Sung-Min Joo

Redistributions of Welding Residual Stress for CTOD Specimen by Local Compression

주성민 * ·윤병현 * ·장웅성 * ·방한서 ** ·방희선 **,† ·노찬승 *** * 포항산업과학연구원 용접센터 ** 조선대학교 선박해양공학과 *** 조선이공대학 선박해양·기계공학과 Abstract When conducting CTOD test, especially in thick welded steel plate, fatigue pre-cracking occasionally failed to satisfy the requirements of standards thus making the test result invalid. Internally accumulated residual stress of test piece has been thought as one of the main reasons. The propagation of fatigue crack, started from the tip of machined notch, which might have propagated irregularly due to residual stress field. To overcome this kind of difficulty three methods to modify the residual stress are suggested in standard i.e. local compression, reverse bending and stepwise high-R ratio method. In this paper not only multi pass welding but also local pre-compressing process of thick steel plate has been simulated using finite element method for clarifying variation of internal welding residual stress. The simulated results show that welding residual stress is compressive in the middle section of the model and it is predominantly increased after machining the specimen. Comparing as-welded state all component of the welding residual stress changing to compressive in the tip of machine notch whereas residual stress of the outer area remain as tensile condition relatively. Analysis results also show that this irregular residual stress distribution is improved to be more uniformly by applying local compression.

Laser Stitch Welding Technology for the Fabrication of Automotive Parts

Abstract Nowadays, the weight lightening of automotive is required as conserving the environment has become a major worldwide issue. To solve this issue, various researches for the use of light materials(Alalloy, Mgalloy)andultra high strength steel as substitutes of the current structural material have been carried out. Application of laser stitch welding to the assembly of automotive produces improvement in strength, lightening of body, higher fuel efficiency, lower production cost as well as reduction in assemble line due to its fast welding speed, superior accessible and weld quality. This process overcomes the shortcomings of the current resistance spot welding such as high electricity consumption, electrode replacement, and economical, technical limitation in design and production method of automotives. Key Words : Laser stitch welding, Weight lightening, Weld quality, Automotives 1. 서 론 최근 자동차의 경량화 및 고성능화를 위한 연구 개발의 가속화와 병행하여 차체의 고강도화, 경량화, 생산성 향상 기술 등이 지속적으로 연구 개발되고 있다. 자동차 산업에서 정밀도가 높은 제품과 부품의 제조과정에 고 에너지 열원의 레이저빔 정밀용접공정 사용이 해외에서 점점 확대되고 있으며, 레이저 빔 용접 기술은 정밀제품의 생산성을 향상시키는 중요한 공정으로 제품의 품질을 결정하는 요인으로 인식되고 있다

A Study on the Prediction of the Optimal Welding Condition for Automotive Steel Sheets in MAG Welding Process

Abstract The optimization of the welding parameters was studied to maximize the weldability and minimize the amount of spatter in the MAG welding of automotive steel sheets under different shielding gas composition ratio. JS-EFSC, JS-SPHC steel plates and Ar mixture gases were used as a substrate and shielding gas for welding respectively. The five welding parameters were selected through preliminary experiments and their effects on the weldability were analyzed. Experiments were performed using the Taguchi experimental method. As results, appropriate range for welding could be achieved. Amount of spatter in 80%Ar+20%CO 2 shielding gas was 20% of that of CO 2 welding. Therefore, in terms of high productivity and welding cost, Ar mixture gas(80%Ar+20%CO 2 ) was recommended as a shielding gas for application of MAG process, indicating the low spatter and good weld quality. Key Words : High productivity, Welding cost, Spatter, Ar mixture gas, CO 2 gas, MAG welding, Automotivesteel sheets