to catch the moving target that materials present. No, there must be changes in the underlying paradigms.
I’d argue that joining must undergo at least three paradigm shifts. First, joining must change from a secondary process to a primary process; becoming as much a part of the creation of the final product as the synthesis of the materials comprising the parts or components of the product, and the simultaneous synthesis of the shape, form, and arrangement of the parts or components. Ideally, material, key component, and assembly or structural system synthesis must become as one; simultaneous, integrated, inextricable, and, perhaps, indistinguishable. Think of the current manufacture of micro- and soon-to-become nanoelectronic systems as the model to strive for more broadly. Second, joining must become the enabling technology to allow a product to be produced, not simply the pragmatic process to produce the product. This may seem like a subtle change, but it isn’t. The process of joining will become a science more than an art, practiced as much or more by physicists and physicians as by helmeted welders and hard-hatted riveters. Third, joining must be the first thing a product designer or systems engineer thinks about, not the last. As an enabling technology, joining will be what lets the design move from concept to reality, not a means for doing the best we can under the circumstances.
Just think about what we’ve all seen in the pages of Materials Today over the recent past. Self-assembling molecules to allow what might amount to breakthroughs in micro- and nanoelectronics, MEMs and NEMs, and tissue engineering. Self-healing materials and, thus, structures that join — or actually re-join — themselves whenever they need to. Both examples have at their roots the joining of materials to produce a functional entity. Whether joining meets the challenges being imposed by emerging and yet-to-be-conceived materials will, without question, determine how far we can go! Joining is no longer the necessary evil in manufacturing; it must soon become the basis for allowing us to evolve as a species. Will the process be ready?
Robert W. Messler is professor of materials joining at Rensselaer Polytechnic Institute and a Fellow of the ASM International and American Welding Institute.