Fazna ravnotežja zlitin iz sistema Al-Mg-Si z dodatki elementov za obdelovalnost : doktorsko delo

Rečnik, Simon (author), Medved, Jože (mentor)

Aluminij in njegove zlitine so ključni materiali za izdelavo strojnih elementov z odrezovanjem, kot so struženje, rezkanje in vrtanje. Čeprav se na splošno dobro obdelujejo, se obdelovalne lastnosti med zlitinami razlikujejo. Najboljšo obdelovalnost dosegajo avtomatne zlitine, razvite za hitro CNC obdelavo. Do začetka 21. stoletja je bil glavni dodatek svinec, pozneje pa predvsem kombinacije svinca z bizmutom in kositrom. Zaradi toksičnosti svinca je Evropska unija že okoli leta 2000 uvedla direktive (ELV, RoHS, REACH), ki omejujejo njegovo vsebnost na največ 0,40 mas. %, direktiva RoHS 3 (2018/740/EU) pa jo je dodatno znižala na 0,1 mas. %. V raziskavi smo preučevali vpliv indija na strjevanje, fazna ravnotežja, termodinamsko stabilnost, kinetiko procesov in tvorbo nizkotaljivih faz v mikrostrukturi avtomatne zlitine EN AW-6026 ter tehnično čistega aluminija EN AW-1370. Dodajali smo ga v elementarni obliki ali kot predzlitino BiIn. Poseben poudarek smo namenili sistemom Al-In, Al-Bi-In, Al-Mg-Si-In in Al-Mg-Si-In-Bi ter vprašanju, ali se indij pojavlja kot samostojna faza ali kot del evtektika. Cilj je bil ovrednotiti, ali lahko indij prispeva k tvorbi nizkotaljivih faz in predstavlja ustrezno alternativo svincu. Indij je bil v sistemu Al-In prisoten v elementarni obliki in kot faza In(Al) – znotraj dveh večfaznih heterogenih zlogov (?-Al + Al8Fe2Si + In(Al)) in (?-Al + Al13Fe4 + In(Al)). V sistemu Al Bi-In je tvoril nizkotaljivo fazo BiIn in fazo BiIn(Al), slednja je bila prisotna v heterogenem zlogu (?-Al + BiIn(Al) + Al8Fe2Si). V sistemih Al-Mg-Si-In in Al-Mg-Si In Bi je bil indij prisoten kot samostojna faza v elementarni obliki ali kot del heterogenega zloga (?-Al + Al5Cu2Mg8Si6 + In). Obdelovalne lastnosti smo preučevali izključno na zlitini EN AW-6026, pri čemer sta osnovna zlitina in zlitina, legirana s svincem, služili kot referenčni. Rezultati raziskav so pokazali, da vsi dodani elementi (svinec, indij, bizmut in predzlitina BiIn) izboljšajo lomljivost odrezkov v primerjavi z osnovno zlitino brez dodatkov. Pri zlitinah, legiranih z indijem ali predzlitino BiIn, so se med postopkom struženja tvorili odrezki ugodnejših oblik, kar je posledica prisotnosti indija v obliki samostojnih vključkov ali kot del heterogenega zloga (?-Al + Al5Cu2Mg8Si6 + In). Nasprotno zlitina, legirana z bizmutom, ni pokazala enakega izboljšanja obdelovalnih lastnosti, kar gre pripisati interakciji bizmuta z magnezijem in posledični tvorbi trde, visokotemperaturne intermetalne spojine Bi2Mg3. Pridobljeni rezultati prispevajo k boljšemu razumevanju faznih ravnotežij v sistemih Al-In, Al-Bi-In, Al-Mg-Si-In in Al-Mg-Si-In-Bi, zlasti kinetike strjevanja, termodinamske stabilnosti ter tvorbe nizkotaljivih faz. Ugotovitve omogočajo nadaljnjo optimizacijo kemijske sestave in poglabljajo razumevanje vpliva alternativnih legirnih elementov na obdelovalne lastnosti aluminijevih zlitin.

• Language: Slovenian
• Keywords: EN AW-6026, EN AW-1370, termodinamski izračuni, fazna ravnotežja, indij, mikrostruktura, mehanske lastnosti, obdelovalnost
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2025
• PID: 20.500.12556/RUL-175105
• COBISS.SI-ID: 254404099
• Publication date in RUL: 16.10.2025

Secondary language

• Language: English
• Title: Phase equilibria of alloys from Al-Mg-Si system with the addition of elements for machinability : ph. d. thesis

Abstract

Aluminium and its alloys are key materials for the production of machine components by machining processes such as turning, milling, and drilling. Although they are generally easy to machine, their machinability varies among different alloys. The best machinability is achieved by free-cutting alloys developed for high-speed CNC machining. Until the early 21st century, lead was the main additive, later primarily in combination with bismuth and tin. Due to the toxicity of lead, the European Union introduced directives (ELV, RoHS, REACH) around 2000, limiting its content to a maximum of 0.40 wt.%, while the RoHS 3 directive (2018/740/EU) further reduced it to 0.1 wt.%. In this study, we examined the influence of indium on solidification, phase equilibria, thermodynamic stability, process kinetics, and the formation of low-melting phases in the microstructure of the free-cutting alloy EN AW-6026 and technically pure aluminium EN AW-1370. Indium was introduced either in elemental form or as the BiIn master alloy. Particular attention was devoted to the Al-In, Al-Bi-In, Al-Mg-Si-In, and Al Mg Si In-Bi systems, focusing on whether indium occurs as a separate phase or as part of the eutectic. The objective was to assess whether indium can promote the formation of low-melting phases while providing a viable alternative to lead. In the Al In system, indium was found in elemental form and as the In(Al) phase within eutectics (α-Al + Al8Fe2Si + In(Al)) and (α-Al + Al13Fe4 + In(Al)). In the Al-Bi In system, it formed the low-melting BiIn phase and the BiIn(Al) phase, which appeared in the eutectic (α Al + BiIn(Al) + Al8Fe2Si). In the Al-Mg-Si-In and Al-Mg-Si-In-Bi systems, indium was present as a discrete phase or as part of the eutectic (α-Al + Al5Cu2Mg8Si6 + In). Machinability was investigated only in the EN AW-6026 alloy, with the base alloy and the lead-containing variant serving as references. The results showed that all additions (lead, indium, bismuth, and the BiIn master alloy) improved chip breakability compared to the base alloy. Alloys with indium or BiIn produced more favourable chip shapes during turning, owing to indium present in elemental form or as part of the eutectic (α Al + Al5Cu2Mg8Si6 + In). In contrast, the alloy with bismuth showed no such improvement, attributed to its interaction with magnesium and the formation of the hard intermetallic Bi2Mg3. These findings improve understanding of phase equilibria in the Al-In, Al-Bi-In, Al-Mg-Si-In, and Al-Mg-Si-In-Bi systems, particularly solidification kinetics, thermodynamic stability, and the formation of low-melting phases. They also support further optimisation of alloy composition and clarify the effects of alternative alloying elements on the machinability of aluminium alloys.

• Keywords: EN AW-6026, EN AW-1370, thermodynamic calculations, phase equilibria, indium, microstructure, mechanical properties, machinability