OTOMOTİV SEKTÖRÜNDE KULLANILAN YENİ NESİL YÜKSEK DAYANIMLI ÇELİKLERİN DİRENÇ NOKTA KAYNAĞI UYGULAMALARI VE KAYNAK BÖLGESİ KARAKTERİZASYONU
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2023-01
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info:eu-repo/semantics/openAccess
Özet
STRENX ve DP çelikleri çift fazlı çelik türü olarak ileri yüksek mukavemetli çelikler (AHSS) grubunda yer almaktadır. Bunlar düşük maliyet, yüksek yakıt verimliliği ve daha fazla yük hacmi taşıyabilen hafif araçların üretimi için otomotiv endüstrisinde kullanılmaktadır. Bu çalışmada STRENX 700 CR ve DP 800 malzemeleri hem alternatif akım (AA), hem de orta frekans doğru akım (OFDA) kullanılarak direnç nokta kaynak (DNK) yöntemi ile birleştirilmişlerdir. Kaynak işlemleri öncesinde birleştirilecek parçaların ısı tesiri altında kalan bölgesine (ITAB) bölgesel ön ısıtma (BÖI) işlemi uygulanmıştır. BÖI sisteminin tasarımı, imalatı ve DNK makinesine adaptasyonu çalışma kapsamında gerçekleştirilmiştir. Ayrıca birleştirme işlemlerinde, klasik DNK yöntemine farklı bir konsept kazandırmak için çeşitli fikstürler tasarlanmış imal edilmiştir. BÖI ve fikstür sistemlerinin eş zamanlı olarak çalışması bilgisayar destekli yazılım ile kontrol edilmiştir. Birleştirme işlemlerinden sonra STRENX 700 CR ve DP 800 çelikleri arasındaki DNK bağlantılarına sertlik, çekme-makaslama, çapraz-çekme ve yorulma testleri uygulanmış ve böylece bağlantıların mekanik özellikleri araştırılmıştır. Ayrıca NDK bağlantılarının mikroyapıları optik mikroskop ile incelenmiştir. Uygulanan bu testlerle mekanik özellik ve mikroyapı arasındaki ilişki ortaya konulmuştur. Mikroyapı çalışmaları sonucunda tüm birleştirmelerde kaynak metali yapısının ferrit ve martenzit fazlarından oluştuğu görülmüştür. Sertlik testlerinde kaynak öncesi ITAB’a uygulanan BÖI işleminin kaynak metali sertliğini bir miktar düşürdüğü, hem kaynak metali hem de ITAB genişliğini arttırdığı belirlenmiştir. Kaynak öncesi ITAB’a uygulanan BÖI işlemiyle çekme-makaslama dayanımında %18, çapraz-çekme test dayanımında ise %13,5 artış bulunmuştur. Çekme-makaslama ve çapraz-çekme testlerinde kopmaların tümü ITAB’dan düğmelenme şeklinde gerçekleşmiştir. Yorulma testlerinde en yüksek yorulma ömrü BÖI işlemi uygulanan ve OFDA ile birleştirilen numunelerde bulunmuştur. Çalışmalar sonucunda OFDA teknolojisi ile birleştirilmiş numunelerin, AA ile birleştirilenlere göre daha iyi mekanik özellikler sergilediği görülmüştür.
STRENX and DP steels are in the advanced high-strength steels (AHSS) group as a dual-phase steel type. They are used in the automotive industry for the production of light vehicles with low cost, high fuel efficiency and higher payload. In this study, STRENX 700 CR and DP 800 materials were joined by resistance spot welding (RSW) method using both alternating current (AC) and medium frequency direct current (MFDC). Before the welding processes, the regional pre-heating (RPH) process was applied to the heat affected zone (HAZ) of the parts to be joined. The design, manufacture and adaptation of the RPH system to the RSW machine were carried out within the scope of the study. In addition, various fixtures have been designed and manufactured to provide a different concept to the classical RSW method in joining processes. Simultaneous operation of RPH and fixture systems was controlled by a computer aided software. After the joining processes, hardness, tensile-shear, cross-tensile and fatigue tests were applied to the RSW joints between STRENX 700 CR and DP 800 steels, and thus the mechanical properties of the joints were investigated. In addition, the microstructures of the joints were examined with an optical microscope. The relationship between mechanical properties and microstructure was demonstrated by these tests. As a result of microstructure studies, it was observed that the weld metal structure in all joints consisted of ferrite and martensite phases. In the hardness tests, it was determined that the RPH process applied to the HAZ before welding slightly decreased the hardness of the weld metal and increased both the weld metal and HAZ width. With the RPH process applied to HAZ before welding, an increase of 18% in tensile-shear strength and 13.5% in cross-tensile strength was found. In the tensile-shear and cross-tensile tests, all of the ruptures occurred in the form of buttoning from the HAZ. In the fatigue tests, the highest fatigue life was found in the samples which were applied RPH process and joined with MFDC. As a result of the studies, it was seen that the samples joined with OFDA technology exhibited better mechanical properties than those joined with AA."
STRENX and DP steels are in the advanced high-strength steels (AHSS) group as a dual-phase steel type. They are used in the automotive industry for the production of light vehicles with low cost, high fuel efficiency and higher payload. In this study, STRENX 700 CR and DP 800 materials were joined by resistance spot welding (RSW) method using both alternating current (AC) and medium frequency direct current (MFDC). Before the welding processes, the regional pre-heating (RPH) process was applied to the heat affected zone (HAZ) of the parts to be joined. The design, manufacture and adaptation of the RPH system to the RSW machine were carried out within the scope of the study. In addition, various fixtures have been designed and manufactured to provide a different concept to the classical RSW method in joining processes. Simultaneous operation of RPH and fixture systems was controlled by a computer aided software. After the joining processes, hardness, tensile-shear, cross-tensile and fatigue tests were applied to the RSW joints between STRENX 700 CR and DP 800 steels, and thus the mechanical properties of the joints were investigated. In addition, the microstructures of the joints were examined with an optical microscope. The relationship between mechanical properties and microstructure was demonstrated by these tests. As a result of microstructure studies, it was observed that the weld metal structure in all joints consisted of ferrite and martensite phases. In the hardness tests, it was determined that the RPH process applied to the HAZ before welding slightly decreased the hardness of the weld metal and increased both the weld metal and HAZ width. With the RPH process applied to HAZ before welding, an increase of 18% in tensile-shear strength and 13.5% in cross-tensile strength was found. In the tensile-shear and cross-tensile tests, all of the ruptures occurred in the form of buttoning from the HAZ. In the fatigue tests, the highest fatigue life was found in the samples which were applied RPH process and joined with MFDC. As a result of the studies, it was seen that the samples joined with OFDA technology exhibited better mechanical properties than those joined with AA."
Açıklama
Anahtar Kelimeler
STRENX 700 CR, DP 800, direnç nokta kaynağı, çekme-makaslama, çapraz-çekme ve yorulma., STRENX 700 CR, DP 800, resistance spot welding, tensile-shear test, cross-tension test and fatigue.
