While composite materials like fiberglass, Kevlar, and carbon fiber are the hot ticket materials in uncompromising (and unforgiving) vehicle construction, metals still sing lead vocals for the basis of most race cars. A basic understanding of metallurgy and metal alloys can go a long way in the understanding of structures and racing mechanics.
While we would all love to stroll down to our local supercar vendor on a Sunday morning and pay cash for the latest unobtanium-constructed ride, few are so lucky as to have the means. What this translates to is most of us toil in or garages, often with inadequate tools and resources to satisfy our speed and performance addiction.
The home race car fabricator is only likely to encounter two or three basic families of metals in their endeavors: aluminum alloys and steel alloys. Those who reach a little farther into the materials science realm of metals will find themselves tangling with beasts far beyond their own hand-working means included among these are some non-ferrous alloys, and the aptly named family of super alloys.
Did You Pay Attention In Chemistry Class?
Metals are divided into two distinct categories — ferrous, and non-ferrous. To understand this relationship we turn to our old chemistry class buddy, the periodic table. The element iron, denoted as Fe is the main ingredient for all ferrous alloys — steel primarily. Non-ferrous alloys may contain trace amounts of iron but are characterized by their lack of this element.
What is an alloy you might ask? Simply put, a metal alloy is just a mixture of elemental ingredients. Multiple metal and non-metal players may contribute to the final substance. Common alloying elements include carbon, manganese, molybdenum, silicon, tungsten, nickel, and chromium, among others.
Most metals we find in the motorsports world hail from the center of the periodic table, and are known as transition metals. According to the Los Alamos National Laboratory, “The transition elements are metals that have a partially filled d subshell.” What this means is that because these elements are not carrying a full electron capacity, they seek out and easily bond with adjacent elements to fill their electron shell.
Nearly all cars are made primarily of steel. Bodies, chassis, roll cages, engine parts, suspension components, and so on. Steel is the product of a union between iron and carbon — with the carbon content generally ranging from .1 to 1.5 percent. The carbon content of a steel alloy greatly influences its mechanical properties.
Tensile strength, malleability, toughness, and hardness are all distinct properties in metallurgy. While strength and hardness may be more familiar concepts others may not. Malleability refers to the ease of bending the material before a plastic (permanent) deformation takes place (think about the difference between a paper clip and a clothes pin spring). Toughness is actually a measure of abrasion resistance, or how well will a given material holds up to repeated friction against another surface — consider here a brake rotor for comparison.
Plain carbon steels are just as they sound, a simple alloy of iron and carbon. Often resulting in very robust and useful alloys such as 1018, 1075, 1084, 1095, and others, these materials can be found in everything from car body skins to cutlery. Where plain carbon steel alloys fall down is in utilizing the beneficial properties of other alloying elements.
Household names steel colloquialisms like “chromoly” are fundamentally a high carbon tool steel that reaps the benefits of several other complementary elements. Lending properties like corrosion resistance and grain structure control chromoly steels like 4130, 4340, 4140, and 4340 contain considerable amounts of chromium and molybdenum — hence the contraction chromoly.
As the most common and commercially available, 4130 was originally produced to provide the aviation industry a stronger alternative to small diameter and thin wall thickness tubing. One of the common myths surrounding chromoly is that it is considerably lighter and stronger than carbon steel alternatives.
Where the confusion develops is that Chromoly is stronger when comparing two like sized coupons — therefore one can utilize a thinner piece of 4130 than mild steel to achieve the same strength. Apples to apples, the weight difference between chromoly and mild steel is relatively negligible.
Emerging techniques in metals processing has given rise to sintered materials like compacted graphite iron (CGI). Found in the construction engine blocks like that of the Cummins-powered Nissan Titan XD. CGI offers the mechanical properties previously not found in a common form of iron.
According the Waupaca Foundry, “The internal porosity solidifies in a manner similar to gray iron with the strength of ductile iron and allow complicated castings. It has the ability to dissipate heat and increased dampening properties.”
What Condition Is My Steel?
Ferrous alloys regularly encountered exist in three basic states of molecular arrangement: annealed, normalized, and hardened. Without delving into the specificities of carbon and iron molecular base structures we can still easily understand what these conditions imply. An annealed metal has been heated to a critical temperature where the molecules that comprise the grain structure of the metal are allowed to relax and align.
The metal is allowed to cool very gradually to prevent the grain structure from returning to a stress-induced state. In this condition steels are more malleable and softer. Normalized is the standard condition in which most metals are purchased. For example typical 4130N roll cage tubing can be heated and allowed to air cool without a change in mechanical properties.
Hardened metals often include a secondary mill process to freeze the molecules in a particular arrangement. Depending on the alloy a given metal may be air hardening, or require an oil or water quench for rapid cooling.
Stainless Steel Alloys
Stainless steel alloys often get misconstrued as falling in the non-ferrous category because some alloys are non-magnetic. Stainless steel does still contain an appreciable amount of iron, but is often more rich in the combined chromium and nickel that lend their impressive corrosion resistance traits. Rarely used as a structural material, save for a few alloys like 17-4 H1150, stainless steel is usually implemented in applications where heat tolerance is a priority.
Exhaust systems are one of the primary sites of stainless steel use in the performance aftermarket. Because of its high resistance to oxidation, and retention of hardness at high temperatures, stainless is the choice for racers unable to spring for super alloys but looking for a step above mild steel with a ceramic finish.
For exhaust systems, 321 stainless is another example of aviation technology creeping into motorsport. Formulated to prevent the propagation of cracks under high vibration environments — this alloy includes .15 percent titanium making it unique among other stainless steel alloys.
Common alternatives to 321 include 304, another non-magnetic stainless steel which is common used in food service applications but lacks the titanium addition. In a separate family we find 409 stainless steel, know as a ferritic alloy, 409 is magnetic. Containing several percent less chromium and nickel means that the corrosion resistance of this alloy will certainly surpass mild steel, but not it’s contemporary stainless alloys like 304 and 321.
Budgets aside 321 is the most desirable stainless for exhaust systems and the like when it comes to automotive applications, with 304 and 409 following respectively.
Far and away the most common non-ferrous alloy we encounter in race cars is aluminum. A metal not found in pure form in nature, it starts life as an ore known as bauxite whereby aluminum is separated from its constituent compounds. In pure form aluminum is not a particularly useful alloy for structural fabrication. With additions of silicon, magnesium, and other elements it becomes the favored lightweight material for otherwise hefty items like cylinder heads, engine blocks, control arms, radiators, fluid tanks, and accessory brackets.
The most common aluminum alloys one will encounter are 6061, 7075, and 5052. These alloys are extremely resilient but each have their appropriate applications. While 6061 is the go to for most general purpose projects 7075 is the brute of the bunch boasting the strongest, hardest, and toughest recipe — where this metal falls down is workability. Considered non-weldable due to its properties, 7075 sees usage almost exclusively as select load bearing, machined billet parts.
Aluminum is celebrated not only for its lightness and corrosion resistance but its ease of machinability, and safety considerations. Consider that an aluminum fuel cell grinding along tarmac in the event of crash — no sparks are generated. Aluminum is praised and cursed by its impressive heat transfer properties. As an extremely effective conductor it is no secret that aluminum fluid coolers are the standard in any performance vehicle.
Similarly to how steel alloys are graded by condition aluminum alloys have their own system of grading. Usually denoted by a “T#” suffix, these codes refer to what temper sub process and by extension properties the alloy has been engineered to include. Examples like T4, T6, and T7 refer to temper of the aluminum and are a grade of harness with T6 being the optimal peak.
Titanium has always had an identity of high-tech and glamor to the mainstream public. With billings like stronger than steel and lighter than aluminum sung from the roof tops who are we to question this scarce metal? It’s no secret that titanium is expensive as a raw material — and that overhead hangs far enough to keep our wallets in the dark when it comes to buying go-fast parts. But what are the drawbacks?
Titanium is a metal known for extreme heat tolerance — able to remain structurally sound at temperatures beyond the melting point of steel. While this is true, it is a myth that this element is lighter than aluminum. With an atomic weight of 47.88 units, and aluminum at 26.98 units, titanium has a long way to go to catch up with its fellow non-ferrous contemporary.
With the atomic weight of iron coming in at 55.85 units, titanium remains an option for the ultra weight-conscious, and those with money burning holes in all their pockets. To this end titanium nuts and bolts, and exhaust systems, remain the most common occurrences of this metal. It is unassuming as stainless steel until heated, when waves of colorful heat-effected zones will ripple across the alloy like some otherworldly sunset.
Working with titanium is no casual task. While it may be cut and bent not too dissimilarly to steel, welding is a whole different operation. While welding ferrous alloys one can usually get away with a single-sided gas shield exposure to prevent impurities and inclusions. When it comes to titanium a chamber or pressure shielding system must be fabricated to ensure both sides of the weld are shielded by inert gas for the duration of the weld.
One drawback to Titanium is it’s well-know lack of malleability. Titanium parts that experience repeated heat cycles and exposure to road surfaces (like exhaust systems) are susceptible to cracking. A crack in a titanium exhaust system means a trip to a precision welding shop rather than the local muffler shop.
Among the rarest and most exotic of the metals are the so called super alloys. Generally reserved for the likes of NASA and Formula 1, these are the most expensive to acquire, most challenging to process, and least likely to be run across by the garage hobbyist or even professional racer. The primary elements that go into creating super alloys include nickel, silicon, and chromium. Manifesting in hushed and proprietary boutique metals like Inconel and the more generic Monel, these alloys take specialty and niche to the extreme.
As the turbine section of a jet engine sees extreme heat and stress, materials to tolerate the abuse had to be generated that offered superior hot mechanical properties than titanium. In today’s age of advanced turbochargers and exhaust technology the same demands have surfaced.
Metal Applications That May Have Slipped Your Mind
Familiar with ‘moly gear oil? Molybdenum is a common additive to gear oils as it lends an ultra-slick lubricating tendency when held in solution at the extremely fine level. Gold is becoming more commonly found in the bespoke engine bays of supercars and uncompromising race cars. Remember that space blanket you had as a kid, or as an adult in your hiking pack? While gold is an excellent conductor for electrical system contacts, it is also an effective reflector of heat, and is used to isolate the heat from one system to another.
Whether you are building a roll cage, exhaust system, or just some bracketry for your car, an understanding of the metals involved will help you down the best path. Even if you never tangle with materials beyond mild steel and 6061 aluminum you can build a tool chest of options for materials should an occasion arise where something special is needed.