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Introduction

Forgingistheprocessbywhichmetalisheatedandisshapedbyplasticdeformationbysuitablyapplyingcompressiveforce.Usuallythecompressiveforceisintheformofhammerblowsusingapowerhammerorapress.

Forgingrefinesthegrainstructureandimprovesphysicalpropertiesofthemetal.Withproperdesign,thegrainflowcanbeorientedinthedirectionofprincipalstressesencounteredinactualuse.Grainflowisthedirectionofthepatternthatthecrystalstakeduringplasticdeformation.Physicalproperties(suchasstrength,ductilityandtoughness)aremuchbetterinaforgingthaninthebasemetal,whichhas,crystalsrandomlyoriented.

Forgingsareconsistentfrompiecetopiece,withoutanyoftheporosity,voids,inclusionsandotherdefects.Thus,finishingoperationssuchasmachiningdonotexposevoids,becausetherearen'tany.Alsocoatingoperationssuchasplatingorpaintingarestraightforwardduetoagoodsurface,whichneedsverylittlepreparation.

Forgingsyieldpartsthathavehighstrengthtoweightratiothusareoftenusedinthedesignofaircraftframemembers.

AForgedmetalcanresultinthefollowing

Increaselength,decreasecrosssection,calleddrawingoutthemetal.

Decreaselength,increasecrosssection,calledupsettingthemetal.

Changelength,changecrosssection,bysqueezinginclosedimpressiondies.Thisresultsinfavorablegrainflowforstrongparts

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CommonForgingProcesses

Themetalcanbeforgedhot(aboverecrystallizationtemperatures)orcold.

OpenDieForgings/HandForgings:Opendieforgingsorhandforgingsaremadewithrepeatedblowsinanopendie,wheretheoperatormanipulatestheworkpieceinthedie.Thefinishedproductisaroughapproximationofthedie.Thisiswhatatraditionalblacksmithdoes,andisanoldmanufacturingprocess.

ImpressionDieForgings/PrecisionForgings:Impressiondieforgingsandprecisionforgingsarefurtherrefinementsoftheblockerforgings.Thefinishedpartmorecloselyresemblesthedieimpression.

DesignConsideration:

Partingsurfaceshouldbealongasingleplaneifpossible,elsefollowthecontourofthepart.Thepartingsurfaceshouldbethroughthecenterofthepart,notneartheupperorloweredges.Ifthepartinglinecannotbeonasingleplane,thenitisgoodpracticetousesymmetryofthedesigntominimizethesidethrustforces.Anypointonthepartingsurfaceshouldbelessthan75fromtheprincipalpartingplane.

Asinmostformingprocesses,useofundercutsshouldbeavoided,asthesewillmaketheremovalofthepartdifficult,ifnotimpossible.

Recommendeddraftanglesaredescribedinthefollowingtable.

Material DraftAngle()

Aluminum 02

CopperAlloys(Brass) 03

Steel 57

StainlessSteel 58

Generousfilletsandradiusshouldbeprovidedtoaidinmaterialflowduringtheforgingprocess.Sharpcornersarestressrisersintheforgings,aswellasmakethediesweakinservice.Recommendedminimumradiusesaredescribedinthefollowingtable.

HeightofProtrusionmm(in)

Min.CornerRadiusmm(in)

Min.FilletRadiusmm(in)

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12.5(0.5)

1.5(0.06)

5(0.2)

25(1.0)

3(0.12)

6.25(0.25)

50(2.0)

5(0.2)

10(0.4)

100(4.0)

6.25(0.25)

10(0.4)

400(16)

22(0.875)

50(2.0)

Ribsshouldbenotbehighornarrow,thismakesitdifficultforthematerialtoflow.

Tolerances:

Dimensiontolerancesareusuallypositiveandareapproximately0.3%ofthedimension,roundedofftothenexthigher0.5mm(0.020in).

Dieweartolerancesarelateraltolerances(paralleltothepartingplane)andareroughly+0.2%forCopperalloysto+0.5%forAluminumandSteel.

Dieclosuretolerancesareinthedirectionofopeningandclosing,andrangefrom1mm(0.040inch)

forsmallforgings,dieprojectionarea6500cm2(100in2).

Diematchtolerancesaretoallowforshiftintheupperdiewithrespecttothelowerdie.Thisisweightbasedandisshowninthethefollowingtable.

Material

FinishedForgingWeightTrimmedkg(lb)

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StainlessSteel,Titanium

1.6(0.064)

5(0.2)

12.5(0.5)

Aproperlubricantisnecessaryformakinggoodforgings.Thelubricantisusefulinpreventingstickingoftheworkpiecetothedie,andalsoactsasathermalinsulatortohelpreducediewear.

PressForgings:Pressforginguseaslowsqueezingactionofapress,totransferagreatamountofcompressiveforcetotheworkpiece.Unlikeanopendieforgingwheremultipleblowstransferthecompressiveenergytotheoutsideoftheproduct,pressforgingtransferstheforceuniformlytothebulkofthematerial.Thisresultsinuniformmaterialpropertiesandisnecessaryforlargeweightforgings.Partsmadewiththisprocesscanbequitelargeasmuchas125kg(260lb)and3m(10feet)long.

UpsetForgings:Upsetforgingincreasescrosssectionbycompressingthelength,thisisusedinmakingheadsonboltsandfasteners,valvesandothersimilarparts.

RollForgings:Inrollforging,abarstock,roundorflatisplacedbetweendierollerswhichreducesthecrosssectionandincreasesthelengthtoformpartssuchasaxles,leafspringsetc.Thisisessentiallyaformofdrawforging.

Swaging:Swagingatubeorrodisforcedinsideadieandthediameterisreducedasthecylindricalobjectisfed.Thediehammersthediameterandcausesthemetaltoflowinwardcausingtheouterdiameterofthetubeortherodtotaketheshapeofthedie.

NetShape/NearNetShapeForging:Innetshapeornearnetshapeforging,forgingresultsinwastageofmaterialintheformofmaterialflashandsubsequentmachiningoperations.Thiswastagecanbeashighas70%forgearblanks,andeven90+%inthecaseofaircraftstructuralparts.Netshapeandnearnetshapeprocessesminimizethewastebymakingprecisiondies,producingpartswithverylittledraftangle(lessthan1).Thesetypesofprocessesofteneliminateorreducemachining.Theprocessesarequiteexpensiveintermsoftoolingandthecapitalexpenditurerequired.Thus,theseprocessescanbeonlyjustifiedforcurrentprocessesthatareverywastefulwherethematerialsavingswillpayforthesignificantincreaseintoolingcosts.

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