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Aluminium engine blocks enhancelight weight-constructionCastings of aluminium are makingan ever more significant contributionto leveraging light constructionpotentials in modern passengercars. The heaviest individual component,the engine block, hasmeanwhile taken on a key role.If we consider the enormous numberof in-line engines produced –four-cylinder and recently alsothree-cylinder ones – the costbenefitaspect must have absolutepriority.The replacement ofgrey cast iron with aluminium forengine blocks therefore presupposeslow-cost concepts.Concept diversity for aluminiumengine blocksContrary to the engine blocks producedso far, mostly made from greycast iron based on a uniform conceptwhich already incorporatesthe cylinder bore surfaces when aperlitic structure configuration ischosen, there are many diverseoptions in the case of aluminium.These relate to the conceptmodules of type of engine blockdesign, alloy, casting method andcylinder bore surface, in which casemanifold incompatibilities haveto be taken into account. Theoverriding aspect is therefore thecompatibility of the conceptmodules. A further constraint forthe concepts which are viable inprinciple results for concreteengine development projects fromthe targets and general specificationswith respect to enginefunction, elemental engine characteristics(number and arrangementof cylinders), annual outputs, costsof castings (and whole base enginesystems), optimisation potential,environmental aspects andrecyclability. This situation hasled to a large number of marketableconcepts.Aluminium engine blocks andhigh-pressure die casting method■ Economic aspectFrom the economic aspect, formass produced engines, highlyautomated casting methods usingsand moulds (“core package process”)whose sequence time isnot bound to the solidification areprincipally competing with highpressuredie castings. In theespecially harshly contested, costsensitivemarket segment of fourcylinderengines, the latter castingmethod is unrivalled in the presentsituation with respect to costeffectiveness.■ Design featuresHigh-pressure die casting methodscompulsorily prescribe mouldabilityin steel. Hence the open deckdesign is mandatory. It is obvious,however, that with aluminiumengine blocks this design findsbetter general acceptance today.A distinctly reduced water jacketdepth and co-moulded cylinderbores have created a variant besidesthe conventional open deckwhich exhibits a sufficiently rigidcylinder area even without a closeddeck, meeting the requirements ofmost in-line engines. For nicheapplications in closed deck design,Movable Preform Casting Stationary die halfdie halfa suitable technology is availablein principle for developments inthe form of DOEHLERCORE, a corewithstanding high pressures andhigh thermal stress.■ PropertiesHigh-pressure die castings provideextremely accurate contours anddistinguish themselves by, incomparison, extremely narrowdimensional, shape and positiontolerances. It should be mentionedthat there are certain disadvantagesinherent in the shot-like diefilling. This leads to gas inclusions(microporosity) in the components.Moreover, a certain tendencytowards local shrinkage-cavityformation can hardly be eliminated.Because the gate always solidifiesin the course of this process, thereis no re-feeding possibility. On theother hand, pressure die castingsreach appreciable strength in thinwallareas. The high-pressure impactmakes for extremely fast heatdissipation. The structure near thesurface is therefore remarkablydense. Low-pore vacuum-assistedhigh-pressure die casting is anelaborate process with complexpressure casting dies for engineblocks, for engine blocks, but it ismore and more widely applied.GateCastingchamberMovabledie halfPreformCastingGateCastingchamberStationarydie halfDifferent gate systems of conventional high-pressure die casting (left) and vertical squeeze casting (right)
■ Delimitation high-pressuredie casting/squeeze castingIn the classic squeeze casting process,in contrast to the conventionalpressure die casting, theengine block is gated in a differentcasting position (high-pressuredie casting: block “lies” in horizontalposition; squeeze casting: block“upright”). The gate is arrangedat the bottom of the side wall(s).However, there is a current tendencyalso to gate the highpressuredie castings on both sides,for which purpose the engine blockhas to be turned by 90° comparedwith conventional dies (the engineblock’s longitudinal axis movesto vertical).Whereas the plunger is guidedhorizontally in conventional highpressuredie casting machines,filling the die at a high speed, inthe case of squeeze casting thisis done rather slowly and in“classic” machines with a verticalmovement. The die is thereforefilled without significant gasinclusions. In contrast to conventionalhigh-pressure die castings(that are conditionally weldable,but do not allow artificial ageingbecause of a concomitantdestruction at temperatures justbelow 500 °C), such componentscan be fully heat-treated andwelded. Most high-pressuredie cast engine blocks are notannealed (volume-stabilised) attemperatures around/somewhatabove 200 °C for cost reasons,although this is feasible in principle.Due to the target conflictgiven by growth-conditionedcylinder distortion in long-termoperation without stabilization(annealing) on the one hand andcylinder distortion through potentialloss of internal stress at operatingtemperature caused just bystabilisation (annealing, at thesame time stress relieving), thissituation has to be checked verycarefully for each specific case.In the case of squeeze casting, dieventing and avoidance of oxideinclusions may constitute certainproblems. Satisfactory die fillingcan be ensured by process optimisationin squeeze casting.Moreover, for squeeze casting thegeneral wall thickness should besomewhat more generous thanwith high-pressure die casting, inorder to safely avoid cold-runningproblems. It is only with the highpressuredie casting method, whichmeans very fast filling of the mould,that extremely thin-walled, shelllikestructures can be cast. This iswhy typically high-pressure diecast engine blocks are comparativelysomewhat lighter. Therigidity deficits due to the materialaluminium are compensated byconspicuous ribs, cambering andpreferably closed-profile elements(e.g. suitable tunnel-like configurationof the oil drain ducts).Conventional cast iron liner forhigh-pressure die casting applicationDue to its special aptitude, thesqueeze casting method is mainlyused for infiltration purposes,which means, for example, for theproduction of local aluminiummatrix composites. In this respect,modern high-pressure diecasting machines with real-timecontrol, i.e. the possibility toinfluence plunger speed andpressure build-up, can today beconsidered equivalent to verticallyoperating squeeze castingmachines.■ Cylinder bore surfaceThe lowest cost option consists insolving the cylinder bore problemof high-pressure die cast engineblocks by means of cast-in greycast iron liners. The respectivearmouring of the cylinder bores isindispensable because the hypoeutecticalloying variants suitablefor high-pressure die casting donot provide the necessary tribologicalproperties. However, the useof cylinder bore liners presupposesa heterogeneous character of theengine block. In the case of greycast iron liners with differingproperties, this is becoming morethan obvious. When casting inliners, the smallest gap (betweenliner and casting) is achieved incombination with high-pressuredie casting, and this is expressedby comparatively good equivalentCross-section of a cylinder with cast-in iron liner(high-pressure die casting engine block)
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