Honing Options for Hypereutectic Aluminum Cylinder Bores
Alusil®, Lokasil®, Silitec®, DiASil, Mercosil, ALBOND® --- it sounds like a foreign language, but these are all trade names or trademarks for what is generically known as hypereutectic aluminum, a new/old material for cylinder bore wear surfaces. Hypereutectic aluminum is both new and old. Its cousins, hypoeutectic and eutectic aluminum, have been used for pistons and connecting rods for a number of years. Hypereutectic aluminum saw one of its earliest applications as the wear surface in an unlined cylinder in some Porsche engines in the 1960's. The 1971 Chevrolet Vega was the first true production automotive engine with a liner-less hypereutectic aluminum cylinder bore as the wear surface. Despite the car's reputation, the cylinder concept was ahead of its time.
No matter what trade name is used for this alloy or how the cylinder was created, this material is something rebuilders should understand because it may represent the future of cylinder technology and will probably start showing up more frequently in rebuild shops.
When properly finished, hypereutectic-aluminum cylinder bores present a surface to the piston rings that's roughly equivalent to glass [Figure one]. The resulting engine has lower friction, excellent sealing, improved dimensional stability, improved heat dissipation, reduced weight, better recyclability, lower manufacturing cost and higher durability - compared to the traditional aluminum block with cast-iron cylinder liners.
Aluminum cylinder evolution.
Since gasoline burned and forced a piston down a cylinder for the first time, aluminum has been the metal of choice when light weight was the most critical requirement for an internal combustion engine. This is as true today as it was in 1902 when the Wright Brothers were unable to purchase a suitable commercial engine for their experimental airplane and built their own, casting the aluminum block.
Automotive OEMs seized upon aluminum for the same reason and found manufacturing advantages, too: lower cost casting processes and easier machining. Aluminum automotive engine blocks are pretty much the norm today, and the standard solution for a cylinder wear surface has been, and still is, a gray iron liner. While low-cost, durable, and easy to manufacture (the key decision points for OEM's), engines using the iron-liner solution have inherent disadvantages in weight, size, thermal conductivity, differential thermal expansion and recyclability.
Consider that a minimum land width between cylinders must still be maintained, even with an iron liner. So the liner-equipped engine is still unnecessarily large, still has differential expansion and reduced heat dissipation issues, still needs a heavier and larger cooling system, etc.
A major advance came in 1971 when GM used Reynolds A390 aluminum alloy in the linerless Vega block. A390 is a hypereutectic alloy saturated with silicon, such that silicon particles are dispersed throughout the alloy similar to chocolate chips in cookies. "Saturated" is the key word. Small amounts of silicon will dissolve in aluminum and become inseparable, but above the saturation point (the "eutectic" point), silicon will precipitate out in crystal form. Typically, this begins to take place at around a 12% silicon concentration, and the hypereutectic cylinder surfaces in use today range from 12% to 20% or more in silicon concentration. Depending on the manufacturer, traces of other elements likely to be in the alloy can include copper, manganese, magnesium, phosphorus and strontium.
After traditional machining of the Vega engines, the factory cylinder surface was produced by chemical etching to create a surface where individual silicon particles protruded a small distance (perhaps 0.00004"/1 µm or a little more back then) above the aluminum cylinder surface. This process in cylinder preparation was, and still is, called the exposure step, whether done by the OEM or rebuilder. The intent is for the piston rings to ride on the silicon particles, not the aluminum cylinder wall.
Hypereutectic aluminum cylinders have evolved considerably since the Vega. And while GM led the way with the Vega engine, today Europe and Japan are leading the trend to the linerless aluminum block. OEM's using the material include Mercedes, Audi, Porsche, BMW, Volvo, VW, Jaguar, Yamaha, and Honda. Manufacturers of power sport vehicles, outboard motors and compressors also use hypereutectic cylinders.
Finishing without chemicals.
For the OEM, chemical etching of the cylinder wall was a non-traditional process and an intermediate step. The OEM wants to pour the block and put it in a transfer line. Chemical etching also became increasingly burdensome as environmental regulations tightened.
Whether it's in a rebuild shop or an OEM plant, the key to preparing the cylinder surface is to expose a tribologically optimized wear surface of silicon that withstands the grinding friction of pistons rings on the bore surface [Figure 2]. This requires relieving a small amount of aluminum from around the silicon particles. The ideal surface should have flats on the silicon crystals and crystal protrusion of 0.00002" to 0.00004" (0.5 to 1.0 µm) above the aluminum, with a minimum of holes (crystals torn from aluminum) and fractured crystals. The end product can be visualized as lily pads (hard silicon) sitting on still water [Figures 3 and 4].
Hypereutectic aluminum crankcases present another obstacle for metal cutting, which has led to advancements in the way the alloy is formulated. Silicon particles quickly destroy cutting tools. Several solutions have evolved. Tooling has improved with PCD and similar materials. Much effort has also been focused on improving tooling life by altering metallurgy to reduce silicon particle size, while retaining the excellent tribological properties of the hypereutectic surface. Based on SEM (scanning electron microscope) and VEECO-Scan studies of early cylinder surfaces and new products on the market today, it appears the silicon crystals have been reduced in size from about 10 µm originally to about 1 µm today, which would materially improve the machinability of the material. [Figures 5, 6, 7]
OEMs also devised ways to localize and limit the use of silicon through the use of cast-in hypereutectic aluminum liners, selective die casting and flame-sprayed coatings. Casting the block around special liners complicates the molding step and production rate. Flame spraying represents an additional process step between the mold and the transfer line.
The Holy Grail is the ideal combination of metallurgy, cutting tools and lowest-cost casting technology that allows machining blocks straight from the mold, and some OEMs have found it.
Finished Cylinder Bore Specifications.
On the rebuilder's side, Sunnen developed a GM-certified method for restoring a factory-quality surface for the Vega engine. Damaged bores were honed oversize using conventional abrasives, followed by an "exposure" step using a special lapping paste and felt honing pads. The process could be used with hand-held portable tools or honing machines. It produced excellent results and was ideal for occasional users. That process has since evolved, thanks to metal-bond diamond abrasives, and today there is a new honing option for OEMs or production rebuilders, as well as the low-volume rebuilder. It should be noted, too, that rebuilders have the option to use a replacement, press-fit hypereutectic aluminum cylinder liner available from Kolbenschmidt, if a cylinder is damaged beyond the point where it can be repaired by over-boring or honing. The honing process described here will work with this replacement liner, too.
In our honing process development work, we found that no two manufacturers of hypereutectic cylinders have identical specifications - similar to the situation with plateau specifications for cast iron. Several block manufacturers have patented manufacturing technologies, so rebuilders can expect to see variety in the alloys and the physical make-up of the cylinder wall.
There are, however, some generally common requirements for honing hypereutectic aluminum cylinders. First is the need for excellent geometry. Cylindricity limits of 0.0005" (0.013 mm) are typical. Limits are also placed on the percentage of fractured or displaced silicon crystals at the surface, which must be free of any torn or folded metal.
Because the silicon crystals are distributed throughout the metal in a homogeneous manner, there will always be some that are nearly machined through and will be displaced from the surface. Specifications typically call for about 80% intact particles.
There must be a minimum of sub-surface fractured material. This is largely a function of the prior boring step and the amount of material removed by honing.
Lastly, the exposed silicon particles must protrude above the base aluminum from 0.000004" to 0.00004" (0.1 to 1.0µm). The exposure height is related to the size of the silicon particles in the alloy. Smaller particle size will mean less exposure height. Average exposure height today is about 0.00002" (0.5 µm).
Assuming the cylinder was bored using high quality machinery and PCD or equal inserts, the honing process will have two or three steps: honing, finish honing and exposure. From a honing standpoint, working with hypereutectic aluminum is somewhat the reverse of working with cast iron - the end result is measured as a desired roughness or peak height of the exposed silicon.
The essence of the honing process for hypereutectic aluminum is to first produce an ultra-smooth, mirror-finish surface with the initial honing steps, then finish with an exposure step that will actually increase the roughness, as measured with a Profilometer, by relieving softer aluminum from around the silicon. The desired end result is an exposed surface of rounded-edge primary silicon particles.
Tooling for the initial steps should be selected according to traditional guidelines for high-precision honing. Machine settings, such as RPM, stroking speed, stroke length, etc. should be similar to those used for ordinary precision work. Feed rates are selected to complement the part geometry and abrasive characteristics. All abrasive should be fully trued to produce 100% surface contact at the diameter corresponding to the finish-honing step. Crosshatch angle is less important than with cast iron and will typically be rather flat (5-10 degrees) due to the slow stroking speed. Instead of a crosshatch, the aluminum cylinder relies on the area between the silicon crystals to hold its oil film. Keep in mind that the goal in the initial steps is produce a very accurate bore with a fine (mirror) finish.
MAN-845 Honing Oil is the minimum requirement and it should be filtered to at least 0.0004" (10 µm), preferably 0.0002" (5 µm). No water-based coolants should be used. In our process development work, we found that high-performance EP oil caused a sludge build-up, which impeded contact with the ultra-fine honing grit used in the exposure step. This is the result of the extreme surface area and high energy found in freshly cut, ultra-fine metal chips. These conditions facilitate far more aggressive chemical activity with the oil additives than would be experienced with larger metal chips.
In most cases, the first two honing steps can be accomplished with conventional or diamond abrasives [Figure 8]. However, because of the high value of these engine blocks and the wide variety of OEM materials and manufacturing methods, it is critical for a rebuilder to know the exact recommendation for reconditioning abrasives or consult a honing abrasive supplier. Some cylinder materials may simply require metal-bonded diamond for all of the steps. Conventional abrasives with bronze guide shoes are unquestionably the most economical option for infrequent work with hypereutectic aluminum. In production or OEM work, diamond is preferable for the first two honing passes.
The first honing step may not be required if the block has been bored with a final finish of <=19 µin. Ra (0.5 µm) If necessary, as a first honing step we recommend removal of 0.001" in. (25 µm), using classic abrasives or a 29 µm diamond, to produce a finish <=19 µin. Ra (0.5 µm).
The second finish-honing step removes 0.0001" (2.5 µm), again using traditional abrasive or a 9 µm diamond to produce a finish <=3.9 µin. Ra (0.1 µm).
The final exposure step requires a new specially developed, elastomer-bond abrasive (XM27), using light honing force. For the exposure step, we recommend tooling with the greatest abrasive surface contact area. This step is based on time, typically 1-1 ½ minutes for 19 µin. (0.5 µm) exposure height. Longer cycle times are not harmful, because the process is somewhat self limiting. It is absolutely critical that honing force or pressure be kept as low as possibly, while still maintaining tool stability. Surfaces shown in the accompanying illustrations were honed with less than 5 lb/in2 pressure.
The elastomer based -stone- [Figure 9] is purpose-designed to overcome three limitations of rigid abrasive in the silicon exposure process. First, the elastomer serves as a cushion, deforming to allow individual abrasive particles to literally bounce over the silicon particles, while still being rigid enough to cut the surrounding aluminum. Second, the elastomer dampens or limits the overall force applied to the abrasive, making the process very forgiving of variations in pressure from the honing machine feed system. The honing tool diametrical expansion does not have to exactly match the rate at which the cylinder is increasing in diameter from stock removal. Third, the elastomer conforms to any taper or out of roundness in the cylinder, allowing it to remove very small (0.00002"/0.5µm per side) amounts of material, uniformly throughout the cylinder. With rigid abrasive, any out of roundness in the bore would result in abrasive cutting pressure variations as the honing tool rotated.
Critical Point - Process Verification.
Any shop planning to do work on hypereutectic cylinders must have a Profilometer® or similar instrument for contact surface texture measurement to verify results. The instrument should produce a trace, not just a readout, and must be capable of Rk, Rpk and Rvk measurements. These engine blocks can cost $4000 or more, so honing without a Profilometer to verify results would be negligent.
Prior to the exposure step, the Profilometer will should show a very smooth surface <3.9 µin. Ra (0.1 µm), which becomes rougher - according to the instrument - after exposure. This is because the instrument senses the exposed silicon crystals as surface finish features (peaks). Several traces of the stylus across the surface may be needed before the stylus hits a silicon particle to verify peak height [Figure 10]. The absence of a peak means you probably need to make another trace. The presence of a peak verifies success. If no peaks are encountered after 8-10 traces, more time on the exposure step is needed.
Hypereutectic aluminum is not yet a mainstream material, and the different alloys and OEM manufacturing methods ensure there is no "standard" to refer to yet. However, the honing techniques outlined here were developed for OEM use and can easily be practiced in rebuilding. Nevertheless, until it becomes as familiar as cast iron, rebuilders may want to proceed with caution, and consult a honing abrasive supplier as needed.