21-04-2012, 10:34 AM
Expansion Coefficient of Aluminum Pistons– Forged vs Cast Pistons:
A Literature Search
Expansion Coefficient of Aluminum Pistons– Forged vs Cast Pistons A Literature Searc.doc (Size: 128.5 KB / Downloads: 31)
Summary: Expansion coefficient is a function of the silicon content of the aluminum alloy.
Forging requires alloy compositions lower in silicon content. These have higher expansion coefficients than cast alloys. So it is not the forging, but the alloy selected that dominates the expansion coefficients and thus the piston to cylinder clearances required. More silicon means lower expansion coefficient. But hardness and brittleness go up as silicon content goes up.
Modern cast pistons have lots of silicon content. These alloys are termed hypereutectic (more than eutectic mixture which is 12% silicon in aluminum).
eu•tec•tic (yōō-těk'tĭk) adj.
1. Of, relating to, or formed at the lowest possible temperature of solidification for any mixture of specified constituents. Used especially of an alloy whose melting point is lower than that of any other alloy composed of the same constituents in different proportions.
2. Exhibiting the constitution or properties of such a solid.
n. 1. A eutectic mixture, solution, or alloy.
2. The eutectic temperature.
Because higher silicon contents make the alloy harder and more brittle, alloys used in forging are necessarily lower in silicon. The resulting alloy is tougher, sometimes stronger, more resistant to detonation damage and more suited to extreme use such as racing. BUT it has a higher expansion coefficient requiring MORE piston to cylinder clearance to allow for the piston to grow when it heats. This is particularly true when using a steel or iron bore since steel and iron have lower thermal expansion coefficients than aluminum.
For aluminum pistons operating in steel cylinders, the key figure of merit is the difference in thermal expansion coefficients between steel and aluminum. I created an Excel spread sheet that computes this value (using an average figure of 12 for steel which ranges from 11 to 13) and then plotted the result which is shown below.
The key is the red line, the difference in thermal expansion between steel and aluminum. Note that it ranges from 12 for zero percent silicon to 8 for 12% silicon (the eutectic ratio) and down to 7 for 15% silicon, a slightly hypereutectic mix. So we can see that the thermal expansion coefficient reduces to 7/12 = 58% in going from no silicon to 15% silicon. It is down to 50% if you go out to 20% silicon.
This suggests that the piston to bore clearance can be reduced by this much if the silicon value is raised from zero to 15%. In other words, silicon content makes a big difference requiring that adjustments must be made in piston to bore clearance as the amount of silicon in the alloy is changed.
The advantages of cast pistons with higher silicon alloy are
• lower expansion coefficient,
• less bore clearance,
• less noise on start up when pistons are cold, and
• better long term wear in “normal” applications because the excess silicon hardens surfaces.
The advantages of forged pistons with lower silicon content are
• greater toughness,
• resistance to cracking, and
• Tolerance of abuse, and detonation
• but at the cost of greater required piston to cylinder clearance to accommodate the higher thermal expansion coefficient.
The following was extracted from one of the articles cited in the attachment (emphasis added).
With Hypereutectic pistons [cast], the primary reason for having all of this free silicon is to reduce piston ring groove wear. This allows piston designers to move the top compression ring much farther up the side of the piston (where combustion temperatures are much hotter), and run much smaller, thinner piston ring lands (the metal section separating the ring grooves).
The conclusion: be cautious with tight piston to cylinder clearance when using low silicon aluminum alloys typically used in forged pistons.
Below are presented articles from various sources adding additional information and background on the topic of forged versus cast pistons. A discussion of hypereutectic pistons is also included.
http://www.carcrafttecharticles/piston_r...index.html
Piston And Ring Technology
Better Materials, Shaving Weight, Improved Manufacturing Precision, Computer-Aided Design, And CNC-Machining Processes
Repeated cycles of searing combustion heat alternating with cool incoming air and fuel, extreme load reversal, thrust forces that slam the pistons into the cylinder walls—it’s quite amazing that aluminum pistons can survive in a performance engine at all. That they do is a tribute to today’s aftermarket material science and advanced manufacturing techniques. Technology has advanced to the point that what just a few years ago were considered custom “race-only” parts are now available at relatively affordable prices for even everyday performance use. Big-time companies like Federal Mogul/Speed-Pro are introducing new-generation lightweight mass-produced parts, while on the extreme high-end, new-tech niche manufacturers—such as HTC Products and CP Pistons—are working with advanced design processes and exotic materials to merge rocket science with race science.
Developments have focused on better materials, shaving weight, improved manufacturing precision, computer-aided design, and CNC-machining processes. Better materials and weight savings go hand in hand; stronger materials can be made lighter without sacrificing durability. And tighter manufacturing tolerances ultimately translate into better, closer-fitting parts that can be installed with reduced clearances for better sealing. Computer-aided design combined with finite element analysis allows “testing” the part before it actually leaves the drawing board, ensuring it is both as light and as strong as possible. With CNC milling, custom pistons are easier than ever to make, with lead time often in days instead of weeks and months. And what used to be custom orders are now in many cases in-stock items.
Pistons
The first step up from the common cast piston is the hypereutectic casting, which is strengthened with additional silicon content added to the aluminum brew. As with a conventional cast piston, a properly designed hypereutectic piston permits relatively tight piston-to-wall clearances, making for less noise under cold-start conditions.
All cast pistons—be they standard or hypereutectic—are made by pouring melted aluminum into a mold that shapes the slug into a piston. In contrast, forged pistons are formed using a giant press that takes a block of metal and pounds it into shape under thousands of tons of pressure. The tooling needed to do this is much more expensive than the tooling used to make a casting, and it wears out quicker. This makes forged pistons more costly than castings.
However, forgings have inherent advantages in terms of density, ultimate strength, and durability. Forging eliminates metal porosity, improves ductility, and generally allows the piston to run cooler than a casting. Within reason, forgings can be lightened without adversely affecting structural integrity. However, forged pistons expand and contract more under changing temperatures, so they traditionally require greater piston-to-wall clearance than cast pistons. In recent years, CNC-machining processes, diamond tooling, and careful attention to piston skirt profiling has given piston makers the ability to finely adjust skirt contact areas for more even loading. Barrel-type profiles now accommodate greater expansion at operating temperature. One result is that today’s short-skirted pistons have better contact areas than the old long-skirt designs, and wear is reduced even as piston-to-wall clearances are tightened up.
Forged pistons are generally made from one of several different aluminum alloys, with each offering different benefits depending on the application. The two most popular alloys are 4032 and 2618. Speed-Pro typically uses VMS-75, which is fairly close to 4032—both contain about 11 percent silicon, which helps with ring groove and skirt durability. These are the best choice for applications expected to have decent longevity, such as street vehicles and entry-level bracket racing and oval track combos. Although 2618 has better high-temperature characteristics, it contains virtually no silicon. This material expands and contracts more, so greater bore clearances are needed to prevent scuffing. Pistons using 2618 are best suited to nitrous, blowers, or higher end race applications where frequent inspection and replacement are not a problem.
A recent innovation is “Ultralloy,” a secret patented ceramic-aluminum alloy presently available from HTC Products, a premier manufacturer and distributor for most brands and types of cranks, rods, pistons, and rings. The silicon particles in Ultralloy have unprecedented uniformity in terms of their size, shape, and dispersion in the aluminum matrix. The new alloy’s strength is on par with titanium (and costs less) and parts can be made much lighter.
One of the most important advantages of forged pistons is what happens at the point of piston failure. Under extreme conditions—like detonation—forgings tend to “go plastic” and fail gradually. You generally have time to replace them before the entire engine is toast. Hypereutectics, although relatively strong in terms of ultimate tensile strength, have less ductility and are prone to fracture when their limits are exceeded. However, a lightweight hypereutectic piston specifically designed for high-performance use can withstand considerable cylinder pressures if the tune is right. When considering which style of piston is right for your application, you should consider how much abuse the piston will see, your budget, and the need to remain competitive in your form of racing. Sustained heat is the biggest piston-longevity limiter.