• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.330-335
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• 2002

Resistance spot welding is one of the most widely used processes in sheet metal fabrication. Quality assurance of welding has been important to increase the productivity. In this study, weld quality estimation using primary circuit dynamic resistance applied to the in-process real-time systems. For quality estimation, factors relating to quality were extracted from the dynamic resistance, measured in the timer. The relationship between these factors and weld quality was determined through a artificial neural network model. This method has the advantage over the conventional one, such as obtaining the quality information without the use of extra devices.

• Journal of Welding and Joining
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• 제18권3호
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• pp.114-121
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• 2000

Since resistance spot welding has become one of the most popular sheet metal fabrication processes, a strong emphasis is being put on the quality of the welds. Throughout the years many quality estimation systems have been developed by many researchers to ensure weld quality. In this study, the process variables, which were monitored in the primary circuit of the welding machine, are used to estimate the weld quality with Hopfield neural network. The primary dynamic resistance is vectorized and stored as five patterns in the network. As the welding is done, the dynamic resistance patterns are recognized and the quality is estimated with the proposed method. Due to the primary process variables, it is possible to utilize this algorithms as an in-process real time quality monitoring system.

• Corrosion Science and Technology
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• 제11권1호
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• pp.29-36
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• 2012

The resistance spot welding of high strength steel degrades the weldability because of its high strength with rich chemical composition and coating layer to protect from corrosion. During the each resistance welding process the electrodes tip reacts with coating layer, then subsequently deteriorates and shorten electrode life. In this study, the Al-coated HPF (Hot Press Forming) steels and Zn-coated TRIP steels were used to investigate the electrode life for resistance spot welding. Experimental results show that the reactivity of Al-coating on HPF steels to electrode tip surface behaviors different from the conventional Zn-coated high strength steels. The electrode tip diameter and nugget size in electrode life test of Al-coated HPF steels are observed to be constant with respect to weld numbers. For Al-coated HPF steels, the hard aluminum oxide layer being formed during high temperature heat treatment process reduces reactivity with copper electrode during the resistance welding process. Eventually, the electrode life in resistance spot welding of Al-coated HPF steels has the advantage over the galvanized steel sheets.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 1988년도 한국자동제어학술회의논문집(국내학술편); 한국전력공사연수원, 서울; 21-22 Oct. 1988
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• pp.370-373
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• 1988

Althouah there have been many attempts to control weld quality in resistance spot welding processes, design method for an on-line feedback controller based upon process dynamics has not been suggested. This is due to the fact that the resistance spot welding is a highly complicated process, whice involves the interaction of electrical, thermal, mechanical and metallurgical phenomena. In this paper, an optimal control method based on FDM model with shunt effect is presented, which can regulate the nugget size, at the same time minimizing the control heat input. Optimal PI gain of the controller were determined by numerical optimization. Simulation results show that, as a result of the proposed optimal control, the weld nugget can be made to approach a desired nugget size with less control heat input than that required for the conventional spot welding process in the face of the shunt effect.

• Journal of Welding and Joining
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• 제31권1호
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• pp.43-50
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• 2013

The spot welds of Transformation Induced Plasticity (TRIP) steels are prone to interfacial failure and narrow welding current range. Hard microstructures in weld metal and heat affected zone arenormally considered as one of the main reason to accelerate the interfacial failure mode. There fore, detailed observation of weld microstructure for TRIP steels should be made to ensure better weld quality. However, it is difficult to characterize the microstructure, which has similar color, size, and shape using the optical or electron microscopy. The atomic force microscope (AFM) can help to analyze microstructure by using different energy levels for different surface roughness. In this study, the microstructures of resistance spot welds for AHSS are analyzed by using AFM with measuring the differences in average surface roughness. It has been possible to identify the different phases and their topographic characteristics and to study their morphology using atomic force microscopy in resistance spot weld TRIP steels. The systematic topographic study for each region of weldments confirmed the presence of different microstructures with height of 350nm for martensite, 250nm for bainite, and 150nm for ferrite, respectively.

• 대한금속재료학회지
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• 제50권2호
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• pp.136-145
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• 2012

Resistance spot welding of TRIP780 steels was investigated to enhance understanding of weld fracture mode after tensile shear testing (TST) and L-shape tensile testing (LTT). The main failure mode for spot welds of TRIP780 steels was partial interfacial fracture (PIF). Although PIF does not satisfy the minimum button diameter (4${\surd}$t) for acceptable welds, it shows enough load carrying capacity of resistance spot welds for advanced high strength steels. In the analysis of displacement controlled L-shape tensile test results, cracks initiated at the notch of the faying surface and propagated through the interface of weldments, and finally, cracks change path into the sheet thickness direction. Use of the ductility ratio and CE analysis suggested that the occurrence of PIF is closely related to high hardness and brittle welds, which are caused by fast cooling rates and high chemical compositions of TRIP steels. Analysis of the hold time and weld time in a welding schedule demonstrated that careful control of the cooling rate and the size of a weld nugget and the HAZ zone can reduce the occurrence of PIF, which leads to sound welds with button fractures (BFs).

• Journal of Welding and Joining
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• 제33권2호
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• pp.47-55
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• 2015

With using adaptive control of the resistance spot welding machine, the advantage on electrode life time for galvanized steels has been addressed. This study was aimed to evaluate the electrode life time of galvanized steels with applying the constant current control and the adaptive control resistance spot welding process for a comparison purpose. The growth in diameter of electrode face was similar for both the constant current and the adaptive control up to 2000 welds. The button diameter was decreased with weld numbers, however, sudden increase in button diameter with use of the adaptive control after 1500 welds was observed. The peak load was continuously decreased with increasing number of welds for both the constant current and the adaptive control. The current compensation during a weld was observed with using the adaptive control after 1800 welds since the ${\beta}$-peak on dynamic resistance curve was detected at later weld time. The current compensation with adaptive control during resistance spot welding enhanced the nugget diameter at the faying interface of steel sheets and improved the penetration to thinner steel sheet.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2007년 추계학술발표대회 개요집
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• pp.199-202
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• 2007

The center floor is a part of underbody of cars. The assembling method is mostly a resistant spot welding. But the resistant spot welding has many demerits in productivity such as low welding speed, large spatter occurring, hard to verify the welding quality. The laser welding has many merits such as high welding speed, high static and dynamic weld strength. So the laser welding has been studied to substituted the resistant spot welding. Therefore, in this study, as a pre-evaluation stage for replacing the resistance spot welding with laser in production of automobile parts, the weldability of cold rolled and zinc coated steel sheets is studied and also evaluated the weld strength by the tensile shear test. And the shape of weld bead appearances and micro structure of the weld bead section is also evaluated.

• 대한기계학회논문집
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• 제10권5호
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• pp.661-669
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• 1986

In this study, the resistance spot welding process of an aluminum alloy was analyzed through the numerical simulation including the electric contact resistance and the heat generation in the electrode. The finite element model was used to solve the electro-thermal responses in weld cycles. The resistance of the contact area was represented as the contact element modeling, but the thermal resistance between the contact surfaces was neglected. Welding tests of Alclad 2024-T3 aluminum alloy were made not only to get the input data for the numerical simulation, but also to compare the numerical results. The contact resistance was determined initially by the contact resistance tests and assumed to decay exponentially up to the solidus temperature. The temperature distributions and dynamic resistance obtained numerically were in good agreement with the experimental results. Numerical results revealed that nugget growth depends mainly on the heat generated in the workpiece and its contact area. The heat generated in the electrode has, however, only a little effect on the nugget growth, and the heat generation in the electrode-workpiece interface is initially high but decrease repidly.

• 한국산학기술학회논문지
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• 제19권3호
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• pp.52-60
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• 2018

최근 자동차 산업에서는 연비향상 및 안전규제 강화에 따라 차량 경량화가 필수적으로 요구됨에 따라 DP강(Dual Phase steel), CP강(Complex Phase steel), MS강(Martensitic Steel), TRIP강(Transformation Induced Plasticity steel), TWIP강(Twinning Induced Plasticity steel) 등과 같은 인장강도 700MPa 이상인 초고장력강(Ultra High Strength Steel)의 적용이 증가하고 있다. 초고장력강을 차체에 적용하기 위해서는 용접공정이 필수적이며, 원가 측면에서 유리한 전기저항점용접(Resistance Spot Welding, RSW)이 차체 용접에서 80%이상으로 가장 많이 적용되고 있다. 초고장력강은 강도향상을 위해 합금원소 함량을 늘이기 때문에 일반적으로 용접성이 열악한 것으로 알려져 있다. 이러한 초고장력강의 저항점용접의 경우 적정 용접조건 영역이 축소되고 용접부에서 계면파단 및 부분계면파단이 발생하는 것으로 보고되어 있어 결함 및 품질을 실시간으로 예측할 수 있는 용접품질 판정 연구가 활발히 진행되고 있다. 이에 따라 본 연구에서는 저항 점 용접을 수행할 때 검출되는 2차 회로 공정 변수를 이용하여 용접부의 동저항을 모니터링하고, 이 동저항 패턴에서 용접 품질 판단에 필요한 인자들을 추출하였다. 추출한 인자들을 상관분석하여 용접 품질과의 상관성을 파악하였으며, 상관성이 높은 인자들을 이용하여 회귀분석을 실시하였다. 이를 근거로 현장 적용이 가능한 회귀 모델을 제시하였다.