Characterization of Al-Zn-Mg-Cu alloys

Aluminijeve zlitine Al-Zn-Mg-Cu spadajo v skupino 7XXX. Te zlitine dosegajo najvišje mehanske lastnosti od vseh aluminijevih zlitin, predvsem zaradi svoje dobre odzivnosti na izločevalno utrjevanje. Uporabljajo se predvsem v letalski, vesoljski in avtomobilski industriji. Izdelki so ponavadi izdelani s postopki preoblikovanja in mehanskimi obdelavami. Za dosego najvišjih trdnosti so ustrezno toplotno obdelani. Trend in potrebe trga pa danes zahtevajo tudi izdelke, izdelane s postopkom litja. To pa je pri teh zlitinah, še posebej pri tankostenskih ulitkih, problematično zaradi slabe livnosti in nagnjenja k pokanju v vročem. V okviru diplomskega dela smo proučili sestave zlitin Al-Zn-Mg-Cu. Dve izbrani zlitini smo izdelali v indukcijski talilni peči in ulili v kokilo za testiranje livnosti in merilno celico za enostavno termično analize. Vzorce smo karakterizirali z diferenčno termično analizo in optično mikroskopijo ter EDS analizo. Mikrostruktura preiskovanih zlitin je kompleksna in jo sestavljajo dendritsko izoblikovani primarni zmesni kristali ?Al in evtektika na osnovi železa (?Al + Al7Cu2Fe) ter V-faze (raztopina Mg2Zn11 in Al5Cu6Mg2 ali Mg(Zn,Al,Cu)2). Znotraj te strukture, se lahko izloča Al2Cu. Lahko pa so prisotne tudi: C14-Lavesova faza, S-faza, in T-faza.Al-Zn-Mg-Cu alloys belong to the aluminium alloys with group sign 7XXX. These alloys achieve the highest mechanical properties of all aluminium alloys, mainly due to their good response to the participation hardening. These alloys are used primarily in the aerospace, space and automotive industries. The products are usually made by means of forming processes and mechanical processing, and of course, in order to achieve the highest strengths, they are adequately heat treated. Today’s the trends and the needs of the market however also require products manufactured by the casting process. These alloys are unfortunately not appropriate for casting (especially for thin-wall castings) due to poor castability and high hot tearing tendency. Within the diploma work, we studied Al-Zn-Mg-Cu alloys. Two selected alloys were manufactured in an induction melting furnace and poured into a mould for testing castability and into a measurement cell for simple thermal analysis. Samples were characterized by differential thermal analysis and optical microscopy and EDS analysis. Microstructure of researched alloys is complex and consists of dendritic formed primary mixed crystals αAl, iron based eutectic (αAl + Al7Cu2Fe) and V-phase (solution of Mg2Zn11 and Al5Cu6Mg2 or Mg(Zn,Al,Cu)2). Eutectic structure can also include Al2Cu precipitate. However, the following phases may also be present: C14-Laves phase, S-phase, and T-phase