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Understanding Today’s Engine Metals
In the early days of the automobile engine, components were typically made from iron and steel either by casting or by forging. Iron is one of the most abundant elements, forming much of the Earth’s crust and inner core.
By Brendan Baker
Metallurgy is one of the oldest sciences in the world it’s a history that can be traced back to the beginnings of civilization. It’s hard to believe that in the thousands of years since the discovery of gold and silver there have only been 84 new metals discovered. In fact, when the automobile was invented not even half of the metals we know today had been discovered.
Although materials have changed quite a bit in the last 100 years of the automobile, we are still casting and forging engine components using similar processes. Where today’s engines differ is in an increasing emphasis on improving fuel economy and emissions. In order to achieve these goals there is a greater reliance on lighter-weight materials such as aluminum for use in cylinder blocks, cylinder heads and other engine components.
Some components such as engine covers and intake manifolds, for example, are now made of exotic materials such as magnesium, titanium and various composite alloys. And many internal engine parts, such as connecting rods, timing gears, oil pump rotors and valve seats and guides, are formed to nearly exact size (meaning very little waste or machining) using powder metallurgy, an alloy blended with several materials to produce the desired properties for the component. Subsequently, OEMs say powdered metal is as durable as steel, and it’s lighter and less expensive to manufacture.
Aluminum has been used for pistons, cylinder heads, blocks and even connecting rods for many years. Aluminum’s main advantage is its light weight, which is up to a third less than cast iron. It also dissipates heat very quickly, which can be an advantage or a disadvantage depending on what you are trying to accomplish. To make horsepower, you want to retain heat in the combustion chamber, but you don’t want an engine to detonate or experience preignition. So for maximum power, aluminum heads are usually the best choice.
A common alloy used for OEM and aftermarket aluminum blocks is 355 with pressed-in or cast-in iron or steel cylinder liners. This can present a challenge when resurfacing an aluminum block with iron or steel liners. Cylinder heads are made with a variety of alloys but one popular choice is A356 with a T6 heat treatment. Ordinary aluminum blocks without liners are too soft to provide adequate wear resistance, so a special high silicon alloy aluminum must be used (typically 16 to 18 percent silicon).
A nickel silicon carbide process for coating cylinder bores was introduced in 1967 by Mahle. Called “Nikasil,” it was initially developed for rotary engines to reduce wear in the aluminum rotor housing. But it also turned out to be a good wear-resistant coating for aluminum piston engines, too. It only needs to be a few thousandths of an inch thick (typically .003˝ to .007˝) to make the cylinder walls wear resistant, and the coating can be reapplied if the cylinders need to be bored to oversize. It retains oil well and allows tighter piston-to-cylinder clearances to reduce blowby.
Aluminum forms an oxide on the surface that has to be broken down chemically with caustic during the cleaning process. The chemical reaction can discolor the aluminum, so that may have to be removed with a de-oxidizer treatment or by blasting with glass beads, aluminum shot or soft media such as plastic or baking soda. Using baking soda eliminates any worries about glass beads or shot being left inside a block or head because it is water soluble and can be easily washed away.
Aluminum can also be cleaned in an oven, but experts say the temperature of the thermal cleaning process has to be limited or it will soften. In general, you should never heat aluminum components to more than 600 degrees F, or allow them to bake at 450 degrees F or higher for more than two hours.
Conventional cylinder heads and engine blocks are made of cast iron, which is considered an iron alloy with more than 2 percent carbon as the main alloying element. In addition to carbon, cast iron must also contain from 1 to 3 percent silicon that when combined with carbon produces a very castable metal.
Cast iron has a much lower melting temperature than steel and is more fluid and less reactive with molding materials. However, it does not have enough ductility to be rolled or forged. Gray cast iron, which is used the most for automotive applications, has about 92 percent iron, 3.4 percent carbon, 2.5 percent silicon and 1.8 percent manganese. It has a tensile strength of 25,000 psi and a hardness of 180 Brinnell.
During the solidification process, carbon becomes graphite, which is one of the main characteristics of the metal’s distinctive properties. The graphite provides excellent machinability (even at wear-resisting hardness levels), dampens vibration, and aids lubrication on wearing surfaces (even under borderline lubrication conditions).
Compacted Graphite Iron
Compacted graphite iron (CGI) is offered by some suppliers in some of their performance aftermarket blocks. But CGI has mainly been used by OEMs, primarily in Europe, for diesel engine applications to handle the higher loads without adding weight because it is stronger than cast iron castings, it can be made thinner and lighter. A CGI block can also be a good choice for performance engines that are supercharged or turbocharged or running a lot of boost pressure and is a good material for engines that run big shots of nitrous oxide. CGI roughly doubles the strength of the casting but adds no additional weight. However, it’s a little more expensive to manufacture and therefore will cost you few dollars more than an ordinary cast iron block.
Metal Matrix Composites
With the OEs’ continued quest to lighten the weight of their vehicles in order to reduce fuel consumption through the use of lighter materials, they have begun looking beyond even aluminum. They have developed cast metal matrix composites (MMC) to help answer the challenge. MMC is an aluminum-graphite particle composite that can be used for pistons and cylinder liners. It reduces friction and gives the aluminum lubricity in low-temperature, low-oil conditions, similar to cast iron.
Today’s stock production aluminum heads are often fitted with powdered metal (PM) seats and guides. PM components are less expensive to manufacture, and they have turned out to be very durable. PM seats, for example, often show little wear at high mileages. Consequently, if you are rebuilding a head with PM seats, the seats may only need a light touch-up.
Powder metal seats are made by mixing together various dry metal powders such as iron, tungsten carbide, molybdenum, chromium, vanadium, nickel, manganese, silicon or copper, etc. This powder is then pressed into a die mold, then subjected to high heat and pressure (a process called “sintering”) to bond together the metals and form a solid composite matrix with very uniform and consistent properties.
The demands of today’s engines can be extreme and the materials must have the right combination of strength and resistance to survive.