• Proceedings of the KWS Conference
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• 2009.11a
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• pp.68-68
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• 2009

Recently, resistance spot welding using Inverter DC is applied to improve the weldability of high strength steel for the auto-body fabrication. In this study, inverter DC spot welding machine, which is developed in the domestic, was evaluated weldability of the galvanized and cold rolled dual-phase steel(tensile strength : 590MPa). The welding conditions (welding time, current and force) were decided by tensile-shear test, and welding strength and nugget size were analyzed by statistical analysis methods which involved dynamic resistance and indentation. The results of the statistical analysis was utilized for real-time estimation of the invisible nugget size and tensile strength. Moreover, it can be achieved without the conventional destructive testing of welds.

• Journal of Welding and Joining
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• v.26 no.5
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• pp.79-89
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• 2008

With the increased use of surface textured steel sheet in body-in-white assembly, resistance spot weldability of these steels is considered to be an important subject. This study evaluated nugget formation and weldability by measuring dynamic resistance with various weld pressure, current, and weld time for steel sheet with two different surface roughnesses. The surface roughness for T-H steel ($R_{a}=1.70\;{\mu}m$) was higher than that for T-L steel ($R_{a}=1.33\;{\mu}m$), and resulted in increased contact resistance and heating for T-H steel spot welding. Therefore, at low weld current and weld cycle ranges, the T-H steel showed better weldability over the T-L steel. The evaluations of weld interface showed that the fusion zone in the T-H steel sheet was continuous in contrast to discontinuous fusion zone for T-L steel sheet at the same welding conditions. A comparison of dynamic resistance and tensile-shear strength (TSS) between T-H and T-L steel sheet suggested that high surface roughness provided larger heating at early cycle of welding and larger TSS.

• Journal of Welding and Joining
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• v.21 no.2
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• pp.76-81
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• 2003

Due to the environmental problem, automotive companies are trying to reduce the weight of car body. Therefore, WP(Transformation Induced Plasticity) steels, which are hish strength and ductility have been developed. The application of TRIP steel to the members has been reported to increase the energy absorption capability. Welding process is a complex process; therefore deciding the optimal welding conditions is an effective method on the basis of the experimental data. However, using a trial-and-error method from the beginning in such a wide area, in order to decide the optimal conditions requires too many numbers of experiments. To overcome these problems and to decide the optimal conditions, response surface methodology was used. Response surface methodology is a collection of mathematical and statistical techniques that are for the modeling and analysis of problems in which a response of interest is influenced by several variables and the objective is to optimize this response. The introduced method was applied to the resistance spot welding process of the TRIP steel and the welding parameters were optimized. (Received December 6, 2002)

• Proceedings of the KIPE Conference
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• 2000.07a
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• pp.568-571
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• 2000

Resistance spot welding is widely employed in a manufacturing process. In recent years the requirement for more sophisticated quality control procedures has been in the mass production industries. The requirements for high productivity and good welding qualities have lead to the development of more widely available microprocessor or computer based control, In this paper the inverter-type power source and welding servo gun are developed.

• Journal of Welding and Joining
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• v.30 no.6
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• pp.68-73
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• 2012

Nowadays, car manufacturers tried to improve automotive fuel efficiency, and applied many high strength steels such as AHSS or UHSS to car bodies. Therefore, the number of steel combinations for the resistance spot welding are dramatically increased and the need for weldability evaluation of these combinations are also required. In this study, we suggest the lobe curve using FEM simulations for DP780 steel with 1.0t, 1.4t. The lobe curves which could expressed weldablity and optimal welding condition were obtained according to 6 steel combinations. There were two combinations for same steel sheet which were DP780 1.0t, DP780 1.4t. Dissimilar steel sheet combination with different thickness was 1.0t and 1.4t of DP780. Different steel combinations were DP780 1.0t and SPRC440 1.0t, and DP780 1.0t and DP590 1.0t. Finally dissimilar combinations was and DP780 1.0t and DP590 1.4t. The trend of low boundary and high boundary variation of lobe curve were analyzed with a viewpoint of the contact resistance and the heat input.

• The Transactions of the Korean Institute of Power Electronics
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• v.20 no.6
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• pp.491-497
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• 2015

This study proposes a new control method for a resistance spot welding inverter to improve weld quality. The proposed method is based on the dynamic resistance characteristics of steel sheets to be welded. A point in the second peak value of the dynamic resistance occurs during one shot of the welding current flow. A constant voltage control is applied from zero to the peak point, and a constant current control is adopted from the peak point to the end of the shot. The mixed mode control of the constant voltage and current guarantees high weld quality. Experiments are conducted with a 5 kA power supply and 0.5 mm steel sheets to compare quality. Experimental results show that weld quality is improved more than 10 times that of the conventional control method.

• Proceedings of the KWS Conference
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• 1990.11a
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• pp.82-86
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• 1990

A new method was investigated to measure the high current in resistance welding processes. A measuring unit was developed by using a strain gage attached on the outer surface of a steel ring. The steel ring was placed around a section of the secondary loop of the welding machine, and was deformed by electro-magnetic forces induced by the high welding current. The circumferential constituent of the ring deformation was then used to obtain a signal voltage proportional to the secondary welding current. The strain gage signal of ring deformation is enough to determine the welding current in resistance spot welding, especially when welded with direct current.

• The Transactions of the Korean Institute of Power Electronics
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• v.8 no.5
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• pp.397-406
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• 2003

Resistance spot welding is widely employed in a manufacturing process. In recent years, the requirement for more sophisticated quality control procedures has been in mass production industries. The requirements for high productivity and better weld qualities have lead to the development of more widely available microcomputer-based control. In this study, the inverter type power source and welding servo gun are developed. As the results, we have some advantages over the previous methods such as pneumatic gun. One of them is that the precise pressure control can be attained during the welding process. In addition, production time and cost can be decreased.

• Journal of Welding and Joining
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• v.30 no.2
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• pp.76-80
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• 2012

In this study, the effect of the welding current on the hardness characteristics and microstructure in the resistance spot welding of 1GPa grade cold-rolled DP steel was investigated, Also, correlation between the hardness and microstructure was discussed. In spite of the change in the welding current, the hardness distributions near weld was similar. the hardness in the HAZ and the fusion zone was higher than that of the base metal and the hardness in the fusion zone was variated with the location. Especially, the hardness of HAZ adjacent to the base metal showed maximum value, and softening zone in the base metal adjacent to HAZ was found. With the increasing of welding current, there were no difference in maximum hardness and average hardness in the fusion zone were, but the hardness of the softening zone reduced. The difference in the hardness in each location of weld due to grain size of prior austenite. The softening of the base metal occurred by tempering of the martensite.

• Transactions of the Korean Society of Automotive Engineers
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• v.13 no.4
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• pp.129-134
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• 2005

There is a strong industrial demand for the development of light-weight vehicle to improve fuel efficiency and dynamic performance. The effective method of achieving the weight reduction is to use low-density materials such as aluminum and magnesium. In applying these materials to the vehicle, it is often required to join dissimilar materials such as aluminum and steel. However, conventional joining method, namely resistance spot welding cannot be used in joining dissimilar materials. Self·piercing rivet(SPR) and adhesive bonding is a good alternative to resistance spot welding. In this study, the impact test of double hat-shaped member made by resistance spot welding, SPR and adhesive bonding was performed. As a result, various parameters of crashworthiness were analyzed and evaluated. Also, the applicability of SPR and adhesive bonding as an alternative to resistance spot welding was suggested.