• Laser Solutions
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• v.9 no.2
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• pp.1-9
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• 2006

In this paper, we investigated the characteristics of fatigue fracture on TB(Tailored Blank) weldment by comparing the fatigue crack propagation characteristics of base metal with those of TB welded sheet used for vehicle body panels. We also investigated the influence of center crack on the fatigue characteristic of laser weld sheet of same thickness. We conducted an experiment on fatigue crack propagation on the base metal specimen of 1.2mm thickness of cold-rolled metal sheet(SPCSD) and 2.0mm thickness of hot-rolled metal sheet(SAPH440) and 1.2+2.0mm TB specimen. We also made an experiment on fatigue crack propagation on 2.0+2.0mm and 1.2+1.2mm thickness TB specimen which had center crack. The characteristics of fatigue crack growth on the base metal were different from those on 1.2+2.0mm thickness TB specimen. The fatigue crack growth rate of the TB welded specimens is slower in low stress intensity factor range $({\Delta}K)$ region and faster in high${\Delta}K$ region than that of the base metal specimens. The slant crack angle slightly influenced the crack propagation of the TB specimen of 2.0+2.0mm thinkness.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1999.10a
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• pp.339-344
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• 1999

For the Tailor Welded Blank sheet used for automobile body panel, the characteristics of fatigue strength and crack propagation behavior were studied. The thickness of specimen was machined to be same (0.9mm+0.9mm) and different (0.9mm+2.0mm). As a base test, mechanical properties around welding zone were examined. The results indicated that there were no significant decreases in mechanical properties, but hardness around welding bead is 2.3 times greater than base material. The fatigue strength was the highest when the loading direction was parallel to the welding direction, which was about 85% of tensile strength of base material. It was decreased by 8.5% when the thickness of specimen and base material was different, and it is increased by 25% when pre-strain was applied. The crack propagation rate was noticeable decreased around welding line and rapidly increased as it passed by welding line. Reviewing the shape of the crack propagation, crack width around welding line was wide around the welding zone due to retardation of crack growth, but it became narrow passing welding line due to decreased toughness.

• Transactions of the Korean Society of Automotive Engineers
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• v.11 no.1
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• pp.143-150
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• 2003

Experimental research has been carried out to investigate the characteristics of the fatigue crack initiation and propagation behavior of Tailor Welded Blank(TB) sheet used for vehicle body. We used three types of specimens which were machined of the same base metal: one is 1.4mm thick, another is 1.6mm thick, and the third(TB specimen) is laser-welded of two specimens(1.4mm and 1.6mm thick ones). The results of tensile and hardness test indicate that the yield strength of the TB specimen is the highest, and the hardness around welding bead is higher than that of base metal. Fatigue strength and fatigue limit of the TB specimen are much superior to those of the base metal up to $10^6$ cycles. The fatigue crack propagation of the heat-affected zone of the TB specimen is slower than that of the base metal. Welding bead has the fastest crack Propagation in the low stress intensity factor range$(\DeltaK)$ region, but the slowest in the high $\DeltaK$ region. The fatigue propagation characteristic of the TB specimen is relatively stable in comparison with that of the base metal in the high ${\Delta}K$ region around over $28MPa\sqrt{m}$.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.249-253
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• 2003

In recent years, developments of light weight vehicle are one of the most important issues in automotive makers. New materials and new processes have been studied on the point of weight saving of chassis worldwide. Associated with materials, applications of high strength steel, aluminium, magnesium are being developed. On the point of new processes, tailored welded blank and hydroforming have been applied. In this paper, focusing to both material and process, we have applied hydroforming process to the engine cradle. In addition to that, three kinds of high strength steel have been applied to the development of light weight material for hydroforming. All the studies have been carried by FEM.

• Proceedings of the KWS Conference
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• 2002.10a
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• pp.606-611
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• 2002

Welding using lasers can be mass-produced in high speed. In the laser welding, performing real-time monitoring system of the welding quality is very important in enhancing the efficiency of welding. In this study, the plasma and molten metal which are generated during laser welding were measured using the UV sensor and IR sensors. The results of laser welding were classified into five categories such as optimal heat input, little low heat input, low heat input, partial joining due to gap mismatch, and nozzle deviation. Also, a system was formulated which uses the measured signals with a fuzzy pattern recognition method which is used to perform real-time evaluation of the welding quality and the defects which can occur in laser welding.

• Transactions of Materials Processing
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• v.15 no.4 s.85
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• pp.289-294
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• 2006

To reduce automobile parts weight, TWB(tailored welded blank) forming is widely used in panel forming. But products used TWB forming process have many defect, wrinkle, crack and springback. So study of TWB forming process character is very important. In this study one of the current problems of TWB forming was analyzed, especially for the try-out process of inner door panel without frame. A comparison was made between actual measurements and prediction of forming analysis for formability and springback. Also a new analysis die model which have additional plane on die surface was proposed to correct result of forming analysis. This proposed method overcomes the difference for TWB forming result between try-out and analysis.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.791-796
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• 2001

Welding using lasers can be mass-produced in high speed. In the laser welding, performing real-time evaluation of the welding quality is very important in enhancing the efficiency of welding. In this study, the plasma and molten metal which are generated during laser welding were measured using the UV sensor and IR sensor. The results of laser welding were classified into five categories such as optimal heat input, little low heat input, low heat input, focus off, and nozzle change. Also, a system was formulated which uses the measured signals with a fuzzy pattern recognition method which is used to perform real-time evaluation of the welding quality and the defects which can occur in laser welding. Weld quality prediction program was developed using previous weld results and statistical program which could show the trend of weld quality and signal was developed.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.10a
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• pp.132-137
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• 2004

In order to reduce automobile parts weight, TWB(Tailored welded blank) forming is widely used in the forming of car panel, such as door inner, side outer panels. In this study, one of the current problems of TWB forming was analyzed, especially for the try-out process of TWB door inner panel without frame. A comparison was made between actual panel measurements and results of forming analysis for formability and springback.

• International Journal of Korean Welding Society
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• v.1 no.1
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• pp.7-12
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• 2001

Real time monitoring has become critical as the use of laser welding increases. Plasma and spatter are measured and used as the signal for estimating weld quality. The estimating algorithm was made using the fuzzy pattern recognition with the area of data that is beyond the tolerance boundary. Also, an algorithm that detects the spatter and the localized defect was created in order to kd the partially produced pit and the sudden loss of weld penetration. These algorithms were used in quality monitoring of the $CO_2$ laser tailored blank weld. Statistical program that can display the laser weld quality result and the signal transition was made for the first stage of the remote control system.

• Proceedings of the KWS Conference
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• 2001.05a
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• pp.189-191
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• 2001

This study introduces the new way for the evaluation criteria of the Mash-Seam Tailored Blank weldability, The materials used are low carbon automotive galvanized and high strength steels and the evaluation of weldability are examined with various thickness. Welding tests were conducted for both similar thickness and dissimilar thickness cases. The criteria developed for optimum welding conditions were based on the relationship among results of die press forming test, weld transverse tensile test, Erichsen test and microhardness measurements. The application of the developed criteria(fracture ratio, strength ratio, etc) in obtaining optimum welding condition revealed that a weld which satisfied ant of the criteria did not fracture during actual die press test.