Tikso dövme proses parametrelerinin çeliklerin mekanik ve mikroyapisal özelliklerine etkisinin incelenmesi
Investigation of the effect of thixo forging process parameters on the mechanical and microstructural properties of steels
Yazar: İSA METİN ÖZKARA
Danışman: PROF. DR. MURAT BAYDOĞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Metalurji ve Malzeme Mühendisliği Bilim Dalı
Derece: Doktora, 2019
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Mikroalaşımlı çelik = Microalloyed steel ; Pres dövme = Press forging ; Sıcak dövme = Hot forging

Özet:
Bu doktora tez çalışmasının amacı yarı katı faz dönüşüm sıcaklıklarında çeliklerin 
tikso dövme ile şekillendirilmesi için gerekli parametrelerin incelenmesi ve en uygun 
parametrelerde  endüstriyel bir prototip ürün üretilmesini sağlamaktır. 
Çeliklerde tikso dövme ile ilgili günümüze kadar laboratuvar ölçekli çok sayıda 
çalışma yapılmış olması sebebiyle, öncelikle tikso dövme için gerekli koşullar 
belirlenmiş ve başarılı olan tüm çalışmalarda belirtilen parametreler üzerinde 
iyileştirmeler yapılmıştır. 
Yapılan çalışmaların en önemli iki özelliği, tikso sıcaklık aralığı geniş metallerin 
kullanılması ve ısıtma ekipmanı olarak indüksiyon sisteminin tercih edilmesidir. 
İndüksiyon sistemi, özellikle metal ve alaşımlarında daha hızlı ve stabil bir ısıtma 
sağladığı için bu sistem tercih edilmektedir. 
Bu çalışmanın özgün yanı, hali hazırda ticari olarak kullanılan bir çelik kalitesinde 
(C70S6) tikso dövme çalışmasının yapılmış olması ve bu çelik kullanılarak üretilen 
biyel kolunun endüstriyel tikso dövme proses parametrelerin belirlenmiş olmasıdır. 
Yapılan bu çalışmada otomotiv sektöründe en çok kullanılan C70S6 ve 100Cr6 mikro 
alaşımlı çelikler tercih edilmiştir. Bu çeliklerin diğer bir özelliği ise, yarı katı faz 
aralığı geniş olması sebebiyle tikso dövmeye uygun çelikler olmasıdır. Özellikle 
100Cr6 yarı katı şekillendirme çalışmalarında en çok tercih edilen mikro alaşımlı çelik 
türüdür. C70S6 ise 100Cr6'ya göre yarı katı faz aralığı dar olmasına rağmen, otomotiv 
sektöründe en çok kullanılan mikro alaşımlı çeliktir. 
Yapılan bu çalışmada öncelikle, mevcut bilimsel çalışmalar değerlendirlmiş ve yarı 
katı şekillendime çalışmalarının bilimsel ve teknik anlamda ortak noktaları 
belirlenmiştir. Bu sayede kullanılacak yöntemler ve teçhizatlar konusunda araştırmalar 
yapılmıştır.  

Deneysel çalışmaların başarılı olabilmesi için öncelikle proses gereksinimleri 
tanımlanmıştır. Bu sebeple, ısıtma düzeneğinin en önemli unsur olduğu farkedilmiştir. 
Yapılan mevcut çalışmalarda farklı indüksiyon sistemleri kullanıldığından, mevcut 
çelik türlerinin yarı kat faz aralığına doğru şekilde ısıtılması için farklı tip indüksiyon  
cihazlarında denemeler yapılmıştır. Bu amaçla öncelikle levha tip indüksiyon cihazı 
kullanılmış ve proses sıcaklığı belirlenerek elde edilen mikroyapılar incelenmiştir. 
Daha sonra ısıtma rejiminin avantajı kullanılarak yatay indüksiyon fırını 
kullanılmasına karar verilmiştir. Ancak yatay indüksiyon cihazında, yüksek 
sıcaklıklarda numunelerin transferinde yaşanan zorluklar ve sıcaklık ve ısıtma 
süresinin kontrollü olmaması sebebiyle sistem geliştirilerek dikey tip indüksiyon 
cihazı kullanılmasına karar verilmiştir. 

Dikey indüksiyon sisteminde her iki çelik için yarı katı faz aralığı belirlenerek, bu faz 
aralığında seçilen sıcaklıklara kadar numuneler ısıtılmış ve su verilerek elde edilen 
mikroyapı farklılıkları incelenmiştir. 
Her iki çelik için farklı tikso dövme sıcaklıkları belirlendikten sonra, otomotiv 
sektöründe biyel kollarında kullanılan C70S6 mikro alaşımlı çeliğinin belirlenen 
parametrelerle tikso dövme ile şekillendirilmesi sağlanmıştır. 
Farklı sıcaklıklarda yapılan denemeler sonrasında, kapalı kalıp sisteminde tikso dövme 
işlemi ile biyel kolu prototipi üretilmiştir. Elde edilen sonuçlar incelenerek, proses 
parametreleri geliştirilmiş ve iyileştirme çalışmaları üzerinde durulmuştur. 
Optimum parametreler ile elde edilen biyel kolu prototipinde çeşitli mekanik deneyler 
yapılmış ve sonuçlar geleneksel sıcak dövme ile üretilen biyel kolu numuneleri ile 
karşılaştırılmıştır. 
Yapılan çalışmalar sonucunda tikso dövme ile üretilen biyel kolunun mekanik 
özelliklerinin, geleneksel dövme ile üretilen biyel kolunun mekanik özelliklerini 
yeterli oranda karşıladığı görülmüştür. 
Tüm bu çalışmalar TÜBİTAK TEYDEB (Proje No: 3130030) projesi kapsamında 
gerçekleştirilmiş ve başarılı şekilde sonuçlandırılmıştır.

Abstract:
The purpose of this doctoral thesis study is to examine the thixoforging parameters  of 
steels at semi solid phase transformation temperatures and to produce an industrial 
prototype product with the most suitable parameters. 
Since there are many laboratory scale thixoforging studies for various steels in the 
literature, the required parameters for the thixoforging have been determined and 
improvements have been made on the process parameters specified in all these 
successful studies. The two most important characteristics of the subjected 
thixoforging studies are the use of metals which have wide thixo temperature range 
and the preference of the induction heating system as an equipment. 
The novelty of this study arises from a commercial grade steel (C70S6) was employed 
and industrial thixoforging process parameters of a prototype connecting rod made of 
this steel was determined. 

In this study, the most commonly used C70S6 and 100Cr6 micro alloyed steels which 
are also preferred in the automotive industry have been preffered for our trials. Another 
feature of these steels is that they are suitable for thixoforging applications due to their 
wide range of semi-solid phase.  
In particular, 100Cr6 is the most preferred type of micro alloyed steel in semi-solid 
forging works and C70S6 is the most widely used micro alloy steeled in the 
automotive industry, although the semisolid phase range is narrow. Based on this 
reason, limited laboratory scale trials was performed by using this type of steel until 
now. 

In this study, firstly, the existing scientific studies were evaluated and the common 
points of the semi-solid shaping studies in scientific and technical terms were 
determined. In this way, researches have been made on the methods and equipments 
to be used.
 
During the study, firstly, semi-solid phase intervals were determined for both steels 
and the samples were heated up to the selected temperatures in this range and the 
microstructure differences obtained by water quenched were investigated. Both, 
numerical and theorical ways were used for identifiying semi solid intervals and phase 
diagrams for both steels. 
 
Especially, ThermoCALC software was used for determining semi solid temperatures 
and intervals, then trials in different induction devices helped us for confirming all 
outputs of ThermoCALC software. 

In order for the experimental studies to be successful, the process requirements are 
defined first. Therefore, it has been recognized that the heating device is the most 
important element. Since different induction systems are used in the current studies, 
different types of induction devices have been tested for the proper heating of the 
existing steel alloys to determine semi solid phase range. 
For this purpose, plate induction device was used firstly and microstructures obtained 
by determining process temperature were examined. This plate induction device is 
already used in forged hand tools production line for hardening of tool surfaces. 
Plate induction device helped us for first trials of thixoforging temperatures which we 
want to reach out. Thanks to this device, the effects of the temperatures mentioned in 
the literature were confirmed on the microstructure of the steels. It was then decided 
to use the horizontal induction furnace using the advantage of the heating regime.In 
addition, homogenous heating can be obtained by horizontal induction furnace by 
using different type of thermocouples.  
Due to some issues on heating control inputs and transfering problem between furnace 
and die tools, vertical induction furnace were designed and further trials were 
performed in this developed system. By using new vertical induction system, different 
thixoforging temperatures were confirmed which was already mentioned by 
ThermoCALC results previously.  

After experimential studies at elevated temperatures, the prototype of the connecting 
rod was produced by thixoforging method in the closed die system. For this purpose, 
1/7 scales of commercial connecting rod was designed for trials and thixoforging was 
done within selected temperatures. In order to manufacture the connecting rods by 
thixoforgingprocess, a fully closed die assembly was designed.In this design, the 
sample workpiece is ﬁrst placed on the punch of the lower die. After closing the upper 
and lower dies, lower punch is moved upward by a mechanical press with a capacity 
of 400 tons into the closed die cavity to form the billet. The connecting rod was ejected 
from the die and cooled down to room temperature in air. 

After determining different thixoforging temperatures for both steels, it was ensured 
that C70S6 micro alloyed steel used for connecting rods in automotive industry was 
shaped by thixoforging with selected heating parameters in the range of 1400-1460°C  
During all experiments, die temperature was controlled in the range of 100 and 150°C. 
At the temperature of 1400°C, the die could not be completely ﬁlled by the material 
and only the big end of the connecting rod could be formed. Increasing the forging 
temperature by about 20°C (1420°C), although a larger volume of the die could be 
ﬁlled by the material, it was still insufﬁcient for obtaining the connecting rod in its 
ﬁnal form and some materials resulted in ﬂash formation around the big end. 
Thixoforging temperature of 1440°C exhibited almost complete formation of the big 
end without any ﬂash formation. At this temperature, however, the small end of the 
connecting rod could not be properly obtained, and a fold formation in the big end 
toward the beam section of the connecting rod was also observed. 
Further increasing the forging temperature to 1460°C to see the effect of much higher 
temperature, although provided material ﬂow in some extent toward the small end, the 
obtained shape was far from the desired geometry of the connecting rod and the ﬂash 
formation around the big end was reappeared.  

According to the results of thixoforging experiments performed in this study, the 
samples thixoforged at 1440°C resulted in better forms of the connecting rod among 
the others. In those samples, liquid volume fraction was estimated around 0.35. On the 
other hand, those samples still contain some imperfections such as incomplete 
formation of the small end and fold formation in the big end toward the beam section 
of the connecting rod, both of which need to be corrected. 
The induction heating device and the mechanical press were located very close to each 
other to minimize the temperature drop as much.  By analyzing the obtained results, 
optimum process temperature was determined due to flash ratio and visual inspection 
results about surface quality. Then, process parameters were developed and 
improvement studies were emphasized for new trials. Thixoforging experiments were 
repeated by using the modiﬁed die design, and it was shown that these imperfections 
on the connecting rod could be eliminated. 
Various mechanical tests and microstuctural examinations were performed on the 
connecting rod prototype obtained by optimum parameters and the results were 
compared with the connecting rod samples produced by conventional hot forging.  
For the microstructural examinations, samples were prepared in the standard 
metallographic procedure and examined by optical microscope and scanning electron 
microscope after etching. A scanning electron microscope equipped with EDS was 
also used for semiquantitative identiﬁcation of the second-phase particles in the 
microstructure of as-quenched samples. Prior austenite grain size was determined 
according to the lineal intercept method, while volume fraction was estimated by the 
point counting method. Interlamellar spacing of pearlite was calculated as the mean 
true spacing in accordance with the procedure proposed. Hardness was measured under 
a load of 10 kg. At least ﬁve measurements were taken on the surface of each sample, 
and the results were averaged. 

Tensile test samples which were machined from the thixoforged connecting rods were 
subjected to tensile test at a crosshead speed of 2 mm/min. Samples which were 
produced by conventional hot forging at 1280°C were also subjected to tensile test for 
comparison. Following the tensile tests, fracture surface of the samples was examined 
by a scanning electron microscope. In order to check the thixoforged connecting rods 
against any defects arising from the forming process and die design, ﬁnite element 
analysis was performed by a finite element analysis software. 
As a result of this study, the mechanical properties of the thixoforged connecting rod 
meet the mechanical properties of conventional hot forging. Thixoforging of C70S6 
steel has been assessed by using a scaled-down model of a commercial vehicle 
connecting rod. The current work has demonstrated that C70S6-grade steel can be 
successfully thixoforged at 1440°C in the form of scaled-down connecting rod without 
excessive ﬂash formation. 

Mechanical test results showed that hardness and strength values after thixoforging 
can be maintained at almost similar levels with those presented by conventional hot 
forging. However ductility is signiﬁcantly decreased due to grain boundary ferrite 
network upon thixoforging. This results tell us for the success of this study and also 
means thixoforging may used for different type of forged parts instead of conventional 
hot forging for several industrial application. 

All of these studies were carried out within the scope of TÜBİTAK TEYDEB  project 
(Project No: 3130030) and were successfully completed.