AM materials reveal their super powers

Additive manufacturing (AM) is known for providing design freedom that can be used to create innovative, efficient component designs, produced without tooling. But tearing up the design rulebook in this way may not be the most disruptive thing about AM - materials may matter just as much as geometry. Laser powder-bed fusion (LPBF) processes familiar materials in an unfamiliar way, producing a unique microstructure that results in new material properties. New alloy formulations and optimised heat treatments can extract even more extreme properties. Designers that use these 'super materials' can exploit a virtuous circle to make stronger, lighter and cheaper AM components, with a superior business case and a corresponding competitive advantage.

This article looks at how LPBF can produce 'super materials' with properties that are distinct from conventional alloys, and how this will disrupt product markets.

AM has been constrained by process economics

In conventional manufacturing, we are used to the concept of economies of scale, in which the unit cost of a component reduces as the production volume rises. This is due to the preponderance of fixed costs associated with tooling that must be defrayed over a production run. This makes tooled production ideal for high volume applications, but less suited to lower volume or more varied production schedules.

By contrast, the economics of AM are very different. Whilst unit costs do fall somewhat with higher production volumes, this effect is much less marked, since much of the fixed cost is associated with the 3D printers, and we simply deploy more printers to make more parts. A consequence of this is that AM is typically at a cost disadvantage compared to tooled processes at higher volumes, but it finds its competitive niche in smaller production batches.

But what if we can reduce the cost of AM? If the additive cost curve is lowered, then clearly more AM business cases will make commercial sense, and the range of applications for the technology will expand.

Before we look at how to go about doing this, it is informative to look at how component weight and cost interact, which brings home why material properties are so critical.

Subtractive manufacturing trade-off between cost and mass

In subtractive manufacturing, the cost of a complex part and its weight are related in an interesting way. Very low weight components are often very expensive, since it gets more and more costly to shave off each last gram of material. In these situations, it is processing costs that dominate, although material costs may also rise if we choose an exotic alloy to help us to achieve our goal. By contrast, the cheapest way to make a part from a processing cost standpoint will involve a quick forming process and minimal machining. But this is likely to leave a lot of extra material on the part, and so the material cost starts to become more significant. The minimum overall cost is achieved when we find the best trade-off between processing complexity and material costs. A typical cost-v-mass curve is shown below.

Image above - relationship between unit cost and part mass for a complex subtractive manufactured component. Part costs increase exponentially as we approach the minimum mass to achieve the part's function. At the other extreme, minimal processing will produce a heavier part, so material costs will rise. The optimum point from a cost perspective will be found between these extremes.

Typical market segmentation

Many industrial products are used in multiple market sectors - they perform the same basic function and there will generally be common performance and reliability standards that all sectors expect. Where market sectors may differ, however, is in the value that they assign to product weight and size. For some customers, the product must have the lightest possible weight and make the smallest possible space claim. For others, space and weight are not at a premium, and cost is the prime driver. In a market served by products made using conventional manufacturing processes, these customers will select different product variants from different positions on the cost-v-mass curve, as shown here.

Image above - market segmentation for products made with subtractive manufacturing. The most weight-sensitive customers pay the most for the lightest product, whilst other sectors are more cost-driven. Refer to Is This the End of Different Fluid Power Products for Different Markets? for how this applies to the market for hydraulic servo valves.

The consequences of these process economics are significant - they drive separate manufacturing supply chains and logistics arrangements to serve these different markets. This situation is pregnant with possibilities for streamlining and simplification should a disruptive approach come along.

AM creates a 'virtuous circle'

AM can be that disruptor, providing alternative process economics. The cost-v-mass curve for additive manufacturing is quite different - the cost of the material in the part and the processing costs are both directly related to part mass.

Image above - additive and conventional manufacturing cost curve comparison

Of course, this is just one possible cost-v-mass curve for AM. Our choice of material will make a difference - a less expensive alloy will result in a flatter line. Similarly, the productivity and automation of our AM machine will govern how long it takes us to build the part, whilst the purchase and running costs of the equipment will determine how much this time costs us, once again affecting the cost-v-mass curve. Post-processing costs also have a role to play. If we can streamline or eliminate downstream finishing, assembly, inspection and testing processes, then this also moves the curve.

Image above - productive AM move the cost-v-mass curve downwards

Whatever the exact position of the cost-v-mass curve, the important point is that there is a virtuous circle in AM:

Lower part mass = lower AM part costs

The properties of our AM material make their impact felt here - if we have a stronger material, then we will need less of it to provide the required strength, so we can reduce the mass of our part and move further down the curve to an even lower cost point. 'Super' AM materials extend the AM virtuous circle to:

Higher material properties = lower part mass = lower AM part costs

Combining high material properties with design for AM means that we may be able to develop an innovative component design that is far enough down the cost-v-mass curve to undercut any existing conventional part in terms of both cost and mass. If we can achieve this, then we have a single product that could serve all market sectors, enabling a simpler product configuration and sales process, reduced inventories and streamlined servicing. In other words: disruption!

Image above - design for AM and high material properties combine to enable disruptive new products that bth out-perform and undercut conventional rivals.

AM 'super materials'

LPBF puts the feedstock alloy through a unique thermal cycle, melting and cooling the material at extreme speed, producing a very different microstructure to that which results from casting or forging processes. This rapid solidification produces a fine cellular-dendritic microstructure, which influences the material’s tensile strength, hardness and fatigue life. In laser melting the primary dendritic arm spacing is generally sub-micron in scale, delivering a combination of high strength and ductility.

Image above – SEM image section through melt tracks showing melt track boundaries and grains containing aligned cells.

Grain boundaries form where the cellular-dendritic growth fronts meet. LPBF's narrow melt tracks and fast cooling rates produce small crystal grains, of the order of a few tens of microns in size, and these frequent grain boundaries restrict crack propagation, boosting fatigue life. More details can be found in Fatigue fundamentals - improvements in AM part durability.

This fine microstructure produces metal with superior inherent properties to the coarser materials produced by other means. However, these 'super powers' are undermined if our LPBF component contains defects. Porosity is like kryptonite to additive parts, creating stress raisers that can fatally weaken them.

Porosity is kryptonite to additive parts

However, new AM machines such as the RenAM 500Q multi-laser system with its intelligent gas flow (see image right), largely eliminate the porosity that has bedeviled previous equipment, finally unleashing the full potential of these 'super' additive materials.

The 'as built' material can be further refined using heat treatment to produce a wide range of material properties, from which we can select the right combination to suit our application. In The heat is on to surpass wrought performance, I reported how the properties of Ti6AL4V produced by LPBF can be tailored using different heat treatments.

This can deliver a range of strengths and ductilities (shown in orange in the following chart) that far surpass conventionally produced material (shown in black) with the same chemistry:

Image above – tensile properties of Ti6Al4V LPBF material produced on RenAM 500Q under various heat treatments, compared with previous published LPBF data (source: Wycisk et al, doi: 10.3389/fmats.2015.00072) and MMPDS wrought standards (source: MMPDS Table 5.4.1.0(c), annealed bar).

We see a similar picture with fatigue performance, where LPBF now matches or exceeds the durability of wrought:

Image above – Ti6Al4V fatigue test comparison of RenAM 500Q (orange), other published LPBF data (red) and wrought (black). References: Renishaw: RenAM 500Q, CONT-60 parameters, 800C / 4 hrs in vacuum furnace. Other LPBF: Wycisk et al, doi:10.3389/fmats.2015.00072. Wrought: from MMPDS Figure 5.4.1.1.8(a), annealed bar

With up to a third more strength, up to double the ductility and offering superior fatigue life to wrought material, LPBF Ti6Al4V components can be designed to use the minimum of material, making them even higher performing and cheaper to produce.

New materials, new possibilities

Most of the materials used in AM today were developed with other manufacturing processes in mind. LPBF's rapid thermal cycle, which can produce high performance in one alloy, can be a hinderance with other metals that cannot handle the stresses that it induces.

Some difficult-to-weld alloys are prone to micro-cracking during solidification or because of re-melting, due to the stresses that build up between different phases and at crystal grain boundaries as the part cools. Such behaviour can make these materials unusable, or else requires costly hot isostatic pressing to close up the defects that are thus produced.

But AM need not be limited by the current palette of materials. Many companies are working on new alloy formulations that either address the processability of current alloy systems, or venture into new domains, using combinations of elements that have never been tried before. The goal here is to provide superior properties whilst working with the characteristics of the LPBF process.

Legacy nickel-based superalloys (NSAs), for example, become increasingly un-weldable as their operating temperature rises. NSA 718, for instance, is widely used and processes very nicely, but its operating temperature is limited to around 650C. Compositions such as NSA 738 or CM247-LC can bear much higher temperatures, but are also much harder to print, limiting their use in AM parts. OxMet Technologies has developed a range of new nickel alloys that combine excellent printability with high temperature performance, providing a viable additive alternative in these more challenging applications.

Image above - new 'alloys by design' developed by OxMet Technologies offer high temperature performance without the printability problems of legacy nickel-based superalloys. Image courtesy OxMet Technologies.

Summary

Additive manufacturing unleashes the super powers lurking within familiar materials. With their unique microstructure, alloys produced by LPBF can out-perform their conventionally-produced counterparts, particularly once post-process heat treatment is also optimised. By avoiding the kryptonite of processing defects, modern AM machines allow these super materials to be deployed to full effect. Meanwhile new alloys with even greater powers are joining the fight.

These new 'super materials' will make as big an impact as the more widely recognised design freedom that AM provides. Combining innovative and efficient component geometries with superior material properties will allow disruptive new products and business models to emerge.

Next steps

Visit www.renishaw.com/amguide for more education resources and to access downloadable versions of LinkedIn articles by Renishaw authors.

Readers may also find the following articles useful:

• How steel can be as light as titanium - and why it matters
• Fatigue fundamentals - improvements in AM part durability
• The heat is on to surpass wrought performance