Titanium - The Metal That Made The SR-71 Possible

We recently explored the fascinating engineering that

made

the SR-71

possible

, detailing its unique hybrid engine cooling systems and much more, but we forgot to explore one of the most fascinating aspects of its design: the exciting new materials science that

made

the sr-71

possible

. The speed of 71 was not limited by the power of its engines, it was limited by the heat that its structure could withstand today we are going to explore

titanium

, a material that made up 93 of the structure of the sr-71, a material that had never been Actually used to its full potential until the SR-71 appeared, we will explore its material properties, how it is manufactured, and how the SR-71 engineers overcame the challenges they encountered when using the innovative new material.

Titanium is one of those words that has entered common language. become synonymous with resistance. Sia compares

titanium

to being bulletproof and yes, at the right thickness it is bulletproof, which is why it is used on the A-10 to protect the pilot, but in reality the stronger titanium alloys are as strong as steel alloys are stronger and their temperature tolerance is actually worse, while aluminum is lighter. What makes titanium special is not its tensile weight or high temperature performance, but a combination of all these material properties that made it perfect for sr-71 when choosing materials for an application. in particular.

We will often refer to something called a material selection diagram where we plot two material properties on the x and y axes, this allows us to see the relative benefits of the materials so we can choose a material according to our needs. Here is a material selection diagram particularly relevant to the aerospace industry with density at the x-axis and strength the maximum pressure it can withstand before breaking at the axis and our top three

metal

material choices for aircraft structures are aluminum, steel and titanium located here here and here extend along the axis and axis because different alloys have different strengths, steel is by far the heaviest, which excludes it from most aircraft structures, but it is still Use where its strength and heat tolerance are needed.

We can also see that aluminum is, in fact, lighter than titanium, but titanium is stronger. than aluminum, a better measure here is the strength-to-weight ratio, a ratio found by dividing the

metal

's strength by its density; After all, we can make an aluminum part stronger by simply adding more material, but if we need to add so much material that the part is now heavier than an equivalent strength part made of titanium, then it's not worth it here, titanium gains its strength to weight ratio or specific strength is better than aluminum, but nowadays very little titanium is used in aviation aircraft, mainly aluminum is used, not titanium, why is that one of the reasons ?

It is really expensive even though titanium is the ninth most common element in the Earth's crust with a weight percentage of 0.6 percent. There is more titanium in the Earth's crust than carbon, an element that no one considers rare, but in its purified form it currently costs about four and a half. By comparison, $1,000 per metric ton of aluminum costs a third of that, $1,000 and a half per metric ton, which itself is a relatively expensive metal as a result of the high-energy electrolysis refining process. That's the current price, which has fallen dramatically since then. The SR-71 was created in titanium, it is expensive because its refinement process is a nightmare.

To make titanium we start with the raw material in the form of titanium dioxide with this chemical formula this oxide mineral called root oil can be found in high concentrations in these dark sandy soils to build the sr-71 the united states needed to purchase large quantities of the ore to the Soviets who had large deposits of rutile to do this they bought the material through shell organizations to hide the final destination of the material if the Soviets had known what they were helping to build they would not have sold that material, however, The United States probably could have purchased the material from mines in Australia.

This is a relatively common raw material and is mainly used as a white pigment for paints and is even found in sunscreen lotions as a pigment that blocks ultraviolet radiation. Our problem begins when we need to separate these two oxygen molecules to obtain pure titanium for iron. or refinement, we heat it in the presence of carbon to force the oxygen to separate from the iron and join with the carbon to form carbon. carbon dioxide with aluminum oxide its melting point is too high, so we dissolve it in a solvent and then use electrolysis to separate the oxygen molecule neither of these methods work with titanium titanium dioxide is thermally stable and resistant to chemical attack in the 1940s the first a reliable process was developed to produce a chemically pure form of titanium called the trace process.

This process made the SR-71 possible. It begins by first converting titanium dioxide into titanium chloride. To do this, titanium dioxide is mixed with chlorine and pure carbon and heated. any leakage of oxygen or nitrogen will ruin the process, so this should be done in relatively small batches in a sealed container. Once the process is completed, we have titanium chloride, then we must purify the titanium chloride from any impurities in the titanium ore by distillation. where we heat the product and separate the titanium chloride using its lowest boiling point, this titanium chloride vapor is introduced into a stainless steel container containing molten magnesium at 1,300 kelvin.

Titanium is highly reactive with oxygen at higher temperatures, so the container must also be sealed and filled with aragon. Here, titanium chloride reacts with magnesium, which itself is an expensive metal to form titanium and magnesium chloride. magnesium. This reduction reaction is extremely slow, taking between two and four days; Then, once the reaction is complete, we must allow the product to cool before removing it. magnesium chloride products through high temperature distillation once again magnesium and chlorine are recycled with electrolysis another energy intensive process at this stage we have a titanium sponge which needs further processing but normally a porous metal as this would simply be heated and compressed into rolls of foil. metal or some other form of useful end product, but titanium, as we said, will react with oxygen and nitrogen if heated to this height, we cannot do that, so the titanium sponge is compressed into an electrode along with the alloy metals needed and then heat is generated through an electrical air current inside another sealed container this form of heat does not require oxygen this melts the electrode to form a large ingot of titanium this process results in an incredibly expensive material that becomes even more expensive as a result of the difficulty engineers encountered in trying to give it its final form, the SR-71 engineers were among the first people in history to make real use of the material in that process, they ended up scrapping a lots of material, some by necessity, some by mistake, sometimes the engineers stumped as to what was causing the problems, but fortunately they documented and cataloged everything that helped find trends in their failures.

They found that spot-welded parts made in summer failed very early in their life, but those made in winter were fine. They finally traced the problem to the end. The fact that the Burbank water treatment facility was adding chlorine to the water they used to clean the parts to prevent algae blooms in the summer, but they removed it in the winter. Chlorine, as we saw before, reacts with titanium, so they started using distilled water from that point on. They discovered that their cadmium-coated tools left traces of cadmium on the bolts, which would cause galvanic corrosion and cause the bolts to fail.

This discovery led to all cadmium-containing tools being removed from the workshop; However, the greatest waste was caused by the lack of appropriate forging presses in the United States. Titanium alloys require much greater pressure to deform during forging than aluminum alloys or steel alloys. The best fodder in The United States at the time could only produce 20 of the pressures needed to form these titanium parts Clarence L Johnson the manager of the Skunk Works at the time advocated the development of a suitable forging press which he claimed would have to be a 250,000 ton metal forming press. Due to these deficiencies and forming capabilities, the final forging dimensions were nowhere near the design dimensions and much of the forming process had to be completed by machining, meaning that most of the material was cut to form the part, resulting in 90 percent of the material being wasted when the raw material costs so much, this type of waste really hurts, to add insult to injury.

Drill bits and other machining tools were being discarded at a rapid rate. Titanium is a difficult material to machine precisely because of its qualities that made it suitable for use in the SR-71. This is a material selection diagram with thermal conductivity on the x-axis. and thermal expansion in the y axis here we can see that titanium has low values ​​for both among the four lowest metals. Its low thermal expansion made accommodating thermal expansion as the plane heated easier, but measures still needed to be taken to prevent it from causing stress. The skin panels were fastened to the underlying structure with oblong holes that would allow the skin to expand and contract without the fasteners causing buckling and the skin over the wing would also be corrugated to prevent deformation during expansion.

This is actually quite remarkable, you can see the sections. which are clearly corrugated here, this did not affect the machining difficulties, but the extremely low thermal conductivity which made metal machining produces a large amount of heat which can damage the tool and cause unfavorable material properties in titanium-like hardening, meaning that the metal under the fresh cut is no harder and therefore even more damaging to the tool, this is usually minimized with coolant, but the low thermal conductivity of titanium means that very little heat is transferred to the coolant To deal with this, lower machining speeds must be used along with large volumes of coolant, which is also expensive, this slows down the heat generation rate and increases the removal rate.

This slower machine speed makes the process incredibly slow, but this is offset by making deeper cuts with each pass, which has the added benefit of cutting beneath work-hardened layers. It is also more sensitive to dull tools since its rigidity is quite low. The machine refers to metals like this as rubbery, they tend to form long chips which can clog the work area and cause all sorts of problems if not handled properly, they can ruin the work surface. and damaged tools, Lockheed engineers gradually learned these lessons and developed better tools for the job when the first version of the SR-71 was being built.

The drill bits used to cut the holes for the rivets could only drill 17 holes before they became unusable and had to be scrapped at the end of the program. SR-71 had developed a new drill bit that could drill 100 holes and then be sharpened for later use at the end. From the program engineers found enough improvements to save $19 million. In the manufacturing program it is quite clear that titanium is expensive and extremely difficult to work with. If aluminum had been an option for the SR-71 with a little added weight, the engineers would have jumped at the opportunity, but aluminum simply can't handle the temperatures. that steel and titanium can this is the material selection diagram showing the specific strength of various metals as a function of temperature this is ultimately what made titanium so attractive for sr-71 titanium alloys They maintain much of their strength up to temperatures as high as 450 degrees Celsius.

The same cannot be said for aluminum. What I find fascinating is that the maximum operating temperature of titanium depends not so much on loss of strength but rather on oxidation. Pure titanium is highly reactive to oxygen, which forms an oxide layer on the outside of the metal. which is brittle, this oxide layer has some benefits as it provides excellentcorrosion resistance, which is why many submarines use titanium to resist saltwater attack, but at higher temperatures this oxide layer and the titanium are soluble in oxygen, meaning oxygen can penetrate through the exterior . oxide layer and diffuses into the metal, causing the oxide layer to grow and eventually helping dangerous cracks to form.

The primary titanium alloy used in the SR-71 was 13 vanadium, 11 chromium and 3 percent aluminum. Both chromium and aluminum form thermally stable oxide layers on the metal. The outer skin of the metal prevents oxygen from diffusing further into the metal and causing it to become even more brittle, raising the metal's maximum operating temperature, while the vanadium acts as a stabilizer for a crystalline structure known as the beta phase. that drives. to a material with higher tensile strength and better formability with heat treatment capability for higher strength IMHO, advances in materials science like this have the biggest knock-on effect on the advancement of human technologies, up to To the point that we name entire eras of human history after the materials we developed during that time during World War II, the development of aluminum alloys suitable for aviation allowed for the emergence of some incredible aircraft and with that some incredible tactics like aerial invasions, a method of invasion that emerged for the first time in the world. war ii i just released the fifth episode of d day logistics on nebula, the streaming platform i created with my youtube friends in this episode i explore the tactics of the allied air landings in normandy i explored the technologies that helped the planes navigate to their drop zones in a pre-GPS era, where airborne troops landed and why, and even explore some of the wooden gliders that were used to transport heavy equipment to the battlefield.

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