Sıcak metal şekillendirmede proses kaynaklı kusurların proses performansına etkisinin deneysel ve sayısal olarak incelenmesi
Experimental and numerical investigation of the effect of process-induced defects on process performance in hot metal forming
- Tez No: 921939
- Danışmanlar: PROF. DR. MESUT KIRCA
- Tez Türü: Yüksek Lisans
- Konular: Makine Mühendisliği, Mechanical Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2025
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Lisansüstü Eğitim Enstitüsü
- Ana Bilim Dalı: Makine Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Katı Cisimlerin Mekaniği Bilim Dalı
- Sayfa Sayısı: 129
Özet
Bu tez çalışmasında, sıcak dövme yöntemlerinden biri olan kapalı kalıpta sıcak metal şekillendirme sürecinde ortaya çıkan proses kaynaklı kusurların (örn. kalıp hizalama hataları, kaçıklık vb.), proses performansı ve nihai ürün kalitesi üzerindeki etkileri deneysel ve sayısal yöntemlerle incelenmiştir. Çalışmada, ardışık iki temel dövme prosesi (yığma dövme ve endirekt ekstrüzyon) bir üretim zinciri olarak ele alınmış; her bir adım ayrı ayrı modellenerek sonlu elemanlar metodu (SEM) kullanılmıştır. Analizlerde Abaqus/Explicit yazılımı ve AISI 4140 çeliği esas alınmış; sürtünme, kalıp ve iş parçası sıcaklıkları, presleme hızı ve Tresca kayma limiti gibi kritik parametreler değişken olarak tanımlanmıştır. Sahadan elde edilen deneysel veriler, modelin doğrulanması için kullanılmış; sonrasında kusurlu kalıp hizalamaları (2 mm kaçık ve 0.1° eğik) gibi durumlar senaryo tabanlı olarak incelenmiştir. Analiz sonuçları, kalıp kaçıklıkları ve uygun olmayan proses parametrelerinin hem nihai geometri üzerinde ölçüsel bozukluklara hem de deformasyon yükü artışı ile kalıp-aşınma riskine neden olduğunu göstermiştir. Ayrıca, optimizasyon çalışmaları sonucunda doğru kalıp sıcaklığı (300 °C), sürtünme katsayısı (0,11), ilk parça sıcaklığı (1200 °C) ve presleme hızının, süreç performansını iyileştirdiği ve ürün kalitesini artırdığı gözlemlenmiştir. Sonuç olarak bu tez, sıcak metal şekillendirme süreçlerinde proses kaynaklı kusurların önlenmesi ve süreç parametrelerinin optimizasyonu için bütüncül bir yaklaşım ortaya koymaktadır. Hem deneysel hem de sayısal analizlerden elde edilen bulgular, üretim verimliliğinin artırılması ve ürün kalitesinin iyileştirilmesi açısından önemli bir rehber niteliğindedir.
Özet (Çeviri)
Hot metal forming is a fundamental manufacturing technique used to produce components with high strength and complex geometries, especially in the automotive, aerospace, and defense industries. Among the many hot forming processes, closed-die forging stands out for its ability to impart improved mechanical properties and refined microstructures to metals, most notably steel alloys. In such processes, a heated billet is placed between dies and subjected to compressive forces, resulting in a near-net-shape part. Despite extensive research on topics such as friction behavior, material flow, and die design, there remains a notable gap concerning the systematic investigation of sequential forging operations under realistic industrial conditions, particularly when process-induced defects—like die misalignment or uneven lubrication—are taken into account. Even minor defects can lead to significant dimensional inaccuracy, increased forging loads, or reduced tool life, underscoring the need for a more holistic approach that links experimental findings with detailed numerical simulations. In this thesis, two successive forging operations—upsetting and backward extrusion—are analyzed as an integrated production chain, providing a more comprehensive understanding of hot forging beyond single-stage studies. Upsetting is a procedure in which the heated billet is compressed along its axis to reduce its height and expand its cross-sectional area, typically improving the internal structure and preparing the workpiece for the subsequent operation. Backward extrusion, by contrast, drives a punch into the preformed workpiece so that the metal flows backward around the punch, forming a hollow or cup shape. Examining these operations in tandem yields practical insights into how process parameters (like die temperature and friction) and defects (such as a die offset) carry over from one stage to the next, thus influencing final part quality. The thesis thus aims not only to document and model these two consecutive processes but also to quantify how small misalignments—on the order of millimeters or tenths of a degree—can drastically alter dimensional outcomes. On the experimental side, the work used a 1500-ton hydraulic press capable of exerting substantial compressive forces suitable for forging steel billets at elevated temperatures. AISI 4140 steel was chosen due to its strong hardenability, high strength, and widespread industrial use in components like gears and shafts. Each billet was heated to around 1200 °C before forging, a temperature that balances reduced flow stress with manageable oxidation. Once heated, the billet underwent the upsetting process, and relevant data—such as load–time curves—were recorded through a high-capacity load cell. Upon completion, the partially shaped component was immediately transferred to a second station for backward extrusion, where additional load–time data were collected. After each step, final dimensions were measured using vernier calipers or coordinate measuring equipment, providing a precise record of part geometry. The experiments served multiple purposes: validating numerical simulations, helping identify optimal process conditions (for instance, appropriate die temperature), and revealing the real-world impact of minor alignment issues introduced deliberately or inadvertently. Turning to the numerical side, the research employed thermomechanically coupled finite element models in Abaqus/Explicit. This solver is especially suitable for problems involving large plastic strains and complex contact conditions. The workpiece was discretized using C3D8R elements, which are eight-node hexahedral elements with reduced integration, while the dies were treated as rigid bodies. Material properties for AISI 4140 were modeled as temperature-dependent, capturing variations in yield stress and hardening at elevated temperatures. The friction model combined Coulomb friction with a Tresca shear limit to better represent interface shear behavior under high normal pressures. Key process parameters—like die temperature (spanning 200–400 °C), initial workpiece temperature (850–1200 °C), friction coefficient (ranging from 0.11 to 0.4), and press velocity—were systematically varied to capture their combined influence on forging loads and final part geometry. Special numerical techniques, including mass scaling and hourglass control, were employed to ensure stable solutions within reasonable computation times, whereas adaptive meshing alleviated excessive mesh distortion in severely deformed regions. Comparisons between the numerical predictions and the experimental observations revealed a high degree of consistency. Specifically, when the die temperature was maintained around 300 °C and the friction coefficient set to around 0.11 (typical for graphite-based lubricants), the load–time curves from simulations exhibited only a 1–3% deviation from the measured values. Furthermore, final geometries—such as the internal diameter or height of the extruded cup—demonstrated minimal mismatches, indicating that the chosen thermomechanical model was capturing the critical aspects of material flow and temperature evolution. Lower die temperatures (e.g., 200 °C) led to rapid cooling of the billet upon contact, elevating the deformation load and risking incomplete filling, whereas temperatures exceeding 400 °C risked over-softening at the die–workpiece interface. Similarly, an initial workpiece temperature of around 1200 °C optimized the forging load and minimized shape defects, whereas significantly lower temperatures (like 850 °C) produced partial fills, higher forces, and potential distortions. A key focus of the research was quantifying the effect of die misalignment. Two realistic misalignment scenarios—a 2 mm horizontal offset and a 0.1° angular tilt—were examined to emulate common production-floor errors. Both experimental trials and simulations indicated that even these modest deviations could cause substantial asymmetry in the part and amplify forging loads in specific regions. In the case of a 2 mm offset, for instance, wall thickness in the backward-extruded cup varied by several millimeters relative to the intended specification, corroborating the notion that an apparently“small”error can translate into large dimensional inaccuracies under heavy forming loads. Likewise, a 0.1° tilt caused a slight slope in the part's final geometry, altering the uniformity of the cup's height and thickness. These findings confirmed that alignment is just as critical a parameter as temperature and lubrication in multi-step forging. From these combined experimental and numerical results, several practical guidelines emerged. First, die temperatures near 300 °C strike a balance between preventing excessive cooling and avoiding detrimental die softening. Second, ensuring the workpiece is uniformly heated to around 1200 °C helps minimize forging loads while maintaining good material flow. Third, friction coefficients around 0.11, typical of graphite lubricants at elevated temperatures, yield stable and predictable results, though variations in lubrication strategies may be explored in future studies. Fourth, replicating realistic press-velocity profiles from actual machine data—rather than imposing a constant velocity—improves the fidelity of simulation outcomes, especially regarding load peaks at the start and end of the stroke. Lastly, meticulous die alignment protocols should be enforced, as minor offsets or tilts can significantly degrade part quality, increase mechanical loads, and complicate subsequent machining or assembly steps. In conclusion, this thesis offers a validated computational framework that accurately reflects the physical realities of sequential forging operations, including upsetting and backward extrusion. By integrating carefully controlled experiments with detailed finite element simulations, it demonstrates that process-induced defects like die misalignment can have a pronounced effect on material flow, forging forces, and final dimensions. The findings thus underscore the necessity for diligent alignment checks, precise temperature management, proper lubrication, and accurately represented press velocity inputs. Collectively, these measures can substantially enhance productivity and part quality while reducing waste and rework. In real industrial practice, paying attention to small details—such as a one-degree tilt or a couple of millimeters in offset—helps avert costly errors, ensuring a robust forging process. Looking ahead, future work could delve deeper into microstructural evolution during sequential hot forging, using advanced damage or fracture models to predict crack initiation under aggressive deformation conditions. Alternatively, the scope could expand to include other high-strength alloys—like titanium or nickel-based superalloys—that demand stricter thermal management. Investigations might also incorporate sensor-based closed-loop control, automatically adjusting alignment or lubrication in real time to compensate for drift in machine settings. In this way, the approach pioneered in this study can be extended and refined, ultimately leading to more predictive and adaptive forging systems that further improve part accuracy and mechanical performance.
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