SC ilavesinin Al-Cu alaşımlarının mikroyapısı ve mekaniközelliklerine etkisi
The effect of sc addition on the microstructure andmechanical properties of Al-Cu alloys
Yazar: KADİR GÖK
Danışman: DR. ÖĞR. ÜYESİ CEVAT FAHİR ARISOY
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Alüminyum ve alüminyum alaşımları, hafiflikleri ve üstün korozyon dirençleri
sayesinde başta otomotiv, havacılık ve savunma sanayii olmak üzere birçok sektörde
kullanılmaktadır. Avrupadaki emisyon kurallarının sıkılaşmasıyla beraber özellikle
yolcu ve mal taşımacılığında kullanılan araçların (uçak, otomobil, vb.) istenilen
emisyon değerlerine ulaşabilmesi için hafiflik kilit bir rol üstlenmiştir. Alüminyum
alaşımları bu taleplerden dolayı kullanımı artan ve üzerindeki araştırmaların
yoğunlaştığı bir alaşım grubudur. Alüminyum alaşımlarının mekanik özelliklerindeki
iyileştirmeler sayesinde gerekli parçaların hafifletilmesi mümkündür, bu durum ile
talep edilen hafiflik sağlanarak araçların emisyon kurallarına uyumuna gayret
edilmektedir. Bu nedenle mevcut malzemelerin iyileştirilmesi gerekmektedir.
Günümüzde endüstride motor parçaları, tekerlekler ve otomobillerdeki bazı yapısal
parçalar başta olmak üzere yüksek yük, stres ve yorulma altındaki parçalarda
alüminyum alaşımları kullanılmaktadır. Alüminyum ve alüminyum alaşımlarının daha
fazla yerde kullanılabilmesi için mekanik özelliklerinin iyileştirilmesi şarttır.
Mikroyapısal değişiklikler, herhangi bir alaşımın mekanik özelliklerini doğrudan
etkileyebilir. Bir döküm alaşımının mikroyapısı genellikle tane incelticiler gibi diğer
elementlerin eklenmesiyle değiştirilir. Tane incelticiler, çoğunlukla endüstride
alüminyum döküm alaşımlarının mekanik özelliklerini iyileştirmek için kullanılır. Son
yıllarda La, Ce, Er, Eu gibi alüminyum döküm alaşımlarında tane inceltici olarak nadir
toprak (RE) elementleri kullanılmakta ve test edilmektedir. Skandiyum, alüminyum
alaşımlarında nadiren kullanılan ve görülen RE elementlerinden biridir. Bu çalışmada,
Al-Cu alaşımları arasında yer alan A206 alaşımına ağrılıkça %0,03, %0,06 ve %0,09
Sc ilavesi ile elde edilen mikroyapısal değişiklikler ve mekanik özellikler endüstride
sıklıkla kullanılan Ti tane inceltici ilavesi ile üç farklı ısıl işlem prosedürü altında
(döküm hali, T6 ısıl işlemi, T7 ısıl işlemi) incelenmiş ve karşılaştırılmıştır. Tane
inceltici ilavesi ile Al-Cu alaşımlarında akma ve çekme mukavemeti hem mikroyapısal
değişim sayesinde artar hem de bu alaşımda gözlemlenen ötektik Al2Cu fazının ısıl
işlem sonrasında tane içine daha homojen şekilde yayılmasıyla bu mukavemet
değerleri maksimum seviyeye çıkmış olur. Bu çalışmadaki amaç ise Sc ilavesinin
denemesiyle Ti tane incelticilerine bir alternatif sunmaktır.
Yapılan literatür taramasına göre, Sc ilavesinin Al-Cu alaşımlarında tane inceltici etki
gösterdiği ortaya konmuştur. Sc ilavesi ile sağlanan tane inceltme etkisi, döküm
sıcaklığında halen katı halde bulunan ve alaşımın katılaşması sırasında heterojen
katılaşma için uygun çekirdeklenme yüzeyi sağlayan Al3Sc intermetaliği ile elde
edilmektedir. Ancak bu intermetalik belirli bir oranın üzerindeki Sc ilavesiyle ortaya
çıkar. Ayrıca yine yapılan çalışmalara göre Al-Cu alaşımlarına belirli bir orandan fazla
Sc ilave edilmesiye W fazı olarak adlandırılan Al-Cu-Sc bileşiği de gözlemlenebilmektedir. W fazının herhangi bir tane inceltme etkisine sahip olmadığı ortaya konmuştur.
Çalışmada mekanik özelliklerin belirlenmesi için çekme testi yapılmıştır.
Mikroyapısal değişimi anlamak için de tüm numuneler optik mikroskop ve SEM/EDS
ile analiz edilmiştir. Yapılan çalışmaların sonuçlarına göre hem Sc hem de Ti
elementlerinin eklenmesiyle A206'nın akma ve çekme mukavemeti arttırılmıştır.
Bununla birlikte, Sc ilavesi içeren alaşımlar önemli ölçüde daha fazla uzamaya
sahiptir. Metalografik inceleme sonucuna göre çalışmada kullanılan Sc oranlarıya
Al3Sc intermetaliği oluşmamıştır, dolayısıyla A206 alaşımında tane inceltme
gerçekleşememiştir. Şaşırtıcı bir şekilde hem Sc hem de Ti ilaveli alaşımlarda tane
boyutu büyürken, dendritik kol aralıkları 0,03 ve 0,06 Sc ilavesiyle azaltılmıştır.
Ayrıca alaşımların soğuma eğrileri çıkarılmıştır. Elde edilen sonuçlara göre soğuma
hızı-tane boyutu ilişkisinde tam bir korelasyon ortaya çıkmıştır. Soğuma eğrilerine
göre Sc ilavesiyle A206 alaşımının likidüs, solidüs ve solvüs sıcaklıklarında azalma
gözlemlenmiştir.
T6 ve T7 ısıl işlemine tabii tutulmuş Sc ilaveli alaşımlarda W fazı gözlemlenmiştir.
Gözlemlenen W fazı küresel yapıdadır. Dolayısıyla çok düşük oranlarda eklenen Sc
ile ısıl işlem uygulandığı takdirde W fazı ortaya çıkabilir.
Sc ilavesinin Al-Cu alaşımlarının mekanik özelliklerine ve mikroyapısal değişimine
etkisi halen tam olarak ortaya konulabilmiş değildir. Bu bağlamda yapılan çalışmalar
bilim dünyasını aydınlatabilmek için çok önemlidir. Her geçen gün büyüyen
alüminyum endüstrisinin gelişimine katkı sağlamak için alüminyum alaşımları üzerine
çalışmaların devam etmesi gerekmektedir.

Abstract:
Aluminum casting has increased considerably over time in line with advancing
technology and needs. Today, aluminum and aluminum alloys are used in many
industries with a wide range of products. Aluminum and aluminum alloys are used in
many sectors, especially in the automotive, aerospace and defense industries for their
lightness and superior corrosion resistance. With the limitations of emission rules in
Europe, lightness has played a key role in reaching the desired emission values,
especially for vehicles used in passenger and goods transportation (airplane,
automobile, etc.). Aluminum alloys are a group of alloys that the researches and the
use are increasing due to these demands. With the improvements in the mechanical
properties of aluminum alloys, it is possible to lighten the necessary parts, and the
demanded lightness can be provided and efforts are made to comply with the emission
rules of the vehicles. Therefore, existing materials need to be improved. Today,
aluminum alloys are used in the industry in parts under high load, stress and fatigue,
especially engine parts, wheels and some structural parts in automobiles. In order for
aluminum and aluminum alloys to be used in more places, it is necessary to improve
their mechanical properties.
Microstructural changes can directly affect the mechanical properties of any alloy. The
microstructure of a casting alloy is often changed by the addition of other elements
such as grain refiners. Grain refiners are mostly used in industry to improve the
mechanical properties of aluminum casting alloys. Grain refining, which is usually
applied by adding master alloys, deliberately suppresses the longitudinal growth of
grains, resulting in the formation of fine and coaxial microstructures. Longitudinally
grown or coarse-grained castings have poor mechanical properties compared to finegrained
and coaxial castings. When an alloy with a finer grain structure is under stress,
dislocation movements are difficult and restricted. Therefore, an increase in yield and
tensile strength can be observed.
In recent years, rare earth (RE) elements have been used and tested as grain refiners in
aluminum casting alloys such as La, Ce, Er, Eu. Scandium is one of the RE elements
rarely used and seen in aluminum alloys. With Sc addition, a mechanism similar to Al-
Ti-B addition, Al3Sc creates a suitable surface for heterogeneous nucleation and helps
to initiate the solidification of α-Al from different points at the same time, so grain
refinement is possible with the addition of Sc. The aim of this study is to bring
alternative solutions to Ti-B grain refiners used in industrial casting of A206 alloy.
The effect of the element Sc as a grain refiner on the mechanical properties and
microstructure of the A206 alloy has been investigated.
In this study, microstructural changes and mechanical properties obtained by the
addition of 0.03, 0.06 and 0.09 wt.% Sc to A206 alloy, which is among the Al-Cu
alloys, were examined in three different heat treatment procedures (as-cast, T6 heat
treatment, T7 heat treatment) were examined and compared with the addition of Ti
grain refiner, which is frequently used in the industry. With the addition of grain
refiner, yield and tensile strength in Al-Cu alloys both increase due to microstructural
change and these strength values are maximized by the homogeneous distribution of
the eutectic Al2Cu phase into the grain after solution treatment.
In the literature review, although there are extensive studies on the addition of Sc to
Al-Cu alloy alone or together with other rare earth elements and transition metals, it
has not focused on its comparison with Ti, which is added to Al-Cu alloys as grain
refiner in the industry. In addition, Al-Cu alloys are used because of their high strength
values, so these strength values cannot be obtained without heat treatment. In some
studies, heat treatment was applied, while in others, examinations were made on the
as-cast state. Applying heat treatment to the A206 alloy is a must for the desired
strength values.
According to the literature review, it has been revealed that the addition of Sc has a
grain refiner effect in Al-Cu alloys. The grain refinement effect provided by the
addition of Sc is achieved by the Al3Sc intermetallic, which is still solid at the casting
temperature and provides a suitable nucleation surface for heterogeneous solidification
during the solidification of the alloy. However, this intermetallic occurs with the
addition of Sc above a certain ratio. In addition, according to studies, Al-Cu-Sc
compound, which is called W phase, can be observed after adding more than a certain
amount of Sc to Al-Cu alloys. It has been shown that the W phase does not have any
grain refinement effect.
In the experimental procedure, computer modeling was done to obtain a tensile test
sample. Then, wooden mold models suitable for this model were made. Sand molds
were prepared with this model. After the alloy was melted in the electric arc furnace,
it was alloyed with Al-2Sc master alloy and then poured into sand molds at
approximately 780 °C. After cooling at room temperature, the samples were separated
for T6 and T7 heat treatments. In both heat treatments, the solution treatment took 22
hours at 540 °C. After quenching, the samples to be treated with T6 heat treatment
were directly exposed to artificial aging at 160 °C for 13 hours. The samples to be
treated with T7 heat treatment were first left to natural aging at room temperature for
24 hours and then artificially aged at 160 °C for 13 hours.
In the study, tensile tests and hardness tests were performed to determine the
mechanical properties. All samples were analyzed by optical microscope and
SEM/EDS to understand the microstructural change. Primary and secondary dendrite
arm spacings were measured. In addition, in order to observe the change in the cooling
characteristic of the alloy, the cooling curves depending on the temperature-time
variables are obtained from the casting temperature of all alloys until they are
completely solidified.
According to the results of the studies, the yield and tensile strength of the as-cast
A206 was increased with the addition of both Sc and Ti elements. Although the yield
and tensile strength of the heat-treated alloys increased in the alloy containing
0.03wt.% Sc, these strength values were decreased with increasing Sc ratio. However,
the heat-treated alloys with the addition of Sc have significantly greater elongation.
With T6 heat treatment, the hardness values of Sc and Ti added alloys increased
compared to the base alloy. The hardness value of the alloy containing 0.09% Sc
addition with T7 heat treatment decreased. This alloy is also the alloy with the highest
elongation value among the T7 heat treated samples.
According to the results of the metallographic examination, Al3Sc intermetallic did not
observed in the Sc ratios used in the study, so grain refinement could not occur in the
A206 alloy. Surprisingly, the grain size increased in both Sc and Ti addition alloys,
while the dendritic arm spacing was reduced by the addition of 0.03 and 0.06wt.% Sc.
According to the cooling curves, a complete correlation emerged in the cooling rategrain
size relationship. The finest grain structure belongs to the base A206 alloy. The
coarsest grain structure belongs to A206-0.09Sc alloy. If we sort by grain size the order
from coarsest to the finest is as follows; A206-0.09Sc, A206-0.06Sc, A206-Ti, A206-
0.03Sc, A206. According to the cooling curves, the cooling rates are as follows, from
largest to smallest: A206, A206-0.03Sc, A206-Ti, A206-0.06Sc, A206-0.09Sc. Also,
a decrease was observed in the liquidus, solidus and solvus temperatures of the A206
alloy with the addition of Sc.
The cooling rate of the A206 alloy decreased with the addition of Sc. The decrease in
the cooling rate can be seen as the reason behind the coarsening of the grain size.
However, surprisingly, a decrease in DAS/SDAS values was observed. In general,
there is a direct correlation between the DAS/SDAS values and the grain size. So,
DAS/SDAS values can be used to have an idea about the grain size. However, in this
study while the grain size increased with Sc addition, a decrease in DAS/SDAS values
was observed. This showed that there was no correlation between grain size-
DAS/SDAS values.
W phase was observed in T6 and T7 heat treated alloys with Sc addition. We can
clearly say that if heat treatment is applied with the addition of Sc even at very low
rates, the W phase may occur. Diffusion of Sc and Cu elements is necessary for the
formation of the W phase. Therefore, this diffusion energy is provided in heat-treated
alloys with holding them long time at high temperature.
Although some researchers in the literature have concluded that the W phase has sharp
edges and corners, the W phase observed in this study is spherical and did not cause
any early crack initiation. It is thought that the W phase needs heat treatment for
complete diffusion to reach the spherical form. By forming the W phase, the matrix
became depleted in Cu and Sc and therefore the alloy became more ductile. As a result,
the yield strength of heat-treated alloys decreased with increasing Sc ratio and the
elongation was greatly increased.
According to the experimental results obtained and the theories in the literature; when
Sc was added to the as-cast A206 alloy, the yield and tensile strength increased
relatively, but when the heat treatment was applied, Sc and Cu diffused into the Al2Cu
phase and formed the W phase. Therefore, the matrix is depleted in terms of Cu and
Al2Cu and becomes ductile. So, T6 and T7 heat-treated alloys with Sc addition can be
used in applications requiring high toughness. The mechanism of formation of the W
phase is not fully understood. Although Sc and Cu diffusion into Al2Cu intermetallics
is the most correct hypothesis so far, there is no definitive conclusion.
The temperature of the liquidus, solidus and solvus curves in the Al-Cu phase diagram
decreased with increasing Sc ratio in A206 alloy. When the curves were drawn
according to the addition of 0.09wt.% Sc, the eutectic temperature decreased by about
6 °C. Due to this decrease, it is thought that the Al2Cu intermetallics diffuse better into
the grain due to the fact that the solution treatment at 540 °C is closer to the eutectic
temperature. This may have contributed to the improvement of yield and tensile
strength.

The effect of Sc addition on the mechanical properties and microstructural changes of
Al-Cu alloys has not been fully revealed yet. Studies in this context are very important
to enlighten the scientific world. Studies on aluminum alloys should continue in order
to contribute to the development of the growing aluminum industry.