Otomotivde yaygın kullanılan saplama bağlantı elemanlarının kaynaklanabilirliğinin altı sigma metodu ile optimizasyonu
Optimization of weldability of stud joining elements commonly used in automotive with six sigma methodology
Yazar: UTKU CAN KUMRU
Danışman: DOÇ. DR. DERYA DIŞPINAR
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Malzeme Mühendisliği Ana Bilim Dalı / Malzeme Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Saplama  bağlantısı ile birleştirme yöntemi, endüstrinin birçok dalında yoğun şekilde kullanılan bir birleştirme yöntemidir. Saplamalar, komponent üzerine çeşitli elemanların montaj yapılmasını sağlayan vida görevi gören ve uygulamaya yönelik özelleştirilebilen bağlantılar olduğundan özellikle bu çalışmanın da yararlandığı sektör olan otomotiv sektörünün araç gövdesi parçaları üzerinde sıklıkla kullanılmaktadır. Seri üretime yatkın ve güvenilir bir yöntem olan saplama kaynağı, otomotiv sektörü haricinde de kazan yapımı, bina ve köprü yapımı, çiftlik ve endüstriyel ekipman imalatı, demiryolu ve vagon yapımı ve gemi yapımı gibi endüstriyel uygulamalarda tercih edilen bir birleştirme yöntemidir. Bunun yanında, askeri uygulamalarda kullanımı arasında füze konteynırları, zırhlı araçlar ve tanklar bulunmaktadır. Yöntem, seri üretime uygun bir ergitme kaynağı yöntemidir ve seri üretim esnasında kaynak parametrelerin hassas kontrolü büyük önem taşımaktadır. Birleştirme için seçilen kaynak parametreleri, saplama özellikleri ve birleştirilecek parçanın malzeme özellikleri, birleştirme sonrası birleştirme bölgesinin mekanik dayanımında önemli değişime sebep olduğundan dolayı birleştirme tasarımının dikkatli yapılmasını zorunlu kılmaktadır. Mekanik özelliklerdeki değişimi minimuma indirmek uygun kaynak prosedürü ile mümkün olmak ile birlikte, proje devreye alma süreci esnasında otomotiv gövdesi gibi çok sayıda malzeme ve saplama bağlantı çifti olan parçalarda her bir çift için kaynak prosedürü oluşturmak mümkün olmamakta ve optimizasyon için istatistiksel yöntemlere ihtiyaç duyulmaktadır.
"Otomotivde Yaygın Kullanılan Saplama Bağlantı Elemanlarının Kaynaklanabilirliğinin Altı Sigma Metodu İle Optimizasyonu" başlıklı bu çalışmada otomotivde kullanılan saplama ve malzeme çiftlerinin haritası çıkartılarak en yaygın kullanılan 8 adet deney  çifti belirlenmiştir. Deney çiftleri 5 farklı saplama sınıfı, 4 farklı sac malzeme sınıfı ve 2 farklı sac kaplama tipini içermektedir. Ardından, deney çiftleri üzerinden altı sigma yöntemi araçlarını kullanarak malzeme ve kaynak parametrelerinin bağlantının mekanik özelliklere olan etkilerinin incelenmesi amacıyla Minitab 18 istatiksel veri analizi yazılımı vasıtası ile cevap yüzey metodolojisi metodu ile istatistiksel analizler gerçekleştirilmiştir.
Deney tasarımı seti, belirlenen parametrelerden seri üretim koşullarında kontrol edilebilir olanlar ile yapılmış ve bu kapsamda üretim koşullarında 112 adet deney gerçekleştirilmiştir. Çekme testleri, veri analizleri ve deney tasarımı çalışmaları sonucunda bağlantının mekanik özelliklerini optimize edecek en ideal koşullar belirlenmiştir.
Cevap yüzey metodolojisi metodunun çıktısı olarak kaynak performansını değerlendirmek için ise, bir "çalışma penceresi analizi" yöntemi kullanılmıştır. Bu yöntem, her faktör için kabul kriterlerini belirler ve tüm çıktıların ilgili kriterleri karşıladığı bölgeyi (yeşil renk) ve herhangi bir çıktının ilgili kriterleri karşılamadığı bölgeyi (kırmızı renk) renkler ile haritalandırarak çıktıyı optimize eden faktörlerin dağılımını belirler. Söz konusu yöntem ile her bir deney çifti için kaynak akımı ve kaynak zamanını referans alan bir çalışma penceresi analizi yapılmış olup, elde edilen sonuç seri üretim şartlarında doğrulanmıştır.

Abstract:
The stud welding method is a joining method that is used extensively in many branches of the industry. Studs are frequently used on vehicle body parts of the automotive industry, which is the industry that this study also benefited from, as they serve as screws that allow the assembly of various elements on the component and can be customized for different applications. Stud welding, which is a reliable and prone method for mass production, is a preferred joining method in industrial applications such as boiler construction, building and bridge construction, farm and industrial equipment manufacturing, railway and wagon construction and shipbuilding in addition to the automotive industry. Besides, its uses in military applications includes missile containers, armored vehicles and tanks. The method is a fusion welding method suitable for mass production and precise control of welding parameters during mass production is of great importance. Since the welding parameters chosen for the joint, the stud properties and the material properties of the part to be joined cause a significant change in the mechanical strength of the joint, the joint design must be done carefully. Although it is possible to minimize the change in mechanical properties with the appropriate welding procedure, it is not possible to create a welding procedure for such an huge number of couples like automotive body parts during the project approval process, and because of that statistical methods are required for optimization.
In this study titled "Optimization of Weldability of Stud Joining Elements Commonly Used in Automotıve With Six Sigma Methodology", the most commonly used 8 type of welding couples were determined by mapping the stud and material used in automotive bodies. In the identification and measurement phase, first of all, the stud types and sheet metal classes to be used in the experiments were determined. The mentioned stud types and steel sheet material classes have been determined based on frequently used references in automotive.
According to the research conducted by CAR Research on 42 different automobiles and 14 parts used in these automobile parts, when the usage rates of different steel classes in automobile parts are examined, dual-phase (DP) steels are the most common among the high-strength steels developed with a usage rate of close to 20%. In conventional high strength steels (HSS), high strength-low alloy steels (HSLA) and bake hardenable steels (BH) are the most commonly used steel grades with 31%, followed by mild steels (MILD). While selecting steels for the welded couples, these classes were used.
With the six sigma approach, the identification phase was completed with the combination of the specified stud type and sheet metal material couples. As a result, 8 different experimental couples were created to be used in the next phase, the measurement phase. The welding couples include 5 different stud classes, 4 different sheet material classes and 2 different sheet metal coating types.
With the SIPOC analysis, the start and end limits of the process were determined and a road map was created for the next step. Inputs include geometric and material properties of studs and sheet metal parts, and welding process parameters. In addition, the cleanliness of the weld area is also considered as an input parameter. The outputs consist of a visual evaluation of the quality of the weld fusion area, the strength of the weld seam after the tensile test, and the evaluation of fracture area whether the joint has fractured from the weld or the base metal after the tensile tests.
The inputs listed in the SIPOC analysis were divided into three main classes as noise inputs, process parameters and standard operations, and the parameters to be used for the design of experiment were determined.
Then, statistical analyzes were carried out with the response surface methodology method using the Minitab 18 statistical data analysis software in order to examine the effects of material and welding parameters on the mechanical properties of the joining using six sigma tools on the welding couples.
The design of experiment (DOE) set was made with the parameters that can be controlled under mass production conditions and within this scope, 112 tests were carried out under production conditions. As a result of tensile tests, data analysis and DOE studies, the ideal conditions to optimize the mechanical properties of the joints have been determined.
In order for the parts to be welded, first of all, sample preparation studies were carried out. Plates of 100 x 100 mm were created by supplying and cutting sheet metal material classes. Stud welding was performed on the plates according to the experimental design for each welding couples.
A "process window analysis" method was used to evaluate the output of the response surface methodology. This method determines the acceptance criteria for each factor and determines the distribution of factors that optimize the output by mapping the region (green color) where all outputs meet the relevant criteria and the region (red color) where any output does not meet the relevant criteria. With the aforementioned method, an operating window analysis based on welding current and welding time was made for each welding couples, and the results obtained were confirmed under mass production condition.
When the response surface analysis was performed on the data determined to be statistically significant, curvature was observed in the experimental pairs of "A", "B", "E", "F", "H" and it shows that the second order effect is strong. It was observed by the curvature of the response surface of each contour that the optimum level between inputs and outputs was not at the extremes of the experimental range but within. Therefore, it was concluded that in order to obtain an optimum welding quality, the current strength and welding time from the input variables should be adjusted in a balanced way depending on each other. When the C, D and G experimental pairs were analyzed, no curvature was observed in the model, indicating that the quadratic effect was not strong. Optimum levels between inputs and outputs were seen at the extremes of the experimental range.
According to the findings from the above examinations, our results showed that welding current is the most important factor on the strength of the weld zone, besides, welding time is another important factor and the best results are obtained with a balanced optimum adjustment of current and time. Although we consider the lift height and plunge amount factors as fixed factors, the fact that high "R2" data are obtained allows it to be evaluated that the effect of these factors is low. The generalizability of the results for the lift height and plunge amount factors is limited to the R2 values and is excluded from the scope of this study.
Besides the tensile loads, the weld area was visually inspected for spatter formation. For each test, the heat input during welding was measured, and a visual inspection was made on the welding area of each test sample. After the correlation analysis, a positive correlation was clearly seen between the heat input and spatter formation, besides, it was observed that spatter formation started when the heat input levels of 400 joules and above were increased regardless of the experimental pairs. In heat inputs between 200 and 400 joules, spatter formation was observed to vary according to the material couples.