0052-0055 1 Technologies-Rolling 6xxx Alloys: Extruded and Rolled Products with aGreatPotentialforGrowth EBNER HICON® roller-hearth furnaces for aluminium plates and sheets. Depending on the range of alloys and the quantity of materials to be processed, these facilities may be designed as continuous or semi-continuous furnaces for recrystallizing, tempering and solution heat treating with a quench immediatelydownstreamofthefurnace(Photo:CourtesyEBNERIndustrieofenbauGmbH) Alloys in the 6xxx family, traditionally used in extrusion, provide mechanical properties that make them interesting for the production of hot and cold by Giuseppe Giordano formed automotive body parts Al-Mg-Sialloysinthe6xxxseriesareknownas heat-treatment alloys showing good mechanical resistance and are used in different sectors, particularly in extrusion. Rolled 6xxx alloys recently underwent a strong development in the automotive sector for the production of hot and cold formed body parts. Tables 1-3 show the chemical composition of some of the most widely used alloys in this series and the standard values of some chemical-physical and mechanical properties. 6xxx alloys: precipitation hardening and ageing The increase of tensile properties occurs mainly through a precipitation process of second phases containing Mg and Si starting from a supersaturated solution. The quantities of Mg and Si present in the solution are of the utmost importance in order to reach a certainleveloftensileproperties,ascanbeseeninTable3,butmoreevidentlyinFigure1,wheretheconstant tensile stress curves Rm (N/mm 2 )crosstheareas 52 - .com PubliTec
0052-0055 2 determined by the composition intervals of the different alloys. By observing Table 1, it may be seen that some alloys have a Mg% to Si% ratio which is close to the stoichiometricvalue(Mg%/SI%=1,73:1,00). These alloys are characterised by a decreased hardnesscombinedwithagoodformabilityandsuitability for extrusion, as well as being easy to temper (this can be done directly at the press). Other alloys are characterised by a Si content higher than that necessary to form Mg2Si. Such alloys are called “excess silicon alloys” and can be distinguished on account of their better and faster response to ageing. In any case, the ageing process begins with the product being kept at a high temperature to obtain a condition of complete solubility which is followed by a sufficiently rapid cooling (quenching) so as to obtain, at a temperature close enough to room temperature, a solid solution, supersaturated in the alloy’s elements. It can easilybeunderstoodthatsupersaturationisonlypossible if the alloy’s elements are soluble in solid solution at high temperatures, but are scarcely soluble at low temperatures, or if their compounds with aluminium or among the elements themselves are scarcely soluble. Unlike what happens with aluminium-copper alloys, historically the first to be studied in relation to the hardening precipitation process, for 6xxx alloys both main alloying elements, Si and Mg, take part in the process with the formation of a precipitate that is the stable final phase. The precipitation scheme is the following: SSSS→separateSiandMgclusters→Guinier-Preston zones→ →β’ (semi-coherent) →β’’ (semi-coherent) →β (cubic, small plaques) Table 1: Chemical composition of some 6xxx alloys used both for extrusion and rolling Tabella 1: Composizione chimica di alcune leghe 6xxx utilizzate sia in estrusione sia in laminazione Alloy ENAW Si% Fe% Cu% Mn% Mg% Cr% Zn% Ti% other each other total Aluminium 6005A 0,50-0,90 0,35 0,30 0,50 0,40-0,70 0,30 0,20 0,10 0,05 0,15 balance 6060 0,30-0,60 0,10-0,30 0,10 0,10 0,35-0,60 0,35 0,15 0,10 0,05 0,15 balance 6061 0,40-0,80 0,70 0,15-0,40 0,15 0,80-1,20 0,04-0,30 0,25 0,10 0,05 0,15 balance 6082 0,70-1,30 0,50 0,10 0,40-1,00 0,60-1,20 0,25 0,20 0,10 0,05 0,15 balance 6106 0,30-0,60 0,35 0,25 0,05-0,20 0,40-0,80 0.25 0,10 0,10 0,05 0,15 balance 6016 1,00-1,50 0,50 0,20 0,15 0,20-0,60 0,25 0,10 0,10 0,05 0,15 balance Leghe 6xxx: estrusi e laminati con un futuro di grande crescita Tecnologie-Laminazione Le leghe della famiglia 6xxx, tradizionalmente impiegate per l’estrusione, offrono caratteristiche meccaniche che le rendono interessanti per la produzione di parti di carrozzerie automotive stampate a freddo e a caldo Le leghe Al-Mg-Si della serie 6xxx sono notoriamente leghe da trattamento termico a media resistenza, impiegate in diversi settori con particolare incidenza in estrusione. I laminati in leghe 6xxx hanno avuto recentemente un forte sviluppo nel settore automotive per la realizzazione di parti di carrozzeria stampati a caldo ed a freddo. NelleTabelle1-3siriportalacomposizionechimicadialcune tra le più diffuse leghe della serie ed i valori standard di alcune caratteristiche chimico fisiche e meccaniche. Leghe 6xxx: precipitazione indurente ed invecchiamento L’incrementodellecaratteristichetensiliavvieneprincipalmente attraverso un processo di precipitazione di seconde fasicontenenti MgedSiapartiredaunasoluzionesolidasovrassatura.LequantitàdiMgeSipresentinellalegasonodi fondamentale importanza per il raggiungimento di un certo livello di caratteristiche tensili, come già rilevabile nella Tabella3,mapiùevidenteinFigura1,dovelecurvedicarico dirotturacostanteRm(N/mm2)attraversanoleareedeterminate dagli intervalli di composizione delle diverse leghe. SesiosservalaTabella1,sipuònotarechealcuneleghehanno un rapporto tra Mg% e Si% prossimo al valore stechiometrico(Mg%/SI%=1,73:1,00). Queste leghe si caratterizzano per una durezza ridotta che Alloy ENAW Density Melting Range E ResistivityT5 g/cm3 °C MPa µOhm/cm 6005A 2,70 605-655 69500 3,5 6060 2,70 615-655 69500 3,3 6061 2,70 575-650 69500 4,0 6082 2,71 570-645 69500 4,2 6106 2,70 610-655 69500 3,5 6016 2,72 570-645 69500 4,5 Table 2: Standard values of some chemicalphysical properties of 6xxx series alloys Tabella 2: Valori standard di alcune caratteristiche chimico fisiche di leghe serie 6xxx August <strong>2016</strong> N° 04 - 53
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