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.

Materials today,joining tomorrow

OPINION

September 200248

...Robert W. Messler, Jr.