Titanium Aerospace Castings

For nearly 50 years, FS Precision has helped to connect families around the globe by supplying the industry with unmatched high strength and low weight near-net and net shape titanium aerospace castings.

lightweight_castings_aerospace

In an industry that extends from here on the ground to the far reaches of our solar system and beyond, FS Precision Tech produces titanium components with the reliability and safety margins to ensure that your family arrives safe and sound – every time. Equipped with FSPT cost-effective titanium castings, mission critical aerospace components resist even the most extreme forces – whether cyclic or static – without the weight penalty associated with conventional metals.

FSPT has invested decades developing and controlling our near-net casting processes to repeatably meet the stringent aerospace requirements for Ti6-4 (Gr. 5) and Ti6-2-4-2.  We have also developed the proprietary FS2S titanium alloy specifically for high stress and high cycle applications to enable components to achieve tensile and fatigue strength equivalent to machined wrought titanium alloys.

The Challenges of Aerospace Design

This article focuses on the natural application of titanium to the aerospace industry, and our processes that make otherwise costly titanium affordable for aerospace applications. Particular attention will be paid to advanced aerospace applications which are already equipped with FSPT titanium castings.

The Weight Penalty for Aerospace Castings

Engineering design for the aerospace industry comes with a unique set of priorities and stringent requirements that must be adhered to. As engineers strive to meet these requirements, weight penalties are always in the forefront their minds. Unnecessary weight significantly impacts both performance and total cost of deployment. While it is relatively straightforward to assert that less weight leads directly to improved performance and cost, let’s talk a little bit about the science and math behind this assertion.

Consider an aircraft in steady level flight. The Thrust (T) imparted on the aircraft must equal the Drag (D) acting on it in order to maintain steady flight velocity. The Lift (L) must equal the aircraft’s Weight (W) in order for it to maintain altitude. Therefore, we have:

[Eq. 1]
D and W

From standard aerodynamic principles, these two relations may be modified to include Dynamic Pressure (q), Surface Area of the aircraft’s Wingspan (S), and Coefficients of Lift (CL) and Drag (CD).

[Eq. 2]
qSCD and qSCL

A small math operation of dividing one equation by the other yields:

[Eq. 3]
T = W/(L/D)

Equation 3 is the mathematical representation of the penalty applicable to unnecessary weight. It is clear that any weight increase must be balanced by an increase in thrust. Additional thrust means larger, more costly engines – and greater fuel costs. What’s more, increased thrust directly leads to increased loads and stresses, which in turn leads to larger structural components and further additional weight.

Thus, the cycle continues; a perpetual balancing act to achieve the optimum combination of performance, strength, reliability, and weight.

Faced with the need to minimize weight, designers might be inclined to use lightweight aluminum as a primary material. And Aluminum is, for many applications, sufficient. The ductile characteristics of aluminum, however – if used in the wrong applications – may allow cracks to propagate in certain high stress/loading environments. This issue leads us to another primary consideration for aerospace design: fatigue strength.

Fatigue strength

In addition to the static loads referenced in Equation 3, designers must also consider tremendous amounts of cyclic loading.

fatigue_failure

From the moment an aircraft’s engines are spooled up – whether the vehicle is for space or suborbital flight – many of its components are exposed to resultant vibrations. Design engineers are, therefore, tasked with designing components that will not suffer from a loss of strength due to cyclic loading – also known as fatigue degradation. This fatigue degradation takes form as aerospace castings are subjected to a pattern of loading and unloading at stresses below ultimate tensile for hundreds of thousands of cycles – or high cycle fatigue.

In the case where components are subjected to the vibrations of flight, problems may begin with crack nucleation at local stress concentrations and imperfections. Once a microscopic crack has been initiated, it may continue to grow as stresses are repeatedly applied through cyclic loading.

Simulation of this loading can be performed in a laboratory environment by applying a distinct Stress Range (ΔS) to a sample for a set Number of Cycles (n), thus producing what is known as an S-n Curve. This curve may then be used to estimate the growth of the Crack Length (a). This growth appears as the Crack Growth Rate (da/dN), or change in length over change in cycles. For predicting fatigue failure, the Stress Concentration (K) for a particular Geometry Function (µ) is established, and then the growth of the crack may be characterized by equations 4 and 5 below:

[Eq. 4]
da/dN = A(∆K)n
[Eq. 5]
∆K=∆S√ πa∝ 

From these relationships, you might recognize that the lower the stress concentrations, the lower the chances are for crack formation and growth. Porosity and voids are notorious contributors to stress concentrations; as are rough and uneven surfaces created from grinding and machining. Soft or ductile materials, such as aluminum, tend to allow cracks to spread more quickly, leading one to believe that the use of higher strength materials should reduce stress concentrations and consequent crack propagation.

If stronger materials lead to improved fatigue resistance, why not opt for high strength steel for all aerospace castings? Circle back to the previous discussion regarding static loading where increased weight comes with a penalty, and the puzzle of aerospace design and material selection begins to become clear.

Design Tradeoffs for Aerospace Metals

Aluminum may be considered for its ease of manufacturing and low weight, and steel for its exceptional strength and fatigue qualities. Many of the benefits of one, however, are absent in the other. Steel alloys far exceed the mechanical properties of aluminum, yet their relatively high weight carries the performance penalties illustrated in equation 3.

This leads to the classic weight vs. strength question where additional aluminum must be used to meet required stress margins, or be substituted altogether by steel. In either case, design engineers must pay the price for this additional strength with a weight penalty.

The exact cost associated with this additional weight varies from craft to craft within the aerospace industry, but consider this: According to Business Insider, the cost of cargo or extra weight for NASA’s Space Shuttle program and the SpaceX Dragon is an impressive $10,000 per pound. Some communications satellite manufacturers have put this penalty at more than $35 per gram – or in excess of $15,000 per pound. Clearly, weight can be equated with money, and FSPT near-net titanium castings reduce that burden while assuring the achievement of mechanical reliability objectives for mission critical system components.

Titanium strength to weight ratio

The Advantages of Titanium

Titanium’s relative high strength-to-weight ratio versus steels and aluminum alloys has paved the way for its prominence in the aerospace field. With a near ideal combination of low weight and high strength, titanium has the unique potential to satisfy many demanding aerospace requirements.

Titanium’s ideal application for aerospace castings stems from its unique atomic structure. Titanium has an atomic mass of 47.8 atomic mass units (amu). Whereas iron, which is known for its high strength properties, has an atomic mass of 55.8 amu. From a pure mass perspective, one might expect a cubic block of titanium to weigh only 17% less than the same sized block of iron. This is not the case, however.  In fact, typical steel alloys weigh nearly 80% more than titanium alloys.  If the atomic mass of the two are so nearly equal, how can this be?

The answer lies in the structure of titanium’s electron shell. A standard molecular measurement of bonded metal atoms is the Metallic Diameter, or the mean distance between the nuclei of the atoms.  For titanium, this measurement is 294 picometers (pm), whereas in an iron crystal structure, the distance is 252 pm.

At the molecular level, titanium – because of its wide electron shell structure –  takes nearly sixty percent more space for a given number of atoms than the space required by the same number of iron atoms.

To appreciate the significance of the atomic electron shell, imagine two trucks slamming into each other on the highway. Visualize the broken glass and the twisted metal.  Hear the screeching brakes and the explosive impact, as you see truck bumpers thrown into the air and clear across the road.

Now consider that those trucks never actually made contact with each other! The physical matter contained within their metallic atoms – the neutrons, protons, and electrons – never made physical contact with the matter in the other vehicle. Instead, the electron shells of the various atoms within the structures were merely forced closer together than normal. The relatively large distances between atomic nuclei allowed for the two vehicles to appear to collide with devastating consequences, but there was never any actual physical contact between the two.

All interactions between the two vehicles were caused by atomic shell electric forces acting against each other. These similarly charged atomic shells – think of two magnets – repelling each other with ever increasing opposing forces as they are brought closer together. The relatively vast amount of distance between atoms combined with the immense strength of these electrical forces are enough to prevent any direct contact between the atoms – in spite of the immense devastation illustrated by the apparent ‘collision’.

You may ask, if there is so much distance between atoms, then what fills all that extra space? One might launch into a discussion of the quantum foam, but that’s beyond the scope of our discussion.  Instead, we will go with the simplified answer: Nothing! In fact, when viewed at the atomic level, material compounds are mostly unoccupied space. Titanium’s atomic structure of wide electron shells increases this empty space as the electrons orbit at a greater distance from their nuclei.

This atomic structure leads titanium to be classified as a “Transition Metal”. Titanium atoms form strong covalent bonds due to the electrons available for bonding – or valence electrons – being present in more than one shell. This allows titanium atoms to share electrons, as opposed to aluminum and steel molecules that form delocalized bonds where multiple atoms are influenced by electron clouds. Titanium’s bonds are stable and exceptionally strong in both pure titanium and in its alloyed forms, allowing it to retain an incredible amount of strength while at the same time remaining comparatively lightweight.

Ti6-4 Grade 5 for Aerospace

The current industry standard for titanium use in aerospace applications is Ti6Al4V, or Grade 5 Titanium (Ti64). Ti64, and titanium in general, is not without drawbacks. The primary drawback being the typically high cost of titanium in its refined form.  In spite of the relative abundance of titanium in the world – it is the 9th most abundant element, and makes up more than 0.5% of the earth’s crust – it is costly to extract and refine into its pure usable form.

In its wrought and machined form, extraordinary amounts of resources are committed to the multiple processes required for initial heating and forming, followed by the associated machining costs and finishing to produce a final product. While these time consuming and costly processes yield an extremely mechanically resilient version of titanium with excellent strength and fatigue properties, they are often not practical due to the cost.

In order to offset these costs, designers have the option to produce titanium castings. However, standard cast Ti64 sacrifices some tensile and yield strength in exchange for reduced machining and material quantity costs. Wrought Grade 5 titanium, for example, typically achieves 140ksi and 130ksi for tensile and yield strength, respectively. Its cast counterpart typically achieves levels of approximately 135ksi and 122ksi, respectively.  Cast Ti64 typically also has a lower fatigue strength than that of wrought Ti64.

At FSPT we believe you should not necessarily be required to settle for any less than you need for your aerospace castings. With that in mind, we set out to develop a titanium alloy to include the cost-effective approach of near-net casting while retaining the mechanical properties of wrought and machined components. This resulted in our advanced FS2S titanium alloy.

The Better Titanium for Strength and Fatigue

FS Precision Tech’s advanced FS2S titanium alloy checks off every mechanical property that an aerospace application requires. Through alloying with aluminum, vanadium, chromium, and other elements, FS2S has improved the mechanical properties for cast titanium to a level significantly higher than that of cast Ti64 (Gr.5), and virtually equivalent to that of machined wrought titanium alloys.

Cast FS2S has ultimate tensile strengths and yield strengths of 148ksi and 135ksi, respectively. With elongation of nearly 40% higher than cast Ti64, FS2S is an ideal material for critical applications where catastrophic mechanical failures might have extreme consequences.

fatigue is critical for titanium aerospace castings

In addition to being on par with fully machined conventional wrought titanium alloys, FS2S has double the strength of typical aluminum alloys, and is 40-50% lighter than conventional steel alloys. These superior properties for aerospace castings are made possible by the unique microstructure of FS2S alloy. Standard Ti64 castings yield a hard and brittle outer layer known as alpha-case, which tends to introduce micro-cracks into the surface structure of castings, and thus can lead to fatigue degradation. The FS2S compound and microstructure, however, yields a beta rich final product which reduces the as-cast alpha case by a factor of two.  This can reduce the amount of costly chemical milling required, as well as reduce the potential for Hydrogen uptake during the chemical milling process.

It is clear that FS2S Titanium Alloy ideally connects all the dots required for our aerospace castings: Low weight, high strength, and fatigue properties in a cost-effective casting process, while achieving the mechanical properties of fully machined wrought Ti64. With our FS2S titanium alloy, designers will reap the benefits of every one of these points, produced using our cost-effective near-net casting process.

If you would like to investigate the potential for applying these benefits for your applications, please download our FS2S Titanium Data Sheet, or contact us now.

FS Precision Near Net Casting

FSPT delivers titanium’s superior strength to weight ratio and fatigue resistance through a process known as near-net titanium investment casting. Our proven method ensures that you receive dimensionally compliant components while reducing material waste and excessive machining that can otherwise drive up the costs associated with fully machined wrought titanium applications.

video titanium investment casting process
Click for video of investment casting process

This cost-effective process begins with our using your design to create an exact wax copy – “pattern” – via an injection molding process. Next, we coat the wax pattern with several layers of a ceramic slurry mix, and allow it to dry.  We then remove the wax, leaving behind a mold in the shape of your complex component design. Finally, fill the mold with molten titanium. The ceramic mold is then destroyed to allow removal of the titanium casting.

Titanium Turbocharger Castings

Typically we then follow the casting process with a series of additional post-cast processes, inspections, and certifications until your cast titanium component is ready for its final quality inspections and release for shipment.

When designing components for the rigors of aviation, two primary concerns tend to be in the forefront of the engineer’s mind are 1) component strength/fatigue and 2) tight tolerances. At FSPT, we systematically address and resolve these and many other potential concerns. Since casting of titanium is the only process we do, we do it very well.

We are AS9100 and NADCAP certified, and adhere to strict aerospace casting specifications of AMS 4991, AMS 4992,  AMS T-81915A, as well as many custom specifications imposed by our large number of aerospace castings customers.

Following removal of the casting from its ceramic mold, it is typically put under extreme temperature and isostatic pressure – up to 15,000 psi – in a process known as hot isostatic pressing (HIP), to collapse any internal voids which may have formed during casting. Following HIP, we perform in-house chemical milling to remove the hard alpha-case layer generated by the high temperature conditions of casting.  This chemical milling is performed by our in-house Lockheed Martin approved chemical milling system.

In their as-cast condition, our casting process can typically hold components to a typical 0.010” – 0.015” tolerance. While we typically achieve net-shape geometries for complex turbo blade profiles, our process is essentially a near-net casting process. In many cases, the cast titanium is just about what you need.  We offer the option of performing finish machining in order to deliver final component precision surfaces if required.

FS Precision implements the highest level of Continuous Improvement and 6 Sigma process control methodologies available in the industry, and we hold ourselves to extremely high quality standards maintained through AMS 2175 and similar aerospace inspection specifications.

Our process of near-net casting and machining ensures that no time or unnecessary expense is wasted machining or converting costly titanium into dirty scrap chips on the floor.  For many of our casting projects, a fully machined version may otherwise have begun as a 100-pound billet, and then been machined down to a finished geometry of five pounds or less. That’s money spent on wasted titanium and excessive machining operations! At FS Precision Tech, we like to say that “We put the air in your parts so that YOU don’t have to.

FS Precision Experience with Titanium Aerospace Castings

Aerospace Valves and Manifolds

As we’ve stressed up to this point, our cost-effective cast titanium is ideally suited for the demanding field of aerospace design. Critical hydraulic systems, for example, operate under especially stringent requirements where any small failure within a valve or manifold can lead to significant damage or a complete aircraft failure.

To prevent these failures, hydraulic systems must withstand thousands of flight hours and all the fatigue and potential damage that accompanies those hours. It was precisely this challenge that caused one of the leading aerospace motion & control companies to partner with FSPT to develop the exact titanium valve components required for a reliable premier aviation anti-icing system.

Titanium castings for airplane anti icing systems

One solution to prevent ice buildup on the leading edges of an aircraft is to provide a steady supply of anti-icing fluid to those edges. However, an effective system must maintain a precise flow-rate of fluid, as too little flow can lead to a wholly ineffective system, resulting in damaging ice buildup. Conversely, too high a flow-rate may potentially lead to corrosive anti-icing fluid damage elsewhere on the aircraft.

Naturally, precision paired with unyielding strength and reliability were crucial for the success of this project. One without the other could easily lead to one of two outcomes; first, an imprecise and leaking valve where free fluid could cause thousands of dollars’ worth of damage. Or second, a valve unable to withstand the continuous fatigue and strength requirements placed on the system, leading to a failure of the anti-icing system.

FSPT delivered on both requirements – and more – with our near-net aerospace castings crafted to deliver unwavering reliability to our customer’s system through our cost-effective quality-certified processes.

FSPT’s rigorous process control and quality systems ensures that every valve serves as a gleaming example of our commitment to providing customers with an affordable avenue to the strength and precision required for the aerospace industry. without the usual weight penalty associated with such benefits. We continue to produce multiple valve castings each and every month for this aviation systems customer, as their success and reliability are assured.

The application of FSPT near-net aerospace castings excels far beyond even the field of aircraft. The beneficial impact of titanium alloys can be felt tenfold when applied to technological marvels that launch beyond our planet.

As previously discussed, the cost of a single pound of weight added to a spacecraft can pin tens of thousands of wasted dollars to a single launch. In addition to the compounded weight penalty, the components within these craft are subjected to incredible loads, both static and cyclical. This leads to a design process where reliability is paramount, strength is critical, and lightweight is a major budgetary and operational consideration. These were the requirements brought to FSPT by a major leader in the race to spaceflight and space tourism when designing their craft’s fuel system.

Naturally the requirements and standards for a spacecraft’s fuel system demand perfection from the quality of material, to its mechanical properties, all the way to the dimensional precision of the final component.

Titanium Aerospace Castings

This system must exactly control fuel flow to accommodate precise adjustments to velocities during the craft’s launch and return from and to Earth. That alone requires aerospace castings that can maintain exceptionally tight tolerances on the order of a few thousandths of an inch, or less.

However, these same components must also stand fast in the face of extreme loading as these craft are launched into space at up to four Gs or more. Such extreme loading places equally extreme stress on the components delivering the craft’s fuel. In addition to these direct loads, cyclic forces come into play to put the fatigue strength of our castings to the test. This leaves the requirement for valves and manifolds that demonstrate superior strength and fatigue properties, while retaining finite control over geometry for precise application.

Achieving these milestones in a cost-effective manner was no small task. However, FSPT’s near-net aerospace casting components managed to do exactly that. Seamlessly blending the mechanical properties of our cast titanium with a proven cost effective process; we continue to produce the intricate channels and geometry required for fuel system valves and manifolds. Our aerospace castings offer creative design flexibility that enable what might otherwise be two or more machined subsystems welded together to be cast as one solid, robust, integral component or system. That’s our cost-effective guarantee at work.

Titanium Casting Mounts & Housings

Fluid control is only one of a multitude of applications for our titanium aerospace castings. The superb fatigue strength of titanium investment castings can also be harnessed to create built-in reliability for critical structural components from the potentially damaging loads that are applied during flight.

Flight Data Recorders, for example, must continue to function regardless of the jolting or shuddering motion that may be encountered during certain mission profiles. Nearly every component within these modules is susceptible to performance loss with exposure to continuous fatigue. What’s more, cockpit data storage units must be designed to survive complete aircraft failures where the majority of other aircraft systems and components will be a complete loss. FSPT’s titanium aerospace castings are currently being utilized to provide the precise and rugged protection these sensitive electronics require.

From the first flight to beyond a million hours, the remarkable fatigue strength of our titanium ensures that the strength of the housing does not yield over time with continuous exposure to cyclic loading. Therefore, in the unfortunate case where the housing must withstand a complete failure, the titanium will be just as strong as the day it was cast.

Titanium Castings for Flight Data Recorder

Additionally, with the use of FSPT FS2S alloy, that strength can be nearly 10% greater than standard aerospace alloys such as Grade 5. For an even greater appreciation of FSPT cast titanium’s blend of fatigue and mechanical strength, look no further than our cast engine mounting structural castings used on the Airbus A400 military aircraft.

Any component designed to be located near, or mounted to the engine must expect a staggering amount of cyclic stresses. Beginning immediately with engine run up, the engine mount will be subjected to a continuous gauntlet of vibration and loading caused by the natural operation of the engine. This fatigue loading is exacerbated by the higher loads associated with extreme aircraft maneuvers such as takeoff and landing.

Engine structural components represent an ideal application for titanium’s unique characteristics. Improved ductility and fatigue strength will ensure no strength loss over time, and that strength guarantees that engine mount castings will offer unyielding support throughout their lifetime.

The Bottom Line

We receive few luxuries during the design process for the aerospace industry. Primary components must possess superb mechanical properties, maintain exceptionally tight tolerances, and all the while keep weight and cost to a minimum. Ordinarily, these requirements would be exclusive of each other, where inefficient machining operations and costly weight penalties might prevail. Our AS9100 certified aerospace titanium castings, however, are no ordinary castings!

FSPT enables the technology to merge high tensile and fatigue strength with reduced weight into a single high quality highly reliable component that minimizes material waste and maximizes your returns. Contact us, and we’ll work directly with your team to develop aerospace castings that meet all your needs to keep your mission critical systems performing reliably!

Some of FS Precision Tech’s casting programs

Titanium Castings for Airbus A400

Situation

Our customer needed to develop a light weight and geometrically complex component for its Airbus A400M military transport aircraft engine mount system.

Opportunity

Our team proposed that a substantial cost savings could be achieved by converting the engine mount components from stainless steel to titanium.

Application

FS Precision’s sophisticated engineering and high precision process capabilities enabled the formation and production of very complex geometry using Titanium alloys.

Results

  • Successful casting design and process control
  • Achievement of a weight savings in excess of 40%
  • Improved component strength and life cycle reliability
  • FS Precision helped this customer to save an estimated $700,000 over the lifetime of this one subsystem program alone.

FS Precision Tech has been selected to produce the internal main structural frame for the Raytheon Missile Systems NATO Evolved Sea Sparrow Missile. This casting represents extremes in surface to volume ratio, and would be a challenge for casting even in steel or aluminum.

sea_sparrow_missile_structure
photo by Rick Rodriguez

With its extreme reactive nature, titanium castings of these types are a magnitude more challenging than all other alloys. With our everyday commitment to six sigma and continuous improvement methodologies, FSPT is managing a multitude of complex process variables to achieve outstanding fill results and dimensional conformance.

As a principle titanium castings supplier for the Lockheed Martin PAC-3 defensive missile program, the world’s largest defense contractor depends upon FS Precision to ensure the safety of our allied troops.