Μελέτη των θερμικών κατεργασιών προϊόντων διέλασης βιομηχανικών κραμάτων αλουμινίου σειράς 6ΧΧΧ, με σκοπό τη βελτιστοποιήση της μηχανικής αντοχής και της ικανότητας απορρόφησης ενέργειας (crashworthiness)

Abstract

The present Thesis deals with the homogenization and artificial aging thermal treatments of 6xxx series alloy extruded profiles aiming at the optimization of the mechanical properties as this can be expressed by a combination of the highest strength and energy absorption. This is required by the automotive industry in order to ensure minimization of the energy transfer inside the passengers’ cabin in the case of a crash.In the frame of the examination, extrusions with four different concentrations of Mg and Si and with two different cross-sections, square and complex with three boxes of different wall thickness, were tested (alloy 1: 0.6% Mg2Si excess Si, alloy 2: 0.8% Mg2Si, alloy 3: 0.8% Mg2Si excess Si, alloy 4: 1.0% Mg2Si, excess Si, Mn containing). More specifically the effects of (i) cooling rate after solid solution treatment, (ii) natural aging and (iii) artificial aging temperature were investigated. The analytical techniques that were implemented included optical and scanning electron microscopy, transmission electron microscopy, tensile testing, compression testing and finite element analysis. The higher the amount of alloying elements the higher the rate of the kinetics of the aging process and the strength in T6 condition. With excess Si, Si crystals are precipitated deteriorating ductility especially when this occurs in precipitate free zones (PFZs). However the Si content is in general enabling the formation of hardening constituents due to the smaller distance the Si atoms have to travel in order to reach the Mg ones. Indicative measurements with TEM revealed 25% increase in the number density of coherent particles by a respective increase in the Mg2Si percentage from 0.6 to 1%. In alloys with 0.6 and 0.8% Mg2Si and excess Si 40 hours aging at 170οC enabled higher yield and tensile strength values than with aging at 200οC. In the balanced alloy with 0.8% Mg2Si aging at 200οC produces superior strength, but it is expected that prolonged aging at 170oC (more than 40 hours) would be more effective. For 6082 alloy with 1.08% Mg2Si slight differences in strength were observed between these two heat treatments.Regarding the preaging conditions the effects of (i) two step aging and (ii) 24 hours natural aging after solution treatment were investigated. 12 hours preaging at 100oC resulted in acceleration of aging kinetics leading to massive non-coherent particles precipitation and increase in strength by 8-13MPa without any compromise on the ductility. Results showed that water quenching (wq) after heat treatment at 550oC for 50 min prevented the non-desirable precipitation of non-coherent phases, retaining a higher amount of alloying elements in solid solution which became available for precipitation in a controlled and efficient way during subsequent artificial aging. On the contrary, air cooling (ac) led to non-desirable, intergranular, mainly, precipitation and formation of precipitate free zones (PFZs), which rendered low ductility, due to the difference in hardness between these narrow zones and the inner part of the grains. Consequently, ac samples exhibited lower strength, slower response to artificial aging, and also grain boundary sensitization which resulted to intergranular cracking in tensile tests. PFZ formation was also detected in wq samples but only in overaged condition (T7). Not naturally aged samples exhibited higher strength values than from the naturally aged (by 8MPa for the alloy with 0.6%Mg2Si and by 22MPa for 1,0% Mg2Si). However not naturally aged samples had lower values of toughness in the post-uniform elongation area. For the evaluation of toughness indices ΤFR, ΤUTS και ΤPI were calculated from σ-ε curves referring to toughness at FRacture or total amount of toughness, toughness at UTS or toughness corresponding to the area of uniform elongation and toughness in Plastic region (post uniform). For alloy 1 (0.6%Mg2Si) ΤFR και ΤUTS values had values 32 και 25J/cm3 in Τ4 και Τ6 metallurgical condition while ΤPI had lower value in T4 than in T6 condition (6 and 10J/cm3 respectively). These values of toughness were higher than those anticipated by aging at 200οC and 220οC. Prolonged preaging step at 100οC led to an increase of ΤFR και ΤUTS values and slight decrease of ΤPI. In ac samples low toughness was expressed by a progressive tendency for converging of ΤFR και ΤUTS with aging progress which indicated that the total amount of toughness resulted from deformation in the elastic and proportionality regions. Similar results were acquired for alloy 2. Alloys 3 and 4 differed in that in WQ condition ΤFR received lower values than in aged condition and a considerable raise and stabilization of toughness value was realized with the completion of preaging stage. ΤPI proved to be a very reliable index for the characterization of samples as more or less ductile even when two samples had comparable ΤFR values and it could be well related with the observed reduction in area from the tensile tests. It also received high values in T7 condition even if ΤFR was decreased and the respective tensile specimens were also very ductile.Comparing samples with and without natural aging it was found that for alloy 1, in underaged condition naturally aged samples had higher ΤFR και ΤUTS values but lower ΤPI due to the higher aging degree while in peak age condition ΤFR and ΤUTS were comparable but ΤPI was higher for naturally aged specimens. For alloy 4 naturally aged specimens had also higher values, while for alloys 2 the opposite was observed. For alloy 3 similar fracture values were found. The results of compression tests proved that extrusions with high strain rate sensitivity indices (m) are expected to present acceptable buckling behavior, high local peak values of compression load and also high mean load values (energy absorption). Shear zones formation and stress concentration during deformation on the secondarily recrystallized grains near the surfaces in 6082 alloy was detrimental to the observed bucking behavior. Aging at 220oC for 6106 alloy improved significantly the buckling behavior, due to massive precipitation and the subsequent neutralization of the effect of the PFZs, while at 200oC buckling mode was also crack free in all examined alloys, because of the equivalent (or less localized) rate of precipitation on grain boundary interfaces and grain interior regions. Useful conclusions were also drawn regarding the maximization of mean load with prolonged preaging step and the destructive buckling mode of ac extrusions due to PFZs formation.In the assessment of crash tolerance properties of extrusions both tensile and compression tests are indispensable. In case that the microstructure includes sensitized areas such as stress concentration sites (corners, weld areas with oxides, non-uniform grain structure.) compression testing constitutes a more effective method for the suitability of the selected material. On the contrary, the calculation of strain during necking from stress-strain curves is a useful and easy way of understanding whether a sample has a tendency to display a more or less ductile behavior. The conclusions from the calculation of fracture toughness from these two tests were also different. The most typical example was the wq specimen in T4 condition, which in the tensile test overcame the other specimens, while in compression test the respective specimen showed the half energy absorption compared with specimens in T6 condition. Finite element modeling of compression test was efficient in reproducing the morphology of the compressed specimens, regardless of the buckling behavior. A good agreement was observed for fracture toughness values and load-displacement curves morphologies obtained from actual tests and the model regarding square cross-section specimens, while inconsistency eventuated for those with the complex cross-section. After constructing the model a series of tests was feasible, including examination of the effect of fracture strain, strain rate and wall thickness of the extrusions. A fracture strain increase resulted in mean load increase, no change in maximum load and limitation of discontinuities formation. Strain rate affected particularly the maximum load and less the mean load, induced shortening of the neighboring peak load distances in the curves and change of the curves mode from regular to irregular. Increasing the wall thickness caused a considerable increase of both mean and maximum load, while the buckling mode was also affected.

Οι θερμικές κατεργασίες διαλυτοποίησης και τεχνητής γήρανσης μετά τη διέλαση σε προϊόντα διέλασης κράματος Al της σειράς 6xxx διαμορφώνουν τις τελικές μηχανικές ιδιότητες και ο κατάλληλος σχεδιασμός τους είναι απαραίτητος για την επίτευξη του επιθυμητού συνδυασμού υψηλής αντοχής και απορρόφησης ενέργειας σε περίπτωση κρούσης. Κατά την διαλυτοποίηση η προσφερόμενη θερμότητα μπορεί να προκαλέσει ανεπιθύμητη μεγέθυνση κόκκου και ελάττωση των μηχανικών ιδιοτήτων. Ο ρυθμός ψύξης από τη θερμοκρασία στερεού διαλύματος έχει άμεση επίδραση στην δυσθραυστότητα των προφίλ. Χαμηλή ταχύτητα ψύξης σχετίζεται με περιορισμό της ολκιμότητας, λόγω σχηματισμού περικρυσταλλικών ζωνών χωρίς κατακρημνίσματα, οι οποίες αποτελούν περιοχές χαμηλότερης αντοχής και ολκιμότητας από το εσωτερικό των κόκκων Al. Η επιδιωκόμενη αύξηση της αντοχής επέρχεται μέσω κατακρήμνισης του μέγιστου δυνατού πλήθους κατακρημνισμάτων β’’ και β΄ και από διαμόρφωση μη ανακρυσταλλωμένης μικροδομής ή μικρού μέσου μεγέθους κόκκου κατά τη διέλαση. Κράματα με περίσσεια Si σε σχέση με τη στοιχειομετρική αναλογία Mg:Si - 2:1 εμφανίζουν κατακρήμνιση κρυστάλλων Si σε περικρυσταλλικές θέσεις με αποτέλεσμα την ελάττωση της ολκιμότητας. Ταυτόχρονα όμως παρατηρείται και επιτάχυνση της κινητικής της τεχνητής γήρανσης λόγω του μικρότερου απαιτούμενου αριθμού αλμάτων των κραματικών στοιχείων για σχηματισμό φάσεων συνοχής. Από τα πειραματικά δεδομένα προέκυψε πως στα κράματα περιεκτικότητας 0,6% και 0,8% κ.β. σε Mg2Si με περίσσεια Si η τεχνητή γήρανση στους 170οC για 40 ώρες επιφέρει υψηλότερες τιμές αντοχής, ενώ στα με 0,8% Mg2Si και 1,0% με Mn και περίσσεια Si οι προτεινόμενες συνθήκες είναι 200οC για χρόνο παραμονής 4 ωρών. Η αποφυγή του σταδίου φυσικής γήρανσης 24 ωρών μεταξύ διαλυτοποίησης και τεχνητής γήρανσης είχε αρνητική ή αμελητέα επίδραση στην ολκιμότητα (ελάττωση διατομής) αλλά θετική όσον αφορά στην αντοχή των προφίλ μέσης κραμάτωσης. Οι δοκιμές κατέδειξαν επίσης πως με τεχνητή γήρανση σε δύο στάδια, δηλαδή με την ενσωμάτωση ενός 1ου σταδίου γήρανσης σε θερμοκρασία χαμηλότερη της κύριας που έπεται, επιτυγχάνεται θετική απόκριση του υλικού στην κινητική της σκλήρυνσης. Από την πραγματοποίηση δοκιμών θλίψης διαπιστώθηκε καλύτερη συμπεριφορά των προφίλ μετά από θερμική κατεργασία στους 200οC συγκριτικά με τους 170οC, καθώς ευνοείται η γενικευμένη, αντί για την περικρυσταλλική κατακρήμνιση. Στα προφίλ με μη ανακρυσταλλωμένη μικροδομή (όπως π.χ. στο κράμα 6082) η ύπαρξη ευμεγέθων κρυστάλλων κοντά στην επιφάνεια οδήγησε, κατά την συμπίεση τους, στην ανάπτυξη ζωνών ολίσθησης και πρώιμων ρωγματώσεων. Η προσομοίωση των δοκιμών θλίψης με λογισμικό πεπερασμένων στοιχείων αναπαρήγε με πολύ καλή προσέγγιση την μορφολογία των συμπιεσμένων δοκιμίων και τις τιμές μέσου φορτίου θλίψης, αποδεικνύοντας πως μπορεί να αποτελέσει ουσιαστικό εργαλείο για τον σχεδιασμό τόσο της κατάλληλης γεωμετρίας όσο και των θερμικών κατεργασιών των εκάστοτε προφίλ για τη βελτιστοποίηση των μηχανικών ιδιοτήτων.