Key design considerations for metal additive manufacturing

The hype that surrounds additive manufacturing is partly understandable given the mind-blowing attributes of these technologies. They seem bounded to open a world of new possibilities for designers in a multitude of applications.

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Metal Additive Manufacturing is a booming industrial sector and an active field of research. Even through it can’t fully replace conventional manufacturing techniques, laser or electron-beam-based metal AM processes have known significant developments in the past years. In some cases, metal additive manufacturing offers a relevant technical-economic solution, for instance for components with an asymmetry between design complexity and production volume.

The sector of medical technologies abounds with applications where additive manufacturing is relevant. For example, fractures heal faster when orthopedic implants incorporate a trauma-specific geometry. Such custom items are characterized by small or unique series of production.

For a wider adoption of metal AM, it is the duty of engineers/designers to leverage unique manufacturing capabilities and obtain optimal designs that justify the investment. Doubtlessly, constraints and capabilities of metal AM are characteristics of the highest importance for designers.

The game-changing capabilities of metal additive manufacturing

Design freedom is the main advantage of additive manufacturing. Unlike milling and turning where planes or cylinders are the easiest and cheapest geometrical entities to manufacture, additive manufacturing is virtually able to produce almost any geometrical structure, due to the slicing of the parts and the manufacture of a section at a time. It requires no tooling and brings flexibility in a production line.

AM’s design freedom allows engineers to design and produce organic shapes or lattice structures, which were impossible to obtain before additive manufacturing. Lightweight and consolidated aerospace components can be obtained when topology/structural optimization is combined with cellular lattice structures. They contribute to higher fuel efficiency and overall reliability.

Metal additive manufacturing has some environmental benefits. Beside design freedom, it is another mainstay of the technology. Indeed, adding material only where the part’s functions require it allows significant material savings. AM machines, however, should be used at sufficient capacity. By maximizing performance with design complexity, significant reduction of environmental impacts during the use stage of products lifecycle is possible. Relevant for components used in mobility applications (automotive and aerospace, particularly), where most of overall environmental impacts occur during their utilization phase.

"Metal AM components can have mechanical properties that exceed those of cast parts and approach the properties of forged partsThis is due to the high cooling rate associated with the layerwise deposition of the melted metal. Grains growth is interrupted right after grain germination, giving rise to a thin microstructure, characterized of a high strength but slightly reduced ductility." [Wohlers 2014]

High-grade metallic feedstocks are available for AM. Commercially pure titanium (CpTi), titanium alloys, nickel superalloys and cobalt-chromium superalloys are widespread. Moreover, the cost of atomization to obtain metallic powder particles reduces the price differences between metallic materials. It provides an incentive for designers to upgrade the chosen material for a certain component in order to improve performance (from aluminum to titanium alloy, for example).

DED: Remanufacture/repair, hybridization and FGM

In powder-bed processes, the scanning trajectories of the energy source are driven by the shape of the section. In direct energy deposition (DED), the nozzle is kept “normal” to the manufacturing surface and material can be deposited on a non-planar metallic substrate. It therefore allows the re-manufacture or repair of expensive components.

To combine the advantages of additive and subtractive manufacturing, hybrid systems use DED in most cases. For example, near-net turbine blades are built and finished directly onto the machined shaft.

Finally, DED allows the deposition of functionally graded materials (FGM) by varying, in real-time, the composition of the feedstock to obtain parts with heterogeneous properties, able to achieve complex behaviors.

Limitations are in the shadow of capabilities

Capabilities and applications of AM have been in the spotlight far more than constraints. The technology has even been qualified as a manufacturing revolution, bound to replace all other techniques, as if everything was now additively producible. It is not.

The real power of additive manufacturing comes out when a designer knows how to take advantage of the process. Unique design freedom triggers in a designer’s mind confusion and unrealistic expectations. The lack of a general design framework for this nascent technique doesn’t help and AM limitations are somehow belittled.

To release the potential of this technology for optimal designs, designers must not cling to old principles but think out of the box of traditional manufacturing. Principles of “Design for Manufacturing and Assembly” (or DfMA) are rather unpractical for AM, not to say crippling.

Basically, limitations of additive manufacturing are either of a physical (process-dependent or not) or digital nature.

Physical and process-independent limitations

They result of the layerwise nature of additive manufacturing:

Processing time is essential in the cost structure of metal printed parts, thus the cost will increase when parts are larger and higher.

Overhangs should be avoided to minimize the need of support structures. For example, circular profiles for horizontal holes are not optimal. Drop or polygon profiles are preferred.

Design should ease the removal of powder particles in internal hollow sections. When internal mesostructures such as lattice cellular structures are designed, they can’t be closed or powder particles will be trapped.

The resolution is a critical consideration for small features. Below a certain dimension, minimal material thickness limits the possibilities.

Open/closed porosities impair structural resistance and are undesired. Porosity largely depends of the material’s particle size distribution (PSD) and process parameters (power and scanning speed of the energy source).

Internal stresses are caused by the thermal gradients. Building parts at elevated temperatures, such as in electron-beam-based AM helps relief internal stresses to improve stability of components.

The anisotropy of material properties results from high cooling speeds. After solidification, material grains grow preferentially in the direction of the build. Optimal laser-electron beam scanning strategies can reduce this effect.

Microstructures variability causes inconsistent material properties. Qualification is a key to a widespread adoption of metal AM in critical applications. It requires the full understanding of the interrelationships between material & geometry, process manipulation (to achieve location-specific control of composition and microstructure) and the exploitation of digital data throughout the planning, additive manufacturing and quality-assurance processes.

The very limited availability of material data imposes significant incertitude and impairs the predictability of numerical simulations. So far, results derived from finite element analysis (FEA) are relevant only when uncertainty is quantified to predict a likelihood of outcomes.

The rough surface quality sometimes imposes post-processing operations. Shot peening is often enough. Depending of part specifications, the need to add post-built finishing increases both cycle time and cost.

Physical limitations, regarding material supply (feedstock)

In a nutshell, there are three primary feedstock (material supply) process types among metal additive manufacturing techniques:

Powder-bed processes:

Support structures have to be removed mechanically and their usage consumes extra material, time and removal effort. While EBM needs less support structures, powder-bed-based additive manufacturing processes generally require the incorporation of additional structures.

Some feature just can’t be built without them. For example, they prevent the molten drops from sinking into the powder bed when overhanging surfaces are manufactured;

These structures don’t only serve as support. They also evacuate heat and thus reduce warping caused by thermal gradients the thermal gradient attributable to improper energy dissipation. Heat is better conducted from the molten pool to the start plate and the machine structure.

In processes with high thermal gradients such as laser melting (SLM/DMLS), additional structures further rigidify the part to reduce distortions. Experience shows that parts with overhang greater than 25-30° can be made without supports.

Support structures also serve to diminish the heat transfer in certain areas. In EBM, the startplate, because of a high heat capacity, plays as a heat sink. The portion of the part in contact with it is therefore rapidly cooled and the resulting thermal gradient makes the contact area prone to distortion.

Part design and orientation must consider the functionality of the parts, as well as the necessity and influence of the support structure. Designers must pay attention to supports (and keep in mind that part orientation also impacts the quality) in order to decrease their density compared to what pre-processing software suggests.

Fabricated component size is limited to the bed size. The deposition rate is low and the size of components is limited directly by the bed size. It restricts powder-bed processes to the fabrication of relatively small parts, generally less than 400 x 400 x 400 mm.

Powder and wire directed energy deposition processes:

Accessibility constraints are key concerns. Directed Energy Deposition (DED) techniques usually operate through a robot-like End-of-Arm Tooling (EOAT). Collisions between the nozzle and the part must be avoided. Due to the mechanism of material deposition, the nozzle must stay normal to the substrate surface and the part is mounted on a rotating platter. This leads to complicated kinematics and the need for advanced algorithms and sufficient preparation to mitigate collision risks.

Sharp corners are troublesome. In addition, the speed of material deposition (and height of deposited material) depends mostly on the nozzle speed and on the rate of material sprayed with the nozzle. Consequently, acceleration and deceleration stages can cause variations of the height of the deposited layer. The repetition of this phenomenon can stop the manufacture (especially if the distance between the nozzle and the surface is too great so the molten drops solidify before landing on the surface). To avoid this, acceleration and deceleration stages must be minimized from the design, by avoiding sharp corners and replacing them by curves for example.

Digital limitations: Design environments and data representation

CAD design environment

Apply mesostructures in real-world products remains challenging. Complex mesostructures are a direct legacy of additive manufacturing. For instance, Netfabb and Within have been focusing on the design and generation of refined structures, to turn solids into lattice structures, for example.

Fragmented design environment. To design for AM, there are way too few options for a convenient and user-friendly environment. Today, preliminary design/engineering, shape optimization and pre-processing operations (support structures, orientation) are mostly achieved in separated environments. It is an impediment, especially for independent designers, with limited resources.

Data representation

Commercially available CAD systems don’t allow the design of graded material structures. No material information is available within the representation which is required for heterogeneous objects. Broader adoption of AM for the deposition of functionally graded materials relies completely on the availability of discretized material information at any point in a model. TNO and other academies have developed methods and tools to let users specify changing material compositions throughout a solid.

STL file format is a de facto standard for AM. While commonly used to convey CAD objects information to machines, it is not suited for the representation of most of the optimized shapes with many features and mesostructures allowed by AM’s design freedom. It leads to very large files and the ability to allocate graded material properties is lacking.

Additive manufacturing: A technology promised to a bright future.

Every AM process is somehow advantageous in certain applications, which justifies its survival until today. Nevertheless, performance factors such as time, cost and quality depend also on machines, material and geometry. Doubtlessly, significant developments in AM technologies in these areas will encourage a wider adoption for direct part manufacturing.

Machine improvements will push the barriers of today’s AM to gain more accuracy, quality consistency and productivity. The pathway to realizing the full potential of additive manufacturing is still long.

 


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