Will additive manufacturing powder processes become obsolete?

Topology optimization patterns of a 2D rectangle under different loading conditions and restrictions (Source: wwwcg.in.tum.de/group/persons/westermann.html)

Fedor Antonov is chief executive officer (CEO) of Russian additive manufacturing (AM) firm Anisoprint. Here, he presents his vision for the future of the industrial AM industry, which focuses largely on composite materials rather than the current favorite—metals.

Perspective
As the 3D printing industry has evolved over the last three decades, two distinct communities have emerged with, I believe, only a small intersection. These different communities can be broadly identified as consumer and industrial, and within the latter community the term 3D printing is rarely used, with an obvious preference for the ‘additive manufacturing’ moniker.

The differences between these two worlds can be debated at length, but this article will focus on the industrial AM sector. It is within this segment that the advanced additive processes are starting to radically change the way companies approach some production applications.

Many promises have been made about AM technologies, not least: the ability to manufacture a new generation of functional, mechanical parts; improving design and certification approaches; and the transformation of the supply and added value chains. In parallel with the process hardware developments, new design techniques such as topological optimization are also evolving, enabling additive processes to fulfill the production of structures with an arbitrarily complex shape.

In this article, I set out to address—and challenge—some of the widely-held views across the AM industry today, as well as future predictions for the direction AM technologies are heading as a universal production technique. Suffice to say, I am not disputing here the power of AM as a supporting technology for prototyping, manufacturing of models and master-models, tools and other auxiliary technological devices.

Industrial AM means metal, right?
The origins of the AM industry date back to the 1980s. At that time, the only available materials were polymers in resin form. Today, polymers still make up more than half of the AM materials market, according to 2016 research from EY. However, there is a perception that the field of application for polymers is fundamentally limited in terms of producing structural parts. Since the 1990s, as the process capabilities have improved, the industry’s attention has increasingly focused on metal AM processes, a logical development if the structural capabilities of polymers are limited. Since then, multiple different metal printing technologies have been developed, largely based on powder bed fusion, binder jetting, direct metal deposition and extrusion techniques. As a rule, in more than 90 percent of cases, metal powders are used as the base materials, requiring a fixed granule size, high quality and fine tolerances.

Powder bed metal machines tend to be huge systems and very expensive (up to several million dollars) and invariably require essential ancillary equipment. The market for such machines is estimated to be less than a thousand per year in quantitative terms. In addition, year-on-year growth is slow, according to 2016 research from CONTEXT. For comparison, current industrial robot sales are estimated to be more than two-hundred-and-fifty-thousand units per year.

There can be no dispute that both users and manufacturers of additive machines are actively looking for new metal technology applications. This, in turn, is driving progress, as barriers are gradually breaking down and global corporations such as GE is investing billions of dollars in the development of these metal technologies, supported by scientists and engineers around the world who are working hard to improve design methods and technologies, develop new materials and optimize processes of metal AM.

In can be frustrating to find that the majority of expert discussions on industrial AM today focus exclusively on metal processes. It seems that the industry does not consider any alternatives to metals in AM for structural load bearing parts, with perhaps a rare exception made for ceramics.

Some of the issues with metal AM are being resolved, such as density (porosity) of the AM parts, in-process quality control, repeatability, etc. Meanwhile, the complexity and cost of the systems continues to grow, the demand largely fueled by investments of ‘giants’. However, there is very little discussion and debate around alternatives to metal, which is what I am looking to address in this article.

Optimization space
I do believe that topology optimization is the key to unlocking the real potential of industrial AM, but it is not all about metal.

Here I attempt to define topology optimization in simple but scientific terms for anyone not familiar with the discipline, with respect to the production of structural parts. The structural nature of any part is determined by its operating conditions, namely, the external loads acting on it, and both its shape and the material it is made from have to meet or exceed its functionality requirements. In mechanical engineering terms, the mass and area forces, temperature and other environmental factors have to accommodate its function (and position relative to the other parts of the structure if part of an assembly). As a result, for a given shape, all these factors determine the field of internal forces in the part—a stress state. The ability of a part to withstand (i.e. not break under) external loads is determined by the ability of the material to resist these internal forces. Obviously, the distribution of internal forces depends on external loads as well as on the shape and material of the part.

The task of any product engineer is to optimize the part for maximum structural functionality and performance (over expected lifetime) while minimizing the mass of the part (this is especially important for the aerospace industry) and the cost of manufacturing. The optimal design also minimizes stresses, with materials that exhibit the same strength throughout the entire part.

Topology optimization, using algorithms, is an iterative digital method of eliminating unnecessary material (mass) from the part where internal forces are absent or negligible. This can often result in parts with very bizarre—and complex—shapes. Shapes that are not conducive to the use of traditional manufacturing methods, such as casting, milling, turning, forging, etc. This is where AM technologies excel, and the layer-by-layer processes are not fundamentally limited by complex geometry. (Figure 1)

There is currently a great deal of R&D taking place in this arena. But, my question is, what if you could optimize not only the shape of the part, but also the internal structure of the material it is made from? The additive powder-bed technologies basically do not allow one to pose the question that way, producing parts with the material as homogeneous as possible, with no pores and inclusions, whose properties are the same, regardless of the direction in which the load is applied. By optimizing just the shape, we miss a lot of opportunities. Although the design is three-dimensional, the material itself remains flat, even linear, or, more precisely one-dimensional.

Designers working on topology optimization often focus on natural, organic structures. Indeed, the results of such optimization often resemble familiar, natural objects—dragonfly wings, trees, snail or turtle shells, etc. (Figure 2)

All these objects, in addition to having a complicated shape, also have a very complex internal structure; indeed, traditional industry has long been adopting such approaches. It is here that I believe composite materials can contribute positively. Composite materials have a heterogeneous internal structure and as a result are used extensively in the aerospace, automotive and civil engineering sectors.

The main characteristic of such materials is the presence of two different phases: matrix, or binder, and reinforcement. The reinforcement resists the main loads, while the matrix makes the individual reinforcement elements work together. In structural composites, high-strength fibers such as carbon, glass or organic are used as the reinforcement element, providing significantly higher specific strength and stiffness compared with any metal.

Matrices are predominantly polymers, although composites based on metallic, ceramic and other types of binders are widely known. The main feature of such fibrous composites is their anisotropic properties. That is, the material response to external loads depends significantly on the direction of these loads. For example, the cCFRP ((continuous) carbon fiber reinforced polymer) strength differs by two orders of magnitude depending on whether the load is applied along the direction of the fibers, or transversely. Traditionally, this feature of composite materials is considered to be one of their main disadvantages, limiting their mass use in most industries. Anisotropy is countered mainly by making so-called quasi-isotropic laminates—or sheet materials—in which the layers of a unidirectional composite are stacked at different angles.

This results in material behavior comparable with metal sheet in a plane stress state, Furthermore, with the exception of the molding step, they are often used in exactly the same way, namely cut, drilled and connected with rivets and bolts, which destroys the integrity of the reinforcing fibers, in turn leading to additional stress concentrations due to the free edge effect and other features inherent in composite materials that reduce the overall strength of the structure.

This is where nature can point us in the right direction. Wood fibers do not break off at the point where the branches grow, but gently warp around, compacting locally. Fibers in the leaves form a complex branched structure of reinforcing ribs, not forming edge to edge on top of each other in several directions, breaking off at the ends. The internal forces, or stresses, that arise in these bodies as a reaction to external loads have a tensor nature, that is, at each point there is a distribution of stress values relative to three spatial directions. The material response at each point is non-uniform, and therefore the material must also be non-homogeneous to optimally work off this response.

A classical isotropic material must be able to withstand the maximum stresses at a given point in a certain direction. In this case, the stresses in the remaining directions can be much smaller, but the strength of the material in these directions is the same, which means that it is redundant. Anisotropic material should allow the properties to be optimized, providing the minimum necessary characteristics in different directions. The simplest example is the rod, which works just in tension along its axis. In this case, all the fibers must be parallel to this axis, and the transverse properties of the material do not matter. Such a rod is the most effective composite part.

Based on these considerations, the most efficient composite design is a structure that consists of a set of rods connected to each other in such a way that each rod perceives only the load along its axis. This is a so-called lattice structure. It is not surprising that most topology optimized structures are lattice structures. In this sense, I would suggest that the use of composite materials is the best option for making such optimized designs. It should also be noted that such designs are successfully used in the production of rocket and satellite composite structures, but the technological features associated with the process of manufacturing such structures impose significant restrictions on their shape as well as the relative alignment and choice of the direction of the ribs.

Real 3D
Here, at the intersection of composite materials and additive technologies, a fundamentally different industrial approach from the powder-bed metal vision of the industry arises. Real 3D, not only by shape, but by internal structure. If you factor in the potential capability to incorporate sensors and transducers, through functional fibers, the option to create adaptive designs by building electronics and special functional components directly into the material during the original production process emerges. Furthermore, this introduces the potential to create smart and self-healing materials and 3D structures in-situ by inserting special components while printing or arranging the delivery of healing agents through dedicated channels inside the material. (Figure 3)

The process itself now ceases to be layer-by-layer, because the direction in which the reinforcing and functional fibers are laid is of fundamental importance. All of these possibilities are unattainable within the framework of a layer-by-layer paradigm of additive manufacturing. In practice, such equipment can be implemented on the basis of fiber-polymer co-extrusion or automated fiber placement. A print head containing extrusion nozzles through which reinforcing or functional fibers and a polymer resin are fed is mounted on a multi-axis manipulator that performs the positioning of the head arbitrarily in space with respect to the part being built.

The part itself can also be placed on the robot or on the bed. The entire process takes place in an enclosed, thermo-regulated chamber to ensure adhesion between the fiber bundles and to eliminate the shrinkage and warpage effects. The fiber volume fraction can be locally varied by means of a separate feed for the reinforcing fiber and resin, while the manipulator must ensure the layup along complex, curvilinear, spatial trajectories. In this way, it is possible to control the degree of anisotropy at each point, ensuring the production of optimal structures of complex shape and internal structure. Additional positioning devices can incorporate electronic and other functional components into the part.

The greatest limitation of this approach is the productivity level, which, I believe, can be resolved in two ways. With the hardware, the number of extrusion nozzles can be increased within a single printing head to increase deposition speed. From the manufacturing point of view, productivity levels can be increased by transitioning from mass production to on-demand manufacturing through the creation of a network of local production cells.

This becomes possible due to the dramatic reduction in the number of parts in the product and, as a result, a significant simplification of the assembly process. Even using a complex example, such as a road vehicle, assembly and even partial production could be carried out directly at the sales location within the framework of a single technological process. This concept is already being explored by Local Motors and demonstrates how virtually any engineering (or even consumer) product could potentially be manufactured. The vision for an on-demand manufacturing approach has been extensively debated across the 3D printing and AM community, along with many other potential disruptive elements where AM is the enabling technology. However, they have not yet been brought to fruition with any existing technology and equipment. Moreover, the potential capabilities of existing technologies already seem fundamentally limited and the prospects for implementing all the described advantages look vague, at best.

The new level
It is my contention that the powder-bed fusion processes with metal materials are not the future of the AM industry. AM technologies are developing at such a pace that many of them are becoming obsolete before they are even fully realized. The traditional technologies for composites manufacturing, such as layup and lamination, will also gradually become obsolete, I believe, as more flexible and more effective methods for composite part production emerge.

The real future of additive technologies is in composite materials with controlled anisotropy, shape and internal structure optimization-based design, multifunctional capabilities and adaptive structures. This will require decentralized autonomous production and increased personalization based on new, smart certification approaches. For those who are not on board the AM train, there is probably no point in trying to catch up with the outgoing layer-by-layer technologies, where certain companies have already achieved some success, have invested a huge amount of resources and great efforts but are yielding low results. To get ahead now, it is necessary to develop additive technologies at a new level—a composite level.