How steel can be as light as titanium - and why it matters

Titanium is often the material of choice for light-weight structural additive manufacturing (AM) components. It combines high strength with low density, making it invaluable for long-life, high-stress applications where weight is critical.

AM parts take full advantage of these properties since they can be designed to provide the required strength with the minimum use of material. Millions of us rely on titanium implants to keep our bodies working, whilst significant parts of efficient modern aircraft are built from the stuff.

So, titanium has a lot to recommend it. But is it always the right material when we want to build high-strength parts with minimal mass? Not necessarily.

This question becomes more pertinent when part cost is a critical factor, since titanium powder typically costs around five times as much as some aluminium and steel powders. The relatively high cost of titanium can price AM parts out of more cost-sensitive markets. But if we could make really light-weight parts out of a relatively inexpensive metal, then it might be possible to serve all market sectors with a single product.

So, is it time to give less fashionable materials like steel another chance? Spoiler alert: the answer is 'yes', but it may not be the steel that you're thinking of.

Thanks to Marcus Pont and Andrew Collins from Domin Fluid Power for their insights and help in preparing this article.

Readers may also want to read Martin McMahon's article The status quo of metal alloys for additive manufacturing to gain an overview of the metallurgy and properties of commonly used AM materials.

The default choice?

It is easy to understand why the versatile Ti6Al4V alloy is perhaps the most commonly used material in metal AM. It has a long history in cast, wrought and forged forms, and so manufacturers have confidence in its suitability for a wide range of demanding applications. Plus, it provides a compelling blend of properties.

Titanium is often described as being as strong as steel but much less dense. With a tensile strength that can exceed 1,000 MPa after heat treatment, it is actually significantly stronger than austenitic stainless steels such as 316L, and yet it is roughly half as dense.

A part built from 316L will be both larger and heavier than a equivalent titanium part. Titanium clearly wins this contest easily.

Specific strength

But when we look at a wider selection of materials, we see some potential rivals. The chart below shows the tensile strengths and densities of a range of alloys that are commonly used in laser powder-bed fusion (LPBF). To the left we see the low-density aluminium alloys, including the widely-used AlSi10Mg as well as higher strength formulations such as A20X and Scandium-Aluminium. To the right, we see the denser iron-, nickel- and cobalt-based alloys. These exhibit a broad range of strengths, with the strongest being a maraging steel that is often referred to as M300 or 1.2709.

We can compare the suitability of these alloys for use in light-weight, high-strength applications by assessing their specific strength. A higher specific strength means that we require less component mass to deliver the required load-bearing capability.

The diagonal orange line on the chart above is a specific strength contour passing through the Ti6Al4V data point. Points along this line exhibit an equal ratio of strength to density. The steeper the contour, the higher the specific strength.

It won't have escaped your notice that one other material - maraging steel - matches and even exceeds Ti6Al4V on this metric. Indeed, if we plot the specific strengths of each material, we can see this clearly:

So, it looks as though maraging steel could be used successfully in light-weight parts, but for this to work, it is vital that part volume is minimised.

Bulky parts demand low-density alloys

The tendency to select low-density materials for light-weight parts has been driven to a large extent by manufacturing constraints. Many conventional manufacturing processes are limited in the intricacy and detail of what they can produce, and so they cannot always be used to produce the most weight-efficient designs. These problems become more severe as part complexity rises. So, if our conventional part has to be bulkier than we would like it to be, then a low-density material is a must.

For instance, casting and forging processes require certain section thicknesses to allow the metal to flow during forming, constraining our ability to make parts as light as we might wish. Stress-bearing members may have to be solid due to the manufacturing process, when a lighter hollow structure would be preferable. If the forming process demands a volume that is larger than the ideal size, then this favours low-density materials over their stronger but denser counterparts.

Image above - the geometries that can be produced by machining are constrained, whereas AM gives us freedom to create organic shapes with internal features

Similarly, where machining is used, the stiffness required to withstand cutting forces may limit how thin walls can be. Furthermore, limitations in where cutting tools can reach often mean that material is too expensive to remove, and so it remains on the component, adding unwelcome mass. For the most weight-critical applications we can deploy 5-axis machining using ball-nose cutters to whittle as much remaining material as we can reach, but this all adds to part costs. When we have exhausted even these techniques and we have to carry around more material than we would ideally like, it had better not be dense.

AM rehabilitates denser materials

Additive manufacturing opens up new possibilities, making it possible to realise near-ideal part designs. Shapes can be organic, walls can be thinner and structures can be hollow or lattice-filled. We can place material just where we need it to fulfil the component's function, and we can omit it from areas where we don't. Parts can be multi-functional so that joints are eliminated and products can be made more compact.

AM rehabilitates neglected materials by taking full advantage of their specific strength. It is now possible to optimise the part design to suit our chosen alloy, using just the amount of material that we need. If our denser, stronger material only requires thin sections or small features to deliver the required performance, then that's fine.

By choosing the alloy with the highest specific strength, whatever its density, we can build the lightest part. In the case of titanium and maraging steel, either could do the job from a weight perspective, but the steel part could be smaller. This may have knock-on benefits in the rest of the product, enabling that to be smaller and lighter too.

The cost factor

We touched on cost earlier and we will return to it now. In some market sectors, cost is the driver and not weight. How does this affect our material choice?

The cost of an AM part is made up as follows:

AM cost = mass of function x cost of mass

For structural parts, the mass of function is the amount of material that is required to provide the required strength. Materials with a higher specific strength will require less mass to bear a load. If two materials have similar specific strengths but different densities (e.g. Ti6Al4V and maraging steel), then the masses of the parts made from each material will be similar, although the designs will differ since the part made from the higher density alloy will have thinner sections and a smaller volume.

The cost of mass comprises two elements, the first of which is the raw material cost. The cost per kg of steel powder is relatively low, and so it has an advantage here. The second element is processing costs, driven largely by the build time. Denser materials generally require more energy to melt each layer and so volume build rates can be slower, but the mass build rates of denser materials are generally higher. Both these factors favour steel.

So, the cost of an AM part made from maraging steel will be lower than that of a part of similar mass made from titanium.

Disclaimer

This is clearly a simplistic analysis - we have only focused on tensile strength and cost. Most material selection decisions require consideration of multiple properties, which may include stiffness, ductility, fatigue, creep, hardness, wear resistance, high temperature properties, corrosion resistance, thermal conductivity, bio-compatibility, embodied CO2, re-cyclability and more. I am not implying that we should always select maraging steel for every high strength application. However, we should not automatically use the same material that was chosen for a subtractive manufactured part when we come to build its AM replacement.

Why this matters

So, a maraging steel part can be smaller and cheaper than an equivalent titanium part. To see why this matters, we first need to look at how the cost drivers of additive and subtractive components compare. Then we will look at the impact this this could have on product markets.

Note: The following analysis is adapted from a Domin Fluid Power case study, Is This the End of Different Fluid Power Products for Different Markets?

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 the manufacturer must go to extreme lengths to remove any excess material, as we have discussed above. It gets more and more expensive to shave off each last gram of material. In these situations, it is the processing cost that dominates, 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 found 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.

In AM, a lighter part is a cheaper part

The cost-v-mass curve for additive manufacturing is quite different. As we have already seen, the cost of the material in the part and the processing costs are both directly related to part mass. There is a virtuous circle in AM:

Lower part mass = lower AM part costs

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 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 - factors affecting the cost-v-mass curve for an AM part

Typical market segmentation

Many industrial products are used in multiple market sectors - they perform the same basic function and there will generally be common requirements such as performance and reliability 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 below.

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.

Impact on product markets

The disruptive impact of AM on a product market depends on to what extent we can make AM products that meet the needs of each these various market sectors. For instance, aerospace customers will be looking for weight reduction and they attach a certain value to weight, so if we can make an AM part that falls into the blue region in the diagram below, then it should be attractive to them.

Whether we can do this depends on the relative position of the two cost-v-mass curves and how far down the AM curve we can move through good product design. AM enables innovative product configurations and efficient designs, so products can be lighter, perhaps even lighter than the current minimum mass, which is why weight-sensitive sectors have been additive pioneers. But expensive materials, long build times and costly post-processing and verification often meant that early AM products were positioned such that their appeal was limited to a few sectors.

Image above - AM pioneers pursued weight reduction, but the economics of early AM processes meant that part costs were often high, making them unsuitable for more cost-sensitive applications.

However, if we select a cost-effective material such as maraging steel, use a productive AM machine and minimise post-processing, then our cost-v-mass curve shifts. If we work really hard on our product design and find more weight savings, then it might even be possible to move to a point where our ultra-lightweight product matches the minimum cost conventional product.

If our AM product design is sufficiently innovative, if we select the right material to build the part from, and if our AM process and post-processing are efficient, then we could serve the whole market with a single product

The impact of this could be profound. Instead of a wide range of different products optimised for each market sector, we just have one. Product configuration and sales processes can be simplified, inventories can be reduced, servicing can be streamlined.

Case study - industrial hydraulics

Domin Fluid Power is a company that has put all of these principles into practice. It has developed a range of direct drive servo valves that are lighter than the best conventional products, and inexpensive enough to satisfy even the most cost-sensitive sectors. Domin has chosen maraging steel to produce innovative valve bodies and spools that are able to withstand high hydraulic pressure in an ultra-compact package.

Image above - production build of servo valve bodies produced on RenAM 500Q industrial AM machine

For more details refer to Is This the End of Different Fluid Power Products for Different Markets?

Summary

Material selection is critical to designing and making a successful AM product. The right material for a subtractive manufactured part is not necessarily the best choice for an AM part.

AM rehabilitates denser, stronger materials - enabling high-performance, compact products. Maraging steel's high specific strength, high density and low cost makes it particularly attractive.

Steel really can be as light as titanium, and the compact, cost-effective AM products made from it could be even more disruptive.

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:

• The status quo of metal alloys for additive manufacturing for an overview of the metallurgy and properties of commonly-used metals
• Boosting AM adoption - the next phase of market growth to understand the changes in AM systems that are driving part costs down