KRİYOJENİK İŞLEMİN, LAZER KAYNAKLI OTOMOTİV SACI BİRLEŞTİRİLMELERİNİN MEKANİK VE MİKROYAPI ÖZELLİKLERİNE ETKİSİ
Yükleniyor...
Dosyalar
Tarih
2023-08
Yazarlar
Dergi Başlığı
Dergi ISSN
Cilt Başlığı
Yayıncı
Erişim Hakkı
info:eu-repo/semantics/openAccess
Özet
Kriyojenik sığ ve derin olarak tanımlanan esasen malzemelerin -80 ?’den daha düşük sıcaklıklarda tutulma işlemidir. Birçok metal üzerine uygulama alanı bulmakla birlikte su verilmiş çeliklere uygulanmasının takım çeliklerinin mekanik özelliklerini geliştirmede çok önemli bir faydası bulunmaktadır. İleri dayanımlı ve otomotiv çelikleri olarak bilinen çift-fazlı (DP) çelikler son 40 yılda ve HSLA çelikleri son 60 yılda otomobil gövde imalatında kullanılan ve yüksek dayanım ile yolcu güvenliği adına çok önemli kazanımlar sağlayan çeliklerdir. Diğer taraftan lazer kaynağı da oldukça yaygınlaşan, yüksek vasıflı kaynak ve kaynak performası açısından önemli avantajlar sağlayan teknolojik her alanda kendine yer bulan bir kaynak metodudur. İlave olarak dolayısıyla bu çalışma iki farklı alanda üstün ve önemli fenomeni kombine ederek yüksek performansa sahip kaynaklı bağlantılar üretilebilirliği araştıran bir çalışmadır. Çalışmalarda kullanılan bu iki farklı çelik sac numuneler kriyojenik işlem uygulanmış ve uygulanmamış olarak iki gruba ayrılmıştır. Kriyojenik işlem uygulanan numunelerde lazer öncesi ve sonrası olarak iki gruba ayrılmıştır. Kriyojenik işlem uygulanmış ve uygulanmamış numuneler her malzeme grubu (DP600, HSLA) kendi aralarında ve birbirleri ile alın pozisyonunda katı hal lazer kaynağı ile birleştirilmiştir. Tüm birleştirmeler çift taraflı olarak dolgu metalsiz gerçekleştirilmiştir. Kaynaklı bağlantılara çekme, sertlik ölçme deneyleri ve metalurjik yapı inceleme ve karakterizasyonları (Optik, SEM görüntü inceleme ve kimyasal analiz, XRD) ile yapılmıştır. Sonuç olarak kriyojenik işlem ile ana malzeme ve kaynak bölgesinde martenzitik ve ferritik yapıların inceldiği tam olarak gözlemlenememiştir. DP600 çeliğine ait XRD sonuçları yapıdaki incelmeyi kristalit tane boyutundaki azalmalar ile bir derece göstermektedir. Şöyleki ana malzemelerin kristalit tane boyutuyla ters orantılı dislokasyon yoğunluğu lazer kaynaklı numuneye kriyojenik işlem uygulanması neticesinde DP600 için %12.67 artış göstermiştir. Genel olarak değerlendirildiğinde hem DP hem de HSLA çeliğinde kaynak metali mikroyapısının martenzitik olduğu söylenebilir. Ayrıca ITAB’da kaynak esnasında oluşan sıcaklık gradyanına bağlı olarak AC1-AC3 arası kritik sıcaklıklarda östenitten ferrit dönüşümü ve kaynak sonrası hızlı soğuma ile östenitin martenzite dönüşmesi ile ferrit martenzit çift fazlı yapı oluştuğu gözlemlenmektedir. Buradaki ferrit martenzit değişimi kaynak metalinden ana malzemeye doğru azalmakta ve neticede ana malzemenin ferrit martenzit ve ferrit perlit oranına benzer oranına yaklaşmaktadır. Kaynak metali sertlikleri ve diğer sertlikler karşılaştırıldığında çift fazlı çelik kaynak bölgesinde ortalama 405 HV sertlik elde edilirken HSLA çeliğinde kaynak metalinde ortalama 335.1 HV sertlik elde edilmiştir. Buna göre ana malzeme sertlikleriyle değerlendirildiğinde DP çelik kaynak metalinde ana malzemeye göre sertlik artışı %101 iken kadar olurken HSLA çeliğinde yüzde %78 olarak gerçekleşmiştir. Kaynak merkezindeki sertlik dağılımları ortalama olarak çok az miktarda düşerken sertlik değerlerinin birbirine daha yakın bir şekilde birbirine yakınlaştığı gözlemlenmiştir. Çekme testi sonuçları, kriyojenik işlemin DP çeliklerin lazer kaynaklı birleştirmeleri ve HSLA çeliklerinin lazer kaynaklı birleştirmelerin dayanımlarını artırırıken sünekliğini düşürdüğü göstermiştir. Diğer taraftan akma dayanımını DP600 için %8,32 ve HSLA çelik grubu için %7,27 artış gerçekleşirken çekme dayanımlarında ortalama olarak DP600 için %4,76 iken HSLA için %3,00 artış gerçekleşmektedir. Dayanımlardaki gelişmeler incelendiğinde akma dayanımında gerçekleştiği gözlemlenmiştir. Uzama miktarlarında ise kaynaklı numunelere sonradan uygulanan kriyojenik işlem ile DP600 gurubunda %38 HSLA grubunda ise %11,87 artmıştır. Ancak ana metale uygulanan kriyojenik işlem ile toplam uzama miktarlarında DP600 grubunda %13,99 HSLA grubunda ise %37,86 azalmıştır.Lazer kaynaklı birleştirilen numunelerin çekme testlerinde hasarlar ana malzemede gerçekleşmektedir.
Cryogenics, defined as shallow and deep, is essentially the process of keeping materials at temperatures lower than -80 ?. Although it finds application on many metals, its application to quenched steels has a very important benefit in improving the mechanical properties of tool steels. Dual-phase (DP) steels, known as advanced strength and automotive steels, are the steels that have been used in automobile body manufacturing in the last 40 years and HSLA steels in the last 60 years and provide very important gains in terms of high strength and passenger safety. On the other hand, laser welding is a welding method that has become very widespread and provides significant advantages in terms of high-qualified welding and welding performance, which finds its place in every technological field. In addition, therefore, this study is a study investigating the producibility of high-performance welded joints by combining outstanding and important phenomena in two different fields. These two different steel sheet samples used in the studies were divided into two groups cryogenically treated and untreated. The cryogenically treated samples were divided into two groups pre-laser and post-laser. Each material group (DP600, HSLA) of cryogenic treated and untreated samples were combined with each other and among themselves by solid-state laser welding in the butt position. All joints are made on both sides without filler metal. Tensile and hardness testing experiments and metallurgical structure examination and characterizations (Optics, SEM image examination and chemical analysis, XRD) were performed on welded joints. As a result, the thinning of martensitic and ferritic structures in the base material and weld zone by cryogenic treatment could not be observed exactly. XRD results for DP600 steel show a degree of thinning in structure with reductions in crystallite grain size. That is, the dislocation density of the base materials, inversely proportional to the crystallite grain size, increased by 2.2% as a result of the cryogenic treatment of the laser-welded sample. When evaluated in general, it can be said that the weld metal microstructure in both DP and HSLA steel is martensitic. In addition, it is observed that a ferrite-martensite biphasic structure is formed with the transformation of austenite to ferrite at critical temperatures between AC1-AC3, depending on the temperature gradient formed during welding in ITAB, and the transformation of austenite to martensite with rapid cooling after welding. Here, the ferrite martensite variation decreases from the weld metal to the base material, and as a result, the rate of ferrite martensite and ferrite perlite of the base material approaches a similar ratio. When weld metal hardnesses and other hardnesses are compared, an average hardness of 405 HV was obtained in the dual-phase steel weld zone, while an average hardness of 335.1 HV was obtained in the weld metal in HSLA steel. According to this, when evaluated with the hardness of the base material, the hardness increase in dp steel weld metal was 101% compared to the base material, while it was 78% in HSLA steel. It was observed that while the hardness distributions in the weld center decreased very little on average, the hardness values became closer to each other. Tensile test results showed that cryogenic treatment reduces the ductility of laser-welded joints of DP steels and HSLA steels while increasing the strength of laser-welded joints. On the other hand, while the increase in yield strength was 8.32% for DP600 and 7.27% for HSLA steel group, while the increase in tensile strength was 4.76% for DP600 and 3.00% for HSLA, the increase in actual strength was observed to occur in yield strength. The amount of elongation increased by 38% in the DP600 group and by 11.87% in the HSLA group with the subsequent cryogenic treatment applied to the welded samples. However, with the cryogenic treatment applied to the base metal, the total elongation amount decreased by 13.99% in the DP600 group and 37.86% in the HSLA group."
Cryogenics, defined as shallow and deep, is essentially the process of keeping materials at temperatures lower than -80 ?. Although it finds application on many metals, its application to quenched steels has a very important benefit in improving the mechanical properties of tool steels. Dual-phase (DP) steels, known as advanced strength and automotive steels, are the steels that have been used in automobile body manufacturing in the last 40 years and HSLA steels in the last 60 years and provide very important gains in terms of high strength and passenger safety. On the other hand, laser welding is a welding method that has become very widespread and provides significant advantages in terms of high-qualified welding and welding performance, which finds its place in every technological field. In addition, therefore, this study is a study investigating the producibility of high-performance welded joints by combining outstanding and important phenomena in two different fields. These two different steel sheet samples used in the studies were divided into two groups cryogenically treated and untreated. The cryogenically treated samples were divided into two groups pre-laser and post-laser. Each material group (DP600, HSLA) of cryogenic treated and untreated samples were combined with each other and among themselves by solid-state laser welding in the butt position. All joints are made on both sides without filler metal. Tensile and hardness testing experiments and metallurgical structure examination and characterizations (Optics, SEM image examination and chemical analysis, XRD) were performed on welded joints. As a result, the thinning of martensitic and ferritic structures in the base material and weld zone by cryogenic treatment could not be observed exactly. XRD results for DP600 steel show a degree of thinning in structure with reductions in crystallite grain size. That is, the dislocation density of the base materials, inversely proportional to the crystallite grain size, increased by 2.2% as a result of the cryogenic treatment of the laser-welded sample. When evaluated in general, it can be said that the weld metal microstructure in both DP and HSLA steel is martensitic. In addition, it is observed that a ferrite-martensite biphasic structure is formed with the transformation of austenite to ferrite at critical temperatures between AC1-AC3, depending on the temperature gradient formed during welding in ITAB, and the transformation of austenite to martensite with rapid cooling after welding. Here, the ferrite martensite variation decreases from the weld metal to the base material, and as a result, the rate of ferrite martensite and ferrite perlite of the base material approaches a similar ratio. When weld metal hardnesses and other hardnesses are compared, an average hardness of 405 HV was obtained in the dual-phase steel weld zone, while an average hardness of 335.1 HV was obtained in the weld metal in HSLA steel. According to this, when evaluated with the hardness of the base material, the hardness increase in dp steel weld metal was 101% compared to the base material, while it was 78% in HSLA steel. It was observed that while the hardness distributions in the weld center decreased very little on average, the hardness values became closer to each other. Tensile test results showed that cryogenic treatment reduces the ductility of laser-welded joints of DP steels and HSLA steels while increasing the strength of laser-welded joints. On the other hand, while the increase in yield strength was 8.32% for DP600 and 7.27% for HSLA steel group, while the increase in tensile strength was 4.76% for DP600 and 3.00% for HSLA, the increase in actual strength was observed to occur in yield strength. The amount of elongation increased by 38% in the DP600 group and by 11.87% in the HSLA group with the subsequent cryogenic treatment applied to the welded samples. However, with the cryogenic treatment applied to the base metal, the total elongation amount decreased by 13.99% in the DP600 group and 37.86% in the HSLA group."
Açıklama
Anahtar Kelimeler
Lazer kaynağı, DP600, HSLA, Kriyojenik İşlem, Laser Welding, DP600, HSLA, Cryogenic Treatment
