Materials today, joining tomorrow

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  • 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 hasnt 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 materials properties or a systems 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.

    Id 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 isnt. 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 weve 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 Instituteand a Fellow of the ASM International and American Welding Institute.

    Materials today,joining tomorrow

    OPINION

    September 200248

    ...Robert W. Messler, Jr.