Hidrolik tahrikli preste kapalı kalıpta sıcak dövme prosesinin sonlu elemanlar yöntemi ile modellenmesi testi ve analizi
Finite element modeling test and analysis of closed die hot forging process in hydraulic driven press
Yazar: UTKU BİÇER
Danışman: PROF. DR. CEVAT ERDEM İMRAK
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Konstrüksiyon Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Hidrolik = Hydraulics ; Modelleme = Modelling ; Proses =   ; Sonlu elemanlar yöntemi = Finite element method ; Sıcak dövme = Hot forging

Özet:
Günümüzde bazı makina parça geometrilerinin karmaşık olması imal edilmelerini kısıtlamakta, imalat süresini ise uzatmaktadır. Sıcak dövme yöntemi ile proses öncesi ısıtılarak şekil verilebilirliği artırılan parçalarda, diğer yöntemlere göre daha düşük kuvvet ve enerjiye ihtiyaç duyularak tek seferde büyük şekil değişimi sağlanabilmektedir. Sıcaklık, basınç, kalıp geometrisi ve yağlama gibi temel parametrelerin parça geometrisine etkisi büyük olduğu için, soğuk şekillendirme kadar hassas parça geometrisi elde edilememektedir. Ancak, parça geometrisine etki eden optimum parametreler belirlenerek dövme prosesi sonrası, final parça geometrisine oldukça yakın parça geometrisi elde etmek mümkündür.
Bu yüksek lisans tez çalışmasında, istenen final parça geometrisine ulaşabilmek için hidrolik pres tahrikli kapalı kalıpta sıcak dövme prosesi modellenmiş ve parametre optimizasyonu yapılmıştır. Sonlu elemanlar metodu ile gerçekleştirilen sayısal modelleme için öncelikle final parçayı elde edebilmek için uygun iş parçası belirlenmiştir. Belirlenen iş parçasından, istenen final geometriyi elde etmek için proses zinciri oluşturulmuştur. Oluşturulan bu proses zincirinde sırasıyla, iş parçasını ısıtma, soğuma, dış tufal giderme, haddeleme, kapalı kalıpta dövme ve soğuma adımları bulunmaktadır. İstenen final parça geometrisini elde edileceği kapalı kalıpta dövme adımında kullanılacak proses parametreleri belirlenmiştir. Bu parametreler sırasıyla; dişi ve erkek kalıp sıcaklığı, presleme hızı, kalıplar ile iş parçası arasında oluşan sürtünme katsayısı, iş parçasının ısıtıldığı ilk sıcaklık ve kalıplar ile iş parçası arasında gerçekleşen ısı iletim katsayısıdır. Gerçekleştirilen literatür çalışmaları doğrultusunda belirlenen her bir parametre için referans sayısal değerler oluşturulmuştur. Bu sayısal değerlere göre, pozisyon kontrollü gerçekleştirilecek simülasyon için proses tasarımı yapılmıştır. Yapılan proses tasarımı ile erkek kalıbın –Z yönünde hareket etmesi gereken mesafe değeri bulunmuş ve böylece referans simülasyon modeli oluşturulmuştur. Referans simülasyon sonucunda istenen final parça geometrisi elde edilememiş, iş parçası dişi kalıp tabanını tamamen dolduramamıştır. Yeniden proses tasarımı yapılarak erkek kalıbın presleme mesafesi artırılarak Ref-144 simülasyonu oluşturulmuş ve analiz tekrarlanmıştır. İstenen doluluk oranına ulaşılarak final parça geometrisine oldukça yakın geometri elde edilmiştir. Ancak erkek kalıbın presleme mesafesi artırıldığı için parçanın boyu istenen final parçanın boyundan kısa kalmıştır. Aynı zamanda iş parçası erkek kalıp ile dişi kalıp arasından akarak sakal oluşturmuştur, bu sebeple final parçayı elde etmek için talaş kaldırmak gerekmektedir.
İstenen final parça geometrisini talaş kaldırmadan elde edebilmek adına referans parametre değerlerinde değişiklikler yapılmıştır. Optimum parametre değerlerini elde etmek için, referans değerler belirli oranlarda artırılıp azaltılarak on farklı analiz daha gerçekleştirilmiştir. Gerçekleştirilen analizlerin 2D geometrileri karşılaştırılarak optimum parametre değerleri belirlenmiştir. Bu parametre değerleri ile Optimum simülasyon oluşturulmuştur. Yapılan analiz sonucunda çok az bir sakal oluşumu ile istenen final parça geometrisine ulaşılmıştır.
Gerçekleştirilen analizler doğrultusunda 2 farklı deneysel çalışma 1250 tonluk hidrolik presle gerçekleştirilmiştir. Birinci proses çalışmasında, talaş kaldırmadan final parça geometrisinin elde edilmesi hedeflenmiştir. Ancak Optimum simülasyon parametreleri ile proses gerçekleştirilemediği için, Referans simülasyon parametreleri kullanılmıştır. İkinci proses çalışmasında ise parçanın dişi kalıp tabanı doluluk oranını kontrol etmek için Ref-144 simülasyon parametreleri kullanılmıştır. Deneysel çalışma ve analizlerin sonucunda oluşan parça geometrilerinin birbirine oldukça yakın olduğu görülmüştür.

Abstract:
Nowadays, the complexity of some machine part geometries limits their manufacturability and prolongs the manufacturing time. With the hot forging method, parts whose formability is increased by heating before the process can undergo a large amount of shape change in a single operation by requiring less force and energy compared to other methods. Since the basic parameters such as temperature, pressure, die geometry and lubrication have a great effect on the part geometry, it is not possible to obtain as precise part geometry as cold forming. However, by determining the optimum parameters affecting the part geometry, it is possible to obtain a part geometry very close to the final part geometry after the forging process.
In this master's thesis, the hot forging process in a hydraulic press driven closed die is modeled and parameter optimized to achieve the desired final part geometry. For the numerical modeling performed with the finite element method, firstly, a suitable workpiece was determined to obtain the final part. A process chain was created to obtain the desired final geometry from the workpiece. In this process chain, the steps of heating, cooling, external descaling, hot rolling, closed die forging and cooling of the workpiece are respectively. In the first step of the modeled process chain, the workpiece is heated. Since the workpiece, which reaches the desired temperature, comes into contact with air for a certain period of time on its way to the external desizing unit, a cooling step was added as the second step. In order to define the process that takes place in the external descaling unit, the external descaling step was created as the third step. For the square billet part, which is subjected to corner rounding after external descaling, the rolling step was added as the fourth step. After rolling, the closed die forging step was modeled as the fifth step for the part to be forged in a closed die. Finally, the cooling step is defined as the sixth step to bring the forged part to room temperature.

The process parameters to be used in the closed die forging step to obtain the desired final part geometry were determined. These parameters are; female and male die temperature, pressing velocity, friction coefficient between the dies and the workpiece, initial temperature at which the workpiece is heated and heat conduction coefficient between the dies and the workpiece. In line with the literature studies, reference numerical values were determined for each parameter. These parameters are 300 °C, 100 mm/s, µ=0,15, 1200 °C and 10 kW/m2K respectively. According to these numerical values, the process design was made for the simulation to be performed with position control. With the process design, the distance value that the male die should move in the -Z direction was found and thus a reference simulation model was created.
As a result of the reference simulation, the desired final part geometry could not be obtained and the workpiece could not completely fill the female die base. The Ref-144 simulation was created by increasing the pressing distance of the male die by redesigning the process and the analysis was repeated. The desired fill rate was achieved and a geometry very close to the final part geometry was obtained. However, since the pressing distance of the male die was increased, the length of the part remained shorter than the desired final part length. At the same time, the workpiece flowed between the male mold and the female mold and formed flash, so it is necessary to remove sawdust to obtain the final part.
In order to obtain the desired final part geometry without machining, the reference parameter values were modified. In order to obtain the optimum parameter values, ten more analyses were performed by increasing and decreasing the reference values in certain ratios. Simulation 1 (S1) and Simulation 2 (S2) were performed by increasing and decreasing the die temperature by 100 °C, respectively. Simulation 3 (S3) and Simulation 4 (S4) were performed by increasing and decreasing the pressing velocity by 50 mm/s, respectively. The coefficient of friction value was defined for die lubrication, µ=0.22 for synthetic oil and µ=0.4 for no lubrication, and Simulation 5 (S5) and Simulation 6 (S6) were performed respectively. Simulation 7 (S7) and Simulation 8 (S8) were performed by increasing and decreasing the initial part temperature by 100 °C, respectively. The heat transfer coefficient was increased and decreased by 2 kW/m2K and Simulation 9 (S9) and Simulation 10 (S10) were performed respectively. Optimum parameter values were determined by comparing the 2D geometries of the analyzes. Optimum simulation was created with these parameter values. As a result of the analysis, the desired final part geometry was achieved with very little falsh formation.
According to the analysis, 2 different experimental studies were performed with a 1250 ton hydraulic press. In the first process study, it was aimed to obtain the final part geometry without sawdust removal. However, since the process could not be realized with Optimum simulation parameters, Reference simulation parameters were used. In the second process study, Ref-144 simulation parameters were used to control the female die base fill ratio of the part.
In the first way, the simulation was created according to the reference parameters, but as a result of the simulation, the desired final geometry could not be achieved because the part could not fill the female die base sufficiently. For this reason, the pressing distance of the press was increased from 142 mm to 148 mm and the simulation was performed again. As a result of the simulation, the part filled the entire female die, but intense flash geometry was formed between the male and female die. When this simulation was examined, it was seen that the step where the part both filled the die base and the least beard formation occurred was the step where the press compression distance was 144 mm. For this reason, Ref-144 Simulation was created and as a result of the simulation, it was obtained that the final geometry can be achieved by increasing the volume of the workpiece to obtain the final geometry and removing the sawdust from the top of the part obtained at the end of the process.
In the second way, the outputs of eleven different simulations, including the reference simulation, were analyzed according to the parameters determined. In order to make a comparison, limit values were determined according to the reference analysis, and the positive and negative aspects of the simulations that were outside these limit values were evaluated. As a result of the evaluation, it was seen that the desired final part geometry could be achieved with Simulation 8, but after the examination, it was observed that this part could not completely fill the die. For this reason, simulations within the limit were examined and parameters that gave better results than the reference parameter were determined. Optimum Simulation was performed with these parameters and the final part geometry was obtained with very little flash formation.
Since experimental work could not be performed with the optimum Simulation parameters, both Reference Simulation and Ref-144 Simulation were performed. While the main purpose of the reference simulation process is to control the die top geometry, the main purpose of the Ref-144 Simulation is to control the achievement of the die base fullness. As a result of the processes performed, it was seen that the part geometries obtained in the simulation and the part geometries obtained in the experimental study were very close to each other.