20 APR-JUN 2008 engine professional

BackgroundAlusil, Lokasil, Silitec, DiASil,Mercosil, ALBOND it sounds like aforeign language, but these are all tradenames or trademarks for what isgenerically known as hypereutecticaluminum, a new/old material for cylinderbore wear surfaces. Hypereutecticaluminum is both new and old. Itscousins, hypoeutectic and eutecticaluminum, have been used for pistons andconnecting rods for a number of years.Hypereutectic aluminum saw one of itsearliest applications as the wear surface inan unlined cylinder in some Porscheengines in the 1960s. The 1971 ChevroletVega was the first true productionautomotive engine with a liner-lesshypereutectic aluminum cylinder bore asthe wear surface. Despite the carsreputation, the cylinder concept wasahead of its time.

No matter what trade name is used forthis alloy or how the cylinder wascreated, this material is somethingrebuilders should understand because itmay represent the future of cylindertechnology and will probably startshowing up more frequently in rebuildshops.

When properly finished, hypereutecticaluminum presents a surface to the pistonrings thats roughly equivalent to glass.The resulting engine has lower friction,

excellent sealing, improved dimensionalstability, improved heat dissipation,reduced weight, better recyclability, lowermanufacturing cost and higher durability compared to the traditional aluminumblock with cast-iron cylinder liners.

Aluminum cylinder evolutionSince gasoline burned and forced a pistondown a cylinder for the first time,aluminum has been the metal of choicewhen light weight was the most criticalrequirement for an internal combustionengine. This is as true today as it was in1902 when the Wright Brothers wereunable to purchase a suitable commercialengine for their experimental airplane andbuilt their own, casting the aluminumblock.

Automotive OEMs seized uponaluminum for the same reason and foundmanufacturing advantages, too: lowercost casting processes and easiermachining. Aluminum automotive engineblocks are pretty much the norm today,and the standard solution for a cylinderwear surface has been, and still is, a grayiron liner. While low-cost, durable, andeasy to manufacture (the key decisionpoints for OEMs), engines using the iron-liner solution have inherent disadvantagesin weight, size, thermal conductivity,differential thermal expansion andrecyclability.

Consider that a minimum land widthbetween cylinders must still bemaintained, even with an iron liner. Sothe liner-equipped engine is stillunnecessarily large, still has differentialexpansion and reduced heat dissipationissues, still needs a heavier and largercooling system, etc.

A major advance came in 1971 whenGM used Reynolds A390 aluminum alloyin the linerless Vega block. A390 is ahypereutectic alloy saturated with silicon,such that silicon particles are dispersedthroughout the alloy similar to chocolatechips in cookies. Saturated is the keyword. Small amounts of silicon willdissolve in aluminum and becomeinseparable, but above the saturationpoint (the eutectic point), silicon willprecipitate out in crystal form. Typically,this begins to take place at around a 12%silicon concentration, and thehypereutectic cylinder surfaces in usetoday range from 12% to 20% or morein silicon concentration. Depending onthe manufacturer, traces of other elementslikely to be in the alloy can includecopper, manganese, magnesium,phosphorus and strontium.

After traditional machining of the Vegaengines, the factory cylinder surface wasproduced by chemical etching to create asurface where individual silicon particlesprotruded a small distance (perhaps 1.0m/0.00004" or a little more back then)

New Honing Options for Hypereutectic Aluminum Cylinder BoresMercury Marine, BMW, Porsche, Mercedes Benz and otherengine manufacturers use a special aluminum alloy cylinder material filled with very hard, glass-like particles.You need to know the dos and donts when honing this alloy.

BY TIM MEARA PHOTOS PROVIDED BY SUNNEN PRODUCTS COMPANY

engine professional APR-JUN 2008 21

above the aluminumcylinder surface. Thisprocess in cylinderpreparation was, andstill is, called theexposure step, whetherdone by the OEM orrebuilder. The intent isfor the piston rings toride on the siliconparticles, not thealuminum cylinder wall.

Hypereutecticaluminum cylinders haveevolved considerablysince the Vega. And while GM led theway with the Vega engine, today Europeand Japan are leading the trend to thelinerless aluminum block. OEMs usingthe material include Mercedes, Audi,Porsche, BMW, Volvo, VW, Jaguar,Yamaha, and Honda. Manufacturers ofpower sport vehicles, outboard motorsand compressors also use hypereutecticcylinders.

Finishing without chemicalsFor the OEM, chemical etching of thecylinder wall was a non-traditionalprocess and an intermediate step. TheOEM wants to pour the block and put itin a transfer line. Chemical etching alsobecame increasingly burdensome asenvironmental regulations tightened.

Whether its in a rebuild shop or anOEM plant, the key to preparing thecylinder surface is to expose atribologically optimized wear surface ofsilicon that withstands the grindingfriction of pistons rings on the boresurface. This requires relieving a smallamount of aluminum from around thesilicon particles. The ideal surface shouldhave flats on the silicon crystals andcrystal protrusion of 0.5-1.0 micronabove the aluminum, with a minimum ofholes (crystals torn from aluminum) andfractured crystals. The end product can bevisualized as lily pads (hard silicon) sittingon still water.

Hypereutectic aluminum crankcasespresent another obstacle for metal cutting,which has led to advancements in the waythe alloy is formulated. Silicon particlesquickly destroy cutting tools. Severalsolutions have evolved. Tooling hasimproved with PCD and similar materials.Much effort has also been focused onimproving tooling life by alteringmetallurgy to reduce silicon particle size,while retaining the excellent tribologicalproperties of the hypereutectic surface.Based on SEM (scanning electronmicroscope) and VICO-Scan studies ofearly cylinder surfaces and new productson the market today, it appears the siliconcrystals have been reduced in size fromabout 10 m originally to about 1 mtoday, which would materially improvedthe machinability of the material.

OEMs also devised ways to localizeand limit the use of silicon through theuse of cast-in hypereutectic aluminumliners, selective die casting and flame-sprayed coatings. Casting the block

Fig. 3: Ideal honed surface imaged bySEM (scanning electron microscope)shows smooth-surface, round-edgesilicon crystals (dark areas) protrudingabove the aluminum base.

Fig. 4: Close-up of crystal from Fig. 3,which shows smooth surface, free offractures or scoring.

Fig. 1 & 2: Two differentillustrations portray theworking surface of ahypereutectic aluminumcylinder wall. Typicalprotrusion of the siliconcrystals is 0.00002".

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2

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around special liners complicates themolding step and production rate. Flamespraying represents an additional processstep between the mold and the transferline.

The Holy Grail is the ideal combinationof metallurgy, cutting tools and lowest-cost casting technology that allowsmachining blocks straight from the mold,and some OEMs have found it.

Finished cylinder bore specificationsFrom the rebuilders side, Sunnendeveloped a GM-certified method forrestoring a factory-quality surface for theVega engine. Damaged bores were honedoversize using conventional abrasives,followed by an exposure step using aspecial lapping paste and felt honing pads.The process could be used with hand-heldportable tools or honing machines. Itproduced excellent results and was idealfor occasional users. That process hassince evolved, thanks to metal-bonddiamond abrasives, and today there is anew honing option for OEMs orproduction rebuilders, as well as the low-volume rebuilder. It should be noted, too,that rebuilders have the option to use areplacement, press-fit hypereutecticaluminum cylinder liner available fromKolbenschmidt, if a cylinder is damagedbeyond the point where it can be repaired

by over-boring or honing. The honingprocess described here will work with thisreplacement liner, too.

In our honing process developmentwork, we found that no twomanufacturers of hypereutectic cylinders

have identical specifications similar tothe situation with plateau specificationsfor cast iron. Several block manufacturershave patented manufacturingtechnologies, so rebuilders can expect tosee variety in the alloys and the physicalmake-up of the cylinder wall.

There are, however, some generallycommon requirements for honinghypereutectic aluminum cylinders. First isthe need for excellent geometry.Cylindricity limits of 0.013 mm (0.0005")are typical. Limits are also placed on thepercentage of fractured or displacedsilicon crystals at the surface, which mustbe free of any torn or folded metal.

Because the silicon crystals aredistributed throughout the metal in ahomogeneous manner, there will alwaysbe some that are nearly machined throughand will be displaced from the surface.Specifications typically call for about80% intact particles.

There must be a minimum of sub-surface fractured material. This is largelya function of the prior boring step and theamount of material removed by honing.

Lastly, the exposed silicon particlesmust protrude above the base aluminumfrom 0.1 to 1.0m. The exposure heightis related to the size of the silicon particlesin the alloy. Smaller particle size willmean less exposure height. 0.5mexposure height is about average today.

NEW HONING OPTIONSBY TIM MEARA

22 APR-JUN 2008 engine professional

Tech TipBoring or rough honinghypereutectic aluminum alloy withvarious cutting insert materials,metal bonded superabrasives oreven common vitrified bondedabrasive tends to crush orfracture an inordinate number ofsurface-available silicon crystalsrequired for load carryingcapacity. As such, it would be aviolation of material properties tosimply bore and honehypereutectic aluminum alloyswithout further surface andcrystal preparation. To that end,boring and honing alone will onlyserve to damage the cylindersurface and create highexpectation of complete slidingseal failure. Surface preparationbeyond finish honing is a must.

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Fig. 5: Surface scans of various hypereutectic cylinder walls producd by different manufacturers and methods. Red patches are silicon and reflect different sizes and distributions of particles that can be found on the market.

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Honing ProcessAssuming the cylinder was bored usinghigh quality machinery and PCD or equalinserts, the honing process will have twoor three steps: honing, finish honing andexposure. From a honing standpoint,working with hypereutectic aluminum issomewhat the reverse of working withcast iron the end result is measured as adesired roughness or peak height of theexposed silicon.

The essence of the honing process forhypereutectic aluminum is to first producean ultra-smooth, mirror-finish surfacewith the initial honing steps, then finishwith an exposure step that will actuallyincrease the roughness, as measured witha profilometer, by relieving softeraluminum from around the silicon. Thedesired end result is an exposed surface ofrounded-edge primary silicon particles.

Tooling for the initial steps should beselected according to traditionalguidelines for high-precision honing.Machine settings, such as RPM, stroking

speed, stroke length, etc. should be similarto those used for ordinary precision work.Feed rates are selected to complement thepart geometry and abrasivecharacteristics. All abrasive should befully trued to produce 100% surfacecontact at the diameter corresponding tothe finish-honing step. Crosshatch angle isless important than with cast iron and

will typically be rather flat (5-10 degrees)due to the slow stroking speed. Instead ofa crosshatch, the aluminum cylinder relieson the area between the silicon crystals tohold its oil film. Keep in mind that thegoal in the initial steps is produce a veryaccurate bore with a fine (mirror) finish.

MAN-845 Honing Oil is the minimumrequirement and it should be filtered to atleast 10 m, preferably 5 m. No water-based coolants should be used. In ourprocess development work, we found thathigh-performance EP oil caused a sludgebuild-up, which impeded contact with theultra-fine honing grit used in the exposurestep. This is the result of the extremesurface area and high energy found infreshly cut, ultra-fine metal chips. Theseconditions facilitate far more aggressivechemical activity with the oil additivesthan would be experienced with largermetal chips.

In most cases, the first two honing stepscan be accomplished with conventional ordiamond abrasives. However, because ofthe high value of these engine blocks andthe wide variety of OEM materials andmanufacturing methods, it is critical for arebuilder to know the exactrecommendation for reconditioningabrasives or consult a honing abrasivesupplier. Some cylinder materials maysimply require metal-bonded diamond forall of the steps. Conventional abrasiveswith bronze guide shoes areunquestionably the most economicaloption for infrequent work withhypereutectic aluminum. In production orOEM work, diamond is preferable for thefirst two honing passes.

The first honing step may not berequired if the block has been bored witha final finish of

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is somewhat self limiting. It is absolutelycritical that honing force or pressure bekept as low as possibly, while stillmaintaining tool stability. Surfaces shownin the accompanying illustrations werehoned with less than 5 lb/in2 pressure.

The elastomer based stone ispurpose-designed to overcome threelimitations of rigid abrasive in the siliconexposure process. First, the elastomerserves as a cushion, deforming to allowindividual abrasive particles to literallybounce over the silicon particles, whilestill being rigid enough to cut thesurrounding aluminum. Second, theelastomer dampens or limits the overallforce applied to the abrasive, making theprocess very forgiving of variations inpressure from the honing machine feedsystem. The honing tool diametricalexpansion does not have to exactly matchthe rate at which the cylinder is increasingin diameter from stock removal. Third,the elastomer conforms to any taper orout of roundness in the cylinder, allowingit to remove very small (0.5m/side)amounts of material, uniformlythroughout the cylinder. With rigidabrasive, any out of roundness in the borewould result in abrasive cutting pressurevariations as the honing tool rotated.

Critical Point Process VerificationAny shop planning to do work onhypereutectic cylinders must have aProfilometer or similar instrument forcontact surface texture measurement toverify results. The instrument shouldproduce a trace, not just a readout, andmust be capable of Rk, Rpk and Rvkmeasurements. These engine blocks cancost $4000 or more, so honing without aProfilometer to verify results would benegligent.

Prior to the exposure step, theProfilometer will should show a verysmooth surface (