Kaynak akım şiddetinin kaynak elektrot ömrüne etkisinin incelenmesi
Investigation of the effect of welding current intensity on welding electrode life
Yazar: ERHAN HALİT ÇEBİ
Danışman: PROF. DR. SALİM ASLANLAR
Sakarya Uygulamalı Bilimler Üniversitesi / Lisansüstü Eğitim Enstitüsü / İmalat Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Makine Mühendisliği = Mechanical Engineering ; Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Otomotiv sektöründe geçmişten beri yeterli mukavemet ve uzama değerlerine sahip yüksek mukavemetli çelikler (HSS) tercih edilmektedir. Otomobillerde yapısal parçalar yüksek çarpma direnci gerektirir, belirli parçalarda bu durum malzeme kırılma tokluğuna ve sac kalınlığına bağlıdır. BIW (metal parçaların birbirine kaynatılarak gövdeyi oluşturmuş hali) araçlardaki en ağır ve en büyük yapıdır. Orta büyüklükte bir binek otomobilin toplam ağırlığının yaklaşık %20 ila 30'unu oluşturur. BIW kütlesini yüksek mukavemet kullanarak azaltmak, önemli ölçüde ağırlık tasarrufu ve gelişmiş çarpışma direnci sağlayabilir. Bu tür uygulamaların örnekleri, darbe kirişleri, B-Pillar takviyeleri ve tampon çubuklardır. HSS, düşük karbonlu çeliklerin mukavemet ve ağırlık oranını iyileştirmek için geliştirilmiştir. Mikro alaşımlama, eritme uygulaması, ısıl işlem veya bunların bir kombinasyonu ile daha yüksek mukavemetler elde edilir. HSS için bu limitler, 255-550 MPa akma mukavemet aralığındadır. Araç ağırlığındaki azalma ise, bir aracın yakıt ekonomisine katkıda bulunmaktadır. Fakat kullanılan çeliklerin kaynaklanması esnasında tercih edilen elektrotların ömrüne dair çalışmalar devam etmektedir. Bu çalışma da fabrika koşullarında çinko kaplamalı HSLA sac malzemenin nokta kaynağı ile birleştirilmesi 300 kez atılarak yapılmıştır. Kaynak prosesi esnasında değişken akım şiddeti altında ve sabit sürelerde denenerek en uygun koşullarda üretimin gerçekleştirilmesi sağlanmıştır. Kaynak denemeleri sonrası elektrodun boyutsal ölçümleri (elektrot boyunda kısalma) incelenerek, elektrotların ömürleri hakkında bilgi edinilmiştir. Sac malzemedeki kaynak çapının boyutsal ölçümü, mikroyapısal incelemeler ve kesitten alınan mikrosertlik yardımıyla kaynak sertliği ile çökme hakkında bilgi edinilmiştir. HX340LAD+Z malzemesinde meydana gelen sıçrantıların tespiti SEM-EDS ve EDX çalışmaları ile ortaya çıkarılmıştır. Boyutsal ölçümler sonucunda kaynak bölgesi ve elektrot yüzeyinin F tipi elektrot ucu ile yapılan deneylerde G tipi elektrot ucu ile yapılan deneylere göre daha iyi sonuç verdiği açıkça görülmektedir. Çekirdek iç ve dış çap farkının artması ısı girdisinin arttığını göstermektedir. SEM-EDS ve EDX çalışmaları ile çok ciddi oranda çinko birikiminin veya yüzeylere transferinin gerçekleştiğini işaret etmektedir. F tipi elektrot ile yapılan denemelerde elementel dağılım genel olarak homojen olmakla beraber yüzeydeki gözeneklerin içerisinde çinkonun bulunması yüzeyde çinko buharının oluşması ve buharlaşma ile beraber gözeneklerde hapsolması neticesinde, yoğunlaştığı ve biriktiği düşünülmektedir. G tipi elektrodun mikrosertlik üzerine etkisi oldukca belirgindir. Sertlik değerleri elektrot tipine göre G, B ve F sırasıyla ortalama olarak artmaktadır. Deneyler ve analizler neticesinde belli bir süre sonunda hangi elektrodun daha verimli olduğu belirlenmiştir.
Anahtar Kelimeler: Direnç Nokta Kaynağı, HSLA, Kaynak Akım Şiddeti, Elektrot Ömrü.

Abstract:
HSLA Steels as a term refers to high strength low alloy steels. Steels in HSS class are preferred due to their high strength.
High Strength Low Alloy Grades. HSLA steels have small amounts of alloying elements such as phosphorus, manganese or silicon added to low carbon (0.02 - 0.13% C) steels to achieve high strength levels. In these steels, higher strength is obtained by rapid cooling to form a very fine ferrite grain size. The solid solution is strengthened with phosphorus, nitrogen, silicon and manganese. Formation of carbides or carbonitrides occurs with vanadium, nickel, and titanium. These steels have better mechanical properties, corrosion resistance and weldability than mild steels [1].
HSS has been developed to improve the strength / weight ratio of low carbon steels. Higher strengths are obtained by micro alloying, melting application, heat treatment or a combination of these. HSS covers a wide variety of properties and strength levels. For HSS, these limits are in the yield strength range of 255-550 MPa.
Using HSS instead of mild steel in an automotive body structure can reduce its weight by about 40% without sacrificing performance. Overall, a 10% reduction in weight can increase a vehicle's fuel economy by about 8% [1].
HSS and AHSS are more difficult to bend than plain carbon steels due to their higher yield strength and lower ductility. This requires more strength, greater bending radius, more mold cavity, and more springback. It may be necessary to remove cutting burrs in the bending area and to straighten corners. Whenever possible, the bending axis should be perpendicular to the direction of rolling. If the bending axis must be parallel to the rolling direction, it may be necessary to use cross rolled material depending on the severity of the bending. Due to the high ratio of yield strength to elastic modulus of HSS and AHSS, a greater amount of springback occurs in the shaped part compared to soft steel.
Structural parts in automobiles require high impact resistance, in certain parts this condition depends on the material fracture toughness and sheet thickness. Most of the structural components in current production are made of steel sheets with a tensile strength of 270 to 540 MPa. A major problem in using higher strength steels for automotive applications is the increased springback of formed parts. For a constant elastic modulus, springback increases with an increase in the yield strength of sheet metal. BIW (body-in-white is the stage in automobile manufacturing where the frame of a car body is assembled, i.e. before painting.) is the heaviest and largest structure in vehicles. It accounts for about 20 to 30% of the total weight of a medium-sized passenger car. Reducing the white body mass using high strength can provide significant weight savings and improved crash resistance. Examples of such applications are impact beams, B-Pillar stiffeners and bumper bars.
The quality of the pressed parts is critical to avoid problems in assembly and final product performance. Quality problems; Formability problems such as excessive compression or splitting, wrinkling caused by excessive stress, and dimensional accuracy problems caused by spring-back caused by elastic recovery. It has been found that controlling metal flow into the mold cavity is crucial for both part quality and dimensional accuracy. Various alternative mold processes are being evaluated for HSS. These focus primarily on better process control and various ways of inducing shape-adjusted tension in pressing. These processes include programmable pot rim to change the force-strike trajectory, draining rods to increase the confining force as the press approaches the sub-base, and flexible coupler technology for local control of the binder area. Adjusting the binding force in the process can provide better quality and improved part consistency.
Resistance spot welding, which is mentioned in the literature as Resistance Spot Welding (RSW), is applied by joining metals by applying pressure. In the method, current is passed through the bonding area for a long time. With the resistance of the welded materials to the current, heating occurs and melting and therefore the weld pool are formed. The weld pool formation is almost stopped by the application of current and pressure is applied and the combination process is completed with cooling under this pressure. The heat generated by time between metal plates is a product of the total resistance, square density of the current and the duration of the welding. Several parameters describe the resistance spot welding process, while the time and available force describe the welding process. The surfaces of sheet metal and the size, shape and thickness of the electrodes greatly affect the weld quality. It is possible to achieve the preferred core diameter by repositioning the welding time perfectly against the current density.
In this study, zinc coated HSLA sheet metal material was joined by spot welding 300 times under factory conditions. During the welding process, it was ensured that production was carried out under the most suitable conditions by testing under variable current intensity and at fixed times. After the welding trials, the dimensional measurements of the electrode (shortening of the electrode length) were examined and information about the life of the electrodes was obtained. With the help of dimensional measurement of the weld diameter in sheet material, microstructural studies and microhardness taken from the cross-section, information about weld hardness and collapse was obtained. Detection of spatter in HX340LAD + Z material was revealed by SEM imaging and EDS studies. As a result of experiments and analysis, it was determined which electrode was more efficient after a certain period of time.
First of all, the HX340 LAD + Z coded sheet materials, which are the basis of the study, were supplied from ERDEMİR A.Ş. in the form of zinc-coated rolls, and the slicing was carried out to YILDIZ KALIP A.Ş. It was carried out in-house. Test certificate of the product obtained from ERDEMİR A.Ş. By choosing HSLA steel as the material, the effect of zinc coating on spot welding has also been investigated.
The sample was cut in the laser cutting device to make the cut product dimensions of 1500 x 900 x 60 mm ready for spot welding. The areas of the sheet plates to be spot welded are divided into sections and made ready for welding. Three different electrode tip forms with a fixed diameter of 16 mm, which are mostly used in the automotive industry and in our factory, have been selected and spot welding has been started.
The parameters used during spot welding of the sheets with the above features are the current (A) provided that the electrode tip form and the welding current application time are constant (ms). Before proceeding to the application experiments, preliminary tests were made on the sheet according to the spot resistance welding electrode tip types, and the parameters that could occur without spatter and adhesion were determined. Experimental studies were grouped and applications were carried out at different currents according to the electrode type. Application values / ranges of preferred parameters were found during spot welding. Experiments were carried out on a 120 kVA spot resistance welding machine from Mactera.
After cutting sheet metal samples containing three hundred electrode tips and 300th spot welding, the laboratories of the Technology and Application Research Center (Afyon Kocatepe University) and Afyon Kocatepe University, Faculty of Technology, Metallurgy and Materials Engineering Department are primarily abrasive. It was cut with cut stone and then polished with bakelite. For the polishing process, starting from 220G and using sandpaper up to 1200G, the surfaces were sanded and then it was brought to a mirror brightness by using 1 µm Alumina powder. After the polishing process, the microstructure was revealed by etching with 1% Nital. N-point source cross-sectional images were carefully taken using Olympus brand optical microscopes and then sent with devices in TUAM and Technology Faculty Research Laboratories for Microhardness measurement. While microstructure imaging was performed with SEM-LEO 1430 VP, additionally, element analysis was performed with OXFORD EDX device. Microhardness measurements of the sheet material taken from the welding interruption, the load applied in SHIMADZU HMV-2 device is 50 g, the time the load is applied is 10 seconds. As a result of microhardness measurements made in triplicate, evaluation was made on the average value. By making dimensional measurements of the sheet material and the electrode, information was obtained about the collapse in the sheet material and the wear on the electrode. In Figure 2.3, a visual about how dimensional measurements are made is presented.
Although there are no serious changes in the welding electrode before and after welding, it is seen that the F type electrode is exposed to the most shape change when the change in size or decrease in the amount of increase is observed. Due to the shape of the G type electrode, rather than having a more flat protrusion and having a more balanced pressing surface as in the B type electrode, the narrowness of the area that facilitates deformation and reduces the core thickness compared to others, can play an important role in the shape change. Further wear or decrease in size of lower electrodes is associated with increased heat input.
Unlike the F and G type electrodes, the high amount of corrosion may be that the electrode is completely zinc coated, allowing the formation of CuZn compound and more easily exposed to mass erosion. Although no investigation has been made in this regard, the absence of Copper on B type electrode surfaces and the presence of only Zinc in very high amounts indicates this direction. It is seen that Copper is encountered for G Type electrode, but due to the low amount of this amount, it is seen that a serious Copper transfer does not occur. Low Copper transfer, when considered in the other two electrode profiles, is a very important factor in reducing electrode wear. Although the elemental distribution is generally homogeneous, it is thought that it condenses and accumulates as a result of the presence of Zinc in the pores on the surface and the formation of zinc vapor on the surface and entrapment in the vaporization pores.
As a result of the physical and microstructural examinations of the welding area and electrode surface, the following conclusions have been reached. In the experiments with F type electrode tip, it is clearly seen that it gives better results compared to the experiments made with the G type electrode tip, which gives good results. Accordingly, it is subject to deformation after a while due to heat accumulation. Weight loss may occur from the electrode material as a result of alloying with Zinc occurring at the welding of steels. The effect of G type electrode on microhardness is quite obvious. The hardness values of the electrode type G, B and F increase on average, respectively.
Keywords: Resistance Spot Welding, HSLA, Welding Current, Electrode Life.