Beryllium-Aluminum Alloys for Optical Applications

Interview conducted by G.P. ThomasMar 26 2014

Chris Huskamp, Director of Technical Business Development at IBC Advanced Alloys, talks to AZoM about cast beryllium-aluminum alloys and the benefits of using these in optical components.

Could you provide a brief introduction to IBC Advanced Alloys and how it's involved in beryllium alloys?

IBC was set up in 2007 and for the beryllium-aluminum sector acquired the assets of what was formally Starmet, which owned the IP for the Beralcast^® family of alloys.

IBC itself has main two arms - The first area works on Beralcast^® alloys in our Engineered Materials division based in Wilmington, Massachusetts. We have a sixty-five thousand square foot facility that's dedicated to the production of Beralcast and other beryllium-aluminum alloys for aerospace, motor sports, precision optics and other high end applications.

We also have a copper division which produces forgings, rings, and a variety of other worked conditions out of copper alloys, including copper beryllium alloys all the way through to high end bronzes and brasses.

Could you provide a bit more information on the Beralcast^® family of alloys?

This family of alloys is material with a high beryllium content, at roughly sixty-five weight percent. They are particularly good for applications that require high specific stiffness, or applications that simultaneously require low density and high modulus.

They're excellent for applications involving mounting sensitive equipment, as the material has some self-dampening characteristics, which helps to minimize the amount of transmitted vibrations.

Beyond that, engineers would look at the high thermal and electrical conductivity of the alloys. They have a very low Coefficient of Thermal Expansion which allows the material to be comfortably used in applications exposed to a wide range of temperatures. For a point of reference, the Beralcast^® CTE is much closer to that of titanium than aluminum.

Could you describe the manufacturing process involved in producing the Beralcast^® alloys?

It’s a very valuable raw material, and the only competitive product form that's really available in the market is a made from powder metallurgy. You have a high cost of manufacture to first melt the materials and then form them into powders.

These powders then have to be handled very carefully and poured in blended amounts up to the preferred chemistry level. Once this has been done, the products are poured into a steel can and finally pressed into a billet at high temperatures and pressures.

This is a very costly way of manufacturing the billets and the reality is you can't make very accurate components to start the manufacturing process, which therefore means a lower yield.

So this opened a window of opportunity, to produce a fairly expensive material that is of benefit in a lot of engineering applications, but of which people really only want to buy what they need. The technology that existed from Starmet that was procured by IBC does just this.

What properties does an beryllium-aluminum alloy have that make it ideal for manufacturing metal mirrors at high grade?

First of all we need to understand the requirements of the application, and in this application a very dimensionally stable substrate is needed. For example, you want something that's not going to move even if you are seeing temperature changes in excess of -65 to 250^o Fahrenheit.

What you want to assure is that within the products thermal service environment you don't see a significant dimensional change or distortion in the optical surface, such that you end up with an error, whether that's for an imaging or for a precision pointing applications.

When choosing the right material, you firstly need a something that's inherently stiff. A material that's highly conductive is also needed, because if you have a material that's poor in conductivity you get hot and cold spots. Another important property is a very low CTE (coefficient of thermal expansion). After all this, the questions to be asked are “will this material meet the mechanical requirements" and "how will it affect the cost?”

One of the issues with traditional methods of producing beryllium-aluminum mirrors is the large 'buy-to-fly' ratio. If you look at an aspheric construction with large curvatures and significant light-weighting, this generates a large block that you have to start with and then machine to a final component. We are familiar with 40:1 ‘buy-to-fly’ ratios in some cases.

What you really want to do is be able to produce an equivalent material, because we know the material performs quite well, in a very net-shaped geometry.

By being able to investment cast the Beralcast^® alloys, we can produce yields as high at ninety seven percent. That means you very nearly achieve a 1:1 buy-to-fly ratio. It makes it a very attractive cost proposition for these companies that are manufacturing product from beryllium-aluminum alloys.

That is the approach that IBC is looking at to work with the mirror manufacturers - to have a very cost effective substrate that meets both the physical and mechanical requirements of systems going into production that have such high cost constraints, because virtually every government entity is under significant downward cost pressure no matter which country you're dealing with.

You’ve talked about the financial benefits of using a different method. Are there any further benefits to bypassing those traditional methods?

With beryllium, we're looking at a substitution for a more cost effective, more 'health and safety' conscious approach to manufacturing that meets the requirements. From a mechanical standpoint, it's a one-to-one substitution.

In a recent paper we performed an initial study evaluating thermal and optical performance and the material performed quite well. IBC’s goal is to broaden the market for beryllium-aluminum materials by helping companies capitalize on the significant performance benefit at a lower cost by investment casting components.

One of the critical considerations for optics applications is that the coefficient of thermal expansion for the Beralcast^® alloys are very similar to that of plated nickel. The normal manufacturing process is a single point diamond turn of the Beralcast^® substrate followed by a nickel plating process. The nickel itself is what is finished to a high polish.

So having a good CTE match between the plated material and the substrate is critical to prevent peeling and delamination type issues. For comparison, if you went with the straight aluminum substrate, the CTE's are so different that you can see blistering and edge peeling as the two materials seperate.

Could you summarize the aims of this paper?

The real target we were aiming for was to be able to produce these components quickly and efficiently with a solid business case underneath.

This allows the end-user to justify implementation without mechanical or physical characteristics deviating significantly from the expected performance of the consolidated powder traditional substrate.

You worked with both Beralcast^® 191 and Beralcast^® 363 to produce the mirrors. Did you find any significant differences between the two? If so, were these expected?

That's a great question! Beralcast^® 191 is a material that responds to a mild heat-treatment, but it has a much higher thermal conductivity because of the silver content. So we would utilize that in narrower range optics.

This means if your exposure ranges are in a much narrower band, you want the thermal conductivity to help you out. So within a system that needs to use the mirror as a way to help radiate heat from the overall package, 191 wold work quite well.

However Beralcast^® 363 is a material that does not respond to heat-treatment. We just do a stretch relief thermal cycle to eliminate any residual casting stresses. This is the same cycle we go through whether the material is for optical structures, mounting structures, and structural airframe component. The result is that you get a very stable substrate throughout the thermal range.

We're now in the process of doing back to back comparisons between 191, 363, and another material which is soon to be launched in the market.

Could you expand on the further work you're looking to do?

We have worked with a premium optics finisher to measure the distortion of an optical image based on the mechanical change in a component. So as you cycle up and down thermally, you're looking at a deviation based in wavelengths.

We're going to go through and do a complete baseline with a slightly different geometry that replicates the baseline test of the substrate exactly. This will be with Beralcast^® 363, Beralcast^® 191, and our new material.

In the future, what are the applications that you hope that this work can be used in?

Traditionally we've used the Beralcast^® materials for a wide range of optical positioning systems and substructures, such as mounts, fold mirrors and bodies.

That's been one of the markets we’ve been able to dominate because they are large open structures and we have a significant amount of machining. Metallic optical substrates is an area that we see as a natural progression of our aerospace work, for which we are AS9100 certified. The optics community has an even tighter regime of controls than aerospace, but we're now identifying this as the appropriate level for us to move into. With investment casting we can utilize rapid prototyping processes to have equitable production quantities of one to quantities that would necessitate procurement of a hard wax investment tool.

The industry now has a effective range that's very usable and it's also scalable. So for instance if a program is in a prototyping phase and needed one component the rapid prototype can be used. That component can then be moved forward after it is tested.

Now there are two options here: 1) either the mirror behaves exactly what they wanted it to and they can move forward into production (if they have a fast paced program they can use rapid prototypes and they can start production immediately), or 2) the customer can wait for the hard tool to be completed and they get first article qualification for the hard tool.

Now if it does not behave as expected and the designer needs to implement a change for some reason, since a rapid prototype was used we have no hard tools. Therefore you can immediately make a change in the design, produce another prototype invest and within very short order, sometimes as little as three weeks, be testing the next generation of the design. This is an outstanding benefit to the design spiral.

About Chris Huskamp

Chris Huskamp is currently the Director of Technical Business Development for IBC Advanced Alloys. He is a Metallurgical Engineer from University of Missouri – Rolla, employed by McDonnell Douglas within their Phantom Works organization upon graduation.

Of his 17 years of work experience, eleven years were spent within The Boeing Company supporting advanced metallic materials and processes transition, direct manufacturing technologies and advanced technology scouting for aerospace programs. Early experiences gained while employed within the foundry industry as a technologist and process metallurgist combine with those from aerospace technology transition roles to provide a unique perspective that he capitalizes on in his current position with IBC.

Chris is a prolific inventor and has successfully contributed to transitioning many new materials and processes into flight capable solutions.